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THE MICEOSCOPE

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FRONTISPIECE.

X6

X490

X77

X140

X138

THE

MICROSCOPE

AND

ITS REVELATIONS

^ / BY THE LATE

0/
WILLIAM B. CAfePENTEE, C.B., M.D., LL.D., F.R.S.

EIGHTH EDITION

IN WHICH THE FIRST SEVEN AND THE TWENTY-THIKD CHAPTERS HAVE

BEEN ENTIRELY REWRITTEN, AND THE TEXT THROUGHOUT

RECONSTRUCTED, ENLARGED, AND REVISED

BY THE EEV.

W. H. DALLINGER, D.Sc, D.C.L., LL.D., F.R.S., &c.

WITH XXII PLATES AND NEARLY NINE HUNDRED WOOD ENGRAVINGS

LONDON
J. & A. CHUECHILL

7 GEEAT MARLBOROUGH STREET

1901

All rights reserved

PEEFACE

Although no changes of so important a character as those which
distinguished the Vllth Edition of this book from the editions
that had preceded it have been necessitated, yet a thorough and
complete revision of the entire text has been made, and everything
of importance to Microscopy which has transpired in the interval
has been noted. This applies to the theory of the microscope as
well as to its use.

We have adopted a classification of microscopes that we hope
may be of value to many in the purchase of a stand, especially as we
also point out with pleasure the great and successful eflfbrts which
English, Continental, and American makers have made within the
last few years to supply good and useful microscopes at a greatly
reduced price.

Invaluable aid and suggestion have been given me by my friend
Mr. E. M. ISTelson, ex-President of The Royal Microscopical
Society, to whom my thanks are due. Mr. Arthur Bolles
Lee has rendered unique service in the section dealing with the
Preparation and Mounting of Objects ; and to Prop. E. Crookshank
I am indebted for valuable and useful help. In the matter of the
Application of the Microscope to Geological Investigation the
Rev. Prof. T. Bonney, F.R.S., has been, fortunately, my valued co-
adjutor. On the subjects of Micro-ciystallisation, Polarisation, and
Molecular Coalescence, I have received the expert advice and help
of Mr. W. J. Pope, F.I.C, F.C.S., &c., Chemist to the Goldsmiths'
Technical Institute, whose large practical knowledge of this depart-
ment of chemistry is widely known.

For the valued help of Prof. A. W. Bennett, M.A., B.Sc,
Lecturer on Botany at St. Thomas's Hospital, and of Prof. F.
Jeffrey Bell, M.A., Professor of Comparative Anatomy and
Zoology, King's College, London, I have, as in the former Edition,
to make my appreciative acknowledgments.

It is hoped that this Edition may, as its predecessors have done,
prove of practical help to many in understanding the scientific use
of the microscope.

W. H. DALLINGER.

London : Maech 1901.

Erratum. — Page 333, eleventh line from the bottom, read 'Plate IV.' not III.

PREFACE

TO

THE SEVENTH EDITION

The use of the Microscope, both as an instrument of scientific research
and as a means of affording pleasure and recreative instruction, has
become so widespread, and the instrument is now so frequently found
in an expensive form capable of yielding in skilled hands good
optical results, that it is eminently desirable that a treatise should
be within the reach of the student and the tiro alike, which would
provide both with the elements of the theory and principles involved
in the construction of the instrument itself, the nature of its latest
appliances, and the proper comditions on which they can be em-
ployed with the best results. Beyond this it should provide an
outline of the latest and best modes of preparing, examining, and
mounting objects, and glance, with this purpose in view, at what is
easily accessible for the requirements of the amateur in the entire
organic and inorganic kingdoms.

This need has been for many years met by this book, and
its six preceding editions have been an extremely gratifying evidence
of the industry and erudition of its Author. From the beginning
it opened the right path, and afforded excellent aid to the earnest
amateur and the careful student.

But the Microscope in its very highest form has become — so far
at least as objectives of the most perfect constrviction and greatest
useful magnifying power are concerned — so common that a mvicb
more accurate account of the theoretical basis of the instrument
itself and of the optical apparatus employed with it to obtain the
best results with ' high powers ' is a want very widely felt.

The advances in the mathematical optics involved in the con-
.sti'uction of the most perfect foi-m of the jji-esent Mici'oscope have
been very rapid dni-ing th(i last twenty years ; and the pi'ogress in
the principles of practical consti'uction and the application of theory

PEEFACE TO THE SEVENTH EDITION vii

has, even since the last edition of this book was published, been so
marked as to produce a revolution in the instrument itself and in its
application. The new dispensation was dimly indicated in the last
edition ; but it has effected so radical a change in all that apper-
tains to Microscopy that a thorough revision of the treatment of
this treatise was required. The great principles involved in the
use of the new objectives and the interpretation of the images pre-
sented by their means, are distinct and vmique ; and unless these be
clearly understood the intelligent use of the finest optical appliances
now produced by mathematical and practical optics cannot be
brought about. They have not rendered the use of the instrument
more difficult— they have rather simplified its employment, provided
the operator understand the general nature and conditions on
which his Microscope should be used. If the modern Microscope be,
as a mechanical instrument with its accompanying optical ajjparatus,
as good as it can be, a critical image — a picture of the object having
the most delicately beautiful character — is attainable with ' low
powers ' and ' high powers ' alike. Microscopists are no longer
divisible into those who work with ' high powers ' and those who
work with ' low powers.' ISTo one can work properly with either
if he does not understand the theory of their construction and the
principles upon which to interpret the results of their employment.
If he is familiar with these the employment of any range of magni-
fying power is simply a question of care, experiment, and practice ;
the principles applicable to the one are involved in the other. Thus,
for example, a proper understanding of the nature and mode of
optical action of a ' sub-stage condenser ' is as essential for the very
finest results in the use of a 1-inch object-glass as in the use of a
2 mm. with N.A. 1-40 or the 2-5 mm. with N.A. 1-60, while it
gives advantages not otherwise realisable if the right class of con-
denser used in the right way be employed with the older -^-^ih inch
or -j^sth inch achromatic objectives, and especially the j^th inch
and ^\y^h inch objectives of Powell and Lealand, of N.A. 1'50.
Without comparing the value of the respective lenses, the best
possible results in eveiy case will depend upon a knowledge of the
nature of the instrument, the quality of the condenser required by
it, and its employment upon right principles.

This is but one instance out of the whole range of manipulation
in Microscopy to which the same principles apply.

In its present form, therefore, a treatise of this sort, preserving
the original idea of its Author and ranging from the theory and
construction of the Microscope and its essential apparatus, embracing
a discussion of all their principal forms, and the right use of each, and
passing to a consideration of the best methods of preparation and

viii PEEFACE TO THE SEVENTH EDITION

mounting of objects, and a review of the whole Animal, Vegetable,
and. Inorganic Kingdoms specially suited for microscopic purposes,
m.ust be essential^ a cyclopaedic work. This was far more possible
to one man Avhen Dr. Carpenter began his work than it was even
when he issued his last edition. But it is practically im.possible
now. It is with Microscopy as with every department of scientific
work — we must depend upon the specialist for accurate knowledge.

In the following pages I have been most generously aided. In
no department, not even that in which for twenty years I haA^^e
been specially at work, have I acted without the cordial interest,
suggestion, and enlightenment afibrded by kindred or similar workers.
In every section experts have given me their imstinted help.
To preserve the character of the book, however, and give it homo-
geneity, it was essential that all should pass through one mind and
be so presented. My work for many years has familiarised me,
more or less, with every department of Microscopy, and with the
great majority of branches to which it is applied. I have therefore
given a common form, for which I take the sole responsibility,
to the entire treatise. The subject might have been carried ovei-
ten such volumes as this ; but we were of necessity limited as
to space, and the specific aim has been to give such a condensed
view of the whole range of subjects as would make this treatise
at once a practical and a suggestive one.

The first five chapters of the last edition are represented in this
edition by seven chaptei'S ; the whole matter of these seven chapters
has been re-wi-itten, and two of them are on subjects not treated in
any former edition. These seven chapters represent the experience
of a lifetiihe, confirmed and aided by the advice and practical help
of some of the most experienced men in the world, and they may be
i-ead by any one familiar with the use of algebraic symbols and the
practice of the rule of three. They are not in any sense abstruse,
and they are everywhere practical.

In the second chapter, on The Principles and Theory of Vision
with the Compound Mici-oscope, so much has been done dui-ing the
past twenty years by Dr. Abbe, of Jena, that my first desire was to
induce him to summarise, for this treatise, the results of his twenty
years of unremitting and marvellously productive labour. But the
state of his lieulth and his many ol>ligations forliade this ; Jind at
length it 1)ec;inie ap[)arent that if this most desiral)l(^ end wei'e to
be secured, ] mu.st re-study witli this ol)ject all the monographs of
tliis author. I summarised them, not without iuixiety ; but that wns
speedily removed, for Dr. Ahbe, with gieat g(fiier()sity, consented to
examine my r(!sults, and has been good enough to write that he has
* read [niy] clear expositions with tlie greatest intei'est ; ' and, after

PEEFACE TO THE SEVENTH EDITION IX

words which show his cordial fi'ieiidliness, he says : ' I find the whole
. . . much more adequate to the purposes of the book than I should
have been able to write it. ... I feel the gi'eatest satisfaction in
seeing my views represented in the book so extensively and inten-
sively.'

These words are more than generous ; but I quote them hei'e
in order that the reader may be assured of the accuracy and
efficiency of the account given in the following pages of the invalu-
able demonstrations, theories, and explanations presented by Dr.
Abbe on the optical principles and practice upon which the recent
improvement in the construction of microscopical lens systems has
so much depended.

It will not be supposed that I implicitly coincide with eveiy
detail. Dr. Abbe is too sincere a lover of independent judgment
to even desire this. But it was important that his views as such
should be found in an accessible English form ; in that form I have
endeavoured to jjresent them ; and in the main there can be no
doubt whatever that these teachings are absolutely incident with
fact and experience. In details, as may appear here and there in
these pages, especially where it becomes a question of practice, I may
differ as to m.ethod, and even interpretation, from this distinguished
master in Mathematical Optics. But our differences in no way affect
the great principles he has enunciated or the comprehensive theory
of microscopical vision he has with such keen insight laid down.

In preparing the remainder of the seven new chapters of this
book I have sought and, without hesitancy, obtained advice and
the advantage of the supj)ort of my own judgment and experience
from many competent men of science, who have shown a sincere
interest in my work and have aided me in my endeavoui'S. But
first on the list I must place my friend Mr. E. M. Nelson. Our
lines of expei-ience with the Microscope have i-un parallel for many
years, although the subjects of our study have been wholly different ;
but the advantages of his suggestion, confirmation, and help have
been of constant and inestimable value to me. He placed his know-
ledge, instruments, and experience at my disposal, fully and without
limit or condition ; and his exceptional skill in Photo-micrography
has enabled me to add much to the value of this book.

To Coimt Castracane I am indebted for valuable suggestions
regarding the Diatomacese, to be used at my discretion ; to Dr.
VAN Heurck I am also under much obligation for his coui'tesy in
preparing Plate XI. of this book, giving some of his photo-micro-
graphic work with the new object-glass of 2*5 mm. N.A. 1'60.
The full description of this plate is given, with some critical remarks,
in the General Description of Plates. To the late and deeply

X PREFACE TO THE SEVENTH EDITION

lamented Dr. H. B. Brady, F.R.S., I am tmcler obligation for
valuable suggestions regarding the Foraminifera.

From Dr. Hudson I have received cordial aid in dealing with
his special subject, the Rotifera ; and to Mr. Albert Michael I am
imder equal obligation for his assistance in regard to the Acarina.

Mr. W. T. Suffolk gave me his most welcome judgment and
advice regarding my chapter on Mounting, and I received also the
suggestions of Mr. A. Cole with much pleasure and advantage.
I have received help from Dr. A. Hill, of Downing College,
Cambridge, and from Professor J. IST. Langley, of Trinity College,
Cambridge — from both of whom special processes of preparation
for histological work were sent.

Mr. Frank Crisp, with characteristic generosity, aided me much
by suggestions of special and practical value ; and Mr. John Mayall,
jun., the present Secretary of the Royal Microscopical Society, has
been untiring in his willingness to furnish the aid which his influence
was able to secure.

To Professor W. Hicks, F.R.S., Principal of Firth College,
Sheiiield, I am indebted for the revision of special sheets ; so also I
owe acknowledgments to Dr. Henry Clifton Sorby, F.R.S., and to
Dr. Groves, as well as to others, whose suggestions, advice, or con-
firmation of my judgments have been much esteemed ; and prominent
amongst these are Professor Alfred W. Bennett, B.Sc, and Professor
F. Jeffrey Bell, M.A., whose constant advice in their departments
of Biology I have received throughout ; while in Micro-geological
subjects I have been aided by the suggestions and experience of
Professor J. Shearson Hyland, D.Sc.

It will be observed that every endeavour has been made to
bi'ing each of the many subjects discussed in this book into conformity
with the most recent knowledge of experts. Many of the sections,
in fact, have been wholly rewritten and illustrated from new and
original sources ; this may be seen in the sections on the History
as well as the Construction and Use of the Microscope and its appli-
ances, as also in those on Diatomacepe, Desmids, Saprophytes,
Bacteria, Rotifera, Acarina, and in the chapters on Microscopic
Geology and Minei-alogy. To the same end nineteen new plates
have been prepared and 300 additional woodcuts, many of which
are also new, and for the iise of the majoiity of those which are
not so, I am indebted to the Editors smd Secretary of tlie Royal
Microscopical Society,

There certainly never was a time when the Mici'oscope was so
gener-ally used as it now is. With many, as already stated, it is simply
an insti'iimont emplf)y<!d foi- elegant and insti'tictive j-elaxation and
amu.sement. For this tliei'e can be nothing but connnendation, l)ut it is

PREFACE TO THE SEVENTH EDITION xi

desirable that even this end should be sought intelligently. The social
influence of the Microscope as an instrument employed for recreation
and pleasure will be greater in proportion as a knowledge of the
general principles on which the instrument is constructed are known,
and as the principles of visual interpretation are understood. The
interests of these have been specially considered in the following
pages ; but such an employment of the Microscope, if intelligently
pursued, often leads to more or less of steady endeavour on the part
of amateurs to understand the instrument and use it to a piirpose
in some special work, however modest. This is the reason of the
great increase of ' Clubs ' and Societies of various kinds, not only in
London and in the provinces, but throughout America ; and these
are doing most valuable work. Their value consists not merely in the
constant accumvilation of new details concerning minute vegetable
and animal life, and the minute details of larger forms, but in the
constant improvement of the quality of the entire Microscope on its
optical and mechanical sides. It is largely to Amateui- Microscopy
that the desire and motive for the great improvements in object-glasses
and eye-pieces for the last twenty years are due. The men who have
compared the qualities of respective lenses, and have had specific ideas
as to how these could becom.e possessed of still higher qualities, have
been comparatively rarely those who have employed the Microscope
for professional and educational purposes. They have the rather
simply ibsecl — employed in the execution of their professional work
— the best with which the practical optician could supply them.
It has been by amateur microscopists that the opticians have been
incited to the production of new and improved objectives. Biit it
is the men who work in our biological and medical schools that
ultimately reap the immense advantage — not only of greatly im-
proved, but in the end of greatly cheapened, object-glasses. It is
on this account to the advantage of all that the amateur micro-
scopist should have within his reach a handbook dealing with the
principles of his instrument and his subject.

To the medical student, and even to the histologist and patho-
logist, a treatise which deals specifically with the Microscope, its
principles, and their application in practice, cannot fail, one may
ventui'e to hope, to be of service.

This book is a practical attempt — the result of large experience
and study — to meet this want in its latest form ; and I sincerely
desire that it may prove useful to many.

W. H. DALLINGER.

London : 1891.

CONTENTS

CHAPTER PAGE

I. ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS .. . 1
II. THE PRINCIPLES AND THEORY OF VISION WITH THE COM-
POUND MICROSCOPE 36

III. THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE . . 117

IV. ACCESSORY APPARATUS 270

V. OBJECTIVES, EYE-PIECES, THE APERTOMETER .... 353

VI. PRACTICAL MICROSCOPY : MANIPULATION AND PRESERVATION

OF THE MICROSCOPE ........ 397

VII. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS . . 438

VIII. MICROSCOPIC FORMS OF VEGETABLE LIFE — THALLOPHYTES . 530

IX. FUNGI 633

X. MICROSCOPIC STRUCTURE OF THE HIGHER CRYPTOGAMS . 665

XI. OF THE MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS . 684

XII. MICROSCOPICAL FORMS OF ANIMAL LIFE — PROTOZOA . . 726

XIII. ANIMALCULES^INFUSORIA AND ROTIFERA .... 763

XIV. FORAMINIFERA AND RADIOLARIA 795

XV. SPONGES AND ZOOPHYTES 855

XVI. ECHINODERMA .......... 884

XVII. POLYZOA AND TUNICATA . . 904

XVIII. MOLLUSCA AND BHACHIOPODA ....... 919

XIX. WORMS 943

XX. CRUSTACEA . 957

XXI. INSECTS AND ARACHNIDA 972

XXII. VERTEBRATED ANIMALS 1017

XXIII. APPLICATION OF THE MICROSCOPE TO GEOLOGICAL INVESTIGA-
TION ........... 1066

XXIV. CRYSTAI-LISATION, POLARISATION, MOLECULAR COALESCENCE . 1094

1137

EXPLANATION OF PLATES

FEONTISPIECE

Fig. 1. X 6 diameters. Horizontal and transverse section of an orbitolite.
Fig. 2. An imperfect or uncritical image of the minute hairs on the lining
membrane of the extremity of the proboscis of the blow-fly x 510 diams., taken
with a Zeiss apochromatic 5 -inch objective of '95 N.A. x 3 projection eye-piece ;
but it was illuminated by a cone of small angle, viz. of O'l N.A., and illustrates
the unadvisability of snaall cones for illumination.

The first obvious feature in the picture is the doubling of the hairs which
are out of focus ; but the important difference lies in the bright line with a
dark edge round the hairs which are precisely in focus. This is a diffraction
effect which is always present round the outlines of every object illuminated
by a cone of insufficient angle. Experiment shows that this diffraction line
always ceases to be visible when the aperture of the illuminating cone is equal
to about two-thirds the aperture of the objective used : but it will become
again distinctly apparent when the aperture of the cone is reduced less than
half that of the objective.

Fig. 3. X 510 diams. A correct or critical image of the minute hairs on the
lining membrane of the extremity of the blow-fly's proboscis. In this picture
the focus has been adjusted for the long central hair. It will be observed that
this hair is very fine and spinous ; it has not the ring socket which is common
to many hairs on insects, but grows from a very delicate membrane, which in
the balsam mount is transparent. This photograph was taken with a Zeiss
apochromatic -J of -95 N.A. x 3 projection eye-piece. The illumination v/as
that of a large solid axial cone of '65 N.A. from an achromatic condenser, the
source of light being focussed on the object.

Fig. 4. Section of cerebellum of a lamb, x 77 diams., by apochromatic 1-inch
•3 N.A. This preparation was courteously supplied to the present Editor by Dr.
Hill, whose imbedding and staining processes for these tissues it beautifully
illustrates.

Fig. 5. Amphipleura pellucida x 1860 diams., by apochromatic ^ 1*4 N.A.
illuminated by a very oblique pencil in oire azimuth along the valve.

Fig. 6. A hair of Polyxenus lagurus, a well-known and excellent test object
for medium powers x 490 diams. by apochromatic j '95 N.A.

Fig. 7. A small vessel in the bladder of a frog, prepared with nitrate of
silver stain, showing endothelium cells, x 40 diams., by Zeiss A. -2 N.A. This
object has been photographed for the purpose of exposing the fallacy which
underlies the generally accepted statement that ' low-angled ' glasses are the
most suitable for histological purposes. The supposition that it is so has
been founded on the fact that the penetration of a lens varies inversely as its
aperture ; therefore, it is said, a ' low-ahgied ' glass is to be preferred to a
wide-angled one, because 'depth of focus,' which is supposed to enable one to
see into tissues, is the end in view.

On carefully examining this figure it will be noticed that it is almost
impossible to trace the outline of any particular endothelium-cell because its
image is confused with that of the lower side of the pipe. In a monocular
microscopical image a perspective view does not exist ; it is better, therefore, to
use a wide-angled lens, and so obtain a clear view of a thin plane at one time,
and educate the mind to appreciate solidity by means of focal adjustment. It
will be admitted that unless one approaches fig. 7 with a preconceived idea of

xiv EXPLANATION OF PLATES

what an endothelium-cell is like, the knowledge gained of it will be small
indeed.

Eig. 8 represents the same structure, x 138 diams., by an apochromatic
f -65 N.A. Here only the upper surface of the pipe is seen, so that the out-
line of the endothelium-cells can be clearly traced. The circular elastic tissue
is also displayed. There is, moreover, an increased sharpness over the whole
picture, due to the greater aperture of the objective.

PLATE I

Eig. 1. The inside of a valve of Pleurosigma angulatum, showing a
' postage-stamp ' fracture, x 1750 diams., with an apochromatic ~ 1*4 N.A. by
Mr. T. F. Smith, and illustrating his view of the nature of the Pleurosigma
valve.

Fig. 2. The outside of a valve of Pleurosigma angulatum, showing a dif-
ferent form of structure, x 1750 diams., with an apochromatic ~ 1'4 N.A. by
Mr. T. F. Smith. These two photo-micrographs demonstrate the existence of at
least two layers in the angulatum.

Fig. 3. Coscinodiscus asteromphalus, x 110 diams., with an apochromatic
1-inch -3 N.A.

Fig. 4. A portion of the preceding, x 2000 diams., to show the lacework
inside the areolations. This lacework is believed to be a perforated structure,
as a fracture passes through the markings. In the central areolation there
are forty-six smaller perforations surrounded by a crown of fifteen larger ones.'
Photographed with an apochromatic ^ 1-4 N.A.

Fig. 5. Aulacodiscus Kittonii, x 270, by an apochromatic 1-inch -3 N.A.

Fig. 6. A small portion in the centre of an Aulacodiscus Sturtii, x 2000,
by an apochromatic ^ 1-4 N.A. Broadly speaking, the difference between the
Coscinodisci and the Aulacodisci lies in the fact that in the former the
secondary structure is inside the primary, while in the latter it is exterior to it.
This definition, however, is not strictly accurate, as it is believed that the fine
perforated structure covers the entire valve, it being only optically hidden by
the primary structure.

The whole of these demonstrations were photographed for the present
Editor by his friend E. M. Nelson, Esq., and have been reproduced from the
negatives by a process of photo-printing.

PLATE II. (Facing p. 274)

ARRANGEMENT OF THE MICROSCOPE WITH A STAND FOR THE MICROMETER
EYE-PIECE, TO SECURE STEADINESS AND ACCURACY IN MEASUREMENT

PLATE III. (Facing p. 286)

ARRANGEMENT OF THE MICROSCOPE AND ACCESSORIES FOR THE EMPLOY-
MENT OF THE CAMERA LUCIDA

PLATE IV. (Facing p. 334)

THE METHOD OF USING THE SILVER SIDE REFLECTOR OR PARABOLOID

PLATE V. (Facing p. 410)

METHOD OF USING DIRECT TRANSMITTED LIGHT M^ITHOUT THE
EMPLOYMENT OF THE MIRROR

Pla'J'ks II. to V. are engraved from photographs, taken at the request of
the Editor by Mr. E. M. Nelson, from the arranged instruments.

1 A section of this diiiiom will In; IohikI i)i (-lie TrtiiiHwIidnfi of tlui Coimty of
Middlesex Natural Hislor/j Sdciclij for IW.), I'latc; 1. fig. '2.

EXPLANATION OF PLATES XV

PLATE VI. (Facing p. 550)

SEXUAL GENERATION OF VOLVOX GLOBATOR. (After Cohll)

Fig. 1. Sphere of Volvox globator at the epoch of sexual generation : a,
sperm-cell containing cluster of antherozoids ; a', sperm-cell showing side-
view of discoidal cluster of antherozoids ; a^, sperm cell whose cluster has
broken up into its component antherozoids ; a*, sperm-cell partly emptied by
the escape of its antherozoids ; 6 b, flask-shaped germ-cells showing great
increase in size without subdivision ; 6-, 6'', germ- cells with large vacuoles in
their interior ; 6^, germ-cell whose shape has changed to the globular.

Fig. 2. Sexual cell, a, distinguishable from sterile cells, b, by its larger
size.

Fig. 3. Germ-cell, with antheroids swarming over its endochrome.

Fig. 4. Fertilised germ-cell, or o5sphere, with dense envelope.

Fig. 5. Sperm-cell, with its contained cluster of antherozoids, more
enlarged.

Figs. 6, 7. Liberated antherozoids, with their flageUa.

PLATE VII. (Facing p. 553)

OSCILLARIACE^ AND SCYTONEMACE^

Fig. 1. Lyncjbya cestuarii, Lieb. x 160.

Fig. 2. Spirulina Jenneri, Ktz. x 400.

Fig. 3. Tolypothrix cirrhosa, Carm. x 400.

Fig. 4. Oscillaria insignis, Thw. x 400.

Fig. 5. 0. Frolichii, Ktz. x 400.

Fig. 6. 0. teuerrima, Ktz. x 400.
These figures are after Cooke.

PLATE VIII. (Facing p. 554)

DESMIDIACE^, RIVULARIACE^, AND SCYTONEMACE^

Fig. 1. Zygosperm of Micrasterias denticulata, Breb. (After Ealfs.)

Fig. 2. Cosmariwn Brebissonii, Men. (After Cooke.)

Fig. 3. Euastntm 2xctinahtm, Bxeh. (After Ralfs.)

Fig. 4. Zygosperm of Siaurastrum hirsutum, Breb. (After Ealfs.)

Fig. 5. S. gracile, Ralfs. (After Cooke.)

Fig. 6. Xanihidiimi aculeatum, Ehrb. (After Ealfs.)

Fig. 7. Bivularia dura, Ktz. (After Cooke.)

Fig. 8. B. dtcra, Ktz. x 400. (After Cooke.)

Fig. 9. Seijtonema nutans, Breb. x 400. (After Cooke.)

Fig. 10. Staurastrwn liirsutum, Breb. (After Cooke.)

PLATE IX. (Facing p. 580)

DESMIDIACE^

Fig. 1. Micrasterias crux-inelitensis, Ehrb. (After Cooke.)

Fig. 2. Clost&rium setaceum, Ehrb. (After Cooke.)

Fig. 3. Desmidium Sioartzii, Ag. (After Cooke.)

Fig. 4. Penium digitus, Ehrb. (After Cooke.)

Fig. 5. P. digitus, Ehrb. (transverse view).

Fig. 6. SpirotcBnia condensata, Breb. (After Cooke.)

Fig. 7. Docidium baculum, Breb. (After Cooke.)

Fig. 8. Gonatozijgon Brebissonii, De Bary, conjugating. (After Cooke.)

PLATE X. (Facing p. 598)

PLEUROSIGMA ANGULATUM
This is a direct photo-micrograph, taken by Dr. R. Zeiss, as magnified 4900
diameters. We direct attention specially to it as giving evidence of the pre-
sence (however originated) of the intercostal markings, which may be seen
with considerable clearness on the right-hand side of the midrib and in the
middle of the valve.

xvi EXPLANATION OF PLATES

PLATE XL (Facing p. 594)

This plate has a twofold purpose. It is designed, first, to justify the
opinions held by Dr. Henry van Heurck upon the structure of the valves of
diatoms, and also to show how the usual microscopical tests present them-
selves when examined with the new objective with N.A. 1-60, lately constructed
by the firm of Zeiss. This objective is believed by Dr. van Heurck to realise
what he considers the highest results of photographic optics, which in his
judgment could only be surpassed by finding a new immersion liquid of still
higher refractive index presenting all the necessary qualities, and which at the
same time would not affect the very delicate flint of which it is necessary to
make the front lens of this objective. This medium he hopes may be some day
realised. Unfortunately, up to this time, no indication permits us to foresee
the discovery of the liquid desired.

The following is the way in which Dr. Henry van Heurck summarises his
ideas upon the structure of the valve : —

1. The valve of diatoms ' is formed by two membranes or thin plates and
by an intermediate septum. By this he understands a plate pierced with
openings. The superior membrane, often very delicate, may be destroyed
in the treatment by acids in the washings, by rubbing, &c. It is possible also
that it sometimes only exists in a very rudimentary state. The majority of
the students of diatoms a.gree in believing that these membranes may be suf-
ficiently permeable to permit of exchange by endosmose between the contents
of the valve and the surrounding outer water, but that these membranes have no
real openings so long as the diatom is living and intact.

2. When the openings of the septum are disposed in alternate rows, then
they take an hexagonal form. When in perpendicular rows then the openings
are square or elongated. The hexagonal form, which is besides so frequent in
nature, seems to be the typical form of the openings of the septum, and it
is found most frequently when the valve is large, destitute of consolidated
sides, and must offer resistance to outside agents. Even in the forms of the
square openings we see very frequently deviations and returns to the hexagonal
type upon certain parts of the valve. It is possible that the septa may be
sometimes composed of many layers, placed one above another, formed succes-
sively and closely united ; but up to this time we have no proof of it, neither
have we met with any form presenting layers placed one above another.

Such, ia brief, is the view held by Dr. van Heurck as an interpretation of
our present knowledge of the structure of the valve of the diatoms. We give
now a description of the objects represented on the plate.

Figs. 1, 2, .3. Ampliipleura pelliicida, Kiitz, 1 and 2, valve resolved into
pearls. Fig. 2 x 2000 diams. Fig. 1 x 3000 diams. Fig. 3. Valve resolved
in striai at about 2300 diams.

Fig. 4. Amphipleura LiiidJieimeri, Gr., x 2500 diams.

Fig. 5. Pleitrosigma ancjulatwn, in hexagons, x (about) 10,000 diams.

Fig. 6. Idem x 2000 diams., illusory pearls which are formed by the angles
of the hexagonal cells when the focussing is not perfect.

Fig. 7. The nineteenth band of Nobert's test plate. This photo-micro-
graph has been made exceptionally with the apochromatic j^ of 1-4 N.A.
The lines being traced upon a cover in crown-glass, the objective of N.A. l-{\
cannot be used here.

Fig. 8. Surirella ijevwia, Ehrb. x (about) 1000 diams.

Fig. 9. Van lleurclda cras^inerris, Lreb. (Frustulia saxonica, Rabh) x 2000
diams.

All the photo-micrographs (except fig. 7) have been done witli tlie new -{j-,-
inch N.A. l-fiO of MM. Zeiss.

These micro-pliotographs have been produced by sunlight in a monochro-
matic form, the special compensating eye-piece 12, and the Abbe condenser of
N.A. 10.

> 'The Structure of the Vtilve of Diiitoms' in llrcmU of llic Jichi'nni Sorir/ij,
V. xiii. IH'.Hi.

EXPLANATION OF PLATES xvii

Covers and slides in flint of l-7'2 ; diatoms in a medium 2-4.

We are bound, however, to note that the condenser used is not corrected in
any way ; its aberrations are enormous. Although the highest admiration must
be expressed for the skill exercised by Dr. van Heurck in these remarkable
photo-micrographs, and the highest esteem for his courtesy to the present Editor
in supplying them, it must not be forgotten that Dr. van Heurck was obliged
to employ an imperfect condenser — a condenser absolutely uncorrected — and
although we can testify to the high quality and fine corrections of at least one
of the lenses of N.A. 1-6, we are convinced that much of its real perfection
in image-forming is destroyed by uncorrected sub-stage illumination. Upon
the corrections and large aplanatic area presented by the condenser and its
careful and efficient employment depends entirely the nature of the image
presented by the finest objective ever constructed ; and as the perfection of the
objective, with a high amplification and a great aperture, is more nearly
approached, the more dependent are we upon perfect corrections in the con-
denser to bring out the perfect image-forming power of the objective. No
image formed by such an objective as that possessing N.A. 1-60 can be consi-
dered reliable until a condenser corrected for all aberrations like the objective
itself is produced ; and so convinced are we of the possible value of this objec-
tive that we trust its distinguished deviser and maker may be soon induced to
produce the condenser referred to.

If, then, by the aid of the chemist we can discover media which will be
of sufficiently high refractive index, and still tolerant or non-injurious to
organic tissues immersed in it, a new line of investigation may be open to
histology and pathology. —W. H. D.

PLATE XII. (Facing p. 597)

ARACHNOiDiscus JAPONicus. (After E. Beck)

The specimens attached to the surface of a seaweed are represented as
seen under a jth objective, with Lieberkuhn illumination : A, internal
surface ; B, external surface ; C, front view, showing incipient subdivision.

PLATE XIII. (Facing p. 651)

BACTERIA, SCHIZOMYCETES, OR FISSION FUNGI

1. Cocci singly and varying in size. 2. Cocci in chains or rosaries (strepto-
coccus). 3. Cocci in a mass (staphylococcus). 4 and 5. Cocci in pairs
(diplococcus). 6. Cocci in groups of four (merismopedia). 7. Cocci in packets
(sarcina). 8. Bacterium termo. 9. Bacterium termo x 4000 (Dallinger and
Drysdale). 10. Bacteriuvi septicmmics hcemorrliagicce. 11. Bacterium xoneu-
vionicE crouposcB. 12. Bacillus subtilis. 13. BacilUis imirisepticus. 14.
Bacillus cliphtlierice. 15. Bacillus tyx^hcsus (Eberth). 16. Spirillum unclula
(Cohn). 17. Spirillum volutans (Cohn). 18. Spirillum cholerce Asiaticce.
19. Spirilhim Obermeieri (Koch). 20. Spirochceta plicatilis (Fliigge). 21.
Vibrio rugula (Prazmowski). 22. Claclothrix Forsteri (Cohn). 23. Claclothrix
diclwtoma (Cohn). 24. Monas Okenii (Cohn). 25. Monas Warmingii (Cohn).
26. Rhabclomonas rosea (Cohn). 27. Spore-formation {Bacillus alvei). 28.
Spore-formation [Bacillus aiithracis). 29. Spore-formation in bacilli cultivated
from a rotten melon (Frankel and Pfeiffer). 30. Spore-formation in bacilli
cultivated from earth (Frankel and Pfeiffer). 31. Involution-form of Crenotlirix
(Zopf). 32. Involution-forms of Vibrio serpens (Warming). 33. Involution-
forms of Vibrio rugxda (Warming). 34. Involution-forms of Clostridium
polymyxa (after Prazmowski). 35. Involution-forms of Spirillum cholerce
Asiaticce. 36. Involution-forms of Bacterium aceti (Zopf and Hansen).
37. Spirulina-form of Beggiatoa alba (Zopf). 38. Various thread-forms of
Bacterium merismopedioides (Zopf). 39. False-branching of Cladothrix (Zopf).

a

XVlll EXPLANATION OF PLATES

PLATE XIV. (Facing p. 664)

PURE-CULTIVATIONS OF BACTERIA

Fig. 1. In the depth of Nutrient Gelatine. A pure-cultivation of Koch's
comma-bacillus (Spirillum cholerce Asiatic^') showing in the track of the
needle a funnel-shaped area of liquefaction enclosing an air-bubble, and a
white thread. Similar appearances are produced in cultivations of the comma-
bacillus of Metchnikoiif.

Fig. 2. 071 the surface of Nutrient Gelatine. A pure-cultivation of Bacillus
typhosus on the surface of obliquely solidified nutrient gelatine.

Fig. 3. On the surface of Nutrient Agar-agar. Pure-cultivation of Bacillus
indicus on the surface of obliquely solidified nutrient agar-agar. The growth
has the colour of red sealing-wax, and a peculiar crinkled appearance. After
some days it loses its bright colour and becomes purplish, like an old cultiva-
tion of Micrococcus prodigiosus.

Fig. 4. On the sttrface of Nutrient Agar-agar. A pure-cultivation obtained
from an abscess {Staphylococcus pyogenes aureus).

Fig. 5. On the surface of Nutrient Agar-agar. A pure-cultivation obtained
from green pus [Bacillus pyocyaneus). The growth forms a whitish, transparent
layer, composed of slender bacilli, and the green pigment is diffused throughout
the nutrient jelly. The growth appears green by transmitted light, owing to
the colour of the jelly behind it.

Fig. 6. On the surface of Potato. A pure-cultivation of the bacillus of
glanders on the surface of sterilised potato.

PLATE XV. (Facing p. 756)

COMPLETE LIFE-HISTORIES OF TWO SAPROPHYTES

(Drawn from nature by Dr. Dallinger)

PLATE XVI. (Facing p. 763)

The various stages of the development of the nucleus in two saprophytic
organisms, as studied with recent homogeneous and apochromatic objectives
both in the several stages of fission and genetic -fusion, indicating karyoki-
nesis, and proving, as established in detail by the text, that all the steps in
the cyclic changes of these unicellular forms are initiated in the nucleus before
being participated in by the whole body of the organism. (Drawn from nature
by Dr. Dallinger.)

PLATE XVII. (Facing p. 792)

ROTIFERS

Fig. 1. Floscularia campanulata.
Fig. 2. Stcphanoccros Eichhornii
Fig. 3. Melicerta ringens.
Fig. 4. Pedalion mirtmi (side view).
Fig. 5. P. mirum (dorsal view, showing muscles).
Fig. 6. Copeus cerherus (side view).

Fig. 7. Philoclina aculeata (side view, corona expanded).
Fig. 8. Male of Pedalion mirum.

All these figures, save fig. 2, are reduced to scale from the beautiful plates
in Hudson and Goss's Piotifera.

PLATE XVIII. (Facing p. 7U7)

KOKAMXNIFERA

Fig. 1. Miliulina semimdum (a and b, lateral aspects).

Fig. 2. AlveoUna Boscii {a, lateral aspect ; b, longitudinal section).

EXPLANATION OF PLATES xix

Fig. 3. Astrorhiza liniicola (a, lateral aspect ; b, portion of the test more
highly magniiied, showing structure).

Fig. 4. Haliphysema Tumanoioiczii, showing the pseucTo-polythalamous foot.

Fig. 5. Ibid, (group of specimens in situ).

Fig. 6. Haplojphragmium agglutinans {a, lateral aspect ; &, longitudinal
section).

Fig. 7. H. nanwn {a, superior aspect; b, peripheral aspect).

Fig. 8. Textularia gravien {a, lateral aspect ; b, oral aspect).

Fig. 9. T. gramen (peripheral aspect).

Fig. 9a. Pavonina fiabelliformis {a, lateral aspect ; b, oral aspect).

Fig. 10. Bulminia spinulosa.

Fig. 11. Chilostomeila ovoiclea (a and b, lateral aspects ; c, specimen
mounted in Canada balsam and seen with transmitted light).

PLATE XIX. (Facing p. 799)

FORAMINIFERA

Fig. 12. Lagena sulcata.
Fig. 13. L. sulcata.
Fig. 14. L. sulcata.

Fig. 15. i. sttZcato (a, lateral aspect ; &, oral aspect).
Fig. 16. Noclosaria raphanus.

Fig. 17. Crisiellaria calcar (a, b, c, lateral aspects).
Fig. 18. Ramulina globulifera.
Fig. 19. R. globulifera.

Fig. 20. Globigerina bulloides (var. triloba, pelagic specimen).
Fig. 21. G. bulloides {a b, c, adult typical shell).
Fig. 22. Rotalia Beccarii.
Fig. 23. Polystomella craticulata.

Fig. 24. Ampliistegina Lessonii {a, superior lateral aspect ; b, inferior lateral
aspect ; c, peripheral aspect).

Fig. 25. Nummulites Icevigata [b, lateral aspect ; c, vertical section).
Fig. 26. Portion of Orbitoides nimvmulitica.

PLATES XX, XXI, XXII

ACARINA

All the figures, except fig. 4, Plate XXII. , are copied from plates drawn by
Mr. A. D. Michael, F.L.S., &c. by the kind permission of the respective
societies that published them. Figs. 1 to 6, Plate XX., and 1 to 3, Plate
XXL, are from ' British Oribatidse,' published by the Eay Society ; fig. 7, Plate
XX., from the ' Journal of the Linnean Society ; ' fig. 4, Plate XXL, and fig. 3,
Plate XXIL, from the ' Journal of the Eoyal Microscopical Society ; ' fig. 5, Plate
XXL, and figs. 1 and 2, Plate XXIL, from the ' Journal of the Quekett Micro-
scopical Club.' Fig. 4, Plate XXIL, is drawn after Fiirstenberg by the Editor.

PLATE XX. (Facing p. 1008)

ORIBATIDiE

Fig. 1. Anatomy of Nothrus tlmleproctus (male, dorsal aspect, x about 60).
The dorsal portion of the chitinous exo-skeleton, and the fat and muscles
which underlie it, have been removed from the abdomen. The internal organs
are shown protruding, as they usually do when the creature is opened, as
though they were too large to be contained in the ventral exo-skeleton. Part
of the Cbsophagus is seen at the top (the brain having been removed). The
preyentricular glands (brown) lie on each side of the cesophagus. The ventri-
culus is coloured pink ; part of it and the whole of the caca are covered with

XX EXPLANATION OF PLATES

botryoidal tissue (yellow). The testes (white shaded with blue) show at the
sides protruding from beneath the alimentary canal.

Fig. 2. Hoploplio7-a magna (female, lateral aspect, x about 50). The chitin
at the side and the fatty tissue and muscles have been removed. Alimentary
canal pink ; Cffica of the ventriculus spotted ; preventricular glands brown ;
supercoxal gland white ; its vesicles yellow ; expulsory vesicle, between
supercoxal and ovaries, grey ; ovary and oviducts white shaded with blue and
yellow. The genital and anal plates are open, and the genital suckers pro-
truding. One maxilla, white, is seen between the legs.

Fig. 3. Tegeoc7-anus latus (female, dorsal aspect, x about 55). Dorsal
exo-skeleton, fatty tissue, and muscles removed. Same colours as before.
Brain (between preventricular glands) blue grey. Mandibles seen from above
and behind, their retractor muscles cut short. The trachea, which are present
in this species, are seen proceeding to their stigmata in the acetabula of the
legs.

Fig. 4. Female genital organs of Cepheus tegeoeranus ( x about 25), Vigt.
Central Ovary, oviducts with eggs, vagina, and ovipositor.

Fig. 5. The SBivae oi Danusiis geiiiculatus { x about 20). The genital plates
and the muscles and tendons which move them, and the genital suckers, are
shown.

These two figures are reduced from the originals.

Fig. 6. Nymph (active pupal stage) of Tegeoeranus hericius ( x about 100)
(carrying its cast dorsal skins).

TYROGLYPHID^

Fig. 7. Hypopial (travelling) nymph of Rliizoglyphus Bobini (ventral
aspect, X 100).

PLATE XXI. (Facing p. 1010)

OEIBATIDiE

Fig. 1. Leiosoma palmieinctwn {x about 40).

Fig. 2. Nymph of same species, fully grown ( x about 55). The central
ellipse with the innermost set of scales attached is the cast larval dorsal
abdominal skin. The other rows of scales belong to the successive nympha-
skins.

Fig. 3. One of the scales more highly magnified.

CHEYLETID^

Fig. 4. Rostrum and great raptorial palpi, with their appendages of Cheij-
letus venustissimus ( x about 150).

MYOBIID^

Fig. 5. Myobia chiropteralis (female, x about 125).

PLATE XXII. (Facing p. 1012)

Claw of first leg of same species, being an organ for holding the hair of
the bat.

GAMASIDA-:
Fig. 2. Gamasus tcrribilis (male, x 30). A species found in moles' nests.

ANALGINA-;

Fig. 3. Freyana Jiclerojnis (male, x about 95, a parasite of the cormorant).
Fig. 4. Sarcoptes ncabiei (the itch mite, x about 150, adult female).

PLATE 1,

XI 750

X1750

X2000

X270

X2000

Collotype Ptg. Co., 282 High Holborn, W.C,

THE MICROSCOPE

CHAPTER I

ELEMENTARY PBINCIPLES OF MICBOSCOPICAL OPTICS

To be the owner of a well-chosen and admu^ably equijjped micro-
scojie, and even to have learnt the general purpose and relations of
its pai-ts and appliances, is by no means to be a master of the in-
strument, or to be able to employ it to the full point of its
efficiency even with moderate magnifying powers. It is an instru-
ment of precision, and both on its mechanical and optical sides
requires an intelligent understanding of jo? incijiles before the best
optical results can be invariably obtained.

We may be in a position, with equal facility, to buy a high-class
microscope and a high-class harp ; but the mere possession makes
us no more a master of the instrument in the one case than the
other. An intelligent understanding and experimental training are
needful to enable the owner to use either instrument. In the case
of the microscope, for the great majority of purposes to which it is
ajDplied in science, the amount of study and experimental training-
needed is by comparison incomparably less than in the case of the
musical instrument. But the amount required is absolutely essen-
tial, the neglect of it being the constant cause of loss of early enthu-
siasm and not infrequent total failure.

In the following pages we propose to treat the elementary
principles of the optics of the microscope in a practical manner, not
merely laying down dogmatic statements, but endeavouring to show
the student how to demonstrate and comprehend the application of
each general principle. But in doing this we are bound to re-
member a large section of the readers who will employ this treatise,
and to so treat the subject that all the examples given, or that may
be subsequently required by the ordinary microscopist, may be
worked out with no heavier demand upon mathematics than the
employment of vulgar fractions and decimals.

In like manner, although we shall again and again employ the
trigonometrical expression ' sine,' its use will not involve a mathe-
matical knowledge of its meaning. The sines of angles may be

B

2 ELEMENTARY PEINCIPLES OF MICEOSCOPICAL OPTICS

found by published tables. A. table to quarter degrees is given in
Appendix A of this book, which will, in the majority of cases,
suffice ; it is not difficult to find such tables as may be required.^

Of course it is more than desirable that the microscopist should
have good mathematical knowledge ; but there are many men who
desire to obtain a useful knowledge of the principles of elementary
optics who are without time or inclination, or both, to obtain the
large mathematical knowledge required.

Now, just as a man who is without any accurate knowledge of
astronomy or mathematics may find time from a sun-dial by applying
the equation of time taken from a table in an almanac, so by the
use of a table of sines the microscopist may reach useful and reliable
resixlts, although he may have no clear knowledge of trigonometry,
physical optics, nor the mathematical proof of formulfe.

All microscopes, whether sinijyle or compoitnd, in ordinary use
depend for their magnifying power upon the ability possessed by
lenses to refract or bend the light which passes through them. Re-
fraction acts in accordance with the two following laws, viz. : —

1. A ray which in passing from a rare medium into a denser
medium makes a certain angle with the normal, i.e. the perpendicu-
lar to the surface or plane at which the two media join, will, on
entering the denser medium, make a smaller angle loith the normal.
Conversely, a ray passing out from a dense medium into a rarer one,
making a certain angle with the normal, will, on emergence from
the dense medium, make a greater angle with the normal.

The ray in one medium is called the incident ray, and in the
other medium the refracted ray.

The incident and refracted rays are always in the same plane.

2. The sine of the angle of incidence divided by the sine of the
angle of refraction is a constant quantity for any two particular
media.

When one of the media is air (accurately a vacuum) the ratio of
these sines is called the absolute refractive index of the medium.
As every known medium is denser than a vacuiim, it follows that
the angle of the refracted ray in that medium will be less than the
angle of the incident ray in a vacuum ; consequently, the absolute
refractive index of any medium is gi'eater than unity.

Fui'thei', the absolute refractive index for any particulai- sub-
stance will differ according to the coloiar of the ray of light employed.
The refi'action is least for the i-ed, and greatest for the violet. Tlie
difference between these refractive values determines what is called
the dispersive power of the sidistance.

This will be understood by fig. 1. Let I C, a ray of Hg] it travel-
ling in air, meet the surface A B of watei- at the point C. Through
C draw N W at right angles to the surface of the water A B. The
line N W is called the normal to the surface A B. The ray I C will
not continue its path through the water in a straight line to Q ; but,
because water is denser than air, it will be bent to R, that is
towards N'. The whole course of the ray will be I C R, of which
tin; [lart J C is cidled the; incident ray, and 0 R the refracted ray.
' Vide, ClmmljfrH's M<illiciii((i.ir(il Tables.

THE LAW OF SINES 3

The angle I C makes with the normal JST N', viz. I C IST, is called the
angle of incidence ; and the angle R C makes with the normal N' IST,
viz. R C N', is called the angle of refraction.

Conversely, if a ray R C, travelling in water, meet the surface of
air A B in the point C, it will not continue in a straight line, but
will be bent to the point I farther away from N". Thus, when a
ray passes from a rai'er to a denser medium it is bent or refracted
tovKcrds the normal, and when it passes out of a dense medium into
a rarer one it is bent oi- I'efi'acted aioay from the normal.

Further, if the shaded portion of the figure were glass instead of
water, the refracted ray R C would be bent still nearer N', and,
conversely, if the ray passed out of glass into air, it would be more

Fig. 1. — The refraction of light. The law of sines.
bent away from the normal than if it had passed out of water into

The angle of incidence I C N is connected with the angle of re-
fraction RON' (as stated above) by what is known as Snell's Law of
Sines. The constant relation between the two sines for two specific
media is called the refractive index of the medium, and is usually
indicated in problems by the symbol fx.

This law, stated with I'eference to the figure, would be :

sine I C IST ,, c .- • ^ c ^

—. =, ^ ^, = u == the retractive index oi water.

sine RON

In 1 0 take any point, P, and from P draw P T perpendicular
to N W . Similarly in R C take any point, F, and draw F H per-
pendicular to N N'.

b2

4 ELEMENTARY PEINCIPLES OF MICROSCOPICAL OPTICS

"P '^r T*^ IT

ISTow, as sine I C N = ,-^, and sine R C N' = -^^7^, then, by
PT
Hnell's law, — /i.

Fir

As any points may be taken in I C and R C if the points had been
more judiciously selected, we might have greatly simplified the above
expression. Thus, if we take two other points, K and E, such that
K 0 = E C, and draw the perpendiculars as before, we shall have

KS

sine I C IST = and sine R C K^ = -— — , and therefore = n.

KG EC E D ^

But as K C = E C by construction, we can write K C for E C
KS

thus : =— ~ = M- K C is cancelled, which leaves ,— - = a
ED ^ ED ^

K-G

As ju can be experimentally determined for any two particular
media, it follows that if one of the other terms is known, then the
remaining term can be found. Thu.s, if p. and the angle of incidence
are known, the angle of refraction can be found ; and if ^( and the
angle of refraction are known, the angle of incidence can be found.
The unknown quantity can be found either geometrically or by cal-
culation when the other two terms are given.

It will, of course, be understood that, for the same medium in
every case, a red ray would be bent or refracted less than a violet
ray. The value thei-efore of jx for a red ray will be less than that of
fx' for a violet ray. As a pi-actical illustration : The refractive in-
dex for a red ray in crown glass is 1"5124 = ^, and for a violet ray
is 1-5288 = /, the difierence being ju' — ^ = -0164.

The refractive index for a red I'ay in dense flint glass is 1'7030
= /i, and for a violet ray is 1'7501 — fi', the difference being /j.' — ^
= •0471.

Consequently thei-e will be a greater diifei'ence between the bend-
ing of the refracted i-ed and violet rays in the case of dense flint than
in the case of ci-own glass, the angle of the incident luy with the
uoi'mal l)eing the same in either case.

Wliere air (more correctly a vacuum) is not one of tlie media,
tlien tlie refi'active index is called the relative i-efractive index.

Tlie normal to a plaiie, surface is always the pei-pendiculai- to it ;
the normal to a spherical snrface is the ludius of curvature. The
angle of the incident ray and the angle of the refracted ray are
^dwavs ni(;asur('d iritlh tlte normal, and not with the sui'face.

Fig. 2 «, /;, shows the normals A, V> to l)oth a, plane and a.
spherical sui'face, C J).

In theca.seof the .spherical surface, 1> is the centre of ciu'\a.tiire, E ¥

PROBLEMS ON EEFEACTIVE INDEX

5

is the incident ray in air, F G the refracted ray in crown glass. The
angle A F E is the angle of incidence, B F G the angle of refraction.
Sine A F E divided by sine B F G is equal to the refractive in-
dex of air into crown glass, or, in other words, the absolute refractive
index of crown glass, /j. ; thus in this particular case :

(Problem) I. :

sin A F E sin 45° -707 3

- = fi.

sin B F G ~ sin 28°

•472

This problem, however, is not actually needed by the reader of
this book, for a table of
absolute refractive indices
is given in Appendix B.

It will be clear from
the above that when the
refractive index, absolute
or relative, of a ray from
any first medium is given,
the refractive index from
the second to the first may
be foiuid.

Thus, the absolute re-
fractive index fj. from air
into glass being given as
3

-, find u

the refractive

■ index from glass into air.

(Problem) II. :

' = 1 = ^ ^
ju 3

9

^

3'

When the absolute
refractive indices of any
two media are given, the
relative refractive indices
between the media can be
found.

Thus, the absolute re-
fractive index yu of crown
glass is 1'5, and the ab-
solute refractive index /i'
of flint glass is 1'6 ; find the relative refractive index ^'" from crown
to flint. \

Fig.

-The normals to a plane and a curved
surface.

(Problem) III.

i"=^ = — = 1-066

h"

1-5

The relative refractive index fj.'" from flint to crown is determined

by (problem) ii. :

,. 1 1

A' =-

1-066

= -938.

6 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

Let us now suppose that iia fig. 2 the ray is travelling in the op-
posite direction, G F in the denser medium will now be the incident
ray, and F E in the rarer medium will be the refracted ray. Now,
if the angle B F G be increased, the angle A F E will also be in-

PlG.

-The phenomenon of total reHexion. (From tht; ' Forces of Nature,'
published by Macmillan.)

creased in a gi-eater proportion, and tlie rny F E will ;ii)[)r();ich tlie
surface F D.

Wlien F E coincides witli F 1), G F is said to be incident at the
ci'itirMl an//k of i\\H uiciWwiw. Wlieu this critical angle is reached,
none of the incident ligiit will pass out of the denser medium, but it

PROBLEMS ON REFRACTIVE INDEX 7

will be totally reflected from the surface C D back into the denser
medium.

A simple illustration of this is shown in fig. 3. It represents
a glass of water so held that the surface of the water is above the
eye. If we look obliquely from below at this surface, it appears
brighter than polished silver, and an object placed in the water has
the upper portion of it brightly reflected.

The action on all light incident on C D in the denser medium
(fig. 2) at an angle greater than the critical angle is precisely the
same in fact as if C D were a silvered mirror.

A critical angle can only exist in a denser medium, for obviously
there can be no critical angle in the rarer medium, since a ray of
any angle of incidence can enter.

Wlien the relative or absolute refractive index of the den8er
medium is given, the critical angle for that medium can be found,
thus: The absolute refractive index of water is l'33 = /i; find its
critical angle 9.

(Problem) IV. : .<■ ^ 1 1 ^r

// l'3o '

B =481° (found by table).

So the sine of the critical angle is the reciprocal of the refractive
index.

The connection between the path of an incident ray in a first
medium and its refracted ray in a second medium is established by
the formula

jx sin (f> ^ fi' sin (f>',

where jj. is the absolute refractive index of the first medium, 0 the
angle of the incident ray in it, /j' the absolute refractive index of
the second medium, ancl (// the angle of the refracted ray in it.

The angle (j> = 45° of the incident ray in the first medium A F E

(fig. 2) and jj. = 1 , ^^ = ^, the absolute refractive indices of both the

A

media, air and glass respectively, being given, find f', the angle of
the refracted ray in glass.
(Problem) Y. 1 :

o- , /u sin 0_1 X sin45°_l x '707 .^-.

m 0 - -^ ^-^-

0' = 28° (found by table).

To put another case. Suppose the angle 0' = 28° (fig. 2, B F G)
is given ; find 0, the refractive indices remaining the same as before.
(Problem) Y. 2 :

Sin 0 ^/ Bin 0-^1-5 X sin28°^]_-5 x -T^l^.^Qg^

fX 1 1

0 = 45° (found by table).

Now, suppose the A side of C D (fig. 2) is crown glass, ju = 1-5,
and the B side of C D is flint glass, / = 1-6. The angle of the
incident ray A F E 0 = 45", find the angle of the refracted ray 0' or
BFG.

8 ELEMENTAEY PEINCIPLES OF MICEOSCOPICAL OPTICS

(Problem) V. 3 :

jx sin (^ 1-5 X sin 45 1-5 x 707 1-0605
^^^¥='^^= Te = r6~ = ~r6" = "^^^'
<p'=i\\° (found by table).

As a final instance. Suppose the ray to be travelling in the
opposite direction, so that G F is the incident ray and B F G, or
^'':=41^°, be given, the media being the same as in the last case,
yu,'=l"6 and ju,= l"5, find ^, or the angle of the refracted ray.

(Problem) V. 4 :

/ sin (p' 1-6 sin 4H° 1-6 x -663 1-0608
^^^ ^=^-;r~=--TF-^ = ^hF-="T5-=-^07 ;

^=45° (found by table).

The importance of the prism in practical optics is well known.

Its geometrical form in per-
spective and in section is shown
in fig. 4.

By means of the above pro-
blems and their solutions we
are now able to trace the diver-
gence of a ray through a pristn.

In fig. 5 let ABC repre-
sent a, prism of very dense fiint
glass whose absolute refi-active
indices /x' for reel light is I'T,
and It!' for blue light is 1-75.
Let the refracting angle BAG
of the prism =50°, and let the
angle of incidence of a ray of
white light D E=45° =9. in
air, //,= 1 . The dotted lines show
Fig. 4. — The geometrical form of the prism, the normals. Then by (problem)
(From the 'Forces of Nature.') y_ j fo^. ^.^^ jig.]^^ ^g ^^^^^ fo^.

the angle of refraction (j)' .

, //, sin (h 1 sin 45° -707
,3n ^'^--,-^=.-^^ =-p7=-416 ;

0'= 24^° (found by table).

And for hlue light

IX sin 0 1 sin 45° -707

«in /'='-;; ^=^5- =r75='^o^ '

0''=23|° (foTuid l)y table).

Now, for the red ray draw E F (fig. 5), 24^° to the normal, and
let it meet the other side of the jn-ism A G in F. At F draw
another noi-mMl.

On the scale of our diiigr;iiii it is not possible, to draw two lin(!s
E F, one foi- the red ray and the other for the blue, for they are too
close together, their angular divergence being only |°. But by

PATH OF LIGHT THROUaH A PBISM 9

measurement it will be found that E F makes, with the normal at
F, an angle (p' of 25^°, and for the blue ray an angle (p" of 26|-°.

It should be remembered, however, that if the refracting angle
of the prism is known, there is no necessity for this measurement,
because it is always the difference between this and the angle of
refi-action before determined, thus 50° — 24^° = 25^°.

B C ^

Fig. 5. — Diagram of deviation of luminous ray by a prism.

V

This ray E F now becomes the incident ray on the surface AC ;
and as the angle it makes with the normal at F is known, and as
the refractive indices remain the same, we can, by (problem) v. 2,
find the angles of refraction for each colour.

If we take red light :

/sinri)' l-7sin25i° l-7x-43 ^^^

Sm d)= '- = :, -^ = :, = -732 ;

' /x i 1

^, the angle of refraction =47° (found by table).
If we take blue lie-ht :

Sin 0=

jiJ' sin (\>" 1-75 sin 261° 1-75 x -442

= •774 ■

IX — 1 — 1 — "*'

0, the angle of refraction = 50|° (found by table).

This dispersion can now be represented in the diagi'am, seeing
that it amounts to 3|°.

In optics it is convenient to use an expression to measure the
dispersive power of diaphanous substances, which does not depend
on the refracting angle of the prism employed. Furthei', in order
that various substances may be compared, their dispersive powers
are all measured with reference to a certain selected ray. (For this
purpose the bisection of the D or sodium lines is the point in the
spectrum often chosen.)

In the crown and flint glasses mentioned on page 4 the dispersion
between the lines C and F, in the spectrum, referred to the bisection
of the sodium lines D, is as follows. Crown glass : — refractive index
bisection of lines D, l-5179=;u; line F, 1-52395=/; line C,
l'51535=/x.'''. Then the dispersive power to

1-52395-1-51535 -0086

: = -0166].

" M-1

■1-5179 — 1

-5179"

lO ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

The values of the same lines for the flint glass are as follows :
I), 1-7174=^; F, 1-73489=m'; C, 1-71055=/^

1-73489 - 1-71055 -02434

M — M

A'

— 1

1-7174

1

'7174

-=-0339.

So the dispersive power of the flint between the lines C and F is
slightly more than twice that of the crown for the same region of
the spectrum. In the above formula the expression yu' — fi" is usually

Avritten h /.i ; in full it is therefore w = — ^- — •

fi-l

Having thus traced a ray

experimentally through a

prism, our next step is to show

that a convex lens is only a

curved form of ttoo stick jjrisms

with their bases in contact,

as is shown in A, fig, 6,

where the curved line shows

the lenticular character and

the shaded elements the two

prisms. A concave lens is in

effect two prisms reversed,

that is, with their apices in

contact, as in B, fig. 6, where,

again, the curved line shows

the form of the lens and the

Fig. 6. — Convex and concave
lenses are related to the
prism.

^

Fig. 7. — Proof that a lens may be considered
as an assemblage of prisms. (From the
' Forces of Nature.')

shaded parts its relation to a pair of prisms. The fact that a lens is,
in effect, as such, but an assemblage of super})osed prisms is seen in
fig. 7, the refi-acting angle of the piisni being more acute as the
pi'incipal axis is approached, and tlie deviation being greater as tlie
angle is more obtuse.

In fig. 8 let O P be the axis in eacli case ; then, from wliat we
have seen, it is manifest that rays parallel to tlie axis falling on the
prisms witli their bases in contact and acting like a convex lens will
be refi'acted towai-ds the axis 0 P, But in the other case, where
tlio [)ri.sms liave their apices together, as in fig. 9, acting as a con-
cjive lens, the light is refracted away from the axis O P,

ACTION OF A PAIR OF PEISMS

II

Fig. 8. — Action of a i^air of prisms with their bases in contact on
parallel light.

Fig. 9. — Action of a pair of prisms with their ax^ices in contact on
parallel light.

12 ELEMENTAEY PRINCIPLES OF MICEOSCOPICAL OPTICS

It must, however, be understood that there is a very miportant
difference between the action of spheiical lenses, lohich is clue to the
different positio7is of the normals.

In the prisms (figs. 8, 9) the incident surface A B is a plane ;
and as the normals are perpendicular to it, they must be parallel to
one another, whether near the base or near the apex. Thus the
normal at E is parallel to the normal at K ; therefore, whatevei-
angle D E makes with the normal at E, H K will make a similar
angle with the normal at K, because the normals are parallel and the
incident rays are 'parallel.

But in the case of a spherical lens the normals are radii ;
parallelism is therefore impossible, and parallel incident rays will
not make equal angles with them, and so the refracted rays will not
be parallel.

This explains how it is that when rays parallel to the axis fall
on the prism (see fig. 8) those which pass through the prisms near
their bases cut the axis nearer the prisms than those which pass
through near the apex.

But in a convex lens the reverse takes place ; the rays passing
through near the middle of the lens cut the axis farther from the
lens than those which pass through the edge of the lens. The
typical form of a biconvex or magnifying lens is shown in fig. 10,

Fig. 10. — Front and edge views of a biconvex lens.
(From the ' Forces of Nature.')

both in perspective, as seen from the edge, and with a full view of
the disc ; while the various forms which for various optical purposes
are given to lenses is shown in figs. 1 1 and 1 2.

Now, if we .study the four following figures, we shall see the
|)riiicipal action of lenses on light incident on theii- surfaces. Fig.
13 .shows that if a radiant is j)laced at the princii)al focus of a con-
verging lens, the rays are rendered parallel ; conversely, if parallel
rays fall (m a converging lens, they are brought to a principal focnis
or point uptm the axis.

Fig. 14 shows that if a radiant be placed /^eyo?//^ the principal

THE FOCI OF LENSES

13

focus of a converging lens, the rays are brought to a focus beyond
the principal focus on the other side of the lens. The nearer the
radiant is to the principal focus, the farther away will be its conjugate
focus from the other principal focus. In other words, there are two
points in the axis such that if the object is one point its focus will
be the other ; these are reciprocal one to the other. These points,

Fig. 11. — Biconvex, plano-convex,
and converging meniscus lenses.
(From the 'Forces of Nature.')

Fig. 12. — Biconcave, plano-concave,
and diverging meniscus lenses.
(From the ' Forces of Nature.')

the focal distances of which can always be calculated, are known as
conjugate foci.

Should the radiant be at a distance from the principal focus equal
to the focal length of the lens (i.e. twice the focal length from the
lens), then its conjugate will be at the same distance from the focus

Fig. 13. — A radiant at the principal focus of a biconvex lens makes the refracted

rays parallel.

Fig. 14.-

-A radiant placed beyond the principal focus causes rays to converge
beyond the x^rincipal focus 011 the other side of the lens.

on the other side of the lens (i.e. twice the focal length from the lens).
In other words, when the object and its image are equidistant on
either side of the lens, they are equal to each other in size, and
are four times the focal length of the lens apart.

14 ELEMENTAKY PEINCIPLES OF MICROSCOPICAL OPTICS

This law forms a ready means of determining the focal length of
a lens. An object is placed in front of a lens, and the distances
between this object and the lens and a sci-een to receive the image
of the object are so adjusted that the image of the object becomes equal
ill size to the object itself. The distance of the object from the screen
divided by 4 gives the focal length of the lens.

If a radiant be placed betwee?i a lens and its principal focus, the
rays on the other side of the lens ai-e still divergent, and will never
meet in a focus on that side. This is seen in fig. 1 5 ; but if they are
traced backwards, as in the dotted lines of fig. 15, they will then

Fig. 15. — Rays diverge when a radiant is placed between a lens and its
principal focus. Focus of divergent rays is virtual.

meet in a point. This is called the virtual conju,gate focus of the
radiant. The principal focus of a concave (or diverging) lens is
shown in fig. 16. It will be seen that the principal focus is not
real but virtual.^ Parallel rays falling on a concave lens a,i-e rendered

Fig. 10. — ' Virtual ' focus of concave lens.

divei-gent on the other side of the lens, and consecpiently c^an never
(;om(! to :\ focus. But if we trace these divei-gent rays backwa,rds,
as ill the dotted lines of fig. 16, we find that they meet in a- point,
and this point is called the virtual principal focus of the lens.

It will be manifest that since the rays in jiassiug tlu'ough Icnsf^s of
various kinds jirc u/aefpudly refracted \^\(^y cMwwtixW wvvi exuctly in a,
single focal point. Tliisgi\'f's rise to wliat isa- mosi, iiiipovtnnt fcnturc
in the Ijchav'ioui' of Icn.scs, which is known as spherical aberration.

Fig.s. 17 Jind IH siiow the refraction of rays of monochroinatic
' A reai iiiiii,g(; can Ije received on a screen, but a virtual image cannot.

SPHEEICAL ABEKEATION

15

light parallel to the axis falling on a plano-convex lens of ci'own
glass. These figures illustrate : (1) Longitudinal spherical aberration
and (2) the focal length of a jolano-convex lens and the point from
which it is measured.

(1) In regard to the former it will be seen that the longitudinal
sphei'ical aberi-ation is greatest in fig. 17, where the parallel rays
of light fall upon the plane surface, and least where, as in fig. 18,
they fall upon the sphei'ical surface. For spherical aberration is the

n

^.

^

r'

\

^-^jT" -^'-^

A~~^

/

:^'-^^^ 1

■R^

/

^

R'

V

Fig. 17. — Spherical aberration.

distance of the focus for any ray passing through a lens froin the
principal focibs of that lens.

Thus in figs. 17, 18, the sphei'ical aberration is F F' for the rays
R"^ R2, and F F'' for the rays R^ R^, and the diffei'ence between the

Fig. 18. — Spherical aberration.

spherical aberration of the I'ays R^ R^ and that of the rays R'^ R^ is
F F'' — F F', which is F' F".

in(fig. 17),a/=-^- y'

Thus

F F' and F F'

7

/

F F' and F F''

in (fig. 18) cV= -

- - -l-, where cf signifies the distances F F',
6 /

F F'' respectively, y the distance from the axis whei'e the incident
ray enters the lens, and f the focus.

(2) In regard to the focal length of a plano-convex lens, it may
be incidentally noted that the focal length in fig. 1 7 is twice the radius,
measured from the vertex A, that is, A F. But in fig. 1 8 it is twice
the radius measured from the point A ; that is, the point F is distant
from the lens twice the radius less two-thirds the thickness of the lens.

It will be seen, then, that the aniormt of sphei'ical aberi'ation is
due to the sha2)e of the lens, and is least in a biconvex lens, when the
radii of curvature are in the proportion of 6 : 1 , lohen the more curved
surface faces the incident light. But when the lens is turned round,
so that the other side faces the incident light, the spherical aberi'ation
reaches a maximum.

It would be well for the student who desires to become familiar
with these facts, without attempting any profound mathematical

1 6 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

grasp of them, to draw such a lens, and trace the paths of. two rays
through it, one near the axis, the other near the edge ; then do the
same with the lens reversed.

Formula for spherical aberration : ^

where / = principal focal length ; y = semi-aperture ; ju = refr.
index ; and ?', — ?"', radii.

3

In an equi-convex of crown, where f^ = -, r ■=■ — r' ■=. f,

■^ 3 /

3 -f

In a plano-convex of crown, where u ^ -, — ?•' = oo, r = ■^-,

1 ' r- 2' '2

7 v^
^ / = — - • ~ . Here parallel rays are incident on the convex

surface. But when pai-allel rays are incident on the plane surface,

3 /" 9 ?/^

/^ = -, r =. oo, — r'=^-,lf=: — . ^ ; consequently the sphe-

. . . ^ J

rical aberration is four times as great (see figs. 17 and 18).

When — r' = oo, and /.t = 1*69, the plano-convex becomes the

form of minimum aberration.

3

2'

In a crossed - biconvex lens, where — r' = 6 r, and jx ^

15 ,,2

^f = — — • '---J,, the parallel rays being incident on the more

curved surface.

Formula for finding the principal focus F of a lens equivalent to
two other lenses whose foci are /", f and their distance apart d :

1 ^1 j__ jZ_

F f f ff

In figs. 5, 8, and 9 we see that when the incident ray D E con-
sists of vihite light, the colours of which it is composed are unequally
refracted ; the two extremes, R (red light) and Y (violet light), being
bent in different directions, the other colours lying between them
in their proper order.

This unequal I'efraction of the different coloiu's takes place in
like manner in sphei'ical lenses, and it is then known as clwomatic
aberration.

The effect of tliis upon the action of a lens is that, if pai'allel wliite
light fiill u])on a convex surface, the most refrangible of its ci)m})onent
rays (wliich, as we have seen, is the violet) will be bi'ought to a focus
at a point somewhat neai'er the lens than the principal focus ; and
the red ray, having the least refrangibility, will be brought to a, focus
at a point fiirther from the lens than its principal focus, which is, in
eflfect, the mean of tlif cliinniatic foci.

' Encyclopedia Brit. vol. xvii.

^ A biconvex lens is Haid to be 'croHsed ' when the radii of itw surfacew are in the
proportion of 1 : 6.

HOW ACHEOMATISM MAY BE OBTAINED

17

This will be fully understood by the aid of fig. 19.

The white light, A A''', falling on the jDeripheral portion of the lens,
is so far dispersed or decomposed that the violet I'ays are brought to
a focus at C, and, crossing there, diverge again and pass on towards
F F ; whilst the red rays are not brought to a focus until they reach
the point D, crossing the divergent violet rays at E E. The foci of
the intermediate rays of the spectrum (indigo, blue, green, yellow,
and orange) are intermediate between these two extremes. The
distance C D, limiting the violet and the red, is termed the longitu-
dinal chromatic aberration of the lens.

If the image be received upon a screen placed at C, violet will
predominate, and will be surrounded by a prismatic fringe in which
blue, green, yellow, orange, and red may be distinguished. If, on
the other hand, the screen be placed at D, the image will have a

Fig. 19. — Chromatic aberration.

predominantly red tint, and will be surrounded by a series of
coloured fringes, in inverted order, formed by the other rays of the
spectrum which have met and crossed.

The line E E joins the points of intersection between the red
and the violet rays which marks the mean focits, or the point where
the dispersion of the coloured rays will be least.

The axial ray undergoes neither refraction nor dispersion, and
the nearer the rays are to the axial the less dispersion do they
undergo. Similarly, when the refraction of the rays is greatest at
the periphery of a lens, there the dispersion will be most. Hence
the peripheral portions of uncorrected lenses are stopped out, and
the centre only often used that the chromatic aberration may be
reduced to a minimum.

Manifestly, therefore, the correction or neutralisation of this
chromatic abei'ration, which is known in optics as achromatism, is a -
matter of the first moment. Multiplied colour foci between C and
D (fig, 19) make a perfect optical image impossible.

It is a question of interest and importance to the microscopist to
know hoio achrom^atism is obtained.

In a prism the amount of dispersion or luiequal bending of
R and Y (fig. 5) depends on two things: (1) the nature of the
glass of which the prism is composed, and (2) the refracting angle
BAG.

If, for example, another prism were taken, made of a difierent
kind of glass, possessing only half the dispersive power of that in
the figure, but with the angle BAG 50°, as in this case, the separa-

c

1 8 ELEMENTAEY PRINCIPLES OF MICROSCOPICAL OPTICS

tion of II and V would only be half as great as that effected by the
prism in the figure.

Then if another pinsm were made of the same material as that
assumed in fig. 5, but with only half the refracting angle, viz. 25°,
the dispersion between R and V would also be but half that repre-
sented. Also a prism having 50° of i-efi-acting angle gives the same
amount of dispersion as that from a prism of 25° of refracting angle,
but of twice its dispersive power.

Under these conditions, when one prism, exactly like another in
angle and dispersive power, is placed close to it in an inverted
position, the dispersion of the first prism is entirely neutralised by
that of the second because it is precisely equal in amount and

opposite in power.
This will be under-
stood by a glance at
fig. 20. But it will
be seen that not only
is dispei-sion reversed,
but refraction also
is neutralised, the
emergent ray being
parallel to the in-
cident ray. Therefore
the equaland inverted
system of prisms can
be of no possible use
to the practical opti-
cian in the correc-
tion of lenses because
the convergence and divergence of rays are both essential to the
construction of optical instruments. The dispersion, in fact, must
be destroyed without nevxtiulising all the refraction.

Suppose we take a prism with an angle of 50°, composed of glass
having a certain dispersive power, and invert next it a piism of 25°
angle, composed of glass having twice the dispersive power of the
formei'. Dispersion will be manifestly destroyed, because it is equal
in amount and opposite in natvire to that possessed by the pi'ism of
50° ; but the prism with an angle of 25° will not neutralise all the
refraction effected by the prism of 50°.

These conditions plainly suggest the solution of the problem, for
part of the convergence is maintained while the whole of the
dispersion is destroyed.

The spherical lenses Avliicli answer to these prisms are a. crown
biconvex, fitting into a flint plano-concave of double the dispci-sive
power-
It has been })ointed out above tliat all the other colours lie in
their pi-oper oi-dei- between the rays II and Y (fig. 5). Let us select
one, green, and i-ej^'csent it l)y (\. Now if C lic^s midway l)etween
R and V in the jirisni of 50° of angle, and also Ix^tween R and V in
the prism of 25° of ,in;^lc. its dispersion will also be neutralised.
This rnejins tlijit when 1 lie disjiersion between the three coloin\s in

Fig. 20. — Recomposition of light by prisms,
the 'Forces of Nature.')

(From

ACHROMATIC OBJECTIVES 1 9

one kind of glass is pi-oportional to their dispersion in the other,
then when any two are destroyed the third is destroyed with them.
This nnfortunately is not the case in practice, because two kinds of
glass having proportional dispersion powers cannot be obtained.
This, however, is what really happens. G may lie midway between
R and V in one kind of glass, but in the other it may lie, for
instance, much nearer R, say a third instead of half the distance
of R from Y. If now the dispersion of R V be destroyed, G will
be left outstanding. If a different angle of prism be chosen, so that
R and G are neutralised, then V must be left outstanding.

This want of proportion in the dispersion of the various colours
of the spectrum in two kinds of glass is termed the irrationality of
the sjMCtrioni, and the colour or colours left outstanding in a corrected
combination of lenses is known as the secondary sjjectrum.

In some subsequent pages we shall have to call attention to the
manufacture in Germany of some new vitreous compounds by the
combination of which with fluor spar the secondary spectrum has
been removed from microscope objectives, and an a'pochromatic
system, of construction has been introduced.

Meanwhile, we may remember that it has only been in compa-
ratively recent times that the construction of achromatic object-
classes for microscopes has been brought about, but the gradual
enlargement of aperture and the greater completeness of the cor-
rections soon after the discovery of achromatism rendered sensible
an imperfection in the performance of these lenses under certain
circumstances, which had previously passed unnoticed, and Andrew
Ross made the important discovery that the use of cover-glass in
mounting minute objects introduced aberration, and that a very
obvious difference exists in the precision of the image, according as
it is viewed with or ivithout a covering of thin glass, an object-
glass which may be perfectly adapted to either of these conditions
being sensibly defective under the other.

He also devised the means of correcting this error, and published
his device in vol. li. of ' Transactions of the Society of Arts ' for 1837.

Fig. 21 will illustrate the effect produced on the corrections of
an object-glass by the interposition of a cover-glass between the
object and the objective.

The rays radiating from the object O in every dii'ection fall upon
the cover-glass C 0 [p. = 1"6). On tracing two definite rays, such
as O A and 0 B, it will be found that they will be refracted to R
and P (shown by the dotted lines of the figure). On their emergence
into air they will be again refracted in a dii'ection parallel to their
first path, and will enter the front lens of the objective at the
points M and N.

Now as M R and N P, produced, meet in Y, it follows that, so
far as the objective is concerned, the rays MR, N" P might have
diverged from the point Y.

Similarly, by tiucing two of the less divergent i-ays fi'om O they
will be made by the refraction of the cover-glass to appear as if
they diverged from X. Therefore, in consequence of the cover-glass
the objective has to deal with vsiyfi v&(^\&ti\\g a^yj^arently froon two clis-

c2

20 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

tinct points, X and Y. If there were no cover-glass all the rays would
divei-ge from 0, and then the objective would require to be perfectly
aplanatic. This word (derived from o = privative, and TrXaraw, to
wander, i.e. free from wandering or error) means, as used by opticians,

Fig. 21.

-The eifect produced by a cover-glass on the corrections of an
object-glass.

that all the rays passing through a lens system are brought to an identi-
cal conjugate focus, as shown in fig. 22. But as affected by the cover-
glass the marginal rays diverge, apparently, from a focus, nearer the,
objective than the central rays ; therefore the objective, to meet this
condition, must be what is called under -corrected ; a condition jDre-
sented in fig. 23, so as to focus both these points at once. Here the

Fig. '22. — Aplanatic system.

Fig. 23.— Under-corrected system.

cui-vature of the surface of the crown lens being increased, the flint
pliino-concave is not sufficiently powerful to neutralise all the
sphei'ical aberration of the crown. As a, consequence the periplieral
rays sire brought to a focus at F', while the centi'al rays pass on to
F. This is what is meant by ' under-coi'rection ' in an object-glass.

In fig. 24 the reverse condition
is presented, for the incident curve
of the crown lens has been flattened,
while that of the flint has been
dot'pcno^d, which increases tlie cor-
rective power of tlie flint, iiiid thus
destroys tlie balance of tlie cf)m-
l.iii;it ion in oilier dii'cctioiis. The I'ays [jassing tlii'ongh the pei'ii)hery
of tlu! coiiiljination will be brought to a focus F', while the centnil
rays will be focussed at F. This is what is known -a'a over-correction.

24. -Over-corrected systf/m.

COLLAK COKRECTION — FOCI OF LENSES 21

An aplanatic objective can be made into an under-corrected
objective by (1) causing the back lenses of which it is coimposed
to approach the front lens. This is the device of Andrew Ross, and
is now effected ^ by means of a special ' collar ' ai-rangement, which,
by the action of a screw, approximates or separates the suitable
lenses. But for this a special device is needed for each objective.
(2) The result can moreover be secured by catcsing the eye-piece to
ap2)roach the objective. This of course is accomplished by the use of
the di"aw-tube, and must be employed with objectives having rigid
mounts.

Closing lenses, that is, bringing them together, whether in the
objective itself or in the microscope as a whole, by shortening the
distance between the eye-piece and the objective, itnder-corrects the
objective, that is, gives negative aberration ; while the separation of
lenses over-corrects or gives positive aberration.

In using the collar correction ^ for a longer body or a thicker
cover-glass the collar adjtistment must be moved so as to cause the
back lenses of the objective to approach the front lens, while for a
shorter body or a thinner cover-glass, the adjustment must be moved
so as to cause theii* separation.

In correcting by tid^e length for a thicker cover shorten the tube,
and for a thinner one lengthen it.

For the benefit of those who aim at work with lenses, that is
such as may be compassed with the aid of the most elementary
mathematics, it may be well to indicate a simple method for the
deduction of the /bci of plano-convex and, biconvex lenses.

In fig. 17 the focus is twice the radius measured from the vertex
A, that is, A F. But in fig. 18 it is twice the radius measured
from the point A, that is, the point
F is distant from the lens twice the
radius less two-thirds the thickness
of the lens.

Similarly, in fig. 25, the focus
of a biconvex lens '\% measured from
the point A ; in other words, F is
distant from the lens the length of
the radius less one-sixth the thick- Fig. 25.— The focus of a convex lens
ness of that lens (nearly) .

Formidoi relating to a biconvex lens. — Where P is one' focus, P' its
conjugate, F principal focus (solar focus, or that for a very distant
object), E, radius of cui-vature for one surface, R' for the other
surface, p the refractive index of the medium, then

11 /I 1 \

p + p/ = (M-l)(^p^+3^,j;

k=(m-i)(r+r/);

F

1 1 _1
p + p/— F

Fig. 25a. — Focus of a concave lens.
See Chapter V.

22 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

Also, if X is the distance of a focus from F, the jt>?'mci^/)aZ focus,
and y, the distance of its conjugate from F', the other princi2)al
fo_cus on the other side, then

or,

X ?/=F F'

xy=Y'\

In an eqviiconvex lens of crown glass if /j = 1"5, F= radius of
curvature. But in a plano-convex lens of crown glass if yu = l"5,
F=twice the radius of curvature.

In the above formula the thickness of the lens has been neglected.
In thick lenses, however, its effect must not be disregarded, even if
only approximate results are i-equired. A very approximate deter-
mination of the principal focal length of an equiconvex lens ^measured
from the surface may be made by subtracting from the result
obtained by the foregoing formula? one-sixth of the thickness of the
lens. (See fig. 25.)

Exaraj)les. — Equiconvex lens of crown glass ju = l"5, r:=.\, thick-
ness=j. By above formula F=^. Subtracting from this one-
sixth of the thickness of the lens, we get F=^ as the distance
between the focus and the surface of the lens. This is only 2^5^ inch
from the truth. If the lens were a sphere it would be necessary to
subtract \ of its thickness.

In the case of a plano-convex lens the principal focus on the
convex side is equal to twice the radius as above, but on the plane
side two-thirds of the thickness of the lens must be subtracted from
it.

In a hemispherical lens of crown glass ^ = 1'5, i'adius=^, thick-
ness=^, the principal focus on the convex side will be one inch
from the curved surface and on the plane side ^ inch from the plane
surface.

In an equiconcave lens the foci are virtual and are crossed over ;
thus, the lens in fig. 25a is equiconcave, the focus F, instead of being-
measured, from A to the right hand, must be measured to the left
hand ; consequently, \ of the thickness must be subtracted from
the focal length in order to determine the distance of F from the
surface of the lens.

A plano-concave lens follows the plano-convex, but the foci are
virtual and ci-ossed over. Fi'om the principal focus on the cui-ved
side subtract |- of the thickness, and from that on the plane side
subtract the whole tliickness of the lens.

Examples. — Equiconcave of dense flint /x = l"75, ra.dius = — \,
thickness \, F by formula = — i ; subtract fi'ona this \ of the thick-
ness of the lens, we obtain — \, which is only jjj, inch too short.

Plano-concave of dense flint /^ = 1 "75, raditis= — !,-, thickness],
F by formula= - §, subtract from this the thickness of the lens.
Tlien F= — ^\- ; this is the focal distance from the plane side. Foi-
the focal distance fi-om tlie cm-ved side subtract j| of the thickness,
then F= — f;;':, which is -^ incli too long.

The pi'incipalfoc'i's of a coiahriiatioii, of ivo or 'more lenses, wliose

THE FOEMATION OF A 'REAL IMAGE'

23

principal foci and distances ai-e known, can be found from the formula
- + _ ,= J by assigning for the value of 2^ the distance of the prin-
cipal focus of the first lens from the second, and so on.

Example. — Parallel rays fall on an equiconvex lens of four inches
focus. Two inches from this lens is another equiconvex lens of
three inches focus. Find the distance of the focal point from this
last lens, to which the rays will be brought. It is evident that the
rays would be brought by the first lens to a focus two inches behind
the second if it were not there. This point, which is negative with
regard to the second lens, must be taken as the value of jo in the
formula. We have, therefore :

9

P'

'A

6

Hitherto our attention lias been confined, in studying the action
of lenses, to the manner in which they act upon a bundle of parallel
rays, or upon a pencil of rays issuing from a radiant point. More-
over, we have considered this point as situated in the line of axis.
But the surface of every luminous body may be regarded as compre-
hending an infinite number of such points, from every one of which
a pencil of rays proceeds, to be refracted in its passage through the
lens according to the laws enunciated. In this way a complete
image, i.e. picture of the object, will be formed upon a suitable
surface placed in the position of the focus.

There are two kinds of image formed by lenses, a real image and
a virtual image.

1. The formation of a real image means the production of a

The formation of a real image.

picture by a lens, or a combination of lenses, which can be thrown
upon a screen ; such are the images of a projection lantern and the
image produced by the camera upon the focussing glass. The manner
in which this takes place will be understood by reference to fig. 26,
where A B is an object placed beyond P, the principal focus of the
aplanatic combination. From every point of A B are rays radiating
at every possible angle. Let A F and A H be two such rays
radiating from the point A. Now if the refraction of these rays be

24 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

traced, in the manner already indicated, throngh the aplanatic com-
bination, it will be found that the rays which before immergence
were diverging are by the refraction of the combination on emer-
gence rendered converging. Thus the ray F C meets H C at the
point 0. The point C is called the conjugate focal point of A, and
wherever there is a focal point there will be an image. Therefore,
at C, there will be an image of A. In the same manner the rays
issuing from every point along A B may be traced, and will be found
to have each one its respective conjugate lying on 0 D, so the con-
jugate of B is at D. Hence it is at once manifest that an inverted
conjugate image of the object A B is formed at C D. Further, it
will be noticed that, although the object is straight, the image of it
is curved towards the lens.

If the object A B had been curved, so that it presented a convex
aspect to the lens, then its conjugate image C D would have been
more curved ; but if A B had been slightly concave towards the lens,
then its conjugate would have been straight.

As before stated, the point C has been determined by tracing
the refraction of two rays,^ A F and A H, through the lens. Another
method is, however, often employed.

In every lens there is a point which is called its optical centre.
This point is such that any ray, which in its refraction through the
lens passes through this point, will emerge in a direction parallel to
its path* before immergence. Now as lenses for graphic and theoreti-
cal purposes are often assumed to be of insensible thickness, it has
become the practice to draw any ray passing through the optical
centre of the lens a straight line. Obviously, if the lens has sensible
thickness the ray cannot be considered a straight line, and in the
microscope, where the lenses are very thick in proportion to the
length of their foci, this method will lead to much error. Of course,
in those cases where it can be taken as a straight line, it saves the
trouble of computing a second ray to intersect the first, as any ray
intersecting the straight line will determine a conjugate focal point.

In the upper jmrt of fig. 26 the two rays, A F and A H, are
traced through the lens to determine the point 0, but in the lower
part of the figui-e only the ray B K is ti-aced, and the intei'section of
this ray by the sti'aight line B D passing through the optical centi'e
gives the point D.

2. An image is said to be virtual when it cannot be received on
a screen. Fig. 27 shows how a virtual image is formed. The
lettei-s are the same as in the preceding figure, so as to show the
jinalogy between the two. The fundamental difierence between
this figure and the last is that the ol)ject A B is placed hetimen P, the
))rincipal focus, and the lens.

We have already seen from fig. 15 that when a I'adiaiit is placed
before a converging lens, and nearer to it than its principal focus,
the rays emerging from the lens are still divergent even after their
i-efraction through the lens ; consequently they will never intersect,

' In the majority of the preceding diiif^rainH the drawing has represented the
facts accurately ; in this instance they are diagrammatic, tlie size of admissible illus-
trations making an accurately traced ray impossible.

FORMATION OF A 'VIRTUAL IMAGE

25

and as thel'e is no focal point, thei-e can be no screen image.
Thus two i-ays i-adiating from the point A of the object A B fall on
the lens and are refracted in the dii-ections A F, AH: these are
divergent and will nevei- meet ; but if the human eye is placed near-
the lens, so that it can i-eceive the rays F and H, the i-ays will be
converged by the lens of the eye, and will be brought to a focal
point in the retina.

Similarly, from every point in A B there will be a corresponding-
retinal point. ISTow if we produce F and H backwards (see the
dotted lines in the figui-e) we shall find that they intersect at the point
C. As the I'ays F and H ai-e precisely identical with luys which
would have divei'ged from the point 0 had it been an entity, the
retinal image therefore will be an image of a non-existent picture
CD.

The method of drawing this is exactly similar to that of the

Fig. 27. — The formation of a ' virtual image.

preceding figure. The rays A F and A H are traced tlii'ough the
lens, and their prolongation backwards (see the dotted lines in the
figure) gives the point C. Also, as in the preceding figure, any
point of the picture can be found by tracing one ray, such as K ;
then the intersection of its backward pi'olongation with a straight
line joining B with the optical centre, produced, will give D.

The points C and D are called the virtual conjugate foci of A
and B respectively. In mathematical optics it appears as a negative
quantity which satisfies an equation, and is a sort of metaphysico-
mathematical truth. In this case the virtual image is convex
towards the lens.

Fig. 27 illustrates the action of a simple microscope. The object
itself is not seen, but the picture presented to the eye is an
enlarged ghost of it. As some eyes can take in rays of less diverg-
ence than others, it might happen that the rays C F, C H, were too
divergent for the observer's eyesight, in which case the lens would

26 ELEMENTAEY PEINCIPLES OF MICROSCOPICAL OPTICS

have to be withdrawn from the object. Similarly, if the observer
were short-sighted, the lens must be placed nearer the object to
render the rays more divergent. Dr. Abbe points out ^ that the
generally adopted notion of a ' linear amplification at a certain
distance ' is, in fact, a very awkward and irrational way of defining
the ' amplifying power ' of a lens or a lens-system.

In the formula N = -- the amplification of one and the same

system varies with the length of Z, or the ' distance of visioai,' and
an arbitrary conventional value of I (i.e. 10 inches, or 250 mm.)
must be introduced in order to obtain comparable figures. The
actual ' linear amplification ' of a system is, of course, different in

Fig. 28. — The amplifying power of a lens.

the case of a short-sighted eye, which projects the image at a dis-
tance of 100 mm., and a long-sighted one, which projects it at
1000 mm. ISTevertheless, the '■ aimplifying power ^ of every system is
always the same for hoth^ because the short-sighted and the long-sighted,
observers obtain the image of the same object under the same vis^tal
angle, and consequently ttte same real diameter of the retinal image.
That this is so will be seen from fig. 28, where the thick lines show
the course of the rays for a short-sighted eye, and the thin lines for
a long-sighted one, the eye in each case being su})posed at the pos-
terior pi-incipal focus of the system.

Tlie other generally ;id()]»ted ex})ression of the power by N =

I

f

may be |iut <iii ;i somewhat more raticmal bji-sis than is generally
done Vjy dcjiniiig the lengtli I (10 inches) not as ' distance of distinct
vision,' but i-athei- as ' di.stance of ja-ojection of tlie image.' As far
as 'distinct visifni ' is assumed for determining tlu^ ;nii])lific;ition,
the valueof X ii;is no real signification at all in i-egard ionii ()l)servei'

' Joiirii. U.M.H. vol. iv. Her. ii. p. 348.

AMICI'S USE OF 'IMMEESION' LENSES 2/

who obtains distinct vision at 50 inches instead of 10 inches, and, in
fact, many microscopists declare the ordinary figures of amplification
to be useless for them because they cannot observe the image at the
supposed distance. It appears as if — and many have this opinion —
the performance of the microscope in regard to magnification
depended essentially on the accommodation of the observer's eye.
This misleading idea, resulting from the common expression, is
eliminated by defining the 10 inches merely as the distance from the
eye at which the image is measured — whether it be a distinct or an
indistinct image. For, if an observer, owing to the accommodation
of his eye, obtains a distinct image at a distance of 10 feet, I may
nevertheless assume a plane at a distance of 10 inches from the eye
on which the distant image is virtually projected, and measure the
diameter of that projection. Now this diameter is strictly the same
as the diameter of that image, which another observer would
really obtain with distinct vision at that same distance of 10
inches.

The only difierence is that in the former case we must take the
centres of the circles of indistinctness instead of the sharp image-
points in the latter case. If the conventional length of ^=10 inches
is interpreted in this way (as distance of projection, independently
of distinct vision) the absurdity at least of a real influence of the
accomnaodation on the power of a microscope is avoided. It becomes
obvious that for long-sighted and for short-sighted eyes the same N
must indicate the same visual angle of the enlarged objects, or the
same magnitude of the retinal image, because it indicates the same
diaraeter of the projection at 10 inches distance.

It was long since pointed out by Amici, that the introduction of
a drop of water between the front surface of the objective, and
either the object itself or its covering glass, would diminish the loss
of light resulting from the passage of the rays from the object or its
covering glass into air, and then from air into the object-glass.
This, which is known as ' water immersion,' was, however, first sug-
gested by Sir D. Brewster in 1813. But it is obvious that when the
rays enter the object-glass from water instead of from air, both its
refractive and its dispersive action will be greatly changed, so as to
need an important constructive modification to suit the new condi-
tion. This modification seems never to have been successfully
effected by Amici himself; and his idea remained unfruitful until it
was taken up by Hartnack, who showed that the application of what
is now known as the im')nersion system to objectives of high power
and large aperture is attended with many advantages not otherwise
attainable. For, as already pointed out, the loss of light increases
with the obliquity of the incident rays ; so that when objectives of
very wide aperture are used ' dry,' the advantages of its increase are
in great degree nullified by the reflection of a large proportion of
the rays falling very obliquely upon the peripheral portion of the
fr'ont lens. "When, on the other hand, rays of the same obliquity
enter the peripheral portion of the lens from water, the loss by re-
flection is greatly reduced, and the benefit derivable from the large
aperture is proportionately augmented. Again, the ' immersion

28 ELEMENTARY PEINCIPLES OF MICROSCOPICAL OPTICS

system ' allows of a greater working distance between the objective
and the object than is otherwise attainable with the same extent of
aperture ; and this is a great advantage in manipulation. Further,
the observer is rendered less dependent upon the exactness in the
correction for the thickness of the covering glass, which is needed
where objectives of large aperture are used ' dry ; ' for as the
amount of ' negative aberration ' is far smaller when the rays which
emerge from the covering glass pass into water than when they pass
into air, variations in its thickness produce a much less disturbing
effect. And it is found practically that ' immersion ' objectives
can be constructed with magnifying powei's sufficiently high, and
apertures sufficiently lai-ge, for the majority of the oixlinary pur-
poses of scientific investigation, w^ithout any necessity for cover-ad-
justment ; being originally adapted to give the best results with a
covering glass of suitable thinness, and small departures from this
in eithei' direction occasioning comparatively little deterioration in
their performance. But beyond all these reasons for the suj)eriority
of the ' immersion system ' is, as will be presently seen, the fact that
it admits into the lens a larger ntimber of ' diffraction spectra ' than
can be possibly admitted by a lens working in air ; and upon this
depends the perfect presentation of the image.

The immersion system has still more recently been advanced upon
by the application of a principle which lies at the root of the optical
interpretation of the images which modei-n lenses present, and
which has greatly increased the value of the microscope as a scientific
instrument. It is an improvement that primarily depends upon a
correct theoretical understanding of the pi-inciples of the construction
of microscopical lenses, and the interpi-etation of the manner in
which the image is realised by the observer. The late Mr. Tolles
was the first to adopt this system, as we point out subsequently ;
but it is to Professor Abbe we are indebted for its practical appli-
cation, through whom it is now known as the homogeneous system.
The word ' homogeneous ' was, however, first applied to microscope
lenses by Tolles (1871), as may be seen in the following passage. . . .
' two hemispherical lenses balsam-cemented, with a diatom or other
small object at the centre, together constituting a nearly homo-
geneous transparent globe' (M. M, J., vol. vi. p. 214). 'The idea
of realising the various advantages of such ' a system by constructing
a certain class of homogeneous objectives had. Professor Abbe
says, ^ ' for some time presented itself to his mind.' ' The matter
assumed, however, subsequently, a different shape in consequence
of a suggestion made by Mr, John Wai-e Stephenson, ... of
London, who independently discovered the principle of homogeneous
immei'sion.' ^

This metliod consists of the i-eplacement of water between the
covering glass of the mounted object and the front svu'face of the
object-glass by a liquid having the same i-efractive and dispersive
power as crown glass. With such a Hnid taking the place of air, it

' On ' Stephenson's System of Homogenous Immersion for Microscopic Objec-
tives' (Abbe), Journ. li.M.S. vol. ii. 187i), p. '257.
2 Ihid.

ABBE'S CONSTRUCTION OF FIEST HOMOGENEOUS OBJECTIVE 29

follows that the correction collar, though still a refinement and aid
in the attainment of the finest critical images, would be a necessity
no more.

The desirability of the construction of a combination of lenses
which would satisfy these conditions was urged by Mr. Stephenson
upon Professor Abbe, and he secured the profound knowledge,
which, as a mathematical optician he possessed, for the complete and
practical solution of the problems involved, and the production of a
remarkable series of lenses, marking a distinct epoch in the progress
of theoretical and practical optics.

He had, in fact, as we have hinted, already approached the con-
sideration of the subject from another point of view, believing that
petrographic work — the study of thin sections of mineral substances —
could be far more efliciently accomplished by the use of homo-
geneous lenses. But in the new aspect in which the problem was
presented by Mr. Stephenson it caixied with it new interest to Abbe,
not only as promising to largely dispense with the ' correction collar,'
but also to greatly enlarge the ' numei'ical aperture,' and therefore
secure a greater resolving power in the objective.

One of the difiiculties was to find a suitable fluid to meet the
necessities as to refraction and dispersion. But after a long series
of experiments Professor Abbe found that oil of cedar wood so
nearly corresponds with crown glass in these respects that it served
the purpose well.

The result of Abbe's calculations based on Mr. Stephenson's sug-
gestion was the construction by Carl Zeiss of a yV^h with a N.A.^
of 1*25 of fine quality, and still higher promise, and subsequently
of a -|^th and a y g-th in. objective of a like character.

It may be well to note that Amici suggested the use of oil
instead of water prior to 1850, and Mr. Wenham again revived
the suggestion in 1870.^ But neither of these is in even a remote
sense an anticipation of the ' homogeneous system ' of lenses as we
now understand it. The ' oil immersion ' in both instances was an
expedient. The principle on which the construction carried out by
Professor Abbe depended was the ' optical ' principle that a medium of
high refractive power gives an aperture greatly in excess of the
maximum (180°) of a dry lens ; while Abbe's explanation, propounded
in 1874, of the important bearing which the diffraction pencils have
on the formation of the mici'oscopic image makes the resolving
power of the object-glass dependent upon the difiraction pencils that
are taken up by it.

All this was unknown or unadmitted by those who had previously
suggested oil as an im'mersion medium, which leaves the homogeneous
system as now employed wholly dependent upon the principles
enunciated by Abbe, arising from the practical suggestion of Stephen-
son and resulting in the beautiful object-glasses of Abbe and Zeiss,
although it is best just to remember that Tolles always maintained
that his immersion objectives had a greater aperture than 180° air

1 The meaning of this expression will be found on p. 49, but the whole of Chap. II.
must be carefully read.

^ Monthly Micro. Journ. vol. iii. p. 303.

30 ELEMENTARY PEINCIPLES OF MICEOSCOPICAL OPTICS

angle. Dr. Royston-Pigott constructed the first aperture table
gi\dng tlie relative values of dry, water, and homogeneous (nascent
pencil) immei'sion objectives ; it is given in M. M. J., vol. iv. p. 26,
(1870).

One of the essential advantages of this system, beyond those
stated, is that by the suppression of sphei-ical aberration in front of
the objective, facilities are afforded for correcting objectives of great
numerical aperture, both in theory and practice, that reduce it to
the level of the problem of correcting objectives of moderate ' angle.'
As a result, stimulated by the manifest advantage to be obtained
and the wants of those engaged in actual research, Messrs. Powell &
Lealand, of London, very soon made a oV^h inch and a g^th inch
objective on the homogeneous principle, with numerical apertures
respectively of ] "38, and during the year 1885 produced lenses of an
excellence impossible to any previous system of -^th inch, xV^^^ inch,
and o\fth inch power, having respectively numerical apertures of
1'50, while 1"52 is the theoretical maximum.

The use of a ' correction collar ' in homogeneous object-glasses
has been dispensed with, correction being obtained by alteration of
the tube length solely, but this miist also be aided in endeavour-
ing to secure the most perfect ' critical images ' by a body-tube pro-
vided with rack and pinion motion ; this should be of the best
quality, and if the object-glass is of perfect construction and of latest
form (apochromatic, q.v.), results never before attainable can be got
with comparative ease.

With such evidence of advance in the optical consti-uction of
microscopes, dependent apparently on such accessible conditions, the
question of what is possible in the futui'e of the instrument no doubt
obtrudes itself; that, however, can only be considei-ed as having
application to the area of our present knowledge and resoui-ces. It
is impossible to forecast the future agencies which may be at the
disposal of the practical optician. To photogi-aph stars in the im-
measurable amplitudes of space, absolutely invisible to Ihe human
eye, howevei- aided, was hai-dly within the pui-view of the astronomei-s
of a quarter of a century ago ; that there may be energies and
methods discoverable by man that will open up possibilities to the
eager student of the miniite in nature which will just as widely
overstep our present methods of ojDtical demonstration, thei'e can be
little reason to question. But it is no doubt true that Math the in-
struments and media now at the disposal of the practical optician
no indefinite and startling advance in microscopic optics is to be
looked for. The ' atom ' is infinitely inaccessible with any conceiv-
able application of all the resources within our reach. But optical
iinpi'ovement of great value, bi-inging natui'e mf)re and more neai-ly
;ind accurately within our ken and reducing more and more cei'tainly
the interpretation of the most difiicvilt textures and constructions
in the mirnitest ac('essil)le tissue to an exact 'method, is cei'tainly
within oui' sight and reach. It is not a small mattei- tliat the homo-
geneous lenses were, in a comparatively short period of timc^, carritid
fi-omaX.A.of 1 "25 to I '.OO ; and this cirricd witii it the capacnty
theoretictdly indicated.

NEW VITEEOUS OPTICAL COMPOUNDS 3 1

High refractive media can greatly reduce the value of even the
wave-length of light, and what is possible in the jDroduction of vitreous
combinations, refractive fluid media, and mounting substances we
may not forecast ; but, judging from the past, we have by no means
reached their limit. At the same time, it may be remembered that
photo-mici-ography, by constantly covering a wider area of applica-
tion with its evei- increasingly delicate and subtle methods, is
more penetrating in the revelation of structui'e than the human
eye.

It may be taken for granted that in the pi'esent state of optical
mathematics the opticians, English, Continental, and American,
have given up the quest of many things fruitlessly sought. Empty
amplification is a folly of lenses of the past. Magnification without
concurrent disclosure of detail is of no more scientific value for the
disclosure of structure than the projection of the photo-micrograph
by an electric arc upon a screen would be. What is needed is an
ever-increasing exactitude in the formation of the dioptrical image.
The imperfection of this at the focal point spi'ings from two causes :
one, as we have just demonstrated, arises from the residual spherical
and chroinatic aberrations, the other takes oiigin in the -loant of homo-
geneity, absolute jyrecision of curve, and perfect centering of the system
of lenses in a combination. This causes the cone of rays proceeding
from the object to unite, not in perfect image points, but in ' light
surfaces of greater or less extent — circles of dissipation ' — which
limits the distinctness of minute details. It is the faults of the ob-
jective that in practice are alone important, and with the crown and
flint glass commonly at the disposal of the optician there are two
great drawbacks to perfection, or rather to an approximation to it.

1. The first arises from the unequal course of the disjMrsion in
crown and flint glass, already described, which makes it impossible
to unite perfectly, with the properties they possess, all the coloured
rays in an image. Absolute achromatism cannot by their means be
attamed, the dispersion at diflferent parts of the spectrum being so
greatly disproportional. It has never been possible to xniite more
than two different colours of the spectrum. The rest, in spite of all
effort, deviate and form the secondary spectrum, leaving, in the very
finest lenses, circles of dispersion not to be excluded.

2. The second defect arises in the impossibility of coi'recting by
means of ordinary crown and flint glass the spherical aberration for
more than oiie colour. If the spherical aberration be removed as
far as may be for the centre of the spectrum, there remains under-
correction for the red, and over-correction for the blue and violet
rays, presenting a want of balance between the chromatic connections
for the central and marginal zones of the objective. Although
perfect chromatic corrections for the central rays may be effected,
giving images of great beauty, the chromatic over-correction for the
peripheral rays with oblique illumination will show the borders of
the image with distinct chromatic fringes.

To compensate these aberrations in the construction of an object-
glass, what is needed is a vitreous material applicable to optical
purposes possessed of such properties that a relatively smaller re-

32 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

fractive index could be united with a higher dispersive power, or a
higher refractive index with a relatively lower dispersive power.
By proper combination of such materials, if they be provided with
ordinary crown and Hint glass to partly remove the chromatic and
spherical aberrations independently of each other, and so to obey
the conditions on which the removal of the chi'omatic difference
depends, these aberrations could be compensated.

All this was seen and fully demonsti'ated and set forth by Abbe
as far back as 1876,^ and lie pointed out that the further perfecting
of the microscope in its dioptrical working was dependent on the
art of glass making ; the production, tliat is to say, of vitreous
compounds possessing different relations of refi^active and disper-
sive power by means of which the secondary spectrum could be
removed.

For practical purposes the matter was in abeyance until 1881,
but since that time Dr. Schott and Professor Abbe, with the active
co-operation of the optical workshops of Zeiss, undei-took the
laborious and prolonged investigation into the improvement of
optical glass, to which we have alluded ; the result has been the
production of ' crown ' and ' flint ' glass possessing exactly the
qualities foreshown as indispensable by Abbe.

By chemical, physical, and optical research of a most laborious
nature, and by spectrometric observations of numerous experimental
fusions systematically carried out with a large vai-iety of chemical
elements, the relation between the vitreous products and their
chemical composition has been more closely investigated.

In the crown and flint glass produced up to the time of these
investigations, the uniformity of property arose from the relatively
small number of materials employed. Aluminium and thallium,
with silica, alkali, lime, and lead, formed the limit. By the use of
more chemical elements, especially phosphoric and boric acid as the
essential constituents of glass fluxes in the place of silica alone, flint
and.crown glass have been produced in which the dispersion in the
different ]mrts of the sjiectrum is nearly proportional ; so that in
achromatic combinations it is now a question of detail and practical
optics to eliminate, almost entirely the secondary spectrum. It is
unfortunate, nevertheless, that a large number of these glasses,
especially those of most value to the optician, have ^^roved to be so
unstable in theii- composition that opticians I'efrain from using them.
It may be hoped that further experiment and research will gi-eatly
reduce this defect. On the other hand, the kinds of glass which
can be used for optical purposes have been so increased in variety
that, while the mean index of refraction is constant, considei'able
vai-iations can l^e given to the dispersion or to tlie i-efractive index
wliile tlie dispersion remains constant. A high index of refraction
is no longer of necessity accf)nii)anied by a. high dis])ersioii in
flint glass, but may l)e retained in ci'own glass with a- low degree
of dispersion.

Tlie [)i'actic;d ciiiix'qiicncc of this is that IidIIi tli<' imperfections

• Hoffman, A. W., Bericht ilher die vmsenschnftlichen Aj^j^arate aitf der Lon-
doner Iviernationalen Aiissfelliniff im Jahre 1876.

ADVANTAGES OF APOCHROMATIC OBJECT-GLASSES ^3

inalienable from an objective constructed of ordinary crown and
flint glass, can be, and have been, eliminated, and the secondary
sj)ectrum. annulled ; it is removed and reduced to a residue of
chromatism of a tertiary character, while the chromatic difference
of spherical aberration can be eliminated or completely corrected
for hvo different colours of the spectrum at once, and therefore
practically for all.

In the lenses formed of the crown and flint glass as used prior
to the new German glass, we were provided with what (in com-
parison with non-achromatised lenses) were called ' achromatic ; '
but in the new system of lenses, which may be ' dry ' or ' homogeneous,'
we have so great a freedom from colour defect as to admit of their
being designated ajwchromatic lenses (d ^privative ; xp'i^i""^ colour ;
a7r6=from, away from ; )(pw/ia=colour).

The practical advantages obtained by this system of object-glass
construction are so great as in delicate researches to be invaluable —
provided always that the work in all its details is of the most perfect
kind. The accidental juxtaposition of lenses of the required curves,
and, relatively, even the careful selection of lenses not homogeneously
related to each other by a unity of purpose and work on the part of
the practical optician, cannot yield perfect results. ' Division of
labour ' is not compatible with perfect results in the making and
building up of an apochromatic lens ; and therefore, in their best
form, these objectives must apparently command a high price. But,
given such an object-glass — which is the production of a thoroughly
competent practical optician — and its advantages, theoretical and
practical, are great.

1 . The aperture of the objective can be utilised to its fall extent.
In the best of the older object-glasses at least one-tenth of the
available aperture was useless ; the inalienable defect in the con-
vergence of the rays prevented a proper combined action of the
outermost zone and the central parts of the aperture, and therefore
by those objectives it has never been possible to realise the amount ^
of resolving power indicated by theory with a given aperture. But
in a well -constructed apochromatic objective — the secondary spec-
trum being removed, and the spherical aberration being uniformly
corrected for different parts of the spectrum — there is a practically
perfect focal concentration of the rays in the image.

2. Increase of magnifying power by means of specially constructed
eye-pieces is also a most important feature of objectives of this class.
The result of this is that great magnifying power can be obtained
by objectives of relatively large focal lengths. We have always
maintained the utility of high eye-piecing under proper conditions,
and with suitable apertures and fine corrections in the objective ;
the physical brightness, we learn from Abbe, in every case depends
only upon the aperture and the total magnifying power ; and it is
of no moment in what way the latter is produced — by means of focal
length of the objective, length of tube, and focal length of eye-piece.

1 Excepting when resolution is effected by light of extreme obliquity. If the
outermost zone of the objective is corrected alone, and that only be employed, at that
limit equally good resolution may be accomplished .

D

34 ELEMENTAKY PEINCIPLES OF MICEOSCOPICAL OPTICS

But he has further shown us ^ that with the best objectives of the
okl construction, and with large apertures, the limits of a completely
satisfactory clearness of image are reached when the SMjoer-amplifica-
tion is four- to six-fold ; that is, when the total magnifying power of the
objective and eye-piece together is four to six times as great as that
obtained with the objective when used by itself as a magnifying
lens. On the other hand, with apochromatic objectives the available
super-amplification — even with the greatest apertures — is at least
twelve- to fifteen-fold, and considerably higher with medium and low
objectives.

3. Achromatism touches almost an ideal jyoint in these objectives.
The images are practically free from colour over the entire area.
This is of great value in photo-micography. The correction errors
of the ordinary achromatic systems are much more powerful as
disturbing influences than in ordinary observation with the eye.

4. In spite of the removal of the secondary spectrum certain
colour deviations of a tertiary nature remained, and are inevitable
in all objectives of great aperture in which the fi-ont lens cannot be
made achromatic by itself. With ordinary achromatic objectives,
from the properties of the glass used, the amount of this is very un-
equal in the central and periphei"al parts, but in the apochromatic
object-glass it is approximately constant for all jyarts of the ope^iini/,
and therefore it allotvs of correction hy the eye-jnece^i a special con-
struction possessing equal but opposite differences of magnifying
power for different colours. The eye-piece is so constructed as to
completely secure the desired result, and, as we have stated above,
images free from colour are obtained.

5. The classification of the eye-pieces for this system of objectives
has been established by Abbe, and depends on the increase in the
total magnifying power of the microscope obtained by means of the
eye-piece as compared with that given by the objective alone. The
number which denotes how many times an eye-piece increases the
magnifying power of the objective, when used with a given body-
tube, gives the proper measvire of the eye-piece magnification, and
at the same time the figures for i"ational numeration. ^

From their propei-ties these are known as ' compensating eye-
pieces.'

The following is a fair typical selection of the objectives and
eye-pieces furnished from the workshops of Carl Zeiss, of Jena, on
this impoT'tant system, viz. :

1 ' On the Relation of Aperture to Power,' Journ. li.M.S. 1883, p. 803.
- ' On Improvements of the Microscope with the aid of new kinds of optical glass '
(Abbe), Journ. li.M.H. 1887, p. 25 et seq.

APOCHROMATIC LENSES

35

Apochromatic Objectives.

Numerical
aperture

Equivalent

focus in

mm.

Initial
magnification

English

equivalent

focus in

inches

Dry. series . . -/

0-30

24-0
16-0

10-5
15 5

1

2
"3

0-65
0-95

12-0
8-0

21
31

1
3
1
3

6-0
4-0
3-0

42
63

83

1
1.

Water immersion .

1-25

2-5

100 ! ^

Homogeneous J
immersion

1-30

30
20
15

83
125
167

1
12

1
16

1-40

3 0
20

83 i
125 Jj

Compensating Eye-pieces for English Bodies.
2 4 8 12 18 27

It is of interest to note that Messrs. Powell and Lealand have
since produced a remarkable lens on the same system, having a N.A.
of 1-50, with a power of -^V^ of an inch. Object-glasses are also now
made by other makers, English, European, and Amei'ican, those
having fluorite in them being termed apochromatic, while others
made of new kinds of glass are called semi-apochromatic. Semi-
apochromats are being daily improved, so much so that some recent
objectives nearly equal apochi'omatic objectives themselves.

T) 2

36 VISION WITH THE COMPOUND MICKOSCOPE

CHAPTER II

THE PBINCIPLES AND THEORY OF VISION WITH THE
COMPOUND MICBOSCOPE

We are now prepared to enter upon the application of the optical
principles which have been explained and illustrated in the foregoing
pages to the coiistruction of microscojies. These are distinguished
as simjile and co'm'pound^ each kind having its peculiar advantages
to the student of nature. Their essential difference consists in this,
that in the former, the rays of light which enter the eye of the
observer proceed directly from the object itself, after having been
subjected only to a change in their course, as we have shown by
fig. 26, which fully explains the action of the simple lens ; whilst in
the compound microscope an enlarged wiaye of the object is formed
by one lens, which image is magnified to the observer by another,
as if he were viewing the object itself. In the compound micro-
scope not less than two lenses must be employed : one to form the
enlarged image of the object, immediately over which it is placed,
and hence called the object-glass ; whilst the other again magnifies
that image, and, being interposed between it and the eye of the
observer, is called the eye-glass. A perfect object-glass, as we have
seen, must consist of a combination of lenses, and the eye-glass is
best combined with another lens interposed between itself and the
object-glass, the two together forming what is termed an eye-piece.
The compound microscope must be the subject of careful and de-
tailed consideration ; but it must be remembered that the shorter
the focus of the simple magnifying lens, the smaller must be the
diameter of the sphere of which it forms part ; and, unless its
aperture be proportionately redxiced, the distinctness of the image
will be destroyed by the spherical and chromatic aberrations neces-
sarily resulting from its high curvatui-e. Yet notwithstanding the
loss of light and other drawbacks attendant on the use of single
lenses of high power, they proved of great value to the oldei- micro-
scopists (among whom Leeuwenhoek should be specially named), on
accotnit of their freedom from the ei'i'ors to which tlie compoiuid
microscope of the old construction was necessarily sulvject ; and the
ajiionnt of excellejit work done by uieans of tliem surprises every one
wlio studies the histoi-y of microscopic^ iiKpiiiy. An iiiipoi-tant im-
provement on the single lens was introduced by Dr. Wollaston, who
devised the doublet^ still known by his name, which consists of two
plano-convex lenses, whose focal lengths ai-e in the pi-oportion of one
to tliree or nearly so, having their convex sides directed towards

PRINCIPLES AND THEORY OF MICROSCOPIC VISION 37

the eye, and the lens of shortest focal length nearest the object. In
Dr. Wollaston's original combination no pei'forated diaphragm (oi-
' stop ') was interposed, and the distance between the lenses was left
to be determined by experiment in each case. A great improvement
was subsequently made, however, by the introduction of a ' stop '
between the lenses, and by the division of the power of the smaller
lens between two (especially when a very short focus is required), so
as to form a triplet, as first suggested by Mr. Holland.^ When
combinations of this kind are well constructed, both the spherical
and the chromatic aberrations are so much reduced that the angle
of aperture may be considerably enlarged without much sacrifice of
distinctness ; and hence for all, save very low powers, such ' doublets '
and ' triplets ' are far superior to single lenses. These combinations
took the place of single lenses among microscopists (in this country
at least), who were prosecuting minute investigations in anatomy
and physiology prior to the vast improvements efiected in the com-
pound microscope by the achromatisation of its object-glasses.

Another form of simple magnifier, possessing certain advantages
over the ordinary double-convex lens, is that commonly known by
the name of the ' Ooddington ' lens.^ The first idea of it was given
by Dr. Wollaston, who proposed to apply two plano-convex or hemi-
spherical lenses by their plane side, with a ' stop ' interposed, the
central aperture of which should be equal to one-fifth of the focal
length. The great advantage of such a lens is, that the oblique
pencils pass, like the central ones, at right angles to the surface, so
that they are but little subject to aberration. The idea was, how-
ever, greatly improved upon by Sir D. Brewster, who pointed out
that the same end would be much better answered by taking a
sphere of glass, and grinding a deep groove in its equatorial part,
which should be then filled with opaque matter, so as to limit
the central aperture ; in other words, Brewster made Wollaston's
plano-convex lenses hemispheres. Such a combination gives a
large field of view, admits a considerable amount of light, and
is equally good in all directions ; but its power of definition
is by no means equal to that of an achromatic lens, and its
working distance is inconveniently small. This form is chiefly
useful, therefore, as a hand-magnifier, in which neither high power
nor perfect definition is required, its peculiar qualities rendering
it superior to an ordinary lens for the class of objects for which
a hand-magnifier of medium power is required. Many of the
magnifiers sold as ' Coddington ' lenses, however, ai-e not really
portions of spheres, but are manufactured out of ordinary double-
convex lenses, and are therefore destitute of the special advantages
of the real ' Coddington.' The ' Stanhope ' lens somewhat resembles
the preceding in appearance, but differs from it essentially in
properties. It is nothing more than a double-convex lens, having
two surfaces of unequal curvatures, separated from each other by a

^ Transactions of the Society of Arts, vol. xlix.

- This name, however, is most inappropriate, since Mr. Coddington neither was,
nor ever claimed to be, the inventor of the mode of construction by which this lens
is distinguished.

38

VISION WITH THE COMPOUND MICROSCOPE

considerable thickness of glass, the distance of the two surfaces from
each other being so adjusted that when the more convex is turned
towards the eye minute objects placed 07i the other surface shall be
in the focus of the lens. This is an easy mode of applying a rather
high magnifying power to scales of butterflies' wings, and other
similar flat and minute objects, which will readily adhere to the
surface of the glass ; and it also serves to detect the presence of the
larger animalcules or of crystals in minute drops of fluid, to exhibit
the ' eels ' in paste or vinegar &c. A modified form of the ' Stan-
hope ' lens, in which the surface remote from the eye is plane instead
of convex, has been brought out in France under the name of
' Stanhoscope,' and has been especially a^^plied to the enlargement of
minute pictvires photographed on its plane surface in the focus of its
convex surface. A good ' Stanhoscope,' magnifying from 100 to 150
diameters, is a very convenient form of hand-magnifier for the
recognition of diatoms, infusoria, &c., all that is required being to
place a minute drop of the liquid to be examined on the plane
surface of the lens and then to hold it up to the light. But no hand
lenses we have yet seen will compare with the Steinheil ' loups ' of
six and ten diameters made by Zeiss, and Reichart's j^ocket loups.
For the ordinary purposes of microscopic dissection single lenses
of fi-om 3 inches to 1 inch focus answer very well. But when higher
powers are required, and when the use of even the lower powers is
continued for any length of time, great advantage is derived from
the employment of achromatic combinations, now made exjjressly
for this purpose b}^ several opticians. The Steinheil combinations
give much more light than single lenses, with much better definition,
a very flat field, longer working distance (which is very important
in mmute dissection), and, as a consequence, greater ' focal depth '
or ' penetration,' i.e. a clearer view of those parts of the object
which lie above or below the exact local plane. And only those
who have carried on a piece of minute and difiicult dissection
through several consecutive hours can appreciate the advantage in
comfort and in diminished fatigue of eye which is gained by the
substitution of one of these achromatic combinations for a single
lens of equivalent focus, even where the use of the
former reverds no detail that is not discernible by the
latter.

Although not strictly its position, it is convenient
here to refer to what is known as the ' Briicke lens ; '
it is much used on the Continent, but does not ap-
])ear in any Englisli treatise we have seen. It has
two a(;liroma,tic lenses for the objective, and a, concave
eye lens. It is illustrated in fig. 29.

To i-emedy the inconvenience of tlie lens being too
clo.se to the object in all but low i)owers, Charles
Clievalier, in his 'Manuel du Micrographe' (1839),
proposed to place abr)\-e a, doublet a, concave achro-
matic lens, the distaiic(( of whicli could l)e varic^d at
pleasure. The effect of this combination is to in(rreasethe magnifying
power and lengtlun tln' f'ucns. Tims ai raiigci], ibis iiistriinient will

Mhbemehsb/

Fio. 29.— The
Briicke lenB.

COMPOUND MICROSCOPE 39

be the most powerful of all simple mici-oscopes, and the space
available for scalpels, needles, &c. will be much greater than
with a doublet alone. The further the concave lens is i-emoved from
the latter, the greater will be the amplification.^ Even in this,
however, Chevaliei* had been anticipated by Professor Joblot in
1718.

This combination, applied to lenses for examining the eye and
skin, allows the use of doublets which leave a considerable distance
above the object, and it is this idea which has govei-ned the con-
strviction of the Briicke lens.

' The lens has a very long focus, and the construction is that of
the Galileo telescope as applied to opera-glasses, but the amplifica-
tion of the objective is much greater than that usually obtained in
opera-glasses. The focus is about 6 cm., and the j^ower three to
eight times. The latter power is obtained by lengthening the tvibe,
by which means the distance between the two lenses is much
enlarged, and the amplification increased without inconveniently
modifying the focus.'

This lens may be used in place of the body of a compound
microscope, when it is desired to dissect or to find small objects, or
it can be adapted to a simple microscope or lens-holder, with from
3 to 8 cm. between the object and objective. But the Briicke lens,
like the Galilean opei-a glass, has a very small field.

Compound microscope. — The compound microscope, in its most
simple form, consists of only two lenses, the ohject-glass and the
eye-glass, and is a Keplerian telescope adapted for viewing very near
objects. The former receives the light -rays direct fr'om the object
brought into near pi-oximity to it, and forms an enlarged but inverted
and reversed image at a greater distance on the other side ; whilst the
latter receives the rays which are diverging from this image, as if
they proceeded from an object actually occupying its position and
enlarged to its dimensions, and brings these to the eye, so altering
their course as to make that image appear far larger to the eye, pre-
cisely as in the case of the simple microscope. It is obvious that,
in the use of the very same lenses, a considerable variety of magnify-
ing power may be obtained by merely altering their position in regard
to each other and to the object. For if the eye-glass be carried farther
from the object-glass, whilst the object is approximated nearer to the
latter, the image will be formed at a greater distance from the object-
glass, and the dimensions of the magnified image will consequently
be augmented ; whilst, on the other hand, if the eye-glass be brought
nearer to the object-glass, and the object removed further from it,
the distance of the image from the object-glass will be less than it
was before, and the dimensions of the magnified image will be
correspondingly diminished. The amplification may also be varied
by altering the magnifying j)Ower of the eye-pieces. In practice,
variations in power must be obtained by altering either the objective
or the eye-piece, or both, and the use of the draw-tube for this
purpose mnst be altogether abandoned, because objectives are

1 Robin, C, Traite du Microscope et cles Injections, 2nd ed. 8vo. pj). 33, 34.
Paris, 1887.

40 VISION WITH THE COMPOUND MICROSCOPE

corrected for a certain length of draw-tube, and, in order that they
may work efficiently, that definite length of draw-tube must be
maintained.

In general it is not advisable to use with an achromatic objective
a greater super-amplification than can be obtained with a 10-power
eye-piece, or with an apochromatic objective that yielded with a
12 or 18 power one.

We shall facilitate the comprehension by the student of the
principles of the modern form of a compound microscope by means
of fig. 30. In this figure the optical portion, that is, the objective and
eye-piece, are drawn to the full size, but the distance between these
has, from the exigencies of space, been much curtailed. A low-
power objective has been specially chosen for simplicity, and a com-
pensating eye-piece (^vide Chapter V.) has been introduced to show
its form and mode of action.

The objective is a copy of an old Ross 1-inch of 1856. The
incident front (that is, the lens on which the incident beams from
the object first strike) is a convex of long radius ; the incident sur-
face of the flint lens of the back combination is a concave of very
long radius, being in fact about twenty inches.

The object F has only rays drawn from one side in order that
a clearer perception of the path of the rays may be seen. This pair
of rays passes from the arrow (object) through the combination of
lenses forming the objective, giving an inverted real image at A B.
This image, in fact, has a convex curve towards the eye-piece : this
is a position that will tend to increase the curvature of the virtual
image C D given by the eye-piece, the inverted image (A B) at the
diaphragm of the eye-piece being the subject of still fui'ther and
often great magnification.

In addition to the two lenses of which the compound microscope
may be considered to essentially consist, it was soon found needful
to introduce another lens, or a combination of lenses, between the
object-glass and the image formed by it, the purpose df this being
to change the course of the rays in such a manner that the image
may be formed of dimensions not too great for the whole of it to
come within the range of the eye-glass. As it thvis allows more of
the object to be seen at once, it has been called the field-glass ; but
it is now usually considered as belonging to the ocular end of the
instrument, the eye-glass and the field-glass being together termed
the eye-piece, or ocidar. Yai'ious forms of this eye-piece have been
proposed by diffei'ent opticians, and one oi- another will be prefei-red
according to the purpose for which it maybe required. That which,
until the construction of the compensation eye-pieces by Abbe, was
considered the most advantageous to employ with achromatic object-
glasses, to the performance of which it is desired to give the greatest
possible effect, was termed the ITuyyhenian, having been employed
by Huyghens foi- his telescopes, although without the knowledge of
all the advantages which its best consti'iiction renders it capable of
affoi-ding. This eye-piece, with others, will be considered in detail
in the chapter (v.) given in part to their considei-ation ; but this
eye-piece consists of two plano-convex lenses, with their plane sides

Fig. 30.— Path of a ray of light through a modern combination of lenses for
compound microscope.

42 VISION WITH THE COMPOUND MICROSCOPE

towards the eye. A ' stop ' or diaphragm, B B, must be placed
between the two lenses, in the visual focus of the eye-glass, which
is, of course, the position wherein the image of the object will be
formed by the rays brought into convergence by their passage
through the field-glass. Huyghens devised this arrangement merely
to diminish the spherical aberration ; but it was subsequently shown
by Boscovich that the chromatic dispersion was also in great part
corrected by it. With the apochromatic lenses of the highest and
best quality (see Chapter Y.) no amount of obtainable eye-piecing, if
it be of the ' compensation ' form, can break down the image. The
editor has tried in vain to break down the image formed by a
24 mm., a 12 mm., a 6 mm., and a 4 mm., all dry apochromatics
by Zeiss, and especially with a Jth by Powell and Lealand. It
is, however, a matter of moment and interest to note that with
good objectives of the ordinary achromatic construction of large
N.A. the compensating eye-pieces give better results than
Huyghenian.

But of the old form of achromatic object-glass it is true of the
majority that they will not bear high eye-piecing. ' B,' \^ inch in
focus, is a convenient and useful eye-piece for viewing large flat
objects, such as transverse sections of wood or of echinus- spines,
under low magnifying powers. A flat large field may be obtained
by means of a Kellner ; but, on the other hand, there is a very
serious falling ofi' of defining power, which renders the Kellner eye-
piece unsuitable for objects presenting minute structural details ;
and it is an additional objection that the smallest speck or
smear upon the surface of the field-glass is made so unplea-
santly obvious that the most careful cleansing of that surface is
required every time that this eye-piece is used. Hence it is
better fitted for the occasional display of objects of the character
already specified than for the scientific requirements of the working
microscopist.

A ' positive ' or Ramsden's eye-piece — in which the field-glass,
whose convex side is turned upwards, is placed so much nearer the
eye-glass that the image formed by the objective lies below instead
of above it — is sometimes used for the jDurpose of micrometry, a
divided glass being fitted in the exact plane occupied by the image,
so that its scale and the image are both magnified together by the
lenses intei-posed l)etween them and the eye. The same end, how-
ever, is also attained with the Huyghenian eye-piece, and it is
doubtful if any advantage is gained by the Ramsden in microscope
work. The coin])ensating eye-piece is also used in conjunction
with the micrometer.

Aperture in microscopic objectives and the principles of micro-
scopic vision. — Jt is now of tJie utmost moment that we should
understand clearly the meaning and importance of 'aperture' in
microscopic objectives, and by that means be led to a pei'ception of
the principles of the most recent and only rational tlieory of micro-
scopic vision. Witliin the last twenty-five years this entire subject
has undei'gone ii ligorous and exhaustive reinvestigation by one
of the most competent jhhI iiuisterly matheuiiiticnl ;i,nd practical

THE FOEMATION OF MICROSCOPIC IMAGES 43

opticians in the world, Professor Abbe of Jena ; and, as a result,
some of the judgments and opinions, as well as what were supj^osed
to be established truths, depending apparently upon the simplest
principles, and not believed to be open to change, have been shown
to be absolutely without foundation ; while pi-inciples hitherto quite
luiknown and unsuspected have been shown to operate and to rest
on clearly demonstrable mathematical and physical bases. The
result has been a complete revolution of what were held to be
fundamental principles of microscopic optics and the theory of
vision with microscopic object-glasses.

Professor Abbe contends that one of the foremost errors relates
to the mode in which microscopic images are formed. It was
assumed that their formation took place on ordinary dioptric
principles. As the camera or the telescope formed images, so it
was assumed that the image in the compound microscope was
brought about. The delicate and complex structure of an insect's
scale or of a diatom were believed to form their images according to
the same precise dioptric laws by which the image of the moon or
Mars is formed in the telescojoe. Hence it was taken for granted
that every function of the microscope was determined by the geo-
metrically traceable relations of the refracted rays of light. We
would nevertheless remark that visibility of detail in, for example,
the moon depends on the aperture of the telescojDe ; of course, what
is known as its ' aperture ' is simply estimated by the diameter of
the object-glass, but accuracy apj)ears to require that n sin u^=^ a
ought to be applied to the telescope. In practice the diameter is
taken conventionally for the sake of simplicity, as it makes no
numerical diiierence, because the sines of small angles such as are
dealt with in the telescope are projDortional to the angles themselves.
The microscope, on the other hand, deals with large angles ; con-
sequently the sine cannot be dispensed with.

But Professor Abbe argues that a close examination in theory
and practice of the conditions of vision with microscopic objectives
shows that such an estimate of apertui'e is wholly wrong in prin-
ciple. The front lens of a -oV'in- objective may be no more than the
^ijyth of an inch in diameter, while a 3-in. objective may have a
diameter of half an inch. Yet it is the smaller lens that has by far
the larger ' aperture.'

Light is dispersed from every point on the surface of an object
in all directions up to ISO'^. Only an extremely narrow pencil of
this can be received by the human eye, a large pencil of light
emanating from the object being lost on each side of what the eye
receives. The apparent problem of practical optics is to be able,
by means of lenses, to gather up and bring to a focus as many of the
unadmitted rays as possible. The general manner in which lenses
act in doing this we have endeavoured in an elementary manner to
show.

Soon after achromatic object-glasses were first made, Dr. Goring
found that the markings on special objects — such as the scales of
the wings of insects — could be seen by some object-glasses, while
with others, although the magnifying power was eqvial, it was im-

44 VISION WITH THE COMPOUND MICROSCOPE

possible to discern them. In every ease the greater ' angle ' was
shown to possess the greater ' resolving ' or delineating power ; and
this led to the important conclusion that power of ' resolution ' in a
lens was dependent upon ' angular aperture.'

This, however, was at a time when only ' dry ' objectives were in
use ; the immersion and homogeneous systems, as we use them, were
unknown.

But (as we shall subsequently see), even with objectives employed
only with air, the angle of the radiant pencil did not afford a trvie
comparison ; when immersion objectives were introduced — objectives
in which water or cedar oil replaced the air between the objective
and the ujDP^i" surface of the cover of the mounted object — the
use of angles of aperture became in the utmost degree misleading ;
for different media with different refractive indices were employed,
and the angle of the radiant pencil was supposed not only to admit
of a comparison of two apertures in the same medium, but also to
be a standard of comparison when the media were different. It
was, in short, believed that an angle of 180° in air represented
a large excess of aperture in comparison with 96° in water and
82° in balsam or oil, denoting, in reality, what was believed
to be the maximum aperture of any kind of objective, which
could not, it was held, be exceeded, but only equalled, by 180°
in water or oil ; in other words, that a radiant pencil has exactly
the same value, when the angles are equal, no matter what the
refractive index of the medium through which the pencil might
be passing.

But to a thorough physical and mathematical study of the ques-
tion such as that in which Professor Abbe engaged, it soon became
apparent that even in the same medium the only exact method of
comparison for objectives — when the fundamental phenomena of
optics (which the older opticians had disregarded) were taken into
account — was not a comparison by the angles of the radiant pencils
only, but a comparison by their sines ; while, when the media are
different, the indices of those media would be found to form an
essential factor in the problem ; for an angle of 180° in air is equal
to 96° in water or 82° in oil ; hence three angles might all have the
same number of degrees and yet denote different values, according
as they were in air, water, or oil.

Thus there might be large divei'gence of aperture in two oi-
more cases while the angle was identical, and from this the greatest
confusion was not only possible but was realised.

A solution of the difficulty was (as we have indicated above)
discovei^ed by Professor Abbe ; and it is to Mr. Frank Cris])'s lucid
exposition of Abbe's elaborate monographs that the Englisli student
is immensely indebted.^ '

The definition of ' aperture ' in its legitimate sense of ' opening '
is slif)wn by Abbe to be obtained wlien we compare the diameter of

' 'On the EKtimation of Aportuic! in the MicroHcope ' (Abbe), Joiirv. It. M.S.
ser. ii. vol. i. :J8H ; ' NoIbh on Aperture, Micrf)scopica] ViHion, and the Vahie of wiclo-
angled ImmerKion Objectives,' ibid. :JOS ; ' The Aperture of Microscope Objectives,'
English Mechariic.

HOW ' APEETUEE ' IS OBTAINED 45

the pencil emergent fi'om the objective with the focal length of that
objective.

It will be desirable to explain somewhat more in detail how
this conclusion is arrived at, as given in Professor Abbe's
papers.

Taking in the first case a siiigle-lens microscope, the number of
rays admitted within one meridional plane of the lens evidently in-
creases as the diameter of the lens (all other circumstances remaining
the same), for in the microscope we have at the back of the lens the
same circumstances as are in front in the case of the telescope. The
larger or smaller number of emergent rays will therefore be properly
measured by the clear diameter ; and, as no rays can emerge that
have not first been admitted, this must also give the measure of the
admitted rays.

Suppose now that the focal lengths of the lenses compared are
not the same — what, then, is the joroper measure of the rays
admitted 1

If the two lenses have equal openings but different focal lengths,
they transmit the same number of rays to equal areas of an image
at a definite distance, because they would admit the same number if
an object were substituted for the image — that is, if the lens were
used as a telescope-objective. But as the focal lengths are difierent,
the amplification of the images is difierent also, and equal areas of
these images correspond to difierent areas of the object from which
the rays are collected. Therefore the higher-power lens, with the ; i i '

same opening as the lower power, will admit a greater number of ■ "'
rays in all from the same object, because it admits the same numbei'
as the latter from a smaller portion of the object. Thus, if the focal
lengths of two lenses are as 2 : 1 , and the first amplifies N diameters,
the second will amplify 2 N with the same distance of the image, so
that the rays which are collected to a given field of 1 mm. diameter

of the image are admitted from a field of ^^ mm. in the first case

and of oTsj ^^^- i^^ ^^^® second. Inasmuch as the ' opening ' of the c

objective is estimated by the diameter (and not by the area), the w,^

(higher-power lens admits i^tu'ce as many rays as the lower jDower, "^ S^^^ ,
because it admits the sam.e number from a field of Act//" the diameter, -^aJ?!*^'
and in general the admission of rays with the same opening ' (\
but different powers must be in the inverse ratio of the focal ' ^ '

lengths.

In the case of the single lens, therefore, its aperture must be
determined by the ratio between the clear ope7iing and the focal
length, in order to define the same thing as is denoted in the telescope
by the ahsohijte opening.

Consider now the compound objective — the most imjjortant case
in the microscope. What is the opening of this composite system ?
We must adhere to the diameter of the admitted cone at that plane
where it has its tdtmiate inaximitm value, which is obviovisly the
diameter of the pencil at its emergence, from the system, or, practi-

46 VISION WITH THE COMPOUND MICROSCOPE

cally, the clear, effective diameter of the hack lens. The emergent
pencil from a microscope -objective converging to a relatively distant
focus has its rays approximately parallel, and the conditions are
once more similar to those of the telescope-objective on the side of
the object. The diameter of this emergent pencil, whether it emerges
from a single lens or from a composite system, must therefore always
have the same signification. The influence of the power on focal
length also remains the same as in the case of the single lens. An
objective with a focal length equal to half that of another admits,
with the same linear opening, twice as many rays as the latter,
because the amplification of the image at one and the same distance
is doubled, and the same number of rays consequently are admitted
by the higher power from a field of half the diameter. And this
will hold good whether the medium around the object is the same
in the case of both objectives or difierent ; for an immersion system
and a dry system always give the swine amplification when the focal
length is the same.

Thus we arrive at the geneiml proposition for all kinds of objec-
tives. First, when the power is the same, the admission of rays
varies with the diameter of the pencil at its emergence. Secondly,
when the powers are different the sa^ne admission requires different
ojjenings in the proportion of the focal lengths, or, conversely, with
the same opening the adinission is in inverse proportion to the focal
length — that is, the objective which has the wider pencil relatively
to its focal length has the larger aperture.

Thus we see that, just as in the telescope the absolute diameter
of the object-glass defines the aperture, so in the microscope the
ratio between the utilised diameter of the back lens and the focal
length of the objective defines its aperture.

This definition is clearly a definition of aperture in its primary
and only legitimate meaning as ' opening ' — that is, the capacity of
the objective for admitting rays from the object and transmitting
them to the image ; and it at once solves the difiicvilty which has
always been involved in the consideration of the apei-tures of
immersion objectives.

So long as the angles were taken as the pi-oper expression of
aperture, it was difiicult for those who were not well versed in
optical matters to avoid regarding an angle of 180° in air as the
maximum aperture that any objective could attain. Hence, watei--
immersion objectives of 96° and oil-immersion objectives of 82°
were looked upon as being of much less apei-tm-e than a di-y objective
of 180°, whilst, in fact, they are all eqaal, that is, they all ti'ansmit
the same i-ays from the object to the image. Tliei-efoi'e, 180° in
water and 180° in oil ai-e miequal, and both ai-e much larger apei--
tures than the 180° which is the maximum that the air objective can
transmit.

If we compare a series of dry Miid oil-iinincrsion objectives, iuid,
commencing with very small aii'-angles, progress up to 180° air-
angle, then taking an oil-immersion of 82° and progressing again to
180° oil-angle, the ratio of opening to power progresses continually

COMPARISON OF OBJECTIVES OF THE SAME POWER 47

also, and attains its maximum, not in the case of the air-angle of
180° (when it is exactly equivalent to the oil-angle of 82°), but is
greatest at the oil-angle of 180°.

If we assume the objectives to have the same power throughout,
we get rid of one of the fiictors of the ratio, and we have only to
compare the diameters of the emergent beams, and can represent
their relations by diagrams. Fig. 31 illustrates five cases of different
apertures of j-in. objectives —
viz. those of dry objectives of
60°, 97°, and 180° air-angle, a
water-immersion of 180° water-
angle, and an oil-immersion of
1 8i0° oil-angle. The inner dotted
circles in the two latter cases
are of the same size as that
corresponding to the 180° air-
angle.

A dry objective of the full
maximum air-angle of 180° is
only able (whether the first sur-
face is plane or concave) to utilise
a diameter of back lens equal to
twice the focal length, while an
immersion lens of even only 100°
(in glass) requires and utilises a
larger diameter, i.e. it is able
to transmit more rays from the
object to the image than any
dry objective is capable of trans-
mitting. Whenever the angle of
an immersion lens exceeds twice
the critical angle for the immer-
sion-fluid, i.e. 96° for water or
82° for oil, its aperture is in ex-
cess of that of a dry objective of
180°.

Having settled the principle,
it was still necessary, however,
to find a proper notation for com-
paring apertures. The astrono-
mer can compare the apertures of his various telescopes by simply
expressing them in inches ; but this is obviously not available to
the microscopist, who has to deal with the ratio of two varying
quantities.

Professor Abbe here again conferred a boon upon microscopists by
his discovery (in 1873, independently confirmed by Professor Helm-
holtz shortly afterwards) that a general relation existed between the
pencil admitted into the front of the objective and that emerging
from the back of the objective, so that the ratio of the semi-diameter
of the emergent pencil to the focal length of the objective could be

Numerical Aperture

1-52

= 180° oil-angle.

Numerical Apertiire

3-33
= 180° water-angle.

Numerical Aperture
1-00
= 180° air-angle
= 96° water-angle
= 82° oil-angle.

Numerical Aperture

•75

= 97° air-angle.

Numerical Aperture

•50

= 60° air-angle.

Fig. 31. — Relative diameters of the (uti-
lised) back lenses of various dry and
immersion objectives of the same
power (q:) from an air-angle of 60° to
an oil-angle of 180°.

48

VISION WITH THE COMPOUND MIGEOSCOPE

expressed by the sine of half the angle of aperture (^t) ^ multiplied
by the refractive index of the medium (n) in front of the objective,
or 71 sin u {n being I'O for air, 1-33 for water, and 1-5 for oil or
balsam).

Fig. 32. — Illustration of the law of consequence for aplanatic systems.

Let 0 and 0* (fig. 32) be the conjugate aplanatic foci of a wide-
angled system ; ic, ~U the angles of inclination of a7iy/ ttoo rays admitted

'- In the original translation of the papers of Professor Abbe from G-erman into
English the G-erman mathematical symbols have been retained. In the summary of

N.A. of objectiye=/u. sin <^=l-5 x •573='

: N.A. of objective.

Angnlaraperture of
objective = 35° +
35° = 70° hi glass,
which is equiva-
lent to the angular

R(T^ /y/T' //'= /■/} aperture of the
60 \^;/' /I -I U co^denser= 60° +

^'sinf^ 60° = 120°i„air.

>'.A. of <;on(lenser=H« sin (j»= 1-0 x -86 = '86

^' sin (/)'= 1-0 X '86 = -86 =N.A. of condenser.

I'lO. Al. — IdtMitity of n sin n (flerman math, form) with ft. sin ^ (English). Also N.A. and
iingnhir aperture.

Abbe'H theoritH and demonKtrations presented in the following pages the Editor has
Hcarcely felt justified in altering this, esjpecially as the German fornj of symbol ob-

EEL ATI VE APERTUKES 49

from the radiant, and to*, U* the angles of the same rays on their
emergence ; then we shall have always

sin U* : sin u* : : sin U : sin u ;

sin U* sin ic* ,

or, =F=r= — : = const. = c;

sm U sm u

that is, the shies of the angles of the conjugate rays on both sides of
an aj^lanatic system always yield one and the same quotient c, what-
ever rays may be considered, so long as the same system and the
same foci are in question.

This proposition holds good for every arrangement of media, and
refi'acting surfaces that may go to the composition of the system, and
for every position of object and image. It is the law upon which de-
pends the delineation of an image by means of wide-angled pencils.

When, then, the values in any given cases of the expression
n sin u (which is known as the ' numerical aperture ' and expressed
by N.A.) has been ascertained, the objectives are instantly compared
as regards their aperture, and, moreover, as 180° in air is eqiial to
1-0 (since ?i=l-0 and the sine of half 180° or 90° = 1-0), we see with
equal readiness whether the aperture of the objective is smaller or
larger than that corresponding to 180° in air.

Thus, suppose we desire to compare the relative aperture of three

tains iu our Universities, and is thoroughly understood amongst University men.
But to those unaccustomed to matliematical formulas confusion might easily arise
from the juxtaposition of different symbols meaning precisely the same thing. To
meet the possible necessity of these this footnote is inserted with an accompanying
diagram to illustrate the identity of ' n sin it ' with ' fj. sin cp.'

The student who has mastered Snell's Law of Sines, given and illustrated on p. 3
(fig. 1), will by a glance at the figure Al on p. 48 understand the meaning and import-
ance of the expression ' N.A.' {numerical aperture) and at the same time will grasp
wherein it differs from ' angular aperture ' (q.v.). He will also perceive how it comes
to pass that an angular aperture of 70° in glass is equivalent to an angular aperture of
120° in air.

In the figure the upper hemispherical lens represents the front of a homogeneous
immersion objective. It is supposed to be focussed on an object in contact with the
lower side of a cover-glass. Between the plane front of the lens and the upper surface
of the cover-glass is a drop of oil of cedar-wood, whose refractive index is I'o, being
thus identical with the cover-glass and the front lens.

It is understood that no slip is used, and that there is nothing between the object
and the front lens of the condenser.

In this case the axis A B is the normal (p. 5, fig. 2) ; on the left-hand side there
is a ray which makes an angle of 35° with the normal in glass issuing into air on the
right-hand side of the normal. By Snell's formula (p. 3) —
/JL sin <\> = fj! sin <p' ;

• , , u sin d) 1"5 x 'STS .„„

sm (t) = c^ i= =86;

fi' 1-0

(^' = 60° (from Table I.)
Therefore the ray on emerging from the under surface of the cover-glass will make
an angle of 60° from the normal.

The dotted lines show the path of the ray luhere the German symbols are used,
n sin u = n* sin u* ;

* n sin 7.(. 1'.5 x '573 „„

sm u* = = , = -86.

n* 1-0

it* = 60° (from Table I.)
Numerical aiJerture, therefore, is the sine of half the angular aperture multiplied
by the refractive index of the medium.

It will be observed that the rays passing through the oil of cedar enter the front
lens without refraction ; this is due to the fact that the media in which the rays are
travelling are of the same refractive index, i.e. they are homogeneous.

E

VISION WITH THE COMPOUND MICROSCOPE

ives, one a dry objective, the second a water-immersion, and

ird an oil-immersion. These would be compared on the

aperture view as, say, 74° air-angle, 85° water-angle, and

[alsani-angie ; so that a calculation must be worked out to

at a due appreciation of the actual relation between them.

however, 'numerical' apertvu^e, which gives '60 for the di-y

ve, "90 for the water-immersion, and 1'30 for the oil-immersion,

elative apertures are immediately apj)reciated, and it is seen, for

instance, that the aperture of the water-immersion is somewhat less

11 that of a dry objective of 180°, and that the aperture of the

oil-immersipn exceeds that of the latter by 30 per cent.

When these considerations have been appreciated, the advantage
possessed by immersion in comparison with dry objectives is no
longer obscured. Instead of this advantage consisting merely in
increased working distance or absence of correction-collar, it is seen
that a wide -angled immersion objective has a lai'ger aperture than
a dry, objective of the maximum angle of 180° ; so that for any of
the purposes for which apei-ture is desired an immersion must
necessarily be preferred to a dry objective.

1. There exists then a definite ratio between the linear opening
and the focal length of a system, which must be entirely indepen-
dent of the composition and arrangement of the system, and solely
determined by the above-mentioned aperture equivalent of the
admitted cone of rays. When the equivalent is the same we have
always the same proportion of opening to focal length, whatever may
be the particular arrangement of refracting media in the system.

2. If the objectives whose apertures are compai'ed work in the
same medium, and admit angles of, say, 60°, 90°, 180°, their aper-
tures are not in the ratios of those numbei's, but are as "50, '70, and
1"0. The 180°, for instance, does not represent three times the aper-
ture of the 60°, but tioice only.

3. If the objectives work in different media, as air and oil, the
latter may have an aperture exceeding that of a dry objective of
180° angle. For with the dry objective the refractive index {ii) and
the sine of half the maximum angle {u) botli=l, so that n sin v,
= 1 also, whilst with the immersion objective n is greater than 1 (say
1'5 for oil), and the angle u may therefore be mucli less than in the
case of the dry objective, and yet the value of the expression n sin i('
(i.e. the aperture) may be greater than 1*0.

The two latter deductions are so directly opposed to what was
accepted by the older opticians and microscopists that a closer if
brief consideration of some of the points which bear upon this brancli
of the subject may here be serviceably summarised.

Take, first, the case of the modiiuii being tlie sajne.

Difi'ei'ence of ajierture involves a diflei'ent (jiiantity of light ad-
mitted to the objective provide<l all other circunistaiices are e({ual.
Hence the question of aperture leads to the consideration of Wxej^hoto-
'fiietrical equivalent of different apei'tures or ajx^i-tinc ;iiigl(\s. It is
not of the essence of the })roljU;m, but it affords an additional illus-
tration of numei'ical aperture, and is thus of gi-eat service in its
exposition. It is manifest that aperture cannot be based on quantity

UNEQUAL INTENSITY OF EMITTED LIGHT

51

of light alone — more light can always be obtained in the image by
throAving more upon the object — but no increase in the amount of illu-
mination can make a dry lens equal in perfoi'mance as regards aperture
to a wide-angled immersion lens.

The popular notion of a pencil of light may be illustrated by
fig. 33, which assumes that there is equal intensity of emission in
all directions, and that the intensity of a jjortion of the pencil taken
close to the perpendicular is identical with that of another portion
of equal angular extension, but more removed from the perpendicular.
On this view, therefore, the quantity of light contained in any given
pencils may be compared by simply comparing the contents of the
solid cones.

When, however, aperture is considered, and the values of n sin u
are worked out for particular cases, they are seen to differ from

Fig. 33. — Diagram showing erroneous inference as to the intensity of emitted rays.

those obtained by estimating in the above manner the amount of
light in the solid cones, and some perplexity naturally arises from
the supposition that the measure of the aperture of the objective does
not correspond to that of the quantity of light which it admits.

All this arises from the mistaken assumption that a luminous
pencil is properly represented by fig. 33.

In the last century (1760) Bouguet^ and Lambert ^ established
the important fact that
with any surface of ■uni-
form radiation the inten-
sity of the emitted rays is
not the same in all direc-
tions. The i^ower of emis-
sion and the intensity of
the rays (i.e. the quantity
of light emanating from
a given surface-element
within a cone of given
narrow angle) varies in
the proportion of the co- „ „. m ■ . -^ j. -x, i • ^^i

(• A 1 (.IT ■ -ciG. o4. — Ihe intensity or emitted rays is not the

S%ne ol the angle of obliqui- game in all directions,

ty under which the ray is

emitted ; in other Avords, in the proportion of the cosine of the angle
of defiection from the perpendicular to the luminous surfiice under

^,^.^35===

!SS°~~!»^

r^l

P,*C" ' \

Traite d'Optiqae sur la Gradation de la Laniiere, 1760.
Photometria, 1760.

E 2

52 VISION WITH THE COMPOUND MICROSCOPE

which the ray is sent out. The rays are more intense in proportion
as they are inclined to the surface which eraits them, so that a pencil
varies in proportion as it is taken close to or is removed from the
perpendicular. A pencil is not, therefore, correctly represented by
fig. 3.3, but by fig. 34, the density of the rays decreasing continuously
from the vertical to the horizontal.

Owing to the different emission in difierent directions, the quan-
tities of light emitted by an element in the same medium in cones of
difierent angle such as w and lo', fig. 35, are not in the ratio of the
solid cones, as would be the case with equal emission, but iii the
ratio of the squares of the sines of the semi-angles so that the squares
of the sines of the semi -angles constitute the true measure of the
quantity of light contained in any solid pencil.

When, therefore, the medium is the same, it is seen that there

Fig. 35. — The unequal emission of rays.

is no conti"adiction between the measure of the aperture of an ob-
jective {n sin ti) and that of the quantity of light admitted by it
the latter being {n sin w)^.

The simplest experimental proof of the unequal emission in
difi'ei-ent directions will be found in the fact that the sun, the moon
the porcelain globe of a lamp or any other bright spherical object
with so-called uniform radiation in all directions, is seen projected
as a surface of equal brightness. If there were equal intensity of
emission in all directions, what would be the necessary result?
Compare two equal portions of tlie surface, one, a (fig. 36), perpen-
dicuhir to the line of vision, and the other, h, greatly inclined.
Eveiy infinitesimal surface-element of h sends to the pupil of the eye
a cone of the same angle u' , as a similar point of « (the sliglit differ-
ence of the distance from the eye being disregarded). If the in-
tensity of the rays were equal as supposed, the whole area h would
Hend to the eye the same quantity of light as the equal area a,
since both areas contain exactly the same numbei- of elements. But
the 'whole quantity of light from h woidd l)e projected upon a
smaller area of the retina tbaii tliat from a (as b a.))p('ars under a,
smaller visual angle, liciiig diiiiiiiisluMl uccoi'diiig to tlic obliquity, oi-

RADIATION OF LIGHT IN DIFFEEENT MEDIA

5,

as 1 : cos w). Consequently, if the assumption were true, b must
appear to be brighter than a, and the sphere wotxld show increasing
brightness from the centre to the circumference. Close to the
margin the increase ought to be very rapid, and the brightness a-
large multiple of that at the centre.

This, as is well known, is not the case, the projection of the
sphere showing equal brightness. The quantity of light, therefoi-e,
emitted from b within a given small
solid cone ^t' in an oblique direction
must be less than that which is emit-
ted from a within an equal solid cone
M in a perpendicular direction, and the
intensity of the rays must decrease in
the proportion of 1 : cos lo when the
obliquity to increases.

As then in one and the same
medium the number of rays conveyed
by a pencil and the photometrical
quantity of light are proportional, this
theorem of Lambert, established for
more than a century, is sufficient of
itself to overthrow the very basis of
the a7igular expression of aperture,
and to prove that, when we are dealing
with one and the same medium ojily,
the angle is not the sufficient expres-
sion, but that it is the siiie of the sevii-
angle which must be taken.

We may pass now to the case of the
media being different ^ as air and oil,
and comparing the aperture of a dry
objective of 180° with that of an oil-im-
mersion objective of 100°, the values of
n sin %i (or the ' numerical ' aperture)
give l"Oforthe former and 1'17 for the
latter, which is therefore represented to
have a larger aperture than a dry ob-
jective of the greatest possible angle.

In this case also considerable per-
plexity has arisen. It has been assumed
that the total amount of light emitted
fr'om a radiant point under a given
fixed illumination must be the same,

whether the point is in air, water, or oil, and that that being so,
the 180° admitted by the dry objective must represent a maximum
quantity of light, a 'whole' which cannot be exceeded, but only
equalled, by a water- or oil-immersion objective. The numerical
aperture notation giving figures in excess of I'O (which represents
180° in air) is consequently supposed to be clearly erroneous and
misleading. Here the whole difficulty lies in the absolutely false
assumption that there is identity of radiation in different media.

Fig. 36. — Diagram of a bright
spherical object emitting
light.

54 VISION WITH THE COMPOUND MICEOSCOPE

In 1864 R. Clausms established, by distinguished research, the propo-
sition ^ that the poioer of emission of a body — in regard to heat as well
as light — is not the same in different media, but varies in the ratio
of the squares of the refractive indices, so that the whole emitted
light from any surface-element of a self-luminous body is increased
in the proportion of 1 : 7^^ when this body is brought from air into a
denser medium of refractive index n. If a glowing body at a con-
stant temperature, such as a bar of iron, could be immersed in a
medium of I'D refractive index in such a way that the surface were
in optical contact with the medium, and the eye of the observer im-
mersed likewise in suitable conditions, the body would be seen brighter
in all directions in the j^i'oportion of 9 : 4 than it appeared in air.

The whole hemisphere of radiation in air is indeed less than the
whole hemisphere of radiation in water or oil, as the squares of the
refractive indices of the media, viz. as 1"0, 1'77, and 2'25.

Thus it is seen that the quantity of light emitted from an object
under a given illumination is not measured by the angle of the
emitted cone at the radiant, nor can it be measured in any way by
means of the angle alone. The quantity depends under all circum-
stances on the 'product of the sine of the semi-angle and the refractive
index of the medium in 'which the object is luminous, and is expressed
by the square of this product, or by the square of the ' numerical
aperture ' of the pencil.

It thus follows that the estimation of the quantity of light is
found to be in complete accordance with the expression of aperture?
We are now prej^ared to advance to another point. It was a
view very commonly held until recently, that the superiority of im-
mersion objectives over dry ones was confined to the case of the
former being used with balsam-mounted objects.

If we have a pencil in air, say 170°, as shown in fig. 37, a dry
objective of large aperture will be able to admit it. If, however, the
object is in balsam, as in fig. 38, it is no longer possible for so large
a pencil to emerge from the balsam. The rays shown by the dotted

Fui. 37.

lines in fig. 38 will be totally reflected by the cover-glass, and only
those within a smaller angle of 82° will pass out. Altliough these
are expanded into 180° on emerging into air, of which the objective
takes up 170°, yet this 170° contains, it is supposed, less light than
the 170° in fig. 37, as it has been 'diluted' by the refraction.

1 ' Ueber die Concentration von Wiirme- und Lichtstrahlen &c.' Fogg. Annaloi
d. Physik, cxxi. 1864.

^ Fig. A 2 gives a good, practical illuKtration of the relative illuminating power of
objectives of varying apertures, and at the same time affords a simple explanation
of the reason why (ii sin '«)- is a measure of this illuminating power. Let the circles
A and B represent the backs of two objectives of the same jjower but of different
apertures ; then the radii C D and E P will represent the angle n sin u (or /x sin </>)
in fig. A 1 (p. 48, note). Now because the areas of circles are to one Another iu the

RADIATION IN AIE AND BALSAM

55

A dry objective was therefoi-e supposed to be placed at a disad-
vantage when used upon balsam-mounted objects, its ajjerture being-
supposed to be ' cut down ' by the balsam, and the advantage of the
immersion objective was considered to rest on the fact that it
restored, in the case of the balsam-mounted object, the same condi-
tions as subsisted in the case of the dry-mounted object, allowing as
large (but no lai'ger) an aperture to be obtained with the former
object as is obtained by the dry objective with the latter.

The error here lies in the assumption of the identity of radiation
in air and balsam. If there were in fact any such identity, the

170° IN AIR

Fig. 38.

conclusion above referred to would, of course, be correct, for if in
fig. 37 the air pencil of 170° was identical with the balsam pencil of
170° (shown by the dotted lines in fig. 38), there would necessarily
be a relative loss of light in the latter case in consequence of so
much of the pencil being reflected back at the cover-glass.

When, however, the increase of radiation with the increase in
the refractive index of the medium is recognised, the mistake of the
preceding view is appreciated. The 170° in air of fig. 37 is not
equal to, but much less than, the 170° in halsatn of fig. 38, and not-
withstanding that a great part of the latter does not reach the

proportion of the squares of tlieir radii, it follows that if we designate the radius by
11 sin u (or fx. sin (p), the area of the circle A will be to the area of the circle B as the
square of the radius of A is to the square of the radius of B, or as [n sin u)'' is to
(»' sin ti'Y.

Fig. a 2. — The backs of two obiectives of the same power but diflferent apertures.

The student will observe that the radius of B is twice that of the radius of A ;
consequently the area of B will be four times as great as that of A ; which means
that, since the numerical aperture of the objective B is twice as great as that of the
objective A, its illuminating power will be four times as great.

56 VISION WITH THE COMPOUND MICROSCOPE

objective in consequence of total reflection, yet the remainder (80°)
which does reach it is the exact equivalent of the air-pencil of fig. 37,
the two air-pencils of 170° being in all respects identical.

The immersion objective, therefore, which is able to receive the
Avhole balsam pencil of 170° (dotted lines in fig. 38), takes up a
greater quantity of light than the air pencil of fig. 37, and so not
merely equals the dry objective but surpasses it.

Let it be specially noted that in dealing with the quantity of
light in connection with aperture, the idea has not been that we have
been engaged with what is in any sense essential, but to remove a
difl&culty felt by many. It must be clear to all that if a greater
aperture signified nothing more than a greater quantity of light, that
is to say, if there were no such specific difierence of the rays which
can be utilised by difierent apertures, as we have demonstrated
above, the whole question woiild be of quite subordinate interest.

Another svibject requiring some further elucidation here is the
' different angular distribution of the i-ays in different media.' The
essence of the idea of ' aperture ' is relative opening. However
defined, its significance can only be appreciated by taking into
account the image-forming pencil emergent from the objective, and
the change in its diameter consequent upon the admission of difierent
cones of light. This diameter afibrds a visible indication of the
number of rays (not mere quantity of light photometrically, which
can be readily varied) which are collected to a given area of the
image, and which must have been gathered in by the lens from the
conjugate area of the object. \^ If the diameter of the emergent jDencil
is seen to be increased, whilst the amplification of the image and the
focal length are unchanged, it is clear that the objective must have
admitted more rays from every element of the object because it has
collected more to every element of an equSiWy enlarged mvagQ. Mani-
festly we get an accurate measure of what is admitted into an objective
by being able to estimate what it emits. It is physically impossible
that a system of lenses should emit more light than it has taken in.

Hence ' aperture ' means the greater or less capacity of objectives
foi' gathering-in rays from luminous objects.

Wlien the admitted j^encil is in the same medium, we see the
additional portions of the solid cone from the radiant, which corre-
.spond to the additional portions of the enlarging opening. But if in
any other case (e.g. where the medium is diflferent) we see that a
certain solid cone. A, from a radiant is transmitted through a certain
opening, «, and that another solid cone of rays, B, cannot be tiuns-
mitted thi'ough the same opening, a, but requires a wider one, /8,
whilst all other circumstances, except those of the radiant, have
lemained the same, we can only conclude that the pencil B must
contain rays which ai-e not contained in A, even if the admitted cone
is not increased in size. For the additional portion {(i-ru) of the
wider opening, /3 conveys rays to the image which are cei'tainly not
conveyed by the smaller opening a. From the radiant only can this
surplus come, and the pencil B which requires the additional o])ening
must embrace more rays, even if it should not he of greater angle.

A given objective may, in fact, collfct the rays from a radiant in

RADIATION IN AIE AND BALSAM 57

air almost to the entire hemisphere, and it then utilises a definite
opening double its focal length. But when the radiant is in balsam
(without any other alteration), the same opening is seen to be utilised
by the rays which are within a smaller cone of not more than 82°,
and rays which are outside this cone require a surplus opening which
is never required for rays in air.

This holds good whether there be refraction or no i-efi-action at
the front surface of the system ; the difference is based solely on the
difference of the medium. Consequently we arrive at the conclusion
that the solid cone of 82° in balsam embraces the same rays which,
in air, are embraced by the whole hemisphere, and every wider cone
in balsam exceeding the 82° conveys 7nore rays from the object than
are admitted by the whole hemisphere of radiation in air.

It follows, therefore, that the same rays which in air are spread
over the whole hemisphere are closed together or compressed in
balsam within a narrower conical space of 41° around the perpen-
dicular, and all rays which travel in balsam outside this cone con-
stitute a stirphcs of 7ieio rays, which are never met with in air — that
is, are not emitted ivhen the object is in air. The loss which takes
place in the latter case can never be compensated for by increase of
illumination because the rays which are lost are different rays
physically to those obtained by any illumination, however intense, in
a medium like air.

In the paper of Professor Abbe there is an elaborate and careful
elucidation of this change in the angular distribution of the radiating
light when the medium is changed ; but to Mr. Crisp's paper on the
same subject, giving an exposition and simplification of Abbe's de-
monstration, the novice will look with the utmost pi-ofit.^ The
following extract will give clearness and emphasis to the above
deductions of Abbe : —

' If we take the case of refraction, then one of the most funda-
mental of optical principles shows that the same rays which in air
occupy the whole hemisphere

are compressed in a medium of 0/^ so" c"

higher refractive index within air ivns^^^^^^ -
a smaller angle, viz. twice the ^^^"^^^ — ^^^

critical angle. If in fig. 39 ^ ^ \suDS

the object is illuminated by an <* so'

incident cone of rays of nearly Pig. 39.— Comparative compression of

82° within the slide, and the light I'ays in two different media.

slide has air above in the first

case and oil in the second, it is obvious that the same ray which is
incident on the object at nearly 41° will always emerge in air at an
angle of nearly 90° {a'), and in oil at nearly 41° {a"), so that the
same rays which in air are expanded over the whole hemisphei'e are
compressed into 82° in oil, and, therefore, rays beyond 82° in oil
must represent surplus rays in excess of those found in the air-
hemisphere.

' If, on the other hand, the case of diffraction is considered, then
Fraunhofer's law shows that the same difiracted beams which in aii-
1 Journ. B.M.S. ser. ii. vol. i. p. 303.

VISION WITH THE COMPOUND MICEOSCOPE

occupy the whole hemisphere (fig. 40) are in oil compressed within
an angle of 82° round the direct beam (fig. 41), so that there is i-oom
for additional beams.'

The unequal equivalent of equal angles becomes, therefore, a de-

FiG. 40. — Diffracted beams in air.

Fi(i. 41.~DilTi'aeted Ijeanis hi oil.

monstrated truth — a truth which is capable of experimental proof by
every owner of a fair microscope.

Any one possessing a dry object-glass of an aperture of 170°, for
example, may readily do so. In this case, a, «, fig. 42, will represent

Fig. 42.

the pencil radiating from an object in air, and capable of being
taken up by that objective. This pencil, on its emergence from the
back lens of the combination, will present a diameter somewhat less
than twice the focal length of the objective presented in fig. 43.
But let the object be now placed in Canada balsam and
covered in the usual way ; the angle of the pencil, by
the greater refractive power of the medium, will be de-
monstrably reduced to 80°, as shown in fig. 44. But it
will be found, on examination of the emergent pencil
from the back lens, that this pencil occupies exactly the
same diameter (fig. 43) as before. The medium in which
the object is has not, of course, altered the poioer of the
and since the diameter of the emergent pencil is the same
in both cases, the ratio of ' opening ' to focal length, which is the
aperture, is the same also. Hence it is seen in the simplest way
that different angles in media of different refractive indices may

170° IN AIR

Fig. 43.

objective

r^

Fig. 44.

denote equal apertures, and equal angles in difi'ereut mediii denote
different apertures.

That ' immersion ' objectives may have greater apertures than
the maxinnnn attjiinable by a dry objective is capable of equally
simple proof 1)y accessible experiment.

If an oil-iniinersioii objective of 122° balsam a-iigli; ))(' taken, and
.so illuminated tliat the whole a])erture is filled with tlie incident rays,
and if we u.se first an object mount(!d in air, we really find tliat we

DIFFEACTION EAYS AEE IMAGE-FOEMING

59

OBJCCTIN AfR ^
SLIDE. \

FRONT LENS

IMMERSION FLUID
i^ COVER CLASS

Fig. 45. — Diagram illustrating difference of emerging
pencil without and with balsam.

have a dry object-glass of nearly 180° angular aperture. This is readily-
seen by fig. 45. By the arrangement presented in the figure the cover-
glass is practically the
first surface of the ob-
jective, for the front
lens, the immersion
liuid, and the cover-
glass are all homo-
geneous, and of the
same refractive index,
and consequently they
form a front lens of
extra thickness. When
the object is close to

the cover-glass the pencil radiating from it will be very nearly 180°,
and the emergent pencil will be seen to utilise so much of the back
lens of the combination as is equal to twice the focal length of the
objective, as shown in the inner circle of fig. 46.

If now we i-un Canada balsam beneath the cover-glass so as to
immerse the object, the pencil taken up by the objective is no longer
180°, but only 122° ; but in spite of that the diameter
of the emergent pencil is larger than it was when the
angle of the pencil was 180° in air, and is represented
by the outer circle in fig. 46. In both these cases
the power is identical ; it follows, therefore, that the
greater diameter of the emergent pencil from the
back of the combination denotes the greater aperture.
of the immersion objective over that of the dry one,
although it possessed an angle of 180°. From this escape is impos-
sible, and it is for this reason that opticians make the back lenses of
their immersion object-glasses larger than those of dry ones of the
same power.

Many further illustrations might be given, but none afibrding
greater facility than the following, viz. : ' Select a good specimen of
Amphipleiira pellucida, and use oblique illumi-
nation, bringing out clearly the striation.

' On removing the eye-piece, placing the
pupil on the air-image of the diatom, and
looking down on the lens, the direct incident
beam will be seen emerging as a bright spot,
and exactly opposite and close to the -margin
a faint bluish light (see fig. 47). If now a
small piece of paper is placed on the back lens
of the objective so as to just cover up the blue
light, and the eye-piece is replaced, the diatom
is still visible, but all the striation which was
imaged by the blue marginal light has entirely
disappeared. The latter must therefore consist
of image-forming rays.'

Enough has thus been advanced to enable the student of even
the elementary principles of modern object -glass construction to

Fig. 46.

Fig. 47. — Back of lens
on removing eye-
piece when A.-pellu,-
cida has been resol-
ved, showing spot of
bright light and faint
bluish spot opposite.

6o VISION WITH THE COMPOUND MICROSCOPE

demonstrate foi' himself that immersion lenses not only possess an
excess of aperture over dry lenses, but that the rays so in excess are
image-forming.

The refractive indices of (cedar) oil, water, and air are respec-
tively 1"52, 1"33, and I'O. 'Angular apertin^e' claimed that the
angles of the admitted pencils to lenses of these three constructions
expressed equal ' apertures.' But this is a fallacy, now so palpable,
but wdiich has exerted an influence so deterrent on the progress
of the construction of our higher object-glasses and condensers,
that its final disappearance as an unjustified assumption which had
crept into the area of theoretical and joractical optics, unverified by
facts and devoid of the wedding garment of deduction, is a triumph
which will make the name of Abbe long and gratefully remem-
bered.

The principle upon which increase of numerical aperture g^ives
increased advantage to an object-glass manifestly needs careful
study and elucidation. We have but to refer to the best w^ork done
by those who have employed the microscope to any scientific purpose
for the past fifty years to discover that there has been an admission,
which has steadily strengthened, that by enlargement of aperture an
increase in the efficiency of the objective, when well made, was
inevitable. During the last thirty-five years this has been especially
manifest. To increase the aperture of an objective vinder the name
of greater ' angle ' has been the special aim of the optician and the
constant and increasing desire of all workers with moderate and
high powers.

The true explanation of this is quite independent of any con-
sideration of apei'tures in excess of the maximum in air, and indeed
of the whole question of immersion objectives. The old view that
all high and excellent results depended on the angle at which the
light emerged from the object, involving some assumed property of
a special kind in the obliquity as such, has been most tenaciously
held ; but it is an x in the problem which has not only never been
demonstrated, but the scientific explanations of all the optical
properties of lens combinations in the formation of images by means
of numerical aperture, prove that it is hopeless to attempt to attach
any value to angle as angle.

About thirty years ago it presented itself to Professor Abbe as
a problem worthy of most careful inquiry as to why gi-eat ' angle '
or obliquity as such gave to objectives an enhanced capacity in the
disclosure of obscure sti'uctui-e. The first step was a consideration
of the grounds on which the theory of the value of ayigle of aperture
rested. But no such basis was found to exist ; no investigation of
the question had been ma,de. It was demonstrated tliat a pencil of
1 70° would show minuter structure than one of 80° in the same
medium ; and from this a, generalisation ]ia,d been made that upon
the obliquity of the 'angle' of light de])ended the delineating power.
It v'as taken as a, salf-eoldent propositio'ih that tlte formation of the
image in the microscope took place in every pa/rticular according to
tfie same dioptric latas hy which im/iges are formed in the telescope,
and it wa,s tacitly taken for gi-aiited that evo'y fniiction of tlic

'ANGLE' AS SUCH OF NO VALUE 6 1

microscope was determined by the geometrically traceable relations
of the refracted rays of light.

A prolonged course of able and exhaustive experiments con-
ducted by Abbe showed that, whilst the old view held good in
certain cases capable of definite verification, yet that the vast
majority of objects, and especially those with which the highest
qualities of an objective are called into operation, the jDroduction of
the microscopic image is wholly and absolutely dependent, not upon
the obliquity of the rays to the olject, as it had been so long and so j
stoutly maintained, but upon their obliquity to the axis of the micro- ^
scojje.

Such coarse objects as require only a few degrees of ajaerture
to disclose them are dependent on ' shadow efiects ; ' but when
extremely minute and delicate structures are to be disclosed small \
apertures are absolutely useless, and mere increase of obliquity of J ■
pencil as such is powerless to alter the result. It can be effected
only by increased numerical aperture, showing that the greater
obliquity of the rays incident on, or remitted from, the object is not,
and cannot be, of itself an element in the superior optical perform-
ance of greater aperture. If it were so, all the results of increased
aperture would be secured by inclining the object to the axis of the
microscojje ; but it may be readily tested that when a given object
cannot be ' resolved,' or its structure delineated, by an objective with
an aperture of 80° in the ordinary position, but can be resolved in
the ordinary position by an objective with an aperture of 90°, no
inclination of the object to the axis of the instrument will enable the
objective of 80° to do the work easily done by one of 90°. This
may be tested by any one possessing the instruments.

As a matter of fact, this so-called but imaginary ' angular grip '
is greater in a wide-angled dry lens than in one of 90° balsam-angle,
and it is certainly cvit down more and more when with one and the
same objective preparations are observed in water, balsam, and
cedar oil successively. If now the angles qua angles are eflfective
in an?/ way, something must be lost by change of angle in this direc-
tion, and something ought to be gained by change in the reverse
direction, other conditions remaining the same. It is needless to
say that all experience and the entire course of proof and reasoning-
given above are diametrically opjDosecl to such conclusions.

Similarly it will be manifest that the conception that ' solid
vision ' or perspective effect in a microscopic image is one of the
consequences of oblique ' angular ' illumination is equally invalid. ■
It assumes that the different perspective views of a preparation 1

examined with the microscope, which correspond to the different I

obliquities, produce the same effects as if they were seen separately i

hj different eyes, as in the case with the binocular microscope. In I

reality, whenever we have the advantage of solid vision, owing to a
different perspective projection of different images, in the microscope
or otherwise, this is solely because the different images are seen by /,' j

different ey en. L.ln microscopic vision there is no difference of jiro- ") ylw^ •)<</ '
jection connected Avith different obliquities ;/ in the binocular micro-
scope there is a diversity of images which are depicted by pencils of

'^ VAXm.4 -

62 VISION WITH THE COMPOUND MICROSCOPE

different obliquities at the object which is a certain kind of perspec-
tive difierence ; but the above and other observations and experiments
show that even here there is essential divergence from the conditions
of ordinary vision.

It is thus plain that whenever aperture is effective in delineation
the mode in which it becomes so is not by means of the obliquity of
the rays to the object ; while it has already been shown that
"increase of light, always concomitant with the use of immersion
objectives, is a relative advantage,(but no part of the explanation of
the superior action of the combination of lenses. Angle is demon -
-strably not the true basis for the comparison of objectives ; it fails
in regard to aperture in general, so far as it has relation to opening ;
it fails equally in regard to the number of rays and the quantity of
light admitted to the system of lenses ; while its failure in regard to
the delineating power of objectives is everywhere seen and admitted.

At the same time it is plain that the cause of increased power of
performance in the objective is directly connected . with the larger
ojyening or ' aperture ' of the immersion and homogeneous systems.
In other words, it becomes clear that something is admitted into the
objectives with'greater apertures which contributes to the formation
of an image, such as objectives of lesser apei'ture cannot form
because their ' openings ' or ' apertures ' cannot admit that ' some-
thing.'j

What this is becom.es explicable by the researches of Abbe. It
is demonstrated that microscopic vision is sui generis. There is,
and can be, no comparison between microscopic and macroscopic
vision. The images of minute objects are not delineated microscopi-
cally by means of the ordinary laws of refraction ; they are not
diojjtrical results, but depend entirely on the laws of diffraction.
These come within the scope of and demonstrate the undulatory
theory of light, and involve a characteristic change which material
particles or fine structural details, in proportion to their minuteness,
effect in transmitted rays of light. The change consists generally
in the breaking up of an incident ray into a gi-oup of rays with
large angular dispersion within the range of which periodic alterna-
tions of dark and light occur.

If a piece of wire be held in a strong beam of divergent light so
that its shadow fall upon a white sui-face, the shadow will not be
sharp and black, but surrounded by luminous fringes having the
colours of the s])ectrum, and the centi-e, whei'e the black shadow of
the wire should be, is a luminous line, as if tlie wire were transparent.
This pheiKjmenon, as is generally known, is due to the infiection of
the diverging i-ays on either side of the wii-e. The inflected rays, in
passing ovei' one edge of the wii'e, meet the I'ays inflected l)y the
(jther edge and ' intei'fei-e,' pi'oducing alternate increase and diminii-
tion of aiii[)litude of oscillation or luiduliitory intensity, and giving
rise to coloiu-ed fringes if whites light is used, iind if homogeneous
light be emj)loyed giving origin to ;dj-crii;il c luiiids of light iiiid dnrk,
the centre always being luminous.

Again, if a disc perfoi-ated with a very smidl hole in tins centre
be held in ;i pencil of diverging light, those undnl;itions which pass

DIFFEACTION PHENOMENA 63

directly through the aperture interfere with those passing obliquely
at the edge of the disc and produce, at certain distances, a dai'k
spot, at other distances increased brightness, on that part of tlie
shadow which is opposite the aperture in the disc ;:'so that light is
supplanted by darkness, and darkness changed to light, by the discord
or concord of the luininous waves. "^

Independently of all experiment, the first jarinciples of undulatory
optics lead to these experimental conclusions. The laws of recti-
linear propagation of the luminous rays of reflection and refraction
are not absohtte laws. They arise from, and depend upon, a certain
relation between the wave-lengths and the absolute dimensions of
the objects by which the waves are intercepted, or reflected, or
refracted.

Taking as illusti'ative the waves of sound, an acoustic shadow is
only produced if the obstacle be many times greater than the length
of the sound-waves. If the obstacle is reduced, the waves j)ass com-
pletely round it and there is no shadow, or if the notes are of higher
pitch, so that the waves are reduced, a smaller obstacle than before
will produce the shadow. In the case of light there are similai'
phenomena. If the obstacles to the passage of light be large in
comparison with the wave-lengths, shadow effects result ; but if the
linear dimensions of objects are reduced to small niultijyles of the
wave-lengths of light, all shadows or similar effects of solidity must
cease. As in the instances given above, light and dark, or maxima
and minima of luminosity, interchange their normal positions by
harmony or disharmony of luminous waves.

It is then by means of difir-action 23henomena that Abbe is
enabled to explain the formation of the images of objects containing
delicate strife or striicture, and requiring large apertures for their
complete or approximate delineation. In the interests of this ex-
position we must here for a moment diverge on slightly personal
grounds. It has been the good fortune of the j^resent editor to obtain
the courteous consent of Dr. Abbe to read and criticise the whole
of the present chapter ; however careful and earnest a student of
such complex and original work as Dr. Abbe has done and recorded
in German and English during the last thirty years or more, it is
impossible to be wholly satisfied with the most sympathetic and
sincere desire to give such work a popular' form unless it should
have been perused and accepted by the author. Di'. Abbe has read
the entire chapter, and, with many generous words besides, relieves
the editor in his consciousness of great responsibility by saying that
he distinctly approves of the ' lively interest and care which (the
present editor) has bestowed on the exposition of his (Dr. Abbe's)
views,' and that he feels '■ the greatest satisfaction in seeing (his)
views represented ... so extensively and intensively.'

But beyond this, an original Avorker like Dr. Abbe would almost
inevitably find, in the course of years, reason for slight verbal and
other more serious modifications of his inferences, explanations, and
views ; and the editor has great satisfaction in being able to put
these modifications where they occur, with the approval of Dr. Abbe.
In the expositions of Dr. Abbe's views on the difl[;raction theory

64 VISION WITH THE COMPOUND MICEOSCOPE

of microscopic vision given up to this time, it has been usual to
state that he held and taught that the mici^oscopic image consists of
tioo swperimjjosed images, each having a distinct character as well
as a different origin, and capable of being separated and examined
apart fi-om each othei'. The one called the ' absorption image ' is a
similitude of the object itself, an image of the main outlines of the
larger parts ; but by the other image all minute structures, striation,
and delicate complexity of detail whose elements lie so close together
as to occasion diffraction plmnomena can alone be formed, because
these coidd not be geometrically imaged. So that in the case of an
object with lines closer than the 2-5V0 *^f ^^ inch apart, the image
seen by the eye is formed, not simply by the central dioptric beam,
but by the joint action of that and the superimposed diffraction
images, and their exact union in the upper focal plane of the objective.
The first of these was held to be a negative image, representing
geometrically the constituent parts of the object ; bvit the second
was considered a positive image because it delineates structure, the
parts of which appear self-luminous on account of the diffraction
phenomena w^hich they cause. It was this ' diffraction image ' that
was said to be the instrument of what has so long been known as
(! / /tiie ' resolving ' power of lenses.

'^ - But Dr. Abbe, with the full light of further investigation and

'\h/ experience, does not hesitate to modify this explanation. He says :

>-<j »'V ' I no longer maintain in principle the distinction between the

', •[ '^..'7 '? " absorption image " (or direct dioptrical image) and the " diffractibiT^

'<• ^* image," nor do I hold that the microscopical image of an object

consists of two superimposed images of different origin or different

mode of production.

' This distinction, which, in fact, I made in my first paper of 1873,
arose from the limited experimental character of my first researches
and the want of a more exliaustive theoretical consideration at that
period. I was not then able to observe in the microscope the dif-
fraction effect produced by relatively coarse objects because my
experiments were not made with objectives of sufficiently long focus ;
hence it appeared that coarse objects (or the outlines of objects
containing fine structural details) were depicted by the directly
transmitted beam of light solely, without the co-operation of diffracted
light.

' My views on this subject have undergone important modifica-
tions. Theoretical considerations have led me to the conclusion
that there must always be the same conditions of the delineation as
long as the objects a,re depicted hy means of traiismitted or reflected
light, whether the objects ai'e of coarse or very fine structure.
Further experiments with a large mici-oscope, having an objective
(jf about twelve inches focal length, have enabled me to actually
^ observe the diffraction effect and its influence on the image, viewing
gratings of not inoi-e than foi-ty lines pei- inch.^

' Diffraction effects may be observed without a microHcope ; they can be easily
demonstrated by observing a lamp-flame throu{(h a linom pocket-handkerchief or a
fine f^anze wire blind. This can be done readily by placing the eye close to the linen
or wire.

tfv'

EECENT MODIFICATIONS OF ABBE'S VIEWS 65

' My present views may be thus expressed : With coai'se objects \
the diffracted (bent off) rays belonging to an incident ray or pencil \
are all confined within a very narroio angular space around that \
Incident ray, and do not appear separated from this except with ',
objectives of very long focns. The ivhole of such a narrow diffraction \
pencil is consequently always admitted to the objective together with
the direct (incident) beam, whatever may be the direction of inci-
dence, axial or oblique. According to the proposition of p. 72 (1)
the image is in this case strictly similar to the object, i.e. the effect,
is the same as if we had a direct delineation by the incident cones
of light alone, and as if the image did not depend at all tipon the
diffractive action of the object. ♦

' If we have a preparation like a diatom — a relatively coarse
object, including fine structural details — or another preparation con-
taining coarse elements and fine ones in juxtaposition/^the total
diffraction effect may be separated (theoretically and practically)
^nto two parts :T{1) that which depends on, or corresponds with, the^
coarse object -■(e.g. the outlines of the diatom) or to the coarse \
elements ; and (2) that depending upon, or resulting from, the fine /
structural detail or the minute elements. The foregoing consideration ^^
applies to (1) : this constituent part of the total diffraction pencil ;

J of the preparation which is admitted to the objective completely,
independently of the limiting action of the lens opening, and hence
the corresponding parts of the object (outlines itc.) are depicted as
if there were a direct delineation, i.e. in perfect similarity — even
with low apertures. Those diffracted rays within the whole diffrac-
tion pencil which ai'e due to the raimde elements are strongly
deflected from the incident beams to which they belong.' ^

According to the less or greater aperture of the objective and
the axial or oblique incidence of the illuminating pencil or cone,
this part of the total diffraction pencil will be subject to a more or
less incomplete admission to the objective, and the corresponding-
image will therefore show the characteristic traces of the diffraction
image, that is to say, change of aspect with different apei'tures and
different illumination, dissimilai-ity to the real structure, and so
forth. Thus we have practically, in most cases, a composition of
the microscopical image, consisting of two superimposed images of
different behaviour. But the diffei'ence is not to be considered one
oi principle, so far as the proditcti on of the image is concerned ; for
it depends solely upon the different angular expression of the diffrac-
tion fans resulting from coarse and from extremely fine elements.^

Resuming, then, our illustration of diffraction phenomena as
applied to the theory of microscopic vision, we would point out that
perhaps the most serviceable illustration for our purpose is a j^late
of glass ruled with fine parallel lines. If the flame of a candle be so
placed that its image may be seen through the centre of the plate, this

1 Letter from Dr. Abbe.

- Thus it apipears that both the ' absorption image ■ and the ' diffraction image '
are now held to be equally of diffraction origin ; but, whilst a lens of small aperture
would give the former with facility, it would be powerless to reveal the latter because
of its limited capacity to gather in the strongly deflected diffraction rays due to the
minuter elements.

66

VISION WITH THE COMPOUND MICROSCOPE

central image will be clear and uncoloured, but it will be flanked on
either side by a i-ow of coloured spectra of the flame which are faintei-
and more dim as they recede from the centre : fig. 48 illustrates this.
A similar phenomenon may also be produced by dust scattered
over a glass plate and by other objects wdaose structure contains very
minute particles, the light suffering a characteristic change in pass-
ing through such objects, that change
consisting in the breaking up of a
parallel beam of light into a group of
rays diverging with wide angle, and
forming a regular series of maxima and
minima of inteiisity of light due to diflference of phase of vibration.

In the same way in the microscope the difii^action pencil origi-
nating from a beam incident upon, for instance, a diatom appears
as a fan of isolated rays, decreasing in intensity as they are further
removed from the direction of the incident beam transmitted through
the structure, the interference of the primary waves giving a number
of successive maxima of light with dark interspaces.

With daylight illumination if a diaphragm opening be intei-posed
between the mirror and a plate of ruled lines placed upon the stage,
the appearance shown in fig. 49 will be observed at the back of the

objective on removing the eye-piece and
looking down the tube of the microscope.
The central circle is an image of the dia-
phragm opening produced by the direct,
so-called non-diffi'acted rays, while those
on eithei' side ai-e the diff"raction images
produced by the I'ays which are bent oiF
from the incident pencil. In homogene-
ous light the central and lateral images
agree in size and form, but in white light
the diffracted images are radially drawn
out with the outer edges red and the
inner blue (the reverse of the ordinai-y
spectrum), forming, in fact, regular spec-
ti-a, the distance separating each of which varies inversely as the
closeness of the lines, being, for instance, with the same objective
twice as far apart when the lines are twice as close.

The formatif>n of the microscopical image is explained by the
fact that the rays collected at the back of the objective, depicting
there the direct and spectral images of the source of light, reach in
their further course the plane which is conjugate to the ol)ject, and
give rise there to an interference phenomenon (owing to the connec-
tions of the undulations), tliis interference effect giving the ultimate
image which is ()l)served l)y the eye-piece, and which therefore
dei)en<ls essentially on the nuinbei- Jiinl distrilaition of the diffracted
beams which enter the objective.

It would exceed tlic limits :iiiil the ()I)ject of this ]i;iiidl)ook to
attempt a tlieoreticjii (Icnioiistraiioii {>['. the action of diffraction
spectra in forming tlie images of fine sti'ucture and striation so as
to jiff'ovd ' resolution.' Tlu)se who desire to pursue this part of the

Fig. 49.

DIFFEACTION EXPERIMENTS

67

subject nmy do so most profitably by the study of the only book in
our language that deals exhaustively with the theoiy of modern
mici'oscopical optics, viz. the translation of Naegeli and Schwendener's
' Microscoj)e in Theory and Practice,' translated and placed within
the reach of English microscopists by the joint labour of Mr. Frank
Crisp and Mr. John Mayall, jun. The experimental proof of the
diffraction theory of microscopic vision lies within the range of our

Fig. 50. — Diffraction sfratiui

Fig, 51. — Diffraction image at back
of lens without eye-piece.

purpose, and the following experiments will suffice to show those who
possess the instruments, and desire the evidence, that to the action
of diffraction spectra we are indebted for microscopical delineation.

The first expei'iment shows that with, for instance, the central
beam, or any one of the spectral beams alone, only the contour of
the object is seen, the addition of at least one diffraction spectrum
being essential to the visibility of the structure.

Fig. 50 shows the appearance presented by an object composed
of wide and narrow lines ruled on glass when Adewed in the ordinary
way with the eye-piece in place, and fig. 51 the appearance presented
at the back of the objective when the eye-piece is removed, the

Fig. 53.

spectra being ranged on either side of the central (white) image, and
at right angles to the direction of the lines ; in accordance with theory,
they are farther apart for the fine lines than for the wide ones.

If now, by a diaphragm at the back of the objective, like fig. 52,
we cover up all the diffraction-spectra, allowing only the direct rays
to I'each the image, the object will appear to be wholly deprived of

p 2

68

VISION WITH THE COMPOUND MICROSCOPE

fine details, only the outline i-emaining, and every delineation
of minute structure disappearing just as if tlie microscope had sud-
denly lost its optical power (see fig. 53).

This illustrates a case of the obliteration of structure by obsti'uct-
ing the passage of the diffraction-spectra to the eye-piece.

The second experiment shows how the appearance of fine structure,
may be created by manipulating the spectra.

Fig. 54.

Fig. 55.

If a diaphragm siich as that shown in fig. 54 is placed, at the back
of the objective, so as to cut off each alternate one of the upper row
of sj)ectra in fig. 50, that row will obviously become identical with
the lower one, and if the theory holds good, we should find the image
of the upper lines identical with that of the lower. On replacing
the eye-piece we see that it is so : the upper set of lines are cloubled
in number, a new line appearing in the centre of the space between
each of the old (upper) ones, and upper and lower sets having become
to all appearance identical (fig. 55).

In the same w^ay, if we stop off all but the outer specti^a, as in fig.
56, the lines are apparently again doubled, and are seen as in fig. 57.

Fig. 57.

A case of apparent creation of structure simiLn- in principle to
tlie foregoing, though more strikiiig, is afforde<l }>y a network of
squares, such as fig. 58, li;i ving sides parallel to the pMge, which gives
the speotrn, shown in fig. 59, consisting of verticnl rows for the
horizont;i] lines and horizontal rows for the vertical ones. But it
is i-cadily se<'n tluit two di;igoii;il rows of spectra, exist at riglit

DIFFEACTION EXPERIMENTS

69

angles to the two diagonals of the squares, just as would arise from
sets of lines in the direction of the diagonals, so that if the theory
holds good we ought to find, on obstructing all the othei- spectra and

Fig. 58.

Fig. 59.

allowing only the diagonal ones to j)ass to the eye-piece, that the
vertical and horizontal lines have disappeared, and two new sets of
lines at right angles to the diagonals have taken their place.

Fig. go.

Fig. 61.

On inserting the diaphragm, fig. 60, and replacing the eye-piece,
we find, in the place of the old network, the one shown in fig. 61,

Fig. 62.

Fig. 6?

the squares being, however, smaller in the proportion of 1 : n/2, as
they should be in exact accordance with theory.

An object such as Pleurosigma angulatum, which gives six

70 VISION WITH THE COMPOUND MICEOSCOPE

diffraction spectra arranged as in fig. 62, should, according to theory,
show markings in a hexagonal arrangement. For there will be one
set of lines at right angles to h a e, another set at right angles to
c a f^ and a third at right angles to g a d. These three sets of lines
Avill obviously produce the appearance shown in fig. 63.

A great variety of other appearances may be produced with this
sarae arrangement of spectra. Any two adjacent spectra with the
central beam (as h c a) will form equilateral triangles and give
hexagonal markings. Or by stopping off all but y ce (or h d f) we
again have the spectra in the form of equilateral triangles ; but as
they are now further apart, the sides of the triangles in the two
cases being as \/ 3 : 1, the hexagons will be smaller and three times
as numerous. Their sides will also be arranged at a different angle
to those of the first set. The hexagons may also be entirely
obliterated by admitting only the spectra g c or g f or hf, &c., when
new lines will appear parallel at right angles or obliquely inclined
to the median line.

By varying the combinations of the spectra, therefore, different
figures of varying size and positions are jDroduced, all of which cannot
of course represent the true structure.

In practice, indeed, it has been proved that if the position and
I'elative intensity of the spectra, as found in any particular case, be
given, what the resultant image will be can be reached by mathe-
matical calculations wholly, and with an exactness that may even to
some extent transcend the results of previous observation on the
actual image of the object whose spectra formed the mathematician's
data.

If P. angtdatum be ilkiminated by central light transmitted
from an achromatic condenser, and examined by means of a homo-
geneous lens of large aperture, Mr. Stephenson points out ^ that
under ordinary conditions it would show, on withdrawing the eye-
piece and looking down the tube, one bi-ight centi-al light from the
lamp with six equidistant surrounding difiraction spectra, pi'odviced
by the lines (' if, indeed, lines they be ') in the object itself. But let
a stop made of black paper, which entirely excludes the central beam
of light, be placed at the back of the objective and close to the pos-
terior lens ; in the stop let six mai'ginal openings be made through
wliich the diffi'action spectra may pass. On examining the image
we find that in lieu of the oi-dinai-y hexagonal markings the valve
appear.s of a beautiful blue colour on a black ground, and covered
with circular spots, cleaily defined, and admitting of the use of deep
eye-pieces.

This is precisely what we leai-n from Abbe that the diffriiction
tlieory involves. In support of this, the philosophical faculty of the
Univer.sity of Jena liad projKjsed as ;i question to the matheiiiiiticMl
students the effect produced in tlie mici'os(U)pe by tliese iutei-fcrencc^
plienoiiiena. One problem was that of the ;i])])e;irauc(' pi'oduced by
six equidisbmt spectrii in a circle ; tliey coi'rcsjKtud precisely witli
tlie spectra of P. anijidat'mii, as accessible to us witli our pi-esent
numerical aperture ; and the dingram of the diffriiction iiiiitge, de-
" Joiirn. li.M.H. vol. i. 187K, p 18(i.

PLEUROSIGMA ANGULATUM

71

ducecl from theory, of what spectra of the given position and inten-
sit}^ of the proposed data should give is seen in fig. 64. But what
seems quite as much to the purpose is, that Dr. Zeiss has produced a
fine photogi'ajDh of P. angidatimi, given in Plate X., where it will
be seen that the details shown in fig. 64 appear.

Let it be clearly understood that this does not pretend to be an
interpretation of the markings of the diatom ; it is only held by
Abbe to be an accurate indication by calculation of what image the
given difi;raction spectra should produce. An optical glass and
media for ' mounting ' and ' immersion ' of immensely greater refrac-
tive and dispei'sive indices — at j^resent wholly inaccessible to us —
must, he contends, be found and emjoloyed before all the diSi'action
spectra of P. angidatum could be admitted to form its absolute and

Fig. 64.

complete ' diflfraction image ; ' but from such spectra as the objective
employed can admit, it is maintained by Abbe that the mathe-
matician can accurately show what the image will be. In the case of
P. angidatum theoiy indicated the 02)fical, but not necessarily the
stricctural existence ^ of smaller markings, shown in fig. 64, between
the circular spots. These had not been before seen by observers ; and
the mathematician who made the calculation (Dr. Eichhorn) had never
seen the diatom under the microscope ; but when Mr. Stephenson
re-examined the object — stopping out the central beam as above
described and allowing the six spectra only to pass — he saw the
small markings, and showed them at a meeting of the Royal Micro-
scopical Society to many experts who were there. They were small
and faint, and no doubt purely optical ; and, we learn from experiment,
may readily escape observation ; but by cai'eful investigation they

1 Conf. Abbe's recent note, pp. 72 et seq.

72 VISION WITH THE COMPOUND MICROSCOPE

are as present to the observer as they are capable of being demon-
strated by calculation to the mathematician.

Clearly, then, on these assumptions and with all other considera-
tions put aside, our finest homogeneous objectives of greatest aper-
ture inevitably fail to reveal to us the real structui-e of the finei'
kinds of diatom valves. We learn that dissimilar structures will
give identical microscopical images when the difference of theii-
diffractive efiect is removed, and conversely siviilar structures may
give dissimilar images when their difli-active images are made
dissimilar. A purely dioptric image answers point for point to the
object on the stage, and therefore enables a safe inference to be
drawn as to the true nature of that object ; but the diffraction or
interference images of minute structure stand in no direct relation
to the nature of the object, and are not of necessity conformable to
it. As Dr. Abbe has already insisted, minute structural details are
not imaged by the microscope geometrically or dioptrically and can-
not be interpreted as images of material forms, but only as signs of
material differences of composition of the particles composing the
object, so that nothing more can safely be inferred from the image
as presented to the eye than the presence in the object of such
structural peculiarities as will produce the specific diffraction pheno-
mena on which the images depend.^

It follows, therefore, that the larger the number of diffracted rays
admitted into the objective the greater is the similarity between the
image and the object. But carefully observe —

(1) Perfect similarity between these dejDends always on the ad-
mission to, and utilisation by, the optical combination of the ivhole of
the diffracted rays which the structure is competent to emit.

For the same i-eason the diffi-action fan of isolated corpuscles oi'
fagella in a clear field must be exactly identical to that of equal-
sized holes or slits of equal shape in a dark backgromid, and theory
shows that there must be a continuous and nearly uniform dissipa-
tioai of diffracted light over the whole hemisphei'e, provided the
diameter of the object is a small fraction of the wave-length of light ;
and this would be so even in a medium of highest known refractive
index. Sach isolated objects can be seen, hoivever minute they may
h^ ; it is merely a question of contrast in the distribiition of light, of
good definition in tlie objective, and of sensibility of the retina.
The diffi'action theoiy does not put a limit to visibility with micro-
scopic objectives ; it simply proves, in theory and practice, what is
the limit of visible separation in fine striation and structure.

In the visible flMgellinn oi Bacterium termo only a, fraction of a
wave-lengtli in dijiineter ajjpeai-s as of considerably increased dia-
meter, even witli m. very wide ;ipertui-e. The image seen is that of
another thread, tlie composition of wliicli theory can })e em[)loyed to

1 See AIjIio'k note, ]). (i"). But we ciiimot ]);iss over in tliis eomiectioii the
remarkable jiajier in tli(; Jm/ni. QnckctL (Uiilj, ser ii. vol. iv. on the ' Sub-stage
Conflenser,' Vjy Mr. Nelson. His |ilioto-inicro^;i'n,])lis illnstratin;,' the mutable diffrac-
tion e/Tectsof the ' small cone ' ol' oblique illumination, as distinct from a ' solid central
cone,' and tlie curious ' ghostly ' dilTraction images of the former, iis distinct fi'om the
immntiilde difTractif)n images of the latter, deserve careful consideration. From
|i. 125 of the i)a[)er tliis matter is carefully discussed.

SIX EQUIDISTANT SPECTEA AS A DIFFEACTION PEOBLEM 73

compute, which would give an exactly similar diffi-action fan, but
abruptly broken off at the limit of the apertui-e. Theory shows that
a thread-shaped object which could yield such a particular diffraction
effect must (without considering other differences) be greater in
breadth than another one yielding the full continuous dissipation of
light ; in other words the actual thread, so inconceivably fine,
belonging to the Bacteiium has produced a ' diffraction effect '
through the microscope, resulting in the appearance of a thread
which is the ' diffraction image.' But this latter is greater in width
than the actual thread or protoplasmic fibre would be could it be
seen directly without the aid of diffraction.

(2) Whenever a portio7i of the total diffraction fan appertaining
to a given structure is lost, the image will be more or less incomplete
and dissimilar from the object ; and in general the dissimilarity
will be the greater the smaller the fraction of light admitted. With
structures of every kind (regular and irregular) the image will lose
more and more the indications of the minuter details, as the peri-
pheral (more deflected) rays of the diffraction pencil are more and
more excluded. The image then becomes that of a different structure,
namely, of one the lohole of tohose diffracted heamis Avould (if it
physically existed) be represented by the utilised diffracted beams of
the structure in question.

At this place it is suitable to point out that Dr. Abbe em-
phasises to the present editor the importance of interpreting the
' intercostal points ' shown by Mr. Stephenson in P. angidatum
(fig. 64) as not a revelation of real structure. ' The fact is that the
image, which is obtained by stopping off the direct beam, will be
'more dissimilar from the real structure than the ordioiart/ imsige.
It has already been shown that the directly transmitted ray is a
constituent and most essential part of the total diffraction pencil
appertaining to the structure ; it is the central maximum of this
pencil. If this be stopped off a greater part of the total diffraction
pencil is lost than otherwise, and the image, consequently, is a more in-
complete one, and therefore more dissimilar than the ordinary image.

' The interest of the experiment in question is consequently confined
to two points, viz. —

i. ' It is an exemplification of the general proj)Osition that the
same object affords different inages if difierent jwrtions of the total
diffraction fan are admitted to the objective.

ii. ' The image in question shows to the observer what would be
the true aspect of that structure which will split up an incident beam
of light into six isolated maxima of second order of equal intensity,
suppressing totally the (central) maximum of the first order, as
fig. 65 ; a structure of such a particular and unusual diffraction effect
is theoretically possible, although it may be probably impossible to
realise it practically. Mr. Stephenson's experiment shows, in fact,
the true projection of the hyjoothetical structure.

(3) ' As long as the elements of a structure are large multiples of
the wave-length of light, the breaking up of the rays by diflraction
is confined to smaller and smaller angles ; that is, all diffracted rays
of perceptible intensity will be comprised within a narrow cone

VISION WITH THE COMPOUND MICROSCOPE

ai'ound the direction of the incident beam from which they originate.
In such a case even small apertures are capable of admitting the
whole. The images of such coarse objects will therefore be always
perfectly similar to the object, and the result of the interference
effect is the sarae as if there were no diffraction at all, and the
object were a self-luminous one.

(4) ' When the elements of a structure are reduced in diameter to
smaller and smaller multiples of the Avave-length which corresponds
to the medium in which the object is, the diffraction pencil originating
from an incident beam has a wider and wider angular expansion
(the diffracted rays are fui-ther apart) ; and when they are reduced

to only a few wave-lengths, not even
I the hemisphere can embrace the niliole

I diffraction effect which appertains to

i the structure. In this case the whole

can only be obtained by shortening
the wave-length, i.e. by increasing
the refractive index of the surround-
ing medium to such a degree that the
linear dimensions of the elements of
the object become a large multiple of
the reduced wave-length. With very
minute structures, the diffraction fan
which can be admitted in air, and
even in water or balsam, is only a
greater or less centred jyortion of the
whole possible diffraction fan corre-
sponding to those structures, and which
could be obtained if they w"ere in a
medium of much shorter wave-length.
Under these circumstances no objec-
tive, however wide may be its aperture,
can yield a complete or strictly similar
image!

It is at points of such extreme
delicacy and moment as this that the
diffraction hypothesis of Dr. Abbe is
so liable to misapprehension and mis-
interpretation, and a- further note fi'om
him relating to the disslmAlarity of the
image in the case of incomplete admis-
sion of the diffi'action pencil will be of
great value hei'e.

i. ' lu the case of i-egulai- periodic
.structures (i.e. equidistant stria-, i-ows ofapertiu-es, 'dots,' and so forth)
tlie distance of the lines apart is, even with an incomplete admission
of the diffracted light, always depicted correctly; that is to say, the
number of tlie lines per incli is n(!ver rh-.iujfed, provided i:he direct beam
(i.(\ the central maximum of the diffi'action fan) is admitted to the
ofjjectlve and, at least one of the next diffracted, rays,., or, in other words,
oiM- nf'llii' iH'Xt maxima of sccinid order. 'I'lic 'i-aiaje of d'lssimihmty

Vm.. c..--

DIFFEACTION THEOKY UNIVEESALLY APPLICABLE 75

is in this case confined to the proportion between the bright and the
dark interspaces of the striation and to the appearance of tlie con-
tours of the stria; .

' If not moi'e than the said two rays of the total difFi-action fan are
admitted, the dark and the light intei-vals are ahoays shown of
approximately equal breadth, even if the I'eal proportion of both
intervals differs much from 1:1; and the dark and bright strife show
always gradiicdly increasing and deci-easing brightness ; in other
words, want of distinct contours.

' This phenomenon shows the typical picture of eveiy regulai-
striation foi- the depiction of which not more than two diffractioji
i-ays can be utilised. For example, Am/phij^leura pellucida, or any
othei' striation which is near to the limit of resolution foi' the optical
system in use, and, therefore, even with oblique light, brings only
one diffracted beam into the objective.

ii. ' Whenever a structure gives rise to a diffraction fan of con-
siderable angular extension, the observation with a central incident
beam or axial light may lose a greater or smaller portion of the
whole diffracted light if the angular expansion of the fan extends to
the aperture of the objective in use. But oblique illumination r)itist
always involve a loss, and this loss is not confined to the external
(jjeripheral) rays of the diffraction pencil (as is the case in central
light), but the portion lost will more and more extend to one full
half of the whole when the obliquity is gradually increased to the
utmost limit, so that the direct beam touches the edge of the aper-
ture. It follows that the images which are obtained with ohlique
light will always be incomplete and not similar to a geometrical
projection of the object ; and generally (though not always) more dis-
similar than those by central light in regard to the minuter details.

' Strictly similar images cannot be expected, excej)t with a central
illumination with a narrow incident pencil, because this is the
necessary condition for the possible admission of the whole of the
diffracted light.'

Let it be noted that these principles of the difiraction theory of
microscopical vision relate to structures of all kinds, whatever may
be their physical and geometrical composition. Irregular structures,
isolated elements of any shape, equally jjroduce diffraction effects,
observed either by transmitted or reflected light, and being either
transparent, semitranspai'ent, or opaque.

The value of a = n sin u indicates the number of rays which
an objective can admit ; the aperture equivalent measures the very
essence of microscopical performance. It measures the degree in
which a given objective is com^^etent to exhibit a true, complete
delineation of structures of given minuteness, and conversely the
proportion of a in different objectives is the exact measure of the
different degree of minuteness of structural details which they can
reach, either with perfect similarity of the image or with an equal
degree of incompleteness of the image, jDrovided that the purely
dioptrical conditions are the same.

' Resolving ' power is thus a special function of aperture. The
limit of visible separation in delicate structure and striation is

7^

VISION WITH THE COMPOUND MICKOSCOPE

determined by the fact that no resohition can be effected unless at
least two diffraction pencils are admitted, and the admission of these
we have seen is absolutely dependent on the aperture of the objective.

The rule given by Professor Abbe for determining the greatest
number of lines per inch which can be resolved by oblique light will
be found (taking any given coloui- as a basis) to be equal to tioice
the momber of unchtlations in an inch niuUi^jlied by the numerical
a'pert%u)'e.

To those who have studied this subject it will be seen that the
' numerical aperture ' here takes the place of what was formerly the
' sine of half the angle of aperture ; ' and it has the effect of giving
the proposition a broader generality. By iTsing the ' sine of half
the angle of aperture,' the proposition is only true with the addition
that the number of undulations be calculated from the wave-length
within the special medium to which the angle of apertvire relates.

In introducing the numerical aperture instead of the sine of the
angle, the latter (the sine) is increased in the proportion of 1 : n
(n standing for the index of the medium), and that has the same
effect as increasing the other factor the number of undulations.

What the colour employed should be is only capable of individual
deter miination, since the capacity for appreciating light varies with
different individuals.

If, for instance, we take •43/x in the solar spectrum as being
sufficiently luminous for vision, we find the maximum — so far as
seeing is concerned — to be 118,000 to the inch (the object, in this
case, being in air) ; but as the non-luminous chemical rays remain
in the field after the departure of the visible spectrum, a photo-
graphic image of lines much closer together might be produced.

'J'liis inipoi'tinit sulijcct can scarcely l)e considered coiiiijU-jte, even
in outline, without a brief consideration, in their combined reliitions,
of apertures in excess of 180° in nir and tlie special function these
apertures possess.

1. Suppo.se any object composed of minute elements in regular
arrangement, such as a. diatom valve ; and, to confine the consideivi-
tion to tlie most simple cjise, sujjpose it illuminated by ;i narrow

APPLICATION OF THE DIFPEACTION THEOEY

77

axial pencil of incident rays. If tliis object is observed in air, the
radiation from every point of the object is, in consequence of the
diffraction effect, composed of an axial pencil S, fig. 66 (the direct
continuation of the incident rays), and a number of bent-off pencils,
81,82, . . . surrounding S.^

If, now, instead of air, the object is immersed in a medium of
greater refractive index, n, it results from Fraunhofer's formula that
the sine of the angle of deflection of the first, second, . . . bent-off

Fig. 67.

beam is rechicsd in the exact proportion of 9^, and the angle is re-
duced also — that is, the whole fan of the diffracted rays is contracted
in comparison with its extension in air. Fig. 67 Avill represent the
case of the same object in oil.

If now any dry objective (with a given angular semii-aperture u)
is capable of gathering-in from air the first, or the first and second,
diffraction beams on every side of the axial pencil, another objective
with a more dense front medium of the refractive index, n, will be
capable of admitting, from within the dense medium, exactly the same
beams (no more and no less), if its angular semi -aperture, v, is less
than u in the proportion :

sin V : sin w = 1 : n,

or

n sm V = sm «-,

all other circumstances — object and illumination — remaining the
same.

For example, a diatom for which the tlistance of the stria? is 0'6 /,/,
will give the ,/irs^ bent-off beam of gTeen light (X = -SS/u) in air under
an angle of 66 "5°. This will be just admitted by a dry objective of,
133° anyidar aperture. In balsam (01 = 1"5) the same pencil will
be deflected by 37 '5° only, and would be admitted, therefore, by an
objective of not more than 75° balsam-angle. Again, if the distance
of the lines should be greater, as I'^jj, the second deflected beam

1 In figs. 66, 67, and 68 S4 and Sg are supposed to be identical with the surfaces,
but are drawn at a slight inclination to them for the purpose of clearness in the dia-
grams.

78

VISION WITH THE COMPOUND MICROSCOPE

would be emitted in air under an angle of 66"5°, but in hcdsam the
third woiild attain the same obliquity. Wliilst now the dry objective
of 133° air-angle cannot admit more than the two first diffraction
beams on each side of the axis, the immersion of 133° balsam-angle
is capable of admitting from balsam three on each side under exactly
the same illumination.^

It follows, therefore, that a balsam-angle of 75° denotes the same
aperture as the larger air-angle of 133°, and a balsam-angle of 133°
a much greater aperture than an air -angle of the same number of
degrees, and in general two apertures of different objectives must be
equal if the sines of the semi-angles are in the inverse ratio of the
refractive index of the medium to which they relate — or, which is the
same thing, if the product of the refractive index multiplied by sine
of the angular semi-aperture {n sin ti) yields the same value for both,
i.e. if they are of the same numerical aperture.

2. Suppose the same object to be observed by a dry objective
of a given air-angle, at first in air uncovered, and then in balsam
protected by a cover-glass. The first case woidd be represented by

Fig. 68.

fig. 66, and the second l)y fig. 68. As we have seen, the group of
diffracted beams from the object in balsam is contracted in com-
parison to that in air in the ratio of the refractive index. But

' Tlio foUowiii!^ are the actual angles represented in the diagrams, viz. :

(Strite = '2'2 ^, wave-length A = '55 jx, medium air k — 1.)
Si = 14° 30'
S„ = 30°0'
St = 48° 36'
S4 = 90° 0'.

Stri8e = '2"'2 jx, wave-length a = "55 fj., medium Ijalsam » -1'.'5.)
S|=y° 36'
S2 = l!>"28
83 = 30'"-' 0'
S., = 4rMK'
85 = 50" '26'
S.i 1)0" ()'.

HOMOGENEOUS VERSUS DRY OBJECTIVES 79

according to the law of refraction, this group, on passing to air by
the plane surfece of the coveiing-glass, is spread out — the sines of
the angles being compared — in the i-atio of the same refractive index.
Consequently the various diflraction pencils, the first, second, . . .
on eveiy side, after their transmission into air, have exactly the
same obliquity which they have in the case of direct emission in
air from an uncovered object.

If now any dry objective of, say, 133° air-angle is capable of
admitting a certain number of these joencils from the uncovered
object, it will admit exactly the same pencils from the balsam-
mounted object. The contracted cone in balsam of 75° angular
aperture embraces all rays which are emitted in air within a cone of
133°.

The aperture of an objective is not, therefore, cut down by
mounting the object in a dense medium, for no ray which could be
taken in from the uncovered object is lost by the balsam-mounting.

3. A comparison of figs. 66, 67, and 68 will show that a cone of
82° within the balsam medium embraces all the diffracted rays
which are emitted from the object in air or transmitted from balsam
to air. This, however, is not the totality of rays which are emitted
in the balsam. The formula of Fraunhofer shows that the number
of the emitted beams is greater in balsam than in air in the same
ratio as the refractive index.

A structure the distance of whose elements equals 2'2/,t emits in
balsam six distinct beams on each side of the direct beam, but in air
onlyyb^w (see figs. 66, 67, and 68); the fifth and sixth are completely
lost in air. A dry objective of an angular aperture closely aj^proaching
180° will not even take in the fourth deflected beam, as this is de-
flected at an angle of 90°. But any immersion-glass of a balsam-
angle slightly exceeding 82° will take in the fourth, and if the
balsam-angle should exceed 112° it will take in the fifth beam also,
provided the object is in balsam, and in optical continuity with the
front of the lens.

Thus, again, it is seen (as was before shown by the purely dioptric
method) that an immersion objective of balsam-angle exceeding 82°
has a wider aperture than any dry objective of maximum angle can
have, for it is capable of gathering in from objects in a dense medium
rays which are not accessible to an air-angle of 180°.

It is, then, by the above facts and reasoning, placed beyond all
dispute —

1 . That a wide-angled ' immersion ' or ' homogeneous ' objective
pos.sesses an aperture in excess of 180° ' angular aperture' in air ;

2. That the great value of this — always manifest practically — is
fully accounted for and explained by the diffraction theory of micro-
scopic vision ; and

3. That 'dry' objectives, so far as regards the perfect delineation
of very minute structures, can only be considered as representing an
imperfect phase of construction. When made by the best hands,
with every precavition and care employed to secure the best possible
corrections, their defects do not lie in the direction of the presen-
tation of false or even partially erroneous and distorted images.

8o VISION WITH THE COMPOUND MICEOSCOPE

Their defects are their inevitable incapacity to open up details in
structure that can be disclosed with relative ease by the inclusion
into an oil immersion, and especially an '■ apochroniatic ' objective of
great aperture, of the all-revealing diffraction beams excluded by the
dry lens of equivalent power.

With dry objectives splendid results have been attained both in
low and high power work ; but all the latter is being advanced upon
by revision with lenses of greater apertiire in a striking manner.
For twenty years we have been urging our best English mici-oscope
makers to enlarge the ' angle ' of our objectives, and employing
them from a -^-inch to a J^-inch focus. We have seen them
advance from dry to water immersion, and from this to oil ; from
^^5-inch, a 3^5 -inch, and a gi^-inch of JST.A. 0"95 each, and re-
spectively to water immersions of IST.A. 1"04 and then to 'oil
immersions' or homogeneous lenses of N.A. 1"38 for the -^g-hxch.
and gL-inch respectively, and ultimately by a -JQ-inch with N.A.
of 1*50; and from that we have progressed to the apochromatic
objectives with compensating eye-pieces.

Now the objectives with which the earlier work done by the
present editor and his colleague, Dr. Drysdale, was effected — to
which allusion is made only as being the instance with which we
have most practical familiai'ity — are still in our possession ; what
was revealed by them fifteen, twelve, or ten years ago we can
exactly repeat to-day ; and in the general featiires of the work — in
the broad characteristics of the life histories of the saprophytic
organisms, minute as they are, revision with objectives of N.A. TSO
and other lenses of the best English and German makers, reveals no
positive error, even in the mimctest of the details then discovered and
delineated. But the later lenses of great apertui-e and beautiful
correctious have opened up structure absolutely invisible before.

Thus, for example, a minute oval organism from the ■j^^^^.th to
the 5-oTTo'tli ^^ ^^ ^"^^^ "^ long diameter was known to possess a
distinct nucleus ; the long diameter of this was from the roth to the
,'-^th of the diameter of the whole body of the organism. In the obser-
vations of fifteen to twenty-five years since the cyclic changes of the
entire oi-ganism were cleai-ly visible and constantly observed ; liut
of the nucleus nothing could be made out save that it a^ypeared to
share the changes in self-division and genetic reproduction, initiated
by the organism as a whole. Biit by lenses of N.A. 1"50 and the
finest apochromatic objectives of Zeiss, especially a most beautifully
cori-ected 3 mm. and 2 mm., structure of a remarkable kind has
hecu demonstrated in the nucleus, and it has been shown that the
initiation, of the great cyclic changes takes place in the nucleus, and
is then shared in by the oi-ganism as a whole. Tn shoi-t, we have
discovered as much concei'uing the ' inaccessil)le ' nucleus — wliich
may be not more than, say, a twelfth of tlie long diameter of tlio
whole organism — by means of lou^-'.r povers, but f/reater apertures, as
\v(j, were able to find cfjiici'iiiing iiic cijinplctc Ixxly oftlic saprojOiyte
with dry objectives.

But in spite of these facts tlicre is a cci-tain class of even higli
power work in biology from wliicli the dry lens can never be dis-

DRY OBJECTIVES OF GREAT VALUE STILL 8 1

missed. It must always be an indispensable instrument in a large
pai't of the woi'k done in the study of the life history of active
living oi'ganisms ; and whatever accessoiies in research on svich
subjects be employed, the main path of accurate and well correlated
discoveiy must he by ultimate and consecutive reference to the
changes of the living organism. But we cannot with any certainty
do this with eithei- a water immersion or a homogeneous objective.
With an active oi-ganism under investigation, we desire, as far as
practicable, to limit the area of its excui-sions ; a cover-glass of not
more than four-tenths or a quarter of an inch in diameter is large
enough when objectives from a o-V inch to a ^^y inch are used,
or when the recent 2 nnn. objective with 27 eye -piece is employed.
To have oil or water on the top of the covei", between it and the
front lens of the objective combination, is, with almost inevitable
certainty, sooner or later, in following the object with counter move-
ments of the stage, to i-each the edge of the cover, and cause the oil
or water above to mingle by capillarity with the minute drop of fluid
under observation, and thus to involve the whole in catastrophe.

To do the main woi'k of studying consecutively the life history
of unknown organisms, dry objectives will and must be iised ; but
in all cases such woi-k must be supplemented by the use of objectives
of great aperture. The details and relations of minute structure
must be studied in one field, and their genei-al origin and sequences
in another. The latter will be ' continuous,' the former will be
•employed as necessity indicates. The diffi-action theoiy of micro-
scopic vision does not invalidate, but in reality, under definable
conditions, directs the employment of ' naii-ow ' apertures. All
depends on the minuteness of microscopic detail. The law has been
enunciated above : the minuter the dimensions of the structui-al
■elements, the wider must be the apertui-e : the lai'gei- the details of
ultimate structure, the nai-rower the ajDcrtui-e that will suffice. This
is true in regard to objects of every kind ; thei'e is no variation in
the conditions of microscopical delineation.

The men engaged in microscopical reseai-ch have difierent aims,
nay, the same woi'ker at difierent times difiiers in the object pursued.
' Ultimate structure ' is not the oiie consideration of the micro-
scopist ; he often, as indicated above, has to take a comprehensive
view of the whole object or objects of his research, apai't from the
mo.st complex and delicate details.

It is folly to suppose that because great apertui'es have been
proved theoretically and practically to be able to open out minute
sti-ucture so pei-fectly, therefoi-e there is no raison cVetre for small
apei'tures. Low amplification cannot render distinctly visible de-
tails beyond a certain limit of minuteness, and wide apertures
cannot be utilised unless there is a concui-rent linear amplification
of the image which is competent to exhibit to the eye the smallest
dimensions which ai-e by optical law toithin the reach and grasj) of
such an aperture.

In the same way great amplification will be useless if we have
small apertures which delineate details of dimensions only capable
of being distinctly seen in a-n image of much lower amplification.

G

82 VISION WITH THE COMPOUND MICROSCOPE

It will be ' emjjty amplification,' because there is nothing in the
image which requires so much power for distinct recognition. If
the p(>%oer be deficient, aperture will not avail ; if the aperture be
wanting, nothing is gained by high power. If the angular aj)erture
of the microscope is such that the delineation of fine lines, whose
intersjaaces are one 'niicron,^ is just possible, it is fruitless labour to
increase the amplification beyond what we know to be sufficient for
theii- obsei'vation. We potentially difierentiate what we are powei--
less to see.

Thus it may be inferred from the difii-action theory, as such, that
wide aperture should accompany high amplification, and modeiute
a|3erture be the accompaniment of low or moderate amplification.
We have observed with great regret that students at our Biological
Schools in these days of low-priced objectives frequently abandon a
fairly good ^-inch objective of suitable numerical aperture, and
obtain in its place a A inch or -^^ inch with scarcely any increase of
numerical aperture, merely for the ease with which amplification is
efiected. But it would be well to remember that high amplification
effects nothing unless accompanied by suitably widened aperture.

The circvimstances on which Avhat has been called ' penetration '
in objectives is dependent will be shortly considered ; ^ it may be
stated here that theory and experience alike show that ' penetration '
is reduced with increasing aperture under one and the same ampli-
fication. As we have indicated, there are many subjects of study
and research presented to the biologist for which he needs as much
' penetration ' as possible. This is always the case where the recog-
nition of solid forms — as the infusoria, for example — is important.
A fair vision of different planes at once is required.^ Indeed the
gi'eatei- part of all morphological woi'k is of this kind ; hei-e, then,^
in the words of Abbe, ' a proper economy of aperture is of equal
importance with economy of power.' ^

Whenever the depth of the object or objects undei- obsei-vation
is not very restricted, and for the pui-poses of obsei-vation we require
depth dimension, low and moderate powers must be used ; ' and no
greatei- aperture should therefore be used than is i-equired for the
effectiveness of these powers — an excess in such a case is a real
daiHHge.' '^

Moreover, in l)iological work — constant ajiplication of the instru-
ment to varied objects — lenses of modei-ate apei'ture sind suitable
power facilitate certainty of action and conserve laboui-. Increase
of aperture involves a diminislied working distance in the objective,
and it is inseparable from a rjipid increase of sensibility of the
ol»jecti\'es foi- sliglit devifitious from tlie conditions of [;erfect cor-
rection. If it be not necessary to encounter the possil)le difticidties
tliese things involve, to do so is to lose v^dual)le moments. These
difficulties, of course, iirc (liminislied by tlic use oC lioiii(igeii(H)us, ;ind

' ^1. micron is IJ.= unnt "'H'- '"'''■ ■foiini. U.M.H. 1S8H, pp. 502 iind .S'iC) ; and
Nature, vol. xxxviii. pp. 221, 244. - Sec i). K\.

•' Abbe'H explanation of the reason ol tlif i ion -stereoscopic pcrccpl ion of tliese is.
}<i\cM (see pp. 93 ct snq.).

' ' Tlie Relation of Aperture to Power,' ■Joiini. ILM.S. series ii. \ol. ii. p. 304.

••• 7///V/.

PENETRATING POWER IN OBJECTIVES 83

especially apocliromatic objectives, but even with these they are
not, in practice, eliminated where the best results are sought.

Employ the full aperture suitable to the jwwer used. This is the
practical maxim taught in effect by the Abbe theory of microscopic
vision.

It has been suggested that all objectives be made of relatively
wide apertures, and that they be ' stopped down ' by diaphragms
when the work of ' lower apertures ' has to be done. But this is
not a suggestion that commends itself to the working biologist. If
there were no other defects in such a method, the fact that the
working distance remains unaltered would be fatal ; and we may
safely adopt the statement of Abbe,^ that ' scientific work with the
mici'oscope will always require, not only high power objectives of
the widest attainable apei'tures, but also carefully finished lower
powers of small and very moderate apertures.

We complete this section with a table of numerical a^pertures,
which will be found on the following ]3age. As already stated, the
resolving powers are exactly proportional to the numerical apertures,
and the expressions for this latter will allow the resolving power of
different objectives to be compared, not only if the medium be the
same in each, but also if it be different. The resolving power for an
objective, when illuminated by a | solid axial cone of white light, is
found by multiplying its N.A. by 70,000, and for monochromatic
blue-green light (Giffbrd's screen) by 80,000.^

The first column gives the numerical apertures from '40 to 1"52.
The second, third, and fourth, the air-, water-, and oil- (or balsam-)
angles of aperture, corresponding to every "02 of N.A. from 47° air-
angle to 180° balsam-angle. The theoretical resolving power in
lines to the inch is shown in the sixth column ; the line E of the
spectrum about the middle of the green (\ = 0'5269^) being taken.

The column giving ' illuminating power,' we have already seen,
is of less importance ; while it must be borne in mind in using
the column of ' penetrating power ' that several data besides

- go to make up the total dej)th of vision with the microscope.

Penetrating Power in Objectives. — Intelligibility and sequence,
more than custom, suggest the consideration of this subject at this
point. The true meaning and real vakie of ' depth of focus,' or what
is known as ' penetrating power,' follows logically upon the above
considerations.

That quality in an objective which was supposed to endow it
with a capacity of visual range in a vertical direction, that is, in the
direction of the axis of vision, has been called ' penetration,' it
being supposed that by this ' property ' parts of the object not in
the focal plane could be specially presented, so as to enable their
perspective and other relations with what lies precisely in the focal
plane to be clearly traced out.

Concerning the manner in which this quality of the objective
operated, there have been most diverse opinions ; indeed, the whole

1 ' The Relation of Aperture to Power,' Journ. B.M.S. series ii. vol. ii. y>. 309.
'^ Journ. B.M.S. (1893), p. 17.

g2

84

VISION WITH THE COMPOUND MICEOSCOPE

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N.A. TABLE Etc. 85

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86

\^TSTON AVITH THE COMPOUND MICROSCOPE

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JOiXJ05(MlOflOrH10XC1^000t~(Mt-CCOCO
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i-l rH 1-1 CM :

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co" cm"" r-T o" oi" oo~ b^ cd" iicT -rf co" c>f cD~ x' t^ io~ co" th" (tT l^^ to" ire" co" rn" cT 1?^ bd" lo" c^f cT 00^

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i Eesolution of Plearosigtita fas-
ciola, 55,000 to 58,000 lines per
inch — >

Resolution of Pleurosigma angu-
> latum, 44,000 to 49,000 lines
per inch when mounted dry.
1 'Dots' shown with axial illumi-
nation and wide cone. Obj.
' apochr. i N.A. -65 . . . ->

Resolution of Nitzschia scalaris in
balsam with apochr. obj, 1 in
N.A. -30 ->

1

88 VISION WITH THE COMPOUND MICKOSCOPE

matter was involved in obscurity. The remarkable insight and
learning of Professor Abbe have, however, found for this important
subject a sound scientific basis.

The delineation of solid objects by a system of lenses is by
virtue of the most general laws of optical delineation, subject to a
peculiai' disproportion in amplification. The linear amplification of
the f%j^A-dimension is, when both the object and the image are in
the same medium (air), found to be always equal to the sqtiare of
the linear amplification of the dimensions at right angles to the
optical axis ; but if the object be in a more highly refracting medium
than air, it is equal to this square divided by the refractive index
of the medium. In proportion to the lateral amplification there is
a progressive, and with high powers a rapidly increasingv_o^'e»"- / ) ") '^
amplijication of the depth of the three-dimensional image. If a '
transverse section of an object is magnified 100 times in breadth the
distance between the planes of parts lying one behind the other is
magnified 10,000 times at the corresponding parts on the axis when
the object is in air, 7500 times when it is in water, and 6600 times
when it is in Canada balsam.

This excessive distortion in the case of high amplifications is not,
however, of itself so complete a hindrance to correct appreciation of
solid forms in the microscopical image as at first appears. The
appreciation of solid form is not a. matter of sensation only ; it is a
mental act — a conception — and, therefore, the peculiarity of the
optical image, however great the amplification, would not prevent
the conception of the solidity of the object so long as salient points
for the consti'uction of a three-dimensional image were found. But
for this the solid object, as such, must be simultaneously visible ;
a single layer of inappreciable depth can convey no conception of
the tho'ee space dimensions possessed by the object.

Owing to the disproportional amplification of the depth-dimension
normal to the action of optical instruments, the visual space of the
microscope loses more and more in depth as the amplification increases,
and thus constantly approximates to a bare horizontal section of the
object.

The visual space, which at one adjustment of the focus is plainly
visible, is made up of two parts, the limits of which as regards the
depth ai-e deteimined in a very different mannei'.

First, the accovimodation of the eye embraces a certain depth,
diffei-ent planes being successively depicted with perfect sharpness
of image on the retina, whilst the eye, adjusting itself by conscious
or unconscious acconmaodation, obtains virtual images of greater or
less distance of vision. This depth of acconnncdation, which plays
the same part in microscopical as in ordinal}- vision, is wholly
determined by the extent of power in this direction possessed by the
partioulai' eye, tlie limits being the gi-eatest and the least distance
of distinct vision. Its exact numerical measure is the differen(H^
lietweeri tlie recijrrocal values of tliese two extreme distances. Tlie
dfptli of distinct vision is directly proportional to tliis numerical
equivalent of the accfnmnodation of the eye, directly [)roi)orti()nal
to tlie i-efractive mcdiimi of tlic object, and inversely proportional
to the sr|ii;ii"<' of tiii' :iiii|ili llcat iciii wlicii referred ;d\vays to the same

PEINCIPLES OF STEREOSCOPIC VISION 89

image-distance. For example, a moderately short-sighted eye sees
distinctly at 150 mm. as its shortest distance, and at 300 mm. as
its longest distance ; then the numerical equivalent of the extent
of accommodation would be equal to ^-J-q mm. ; the calculation for
an object in air would give a depth of vision by accommodation
amounting to

2-08 mm. \Yith 10 times amplification

0-23 „ 30

0-02 „ 100

0-0023 „ 300

0-00021 „ 1000

0-00002 „ 3000

These figures are modified by the medium in which the object is
placed and by the greater or less shortness and length of vision.

Secondly, the perception of depth is assisted by the insensi-
bility of the eye to small defects in the union of the rays in the optic
image, and therefore to small circles of confusion in the visual image.
Transverse sections of the object which are a little above and below
the exact focal adjustm.ent are seen without prejudicial efiects. The
total effect so obtained is the so-called penetration or depth of focus
of an objective. This may be determined numerically by defining
the allowable magnitude of the circles of confusion in the micro-
scopical image by the visual angle under which they appear to the eye.
It is found that one minute of arc denotes the limit of sharply
defined vision, two to three minutes for fairly distinct vision, and five
to six minutes the limits of vision only just tolerable. This being-
determined, the focal depth depends only on the refractive index of
the medium in wdiich the object is placed, the amjDlification, and the
angle of aperture, and it is directly proportional to the refractive
index of the object medium, and inversely proportional to the
' numerical aperture ' of the objective, as also to the first power of
the amplification. These assume the visible angle of allowable
indistinctness to be fixed at 5', the aperture angle of the image-
forming pencils to be 60° in air ; the depth of focus of an object in
air will then be —

0*073 mm. for 10 times amplification

0-024

30

0-0073 ,

, 100

0-0024 ,

, 300

0-00073 ,

, 1000

0-00024 ,

, 3000

By limiting or enlarging the allowable magnitude of indistinctness
in the image we correspondingly modify these figures, as we should
do with media of different refractive indices and increased aperture -
angle.

It is plain, then, that the actual depth of vision must always be
the exact sum of the accommodation depth and focal depth. The
former expresses the object space through which the eye by the play
of accommodation can penetrate and secure a sharp image ; the latter
gives the amount by which this object-space is extended in its
limits — reckoning both from above and below — because without per-
fect sharpness of image there is still a sufiicient distinctness of vision.

90

VISION WITH THE COMPOUND MICROSCOPE

As the amplification increases the over-amplification of the
depth -dimension presents increasing! j unfavourable relation between
the depth and width of the object-space accessible to accommodation.
When low powers are employed we have relatively great depth of
vision, because we have large accommodation-depth ; but as we pass
to medium powers, the acconnnodation-depth diminishes in rapid
ratio, becoming equal to only a small depth of focus ; while when
the magnifying power is greatly increased the acconnnodation-depth
is a factor of no moment, and we have vision largely, indeed almost
wholly, dependent on depth of focvis.

The following table shows the total depth of vision from ten to
3,000 times :—

Amplification

Diameter of
Field

Accommoda-
tiou Depth

mm.

Focal Depth

Depth of Vision,

Ac commodatiOD

Depth, aud

Focal Depth

Ratio of Depth

of Vision to

Diameter of

Field

mm.

mm.

mm.

1

10

25-0

2-08

0-073

2-153

11-6

30

8-3

0-23

0-024

0-254

1 1

32-7

100

2-5

0-02

0-0073

0-0273

1
91-6 1

300

0-83

0-0023

0-0024

00047

1
176-6

1000

0-25

0-00021

000073

0-00094

1
266

3000

0-083

0-00002

0-00024

0-00026

1
319

It has been pointed out by Abbe that this over-amplification of
depth-dimension, though it limits the direct appreciation of solid
forms, yet is of great value in extending the indirect recognition of
space i-elations. When with increase of magnifying powei* the depth
of the image becomes more and more flattened, the images of difl:ei-ent
planes stand ovit from each other moi-e perfectly in the same i-atio,
and in the same degree are clearer and more distinct. With an
increase of amplification the microscope acquires increasingly the
property of an optical microto'iiie, which pi-esents to the obsei-ver's
eye sections of a fineness and sliarpness which would be impossible
to a meclianical section. It enables the observer, by a sei-ies of
adjustments for consecutive planes, to construe the solid foi-ms of
the smallest natural objects with the same certainty as h(^ is
accustomed to see with the naked eye the objects with wliic-li it is
concerned. This is a lai-ge advantage in the general scientific use
r)f the instrument ; a greater gain, in fact, tlian coidd be expected
(Vom the a])plicati()n of stereosc()])ic observaticm.

Stereoscopic Binocular Vision. — This subject has Ijecn elaborately
(•()ii>id<-icd iiiid jiait i;ill\- (•.xpoiiiidcd and deiiioiistrated by Pi'ofessor
Althe; his exposition diflei-s in soKie important ]iarticidars f)'om
tliat of tlie oi-iginal aiitlior of tliis book, l)utin its present incomph^te

STEEEOSCOPIC BINOCULAE VISION 91

forms it appeal's to the editor to be the wiser way to allow Di'. Cai'-
peiitev's ti'eatiiient of the subject to stand, and to place below it as
complete a digest of Pi-ofessor Abbe's theory and explanation of the
same subject as the data before us will admit.

The admii-able invention of the stereoscope by Pi-ofessor Wheat-
stone has led to a general appreciation of the value of the conjoint
use of both eyes in conveying to the mind a notion of the solid forms
of objects, such as the use of either eye singly does not genei-ate with
the like certainty or eifectiveness ; and after several attempts,
which were attended with vai-ious degrees of success, the principle of
the stereoscope has now been applied to. the microscope, with an
advantage which those only can trvily estimate who (like the Author)
have been for some timje accustomed to woi'k with the stereoscopic
binocular ^ upon objects that are peculiarly adapted to its powei'S.
As the result of this application cannot be rightly understood witli-
out some knowledge of one of the fundamental principles of binocular
vision, a brief account of this will be here introduced. All vision
depends in the first instance on the formation of a picture of the
object upon the retina of the eye, just as the camera obscura forms
a pictui-e upon the ground glass placed in the focus of its lens. But
the two images that are formed by the two eyes respectively of any
solid object tha,t is placed at no great distance in front of them are
far from being identical, the perspective projection of the object
varying with the point of view from which it is seen. Of this the
reader may easily convince himself by holding up a thin book in
such a position that its back shall be at a moderate distance in front
of the nose, and by looking at the book, first with one eye and then
with the other ; for he will find that the two views he thus obtains
are essentially different, so that if he were to represent the book as
he actually sees it with each eye, the two pictui-es would by no
means correspond. Yet on looking at the object with the two eyes
conjointly, there is no confusion between the images, nor does the
mind dwell on either of them singly ; but from the blending of the
two a conception is gained of a solid projecting body, such as could
only be otherwise acquired by the sense of touch. Now if, instead
of looking at the solid object itself, we look with the rigftt and left
eyes respectively at pictures of the object, corresponding to those
which would be formed by it on the i-etina^ of the two eyes if it were
placed at a moderate distance in front of them, and these visual
pictures are brought into coincidence, the same conception of a solid
projecting form is generated in the mind, as if the object itself were
thei'e. The stereoscope — whether in the forms originally devised by
Pi'ofessor Wheatstone or in the popular modification long subse-
quently introduced by Sir D. Brewster — simply serves to bring to
the two eyes, either by reflexion from mirrors or by i-efraction
thi'ough prisms or lenses, the two dissimilar pictui'es which would
accurately represent the solid object as seen by the two eyes respec-

^ It has become necessary to distinguish the binocular microscope which gives
true stereoscopic effects by the combination of two dissimilar pictures from a
binocular which simply enables us to look with both eyes at images which are
essentially identical (p. 106).

92

VISION WITH THE COMPOUND MIOIIOSCOPE

tively, these being thrown on the two retinee in the precise positions,
they woukl have occupied if formed there direct from the soHd
object, of which the mental image (if the pictures have been correctly
taken) is the precise counterpart. Thus in fig. 69 the upper pair of
pictures (A,B) when combined in the stereoscope suggest the idea of
R. jyrojecthig truncated pyi-amid, with the small square in the centre
and the four sides sloping equally away from it ; whilst the combi-
nation of the lower pair, C, D (which are identical with the upper,
but are transferred to opposite sides), no less vividly brings to the
mind the visual conception of a receding pyramid, still with the small
square in the centre, but the four sides sloping equally towards it.

Thus we see that by simply crossing the pictures in the stereo-
scope, so as to bring before each eye the picture taken for the other,,
a 'conversion of relief ' is produced in the resulting solid image,,
the projecting parts being made to recede and the receding parts
brought into relief. In like manner, when several objects are com-

FiG. 69.

1:)ined in the same crossed pictures, their apparent relative distances
are reversed, the remoter being brought nearer and the nearei'
cari'ied backwards ; so that (for example) a stereoscopic j)hotograph
representing a man standing in front of a mass of ice shall, by the
crossing of the pictures, make the figure appear as if imbedded in the
ice. A like conversion of relief may also be made in the case of
actual solid objects by the use of the pseudoscope, an instrument
devised by Professor Wlieatstone, which has the efi'ect of reversing
tlie pei'spective projections of objects seen througli it by the two
eyes respectively ; so that the interior of a Imsin or jelly-mould is
made to appear as a projecting scjlid, whilst tlie extei'ior is lna,d(^ to
appeal- liollow. Hence it is now customM-ry to speak of stereoscopic
\ision as tliat in wliich tlie conception of the true naturnl relief of an
object is called up in the mind by the noi-mal coml^iiiiition of the
two perspective projections formed of it l)y the right and left eyes
i-espectively ; wliilst by psendoscopic vision we iiienii that ' conver-
.sion of relief ' whifh is produced by the combination of two reversed

CARPENTEE'S r. ABBE'S VIEW OF STEREOSCOPIC VISION 93

perspective projections, whether these be obtaiaiecl directly from tlie
object (as by the pseudoscope) or from ' crossed ' pictures (as in the
stereoscope). It is by no means every solid object, however, or every
pair of stereoscopic pictures which can become the subject of this
conversion. The degree of facility with which the ' converted ' form
can be apprehended by the mind appears to have great influence on
the readiness with which the change is produced. And while there
are some objects — the interior of a plaster mask of a face, for ex-
ample— which can always be ' converted ' (or turned inside out) at
once, there are others which resist such conversion with more or less
of persistence.^

JSTow it is easily shown theoretically that the pictui'e of any
projecting object seen through the microscope with only the right-
hand half of an objective having an even moderate angle of aperture,
must differ sensibly from the picture of the same object received
through the left hand of the same objective ; and, further, that the
diflerence between such pictures must increase with the angulai-
aperture of the objective. This difierence may be practically made
apparent by adapting a ' stop ' to the objective in such a manner as
to cover either the right or the left half of its aperture, and then by
carefully tracing the outline of the object as seen through each half.
But it is more satisfactoiily brought into view by taking two photo-
graphic pictures of the object, one through each lateral half of the
objective ; for these pictures when properly paired in the stereo-
scope give a magnified image in relief, bringing out on a large scale
the solid form of the object from which they were taken. What is
needed, therefore, to give the true stereoscopic power to the micro-
scope is a means of so bisecting the cone of rays transmitted by the
objective that of its two lateral halves one shall be transmitted to
the right and the other to the left eye. If, however, the image thus
formed by the right half of the objective of a compound microscope
were seen by the right eye, and that formed by the left half were
seen by the left eye, the resultant conception would be not stereo-
scojnc but jJseudoscojnc, the projecting parts being made to appear
receding, and vice versa. The reason of this is, that as the microscope
itself reverses the picture, the rays proceeding through the right and
the left hand halves of the objective must be made to cross to the
left and the right eyes respectively, in order to correspond with the
direct view of the object from the two sides ; for if this second
reversal does not take place, the effect of the first reversal of the
images produced by the microscope exactly corresponds with that
produced by the ' crossing ' of the pictures in the stereoscope, oi- by
that reversal of the two perspective projections formed direct fi'om
the object, which is effected by the pseudoscope. It was from a
want of due appi-eciation of this principle (the truth of which can
now be practically demonstrated) that the eailier attempts at pro-
ducing a stereoscopic binocular microscope tended rather to jDroduce
a ' pseudoscopic conversion ' of the objects viewed by it than to
i-epresent them in this true relief.

1 For a fviller discussion of this subject see the Author's Mental Physiology,
§§ 108-170.

94 VISION WITH THE COMPOUND MICROSCOPE

In contradistinction to this exjilanation of binocular vision Dr.
Abbe, as we have seen, has demonstrated that oblique vision in the
microscope is wholly unlike ordinary vision ; there is, in fact, no
perspective. \ The perspective shortening of lines and surfaces by
oblique projection is entirely lost in the microscope^and, as a con-
sequence, it is contended that the special dissimilarity which is the
raison cVetre of ordinary stereoscopic efieets does not exist, but that
an essentially different mode of dissimilarity is found between the
two pictures. The outline or contour of a^ microscopic object is
unaltered, whether viewed by an axial or an oblique pencil ; there is
no foreshortening, there is simply lateral displacement of the images
of consecutive laj'ers. But Abbe contends that, whilst the manner in
which dissimilar pictures are formed in the binocular microscope is
different from that by which they are brought about in ordinary
stereoscopic vision, yet the activities of the brain and mind by which
they are so blended as to give rise to sensations of solidity, depth,
and perspective are practically identical.

The fact that lateral displacements of the image are seen in the
microscope depends on a peculiar property of microscopic amplifica-
tion, which is in strong contrast to the method of ordinary vision.
It depends entirely on the fact, enunciated above, that the amplifi-
cation of the depth is largely exaggerated, f Hence solid vision in
, ^ ' the binocular microscope is confined to large and coarse objects, the
/ dimensions of which are large multiples of the wave-length] It

therefore follows that when moderate or large apertures have ^o be
employed — that is to say, whenever delineation requires the employ-
ment of oblique rays — the elements of the object are no longer
depicted as solid objects seen by the naked eye oi- through the telescope
would be depicted ; nevertheless the brain ai-ranges them so that
the characteristics of solid vision are still presented.

Pi'ofessor Abbe demonstrates ^ that in an aplanatic system pencils
of different obliquities yield identical images of every plane object,
or of a single layer of a solid object, i This is truer however^ large the ^
aperture maybe. ' ] X J, '{tyt fir^ '''ize'>ioiWl''r;- lo i ^5^." f (i-ZiiU,'! JC 'i'''h'l %' )

This carries with it, as we have said, a total absentee of perspec- '
tive and an essential geometrical difference between vision with the
binocular microscope and vision with the unaided eye.

An object, not quite flat, as a curved diatom, when observed with
an objective of wide apertui-e will present points of great indistinct-
ness. This has been by some supposed to arise fi-om the assumption
that there was a dissimilai'ity between the images formed by the
axial and oblique pencils ; but this'is not so. It is wholly expli-
cable by the fiict that the d('i)th of the object is too great for the
small depth of vision attendant u])on a large aperture. ^Icj^c/ (l li/^U^- ^[O

It will be seen, then, tliat so long Tisthe deptli of the ol)ject is
within tlie limits of the depth of visicm, cori'esjxaiding to tlie aperture
and amplification in use, we obtain a distinct parallel projection of
all the successive layers in one ciiininoii plmic pei-pendicular to the
axis of the micro.'-cope — ji groiind )il;iii, as it wei-e, of the object.
Manifestly, then, since d(*))1li of \ ision deci-eases with increasing

' Jtiiini. II. M.S. series ii. vol. iv. iip. 21-24.

^j;^^-

ABBE OX STEREOSCOPIC VISION «,' ' ■''IQ'^

iipertui'e, good delineation with thesexmijSt be confined to thinner
objects than can be successfully employed/with objectives of naiTow
apertures. -<(^ /\

Stereoscopic vision with the microscope, therefore, is due solely'
to difference of projection exhibited by the diffei-ent parallactic dis- t . y, ,.^ ^.
placements of the images of successive layers on the common ground 1 ~ |,J',, ^
plane and to the ^^e'i'ception of depth, not to the delineation of the ; '■'^'
plane layers themselves. For, if there were dissimilai- images per- {
ceptible at different planes, the out-of-focus layer's must_ ajjp^ar con- /
fused and no vision of depth would be possible. ( «b-2Jc_ ;^.Kii}-\~ U-'-v'

Now stereoscojDe vision requires, as shown by Dr. Carjjenter, iha€ i

the delineating pencils shall be so divided that one portion of the
admitted cone of light is conducted to one eye and another portion to
the other eye. If this division of the image is effected in a symmetri-
cal way, the cross section of, e.g., a circle must be reduced to two
senaicircles representing one of these two arrangements seen in
0 and P, fig. 70.

% 0 P P ^ <Jl

Fig. 70.

Dr. Abbe's theory is that the only condition necessary for ortho-
scopic effect in any binocular system is that these semicircles or
their equivalents should be depicted according to diagram 0, fig. 70,
and for jiseiidoscojjic effect according to diagram P in the same figure ;
and he demonstrates that all other circumstances, such, e.g., as the ■

crossing of the images, are wholly immaterial. oy i

/ Orthoscopic vision is always obtained when the right half of the \ ^
riglit pupil and the left half of the left pupil oiily are employed ; / *
pseudoscopic vision in the opposite conditions. ) f'^It is quite indif-
ferent whether the effect is obtained with crossing or non-crossing
rays, whether the image be erect, or invei-ted, or semi-inverted, and-
whatever may be components of the optical arrangement.^ y^f^t Ivfc-

The observant readei- will perceive that it is at this point that "■
there is a radical divergence from the interpretation given by Dr. ,0".
Carpenter, who, as we have seen above, insisted that orthoscopic / jYT^Vv^
vision is not to be obtained in a binocular with non-erecting eye-pieces
unless the axes of the two halves of the admitted cone cross eacJh
other.

Of course we must keep cleai'ly befoi-e us the fact that in micro-
scopic vision it is not the object but its virtual image only that we
see. This apparently solid image is placed in the binocular micro-
scope under circumstances similar to those of common objects in
ordinary vision. Clearly, th^i, it is the perspective projections of
this image which require to be compai-ed to the projections of solid
objects in ordinary vision, in respect to which the criteria of ortho-
scopic and pseudoscopic vision have been defined. But it can be i '}~-i -
geometrically demonstrated that right-ej'e perspective of the ap- [ / (\"
parently solid image is always obtained from the right-hand portion of \ ly^viP'^
the emergent pencils, left-eye perspective from the left-hand j)ortion J-

g6 VISION WITH THE COMPOUND MICEOSCOPE

t1//vv:£

Fio. 71.

BINOCULAR MICROSCOPES^ 97

aiul it is quite immaterial, as regards this result, lohich jiortion of the\
emergent rays is admitted hy the right or the left jjart of the objective. J

The manner in which the delineating pencils are transmittea
through the system may be such as to reqvxire crossing over of the
i-ays from the I'ight-hand half of the objective to the left eye-piece,
and vice versa. But it is not essential to binocular effect. In the
Wenham and Nachet binocular (pp. 98, 99) crossing over isrequirecl
because the inversion of the pencils is not changed by two reflexions.
If the delineating pencils have been reflected an even member of times
in the same plane, it will be necessary for the rays to cross ; but if
they have been reflected an odd number of times, it is not only un-
necessary, but is destructive of orthoscopic effect, provided ordinary
eye-pieces (non-erecting) are employed. Hence in the Stephenson bi-
nocular it is not only not required, but would give pseudoscopic effect.

Principal Forms of Binocular Microscopes. — The first binocular
of a j^ractical character was the ai-i-angement of Professor J. L.
Riddell, of New Orleans. It was devised in 1851 and constructed in
1852, and a description of its nature and its genesis was given by
him in the second volume of the first series of the ' Quarterly Journal
of Microscopical Science' in the year 1854.^^

A representation of his original instrmnent is presented in fig. 71,
<ind the arrangement of the prisms by which the binocular effect was
obtained is shown in fig. 72.

It will be seen that the pencil of rays emerging from the back
lens of the combination / is divided into t,wo, each half passing re-
spectively into the right and left prisms ; the path of the rays is
indicated at a, 6, c, d, the object being at 0.

To secure coincidence of the images in the field of view for
varying widths betAveen the eyes Professor Riddell devised (1) a
means of regulating the inclination of the prisms by mounting them
in hinged frames, so that, while their lower edges, near a, fig. 72,
remain always in parallel contact, the inclination of the intei-nal
reflecting surfaces can be varied by the action of the milled head in
front of the prism box ; (2) the lower ends of the binocular tubes
are connected by travelling sockets, moving on one and the same
axis, on which are cut corresponding right- and left-handed screws,
so that the width of the tubes may correspond with that of the
prisms ; and (3) the upper ends of the tiubes are connected by racks,
one acting above and the other below the same pinion, so that right-
and left-handed movements are communicated by turning the pinion.

This instrument could only be used in a vertical position, as
shown in the figure (71). The two prisms jn fig. 72 correct the in-
version of the image in a lateral direction, two more prisms are
needed to correct the inversion in the vertical direction. These
Professor Riddell placed above the eye caps, but now they are placed
immediately above the binocular prisms, fig. 78.

This system of binocular excited much interest in England im-
mediately after its publication, and Mr. Wenham in London and
MM. Nachet, of Paris, soon suggested and devised a variety of
binocular systems.

1 P. 13.

H

98

VISION WITH THE COMPOUND MICROSCOPE

Nachet's Binocular was early in the field, but was not a
practical construction on account of the parellelism of its tubes, and
is not now advocated by its inventor or adopted by opticians of any
country.

Wenham's Stereoscopic Binocular. — All these objections are
ovei-come in the admirable ai-i-angement devised by the ingenuity of

Mr. Wenham, in 1860 (Trans. Microscopi-
cal Soc. of London, vol i. N.S. p. 15), in
whose binocular the cone of rays pro-
ceeding upwards from the objective is
divided by the interposition of a prism
of the peculiar form shown in fig. 73, so
placed in the tube which caiunes the objec-
tive (figs. 74, 75, a), as only to interrupt
one half, a c, of the cone, the other half,
a b, going on continuously to the eye-
piece of the principal or right-hand body,
R, in the axis of which the objective is
placed. The interrupted half of the cone
(figs. 73, 74, a), on its entrance into the
prism, is scarcely subjected to any refrac-
tio7i, since its axial ray is perpendicular-
to the surface it meets ; but within the prism it is subjected to two
reflexions at b and c, wdiich send it forth again obliqxiely in the line

Fig. 73. — Wenham's prism
(1860).

Fig. 7-1. Fig. 75.

Wenham's stereoscopic Ijinociilaf microscope (18G0).

dUnviirds the eye-piece of the secoinhny or left-hand body (fig. 74,
L); ;iiid since at its ciiicrgfiice its a.\i;il r.iy is again pcriiondicular

WENHAM'S BINOCULAR PRISM 99

to the surface of the glass, it suffers no more refi-action on passing
out of the prism than on entering it. By this arrangement the
image received by the right eye is foi-mecl by the rays which have
passed through the left half of the objective, and have come on
without any interruption whatever ; whilst the image received by
the left eye is formed by the rays which have passed through the right
half of the objective, and have been subjected to two reflexions within
the prism, passing through only two surfaces of glass. The adjustment
for the variation of distance between the axes of the eyes in different
individuals is made by drawing out or pushing in the eye-pieces, which
are moved consentaneously by means of a milled head, as shown in
fig. 75. Now, although it may be objected to Mr. Wenham's method
(1) that, as the rays which pass through the prism and are obliquely
reflected into the secondary body traverse a longer distance than
those which pass on uninterruptedly into the principal body, the
picture formed by them will be somewhat larger than that which
is formed by the other set ; but this can be easily compensated for
by (a) altering the power of one of the eye-pieces, {h) by increasing the
tvibe length of the direct tube ; and (2) that the picture foi'med by the
rays which have been subjected to the action of the prism must be
inferior in distinctness to that formed by the uninterrupted half of
the cone of rays ; these objections are found to have no practical
weight. For it is well known to those who have experimented
upon the phenomena of stereoscopic vision (1) that a slight differ-
ence in the size of the two pictures is no bar to their perfect com-
bination ; and (2) that if one of the pictures be good, the full effect
of relief is given to the image, even though the other picture be
faint and imperfect, provided that the outlines of the latter are
suflSciently distinct to represent its perspective projection. Hence
if, instead of the two equally half -good pictures which are obtainable
by MM. Nachet's original construction, we had in Mr. Wenham's
one good and one indiffeo'ent picture, the latter would be decidedly
preferable. But, in point of fact, the deterioration of the second
picture in Mr. Wenham's arrangement is less considerable than
that of both pictures in the original arrangement of MM. Nachet ;
so that the optical performance of the Wenham binocular is in every
way superior. It has, in addition, these further advantages over
the preceding : First, the greater comfort in using it (especially foi-
some length of time together), which results from the convergence
of the axes of the eyes at their visual angle foi- moderately near
objects ; secondly, that this binocular ari-angement does not necessi-
tate a special instrument, but may be applied to any microscope
which is capable of carrying the weight of the secondary body, the
prism being so fixed in a movable frame that it may in a moment
be taken out of the tube or replaced therein, so that when it has
been removed the principal body acts in every respect as an ordinary
microscope, tlie entire cone of rays passing uninterruptedly into it ;
and thirdly, that the simplicity of its construction renders its de-
rangement almost impossible.^

1 The Author cannot allow this opportunity to pass without expressing his sense
of the liberality with which Mr. Wenham freely presented to the public this im-

h2

lOO

VISION WITH THE COMPOUND MICEOSCOPE

Fig. 76. — Eiddell's binocular
prisms, as applied by Mr.
Stephenson (1870).

Stephenson's Binocular. — A new form of stereoscopic binocular
has been introduced by Mr. Stephenson/ which has certain dis-
tinctive features, and at the time Mr. Stephenson devised it he was
entirely unaware that any part of the
method he employed had been used by
another. He had, however, independently
conceived Riddell's device for dividing the
beam as a part of his very ingenious in-
strument. This he discovered and acknow-
ledged about three years after the full de-
scription and completion of his binocular.^
The cone of rays passing upwards from the
object-glass meets a pair of prisms (A A,
fig. 76) fixed in the tube of the microscope
immediately above the posterior combina-
tion of the objective, so as to catch the
light-rays on their emergence from it ;
these it divides into two halves and be-
haves as described in the Riddell prisms,
which, in fact, they are. As the cone of
rays is equally divided by the two prisms,
and its two halves are similarly acted on,
the two pictures are equally illuminated,
and of the same size ; while the close ap-
proximation of the prisms to the back lens of the objective enables
•even high powers to be used with very little loss of light or of
definition, provided that the angles and surfaces of the prisms are

worked with exactness ; and as the two
bodies can be made to converge at a
smaller angle than in the Wenham ar-
I'angement, the observer looks through
them with more comfort. But Mr. Ste-
phenson's ingenious arrangement isliable
to the gi-eat drawback of not being-
convertible (like Mr. Wenham's) into
an ordinary monocular by the with-
drawal of a prism, so that the use of
this foi-m of it will be probably re-
stricted to those who desire to work
with i\ binocular when employing high
powei's.

]:)ut one of the greatest advantages
attendant on Mr. Stephenson's con-
struction is its capability of being coml)ined with an erectimj
arrangement, whicli renders it applicable to purposes for which
the Wenham binocular cannot be conveniently used. By tlie in-
terposition of a plane silvered iniri-or, or (still l)ettei-) of a refle(;ting

Fig. 77.

—Stephenson's erecting
prism (1870).

portant invention, V)y which, there cuii lie no doubt, he niij^
fited if he had chosen to retain the exclusive ritrht to it.

' Montlily Microscopical Journal, vol. iv. (1870), p. 01, ami

- Ibid. vol. X. J). 41.

it li

i lar<,'(;ly jiro-
(1872), p. 107.

STEPHENSON'S BINOCULiJl 10 1

prism (fig. 77), above the tube containing the binocular j)rism.s,
each half of the cone of rays is so deflected that its image is reversed
vertically, the rays entering the prism through the surface 0 B, being-
reflected by the surface A B, so as to pass out again by the surface
A C in the direction of the dotted lines. Thus the right and the left
half-cones are directed respectively into the right and the left
bodies, which are inclined at a convenient angle, as shown in fig. 78 ;
so that^ — -the stage being horizontal — the instrument becomes a most
useful compound dissecting microscope, and as thus arranged by
Swift, with well adjusted rests for the hands, has but few equals for
the purposes of minute dissections and delicate mounting ojoerations ;
indeed, the value of the erecting binocular consists in its applica-
bility to the picking out of very miniite objects, such as Diatoms,
Polycystina, or Foraminifera,
and to the prosecution of /^^^^^^^

minute dissections, especially ^^^""''^^^0"^"-"-^^
when these have to be carried on ^^ ^^~^-..^^^^^~\„^^

in fluid. iSTo one who has only (<^^fe»,^ ^""~~\!^~~--^

thus worked monocidarly can ^-^ ^''''*'*^^^ ^^^\

appreciate the guidance derivable ^"'"-^■^^^^ l\,ss\

from hinocular vision when once C^^^^^^^'~^^^^:^^

the habit of working with it has ^"Bj^^^oS^My

been formed. ^^^^^^^^^^

Tolles's Binocular Eye-piece. l^^^^^^ol

An ingenious eye-piece has been ^^^^^x^ll'''''™'''5

constructed by Mr. Tolles (Boston, ,^i=JlL<^^ I^^''^

U.S.A.), which, fitted into the ^^^^^^\CJ li

body of a monocular microscope, ||| IT ^^

converts it into an erecting stereo- Fig. 78.— Stephenson's erecting

scopic binocular. This conversion binocular (1870).

is eflTected by the interposition

of a system of prisms similar to that originally devised by MM,
Nachet, but made on a larger scale, between an ' erector ' (re-
sembling that used in the eye-piece of a day-telescope) and a pair
of ordinary Huyghenian eye-pieces, the central or dividing prism
being placed at or near the plane of the secondary image formed by
the erector, while the two eye-pieces are placed immediately above
the two lateral prisms, and the combination thus making that
division in the pencils forming the secondary image which in the
Nachet binocular it makes in the pencils emerging from the objective.
A.S all the image-forming rays have to pass through the two surfaces
of four lenses and two prisms, besides sustaining two internal re-
flexions in the latter, it is surprising that Professor H. L. Smith, while
admitting a loss of light, should feel able to speak of the definition
of this instrument as not inferior to that of either the Wenham or
the Nachet binocular. It is obviously a great advantage that this
eye-piece can be used with any microscope and with objectives of
high power ; but as its effectiveness must depend upon extraordinary
accuracy of workmanship its cost must necessarily be great. ^

1 See American Journal of Science, vol. xxxviii. (1864), id. 111. and vol. xxxix.
(1865), p. 212 ; and Montldy Microsc. Journal, vol. vi. (1871), p. 45.

I02

VISION WITH THE COMPOUND MICROSCOPE

A form of this binocular eye-piece was made by Professor Abbe
with the ingenuity and thoroughness characteristic of the firm of
Zeiss ; but in spite of its beauty as an optical instrument, and its use-
fulness as applicable to any tube, and especially the shorter tubes
to which the Wenham
binocular could not well
apply, the double image
in the right-hand tvibe
was most conspicuously
apparent, greatly inter-
fering with the perfection
of the stereoscopic image.
On this account chiefly it
has not come into general
use. We are nevertheless
indebted to the firm of
Zeiss for the introduction
of a very satisfactory
form of binocular instru-
ment, of which we can
speak with unconditional
praise. It is designated
as Greenough's binocular
microscope, and we can
confidently afiirm that
it furnishes an accurate
solid and withal an erect
image, so that for all the

^v>-^^2^^,

Fk;. 79. — (irc'(j)ioiigli's binociiliir iiiicroscopo (18i)7j.

purposes for wliich tlie use of t\w Ijinocular is at })resent desir;il)le it ac-
<;omp]ishes wluit is sought, and will be found invaluable for zoologists,
botanists, and embi-yologists. 'J^lio mici-oscope is sliown in fig. 79,

GEEEKOUCtH'S binoculae miceoscope

103

and lias been constructed by means of a combination of Pwro pi-isms
with a compound microscope of the usual optical type ; it possesses
many of the advantages of the compound micro-
scope, but inevitably loses light by the passing of
the ray throvigh so many pi'isms, yet by means of
the Porro prisms the inverted image is rendered
erect. This may be practically illustrated by fig.
80, which shows that the i-ays of light in passing
ffom the object to the eye undei-go foui' succes-
sive reflexions at the svirfaces of the pi-isms and
emerge from the last prism with undiminished
intensity. The prisms, it will be seen, have the
eflfect of erecting the inverted image formed
by the object-glass. But in this microscope
binocular vision is obtained, not as in the usual
form of binocular microscope, by the subsequent
division of a pencil of light passing through
one object-glass ; but two complete microscopes,
each having its own objective and eye-pieces,
are simultaneously directed upon the object.
This secures perfect stereoscopic (orthomorphic)
vision, but of course no power higher than
1^ inch can be employed. The path of the rays
is more clearly seen in fig. 81, giving a diagram Fig. 80.— Showing the
by Mr. ^-elson with one of the prisms turned ^^^^^^ ^^
round 90° to make clearer the action of the rays (1894).
prisms on the ray. It is well to note that,

when two of these erectors with a double objective binocvilar are
used, the distance between the eyes can be compensated for by
merely turning the erector adaptors round in the microscope tube.

This method of erection, which is both valuable and practical, was'
first described in Zahn's ' Oculus Artificialis ' (1702), only reflectors
were used instead of prisms, but the path of the rays is diverted in
precisely the same way as with the Porro prisms.

The stereoscopic binocular is put to its m.ost advantageous use
when applied either to opaque objects of whose solid forms we are
desirous of gaining an exact appreciation or to transjiarent objects
which have such a thickness as to make the accurate distinction
between their nearer and their more remote planes a matter of im-
portance. All stereoscopic vision with the microscope, so far as it
is anything more than mere seeing with two eyes, depends, as already
seen, exclusively upon the unequal inclination of the pencils which
form the two images to the plane of the preparation, or the axis of
the microscope. By uniform halving of the pencils — whether l^y
prisms above the objective or by diaphragms over the eye-pieces —
the difference in the directions of the illumination in regard to the
preparation reaches approximately the half of the angle of apei'tui'e
of the objective, provided that its whole aperture is filled with rays.
By the one-sided halving we have been considering, the direct image
is produced by a pencil the axis of which is perpendicular to the

I04

VISION WITH THE COMPOUND MICEOSCOPE

plane of the prepai^ation, and the deflected image by one whose axis
Ts inclined about a fourth of the angle of aperture.

With low powers, which allo-w of a relatively considerable
depth-perspective, the slight difference of inclination, which remains

in the latter case, is quite sufficient to

^^ y j)roduce a very marked difference in

the perspective of the successive layers
in the images. But with high powers
the difierence in the two images does
not keep pace — even when both eye-
pieces are half covered — with the in-
crease of the angle of aperture, so long-
as ordinary central ilkmiination is
used. For in this case the incident
pencil does not fill the whole of the
opening of the objective, but only a
relatively small central part, which,
as a rule, does not embrace more than
40° of angle, and in most cases can-
not embrace m.ore without the clear-
ness of the microscopic image being"
affected and the focal depth also being
unnecessarily decreased. But as
those parts of the preparation which
especially allow of solid conception
are always formed by direct trans-
mitted rays in observation with ti-ans-
mJtted light, it follows that under
these circumstances the difference of
Pig. 81.--Simpler illustration of the ^j^^ ^^^ • ^ -^ founded, not on the

path of the ray with one prism ° i f. i i •

turned through an angle of 90° to whole aperture-angie of the objec-
make the path of the rays clearer, tive, but on the much smaller angle

of the incident and directly trans-
mitted pencils, which only allow of relatively small differences
of inclination of the image-foi'ming rays to the preparation. It is

evident, however, that when objectives
of short focus and correspondingly large
angle are used, a considerably greater
differentiation of the two images with re-
spect to parallax can be pi'oduced if, in
place of one axial illuminating pencil, two
pencils are used oppositely inclined to the
axis in such a way that each of the
images is produced l)y one of the pencils.
Tliis kind of doubki illumination, though
it cannot be ol)tained l)y the simple
mii-roi', can be easily produced by using
witli the condenser a diaphragm with two
openings (fig. 82), placed in the diaphragm stage under the con-
denser. We tlien have it in our power to use, at pleasure, pencils
of narrower or wider aperture and of greater' oi' less inclination

Fk;. wi.

Fig. 83.

POWELL AND LEALAND'S HIGH-POWER BINOCULAR 105

towards the axis by making tlie openings of different width and
different distance apart.

With diaphragms of this form (which can easily be made out of
cardboard) the larger aperture angles of high-power objectives may
be made use of to intensify the stereoscopic effect without employing
wide pencils, which are prejudicial both as diminishing the clearness
of the image and the focal depth.

Of course with this method of illumination both eye-pieces must
be half covered in order that one image may receive light only from
one of the two illuminating cones, and the other only
from the other. The division of light in both the aper-
ture-images will then be as shown in fig. 83 ; and it is
evident that in this case the brightness of the image for
both eyes together is exactly the same as would be given
by one of the two cones alone without any covering.

The method of illumination here referred to — which was origi-
nally recommended by Mr. Stephenson for his binocular microscope —
has, in fact, proved itself to be by far the best when it is a question
of iising higher powers than about 300 times. It necessarily requires
very well corrected and properly adjusted objectives if the sharpness
of the image is not to sviffer ; but if these conditions are satisfied it
yields most striking stereoscojDic effects, even with objectives of 2 mm.
and less focal length, provided the preparation under observation
presents within a small depth a sufficiently characteristic structure.

Non-Stereoscopic Binoculars. — The great . comfort which is ex-
perienced by the microscopist from the conjoint use of both eyes has
led to the invention of more than one arrangement by which this
comfort can be secured when those high powers are required which
cannot be employed with the ordinary stereoscopic binocular. This
is accomplished by Messrs. Powell and Lealand by
taking advantage of the fact already adverted to, that
when a pencil of rays falls obliquely upon the sur-
face of a refracting medium a part of it is reflected
without entering that medium at all. In the place
visually occupied by the Wenham prism, they in-
terpose an inclined plate of glass with j)arallel sides,
through which one portion of the rays proceeding up-
wards from the whole aperture of the objective passes
into the ■principal body with very little change in its
course, whilst another portion is reflected from its sur-
face into a rectangular prism so placed as to direct it
obliquely upwards into the secondary body (fig. 84).
Although there is a decided difference in brightness be-
tween the two images, that formed by the reflected rays
being the fainter, yet there is marvellously little loss of Fig. 81. (1865.)
definition in either, even when the 50th of an inch objec-
tive is used. The disc and prism are fixed in a short tube, which can
be readily substituted in any ordinary binocular microscope for the
one containing the Wenham prism. Other arrangements were long
since devised by Mr. Wenham,^ and subsequently by Dr. Schroder,
1 Transactions of the Microsc. Soc. N.S. vol. xiv. (1866), p. 105.

I06 VISION WITH THE COMPOUND MICKOSCOPE

for securing binocular vision with the highest powers. We have used
the lattei' of these with perfect satisfaction, but all that is required
is at the disposal of the student in the arrangement of Powell and
Lealand.

To those who have used these forms of binocular habitually it
has been a frequent source of surprise and perplexity that, although
theoretically such a form as that of Powell and Lealand's is non-
stereoscopic, yet objects studied with high powers have appeared as
if in relief, and the effect upon the mind of stereoscopic vision has
been distinctly manifest. The Editor was conscious
I I of this for many years in the use of the Powell and

#^M Lealand form, with even the -^^th of an inch power
\^ of the achromatic construction ; at the time he inter -
1 j preted it as a conceptual effect ; but it always arose

('•■'% ^^ when the pupils fell upon the outer halves of the
VJ. ^W Ramsden circles. The explanation, Dr. A. 0.

j i Mercer considers,^ is due to Abbe. Since (fig. 85)

^^ ^i when the eye-pieces are at such a distance apart that
»■ »|| the Ramsden circles correspond exactly with the
i i pupils of the eyes, centre to centre, the object appears

' ' flat. But if the eye-pieces be i-acked down, so as

Fig. 85. to be nearer together, the centres of the pupils fall

upon the outer halves of the Ramsden circles and we
have the conditions of orthoscopic effect ; while if they be racked up
so as to be more sepai^ated, the centres of the pupils fall on the inner
halves, and we have pseudoscopic effect.

The Optical Investigations of Gauss. — Before leaving this section
of our subject, in which we have endeavoured, with as much clear-
ness as we could command, to enable the general i-eader to com-
prehend with intelligibility the principles of theoretical and ajyplied
ojytics as they relate to the microscope, we believe we shall serve
the higher interests of microscopy, and the Avants or desires of the
more advanced microscopical experts, if we endeavour to present in
a form either devoid of technicality or with inevitable technicalities
explained a ge'tieral outline and then an application of the famous
dioptric investigations of Gauss, an eminent German mathematician,
who, amongst many other brilliant labours in applied mathematics,
expou'iuled the laws of the refraction of light in the case of a co- axial
system of spherical surfaces, having media of various refractive in-
dices lying between tliem.

Although the assimiptions upon which the formula? of (iauss
rest are not coincident with the conditions presented by the lens-
combinations which are employed in the constiaiction of modern
objectives of gi-eat aperture, the results, nevei-theless, fui'nish an
admirable presentation of the path of the I'jiys and the positions of
cjirdinal points, even in the mici'oscope as we know and use it.

We remember that the mici'oscf)pe is lai-gely used in England
and America by men who can oidy employ it in their more or less
brief i-ecessions from pi'f)fessional and cominei'cial pui-STiits, bixt who
often employ it with euthusiasm and intelligent purpose. Much

I Jdiirn. Jt.M.S, Hor. ii. vol. ii. ]). 271.

DIOPTRIC INVESTIGATION BY GAUSS

107

scientific work may be done by snch men, and it will promote the
accomplishment of this, in our judgment, if the frequently expressed
desire be met which will enable
such students to understand in a
general but thoroughly intelli-
gent manner the principles in-
volved in the employment of
systems of lenses.

Many such either have scanty
knowledge of algebra, or in the
continuous pressure of other
claims have lost much that they
once possessed. We believe that
in these cases the following ex-
position of the dioptric system
of Gauss, with a following ex-
ample worked out in full and
with every step made clear, will
be of real and practical value.
Without some intelligible under-
standing of the ultimate prin-
ciples of the microscope no re-
sults of the highest order can, at
least with moderate and high-
power lenses of the best modern
construction, be anticipated. On
this ground we commend the
study to the earnest reader.

Let R N, S W (fig. 86) be
the spherical surfaces of a lens
of density greater than air, and
let P R S j9 be the course of a
ray of light passing through it ;
C, C, the centres of the spherical
surfaces.

Let PR, R S be produced
to m.eet the perpendiculars
through C and C in A and A'.

Let C R=r, C S=r',2 ^=
index of refraction out of air
into the medium. NN'=cZ, the
thickness of the lens. IST R^6,
]Sr' 8=5''. These may be con-
sidered as straight lines.

Let the equation to P R be
2/-6=77?(a;— ON) . . (1)

Let the equation to R S be
9/—b =im'{x—0 N) . . (2)

1 This figure is greatly exaggerated for the sake of clearness.

- If either of the curvatures be turned in the opposite direction the sign of the
corresponding r must be changed.

ro8 VISION AYITH THE COMPOUND MICEOSCOPE

or, y—h'=m'{x—OW) . . (3)

Let the equation to S^ he 'y — b^=m^'[x — 0^') . . (4)

From (2) and (3)

h'-b=m' {OW—0'N)=m'l^'l^=mUl . .' . ■ (5)

Now sin 0 R A=/x . sin C R B ;

or, -^-— . sin 0 A R=^ . — — . sin 0 B R.

C R G R

Now C A and 0 B are the vakies of y in equations (1) and (2)
when 33^0 C ;

.-. C A=& + m(OC-ON) = & + mr;

and similarly Clti=b + m'r;

.-. (5 + m9-) sin C A R=/x {b + m' r) sin GBR.
Now OAR, GBR do not in general differ much from each
other, so that for a first approximation we may consider them to be
equal.

.•. b + m r:=fx(b + 'm' r), i.e. fx m'=')n — ~ . b.

Let — ^=:u; then fx on' ^m — b u . . . (6)

Similarly, sin G' S B' =ytx . sin G' S A' ;

_^.smGB'S=^^

and, as before,

C' W=b' + m''r', C A':=^b' + m/ r' from equations (4) and (3) ;
.•.as before we may take

b' + 7n" r'=fi {b' + m' r'), or ixm'=m" — ^ ^ b' .

(-1

r'

Let C— __ = ?/, then ix')n'-=m" — b' u' . . . (7)'

r

From (5) and (6) b'=b+ cl=b [i _ ) -|- — .

fX \ ft J fX

,, this and (7) m,"=^im' + b u' ( 1 — - ) j_

A fx J fx

and from ((5) z=.iii — b >i,-\-b n! (1 — -) +

='"' [' + -,-) + ''{"-•"- , )■

Assume

rZ d /f, d 9(/ d 'II, 'iif

- ^k I — =71+ ^=L 'III — 'n, — =/(;

IX IX ■" fX ' ^i

then b'=(i J>-\-Ji.iii,\ , / 7 7 1 /Q\

,, ,1,1 >, \vlier(w// — A, /,;= 1 . . (8>

Nfnv let X, Y be the coordinates of P, the point from which the
ray of light proceeds ;

then by (1) b=Y-vi (X— O N) ;

DIOPTRIC INVESTIGATION BY GAUSS
substituting in (8)

109

whence

m" — h Y

h'=(jY+m (A-(/.X-ON);
m"^kY + m (/-^■X-ON);

Now substituting in (4) the equation to the i-efi-acted i-ay
becomes

or by (8)

■' ./-A;(X-OX) V Z-A;(X-OX)/

First: If X be taken such that l—k (X-0X) = 1, i.e.

(9)

x = ox~

then when

T

= 0 E, suppose ;

;^.-^0 W-h + g ^""i =0 W + 1-^=0 E', suppose,
K k,

.//=: Y, or P and p are equally distant from the axis.

Also, if Y ^ 0, y = 0 ; or if a ray proceed from E, it will fifter

^,2,// ^ y

refraction pass through E^ Also m = -— =- --— - = ml' that is,

Z — A; (X — OX)

the ray will be equally inclined to the axis Ijefore and aftei' refrac-
tion.

E and E' are called the ' principal points.'

du'
\-l

O E = 0 X

OX +

k

= ox +

cl%(I

d u a'

fj. [u' — u) — d to til '

O E'= 0 W + —-j_ ^ =0 X' +

%i — u —

OX' +

d%i

[X {ii' — to) — d u u'

Secondly : If m" = 0, or the ray be parallel to the axis after
refraction, we have from (8)

h = — _ m, and the equation to the incident ray becomes

?J + j m

K

{x - O X), or .y = m (x-01^- ^\

no VISION AVITH THE COMPOUND MICEOSCOPE

dii'
I 1 + —

.•.wheny = 0,.x- = ON+^, = ON+- ^,

%{, — It, —

= 0 F, suppose. "

If m = 0, or the ray be parallel to the axis before refraction, we
have from (8)

h' =■ g h =-i m", and the equation to the refracted ray becomes

?/ — -L m ' =

A-

{x — O N'), or 2/ = nV' (x - O N' + ^"j ;

d u
.-. when 2/ = 0, x = 0 N' - f = O N' ^^ ^

= O i^ , suppose. fx

F and F' are called the ' focal points.'

O F = O N + ^

0 F = 0 N' -

yU {ll' — Xl) — d U u'

fi — du

fi {u' — i() — duu']
The focal distance -/=OF-OE = OE'-OF'
^ M ^ 1 .

/u {%i' — %i) — d u u' k

Similarly, it may be shown that if there be two lenses, and sub-
script numbers refer to the first and second lens respectively, while
E, E', F, F' refer to the entire system, and if

a = 0E.2-0E/,
^1 = — 7^ = A'l 0'/ — "i) — (h "i 'ih\

^'2 = - ^ = ^2 0^2' — ^^2) — (h ^2 «2',

OE = OE.V ^'^"^

OE' = OE./-

OF = OE,+ ---^i-^^^ + ^/f— ■)

A'2^1 + /^l V2 + OViV^
^2(/^l^J^l)

0F' = 0E2' —

We are now pi-epai-ed to loork out an example of the Gauss s/jstem,
by tracing a ray through two or more lenses on an axis, showing how
any conjugate may be found through two or more lenses on that axis.^

' Rememberiiif^ our object, and the assumed conditions of some for whom we
write, we do not liesitate to preface tin's with tlie following,' notes to remind tlie
reader of the sense attached to certain mathematical expressions.

00 means infinity. A i)lane surface of a lens is considered a splierical surface of
an infinite radius. Any number divided by<»-=0 any number divided byO — co;

EXAMPLE AFTER GAUSS III

The Gauss system of tracing a ray through two or more lenses
on an axis illustrated by means of a worked-out example.

Two lenses, 1 and 2, fig. 87, or an axis x y are given. Iso. 1 is
a double convex of crown \ inch thick, the I'efi-active index /x being
4, the radius of the surface A is | and that of B 1 inch. No. 2 lens
is a plano-concave of flint -jL. inch thick, the refractive index yu being
;?-, the radius of the surface C is |-, and the surface D is plane. The
distance between the lenses, that is, from B to C measured on the
axis, is J inch. The problem is to find the conjugate focus of any
given point V.

In order to accomplish this two points have first to be found with
i-egard to each lens. These points are called principal points (see
PP', QQ'' in fig. 87). When the radii of curvature r and r', d,
the thickness, and ^^ f-ioi the refractive indices of the respective
lenses,^ are known, the distance of these points from the vertices, i.e.
the points where the axis cuts the surfaces of the lens, can be foimd.
Thus by applying Professor Fuller's formulfe to lens 1 the distance of
P from the vertex A can be determined — seep. 115 (i) — similai-ly P'^
from B — p. 115 (ii). In the same way the points QQ'' from C and D
in lens 2 can be measured off — (v) (vi), pp. 115, 116.

It must be particulai-ly noticed that in measiu'ing ofi" any dis-
tance if the number is + it must be measured from left to right,
and if — from right to left. Thus in (i) p. 115 because the sign of
•158 is + P lies '158 of an inch to the right of A. And in (ii)
because "21 is — P' lies "21 of an inch to the left of B. The same
rule applies to the radii ; thus the radius of A, being measured from
the vertex to the centre or from left to right, is + ; but the radivis
of B, being measured from the vertex to its centre or from right to
left, is — . Similarly with the concave surface, C being measured
from right to left is — .

In both the examples before us the points PP', QQ' fall inside

any number multiplied by 0 = 0. =■= plus, or minus, or multiplied by any number is-

still CO.

The following are the rules for the treatment of algebraical signs :

In the multiplication or division of like signs the result is always j^his ; but if

the signs are dissimilar it is always minus.

In addition, add all the terras together that have a plus sign ; then all the terms

with a minus sign ; subtract the less from the greater and affix the sign of the

greater. Example :

+ 3-4 + 2-5=-4.

In subtraction change the sign of the term to be subtracted and then add in
accordance with the i^revious rule. Example ;

-3

+ 2_

-5
An example occurs in the annexed equations (x) and (xi), p. 116, of — "^ — = +,
but then the + is changed into a — by the negative sign in front of the fraction.
In (xii), p. 116, however, there being a + in front of the fraction, the result remains
positive.

1 In the worked-out example no distinction has been made between the r, r' of
one lens and the r, r' of the other lens, as well as of /j. and d, because when the
X^rincipal points and focal length are determined for one lens those expressions are
not again needed, so the same letters with different values assigned to them may be
equally well used for the next lens. Too many different terms are apt to confuse
the student, while those who are familiar with mathematical expressions will under-
stand the arrangement.

112 VISION WITH THE COMPOUND MICROSCOPE

their respective lenses, but it does not follow that they will do so in
«very instance. In some forms of menisci, for example, they will fall
-outside the lens altogether.

With regard to the focus of the lens it follows the same rule ;
thus, /in lens 1 is measured to the left fi'om P, and/' to the right
from P' ; similarly in lens 2,/'' is measured to the right from Q,
And/"' to the left from Q'.

Having determined the focal length of each lens, the distance
between the right-hand principal point of the first lens P' and the
left-hand principal point of the second lens Q must next be found. It
manifestly is the distance of B from P' + the distance B C between
the lenses, Q being at the point C. Therefore,

P'Q=-21 + -25=-46=a.

"Wlien these three data have been obtained — that is, the focal
length of each lens, and the distance between them — we are in a
position to apply the formulas (ix) and (x), p. 116, to find the principal
points E and E' of the combination.

In selecting the value of the focus to be put into the equations
for both lenses, the last must be taken, that is, in lens 1 (iv) or
+ ■94,7, and in lens 2 (viii), or — 1-875.

It will be noticed that the value of E being negative, it will be
measured '314 inch to the left from P. Similarly, E' is measured
•622 inch to the left from Q',

(ft also is 1-28 to the left from E, and 0' 1-28 to the right from E'.

These four pouits, E E' and f (p', are called the cardinal points
of the combination.

Here it must be observed that in this work it has been necessary
for want of space to restrict the problem to dry lenses, that is, to
those cases where the ray emerges from the combination into air, the
same medium in which it was ti'avelling on immergence. It is on
that account that the values of 0 and ^' are the same.

Having now obtained the four cardinal points, we may at once
proceed to find the conjugate of x.

Let X equal the distance of the point x from the focal plane 0,
and y the distance of its conjugate from (p' . Then by formula (xiii)

xy = (f^, and as « = 1 inch, y =■ — '— = 1-6384.

This numerically determines the position of the conjugate plane.
If the rays incident on the combination are parallel, then xz= oo,

;iiid y = I =0, which means that y is coincident with (j>' .

The following is the graphic method of finding the conjugate of
V, From Y, fig. 87, draw a line ])arallel to the axis to meet E', and
fi-om the point where it meets E' di-iw ;i line; through N, the point
where </»' cuts the axis, to W.

From V draw another line through M, the point where 0 c-uts
the axis, to meet E, and fi-0}ii the })oi)it where it meets E draw a
line pai-allel to the axis, cutting the other line in W. W will 1)0 the
conjugate of V, wliich was required.

A PEACTICAL EXAMPLE AFTEK GAUSS

113

If it is required to find
the conj ugate of a, i-ay pass-
ing through three lenses on
an axis, two of the lenses
must be combined and their
four cardinal jDoints found.

The principal points
and the focal length of the
third lens must then be
calculated, and then com-
bined in their turn by
formulfe (ix), (x), (xi), and
(xii), p. 116, with the car-
dinal points of the double
combination. 8 is taken as
the distance of the first
principal point of the com-
bination, nearest the third
lens, to the second principal
point of the lens, nearest
the combination. A fresh
set of cardinal points is de-
termiined in this manner
for the three lenses.

So also with four lenses ;
the cardinal points of each
pair being found, they ai»e
combined by the same
formulae, and new cardinal
points for the whole com-
bination of four lenses are
obtained. Similarly, the
cardinal points of five, six,
or any number of lenses
can be found and the con-
j ugate of any point local ised .

Finally, no one need be
discouraged by the appear-
ance of the leng-th of the
calculation ; the example is
given in full, so that any
one acquainted only with
vulgar fractions and deci-
mals can work it, or any
other similar problem, ovit.

In lens No. 1, for in-
stance, the numerators of
the fractions are all very
simple, and the denomina-
tors of the four equations
are all alike ; so, too, in

114 VISION WITH THE COMPOUND MICKOSCOPE

the equations for ISTo. 2 and in those for both lenses. Further, / is
the same ix^sf'^f'" as f", and (\>' as ^. Hence the problem is much
shorter than it looks.

If the conjugate of a point on the axis is only required, and if
the principal points and foci of each lens have been determined, it
will not be necessary to enter into the further calculation to find E,
E' and <^, cp', the cardinal points of the combination.

The method of procedure is as follows : If x is the given point,
its distance from/", the focus of lens No. 1, must first be measured.
Call this distance x. Then the distance of o its conjugate from the
other focus, f, supposing lens No. 2 to be removed, can be found by
formula

0 X- = /2, 0 = ■^,

/2 = -897, a; = 1-65;

•897 f,,.,

.-.0= -- = -543.
1-65

This is the distance from/' to o.

As the distance from a; to/" is positive, the distance between f
and 0 is also positive ; so o is to the right of /^

Before proceeding it will be as well to examine other possible
•cases which might occur.

Suppose that x was at the point f, then x would equal 0, and
0=00 ; that is, o would lie at an infinite distance from f. If, on
the other hand, the point x was to the right of f, x would be nega-
tive, and 0 would be also negative, because f^ is always j)ositive ;
0 would then be measured ofi" to the left of f, and the conjugate
would be virtual. This means that there will be no real image,
because the rays will be divergent on the/"' side of the lens, as if
they had come from some focus on the / side of the lens. But to
return. The point o having been found to be the conjugate of x,
■due to the sole influence of No. 1 lens, we have next to measure the
distance between o and/'', and, by applying the same formula, find
the distance of its conjugate from f", owing to the exclusive efiect
of No. 2 lens now replaced. This distance of" may be found thus:

P' o=PV+/' o = -947 + -543=l-49 ;
p/ f"=-^' B + B C + Q/''=-21 +-25 + l-875=2-335 ;
p/ //'_p/ o=o/''=2-335-l-49 = -845.

Calling this distance O, then, by formula y 0=/" '^, we shall find

f" ^ 3'515
the distance of y h-omf"', which we shall call //. y= = ' q.^

:=4"16, which is positive ; therefore y lies 4* 16 inches fromy"' to the
right liand. y is therefore the conjugate of x, due to the influence
of both lenses 1 and 2. Similarly, the conjugateof any point on the
uxis may be found through any number of lenses.

Lens No. \ : Data. — Radius A = - = /• ; radius 15 = — 1 =/•'';

4

I ."')

foci, y, f'\ thickness = ^ = r^ ; /i = ^ ; P = jii'iiiciiial point meii-

A PEACTICAL EXAMPLE AFTER GAUSS I I 5

sured from A ; P'= principal point measured from B
•^ 1 -^ 1

r 3 3' r' _l 2

4

/ / X 3 / 1 2\ 7 ; , 1 2 1 1

f, K-«)=^ (-2-3)= ~ 4 ' '^ '' '' =2^3^ -2= - 6

7 1 19

// («' — h) — f^ u u' = — 4 + Q= ~12~ — 1"'5B3 ;

1 1

. A _L

/i (?«.' — ii) — dull'

du' , 2^ 2 3

12

=A + -158 (i)

1 2

12

=B— -21 . . . . . (ii)

3

/=p+ ^ p , — L,=:P_^

iii{ii'-v)-dmi'-^ '^ _^^ 19

12

=p_-947 (iii)

3

2 1ft

// __ p/ M ^ p' P^ 4-

•^ ^(V-m)-cZwm' 19 "^19

r2

=P' + -947 (iv)

9
Lens No. 2 : Data. — Radius C = — q=»" ; radius D = oo = r';

o

1 8

foci, f"if"''i thickness =--|^= fZ ; /^ = -^ ; Q := principal point

measured from C ; Q'= principal point measured from D.

8_i 8-1

j^^J i _ _ A . ,,' =^-1^=5 ^ _ 0 •

^'' r ~ 9 15 ' r' cc '

~8

x8/ 8\64,,1 8^^

^(---)=5(0 + r5)=75'^^""=10X-15>^0=0;

, , . 64 64 OP,.,

ju (^(- —u)—d'u u =^g — 0 = ;j,g— "oo-^ ;

Q=C+ -,o— !--^i ,= C+,?,=C + 0 . . . (v)

F (*'■ — '0 — duu 64

75

i2

Il6 VISION WITH THE COMPOUND MICROSCOPE

Q' = ^"^ + uK-w)-cariV~ "^ 64 -^ 16

75
= D — -0625 (vi)

yU (■«' m) fZ U llf

f" = Q + -,... ..^ .;Trr:.= Q +64 =^ Q+y^ = Q + 1-875 (vii)

75

/" =Q' - .... ..^ ..■..=Q-64=Q-

U ('»/ — ■?(.) — f^ U tl'

75
=Q'— 1-875 (viii)

Both Zewses.— Distance apart = B C =^=-25 ; P' Q = -21 + -25

= -46 = a ; /= focus of No. 1 lens = -947 ; /' = focus of No. 2
lens = — 1-875.

E^P , ^f - p + -^6 X -947 _ -436

f-^f'-h "^ -947 - 1-875 - -46 ■^_ 1-388
= P — -314 (ix)

E' = O' - ^-/' = O'- _^6_^ -Jl^^^
f^-f -S ^ -947 - 1-875 - -46

= Q'-^T?^=Q'--62i . . . (X)

<^-E- ff -E- -947 X- 1-875
^ / + /'-^ -947 - 1-875 --46

= E tlL-^ = E - 1-28 . . . (xi)

— 1-388 ^ :'

./ .= E' + - , //' = E' + _J41_x_- 1-875

/+/'-g -947- 1-875 - -46

. y = f-^ ; y = t', = l^^= 1-6384 . . . (xiii)

X 1-0

117

CHAPTER III

THE HISTOBY AND DEVELOPMENT OF THE MICBOSCOPE

The historic progression of the modern microscope from its earliest
inception to its most perfect form is not only full of interest, but is
also full of the most valuable instruction to the practical micro-
scopist. In regard to the details of this, our knowledge has been
greatly enriched during recent years. The antiquarian knowledge
and zeal in this matter possessed by Mr. John Mayall, jun., and
the unique and valuable collection of microscopes made by Frank
Crisp, Esq., LLB., ranging as they do through all the history of
the instrument, from its earliest employment to its latest forms,
have furnished us with a knowledge of the details of its history not
possessed by our immediate predecessors.

We may obtain much insight into the nature of what is indis-
pensable and desirable in the microscope, both on its mechanical and
optical sides, by a thoughtful perusal of these details. It will do
more to enable the student to infer what a good microscope should
be than the most exliaustive account of the varieties of instrument
at this time produced by the several makers (always well presented
in their respective catalogues) can possibly do. Availing ourselves
of the material placed at our disposal by the generosity of these
gentlemen, we shall therefore trace the main points in the origin
and progress of the microscope as we now know it.

Mr. Mayall ^ gives what we must consider unanswerable reasons
for looking upon the microscope, ' as we know and emjDloy it,' as a
strictly modern invention. Its occurrence at the period when the
spirit of modern scientific research was asserting itself, and when
the necessity for all such aids to physical inquiry and experimental
research was of the highest value, is as striking as it is full of
interest.

It may be held as fairly established that magnifying lenses were
not known to the ancients, the simplest optical instruments as we
understand them having no place in their civilisation.

A large number of passages taken from ancient authoi'S, and
having an apparent or supposed refei'ence to the employment of
magnifying instruments, have been collected and carefully criticised,
with the result that all such passages can be explained without in-
volving this assumption.

We learn from Pliny the elder and others, that crystal globes
filled with water were employed for cauterisation by focussing the

1 Cantoi' Lectures on the Microscope, 1886, j)- 1-

Il8 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

sun's rays as a burning-glass, and that these were used to produce
ignition ; but there is no trace of suggestion that these refracting
globes could act as magnifying instruments.

Seneca (' Qufest. Kat.' i. 6, § 5) states, however, that ' letters
though small and indistmct are seen enlarged and more distinct
through a globe of glass filled with water.' He also states that
' fi'uit appears larger when seen immersed in a vase of glass.' But
he only concludes from this that all objects seen through water
appear larger than they are.

In like manner it could be shown that Archimedes, Ptolemy,
and others had no knowledge of the principles on which refraction
took place at curved surfaces.

Nor is there any ancient mention of spectacles or other aids to
vision. Optical phenomena were treated of; Aristotle and the Greek
physician Alexander dealt with myopy and presbyopy ; Plutarch
treated of myopy, and Pliny of the sight. But no allusion is made
to even the most simple optical aids ; nor is there any reference to
any such instruments by any Greek or Roman physician or author.
In the fifth century of the Christian era the Greek physician Actius
says that myopy is incurable ; and similarly in the thirteenth
century another Greek physiciait, Actuarius, says that it is an in-
firmity of sight for which art can do nothing. But since the end of
the thirteenth century, which is after the invention of spectacles,
they are frequently referred to in medical treatises and other works.

If we turn to the works of ancient artists we find amongst their
cut gems some works which reveal extreme minuteness of detail and
delicacy of execution, and some have contended that these could
only have been executed by means of lenses. But it is the opinion
of experts that there is no engraved work in our national collection
in the gem department that could not have been engraved by a
qualified modern engraver by means of unaided vision ; and in
i-eference to some very minute writing which it was stated by Pliny
that Cicero saw, Solinus and Plutarch, as well as PHny, allude to these
marvels of workmanship for the purpose of proving that some men
are naturally endowed with powers of vision quite exceptional in
their excellence, no attempt being made to explain theii- minute
details as the result of using magnifying lenses.

These and many other instances in which reference to lenses
must have been made had they existed or been known ai-e con-
clusive ; for it is inconceivable that even simple dioptiic lenses, to
say nothing of spectacles, microscopes and telescopes, could have
been known to the ancients without reference to them having been
roade liy many wi-iters, and es^jecially by such men as Galen nnd
Pliny.

The eai'liest known reference to tlie ixivention of spectacles is
found in a manuscript dating from Florence in 1299, in which the
writer says, ' I find mys(df so pressed by age that I can neither
read nor write witliout tliose glasses they cnll spectacles, Intely in-
vented, to the great advantage of poor old men when their sight
grows weak.' ^ Giordano da Rivalto in 1305 says that the invention

1 Sinith'H Optics, Cainbriflge, 1738, 2 vols. ii. pp. 12, 13.

A ' LENS ' FEOM SARGON'S PALACE

119

of spectacles dates back 'twenty years,' which woukl be about 1285.
It is now known that they were invented by Salvino d'Armato degli
Armati, a Florentine, who died in 1317. He kept the secret for
profit, but it was discovered and published before his death. But
there is a singular evidence that a lens used for the j^urpose of
magnification Avas in existence as early as between 1513 and 1520,
for at that time Raphael painted a portrait of Pope Leo X. which
is in the Palazzo Pitti, Florence. In this picture the Pope is drawn
holding a hand magnifier, evidently intended to examine carefully
the pages of a book open before him. But no instruments com-
parable to the modern telescope and microscope arose earlier than
the beginning of the seventeenth century and the closing years of
the sixteenth centur}" respectively.

It is, of coui-se, known that there is in tlie British Museum a
remarkable piece of rock crystal, which is oval in shape and ground
to a plano-convex form, which was found by Mr. Layard during the
excavations of Sargon's Palace at
Nimroud, and which Sir David
Brewster believed was a lens de-
signed for the purpose of magni-
fying. If this could be established
it would of course be of great
interest, for it has been found
possible to fix the date of its pro-
duction with great probability as
not later than 721-705 B.C.

A drawing of this ' lens ' in two
aspects is shown in figs. 88 and 89,
and we spent some hours in the
careful examination of this piece
of worked rock crystal, which by
the courtesy of the ofiicials we were
permitted to photograph in various
positions, and we are convinced

that its lenticular character as a diojotric instrument cannot be made
out. There are cloudy stria? in it, which would prove fatal for
optical purposes, but would be even sought for if it had been intended
as a decorative boss ; while the grinding of the ' convex ' surface
is not smooth, but produced by a large number of irregular facets,
making the curvature quite unfit for optical purposes. In truth,
it may be fairly taken as established that there is no evidence of any
kind to justify us in believing that lenses for optical purposes were
known or used before the invention of spectacles.

From the simple spectacle-lens, the transition to lenses of shorter
and shorter focus, and ultimately to the combination of lenses into
a compound form, would be — in such an age as that in which the
invention of spectacles arose — only a matter of time. But it is
almost impossible to fix the exact date of the production of the first
microscope, as distinguished from a mere magnifying lens.

There is nevertheless a consent on the part of those best able
to judge that it must have been between 1590 and 1609 ; while it is

Fig. 89. — An Assyrian ' lens ' (

I20 THE HISTORY AND DEA^ELOPMENT OF THE MICROSCOPE

probable (but by no means certain) that Hans and Zacharias Janssen,
spectacle makers, of Miclclelburg, Holland, were the inventors. But
it would appear that the earliest microscope was constructed for
observing objects by reflected light only.

At the Loan Collection of Scientific Instruments in London in
1876 an old microscope, which had been found at Middelbvirg, was
shown, which, Professor Harting considered, might possibly have
been made by the Janssens. It is drawn in fig. 90, and consists of
a combination of a convex object-lens and a convex eye-
lens, which form was not published as an actual con-
struction until 1646 by Fontana, which, as Mr. Mayall
points out, does not harmonise with the assumption
that this instrument was constructed by one of the
Janssens.

It is strictly a com.pound microscope, and the dis-
tance between the lenses can be regulated by two
draw-tubes. There are three diaphragms, and the eye-
lens lies in a wood cell, and is held there by a wire ring
sprung in. The object-lens, a, is loose in the actual
instrument, but was originally fixed in a similar way
to h.

It cannot be an easy task — if it be even a pos-
sible one — to definitely determine upon the actual indi-
vidual or individuals by whom the compound micro-
scope was fii'st invented. Recently some valuable

Fig. 90.
' Janssen's '
conipound

""°5°^°°P® evidence has been adduced claiming its sole invention
for Galileo. In a memoir published in 1888 ^ Professor
O. Govi, who has made the question a subject of large and continuous
research, certainly adduces evidence of a kind not easily waived.

Huyghens and, following him, many others assign the invention
•of the compound microscope to Cornelius Drebbel, a Dutchman, in
the year 1621 ; but it has been suggested that he derived his know-
ledge from Zacharias Janssen or his father, Hans Janssen, spectacle
makers, in Holland about the year 1590; while Fontana, a Nea-
politan, claimed the discovery for himself in 1618. It is said that
the Janssens presented the first microscope to Charles Albert, Arch-
duke of Austria ; and Sir D. Brewster states, in his ' Treatise on the
Micro.scope,' that one of their microscopes which they presented to
Prince Maurice was in 1617 in the possession of Cornelius Drebbel,
then mathematician to the Court of James I., where 'he made
mici-o.scopes and passed them oflfas his own invention.'

Nevertheless we are told by Yiviani, an Italian mathematician,
in his ' Life of r4alileo,' that ' this great man was led to the discovery
of the microscope from that of the telescope,' and that 'in 1612 he
sent one to Sigismund, King of Poland.'

We now i-eceive evidence thi-ough the i-esearches of Govi that
the invention was solely due to Galileo in the year 1610. Professor
Govi understiinds by ' simple microscope ' an instrument ' consisting
of a single leiis oi- mirror,' and by 'compound microscope' one ' con-

' Atti It. Acad. HcA. Fis. Nat. Napoii, vol. ii. Heries ii. ' 11 microscopio compoBto
inventato da Galileo,' Journ. B.M.S. Pt. IV. 1889, p. .574.

DID GALILEO INVENT THE COMPOUND MICROSCOPE? 121

sisting of several lenses or a sviitable combination of lenses and
mirrors.'

In a pamphlet pviblishecl in 1881, treating of the invention of
the binocular telescope, Govi pointed out that Chorez, a spectacle
maker, in 1625, used the Dutch telescope as a microscope, and stated
that with it ' a mite appeared as large as a pea ; so that one can
distinguish its head, its feet, and its hair — a thing which seemed in-
credible to many imtil they witnessed it with admiration.'

To this quotation he added : —

' This transformation of the telescope into a microscope (or, as
opticians in our own day would say, into a Briicke lens) was not an
invention of the French optician. Galileo had accomplished it in the
year 1610, and had announced it to the learned by one of his pupils,
John Wodderborn, a Scotchman, in a work which the latter had
just published against the mad " Peregrinazione " of Horky. Here
are the exact words of Wodderborn (p. 7) : —

' Ego nunc admirabilis huius perspicilli perfectiones explanare
no conabor : sensus ipse index est integerrimus circa obiectum pro-
prium. Quid quod eminus mille passus et ultra cum neque videre
iudicares obiectum, adhibito perspicillo, statim certo cognoscas, esse
hunc Socratem Sophronici filium venientem, sed tempus nos docebit
et quotidianee nouarum rerum detectiones quam egregie perspicillum
suo fungatur munere, nam in hoc tota omnis instrumenti sita est
piilchritudo.

' Audiueram, panels ante diebus authorem ipsum Excellentissimo
D. Cremonino purpurato philosopho varia narrantem scitu dignissima
et inter csetera quomodo ille minimoi'um animantium organa motus,
et sensus ex perspicillo ad vnguem distinguat ; in pai'ticulari autem
de quodam insecto quod utrumque habet oculum membrana crassius-
cula vestitum, quae tamen septe foraminibus ad instar larvas ferret
militis cataphracti terebrata, viam prsebet speciebus visibilium. En
tibi [so says Wodderborn to Horky] nouum argumentum, quod per-
spicillum per concentrationem radiorum multiplicet obiectu ; sed
audi prius quid tibi dicturus sum : in C8eteris animalibus eiusdem
magnitudinis, vel minoris, quorum etiam aliqua splendidiores habent
oculos, gemini tantum apparent cum suis superciliis aliisque partibus
annexis.' •

To this Govi adds : —

' I have wished to quote this passage of Woddei'born textually,
so that the honour of having been the first to obtain from the Dutch
telescope a compotmd microscope should remain with Galileo, which
the latter called occhialino, and that the gloiy of having reduced the
Keplerian telescope to a microscope (in 1621) should rest with
Drebbel. The apologists of the Tuscan philosopher, by attributing
to him the invention of the microscope without specifying with what
microscope they were dealing, defrauded Drebbel of a merit which
really belongs to him ; but the defenders of Drebbel would act un-
justly in depriving Galileo of a discovery which incontestahly was his.'

I turn now to Wodderborn's account, published in 1610 (the
date of the dedication to Henry Wotton, English Ambassador at
Venice, is October 16, 1610), which reads thus : —

122 THE HISTORY AND DEVELOPMENT OF THE MICEOSCOPE

' I will not now attempt to explain all the perfections of this
wonderful occhiale ; our sense alone is a safe judge of the things
which concern it. But what more can I say of it than that by
pointing a glass to an object more than a thousand paces off, which
does not even seem alive, you immediately recognise it to be
Socrates, son of Sophronicus, who is approaching ? But time and
the daily discoveries of new things will teach us how admirably the
glass does its work, for in that alone lies all the beauty of that
instrument.

' I heard a few days back the author himself (Galileo) narrate to
the Most Excellent Signor Creraonius various things most desirable
to be known, and amongst others in what manner he perfectly dis-
tingviishes with his telescope the organs of m^otion and of the senses
in the smaller animals ; and especially in a certain insect which has
each eye covered by a rather thick membrane, which, however, per-
forated with seven holes, like the visor of a wari-ior, allows it sight.
Here hast thou a new proof that the glass concentrating its rays
enlarges the object ; but mind what I am about to tell thee, viz. in
the other animals of the same size and even smaller, some of which
have nevertheless brighter eyes, these appear only double with their
eyebrows and the other adjacent parts.'

After reading this document Govi judges that it is impossible to
refuse Galileo the credit of the invention of a cofiipoutid microscope,
in 1610, and the application of it to examine some very minute
animals ; and if he himself neither then nor for many years after
raade any mention of it publicly, this cannot take away from him. or
diminish the merit of the invention.

■ It is not to be believed, however, that Galileo after these fii'st
experiments quite forgot the microscope, for in preparing the
' Saggiatore ' between the end of 1619 and the middle of October,
1622, he spoke thus to Lotario Sarsi Segensano (anagram of Oratio
Grassi Salonense) :—

' I might tell Sarsi something new if anything new could be told
him. Let him take any substance whatever, be it stone, or wood,
or metal, and holding it in the sun examine it attentively and he
will see all the colours disti-ibuted in the most minute particles, and
if he will make use of at telescope arranged so that one can see very
near objects, he will see far more distinctly what I say.'

It will not therefore be surj)rising if, in 1624 (according to
some letters from Rome, written by Girolamo Aleandro to the
famous M. de Peiresc), two microscopes of Kuffler, or rather Drebbel,
having been sent to the Cardinal of S. Susanna, who at first did not
know how to use them, they wei'e shown to Galileo, who was then
in Rome, and he, as soon as he saw them, explained their use, as
Aleandro wi-ites to Peiresc on May 24, adding, ' (^alileo told me
that he liad in\'ented an occhiale which magnifies things as mucli
as 50,000 times, so that one sees a fly as lai'ge as a hen.'

This assei'tion of Galileo, that he had invented a telescope which
magnified 50,000 times, so that a fly appears as big as a hen,
mu.st, without doulil. be referred to the year 1610, and from the
niea.sure given (jf the amplification by the solidity or volume the

GALILEO'S 'OCCHIALE' 1 23

linear amplification (as it is usually expi^essed now) would have
been equal to something less than the cubic root of 50,000 — that is,
about 36 — and that is pretty fairly the relative size of a fly and
a hen.

Aleandro's letter of May 24 (1624) does not state at what time
Galileo saw the telescope and explained the use of it, but another
letter of Faber's to Cesi, amongst the autograph letters in . the
possession of D. B. Boncompagni, says (May 11) : 'I was yesterday
evening at the house of our Signor Galileo, who lives near the
Madalena ; he gave the Cardinal di Zoller a magnificent eye-glass
for the Duke of Bavaria. I saw a fly which Signoi' Galileo him-
self showed me. I was astounded, and told Signor Galileo that he
was another creator, in that he shows things that until now we
did not know had been created.' So that even on May 10, 1624,
Galileo had not only seen the telescope of Drebbel, and explained
the use of it, but had made one himself and sent it to the Duke of
Bavaria.

We lack documents to show how this microscope of Galileo was
made, that is, whether it had two convergent lenses like those of
Drebbel. A letter of Eeiresc of March 3, 1624, says that 'the
effect of the glass is to show the object upside down . . . and so
that the real natural motion of the animalcule, which, for example,
goes from east to west, seems to go contrariwise, that is, from west
to east,' or whether it was not rather composed of a convex and a
concave lens, like that made earlier by him, and used in 1610, and
then almost forgotten for fourteen years.

It is, however, very probable that this last Avas the one in
question, for Peiresc, answering Aleandro on July 1, 1624, wix)te : —
' But the occhiale mentioned by Signor Galileo, which makes flies
like hens, is of his own invention, of which he made also a copy
for Archduke Albert of pious memory, which used to be placed on
the ground, where a fly would be seen the size of a hen, and the
instrument was of no greater height than an ordinary dining-room
table.' Which description answers far better to a Dutch tele-
scope used as a microscope, in the same way exactly as Galileo
had used it, rather than to a microscope with two convex
lenses.

One cannot find any further particulars concerning Galileo's
occhialini (so he had christened them in the year 1624), either in
Bartholomew Imperiali's letter of September 5, 1624, in which he
thanks Galileo for having given him one in every %oay perfect, or in
that of Galileo to Cesi of September 23, 1624, accompanying the
gift of an occhialino, or in Federico Cesi's answer of October 26, or
in a letter of Bartholomeo Balbi to Galileo of October 25, 1624,
which speaks of the longing with which Balbi is awaiting ' the little
occhiale of the new invention,' or in that of Galileo to Cesar Marsili
of December 17 in the same year, in which Galileo says to the
learned Bolognese ' that he would have sent him an occhialino to
see close the smallest things, but the instrument maker, who is
making the tube, has not yet finished it.' This, however, is how
Galileo speaks of it in his letter to Federico Cesi, written from

124 THE HISTOEY AND DEVELOPMENT OF THE MICEOSCOPE

Florence on September 23, 1624, more than three months after his
depart m-e from Rome : —

' I send your Excellency an occhlcdino, by which to see close the
smallest things, which I hope may give you no small pleasure and
entertainment, as it does me. I have been long in sending it, because
I could not perfect it before, having experienced some difficulty in
finding the way of cutting the glasses perfectly. The object must
be placed on the movable circle which is at the base, and moved to
see it all, for that which one sees at one look is but a small part.
And because the distance between the lens and the object must be
most exact, in looking at objects which have relief one must be able
to move the glass nearer or further, according as one is looking at
this or that part ; therefore the little tube is made movable on its stand
or guide, as we may wish to call it. It must also be iised in very
bright, clear weather, or even in the sun itself, remembering that the
object must be sufficiently illuminated. I have contemplated very
many animals with infinite admiration, amongst which the flea is
most hol-rible, the gnat and the moth the most beautiful ; and it was
with great satisfaction that I have seen how flies and other little
animals manage to walk sticking to the glass and even feet upwards.
But your Excellency will have the opportunity of observing thousands
and thousands of other details of the most curious kind, of which I
beg you to give me account. In fact, one may contemplate endlessly
the greatness of Nature, and how subtilely she works, and with what
unspeakable diligence. — P.S. The little tube is in two pieces, and
you may lengthen it or shorten it at pleasure.'

It would be very strange, knowing Galileo's character, that in
1624, and after the attacks made on him for having perhaps a little
too much allowed the Dutch telescope to be considered his invention,
he should have been induced to imitate Drebbel's glass with the two
convex lenses, and have wished to make them pass as his own invention,
whilst he had always used, and continued to use to the end of his days,
telescojDes with a convex and a concave lens without showing that
he had read or in the least appreciated the pi-oposal made by Keplei-,
ever since 1611, to use two convex glasses in order to have telescopes
with a large field and more powerful and convenient.

In any case it is impossible to foi-m a decided opinion on such a
mattei-, the data failing ; but the very fact that from 1624 onwai-ds
Galileo thought no moi'e of the occhicdino (pi'obably because he found
it less powei-ful and less useful than the occhicde of Drebbel), as he
had not occvipied himself with it or had scarcely remembered it from
the year 1610 to 1624, seems sufficient to show that the occhUdino,
like the microscope of 1610, was a small Dutch telescope with two
lenses, one convex and one concave, and not a reduced Keplei'ian
telescope like that invented by Drebbel in 1621.

The name of microscope, like that of telescojie, originated with
the Academy of the Lincei, and it was (iiovanni Fa])ei- who invented
it, as sliown l)y a letter of his to Cesi, written April 13, 1625, and
which is amongst the Lincei letters in the possession of J). B. ]3on-
coiiipagni. Here is the passage in Faber's letter : —

' I only wish to say this more to your Excellency, that is, that

GALILEO THE INVENTOR OF THE MICEOSCOPE IN 1610 1 25

you will glance only at Avliat I have wi'itten concerning the new in-
ventions of Signor Galileo ; if I have not put in everything, oi' if
anything ought to be left unsaid, do as best you think. As I also
mention his new occhlale to look at small things and call it mici-o-
scope, let your Excellency see if you would like to add that, as the
Lyceum gave to the first the name of telescope, so they have wished
to give a convenient name to this also, and rightly so, because they
are the first in Rome who had one. As soon as Signor Rikio's
epigram is finished, it may be printed the next day ; in the mean-
while I will get on with the rest. I humbly reverence your Excel-
lency.— From Rome, April 13, 1625. Your Excellency's most
humble servant, Giovanni Faber (Lynceo).'

The Abbe Rezzi, in a work of his on the invention of the micro-
scope, thought that he might conclude from the passage of
Wodderborn, reproduced above, that Galileo did not invent the com-
pound microscope, but gave a convenient form to the simple micro-
scope, and in this way as good as invented it, for the Latin word used
by Wodderborn, j»CT"s;:>ici7i'«7ft, 'signified at that time, it is clear,' Rezzi
says, ' no other optical instrument than sjDectacles or the telescope,
never the microscope, of which there is no mention whatever in any
book published at that time, nor in any manuscript known till then.'

But Rezzi was not mindful that on October 16, 1610, the date
of Woddei-born's essay, the name of microscope had not yet been
invented, nor that of telescope, which, according to Faber, was the
idea of Cesi, according to others of Giovanni Demisiano, of
Cephalonia, at the end, perhaps, of 1610, but more probably at the
time of Galileo's journey to Rome from March 29 to June 4, 161L
If, therefore, the word microscope had not yet been invented, and
if the telescope, or the occhlale as it was then called, was by all
named perspicillimn, one cannot see why Wodderborn's perspicillum
cannot have been a cannocchiale (telescope) smaller than the usual
ones, so that it could easily be used to look at near objects, but yet
a cannocchiale with two lenses, one convex and one concave, like the
others, and, therefore, a real compound microscope, - although not
mentioned by that name either by Wodderborn or others. And,
besides that, how could it be that Wodderborn beginning to treat
' admirabilis huius perspicilli,' that is, of the telescop>e in the first
line, should then have called p)erspicill%mn a single lens in the eleventh
line of the same page ? Rezzi's mistake is easily explained, remem-
bei'ing that he had not under his eyes Woddei-born's essay, but only
knew a brief extract reported by Yenturi.

It thus appears as in the highest degree probable that Galileo,
in 1610, was the inventor of the compound microscope ; it was
subsequently invented, or introduced, and zealously adopted in
Holland; and when Dutch invention penetrated into Italy in 1624
Galileo attempted a reclamation of his invention (which was undoubt-
edly distinct from that of Drebbel) ; but as these were not Avarmly
seconded and responded to abroad he allowed the whole thing to
pass. Nevertheless the facts Govi gives are as interesting as they
are important.

In regard to the discovery of the simple lens Govi points out

126 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

tliat after the year 1000, minds having reoj)ened to hope and in-
tellects to study, there began to dawn some light of science, so that
in 1276 a Franciscan monk, Roger Bacon, of Ilchester, in his ' Opus
Majus,' dedicated and presented by him to Clement lY., could show
many marvellous things, and amongst these the efficacy of crystal
lenses, in order to show things largei', and in this wise he says make
of thena ' an instrument useful to old men and those whose sight is
weakened, who in such a Avay will be able to see the letters suf-
ficiently enlarged, however small they are.' As long as no documents
anterior to him are discovered, Roger Bacon may be considered the
first inventor of convergent lenses, and thei'efore of the simple inicro-
scope, however small the enlargement by his lenses may have been.
As, however, that mian of rare genius, the initiator of expeii-
mental physics, had bi'ought on himself the
hatred of his contemporaries, they kept him
for many years in prison, then shut him up
in a convent of his order to the end of his
long life of nearly eighty years. His writings
had to be hidden, at least those treating on
natural science, to save them from destruc-
tion, and so the invention of lenses, or the
knowledge of their use to enlarge images and
to alleviate the infirmities of sight, remained
unknown or forgotten in the pages of the
famous ' Opus Majus,' which only came to
light in 1733 by the care of Samuel Jebb, a-
learned English doctor.

A Florentine, by name Salvino degli
Armati, at the end of the thirteenth century
(? 1280) (in Bacon's lifetime), had therefore
the glory of inventing spectacles, and it was
a monk of Pisa, Alexander Spina, who sud-
denly charitably divulged the secret of their
construction and use.

Perhaps Salvino degli Ai-mati and Spina
really discovei-ed moi-e than Roger Bacon had
discovered ; that is, they found out the use
of converging lenses for long-sighted people, and of diverging lenses
for short sight, whilst the English monk had only spoken of the
lenses for long sight, and perhaps they added to this first inven-
tion the capability of varying the focal lengths of the lenses accord-
ing to need, and the other of fixing them on to the visor of a cap to
keej) them firm in front of the eyes, or to fasten them into two
circles made of metal, or of lione joined by a small elastic bridge
over the nose. However it may be, the discovery of spectacles, or,
as it mjiy be called, of the s-imple wicroscope, may be efjually divided
betwen Roger BacoJi and Salvino degli Armati, leaving es[)ecially to
the lattei- the invention c)f spectacles.

The ear-liest known illustration of a simple microscopic is given
by ])e.scai'tes in his ' I)ioj)ti'i(]ue ' In I (i;{7 : fig. iM reproduces it. It
i.s pi-actically identical with oni' devised by Liebej-kiihn a centuiy

Fig. 91. — Descartes' simple
microscope with reflector
(1G.37).

GALILEO'S ' AND CAMPANI'S MICROSCOPES

127

after and shown on p. 139. A lens is mounted in a central aperture
in a polished concave metal reflectoi\ Descartes appai'ently devised
another and much more pretentions instrument, but it appears im-
practicable and could never
have existed save as a sugges-
tion. But he appears to have
been the first to publish figures
and descriptions for grinding
and polishing lenses.

In the Museo di Fisica there

Fig. 92. — Galileo's microscopes.
? Campani or later.

Pig. 93. — Campani's microscope (1660)?

ai'e two small microscopes which
it is affirmed have been handed
down from generation to gene-
I'ation since the dissolution of
the Accademia del Cimento in
1667, with the tradition of
having been constructed by
Galileo. They are shown in

fig. 92, but from the superiority of construction of these instru-
ments it is very improbable that they belong to the days of Galileo,
who died in 1 642 ; and there is a specially interesting compound

128 THE HISTORY AND DEVELOPMENT OF THE MICEOSCOPE

microscope, by Giuseppe Campani, which was published first in 1686,
which is presented in fig. 93 ; its close similarity to ' Galileo micro-
scopes ' is plainly apparent, making it still more improbable that
these could be given a date prior to 1642.

In a journal of the travels of M. de Monconys, published in
1665, there is a description of his microscope which is of much
interest. He states that the distance from the object to the first
lens is one inch and a half; the focus of the first lens is one inch ;
the distance from the first lens to the second is fifteen
inches ; the focus of the second lens, one inch and a half;
distance from the second to the third, one inch and
eight lines ; the focus of the third lens, one inch and
eight lines ; and the distance from the eye to the third
lens, eight lines.

This would form the data of a practical com-
pound microscope with a field lens ; and as Mon-
conys had this instrument made in 1660 by the
' son-in-law of Viselius,' it becomes probable in a
very high degree that to him inust be attributed
the earliest device of a microsco2)e loith a field-
lens.

In 1665 Hooke published his ' Micro -
graphia,' giving an account and a figure of
his compound microscope. He adopted
the field-lens employed by Monconys and
gives details as to the mode and object

i''l(,. :jl.— ilookl S KJllllMHUld llU<10S(C)p( (](i(i'>]

DIVINI'S COMPOUND MICEOSCOPE

129

riii!

M

of its employment, which are at once interesting and instruc-
tive ; for they show quite clearly that it was not employed by him
to correct the sphei'ical aberration of the
eye-lens, but merely to increase the size of
the field of view. He tells us that he used
it ' only when he had occasion to see much
of an object at once. . . . But whenevei-
I had occasion to examine the small parts
of a body moi'e accurately I took out the
middle glass (field-lens) and only made use
of one eye-glass with the object-glass.'

Fig. 94 is a reproduction of the original
drawing, and the general design appears
to be claimed by Hooke. There is a ball-
and-socket m.ovement to the body, of
which he writes : ' On the end of this arm
(D, which slides on the pillar C C) was
a small ball fitted into a kind of socket
F, m.ade in the side of the brass ring G,
through which the small end of the tube
was screwed, by means of which contri-
vance I could place and fix the tube in
whatsoever posture I desired (which for
many observations was exceedingly neces-
sary), and adjusted it most exactly to any illiiiiilKi
object.'

It need hardly be remarked that, useful
as the ball-and-socket joint is for many
purposes in microscopy, it is not advan-
tageously em.ployed in this instrument.

Hooke devised the powerful illuminat-
ing arrangement seen in the figure, and
employed a stage for objects based on a
practical knowledge of what was required.
He described a useful method of estimat-
ing magnifying power, and was an in-
dustrious, wide, and thoroughly practical
observer. But he worked without a
mirror, and the screw-focussing arrange-
ment seen in the drawing must have been
as troublesome as it was faulty. But as a
microscopist, Hooke gained a European
fame, and gave a powerful stimulus to
microscopy in England.

In 1668 a description was published
in the ' Giornale dei Letterati ' of a com-
pound microscope by Eustachio Divini,
which Fabri had previously commended.
It was stated to be about 16^ inches

high, and adjustable to four different Fig. 95.— Divini's compound
lengths by draw-tubes, giving a range of microscope (1668).

Pl'

"■^^"

i:;0 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

magnification from 41 to 143 diameters. Instead of the usual bi-
convex eye-lens, two plano-convex lenses were applied with their
convex surfaces in contact, by which he claimed to obtain a much
flatter field. Mr. Mayall found in the Museo Copernicano at Rome
a microscope answering so closely to this descrijation that he does
not hesitate to refer its origin to Divini. He made the sketch of

it given in fig. 95.
But the optical con-
struction had been
tampered with and
could not be esti-
mated.

Oherubin d'Orleans
published, in 1671, a
ti'eatise containing a
design for a micro-
scope, of which fig.
96 is an illustration.
The scrolls wei-e of
ebony, firmly at-
tached to the base
and to the collai-
encircling the fixed
central portion of the
body-tube. An ex-
terioi' sliding tube
carried the eye-piece
above on the fixed
tube, and a similar
sliding tube cari-ied
the object-lens below,
these sliding tubes
serving to focus the
image and regulate
(within cei'tainlimits)
the magnification.
He also suggested a
screw arrangement
to be applied beneath
the stage foi* focus-
sing. He devised, or
recommended, seve-
ral c()ml)inati(ms of
lenses for the optical
l)art of the inici'o-
foiu' separate lenses,

Fio. :)(',.

-Cln'rubin d'Orleans' componnrl microscope
(1071).

scope, and i-efers to coml)inations oH tliree or

by which objects coidd be seen erect, which hi^ considered ' much to

Tje jjreferred.'

He also inveiitiMl m liinociil.-n- Conn of iiiici'oscope aiiil piihlished
it in his woi-k, ' La Visi(jn Parfaite,' in 1677. It consisted of two
comjtcjiind microscopes joined together in one setting, so as to be

EARLY BINOCULAR MICROSCOPE

131

applicable to both eyes at once ; a segment of each object-lens
(supposed to be of one-inch focns) was ground awa.y to allow the
convergent axes starting from the two eyes to meet at about 16
inches distance at the common focus. Mechanism was pi-ovided for
regulating the width of the axes to correspond with the observer's eyes.

Fig. 97, showing the optical construction, is copied from the
original diagTam (' La Vision Parfaite,' tab. i. iig. 2, p. 80). Accord-
ing to the arrangement of the lenses as shown in tlie figure a pseudo-
stereoscopic image would have been obtained.

A drawing of this binoculai', as known to Zahn, was given in
the first edition of his ' Oculus Artificialis ' in 1685 (Fundamen III.
p. 233), and is reproduced in fig. 98.

k2

132 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

In 1672 Sir Isaac Newton communicated to the Royal Society
a note and diagram for a reflecting microscope ; we have, however,
no evidence that it was ever constructed. But in 1673 Leeuwen-
hoek began to send to the Royal Society his microscopical discoveries.
Nothing was known of the construction of his instruments, except
that they were simple microscopes, even down to so late a period as
1709. We know, however, that his microscopes were mechanically
rough, and that optically they consisted of simple bi-convex lenses,
with worked surfaces mounted between two plates of thin metal
with minute apertures through which the objects were directly seen.
At his death Leeuwenhoek bequeathed a cabinet of twenty-six of his
microscopes to the Royal Society ; unhappily, they have mysteriously

disappeared. But Mr. May-
all was enabled to figure one
lodged in the museum of the
Utrecht University, which is
given in figs. 99 and 100 in
full size. The lens is seen in
the upper third of the plate.
It has a J-inch focus. The
object is held in front of the
lens, on the point of a short
rod, with screw ai'range-
ments for adjusting the
object under the lens.

Many modifications of
this and the preceding in-
struments are found with
some early English foi-ms,
but no important consti'uc-
tive or optical modification
immediately presents itself.
But some ingenious arrange-
ments are found in the
simple microscopes devised
by Musschenbroek in the
early yeai-s of the eighteenth
century.
Grindl figured a microscope in his ' Micrographia Nova ' in
1687, in which optical modifications arise. Divini had, as was
stated, combined two plano-convex lenses, with theii- convex surfaces
facing, to form an eye-piece: this idea was carried further in 1668
V)y a London optician, who used two pairs of these lenses ; Grindl
did this also, but in addition he used two similar (but smaller) lenses
in the .same manner as an objective. The form of the microscope
itself was copied from tliat of Cherubin d'Orlcans (lig. 97), but was
iiir)dified by the ap])liciiti()ii of an extei'nal screw.

In 1691 Boiiiiuiius modified preceding ari'angements l)y devising
a means of clipping the object between two ])lates ])ressed away from
the object-leiis by a spinil spring, the luciissing lieing tlien eHected
by a ' .sciew-bari-el.'

Fig. 99. Fig. 100.

Leeuwenhoek's microscope (1073).

134 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

This system of focussing was employed in a more practical form
by Hartsoeker in 1694 and was adopted by Wilson in 1702. It
became a very popular form for the microscope in the eighteenth
century.

We are indebted to Bonannus also for originating a horizontal
form of microscope, which is interesting and which, in a drawing of
the instrument, is shown to possess a sub-stage wmpoimd condenser
fitted ivith focussing arrangements for illimninating transparent
objects. There was great convenience in using the microscope in a
horizontal position with a lamp and condenser in the same axis,
especially as all the compound microscopes previously constructed
had been employed vertically, or had been directed towards the sky
for purposes of illumination. Remarkably crude as the mechanism
apj)ears, it is a very early instance of the use of what has become —
though slowly and late on the continent — -a now universally acknow-

FiG. 102. — Hartsoeker's simple microscope (1694).

ledged optical ai-rangement indispensable for the best results, viz. a
compound condenser fitted with focussing mechanism for illuminating
transparent objects. The picture of the entire instrument is shown
in fig. 101.

In Hartsoeker's microscope ' tlie lens-carrier A B, fig. 102 (on
which tlie cell P, containing the lens, is screwed), screws into the
body O C, Q J) at 0 Q ; tlie thin brass plates E and F fit within the
body, the portions cut out allowing them to slide on the short })illa,rs
0 C and Q I), and tlie spiral spring ])i'essing them towards C I) ;
the object-slides, or an animalcule cage (! H (hinged at a, h to allow
the lid Ci to fit into H, enclosing tlie objects between strips of talc),
slide; between the i)lates E and F when in position, and the " screw-
bari-el "IK fits into the screw-socket CI) and i-egulates the focus-
sing; a condensing lens, N", fits, (m a second " screw-barrel," L M,
which is applied in tlie screw-socket of I K. This arrangement of

HARTSOEKER'S MICROSCOPE

135

the condensei' is better than the plan adopted by Wilson, as it allows
the illumination to be focussed on the object independently of the
focal adjustment of the object to the magnifying lens ; whereas in
Wilson's microscope, the condenser being mounted in I K, without
facility of adjustment, remained at a fixed distance from the object,
and hence the control of the illumination was very limited.'

Another microscope dated 1702 is shown in fig. 103 as drawn by
Zahn in his ' Oculus Artificialis.' Fig. 103 presents a back view of
it and shows an oval wooden plate ; on the other side of this is a

similar plate which holds the lens
in such a position that it is oppo-
site the aperture A. Between the
two plates there is a rotary multiple
object holder shown in fig. 103a M
N, the object being inserted in the
apertures in the circumference of
the disc. Focussing is accomplished

Fig. 103 |1702>.

Fig. 103a (1686).

by means of the milled head B which is attached to a screw regulating
the distance between the two plates, one of which carries the lens,
the other the rotary object holder. The point worthy of note in
this instrument is the I'otating wheel of graduated diaphragms
A, C, D, E, placed on the side away from the lens. This is the first
instance of a useful appliance surviving in our present microscopes.
In Harris's ' Lexicon Technicum ' (1704, 2 vols, fol.), under the
word microscope, Marshall's compound microscope (fig. 104) is
described and figured. Several important innovations in micro-

I o HN Mar s h al l's

New Invented

DoubleMicroscope,

For Viewing the

Circulation oF the Blood

Made &-• Sold bv him af (he Archimedes &S
Golden Spe<aacles in Lu<teaie Street.

7'Lu. ihu under y-ubriLAitcrcfctjpt

B l.mj icJut / Juirl /,•,//<

Fio. 101 (17(14;.

HERTEL'S MICKOSCOPE

137

Fig. 105. — Hertel's microscope (1716).

138 THE HISTOEY AND DEVELOPMENT OF THE MICEOSCOPE

scopical construction were here embodied. (1) A fine-adjustment
screw F is connected with the sliding socket E, supporting the arm
D, in which the body-tube is screwed ; the focussing could thus be
controlled in a far more effective manner than hj any system pre-
viously applied to a large microscope. The previous systems involved
the direct movement of the body-tube either by rotating in a screw-
socket (as in Hooke's) or by sliding in a cylindrical socket (as in
Divini's and Cherubin's) ; in a few instances the object was moved

Fig. lOf).— M. ,To))Iot' s iniei-oscope (1718).

in relation to tlie ol)ject-lens, l)iit all these plans were more or less
defective, especially with microscopes of large dimensions. Marsliall's
system was a distinct mechanical improvement, for the object could
now be viewed during the actual process of focussing, as the image
would remain steadily in the field. (2) A fork, N N, is here applied
with a tliumb-screw clami), O, on tlu; pillar itself. (3) Hooke's ball-
and-socket joint, which was ap[)lied to the arm 1, is here shifted to
the lower end of the pillar, wliere it would give the movements of
inclination to tlie whole microscojje insteail of to the body tube only.

DE. LIEEEEKUHN'S MICROSCOPE

139

-Lieberkiihn's microscope (1739).

as in Hooke's ; tlie ball L could be tightly clamped by the screw
collar M, in which slots were cut to give spring. (4) A condensing
lens on jointed arms appears ; this probably was the first application
of such adjustments to the con-
densei'. Fi-om the singulai- posi-
tion of the candle beneath the
condenser, we may infer, without
doubt, that the mirror was still
unknown as a microscopical ac-
cessory in England.

In fact, in no microscope up
to this time has there been any -p^^._ ^qj
trace of, or reference to, a mirroi- ;
but in 1716 Hertel employed it and introduced some other consider-
able modifications. The general appearance of the instrument as
originally figured by Hertel is given in fig. 105. Not only have we
the mirror below the stage, but also above
the stage a concave metal mirror reflecting
light through a condenser on the object,
while the stage has focussing movement by
the right-hand ornamental 'butterfly' nut,
and is capable of movement to and from the
pillar by the middle nut, and also of rotary
movement by the left-hand nut. These two
last movements form what is now known as
a ' mechanical stage.' The body-tube is
hinged and is inclined by a screw-sector
mechanism. A distinct advance on the simple
microscopes which had preceded it was made
by one devised by M. Joblot, and illustrated
in fig. 106. The ornamental plate holds the
lens, the focus being adjusted by the nut and^
screw ; the j)late next to the ornamental one
is a concentric rotary stage, of good mechanical
quality. The tube A was called by Joblct
'the Canon,' and was lined with black cloth
or velvet, and has a diaphragm at each end.
These diaphragms are movable, which was
practically a considerable optical benefit.

In 1738 Dr. N. Lieberklihn devised,
what had been employed in principle by
Descartes a century before,^ the instrument
that has ever since been known by his name,
and which is still of considerable value to the
microscopist. Fig. 107 is a rep)roduction
from the earliest drawing known of Lieber-
kiihn's microscope. A A is a concave mirror
of silver ; from its form the light is reflected

from it to a focus on the object C. The mirror is pierced in the
centre at B, and the lens, or object-glass, is inserted and adjusted,

' See pp. 126-7.

Fig. 108 — Culpeper and
Scarlet's microscope (1738

140 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

the eye being placed behind in the direction D at any point the
single lens or a combination might require.

Cnlpeper and Scarlet's microscope requires a note, and is illus-
trated in fig. 108. It was inappropriately designated a 'reflecting'
microscope, but this arose merely from the fact that it was the first
English model which employed an illuminating mirror. It was,
however, a dioptric, not a catoptric instrument, and is figured in
Dr. Smith's ' Opticks,' 1738.

' A Pocket Reflecting Microscope ' was figured by Benjamin
Martin in his ' Micrographia Nova' in 1742, having the interesting-
feature of a micrometer eye-piece depending on a screw with a certain
number of threads to the inch, and by which accurate measurements
could be taken. It was called a reflecting microscope because it had
a mirror fitted into its cylindrical base ; but it was, in reality, a
compound refracting form, and appears to have a good claim to have

been the original from whence the
modern ' drum ' microscopes were
taken.

Wilson devised a simple
' screw-barrel' microscope in 1702,
and Baker describes and figures
in 1742 the Wilson model
mounted on a scroll standard and
with a mirror mounted on the base
in a line with the optic axis. Fig.
109 reproduces the drawing of
Adams.

But Martin originated a large
number of improvements both in
the optical arrangements and the
mechanism of the microscope, and
was an excellent maker. He ap-
plied rack-and-pinion focussing ad-
justments, to the compound micro-
scope he added inclining move-
ments to the pillar carrying the
stage and mii'i-oi-, and he fui-nished
the stage with rectangular movements.

It was to this maker that the late Professoi- Quekett was
indebted for an early mici-oscope, of which he evidently to the last
thought highly, and which was subsequently purchased by the Royal
Microscopical Society. A drawing of this instrument is given in fig.
110, and should be described in Quekett's own words. He says :
' It stands about two feet in height, and is supported on a tripod
base, A ; the central part or stem, B, is of triangular figui'e, having
a i-ack at the back, upon which the stage, 0, and frame, I), sxxpport-
ing the mirror, E, are capable of being moved up or down. The
compound body, F, is thicf indies in diameter ; it is composed of
two tubes, the inner of w liicli contains the eye-piece, and can be
raised oi- dej)]"essed by rack and ])inion, so as to increase or diminisii
the magnifying power. At the base of the triangular bai- is a cradle

Fig. 109. — Wilson's simj)le microscope
on scroll standard (as made by
Adams, 1746).

QUEKETT'S MIOEOSCOPE

141

joint, G, by which the instrument can be inclined by turning the
screw-head, H [connected with an endless screw acting upon a worm-
wheel]. The arm, I, supporting the compound body, is suj) plied
with a rack and pinion, K, by which it can be moved backwards and
forwards, and a joint is placed below it, upon which the body can be
tui'ned into a hoi-izontal position ; another bar carrying a stage and
mirror can be attached by the screw, L N, so as to convert it into a
horizontal microscope.
The stage, O, is provided
with all the usual appa-
ratus for clamping ob-
jects, and a condenser
can be applied to its
under surface ; the stage
itself may be removed,
the arm, P, supporting
it, turned round on the
pivot C, and another
stage of exquisite work-
manship placed in its
stead, the under surface
of which is shown at Q.
' This stage is strictly
a m.icrometer one, hav-
ing rectangular m.ove-
ments and a fine ad-
justment, the move-
ments being accom-
plished by fine-threaded
screws, the milled heads
of which are graduated.
' The mirror, E, is a
double one, and can be
raised or depressed by
rack and pinion ; it is
also capable of removal,
and an apparatus for
holding large opaque
objects, such as minerals,
can be substituted for it.
The accessory instru-
ments are very numer-
ous, and amongst the
more remarkable may
be mentioned a tube, M,
containing a speculum, which can take the place of the tube, R, and so
form a reflecting microscope. The apparatus for holding animalcules
or other live objects, which is represented at S, as well as a plate of
glass six inches in diameter, with four concave wells ground in it,
can be applied to the stage, so that each well may be brought in
succession imder the magnifying power. The lenses belonging to

FiG.l]0.-

-Mai'tin's large universal microscope as used
by Quekett (1780J.

142 THE HISTOKY AND DEVELOPMENT OF THE MICEOSCOPE

this mici'oscope are twenty-four in number ; they vary in focal
length from four inches to one-tenth of an inch ; ten of them are
supplied with Lieberklihns. A small arm, capable of carrying single
lenses, can be applied at T, and when turned over the stage tlie in-
strument becomes a single microscope ; there ai-e foui- lenses suitable
for this purpose, their focal length varying from y^^^^ ^^ 4^ijth of an
inch. The performance of all the lenses is excellent, and no pains
appear to have been spared in their construction. There are
numei-ous other pieces of accessory apparatus, all i-emarkable for the
beauty of their workmanship.' ^

Benj. Martin not only in this way greatly advanced the
mechanical ari-angements of the microscope, but he improved the
optical part. He used a Huyghenian eye-piece on the telescope
formula, where the focus of the eye-lens was that of the field-lens
3, and the distance between them 2 ; but instead of employing a
single eye-lens he broke it up into two of equal foci, that nearest the
eye being a ' crossed ' lens, and the other a plano-convex, the steeper
convexities of these lenses being towards each other. In addition to this
he placed at a short distance above the nose piece an equi-convex lens
of 5^ inches focus ; this acted as a back lens to all the objectives,
so that when an objective was changed it was really only the front
lens of a compound objective that was altered.

Cuff designed and made a microscope, in 1744, which Baker
figured and described in his ' Employment for the Microscope ' in
1753, wdiich possessed several conveniences and improvements. Not
the least of these is that which gives greater delicacy to the fine ad-
justment than is found in any preceding model. It was subse-
quently further improved by the addition of a cradle joint at the
bottom of the pillar by Adams. Cuff also designed a simple form of
micrometer.

There were three designs of microscopes by fleorge Adams, of
London, in 1746 and 1771, which have many points of interest, but
scai-cely contribute enough of distinctive improvement to tlie modei-n
foi'ms of the microscope to detain us long. That designed in 1771 is
figured in the Adams ' Micrographia Ilkisti-ata,' and is i-eproduced
in fig. 111.

In this instrument Adams claims to have embodied a, number of
improvements on all previous constructions. He applied ' two eye-
glasses at A, a thii-d near B, and a foui-th in the conical part between
B and C,' by which he increased ' the field of view and of light ; '
draw-tubes were at A and B, by which these lenses could be separated
moi-e oi- less, but the probability is veiy gi-eat that these were
simply copied fi-om the inqn-ovements of a. like kind devised by B.
Martin and descriVjed above. He also arriinged the object-lenses, or
' buttons,' a and i, to be coiiibined ; seven ' Imttons ' wei-e provided,
'also six silver specula [' Liebcrkiihns'] higldy ])olished, each having
a magnifier adaj^tefl to the focus of its coucavity, one of which is
represented Jit 6,' and the 'buttons' could also be nscil \vit]i 'any
one of these speculti' by means of the adaptei', d.

' A Practical TrrialUe on tha Uncof the. Mlrrosrojir, lird.cd. [joiidon, THr>'), Hvo,
pp. '25, 26.

THE Variable Microscope

By George Adams ^''^d?.-^^ StreetJ^ONDON

Fig. Ill (1771).

144 THE HISTOKY AND DEVELOPMENT OF THE MIGEOSCOPE

The body-tube, ABC, with its arm, F (in which it screwed at/),
and stem attachment with the fine adjustment were clearly modifi.ed
from a design which CufF originated. The large ivory head, I,
actuated a pinion and rack for raising or depressing the body-attach-
ment on the stem, but as there was only one slide the rackwork
could not be used unless the fine adjustment was first put out of action
by unclamping it. The stage and mirror were adjustable on the stem.
The large ratchet-wheel controlled by the pinion-handle, S, gave the
required inclination to the stem.

Nos. 1 and 2 were ivory and glass 'sliders' for objects, to be
applied in the spring-stage No. 3 fitting at T ; the ' hollow at K [No.
3] is to receive the glass tube No. 10.' No. 4 was a diaphragm called
a cone, from its conical shape ; this was inverited by Baker in 1743,
and was used in all microscopes up to about 1820, when the wheel of
diaphragms was re-invented by Mons. Le Baillif of Paris fitting in
the lower end of No. 3, ' to exclude some part of the light which
is reflected from the mirror Q.' The forceps. No. 5, could be placed
' in one of the small holes near the exti'emities of the stage, or in the
socket, R, at the end of the chain of balls No. 6.' No. 6 was an ai-m
composed of a series of ball-and-socket joints, similar to the system
employed by Musschenbroek, by Joblot, and by Lyonet, and was in-
tended to be applied at W, when the stage was removed. No. 7 was
a box of ivory in which discs of talc and brass rings were packed ;
No. 8, a hand-magnifi.er ; No. 9, a sliding arm lens-carrier fi.tting on
Z, when the instrument was required to be used as a simple micro-
scope ; No 11, a rod of wire with spiral at the end for picking up
soft objects from bottles &c. ; and No. 12, an ivory disc, black on
one side and white on the other, fitting at T, to carry opaque
objects.

To use the instrument as a, simple microscope the body-tube,
ABC, was removed from the ring, F ; the lens-carrier. No. 9, was
placed on Z, and a lens with reflector, E, screwed in the ring, c ;
the ball-and-socket arm. No. 6, was applied at W, by the part X,
and the object held by either of the forceps could be tui-ned and
viewed as desired. For dissections &c. the stage could be screwed
on at F, and a glass plate applied at T.

One of the best examples of this design has a nose-piece with a
slide carrying three objectives — one of the first arrangements of
'triple nose-piece,' or, indeed, of changing nose-piece for objectives
(as distinguished from simple lens-carriers) that have been met with.
A microscope devised by Dellebarre was made the svibject of a
special report to the ' Academie des Sciences' in June 1777, but
there is nothing in it deserving special consideration in com])arison
with contemporary or even anterior forms as bearing upon the evo-
lution of the microscope as we now know it. In fact, up to tlie time
wlieii achromatism exeiied so powerful an influence upon the form
;iiid construction of tlie instrument, there is no microscope tliat calls
for fni'ther consideration save (me — by a,n Englisli maker named
Jones — it was called Jones's ' Most Aj^proved Compound Microscope
and Apparatus,' and altliougli. in piinciple, it does not differ from
Adams's in.strument, fig. Ill, it yet presented differences of detail.

JONES'S MICROSCOPE

145

Its date was 1798, and is seen in fig. 112, which is taken from the
original figure in Adams's ' Essays on the Microscope.'

The base is a folding tripod, and the stem inclines upon a
compass-joint on the top of the pillar. Mr. Mayall justly remarks
that this was the best system devised up to this date. The arm
carrying the body-
tiibe can be rotated
on the top of the limb
E, and is also pro-
vided with a rack and
pinion D, An extra
carrier, W, is pro-
vided for special pur-
poses pivoting at S,
so that objects will
remain in the optic
axis though the stage
be moved in arc.
There are also clips
provided for the
stage. There is a
condenser at U,
which slides on the
stem by the socket u.
The mirror also slides
on the stem. There
is provided a rotating
multiple disc, P, of
object-lenses, and a
brass cell contains a
high power, of ^^ or
^Ijj inch focus, which
on the removal of the
lens-disc can be
screwed into the
nose-piece.

There were also
designed some inte-
resting forms of re-
flecting microscopes,
to the details of which
we can aflbrd no
space, their influence
having been of no
value in the develop-
ment of the microscope as we know it. There was a reflecting
microscope suggested by Sir Isaac Newton in 1672, and one was
devised on the principle of the Gregorian telescope by Barker in
1736 ; another of the Cassegrainian form was made in 1738 by Smith,
which was, perhaps, the most perfect of the Catoptric forms.

An ovitline of its construction and the path of the light-beams is

L

JONES'S MOST IMPROVED COMPOVSI)
MlCnoscuPE AND /IPPARATVS

Fig. 112 (1798).

146 THE HISTORY AND DEVELOPMENT OF THE MICEOSCOPE

given in fig. 113. It was for examining transparent objects and v^as
similar to the OassegTainian telescope, but Avith an extra long eye-
piece tube to permit the focussing by movement of the eye-lens.
The object was placed at M IS" ; the image was taken up by the
concave, reflected on the convex, and again reflected to the eye-lens.
He advised the use of a condensing lens for the illumination, to pre-
vent ' the mixtvire of foreign rays with those of the object,' otherwise
the instrument gave confused images of distant objects when it was
used as a microscoj)e.

Even without a condenser there are good images attainable with
this instrument, but with the condenser they would be, of course,
improved.

We have not followed in any detail the forms of simple micro-
scopes as they presented themselves, but in 1755 a form was made
by OuflTthat can only be regarded as "the precursor of the most com-

^^

Fig. 113. — Smith's reflecting microscope (1738).

plete and perfect of our simple dissecting microscopes : it is shown
in fig. 114. A disc of plane glass, C, or a concave, M, was applied,
on the stage of which dissections &c. could be made ; a mirror, I,
was fitted in a gimbal with a stem sliding in a socket in the pillar ;
the lens-carrier, F, alone, or with Lieberkiihn, F, screwed in a ring
on the end of a liorizontal arm, E, sliding through a socket, attached
to a vertical rod, D, sliding and rotating in a socket at the back of
the pillar for focussing ttc. This motion of the lens over the object
became very popular and was employed in nearly all microscopes up
to the time of the establisliment of achroinatism ; the last microscope
so fitted was that designed by Mr. W. Valentine and made by
Andrew Ross 1831. The movement in arc lasted iinu-h longer, and
the last remnant of it is still to be found in Powell's No. 1.
The pillar screwed on the lid of the box, within whicli the instru-
ment was pa(;kcd with sundry accessories.

Jt was to the disco\ery of achromatism as apjilicd to microscopic

THE EISE OF ACHROMATISM

147

object-glasses that we must attribute the sti-ictly scientific value and
progress in development of this now extremely valuable and beauti-
ful instrument. An exhaustive account of the earliest discovery
and progressive application to our own day of achromatism, so fai-
as it can be given in this treatise, will be found in the chapter on
objectives. We can here only attempt, for the sake of completeness,
a very broad outline of the facts.

Martin appears to have constructed an achromatic objective in
1759, but no results of practical value were obtained, Martin having
formed the judgment that his achromatic inicroscope was not equal
to a reflecting microscope with which he compared it. But it cer-
tainly gives him a place of interest in the history of the achromatism
of object-glasses for the microscope.

Fig. 114. — Ellis's aquatic microscope (1755

In 1762 Euler began to discuss the theory of achromatic
microscopes, and in 1771, in his ' Dioptrica,' he entered iipon the
subject at more considerable length. A pupil of his, named Nicholas
Fuss, published in St. Petersburg, in 1774, a volume entitled ' Detailed
instruction for carrying lenses of different kinds to a greater degree
of perfection, with a description of a microscope which may pass for
the most perfect of its kind, taken from the dioptric theory of
Leonard Eulei', and made comprehensible to workmen by Nicholas
Fuss.' This was translated into German by Kliigel in 1778, but no
I'esult of these discussions of the theory of achromatism can be
discovered earlier than 1791, when Francois Beeldsnyder made an
achromatic objective which was presented by Harting to the museum
of the University of Utrecht ; but it was far from satisfactory. It

l2

148 THE HISTOEY AND DEVELOPMENT OF THE MICKOSCOPE

was composed of two biconvex crown-glass lenses, and a biconcave
flint lens placed between them.

0. Chevalier tells ns 1 that between 1800 and 1810 M. Charles, of
the ' Institut,' Paris, made small achi-omatic lenses ; but they were
too imperfect to be of real service. In 1811 Fraunhofer made
achromatic doublets with no great success; and in 1823-4 an achro-
matic microscope was made by the Messrs. Chevalier, with four
doublet lenses arranged according to a plan devised by Selligue.
Their ' Microscope d'Euler ' followed, and in 1827 Amici constructed a
hoi-izontal microscope on achromatic principles, which was spoken

well of. Bu.t while
up to a very recent
date it was common
to assert that the first
to suggest the plan
of combining two,
three, or four plano-
convex achromatic
doublets of similar
foci, one above the
other, to increase the
power and aperture,
was Selligue in 1823,
it is now known that
this had been antici-
pated by Marzoli (ch.
V. 353). Selligue's
plan was carried into
execution by the
Messrs. Chevalier.
The iuvsti'ument em-
bodying this plan is
shown in fig. 115.

In a report to the
Academie Royale des
Sciences, the well-
known mathema-
tician Fresnel says,
concerning this mi-
croscope, that in comparing the objectives with those of one of Adams's
best non-achromatic instruments — that up to a magnification of
two hundred times — Selligue's was decidedly superioi- ; but beyond
that mMgnification there was no superiority in the achromatic form,
and he prefei-red Adams's form foi- jjrolonged oljscrvations l)ecause
it gave a larger field tlian St^Uigne's.

The meclianisiu of tliis micro.scope was similar to the English
model of Jones, si lown at fig. 112. The focussing was by rack and
pinion acting on the stage, the pinion travelling with the stage on
the rack. Two draw-tubes, A and B, were applied within the
body-tube, C, llic u[)per one having a biconcave hms, S, at tlm
' ])fH MirroHco'pea, Paris, 1839, p. 80.

115. — Selligue's aclivomatic microscope (1823-4).

MODEL STAI^DS FOE ACHEOMATIC OBJECTIVES

149

lower end, serving as an amplifiei", which was probably the first
application of a ' Barlow lens ' to a microscope.

Illnmination for opaque objects was accomplished by a lenticulai-
pi'ism, P, which was gimballed, and connected with a ring embracing
the body tube.

We learn from Fresnel that the range of magnification was fi'om
40 to 1,200 diameters.
The object-glasses were
composed either of two
doublet systems for low-
power woi'k or of four
doublet systems all
screwed together for
high-power work, and
two oculars were pro-
vided of difierent power.

It is interesting to
place one of the earliest
known English models
of the achromatic micro-
scope beside that of Sel-
ligue. It was made by
TuUy the optician, of
London, who at Dr. Gor-
ing's instance had been
working at the achroma-
tising of the microscope.
Selligue's is a manifest
modification of one of
the best forms as made
by Adams, Jones, or
DoUond. Tully made the
microscope figured in
116 from the working-
drawings supplied by
Mr. J.J. Lister, who saw
that great accuracy of
workmanship and com-
plete steadiness in the
stand were needful for
achromatic microscopes,
and to this end they
adopted struts, such as
were used in telescopes,
connecting the body-tube with the base. The instrument is shown in
fig. 116. He also provided mechanical movements to the stage, but
no fine adjustment was applied. There was a sub-stage provided with
a rotating disc of graduated diaphragms. This microscope was made
in the year 1826 by Tully, but it was made from working drawings
supplied by Mr. J. J. Lister, who therefore is responsible for the entire
design. The sub-stage held a combination of lenses for a condenser.

As compared with single lenses of equal power, fi'om which so

Fig. 116.-

-Lister's achromatic microscope made by
Tully (1826).

I50 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

much light was inevitably stopped out by the small cliaphvagm that
it was needful to use in order to secure a fair image, the objectives
vised with this instrument gave a vast increase of light by permit-
ting the employment of the full aperture.

An extremely interesting instrument by C. Chevalier, made very
probably not long after 1824, and bearing much resemblance to that
of Selligue, is shown in fig. 117. It is provided with a revolving
disc of diaphragms applied below the dark chamber under the stage,

and this is a plan which obtained
a permanent place in the micro-
scopes of the future.

The report of Fresnel con-
cerning Selligue's achromatic
mici'oscope determined Professor
Amici, who for nine years had
abandoned his experiments on
achromatic object-glasses, to re-
commence them in 1826, and in
1827 he exliibited in Paris and
in London a horizontal micro-
scope. The real novelty shown
in it was the application of a
right-angled prism immediately
above the objective to deflect
the rays through the horizontal
body-tube. The object-glasses
were composed of three lenses
superposed, each having a focus
of three lines and a greatly in-
creased aperture. It had also
extra eye-pieces by means of
which the amplification could he
increased.

Meantime the subject of
achromatism was engaging the
attention of the most distin-
guished English mathematiciaiis.
Sir John Hei-schel, Sir George
(then Professoi') Airy, Professor
Barlow, Mr. Coddington, and
.several others, woi-ked more or less at the general subject. Cod-
dington alone, however, confined his attention to the microscope,
and his woi-k was limited to tlie eye-piece. Also, for some years,
Joseph J. Listei- liad ])een earnestly woi-king experimentally and
mathematicMlly on the same subject, and he (hscovered certain })ro-
perties in an achi'omatic combination, wliicli wei'e of inipoi'tance,
altJiough they had not been before observed.' In J 829 a, pajier
from Lister was received and pul^lished by the Royal Society, ^
and putting the principles it laid down into practice, Li.ster was
enabled to obtain a combination of lenses capable of transmitting a
• Vide ObjcctiveB, di. v. p. SHS. ^ Trans. Boy. Flor. for 1820.

FlCr

117. — C. Chevalier's achromatic
microscope (circa 1824).

Fig. 118. — One of Eoss's early microscopes designed by W. Valentine (1831)

152 THE HISTOEY AND DEVELOPMENT OF THE MICEOSCOPE

L-ected field. This

and its

pencil of 50° with a large corrected tielcl. inis paper
results exerted a very powerful influence on the immediate improve-
ment of English achro-
matic object-glasses, and
formed a permanent
basis of advancement for
the microscope, not only
in its optical, but also
indirectly in its me-
chanical construction
and refinements.

For convenience, at
this point we may ad-
vance a little in order
to complete our brief
outline of the mechani-
cal apjDlication of achro-
matism to object-glasses.
Mr. A. Ross became
practically acquainted
with the principles of
achromatism as applied
to combinations of lenses
in woi'king with Pro-
fessor Bai4ow on this
subject, and having ap-
plied Lister's principles
with great success, he
discovered, as we have
already pointed out in
Ch. I.,^ that by covering
the object under exami-
nation by a thin film of
glass or talc the correc-
tions were disturbed if
they had been adapted
to an uncovered object ;
and we have seen that
it was in 1837 that Ross
devised a simple means
of correcting this. He
was an indefatigable
worker in the interests
of the advancement of
the mechanical as well
' as the optical side of the
microscope. Fig. 1 1 8
pi'esents a form f)f
microscope, from an extant example which was designed by W.
Valentine of Nottingham in Miirch 1H31 and made; 1)y Andrew Ross.

Fig. 119

-Pritchard's microscope with
fine adjustment (1835).

Continental

A EOSS'S ' LISTEK ' MODEL

153

The stage is actuated in diagonal directions on eitliei' side of the
stem. A Pritchard microscope probably made by Ross is shown in
fig. 119. It is not at all like fig. 118. The stage movement is
by rack and pinion and not by screw as in fig, 118, but it will
be seen that it has also a curious spiral fine adjustment, which is
plainly an uncovei-ed ' Continental ' form, either adopted in England
from G. Oberhauser, or it may
have even pi-eceded it. It is
interesting to note, however,
that the sub-stage arrange-
ments in both these instances
are the same as those employed
by Wollaston in connection
with his celebrated doublets,
an account of which was given
in the Philosophical Transac-
tions of that date.'-

The Ross form cannot be
inclined, nor can the Prit-
chard ; and ' the fine adjust-
ment in the former is effected
by means of a long screw
passing up the pillar and act-
ing on a triangular sheath,
within which the stem is
applied, to move with rack and
pinion, the top of the stem
being hollow to receive either
the cross-arm support for the
single lens or the limb of the
compound body. The screw
is actuated by a large, gradu-
ated, milled head below the
tripod.'

The stage has supports
evidently to enable dissection
to be effected without flexure
by the weight or pressure of
the hands, which makes it
clear that it is the Valentine
microscope that is referred to,
as may be seen by refei-ence
to fig. 118. Rectangular me-
chanical movements are employed acting diagonally on either side of
the stem by rather fine screws, so that the motions are slow.

But A. Ross at an early period worked out a 'Lister' form of
microscope, with the limb supporting the body-tube. He applied a
fine adjustment in this to act upon the nose-piece only, which, as
we shall subsequently see, is a very infeiior method. This instru-
ment dates from 1839, and is shown in fig. 120. In 1842 he
1 Tracts. Boy. Soc. 1829.

Fig. 120. — A Eoss microscox^e (1839).

l''i(;. 121.— 11. rowcH'H microBcope, purchased by R.M. Society in 1841.

PEINCIPAL MODERN STANDS I 55

changed the form to that shown in fig. 123, p. 158. Ross tried
various modifications of this fine adjustment and model, but from
about 1843 he worked only at the level' method as applied to the
nose-piece thi-ough the ' cross ai'm ' and brought it to a relatively
high state of j^erfection. But the full possibilities of this niethod,
as concerned its sensitiveness, were never utilised by Ross, and it
was Hugh Powell who first j^i-^blished an account of his long lever
fine adjustment in the ' London Physiological Journal,' November
1843. The published account of Ross's long lever fine adjustment
did not appear until a month later, viz. December 1843.

In 1835 Powell made a microscope with an extremely delicate
fine adjustment applied to the stage. The mechanism and the
woi-kmanship were excellent (w^e give a drawing of a later form of
the instrument at fig. 121), and this fine adjustment is one of the
slowest and steadiest as yet made. In one we have measured the
movement only amounts to -^-l-^- of an inch for one revolution of the
milled head ; this is six times slower than the fine adjustment applied
to the best Continental microscopes. The disadvantage of this fine
adjustment is that it slightly disturbs the focus of the sub-stage con-
denser ; therefore, if the fine adjustment is much moved, the sub-
stage condenser will require refocussing. The movement usually
required is so slight that the refocussing of the condenser is seldom
required.

James )Smith also made an instrument on an entirely new
plan. It is illustrated in fig. 122, being the first model made
by this firm in this form, and it has many features of interest
from the point of view of our present requirements. But after
we have once secured steadiness, the crucial points in a microscope
are the quality of the fine adjustment, and the delicacy, fiinmess,
and ease with which we can centre, focus, and otherwise modify
the sub-stage illumination. To the former certainly this model
does not contribute.

We are now prepared to examine and endeavoiu- to judge im-
partially fi-om a practical point of view the merits of the princijDal
English, Continental, and American models which are offered to
the microscopical public. It is impossible, no less than it is unde-
sirable, to attempt to describe all the microscopes of every maker,
or even the principal forms made by the increasing multitude of
opticians. We have sought no opticians' aid ; we have cai'efully
examined all the forms that lay any just claim to presenting an
instrument which meets the full requirements of modern microscopy ;
and, although we have reason to know that the judgments we express
are shared by the leading experts of this country, we take the sole
responsibility for these judgments. Having sought for twenty years
the best that could be produced in microscopes an^l objecti^•es our
judgment is given with deliberation and Avholly in the interests of
science.

In examining the principal modern microscopes we shall point
out whatever is of absolute imjDoi-tance oi- relatives value ; and the
absence or presence of this in any form pi-ovisionally selected is all
that the readei- will need to enable him to become convinced of our

IS6 THE HISTORY AND DEVELOPMENT OF THE MICEOSOOPE

estimate of the value of such an insti-UBient, whether the form be
illustrated in these pages or found in the catalogues of the
makers.

I'll,. 12'.'. — li.ini'S Siiiitli's iii'ci-osco|)(; (18U!)).

STEADINESS OF THE MICEOSCOPE 1 57

With this object l^efore us we shall facilitate its attainment by
at once consideidng what are the essentials of a good mici^oscope.
AVliat are the attributes of the instiannent without the possession of
which it cannot meet modern requirements 1

T. Steadiness is absolutely indispensable : this would, in fact,
appear to be obvious. But we are bound to admit that it is, in what
sometimes claim to be stands of the first class, disregarded ; and when
the height of the centre of gravity in the English and American
stands of the first class is considered, this is a fatal mistake.

It is pointed out in the section on micrometry ^ and drawing
that the optic axis of the microscope should be ten inches from the
table ; therefore a first-class microscope whose o^^tic axis when
placed horizontally is either more or less than this is found wanting
in a material point. But to possess this characteristic it must have
a high centre of gravity.

Now it is possible to secure steadiness by (1) weight or (2)
design. ■ The Continental method has invariably been weight. The
pillar of the instrument is fixed to a cumbrous metal foot of horse-
shoe form, which beai'S so high a ratio to the whole remainder of
the instrument that it is usually steady. This secures the end
certainly, but by coarse and unwieldy means. It pi'omises little
for the instrument as a whole.

What is wanted is the maximum of steadiness with the minim.um
of weight. An old plan designed by Cuff, circa 1765, of rotating the
foot below the pillar has been frequent!}" reinvented. It was used
by Adams 1771, by Ross 1842, by Sidle and Poalk in America 1880,
by A. McLaren 1884, and recently again by Ross. This is a very
simple method of obtaining great stability for the instrument when
in either the vertical or horizontal jDositions. An instance of this form,
made by Andrew Ross in 1842, is given in fig. 123 : the foot is seen to
be circular, with a vertical pillar attached eccentrically to it, and the
base rotates, securing stability in eithei- a vertical or inclined position.

Palpably, the mechanical compensation for the difiiculty of an
elevated centre of gravity is an extended base. The leading fault
of many stands claiming the first rank is their narrowed bases. A
broad base, resting on three points only, and these plugged with
cork, is the ideal for a perfect instriiment.

II. ISText in order to the stand of the microscope comes what is
known as the body of the instrument — the tube or tubes for receiv-
ing the objective at one end and the eye-pieces at the other. The
tube of the monocular is always provided with an inner tube called
the drav.i-tuhe. In a first-class instrument this latter should always
be provided with a rack-and-pinion motion, and should have a scale
of from two to three inches, divided into tenths or millimetres. This
enables the operat^or the more accurately to adjust apochromatic ob-
jectives so sensitive, for their best action, to accurate adjustment of
tube-length. In fiict, it is always important to remember that ob-
jectives are corrected for a special tube-length ; that is to say, for
the foimation of the image at a certain definite distance.

1 Chapter IV.

158 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

There are, however, two kinds of tube-length ; (1) an 02)tical anrl
(2) a mechanical.

The optical tuhe-length is measured from the posterior principal
point of the objective to the anterior principal point of the eye-piece.

The mechanical tuhe-length should be measured from the top of
the tube into which the eye-piece fits, and upon which the bearings

Fifi. ]2:-3.— Old Ross stand (1842), rotating foot below the pillai-. From tlie cabinet
of the Royal Microscopical Society.

of the eye-piece rest to IIk- cud oi' tlic nose-piece into wliich the
objective is screwed.

Unfortunately different makers estimate tu))e-leiigth differently
and take different points fiom which to make their measurements.
Looking at the mattci- Iti'oadly, thei-e are two estimates for tube-
Ifmg'th in practical ii.se : tJicse are the English and the Continental.

THE 'BODY' OF THE MICEOSCOPE 1 59

What was formerly known as the English standard tube had an
optical length foi- high and moderate power objectives of ten inches ;
with low powers, however, it was less. The mechanical tube-length
was 8 1 inches.

Professor Abbe, in constructing his apochromatic objectives for
the English body, has taken the mechanical tube-length at 9' 8
inches = 250 mm. ; and the optical tube-leng-th at 10' 6 inches
:= 270 mm. This has caused an increase in the length of the English
standard tube, since all good microscopes are made to work with
these objectives ; and the addition of a rack and pinion to the ' draw-
tube ' becomes of great practical value.

The tube-length of the Continental mechanical tube is 6'3 inches
= 160 mm., and the optical tube-length is 7"08 inches = 180 mm., and
some Continental objectives can only be accurately adjusted on an
absurdly short tube of 4| or 5 inches.

The question has been asked, ' Which is the better of these two
differing tube-lengths ? ' So far as the image in the instrument is
concerned, there is not much difference. It is of little importance
whether the initial magnifying power of an objective be increased
by a slightly lower eye-piece used at a longer distance oi- a slightly
deeper (higher) eye-piece at a shorter distance. But it is of practical
importance to note that a small difference of tube-length produces a
greater effect on adjustment xoith a short body than with a long one.
Critical work is carried on in this country to 2^ mm. adjustment
on the long tube ; with a short tube the delicacy would be greater.-
A difference of 5 mm. on a short tube is equivalent to the difference
between a good and a bad objective. When small cones of illumina-
tion are used lenses are far less sensitive, but, on the other hand,
they are not doing their work. Biologists in a vast majority of cases
use a high power insufficiently worked ; thus a ;^-inch objective with
a small cone is used in place of a 1-inch objective, and an oil im-
mersion -jJ^-inch objective with small cone is used to do what a J-inch
would have done. The oil -j^^^^-inch objective is never fully utilised,
and the objects that it will show if properly used are never seen.
The principal difference, however, between the long and the shoi't
body as affording a datum for their respective values is that when
a short body is used by a person having normal accommodation of
sight, the stage of the microscope cannot be seen unless the head is
removed from the eye-piece, whereas with the long body the eye
need not be taken from the eye-piece at all, as the stage can be seen
with the unused eye. We are informed by a highly competent
German optician that short sight is the most common form of vision
amongst German microscopists. This, of coui-se, for Gei-mans so far
alters the case, but it does not apply in this country. The diameter
of the body tube is also a matter of importance, because when a
microscope is used for photomicrography it is essential that it
should have a body with a large diameter.

III. Arrangements for focussing stand next in order of import-
ance. Every microscope of the first class is provided with two
arrangements for focussing, one a coarse adjustment, acting rapidly,
and the other a fine adjustment, which should act with great delicacy

l6o THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

and precision. A good 'coarse adjustment' or primary movable
part of the instrument is of great importance. The first requisite is
that the body or movable part shovild move easily, smoothly, but
without ' shake ' in the groove or slot or whatever else it slides in.
We have found in practice that a bar shaped like a truncated prism
sliding in a suitable groove acts best and longest. But a bar planed
true and placed in a groove plovighed to suit it is not enough. The
inevitable friction determines wear, and this brings with it a fatal

Fui. 124. — Diagonal rack and twisted pinion devised in 1881.

'shake.' All sucli grooves, whicli are usually v-shaped, should be
cat and sprauy on one side, so that by ' tightening up ' the V's by
means of screws the bar or liml) is again finuly gri})p6d. Further, the
bar should not ' bear ' for its whole length along the groove, but only
on points at either end and in the middle. Powell introduced these
pi'ime es.sentials to ji, good ' coiirs(i adjustment' more than 60 yefu-s
ago ; yet wliat thousands of instruments in whicli tlxese principles
liave not been applied have been, l)y sheer friction wear, soon
changed into u.seless ])i'iiss since then ! But instruments made by

FOCUSSINa ARRANGEMENTS

i6i

this firm are as good after thirty years' use as they were when

new.

Frequently bad workmanship is concealed by the free employment
of what is known as ' optician's grease ' and an over-tightening of the
pinion, driving its teeth into the rack, which, of course, speedily
ends in disaster.

If we desire to practically test this part of a microscope, we
must remove the pinion, take out the bar, clean off the ' optician's
orease ' with petroleum from both bar and groove, oil with watch-
maker's oil, and replace the bar in the groove, and before refixing
the pinion see if it slides smoothly and without lateral shake.

What has been said about the ' springing ' of the bar in this special
instance applies equally to all moving parts, in stage and sub-stage
movements, and wherever constant friction is incurred ; equally

applicable, too, is the

lubricant we suggest.
An instrinnent left
unused in its native
' grease ' for twelve
months becomes so im-
mobile in most of its
parts by the hardening
of its ' normal ' lubri-
cant that motion be-
comes a peril to its future
if persisted in in that
condition.

If a ' coarse adjvist-
ment ' be what it should
be, all lower powers

Fig. 124a. — Nelson's ' stepped ' rack, invented in 1899.

should be exclusively and perfectly focussed by it, and with the
highest powers objects should be found and focussed up to the point
of clear visibility.

The exceedingly useful method of ' diagonal rack and twisted
pinion' was introduced by Messrs. Swift and Son about 1880 and
has since been universally adopted. Its mode of operation is seen
in fig. 124, a sectional drawing of this part of one of Swift's niicro-
scopes. The advantages gained by this method are due to the tAvist
in the pinion being a shade steeper than the diagonal of the rack, by
which expedient there is more gearing contact between rack and
pinion, which prevents ' loss of time ' and obviates the necessity for
unduly forcing the teeth of this pinion into those of the rack.

Mr. Nelson has had made by Messrs. Watson and Sons a still
better form of rackwork. It is what is called a 'stepjaed' rack (not
of the diagonal, but of the straight type). In this very admirable
form two parallel racks engage in the same pinion ; one rack, how-
ever, is placed so that its teeth are stepped an amount equal to the
' back -lash ' behind those of the other, e.g. j f of the pitch.

These racks have to be cut together and fixed in the position
they were cut ; the object of this plan is that one of the racks shall
be in action when the bar is racked up, and the other when it is

M

1 62 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

racked clown ; so that if the racks are properly placed relatively to
one another ' loss of time ' is impossible ; and the result is obtained
without forcing the teeth of the pinion into the rack. If the teeth
are true, the friction is of the least, and the smoothness and firm.-
ness all that can be desired. But what gives great value to this
form of rack is that any loss of time as the result of wear can be
taken up by a slight alteration of the position of the second rack.
The arrangement is shown in iig. 124 a, and it will be seen that at
the top of the right-hand rack as we look at the illustration there is
a small screw. Now the racks are set side by side, one being fixed
finally. The pinion is then made to woi-k freely and smoothly with
this one rack ; the second rack is then introduced, and is provided
with slots and clamping screws, and its position is gradually altered
in the slots in a vertical direction by means of this small screw over
the right-hand rack until the smoothest position of action is secured.
The clamping screws are then tightened and the rackwork becomes
fixed ; and subsequent irregulaiity in it is at once corrected by the
small screw to which we have referred.

When the best position is found the teeth of the two racks, as
Ave have stated, will not be in a line, but those of the loose rack will be
found to occupy a position slightly below the teeth of the fixed
one.

There is a defect in either microscope or microscopist if the
' fine adjustment ' is resorted to before the object is focussed into
clear view, even with the highest powers.

The Fine Adjustment. — This part of the modern microscope
possesses an importance not easily exaggerated, and deficiency or
bad piinciple in the construction of this makes not only inferior,
but for critical purposes absolutely useless, what are otherwise
instruments of excellent workmanship and real value.

There are two kinds of fine adjustment usually employed : —

i. Those which smiiyly move the nose-piece which i-eceives the
objective.

ii. Those which move the ivhole hody, or the whole body including
the coai-se adjiTstment.

All constructions of the second class formei-ly pi-oved impracti-
cable, and even pernicious. They inevitably broke down just as the
purchaser, by practice, began to realise the value of perfect action.
With a large expei-ience of stands of eveiy class, we ai-e obliged to
say that generally with one or two years of work they lost whatever
value they at first possessed.

To this broad statement there are possibly two or three excep-
tions, viz. (Swift's side lever and Cam[)])eirs differential ^ screw and
Watson's long lever, to wliicli we shall subsequently refer.

It is, however, upon tlie model above refei'red to, with all its
i-adicul and glaring imperfections, tliiit tlie iiiajoi-ity of Continental
microscopes liave been l)uilt.

A screw with an extremely fine thread, and therefore of exti-emely
shallow incision — a micrometer screw in fact — Jims to hear the strain of

' The differential screw fine adjustment was first snggesteil ))y Dr. Goring in
]8'j0. It was subsequently made by Nobert about 18(5.5.

IMPEEPECT MODERN MODELS

163

lifting and lowering the entire loeight of the body, with its coarse ad-
justment, lenses, and. so forth ; while the sole object of the adjustment
should be to give a delicate, almost imperceptible, motion to the
object-glass alone. It needs no great experience to foresee the inevi-
table result ; the screw loses its power to act, and something incom-
parably worse than a tolerable coarse adjustment is left in its place.

Yet it is the Con-
tinental model that
has become the dar-
ling of English labo-
ratories, and that still
receives the appreci-
ation of professors
and their students.
True they answer in
the main the purposes
sought — the exi-
gencies of a limited
course of practical in-
struction. But how
many of those who
receive it are the
medical men of the
future, and to whom
a microscope — not of
necessity a costly one
— of the right con-
' struction would be
of increasing value
through a lifetime 1

Almost any in-
strument, however
inferior, could be em-
ployed successfully
with a ^-inch object-
ive of '■ low angle ' (to
give it what has been
called ' the needful
penetration' for his-
tological subjects !) to

obtain an image corresponding to a figure in a text-book of, say,
a Malpighian corpiiscle, or a section of kidney, brain, or spinal cord.
The quality of a fine adjustment is never tested by these means,
for, in point of fact, a delicate fine adjustment is not even necessary.
We wiite in the interests of microscopical research. It certainly
may be taken for granted that the end sought is not simply to use
the microscope to verify the illustrations of a text-book, a treatise,
or a course of lectures ; without doubt it is a subsidiary purpose ; but
the larger aim is to inspire in the young student confidence,
enthusiasm, and anticipation in the methods and promise of histology
and (dl that it touches. But for this there must \)& potentiality (wi th-
ai 2

Fig. 125. — Eoss-Zentmayei- model (1878).

164 THE HISTOKY AND DEVELOPMENT OF THE MICROSCOPE

out costliness) in the mechanical and optical character of the micro-
scopes commended and approved.

A low-priced student's microscope of good workmanship and
perfect design could easily be devised if the demand for it ai-ose.
Indeed, quite recently a certain class of students' microscopes have
been improved greatly ; this has been a concomitant of the science of
bacteriology, which has compelled the use of the sub-stage condenser.
We have said enough of the value of this instrument in a succeeding
chapter, but until recent years histologists did not use it because it
was not used in Germany or with German instruments ! Its present
use, nevertheless, has had the effect of improving the definition
obtained by the objectives used by students generally. Some who
perceive this, endeavour to attribute it to the improvement effected
in modern objectives, but this is not the case ; the objectives in
many cases are not even new, and until the introduction of the Jena
glass ^ the ordinary students' objectives were not really so good as
the English objectives of forty-five years ago. But it could easily
be shown that one of these early objectives, used as it always was
with a condenser, would surpass in the sharpness of its definition the
majority of those now supplied to 'students ' with Continental models.

But it must not be supposed that it is onlt/ the Continental
model that is deformed by the adoption of this radical error in the
' fine adjustment ' with which we are dealing. Even during the last
twent}'' years it has been applied to some of the most imposing and
expensive instruments made in England and America on what is
known as the ' Lister ' model. This model has one supreme virtue,
in the possession of a solid limb. This may take many distinct
forms, but it is sufliciently represented in fig. 125, where it will be
seen that the ' limb,' which is swung between the pillars, and which
carries the body-tubes and the fine adjustment, is in one solid piece.
If nothing were sacrificed this would be a boon. Formerly, this
model was supplied with a fine adjustment which only moved the
nose-piece, but on a principle which we shall see was wrong, and
fi'om its imperfections it was abandoned, and the solid Lister arm was
cut, and the whole body and its coai'se adjustment was j^ivoted on the
lever of the fine adjustment. Thus its normal virtue (a solid limb) was
sacrificed, and a ' fine adjustment,' doomed to failure, was given to it.

A complex roller, a wedge, and a differential sci-ew have in turn
been .since emplo}'ed to redeem this instrument from the failure that
lia-d overtaken it. Partially, or completely, each has failed. The
(Uffei'ential screw certainly comes theoretically nearest to success
with this form of insti-ument. But at the outset this is the case
only wliere it wholly abandons the lifting and lowei/ing of the l)ody-
tube Arc. by the :ic-.tion of a ' fine adjustment,' and its motion is only
brought into (jperation upon the equivalent of a nose-})iece.

JViC form of differential screio hrought into practical operatio'it
hij the Rev. J. Caniphell, of Fetlai;, Sliethind, was a,do[)ted by Swift
and Son in IHOl, but liad Ijeen exliibit(Ml in a stand inade ]jy Baker
in tlie year 1880 at tlie Quekett Micro. Club.^ Its ol)ject is to su})-

' Vide Chaptei- 1.

•■' Journ. Q.M.C. ser. 2, vol. ii. pp. 283 and 287 (188(iJ.

THE ilNE ADJUSTMENT

165

plant the direct-action screw, where the form of the microscope may
appear to make that a necessity. This has been the case with the
Continental model. It was applied by its inventor to a microscope
made by himself, and was brought before the Quekett Club by Mr.
E. M. Nelson.

It is very simple, and is made by cutting two threads in the
micrometer screw. Fig. 126 wdll illustrate the exact method. D is
the milled head of the direct-acting screw. The upper part, S, of
the sci-ew has (say) twenty threads to the inch, and the lower part, T,
twenty-five threads to the inch. B is the fixed socket forming part
of the limb of the microscope, and H is the travelling socket con-
nected with the support of the body-tube. The revohition of D
causes the screw thread S to move up and down in B at the rate of

Fig. 126.— Campbell's
differential screw
fine adjustment
(1886).

Fig. 127. — Zeiss's usual ' new ' fine adjustment (1886)

twenty turns to the inch, whilst the screw thread T causes the
travelling socket H to move in the i-everse direction at the rate of
twenty-five tiu-ns to the inch. The combined effect, therefore, of
turning D twenty revolutions is to. raise or lower T, and with it the
body -tube ith of an inch, or i^th of an inch for each I'evolution.
The spiral spring below H keeps the bearings in close contact.

Of course any desired speed can be attained by proper combina-
tion of the threads : thus 32 and 30 would give ^^-oth of an inch
for each revolution, and 31 and 30 would give -g-^o^th of an inch.

This screw has provided for the Continental model what Swift's
vertical lever has done for the Jackson model ; Mr. Baker, of
Holborn, has adopted it and with very satisfactoiy results ; for it
has passed through that most crucial of tests for a fine adjustment,
its employment in photo-microgTaphy, with excellent results ; and

l66 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

we hope that it may become the general fine adjustment for this
form of microscope in place of the old form of direct-acting screw.

In contrast and comparison with Campbell's difierential screw
we may put the principle on which the usual simplified construction
of the fine adjustment of the Zeiss stands rests. ^ In fig. 127 the
triangular bar C is screwed firmly to the stage ; on it raoves a hollow
piece B, which is connected inseparably with the arm A carrying the
tube. At its upper end C is cut away for about 15 mm. and B
hollowed out at a corresponding place so that space is obtained for a
spiral spring. This spring bears below against the hollowed-out
part of B, its upper end being connected with the projections of the
piece E screwed into C. The piece B is closed above by the cap F, iii
which is the female screw. On the top of the micrometer screw is
fitted a bell-shaped head, and at its lower end is a small nut for
preventing over-screwing. The lower end of the screw is rounded ofi"
and bears against the flat surface of a hard steel cylinder let into E.

Clearly, when worked, the screw remains in the same place,
bearing against C. The female screw, on the other hand, moves ovei'
it, raising and lowering the tube carrier B A connected with it. By
its own weight A B counteracts the rise and thus supjDlies the place
of the strong spiral spring formerly employed. The weak spring-
here adopted acts in the same direction as the weight of A B, and
serves to assist the latter when the upper part of the microscope is
placed horizontally.

Our appreciation of all that is done by the great firm of Zeiss we
need not reiterate ; it is well known ; but our opinion of the form of
stand adopted by these opticians we freely expressed, and we believe
justified in the last edition of this book ; but it is well to get the
oi^inion of one who with practical knowledge would certainly not be
prejudiced against the Continental stand. Dr. H. E. Hildebrand
says ^ that in teaching establishments, where as many as two
hundred microscopes maybe used, the weak points of the Continental
stand are soon brought to. light. The fine adjustment screw soon
becomes unsteady (an inevitable consequence of the weight so fine a
screw has to carry), the prism suffers bending or rotation, the prism
flange or the hinge-block under the object stage loosens its connec-
tion with the stage plate, etc. &c., all of which and much more, as we
ijelieve, is the I'esult of the adaptation of a simjjle and primitive form
to complex iippliances for wliich it was never designed or intended.

It is, however, an admirable characteristic of the firm of Zeiss,
that while they adhere doggedly to tlie old Continental model, they
are continuously putting foi-th their ingenuity and skill to counter-
act wliat are shown to Ije its defects. In tlieii; best usual form the
speed of the fine adjustment is , ,', j inch for each revolution of the
milled head. This is undoubtedly too I'apid, but it could scarcely be
made a finer screw, because, as we have seen, it had the coarse
adjustment and tube to lift, and the wear and tear on so fine
a thread inconstant use led in lapiil railnrc I3ut tlic fii'iii lias

^ This form was introduced in 1S8G, and was a great improvemoiit on its pre-
decessor, which was mechanically bad. Vide It. M.S.J. 1880, p. 1051.
* Zeitschr. f. wise. Milcr. xii. flSU.';) pj). 14.')-54.

Fig. 128 (1898).

1 68 THE HISTOEY AND DEVELOPMENT OF THE MICKOSCOPE

introduced a very complex but very remai-kable modification of their
fine adjustment which is intended to obviate both the above defects.
It is a model ostensibly constructed for photo-micrographic
purposes, but if successful will speedily be applied to all their stands.
Tlie entire microscope is shown in fig. 128, while a vertical section of
the fine adjustment is presented in fig. 129, and a gTOund plan of
the same in fig. 130. A point which seems to be considered of
importance to some German microscopists is the provision of a
handle by means of which the instrument may be readily moved,
and with the provision of this the usu.al large milled head controlling
the fine adjustment has been displaced. This is shown at H in fig.

Fig. 129 (1898).

128. But with the accomplishment of this thei-e was a great desire
to bring about what we have so often endeavoured to show was an
indispensal)le necessity in the beautifid productions of Jena, viz. that
tlie fine adjustment should not luive the biu'den of carrying the
coar.se adjustment and the tube. They have not succeeded in doing
this ; the weiglit of the coarse adjustment and tube is still on
the fine micrometer sci'ew. They have diminished the weight
that 'the fine adjustanent has t(j sup])ort l)y making the body and
draw-tube of aluminium. The fine adjustment is placed close beliind
the coarse one, Vjotli Ijeing fastoneil (juitc iiiilcpcndently, so tliat in

THE FINE ADJUSTMENT

169

fact the object holder can be made to receive, and the optical apj)a-
ratns arranged to examine, preparations of almost any required size.
To accomplish this H (fig. 128) is made hollow, and in place of
the usual triangular ' conductor' of the fine adjustment, a swallow-
tail-shaped slide F (figs. 129, 130) is placed, the upper part of which
is hollowed out to receive the spiral spring U (fig. 129). The lower
part of this is also hollowed and conceals the long box which receives

Fig. 130 (1898).

the micrometer screw M (fig. 129). The pressure of the spii-al
spring is in the direction of the axis of the micrometer screw, which
works against a hardened point shown at D., fixed on the dust-tight
under-cover of H (fig. 128). This ' conducting slide ' F (fig. 129)
is firmly screwed to the jDart carrying the coarse adjustment, and the
aluminium tube T is connected in the usual manner with therackwork.
To avoid what appears to have been considered a peril in the
exposure of the milled head carrying the fine adjustment screw in the
usual form of the
Zeiss stand, Dr.
Czapski caused the
fine adjustment to
be placed in the
hollow of the up-
right H (fig. 128),
so that the screw
itself is complete-
ly removed from
direct contact with
the hand ; the
turning of the
' micrometer ' or
fine adjustment screw only takes place by means of the motion of
the small milled heads WW (figs. 128 and 130) which work the
endless screw E (fig. 130). This engages the wheel S, which being
festened on to the flange of the fine adjustment screw, replaces or

Fig. 131. — Eeicliert's new patent lever fine adjustment (1899).

lyo THE HISTORY AND DEVELOPMENT OF THE MICEOSCOPE

rather supplants the usual milled head ordinarily placed at the to]3 of
H ( fig. 128). One consequence of this is that the speed of the fine
adjustment is slowed down so much that while Zeiss stands of the

Y\(.. 1:^.2.— Watson's lever liuc iuljiisliiieiit ( iHWj.

usuiil form give only n'nth inch for a revolution of the milh'd liciid
of the oi-dinaiy micrometer head, this form of fine adjustment gives
yg-th iiifli foi- ;i rcvolntioii of the siii;d1 milled liciids WW (figs. 1 2H,

THE PINE ADJUST3IENT

171

130). That this is an advantage of a very high order — if experience
proves it to be a practical method — there can be no doubt. Moi'e-
over, the weight which this newly arranged micrometer screw has
to lift is, as the firm informs us, only one-fifth of that which was borne
by the older foi-m, and there are special arrangements made to pre-
vent this delicate constiaiction from being overscrewed either way.

The mechanical stage of this mici-oscope has some featui'es woi'thy
of note. It will be seen that the milled heads which work the stage
are on Tarrell's plan, but the outei- head gives transverse movement
to the stage plate instead of verti-
cal movement. The pitch of the
screw on this pinion is fine, so that
the motion is slow. The vertical
movement which is actuated by the
inner pinion head is on altogether
a novel plan. The motion is one in
arc, this stage plate being pivoted
on the left-hand side ; the circular
portion on the right-hand side
has rack teeth cut in it into which
a pinion is geai-ed. This pinion
has a toothed wheel fixed to it,
which engages an endless screw
attached to the pinion that carries
the inner pinion head.

The speed of the object at the
centre of the stage is about half
that of the rack, because the
object is placed about halfwa}'
betw^een the rack on the right and
the pivot on the left hand side of
the stage.

The stage is concentric Avith
simple non-mechanical rotation ; it
can be clamped in any desired po-
sition by a small screw at the side
of the stage (not shown in the
figure) .

We may now describe the ex-
ceedingly simple, and as we think
beautiful because essentially prac-
tical, fine adjustment invented by
Reichert, which we believe will
prove itself the most useful and
conservative adjunct ever devised
to make the Continental stand of service for high -class work with-
out increasing its expense or reducing its value in ordinary work.
It consists in adapting in a very ingenious manner a lever of the
second order to the usual direct acting screw. It will be seen by
fig. 131, which represents this part of the microscope open at B
and closed as in use at A. The micrometer screw presses on two

Fig. 133. — Swift's iDatent fine adjustment
(I88I;.

172 THE HISTORY AND DEVELOPMENT OF THE MICEOSCOPE

levers, li, h, which in turn press the arched piece with its appendix f
on to the prism support. The principal screw has three threads
to the millemetei', which by the levers is reduced by about one
third. The pointer for reading the raicronieter scale on the
milled head is conveniently arranged so that it can be changed to any
figure on the scale. The speed of the adjustment is 2x0th inch to
one revolution of the milled head.

We may now profitably consider the best forms of fine adjust-
ment that apply to the Lister model, and one of the steadiest and

Fig. 134. — Nelson's model with Swift's fine-
acljustment screw to the left hand (1882).

Fig. 135 (1885).

rno.st delicate of these is that devised by Messrs. Watson and Sons.
The entire body is raised or lowered by means of a milled head fixed
to a screw lutving m hardened steel point, acting (m a lever with
hardened Jiiid highly polished contact surfaces, against a point
attached tf) the body-slide, in a perfect dovetailed fitting, about
2i inches long. This is seen in the section shown in fig. 132. By
tiTi-ning the milled liciid the h;ird steel lever 13, which hiis its fulcrum

THE FINE ADJUSTMENT 1 73

at 0, raises ov lowers the bod}' with great smoothness and with the
gi-eat delicacy of ^ i-jyth inch for every revolution of the milled he;\d,
and therefoi'e capable of yielding good service with the highest power
objectives.

We may now direct our attention to the/oo'mer of the two divisions
into which we have separated the various kinds of fine adjustment,
viz. that in which the nose-jnece only is controlled by the adjustment
screw.

Sivift's vertical side lever is one of the new forms of fine adjust-
ment worthy of careful trial ; it has in it elements of great merit.
It can. however, only be applied to the Lister model, and with the
adjustment described above certainly places this form of microscope
beyond the danger that some years ago promised to have proved its
extinction as a first-class microscope.^

The first form of this adjustment (1881) was sound in principle and
ingenious in construction, and although the patentee inti'oduced a
modification ^ of it (1885), we believe the original form, which he still
makes, to be the best, because it only acts on the nose-piece while the
modification acts on the body-tube.

The early form employed by Swift avoided what had been a sheer
necessity of all successful fine adjustments of this type, viz. the
accuracy and perfection of the fitting of the nose-piece tube. This
was done, as shown in fig. 133, by attaching a vertical prism-shaped
bar, A, to the nose-piece, and sliding this in V-grooves in a box
at the back of the body. A horizontal micrometer screw with a
milled head, F, acts on a vertical bent lever, D, on which a. stud, E,
fixed to the prism bar bears.

There is also an adjustment for tightening up the prism bar in
the V-grooves, B B. Side-shake and ' loss of time ' are impossible
with this form of adjustment ; while the power to ' tighten up ' by
means of the capstan-headed screws enables wear and tear to be
compensated. It is obvious that the slowness of the motion is here
controlled by three factors : (1) the length of the lever, D; (2) the
distance of the lifting-stud, E, from the pivot or fulcrum ; and
the pitch of the screw-thread on F.

Manifestly, where a siVZe-lever fine adjustment such as this is
employed it should be, as it now always is, placed on the left-h-ixnd
side of the operatoi- : we can readily focus with the left hand, and
leave the right hand free for moving the slip and efiecting other
adjustments. Ambi-dexterity is not at present a common gift, and
to have the right hand fi'ee is important. This was pointed out by
Mr. Nelson when this fine adjustment was first introduced, and he
had a student's microscope constructed with the micrometer milled
head on the left side, as in fig. 134. It is manifest, however, that
it would greatly improve this adjustment if the screw-pinion
were carried right through and a milled head placed on both the
right and the left sides of the body.

An eai'ly form of a nose-piece-controlled fine adjustment was em-
ployed by Andi'ew Ross. It was applied to a microscope having a

1 Journ. B.M.S. (1881) p. 297, fig. 43.

2 Journ. B.M.S. (1885) p. 120 and (1886) p. 1043, fig. 207.

174 THE HISTOEY AND DEVELOPMENT OF THE MICEOSCOPE

bar uiovement. It consisted of a lever of the second order inserted
within the bar, and actuated by a micrometer screw with a milled
head at one end, the fulcrum being at the other, and the nose-
piece between them. This served admirably in the days of low-
angled objectives ; but there were two faults belonging to it : one
was that the tube of the nose-piece had not a sufficient lengi;h of
bearing and was liable to a lateral shake ; the other was that the
adjustment screw, being near the middle of the bar, involved tremor.

The application of this principle in its very highest and most
perfectly px"actical form was invented by Powell. His instrument
also had a bar moveraent : but the bar being of relatively great
length, he employed a lever of the first order, the micrometer-screw
being at one end, the nose-piece at the other, and the fulcrum between
them. The ratio of the arms of the lever was 4:1; and the screw
is so arranged that a complete revolution of the milled head is equal
to the 2-g-oth of an inch. The position of the screw is immediately
behind the pivot on which the bar turns, and this precludes the possi-
bility of the impartation of vibration to the body ; and, as the nose-
piece tube is very long, and only bears on three points at either end,
this adjustment is the steadiest, the smoothest, and the most reliable
for all objectives of any of the several devices which have come before
us during the last twenty years. In fact, this fine adjustment has
held an unrivalled position for the past fifty years (fig. 157).

The fine adjustment that was employed as its rival on the earlier
forms of the Lister model was known as the short-side lever, and
it was sometimes employed in the commoner bar-movement micro-
scopes. Its position and character will be seen on the right-hand
side of the body of the Smith model, fig. 122. In the light of
what we now need, we ai-e bound to say to the intending pur-
chaser of a microscope, ' Avoid it ; ' it is bad alike in design and
construction. The screw is so placed that tremor is inevitable in
the body when it is touched, while the nose-j)iece tube is so short
that steadiness of movement does not belong to it. It is only that it
was concurrent with the belief in ' low angles,' andcousequent 'pene-
tration ' in objectives (with which no critical work could be done),
that it is possible to account for the toleration for so long in num-
bers of English microscopes of this wholly inefficient adjustment.

From the foi-egoing we learn that there are thi-ee types of micro-
scope models for which a suitable fine adjustment has been found.

i. The bar movement model, for which Powell's first order of
lever is the perfect method.

ii. The Lister model, for which Swift's vertical lever and
Watson's long horizontal lever are the best forms known.

iii. The Continental model, foi- which Cam])be]rs diffei-ential
screw is the most smooth and delicate device yet suggested, iniless
v\-e take into consideration tlic bcantifid h'xcr fine iidjustment of
Keichei't.

The full value of delicacy in tlie fine adjustment can of course
only be fidly appreciated by the expert. A tolerable speed may be
permitted in this adjustment wlien uncritical images with small
ilhniiiniiiing cones are used, because objectives so'used are far less

THE MECHANICAL STAGE

175

sensitive to focal adjustment. When, however, a critical image is
obtained with a | cone the conditions are changed and an objective
with a wide aperture becomes excessively sensitive to minute focal
alterations. Hence the need with the highest class of microscopic
investigation of at least as slow an action as can with safety to the
mechanism be secui'ed, and therefore comes out the danger of
burdening the screw of the fine adjustment with a fraction of an
ounce of lifting more than can be avoided.

Fig. 136. — Watson's new stage (1898).

So far as we can ascertain the speeds of the several fine adjust-
ments now within the reach of the worker, they are as follows, viz. :

Speed for one revolution of the
milled head in fraction
Model of an inch

Bausch and Lomb Jjst = two threads to 1 mm.

Reichert (old form) ..... J^th

Zeiss (ordinary)

Powell .......

Baker and Swift (Campbell differential screw)
Pieichert new patent .....

Swift vertical lever .....

Watson's long lever .....

Zeiss's new endless screw arrangement for
photo-micrographic stand

Y^st = four threads to 1 mm.
o^th

4-th

lY. The stage of the microscope will next call for considera-
tion. What is known as a mechanical stage must be a part of every
first-class microscope ; but by this we mean one of perfect work-
manship and construction, otherwise it is an impediment and not a
help.

To this end we w-ould say at the outset there must be thoroughly
well-made movements. The employment of levers, cams, and that
class of stage-gear is in practice, for critical purposes, a mere

176 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

mechanical mockery. Better trust to and educate the fingers to
move the object than be beguiled by any such practically tormenting
delusions. They are simply impossible as accompaniments of a
first-class microscope.

The principle upon which alone a perfect mechanical stage can
be constructed, so as to work smoothly without ' loss of time,' and en-
dure constant use without failure, must
be the employment of jorism-shaped
plates sliding in sprung V-shaped
grooves, and bearing only on four points.
We may test the mechanical quality
of the movements of a stage, as in the
case of the coarse adjustment, by re-
moving the parts, cleaning them, and
I'eplacing them, when they should work
smoothly and without shake. Wliere
the sliding parts are tightened into
easily fitting and merely ploughed
grooves by pressing the pinion into the
rack, the desirable resiilt of smooth
working and instant responsiveness of
sliding plates to milled heads will not
present itself.
But besides the perfect action of the sliding parts, a perfect
mechanical stage should have equal speed of motion vertically and
horizontally. A common fault is that the speed ol the rackwork
giving vertical motion is greatly in excess of that of the screw giving
lateral or horizontal motion. If, for example, a pinion has eight
leaves, and the rack it works has twenty-four teeth to the inch, then
three turns of the milled head (and pinion) would cause one inch of
movement to the stage. In order, therefore, to get the same rate of
movement in the lateral motion, the screw should be so pitched as
only to move the stage through an inch with three I'evolutions of the
milled head.

Fig. 137 (1898).

Fig. 188 (1898).

It is most desirable that the pinions should he fixed, not movable
with the movements of the stage, and the milled hejuls cai-iyiug tlu;
resjjective [larts sh.oidd. he as near to each, oth,er as posslhle. The l)est
form is that of Turrell's, devised in 1882, where one (a, screw) is
hollow, and the other (a pinion) passes tln-ougli it ; tliis [)ermits ])otli
to be turned at tlie same time witli one liand, giving a, diagonal

THE MECHANICAL STAGE. HOW TO FOCUS lyj

motion, as well as the separate rectangular ones, and gives great
facility for instantly producing any motion required without remov-
ing the hand from its position ; a most desirable attribute of a stage
when the rapid movements of a living and minute organism are
being followed.

It still further enhances such a stage if a pinion is carried right
through the stage with a milled head at each end.

A new stage devised by Watson has in it some features of interest,
a principal one being that the milled head controlling the horizontal
movements is in a fixed position ; in other words, does not travel
with the plate. This is shown in fig. 136. A is a ball upon which the
turning of the screw takes place ; it will be noticed that this ball has
a groove in it into which grease or dust can drift without afiecting
the motion. The cap B covers the ball when fitted together. The
manner in wliich verniers are fitted is shown at D, D, and the screw
for adjusting the vertical rack movement of the stage is shown at C.
Fig. 137 shows the manner in which the plate E is attached to the
stationary screw; while fig. 138 indicates the careful manner in
which this stage is sprung to counteract continuous wear. The saw-
cuts shown are compressed by means of screws which are situated at
the points F F, G G, and any amount of wear can be corrected by the
use of these screws in these slots.

The aperture in the stage should always he large, not less than 1|-
inches in diameter. There ought always to be space enough above
the ordinary slip when it is in position to pei'niit of the easy inser-
tion of the index finger, for by its proper use, focussing with the
highest powers may be greatly facilitated. The object is to raise or
lower the slip, as the objective approaches the object, so as to dis-
cover how nearly it may be to contact with the front lens of a high
power in approaching focus. The focal distance should always be
felt, and not sought with the eye.

Let it be supposed that we are using a dry object-glass with a full
aperture, and consequently short working distance. With the right
hand the coarse adjustment is worked ; with the elbow of the left
arm on the table, the second finger of the left hand resting on an
immovable part of the stage, which steadies the whole hand, the
index finger should rest lightly on the edge of the slip, and the
thumb be so placed as to graze the objective as it advances towards
the slip. The touch of the thumb indicates whether the objective
is an inch ofi" or only a quarter of an inch away from the cover of
the slip. The movement of the coarse adjustment may be rapid up
to Jth or |-th of an inch, but after this there must be a cautious but
steady advance. The body may be racked down until by gentle
upward movement the slip is found to touch the front of the objective ;
then pi-oceed cautiously by delicately lifting the slip from time to
time, by doing which we can proceed in perfect safety until the focus
of the object is obtained. In this way focvissing becomes easy and
rapid, a matter of touch, and not of discontinuovis procedure to
' discover where the fi-ont of the lens is '^ — a search requiring a hand
glass and often, with its cumbrousness, considerable loss of time.
The above simple plan with brief practice will enable the operator to

'^\%;>^'«.'''^VK--''^

Fid. 130. — Zeiss pli/)to-minor(r,xpliif statifl (180'')).

THE MECHANICAL STAGE 1 79

focus an object in the field with a o'^j- inch objective in ten or
twelve seconds.

If a perfect mechanical stage cannot be obtained, take no middle
course, have a firm, loell-made plain one with a sm,ootlily sliding ledge.
The stage should be large, and the ledge should glide with perfect
ease and without catching when gently pushed from one corner. For
this pui'pose the side-guides should be long, and only the ends of the
bar should bear on the stage. The aperture should be as in the
mechanical stage, and for the same reason.

Mr. ISTelson suggested a stage of large size, Avhich should have a
1^ or 1| inch aperture bored in it, and then have the intervening-
brass between it and the front taken away, so that the stage
assumes a horse-shoe form. This is thoroughly efficient, and the
principle is seen in fig. 134.

It is a matter of great interest to English microscopists to note
that their German collaborate urs in Germany and the leading-
German makers have not only surrendered to the sub-stage condenser,
and even in its achromatic form, but that at length they have also
adopted the mechanical stage ; the latest form adopted by Zeiss is
figured in the accompanying illustration which shows the complete
instrument (fig. 139). We specially call attention to it here, as it has
Turrell heads, mai'ked H V, and a rotating stage of 4 inches diameter.

It must, however, be noted that the usual Continental model
adopts a small stage with a ^-inch aperture and two fixed spring
clips with no sliding ledge ; that is, wanting almost everything-
required to do good modern work.

One of the most practical rules for the young microscojoist in
this relation is, ' Have your mounted slide in a fixed position, but
never clij) it if it can possibly be avoided.'

In addition to perfect rectangular movements a first-class
microscope should have concentric rotary/ motion to the stage. This
is usually effected by rack and pinion, but it is at times desirable
to move it with greater rapidity than this admits of. In very well
made instruments the pinion engages the rack so lightly that this
rapid motion may easily be given to it. In others the pinion can be
disengaged and rapid raovement effected.

The centre of rotation of the stage should be closely approximate
to coincidence with the optic axis, so that in rotation the object
should never be out of the field when a fairly high power is used.
Elaborate rectangular centring gear has been used by some makers,
and is found in some high-class instruments ; but this is not needful,
for all that is really required is to rotate an object without losing it.
In fact exact centring would have to be readjusted for every separate
objective if it were needed. But any slight departure from the
axial centre can be much more readily met by bringing the object
into centre by the mechanical stage.

There are fiour movements in every microscope which shoidd he
graduated : these are (1) the milled head of the fine -adjustment
screw ; (2) the stage movements for finders ; (3) the extension draw-
tube carrying the eye-piece ; and (4) the rotation of the stage.
Divided arcs are imposing, and to the multitude look ' scientific ;, '

N 2

l80 THE HISTOEY AND DEVELOPMENT OF THE MICKOSCOPE

but in practice tliey are superfluous in the most complete instrument
beyond those indicated.

There is a simple form of attachable mechanical stage now em-
ployed by many, and we think with advantage, when the cost of a
complete raechanical stage must be forgone. This consists of a clip
to receive the object, made of glass or .brass, so arranged that the
friction shall be reduced to a minimum.

Such an attachable stage can be made to work with remarkable
smoothness ; and since some persons have not suflficient delicacy of
touch to move so small and thin an object asa 3 X 1-inch slide upon
the stage with steadiness and precision, it is in favour of the super-
stage that it is larger, moves easily, and can be furnished with
convenient points of hold-fast for the hands, and consequently is
more manageable. Against its employment is the fact : 1st, that
the slide is clipped into a rigid position ; and 2ndly, that the aper-
ture is often too small to admit of the employment of the finger in

Fig. 140. — Swift's attachable meclianical stage (1894).

moving the slide to assist in rapid focussing. But these are defects
which are rapidly disappearing.

Amongst those that claim the attention of the microscopist is
that of Messrs. Swift and Son, shown in fig, 140. It can be adapted
to most microscopes ; it is easily applied and removed, leaving the
.stage, if required, free. The up and down motion is effected by a
milled head below the stage. The lateral movement is produced by
two endless screws engaging in woi-m-wheels fixed to smooth rollers.
The lower edge of the slide I'ests on these, and is kept in gentle
apposition Avith them during tiuverse by a third smooth roller at the
free end of a curved spring as shown in the figure. This is readily
turned aside when changing the object. In its most recent form we
have used this stage with comfoi-t and pleasure.

Another of these stages, made by Baker from designs by Mr.
Allen, is shown in fig. 141, which in its latest form is so arranged
that the widtli of space l^etween the i-est and the spring clip can be

THE MECHANICAL STAGE

18 1

enlarged so that a much wider preparation than the usual one inch
may be worked with great facility on this stage. The method of
attachment practically makes the mechanical stage one with the stage
of the microscope, as it is in contact with the fixed stage throughout
its entire length, and is clamped at the lower end to the top, and at
the upper end to the bottom of the stage.

Both the rectangular movements are effected by i-ack and jDinion,
the vertical one of which carries a bar (fixed as to horizontal movement)
against which the slide is pressed by a spring clip, and upon which
is mounted the rack and pinion for the horizontal movement ; the
end which presses upon the slip is tipped with cork in order to grip
the slide, and move it along the fixed bar ; when the milled head is
rotated, the slide actually rests on two small raised surfaces at either
end of the bar to minimise friction. This is without question a well-
made practical and use-
ful stage. Amongst
stages of this kind, how-
ever, the most original
and useful has been de-
vised by Mr. Nelson.
As seen in fig. 142, the
sliding bar has been
slotted and a movable
piece, which may be
called the shuttle, has
been fitted in the slot ;
this shuttle has a dia-
gonal rackwork at the
back, and a vertical
spiral pinion gears in it,
as is shown in fig. 143.
Above this pinion there
is a horizontal bevel
wheel which is geared
by friction to a vertical
wheel fixed on the usual
horizontal pinion. The cock which holds, and is close to, the vertical
bevel wheel in fig. 143 is slotted underneath, a capstan-headed screw
(not shown in the figure) is fitted for the purpose of compressing this
spring part ; the amount of friction between the copper bevel wheels
can therefore be regulated at will. This capstan-headed screw is
placed some distance from the bearing, so that the length of the bar
between it and the bearing may form a stiff spring ; this renders the
motion equable. It will be noticed, therefore, that the transverse
movement is confined to the sliding bar. This sliding bar can be
removed so as to leave the stage perfectly plain. The heads of the
pinions which control the vertical movement have been kept below
the level of the stage so as to be out of the way of culture plates.
Three and a half inches of transverse movement is given to this
stage, and the manner of the holding the clip is quite new and
eminently serviceable. On the shuttle there are two sliding pieces,

Fig. 141. — Baker's attachable stage (1898).

1 82 THE HISTORY AND DEVELOPMENT OF THE MIGEOSCOPE

Fig. 142. — Nelson's new mechanical stage (1897).

Fig. 143. — Nelson's new mechanical stage (1897).

Vi(, 111. — NcIkoii'h Jiew nieclianiciil stage (1888).

THE MECHANICAL STAGE

183

and these hold the slip by the two lower corners, as seen in fig. 142 ;
and this mode of gripping allows for the employment of the in-
valuable method of touch on the edge of the slide for discovering
working distance and focus. A plain sliding bar may be substituted
for the mechanical bar ; this forms a semi-mechanical stage as shown
in fig. 144. The mechanical movement being only imparted to the
lugs at the side of the stage, the bar may be moved by the hand by
sliding as in an ordinary plain stage without the employment of the
mechanical raovement.

The stage is of aluminium, and its size is 4^ x 7 inches.

Another attachable stage having many advantages is made by
Reichert and shown by fig. 145. It can be used with any instrviment
of the Continental type, is very carefully made, and the scales

Fig. 145. — Eeichert's attachable stage. (About half natural size.) (1892.)

attached are divided to read by means of a vernier to 0*10 mm., and
the range of movement is an inch in both directions.

An attachable mechanical stage is also made by the Bausch and
Lomb Optical Company of Rochester, New York, having great
merit and some special points ; and this firm is in advance of all
other makers that we know of in making an attachable revolving
mechanical stage.

There is much similarity to the American mechanical stage in one
made by Carl Zeiss and illustrated infig. 146. Of course the principle,
as primarily in all the others, is that suggested by the late Mr. Mayall,
and afterwards by Reichert. Two sliding pieces, mounted at right
angles to one another, are moved by means of two milled heads, S, T.
They pass along millimetre scales which serve to record any particular
position.

The demand for these attachable stages is, we presume, consider-

1 84 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

able, for they are made by most leading opticians. The last mechanical
stage we illustrated is by Messrs. R. &, J. Beck, which is illustrated
in fig. 147. It has vertical rack and pinion and horizontal screw
motions with graduated finer divisions.

To Messi'S. Bausch and Lomb, however, we are indebted for the
introduction of an attachable stage in which the iris diaphragm is on
the plane of the stage. We illustrate this in fig. 147a. Its use with
a condenser we do not commend. But especially when the illumina-

FiG. 146.

5 full size.) (1895.)

tion is daylight, and veiy critical results are not sought, it will be
useful, and is admirably made.

V. The sub-stage is scarcely second in importance in a first-
class mici'oscope t(j tlie stage itself. It is intended to receive and
enable us to use in tlie most etticient mannei- the optical and other
apparatus employed to illuminate the objects suitalily with the
various powers found needful. Upon this much of the finest
critical work with the modei-n microscope depends.

To accomplish this a good sub-stage must have rectangular
movements, and a rack-and-piniou focussing adjustment.

THE SUB-STAGE

185

The vei'tical and lateral movements need not be as elaborate as
those of the stage, since only a small movement in each direction is
required. The object is to secure a centring motion, a motion that
will make the optical axis of the sub-stage combinations contin^ious
with the optical axis of the objective. It must therefore be a steady
motion ; the sub-stage must move decisively, and must rigidly re-
main in the position in which it is left.

A bad sub-stage moA'es in jei-ks, and is liable to spring from the
position intended to be final.

It is not needful that the motion should be in right lines ;
motion in arcs whose tangents intersect at right angles are quite as
efficient. A steady, even, reliable motion that will enable a centre
to he found is all that is required.

Fig. 147. — Bock's mechanical attachable stage (1896).

The focussing adjustment must be smooth, steady, and firm, acting
readily and remaining rigid. The recent employment of achromatic
condensers of wide apertures has led such critical workers as
Mr. E. M. Nelson to suggest a fine adjustment to the sub-stage.
There are times when it is a great luxury and a facile path to
delicate and desirable results ; but it may be quite simple, a direct-
action screw of fine thread, or a cone which the revolution of
a screw pushes horizontally forward upon the bottom of a sliding
bar to which the sub-stage is fixed, or an inclined plane acting
in a slot in the same way. In fact, any simple device for focussing
the condenser more slowly than the rackwork will do, pushing the
condenser up to, or causing it to recede from, the vmder surface of
the slide with sufficient delicacy. But no means should be employed

1 86 THE HISTOKY AND DEVELOPMENT OF THE MICKOSCOPE

for this end which will imperil the absolute firmness of the sub-stage,
or else more will be lost than can be gained. The need of such a
device for the most delicate and critical microscopical work is shown
plainly by the fact that during the past few years several ingenious
and practical devices have been used, nearly every principal Eng-
lish maker employing a method of his own. The first arrangement
was made in Powell and Lealand's sub-stage and is shown in fig. 148.
The nature of this device, which was suggested by Mr. Nelson, will
be readily understood. It does not interfere Avith the general

Fig. 147a. — Attachable stage with diaphragm in the plane of the stage. Top
view and cross section showing construction of stage and attachment of iris dia-
phragm.

mechanical arrangements of the sub-stage ; it will be seen that the
milled head A controls a screw spindle terminating in a steel cone B.
On rotating A, B tui-ns, and with a veiy slow motion forces up (oi-
releases as the case may be) a pin C, inserted in the base plate E of
the sub-stage. The motion of 0 caii-ies with it the condenser. At
light angles to and forming part of E at the back an inner sliding
plate works against a spi-ing iit the upper- end between bearings F at
each side, which are fixed u|ioii the usual racked slide I) of the sub-

THE SUB-STAQ-E

187

stage ; the inner sliding plate is the essential addition to the usual
racked slide, in the application of the new fine adjustment to the
sub-stage. The range of motion is about ^th in. — the difierence in
radius between the smaller and larger ends of the steel cone.

A very simple and practical device for the same purpose was
suggested by Mr. G. C. Karop, who knew that if the best possible
resolutions are required, the image of the flame given by the con-
denser should be as accurately adjusted in the focal plane as the
object itself. This arrangement of Mr. Karop's, admirably suited
to the stands of Messrs. Swift and Son, was patented by that firm.
It consists in the adaptation of their well-known 'climax' or
' challenge ' fine adjustment to the slide carrying the sub-stage ; but
it is actuated by a milled head borne on the spindle to which is con-
nected the coarse rack motion. As will be seen in fig. 149, it is a
lever actuating a stud fixed to the dovetailed slide which carries the

Fig. 148. — Fine adjustment to sub-stage. Fig. 149.— Karop's fine adjustment for sub-
Powell (1882). stage, made by Swift (1892).

sub-stage. The extreme end of the lever is not acted upon by a fine
screw, but there is a cylindrical pin one end of which engages the
point of the lever, the other the face of the inner milled head ; the
milled heads resemble the Turrell stage arrangement, but the inner
milled head works on a screw on the stem of the outer milled head ;
when the inner milled head is turned it traverses the stem of the
outer one, and pressure by the S-shaped spi'ing in the fig. causes the
stud to slowly raise or lower, as may be desired, the sub-stage which
carries it. One complete turn of the inner head presses the sub-stage
the x^th in. So that small fractions of this may be easily obtained,
and it is an advantage that the milled heads of both movements
are so close to each other.

Messrs. W. Watson and Sons have also devised a useful arrange-
ment to serve the same end. As applied to their Van Heurck
microscope it is shown in figs. 150 and 151. A is a controlling
milled head, B the lever which is seen from the side in fig. 150

1 88 THE HISTOEY AND DEVELOPMENT OF THE MICEOSCOPE

and from the front in fig. 151. This is brought round at one end
at right angles to the front. The fulcrum of this lever is at C, and
it fits under the j)in D which is attached to a dovetailed piece, having'
at the back of it enclosed in a metal casing the counteracting spring

Fig. 150. Pig. 151.

Watson's sub-stage fine adjustment (1899).

shown in fig. 151 ; when, therefore, the lever is depressed at B, the
sub-stage is raised at D and vice versa. The milled head A is placed
at the side of the stage of the microscope towards the back slightly

higher than the surface of the
stage.

The fine sub-stage adjust-
ment of these makers as applied
to their ' Royal ' microscope is
shown as it is in its complete
form in fig. 152.

Another sub-stage fine ad-
justment has been devised by
Baker, which, we are of opinion,
it will be of advantage to the
student to understand. It em-
ploys the difiei-ential screw, and
by this means obtains a very
slow movement. The student has
already understood that the prin-
ciple of this screw is the cutting
of two threads of a different ' pitch,' one at either end of the screw,
the proportion of one to tlie otlier determining the amount of move-
ment. The tlii'eads found most suital)le for their sub-stage fine ad-
justment were 40 :in(l 50 to the inch. In fig. 153 the screw A C

Fig. 152. — Sub-stage fine adjustment com
plete in 'Royal' microscope (1899).

THE SUB-STAGE

189

has 40 threads to the inch, and works thi'ough an immovable fitting,
the thread is discontinued at C, and from C to D a screw having 50
threads to the inch is cnt, working through a fitting E. If now
the milled head F be rotated 40 times, the screw A C will have
travelled one inch. So will the screw C D as it is cut on the same
stem, but it would take 50 i-evolutions of screw C D to travel one
inch through the fitting E, hence the fitting E must have been
carried up bodily the remaining 10 revolutions — that is to say, -1-th

Fig. 153. — Baker's fine adjustment to sub-stage (1888).

of an inch — therefore one revolution raises the fitting E ^w^^ of an
inch.

The fitting E is attached to the sub-stage G through a slot cut in
the cover of the adjustment ; the cover is also grooved on either side
to receive that part of the sub-stage H which insures the true
vertical movement so essential with this screw.

It is almost a matter of compulsion to refer hei-e to a com-
paratively recent arrangement known as a sioinging sub-stage, which
is, as its name implies, a sub-stage so arranged as to be capable of

I90 THE HISTOKY AND DEVELOPMENT OF THE MICEOSCOPE

being moved laterally out of the axis in an arc which has the object on
the stage for its centre.

The sole purpose of this is to secure oblique illumination, which
practically, at the time the swinging sub-stage was devised, meant
obtaining a more obliqiie pencil than the condensers then provided
could command ; and since this also meant sending into the object a
small portion of a cone of light in otie azimuth, many tacitly assu^med
that this alone was taken to be ' oblique illumination.' But whatever
sends oblique light through an object into the objective is an oblique
illuminator. Two condensers may have numerical apertures of 1'4
and 1*5 respectively; a stop behind the back lens in each has a
narrow sector cut out, representing the conditions of the so-called
' oblique illuminators ; ' by the former we get an oil angle of 134° 10',
by the latter a similar angle of 161° 23'. These sectors of the cone
of light of 67° 5' and 80° 41' respectively are in every sense
' obliqxle illuminators,' and the one more oblique than the other.

Whether or not it is needful or best to use su.ch a sector is
scarcely an open question; it is manifest that- by taking the stop
with its sector away from each condenser and sending in the complete
cone of light formed by the condenser, we are still using oblique
ilhominators, but the obliquit-i/ is in all azimuths.

There can be no doubt that a large aperture in a condenser
provides the microscopist with far greater wealth of resource than an
oblique illuminator in one azimuth can ever give him. A condenser
with an oil angle of 161° 23' is much more valuable than even the
semi-angle obtained by a mere section of a luminous cone. The
po¥/er to utilise the entire cone is a gain of the highest order.

It will be manifest to all that we want concentration as well as
obliqu.ity.

Ordinary concenti^ation depends upon the poiuer of the condenser.
If it is required to concentrate the light from the edge of the flame
of a paraffin lamp upon an Amphipleura pellucida, the condenser
must be at least a ^th inch or Jth inch in power, which will give an
image of the flame nearly the same size as the object. The amount
of light which is concentrated upon that object will of course depend
upon the aperture of the condenser. An oblique cone of gTeat in-
tensity is here what is needed ; the illuminating cone should be
equal and conjugate to that which exists between the object and the
objective.

Now it is certain that this condition cannot be met by an ' oblique
illuminator' of the kind commonly under.sood by. that name ; to get
immei-sion contact, which is of course a sine qtia non, we must employ
a hemispherical button — oi- one greater tlian a hemisphei-e — placed
in immersion contact with the under sui-ftice of the slide. This may
be illuminated ]jy a beam ivo\\\ a dry combination, made oblique by
the sub-stage l)eing swung otit of the axis. Granted tliat the angle
is attained Avhich can be got with a condenser of gi'eat apei-tui-e, we
manifestly obtain only a portion, and an attenuated and small poi--
tion, of tlie light given in every, or at will any, aziiriuth by the con-
denser.

Theoretically i)erfect ilhuniiiation of;in objective, for example,

THE MIRROR I9I

a ^th of IST.A. 1'4 or 1-5, would be obtained by using a precisely
similar objective as a condensei", with its back lens stopped down by
a slotted stop, the slot being of the size of the peripheral sector re-
quired to be illuminated. The cone of illumination would precisely
equal that taken up by the objective, and would be of maximum
intensity.

Now these conditions are more nearly approached by a high -class
achromatic condenser of great aperture and of homogeneous consti'uc-
tion than by any other means.

The value of oblique illumination is not here in question ; what
we believe clearly shown is that, however much may have been done
by oblique illuminators dependent on swinging sub-stages, and the
like, the same things can be better done loith i'jninersio7i condensers of
great apertures and perfect corrections.

The swinging sub-stage, with these considerations — as well as all
other 'oblique illuminators' of its order — is a useless and defective^
not to say deceptive, adjunct to the microscope ; and this judgment
has so far obtained amongst practical microscopists as to cause the
virtual disappearance of the swinging sub-stage. It has no valid
function — is unfruitful specialisation in fact — which does not pro-
mote the progress of either the instrument or the worker.

And this will apply to those complex forms of microscope known
as 'radial,' 'concentric,' and those provided with stages that revolve
or ' turn over ' in an axis at right angles to the optical axis of the
microscope.

In addition to the features enumerated hitherto, a complete sub-
stage shoidd also he provided toith a rach-and-pinion rotary motion ;
that is only really needed in order to use the polar [scope. For the
purposes of its successful employment this is important, but other-
wise its use is very limited.

YI. The mirror is also an indispensable part of a complete
microscope. In a first-class stand it should be plane and concave
and from 1\ to 3 inches in diameter. It may be mounted on either
a single or a double crank arm. In any microscope, if there be
only one mirror, it should be concave. This mirror, from its curve,
has a focus, a point in which the reflected rays all meet ; and the
mirror should not be fixed, but so mounted that it may be focussed
on the object.

The plane mii'ror is sometimes found to give several reflexions of
a lamp flame at one time ; we find a very efiicient explanation of
them in a paper by Mr. W. B. Stokes in Vol. YI. of the second series of
the Journal of the Quekett Micro. Club, p. 322 (1 896). His idea oftheii^
origin is explained in fig. 154. A is the glass surface, B the silver
surface, 0 the object, and E the eye. In the direction 1, 2, 3 appear
the first three images. ISTo. 1 is from the glass surface, ISTo. 2 from
the silver, and No. 3 is from the silver and air stu'feces.

Move a card along A towards 1, and No. 3 disappears first. No. 2
immediately after, and No. 1 when the card reaches that point.
This being their origin it may be asked how the images can alter
their position when the mirror is revolved in the plane of A. They
cannot ; the mirror A B has parallel surfaces, but microscope mirrors

192 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

Fig. 154.

are not completely parallelised ; they may be regarded as wedges.
With that fact before us we can see how images approximate and
retire when the mirror is revolved. Let the sm^faces A and B,
fig. 155, have an inclination of 1° ; then, viewing a small object at E
(close to the eye), one image appears towards 1 — i.e. at right angles
to A — and another in the direction E 2, 1^° from E 1, which, after

being refracted to 1 ° in the
glass, is reflected at right
angles from surface B.

If this mirror is re-
volved in the plane of A,
of course No. 1 image will
remain still, and No. 2 and
subsequent images will re-
volve with the mirror
round No. 1.

If we exaggerate the
wedge shape of our mirror,
we can see that at a par-
ticular angle these images
can be made to superim-
pose. In fig. 156 let the
signs be as before, and the
images whose rays pass re-
spectively from 0 to 1 and
2 ^ will be reflected to E as
one image. The images
vary in size owing to the
various distances. No. 2
is the brightest except at
great obliquity.

In practice we find that
these images may be obvi-
ated by rotating the mirror
in its cell until a certain
]Doint is reached where all
the images will be super-
imposed. All mirrors should
be so mounted as to admit
of this rotation.

The present Editor is
gi-eatly in favour of the em-
■ployment of a rectangular
])rism cut with care and precision. We get by this means total
reflexion and no double reflexions ; and he believes that finer images
can be obtained by its mefins than with the plane mii'i-or. It may
be mounted in the place of the phme mirror — that is to say, the
concave mirror may be as usual in its cell — and in the otlier cell,
which would have received the pbuie mirror, the i-ectangular prism
may be mounted and be c;i|)fib]e of rotatif)n as the plane mirror
would have been.

Jt .sliDuli], however, be noted tJuit this applies only when the

£ —

Pig. 155.

Fjg. 156.

A TYPICAL MODERN STAiND

193

Fig. 157.— Powell and Lealand's No. 1 stand (1872)

194 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

light is required to be reflected at an exact right angle. It is of the
greatest service when the mici'Oscoj)e is of necessity used in a rigidly
upright position.

If it be used for angles other than right angles, there will be
refraction as well as reflexion ; and as the necessary decoraposition
of the light into a spectrum will accompany the refraction, care must
be exercised to see that the rays emerging from the prism are at
right angles to those incident to it, and that the areas of the square
faces of the pi-ism are sufficiently large to have inscribed within
them a circle equal to the back lens of any condenser used.

Some employ what has been known as a ' tvhite cloud illuTninator,'
that is, a disc of plaster of Pai-is, or opal glass with a polished
surface. But a disc of finely ground glass dropped into the diaphragm-
holdei- of the condenser will give a precisely similai- result.

Mr. A. Michael has, however, pointed out the curious fact that an
opalescent mirror becomes an inexpensive and excellent substitute
for a polarising prison.

Typical Modern Microscopes. — We are now in a position to care-
fully inspect the characteristics of the chief forms of microscope
which the modern manufacturers of England, the Continent, and
America offer to the microscopist.

We confine ourselves to the chief models, indicating more or less
suggestively their merits or defects. We neither discuss all the
instruments of any maker nor in eveiy case even one instrument of
some makers. This would involve simple repetition in the main
features. The reader can comj)are for himself the microscope of any
given maker from whose catalogue he proposes to select, and can
discover by comparison its incidence or otherwise with the type
given here to which it corre^j)onds.

Beginning with the highest types we place first on the list Powell
and Lealand's No. I. This insti'ument may claim a seniority ovei'
all the foremost instruments, because for nearly fifty years it has
practically remained the same. All its piincij^al features were
brought to their present perfection netirly fifty years ago, while all
other microscopes during this period have been redesigned and
materially altei'ed over and over again. This is no small commenda-
tion, for during that period, as the reader so well knows, the aper-
tures of objectives have been enormously enlarged, and with this
has come a great increase of focal sensibility. As a i-esult the
majority of the microscopes of forty years ago are absolutely useless
for the oljjectives of to-day, but the focussing and stage movements
of Powell and Lealand's microscope still hold the first place.

Fig. 157 represents the instrument in its monocular form. Tlie
foot of the stand is a ti'ipod in one casting ; it has an extended base
of 7 X 9 inches, forming at once the steadiest and the lightest foot
of any existing microscope. The feet are lalugged with cork, and
vvlien the Ijody is in a hoiizontal position the optic axis is (as it
should be) 10 incluis from the tiible.

'V\u'. coarse adjiistincnt is effected by a, bai', consistiiig of a, mas-
sive gun-metal tnmcated prism in form, wiiicii hears only on a.
nai'i'ow part Jit tlio angles. Jt extends sulficieutly to focus a,

POWELL ANI> LEALAND'S BEST STAND 1 95

4-incli objective. The arm which cai'ries the body is of unusual length
for the type it represents ; but this gives a large radius from the
optic centre of the instrument, and makes the complete rotation of
the stage easy. Gi'eat eflbi'ts have been made to accomplish this in
other instruments. The older Ross form fi'om the shortness of the
arm only allowed of a tAvo-thirds i-otation, and in the Lister model
many different devices have been tried, the latest being the placing
of the stage pinions in a vertical position above the stage, which is
an unquestionable eri'or.

The rotation of the stage in the Powell and Lealand model is by
means of a milled head most conveniently placed, and the divided
circle is on a plate of silver.^ It will also rapidly rotate by hand.

The arm is on a pivot, which allows it to be turned away from
the stage altogether, and, as we have already indicated, the length
of the arm lent itself to the use of a longer lever for the fine adjust-
ment (p. 174). The milled head is placed behind the strong pivot
of the arm, where vibration is impossilDle, and it is in an easy and
natural position for the access of either hand.

The body may be, with great ease, entirely removed from the arm ;
this makes the use of the binocular or monocular body or of a short
or long body a matter of choice, while it gives access for cleaning and
other purposes to the nose-piece tube, as well as for the insertion
and focussing of the lens used with an apertometer,^ or an analysing
prism. So also it is of service in low-power photo-micrography.

We have already referred to the stage of this instrument ;
liut it may be briefly stated that it is large, has complete rotation,
it has one inch of i-ectangular motion, being graduated to the i^th
inch for a finder. There is the samx speed in the vertical and the
lateral movements, and the pinions do not altei- their positions. The
aperture of the stage is amply large.

The ledge of the stage has a stop placed on its left-hand
side ; this is held by a screw, but is removable at pleasure.
Two massive brackets under the stage remove all possibility of
Hexure,

The sub-stage has rectangular movements by screw in either dii-ec-
tion, as well as a rotary movement by pinion. The coarse adjust-
ment is by rackwork, and Sijliie adjustment is added when desired.
Fig. 158 illustrates this stage, showing its under side in order to
enable the fine adjustment to be seen.

The vertical and upper horizontal milled heads are centring-
screws, acting at right angles to each other, while the diagonal screw
to the left is the milled head, which causes the stage to rotate, the
whole acting with gi-eat smoothness and accui'acy, also enabling the
operator to centre with complete precision, while, as we have
already seen (pp. 187 and 196), the milled head A works by an
advancing cone the fine adjustment to this stage.

The mirror is plane and concave, with double-jointed arm.

The finish and workmanship of this instrument are of the highest
order. The seen and the unseen receive equally scrupulous care.

1 This is now made of platinum if desired, and thus tarnish is obviated.

2 Chapter V. p. 337.

o2

196 THE HISTORY AND DEVELOPMENT OF THE MICEOSCOPE

The present Editor has had one of these microscopes in constant, and
often prolonged and continuous, use for over twenty years, and the
most delicate work can be done with it to-day. It is nowhere
defective, and the instrument has only once been ' tightened up ' in
some parts. Even in such small details as the springing of the
sliding clip — the very best clip that can be used — the pivots of the
mirror, and the carefully sprung conditions of all cylinders intended
to receive apparatus, all are done with care and conscientiousness.

An instrument of this kind may be made to appear perfect to
the eye, but at the same time may lack some most important elements
as a finished instrument. But this is an instrument of the highest
order as such, and at the same time a very fine specimen of highly
finished brass work.

A note must he made before
leaving this microscope upon the
size of the tubes in the body and
the sub-stage.

Powell and Lealand were the
only makers whose gauge of
tubing had a raison cVetre ; the
size of the tube was such that it
would take in a binocular body a
Huyghenian 2-inch eye-piece,
having the lai'gest field-glass pos-
sible. The size of this field-glass
depends on two factors.

1. The distance between the
centres of the eyes.

2. The mechanical tube-
length.

In order that the binocular
may suit persons with ' nari-ow
centres ' to their eyes, the dis-
tance between them should not be greater than 2-^ inches. The
mechanical tube-length is 8| inches for the standai'd tube. When
the eye-pieces were 'home' in their places in the tubes they just
touched each other, the inner sides of the binocular tubes being cut
away ; so under the above conditions a largei- field than is thus
obtained is simply impossible. The size of the field-glass detei'-
mines the size of the eye-piece, and that was made to fix the
diameter of the body-tube.

Veiy wisely these makers made the tube of the sub-stage the
same size, so as to have one gauge of tubing throughout. This
allows a Kellner or other eye-piece to be used as a condenser, th\is
reducing the number of adaptei-s.

Lately this fii-m have altei-ed theii; sub-stage tube to a gauge
recommended by tlie Royal Mici-()sco2)ical Society. This involves
an adapter where tlie su})-stage ajjpaiutus was a,da|)ted to the ohl
gauge, or when an eye- piece is used as a condenser; as the size is
too large for a Inuoculai-.

The ItoHH laodel, in its (■(^mplctcst form as left l)y Andrew Ross,

Fig. 158. — Powell and Lealand's sub-stage
with fine adjustment (1882).

T. BOSS'S MICEOSCOPE

197

except specially oi'dered is never made by this firm, but for its
qualities and historical relations it is of much interest. It was

Fig. 159.— The model by T. Boss (1862).
very similar to the model by T. Ross shown in fig. 159. A. Ross's
first model had a triangular bar, was monocular, possessed no
proper sub-stage, the condenser was attached to the main stage.

198 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

which was without ari'aiigement for rotation ; and. the mii^ror was
not jointed. The model of T. Ross had, as will be seen, a bar move-
ment, with a foot foiined of a triangular plate to which were bolted
two parallel upright plates to carry the trunnions of the microscope.
The fine adjustment is a lever of the second order, with the milled
head in the middle of the bar, which involves tremor, and the tube of
the nose-piece is short, making shake possible.

The stage movements are of unequal speed, the lateral move-
ment being slower than the vertical. There is no finder, and the

rotation of the stage is
but partial. The sub-
stage and mirror are
good. It was a com-
manding instrument in
its day, and was of ex-
cellent workmanship
and finish ; but it was
not equal to the stiain
of critical work with im-
mersion objectives of
great aperture. Never-
theless the defects of this
stand could have been
readily corrected. With
a more extended base, a
better arrangement of
the fine adjiistment, a
mechanical stage con-
structed on better prin-
ciples, and the rotation
made complete and con-
centric— which it was
not — this would have
been, even for our pre-
sent requirements, an
admirable insti-ument.

This impoi'tant fiini
were otherwise advised,
however ; and, instead
of coi'recting the eri'ors
of the insti'ument whose
history they had made, they designed an entirely ne-w model in whicli
a Lister limb was substituted for the bar movement. Fig. 1 60 illus-
trates this form of the instrument, from which it will be seen that the
foot also was changed for the woi-se ; the base was not suHiciently
extended, and the hindei- part of the foot was too large, so that it
sometimes rocked on four points, because the hinder ptirt wa« too
wide — a fiat surface, in fact. A true tripod will stand fii-m on an
uneven table, but this form will not. It is a form frequently used by
various makers now, and is known its the ' bent claw.' Tt is a bad
design, and may be, as it hjis been, easily thrown over laterally. It

Ross-Zentmayer model (1878).

THREE CtEEAT TYPES OF MICKOSCOPE 1 99

Avas, however, eventually cast in one piece, which gave it a solidity
which the former did not possess.

The introduction of the Lister limb brought its inevitable
troubles — notably, with the fine adjustment — to which we have fully
referred under that head. But in the Ross-Zentmayer model, a
later form, the body and the coarse adjustment were both cariied by
the fine-adjustment lever and screw.

This form could not — as it did not — long jDrevail. Its existence
was ephemeral, and in its place was put a modification of the form
devised by Zentmayei-, known subsequently as the Ross-Zentmayer
model. This was the Ross- Jackson instrument -with a ' swinging
sub-stage.' This instrument is illustrated in fig. 161. It will be
seen that the foot is a true tripod, consisting of a triangular base
with two pillars rising from a cross-piece, which carried the trun-
nions.

Here it may be as well to point out the differences which exist
between the three great types of microscope, viz. the bar move-
ment, the Lister limb, and the Jackson limb. In the bar movement
we find a transverse bar uniting the lower end of the body to the
coarse adjustment bar (figs. 157, 159). In the Lister the body is
supported through a greater or less portion of its entire length,
the limb being formed of one solid casting (figs. 160, 161, 162, 167).
In the Jackson the dovetailed groove which carries the sub-stage
slide is included in the casting, and the groove for the coarse ad-
justment of the body, as well as that for the sub-stage, is ploughed in
one cut (fig. 165). Jackson also designed the double pillar foot
(fig. 161).

We have already assessed the value of a swinging sub-stage and
found that in our judgment it is at best redundant and really
adverse to the accomplishment of the best scientific work.^ No
microscope is complete without a good condenser ; all and much more
than all that can be done by a swinging sub-stage can be done with
a slotted stop at the back of the condenser. This elaborate appen-
dage is therefore without justification. Yet in the impatience for
large illuminating apertures, which v:ere not at that time provided hy
condensers, this phase of pseudo-illumination was carried to a still
greater and more elaborate development in the production of a con-
centric onicroscope. This was a Ross-Wenham, known as the radial
microscope. But elaborate and costly as it was it never justified its
existence, and like the whole group of ' concentric ' and ' radial '
microscopes, it has passed away simultaneously with the abolition of
' oblique illumination,' and is to-day a not very interesting curiosity
in the history of the modern microscope.

A large and extremely well-finished stand is made by Messrs.
Watson, known as the Van Heurck microscope in its best form : it is
illustrated in fig. 162. The body has two draw tubes, one of which
is actuated by rack and pinion, and the other sliding inside it so
that a range of body length varying fi-om 142 mm. to 300 mm. can
be obtained. The coarse and fine adjustments have very wide
bearings, and the exact relationship of the pinion to the rackwoi-k

1 P. 188 et seq.

Fig. 161 (1878).

^~iiiilK

i

Fict. 160. — Section of bearings
and fittings of pinion.

Fig. 162.— The grand model Van Ileurck. Watson and Sons (1895).

202 THE HISTORY AND DEVELOPMENT OF THE MICEOSCOPE

is established by means of a block of metal Avhicli fits upon the
pinion shaft and is pressed or released by means of the two screws
provided for the purpose. This is shown in section in fig. 163,
where the pinion is P, the anti-friction block IST, and one of the
adjusting screws M. The perspective view of the coarse adjustment
showing the adjusting screws is given in fig. 164.

The stage can be completely rotated and has mechanical move-
ments on the Tarrell principle, both milled heads being on one axis.
The sub-stage has a fine adjustment, and the plane mirror is care-
fully worked by hand, while exceptional rigidity for the whole stand
is obtained by a special system of construction, and the tripod, which
is shod with cork, has a spread of ten inches.

A high-class stand of distinguished merit is made by the firm of
Baker of Holborn. It is illustrated in fig. 165, is made with great

Fig. 164. — Complete view of Watson's coarse adjustment (1895).

care and is an instrument of precision. It is mounted on a solid tripod
with slotted toes so that it can be fii'mly clamped to the baseboai'd
of a photo-micrographic apparatus. The body is mounted on a mas-
sive limb in one piece thi-oughout, and on to this the stage and
sub-stage are mounted ; in this way the chance of derangement of
the optic axis is reduced to a minimum. The body has diagonal
rack-and-pinion coarse adjustment actuated by very large milled
heads, making a slow movement easy. The fine adjustment carries
the body tube only each revolution of the graduated milled head,
being equal to the -.jlrd^^ of an inch ; the Campl)ell differential screw
being employed, and the milled head being ])laced at the lower end
of the body. The Ijody can l)e extended to 300 mm. and closed to
1 50 mm. The mechanical stage is woi'ked on the Turi'ell method
by .stationary milled heads woi-king on a common centre commanding
oV>li<jue as well as rt'ctangulai'iiioveinents ; the rectangidar movements
on divided .silver plates for recording positions ; and complete I'ota-
tion cun be secured, either 1)y luiiid or l)y rack and ])inion, and can
be at !Miy })(Mnt clamped.

The .siib-st,a<'e has icc1;iiii;iil;ii' iiiecJiaJiical movements controlled

C. BAKEE'S MODEL

203

by fixed milled heads, and all fittings are sprung and have adjusting-
screws to compensate for weai-, and fine adjustment by Campbell diffe-
rential screw which Mr. Baker adopted for these microscopes in 1888.

Fig. 165.— C. Baker's Model (1895j.

Swift and Son formerly made two instrvnnents of the first class,
one having a bar movement similar to that of Andrew Ross, the
other a Lister similar to Beck's. The principal difference was that
the foot was of the ' bent claw ' form. We have already seen that

204 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

by their invention of the vertical lever fine adjustment (figs. 133 and
135) Swift and Son have made possible a useful future for the Lister
limb ; and their model of this form is shown in fig. 166, known as
the ' Best " Challenge " Microscope.' It has a beautifully made coarse
adjustment, the special fine adjustment invented by this firm, a cir-
cular rotating stage moved by rack and pinion or by hand, and is
provided with divided silver plates to the rectangular movement.
The sub-stage is complete for centring as well as focussing, and has
i-otary movement for use with polariscope. The stand is a firm
form of tripod, and the mirrors ai-e well worked and mounted on a
double crank.

All the movable parts of Swift's instruments are sprung on Powell
and Lealand's method, and the movements are smooth and sound.
Many stands had been devised by American oj^ticians up to the time
of the publication of our last edition of this work, but they were
based upon one or other of the great English models, and the modi-
fications, whether for good or evil, were adopted into the then modi-
fications of the older English types, and were incidentally described.
It should be remembered that Zentmayer, of Philadelphia, devised
the model from which the Ross -Zentmayer was finally formed. Its
principal feature was to obtain oblique illumination in one azimuth
by the swinging stage which we have emphatically shown in this, as
we did in the last edition, to be a pei'nicious adjunct for practical
purposes. The fine adjustment of this instrument was most defec-
tive. Tolles, again, who wholly deserves the very high reputation he
attained, made an instrument in which he mounted the stage on a
disc ; near the edge of this disc the sub-stage is made to travel in a
groove carrying the condenser, or dry combination, in an arc round
the object as a centre. This was only another elaboration of the
same swinging sub-stage.

In later constructions of this form, Tolles first used the mechanical
stage actuated by two pinions vertical to the sttrface of the stage,
and subsequently adapted by Ross. The fine adjustment in this
instrument had the fatal defects characteristic of its form.

Bulloch, anothei' American maker of note, made some modifica-
tions in the Zentmayer model, but they were in the interests of the
swinging sub-stage, and, although no doubt ingenious, mvist pass
with this transient form of tlie microscope.

A modification of this stand was devised by Bulloch ; it presents
no special point, save the employment of a Gillett condenser with
tlie diaphiugm drum, above the lenses I

A later development of this form of instrument by the same
maker was made some years aftei-, l)ut tlie chief difi'ei-ence consists
in the adoption of a stage in which the milled heads stand ujxm
the stage, which is the I'cverse of jin advance. Since, however, the
swinging sul)-stage form of instrument has been entirely superseded,
Aniei-ican makers liave iidoptcd, wit.li very sliglit modifications in
form, none in ])i-inciple, tlie CDntiiicntal stand, wliich is made with
a(hiiiral)le precision and conscicfutious care, ])ut still I'etains its chief
f(!atures. It may tlierefore 1)(^ of sei'vice to consider the principal
lecent modifictitions of tlie Continental stand, so that they may be

SWIFT'S BEST CHALLENGE MICEOSCOPE 205

Fig. 166.— Swift's best cliallengejiiicroscope (1881).

206 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

fairly compared with equally recent American adaptations of tlie same
microscope ; and then endeavour, after examining instruments of
a lower class, to give a dispassionate estimate of this model as com-
pared with that of the highest-class English type.

Amongst Continental makers the firm of Zeiss has taken a fore-
most position and has secured a well -deserved world-wide fame.
Their largest microscope is shown in fig. 167. It is a model of fine
workmanship and has been adapted with singular ingenuity to the
reception of all their accessory apparatus. The uj)per body is
inclinable from the vertical to the horizontal position. It is
provided with coarse rack-and-pinion adjustment, and fine adjust-
ment by means of a direct acting micrometer screw with divided
head. The sub -stage takes all the apparatus provided by this firm,
and in addition it may, by means of a small lever, be swung out of
its central position, ' so as to facilitate rapid transition to illumina-
tion with the cylinder-diaphragm,' while below the condenser is a
movable iris diaphragm fitted with a rack-and-pinion movement to
throw it out of the centi-e, and which can be rotated about the axis
or entirely swung out.

The circular object stage rotates (not by rack and pinion), but
has centring screws. The apertui-e in the stage has received a
more oval form. The rack-and-pinion rectangular movements are
1^-in. vertical and 2-in. lateral ; the milled heads are small but
efficient and work smoothly. That for transverse movement being-
placed upon the top of the stage.

Reichert, of Vienna, makes a stand which in the main cor-
responds with that of Zeiss, and we are enabled to speak with
confidence of the high quality of the workmanship) ; but in illustra-
tion we choose not the 1a stand but the large stand know^n as IIb,
an illustration of which is given at fig. 168. Our object in choosing
this instrument is that it combines every essential of the 1a stand,
and in addition is furnished with the new lever fine adjustment,
invented so recently by Reichei-t, and of whose value we have
already given our ju^dgment. It will be seen that on the part of the
body which the fine adjustment milled head crowns there is a
protrusion on the right and left hand side of the pillar. This is the
only addition outwardly that the new fine adjustment makes needful.

A very high-class microscope is made hj Leitz of "Wetzlar, which,
while it i-etains the principal features common to all microscopes
based on the Continental model, Iims yet qualities peculiar to itself,
nud obtains by means of wf)rkmiinship and ingenuity the most ad-
mirable I'esults attaiiiJible from the model on which it is based. It
is inclinable with a liinged joint and clanqjing lever ; and the stage
is provided with a revolving centi-ing talile. The mechanical stage
is the 'attachable' one ah-eady described, and the adjiistment of the
objective is by i-ack-and-pinion coarse adjustment, and by a fine
adjustment depending on a micrometer screw pi-ovided with a
divided screw head. The draw-tube is fnnn'shed with a millimeti'e
scale. The sub-stage is planneil un tlic princi[)le of the Zeiss
instrument and will receive the illtiiiiinating appjii'iitus as devised
by A\)h(i, which is worked by luck-iiiid-pinion adjustments, which

Fig. 167.— Zeiss's largest and complete stand (1895).

208 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

also raise and lower the iris diaphragm and provide it with possible
oblique or eccentric movements ; and it is furnished with objectives

I-'k). lOH. — Oik; oi l'v,i-i(--li(;rl'H larK*' kIiukIs (JIlij witli ii(;vv lever lin<' ailjustmoiit
fitted (189'.)).

Fig, 169. — Leitz's most comj)lete stand (1893).

2IO THE HISTOEY AND DEVELOPMENT OF THE MICEOSCOPE

ami eye-pieces that give it magnifying powei'S ranging from 15 to
1,500 times. This instrument is shown in fig. 169, and with the two
stands immediately preceding it furnishes us with a fair view of
the principal and latest types of the Continental microscope fitted
with the apparatus essential to the production of good work.

But another most interesting model of Reichert's has just been
finished which, from its size and approximation to the English
stand in some important points, we are consti'ained to notice as
these sheets are passing through the press. The instrument is
illustrated in fig, 169a. The height of the stand in the position
illustrated is 16^ in. The distance between the foot and the stage is
3^ in. The sub-stage is jDrovided with centring screws, and is raised
and lowered by rack and pinion. The mirror can be readily moved
towards or away from the sub-stage or can be entirely removed.
The tube length with both tubes (A A') extended, including the nose-
piece, is 10^ in. The stage is mechanical, and the circle is divided into
360 degrees ; both the horizontal and vertical motions of the stage have
scales read by verniers. The object is fixed on the stage by spring
fittings. The fine adjustment has two speeds of motion by two screws,
the one 0"3 mm., the other 0"1 mm. per revolution, shown at M M'.
The draw-tube has a divided scale and is moved by rack and pinion.

We may now with advantage consider the diflerent classes of
microscopes manufactured by the opticians of Europe and America.
To do this without prejudice and with efliciency it is necessary to
designate the characters which should distinguish each class.

Microscopes placed in Class I. possess —

1. Coai'se and fine adjustments.

2. Concentric rotation of the stage

3. Mechanical stage.

4. Mechanical sub-stage.
Class II.

1. Coarse and fine adjustments.

2. Mechanical stage.

3. Mechanical sub-stage.
Class III.

1. Coarse and fine adjustments.

2. Plain stage.

3. Mechanical sub-stage.
Class lY.

1. Coarse and fine adjustments.

2. Plain stage.

3. )Sub-.stage fitting (no sub -stage).
Class y.

1. Single adjustment (coarse or fine).

2. Plain stage.

3. With or without sub-stage fitting (no sub-stage).
This cla.s.sification applies also to portable mici'osco})es.

The recent microscopes of the best American makers are
characteri.sed by the highest quality of wf)rkin;nisln'p and abundant
ingenuity, but the Continental model is confessedly made their founda-

RECENT AMERICAN MICROSCOPES

211

tioii. In the last edition of this work it was shown that American
opticians made their first-class microscopes with swinging sub-stages,
and we then pointed out that these were not only without value,

Pig. 169a (1900),

p2

212 THE HISTOEY AND DEVELOPMENT OF THE MICKOSCOPE

but injurious to the best work possible to a good instrument. In
the interval the swinging sub-stage has been given up, even by its
most ardent advocates ; but at the same time in the majority of
cases they have abandoned the sub-stage proper and adopted the
Continental condenser fitting instead. In fact, the American
ojDticians have chosen almost exclusively, as the basis of their stands
of every class, the microscope that has been so long in A^ogue on
the Continent of Europe.

It will suffice to take examples of the unexceptionally beautiful
work of the two leading opticians of America — The Bausch and
Lomb Optical Company and The Spencer Lens Company. An
illustration of the best instrument, known as the ' Grand Model,' of
the former of these opticians is given in fig. 170. It is designated
a ' Continental Microscope,' but is not a mere copy of the best work
of Germany or France. The body-tube is large, and the horseshoe
base, of Continental fame, is said by the makers to be improved b}^
the ' back claw ' being prolonged ' so as to virtually form a tripod
base,' and it is commended as ' extra heavy.' From the figure, how-
ever, it would appeal' to be the extra weight rather than a pro-
longed claw that imparts the steadiness. The body is supported
on a pillar of two massive columns. The stage is large, and rotates
with centring screws. ' The heads of the centring screws are
provided with graduations and index, and with a series of lines
recording the number of revolutions of the screw,' so that the
position of any given object may be recorded and thus be referred to
again if the microscope should have been used for othei' work in the
interval. The mechanical stage is woi'ked by one milled head at
the side and the other at the top of the stage, the latter position (as
we pointed out in the last edition of this book when referring to the
Tolles mechanical stage) being one in ,which the efficiency of the
mechanism is reduced to its lowest value. We have long advocated
the adoption of Turrell milled heads as employed in Powell's No. 1
stand ; they give the woi-ker power to effect not only rectangulai'
but diagonal movements, and, without displacing the fingers, to
work the stage in all directions. We ai'e pleased, as we have
pointed out, to note that the eminent firm of Zeiss have a,dopted
these in their best stand (fig. 139).

The sub-stage is composed of three parts, arranged one above the
other. This sub-stage, with the parts separated to show their
construction, is presented in fig. 171. The upper part is a ring
carrying a removable iris diaphragm, so arranged as to come
directly into contact witli the under part of the object slide. The
middle section of flic sub-stage is movable vertically on the main
sub-stage axis, and carries an Abbe condenser of 1*20 N.A., which
can be swung laterally to the left of the instrument so as to put it
out of optical use ; but on the other hand it can at will be tlu-own
V)ack into position and placed in oil contact with the object slide with-
out altering the position of the upper ii-is diapliragm. Tlie third and
lowest section of tlie sul)-stage cairies the large ii-is diajihi'agm used
I)(;low the condenser. 'I'lius it is clear that the whole can be used
together, or anv oin' of llu' three sections can be worked sejjarately.

Fig. 170.— Bausch and Lomb's 'Grand Model' (1897).

214 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

We note one aclrairable feature of the mechanical finish of the
microscopes of this firm, which is, that they avoid sharp angles and
knifelike edges to all their instruments. This looks a trifie, but the
use of the microscope with saprophytic, pathogenic, or other infective
material requires the utmost caution that the skin of the hands
should be unbroken, and there can be but little doubt that all
unconsciously the edges and corners of microscopes finished to the
just pride of the meclianic do often break the skin, and are wisely

Fig. 171. — Bauscli and Jjomh's sub-stage, separated to show construction.

and liHp[)ily worked into rounded edges in the instruments of these
distinguished niiikers, and, we mny add, witliout the slightest loss
of tliat appeariiiice of high finish which has always been cori'elativ(^
witli tlie manufacture of microsco))es.

If we now look at the No. 1 .stand of the Spencer Lens Company,
of liiiffalo, N.Y., we .shall find Mgain that the mo(l(^l of Oberhauser
is adhered to and the instrument is of tlic lliird class. This
microscope is illustrated in fig. 172. It is bciutiriilly made, and
the horseshoe biise 1ms m slill longci' 'cliw' tli;iii tlioscof l>;ins(;li,

Fig. 17-2.— The Spencer Leiis Company's Continental form No 1 (1896;

Fio. 173. Wutsou's Edinburgh Student's; wtund "H ' (vvitli horsi^slioe foot 1880,
with tripod foot 1893;.

EECENT AMEKICAN MICKOSCOPES

217

to give the stability required in utilising the hinged joint foi-
inclination of the body, which stands on a strong unial pillar.
The sub-stage is movable by a quick screw ; in other featui-es it
resembles the majority of the microscopes of the type to which it
belongs ; it is, however, distinguished by rovxnded in contrast
to sharp and pointed corners and edges ; and, although the form
presented has a plain stage with clips, it can be furnished with
a circular revolving centring stage, or Avith an ' attachable ' stage
made by the Spencer Lens Company, having all the advantages of
the several forms of these pieces of apparatus ali'eady described.

Fig. 174.— Baker's Model, No. 2 (1898). ,

We note with some surprise that such accomplished manu-
facturers and opticians have indicated, so far as we can discover, no
advance in their sub-stage condenser beyond that of the now old
achromatic of Abbe, and that there is no evidence before us of their
employment of a sub-stage fine adju.stment, both of which have been
found of such great practical value in England, and which have been,
as we shall shortly show, adopted for the more critical microscopical
work by the Messrs. Zeiss, the leading optical firm of the Continent.

2l8 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

Second-class microscopes are made in great variety by English
makers.

One of the finest examples of this class of microscope at present
brought within the reach of the average student's means is that
known as the ' Edinburgh Student's Microscope " H," ' by the fiinii
of Watson and Sons. It is the most complete of a series of similar
stands varying in cost and completeness. It is illustrated in
fig. 173, where it will be seen that it has the first prime requisite, a
rigid foundation combined with lightness — a tripod having a spread
of 7 inches — and it is also possessed of a well-constructed mechanical
stage which is built with the instrument, an advantage over the
best ' attachable ' stage.

It is essentially a student's microscope, and although of so low a
price is not only a specimen of the best workmanship, but is also
extremely complete and represents an advanced type of construction
capable of doing all ordinary and much experimental work.

Belonging to this class is an instrument by Baker known as
his Model, No. 2. It is smaller than the 'A' stand of the same
type and is simplified, but is capable of doing the most refined and
critical woik. It is illusti-ated in fig. 174. The coarse and fine
adjvTstments are the same. The mechanical stage has rectangular
movements of one inch ; the Tun-ell arrangement is not adopted ;
but the whole stage can be rotated through an arc of 300°. The
sub-stage has diagonal rack and pinion focussing movements with
centring screws, and can be supplied with every improvement
applying to the adjustment of the sub-stage. Taking this instru-
ment MS a Avhole — the thoroughly ]3ractical character of the model,
the high quality of the workmanship, the fact that it will take all
the optical apparatus of the best model, and that all fittings are
sprung and possessed of adjusting screws to compensate for wear —
we have in this microscope one of the very best of its class.

Powell and Lealand make an instrument of this class, having
a quality of work not second even to their large stand. It is
illustrated in fig. 175. The tube length is the same, but the stage
and the foot are smallei- than in the lax-ge instrument. There is
no rotary movement to the sub-stage, and its centiing is done by
the crossing of sectoi-s and not lines at i-ight angles ; but this is in
no way a defect. All the movements and adjustments are other-
wise as in No. 1.

Bakei', of Holborn, makes a veiy admirable and viseful instru-
ment of this class known as his D.P.H. microscope. No. 1. It
has a diagonal i-ack rrnd pinion coarse movement, a micrometei- screw
and lever fine adjustment, giving a movement of ^,-i r/ of an inch for
each revolution f)f the milled head ; a draw-tube, every 10 mm. of
which is engravf'd with a ring, extending to 250 mm. and closing
to 150 mm., thus ;i]lo\\iiig tlie use of either English or Continental
objectives; it possesses;! mechiinical stage givingamovementof 25 mm.
in either direction, graduated to \ mm. ; themill(H] head of the trans-
verse motion is l)elow the level of the toj) plate, and as the other is
removai)]e liirge cidtui^e jilates (;an l)e exnniin(;d, the distance from
optic axis to liml) {'!], in.) nlluwing of tlicir easy miiiiipulntion ; the

BAKEE'S NEW MICEOSCOPES

219

top plate is provided with three adjustable stops, so that the centre of
a 3 X 1 or 3x1^ slip is identical with the optic axis when both
the rectangular movements are at the centre of their travel, thus
enabling any desii'ed field to be recorded ; the stage clips are

Fig. 175 (1852j.

mounted on two of these stops, all of which are removable ;
a centring sub-stage of universal size (1'527 in.) with diagonal rack
and pinion focussing adjustment, plane and concave mirrors ; the
whole mounted on a solid tripod stand, with a bracket to support the

220 THE HISTORY AND DEA^ELOPMENT OF THE MICEOSCOPE

instrument in a horizontal position for photo-micrographic woi-k.
Tlie microscope is illustrated in fig. 176,

A modification of this instrument was brought out as these
pages are passing through the press, which is entitled to rank as a
first-class instrument. It is known as the R.M.S. 1-27 gauge
microscope, and is illustrated in fig. 177. It has a diagonal rack
and pinion coarse movement, and a micrometer screw and lever
fine adjustment giving a movement of O'll mm. (2^5 in.) for each
revolution of the screw, the m.illed head of which is divided into

Fig. 170. -Baker's D.P.H. stand No. 1 (189'J).

ten parts, eacli division being numbered. It also possesses two (ir;iw-
tubes engrjived in mm., every tenth numbei-ed, one of whicli is
pi'ovided with rack and pinion adjustment, so that objectives m;iy
be corrected for the thickness of the cover glass, &c., by the alteration
of the tube length ; these draw-tubes extend to 250 mm., and close to
120 mm., eitlier English or Continental objectives can be used ; this
microscope has a rotating mechanical stage giving a movement of
2o mm. (1 in.) in eitlier direction graduated to ^ mm. (.',j in.) ;
tlir- milled ]ie;id of tlie transverse motion is Iji'Iow tlic level oftlie

BAKER'S LATEST MICROSCOPE

221

top plate, and the other being removable a lai'ge flat stage becomes
available if required ; the top plate is provided with three stops,
adjustable, so that the centre of a 76 mm. X 25 mm. (3 in. x 1 in.)
oi''76 mm. x 38 mm. (3 in. x l-^ in.) slip is identical with the optic
axis when both the I'ectangulai' movements are at the centre of their
travel, thus enabling any desired field to be recorded ; the stage

Fig. 177.— Baker's E.M.S. 1-27 gauge microscope (1900).

clips are mounted on two of these stops, all of which are removable.
It has a centring sub-stage provided with diagonal rack and pinion
focussing movement, and a fine adjustment, the milled head of which
is so placed that both adjustments caii be conveniently conti-olled
without shifting the hand, and it is provided with plane and con-
cave mirrors, and the mici'oscope is mounted upon a solid ti-ipod
stand, with a bracket to support the instrument in a horizontal
position for photo-micrographic work.

222 THE HISTORY AND DEVELOPMENT OF THE MICEOSCOPE

All the fittings are sjDrung and have adjustmg screws to compen-
sate for wear.

Coming now to Third-class microscopes, we note that the dis-
tinguished American fiim, Bausch and Lomb, make a very useful
instrument which must be placed in this class. It is intended as

Pig. 178. — Bausch and Lomb's C.A.3, microscope (1897).

a high-class laboratory instrument for advanced work and for use
in independent researches. It is designated by the firm as the C.A.S.
It has a large stage, but in our judgment this would be greatly im-
proved by being furnished with the horseshoe opening so valu-
able for hand focussing as a preliminary in the use of high powers
and immersion lenses. Of course the mechanical stage of the

THIED-CLASS AMERICAN MICEOSCOPES

■^-^0

firm can be added. The sub-stage is the new and complete one of
the makers, ai-ranged foi- doing critical work ; the fine adjustment

Fig. 178a. — Eeicliert's 'Austrian' Baugii stand (1899).

is by micrometer screw ; the weight of the body is balanced, the
makers tell us, by a spii-al spring which, they believe, subjects the fine

224 THE HISTOKY AND DEVELOPMENT OF THE MICKOSCOPE

micrometer screw only to the friction of the adjustment — and, of
course, it is to be noted that the screw is not an extremely fine
one ; and the makers have evidence of the durability of the adjust-
ment, as aftei- five years of use they have had no single instance of
its breakdown. The coarse adjustment is by diagonal rack and
pinion ; the draw-tube is graduated. It is beautifidly made, and is
by no means an expensive instrument. We illustrate it in fig. 178.

A well-made and remarkable little instrument of the class we
are considering is manufactured by Reichert, of Vienna, known as the
Austrian stand. It is illustrated in fig. 178a. It is the most
modified of all the microscopes we know based on the Continental
model ; it certainly approximates in several points to the English
type. It has a specially extended and steady horseshoe foot, and is
the only strict Continental form with the axis so high up. The re-
sult is that the body is balanced when in a horizontal position. The
coarse adjustment is by spiral rack and pinion with milled heads.
The fine adjustment is Reichert's recent patent, giving extreme
delicacy to the movement, and having a movable pointer, i, for
reading divisions on the micrometer screw. It is provided with a
double rack draw-tube shown at B, it carries the Abbe condenser in a
sub-stage that focusses by a screAv at the side, and centres by the
screw-heads, «, a'. In its most complete form it is remarkably
low-priced, and certainly will meet a demaiad, especially as the
English method of compensation for wear and tear is adopted.
This, indeed, is the case with all but the lowest-priced instruments
of this maker, and we believe him to be the only Continental
manufacturer who has adopted the sprung slots and screws so long
used with success by English makers for compensating wear. We
should have suggested slotting the edges of the stage for sliding
the object-holder or ledge, but we learn from the maker that this
is to be done in all futui'e instruments ; all but the smallest stands
Reichert is willing to provide with English pattern sub-stages
fitted with centring screws of the standard size, and condensers are
mounted to suit these.

Another instrument of the same class and general designation,
made by Messrs. Watson and tSons, and distinguished as ' G,' is shown
in fig. 179. It is identical in build with the C model, but the
stage is plain, and it has only a tube fitting for a sub-stage appa-
ratus ; the workmanship is of the same order, the movements as
delicate and true, the adjustments as reliable, but the price is only
one-half that of the more complicated form.

Amongst the same class of instruments must be placed another
by Messrs. Swift and Son. It is known as an ' Improved " Wale's "
Microscope.'

Mr. George Wale, of America, devised in 1879 a plan of great
merit for the stands of microscopes. The ' limb ' which carries the body
and the stage, instead of being swung by pivots — as ordinarily — on
the two lateral supports (so that the balance of the microscoj^e is
greatly altei-ed when it is much inclined), has a circular groove cut
on either side, into which fits a circular ridge cast on the inner side
of each support, as shown in fig. 180. The two supports, each

WATSON'S 'G-' MICKOSCOPE

225

having its own fore-foot, are cast separately (in ii^on), so as to meet
to form the hinder ' toe,' where they are held together by a strong-
pin ; while by turning the milled head on the right support the two

Fig. 179.— Watson's Edinburgh Student's; stand 'G- ' (1893j.

226 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

are drawn together by a screw, which thus regulates the pressure
made by the two ridges that work into the two grooves on the limb.
When this pressure is moderate, nothing can be more satisfactory
than either the smoothness of the inclining movement or the
balancing of the instrument in all positions ; while, by a slight

Pig. 180. — Swift's improved 'Wale's' microscope (1881 and 1883).

tightening of the screw, it can be firmly fixed either horizontally,
vertically, or at any inclination. The ' coarse' adjustment is made
by a smooth- working rack ; the fine adjustment is Swift's patent
described on p. 172 (fig. 135), and the attachable mechanical stage of
this firm can be readily added (as in fig. 180), but in the best and

LEITZ'S ENGLISH FOKM OF MICEOSCOPE

227

most complete foi'm of the instrument a large mechanical stage is
fitted, and sub-stage apparatus supplied.

Leitz, of Wetzlar, provides a vei-y useful instrument of the same

Fig. 181.— Leitz's 1a stand (1898).

q2

228 THE HISTOEY AND DEVELOPMENT OF THE MICEOSCOPE

class. It has a tripod base on the English model, and is a thoroughly
steady instrument ; it has rack and pinion movement to the coarse
adjustment, and sub-stage ; the draw-tube has a mm. scale, and a fine
adjustment of the usual Continental type, and all the latest adaptations
for sub-stage illumination. The instrument in its simplest form is
remarkably low-priced, and the more important apparatus can be
added to it as required. It is illustrated in fig. 181.

Beck's third-class microscope is shown in fig. 182. It has a good
tripod foot with a single pillar. The Jackson model is used, but
a peculiar fine adjustment is employed, the lever being placed below
the stage, the position of the screw being immediately behind the
pillar which supports the limb, and where it is easy of access. The
body is not affected by vibration when it is touched. The lever is
of the second order, and it draws down the body limb and coarse
adjustment. In fact, save in its fine adjustment, this form ap-
proximates somewhat to the Continental model. The fine-adjust-
ment lever is i-ather short, but it will be found to be much steadier
and slower than the direct-acting screw.

The stage is plain, without mechanical movements ; but it has a
movable glass stage over the principal stage ; to this the slip is
clipped, and the whole s\iper-stage of glass is moved with ease
over a fair area. The aperture in the glass stage is not large
enough ; it should be cut right through to the front, which would
much increase its usefulness. *

This instrument also has a sub-stage with rack and centring
movements.

- Swift and Son's earlier third-class microscope in its most
suitable form dates from about the time of the vertical lever fine
adjustment patented by that firm {q.v.) It was first made from the
designs of Mr. E. M. ISTelson, and it presented three distinctive
features : —

(1) The milled head of the fine adjustment was placed on the
left-hand side of the limb.

(2) The stage was of a horseshoe form, the apei^ture being
entirely cut out to the front of the stage ; and

(3) The body-tube, which was of standard size, viz. 8| inches,
was made in two pieces, which not only secured portability, bvit also
permitted the use of both long and short tubes.

This instrument is ilhistrated in fig. 135. It was also possessed
of a cheaply made and fairly good centring sub- stage, to carry
Powell and Lealand's dry achromatic combination fitted with a tvirn-
out rotary arm to carry stops. The sub-stage was made by adapting
Swift's centring nose-piece, and providing it with a rack and pinion
focussing arrangement, as illustrated in fig. 183. There was also a
graduated stage-plate and sliding bar, a plan devised by Mr.
Wright for a finder. The eye-pieces were provided with rings, like
Powell and Lealand's, outside the tube to govern the depth which
each should slide into the draw-tube, by which means the diaphragm
is in the same place whatever the depth of the eye-piece employed,
and it was consti-ucted to do critical work with the highest
powers.

Pig. 182. — Messrs. E. and J. Beck's third-class microscope (1888).

2^0 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

Another form of this instrument has more recently been intro-
duced by the firm of Chas. Baker, of Holborn, London. It arose in
a suggestion by Mr. Nelson that this form should be adapted to the
Campbell differential screw fine adjustment, making a good quality
third-class microscope. It should be noted that the differential
screw permits of slow action being obtained by means of coarse
threads ; it is therefore very strong. In the ordinary Continental
form of direct-acting fine-adjustment screw, if the motion is slow,
the thread mxijst be fine. Hence in forms where the fine adjustm.ent
is made to lift the body, the differential screw is of great value.

Further, it proved on testing that the Campbell diflerential screw
was equal to the m^ost critical work, and could be used in photo-
micrography. As a result several additions were made, such as
rack and pinion focussing and rectangular movements to the sub-
stage and a rack- work arrangement to the draw-tube. Subse-
quently a larger and heavier instrument was made, having a \ inch
more of horizontal height. In this model the milled head of the
differential screw is placed below the arm, instead of above it, which

is an improvement for
photo-micrographic pur-
poses, and no special
detrim.ent in ordinary
work ; and, if required,
a differential-screw fine
adjustment can be fitted '
to the sub-stage. A
rotary stage is also some-
times put to this instru-
mient, but those which
we have seen have not
given the aperture sufii-
cient dimensions for
modern focussing.
This instrument in its complete form, as suggested by Mr. Nelson
and devised by Baker, gave origin to an entirely new group of
microscopes, which aimed chiefly at supplying the student with
relatively inexpensive instruments, but which at the same time
should possess all the qualities and be capable of receiving all the
apparatus needful for an efficient use of the microscope. One of the
higher forms arising in this new departure is the instrument shown
at fig. 177, and, with the Campbell screw fitted behind the mirror
for the fine adjustment of the condenser, is a very attractive and
useful microscope, and may be safely recommended to the amateur
and the student.

Two microscopes by Ross certainly deserve the attention of the
student seeking a reliable instrument belonging to the class we are
considering. They are both known as ' Ross's New Bacteriological
Microscope.' The Avork of this long established firm, it is needless
to say, is of the veiy finest quality ; and these microscojDes are pro-
vided with all the required adjuncts for the work they specify. The
stage is of horseshoe form ; the fine adjustment is sensitive and firm.

Pig. 183.-

-Centring nose-piece used as sub-stage
(1881).

ROSS'S RECENT MICROSCOPES

231

The principal difference between the two instruments is in their
respective stands. The one shown in fig. 184 gives a Avider spread
to the tripod base than usual, securing greater stability ; but this does
not involve great space in packing, becaiise the hind ' toe ' of the

Fig. 184 — -Ross's uew (tripod) bacteriological raicroscope (1898).

tripod is made to fold forward between the two fixed front toes when
not in use.

The other similar instrument is on a circular foot, to which is
screwed a stout supporting pillar ; the upper part is attached to
this by a substantial compass-joint ; but the pillar is fixed on the mar-
gin of the ring, thus bringing the whole weight centrally upon the

232 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

foot when the instrument is in an upright position. When inclined,
the centre of gravity is again brought directly over the foot, as
shown in fig. 185, by rotating the pillar upon a reliable fitting at
its base, so that absolute steadiness is secured. This is a revival

Fig. 185. — Eoss's new bacteriological microscope (1894).

of an old form made in 1760 by J. Cuff, adapted by A. Ross in
1842, and now again used by the same firm [vide fig. 128).

Ross also manufactures an ' Educational ' microscope having
considerable merit, which may fairly be placed in this class. It

MICEOSCOPES OF THE FOURTH CLASS

233

is presented, on a small scale, in fig. 186. It is admirably made,
and provides all that is required in coarse and fine adjustments ;
it is also provided with admirable sub-stage arrangements, and is
placed on a stand that, while it is of horseshoe pattern, has the
hind 'toe' lengthened considerably, and is made so that the foot
can reverse as in the illustration, and lock, thus making a pei'fect
balance for the body, however it may be inclined. This admirably
made instriunent is considerably under 5/. in cost.

Beck's ' British Student's ' microscope is of this class, as is also the
' Star ' microscope by the same makers. The former has a firmly made
tripod, as fig. 187, i^epresenting this instrument, shows. It has a
spiral rack and pinion coarse adjustment, a fine-adjustment, a draw-
tube with mm. scale, and a focussing sub-stage which swings out when
not in use. The present
Editor can speak highly of
this instrument for elementary
class work, and with good
workmanship its price is ex-
ceedingly low. The ' Star '
microscope is also a very re-
markable instrument, sufii-
ciently so to justify us in
departing from a rule to
point out that with two eye-
pieces, two objectives — a
^-inch and a :g^-inch — and an
iris diaphragm, the whole,
placed in a cabinet, is sold for
U. 15s.

We come now to micro-
scopes of the fourth class.

A small, compact, and
thoroughly useful microscope,
specially adapted for medical
students and Biological
Schools, is made by Swift and
Son, and known as their
New Histological and Phj^sio-

FiG. 186. — Eoss's educationa] microscope
(1898).

logical Microscope.' In its simplest form it is shown in fig. 188.
The stand is a firm tripod, the optical tube slides in a cloth-lined
fitting, the fine adjustment may be the diflferential screw actuated
by a large milled head, and capable of work with at least a -jl^-th-inch
objective. It is beatitifully swung, and is firm in lany position.
The stage is large, and has the horseshoe opening. There are
several grades of this instrument, involving more or less complexity
and apparatus ; but it was designed to meet, and we believe does
meet, the needs of students who want a strong, practical, and well-
equipped instrument at a very moderate price.

Another instrument of this class deserving the highest commen-
dation, and offering the student much more for the outlay involved
than we could have thought possible twenty years ago, is ' The

234 THE HISTOEY AND DEVELOPMENT Oe THE MICEOSCOPE

Fram' microscope of Messrs. Watson and Sons. We illustrate it in
fig. 189. It is strong and rigid, and its workmanshijj is of the
highest order. It has a completely steady tripod foot with a spread

Fig. 187. — Beck's British student's microscope (1898).

A STAND BY SWIFT

235

of 7 inches, and its steadiness is unaflected in whatever position the
bod)'^ may have to be inclined. The coarse adjustment is a diagonal

Fig, 188. — Swift's histological and physiological microscope (1894).

236 THE HISTOKY AND DEVELOPMENT OF THE MICROSCOPE

rack and pinion, while the fine adjustment is the now celebrated
lever employed by this firm. One revolution of the milled head

I'iij. 189. — Watson's ' Fram ' microscope (1898)

moves the body the g-^th of an inch. As we have seen (p. 170,
fig. 132), this adjustment is sound in principle, and in practice all

FIFTH AND SIXTH CLASSES OF MICEOSCOPES 237

that need be desired. The stage has the horseshoe-shaped aperture.
The sub-stage fitting, as shown in the illustration, may be turned
aside out of the optical axis, and a compound sub-stage may be made
with the instrument if desired. Throughout, the working parts are
sprung, and wear may be compensated by adjusting screws. We
cannot speak too highly of the enterprise and skill shown in the design
and manufacture of this instrument ; and yet the student will find
that, good as it is, it is one of the least costly instrviments of its class.
There is a microscope manufactured by Messrs. Zeiss, known as
' Stand YI. A,' which comes to about the same cost as the above, and
which we illustrate in fig. 190. It is of course a strictly Continental
form, having a fixed stage 3| ins. square. The coarse adjustment is
by rack and pinion, and the fine adjustment is the usual micro-
meter screw of these makers. The stand is inclinable, and it is
provided with mirrors and a cylinder diaphragm which slides in a
sleeve fixed below the stage capable of receiving the illuminating
apparatus. It is, of course, made with the accuracy and good
quality of woikmanship for which this firm is noted.

Fifth and sixth classes of microscopes are made by the best
makers, and it is a little notable that the best of these classes was made
by the late Hugh PoAvell, whose maxim was that a microscope with
only a good coarse adjustment was to be preferred to one having an
indifierent fine adjustment with a sliding tube for the coarse adjust-
ment.

This stand is of cast iron, with a flat trip)od, having a single
pillar to which is jointed the Jackson body. The focussing is
admirable ; the stage is of an excellent form, being 4^ x 3^ inches,
and is supplied with a beautifully made sliding ledge, which will
move easily and firmly with pressure from one side only.

The stage is fastened to the upper side of two brackets which
are cast in one piece with the limb ; on the under side of these
brackets there is another plate which holds the sub-stage tube.

This instrument is supplied with large plane and concave
mirrors ; and, considering that it constitutes a sixth class of
microscope, has very m.uch in its favour as a secondary instrument
for the work-table. Like all these makers' instruments, the feet are
plugged with cork ; and we know of some of these microscopes that
have been in use for forty years, and are still the trusted 'jomniey-
men ' instruments of mounters and other workers of various orders
in many departments of microscopy.

Some of the modern forms of these two classes of microscope
deserve, on behalf of beginners with limited means, some considera-
tion. A thoroughly good but extreraely simple microscope of the
fifth class is made by Watson and Sons ; it is illusti-ated in fig. 191.
It was designed for educational purposes ; the workmanship is of the
finest quality, but the instrument is not provided with a fine
adjustment ; it relies on a very perfectly made diagonal rack and
pinion coarse movement. From practical use we can speak in the
highest terms of the delicacy of this focussing arrangement, with
which we have with ease used powers up to ^ inch, and often have
used it with a i-^-va.. objective. The stage is large, the body has a

238 THE HISTOEY AND DEVELOPMENT OF THE MICEOSCOPE

draw-tube, can be inclined, and it is a steady useful microscope. It
can be obtained complete in a case with one eye-piece for the sum
of 21. Is. M.

Fig. 190.— Zeisb's stand VI.a (1898).

LOW-PRICED STANDS

239

Bausch and Lomb manufactui-e an instrument which abandons
the coarse adjustment, but provides a fine adjustment of good

Fig. 191. — Watson's school microscope (1899).

240 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

quality, and is thoroiighlj well made, its object being to meet the
wants of schools and elementary workers. We believe, however,

Fig. 192.— Reichert's stand No. 15 (1890).

for many reasons, that it is better to rely on an excellent rack and
pinion coarse adjustment for such a purpose. This instrument is
remarkable as meeting a distinct demand, for though of excellent

EEOENT AMERICAN MICROSCOPES

241

workmanship it is sold for twenty shillings. We illustrate it in
fio-. 193.

Fig. 193. — Bausch and Lomb's lowest-priced microscope (1897).

Reichert, of Vienna, manufactures an instiument of the same
class with a good coarse adjustment only, built on a tripod, and of
almost equally low price. But amongst the sixth class of micro-

R

242 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

scopes none is more remarkable for its strength, good form, and
excellent finish than the one we show in fig. 194, made by Leitz. Its
coarse adjustment is capable of doing very delicate work, and it is a
thoroughly steady instrument, and is admirably adapted to elemen-

FiG. 194. — Leitz's school microscope.

tarygwork and school use, and, whilst its finish and work are admirable,
ib is sold for II.

A really beautiful instrument of the same class is made by
Reichert, designated ' Stand No. 15,' which is illustrated in fig. 192.
It is admirably made, and the maker, as we think, wisely, has thrown

A GOOD LOW-PEICED MICKOSCOPE BY LEITZ

243

the best possible work into a spiral rack-ancl-pinion coarse adjust-
ment which works with great accuracy and smoothness, and has
dispensed with a fine adjustment. Its construction is neat, but it is

Fig. 195. — Powell and Lealand's jportable microscope (1848).

one of the most rigid of this class of microscope which we have seen -
or used ; this instrument is sold for twenty- five shillings. But the
maker has adopted Mr. Nelson's plan, using a Steinheil magnifier to
be mounted as a sub-stage condenser, and if a simple iris diaphragm

E 2

Pin. 190. — Swift'f? portable histological microscope (1894).

POETABLE MICROSCOPES

245

be iised with this, there are very few but will be astonished at the
beautiful results attainable. Certainly, since the last edition of this
book was published, large and successful efforts have been made to
supply to those who need them cheap but thoroughly good micro-
scopes.

Portable Microscopes. — Microscopes that may be readily taken
from place to place, and which are yet provided with the arrange-
ments required for using
the principal apparatus, are
of importance in some in-
vestigations, and are de-
sirable by the majority of
those who have a living
interest in microscopic
work.

The eai-liest and still
the best form of this kind
of microscope was made by
Powell and Lealand. As
opened for use it is illus-
trated in fig. 195 ; but the
tripod foot folds into what
becomes practically a single
bar, and is bent by means
of a joint to occupy the
least space. The body un-
screws, and the whole lies
in a very small space, giving
at the same time fittings in
the cabinet for lenses, con-
densers, and all needful
apparatus. The coarse and
fine adjustments to the
body are as in the No. 1
stand, so are the stage

movements ; and the sub-stage has rack-and-pinion movements and
rectangular sector centring, while all the apparatus provided with
the largest instrument can be employed with it. We have used
this instrument for delicate and critical work for twenty years, and
there is no falling off' in its quality ; and, when packed with the
additional apparatus required, the case is 12 x 7 x 3 inches.

Swift and Son have arranged their Histological microscope
(fig. 196) as a portable instrument, to which from its peculiar con-
struction it readily lends itself, and must be placed in the third
class of portable microscopes.

Mr. Rousselet has designed an admirable little instrument of
portable form but of the sixth class. It is binocular. The tripod
folds ; the stage is plain, with a sliding ledge. The condenser
focusses by means of a spiral tube, within which an inner tube
slides, carrying stops, diaphragms, &c. The mirror is jointed so as to
be used above the stage, and, as its focus is only 1^ inch, can be

Fig. 197. — Baker's diagnostic travelling
microscope (1896).

246 THE HISTOEY AND DEVELOPMENT OF THE MICEOSCOPE

used as a side reflector. It is also arranged so that eye-pieces
with large field-glasses may be employed. It packs in a box
10^ X 5^ X 3^ inches, and weighs 6 pounds complete.

Fig. 198. — Bausch and Lomb's portable microscope (1898).

Baker now makes a small useful instrument for travelling called
* the Diagnostic ' microscope, designed by Surgeon-Major Ross,
medical superintendent, Indian Army Medical Department.
Fig. 197 illustrates it. The tripod stand is firm, but readily

BAUSCH AND LOME'S PORTABLE MICROSCOPE

247

folds. It is provided with sliding tube, coarse, and micrometei"
screw fine adjustments, a good draw-tube and thoroughly useful
stage, a tubular sub-stage with plane and concave mirrors. It is
packed in a leather case with shoulder strap and loops for a
military belt, or a handle, and this case, with three objectives
and extra eye-piece, occupies 11 x 3^ x 3 inches. It can also
be arranged for a sub-stage carrying a condenser and iris dia-
phragm, and is exceedingly compact and well made.

A very old device has iDeen utilised by Messrs. Bausch and Lomb
for a new portable stand, that, namely, of making the case or box the
foot of the instrument. The microscope itself is, in every other respect
save size, the same as their ' New ' stand shown in fig. 193 ; but the
addition is made of a clamping screw, to prevent the main tube from

Fig. 199. — Bausch and Lomb's portable microscope packed (1898).

dropping or turning. An illustration of this microscope is given, as
set up for use, in fig. 198. It will be seen that a double nose-piece
may be used, and it is provided with a useful condenser, the sub-
stage having a screw focussing adjustment, and an arrangement for
swinging this out of the optic axis. The microscope is rigid, but
can be inclined at any angle by raising the cover of the case as in
the figure. It can be closed into the box with its double nose-
pieces in position, and its sub-stage and condenser ready for use.
The size of the case complete is 8| x 5| X 2^ inches, and its weight
is 3f pounds.

Microscopes employed for the purpose of minute dissection are
of considerable importance in certain kinds of work. Many instru-
ments specially adapted are made, although the majority are
arranged for simple lenses. But an instrument of great value.

248 THE HISTORY AND DEVELOPMEXT OF THE MICEOSCOPE

arranged for use with conijyound lenses, has been devised by employing
the binocular of Mr. Stephenson. This instrument is illustrated in
fig. 200. It is made by Swift and Son. The stage may be enlarged
as a dissecting table, with special rests for the arms. The objective
and binocular part of the body remain vertical and focus vertically
by a rack-and-pinion coarse adjustment, there being no fine adjust-
ment. The bodies above the binocular prisms are suitably inclined,
mirrors being placed inside them to reflect the image. This reflec-
tion also causes the erection of the image, which is valuable to the
majoi'ity engaged in insect dissection or the dissection of very
delicate and minute organisms or organs.

Another type of dissecting microscope has been introduced (as
we have seen on pp. 102-4) by the firm of Zeiss; it is known
as Greenough's Binocular Microscope, and possesses valuable
and interesting features, and has been prepared to facilitate the
examination, dissection, and preparation of eggs, larvse, and
other solid objects by furnishing a true stereoscopic and erect
image. Hence it is most useful for zoologists, botanists, and
embryologists. To accomplish this purpose a combination of Porro
prisms with a compound microscope of the usual optical type has
been efiected. We have said enough of this instrument in an
earlier page, and merely recall its adaptation to dissecting purposes
by the illustration furnished in fig. 201, and we would remark that it
is only when two such complete microscopes, each having its own
objective and eye-pieces, are simultaneously directed upon an object
that the truest stereoscopic images can be obtained.

Only comparatively low powers can be used with this instrument,
but this is no defect, for with such powers alone would the work it
is intended to do be accomplished ; but two special eye-pieces of
dififerent powers, corresponding to Huyghenian eye-pieces 2 and 4,
are prepared for this microscope ; they are known as orthomorphic.
The magnifications resulting from the combination of these eye-
pieces with the objective are respectively 25 and 40.

We have now to consider the most primitive stands adopted for
simple microscopes. That in the form of a bull's-eye stand is the
least complex form possible. This instrument holds an intermediate
place between the hand magnifier and the complete microscope,
being, in fact, nothing more than a lens supported in such a manner
as to be capable of being readily fixed in a variety of positions
suitable for dissecting and for other manipulations. It consists in
its best form of a circular foot, wherein is screwed a short tubular
jDillar (fig. 202), provided with a rack-and-pinion movement, and
carrying a jointed arm movable in many directions by ball-and-
socket and other joints, 6, c, e, but capable of being clamped by
thumb-screws or milled heads, a, b, e ; one end of this arm carries a
joint, to which is attached a ring for holding the lenses. By
lengthening or shortening the pillar, by varying the angle which
the arm makes with its summit, and by using the various joints,
almost any position and elevation may be given to the lens that can
be required for the purposes to which it may be most usefully
applied, care being taken in all instances that the ring which carries

LENS-HOLDEKS

249

the lens should (by means of its joint) be placed horizontally. The
lenses now most suitable for such a holder are those constructed

Pig. 200.— Stephenson's binocular by Swift (1887).

upon the Steinheil formula, composed of three cemented lenses
forming a system which gives relatively long working distances
with large flat field. As made by Zeiss they magnify 6, 12, 20, and

250 THE HISTOEY AND DEVELOPMENT OF THE MICKOSCOPE

30 times, and, employed in such a stand as fig. 202, they are ad-
mirably adapted for picking out minute shells or for other similar
manipulations, the sand or dredgings to be examined being spread
upon a piece of black paper, and raised upon a book, a box, or some
other support to such a height that when the lens is adjusted
thereto, the eye may be applied to it contintiously without unneces-
sary fatigue. It will be found advantageous that the foot of the
microscope should not stand upon the paper over which the objects
are spread, as it is desii'able to shake this from time to time in order
to bring a fresh portion of the matters to be examined into view ;

Fig. 201. — Greenough's binocular, arranged as a dissecting microscope (1897).

and, generally speaking, it will be found convenient to place it on
the opposite side of the object, rather than on the same side with
the observer. In a suitable position these lenses with their holder
may be most conveniently set for the dissection of objects contained
in a plate or trough, the sides of which, being higher than the lens,
would prevent the use of any magnifier mounted on a horizontal
arm. Although the uses of this little instrument are greatly
limited by its want of stage, mirror, &c., yet, for the class of pur-
poses to which it is suited, it has advantages over perhaps every
other form that has been devised. Where, on the other hand,

LENS-HOLDERS

251

portability nifiy be altogethei- sacrificed, and the instrument is to be
adapted to tlie making of large dissections under a low magnifyino-
power, some such form as is represented in fig. 203 constructed by-
Messrs. Baker, on the basis of that devised by Professor Huxley for
the use of his Practical Class at South Kensington, will be found
decidedly preferable. The framework of the instrument is solidly
constructed in mahogany, all its surfaces being blackened, and is so
arranged as to give two uprights for the support of the stage and
two oblique rests for the hands. Close to the summit of each of these
uprights is a groove into which the stage-plate slides ; and this may
be either a square of moderately thick glass or a plate of ebonite,
having a central perforation into which a disc of the same material
may be fitted, so as to lie flush with its surface, one of these beino-
readily substituted for the other, as may best suit the use to be

Fig. 202.— Zeiss's lens-holder.

made of it. The lens is carried on an arm working on a racked
stem, which is raised or lowered by a milled-head pinion attached to
a pillar at the further right-hand corner of the stage. The length
of the rack is sufiicient to allow the arm to be adjusted to any
focal distance between 2 inches and 1 inch. But as the height of
the pillar is not sufficient to allow the use of a lens of 3 inches
focus (which is very useful for large dissections), the arm carrying
the lenses is made with a double bend, which, when its position is
reversed, as in the dotted outline (which is readily done by unscrew-
ing the milled head that attaches it to the top of the racked stem),
gives the additional inch required. As in the Quekett mici^o-
scope, a compound body may be easily fitted, if desired, to a separate
arm capable of being pivoted on the same stem. The mirror frame

252 THE HISTORY AND DEVELOPMENT OF THE MICEOSCOPE

is fixed to the wooden basis of the instrument, and places for the
lenses are made in grooves beneath the hand-supports. The ad-
vantages of this general design have now been satisfactorily de-
monstrated by the large use that has been made of it ; but the
details of its construction (such as the height and slope to be

Fig, 203. — Laboratory dissecting microscope (1876).

given to the hand-rests) may be easily adapted to indi^ddual require-
ments.

A very simple and well-known form of dissecting microscope is
made by Messrs. Bausch and Lomb. It is shown in fig. 204. Its
form is self-explanatory : a plain glass stage, and a mirror at a suit-
able angle giving abundant light, capable of being replaced by

Fig. 204. — Bausch and Lomb's (Barnes) dissecting microscope (1896).

a white or black enamelled background, suitable rests for the
arm, and a sliding holder for the lenses. It is these latter that
are special : they are designed for^ the instrument. They are
doublets, which undoubtedly give a large aplanatic field and fine
definition.

But the very best form of dissecting microscope for simple lenses

A GOOD DISSECTING STAND BY ZEISS

253

which we believe to be at present constructed is made by Zeiss.
We illustrate this form, fig. 205. It has a large firm stage 4 inches
square and 4^ inches from the table, to which wooden arm-rests can
be attached or not, as may be desired. Only one is attached in the

illustration, and the points of attachment of the otherTaie seen.
The stage has a large opening, 3 x 3| inches, into which can be
placed either a flat brass jjlate or a glass substitute, or a metal plate
with a half- inch hole in it. Underneath the stage ai-e black and
white screens, which can readily be turned aside by the use of the

2 54 THE HISTORY AND DEVELOPxMENT OF THE MICROSCOPE

milled heads, A. The arm, which is focussed by an excellent spiral
rack- work adjustment, carries either a Zeiss dissecting microscope,
which, with and without its concave eye-lens, yields six different
powers, varying from 15 to 100 diameters, or the arm will receive
the very fine Zeiss-Steinheil simple magnifiers.

The instrviment is provided with a lai'ge plane and concave
mirror on a jointed arm. The utility of this simple microscope is
very great, and we do not hesitate to pronounce it the best thing of
its class we have ever seen.

The Continental Model. — Our one purpose in this treatise is to
endeavour to promote what we believe to be the highest interests of
the microscope as a mechanical and optical instrument, as well as
to further its application to the ever-widening area of physical
investigation to which, in research, it may be directed. To this end
throughout the volume, and especially on the subject of the value
and efficiency of apparatus and instru.ments, we have not hesitated
to state definitely our judgment, and, where needed, the basis on
which it rests. Incidentally we have expressed perhaps more than
once our disapproval, and, with ourselves, that of many of the leading
English and American microscopists, of the form of microscope, known
as the Contiiiental model ; we believe it is not needful to say that we
have done this after many years of careful thought and varied
practice and experience, and, so far as the human mind can analyse,
without bias. It is not where a microscope is made that the
scientific microscopist inquires first, but where it is made most
perfectly, and we cherish strong hopes, in the interests of the science
of microscopy, that so enterprising and eminent a fii-m as that of
Zeiss, of Jena, will bring out a model that will comport more com-
pletely with the needs of modern microscopical research than even
the best of the models that they now produce. It is to this house,
under the cviltivated guidance of Dr. Abbe and Dr. Czapski, that
we are indebted for the splendid perfection to which the optical side
of the microscope has been recently brought ; and when we know
that the ' Continental model ' has, in the hands of the firm of Zeiss,
passed from an instrument without inclination of the body into an
instrument that does so incline, and from an instrument without
sub-stage or condenser into one provided with the latter of these
absolutely indispensable appendages, and finally from an instrument
with a perfectly plain stage with ' clips ' into what is now a stage
with mechanical movements — we can but hope that these concessions
to what has belonged to the best English models for over forty years
may lead to an entire reconstruction of the stand — a wholly new
model — intended to meet all the reqviirements of modern high-class
work in all departments, and with a fine adjustment of the most
refined class. We cannot doubt, if this were so, that the same
genius which has so nobly elevated the optical requirements of the
instrument would act with equal success on its construction and
mechanism. We have been told in the friendliest spirit, by one
deeply interested in the Continental stand, and a master in optical
knowledge, that on the Continent the microscope is ' actually almost
exclusively used' in a vertical position. Nevertheless we know

CONTINENTAL V. ENGLISH MODEL 255

what elaborate arrangements have been made to enable the body to
be inclined in all the better models, and surely the English stand is
as capable of being used in this position as the most primitive Con-
tinental instrument ; but the doubt we have is as to whether the
most primitive Continental stand possesses the same primal adapta-
bility to all the modern optical and mechanical improvements of
the microscope as is possessed by the English stand. It is said
that ' the Continental microscope has closely followed the wants of
the microscopist, and that in its mechanical arrangements it has kept
pace with the increasing improvement of the optical parts, without
outrunning them, as has been the ca,se with many English forms of
construction.' With the deference and good feeling with which we
receive this statement we are bound to say that it does not present
itself as historical. The mechanical parts have not in reality kept pace
with the optical improvements, for when apochromatic lenses of 0"95
N.A. to r4 N.A. are used with large illuminating cones they become
so sensitive to focal adjvistment that the Continental fine adjustment
(the best form of which has hitherto been used by Zeiss) is not
sufiiciently slow to permit of accurate focussing in highly critical
work. Applications have, for instance, been made to Powell,' asking
him to increase the slowness of his fine adjustment, which is now
twice as slow as the best Continental form. But perhaps the
clearest evidence is found in the fact that, while we are passing this
book throvTgh the press, two striking proofs of Continental conviction
that their fine adjustment should be rendered slower and more sen-
sitive are given, first, by the beautifully simple and, as we believe,
most admirable invention of Reichert, adapting a lever movement
to his stands {vide p. 169, fig. 131), by which he makes the fine
adjustment more than three times as slow as the best hitherto used
on the Continent ; while the firm of Zeiss themselves, in their
newest model (p. 167, fig. 128), have by another method sur-
passed all other makers ; and, as I learn by the courtesy of the
firm, 'the micrometer screw of this new stand is adjusted for
eg-g-th of an inch for each revolution of the milled head ' (figs. 129,
130). ,

"We cannot but believe that this is the best evidence we can
have of the validity of our contention in the last edition of this
book that the Continental fine adjustment was too coarse or quick
for the almost perfect objectives and eye -pieces they themselves had
given to the world.

We have written throughout this book too frankly of the eminent
sei-vices of Messrs. Zeiss, to the furtherance of the interests and pro-
gression of the microscope as a scientific instrument, to be misunder-
stood in making a plain estimate of the quality of the model on which
their elaborate and in some senses beautiful stands are built. It
will be seen that we everywhere justify our judgments by plain and
easily comprehended reasons, and the very eminence of the makers
renders it incumbent that practical microscopists should, without a
shade of bias, assess the value of a stand which is certainly not
built on lines that contribute to a higher and still more efi^cient
microscopy.

256 THE HISTOEY AND DEVELOPMENT OF THE MICEOSCOPE

At the same time we do not blind ourselves to the fact that
an English raarket for the ' Hartnack ' model has had very
much to do with the joerpetuation of the errors which that form
contains.

The reason of this it is not difficult to trace. The mchictive
method advanced but slowly, in practice, upon the professional
activities, and even the professional training, of medical men. The
country which was the home of Bacon and Newton and Harvey and
Hunter theoretically accepted, but was not quick to apply, the
methods of induction to the work of its medical schools. Theory and
emj)iricism held a powerful place in both the teaching and practice
of medicine in England until the earlier years of the present century.
Medicine was absolutely unaffected by Bacon until the latter half of
the seventeenth century. It was not until the early years of this cen-
turj that the modern school of medicine began its beneficent career.
But at that time the microscope — one of the most powerful instru-
ments which can be thought of in the application of experimental
and deductive methods to the science of medicine — was looked upon
and treated by the faculty as a j)hilosophical toy, a mere plaything
for the rich dilettante. But in spite of this the microscope was
brought gradually to a high state of perfection, and by the end of
the first third of the century was remarkably advanced as a practical
instrument, all its essentials being more or less completely developed.
Meanwhile, on the Continent the microscope was regarded by the
Faculty as a scientific instrument of great and increasing value,
being used to good purpose in making important discoveries in
anatomy, histology, and biology generally.

This was gradually realised iyi this country, and there arose
slowly a desire to employ the same instrument in England. But,
although English instruments of the most practical and relatively
perfect kind, representing the large experience of many careful
amateurs, were easily accessible to our medical men in their own
country — because it was on the Continent that the investigations
referred to had been made — it was nothing less than the Continental
microscope that was sought after and obtained. We have been told,
indeed, that ' the development of the English stands has not
depended on the wants of the microscopist,' but has been the result
of ingenuity and invention. To this we simply say that it may be
true that their development has not depended on the immediate
wants of the microscopist, but was in many cases the result not of
ingenuity so much as of powerful insight and foresight. And how
often have these anticipations been idealised ! Because early obser-
vations of a histological character (and therefore of a nature to lie
beyond the sphere of the lay amateur) had been successfully made
with a certain form of microscope on the Continent, it was practi-
cally argued that this must be the most suitable instrument for such
a purpose ; but this was an inference made without knowledge of or
reference to the well-known English models.

Let us carefully examine this instrument. The typical form
was that made by Hartnack. Seen in its jDi-imitive state, we have
it in the catalogues of all the Continental makers — Zeiss, Leitz,

COMPAEISON OF CONTINENTAL AND ENGLISH MODELS 257

Reichert, and the rest. It is a non-inclining instrument, with a
shoi't tube on a naiTow horseshoe foot, in which steadiness is
obtained b}^ sheer weight. It has a sliding-tube as a coarse adjust-
ment, and a dii-ect-acting screw for the fine adjustment. The stage
is small, and the aperture in it is relatively still smaller, of no service
in I'eaching the focus of an object by touch with a high power. It
is provided with spring clips, and a diaphragm immediately below
the stage, and a concave mirror. Now it has been said that the fact
that the Powell stand, e.g. of forty-five years ago, adapts itself without
material change to the most modern appliances would be looked
upon by the German student as being ' no commendation,' because
it would mean that they were more elaborate than was necessary,
but what are the facts ? Let us take an Oberhauser of 1837, and
compare it in one essential particular only with a very early Powell,
designed in 1834. It was a stage-focussing instrument. As a fact
the Oberhauser will not focus a low-angled j-inch objective properly ;
tlie fine adjustment works in jerks, and the lateral movement causes
the object to go out of the field. The Powell will now work an
apochromatic of 1*4 IST.A. oil immersion with accuracy and precision ;
but if an apochromatic oil immersion of 1 "4 were placed on the Ober-
hauser it would be at great risk to the objective. Now even in early
days accurate focussing was surely a vital matter, and the foresight
that could anticipate what might require more delicate focussing than
the objectives then in use was wise, and to the student profitable.
The Powell No. 1 stand, as it is now, was m the main constructed in
1849. so far as regards tripod foot, limb, coarse adjustment, and fine
adjustment with Turrell stage. The alterations that have been
introduced have been the concentric rotary stage (1861), and the
present form was manufactured in 1869.

A sub-stage condenser was rarely used, because up to a compara-
tively late date (1874) it was regarded by many on the Continent
as a mere elegant plaything ; its true value was not perceived.

On this model all the microscopes of the firm of Zeiss, of Jena,
are constructed, as they are used almost exclusively on the Conti-
nent, and are regarded in many of the universities and medical
schools, both here and in America, as possessing all the qualities
required for the best biological research.

If we examine the finest of these instruments made uj) to 1885,
we are impressed, as we always are, with the beauty and care of the
workmanship and finish of this firm ; but there is the same heavy
horseshoe foot, steady enough while the instrument is non-inclining,
only needlessly heaA^y, requiring common ingenuity alone to get
equal steadiness with one-fourth the weight. But since this instru-
ment has been adapted to the English form by being made to incline
to any angle up to the horizontal, the foot but insecurely balances
the instrument, and it is not difficult, as it is not uncommon, to topple
it over. Indeed in their photo-mici-ographic outfit the Messrs.
Zeiss practically see this, for they supply another foot to ivhich the
microscojje is clamped. Messrs. Bausch and Lomb tell us that the
foot of their ' B B ' Continental mici-oscope is ' heavily leaded to ensure
greater stability.' Sidle and Poalk (1880) and McLaren (1884), and

s

258 THE HISTORY AND DEVELOPMENT OF THE MICEOSCOPE

now Ross, adopting this foot, employ tlie added mechanism of the
revolution of the pillar on the foot (an old device) to secure stability
at all inclinations {vide fig. 185, p. 232). Surely if the horseshoe
foot were satisfactory for the inclining microscope these modifications
would not have been deemed needful. Besides which we note that
for the same purpose the C :)ntinental maker, whom we venture to
think very alert to the true needs of modern microscopy, Reichert,
prolongs the projecting ' toe ' of the horseshoe, giving it almost a
tripod form.

It must not be forgotten that this want of balance is with the
short, not the long body.

The diameter of the tube is small, being slightly over seven-
eighths of an inch. No doubt a low-power eye-piece with a large
field is extremely useful as a finder, but this advantage is completely
lost with the original small Continental tube. That this is seen to
be a disadvantage would appear certain, because the photographic
microscope model of Zeiss has a larger hody-tuhe ; and in their recent
' Appendix ' to their latest catalogue they admit that for certain pur-
poses other stands made by them, ' owing to the limited diameter
of their tubes, cut off the field ; ' a significant fact for those who
would narrow the English body, when it is remembered that Powell's
is, and has been, suitable for all purposes without alteration, and
long, short, and binocular bodies are interchangeable.

At the date of the publication of our last edition, out of eighteen
models ten were made with inclining bodies, and three had sliding
coarse adjustment. But in the twelve models for 1889 ten incline,
while only two are rigid, and eight have rack-work, against four
having sliding tubes for coarse adjustment ; but in the current
catalogue of Messrs. Zeiss six out of eight models have inclining
bodies, two are rigid, and one has sliding coarse adjustment. This
is a manifest, if slow, conformity of the primitive model to the
English type, and hardly supports the affirmation ' that (during the
last forty years) the Continental microscope has closely followed the
wants of the microscopist.'

The direct-actmg screw, only slightly modified, obtains universally
in these models. We have already plainly said that this is not suf-
ficiently delicate in its action for critical work with an apochromatic
objective of 1"4 or 1 '5 numerical aperture, especially as a micrometer
screw with a necessarily delicate thread is bound to carry the com-
bined weight of the body, limb, coarse adjustment, and the
opposing spring ; that it will wear loose under the stress of
constant work is inevitable, and thus its utility must be wholly
gone.

The 1889 model has a new form of fine adjustment, the alteration
being that the micrometer screw acts on a hardened steel point. This
may cause it to work smoother ; but as no weight is taken off, there
is difficulty in discovering any reason for its admitting of more
prolonged use without injurious wear. In support of this is the
fact that in the new photographic stand made by this celebrated
firm, with so extreniely delicate a fine adjustment (fig. 129), we
have learned through their English representatives that only one-

CRITICISM OF MECHANICAL PARTS 259

fifth of the amount lifted by the micrometer screw of the 1889
model is lifted by the same screw in the new model. It should be
remembered that few makers of microscopes in England, though
the)' may be for class and school purposes, if they use a fine adjust-
ment at all, use anything less delicate than the Campbell differential
screw ; although it seems on the Continent to be believed that the
direct-acting micrometer screw of the Continental form is still in
vogue.

It must be plain that a screw of x^oth inch to a revolution
cannot bear for long the heavy strain of the body of a microscope.
The remodelling of Zeiss fine adjustments in 1886 undoubtedly
improved their construction and quality of work ; but so fine a steel
thread is not meant to carry weight and strain. This applies to all
delicate instruments of precision.

The stage of this instrument, in common with all built on the
same model, has three fundamental errors of design : —

i. The stage is so narrow that the edges of the 3x1 slips are, in
some Continental stands, allowed to project over the edges. Messrs.
Zeiss have profitably departed from this fault by giving to theii-
larger stands a stage in size more like the English type.

ii. The stages have an aperture so small as to limit their useful-
ness in focussing with high powers.

iii. Instead of a sliding ledge they provide what still more
efficiently militates against easy and rapid focussing, viz. spring-
clips. It is unfortunate that no stage on this model admits of the
use of the finger to aid in reaching the focus. This gentle tilting
lip of the object, as we approach the focal point, would save hundreds
of cover-glasses and objective fronts — and we have reason to know
that not a few are broken with this form of stage ; but we have never
seen put forward, and do not know, a single reason in justification
of a small aperture in the stage.

Another important point is the absence of rotation in the
ordinary Continental stand. True rotation is a strictly English
feature, which has been in use and carefully constructed for many
years. And its value is great ; it is an indispensable adjunct to
practical work.

Messrs. Zeiss, some twenty years since, copied the Oberhauser
form of rotation for the stage ; they did this by making the body
and limb solid vnth the stage, so that the whole rotates to-
gether.

Practically there is only one point in favoui' of such a move-
ment, and that is, that the object remains exactly in the same
position in regard to the field. But against this arrangement there
is —

1 . The liability of throwing the optic axis above the stage out of
centre with that below the stage, and this though the workmanship
be, as it is, of the highest order.

2. The rotation of a microscope object for ordinary examination
is really unimportant, as there can be no top or bottom to it. Even
for oblique illumination it is not required, as it is always easiei' to
rotate the illuminating pencil. The only instances in whiqh rotation

s2

26o THE HISTOKY AND DEVELOPMENT OF THE MICROSCOPE

of the object is impoi^tant are : (a) When the object is polarised, and
then it is a distinct disadvantage not to be able to rotate the object
independently of the body which carries the analyser. In short, the
stage rotating independently of the body would be preferable because,
if it is required to rotate the object on a dark polarised field, the
polarising and analysing prisms caii be set at the jJt'ojjer angles, and
then the object rotated without disturbing the relative positions of
the prisms.

But this cannot be done with the arrangement of the Zeiss
model, which rotates body and stage. The fii'm have, however,
more recently introduced a rotating stage based on the English
model, and we are glad to give our testimony to its admirable
workraanship and perfection of centring. The contention, however,
that we think in all friendliness is sustained, is that the charac-
teristics of the English model were not superfluous, and that the
Continental model has only too slowly followed the requirements
discovered and used by the makers of the best English models so long
ago.

(/(3) For 'photo-'\nicrocjraphic purposes. — In this case, in the Zeiss
stand, the head of the fine-adjustment screw is geared to the focussing-
rod ; so, manifestly, rotation of the body becomes impossible.

Thus, by adopting rotation in the form chosen, the highest ends
for which the microscope stage should revolve cannot be accomplished,
and the newer form of stand must be adopted.

The sub-stage is often quite wanting in the common Continental
forms. This was true of the Hartnack stands, with rare excep-
tions ; the Nachet instruments were provided with an elementary
form.

As we have seen, until quite recent times, the condenser taas-
regarded on the Continent as a super fltious, if not a foolish, ajopliaiice ;
but that prejudice has been killed by the light thrown on the whole
question by (1) the chromatic (1873), and now (2) the achromatic
condenser of Abbe, and finally (3) by the ' centring achromatic
condenser,' only just made accessible by this firm. This condenser is
not only focussed by the rack-and-pinion movement, but also by
means of a special fine adjustment for bringing out its most delicate
results. But even a condenser was in use in England in the year
1691 (vide fig. 101, p. 133), and the best work in England since the
invention of achromatism has never been done without one.

In the mounting of the Abbe condenser every possible ingenuity
has been displayed to make it do its work withotit a sub-stage ; but
a permanent centring and focussing sub-stage, into which this optical
arrangement could, amongst others, fit, might be made with half the
labour, ingenuity, and cost. But rather than this, we have in the
less recent forms the condenser made to slide on the tail-piece, and
to be jammed with a screw.

It has therefore neither centring nor focussing gear ; but, striking-
as it may appear, a diap)hvagin, which cannot be used with, and is no
part of, the condenser, is siqjplied in a stand not of the most recent,
but of comparatively recent make, toith mechanical centring and
rack-work focussing movements ! That is to say, the delicate centre

THE PUECHASE OF A MICROSCOPE 26 1

of an ojJtical combination might in that instrument ifafe care of itself,
but a diaplii-agm ajoei-ture must be centi-ed by mechanism and
focussed by I'ack.

We know that the idea involved in a rack-work diaphragm is
the graduation in the angle of the cone of illumination from the
plane mirror by racking a certain-sized diaphragm up or down.
But this can be better done by an iris diaphragm, or perhaps more
perfectly still by a wheel of diaphragms.

Now, in reality nothing is so important as the centring and
focussing of the condenser, after we are once provided with perfect
objectives ; and any mechanical ai-rangement that would enable us
to perfectly centre an iris diaphragm or a wheel of diaphragms
would enable us to centre the condenser. For the racking and
centring of condensers there was, until very recent times, nothing
in the best stands, of what is doubtless the largest and most
enlightened house for the manufacture of microscopes in the
world, to supply this indispensable need which the modern con-
denser involves.

We observe with pleasure advances in every direction in which
we have called attention to defects. The more recent instruments
are marvels of ingenuity; we present, in fig. 167, the latest and
finest form of Zeiss' s best microscope.

There is no fault in the workmanship ; it is the best possible.
The design only is faidty; there is nothing to command commenda-
tion in any part of the model ; and, seeing that the Messrs. Zeiss
have now progressed so far as to furnish their first-class stand with
the English mechanical movement, and even stage rotation, and fine
adjustment to their newest and best sub- stage condenser, we can
but believe that the advantages of these improvements will make
plain the greater advantage that would accrue from an entirely new
m-odel. To all who study carefully the history of the microscope
and have used for many years every principal form, it will, we
believe, be manifest that the present best stand of the best makers
of the Continent is an over-burdened instrument. Its multiplex
modern apj)liances were never meant to be carried by it. The
attempt to combine a dissecting microscope with an observing
microscope required to do the most critical work is not, we submit
with all friendliness, compatible.

The Purchase of a Microscope. — A desire to possess a good but
not costly microscope is extremely common, but as a rule the
intending pm-chaser has little knowledge of the instrument, and
does not profess to know what are the indispensable parts of such an
apparatus, or what parts may, in the interests of economy and his
special object, be dispensed with, leaving him still possessed of
a sound and well-made instrument. W^e may briefly consider this
matter.

The first question to be asked when a microscope is to be pur-
chased is, ' What is the order of importance of the various parts of
a microscope 1 ' In answering this query it will be to some extent
true that subjectivity of judgment will appear. But we believe
that the following table of the relative order of importance of the

262 THE HISTOEY AJND DEVELOPMENT OF THE MICEOSCOPE

parts of a microscope will commend itself to all workers of large and
broad experience : —

1. A coarse adjustment by rack and pinion.

2. A sub-stage.

3. A j&ne adjustment.

4 . Mechanical movements to sub-sta ge, i.e. focussing and centring .

5. Mechanical stage.

6. Rack-work to draw-tube.

7. Finder to stage,

8. Plain rotary stage.

9. Graduation and rack- work to rotary stage.

10. Fine adjustment to sub-stage.

11. Rotary sub-stage.

12. Centring to rotary stage.

This table gives in order the relative values of the several parts ;
thus a microscope with a rack-and-pinion coarse adjustment and a
sub-stage is to be preferred before a microscope with a rack-and-
pinion coarse adjustment, define adjustnieiit, hut no sub-stage. Or a
microscope with a coarse adjustment by rack and pinion, a sub-stage,
and a fine adjustment, is to be preferred before one with the same
coarse adjustment and a mechanical stage movement, but no sub-
stage or fine adjustment ; and so on. The last item is of least
importance, and the impoi-tance of all the others is in the order of
their numeration.

Another matter of some significance to the tyro is the relative
value, from the point of view of time consumed, and therefore of
j)rime cost, in producing the several kinds of microscopes. The
No. 1 stands of half a dozen makers may be near the same cost, but
may nevertheless have involved the consumption of very different
quantities of the highest class of skilled labour in their production.

Manifestly the first thing to be looked at in a microscope making-
any pretensions to quality is the character of the workmanship ; and
this should carry with it the question how much machine, and how
much hand work and fitting there is in it. Arcs graduated on
silver, for example, are very attractive, and with many are most
impressive ; but they are simply machine work, and quite inex-
pensive.

In the two great types of models, the bar movement and the
Jackson limb, the bar movement involves more than double the
actual hand-fitting ; while a fine adjustment with a movable nose-
piece takes twice the fitting of one in which the whole body is moved
by the fine -adjustment screw. In the same way a mechanical stage
which is made of machine-planed plates, sliding in a machine-ploughed
groove, is much less costly in time and quality of labour than a hand-
made sprung stage. So a sub-stage having a movable ring pressed
by two screws against a spring has very far less work, and work of
a lower class, than one with a true rectangular centring movement.

It will follow, then, that a Jackson-limbed microscope with no
movable nose-piece, with a machine-made mechanical stage and a
movable ring for sub-stage, will not have involved more, perhaps,
than a third of the skilled work which must be expended on a well-

SPECIAL MICROSCOPES

263

made insti'imient of the same size with a bar movement. But if we
comjjai-e the range of prices as presented by English and American
makers, we rai'ely find an equivalent diffei'ence in cost.

Then the tyro will be wai-ned by this not to purchase a ^pretentious
insti'ument with a bai' movement and mechanical stage for, say, 5^.

But if a loio-priced
instrument is to he
purchased, if, as is
almost certain, it be a
Jackson model, see
that it has a rack-
work coarse adjust-
ment, eschew the short-
lever nose-2nece, and
have a diflferential
screw fine adjustment,
a large plain stage,
and an elementary
centring sub-stage.
Such an instrument
should be obtained for
U. 10s.

Although not fre-
quently used, it would
be doing our work im-
perfectly not to refer
to a form of micro-
scope devised for
chemical purposes by
Messrs. Bausch and
Lomb. The object of
Prof. E. Ghamot, of
the Cornell University,
in inducing these op-
ticians to make this
microscope was, he
says, to enable the
chemist who had
mastered the use of the
microscope ' to employ
the elegant and time-
saving methods of
micro-analysis,' thus
giving him ability ' to
examine qualitatively

Fig. 20G. — Microscope tor chemical purposes (1897).

the most minute amounts of material with a rapidity and accuracy
which are truly marvellous, not to speak of the many substances
for which no other method of identification is known.'

An ilkistration of this instrument is given in fig. 206. It will
be observed that it follows the Continental model ; ' since in all the
work for which it is intended the stand is always used in an upright

264 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

position,' it is not provided witha jointed pillar to secvu^e inclination.
The coarse adjustment is by rack and pinion ; the fine, by the usual
micrometer screw of this firm. The stage is circular and rotates,
being j^rovided with centring screws, and its margin is graduated
into degrees for measuring crystal angles. Except foi- this graduated
circle the stage is faced with hard rubber. The sub-stage is adjiist-
able by means of a quick -acting screw. This is fitted with polaiising
apparatus, consisting of a large Nicol prisra so mounted that by
means of a pin fitting into a slot in the sub-stage the prism can
always be replaced in exactly the same position, and rotated with a
circle graduated in degrees ; or it can be swung aside when polarised
light is not needed. The analysing Nicol prism is also provided
with a graduated circle, and is so mounted that it fits over and above
any eye-piece. The draw-tube of the microscope is furnished with
a small projecting pin, which fits into a slot cut in the bottom of
the tube-mounting of the analyser. This slot lies in the same
vertical plane as the zero points of the analyser, the polariser, and
the stage. The zero points of the two former are arranged as usual
for the position of crossed Nicols ; hence, when the polariser is in
position and at zero, and the analyser is at zero and is in position
by its pin and slot, the Nicols are crossed without further adjust-
ment ; this, of course, saves much time. But it is clearly a simplified
petrological microscope ; it is not intended for petrological or
mineralogical work, it is simply an instrument made at a very
low price, but stated by Prof. Chamot to be competent for all
chemical work or food examinations.

An equally important special form of microscope has been made
by Reichert for the examination of metals. ^ Fig. 207 shows this
instrument made according to the instructions of Dr. A. Rejto, of
Budapest. In general appearance it resembles the ordinary horse-
shoe stand, but it has no mirror, and the stage, which is made
adjustable in height, may also be removed altogether.

With very low powers the specimen may be illuminated by
diiFused daylight or artificial light falling freely upon its surface.
With higher powers an illuminator is used which fits the txibe of
the m.icroscope, and is provided with an extension to receive the
eye-piece. The illuminator consists of a thin plate of glass placed
at an angle of 45° with regard to the axis of the tube, and of a con-
densing lens whose focal length is equal to the sum of distances
between the lens and the plate of glass, and between the latter and
the object.

The question of illumination is a very important one, to whic
great attention is to be devoted.

As source of light the ' Auer,' a triplex burner, adjustable in
height, may be recommended ; ^ it is placed at a distance of one
metre from the illuminator. The flame is surrounded by an iron
or asbestos cylinder, with only the necessary aperture for illumination
of the object. The source of light should be at exactly the same
level with the lens, b, of the illuminator. On removing the eye-

1 Central- Zei tuv g fiir Optik und MechaniJc, No. 17, 1897.
- Supplied by Reicliert.

SPECIAL MICKOSCOPES

265

piece and looking through O c, it will generally be found that the
microscopical field is not evenly illuminated ; the light should then
be lowered or raised until perfectly uniform illumination is
obtained.

The beam of light received by the lens, h, is made to converge, and

Fig. 207. — Eeichert's microscope for the examination of metals (1897).

is reflected downwards, in the direction of the axis of the instrument,
by the glass-plate, a. It is then condensed upon the object bj the
lenses of the objective itself. The illuminated object sends back a
portion of the light, which passes through the objective and the plate
«, reaching the eye at 0 c.

The object to be examined should have two parallel surfaces, so

266 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

that it may be placed on the stage of the microscope in a perfectly
horizontal position. With a view of compensating for small de-
ficiencies in the parallelism of the two surfaces, the stage is provided
with the screws, S S, by which means it may be tilted, and the upper
surface of the object made to lie in a truly horizontal plane, which
of course is necessary in order to place the entire field in the focus
of the instrument. The stage is a mechanical one, the milled heads,
Y" and T"", imparting to it a forward and backward movement and
a lateral movement respectively.

After the source of light has been placed in the most desirable
position for the examination of a certain specimen, if a sample of
different thickness be j)laced on the stage, the microscope must be
lowered or raised, with the result that the light is no longer in the
proper position and must again be adjvisted. To avoid this trouble-
some manipulation, the stage of the microscope is made adjustable
in height by turning the milled head T". When the object is too
thick to be placed on the stage, the latter may be turned to one side
and the preparation laid on the foot of the microscope. For still
larger pieces of metal, the stage may be removed altogether, the
body of the instrument turned around 180°, and the metal placed on
the table by the side of the stand ; or the body of the microscope is
connected directly with its foot, for which purpose the intermediate
piece bearing the stage must be removed.

Prof. Rejto's method for the j^reparation of the sample is as
follows : —

The piece of metal to be examined has two of its sides planed ofl'
and made parallel. The upper surface is polished until it is free
from scratches. It is then washed with absolute alcohol, and wiped
with a soft clean cloth in order to remove all fatty substances. The
polished sui-face is next surrounded with a layer of wax so as to form
a rim projecting a little above the surface. Being placed horizon-
tally, pure concentrated hydrochloric acid is poured over it to a
depth of about three millimetres, and allowed to act for five minutes.
It is then poured off, and the surface covered with concentrated
ammonia. The wax is removed, and the surface wiped dry with a
soft cloth. A little oil is next poured over it and allowed to remain
for fifteen minutes.

It is then dried again and rubbed on a piece of chamois leather
until it assumes a shiny appearance.

When large pieces of metal are to be examined, small portions
m.ust be polished by hand and etched as described above.

Figs. 208 and 209 are photomicrographs taken with this instru-
ment, which are self-explanatory of the nature of the work it does.

Tank microscopes (also called aquarium microscopes) have, for
certain kinds of work, a value of their own. They may be used
with low powers outside the glass or above the water ; or the
object-glass may be protected by a water-tight tube outside it, and
with a disc of glass fixed (also water-tight) into that end of the tube
which stands below the front lens of the objective, at a proper
distance for the focus, may then be phmged into the aquarium.
Indeed, the tube of the instrument may be so protected as to work

TANK AND AQUAEIUM MICROSCOPES

267

for some deptli, and have some laiige in the watei' of a good-sized
tank.

A beautiful instrument of this class has been devised by Mr. J.
W. Stephenson for the examination of living objects in an aquarium.
A brass bar is laid across the aquarium as shown in the woodcut

Fig. 208. — Wrought iron magnified 250 diameters.

Fig. 209. — Ordinary steel magnified 250 diameters.

(fig. 210). To adjust it to aquaria of different widths the support
on the left is made to slide along the bar, and it can be clamped at
any given point by the ujjper milled head. The milled head at the
side, by pressing on a loose plate, fastens the bar securely to the
aquarium.

268 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

Between the ends of the bar slides an ai'm carrying a sprung
socket, and the arm can be clamped at any given point of the bar.
Throiigh the socket is passed a glass cylinder, cemented to a brass
collar at the upper end, and closed at the lower by a piece of cover-
glass. Into this cylinder is screwed the body -tube of the microscope
with eye-piece and objective, which are thus protected from the
water of the aquarium. The microscope is focussed by rack and
pinion (milled head just below the eye-piece), and in addition the
objective is screwed to a draw-tube, so that its position in the cylinder
may be approximately regulated.

The arm of the socket is hinged to allow of the microscope being

Fig. 210.

inclined in a plane parallel to the sides of the aquarium. The lower
milled head clamps the hinge at any desired inclination.

The socket also rotates on the arm, so that the microscope can be
inclined in a plane parallel to the front of the aquarium. Thus any
point of the aquai-ium can be reached.

As an adjunct, and admirable aid to the student of the tank and
pond, as well as a simple and easy means by which specific forms of
microscopic life may be found and readily taken, we call attention
to the tank microscope of Mr. C. Rousselet. It is illustrated in fig.
211 and scarcely needs further description.

One of Zeiss's Steinheil aplanatic lenses, to which we have

MR. EOUSSELET'S TANK MICEOSCOPE

269

referred, is carried on a jointed arm, which is clamped to the tank,'
the tank being nowhere deeper than the i-ange of focus of the lens
employed. The arm moves on a plane parallel to the side of the
tank, and the lens is focussed by means of a rack and pinion,
ai'i'anged upon the body
of the clamp, as seen
upon the left-hand
corner of the figure.
The following points
will recommend them-
selves to those who are
in the habit of looking
at their captures with
the pocket lens in the
ordinary way : —

When an object of
interest is found, it can
be followed with the
greatest ease and taken
up with a pipette, both
hands being free for this
operation.

It so frequently
happens that a minute
object is lost simply by removing the pocket lens for an instant to
take up the pipette ; in the above apparatus the lens remains in the
position in which it has been placed. By a new pi'ocess glass tanks
ai-e made with melted seams ; these cannot possibly leak, and are to
be preferred to those with the ordinary cemented joints.

1 We jprefer to have a stand or ' rest ' for the tank, and on one side of this a firm
pillar to which (and not to the side of the aquarium) the jointed arm is clamped.
This enables shallower and deeper tanks to be employed without shifting the rack
carrying the lens.

Fig. 211. — Eousselet's aquarium inicroscoj)e.

270

CHAPTER IV

ACCESSOBY APPABATUS

This chapter on apparatus accessory to the microscope might be
easily made to occupy the whole of the space we propose to devote
to the entire remainder of the book ; the ingenuity of successive
microscopists, and the variety of conditions presented by successive
improvements in the microscope itself, have given origin to a
variety of appliances and accessory apparatus that it would be futile
in a practical handbook to attempt to figure and describe. We pro-
pose, therefore, only to descinbe, and to explain the mode of success-
fully employing, the essential and the best accessories now in use,
neglecting, or only incidentally refei'ring to, those which are either
supplanted, or which present modifications either not important in
themselves or accounted for by the fact of their production by
different opticians.

I. Micrometers and Methods of Measuring Minute Objects. — It
is of the utmost importance to be able with accuracy, and as much
simplicity as possible, to measure the objects or parts of objects that
are visible to us through the microscope.

The simplest mode of doing this is to project the magnified
image of the object by any of the methods described under
' Camera Liicida and Di-awing.' We carefully trace an outline of
the image, and then, without disturbing any of the arrangements,
remove the object from the stage, and replace it with a ' stage micro-
meter,' which is simply a slip of thin glass ruled to any desired scale, •
such as tenths, hundredths, thousandths of an inch and even less.
Trace now the projected image of this upon the same paper, and the
means are at once before us for making a comparison between the
object and a known scale, both being magnified to the same extent.
The amoixnt of magnification in no way affects the problem. Thus,
if the drawn picture of a certain object exactly fills the interval
between the drawing representing the "01 inch, the object measures
the "01 inch, and whether we are employing a magnifying power of
a hundred or a thousand diameters is not a factor that enters into
our determination of the size of the object. In fact, all drawings of
microscopic objects are rendered much more practically valuable by
having the magnified scale placed beneath them, so that measure-
ments may at any time be made.

In favour of the above method of micro-measurement, it will be
noted (1) that no extra apparatus is required, (2) that it is extremely
simple, and (3) that it is accurate.

MICROMETER EYE-PIECES

271

The most efficient piece of apparatus for micro-measurement is
without doiibt the screw-micrometer eye-piece ; it was invented
by William Gascoigne in 1639 for telescopes, and if well constructed
is a most valuable adjunct to the microscope. It is made by
stretching across the field of an eye-piece two extremely fine pai'allel
W'ires, one or both of which can be separated by the action of a
micrometer screw, the circumference of the brass head of which is
divided into a convenient number of parts, which successively pass
by an index as the milled head is turned ; it is seen in fig. 212, B.
A portion of the field of view on one side is cut ofi" at right angles
to the filaments by a scale formed of a thin plate of brass having
notches at its edge, whose distance corresponds to that of the threads
of the screw, every fifth notch being made deeper than the rest to
make the work of enumeration easier. Formerly one filament was
stationary, the object being brought into such a position that one
of its edges appeared to touch the fixed wire, the other wire being
moved by the micrometer screw until it appeared to lie in contact

Fig. 212. — The micrometer eye-piece.

with the other edge of the object ; the number of entire divisions
on the scale then showed how many complete turns of the screw had
been made in the separation of the wires, while the number of
index points on the edge of the milled head showed the value of the
fraction of a turn that might have been made in addition. Usually
a screw with 100 threads to the inch is employed, which gives to
each division in the scale in the eye-piece the value of tw^^^ of ^i"i
inch, whilst the edge of the milled head is usually divided into 100
parts.

Both wires or filaments have since been made to move, a screw
and divided head being fixed to the stationary wire. There is no
advantage in this plan, and it involves needless complexity in calcu-
lation. The best method, there can be no doubt, is the one emj)loyed
by Mr. ISTelson, which is to have one thread fixed, but not in the
centre of the eye-piece, but five notches in the scale from the centre
on the side furthest from the screw-head. This not only permits of
a much larger object being spanned, but also keeps the average of
measurements in the middle of the ' field.' This is not only

272

ACCESSOEY APPAEATUS

convenient but important, because the magnification is not uniform
throughout the field. If the power employed is high, in order to
efiect the span of the great magnification, one wire (the fixed central
one) will be in the middle of the field, the other at the margin, and
the comparison will not be true on account of the unequal magnifi-
cation of the eye-piece throughout the field, whereas if the wire be
placed five notches on one side, both measurements are brought more
within the centre of the field.

Messrs. Zeiss now make a Ramsden micrometer eye-piece. ' It is
provided with a glass plate with crossed lines, which together with
the eye-piece are carried across the image formed by the objective
by means of the measuring screw, so that the adjustment always
remains in the centre of the field of view.

Fig. 213,

Fig. 213 illustrates this instrument, complete and in longitudinal
section.

Each division on the edge of the drum corresponds to 0*002 mm.
Whole turns are counted on a numbered scale seen in the visual
field, and the image may be measured up to 8 mm.

A modification of this instrument, facilitating both accuracy and
simplicity, was in 1890 devised by Mr. Nelson,^ of which we think
highly, and of which we give an illustration in fig. 214.

This screw micrometer eye-piece differs from those of the old
form mainly in two respects : first, the optical part is compensated ;
secondly, the micrometer part with both webs can be made to
traverse e7i bloc the field of the eye-piece by screw motion.

More particularly speaking, the instrument consists of two parts :
1 Joitrn. B. M. S. 1890, p: 508.

THE BEST FORM OF MICROMETER EYE-PIECE

27:

one, M flat rectangular box containing the fixed and movable webs,
the mici'ometer screw, and divided head complete ; the other pai-t
may be called an ' eye-piece adapter,' with sm outer case to hold the
above-mentioned rectangular box.

The flat inner box has a screw attached to it which engages with
a head on the exterior of the outer box. This gives about one inch
of screw movement to the inner box, which causes the webs to
traverse the field of the microscope. It must be remembered that
this in no way afi'ects the movement of the movable web from the
fixed, which can alone be accomplished by turning the graduated
micrometer head as in the old form.

The ' eye-piece adapter ' portion of the instrument is, as its name
implies, merely an adapter to take the optical part of positive com-
pensating eye-pieces of various powers.

Immediately below the web is an iris diaphragm. This permits
a diaphragm to be used suitable to the power of the eye-piece
employed. A guiding line at right angles to the webs has been
added. Care must be taken to observe that when the movable web
coincides precisely with the
fixed web, the indicator on the
graduated head stands at zero.
If this is not the case, the
finger screw must be loosed,
which will liberate the gradu-
ated head, and then it can be
placed in its proper position
and fixed. This is of universal
application to all screw micro-
meters.

Four points are gained by
this arrangement : —

(1) The compensating eye-
piece yields far better defini-
tion when measuring with apochromatic objectives than either the
Huyghenian or Ramsden forms.

(2) Diflferent-powered eye-pieces can be employed.

(3) By means of the screw which moves the micrometer webs
across the field it is possible to perform measurements with the webs
equidistant from the centre of the field, and thus eliminate errors
due to distortion.

(4) The preceding advantage is secured without sacrificing the
benefit of a fixed zero web.

Messrs. Zeiss have since adapted the compensating eye-piece to
their best screw micrometer.

To use the screw micrometer loith success it should not be insei'ted,
as the custom has been, like an ordinary eye-piece into the tube of
the microscope, but it should have a firm stmul quite indepetideiitly,
pi-e venting actual contact with the body-tube.

Plate II. gives the mode of its employment, the illustration being
made from a photograph by Mr. Nelson. The micrometer eye-piece,
it will be seen, is fitted into a stand wholly independent of the

T

Fig. 214. — Nelson's new form of screw
micrometer eye-piece.

274 ACCESSORY APPARATUS .

microscope. This consists of a strong upright, fitted into a massive
tripod or circular foot. The foot in either case only rests on three
points ; the uj)right is capable of telescopic extension by a clamping
tube ; a short tul)e which takes the eye-piece is fixed to this upright
by a compass joint.

To vise it, the object to be measured is placed in position, and
the microscope inclined in the usual way. The ordinary eye-piece
is removed, and the separate stand with the micrometer in its place
is put in front of the microscope, the extension tube being raised or
lowered until the tube at the top of it, carrying the micrometer, is
made continuous with the tube of the microscope, as seen in the
drawing. It is well to leave from ^th to i%ths of an inch of space
between the body-tube and the micrometer tube. It will be noAv
needful to employ corrections to compensate for the increased length
of tube. If the objective be provided with a ' correction collar ' the
adjustment must be re-corrected ; but if it is not so jDrovided the tube
of the microscope must be shortened exactly as much as the tube
carrying the micrometer will have lengthened it.

By this arrangement it will be found that manipulation can be
effected without the vibration of the microscopical image which is in-
evitably the result of the revolving of the micrometer screw head when
the micrometer eye-piece is placed, as it usually has been, in the body-
tube of the microscope. The consequence is that much more minute
spaces can be measured, and with much greater accuracy. Mr.
Nelson has repeatedly spanned the xoVoth of an inch by means of
a stage micrometer in the focus of the objective ; this was replaced
by a mounted specimen of AvijyJiijyleura pellucida, and he has counted
ninety-six lines in the j^^y th of an inch by making the movable wire
pass successively over them until the fixed wire was reached. By
similar means the Editor has measured single objects less than the
1 Woooth of an inch.

It will have been premised by the careful reader that the stage
micrometer must be used in every set of measurements ; at least we
would strictly emphasise this as the only accurate and scientific
method. It has been advised that a record of comparisons with the
vai-ious lenses in the possession of the microscopist should be made
once for all. We decidedly deprecate this method, unless it be in
such utterly valueless work, as is sometimes done, whei'e lenses are
uncorrected and accuracy of tube-length forgotten or ignored. The
correction of an objective and the tube-length ought to vary with
every object, and therefore a comparison of the stage-micrometer
and the screw-micrometer should be made with every set of measure-
ments.

Moreover, the majority of stage micrometers exhibit very con-
siderable discrepancies in the several intervals between the lines ;
it is well in the interests of accuracy to take the screw value of each
under a high power, find the value of the average, and then note the
particular space or spaces that may be in agreement with the average
and always use it. An illustration will make this clear.

Zeiss provides a stage micrometer of 1 mm. divided into "1 and

2;<l

TO OBTAIN THE VALUE OF A MICROMETEE INTERVAL 275

•01. The following are the actual values obtained for each of the "05
divisions, viz. : — n.j^^

8-37
8-38
8-38
8-36
8-36
8-58
8-33
8-31
8-47
8-33
8-33
8-38
8-44
8-38
8-40
8-37
8-40
8-25
8-38

2p)16-76jZ>

8 "38 mean value.

In this instance it will be seen that the last divitjion, 8"38, agrees
with the mean, and is the best for all future use.^

Having thus obtained a screw-micrometer value for a certain
known interval, the screw-micrometer value for any other object
being known, the size of the object may be found by simple propor-
tion ; thus, viz. if 8"38 is the screw-micrometer value for '05 mm.
and 6"45 that for a certain object, the size of the object is

(i) 8-38 : 6-45:: -05 : a; mm. ;

6-45 X -05

8-38

'=•0385 mm.

If the answer is required in fractions of an English inch, all that
we need remember is that 1 inch=25^4 mm. ; then

(ii) 8-38 : 6-45:: ^- :.« inch;

^ ' 25-4

6-45x^00197 -0127 nniKir- 1

x=^ — = = -001515 inch.

8-38 8-38

If the stage-micrometer is ruled in fractions of English inches,
then suppose the screw-micrometer value for ^wo'tli inch =• 4" 2 57,
and that for the object =6-45 as before.

(iii) 4-257 : 6-45 : : -001 : x inch ;

6-45 X -001

4^257

:=-001515 inch.

^ In tLe number given for screw value the whole number stands for a complete
revolution or number of revolutions of the screw head, and the decimal, the portion
of a revolution read oif beyond this.

T 2

2/6

ACCESSOEY APPARATUS

If the answei- is required in metrical measurement, then as 1
inch = 25-4 mm.,

(iv) 4-257 : 6-45 :: (-001 x 25-4) : .-^imm.

6-45 X -0254 -1638

4-257

4-257

= -0385 mm.

In this connection it will be as well to give two examples of
scale comparison which are sometimes required. Thus you have a
certain intei-val on a metrical stage micrometer which you know to
be accurate, and you wish to compare an English stage micrometer
with this scale in order to find out which particular interval of yg^Q^-
inch agrees with it. Suppose "05 mm. = 8-38 screw value as above,
then all that is necessary is to find the point to which the screw
micrometer m^ust be set in order that it may accurately span the
1 inch. Take 1 inch=25-4 mm. as before ; then -001 inch=

1000

•0254.

(v)

•05 mm. : ^0254 mm,
•0254x8-38

,«:= — ■ =

-05

: : 8-38 : x screw value :
■A- 257 screw value.

Conversely, if a metrical scale is to be compared with an accurate
English one where 'OOl inch =: 4- 257 screw value, then the screw
value for -05 mm. may be found thus : -001 inch=^0254 mm.
(vi) "0254 mm. : "05 mm. : : 4^257 : x screw value ;
•05x4-257

•0254

- = 8"38 screw value for "05 mm.

A cheap substitute for the screvi micrometer has been devised by
Mr. G. Jackson. It consists in having a transpai-ent arbitrary scale

inserted into an or-
dinary Huyghenian
eye-piece in the focus
of the eye-lens, so
that it will be in the
same plane as the
magnified image of
the object to be
measured. It is seen
in fig. 215. The
method of using it is
precisely similar to
that of the screw
micrometer ; the

value of xwo ii^^ch or
y^y mm., as the case
may be, is found in
tei-ms of the arbitrary
scale. The value of

Fig. 215. — Jackson's eye-piece micrometer.

the object in terms of the same scale is also found, and comparison
made accordingly. All that need be done is to substitute the terms
of the arbitrai'y scale for screw values in the pi-eceding examples, and
they will meet the case.

ESTLVIATINGr THE EDGES OF MINUTE OBJECTS 277

The arbitrary scale should be capable of movement by a sci-ew,
othei'wise the appliance is liai-dly as accuiute as the first method of
micrometry by simple drawing desciibed above.

Of all the methods of micrometry the most accurate is that
performed by photo-micrography. A negative of the object to be
measui'ed is taken, and then, without any alteration in tube- or
camera-length, the magnified image of the stage micrometer is pi'O-
jected on the ground glass ; this is spanned by means of a pair of
spring dividers. The negative film is then scratched by these
dividers. Then you are in a position to make the most accurate
measurement the microscope is capable of yielding.

It is exceedingly important, when performing micrometric
measurements, to remember that the precise edges of all objects in
the microscope are never seen. Consequently it is impossible to
ascertain from what point to what point the measurement is to be
made.

This, while hardly afl:ecting large and coax'se objects, becomes
supremely important with small objects.

Instead of a real edge to an object you get diffraction bands.
These bands alter with focus, and also to a greater extent with the
angle of the illuminating cone as well as with the apertui'e of the
objective. Hence it ensues that the accurate micrometry of delicate
objects presents one of the most difficult matters encountered in
practical microscopy. At the present time opinions differ greatly
as to the treatment of particular cases.

The following plan of Mr. Nelson's is the outcome of along series
of experiments : —

1. The focus and adjustment to be chosen may be termed that of
the ' black dot ' (see Elimination of Errors of Interpretation) ;
in other words, if the object were a slender filament it would be
represented white with black edges. These black edges are due to
diffraction. If the filament is very slender and the illuminating cone
small, there may be seen a white diffraction edge outside the black
one, and peiliaps another faint black one outside that again.

2. Reduce as far as possible the extent of these diffraction bands
by («) using an objective with as large an apei'tui'e as possible ; (b)
by using as large an illuminating cone as possible.

3. Measure from the inner edge of the inner diffraction band to
the inner edge of the inner diffraction band on the opposite side.

4. But if the diameter of a hole be required, then the measure-
ment must be made from the outer edge of the outer black diffraction
band to the outer edge of outer diffraction band on the opposite side.
It must not be forgotten, however, that these rules only apply for a
particular focus and a particular adjvistment.

II. The Camera Lucida and its Uses. — There are a large numbei-
of contrivances devised for the purpose of enabling the observer
to see the image of an object projected on a surface upon which he
may trace its outlines, but they resolve themselves practically into
two kinds, viz. : —

1. Those intended for use when the microscope is in a horizontal
position.

2/8 ACCESSOEY APPAEATUS

2, Those provided for it when used in a vertical position.

We shall describe what we consider the most practical forms of
each.

In point of antiquity Wollaston^ s camera Xiicida claims the post
of honour ; but to use it the microscope must be placed in a hori-
zontal position. Its general form is shown in fig. 216. The rays
on leaving the eye-piece, above which it is fixed by a collar, enter a
prism, and after two internal reflections pass upwards to the eye of
the observer. It is easy to see a projection of the microscopic image
with this instrument, but it is when we desire at the same time to
see the paper and the fingers holding the pencil that the difficulty
begins. The eye has to be held in such a position that the edge of
the prism bisects the pupil, so that one-half of the pupil receives
the microscopic image and the other half the images of the paper
and the hand employed in drawing. If this bisection is not equal,
too much of one image is seen at the expense of the other. This
was in some sense supposed to be compensated by the use of lenses,
as seen in the figure ; but the difiiculty of keeping the eye precisely
in one position has caused this instrument to fall into disuse, several

cameras being now devised free from
this defect. It has nevertheless one
special point in its favour — it does
not invert the image, causing the

Fig. 216. Fig. 217. — Simijle camera.

I'ight to be turned to the left, and vice versa. This is an advantage
the value of which we shall subsequently see.

A simple camera was made by Soemmering by means of a small
circular reflector, usvially made of highly polished steel, which is
placed in the path of the emergent pencil at an angle of 45° to the
optic axis, thus reflecting rays from the image uj)wards. The
instrument, though rarely used now, is shown in fig. 217, and slides
on to the eye-piece. The reflector must be smaller than the pupil
of the eye, because it is through the peripheral portion of the pupil
that the rays, not stopped out by the mirror, come from the paper
and pencil. Hence, as in the case of Wollaston's camera, the pupil
of the eye must be kept perfectly centred to the small reflector.
As there is but one reflection, the image is inverted, but not trans-
posed. To see the outline of the image as it is in the microscope,
the drawing must be made upon tracing paper, and inverted, looking
at it as a transparency from the wi^ong side.

There is considerable variety in the experience of diflferent
microscopists as to the facility with which these two instruments
can be used. The difference in all probability depends on the

CAMERJE LUCID^

279

Fig. 218.
Beale's camera.

greater normal diameter of the pupils of the eyes of some observers
in comparison with that of others'.

Dr. Lionel Beale devised one of the si'mjilest cameras, which has
the advantage of being thoroughly efficient. It consists of a piece
of tinted glass placed at an angle of 45° to the optic
axis, in the path of the emei'gent pencil. The idea
was first suggested by Amici, but he employed un-
coloured glass ; Dr. Beale made it practical by the
employment of tinted glass. The first surface of the
glass reflects the magnified image upwards to the eye,
the paper and pencil being seen through the glass.
Tn its simplest form it is seen in fig. 218. The glass
is tinted to render the second reflection from the internal surface
of the glass inoperative. The reflection of the image is identical
with that of Soemmering' s.

Another camera lucida of some merit is that devised by Amici,
and adapted to the horizontal microscope by Chevalier. The eye
looks tln-ough the microscope at the object (as in the ordinary view
of it), instead of looking at its projection upon the paper, the image
of the tracing point being projected upon the field — an arrangement
which is in many respects more advantageous. This is efifected by
combining a perforated silver-on-glass mirror with a reflecting
prism ; and its action will be understood by the accompanying
diagram (fig. 219). The ray a b proceeding from the object, after
emerging from the eye-piece of
the microscope, passes through
the central perforation in the
oblique mirror M, which is placed
in front of it, and so directly
onwards to the eye. On the other
hand, the ray a', proceeding up-
wards from the tracing point,
enters the prism P, is reflected
from its inclined surface to the
inclined surface of the mirror M,
and is by it reflected to the eye
at b', in such parallelism to the
ray b proceeding from the object
that the two blend into one
image.

A valuable and simple little
camera was devised by Mr. E. M.
Nelson in 1894.^ It takes into Fm. 219.

account the fact that while that

form known as Beale's neutral tint (fig. 218) has been of great
value and persistence, it is yet a defective form ; the microscopic
image as received at the eye-piece is inverted and transposed.
Beale's camera corrects the inversion, while it leaves the tiunsposi-
tion unaltered ; therefoi'e all the objects drawn with this camera
are unlike the originals. In illustration place the letter F o^^
1 Journ. B: M. S. 1895, v. 21 et seq.

28o

ACOESSOEY APPARATUS

the stage in the position as here printed ; when examined by the
microscope it will appear thus j . In order to look at this letter as
the original, all that we have to do is to tui-n this paper round.
But this object, as drawn by a Beale's camera, will appear "^j and no
turning of the paper can cause it to appear as the original ; it will
only become so when it is viewed as a transjDarency from the other
side of the paper.

This is, of course, impoi'tant in many matters with which the
microscopic biologist is concerned.

In many forms of camera this difficulty has been overcome by
reflecting the image of the paper and pencil down the tube of the
microscope. The drawing there made will be inverted and trans-
posed, but by turning the picture round we at once get a correct
representation of the object itself.

The new camera devised by Mr. Nelson consists of a right-angled
prism or small glass miri-or fixed at an angle of 45° to an eye-piece
cap. This, when the microscope is placed in a horizontal position,
reflects the rays horizontally and at right angles to the optic axis ;
these rays then fall on a piece of neutral-tint glass placed at an
angle of 45° to those rays so as to I'eflect them upwards to the eye.

The mirror corrects the transposition, and the neutral-tint the
inversion ; an erect image is therefore seen on the table. The neutral-
tint glass is mounted on a pivot so that it may be turned round at
a right angle ; this adapts the instrument for use with either the
right or left eye. Should the light be too strong, it must be
m.odified by screens, not by change of focus in the condenser, assum-
ing that the perfect image has been obtained.

On the important subject of the inversion and transposition of
microscopic images brief but valuable data are given and put in the
clearest light, thus : —

3 4

Image projected on sci'een Image seen through

or on sensitive plate. ground glass.

1 2

Object on the Image seen through /
stage. the eye-piece.

F d

Image seen through Woll-
aston's camera.

Image projected on table
by 45° mirror or right-
angled prism, as devised
by C. W. Cooke.

F

6

Image seen through
Beale's neutral tint or
Soemmering's reflector.

Image seen through Nel-
son's camera.

The instrument referred to in (7) of the above table of inversion
and transposition in mici'oscopic images is a somewhat distinct form
of camera called by Mr. Conrad W. Cooke, who devised it in 1865, a
' Micrographic Camera.' The pi'ojection of the image is dependent
on a silvered mirror fixed at 45°, or a right-angled prism. By the
aiTangement of this insti'ument an image can be thrown on a sheet
of paper placed in a hoiizontal position, so that one can readily trace

ABBE'S CAMERA LUCIDA 28 1

on the paper the outKnes and details of the image with ease and
accuracy ; only it must be i-emembered that the mirror or prism
ei-ects the inverted image (No. 2 in the above table), but its trans-
position is due to the fiict of its not being viewed as a transparency.

This instrument is also useful for the purpose of demonstrating
where two or three pei'sons may at the same time examine the
image, and it can be used on many opaque objects, and objects pre-
sented by dark ground illumination ; but to use it the external light
must be carefully screened fi-om the observer.

Coming now to the second group of cameras, thei'e stands first on
the list an insti'ument devised by Professor Abbe ; although, like
many ' new ' apparatus for the microscope, the idea it embodies is
not a new one, but was suggested for micrometric purposes by Mr.
G. Biu'ch in 1878 {,Tourn. Quek. Micro. Club, v. p. 47). We have
used this admirable instrument with complete success.

The accompanying drawing (fig. 220) will at once show the
simplicity of its action. The image of the paper and pencil coming,
say, in a vertical direction (Sj fig. 220), is reflected by a large mirror

Fig. 220. — Abbe's camera lucida.

in a horizontal dii-ection, W, to a cube of glass which has a silvered
diagonal plane with a small circular hole in it in the visual point of
the eye-piece. The microscopic image is seen directly through this
aperture in the silvering of the prism, while the silvered plane of
the prism transmits the image of the paper and the operator's fingers
and pencil. By the concentricity thus obtained of the bundle of
rays reaching the eye from both the microscope and the paper, the
image and the pencil with which it is to be drawn are seen coinci-
dentally without any straining of the eyes.

This instrument requires the paper to be placed in a plane
parallel to that of the object ; thus, if the microscope is vertical the
paper must be horizcjntal, and vice versa, and it presents the image
precisely as it is seen in the microscope. For the piu-pose of drawing
simply, and where the observer has had no experience in the use of
a camera lucida, we should be inclined to recommend this one as the
instrument presenting to the tyro the greatest facility. But there
is a use to be made of the camera lucida to wdaich this one does not so
readily lend itself, which is none the less of great importance ; that is.

282

ACCESSOEY APPAEATUS

the determining of the magnifying power of objectives. It is manifest
that the distance between the paper and the eye of the observer
cannot be so readily determined in this case as in those forms of the
instrument where the image of the paper and pencil is seen direct.

The same apparatus arranged so that the prism casing together
with the mirror may be swung back while the clamping collar
remains on the tube in its adjusted position, is shown in fig. 221.
The mirror has a surface of 75 x 50 mm. (3 x 2 in.), and may be
inclined at any angle between the horizontal plane and 45°, the
latter position being marked by a stop. The length of the arm
supporting the mirror being 10"5 cm. (4 in.), it is only with very
large drawings necessary to incline or raise the drawing surface.

But the latest modification of this instrument is shown in figs.
222 and 223, where it will be observed that the camei^a is attached
to the tube by means of the clamping-ring K, and the Abbe double

Fig. 221. — Abbe's camera, improved.

prism is centred by means of the screws L and H. The brightness
of the drawing surface and the microscopic image is respectively
regulated by a cap E. encasing the prisms, which is pi'ovided with a
clear opening and five moderating glasses of varying degrees of
density, and by an eccentric disc B pivoted below the prisms, which
is also provided with a clear opening and five moderating glasses.

In order to completely utilise the increased cone of emerging
rays obtained with low magnifications, the usual prism, having in its
silvering an aperture of 1 mm., can quickly and conveniently be
exchanged foi' another with an ajDerture of 2 mm.

The prism, together with the moderating glasses, may be turned
aside abou.t the vertical pin Z into the position indicated by the
dotted lines shown in fig. 222. When the prism is returned to its
original position it is fixed by a catch, which is not externally
visible.

In the use of a good drawing apparatvis (1) the light fi^om the

LATEST FORMS OF ABBE'S CAMERA LUCIDA

28-

image must not to any sei-ions extent be weakened by the light from
the di-awing materiaL (2) The image of the drawing paper must

Fig. 22'2. — Latest modification of Abbe's camera

reach the eye with the least possible intensity and be coaxial with the
microscopic image. (3) Thei'e should be an ai^rangement by which

Fig. 223.

the relation of the intensities of these two images can be modified
to suit each other. (4) The apparatus must be adjustable in height

284 ACCESSORY APPAEATUS

and capable of being centred in its horizontal plane. (5) It should
be possible to easily separate the apparatus from the eye-piece and
replace it again in its former position at will. (6) The image of the
plane of the drawing, and the image of the microscopic object pro-
jected on it, must be seen with the apparatus without distortion.
As regards the fii'st two conditions the arrangement of the original
Abbe camera is adojjted, viz. two rectangular- prisms with the hyjao-
tenuses cemented together, of which one is silvered, with a small
portion of the silver deposit in the centre taken away, and with these
a second mirror A, fig. 222, for transmitting the image of the plane
of the drawing to this prism. But since one and the same prism,
with a fixed opening in its silver deposit, cannot suffice for all
purposes and changes of magnification, an arrangement is added by
which the prism P, fig. 223, with its fastening, can be easily taken
out of the apparatus and replaced by another with an opening of
different size.

With respect to the third condition securing a due relation
between the intensities of the two images, an arrangement of two
smoked-glass wedges was made to move over each other so as
to form a plate of continuously varying thickness. This was most
satisfactory but too costly, so smoked-glass plates were employed
and set in the cylindrical wall of a small cap, R, figs. 222, 223, which
was simply placed over the prism. Each smoked glass in turn can
be interposed in the path of the rays by turning the cap on its
upper edge until a small pin engages in a corresponding small hole on
the lower edge of the cylinder. There are five smoked glasses of
different densities of coloiu-, while one aperture is left empty.

The adjustment in height is satisfied by the apparatus being-
attached to the body-tube by means of a clamping screw, while the
adjustment from side to side is eff"ected by the prism, together with
the cap and smoked-glass disc, being centred from front to back by
means of a screw, H, figs. 222, 223, working through a spring socket,
and from right to left by means of a second screw L, against which
works a counter-spring not shown in the figures.

In order to pass conveniently from observation through this ap-
paratus to observation through the fi-ee eye-piece, the prism with its
diaphragm arrangement can be i-otated to one side about a vertical
pin Z ; the return of the pi-ism to its central position is marked by
a sjjring catch. To obtain drawings fi-ee from distortion, a di'awing
table similar to that described by Dr. Bernhard ought to be
employed.'

This useful instrument has, however, been modified and made
simpler by more than one optical firm. Messrs. Swift have con-
structed a very handy and easily applied form, which is so arranged
that the microscope may be employed with it not only in the
vertical but also in an inclined position. It is illustrated in fig. 224.

This camera lucida is precisely on the same principle as the Abbe
form used for the same pui-pose, but being manifestly less bulky it is
far more convenient and easier to use, although less efficient for very
careful work.

1 Zeitschr. f. tviss. Mihr. xi. (1894), pp. 289-301.

ENGLISH AND AMERICAN MODIFICATIONS

285

When this foi-m of camei-a is used, the papei- upon which the
object is received should be tilted to the same plane as the stage of
microscope upon which the object rests, as this will prevent any
marginal distortion.

Another exti-emely good and easily applied
modification of the Abbe form is manufactured by
Bausch and Lomb, and is illustrated in fig. 225.
The Abbe prism is used as in the large Abbe
di-awing camera ; the mirror is reduced in size
and is fixed. The path of the light is seen to be
the same as the white dotted lines and arrows
show, as in the complete form of Abbe ; and the
camera may be swung back when not in use, as
shown in the dotted outline. We can testify that
the image off both object and pencil-point are clear,
and this instrument can be used with most eye-pieces ; but cannot for
complete results be counted equal to the drawing camera of Abbe.

The Editor has used with great facility and success a camera devised
by Dr. Hugo Schroder, and produced by Messrs. Ross. It is figiu^ed
at 226, and consists of a combination of a right-angled prism (fig.
227) ABO, and a rhomboidal jorism D E F G, so arranged that when

Fig. 224. — Swift's
camera lucida ou
the Abbe principle.

Fig. 225. — Bausch and Lomb's modification of Abbe's camera.

adjusted ver}- nearly in contact [i.e. sepai-atecl by onh^ a thin sti'a-
tum of air) the faces B C and D E are parallel, and consequently
between D E and B E' they act together as a thick parallel plate of
glass through which the drawing pajoer and pencil can be seen.
The rhomboidal prism is so constructed that when the face G F is
applied at right angles to the optic axis of the mici'oscope, the axial
ray H passes without refraction to I on the internal face E F ;
whence it is totally reflected to J in the face D G . At J a part of

ACCESSORY APPARATUS

the ray is reflected to the eye by ordinary reflection in the direction
of J K, and a part transmitted to J' on the face A 0 of the right-
angled prism. Of the latter a portion is also reflected to K by
ordinary reflection at J'. The hypotenuse face A C is cut at such
an angle that the reflection from J' coincides with that from J at
the eye-point K, thvas utilising the secondary reflection to strengthen
the luminosity of the image. The angle G is arranged so that the
extreme marginal ray H' from the field of the B eye-piece strikes
ujjon D G at a point just beyond the angle of total reflection, the
diffraction bands at the limiting angle being faintly discernible at
this edge of the field. This angle gives the greatest amount of
light by ordinary reflection, short of total reflection.

In use, the microscope should be inclined at an angle of 45°, and
the image focussed through the eye-piece as usual ; the camera is
then placed in position on the eye-piece, and pushed down until the
image of the object is fully and well seen. The drawing paper
must be fixed upon a table on a level with the stage immediately

Fig. 226. — Schroder's Pig. 227. — Diagram explaining Schroder's camera hicida.

camera lucida.

under the camera. The observer will then see the microscopical
image projected on the paper, and the fingers carrying the pencil
point will be clearly in view, the lohole jjupil of the eye being-
available for both images, the diaphragm on the instrument being
considerably larger than the pupil. The eye may be removed as
often as required, and, if all is allowed to remain without alteration,
the drawing naay be left and recommenced without the slightest shift-
ing of the image.

If a vertical position of the microscope be needful, this may be
done by inclining the table and drawing paper to an angle of 45°
either in front or at the side of the microscope. For accurate
drawing, in all azimuths, the drawing paper should of course coin-
cide with the plane of the optical image. When the paper is in its
proper position, the limiting circle of the field of the microscope
will be projected as a true circle, but if otherwise it will appeal-
elliptical. It is recommended that a circle about the size of the
field be drawn upon the papei', and its coincidence with the projected
field compared.

THE USE OF THE CAMEEA LUCID A 287

This camera may be used with a hand-magnifier, or with simple
lenses used for dissection and other purposes.

With one or other of the foregoing contrivances, every one may
learn to di-aw an outline of the microscopic image ; and it is extremely
desirable for the sake of accui-acy that eveiy representation of an
object should be based on such a delineation. Some persons will use
one instrument more readily, some another, the fact being that
there is a sort of ' knack ' in the use of each which is commonly
acquired by practice alone, so that a person accustomed to the use
of any one of them does not at first work well with another.
Although some persons at once acquire the power of seeing the
image and the ti-acing point with equal distinctness, the case is more
frequently otherwise ; and hence no one should allow himself to
be baffled by the failure of his first attempt. It will sometimes
happen, especially when the Wollaston prism is employed, that the
want of power to see the pencil is due to the faulty position of the
eye, too large a part of it being over the prism itself. Wlien once
a good position has been obtained, the eye should be held there as
steadily as possible, until the tracing shall have been completed. It
is essential to keep in view that the proportion betAveen the size of
the tracing and that of the object is aflfected by the distance of the
eye from the paper ; and hence that if the microscope be placed
upon a support of different height, or the eye-piece be elevated or
depressed by a slight inclination given to the body, the scale will be
altered. This it is, of course, peculiarly important to bear in mind
when a series of tracings is being made of any set of objects which
it is intended to delineate on a uniform scale.

A valuable adjunct to a camera lucida is a small pai'affin lamp,
seen to the left of plate III., which illustrates the correct method 01
using the camera lucida. This lamp is simple, and is capable of being-
raised or lowered, fitted with a paper shade, for a great deal of the
success attendant on the use of the camera depends on the relative
illumination of the microscopic image on the one side, and of the paper
and fingers and pencil of the executant on the other. It is not a
matter to be determined by rules ; personal equation, sometimes
idiosyncrasy, determines how the light shall be regulated. Many
finished micro-draughtsmen use a feeble light in the image and a
strong light on the hand and paper, and others equally successful
manipulate in the precisely reverse way. But upon the adjustment
of the respective sources of light to the personal comfort of the
draughtsman will depend his success.

Care must be exercised in this work in the case of critical images.
These must not be sacrificed either by racking the condenser into or
out of focus, or by reducing its angle by a diaphragm. If the in-
tensity of the light has to be reduced, it must be done by the inter-
position of glass screens, and this is beautifully provided in Abbe's
camera. The illustration of how the various apparatus for the use
of the camera lucida should be disposed, given in plate III., may be
profitably studied. Both mirror and bull's-eye are turned aside,
and the hand and pencil are illuminated by the shaded lamp.

The lamp illuminating the image is seen, with such a screen of

288 ACCESSOKY APPARATUS

coloui^ed glass as may be found needful, and the lamp illuminating
the paper and pencil, and carefully shaded above, is also seen at the
eye-piece end of the body-tube. Often, if the image is too bright,
we find that bringing the lamp down to illuminate the paper more
intensely suffices If not, use screens ; the illuminating cone must
not be tampered with.

III. The Determination of Magnifying Power is an important
and independent branch of this subject. For this purpose, and for
the reason given above, Beale's neutral-tint camera ^ is eminently
suitable — indeed, is the best. We can easily and accurately measure
the path of the ray from the paper to th-e eye. What is necessary is
to project the image of a stage micrometer on to an accurate scale
placed ten inches from the eye-lens of the eye-piece. There must be
complete accuracy in this matter.

We can best show how absolute magnifying power is thus deter-
mined by an example.

Suppose that the magnified image of two YgL^ths of an inch
divisions of the stage micrometer spans i^o^ths of an inch on a rule
placed as required ; then

(i) "002 inch : "8 inch :: 1 inch : x power ;

X — =400 diameters ;

•002

for it is obvious that under these conditions one inch bears the same
proportion to the magnifying power that i-^ou^l^^ °^ ^^^ inch bears to
j^ths of an inch.

Suppose, now, as it sometimes happens, that the operator is pro-
vided with a metrical stage micrometer, but is without a metrical
scale to compare it with, there being nothing but an ordinary foot-
rule at hand.

Let it be assumed that the magnified image of two y^ mm. when
projected covers yQ inch ; then, as there are 25'4 mm. in one inch,

(ii) -02 mm. : (-8 inch x25-4) :: 1 : x power ;

•8 X 25-4 X 1

x:= =1016 diameters.

■02

If the reverse is the case, viz. that you have an English stage
micrometer and a metrical scale, then, if the magnified image of two
J oLj-jjths of an inch spans 18 mm.,

(iii) -002 inch :-^^ ::!:«.;

a;= =354"3 diameters.

•002

The above results indicate the combined magnifying power of the
objective and eye-piece taken at a distance often inches. The arbi-
trary distance of ten inches is selected as being the accommodation
distance for normal vision.

The magnifying power, however, is very different in the case of

1 Page 279.

TO FIND THE INITIAL POWEE OF A LENS 289

a myopic observer. Let us investigate the case of one wliose accom-
modation distance is five inches.

Here he will be obliged, in order to see the object distinctly, to
form the virtual image from the eye -piece at a distance of five inches.
To do this he must cause the objective conjugate focus to approach
the eye-lens ; consequently he must shorten his anteriol" objective
focus. In other words, he must focus his objective nearer the
object. This will have the efiect of causing the posterior conjugate
focus to recede from the objective towards the eye-lens, and the fact
of bringing the inverted objective image nearer the eye-lens brings
also the virtual image of the eye-lens nearer.

Shortening the focus of the objective has the effect of increasing
its power ; but as this alteration is proportionately very little, the
increase in power is very small ; but the shoi'tening of the eye-piece
virtual from ten to five inches has the effect of nearly halving its
power. Consequently the combined result of the eye-piece and
objective, in the case of halving the eye-piece virtual, is to nearly
halve the power of the microscope. The increase of the objective
power is practically so small that it may be neglected.^ In pi'actice
it is found by us that if the image is projected on a ground-glass
screen ten inches from the eye-piece, the image is nearly the same
size whether focussed by ordinary or myopic sight. This is in
harmony with Abbe's demonstration that both images are seen
under the same visual angle. But, on the other hand, if a myopic
sight compares the image with a scale, the magnification will be
less than with ordinary vision, because the observer with myopic
sight must bring the scale to a shorter distance than ten inches
in order to see it.

To find the precise initial power of any lens, or to find the exact
multiplying power of any eye-piece, is not so easy. A laborious
calculation, involving the knowledge of the distances, thickness, and
refractive indices of the lenses, is required. But a very approximate
determination, sufficiently accurate for all jDi'actical purposes, may
be easily made, especially if one has a photo-micrographic camera at
hand. The principle is as follows : —

Select a lens of medium power — a ^-inch is very suitable. Now,
with the microscope in a horizontal position, and with a powerful
illumination, project the image of the stage micrometer on to a screen
distant five feet, measured from the front lens of the objective. If no
photo-micrographic camera is at hand, it will be necessary to perform
the experiment in a darkened room, shading the illuminating source.
Divide the magnifying power thus obtained by 6 ; the quotient will
give the initial power of the lens at ten inches to a A'ery near approxi-
mation.

The reason why the result is not perfectly accurate is that the
ten inches must be measured from the posterior principal focus of
the lens, and that is a point which is not given. But in the case of
a power such as a ^, it is, in practice, found to be very near the front
lens of the objective. So by taking a long distance, such as five feet,

1 English Mechanic, vol. xlvi. No. 1185. Article 011 measurements of magnifying
power of microscope objectives, by E. M. Nelson.

U

290

ACCESSOKY APPAEATUS

the error introduced by a small displacement of the posterior prin-
cipal focus does not materially amount to much.

There is a further error introduced by the appi-oximation of the
objective to the stage micrometer in order to focus the conjugate at
such a distance, but this is small. We can see, therefore, that this
error tends to slightly increase the initial magnifying power.

The initial power of the J being
found, and its combined magnifying
power, with a given eye-piece, being-
known, the combined power divided
by the initial power gives the multi-
plying power of the eye-piece. Care
must be of course taken to notice the
tube-length ^ when the combined power
is measvu-ed. The initial power of any
other lens may be found by dividing
the combined power of that lens with
the eye-piece, whose multiplying power
has been determined, by the multiplying
power of that eye-piece.^

Nose-pieces. — The term ' nose-piece'
primarily means that part of a microscope into which the objective
screws, but the term is also applied to various pieces of apparatus
which can be fitted between the nose-piece of the microscope and
the objective. There are, for instance, rotating, calotte, centring,
changing, and analysing nose-pieces.

Nose-pieces, although thought to be so, are not a modei-n idea ;
our predecessors of a century ago employed similar means. Mr.
Crisp has recently acquired a microscope which possesses a double
arm, at the end of which is a cell for receiving different lenses.
This cell fits over the end of the nose-piece, and so keeps the several
objectives which may be insei-ted in position. It dates, in all proba-
bility, from the end of the seventeenth or the early part of the
eighteenth century.

But in the early days of the microscope rotating discs of objec-
tives, as shown in fig. 228 (or, perhaps, older still, a long dovetailed

Fig. 228.— Eotating disc of
objectives. Benj. Martin
{circa 1776).

Pig. 229.— Sliding plate of objectives. Adams (1771).

slide of objectives, such as fig. 229 shows), were frequently
em^ployed.

It is continually desirable to be able to substitute one objective
for another with as little expenditure of time and trouble as possible,
so as to be able to examine under a higher magnifying power the
details of an object of which a general view has been obtained by

1 English Mechanic, vol. xxxviii. No. 981, ' Optical Tube-length, by Frank Crisp.

2 Ibid. vol. xlvi. No. 1178, ' Measurement of Power,' bytE. M. Nelson.

NOSE-PIECES

291

means of a lower ; or to use the lowei- for the pui-pose of finding a
minute object (such as a particular diatom in the midst of a slideful)
which we wish to submit to higher amplification. Tliis was con-
veniently efiected by the nose-piece of Mr. C. Brooke, which, being-
screwed into the object end of the body of the microscope, carries
two objectives, either of which may be brought into position by
turning the arm on a pivot. This is shown in fig. 230.

The most generally useful of
all nose-pieces now in use are
the rotating foi-ms, Avhich enable
one to carry two, three, or four
objectives on the microscope at
one time, and by mere rotation
each is successively bi-ought
central to the optic axis, seen in
figs. 231, 232, 233, as supplied
by Messrs. Beck. It is almost
unnecessary now to point out the
disadvantage of those older and
straight forms which involved
the danger of knocking out the
front lens of the objectives by
bringing it into contact with some part of the stage while the
other objective was being focussed. This objection was entirely
removed by the introduction of the bent form by Messi'S. Powell and
Lealand, and adopted in the forms shown in figs. 231-233. There can

Fict. 230. — Brooke's uose-
piece, as made by Swift.

Fig. 231.

Fig. 232.

be no doubt that for ordinaiy dry lens woi'k some such device is im-
perative. Some, however, who do a very large amount of microscopical
work prefer to use two microscopes ; the one a third- or fourth-class
microscope, with only a coarse adjustment and a 1-inch objective and
miri-or, the othei- having a coai-se and fine adjustment and a ;|-inch
objective, with a simple form of condenser and plane mirror, all fine
and higher-power work being left for a special microscope.

The one di'awback to the use of a rotating nose-jjiece is the exti-a
weight it throws upon the fine adjustment. As this subject is fully

u2

292

ACCESSOEY APPAEATUS

treated under the heading of 'Microscope,' no more will be said at
present than that a double nose-piece is to be preferred to a triple,
and a quadruple need not be entertained for a delicate instrument
when made of ordinary metal, unless it is required to find out in how
short a time a fine adjustment may be ruined ; for let it be noted
that a 2-inch, 1-inch, ^-inch, and J-inch objective of English make
weigh together 8^ oz. without any nose-piece. But Messrs. Watson
and Son have devised and made in aluminium a dust-proof triple
nose-piece, which, where it is required to be used, reduces the objec-
tions to its emplopiient to their minimum, and not only in greatly
reduced weight, laut in other ways, makes its use more feasible
without strain upon the fine adjustment or dangei- of injury to the
objectives. In many nose-pieces, if the objectives should be acci-
dentally left so that neither of them is in the optical axis of the
microscope, there is nothing to guard the back lenses of the objec-
tives from dust and moisture. Messrs. Watson devised a dust-

PiG. 234. — Watson's dust-iDi'oof aluminium nose-i^iece.

Fig. 235. — Section of the above.

proof arrangement, consisting of an upper and an undei- disc, having
a spherical curve ; to the lower disc are fitted thi-ee small screw
tubes which receive the objectives. This plate rotates upon a centre
pin, and as each objective is brought into the optical axis of the
microscope its axial coincidence is indicated by a spring catch. The
edge is covered with a metal rim, making it dust-proof. The weight
of the ordinary brass nose-piece is 4| oz. ; the weight of this one is
1| oz. Similar instruments are made by other makers, but the
dust-proof arrangement and the extreme lightness are, so far as we
know, chai-acteristic of the instrument of Messrs. Watson. We
illustrate this nose-piece complete in fig. 234, and in an enlarged
section in fig. 235.

For the propei- use of a rotating nose-piece the length of the
objective mounts should be so arranged that when the objective is
changed little focal adjustment will be necessary.

An excellent calotte iiose-piece for four objectives is made by

CHANGING NOSE-PIECES 293

Zeiss ; this is so arranged that only the optical poi'tion of the objec-
tive is screwed into the nose-piece. This plan much lightens it, so
that the nose-piece and the four lenses weigh 3| oz., or only 1 oz.
more than an English ^-inch with a screw collai-, and j oz. more than
an English Vinch of wide angle.

A centring nose-piece has been made with the view of placing
any objective central to the axis of rotation of the stage. It is, of
course, much cheaper to centre an objective by means of a nose-piece
to the axis of rotation of the stage than to centre the rotary stage
to the objective. This, like all other adapters, is an additional
weight ; but here there is very little to be gained by it, for if the
rotary stage is well made any objective will be sufficiently centred
for all practical purposes. Mr. Nelson, as we have seen, pointed
out, at a time when the sub-stage was costly, that such a nose-
piece turned upside down, with a turn-out rotating ring for stops, &c.,
fitted below, made a very efiicient rectangular centring sub-stage
at a small cost. Sub-stages are now quite common and chea]3, and
centring nose-pieces are seldom used for any purpose.

Next to the rotating, probably the changing nose-jnece is the
most important. We do not know from whom, and when, the idea
of an arrangement by which an objective could be rapidly attached
or detached originated ; but certain it is that the idea is admirable,
and one which is scarcely yet as fully appreciated as it should be.
It will be quite impossible to go through a tithe of the appliances
which have been invented for this purpose ; it will be sufficient to
lay down some principles, and mention a few in which those prin-
ciples are fulfilled.

The first principle is that the objective or nose-piece, adapter,
or whatever else is used, should ' face up.' This means that a flange
turned true in the lathe should ' face up ' to the flat side of the nose-
piece, which has also been turned true. This ' facing up ' should be
made tight by a screw, inclined plane, or wedge, kc. Unless this
is done you have no guarantee that the axis of the objective is
parallel to that of the body. Therefore all those appliances which
merely grip the objective, or an adapter screwed on to the objective,
are simply of no value. Secondly, the appliance, whatever it is,
should be light.

Nachet's changing nose-piece, which fulfils none of these con-
ditions, cannot be called good. The nose-piece is large and heavy,
even for the small objective it is intended to take, the screws of
which are -/g- only in diameter, against the \^ of that of the Society.
The objectives are held by a spring clip on a small flange. Of course,
screw-collar adjustment with such a device would be simply im-
possible. Zeiss's sliding-objective changei' is most elaborate and
efficient, although, as we think, much heavier than it need be. It
consists of a grooved slide which screws on to the nose-piece. On
each objective is screwed an adapter to slide into the grooved nose-
piece. These adapters, which are wedge-shaped and ' face up,' have
two novel features, the first being that they are each fitted with
rectangular centring adjustments, which j)ermit the objectives
to be centred to one another ; and the second is that they have

294

ACCESSOEY APPAEATUS

adapters to equalise the length of the objectives, so when a change
of objectives is made little change of focal adjustment is requii-ed.
Figs. 236, 237 show the natui*e of this arrangement. In Nelson's
changing nose-piece a small ring with three studs is screwed on to
the objective ; a nose-piece is screwed on the microscope, having
three slots and three inclined planes. Therefore, by j)lacing the studs
into the slots and giving the objective a quarter of a tiu^n, the
studs run up the inclined planes, thus causing the flanges to ' face
up ' tightly.

Mr. Nelson has pointed out a far better and simpler method
which dispenses with all extra apparatus.

Three portions of the thread in the nose-piece of the microscope
itself are cut away, and also three portions on the screw of the

Fig. 236. — Zeiss's sliding-objective
changer, with objective in position.

Fig. 237.-

-The objective detached from
the body-sHde.

objective. Those portions where the thread is left on the objective
pass through those spaces in the nose-piece where it has been cut
away. The screw engages just as if the whole screw were there, and
the objective faces up in the usual manner. This plan in no way
injures eithei' the microscope or the objectives for use in the ordinary
way; thus uiicut objectives will screw into the nose-piece, and cut
objectives will screw into an uncut nose-piece. This plan is similar
to that employed in closing the breech of guns, and it was seeing one
of them in 1882 which suggested to Mr. Nelson to adapt the same
principle to the microscope. Subsequently it has been found that
in 1869 Mr. James Vogan had proposed much the same plan, only
cutting away two portions instead of three ; it is curious that such
an excellent idea Avas allowed to drop.

An analysing nose-])iece is that which carries a Nicol's analysing

FINDEES 295

prism for polariscope purposes. In some the prism is fixed in the
nose-piece, whereas it ought to be capable of rotation. Lastly we
have a vPA^olving nose-piece for the pm^pose of testing objectives.
Mr. Nelson, in a paper read before the Quekett Microscopical Club,
February 1885, stated that he had observed that certain objectives
performed better when the object was placed in a definite azimuth.
With a view to eliminate any possible alteration which might arise
from the revolution of the object with regard to the light, he had
designed a revolving nose-piece which enabled the objective itself to
be revolved true to the optic axis when any imperfection in its
perfoi'mance in a particular azimuth could be immediately noted.
This plan had, however, been previou.sly in use by Professor Abbe
for a similar purpose, but not, as we believe, made piiblic.

Finders. — A finder is a very important and valuable addition to
a microscope. By its means the position of any particular object or
part of an object in a mount can be noted, so that it may be found
again on any subsequent occasion. In working on a microscope
without a finder it frequently happens that in the prosecution
of special research, or in the examination of unknown objects,
something is seen which it would be of the utmost value to recur to
again ; but the amount of time loSt in transferring the object to a
stand with a finder is so great that most experienced microscopists
do all their search and general work on their best instruments with
finders.

The usefulness of the finder has caused a large number to be de-
vised ; but, as in all cases, we consider only those which we believe
embody the best jDractical jDrinciples.

The first, and by far the best, is the graduation of the stage
plates of a mechanical stage by dividing an inch into 100 parts, both
on the vertical and horizontal plates. The vertical stage-plate will
then indicate the latitude, and the horizontal plate the longitude of
the object, the slip being always pressed close home against a prepared
stop. For many years Messrs. Powell and Lealand have supplied
their No. 1 stand with this kind of finder ; and its permanent position
and ease in use not only give greater facility in special researches,
but in reality attach a new value to every slide in the cabinet. Such
a worker at critical images as Mr. Nelson has weeks of close work
' logged ' on the labels of his slides. A still better plan is to ' log '
in books in which the slides are numbered. The result is that the
labour of days and weeks can be in a moment recalled for demon-
stration ; and so accurate is this method that an object so small as
a Bacterium terr}io or a specified minute diatom in a thickly scattered
mounting may be at once, and as often as we please, replaced in the
field with even high powers.

These finders of course are only suitable for the microscope on
which the ' log ' was taken. It is beneficial, and even needful at
times, to interchange specimens or refer an object to an expert at a
distance. In that case a minute dot may be placed on the cover, or
a single selected diatom or other object may be fixed upon and its
latitude and longitude as read on the microscope of the sender marked
on the slide. If the receiver then places this on his microscope and

296 ACCESSOEY APPARATUS

centres it, the differences in latitude and longitude maybe noted, and
will give the constants for the coi-rection which must be added to
or subtracted from the figures given by the sender.

Mr. Nelson has made some very practical suggestions touching
the improvement of finders. He suggests, what we heartily accord
with —

1. That the stage-stop shall be always on the left hand of the
stage.

2. That the zero of the horizontal graduation shall be on the left
hand of the scale.

3. That the zero of the vertical graduation shall be on the top of
the scale.

4. That when the finder is placed to 0, 0, a spot marked on the
bottom edge of a 3 x 1 inch brass template two inches from the stop
shall be in the optic axis of the instrument. In other words, the
latitude and longitude of the centi-e of a 3 x 1 inch glass slip shall
be 50, 50.

5. That the division shall be in -j-^ths of an inch, and the scales
one inch long.

If these very simple suggestions were adopted generally, an object
found on one microscope could be easily found on any other. This,
like the ' (Society's screw' for object -glasses and a universal sub-stage
fitting, deserves, in the interests of international microscopy, the
consideration of opticians.

In practical ' logging ' the use of a hand lens will enable the ob-
server to read by estimation very accurately ; half a division can be
very approximately judged of, and this is as close as will be required
with the highest powers. We have found, for very delicate work,
that we could log with advantage between the divisions, thus : say
' long. 41 ; ' but if slightly over, but not an estimated half, ' 41 + ; ' if
half, ' 41^ ; ' if more than this, but less than 42, it is logged ' — 42.'
For logging piu'poses the lens we recommend is one of Zeiss's 'loups,'
magnifying six diameters. They are admirable instruments, and
are furnished with a handle, which may be used or not at the will of
the worker.

The other finder we desii'e to consider is called after its inventor,
and is known as ' Maltwood's finder.' ^

It consists of a micro-photograph, one square inch in size, divided
into 2,500 little squares, so that each is 5^-oth inch square. Each
square contains two numbers, one indicating the latitude and one
the longitude. To log any object the slide containing the object
must be removed and the slip holding the micro- photograph substi-
tuted for it ; then the figure in the square wdiich most nearly agrees
with the centre of the field is noted. Of course, both the object and
the Maltwood finder must be carefully made to abut against the
stop.

There are two drawbacks to this finder.

1. The divisions are not fine enough, so that it is only suitable
for low powers.

2. The removal of the slide, and its substitution by the Maltwood

1 Trans, of the Micro. Soc. new series, vol. vi. 1858, p. 59.

DIAPHRAGMS

297

finder, render it extremely unhandjMvhen using an immersion objec-
tive, all the moi-e so if the condensei- happens to be immersed as
well.

If the Maltwood finders are made alike, they are then, of course,
interchangeable.

Diaphragms. — There are three kinds of diaphragms in use.
First, the commonest form is that of a rotating disc of sevei-al aper-
tures graduated as to size. Secondly, a series of separate small
discs of metal, with a single central aperture, which fits in a suitable
carrier. Thirdly, there is what is known as the ' iris ' diaphragm,
which is shown in fig. 238. Upwards of 30 years ago it was applied
to the microscope by Beck ; it has since been brought to great per-
fection, some being made with as many as sixteen leaves ; all makers
now provide them. In whatever form the diaphragm may be which
is for use with the mirror, it is important that it should not be
placed too near the object, as then its position lies so near the apex
of the cone of illummation
that it will not cut it unless
the hole be exceedingly small.
A very small diaphragm aper-
ture is objectionable, as it is
liable to introduce difi;ractional
efiects. Therefore it is better
to use a larger aperture
further away from the stage
than a pin-hole near the
stage. When a diaphragm is
used in connection with a
condenser, it should be placed
just behind the back lens, and
never above the front lens.
Calotte diaphragms placed
close under the stage, and
which have been much in use
lately, both here and on the Continent, are a mistake for critical
work.^

A very good way of cutting down a cone from a mirror is to
have the diaphragm fitted in the sub-stage, so that it can be made
to advance or recede from the object. The advantage thus gained
is that one aperture is made to do fhe duty of several. It also
permits of careful adjustment.

The iris diaphragms are so comparatively inexpensive, that they
have superseded foi- general work and ordinary purposes all others ;
but whatever diaphragm is used it should loork easily. Iris dia-
phragms work sometimes so stifiiy that the mici-oscope may be moved
before the diaphragm. So, too, with the diaphragm wheels ; some
require a pair of pliers before they can be rotated. This is easily
accounted for when we examine the way in which they are fixed. The
usual method is to screw the wheel to the under side of the metal stage.
Now, if there are neither washers nor a shoulder to the screw, it is
'^ Quekett, Micro. Journ. vol. iv. p. 121 et seq.

Fig. 238. — Zeiss's iris diaphragm.

298 ACCESSORY APPARATUS

more than pi-obable that when the diaphragm is rotated it will
screw lip and jam. The purchaser may easily observe a matter of
this kind. Cylinder diaphragms, which were invented in 1832 by
C Varley, are much used on the Continent ; they are also often made
into iris forms. Also diaphragms with a very minute circular hole
in the line of the optical axis are largely used just behind the
object -slip. These are employed with the mirror only (without
condenser) and with dayliglit alone. The object of this method
of illumination being to render very translucent objects visible by
increasing the size of the black diflfraction bands at their edges, it
is, as befoi'e stated, of no use for critical work.

Condensers for Sub-stage Illumination.^ — This condenser is an
absolutely indispensable pai't of a complete microscope. Its value
cannot be overrated, for the ability of the best lenses to do their
best work, even in the most skilful hands, is determined by it.
Perfection in the corrections of object-glasses is indispensable ; but
those who suppose and affirm that this is all that we need — that the
objective is the microscope — cannot understand the nature of modern
critical work. The importance of it could not have been realised in
the sense in which we know it in the earlier dates of the history of
the instrument ; but at as early a period as 1691 we pointed out
(p. 134) that a drawing of Bonanni's horizontal microscope showed
the presence of a condenser. It is, in fact, of soine interest to note
how our modern condensers gradually arose.

The microscope that amongst the older forms (1694) appears
most efficient and suited for the examination of objects by trans-
mitted light was that of Hartsoeker (p. 134, fig. 102). It will be
remembered that it was furnished not only with a condenser, but with
a focussing ari'angement to be used with it, which was not in any
way affected by a change of focus in the object. This is a feature
which, although not then important, is of the utmost importance now.
In the correction of dispersion in the lenses employed in the
dioptric form of mici'osco^De so much difficulty was experienced that
several efforts were made to produce catoptric forms of the instru-
ment ; the most successful of these was that of Dr. Smith, of Cam-
bridge, in 1838 ; but this and all other forms of reflecting microscope
had but a brief existence, and passed for ever away. To the improve-
ment of simj^le lenses much of the earlier progress of microscopic
investigation is attributable ; and that known as ' Wollaston's
doublet,' devised in 1829, was a decided improvement in all respects.
It consisted of two plano-convex lenses ; but this was again improved
by Pritchard, who altered the lens distances and placed a diaphragm
between the lenses. When the object was illuminated with a con-
denser this formed what was the best dioptric microscope of
pre-achromatic times.

Good results, within cei'tain limits, may be obtained by means of
the best Pritchai-d doublets. With a tij^l^ inch the sui'face of a
strong Podura scale may be seen as a surface symmetidcally scored
or engraved ; but the Editor has never himself been able to reveal the

1 The word ' condenser ' throughout this work is appHed to optical appliances for
the sub-stage ; what is known as the ' bull's-eye ' is not called a ' condenser.'

EAELY CONDENSERS 299

' exclamation ' marks, and as this is the expei'ience of the majority
of efficient experts, it may be taken that no resolution of these was
accomplished in pre-achi'omatic days ; these lenses, in fact, over-
lapped the discovery of achromatism.

But the practical results of the use of achromatic lenses soon led
experienced men, understanding their theory and practice, to perceive
that if it wei-e good for the lenses which formed the image, it was
also good for the condenser. Thus Sir David Brewster in 1831 ad-
vocated an achromatic condenser in these remarkable words, viz. : ' I
have no hesitation in saying that the apparatus for illumination
•requires to he as perfect a s the apjxir at us for visio7ijS,nd on. this -account
I would recommend that the illuminating lens should he perfectly
free from chromatic and spherical aberration, and that the greatest
care be taken to exclude all extraneous light, both from the object
and from the eye of the observer.' This is a judgment which every
advance in the construction of the optical part of the microscope, as
used by the most accomplished experts, has fully confirmed.

We have no knowledge, from an inspection of the piece of
apparatus itself, of the construction of the compound sub-stage con-
denser of Bonanni (fig. 101); it does not appear to have attracted
much attention, and of course it was quite imjDossible to secure a
critical image by its means. It was focussed on the object merely
to obtain as bright an illumination as possible, in order that the
object might be seen at all.

In the condenser iised by Smith in his catoptric microscope
(fig. 113) we have the earliest (1738) known condenser, by means of
which a distinction between a ' critical ' image — that is, an image in
which a sharp, clear, l:)right definition is given throughout, free from
all ' rottenness ' of outline or detail — and an ' uncritical ' or imperfect
image could be made. It was not, apparently, at the time it was
first used, considered to be so important as we now know it to be ;
and it is probable that the mode of focussing the light upon the
object by its means was to direct the instrument to the sky with
one hand and to use the biconvex condenser with the other. In
1837 Sir D. Brewster writes of it with appreciation, saying that
' it performs wonderfully well, though both the specula have their
polish considerably injured. It shows the lines on some of the test
objects with very considerable sharpness.'

No advance was made on this condenser for nearly a century.
In 1829 Wollaston recommends the focussing of the ionage of the
diaphragm by means of a plano-convex lens of | of an inch focus
upon the object, and Goring in 1832 says concerning it : ' There is no
modification of daylight illumination superior to that invented by
Dr. Wollaston.' But Sir D. Brewster objected to tliis, contending
that the source of light itself should be focussed u.pon the object. He
prefei-red a Herschelian doublet placed in the optic axis of the micro-
scope. But, whilst there is a very clear diflTerence between these
authorities, we can now see that Ijoth were right.

Goring, who was also a leader in the microscopy of his day, used
diffused daylight, and as the lens he employed was a plano-convex
of I of an inch focus, the method of focussing the diaphragm was as

300

ACCESSORY APPARATUS

good as any other, because the diaphragm was placed at a distance
ti-om the lens of at least five times its focus, so that the difference
between diaphragm focus and ' white cloud ' focus, or the focussing
of the image of a white cloud upon the object, was not very great.
But Bi'ewster was writing of a flame from a saucer of burning spirit
and salt when he insisted on the bringing of the condenser to a
focus on the object, and in this he was, beyond all cavil, right.

In 1839 Andrew Ross gave some rules for the illumination of
objects in the ' Penny Cyclopaedia.' These were : —

1 . That the illuminating cone should equal the aperture of the
objective, and no more.

2. With daylight, a white cloud being in focus, the object was
to be placed nearly at the apex of the cone. The object was seen
better sometimes above, and sometimes below, the apex of the cone.

3. With lamplight a bull's-eye is to be used to parallelise the
rays, so that they may be similar to those coming from a white
cloud.

Of the old forms of condenser, that devised by Mr. Gillett was,

there can be no doubt, the
best. It was achromatic, and
had an aperture of 80°. Fig.
239 illustrates it. It was
fitted with a rotating ring of
diaphragms placed close be-
hind the lens combination.
This was formed, as the figure
shows, by a conical ring with
apertures and stops. The
large number of apertures
and stops it would admit,
provided they are care-
fully 'centred,' are of great
value in practical work ;
and the fact that they are
so placed as not to inter-
fere with the stage, makes
this arrangement of dia-
j^hragms and stops an excellent one, and it is not clear why it has
fallen into disuse.

It had been the custom to recommend the use of this instrument
racked either toithin or vnthout its focus. Oarjaenter emjoloyed it
without, and Quekett within, and one or other of these methods was
general. But in the use of good achromatic condensers with high-
power woi'k it soon became manifest to practical workers that it is
only when, as Sir David Brewster pointed out, the source of light is
focussed hy the condenser on the object that a really critical image
is to be obtained. And Mr. Nelson i-eadily demonstrated this fact
even with the condenser Gillett had devised.

The next condenser of any moment is a most valuable one, and
constitutes one of the great modern improvements of the microscope.
It was an achromatic condenser of 170° devised and manufactured

Fig.

239.-
'Ho;

-Gillett's condenser, from
;g on the Microscope.'

POWELL AND LEALAND'S CONDENSER

301

by Messrs. Powell and Lealand. We have used this instrument for
thirty-five j^eai-s on eveiy variety of subject, and we do not hesitate
to affii'm that foi- general and ordinai-y ci'itical work it is still un-
surpassed. Fig. 240 illustrates this apparatus. The optical com-
bination is a ith of an inch powei', and it is therefore more suitable
for objectives fi'om a Jth of an inch and upwards ; but by removing
the front lens it may be used with objectives as low as one inch.

Having given to this condensei' so hie'h a place amone'st even
those of our immediate times, it may be well to specify what the
i-equii'ements are which a condenser employed in. critical work with
high powers should meet. It is needful that we should be able
(1) to obtain at will the largest 'solid' cone of light devoid of
spherical aberration.^ Directly spherical aberration makes itself
apparent the condenser fails ; that is, when, on account of under-
correction, the central rays ai'e brought to a longer focus than the
marginal rays, or when, because of over-correction, the marginal
rays have a longer focus than the central.

But (2) it is also an absolute essential that if a condenser is to
be of pi'actical service it must have a
woi-king distance sufiiciently large to
enable it to be focussed through
ordinary slips. It would be an
advantage if all objects mounted
for critical high-power woik were
mounted on vslips of a fixed gauge, say
•06 inch, which would be ' medium,'
•05 inch being accounted ' thin,' and
•07 inch ' thick.'

It is plain, however, that to
combine a large aperture with a
great working distance the skill of

the optician is fully taxed, for this can only be accomplLshed («) by
keeping the diameter of the lenses just large enough to transmit
rays of the required angle and no more ; (6) by working the convex
lenses to their edge ; (c) liy making the flint lenses as thin as
possible.

IS^ow it is due to the eminent firm whose condenser we have
been considering with such appreciation to say that the condenser
referred to {d) did, when it was first devised and for many years
after, transmit the largest ' solid ' cone free from spherical aberra-
tion ; (e) that it has the greatest working distance ; {/ ) that its
chromatic aberrations are perfectly balanced. In the possession of
these three essential qualities it stood unrivalled for upwards of
thirty years.

The removal of the front lens of this condenser, which may be
I'eadily unscrewed, reduces it in power and angle, and therefore
makes it suitable for objectives of lower power. This, however, is
rather an adaptation involving compromise than an ideal condenser

1 This is one of the many expressions which are inevitable to the practical use of
aijparatus ; it is simply convenient, and means a. full cone of light — a cone with none
of its rays stopped out.

240. — Powell and Lealand's
condenser.

^02

ACCESSORY APPARATUS

Fig. 241.— Swift's apo
chromatic (1899) con-
denser, N.A. 0-95.

for low powers. When the highest class of work has to be done it
is needful to have condensers suited to tlpe jyoioer of the objective used.

A dry apochromatic condenser of merit is made by Swift and
Son ; it has a IST.A. of 0'95 and an aplanatic cone approximating
0'92, and works with ease through any object-
slide, but is corrected to do this by thinning
the front lens and setting the front and back
combinations further apart than would be the
case if they were used as an objective. The
lower combination has a large, clear aperture.
The optical part of this instrimient is shown in
fig. 241 ; we have used it, and find it a tho-
roughly practical and serviceable condenser.

Before the introduction of the homogeneous
system, and the production of such great aper-
tures by Powell and Lealand as a 1*5 in a ^th,
a iVth, and a ^th. of an inch focus, the cone
transmitted by Powell's dry achromatic condenser was as large as
could be utilised. But with apertures such as these, and because
of the subsequent introduction of the apochromatic system of lenses,
much larger cones were required. To meet this necessity Powell
and Lealand, at the txrgent suggestion of English experts, made first
a chromatic condenser on the homogeneous system ; but this was
subsequently succeeded by an achromatic instrument of great value
on the same system.' This combination consisted of a duplex front
with two doublet backs ; it is nearly of the same power as their
dry achromatic condenser, but is of much greater aperture. It was
brought afterwards to a very high state of perfection, having an
aperture of 1 "40, and will work through a mounting slip of '07, and
for aperture and working distance is, like its dry predecessoi-, quite
unappr oached .

Messrs. Powell and Lealand have produced an entirely new
condenser, strictly apochromatic, employing a fluorite lens in the
combination, and j)resenting features in the highest degree desirable.
We find its IST.A. to be 0'95, its focal length long enough for a thick
slip, its aplanatic aperture '9. We have found it of the iitmost
practical value in critical work, and this valuable apparatus has
been greatly increased in efficiency by the application of a device
by Mr. E. M. Nelson, providing it with a correction collar, which
can be used with the utmost ease, no matter in what position the
microscope may be. It is similar in practice to the correction
collar of an ordinary objective; it has a steeper spiral slot, and
only half a revolution of movement ; a long arm is fixed to the
collar, so that it may be conveniently reached by the fingei-. The
whole condenser is represented in fig. 242, and the arm for moving
the correction collar is seen on the light of the optical tube ; it
turns at the slightest touch, and the collai- moves only the back
lens of the combination, leaving the mount rigid.

The object of this correctional movement is primarily to increase
the maximum aplanatic aperture of the condenser ; this is effected
by separating the lenses. If the back of a wide -angled objective be

NELSON'S CONDENSER ' COEEECTION COLLAR'

303

examined when an object is illuminated by the full aperture of the
condenser, the edge of the flame being in focus, it will be noticed
that the illuminated
portion of the back lens
will be oval and pointed
instead of circular.
Also that Avhen the
condenser is racked up,
although the exteiior
shape of the illuminated
portion will become
more circular, two dai'k
patches will appeal- on
either side of the centre,
showing the opei'ation
of the spherical aber-
ration of the condenser.
If under these circum-
stances the lenses be
separated hy means of
the collar adjustment, the black spots ivill be closed up, and a circular
and evenly illuminated disc toill ajjpear. This is a distinct optical
gain, and will enable the observer to see more than he could
have seen before. Mi'. Nelson made this manifest on the examina-
tion of a well-known diatom, Navicida major. If examined in its
' principal view,' two vertical stripes will be seen running down the
centre of the hoop (fig. 243, a) ; these can easily be resolved into
striee with a ;^-inch objective, but the probability is that these stri?e
are not the real structure but rows of minute j)erforations incom-

FiG. 242.-

-Nelson's correction collar to Powell's
apoclu-omatic condenser.

\

Fig. 243.

Fig. 244. — Watson's oil-immersion condenser.

pletely resolved (fig. 6) ; by using the condenser with the collar
correction these stri?e were resolved by means of the enlarged
aplanatic cone it produced, as shown in c.

Another advantage of the correction collar is that it enables the
worker to determine most delicately the size of the illuminating
cone, and so to recoi-d it that it can be with facility exactly I'esumed
at any time (Journ. R. Mio'o. Soc. 1895, pt. ii. p. 231-2).

One of the most valuable condensers introduced by any maker
lately is an oil-immersion one by Messrs. Watson and )Sons. It has
special claims upon the attention of those who work with high

304

ACCESSORY APPARATUS

Fig 245 — Watson s pai achromatic condenser.

powers, for we know of no similar instrument that yields so large a
' solid cone ' of illumination. The construction is an unusual one,
the corrections for both spherical and chromatic aberrations being
effected by means of a cemented triple back lens, as is shown in the
illustration of the optical system in fig. 244. The only flint glass
used in it is the middle of the triple back. The total niunerical
apei'ture is 1'33, the aplanatic aperture being in excess of 1'25.
The magnifying power is J inch, and the clear aperture at the back
of the lens is ^^ths inch, and it works through a slip of '073
thick.

With the front lens removed it is an efficient dry condenser for
medium powers, magnifying fths inch, with a total 'N.A. of -56, the

aplanatic aperture being-
over -5. It is mounted
like their ' Parachi-o-
matic condenser ' shown
in fig. 245, which is also
a very useful instru-
ment, with a total N.A.
of I'O, a power of |^ths
inch. It is shown here
principally for the
mounting, which is
identical with that used
with fig. 244. The
collar into which the optical pai-t fits carries an iris diaphragm ; on
the diametrical edge of this is engraved a scale showing the IST.A. at
which the condenser is working when the iris diaphragm is in a
given position. We have used this condenser with much pleasure
and profit, and can commend it as a truly valuable instrument and
yet remarkably low in price.

A. condenser satisfying modern necessities has also recently been

made by Messrs. R. and J.
Beck, which we illustrate in fig.
246 in its complete condition.
The optical combination consists
of four systems of lenses, the
front of which is a hemisphere,
with three combinations behind,
and the whole is constructed on
the jjrinciple of an oil-immersion
objective. The IST.A. varies from
1"35 to 1"4, and the aplanatic
cone is about 1"3 JST.A., the
working distance being fully "06.
We can speak highly of this
instrument ; it is in our judg-
ment the best condenser ever
made by this firm.
Another condenser has been made recently by the same firm,
with N.A. of I'D, the maximum obtainable without immersion

P^

Fig. 24(5. — Beck's new achromatic
condenser.

SUB-STAGE CONDENSERS

505

contact. Its aplanatic aperture is -9 N.A. TO. We illustrate this
form in fig. 247.

Fig. 247.— Beck's condenser with N.A. I'O.

It is with great pleasui-e that we are able to announce the
pi'oduction, by the firm of Zeiss, of a ' centring oil-i'mmersion achro-
matic condenser^ oi N.A. 1*30. This is what we have long desired
to see, and we have used it with admirable results. It gives a large
illuminating aplanatic cone, hence very oblique illumixiating rays.
The centring ar-
I'angement is the
same as that of the
achromatic conden-
ser of the same fii-m
having 1-0 N.A. It
is supplied with an
iris diaphragm of
the most pei'fect
workmanship, and
the condenser is
focussed not only by
I'ack - and - pinion
movement, but also
hjj means of a special
fine adjustment ; this
is accomplished by
the aid of a i-otating

ring provided with a difierential thread, as will be seen by examining
the illustration we give in fig. 248. This allows the condenser to
be easily focussed ' at intervals of about 0"01 mm.' ' By means of
this fine adjustment the condenser may be focussed up to about
1 mm.'

Messrs. Swift and Son make a panachi-omatic dry condenser
having a N.A. I'O, an aplanatic cone of 0-93, and it works well when
a critical image is desired. It is well corrected for colour ; they
also make a panaplanatic oil-immersion of N.A. 1-40, with an
aplanatic cone of 1'25. The new optical glass is used throughout
the system. It is mounted in an adjustable cell, if desired, for
correcting the variations in the thickness of the glass slide. The

X

Fig. 248. — Zeiss's centring oil-immersion achromatic
condenser (1899).

3o6

ACCESSOKY APPARATUS

Fig. 249.

Baker's new achromatic condenser
N.A. 1-0.

iris diapliragm supplied Avith this condenser is graduated to show
the N.A. when greater accuracy is required, but the still more
Mccui-ate method of employing fittings with sepai-ate discs with theii'
N.A. marked on them is also supplied by the makers.

A very complete achromatic condensei- is now made by Baker of
Holborn. This condenser is a modification of the well-known Abbe
form, in that the diameter of the component lenses is considerably
smaller : this reduction in the size of the lenses, allowing, as it does, of

greater freedom of move-
ment of the mechanical
stage, has been efiected
without in any way de-
creasing its optical effi-
ciency ; on the contrary the
aplanatic aperture has been
increased, thus rendering
it especially suitable for use
with high powers. The
total aperture is N.A. I'O,
of which N.A. 0-90 is
ajilanatic : the diameter
of the back lens is 22 mm.
(-}-^in.) and the power of the condenser as a whole is 10 mm. ( ^^ in.)
with a working distance of2'5mm. (^Lin.): with the front lens
removed for low-power work the power is reduced to 20 mm. (-^^ in.),
and the working distance, which is calculated with the lamp flame
at ten inches, is increased to 10'5 mm. [^ in.).

The above is mounted in the usual sub-stage fitting of universal
gauge with iris diaphragm and carrier with dark-ground stops, as
shown in the illustration of it in fig. 249.

It is essential for ideal illumination with transmitted light
(1) that the illuminating axial cone should be approximately equal
to the aperture of the objective used ; (2) that the object should be
placed at the apex of this cone.

If an objective breaks down with this ideal illumination, which
is very probable, we must be content to sacrifice the ideal ; or, as
is also exceedingly probable, if the object under examination lacks
contrast, the ideal method must be modified. But if we have a
suitable object and a perfect objective, it is the strong conviction
of some leading experts that, as we increase the cone in aperture, we
increase the perfect I'endei'ing of the image, until the point is reached
where the cone from the condenser is equal to the aperture of the
objective. This ideal can be realised with fine apo- and semi-
apochromatics iip to '3 to "4 N.A. With the most perfect objectives
of the present day of '5 N.A. and upwards we find in practice that
the best results are obtained when a cone of light is used which, on
the removal of the eye-piece, is found to occujjy three-quarters of
the area of the back lens of the objective.

No condenser is suificiently free fi-om spheiical aberi'ation to
transmit a cone equal to its otvn apertui'e. Condensers are all more
or less under-cori'ected, and consequently focus their centi'al rays at

EFFECTIVE APEKTURE OF CONDENSER 307

a greater distance than their marginal rays. If we rack up the
condenser so that the marginal rays are focussed on the object, the
focus of the rays which pass through the centi-e will be beyond the
object.

It is well known to those practised in microscop}^ that, in the
case of a narrow cone from a well-stopped-down condenser — that is,
a condensei- used with diaphragms of i-elatively small diameter — the
illumination is at its greatest intensity when the object is at the
apex of the illuminating cone, and, if the condenser is racked either
up or down, the intensity of the illumination is rapidly diminished.
But in the case of a condenser with great aperture, if it be racked
up, the marginal rays will haA'e their full intensity, while those which
pass through the central portion of the condenser will have a
diminished intensity.

The extent to which this will take place will be wholly dependent
on the amount of under-correction present in the condenser. In
some condensers the under-correction is so serious that to obtain a .
wide or even a moderate cone we so enfeeble the central cone as to
reduce it almost to a mei-e annular illumination, which is not a desir-
able quality.

It will be seen, then, that the aperture of the cone of light trans-
mitted by a condenser plays a very important part in giving ci-itical
quality to an image with different objectives. We should therefore,
to use a condenser accurately, be able to determine the aperture of
the cone we are using.

We may measure the total apertiu'e of a condenser just as we
do that of an objective, viz. by means of Abbe's apertometer.^ But
the effective aperture cannot be measui-ed in that way ; that is to
say, the ajjerture of the largest aplanatic cone (or cone fi-ee from
spherical aberration) the condenser is capable of giving, cannot be so
discovered.

To do this, place the condenser in the sub-stage and an objective
on the nose-piece ; focus hoth upon an object. Let the edge of the
lamp-flame be used, and so arrange the focus of both optical com-
binations that the edge of the clear image of the lamp-flame falls
centrally upon the object. Now move the object just out of the

Fig. 250. Fig. 251. Fig. 252. Fig.253.

field, remove the eye-piece and examine the back of the objective,
and if the apei'ture of the aplanatic illuminating cone is greater than
that of the objective it will show the back lens to be full of light (fig.
250). Therefore, if the aperture of the objective is "5, we know that
the aplanatic illuminating cone cannot be less than -5. If now we

' Chapter v.

x2

308 ACCESSOEY APPAEATUS

close the diaphragm so that the image of it just appears at the back
of the objective, we are able to determine the aperture of the illumi-
nating cone with that given opening in the diapln-agm ; thus in fig.
251 it is a trifle less than '5 N.A.

In a similai- manner the apertures of the other diaphragm open-
ings can be determined.

Now let the diaphragm be opened to the full aperture, and an
objective with a wider aperture, say 'OS, be used. It will perhaps
be found that before we are able to fill the back of the obj ective with
light by racking up the condenser, two black spots will be formed on
either side the middle of the disc. When we reach the disc of light
that is largest (fig. 252), any further racking up causes the apj)eai--
ance shown in fig. 253. The last point before the appearance of the
black sjjots indicates the largest aplanatic ap>erture of the condenser, and
is the limit of the condenser for critical ivork}

There are many other condensers of more or less merit and use-
fulness than those which we have already described and illustrated ;
but for most recent lenses, and for the finest critical results, we have
given them as full a representation as can be fairly desired. But
there are still some foi-ms that either from their own peculiar value
or their historic importance desei've consideration.

A condenser knoion as the ' Webster ' was first made in 1865, and
is still a very useful one for low j^owers. It is the same as that
made by Swift, but without the middle combination. Its angle is
less, and its range is not so extensive ; but its chief commendation in
possessing these qualities is that, having one combination less than
Swift's, it is of necessity lower in pi'ice, and on that account will be
welcome to some workei's.

In its present form it reverses its primary construction. It is
now made with a double front and a single back, instead of a single
front and a double back.

A chromatic condenser which has been very lai-gely used in
England and America, and which has secured a great deal of com-
mendation, is that of Professor Abbe. The optical productions of
Abbe are too well known and too valuable as a rule to make it
needful to be other than pei'fectly fi-ank concerning so important a
piece of apparatus as this ; and there can be no doubt that the wide
popularity of this instrument is due, not so much to intrinsic mei'it
as to the fact that it has been employed much by those who,
jareviously ignorant of the value of any condenser, have at once
perceived the enhanced value of the I'esults yielded by its means.

To those who have made the scientific use of the microscope a
careful study in England it has been a persistent source of regret
that it was so long and pertinaciously taught that the ' cori-ect '
histological microscope must be of the Hartnack type, and that it
shovild be used with naiTow-angled dry lenses, perhaps a -^th-inch
focus, and no illumination but that afibrded by a small concave
mirroi-, the focal point of which is exti'emely douljtful oi- vmknown,

- ' The Back of the Objective and the Condenser.' E. M. Nelson, Eng. Meek.
vol. xlviii. No. 1234, 1888.

IMPOETANCE OF APLANATIC CONDENSEH 309

and in practice wholly disregarded. ISTo donbt a stndent instructed
on these lines would be astonished indeed when he exchanged such a
practice foi' the illumination and improved image afforded by an
Abbe condensei-.

Usually such exchange of illuminating method presages an ex-
change of instrument, for the scientifically imperfect and wholly
unsatisfactory 'tool 'that is in the majority of cases put into the
hands of the medical student will not lend itself even to an Abbe
condenser.

The fact is that a large part of the admiration that has been ex--
pressed for this condenser has resulted, not from a comparison of its
results loith those of other high-class
achromatic condensers, but of images
obtained without any sub-stage optical
arrangements at all, placed in contrast
with the results obtained by using this
condenser against the same objective
when used without its aid. But that
even these images are entirely inferior
to the images obtained by the higher Fig. 254.— Optical arrangement of
order of achromatic condensei'S we only Abbe's chromatic condenser,
require the practical testimony of

Professor Abbe to prove ; for he has since produced an achromatic
condenser of much merit, to which we give considei'ation below.

In its most perfect form this chromatic condenser of Abbe's con-
sists of three single lenses, the front being hemispherical, and the
two lower lenses form a Hei'schelan doublet. This combination is
shown in fig. 254, and the general form of the instrument, as apj)lied
to Zeiss's own microscopes, is shown in fig. 255.

The power of this condenser is low, and its apei'ture is very large
(1"36); hence, beyond the fact that it is not achromatised, it has
enormous spherical aberration. The distance between the foci of
the central portion and of a narrow annular zone whose internal
diameter is f th inch is 4Vth inch. Its aplanatic apei'ture is therefore
only -5. ISTow, whilst it is a gain of no inconsiderable character to
have an achromatised condenser, yet the point of vital importance is
that it should be aplanatic ; the best condenser is always that which
will transmit the largest aplanatic cone. At the close of this section
we furnish a table of the relative qualities of the condensers of the
best construction now accessible to the microscopist, and a reference
to this will show that Powell and Lealand's dry achromatic (fig. 240),
with the toi^ removed, is in this respect as efficient as this form of
Abbe's.

This condenser can be used either dry or homogeneously ; but of
course with objectives of greater aperture than 1"0 tlie base of the
slide should always be in oil contact with the condenser.

It gives the principal modifications from direct to oblique illu-
mination with transmitted light by changing and moving a set of
diaphragms placed in a movable fitting, and the diaphragm may be
moved eccentrically to the optical axis of the condenser by moving
the milled head. It gives dark-ground illumination with objectives

3IO

ACCESSOEY APPARATUS

of "5 jST.A. ; for such illumination, in fact, it is perhaps the best
illuminator extant, and shows objects on a dar-k ground with
sparkling brilliancy, and may be used with polarised light.

A chromatic condenser, somewhat similar in construction to this,
and of low price, is made by Messrs. Powell and Lealand ; but it is
of much higher power, so that the distance between the foci for
the central and peripheral rays is not so great, and on this account
it yields a somewhat larger aplanatic cone. This instrument with
its diaphragms is shown in fig. 256. It is more convenient in form,

Fig, 255. — Abbe's chromatic condenser as applied to the Zeiss microscopes.

and can be handled and adjusted with greater facility, than that ol
Abbe. The size of their respective back lenses is significant in this
regard, that of Powell's being ^%r inch, and that of Abbe's being
1 -^g inch. This instrument of Powell's, if fitted in the usual way,
would be now a very efiicient instrument of its kind and cjuality.
The particulai' quality of oblique illumination was in fact still
furthei- advanced by a modified form by the same makei-s known as
Powell's truncated condenser, which gives great obliqixity with
abundance of light, but it is as a matter of course very chromatic.
The diaphragms (fig. 256, A) have a central aperture for the

POWELL'S CHROMATIC, AliBE'S ACHROMATIC CONDENSER

I [

5^ ♦

purpose of centring, and the movement is made by means of an
oiitei' sliding tube ^>, with a slot at the top in which the aim A fits,
and anothei- ai-m, B, is placed at the lowei' end so as t6 give ready
command of the i-otation. This plan allows of the use of one or two
oblique pencils incident 90° apart in azimuth. The condenser thus
mounted is only intended as an oblique illuminator. It forms one
of the best of the very cheap condensers when it is mounted in a
plain tube mount with a ledge to hold the diaphragms. D is the
optical part of the condenser placed immediately above the dia-
phragms and in oil-immei'sion contact with the base of the slide.
The circular diaphragm is fixed into the inner tube attached to the
sub stage tube C, just below the position of the arm A ; the other
diaphi'agm is screwed to it by a screw in the eccentric hole, shown
in each. It will be seen that when the diaphragms are placed
together in this manner the ^^^

movement of the arm will
produce the changes in the
light as above mentioned.

As we intimated above,
Professor Abbe subsequently
produced an achromatic con-
denser, ostensibly for use in
high-power photographic
woi-k, but in fact of much
more general utility. It
consisted of a single front
with two double backs, and
it projects a sharja and per-
fectly achromatic image of
the soiu'ce of light in the
plane of the object. Its
power is low, being \ inch
focus, and it has a total
aperture of TO. Its gi'eat
superiority over the chi-o-
matic form is that it trans-
mits a much larger aplanatic cone than that ; for whereas the former
gave only an aplanatic cone of '5, this instriiment yields a similar
cone of "SS. But we have already expressed our pleastire that even
this form has been surpassed by the high C|uality condenser illustrated
in fig. 257. Like its predecessor, it is large and heavy ; and, with
gieat deference and respect to our Continental neighbours, we would
suggest that this is a too general characteristic ; the back lens in this
case is more than an inch in diameter, while barely f of an inch is
utilised when it is transmitting its largest cone. A very excellent
modification in fitting it to English microscopes has been made by
Mr. Charles Baker, the optician, which is shown in fig. 258, where
it will be seen that the fitting for stops is conveniently placed, and
an iris diaphragm can be used with great ease below this. This
' turn-out ' arm cairies a disc of metal to receive the diaphragms,

Fig. 256. — Powell and Lealand's
chromatic oil condeDser (1880).

312

ACCESSORY APPAEATUS

stops, &c. Ovei" this is fitted a ring into which screw adaj^ters, which
will allow other condensers to be used on the one mechanism.

The metal disc should have a central aperture as large as the
largest back lens of any of the combinations to be used with the
mount. It should be thick enough to receive two stoj)s or dia-
phragms at a time. This power to alter a diaphragm or stop so as
to secure any required arrangement of apertures and stojis without

Fic '257. — Abbe ^ dcluoinatic condenser

in the least disturbing any of the adjustments of the condenser is a
practical gain of a very valuable kind.

Diaphragms should be marked with the numerical aperture they
yield, and stops should be marked with the numerical aperture of
the cone they cut out. Empii'ical numbers are misleading and

valueless. This special mark-
ing need not involve two sets
of diaphragms with two con-
denser combinations, one for
high and the other for low
powers ; the diflerent numeri-
cal apertui'es for each may be
marked on either side of the
diaphragm or stop. Memory
cannot fail if we make the
loiner side of the diaphragm
indicate the apertures for the
lower-powei' condenser, and
vice versa.

We may note that for
dark-ground work, stops
should be placed close to the 1)ack lens of the condenser, and in the
case of a diaphragm — which is less important — an inch of distance
should not be exceeded. This condensei- gives dark-ground illumi-
nation with objectives of "5 N.A. ; for such illumination it is one of
the best illuminators extant.

Fig. 258. — Baker's fitting for Abbe's acliro-
matic condenser used in Englisli micro-
scopes.

A SIMPLE COJsDENSEK 313

The ii'is diapliragin is for general piirposes more convenient than
the usual circular plate, but it has the drawback of being incapable
of setting to any exact size. A delicate point in an image, caught
with a cei'tain-sized diaphi-agm, is not i-egained with ease and cer-
tainty with the iris,' and may involve much patience and lalioui- ;
but a well-made large plate of gi'aduated diaphi'agms will Avholly
remove this difficulty. Moreover, for testing object-glasses it is
supi-emely impoi'tant that a metal diaphragm be used, so that the
conditions of illumination may be i-eadily and accurately I'eproduced.

It may be of service to those who are unable or indisposed to
spend considerable sums upon condensers to state that an excellent
achromatic condensei- can be made by placing a Zeiss ' aj)lanatische
Lupen ' on Steinheil's formula in the sub-stage.^ This plan has
been adopted in one of Reichert's stands, as we have seen.
These are made in two difierent powers, viz. 1 inch and 1^ inch, and
we can fully testify to their lieing the most useful hand-lenses for
ordinary work that can be employed. Gi-eat credit is due to Dr.
Zeiss for bringing out such excellent achromatic lenses at so low a
price, and so meeting a want long and genei'ally felt. Excellent
forms of triplet lenses answering a similar purpose are made by
Bausch and Lomb after the calculations of Professor Hastings, and
most leading makers, Continental and English, make similar magni-
fiers to those of Zeiss. An achromatic loup of this kind is almost an
indispensable accompaniment of a microscopic outfit, and, if a tube to
receive it be ari-anged in the sub-stage, these lenses make really ex-
cellent condensers for low powers. It need not have a centring sub-
stage, but only a central fitting. It is not of course qualified to
supplant the condenser of larger and moi-e perfect instruments, but it
is capable of I'aising students' and othei' simple mici-oscopes to a much
higher level.

Without a condenser the microscope is either (by construction)
not a scientific insti'ument, or it is an insti'ument unscientifically
used. It becomes a mere ' magnifying glass.' It is the adaptation
for and use of a condenser — though as simple as a hemispherical lens
fitted into a stage plate — that raises it to a microscope.

We have already referred to the nature of the mechanical
arrangements needful foi' the condenser in a general way (Chapter
III., pp. 185-190) ; we may add here that the simplest form of sub-
stage being a tube fixed centi-ally in the optic axis of the microscope,
the simplest form of condenser-mount will be a tube sliding into
this. It must not screw, it must push, and there should be a little
below the back lens a shouldei' to hold the diaphragms, stops, glasses,
&c. Centring gear is not necessary with students' and elementary
microscopes. The slight displacements due to varying centres of

1 It will be urged that apertures can be exactly reproduced with the iris in
photograi^hic lenses ; why cannot they, therefore, in the case of the microscope ?
The answer is (1) that with wide-angled condensers a very slight difference in the
aperture makes a very great difference in the angle ; a similar difference would be
inappreciable in the case of a photographic lens. (2) It is in small apertures such
as are seldom used in photographic lenses where the difficulty arises in the case of
the microscope. (3) It is in the small apertures that the iris fails to respond to
the movement of the lever.

- Journ. Q?(efceWilf?c. C^HZ;,vol.iT.ser.ii.p.77(1889),onZeiss'sloup. E.M.Nelson.

314 ACCESSORY APPARATUS

different objectives will with such microscopes prove of no moment
if the sub- stage is once foi- all carefully fixed centrally in the axis.

What we require to do is to centre the image of the lamp flame,
as seen with a low-power lens through the condenser, so that it
stands in the middle of the field. This can be done by moving the
lamp or the mirror, and until this is satisfactory the best results
cannot be obtained. To obviate the inconvenience of having to re-
move the combination in order to alter a diaphragm ^ or stop in
this simple mount an internal sliding tube may be used. It will be
a fui-ther advantage to have a separate cell to fit into the bottom of
the sliding tube to receive coloured glasses ; a spiral slot-focussing
arrangement may be added with advantage to this kind of mount,
acting like a pocket pencil. For students' and elementary micro-
scopes— still so often and so unwisely without condensers — this is a
most inexpensive and most convenient arrangement.

An epitome of its principal points may be of service.

1. A sub-stage tube fixed centrally to the body of the microscope.

2. A spiral slotted tube to push into (1).

3. A tube carrying the optical combination of the condenser
sliding into (2), with a pin moving in the spiral slot.

4. A long tube carrying the diaphragm and slots sliding into (3).

5. A cell carrying coloured glasses sliding into the bottom of (4).
Condensers require S2:)ecia,l moiijiiting for use vnth the polariscope.

Then at least two ' turn-out ' rotating rings are required to hold
selenites. Swift makes an ingenious mtdtum in parvo mount for
employing, amongst other things, the condenser with the polai-iscope,
to which we call attention in describing the polariscope. But we
know of no plan equal to that found in the best stand of Powell and
Lealand. The sub-stage has a double ring, one placed concentrically
within the other. The inner one revolves by a milled head and
receives the usual sub-stage apparat^is. The outer one receives a
mount of three selenites which revolve, and are placed on ' turn-out '
arms. On the upper part of this mount of selenites is a screw, which
receives the optical combination of their diy achromatic condensei-.
When this is screwed in its place we have a condenser of the first
order, with a mount of three plates of selenites taking the place of a.
mount of diaphragms, (fee. I^ow from the under pai-t of the sub-
stage into the inner and revolving ring is fitted the polariser, and
this leaves little to be desired in practice.

We would advise the microscopist to avoid condenser mounts
which carry their own centring movements apai't from the sub-
stage. . It is with regret that we find that this plan has been adopted
in Abbe's new achromatic condenser. It is manifestly better to fit
the rectangular movements to the sub-stage, and then they become
available for all the apparatus employed with the sub-stage. A plan
which I'equires that each piece of sub-stage ajjpaiatus which needs
centring should be provided with separate fittings for this purpose
can have nothing to recommend it.

1 In the technical language or usage of microseopists a diaphragm means a hole
or aperture ; thus a ' large diaphragm ' means that the opening in the diaphragm
plate, disc, or iris is large. A ' stop ' is an opaque disc stoppiing out central rays.

A COMPAEISON OF CONDENSERS

315

We give below a list presenting the most inipoi'ttint features of
the most important condensers, which we believe will be of service
to the student and worker.

The aplanatic aperture given in the table means the N.A. of the
gi-eatest solid cone a condenser is capable of transmitting, the
source of light being the edge of the flame placed in the axis.

The cone transmitted by any condensei- is assumed, for pi-actical
purposes, to be a solid one, so long as the image seen at the back of
the object-glass when the eye-piece is removed (the condenser and
flame being centred to the optic axis of the objective, and the source
of light focussed by the condenser on the object) presents an un-
broken disc of light.

The moment, however, the disc breaks, that is, black spots
appear in it, or its peripheiy breaks away from its centre, then, as
we have shown above, spherical aberration comes into play, and the
limit of aperture for which that condenser is aplanatic has been ex-
ceeded.

Aplanatic

aperture

Power

if.A.

•8

I

5

•8

1
5

•5

i

•24

a

•5

4

10

•22

1

•5

1
3

•3

t

•7

1

1-1

i

•8

1

•4

4=

•65

i
2

•28

1

•5

3

•9

1

•32

1

•9

1

¥ ■

1^3

i

•92

1

4

•93

i

1^30

■ i <

•95

f

1-25

i

•95

i ■

Condenser

Total
aperture

N.A..

1. Powell and Lealand's dry achromatic (1854) ^99

new formula (1859) . -99

2. ,, ,, ,, top lens removed

3. ,, ,, ,, bottom lens only

4. Swift's achromatic (1868) .... ^92

5. „ „ top lens removed . . ! —

6. Abbe's chromatic (3 lenses) (1873) . . | 1^36

7. „ „ top lens removed . . ! —

8. Powell and Lealand's chromatic (Abbe's

formula) (1880) ' . 1^3

9. Powell and Lealand's oil achromatic (1886) 1^4

10. „ „ „ „ „ used dry 1-0

11. „ ,, „ „ top lens removed —

12. Abbe's achromatic (1888) .... ^98

13. ,, ,, top lens removed . —

14. Powell and Lealand's low -power achro-

matic (1889) ^83

15. Powell and Lealand's apochromatic (1891) . '95

16. Zeiss's ' aplanatische lupen,' large field

(Steinheil formula) ..... —

17. Beck's achromatic, dry (1883) ... 1^0

18. „ oil achromatic (1900) ... 1-4

19. Swift's apochromatic, dry (1892) . . . ^95

20. „ panaplanatic, dry (1897) ... 1^0

21. „ „ oil (1898) ... 1^4

22. Watson's panachromatic, drv (1898) . . 1^0

23. „ „ oil (1899) . . 1^33

24. Zeiss's oil achromatic (1899) . . . 1^30

25. Baker's semi-apochromatic dry (1900) . 1-0

The values of the first sixteen and of ISTos. 22, 23, and 25 have
been obtained from actual measurements ; the others ai-e from the
estimates of the makei's.

The limit given in the table is for the edge of the flame as a

3l6 ACCESSOEY APPARATUS

source of light. When, however, a single point of light in the
axis is the source, the condenser will be much more sensitive, and
a lower value for the aplanatic apertui-e than that given in the table
will be obtained. But as a single point of light is seldom, if ever,
practically used in mici'oscopy, it was deemed better to place in
the table a practical rather than a theoretical and probably truer
result.

It has been stated that the best dark grounds are obtained when
a stop is used which is of just a sufficient size to give a suitable dark
field and no more.

When such a sto^D has been chosen, and excellent results are ob-
tained with, say, balsam-mounted objects, if, in the place of this,
living animalcules in water be examined, it will probably be found
that a dark field can no longer be obtained.

Foi- animalcules in water and ' pond life ' generally a stop lai'ger
than that employed for ordinary objects will be necessary.

Other Illuminators. — In the course of the history of the micro-
scope a large number of special pieces of apparatus have been devised
for the purpose of accomplishing some real or supposed end in illumi-
nation. Many of these have proved wholly impracticable and had a
mere ephemeral existence ; many more never accomplished the end
for which they were supposed to be constructed ; and a still larger
number have been superseded by high-class condensers.

The great majority of these illuminators were devised for the
production of oblique light. In. the sense in which it was employed
a few years ago, it is rendered needless by condensers of great aper-
ture. All the obliquity at present needed can be obtained with good
^iondensers.

To give completeness to this part of our subject it is needful to
refer to the spot-lens and the paraboloid, although they are only
serviceable for very low jiowers, such as 3-inch to 1^-inch objec-
tives, and for use with higher powers they are superseded by the
condenser.

A spot lens is a condenser with a permanent axial stop fixed in
it to cut off the central rays for the purpose of obtaining a dark
ground upon which the illuminated object lies. Its use is very
beneficial in low-jaower work. Large insect preparations are pro-
bably bettei- shown with this device than with any condenser, but
when the moderate poweis are brought into operation the condenser
at once makes manifest its superior qualities.

The paraboloid, or parabolic illuminator, as devised by Mr.
Wenham, and subsequently improved by Mr. Shadbolt, ingenious
and beautiful instrument as it is, comes undei- the same category.
It consists of a paraboloid of glass that reflects to its focus the rays
which fall upon its internal surface. A diagrammatic section of
this insti'ument, showing the course of the rays through it, is given
in fig. 259, the shaded portion representing the paraboloid.^ The

1 A parabolic illuminator was first devised by Mr. Wenham, who, however,
employed a silver speculum for the purpose. About the same time Mr. Shadbolt
devised an annular condenser of glass for the same purpose (see Trans. Micro.
Soc. ser. i. vol. iii. 1852, pp. 85, 132). The two principles are combined in the glass
paraboloid.

PARABOLIC ILLUMINATOE

317

parallel rays v r' r'' (fig. 259), entering its lower surface perpendicu-
larly, pass on until they meet its parabolic surface, on which they fall
at such an angle as to be totally reflected by it, and are all directed
towards its focus, F. The top of the paraboloid being gi-ound out into
a spherical curve of which F is the centime, the rays in emerging fi-om
it undergo no refraction, since each falls perpendiculai-ly upon the
pai't of the surface through which it passes. A stop placed at S
prevents any of the rays reflected upwards by the mirror from
passing to the object, which, being placed at F, is illuminated by
the rays reflected into it from all sides of the paraboloid. Those
rays which pass through it diverge again at various angles ; and if
the least of these, G F H, be greater than the angle of aperture of
the object-glass, none of them can enter it. The stop S is attached
to a stem of wire, which passes vertically thi'ough the paraboloid

Pig. 260.— Parabolic
illuminator.

Pig. 259.

and terminates in a knob beneath, as shown in fig. 260 ; and by
means of this it may be pushed upwards so as to cut ofl^ the less
divergent rays in their passage towards the object. It is claimed
that this instrument has gi'eat capabilities of giving dark-gi-ound
illumination with lenses of ' wide apiertui'es ; ' but that has application
to the lenses contemporary with its introduction, and not to wide
apertures as applied to the lenses of to-day. In comparison with
what can be done with condensei's it sufiers greatly after we pass
the i-inch objective, although it does give excellent results with
very low powers such as 1-inch, 1^-inch, 2-inch, and 3-inch
objectives when emjDloyed to illuminate large objects such as whole
insects, because this in.strument gives more diffusion of light over
the whole of a large object than a condenser does.

Polarising' Apparatus. — In order to examine transparent objects
by polarised light, it is necessary to employ some means oi polarising

3i8

ACCESSORY APPAEATUS

the rays before they pass through the olaject. and to apply to them,
in some part of their course between the object and the eye, an
analysing medium. These two requirements may be provided for
in different modes. The polariser may be either a bundle of plates
of thin glass, used in place of the mirror, and polarising the rays by
reflexion ; or it m.ay be a ' single image ' or ' Nicol ' prism of Iceland
spar, which is so constructed as to ti-ansmit only one of the tAvo
rays into which a beam of ordinary light is made to divaricate by
passing through this substance. Of these two methods the ' Nicol '
prism is the one generally preferred, the objection to the reflecting
polariser being that it cannot be made to rotate. This polarising
prism is usually fixed in a tube, and is shown in a simple form in
A, fig. 261 ; it is usually employed in a sub-stage which rotates by a
rack-and-j^inion arrangement, so that rotation of the prism is easily
effected. For the analyser a second ' Nicol ' j^rism is usually em-
ployed ; and this, fixed in a short tube, may be fitted into a collar
interposed between the lower end of the body and the objective,
as is shown in B, fig. 261. The prism in this fitting can also

be rotated by the fingers
grasping and giving circular
motion to the inner fitting of
B, and it is always important
that the polarising prism
should be large, so as not to
act as a diaphragm to the con-
denser, thus cutting off the
light when it is used ; for the
polarising apparatus may be
worked in combination either
with the achromatic con-
denser, by which means it may be employed with high-power
objectives, or as a ' dark-ground ' illuminator, which shows many
objects — such as the horny polyparies of zoophytes — gorgeously
projected in colours upon a dark field.

For bringing out certain effects of colour by the use of polarised
light it is, as already stated, desirable to interpose a plate of selenite
between the polariser and the object; and it is advantageous that
this should be made to revolve. A very convenient mode of efiecting
this is to mount the selenite plate in a revolving collar, which fits
into the uppei' end of the tube that receives the polarising prism.
In order to obtain the greatest variety of coloration with different
objects, films of selenite of different thicknesses should be employed ;
and this may be accomplished by substituting one for another in the
revolving coUai-. A still greatei- vaiiety may be obtained by mounting
three films, which separately give three different colours, in collars
revolving in a frame resembling that in which hand-magnifiei-s are
usually mounted, this frame being fitted into the sub-stage in such
a manner that either a single selenite, or any combination of two
selenites, or all three together, may be brought into the optic axis
above the polarising prism (fig. 262). As many as thirteen different
tints may thus be obtained. "When the construction of the mici'o-

FiG. 261. — Polarising apparatus.

POLARISING APPARATUS— RINGS AND BRUSHES

319

scope does not readily admit of the connection of the selenite plate
with the polaiising j^i'ism, it is convenient to make use of a plate of
brass (fig. 263) somewhat larger than the glass slides in which
objects ai-e oixlinarily mounted, with a ledge near one edge for
the slide to i-est against and a large circular apertui-e into which
a glass is fitted, having a film of selenite cemented to it ; this
' selenite stage ' or object-carrier being laid upon the stage of the
microscope, the slide containing the object is placed upon it, and, by
an ingenious modification contrived by Dr. Leeson, the ring into
which the selenite plate is fitted being made movable, one plate may
be substituted for another, whilst rotation may be given to the ring
by means of a tangent-screw fitted into the brass plate. The
variety of tints given by a selenite film under polarised light is so
gi-eatly increased by the interposition of a rotating film of mica that
two selenites — red and hlue — with a mica film, are found to give the
entire series of colours obtainable from any number of selenite
films, either separately or in combination with each other.

The comj^act appai'atus made by )Swift as a genei-al sub-stage
illuminator is useful and commendable,
and is capable of adaptation to most
English microscopes. It is shown in fig.
264. The special advantage of this con-
denser lies in its having the polarising

K=^

Fig. 262.

Fig. 263.

prism, the selenite and mica films, the black ground and oblique-
light stops, and the modei'ator all brought close under the back lens
of the achromatic ; whilst it combines in itself all the most important
appliances which the sub-stage of a good moderate microscope can
require.

Rings and Brushes. — Mr. Nelson has pointed out (' Journ.
R.M.S.,' 1892) that it is remarkable the microscopical text-books
give no account of the method of viewing the rings and brushes
which certain minerals show under polarised light. If the instru-
ment be set up as if for viewing ordinary polariscope objects, not a
ring or a brush will be seen.

The whole point lies in the fact that it is a wide-angled telescope
that is required, and not a microscope. Once this is recognised the
whole matter is simple. As the microscope has to be turned into a
wide-angled polarising telescope, all that is necessary is to screw a low
power on the end of the draw-tube, as in fig. 265 . As the light requires
to be passed through the crystal at a considerable angle, a wide-
angled condenser should be employed, but it need not be achromatic.

!20

ACCESSOKY APPARATUS

The objective most suitable is a jAyths of -65 K.A. ; but a ^th of '71
N.A., or a ^rd of -65 N.A. will do equally well, as the whole of the
back lens of the objective should be visible thi-ough the analysing
' Nicol ; ' the back lens of the objective must not be too large, thus a
^ inch of "65 N.A. would not do so well. The analysing prism may
be placed either where it is in the drawing or above the eye-piece.
Practically it works very well above the
objective, which is the position it occupies
in ' ordinary microscopical outfits.'

For the draw-tube a 2 -inch objective
and a B or C eye-piece will answer
admirably.

Fig. 264. — Swift's illuminating and polarising
api^aratus.

Fig. 265. — In this diagram P
is the polarising prism in
the sub-stage, C sub-stage
condenser. On the stage
M mineral. On nose-piece
O' objective Tjjths "64 N.A. ;
A analysing prism. In the
draw-tube, O^ objective 2
or 3 in. H, Huyglienian
eye-piece.

_ For setting up the instrument it is better, before screwing the
objective in the end of the draw tube, to centre the light in the
usual manner, the ' Nicols ' being turned so as to give a light field.
Next fix the objective in the draw-tube, open the sub-stage con-
denser to full aperture, and put the mineral on the stage. Rack

MONOCHROMATIC ILLUMINATION

321

clown the body, so that the objective on the nose-piece nearly
touches the crystal ; then focus with the draw-tube exclusively. The
sub-stage condenser should be racked up close to the under side of
the crystal.

The use of monochromatic light is freqxiently desirahle in micro-
sco'pic loorh, especially blue light, although of less moment than
in pre-achromatic days. The usual method of obtaining coloured
light is to pass sunlight through coloured glass, oi- through a
coloured solution, such as the ammonio-sulphate of copper ; but this
is a most impei'fect and unsatisfactory method, and does not give
moJiochromatic light. This most valuable mode of illumination has
been made possible by the use of what is now known as the Gifford

screen, from the name of its
inventor, Mr. J. W. Gifford ;
and when artificial light is
u.sed one of these screens
•should be interposed between
the lamp and the sub-stage
condenser. It is shown in fig.
266, and consists of a glass
trough, about 3 inches long by
2 inches broad and i^o^ths deep,
filled with a solution of methyl
gi-een and glycei-in mixed

E 5

Fig. 266. — Gifford screen with an adjustable
stand.

Fig. 267. — Gii^ord's F-line mono-
chromatic light screen.

warm. Now this solution passes a little band of infra red, which
must be cut out. To do this a piece of signal green glass just fitting
the trough is placed in it.

A piece of ordinary commercial signal green would cut out too
much light, and render the sci'een too opaque ; therefore it is
I'equisite to have this signal green glass worked down to about half
its thickness, so that only the infra red passed by the methyl
green is cut out, and nothing more. This screen is called an
F-line screen, because the F line is in the centre of the band passed
by it. The band for general microscopical purposes may usefully
extend from E to G. The importance of this screen cannot be held
too high by the modern microscopist. It makes semi-apochromatic

Y

322

ACCESSOEY APPAEATUS

objectives equal to real apochromatics, and it sharpens the images
yielded even by the latter, whilst it increases resolving power in all

lenses, and amelior-
ates the strain often
felt by workers who
have not before used
it.

The cell contain-
ing the solution and
worked glass may
either have its uppei-
end sealed hermeti-
cally with paraffin, or
be simply carefully
corked ; the latter
plan, if the cork is
carefully made, ad-
mits of the easy
opening of the' cell
and renewal of the
fluid. A diagram-
matic illustration of
the effect of the use
of the screen is giA'en
in fig. 267, which
represents tlie band
of colour passed
through the F-line
screen. The green
is represented by the
horizontal lines, and
the blue, in which
the F line is situated,
by the diagonal lines..
The cell itself ife
prepared by the Ley-
bolds process, and is
fused at the joints
and nevei- leaks ; a
still simpler and less
expensive means of
making such a filter
has been devised by
Dr. A. Meithe, pro-
fessor of spectral
analysis at Berlin.
The filter consists of
a trough containing
I of an inch in thickness of saturated solution of acetate of copper
filtered ; a variation in the thickness of the troughs or tanks is
desirable, bxit the results are excellent.

MICRO-SPECTEOSCOPE 323

Equally perfect monochi'omatic illumination can be obtained by
prismatic dispersion.

A method of approximating to monochromatic illumination has
been devised by Mr. Nelson which answers admirably with an
ordinary ^-inch wick paraffin lamp. Briefly, the rays proceeding
from the i-adiant are passed through a slit, as in fig. 268, and
dispersed by a prism of glass, and by means of a second slit any
poi'tion we wish may be selected from the spectrum to be used for
the purpose requii'ed.

First an image of the edge of the flame is focussed upon the slit
by means of a bull's-eye consisting of three lenses ; next the slit is
placed in the principal focus of a lens known as a Wray 5 x 4 R R,

woi-king at ^!— (this lens is not shown in the cut). In the parallel

beam from this lens and close to it is placed an eqviilateral prism of
dense flint set at minimum deviation. Close to the prism is placed

f
another Wray 5 x 4 R R, Avorkino- at * - . If a cardboard screen be
•^ '^ 5-6

held at the principal focus of this lens, there will be seen a spectrum
brilliantly illuminated. A slit -jV^h inch in diameter is cut in the
cardboard screen, through which the requii'ed colour is allowed to
pass to the mirror of the microscope, thence to the sub-stage con-
denser. For visual Avork blue green is the best, but for photo-
graphic work blue would be chosen unless orthochromatic work
required a colour lower down the spectrum.

Sorby-Browning Micro-spectroscope.' — Wlien the solar ray is
decomposed into a coloured spectrum by a prism of sufficient disper-
sive power, to which the light is admitted by a narrow slit, a
multitude of dark lines make their appearance. The existence of
these was originally noticed by Wollaston ; but as Fraunhofer first
subjected them to a thorough investigation and mapjjed them out,
they are known as Fraunhofer lines. The greater the dispersion
given by the multiplication of prisms in the spectroscope, the moi'e
of these lines are seen ; and they bear considerable magnification.
They result from the interruption or absorption of certain rays in
the solar atmosphere, according to the law, first stated by Angstrom,
that ' rays which a substance absorbs are precisely those which it
emits when made self-luminous.' Kirchhofi^ showed that while the
incandescent vapours of sodium, potassium, lithium, kc. give a
spectrum Avith characteristic bright lines, the same vapours intercept
portions of the spectrum so as to give dark lines at the points where
the bright ones appeared, absorbing their own special colour, but
allowing rays of other colours to pass through. Again, when ordinary
light is made to pass through coloured bodies (solid, liquid, or
gaseous), or is reflected from their surfaces so as to affect the eye
with the sensation of colour, its spectrum is commonly found to
exhibit absorption hands, which differ from the Fraunhofer lines not
only in their greater breadth, but in being more or less nebulous or

1 For general information on the spectroscope and its uses the student is referred to
Professor 'Roscoe' '^Lectures on Spectrum Analysis, ovtlie translation of Dr. Schellen's
Spectrum Analysis, and How to use the Spectroscoije, by Mr. John Browning.

Y 2

)24

ACCESSORY APPARATUS

cloudy, so that they cannot be resolved into distinct lines by magni-
fication, while too much dispersion thins them out to indistinctness.
Now, it is by the character of these bands, and by their position in
the spectrum, that the colours of different substances can be most ac-
curately and scientifically compared, many colours whose impressions
on the eye are so similar that they cannot be distinguished being
readily discriminated by their spectra. The purpose of the micro-
spectroscope ^ is to apply the spectroscopic test to very minute
quantities of coloured substances ; and it fundamentally consists of
an ordinary eye-piece (which can be fitted into any microscope) with
certain sjDecial modifications. As originally devised by Dr. Sorby
and worked out by Mr. Browning, the micro-spectroscope is con-
structed as follows (fig. 269) : Above its eye-glass, which is achro-
matic, and made capable of focal adjustment by the milled head, B,
there is placed a tube. A, containing a series of five prisms, two of
flint glass (fig. 270, F F) interposed between three of crown
(0 C C) in such a manner that the emergent rays, ?* ?•, which have
been separated by dispersion, leave the prisms in much the same

dii'ection as the immergent I'ay
entered it. Below the eye-glass,
in the place of the ordinary stop,
is a diaphi'agm with a narrow slit
which limits the admission of light
(fig. 269) ; this can be adjusted in
vertical position by the milled head,
H, whilst the breadth of the slit is

Fig. 269. — Micro-spectroscope.

Fig. 270.

regulated by C. The foregoing, with an objective of suitable power,
would be all that is needed for the examination of the spectra of
objects placed on the stage of the microscope, whether opaque or
transparent, solid or liquid, provided that they transmit a sufficient
amount of light. But as it is of great importance to make exact
compai'isons of such artificial spectra, alike with the ordinary or
natural spectrtim and with each other, provision is made for the
formation of a second spectrum by the insertion of a right-angled
prism that covers one half of this slit, and reflects upwards the light
transmitted through an aperture seen on the i-ight side of the eye-
piece. For the production of the ordinary spectrum, it is only
requisite to reflect light into this aperture from the small mirror, I,
carried at the side ; whilst for the production of the spectrum of any
substance through which the light reflected fi'om this mirror can be
transmitted, it is only necessary to place the slide carrying the
section or crystalline film, or the tube containing the solution, in

1 We do not make the change, lest complications should arise; but we think it
would be more harmonious with analogy to call this instrument the sjpectro-micro-
sco'pe.

USE OF THE MICEO-SPECTEOSCOPE

325

the frame, D D, adapted to receive it. In either case this second
spectrum is seen by the eye of the observei- alongside of that pro-
duced by the object viewed through the body of the microscope, so
that the two can be exactly compared.

The exact position of the absorption bairds is as important as
that of the Fraunhofer lines ; and some of the most conspicuous of
the latter afford fixed points of reference, provided the same spectro-
scope be employed. The amount of dispersion determines whether
the Fraunhofer lines and absorption bands are seen nearer or
farthei" apart, their actual jjositions in the field of view varying
according to the dispersion, while their relative positions are in
constant proportion. The best contrivance for measuring the
spectra of absorption bands is
Browning's bright-line micro-
meter, shown in fig. 271. At R
is a small mii-ror by which light
from the lamp employed can be
reflected through E D to the
lens C, which, by means of a
perforated stop, forms a bright ,^^
pointed image on the surface of \
the upper prism, whence it is
reflected to the eye of the ob-
server. The rotation of a wheel
worked by the milled head, M,
carries this bright point over the
spectrum, and the exact amount
of motion may be read off to
loQooth inch on the graduated
circle of the wheel. To use this
apparatus, the Fraunhofer lines
must be viewed by sending bright
daylight through the spectro-
scope, and the positions of the
principal lines carefully measured,
the reading on the micrometer-
wheel being noted down. A
spectrum map may then be drawn Fig. 271.-Bright-line spectro-micrometer.
on cardboard, on a scale of equal

parts, and the lines marked on it, as shown in the upper half of
fig. 272. The lower half of the same figure shows an absorption
spectrum, with its bands at certain distances from the Fraunhofer
lines. The cardboard spectrum map, when once drawn, should be
kept for reference.^

A beginner with the micro-spectroscope should first hold it up to
the sky on a clear day, without the intervention of the microscope,

1 Mr. Swift has devised an improved micro- spectroscope, in which the micro-
metric apparatus is combined with the ordinary spectroscopic eye-piece, and two
spectra can be brought intcf' the field at once. Other improvements devised by
Dr. Sorby and a new form devised by Mr. F, H. Ward have been carried into

execution by Mr. Hilger.
and vol. ii. 1879, p. 81.)

(See Journ. of Boy. Microsc. Soc. vol. i. 1878, p. 326,

326

ACCESSOKY APPARATUS

and note the effects of opening and closing the slit by rotating the
screw, 0 (fig. 269) ; the lines can only be well seen when the slit is
reduced to a narrow opening. The screw H diminishes the length
of the slit, and causes the spectrum to be seen as a broad or a narrow
ribbon. The screw E (or in some patterns two small sliding knobs)

0 10 3,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 100 110 120 1,30 140

HTmlTTTTTmlTmlTmlTmlTmlmrlTml— llmlmlTml— Irml— I—I— I— I— I— ^

Fig. 272. — Upper half, map of solar spectrum, sliowing Frauuhofer lines. Lower
half, absorption spectrum, showing position of bands in relation to lines.

regulates the quantity of light admitted through the square aperture
seen between the points of the springs, D D. "Water tinged with
port wine, madder, and blood are good fluids with which to com-
mence this study of absorption bands. ^ As each coloiu' varies in
refrangibility, the focus must be adjusted by the screw B, fig. 269,

according to the part of the
spectrum that is examined.
When it is desired to see the
spectrum of an exceedingly
minute object, or of a small
poi'tion only of a larger one,
the prisms are to be re-
moved by withdrawing the
tube containing them ; the
slit should then be opened
wide, and the object, or part
of it, brought into the centre
of the field. ; the vertical and
horizontal slits can then be
partly shut so as to enclose
it ; and if the prisms are
then replaced and a suitable
objective employed, the re-
quired spectrum will be seen,
unafifected by adjacent ob-
j ects . For ordi nary observa-
tions objectives of from two
inches to |-inch focus will be found most suitable ; but for very
minute quantities of material a higher power must be employed.
Even a single I'ed blood-corpu.scle may be made to shoM^ the

1 A series of specimens, in small tubes, for the study of absorption-spectra,_ is
kept on sale by Mr. Browning ; and the directions given in his How to work with
the Micro-spectroscojpe should be carefully attended to.

Fig. 273.

USE OF THE MICEO-SPECTEOSCOPE

327

characteristic absorption bands represented (after Pi'ofessor Stokes)
in fig. 273.1

For the study of coloured liquids in test-tubes or small cells, the
binocular spectrum mici'oscope, described by Dr. Sorby in the ' Pro-
ceedings of the Royal Society,' No. 92, 1867, p. 33, is extremely
convenient.

The spectral ocular by Zeiss is another and a very perfect form of
the mixyro-spectrosGope. This is an opinion expressed by Dr. Sorby
and other expei-ts, and it is manifest in the character of the in-
strument. Fig. 274 represents a sectional view of the instrument.
It will be seen that the lower part is an ordinary eye-piece with its
two lenses, but in place of the ordinary diaphragm there is a slit
adjustable in length and breadth, shown in fig. 275. By studying
this figure the method of adjustment with two screws, F and H,
and the projecting lever, which carries a reflecting prism, can be

Fig. 274.

Fig. 275

readily understood. The upper part of the instrument swings
about the pivot, K, so that by opening the slit the eye-piece can be
used for focussing an object, the slit being the diaphragm. The
upper portion contains the prisms, and also a scale in the tube, N,
which is illuminated by the mirror, 0. The image of the scale is
reflected from the upper surface of the last prism to the eye, and
when properly adjusted gives the wave-length of the light in any
part of the spectrum. There is also a supplementary stage, not
shown in the figure, upon which a specimen can be placed, and its
light thrown up through the slit by reflection from the prism on the
lever shown in fig. 274, alongside of the light from the object on the
stage of the microscope, thus enabling the spectra from the two
sources to be directly compared.

1 For further information on ' The Spectrum Method of Detecting Blood,' see an
important paper by Dr. Sorby in Monthly Microsc. Joicrn. vol. vi. 1871, p. 9.

328 ACCESSORY APPARATUS

The Method of using the Micro-spectroscope. — The objects to be
investigated are of two sorts, liquid and solid. Colouring substances,
as chlorophyll, the colouring matter of hair, blood, &c., will fre-
quently come under micro-spectroscopic investigation in the form of
a, solution. In general we need scarcely say anything concerning
the preparation of the solution. In reference to the chlorophyll of
the phanerogams especially, the particular part of the plant from
which the prejDaration is to be made, as, for instance, the foliage
leaves, is put for a short time in boiling water, then quickly dried
by means of bibulous paper, and then immersed for a longer time in
absolute alcohol, ether, or benzole in a dai-k place, for the purpose
of extracting the chlorophyll colouring matter. The concentration
of the solution thus produced, which influences the intensity of the
absoi'ption s]jectrum and the number and length of the absorption
bands, depends naturally upon the time during which the material is
in the extracting medium, as well as on the quantity of the material.

Commonly also a solution of less concentration will give the same
intensity of spectrum if a siiificiently thick layer of it be used. The
solution can generally be examined in an ordinary test-tube. The
test-tube is filled and carefully corked, and then laid on the stage of

as the case ln^^ be Foi the Ixttei puipose (bunging liquids befoie

Fig. 276.

the opening of the comparison prism) a small open trough of glass,
with two parallel glass plates, is very useful. For exact investiga-
tions, however, the trough-flask is preferable. It is a flask whose
two sides, back and front, are parallel, furnished with a carefully
fitted ground-glass stopj)er. It should be filled quite full of the
solution and then laid with its broad side on the stage. It is
especially indisj^ensable when we wish to study the combination
spectrum of two solutions. In that case two flasks are filled each
with a difierent solution, and both laid upon the stage, one upon
the other. For the purpose of examining small quantities of
any liquid, a sufiicient depth being obtained with very little
material, vertical glass tubes attached to horizontal plates are used,
as proposed by Mr. Sorby and shown in fig. 276. The narrow tubes
are made of vai-ious lengths from sections of barometer tubing, in
order to j)i"esent difierent thicknesses of the contained fluid, the
broad tube being higher on one side than the other, and thus con-
stituting a wedge-shaped cell, which, when filled and closed by a
thin cover-glass, Avill present a varying thickness of fluid for study
and comparison. If the object to be investigated is not a solution,
but a preparation of the kind which we commonly employ in micro-
scopic inquiries, we must first of all bring it into the focus of the
objective system. To do this we must first remove the tube bearing

ILLUMINATION BY REFLECTION

329

the i^-isms, open the slit somewhat, and use the apparatus as a
simple ocular. If one has to deal with a small object which does
not entirely fill the slit, but allows rays of light to come in past
it and distui-b the spectrum, he should turn the comparison prism
so as to shut up some of the slit, without, however, letting in the
light upon it, and then bring the object up near to it, and from the
othei- side push up the shortening apparatus as close as is necessary.
On the othei- hand, should the object consist of a number of single
minute grains, which would cause to be drawn across the spectrum,
in the direction of its length, perpendicular to the Fraunhofer lines,
a like number of dark lines,
one must adjust the micro-
scope so that the object will
be a little out of focus, some-
what above or beloAv the true
focus. In this w^ay we shall
get a uniform spectrum. The
spectrum can also be improved
in some other cases by like-
wise throwing the object
somewhat out of focus.

Illumination by Reflec-
tion.— Objects of almost ever}'
description will require at
times to be examined and
studied by what is called re-
flected light ; the light in
this case is thrown clown upon
the object by various de\'ices,
and is reflected upwards
through the objective. This
has been called ' opaque illu-
mination,' which, however,
is not a comprehensive, nor
even an accurate designation.
Only a small proportion of
the objects examined in this
way ai-e opaque ; the same
diatom, for examj^le, may
often with advantage be ex-
amined with transmitted

light, being transparent, and again by means of an illumination
thi'own upon, and reflected up frora, its surface ; also a condenser
with a central stop, when used for a dark ground, shows objects by
reflected light, but it is manifestly not ' opaque illumination.' The
designation of this method of illumination is consequently more
accommodating than accurate.

There are two very simple means of obtaining this superficial
illumination when low powei'S are employed. The first is the
' bull's-eye ' (which is nowhere in this work called a ' condenser ; '
this would, as it often has done, lead to confusion ; it is enough to

Pig. 277. — The English form of bull's-eye.

330

ACCESSOKY APPAEATUS

designate it as we have done). It is a plano-convex lens of short
focus, two or three inches in diameter, mounted upon a separate
stand in such a manner as to permit of its being placed in a great
variety of positions. The mounting shown in fig. 277 is the usual
adopted in England ; the frame which cai-ries the lens is borne
at the bottom upon a swivel joint, which allows it to be turned in
any azimuth ; whilst it may be inclined at any angle to the horizon,
by the revolution of the horizontal tube to which it is attached,
around the other horizontal tube which projects from the stem. By
the sliding of one of these tubes within the other, again, the hori-
zontal arm may be lengthened or shortened; the lens may be
secured in any position (as its weight is apt to drag it down when

Pig. 277a.

it is inclined, unless the tubes be made to work, the one into the
other, more stiffly than is convenient) by means of a tightening
collar milled at its edges ; and finally the horizontal arm is
attached to a spring socket which slides up and down upon a vertical
stem.

A good form of the bull's-eye is made by Leitz, and is illustrated
fig. 277a. All the required movements are pi'ovided for, but in a
difierent way ; the clamping screws are by means of usvial milled
heads.

The plane side of the bull's-eye should be turned towards the
object. Some microscopists like to have their bull's-eye attached to
some part of the microscope ; but if this is done, care must be taken
to attach it to a fixed part of the microscope, and not to either the

THE USE OF THE BULL'S-EYE 33 1

mechanical stage or to the body, as is so often done. If it is fixed
to the mechanical stage, when the object is moved the light will
require to be readjusted, to say nothing of the probable injury to
the stage by the weight of the bull's-eye. If it is fixed to the
body the light will be displaced when the focus of the objective is
altered. Hence the bull's-eye should either have a weighted separate
stand, or be attached to the stand oi- holder of the lamp or other
illuminant.

The optical effect of such a bull's-eye differs according to the
side of it turned towards the light and the condition of the rays
which fall upon it. The position of least spherical aberi-ation is wdren
its convex side is turned towards parallel or towards the least diverging
rays ; consequently, when used by daylight, its plane side should be
turned towards the object, and the same position should be given to it
when it is used for pi-ocuring converging rays from a lamp, this being-
placed four or five times farthei- off on one side than the object is on
the other. But it may also be employed for the j)urpose of reducing
the diverging rays of the lamp to parallelism, for use either with the
paraboloid, or with the parabolic speculum to be presently described ;
and the plane side is then to be turned towards the lamp, which must
be jslaced at such a distance from the bull's-eye that the rays which
have passed through the latter shall form an inverted image of
the lamp flame on the wall or a distant screen. For viewing minute
objects under high powers, a smaller lens may be used to obtain a
further concentration of the rays already brought into convergence
by the bull's-eye. An ingenious and effective mode of using the
bull's-eye for the illumination of very minute objects under higher-
power objectives has been devised by Mr. James Smith. The micro-
scope being in position for observation, the lamp should be placed
either in the front or at the side (as most convenient), so that its
flame, turned edgeways to the stage, should be at a somew^hat loimr
level, and at a distance of about three inches. The bull's-eye should
be placed between the stage and the lamj), with its plane surface
uppermost, and with its convex surface a little above the stage.
The light entering its convex surface near the margin turned towards
the lamj) falls on its plane surface at an angle so oblique as to be
almost totally reflected towai'ds the opposite margin of the convex
surface, by which it is condensed on to the object on the stage, on
which it should cast a sharp and brilliant wedge of light. The ad-
justment is best made by first placing a slip of white card on the
stage, and, when this is well illuminated, substituting the object
slide for it, making the final adjustment while the object is being-
viewed under the microscope. No difficulty is experienced in
getting good results with powers of from 200 to 300 diameters, but
higher powers recjuire careful manipulation and yield but doubtful
results.

The second simple method of securing this illumination is to have

the concave mirror of the microscope capable of being used above

the stage, ^ so that the source of light may by its means be focussed

on the object. ISTeither of these plans will answer for other than low

1 See Journ. Boy. Microsc. Soc. vol. iii. 1880, p. 398.

332 ACCESSOEY APPARATUS

powers, where there is plenty of room for the light to pass between
the objective and the object. The ingenious use of the bull's-eye
employed by Mr. James Smith, as detailed above, increases the j)ossi-
bility of magnification, but it needs practice and care. With the
great improvement which has been effected in objectives and con-
densers the need of a bull's-eye which should give the minimum of
aberration has become a desideratum ; and Mr. JSTelson has calculated
and had constructed a doublet bull's-eye which gives admirable
results. There are described in most treatises on optics doublets
devised by Herschel which are said to be of ' no aberration.' Mr.
Nelson has shown (' Journ. Q. M. S.,' vol. vi. ser. ii. p. 197, 1896)
that they are by no means free from spherical aberration, and that
their forms are siich as will not even yield a minimum amount of
such aberration ; also that there is a numerical error in the focal
length of the high-power doublet. He has computed that the spheri-
cal aberration in the Herschel doublets amounts to — -296 "i-, and
he gives the following formula for a combination, the sjDherical
aberrationof which is — '207— ; or 30 per cent, less than in either of

those proposed by Sir John Herschel.

Boro-silicate glass, Jena catalogue No. 5 ; /i:=l'5] .

^1^=64-0.
ill

1st lens crossed, r= + 2"359'| ■,. ,0-1.

1r r\ rr o r ciiame tjer z * i \
5-078) '

2nd lens meniscus, ■?■= + 1'280) t ', i <-,
' , .5,0^ f diameter I'o.

s= + 3 •434 J

Distance between the lenses '05, equivalent focus 2'0, working-
distance or back focus 1"55, total aberration — '1035, clear aperture
2-0, angle 62°. The second Gauss point of the combination is close
to the posterior surface of the crossed lens.

As there are some microscopists who might require a combina-
tion of this kind, but with a different focal length, and who are
unable to transpose the formula, the following rule may be of use.
Halve all the radii and diameters and multijoly the results by the focal
length that is required. Example. — Required a doublet' on this
formula with 3^ inches of equivalent focus. Halving the data for
the crossed lens in the given formula, we have ?■:= -f-l"1795,
s= — 7'539, diameter 1'05; multiplying these results by 3^ we
obtain r=+ 4" 128, s= — 26'386, diameter 3-7. Treat the meniscus
in the same way ; the lens distance may with advantage be kept
•05.

The following bull's-eye is not so expensive to manufacture, and
may on that account be preferred to the doublet of minimum aber-
ration just described. Its form, though of mitiimum aberration for
two j^lano-convex lenses, j^ossesses 43 per cent, more aberration than
the former. It will on this account not be possible to obtain such
an even and unbroken disc of light with this form of bull's-eye as
with the other. The data are as follows.

NELSON'S COMPUTATION

Glass, boro-silicate, the same as before.

"" "" f diameter 2"1 ;

s =00 )

/= + 1-631

diameter 1'9.

Distance of lenses apart '05, equivalent focus 2-0, working dis-
tance 1-50, angle 60°.

It is illustrated in a mounted form in fig. 278. Combinations
having different foci may be constructed in the same manner as in
the example above.

An illuminator not so well known, or at least so much used, as
its merits justified, is Powell and Lealand's small bull's-eye of | inch
focus, which slides into an adapter fixed into the sub-stage, and
susceptible of its rack motion up and down. The object is placed
on a super-stage, and lies considerably above, but parallel with, the
ordinary stage. The bull's-eye, capable thus of
being raised or lowered, and of being moved by
sliding away from or close to the mounted object,
has its plane side placed against the edge, and at
right angles to the plane of the slip. By this
means illumination of great obliquity can be
obtained, and very surprising effects secured even
Avith high powers. It was much used by the Editor
and Dr. Drysdale in their earlier work on the
saprophytic organisms, and, in the days before
homogeneous lenses, helped us over many diffi-
culties of detail. It was the first illuminator to
actually resolve the Amphipleura pellucida. It
could be very easily obtained with a student's Fig. 278.— Bull's-eye
microscope provided with Nelson's open stage,^ for 9^ good but not the
on this the bull's-eye could be placed against the vj'g'ea bv^r.^Nel-
edge of the slip without any special apparatus or son.
fitting.

Another and popular method of ' opaque illumination ' is by
means of a specialised form of mirror, generally of polished silver,
called a side reflector, and fixed, as in the case of the bull's-eye, and
for the same reasons, to an immovable part of the microscope.

The manner of employing this I'eflector, as provided with Powell
and Lealand's best stand, is seen in Plate i±J;r The ai-m of the
side reflector is fixed to an immovable jjart of the stand, and is thus
unaffected by the i^acking up or down of the body. The lamp placed
on the right of the observer is set at such a height that its beams'
fall full upon the refiector ; this, by means of a ball-and-socket
joint, can be easily manipulated until the full image of the flame is
caused to fall upon the object. For the same purpose a parabolic
spectdum is commonly employed, mounted either on the objective,
as in Beck's form, fig. 279, or on an adapter, as in Crouch's, shoAvn
in fig. 280, where a collar is interposed between the lower end of
the body of the microscope and the objective seen at A. This is not

1 Fig. 134.

tt

334

ACCESSORY APPARATUS

a coinniendahle plan, for it increases the distance between the ob-
jective and the Wenham binocular prism ; and as the binocular is
specially suited for the kind of object usually examined with this
speculum, this increased distance, acting detrimentally on the be-
haviour of the binocular prisms, and causing the available racking
distance for the focus of objectives of very low power to be shortened
by the width of such collar, is to be avoided.

The best plan without doubt is to attach the speculum to a fixed

Fig. 279.

Pig. 280.

part of the stand, as is done in the Powell and Lealand, the Ross,
and the Beck stands.

A modification of the paraholic reflector xoas devised hy Dr. Sorby,
and has proved to be very useful in certain investigations, such as
the microscopic structure of metals. It consists of a parabolic
reflector, in the centre of which, in a semi-cylindrical tube open in
front, is placed a small plane reflector which covers half of the
objective, and throws the light directly down upon the object and

back through the other half. It is
shown in fig. 281 with the cylinder in
place, and in the dotted lines with the
same tui-ned out. This arrangement
allows of two kinds of illumination,
oblique and direct, being readily used,
as the plane reflector is attached to an
arm so that it can be swung out of the
way when not required, as shown in the figure.

Dr. Sorby was able to get results in the examination of polished
sections of steel not otherwise attainable.

jS^o opaque ilhnnination, however, has yet surpassed the venerable
Lieberkiihii ; the best experts fi'eely admit that the finest critical
images to be obtained by this method of illumination are secui'ed
by the Lieberklihn. This mode of illuminating opaque objects is
by means of a small concave speculum reflecting directly down ujion
them to a focus the light reflected wp to it from the mirror ; it was

Fig. 281. — Sorby's modification of
the parabolic reflector.

LIEBEEKUHN — ITS DRAAVBACKS 335

formerly much in use, but is now comparatively seldom employed.
This concave speculum, termed a ' Lieberkiihn,' from the celebrated
microscojjist who invented it, is made to fit upon the end of the
objective, having a pei-foi-ation in its centre for the passage of the
rays from the object to the lens ; and in order that it may receive
its light from a mirror beneath (fig. 282, A), the object must be so
mounted as only to stop out the centi-al poi'tion of the rays that are
reflected upwards. The curvature of the speculum is so adapted to
the focus of the objective that, when the latter is duly adjusted, the
rays reflected up to it from the mii-ror shall be made to converge
strongly upon the part of the object that is in focus ; a separate
speculum is consequently required for every objective.

It has tioo manifest dravjhacks : the first one, that of reqidring a
seixirate Lieberkiihn for each objective, is a difliculty which in the
nature of things cannot be overcome. The radius of the Lieberkiihn

Fig. 282.

must alter with the focus of the objective employed, and each should
have a certain amount of play on the objective to allow for slight
altei-ations of focus ; for if we employ jDarallel rays it is obvious that
the Lieberkiihn will focus nearer to the object than if divergent
rays are used. This is met by an allowance being made to com-
pensate it on the tube which slides the Lieberkiihn on to the nose
of the objective.

The second drawback has reference to the Sjyecial way in %nhich
objects have to be moxinted in order to be suitable for the Lieberkiihn.
This could be easily avoided if professional and other mounters
would attend to the following simple suggestions : —

1 . Slides should never be covered with paper ; it is without use,
and fails as an ornament ; and opaque glass slips should be entirely
avoided.

2. The ring of cement should not be made of greater widtli than
is necessary for security.

336 ACCESSORY APPARATUS

3. A stop of paper or varnish should never be placed behind an
object.

Let every opaqvie mount be also a ti'ansparent one, since it is
often most useful to examine an opaque object afterwards by trans-
mitted light. The stop should always be a separate one ; this may
be a disc on a pin held in the sub-stage, or, what is still simpler, a
piece of moderately thick ' cover ' glass, cut to the 3x1 inch size,
or rather shorter, should have a small disc of Brunswick black put
on it centrally on the ' turn-table,' ^ and this may be placed under
the slide when the Lieberkiihn is to be used. There may be two oi-
three such slips with stops of different sizes ; in this way every
mount may be examined either with the Lieberkiihn or by directly
transmitted light, and of course by having a largei- stop the same
object may be examined by any kind of reflected light. Many a
valuable pi-eparation has been spoiled by placing a stoj) on it which
cannot be removed.

4. It would be a most appreciable benefit to the cause of micro-
scopy, as we have already hinted, if a uniform gaiige of thickness of
slip and diameter of cover-glass were adopted. For the thickness
of the slip, the ^th of an inch would prove most suitable, and for
the diameter of the cover-glass | of an inch would be most con-
venient, and if the thickness of the cover-glass were uniformly from
•006 to "008 the gain would be still greater. Certainly no mount
ought to be finished without the thickness of the covei'-glass being
marked in diamond j^oint upon it, and a narrow ring of shellac
cement should be put round every cover-glass where there is even a
probability that a homogeneous lens will be admissible in examining
the object mounted.

Very minute cover-glasses — such as those i^gths of an inch in
diameter — are to be wholly condemned. They do not allow the
conditions required by modern mici'oscopy, being adverse to the
employment of oil-innnersion lenses in anything like the most
efficient way.

Lieberkiihns can be used with objectives as high as j of an inch
focus of '77 N.A. For higher powers than this a pei-fectly flat
speculum may replace the conical form, being illuminated by a
condenser with a stop, and racked up well within its focus. The
oblique annular ring of light falls on the flat speculum, and is then
reflected on the object.

The light suitable for illumination by Lieberkiihn may be either
the flat of the lamp flame, reflected by the plane mirror, or the edge
of the flame, the rays being rendered parallel by a bull's-eye, and
reflected from the plane mirror to the Lieberkiihn.

There is one other kind of reflected illumination em-
ployed, produced by the vertical illuminator, which, although it
has been in use for some years, has received an accession of value
from the employment of immersion lenses. The earliest device foi-
accomplishing this was invented by Professor H. L. Smith, of
Geneva, U.S.A.

The principle of this illuminator is to employ the objective as

' Chapter vii.

VERTICAL ILLUMINATOE

ZZ7

its own ilkiminator ; which Professor Smith did by means of a
.specuhim. A pencil of light was admitted from a lateral aperture
above the objective and then reflected downwards vipon the object
through the lenses by means of a small silvered speculum placed on
one side of its axis.

Messrs. R. and J. Beck, in place of a speculum, employ
A disc of cover-glass. The cover-glass is mounted on a pin, B,
fig. 284, in order that it may be rotated, and oblicpie light obtained
by the milled head,/. A, fig." 284. _

Powell and Lealand's method is to fix a piece of glass, toorked
flat, at an angle of 45° to the optic axis, with a rotating diaphragm
in front of the aperture admitting the light.

Fig. 283.

Pig. 284.

To use these instrtmients the edge of the lamp flame should be
placed in front of the reflector, so that the rays may be reflected on
to the back lens of the objective in a line parallel to the optic axis.
The distance from the lamp to the reflector must exactly equal the
distance from the reflector to the diaphragm of the eye-piece in a
positive eye-piece, or the eye-lens of a negative eye-piece, otherwise
the rays will not be focussed on the object.

This illumination is only suitable for objects mounted dry on the
cover, and with immersion lenses. No good result was ever obtained
until the immersion lenses were brought into use, but it is now
largely used in the examination of metals. The microscope adapted
to its employment is shown in fig. 207.

Of all the light which is caused to pass out of the front lens of
the objective, through the oil and into the cover-glass, that which
has an obliquity less than the critical angle for glass (41°) passes
through the cover and object and is lost; but all the light which is
of greater obliquity than the ciitical angle for glass is totally reflected

z

338 ACCESSOEY APPAEATUS

fi'oni the tinder surface of the cover-glass, and conies back through
the oil and the objective to the eye-piece and the eye ; they are, in
fact, all optically continuous, so that the upjoer surface of the cover-
glass has ceased to exist optically, the only reflexion being from its
inner surface. It is here, therefore, that the oil-immersion system
gives a new value to this illuminator, by this means enabling it to
utilise a larger aperture otherwise unavailing.

When this illumination is employed, if the eye-piece be removed
and the back of the objective be examined, it will be seen that all
that portion of the back of the objective whose aperture exceeds I'O
is brilliantly illuminated. This annulus represents, andis produced by,
the excess of apertiu^e beyond the equivalent air angle of 180°, of
which it is also a measure. The internal dark space is of the exact
diameter of that of a dry objective of the same focus, and is the
maximum space which it can itself utilise on a dry object by trans-
mitted light.

By means of this instrument carefully used, some difficult tests
and lined objects have been resolved ; but its principal use at the
present day is for the examination of metals, and it is eminently
serviceable in determining whether any dr}'-mounted object is in
optical contact with the cover-glass or not. If it be not so it is in-
lisible with the vertical illuminator. So also it is instructive to
examine the backs of objectives of various apertures with this mode
of illumination. A dry objective will be wholly without the bright
annulus, while aninnnersion of 1"1 N.A. will have a narrow annulus,.
and that of 1*4 or 1'5 a broad and still broader one. In this way,
by practice, a fail- appi'oxiiiiation to the apertui-e of an objective may
be obtained.

It is not the absolute size of the annulus, but the relation of the
size of the annulus to that of the whole back, that must be estimated.
Thus |-th of N.A. 1'2 will have as broad an annulus as yV'^h of
1"4 N.A., but the diameter of the back of the g-tli is, of course, much
larger than that of the iWth, and this involves the necessity for a
relative compaiison.

Appliances for the Practical Study of Living and other Objects
with the Microscope. — Stage-forceps and Vice. — For bringing under
the object-glass in different positions such small opaque objects as
can be conveniently held in a pair of forceps, the stage -f or cejjs (fig.
285) supplied with most microscopes jirovide a ready means. These
are mounted by means of a joint upon a pin which fits into a hole
either in the corner of the stage itself or in the object-platform ; the
object is inserted by pressing the pin that projects from one of the
blades, whereby it is sepaiuted from the other ; and the blades close
again by their own elasticity, so as to retain the object when the
pressure is withdrawn. By sliding the wire stem which bears the
forceps through its socket, and by moving that socket vertically
upon its joint, and the joint horizontally upon the pin, the object
may be brought into the field pi'ecisely in the position required ;
and it may l)e tui-ned round and round, so that all sides of it
may be examined, by simply giving a twisting iiioA^ement to the
wire stem. The other exti'emity of the stem often bears a small

STAGE APPLIANCES

339

Fig. 285.— Stage-forcepf.

Fig. 286.— Stage-forceps.

brass box filled witb coi-k, and perforated with holes in its side,
seen in fig. 286 ; this aflbrds a secure hold to common pins, to the
heads of which small objects can be attached b}- gum, or to which
discs of cai-d, ifec, may be attached, whereon objects are mounted
for being viewed with the Liebei'klihn. This method of mounting
was formerly much in vogue, but has been less employed of late,
since the Liebei'kiihn has unfoi-tunately fallen into comparative
disuse. The forceps in fig. 287 are also often of great practical value,
and are adjusted for holding by a screw. That which is known as
the stage-iHce, for the
piu'pose of holding small
hard bodies, such as
minerals, apt to be jerked
out by the angular motion
of the blades of the for-
ceps, or very delicate
substances that will not
bear rough compression,
is very useful, and is seen
in fig. 288. The stage-
vice fits into a plate, as
is the case with Beck's
disc-holder, fig. 289, or
it may simply drop into
a stage fitting, as in tlie
figure.

For the examination
of objects which cannot
be conveniently held in
the stage-forceps, but
which can be temporarily
or permanently attached
to discs, no means is
comparable to the disc-
holder of Mr. R. Beck
(fig. 289) in regard to
the facility it aflfords foi-
presenting them in every
variety of position. The
object being attached by
gum (having a small
quantity of glycerine
mixed with it) or by
gold size to the surface of a small blackened metallic disc, this is
fitted by a short stem projecting from its under surface into a
cylindrical holder ; and the holder carrying the disc can be made to
rotate around a vertical axis by turning the milled head on the
right, which acts on it by means of a small chain that works through
the horizontal tubular stem ; whilst it can be made to incline to one
side or to the other, until its plane becomes vertical, by turning
the whole movement on the horizontal axis of its cylindrical

z 2

Fig. 287.
Three-pronged forceps, screw adjustment.

Fig. 288.— The stage-vice.

Fig. 289. — Beck's disc-holder.

340 ACCESSOEY APPAEATUS

socket.^ The supporting plate being perforated by a large aperture,
the obj ect may be illuminated by the Lieberkiihn if desired. The discs
are inserted into the holder, or are removed from it, by a pair of
forceps constructed for the purpose ; and they may be safely put
away by inserting their stems into a plate perforated with holes.
Several such plates, with intervening guards to prevent them from
coming into too close apposition, may be packed into a small box.
To the value of this little piece of apparatus the Author can bear
the strongest testimony from his own experience, having found his
study of the Foraminifera greatly facilitated by it.

Glass Stage-plate. — Every microscope should be furnished with
:a piece of plate glass, about 3^ in. by 2 in., to one margin of which a
narrow strip of glass is cemented, so as to form a ledge. This is
• extremely useful, both for laying objects upon (the ledge preventing
them — together with their covers, if used — from sliding down when
the microscope is inclined), and for preserving the stage from injury
by the spilling of sea- water or other saline or corrosive liquids when
;Such are in use. Such a plate not only serves for the examination
of transparent, but also of opaque objects ; for if the condensing
lens is so adjusted as to throw a side light uj)on an object laid upon
it, either the diaphragm plate or a slip of black paper will afford a
'dark background ; whilst objects mounted on the small black discs
■suitable to the Lieberkiihn may conveniently rest on it, instead of
being held in the stage-forceps.

Growing Slides and Stages. — A number of contrivances have been
devised of late years for the purpose of watching the life histories of

Fm. 290.

minute aquatic organisms, and of ' cultivating ' such as develop and
multiply themselves in particular fluids. One of the simplest and
most effective, that of Mr. Botterill, represented in fig. 290, consists
of a slip of ebonite, three inches by one, with a central aperture of
three-fourths of an inch at its under side ; this aperture is reduced
by a projecting shoulder, whereon is cemented a disc of thin glass,
which thus forms the bottom of a cell hollowed in the thickness of
the ebonite slide. On each side of this central cell a small lateral cell
communicating with it, and about a fourth of an inch in diameter, is
drilled out to the same depth ; this serves for the reception of a supply

1 A small pair of forceps adapted to take up minute objects may be fitted into
the cylindrical holder in place of a disc.

GEO WING SLIDES

341

Fig. 291.

of water ov other fluid, which is imparted, as required, to the central
' growing ' cell, which is completed by placing a thin glass cover over
the objects introduced into it, with the interposition of a ring of thin
paper, oi- (if a gi-eatei" thickness be i-equired) of a ring of cardboard
or vulcanite. If the fluid be introduced into one of the lateral cells,
and be drawn ofl" fi-om the othei's — either by the use, from time to
time, of a small glass syringe, to be hereafter desci-ibed, or by
threads so arranged as to produce a continuous drij) into one and
from the other — a constantly renewed supply is furnished to the
central cell, which it enters on one side and leaves on the other,
by capillary atti'action.

Dr. Lewis's and Br. Iladdox's groiving slides are shown in figs. 291
and 292. Two semicircles of asphalte varnish are brushed on the
slide, one being I'ather larger than the other, so that the ends of one
half-circle may over-
lap the other, but not
so closely as not to
permit the entrance
and exit of air. When
nearly dry a minute
quantity of growing
flviid is placed in the
centre, upon which
a few spores are
sown, a cover-glass

being placed over it, which adheres to the semi-dried varnish. The
slide should be placed under a bell-glass, kept damp by being lined
with moist blotting-paper.

Dr. Maddoxs growing slide will be understood from the annexed
sketch, fig. 292. The shaded parts are pieces of tinfoil fastened with
shellac glue to a glass slide. The
minute fungi or spores to be
grown are placed on a glass cover
large enough to cover the tinfoil,
with a droplet of the fluid re-
qriired. This, after examination
to see that no extraneous matter
is introduced, is placed over the
tinfoil, and the edges fastened
with wax softened with oil, leav-
ing free the spaces, X X, for

entrance of air. Growing slides of this description couhl be made
cheaply with thin glass instead of tinfoil.

Dallinger and Drysdale's Moist Stage for ContinuoiLs Observa-
tions.— It is needful in working out the life histories of minute
forms to be able to keep the organisms in a normal and un-
disturbed condition for sometimes weeks at a time ; only a small
drop of fluid containing the organism can be under observation, and
this, without proper provision, is constantly evaporating. To
prevent this, and still to employ very high powers in prolonged
study of a given organism, is the object of this device. It consists of

Pig. 292.

X X

-Maddox's growing stage.

342

ACCESSOKY APPAEATUS

a plain glass stage, fig. 293, a, a, so fitted as to slide on in the place
of the ordinary sliding stage of a Powell and Lealand or Ross stand.
It is thus susceptible of the mechanical motions common to those
stages. Its foundation, fig. 293, a, a, is plate glass, about the tenth
of an inch thick, in order to give it firmness. But this is too thick
to work through with a condenser and high powers, and therefore a

Fig. 293. — Dallinger and Drysdale's moist continuous growing stage.

circular aperture, h, is cut through it, and a thin piece of good glass,
c, d, e,f, is fixed over the under surface of it with Canada balsam ;
this may be as thin as the condenser may require. At the end of
the arm a, which extends some distance beyond the stage to the
right of the reader, but, when the arrangement is set up on the
microscope, to the left of the operator, a brass socket with a ring
attached is fixed with marine glue. It is marked in the drawing
g, g, g. The object of this ring is to hold a glass
vessel, fig. 294, about 1| or 2 inches deep. It
simply drops in, and the top, a, being slightly
larger than the ring, g, fig. 293, it is prevented
from slipping through.

Let us suppose the stage
to be in its position on the
microscope, and the vessel,
fig. 294, inserted in this
manner into g^ fig. 293. A
piece of good new linen is
now cut to the shape drawn
in fig. 297, the part a being-
long enough to reach to the
end of the glass stage, and then at h bent over, leaving the part in
the vessel, fig. 294, which is inserted into g, fig. 293. Its position
is indicated in fig. 293 by the dotted lines. A, A, h, etc. But before
it is laid uj)on the stage a circular aperture, d, fig. 297, is cut out,
which inust be much larger in diameter than the covering glass
which it is intended to use. We therefore employ small covers.

Fig. 294.

Fig. 295.

GEO WING STAGE FOR CONTINUOUS WORK

343

The glass with the flap of linen in it is now filled with watei-,
iind the linen is wetted and wrung so as not to di-ip, and the whole
is very soon, by capillary action, constantly and evenly wet. A
drop of the fluid to be examined must now be placed at k, fig. 293,
and the covering glass, i, must be laid on. It will be seen that
there is a broad, clear space between the covering glass and the linen.
We now w^ant to form a cliamber into whicli the object-glass can be
inserted, and which shall enclose a poi-tion of the constantly wet
linen, and be to a very lai'ge extent air-tight. The consequence
will be that the evaporation within the chamber will be always
greater in quantity from the linen, on account of its continual
renewal, than it can be from the film of fluid.

Indeed, the moisture in the chamber is so great under favoui-able
circumstances that it rather increases than allows a diminution of
the film of fluid. The manner

efiect this is
piece of gflass

Fig. 296.

in which we
simple. A

tubing, about 1^ inch in dia
meter, is cut to about ^ of an
inch in length. At one end
of this a piece of thin sheet
caoutchouc is firmly stretched,
and a small hole is made in
its centre. Fig. 295 gives a
drawing of it ; a is the piece
of glass tubing, h is the
stretched elastic film, which is
securely tied on by means of a groove in the glass at d, and c is the
aperture. The bottom edge, e, should be carefully ground. This is
laid in the position in which it is looked at in the drawing, on the
linen of the stage, the aperture c being over the centre of the cover-
ing glass. The object-glass is now racked down through the small
hole, c (fig. 295), and adjusted to focus. The caoutchouc should be
thin enough to afford no impediment to the action of the fine
adjustment, when it will be seen that it clasps the object-glass by its
elasticity at the aperture ; and the gentle pressure forces the under
edge of the chamber upon the linen, so that little or no air is
admitted, while if the under edge of the chamber be carefully
ground it will suffer the stage, linen and all, to move under it when
the milled heads for working the mechanical stage are in action.

A drawing of the apparatus in working order is given in perpen-
dicular section at fig. 296. The parts a, a in this figure represent
the glass stage corresponding to ct, a, fig. 293 ; h in both figures
stands for the round aperture in the thick glass ; 6, in fig. 296, cor-
responds to the thin glass wiaich covers this aperture, marked c, d,
e, f in fig. 293 ; but in the form of this device now used by the
Editor the thin glass floor is cemented to the bottom of the plate
glass, a, a, thus making a cell equal to the thickness of the whole
stage. The linen is marked in dotted lines in both figures : cZ,
fig. 296, represents the covering glass, ?', in fig. 293 ; e, e, fig. 296, is
the piece of glass tubing shown in fig. 295; /,/, fig. 296, is the

344

ACCESSORY APPAEATUS

stretclied caoutchouc seen at h in fig. 295, with the object-glass y,
penetrating and tightly filling up the aperture c in the figure, thus
forming the moist chamber, cA, ch, by enclosing 23arts h, h, fig. 296,
of the linen, which from the glass vessel to the left of the stage is
by capillarity always renewing its moisture ; and with h, fig. 296,
sunk as a cell, by the attachment of the thin glass floor to the undei-
side of the stage, as described above, this annular flap of linen over-
hangs, but does not lie upon, the floor on which the drop of fluid
with its living inhabitants is placed. This is a great seciu-ity
against accidental flooding.

It will be seen that the microscope must be vei-tical ; but there
is no inconvenience arising irom this if it be placed on a sufiiciently
low support, and it will be found in practice that it may be worked
for a long time without any other change in the arrangement than
the screwing up or down of the fine adjustment. The difiiculties in
working are few, and can be best discovered and overcome in
practice.

Dr. Dallinger^ s Thermo-statiG Stage for Contiwaous Observations
at High Temperatures. — It frequently happens that, either for the pur-

r"^

a. f^

(i)

~

Fig. 297.

pose of experiment or the study of special organisms, the student
needs a similar continuous stage to the above, but one in which
varying temperatures may be obtained and kept at any point static
at the will of the operator. This is very satisfactorily accomplished
by the following device : The stage was made as described above,
but it was made hollow and water-tight. The whole stage is seen
in persjDective in fig. 298. At A, a b are two grooved pieces of solid
metal which permit the stage to slide on to the stage of an ordinary
microscope, and partake of the mechanical movements efiected by
the milled heads ; B is a vessel for water with a thermometer a
of sufiicient delicacy foi- indicating the temperature ; 6 is a mer-
curial regulator, carefully made, but of the usual j)attern ; c brings
the gas from the main ; d conveys as much of the gas as is allowed
to escape from between the top of the mercury and the bottom
of the gas delivery tube to the burner e. The regulation of this
apparatus so as to obtain a static temperature, as is well known, is
a matter of detail depending chiefly on the careful use of the mer-
curial screw-plug f and the height and intensity of the burner e.
A temperature quite as accurate as is needed for the purpose
required can be obtained.

WAEM CONTINUOUS MOIST STAGE

345

The stage A is placed in position on the instrument, and two
openings in this hollow stage at c d (A) are connected with two
similar openings in the water vessel, viz. g h (B). The whole is
carefully filled with water and raised to the requii'ed temperature
and regulated.

The manner in which it accomplishes the end desired is as follows :
On the centre of the stage (A) will be seen a small cylindei- of glass ;
this is ground at the end placed on the stage, and covered with a
sort of drumhead of indiarubber at the upper end. By examining*
C Avith a lens it will be seen that a cell is countersunk into the
upper plate of the hollow stage at e", and a thin plate of glass is
cemented on to this. At e another disc of glass is cemented water-
tight, so that a film of warm water circulates between the upper and

Fig. 298.

under surfaces of this glass aperture. A glass cup is placed in the
jacketed receptacle / (A and C), and this also is filled with water.
A piece of linen is now laid on the stage (A, g) with an aperture cut in
its centre slightly less than the countersunk cell in which the glass
disc e" is fixed, and a flap from it is allowed to fall over into the glass
vessel y (A and C). Thus by capillarity the water is carried constantly
over the entire face of the linen. But the glass cylinder seen in A is
made of a much larger apertvire than the cell and the ojDening in the
linen, and consequently a large annulus of the linen is enclosed within
the cylinder. The drop of fluid to be examined is placed on the small
circular glass plate, and covered with the thinnest glass, the drum-
head cylinder is placed in position, the point of a high-power lens
is gently forced upon the top of the indiarubber through a small

346 ACCESSOKY APPAKATUS

aperture, tluis forcing the lower ground surface of the cylinder upon
the linen, and making the space within the closed cylinder j^racti-
cally air-tight, but still admitting of capillary action in the linen.
Thus the enclosed air becomes saturated.

By complete circulation the water in the vessel e (A) is but
slightly below that within the jacket of the stage, and thus the
vapour as well as the stage is near the same thermal point.

For the admission of illumination and for allowing the use of
various ilhnninating apparatus, a large bevelled aperture e (C) is
made between the lower and upper plates of the stage jacket, which
is found to supply all the accommodation needed.

There are many other forms of hot stage having various special
purposes, and some of general application ; a good account of these
will be found in the ' Journal Roy. Micro. Soc' vol. vii. ser. ii.
pp. 299-316 and in subsequent volumes.

The Live-box and Compressors. — What is now so well known
even to the tyro as the ' live-box ' was originally devised by Tully,
and it was afterwards improved by Yarley, who, in the place of a
level disc of glass for the floor, as well as the top of the ' box,'
hevelled a piece of thick glass and burnished it into the top of the
tube, whei'e it foi'med the floor of this ' animalcule cage ; ' this
prevented the draining ofi' of the water at the edge by capillary

Pig. 299.

^attraction. But in that form a condenser cannot be used successfully
with it, and therefore a dark ground cannot be employed. But as it
is Rotifei'a and Infusoiia generally that constitute the raison cVetre
for this piece of apparatus, and as a dai'k ground gives results of
high value — to say nothing of their beauty — with these forms, it
lost much of its value.

Mr. Rousselet has overcome these difficulties by a device which
is shown in fig. 299.

In this the glass plate bevelled for the floor is somewhat reduced
in diameter, but the outei' ring is enlarged sufiiciently to allow any
high power to focus to the very edge of this glass floor. An object
lying anywhere over the floor can be reached by the condenser from
below, and by both high and low powers from above, and when well
made it acts admirably as a compressor. A drop of water so small that
a rotifer may be unable to swim out of the field of view of a ^-inch
objective can be readily ai-i-anged with it ; and a little ^r&cticQ
enables the operator to employ it for many useful pui-poses in the
study of ' pond life.'

The compressor or compressorium is a more elaborate device,
somewhat of the same kind, but ai-ranged to give the operatoi-
more accui-ate conti-ol over the amount of pi-essvxre to which the
object is subjected. Mr. Rousselet has constructed one of very

C03IPEESS0RS

547

efficient form ; we illusti-ate it in fig. 300, liut on a reduced scale.
The bevelled glass in this also is kept small, with respect to the
size of the cover-glass, and it acts with perfectly pai-allel pressure
between the two glasses, which in delicate woi'k is of considerable,
importance.

The cover-glass is held on an arm which screws down on a vertical
post against a spring ; as the screw is raised the spiing i-aises the
covei'-glass, and by an ingenious sjDi'ing catch it is kept centi'al with
the glass-plate floor. This can nevertheless be released, and the entire
cover can be turned aside to put on a
fresh object, clean, and so forth. It is
simple, light, and, being parallel, can
be used with the highest powers.

Messrs. Beck and Co. have for
many years made an admirable parallel
compressor, but its weight and cost
were somewhat prohibitive of its use
generally ; the firm have noAV overcome both difficulties by the intro-
duction of a new form which is most useful and fully accomplishes
its work.

This compressor was designed by Mr, H. R. Davis, and is
specially intended for the examination of living objects. It consists,
as shown in fig. 301, of a lower ebonite plate A, which has a circular
hole in the centre, and which is recessed to receive a circular brass
ring B. This ring rests loosely in the recess. On the recessed
portion of this plate A is carried an oblong thin glass which is
held in position by two screws, one of which appears at C Two
end plates D D slide on to the plate A, and hold the ring B loosely
in position, allowing it to be revolved by means of its milled fiange,
Avhich projects at E. Within the ring B is screwed a brass disc F
Avhich carries the upper thin glass which is attached by the screws

Fig. 300. — Rousselet's compressor.

G CI

Fig. 301. — Beck's new compressor.

Ct G. The screws G G and C, fitting into holes in the lower plate
A and the disc F respectively, prevent the disc from revolving, and
when the ring E is turned, the two thin glasses are moved towards
or away from one another.

The slides D D and the i-ing B, together with the disc F, are
removed for ai^ranging the object on the lower cover-glass, and

348

ACCESSOKY APPAKATUS

when replaced by revolving the ring at E, any desired amount of
compression may be obtained. The object having been ari-anged,
either side may iDe examined with equal facility, as the compressor
is reversible.

When a very small object is to be examined a small circular
cover-glass should be cemented with Canada balsam to the lower
cover-glass, and the object is thus confined to the centre of the field.
The zoophyte trough is a larger live-box differently constructed.
The form that has proved one of the best up to our own day was
introduced by Mr. Lister in 1834, and is well known. It is depicted
in fig. 302, being formed of slips of glass, and has a loose horizontal
plate of glass equal to the inside lengiih of the trough, so that it
may be moved freely within it, also a slip of glass that will lie on
the bottom and fill it, with the exception of the thickness of this
loose plate. To use it, the slip is put upon the bottom, the loose
plate is placed in front of it with its l)ottom edge touching the
inside of the front glass, a small ivory wedge is inserted between
the front glass of the trough and the uj)per part of the loose vertical

plate, which it serves to press
backwards ; but this pressure
is kept in check by a small
strip of bent whalebone,^
which is placed between the
vertical plate and the back
glass of the trough. By
moving the ivory wedge up
and down, the amount of space
left between the upper part
of the vertical plate and the
7 front glass of the trough can
be precisely regulated, and as
their lower margins are always
in close apposition, it is evi-
dent the one will incline to the other with a constant dimintition
of the distance between them from above downwards. An object
dropped into this space will descend until it rests between the two
surfaces of glass, and it can be placed in a position of great conveni-
ence for observation.

By very little contrivance these troughs with their contents may
be kept, when not under examination, in much larger aquaria, ob-
taining the advantage of aeration and coolness.

Ml'. Botterill devised a trough which is made of two plates of
vulcanite or metal which screw together, and between them are two
plates of glass, of the proper size, of any desired thickness, kept
apart by half a ring of vulcanised indiarubber, the whole being
screwed tightly enough together by three milled heads to prevent
leakage. But leakage or the fracture of glasses is not uncommon
with this otherwise convenient form.

An excellent, though shallow, trough was made by Mr. C. G.
Dunning, which we illustrate in fig. 303. The lower plate or trough
1 Watch-spring or other elastic metal should not be used, on account of oxidation.

Fig. 302.

A SHALLOW TEOUGH

549

proper is made of metal. 3 inches long by 1^ wide and about j'^
thick, with an oval oi- oblong pei-foration in the centre, and the
under side is recessed, as shown in fig. 303, B. In this recess is fixed,
liy means of Canada balsam oi- shellac, a piece of stout covering glass,
foi'ming the bottom of the cell, the recess being sufficiently deep to
prevent the thin glass bottom from coming into actual contact with
the stage of the microscope oi- with the table when it is not in use.
Two pieces are provided near the bottom edge of the cell : the cover
(fig. 303, C) is formed of a piece of thin brass, I'ather shorter than
the trough, but about the same width ; it has an opening formed in
it to correspond with that in the trough, and under this opening is
cemented a piece of cover-glass. The cover-plate is notched at the
two bottom corners, and at the two top comers are formed a couple
of projecting ears. In order to use this apparatus it must be laid
flat upon the table, and filled quite full of water. The object to be
examined is then
placed in the cell, and A w////mmi^^=
may be properly ar-
ranged therein ; the
cover is then loAvered
gently down, the two
notches at the bottom
edges being first
placed against the
pins ; in this way the
superfluous water will
be driven out, and the
whole apparatus may
be wiped dry. The
capillary attraction,
assisted by the weight
of the cover, will be
found sufiicient to
prevent any leakage ;
and the pins at the
bottom prevent the cover from sliding down when the microscope is
inclined. This zoophyte trough possesses two important qualities :
first, it does not leak ; second, it is not readily broken without gross
carelessness. The shallowness may be overcome by placing an ebonite
plate with the required aperture between the two mounted glasses.

Infusoria, minute algje, &c., however, can be well seen by
placing a di'op of the water containing them on an ordinaiy slide,
and laying a thin piece of covering glass on the top ; and objects
of somewhat greater thickness can be examined by placing a loop
or ring of fine cotton thread upon an ordinary slide to keep the
covering glass at a small distance from it ; and the object to be ex-
amined being placed on the slide with a droj) of water, the covering
glass is gently pressed down till it touches the ring. Still thicker
objects may be viewed in the various forms of ' cells' hereafter to
be described, and as, when the cells are filled with fluid, their glass
covers will adhere by capillary attraction, provided the superfluous

Fig. 303.

350 ACCESSOEY APPAEATUS

moisture that sui'roiiiids their edges be removed by blotting papei-,
they will remain in place when the mici-oscope is inclined. An
annular cell, that may be vised either as a ' live-box ' or as a ' grow-
ing slide,' has lately been devised by Mr. Weber (U.S.A.). It is a
slip of plate-glass, of the usual size and ordinary thickness, out of
which a circular ' cell ' of | inch diameter is ground, in such a
manner that its bottom is convex instead of concave, its shallowest
part being in the centre and the deepest round the margin. A
small drop of the fluid to be examined being placed upon the central
convexity (the highest part of which should be almost flush with the
general sui-face of the plate), and the thin glass cover being placed
upon it, the drop spreads itself out in a thin film, without finding-
its way into the deep furrow around it ; and thus it holds-on the
covei'ing glass by capillary attraction, while the furrow serves as an
air-chamber. If the covei- be cemented down by a ring of gold size
or dammar, so that the evaporation of the fluid is prevented, either
animal or vegetable life may thus be maintained for some days, or,
if the two should be balanced (as in an aquarium), for some weeks.
Dipping Tubes. — In every operation in which small quantities
of liquid, or small objects contained in liquid, have to be dealt with
by the microscopist, he will find it a very great convenience to be
provided with a set of tubes of the forms represented in fig. 304,
but of somewhat larger dimensions. These were formerly desig-
nated ' fishing tubes,' the purpose for which they were originally
devised having been the fishing out of water fleas, aqviatic insect
larvje, the larger animalcules, or other living objects distinguishable
either by the unaided eye or by the assistance of a magnifying glass
fi-om the vessels that may contain them. But they are equally
applicable, of course, to the selection of minute plants ; and they
may be turned to many other no less useful purposes, some of which
will be specified hereafter. When it is desired to secure an object
which can be seen either with the eye alone or with a magnifying
glass, one of these tubes is passed down into the liquid, its upper
orifice having been previously closed by the forefinger, until its lower
orifice is immediately above the object ; the finger being then re-
moved, the liquid suddenly rises into the tube, probably carrying
the object up with it ; and if this is seen to be the case, by putting
the finger again on the top of the tube, its contents remain in it
when the tube is lifted out, and may be deposited on a slip of glass,
or on the lower disc of the aquatic box, or, if too copious for either
receptacle, may be discharged into a large glass cell. In thus
fishing in jars for any but minute objects, it will be generally found
convenient to employ the open-mouthed tube C ; those with smaller
orifices. A, B, being employed for 'fishing' for animalcules, etc., in
small bottles or tubes, or for selecting minute objects from the cell
into which the water taken up by the tube 0 has been discharged.
It will be found very convenient to have the tops of these last
blown into small funnels, which shall be covered with thin sheet
indiarubber, or topped with indiarubber nipples, which by com-
pression and expansion can then be regulated with the greatest
nicety.

DIPPING TUEES

ISI

A

n

In dealing Avitli minute aquatic objects, and in a great variety
of other manipulations, a small glass syringe of the pattern repre-
sented in iig. 305, and of about double the dimensions, will be
found extremely convenient. When
this is firmly held between the fore
and middle fingeis, and the thumb
is inserted into the ring at the
summit of the piston-rod, such
complete command is gained over
the piston that its motion may be
regulated with the greatest nicety ;
and thus minute quantities of fluid
may be removed or added in the
various operations which have to be
performed in the preparation and
mounting of objects ; or any minute
object may be selected (by the aid of
the simple microscope, if necessary)
from amongst a number in the same
drop, and transferred to a separate
slip. A set of such sp-inges, with
points drawn to different degrees of
fineness, and bent to different curva-
tures, will be found to be among the
most useful ' tools ' that the work-
ing mici'oscopist can have at his
command. It will also be found
that if a dipping tube with a glass
bulb have an indiarubber hollow
ball or teat attached to the top of
it, it will act, for the majority of
purposes, as Avell as a syringe.

Forceps. — Another instrument
so indispensable to the microscopist
as to be commonly considered an
appendage to the microscope is the
forceps foi' taking up minute objects ;
many forms of this have been devised, of wliich one of the most con-
venient is represented in fig. 306, of something less than the actual
size. As the forceps, in marine researches, have continually to be

Pig. 304.— Dip-
I)ing tubes.

Pig. 305.— Glass
syringe.

Fig. 306.

plunged into sea- water, it is better that they .should be made of brass
or of German silver than of steel, since the latter rusts far more
readily ; and as they are not intended (like dissecting forceps) to
take a firm grasp of the object, but merely to hold it, they may be
made very light, and their spring portion slender. As it is e,ssential ,

352

ACCESSOEY APPARATUS

howevei", to their utility that their points should meet accurately,
it is well that one of the blades should be furnished with a guide-pin
passing through a hole in the other.

Most microscopists have at some time experienced the danger
that is imminent to their instruments and mountings when exhibit-
ing delicate objects with high power in mixed assemblies, arising
from the inadvertency or want of knowledge
of some visitor, who may do terrible mischief
by innocently using the coarse adjustment.
Messrs. Ross made an arrangement by which
the coarse adjustment could be ' locked ' at a
given point ; but an equally useful and simpler
method was long ago devised by Messrs.
Powell and Lealand, who used a deep ring, as
is shown in fig. 307. This ring has two pins
and a screw projecting inwards. When the
screw is withdrawn, the rings can be slipjjed
over the milled heads of the coarse adjust-
ment, and by screwing the small screw 'home '
the ring cannot be withdrawn ; but as they
are loose upon the milled heads, the latter
cannot be brought into action ; the rings simplj' revolve upon the
heads without bringing them into play.

Other forms of the same appliance have been made by this firm ;
and Messrs. Beck have made these rings with slight modifications
more recently. They are the most efficient means of countei^acting
the danger incident on public exhibition of delicate objects under
high powers.

The foregoing constitute, it is believed, all the most important
pieces of apparatus which can be considered in the light of accessories
to the microscope. Those which have been contrived to afford
facilities for the preparation and mounting of objects will be described
in a future chapter (Chapter YI.).

Fig. 807. — Powell and Lea-
land's protecting ring for
coarse adjustment.

353

CHAPTER V

OBJECTIVES, EYE-PIECES, THE APEBTOMETEB

It is manifest that everything in the form and construction as well
as in the nature of the optical and mechanical accessories of the
microscope exists for, and to make more eificient, the special work
of the objective, or image -forming lens combination, which constitutes
the basis of the optical properties of this instrument.

The development of the modern objective, as we have already
seen, has been very giudual ; but there are definite epochs of very
marked and imj)ortant improvement. Our aim in the study of
objectives is practical, not antiquarian, and we may avoid elaborate
researches on the subject of non-achromatic lenses and reflecting
specula, which have been sufficiently indicated in the thii-d chapter
of this volume. We may also pass over the earlier attempts at
achromatism; the true history of the modern objective begins fromi the
tivie that its achromatism had been finally worked out.

The first movement of a definite character towai-ds this object
was made, it has been recently shown,^ so early as 1808 to 1811 by
Bernardino Marzoli, who was Curator of the Physical Laboratory of
the Lyceum of Brescia. Mr. Mayall discovered a reference to this
efibrt to make achromatic lenses, and, through the courtesy of the
President of the Athenseum of Brescia, discovered that Marzoli
was an amateur optician, that he had taken deep interest in the
application of achromatism to the microscope, and that a paper of his
on the subject had been published in the ' Commentarj ' for the yeai-
1808, and that he had exhibited his achromatic objectives at Milan
in 1811 and obtained the award of a silver medal for their merits
under the authority of the Istituto Reale delle Scienze of that city.
One of these objectives was found to have been ' religiously pre-
served,' and was generously presented in 1890 by Messrs. Tranini
Brothers to the Royal Microscopical Society of London. With it
was forwarded the ' Processo Yerbale,' or official record of the awards,
notifying Marzoli's exhibits and the awai'd of a silver medal, and
the actual diploma, dated August 20, 1811, signed by the Italian
Minister of the Interior.

Marzoli's objective was a cemented combination, having the plane
side of the flint presented to the object ; and if this was a part of
the intended construction, of which there appears small i-oom foi-
doubt, Marzoli preceded Chevalier in this, as we shall subsequently
see, very practical improvement.

1 Journ, Roij. Mic. Soc. 1890, p. 420.

A A

354 OBJECTIVES, EYE-PIECES, THE APERTOMETEIi

It has been, however, customary to accredit the first practicable

attempts to achromatise object-glasses to M. Selligues. In 1823

he suggested to M. Chevalier to superimpose two, three, or

four achromatised plano-convex ' doublets,' that is to say, pairs

of lenses. These objectives had their convex surfaces presented to

the object, which gave them four times as much spherical aberration

as would have been the case had their j^ositions been reversed,^ and,

as we have just seen, Marzoli reversed them. This necessitated an

excessive reduction of the apertures, which, nevertheless, still too

manifestly displayed an obtrusive aberration. Yet the conceirtion

of an achromatised combination had been embodied in an initial

manner. In 1825 M. Chevalier perceived the exact nature of the

mistake made by M. Selligues, and made the lenses of less focal

length and more achromatic, and inverted them, placing the plane

side of the flint towards the object.

It is somewhat important, as it is interesting, to note that the

idea of the superposition of a combination of lenses did not originate

from theoretical considerations of the optical principles involved.

It is scarcely conceivable that where there was manifest ignorance

of the position of a plano-convex lens for least spherical aberration

(a principle now thoroughly understood) there could have been in-

sihgt enough either to detect the presence of the two aplanatic foci

01- to discover a method of balancing them

by inductive reasoning. Everything in the

history points to happy accident as the primal

step in achromatised objectives, and this, with

vei'y high probability, applies to the work of

Chevalier, for Selligues' attempt was a blunder

ono m 11 . 1 affainst the commonplace knowledge of his

Fig. 308. — Tully s achro- p -"^ *=

matic triple. time.

The form of three superimposed similar
achi"omatic doublets is precisely the combination of the French
' buttons,' which have been sold in thousands until quite recently,
many of them being mounted as English objectives.

At the suggestion of Dr. Goring, Mr. Tully, in this country,
without any knowledge of what was being done on the Continent,
made an achromatic objective in 1824. This was a single combina-
tion, being an achromatic uncemented triplet. It was, in fact, a
miniature telescope object-glass, and is illustrated in fig. 308. Two
lenses made on this princijDle by Tully, having j^ and ^ foci, were
found in practice too thick, and in many ways imperfect ; and he
was induced to make another single triplet of y'iy focus and 1 8° aper-
ture, and its performance was said to be nearly equal to that of
theT^y.

Subsequently a doublet was placed in front of a similar triplet of
somewhat shorter focus, forming a double combination objective of
38° aperture. This was pronounced to be a great advance ujjon all
preceding combinations, even those which had been pi'oduced upon
the Continent.

A note of Lister's at this time upon the objectives of Chevalier
1 Chapter I. '

LISTER'S DISCOVERY

355

is of interest. He found them much stopped doAvn, and in one
instance he opened the stop and improved the effect. Lister says :
' The French optician knows nothing of the value of apei-ture, but
he has shown us that fine perfoi-mance is not confined to triple
objectives ; and in successfully combining two achromatics he has
given an impoi'tant hint — probably without being himself acquainted
with its worth — that I hope will lead to the acquisition of a pene-
ti-ating ^ powder gi-eater than could ever be reached with one alone.'

At this time Professor Amici, of Modena, one of the leading
minds who assisted in giving its foi'm to the modern microscope, had
been bafiled by the difficulties presented by the
problem of achromatism, and had laid it aside
in favour of the reflectiiag microscope, but he
now returned to the practical reconsideration of
the production of an achromatic lens. As a
result he appears to have constructed objectives
of greater aperture than those of Chevalier.
He visited London in 1844, and brought with
him a horizontal microscope, the object-glass
being composed of three doublets, which pro-
duced a most favourable impression.

Meantime, in this country, Mr. Lister
brought about an important ejjoch in the evo-
lution of the achromatic object-glass by the dis-
covery of the two aplanatic foci of a combination.
It had occupied his mind for several years, but
in January 1830 a very important paper was
read to, and published by, the Royal Society,
written by him, in which he points out how the
aberrations of one doublet may be neutralised
by a second.

As the basis of a microscope objective, he
considers it eminently desirable that the flint
lens shall be plano-concave, and that it shall be
joined by a permanent cement to the convex
lens.

For an achromatic object-glass so constructed
he made the general inference that it will have
on one side of it two foci in its axis, for the
rays proceeding from which the spherical aber-
ration will be truly corrected at a moderjite
aperture ; that for the space between these two

points its spherical aberration will be over-coi'rected, and beyond
them, either way, under-corrected. ,

Thus, let «, b, fig. .309, represent such an object-glass, and be
roughly considered as a plano-convex lens, with a curve, a c b,
running through it, at which the sphei'ical and chromatic errors
are corrected which are generated at the two outer surfaces, and
let the glass be thus free from aberration for rays,/, d, e, g^ issuing

^ ' Penetrating ' meant ' resolving ' power in those days ; he alludes, therefore, to
increase of aperture.

A A 2

Fig. 309. — The two
aplanatic foci of an
optical combination.

356 OBJECTIVES, EYE-PIECES, THE APEETOMETER

from the radiant point, f, h e being a normal to the convex
surface, and i d to the plane one — under these circumstances the
angle of emergence, g e h, much exceeds that of incidence, f d i,
being probably almost three times as great.

If the radiant is now made to approach the glass, so that the
course of the ray, fd e g, shall be more divergent from the axis, as
the angles of incidence and emergence become more nearly equal
to each other, the spherical aberration produced by the two will be
found to beai- a less proportion to the opposing error of the single
correcting curve a c b ; for such a focus, therefore, the rays will be
over-corrected. But if / still aj)proaches the glass, the angle of
incidence continues to increase with the increasing divergence of
the ray, till it will exceed that of emergence, which has in the mean-
while been diminishing, and at lengtih the spherical error produced
by them will recover its original proportion to the opposite error of
the curve of correction. Whenyiias reached this point /"'' (at which
the angle of incidence does not exceed that of emergence so much as
it had at first come short of it), the rays again pass the glass free
from spherical aberration.

If y* be carried hence towards the glass, or outwards from its
original place, the angle of incidence in the former case, or of
emergence in the latter, becomes disproportionately effective, and
either way the aberration exceeds the correction.

How far Lister's discoveries were affected by Amici's work it is
now quite impossible to say ; there can be but little doubt that some
influence is due to it, but it is equally clear that a profound know-
ledge of the optics of that time was the only foundation upon which
the facts in Lister's paper could have been built. He was a man of
application and an enthusiast, and it was inevitable that he should
exert a powerful influence upon the early history of the optics of the
microscope. This is the more certain when we remember how few
were the men at that time who knew in any practical sense what a
microscope was ; and we find that in 1831, being unable to find any
optician who cared to expeiiment sufficiently. Lister taught himself
the art of lens-grinding, and he made an objective whose front was
a meniscus pair, with a triple middle combination, and the back a
plano-convex doublet. He declared this to be the best lens of its
immediate time, and it had a woi'king distance of • 1 1 .

One of the immediate consequences of the publication of Lister's
3)aper was the rapid production by professional opticians of achromatic
•objectives. The data supplied by Lister proved to be of the highest
value in the actual production of these, and the progi'ess of improve-
ment was, in consequence, and in comparison with the time imme-
diately preceding, remarkably i-apid.

Andreio Ross began their manufacture in 1831. He was followed
by Hugh Powell in 1834, and in 1839 by James Smith. It is of
moi-e than ordinary intei-est to study in detail the work of this im-
mediate time, and the following table giving a list of objectives, with
their foci, apertures, and mode of consti-uction, with the dates of
their production, will give a fair idea of the work of Andrew Ross
in the manufacture of eai-ly lenses. He was the earliest of the three

PEIMITIVE FOEM OF LENS CORKECTION

357

English makers, and undoubtedly caii-ied the palm both here and on
the Contment for the excellence of his objectives.

inch 14° two doublets, 1832. Made for Mr. E. H. Solly.

„ 18° single triple, 1833.

,, 55° three pairs, 1834. This belonged to Professor Quekett,

" nXo \ triple front and two double backs ,„,, °

,',' 44°

„ 63° „ „ „ J>1842

„ 74°

, Lister's formula

t>===^

Fig. 310.— a i-in.
combination by
Andrew Ross.

Examples of these old lenses are extant and in perfect preserva-
tion, and for correction they are compai'able without detriment to
any ordinary crown and flint glass achromatic of the same aperture
of the present day.

An example of the construction of the J-inch focus objective of 55°,
consisting of three pairs of lenses arranged with their plane sides
to the object, the position of least aberration, is shown
in fig. 310. The foci of these three pairs are in the
proportion of 1 : 2 : 3. In 1837 this maker had so
completely coi-rected the errors of spherical and
chromatic aberration that the circumstance of covei"-
ing an object with a plate of the thinnest glass was
found to disturb the coi'rections ; that is to say, the
corrections were so relatively perfect that if the
combination were adapted to an uncovered object,
covering the object with the thinnest glass intro-
duced refractive disturbances that destroyed the high quality of the
objective.^

Lister's paper of 1830 gave the obvious clue to a method of
neutralising this ; that is to say, by lens distance ; and Ross applied
this correction by mounting the front lens of
an objective in a tube which slid over another
tube carrying the two other pairs. A very
primitive form of this lens correction is afforded
lis by a g^-inch objective made by Andrew Ross
in 1838. It belonged originally to Professor
Lindley, the second President of the Royal
Microscopical Society, and was presented to the
society by his son, the Master of the Rolls, in
1899. An illustration of this lens is given in
fig. 311. The tube carrying the front lens
slides on an inner tube ; it can be clamped in
any position b}^ the screws at the sides ; the
line in the small hole in the front indicates its
position, and is the prototype of the ' covered '
and ' uncovered ' lines of later times.

The larger cylinder at the base is the lid of its box upon which
it is standing.

Subsequently this ari'angement was modified by the introduction

1 Vide ChaiDter I.

Fig. oil. — Primitive
form of lens correc-
tion (1838).

3:58

OBJECTIVES, EYE-PIECES, THE APERTOMETER

of a screw arrangement, as in fig. 312. The front pair of lenses is
fixed into a tube (A) which slides over an interior tube (B) by which
the other two paii'S are held ; and it is drawn up or down by means
of a collar (C), which works in a furrow cut in the inner tube, and
upon a screw-thread cut in the outer, so that its revolution in the
plane to which it is fixed by the one tube gives a vertical movement
to the other. In one part of the outer tube an oblong sht is made,
as seen at D, into which projects a small tongue screwed on the
inner tube ; at the side of the former two horizontal lines are
engraved, one pointing to the word ' uncovered,' the other to the
word 'covered;' whilst the latter is crossed by a horizontal mark,
which is brought to coincide with either of the two lines by the
rotation of the screw-collar, whereby the outer tube is moved up
or down. When the mark has been
made to point to the line ' uncovered,'
it indicates that the distance of the lenses SBlk /M^^

Fig. 312. — Section of adjiistmg object-glass.

Pig. 313. — Present collar
correction.

of the object-glass is such as to make it suitable foi- viewing an
object without any interference from thin glass ; when, on the other
hand, the mark has been brought, by the revolution of the screw-
collar, into coincidence with the line ' covered,' it indicates that the
front lens has been brought into such proximity with the other two
as to produce an 'under-correction' in the objective, fitted to
neutralise the ' over-correction ' produced by the interposition of a
glass cover of extremest thickness.

This method of collar correction served the purposes of micro-
scopy for upwaixls of thirty years, but when more ciitical investiga-
tions were undertaken and objectives had more aperture given to
them it was found that the method had two great faults.

The first was that the ' covered ' and ' uncovered ' marks were
too crude. To remedy this, tho screw collar was graduated into
fitfy divisions, a device introduced by James Smith in 1841 so that

THE 3I0DERN USE OF COLLAR CORRECTION 359

intervals between the points 'covered' and ' uncovered ' might be
recorded.

The second, a more serious defect, was the movement of the
fi'out lens while the back I'emained i-igid with the body of the
mici'oscope. The detriment of this ai'i-angement was that in cor-
recting a wide-angled, close-Avoi-king objective thei-e was a dangei- of
foi-cing the front lens through the cover-glass by means of the collar
correction.

Kow the ai'i-angement as shown in fig. 313 enables the front
lens to maintain a fixed position, while the correctional collar acts
on the posterior combinations only. This device was introduced
by Mr. F. H. Wenham in 1855.

On the Continent it has been the practice to graduate the cor-
rectional collar in terms of the thickness of the cover-glass in deci-
mals of a millimetre. Thus if a cover-glass be 0"18 mm. thick, the
correctional collar should be set to the division marked O'lS.

In England, on the contrary, the divisions are entirely empiri-
cal, so that the operator has to discover for himself the proper
adjustment. It is not to be supposed, however, that the English
method is unscientific, for when an operator becomes expert he
would never for an instant think of adjusting by any other indi-
cation than that aflbrded by his own eye and expei-ience. This is a
very important point, because the interpretation of structure to a
great extent dej)ends on accurate adjustment of the objective, and
it would be folly to sujDpose that an eminent observer would sur-
render his judgment to the predetermination of theory embodied in
what must be the imperfections in even the most conscientious and
thorough woi'k which gives a practical form to such theory. In
fact, it is the test of accurate manipulation that, however the collai-
correction be disturbed, the microscopist will, in getting a critical
image of the same object, always, by the quality of the image he
obtains, bring the correction to within the merest fraction of the
same position, although the correction collar and its divisions are
never looked at until the desired image is obtained.

The fact that the over-correction caused by the cover-glass was
discovered in England, and that means were at once found for its
coi'i'ection, while no similar steps were taken on the Continent, is a
sufiicient evidence of the advanced position of this country in practi-
cal optics at that time.

This subject of under- and over-correction is one of large impor-
tance^ and it may be well at this point to enable the tyro to clearly
understand, by evidence, its nature, although loliat it is has been
fully shown in Chapter I. Take a single lens — the field-lens of a
Huyghenian eye-piece will serve admirably — and hold it a couple
of yards from a lamp flame ; the rays passing thi-ough the peri-
pheral portion of the lens will be found by experiment with a card
to be brought to a focus at a p)oint on the axis ''nearer the lens than
those p>assing through the centre. This is tonder-correction, vide fig. 23,
p. 20. The same experiment should be repeated with the plane
.side and the convex side of the lens alternately turned to the flame.
In the former case, when the image of the flame is at its best focus,

360 OBJECTIVES, EYE-PIECES, THE APERTOMETEE

it will be surrounded by a coma, and even tlie portion of tlie flame
which is in focus will lack brightness. But with the convex side to-
toarcls the flame it will be found that in the image on the card the
coma is greatly reduced, and the image of the flame brightened.
The reason for this is, as already stated, that the spherical aberration
is four times as great when the convex side of the lens is towards
the card.

The practice of these simple tests will be most instructive to.
those unfamiliar with the optical principles on which an objective is
constructed. They make plain that an over-corrected lens is one
which brings its j^eriiyheral rays to a longer focus than its central, vide
fig. 24, p. 20. But a cover-glass produces over-correction, therefore
the means emj)loyed to neutralise the error is by the imder-cor-
rection of the objective. If, however, the objective employed
should be unprovided with such means of correction, the eye-piece
must be brought nearer the objective, which will effect the same
result.^

Still confining our consideration to the year 1837, we find that a
further improvement was made by Listei', who employed a triple
front combination. This consistedof two crown piano-con vexes with
a flint plano-concave between them. The result of this was the
increase of the aperture of an inch-focus objective to 22°.

An illustration of the mode of construction of these lenses is
given in fig. 314, which is di-aw^n from an early ^-inch objective by
Andrew Ross, having bayonet-catch correction adjustment. In 1842
a ^-inch of 44°, a J^-inch of 63°, and a ^-inch of 74° were made
^^pon the same lines. The method for computing these fronts is-
given by Mr. Nelson in the 'Journ. R. M. S.,' 1898, p. 160 et seq.

In 1841 the Royal Microscopical Society ordered a microscope
from each of the before-mentioned leading opticians. The objectives-
supplied with these are still extant, representing with moral certainty
the very best work of the several makers ; they are consequently
valuable as reliable specimens of the best work of the period.

The objectives supplied by James Smith have the peculiarity of
being separating lenses.

The lowest power is about l|^-inch focus. Wlien this is used
alone a diaphragm is slid over the front to limit the aperture, but
we are unable to say what that limit was, since the diaphragm has
been lost. By placing another front where the diaphragm would
have been, the new combination becomes an -^^^-inch focus, while^
yet another front may be substituted, making the objective a ^-inch
focus. This latter front consists of two pairs, and it is provided
with a graduated screw-collar adjustment which separates these
pairs, but the arrangement is of a very j)rimitive order.

This object-glass will di^dde the podura marks in a milky field
with a full cone, and the field is much curved.

There is also a separating IJ-inch and §-inch which is good
while the f^-inch and the j-inch may be considered fair.

The lenses supplied by Andrew Ross are a good 2 -inch and a

1 Under-covrection is also known as ' positive aberration ; ' over-correction as
negative aberration.'

TEIPLE BACK COMBINATION ^6 1

fair 1-incli, but we have seen a better than this of about the same
period.

Hugh Powell supplied a 1-inch of good quality, and a ^, ^, ^,
jiy-inch fairly good. The apertures of the ^ and the j\;-inch are of
course very low.

On the whole it may be said that the coi-rections are well
balanced in the lower lenses, and the apertures moderate ; but when
we come to the higher powers it is the deficiency of aperture that
becomes so oppressively apparent. In 1844 Amici made a ^-inch
objective of 112° and brought it to England. It was understood
that extra dense flint was employed in the construction of this-
objective ; but this is perishable ; and Mr. Ross altered slightly the
curves of Amici's construction, and with ordinary flint succeeded
in extending the aperture of a |^-inch objective to 85°, or "68 IST.A.,
and a -j-Vinch objective to 135°, or "93 JST.A. Of this latter it was
aflirmed that it was ' the largest angular pencil that could be passed
throvigh a microscope object-glass.'

In 1850 object-glasses were made with a trij^le back combination ;
these were attributed to Lister ; but it is also affirmed that they

Fig. 314.— An early Fig. 315.— A triple- Fig. 316.— A siugle-

g-in. combination back combina- front combination

by A. Boss. tion by Lister (or by Wenham.
Amici ?).

were the joreviovis device of Amici. It may well be a disputed point,
for it is quite certain that this device brought the dry achromatic
objective potentially to its highest perfection. The combination is
illustrated in fig. 315, and under the conditions of its construction it
may be well doubted if anything will ever surpass the results-
obtained by English opticians in achromatic objectives constructed
with this trij)le front, double middle, and triple back combinations^
apart from the use of the new kinds of Jena glass. For the method
of computing the triple back, vide ' Journ. R. M. S.,' 1898, p. 160 etseq.
It may be noticed that TuUy's objective had a triple back, but it was-
not the result of intended construction ; it was a fortunate combina-
tion the real value of which was neither understood nor appreciated,,
and as a consequence its existence was evanescent.

In this same year Wenham produced another modification of the
achromatic objective of considerable value, but more to the manu.-
facturer than the user of the microscope. It consisted of a single
front ; the combination is seen in fig. 316, which, it will be seen, is a
simpler construction, but this did not affect in the least the price of
the objectives produced. Subseauently, however, the form was-

362 OBJECTIVES, EYE-PIECES, THE APEETOMETER

adopted on the Continent for low-priced objectives, which led to a
reduction of the cost of English objectives of the same construction.

Manifestly, the single front lessened the risk of technical errors,
but we have never been able yet to find a single front objective of
the old achromatic dry construction which has shown any superiority
over a similar one possessing a triple front.

The single front employed with two combinations at the back
was the form in which the celebrated water-immersion objectives
of Powell and Lealand were made. It was by one of these that the
striae on ATnphijjleura pellucida were first resolved. Indeed, what is
known as the tvater-immersion system of objectives, devised by
Professor Amici, was the next advance upon the old form ; it should,
however, be remembered that as early as 1813 achromatic water -
immersion lenses had been suggested by Sir D. Brewster, but it was
an advance the optical principles of which were certainly not at the
time understood.

In Paris, Prazmowski and Hartnack brought these objectives to
gi-eat perfection, and were enabled to take the premier place against
all competitors at the Exhibition of 1867. The next year, however,
Powell and Lealand adopted the system, and in tuxii they distanced
the Paris opticians and produced some of the finest objectives ever
made. Their ' New Formula ' water-immersions were made after
the fine model of Tolles referred to below, and had a duplex front,
a double middle, and a triple back. In 1877, when the water-
immersion system touched its highest point, apertures as great as
1'23 were reached; and in America, Spencer, Tolles, and Wales
produced some extremely fine lenses of large apertvire.

During the year 1869 Wenham experimented with and sug-
g'ested ^ the employment of a duplex front ; that is to say, a front
combination made up of two uncorrected lenses in contradistinction
to an achromatised pair. An illustration of the plan suggested is
given in fig. 317, which hardly appears to us as a practicable form,
and which certainly was never brought to perfection or put into
practice.

But in the month of August, 1873, Tolles actually made, on
wholly independent lines, a duplex front formula for a |- glycerine
immersion of 110° balsam angle, which passed into the possession of
the Army Medical Museum at Washington. There can be little
/doubt but this objective would have produced a much deeper im-
pression but foi- the fact that it was in advance of its immediate
time.

Tolles, as we have hinted above, used the duplex front in the
construction of some of his immersion objectives, and was followed
in this by the best English makers, and, in the case of a celebrated
■g-inch purchased by Mr. Crisp, Tolles was able to reach a balsam
angle of 96°.

At the time that the water-immersion lenses were being con-
sti'ucted by rival opticians with increasing perfection, the great
theory of Pi-ofessor Abbe concerning microscopic vision, the impor-
tance of difii'action spectra, and the i-elation of aperture to power
1 Monthly Micro. Journ. Vol. I. p. 172.

THE INFLUENCE OF THE DIFFRACTION THEORY

\6:-

was entirely unknown. In the absence of this knowledge wholly
mistaken value was attached to poioer per se in the objective.

With a focus as shoi't as the ^jl^-inch, it was not uncommon to
find apertures less than 1-2, while objectives of J5, -^jj, 5V5 -i^cl even
higher powers, were made with extremely reduced apertures. This
was done in the intei-ests of the common belief that ' power ' —
devoid of its suitable concui'rent apei'tui-e — could do what was so
keenly Avanted.

This impi-ession, however, was far from universally i-elied on ;
thei-e were several earnest workers who, without being able to
explain, as Abbe subsequently did, why it was so, still ui-ged the
opticians, in the manufacture of every new power, especially the
higher ones, to produce the largest possible amount of aperture;
and the evidence of this is still to be found in the objectives they
then succeeded in obtaining. But there can be no doubt that a
I'eckless desire for magnifying power, all other considerations apart,
greatly obtained ; and the opticians were able to encourage it, for it
is far easier to construct an objective of high power and low aperture
than it is to make a low power with a large aperture.

Fig. 317.— a suggested
combination by Wen-
ham, 1869.

Fig. 818. — Combina-
tion for ' homoge-
neous ' immersion
by Abbe.

Fig. 319. — Diagram of
apochromatic com-
bination.

Thus a ^-inch of 0"65 N.A. will be far more expensive, and pro-
bably not as well corrected, as -g of 0"7 N.A. The ^-inch objective,
even if a good one, is sure to exhibit spherical aberi-ation, while the
^ of low aperture will show many minute objects with considerable
cleai'ness, especially if a compai'atively nai-row illuminating cone be
used.

This difference becomes still more conspicuous as the difference
between aperture and power grows relatively greater, until we obtain
ultimately an amplification more than useless from its utter inability,
on account of deficiency of aperture, to gi'asp details.^

Up to 1874, however, there was an entire absence of knowledge,
even on the part of the leaders in microscopic theory, art, and
practice, as to the real optical principles that enabled us to see a
mici-oscopic image, and consequently to understand the essential
requirements to be aimed at in the best form of microscope. But in
1877 Abbe's great Diffraction Theory of Microscopic Vision appeared,
which has led to changes of incomparable value in the principles of

1 Vide Chapter II.

364 OBJECTIVES, EYE-PIECES, THE APERTOMETER

construction of objectives and eye-pieces, and, as a consequence, has
to some considerable extent given a new cliaracter to the entire in-
strument. Its promulgation has indeed inaugurated an entirely
new epoch in the construction and use of the microscope.

The genei-al character and the details of Abbe's theory are given
in the second chaj)ter of this ti-eatise ; but its jji'actical bearing upon
the theory and application of the optical part of the instrument was
soon manifest ; for in 1878 the homogeneous system of immersion
objectives ^ ivas introduced as a logical outcome of the diffraction
theory of microscopic vision. A formula for a ^-inch objective on
this system was prepared by Abbe, to whom, we learn from himself,
it had been suggested by Mr. J. W. Stephenson, of the Royal
Microscopical Society.^ It has been already shown '^ that the homo-
geneous system was so called because it employed the oil of cedar-
wood to unite the front lens of the objective to the cover-glass of
the object, in the same way as water had been employed in the
ordinary immersion system. ; bvit as there was a practical identity
between the refractive and dis]3ersive indices of the oil and those
of the crown glass of the front lens, the rays of light passed
through what was essentially a homogeneovis substance in their
path across from the balsam-mounted object to the front lens, and
a homogeneous system of objectives took the place of the previous
water immersions.

This was the first great step in advance in optical construction
and application following the theory of Abbe.

As often happens in matters of this kind, there had been an
apparent anticipation of this system of lenses by Amici as far back
as 1844 ; but it is very apparent that Amici employed the oil of
aniseed without any clear knowledge of the principles involved in
the homogeneous system, being wholly unaware of either the increase
of aperture involved or the cause of it. But this cannot be said of
Tolles, of New York. We have pointed out that, as early as 1873,
he made a -j\y-inch, and subsequently, in the same year, a l-inch
objective, each with a duplex front to woi'k in soft balsam, and with
a N.A. of r27. These objectives were examined by the late Dr.
Woodward, of the Army Medical Department, New York, and with
that examination were allowed to drop. For Tolles as an original
deviser of a practical homogeneous system this was unfortunate ; for
the actual introduction of the system in a form capable of universal
application, and worked out in all its details in an entirely inde-
pendent manner, we are wholly indebted to Abbe.

The i^rinciple was not, nevertheless, so readily and warmly
adopted in England on its first introduction as might have been
anticipated. This arose partly, however, from the fact that water
immersions had been brought to so high a point of excellence by
Messrs. Powell and Lealand that the early homogeneous objectives
were not possessed of more aperture, and were not sensibly
superior to the best immersions made in England.

The homogeneous objectives were made with duplex fronts and

\ Chapter II. 2 P. 27 ; also Journ. Boy Micro. Sac. Vol. II. 1879, p. 257.
^ Chapter I.

THE EXCLUSION OF THE SECONDARY SPECTEUM 365

two double backs. A general diagram of their mode of construction
is given in fig. 318.

So long as crown glass was employed in their manufactui'e, and
the anterior front lens was a hemisphere, it appeared that N.A. 1"25
to 1'27 was the aj^erture limit they could be made to reach.
Messi'S. Powell and Lealand, however, hy making the anterior front
lens greater than a hemisphere, increased the aperture of a j'^-inch
objective to 1-43 N.A.

This front, from being greater than a hemisphere, presented
difficulty in mounting ; this was at fii-st overcome by cementing its
plane sui'face to a thin piece of glass, which was then fixed in the
metal. Eventually, however, this form of construction was changed
by these makei-s in a very ingenious manner ; so to speak, they
entirely inverted the combination, and accomplished the end by
mahing the front of flint. By this means they obtained apertures
Avhich have not as yet been equalled by any other makers, reaching
in a ^, a ^^, and a 2tt ^ N.A. of 1"50 out of a theoretically possible
aperture of 1'52. Professor Abbe has since, it is true, made an
objective with a numerical aperture of 1"63, but this requires the
objects to be mounted and studied in a medium of corresponding
i-efractive index, and consequently, in the present state of oui- know-
ledge of the subject of media, not applicable to the investigation of
ordinary organic structures — certainly not of living things.

These objectives fully occupied the microscopist until 1886, when
the most important epoch since the discovery and application of
achromatism was inaugurated.

We have already pointed out in detail ^ that it was the gi-eat
defect of the ordinary crown and flint achromatics that tivo colours
only could he combined and that the other colours caused out-of-
focus images, which appeared as fringes round the object. This was
what was known as the residuary secondary s-pectrum.

In like manner, it has been shown that it was not possible in the
flint and crowii achromatic to combine two colours in all the zones
of the objective, so that if two given colours are combined in the in-
termediate zone they will not be combined in the peripheral and
the central portions of the objective.

These phenomena, it has been pointed out,^ arise from what is
known as the irrationality of the spectrum. To correct this we have
seen that Drs. Abbe, Schott, and Zeiss directed their attention to
the devising of vitreous compounds which should have their dis-
persive powers proportional to their refractive indices for the various
parts of the spectrum. Only by these means could the outstanding
errors of achromatism be corrected.

It is therefore a fact that the old flint and crown objectives,
whether for the microscope, the telescope, or the photographic
camera, are, strictly speaking, neither achromatic nor aplanatic.

Glass whose properties far more nearly approximated the theo-
retical requii-ement than any previously attainable having been
manufactured by the Jena opticians,^ Abbe was able to pi-oduce
objectives entirely cleansed of the secondary spectrum. From calcu-
1 Chapter I. - Cliapter II.

366 OBJECTIVES, EYE-PIECES, THE APEETOMETER

lations of a most elaborate and exhaustive kind made by Dr. Abbe,
objectives are made by Zeiss which not only combine three parts of
the spectrum instead of two, as formei-ly, but are also ajjlanatic-
for two colours instead of for one. This higher stage of achromatism
Abbe has called ajjochromation.

A general plan of the corstruction of an apochromatic objective
a;s made by Zeiss is shown in fig. 319, which, it will be understood, is
diagrainmatic, but sufficiently illustrates the elaborate corrections by
which the perfect results given by these objectives are accomplished.
But, in addition to their form of construction and the special ojDtical
glass of which they are composed, it is now known that they owe
much of their high quality to the use of fluorite lenses amongst the
combination. Fluorite is a mineral which has lower refractive and
dispersive indices than any glass that has yet been com]3osed, and
therefore by its introduction the optician can reduce the spherical
and chromatic aberrations greatly below that reached by achromatic
combinations of the known type.

It is a somewhat depressing fact that fluorite is very difficult to
pi'ocure in the clear condition needful for the optician, but from what
we have seen the optician can do in the manufacture of glass, we
may hope that an equivalent of this mineral in all optical qualities
may be discovered.

The medium for mounting and immersion contact has, of course,
to be of a corresponding refractive and disj)ersive index in all ob-
jectives of great aperture, and it is insisted by Abbe that the glass of
which the mount is made, both slip and cover, must, when the limit of
reflection by crown glass is passed by the objective, be of flint glass.
This he presents as a sine qua non in the case of the new objective
made a few years since by the house of Zeiss, and a specimen of
which has been generously given by the firm to the Royal Mici-o-
scopical Society. This glass has a numerical aperture of TGS; in
a subsequent chaj)ter on the present state of our knowledge as to
the ultimate structure of diatoms we are enabled to present the
i-esults of some of the photo-micrographs produced by its means.
But it may be noted that very much will depend upon the IST.A. of
the illuminating cone which can be employed with it — not theoreti-
cally, but practically, and it is for practical purposes of no value to
the student of minute life, because the highly refractive and dis-
persive medium needed to make the object mounted homogeneous is
destructive of life, and even of organic tissues. Such value as it
may have is therefore confined entirely to the examination of
silicious and other indestructible organic or inorganic jDroducts.

Before leaving this part of our subject we note with pleasure
that Mr. Nelson has computed a triplex front of minimum aberra-
tion suitable for an oil-immersion condenser. "VVe illustrate it
in fig. 320. The data for this are as follows, viz. : —

0 is the object and Y its virtual image ; the hyperhemispherical
front is aplanatic for these two points. The scale of the drawing is
arranged so that the distance of the vertex A of the front lens to
the object O is one inch. The three lenses are made of borosilicate
glass, No. 5 in the Jena catalogue, ju = l'51 ; and a,s the recijorocal of

IMMERSION FEONT FOR CONDENSER BY NELSON 167

the dispersive power is 64-0, the chromatic aberration of the triplet
is very small. Moreover the glass is hard and perfectly safe to use.

Radii : curve A= + '602
B= 00

0= + 3-434
D= + l-280
E=-15-078
F= + 2-359
Diameters : lens FE = 2-45

DC = 2-1
Distance between surfaces : ED='05

OA=-03

Fig. 320. — Nelson's new immersion front for a condenser.

Thickness AB=-683.

Working distance BO='317.

Diameter of the plane surface B of front lens= 1-192, AO:=1-0,
AV=1-51.

The angle 2=62°. and ^=35° 47'; the numerical aperture of
the combination is therefore 1-33 IST.A.

The front lens AB is aplanatic ; the spherical aberration of the

next two DC, FE only amounts to —-214 "^. The back correcting

368

OBJECTIVES, EYE-PIECES, THE APERTOMETER

lens, which might he a triplet, will require to have +'214 d-

of spherical aberration to render the whole combination ajjlanatic.

On the whole, and for the purposes of practical and prolonged
biological investigation, it is to the dry apochromatics that we ai-e
most indebted, and from their use we shall derive the largest benefit.

As no subject is really of more importance than a clear undei--
standing of the diflference of action of chromatic, achromatic, and
apochromatic lenses, we venture to present a diagrammatic illustra-
tion, which, while not strictly accurate, will carry with it no error,
as a popular illustration of this important subject.

In fig. 321, 1, 2, 3, we have representations, as truly as they can
be drawn, of zones of equal light ; that is to say, the peripheral zone
will transmit an amount of light equal to that given either by the
intermediate zone or the central circle. Let them therefore be
called equUucent zones.

/.

If we assign a numerical value for the visual intensity of the
w^hole spectrum, say 100, made up of the following parts, viz. : —

Red ........ 15

Orange-yellow 40

Yellow-green . 30

Blue 15

then if in any one of the equilucent zones the whole spectrum is
brought to a focus, we shall have for that zone 100 as its effective
value.

But the entire object-glass is divided, as in the diagram, into
three equilucent zones ; consequently 300 will represent the value of
the whole lens, pi'ovided the whole of the spectrum is brought to the
same focus.

By referring to the diagrams we see that in a non-achromatic
lens (fig. 321, 3) we shall get only 40, because only one part of the
spectrum is brought to the focus in its intermediate zone ; and as
spherical aberration causes the light wliich passes through the other
zones to be brought to other foci, they for all practical purposes might
be stopped out.

In the achromatic lens we have (fig. 321, 1) in the intermediate
zone two parts of the spectrum combined, as 40-1-30^70, and one

ZEISS'S APOCHROMATICS 369

in each of the other zones is also brought to the same focics, say 30
in the outer zone, and 40 in the centre circle. The result is that
the whole achromatic lens gives a total of light, on the principle stated
above, of 30 + 70 + 40;= 140. In the apochromatic system, how-
ever (fig. 323, 2), we find in the intermediate zone three jDarts of the
.spectrum united ; that is to say, 40 + 30 + 15 = 85 ; and two in each
of the others, say, 40 + 30=; 70. Thus an apochromatic objective
will give 70 + 85 + 70=225.

Kecalling the suppositions we have made for the pui'pose of this
graphic presentation of a difficult subject, it will be seen that a non-
achromatic objective would give 40, an achromatic 140, and an
apochromatic 225 out of a possible total of 300.

This illustration might be exceeded in severe accuracy, but
scai'cely in simplicity, and it suf&ciently explains from this point of
view alone the vast gain of the apochromatic system.

It is interesting to note that, while the microscope in its earlier
form took its powei'ful j)osition by borrowing achromatism from the
telescojje, it has now led the way to the apochromatised state, which
Avithout doubt it will be the work of the optician, in constructing
the telesco]3e of the immediate future, to follow.

We would beg the i-eader to bear in mind in the purchase of
objectives that, whilst the vitreous compounds with which Abbe's
beautiful objectives are constructed are now accessible to all opticians,
and whilst without these Abbe's objectives could never have been
constructed, yet it does not by any means follmv that because an
objective is made tinth the Abbe-Schott glass it is therefore ajm-
chromatic ; the secondary spectrum must be reimoved, and the spherico-
chromatic aberration balanced^ or it is ' apochromatic ' only by mis-
nomer. It is another feature of these objectives, which it is import-
ant to note, that they are so constructed that the upper focal j)oints
of all the objectives lie in one plane. Now as the lower focal points
of the eye-pieces are also in one plane, it follows that, whatever eye-
piece or whatever objective is used, the optical tube-length will
remain the same.

Professor Abbe has found ^ that in the wide-aperture objective
of high power there is an outstanding ei'ror which there is no
means of removing in the objective alone, but, as we have already
explained, this is left to be balanced by an over -corrected eye-piece.
As this peculiarity jDei-tains only to the highei- powers, a correspond-
ing error had to be intentionally introduced into the lower powers in
order that the same over-corrected eye-pieces might be available for
use with them.

It appears worthy of note in this relation that one of the best
forms for the combination of three lenses is that knowTi as Steinheil's
formula, which consists of a bi-convex lens encased in two concavo-
convex lenses. It will be observed by reference to the figure illustrat-
ing the apochromatic lens construction (fig. 319) that this is largely
made use of. In some instances the encasing lenses possess sufficient
density, with regard to the central bi -convex lens, to altogether over-
power it, the result being a bi-convex triple with a negative focus.

1 Chapter II.

B B

370 OBJECTIVES, EYE-PIECES, THE APEETOMETER

It is another distinctive feature of the 3 mm. objective that it
has a trijjlex front ; thus Zeiss's 3 mm. (= |- inch focus) had the
errors from three uncorrected lenses balanced by two triple backs,
i.e. nine lenses taken together, but it has since been constructed on a
different formula.

The foci of the set of apochromatic lenses now made by Zeiss
are integral divisions of what may be termed a unit lens of 24 mm. ;
24 he chooses as a means of avoiding the inconveniences inseparable
from the use of the decimal system, i. The unit lens is therefore a
little higher than 1 inch in power. In the series of dry lenses there
are two powers of the same aperture. Thus 24 mm. and 16 mm.,
corresponding to English 1 inch and |^ inch, each has an aperture of "3 ;
a 12 mm. and 8 mm. := English \ inch and ^ inch, have each an
aperture of '65 ; while a 6 mm. and a 4 mm. = J inch and ^ inch,
have both an aperture of '95.

There are also water- immersions : a 2'5 ram. = --^^ inch, with
IST.A. 1'25, and two oil-immersions respectively 3 mm. and 2 mm.
= ^ inch and yV inch, both being made either with 1"3 or
1-4 N.A.

Apart from these, intended to be used for photographic purposes
without an eye-piece, is a 70 mm. = a 3-inch, also a 35 mm. or
IVinch objective.

With the exception of the 6 mm., 4 mm., and 2'5 mm. objectives,
which have the screw-collar adjustment, this series have rigid mounts,
correction being secured by alteration of the tube-length.

The performance of these lenses, as they are now made, is of the
very highest order. They present to the most experienced eye unsur-
passed images. They are connected with a delicate perfection which
only this system, couj)led with technical execution of the first order,
can possibly be made to produce. The optical jDolish, the centring,
the setting, and the brasswork certainly have never been surpassed.

It is a matter also woi'thy of note that Zeiss's apochromatic
series of objectives are true to their designations as potoers. The
^-inch is such, and not a '^^-inch designated ^-inch. This was
equally true of the early achi-omatics. A. Ross produced a ^-inch
under that name. One now before us, made fifty years ago, has an
initial power of 41 ; and that of ^ inch has an initial power of 21.
But modern achromatics of fair aperture are always greatly in
excess of their designated power ; |- are nearly ^-inch. A ^-inch
of 40° has an initial power of 25, and is a f^-inch ; y^-inch
objectives are in reality ^rinch ; and ^-inch objectives of 90° and
upwards have initial powers of 50 instead of 40, which they should
have, so that they are in reality -Iths ; some in fact — by no means
uncommon — have an initial j)ower of 60, and are actually ^th-inch
objectives.

This is explicable enough from the maker's point of view ; it is
far easier to put j^oiver into an object-glass t?tan aperture. It is

1 Although the foci of the lenses are expressed in integers, with the single excep-
tion of the water-immersion 2'5 mm., there are inconvenient decimal fractions in the
initial magnifying power of all the series except those of 2' 5 and 2 mm. focus.

HISTOLOGICAL ADVANTAGE OF HIGH POWER 37 1

easier to make a ^-inch of 100° than a ^ with 100° ; the result is
that low powers with suitably wide apertui-es are costly.

In the Zeiss apochromatic series of objectives the 24 mm. of "3
N.A. and 12 mm. of -65 N.A. may be considered as lenses of the
very highest ordei- ; the relation of their aperture to their power is
such that everything which a keen and trained eye is capable of
taking cognisance of is resolved when the objective is yielding a
magnification equal to twelve times its initial j^ower ; for this purpose
an objective must have 0'26 IS". A. for each hundred diameters of
combined magnification. Undei- these conditions an object is seen in
the most perfect manner possible. In this connection Mr. Nelson
has suggested ^ that the term ' optical index ' should be added to
that of the numerical aperture. The optical index or O.I. is the
ratio of the numerical aperture ( x 1000) to the initial magnifying-
power. Thus the numerical aperture of the Zeiss apochromatic
24 mm. is "3, and its initial jDower 10. Then its O.I. is ^y^ = 30. The
O.I. of the 12 mm. apochromatic of -65 N.A. is 6^"= 31. That of
the ^ homogeneous immersion of 1 "4 N. A. is J-|-|-:= 17. Compare
now these figures with an old water-immersion giy of 1"1 N.A. Vs'V*'
= 2"0. The value of these figures will be apparent when we
remember that any lens used with a 10 power eye-piece must have
an O.I. of 26 to resolve all detail visible to a keen eye.

The optical index therefore tells us that the --"q water-immersion
of 1"1 IST.A. had a vast amount of empty magnifying power, while on
the other hand the 24 and 12 mm. wdll both stand a higher eye-
piece than 10 ; nay, even require it before the detail resolved by them
is made visible to the eye. It also shows that the g-of 1"4 N.A. will
stand a higher eye-piece without arriving at an empty magnifying
power than the iV^^ ^"^ N.A., whose O.I. is ll'O.

As it is more difiicult to put aperture into a lens than power, the
O.I. becomes also an index of the money value of a lens. Thus the
^ mentioned above that had an initial magnifying power of 60 and
N.A. of '8 ought to be a cheaper lens than a true \ with an initial
magnifying joower of 40 and a, N.A. of "9, their optical indices
being 13 and 22 resjoectively. The limit of combined power for best
definition with any objective of any given aperture may be found by
multiplying its N.A. by 400. Example : The limit of power for best
definition with a |^ of '3 N.A. is 120 diameters. The converse rule
may be stated thus : The ideal N.A. for any objective whose initial
power is known can be found by multiplying its power by '025.
Example : The ideal N.A. for a \ of power 20 is 20 x "025 = "5 N.A.

It may be well for the student to prove this, which may be
readily done.

Take a suitable object, such as a well-prepared proboscis of a
blow-fly, and examine it under critical illumination with the 24 mm.
•3 N.A. (= 1-inch) objective, and a 12 compensating eye-piece.
Note with close attention every particular of the image : the
resolution of the points of the minute hairs, the form of the edges of
the cut suctorial tubes, the extent of the surface taken into the
' field,' and the relation of all the parts to the whole.

1 Jcmrn. B. M. S. 1893, p. 12.

bb9.

1^2 OBJECTIVES, EYE-PIECES, THE APEETOMETER

Now change the objective for the 16 mm. '3 N.A. (= f, but
with the same aperture). Nothing more is to be seen ; the most
dexterous manipulation cannot bring out a single fresh detail ; the
resolution is in no sense carried farther ; the cut suctorial tubes were
in fact, in our judgment, better seen with a lower power, while with
it all of course a smaller extent of the object occupies the ' field.'

It can in fact be scarcely doubted that the j)icture presented
by the § is a distinct retrogression in every sense compared with
that presented by the 1-inch when both are equally well made
and have equal apertures, viz. '3. But beyond all this, v^hatever
may he done by the 16 mm. '3 N.A. can be accomplished in an
eqvially satisfactory manner by removing the 12 eye-piece and
replacing it, with practically no other alteration, by [an 18 eye-piece;
and still higher results can be obtained without the slightest detri-
ment to the image by vising an eye-piece of 27.

Not less interesting and convincing will it be to examine the
same object with a 12 mm. "65 N.A. (= ^-inch), and an A Zeiss
achromatic of "20 N.A. (= frds inch), using a 12 eye-piece. Those
who may still retain some conviction as to the value of ' low-angled
glasses to secure penetration ' can want no further evidence of its
entire fallacy than such a simple experiment affords.

For those who prefer it, a true histological object maybe selected.
We choose a portion of a frog's bladder treated with nitrate of
silver, in which are some convoluted vessels, enclosed in a muscular
sheath wdiich had contiacted.

This object is presented by photo-micrograph in figs. 7 and 8 of
the frontispiece. In fig. 7 the vessel in the frog's bladder is seen
Tjy a Zeiss A '2 N.A. magnified 140 diameters. The object of the
photogi-aph is to expose the fallacy which underlies the generally
accepted statement that low-angled glasses are the most suitable for
liistological purposes. The assumption is founded on the fact that
the penetration of a lens varies inversely as its aperture, and it is
taken for granted that ' depth of focus ' will be obtained, not to be
secured by large apertures, and therefore it is taken for granted
that we are enabled to see into the structure of tissues.

In examining the illustration (which will with advantage permit
the use of a lens) it will be seen that scarcely an endothelium cell can
be clearly seen. A sharp outline is nowhere manifest, because
the image of one cell is confused with the outlines of others upon
which it is superposed. We have seen that there is no perspective
proper in a microscopic image ; therefore it is better to use high
apertures in objectives, and obtain a clear view of one plane at one
time, and train the mind to appreciate perspective by means of focal
adjustment.

It will be admitted that no clear idea of what an endothelium
■cell is can be obtained from fig. 7.

But fig. 8 (frontispiece) repi-esents the same structui-e slightly
less magnified (x 138) by means of an apochromatic \ N.A. •65.
Here only the uppei* surface of the tube is seen ; but the endothe-
lium cells can be clearly traced, and a sharp definition is given to

HISTOLOGICAL ADVANTAGE OF LARGE APERTURE T^y ^

every cell. The circular elastic tissue is also displayed, while the
whole image hiis an increased sharpness and perfection.

Thus, with the objective (A "20 IST.A. = §rds inch) of lower
aperture, the endothelium cells can he seen ; but when the image is
compared with that of the objective of wider aperture ("65 N.A.),
the former image is found to be dim and ill-defined. The muscular
sheath is so ill-defined that it would not be noticed at all if it had
not been clearly revealed by the objective of wider aperture. But,
on the other hand, the objective of gi'eater apertui-e not only shows
the muscular sheath, but it also shows the elongated nuclei of the
muscle cells ; and at the same time brings out the convoluted vessels
lying in the muscular sheath as plainly as if it were an object of
sufiicient dimensions to lie upon the table appealing to the unaided
eye.

We have pointed out in the proper place,^ that although ' pene-
trating jDOwer ' varies inversely as the numerical aperture, it also
varies inversely as the square of the power.

Now, from what we know of histological teaching in this country,
we do not hesitate to say that a histologist would not have attempted
to examine the above object with even a Zeiss A objective. He
would have advised the use of ' the ^-inch,' of, perhaps, '65 aperture ;
but by so doing he would have secured only one-third of the pene-
trating power quel aperture, and one-seventh of the penetlating powder
quel power.

It is manifest,' then, that pursuing this course in the histological
laboratory defeats the end sought, and which it is so desirable to
attain.

It is absolutely unwise to use a higher povjer than is needful.
A ;^-inch Avhere a ^-inch would answer involves loss in many w^ays,
ancl would never be resorted to if the aperture of the lenses employed
were as great as the pov.^er used legitimately pernnitted?

A given structure, to be seen at all, must have a given aperture ;
to obtain this, as objectives now made for laboratory purposes run,
they are obliged to use too high a power. The result is that in seek-
ing to avoid what is accounted the loss of ' penetrating power ' at
an inverse ratio to the aperture, it is forgotten that we are losing it
inversely as the sqtcare of the power !

Moreover, the two apochromatic objectives we have already
referred to as test lenses are equally able to show the value of
apochromatism, not so much on account of the removal of the
secondary spectrum as for the reduction of the aberrations depend-
ent on the irrationality of the spectrum in ordinary achromatics.

Use the 12 mm. '65 IST.A. objective. Place a diatom in balsam
in the focus of it on a dark ground ; the diatom will shine with a
silvery whiteness, and the image will be wholly free from fog.

Now take one of the best achromatics obtainable of ^-inch
focus of 80° (almost certainly a j^*j in power) and examine the same
diatom in the same circumstances ; it will be bathed in fog. If,
however, the achromatic objective is an exceptionally good one, and
we reduce its apertru'e to 60°, we shall get a fair picture of the
1 Chapter I. ^ Chapter II.

374 OBJECTIVES, EYE-PIECES, THE APERTOMETER

diatom — one indeed that was considered critical until that with the
apochi'omatic was seen. But in comparison it is dull and yellowish.
From which it follows that an exceptionally fine achromatic ^-inch
of 60° or "5 IST.A. will not sufier comparison of the image it yields
with that of an apochromatic ^-inch of "65 K.A.

Speaking generally on the whole question, then, it would be the
utmost folly for histologists or opticians to shut their eyes to the
magnificent character of the series of dry apochromatics of Zeiss,
I'anging from 1 inch (24 mm.) to ^ inch (4 mm. '95 N.A.). They
are the most perfect and eificient series of objectives ever placed in
the hands of the worker ; and, unless English lenses on a truly
apochromatic principle and equal quality are produced, it must be
to the detriment of either the opticians or the woi-kei'S of this
country.

Nor need it be supposed that the production of objectives
approximate to these must be costly ; great steps have been taken
lately in the reduction of their cost. The manufacture of the Jena
glass has indeed wrought an entire change in the character of
objectives now produced ; and although the very finest and most
costly apochromatics having fluoi-ite used in their construction still
hold an unrivalled position, yet the new glass admits of corrections
so nearly perfect that some stronger word than achromatic appeared
to be needed, and the word semi-apochromatic has crept in and
undoubtedly designates a most valuable and far from costly set of
lenses of all powers. It is Leitz, of Wetzlar, that has first and
efficiently attacked this problem and provided the student whose
means are limited with objectives of a very high class, and which
come i-emarkably near to the best apochromatics. We would
specially call attention (wholly in the interests of students) to IsTo. 3
(|-inch ISr.A. 0-28) at a cost of 15s. No. 5 is an equally valuable
and admirable objective which is a J-inch 0'77 N.A., the price of
which is 25s., and it comes so neai- to an apochromatic as to require
expert judgment to discover that it is not. He also makes a dry
-tV-inch N.A. 0-87 and a dry \ of -82 N.A. at a cost of 3^., which
is a very low price for so good a piece of optical work. Also an
oil-immersion ^^Q-inch N.A. 1'30 is sold for 3^. 15s. This glass is
corrected for the long tube, and a similar iVth N.A. 1"30 for
5^. resolves secondary diatom structure well, and it is hardly dis-
tinguishable from an apochromatic lens ; and we can attest, from
personal investigation, the value of each of these, which are only
selections from a considerable sei-ies, all of which we have found
to be reliable, and, when examined in numbers, very few indeed
ai-e below the standard quality. But such work is so much needed
that it is not likely that, with the glass accessible to all, it will
remain the peculiarity of one maker ; hence we find that Reichert
follows Leitz so closely in quality and price that it is not easy to
distinguish the semi-apochi'omats of one maker from the other.
Reichert's No. 3 (|-inch N.A. 0-30) is 17s., his 7a (an admirable
lens) i-inch N.A. 0'87 is 1/. 16s. He makes a high-class oil-
immersion Y^-inch N.A. TSO for 8^. And of apochromatic lenses
he makes a :r]-inch N.A. 0'30 and a ^-inch N.A. 0-95 for U. each.

AMERICAN OBJECTIVES — EYE-PIECE 375

which, so far as we have seen them (and we have examined
many), are excellent. Reichert's semi-apochromatic ^ is also a fine
and useful lens, and his jW-inch apochromatic N^.A. 1"30 has
qualities fitting it for use in any kind of reseai'ch.

But we confess that it is a matter of most pleasant sui-prise to
us to find that the great Ameiican fii-m of Bausch and Lomb are
putting u]3on the English market objectives tliat fairly compete
with the above in the lowness of their price, while their optical
quality and mechanical woi-k ai-e of the best order. We have
examined these lenses with much pleasure ; they are from the com-
putations of Professor Hastings, and, considering the fact that they,
in all the higher powers especially, ai-e so loAv-priced, their correc-
tions and high quality are beyond all praise. We would specially
call attention to a f -inch, a ^-inch, and a ;iV-inch which we have
examined thoroughly and with approval that needs no quali-
fication when it is remembered that the most advanced Continental
opticians have not touched a lower price.

Messrs. R. and J. Beck are making good objectives, oil-immer-
sion and other, and one of their ^V oil-immersions is sold at the
strikingly low figure of Al.

Messrs. Swift and tSon are making a large number of objectives,
especially apochromats and semi-apochromats, and they have long
striven to supply the student with high-quality lenses at the lowest
possible price. There can be no doubt that the whole secret of
success in this matter is dependent on a sufficiently large series of
experiments to determine on the right kind of glass, so as to produce
the highest order of ' semi-apochromatism.'

Messrs. Watson and Sons have commenced the manufacture of a
new series of objectives based on original computations. These
promise exceedingly well. We have examined the ^-inch and the
^-inch. We find that their initial powers are 21 diameters 0"45 I^.A.,
and 40 diameters 0*74 N.A., and they depend for aplanatic results,
which are admirable, on a triple back lens. The objectives, we
believe, will be valuable as a series when complete. They do not
claimL to be amongst the very low-priced lenses ; but they claim,
and we believe they will possess, some of the best qualities which
should be aimed at in microscopic object-glasses.

These facts are of importance to the medical student and to
opticians generally. By apochromatised and semi-apochromatised
objectives of the highest order the work of present and future
microscopy will be done — that is inevitable. To thoroughly under-
stand what its very best i-esults, theoretically and practically, must
be becomes the imperative aim of the optician who would be
abreast of the direct wants of his time ; and to produce the nearest
to these in objectives and eye-pieces at the lowest possible pi'ice
is, apart from all other issues, to be a direct benefactor of true
science.

The Eye-piece. — The eye-piece, sometimes called the ocular, is an
optical combination, the purpose of which is so to refract the diverg-
ing pencils of rays which form the real object-image that they may
all arrive at the pupil of the observer's eye. They have also to form

Z7^

OBJECTIVES, EYE-PIECES, THE APEETOMETEK

a virtual image of the real image which is presented to them as the
object. For this purpose a combination is indispensable, but this
may be varied. There are ordinary and special eye-pieces. Those
in ordinary use separate into two di-\isions : (1) positive eye-piesces
and (2) negative eye-pieces. These are easily distinguished ; with
a positive eye-piece we can obtain a virtual image of an object by
using it as a simjjle microscope, because its focus is exterior to itself.
This cannot be done with the negative eye-piece^ because its focus is
within itself.

The eye-piece in common vise is negative, and is generally known
as Huyghens's, and sometimes as Campani's. Monconys appears to
have been the first (1665) to supply the field-lens to the eye-lens of
the microscope, and Hooke in 1665 adopted his suggestion ; but
how far Monconys was indebted for this to the compound eye-piece
attributed to Huyghens cannot now be determined.

This instrument, as commonly used in a telescope, consists of
an eye-lens and a field-lens, each being plano-convex, having their
convex sides towards the object, their foci being in the ratio of

Fig. 322. — Huyghenian eye-piece.

Fic+. 323. — Kelhier eye-piece.

3 : 1, and the distance between them being equal to half the
sum of their focal lengths, a diaphragm being placed in the
focus of the eye-lens. In a microscope a difl'erent ratio and lens
distance is employed, the fact being that difierent tube lengths
require different formulae. The general form of a Huyghenian eye-
piece is shown in longitudinal section in fig. 322. This makes a
very convenient form of eye-j^iece of 5 and 10 magnifying jjower ;
but when the power much exceeds this last amount the eye -lens
becomes of deep curvature and short focus, so that the eye must be
placed uncomfortably near the eye-lens. This, however, is its chief
defect, and it may fairly be considered the best ordinary eye-piece.

Another negative eye-piece is that known as the Keliner, or
orthoscojnc. This consists of a bi-convex field-glass, and an achromatic
doublet meniscus (bi-convex and bi-concave) eye-lens. A vertical
section of one so constructed is seen in fig. 323. These eye-pieces
usually magnify ten times, and the advantage they are supposed to
give consists in a large field of view ; but they are not good in practice
for this very reason ; they take in a field of view greater than the

NEW HUYGHENIAN EYE-PIECE

377

objective can stand, and as a rule even the centime of the fiekl will
not bear comparison in sharpness with the Huyghenian form.

Mr. Nelson has recently computed and had made a Huyghenian
eye-piece on a wholly new formula ^ which has the field reduced by
about 7 inches, yet we can testify that in use it gives exceedingly
sharp images, and what surprises the accustomed worker is that it
acts admirably in the place of ' compensated ' eye-pieces, giving
results that often not only equal but surpass these.

The j)ower of this eye-piece is 12 ; equivalent focus, '8, corrected
for the English tube (p — 9-5).

Fig. 324 is enlarged twice.

/I

Fig. 324. — Nelson's new formula Huyghenian eye-piece.

Data: CtUiss, borosilicate crown, /i = l'51, j'=64"0, Jena cata-
logue ISTo. 5.

Field-lens, biconvex 5'= + -94) -,. , rx
r. r^AT diameter -So.

30.

s=-2-94

Eye-lens, biconvex r'=+ •34) -,. ,
•' ' / 1 m r diameter ■.:

s = — rui )

Distance of eye-lens from field-lens, measured from their sur-
faces, '97.

Distance of diaphragm from surface of field lens, •48.

Diameter of hole in diaphragm, '26.

Power, 12 ; equivalent focus, '53, corrected for the Continental
tube (p:=6^3).

Data : Glass, same as before.

Field-lens, biconvex 9"= -+- •GS] -■• , , .ok
1 r\c^ [ ciiameter ' o^ ,
s= — r98)

Eye-lens, biconvex ?•'=+ •22) ,. , ^^
•' ' / r>/^ I diameter -20.

s'^— •66)

Distance of eye-lens from field-lens, measured from their sur-
faces, '66.

Distance of diaphragm from surface of field-lens, •34.

Diameter of hole in diaphragm, •IB.

These eye-pieces should enter the tube of the microscope as far
as their diaphiagms.

Positive Eye-pieces. — In the early compound microscopes the

1 J.B.M.S. 1900, p. 165.

378 OBJECTIVES, EYE-PIECES, THE APEETOMETER

■eye-pieces were all positive ; that is to say, they consisted of a single
bi-convex eye-lens and no field-glass. The definition with this must
have been most imperfect ; the addition of a field-lens, though it
were a bi-convex not in the correct ratio of focus nor the theo-
retically best distance, must have been considered a great advance.

In this way matters rested, however, until the theoretically
perfect Huyghenian form was devised. Object-glasses have been
used as eye-pieces, and all forms of loups or simple microscopic
lenses have been employed for the same pur-
pose. Solid eye-pieces have also been used
both in England and America, but with no
i-esults that surpassed a well-made Huy-
ghenian combination ; but the best form of all
of the combinations which have been ti-ied by
us as positive single eye-pieces are the Stein-
heil trij)le loups ; a section of one of these is
pjg g25. seen in fig. 325. This combination also forms

one of the best lenses for projection purposes
■ever constructed. But a positive eye-piece was devised by Ramsden,
consisting of two plano-convex lenses of equal foci ; the distance
being equal to two-thirds the focal length of one. The diaphragm
was of course exterior.

Abbe's Compensating Eye-pieces. — We have already given a
general description of the nature and action, in connection with the
apochromatic objectives, of this form of eye-piece.^ In the section
-above on objectives we have referred to the fact that these eye-pieces
are over -corrected ; this may be easily seen by observing the colour
at the edge of the diaphragm, which is an orange-yellow. If we
compare this with the colour in the same position with a Huyghenian
eye-piece, this will be blue, being seen through the simple uncorrected
eye-lens.

There are three kinds of compensating eye-piece as designed by
Abbe. These are: —

1. Searcher eye-pieces.

2. Working ,,

3. Projection ,,

1 . The searcher forms are negatives of very low power, intended
only for the purjaose of finding an object ; they consist of a single
field-lens and a doublet eye-lens.

The iDorking forms are both positive and negative. The eye-piece
for the long tube has a triplet eye-lens ; but the remainder, viz. 8,
12, 18, and 27, when first introduced, were all positive. The 8 was
subsequently, however, changed for a negative. Having used both,
we ai'e glad to learn that it is made now both positive and negative.
It may be convenient to have the 8 a negative like the 4, but with
regard to the 12, 18, 27 it is important that they should be positives.

These positive foi^ms are on a totally new plan, being composed of
a triple with a single plano-convex over it ; the diaphragm is, of course,
exterior to the lens (fig. 326)^ With these the definition is of the
finest quality throughout the field, which has been i-educed to about
6 inches. They present the admirable condition that with the deeper

1 Chapter I. p. 33.

'• HOLOSCOPIC " EYE -PIECES

379

powers the propei- position of the eye is further from the eye-lens
than is the case with those of the Hu^^ghenian consti-uction ; which
makes it as easy to use an eye-piece of as gi-eat a power as 18 oi' 27
as one of 4 or 8.

The field of these eye-pieces has, as we believe, been very wisely
limited to five or six inches. The attempt on the pai-t of English
opticians to give to our eye-pieces fields reaching eighteen inches is
an error. A microscopic objective with the lowest aperture has the
field greatly in excess of any other optical instrument ; and to deal
with such eccentrical pencils as must be engaged by an eye-piece
Avith a field of eighteen inches is a strain not justified by what is
gained.

The jDOwers of the working eye-pieces are also arranged in a new
way. The multiplying powers for the long tube are 4, 8, 12, 18, 27 ;
it will be seen at once, therefore, that they bear no definite ratio to
one another, and if we seek to simplify the focal lengths we are, by
the employment of the metrical system, confronted with decimal
fractions. But Avithout further elaboration it may be well to say
that 12 is the most generally useful eye-piece,
and if only one compensating eye-piece is to be
selected, there can be no question, from a prac-
tical point of view, but this is the best to em-
ploy. The 4 is too low, and
the 27 is too high for general
purposes, and the 8 and 18 are
sufficiently near the 12 to give
the latter the advantage in
general work. We cannot,
however, refrain from the ex-
pression of the opinion that a
series of 5, 10, 20, or 6, 12, 24
powers would be in many senses
more useful, and would ofier
facilities in application not se-
cured by the series of Abbe now in use.

It may be well to give further emphasis to the fact that this con-
struction of eye-piece is not only essential to the proper work of
apochromatic objectives, but they greatly enhance the images given
by ordinary achromatic lenses ; and it may be noted that the 8, 12,
and 18 eye-pieces for the short tube are identical with 12, 18, 27
for the long tube. The 4 eye-piece for the short tube makes a very
suitable 6 power for the long tube.

A new series of eye-pieces has been recently introduced by
W. "Watson and Sons, to which they have given the trade name of
' Holoscopic' What is held to be a very simple method is employed
for rendering them either over- or under-corrected, and therefore
suitable for either apochromatic or the ordinary achromatic objectives.

This eye-piece is of the Huyghenian type, but unlike the ordinary
pattern the eye-lens, together with the diaphragm, is mounted in a
tube which slides telescopically in the body of the eye-piece, at the
lower end of which the field-lens is fixed. This is shown in fig. 327.
When the sliding tube is pushed home as far as it will go, the eye-

FiCt. 826.— Abbe's
comp e n s a t i n g
eye-piece of 12
power.

Fig. 327. — Watson's
holoscopic eye-
piece.

38o

OBJECTIVES, EYE-PIECES, THE APERTOMETER

piece is an undei'-corrected one and suitable for use with the
ordinary achromatic objectives ; by drawing oiit the sliding tube
and so increasing the distance between the eye and field lenses, the
so-called over-correction, which is associated with the compensating
eye-pieces, can be obtained in varying degree according to the
amount of extension. A scale is provided on the sliding draw- tube
for registering any desired position.

There ai-e theoretically two distinct advantages Avith this
eye-piece : —

(1) It obviates the necessity for being provided with both Huy-
ghenian and compensating eye-pieces, because it performs the
functions of both.

(2) It will have been observed that with some objectives the
compensating eye-piece has apjoeared to possess too much over-
correction, producing the feeling in the mind of the worker that if
it were possible to vary the correction of the eye-piece a little a
better image could be produced ; this can theoretically be done with
the new ' Holoscopic ' eye-piece, but we prefer a definitely com-
pensated, or an ordinary eye-piece.

The initial magnifying powers of this series of eye-pieces are : —
For the 160 mm. tube length 5, 7, 10, and 14 diameters.
„ „ 250 „ „ ^ 7, 10, 14, „ 20

For the English tube length, where the diameter of the eye-piece
fitting of the microscope permits of it, specially
large field-lenses are used.

The cost is very little greater than that of the
ordinary Huyghenian eye-pieces.

The projection eye-piece is mainly intended
for photo- micrography, but it is also useful for
drawing and exhibition purposes. It is a negative,
with a single field-lens and a triple projection-
lens. The projection-lens is fitted with a spiral
focussing arrangement in order that the diaphragm
which limits the field may be focussed on to the
screen or paper. The field of this eye-piece is
small, but its definition is exquisitely sharp.

It may not be generally known that good
photo-micrographs can be obtained by projection
with the ordinary compensating working eye-
pieces, but this is a fact worthy of note.

It will perhaps be of practical utility if we
append a table indicating the focus of the com-
pensating eye-pieces when used with the long and
the short body.

Special Eye-pieces. — The most important of
these, the micrometer eye-piece, we have already
considered, so far as its application to micrometry is concerned.^
Its optical character may be properly considered here. If it is a
negative eye-piece the micrometer is placed in the focus of the eye-
lens ; but if a positive combination, it is placed in the focus of the
eye-piece itself. The Ramsden foi-m described above is thoroughly

1 Chapter IV.

Fig. 328.— Zeiss's
projection eye-
piece No. 2.

SPECIAL EYE-PIECES

Focus of Eye-jiieces for Long Body.

Power

2

4

8

12

18

27

Focus in mm.

135

67-5

33-7

22-5

15

10

1 „ inches .

5-3

2-6

1-33

•89

•59

•39

Focus of Eye-pieces for Short Body.

Power

2

4

4*

6

8

12

18

Focus in mm. .

90

45

45

30

22^5

15

10

„ inches

3^54

1^77

1-77

1^18

•89

•59

•39

Projection Eye-incces. — 2 iov short and 3 for long bodies = 90 mm. or 3-54
inches ; 4 for short and 6 for long bodies = 45 mm. or 1-77 in.

suited for this purpose, but a negative form is often employed, the
micrometer being placed inside the eye-piece in the diaphragm, i.e.
the focus of the eye-lens.

In order that the micrometer may be suscej^tible of focus for
various sights, it is necessary that the eye-lens in the case of a
negative eye-piece, and the whole eye-piece in the case of a jDositive
one, should be mounted in a sliding tube ; and one with a spiral slot
will be preferable, since it makes the work of focussing both facile
and accurate.

If only one micrometer eye -piece is used it should be of medium
power, such as iTj-inch focus; but it is an inexpensive and a useful
plan to have an additional set of lenses to sci'ew on to the same mount,
so as to make the eye-piece, say, a §-ineh focus.

Spectroscopic, polarising, goniometer, and binocular eye-pieces
are each treated under their respective subjects.

Quekett's index eye-piece is one which has a pointer placed at
the diaphragm, so constructed that it can be turned in or out of the
field, and is used to point to the position of an object.

A good plan, ivhen the magnification is great, is to have a dia-
phragm tmth a small apert%ire to drop into the eye-piece and dimi-
nish the field of view. This not only makes the object to be pointed
out more easily accessible to the eye, but — as we have by many
years of observation proved — it aids in close observation upon minvite
objects by cutting oft' a large area of light without altering the in-
tensity of what remains, and so makes close observation more easy.

Diaphragms with a square aperture are fitted into eye-pieces for
the purpose of counting blood-corpuscles in a definite area. The
hole in the diaphragm must be adjusted foi- a definite tube length
and for use with a definite objective and used with no other.

As it is directly associated with the eye-piece, we shall find no
better place to note the curious and hitherto unexplained fact, that
when resolving stria? or lines with oblique light the eftect is much
strengthened l)y placing a NicoTs analysing jjrism over the eye-
jnece.

Testing Object-glasses. — It will have been noted by the attentive

382 OBJECTIVES, EYE-PIECES, THE APERTOMETER

reader that many of the more important qualities of objectives
are determined by the principles of their construction, and become
in fact questions simply of the quality of the workmanship involved
in producing the optical and mechanical parts of the object-glass.

The quality of the workmanship may be tested by technical
means described below, and by that subtle power which comes with
experience. This can only be imparted through the paths of labour
and experiment, by which in every case it is reached. But, granted
that an object has been illuminated in an intelligent and satisfactory
manner, the first complete view of the image (which must of course
be a thoroughly familiar one) will enable the expert to come to a
conclusion as to the quality of a given objective. The character of
the image to the expert determines at once the character of the
lens. This is the more absolute if a series of eye-pieces (up to the
most powerful that can be obtained) are at hand. JSTothing tests
the quality of an objective so uncompromisingly as a deep eye-piece.
For brilliancy of image a moderate power of eye-piece is of course
best ; but the capacity of the object-glass is clearly commensurate
with its ability to endure high eye-pieces without loss of chai'acter,
and even sharjjness in the image. Unless the objective be of high
quality, the sharjDness of the image gradually disappears as the more
powerful eye-pieces are used, until at last either all or part of the
image breaks up into the ' rotten ' details of a coarse lithograph.

A lens finely corrected (with lai^ge aperture) will bear the deepest
eye-piecing with no detriment. The 24 mm. and the 12 mm. of Zeiss
will sujffer any eye-piecing accessible to the microscopist without the
smallest surrender of the sharpness of the image. We have in fact
tried in vain to ' break down the image ' yielded by these objectives.

This mode of testing is of course to a large extent subjective, or
at least is controlled by incommunicable judgments. It is most
important therefore to have a mode of judgment that shall be acces-
sible to the beginner and the interested amateur. Dr. Abbe has
proposed a method which is at least accessible to all.

In ordinary practice microscope objectives, if tested at all by
their possessors, are simply subjected to a comparison of perform-
ance with other lenses tried upon the same ' test-objects.'

The relative excellence of the image seen through each lens may,
however, depend in a great part upon fortunate illumination, and
not a little upon the experience and manipulative skill of the ob-
server ; besides which any trustworthy estimate of the performance
of the lens under examination involves the considei'ation of a suit-
able test-object, as well as the magnifying power and aperture of the
objective. It is knowing what is meant by a ' critical image,' and
being able to discover whether or not a given objective will yield it.
Clearly all tests of optical instruments, which are not capable of
numerical exjjressiooi, must be comparative. Magnifying poioer can
be measured numerically ; it is not comparative. In the same way
resolving jjower is mathematically measurable ; so is penetrating
poioer. But definition and hrilliancy of image, and evidence of
centring, can have no numerical expi'ession ; they are consequently
comparative.

TESTING OBJECTIVES 383

The structure of the test-object should be well known, and the
value of its ' markings ' — if intended to indicate microscopical dimen-
sions— should be accurately ascertained, care being taken that the
minuteness of dimensions and general delicacy and j^Pi'fection of the
test-object should be adapted to the power of the lens. A fairly
correct estimate of the relative perfoi-mance of lenses of moderate
magnifying power may doubtless be thus made by a competent
observer ; but it is not possible from any comparisons of this kind
to detei-mine what may or ought to be the ultimate limit of optical
perfoi-mance, or whether any particular lens xindei' examination has
actually reached this limit.

Assuming the manipulation of the instrument and the illumina-
tion of the object to be as perfect as possible, and further that the test-
object has been selected with due appreciation of the requirements of
perfect optical delineation, a fair compai'ison can only be di'awn be-
tween objectives of the same magnifying power and aperture. Which
of two or more objectives gives the better image may be readily
enough ascertained by svich comparison, but the values thus ascer-
tained hold good only for the particular class of objects examined.
The best performance realised with a given magnifying power may
possibly exceed expectation, yet still be below what might, and
therefore ought to be obtained.

On the other hand, extravagant expectations may induce a
belief in performances which cannot be realised. The employment
of the test-objects most in use is moreover calculated to lead to an
entirely one-sided estimation of the actual working power of an
objective — as, for example, when 'resolving power' is estimated by
its extreme limits rather than by its general efficiency, or ' defining
power ' by extent of amplification rather than by clearness of outline.
So that an observer is tempted to affirm that he can discern through
his pet lens what no eye can see or lens show. This happens chiefly
with the inexperienced beginner, but not unfrequently also with
the more experienced worker who advocates the use of great amplifi-
cation, in whose mind separation of detail means analysis of struc-
ture, and optically void interspaces prove the non-existence of any-
thing which he does not see.

As much time is often lost by frequent repetition of these com-
petitive examinations (which, after all, lead to no better result than
that the observer finds or fancies that one lens jjerforms in his hands
more or less satisfactorily than some other lens), it seems worth
while to consider the value of a mode of testing which can be readily
applied whatever its value may be. A short and easy method of
testing an objective — not by comparison with others only, but by
itself and on its own merits — afibrds not only the most direct and
positive evidence of its qualities to those who are more concerned
in proving these instruments than using them, but also yields to
the genuine worker the satisfying conviction that his labour is
not frustrated by faulty construction and performance of his instru-
m.ent. It is, however, to be borne in mind that the microscoj)ist, in
any scrutiny of the quality of his lenses which he may attempt, has
no other object in view than to acquire such insight into the optical

384 OBJECTIVES, EYE-PIECES, THE APERTOMETER

conditions of good performance as will enable him to make the best
use of his instrument, and acquire confidence in his interpretation
of what he sees, as well as manipulative skill in examining micro-
scopical objects. To the constructor and expert of optical science
are left the severer investigations of optical effects and causes,
the difficulties of technical construction, the invention of new lens-
combinations, and the numerous methods of testing their labours
by delicate and exhaustive processes which require special aptitude
and lie entirely outside the sphere of the microscopist's usual
work.

Professor Abbe's mode of testing objectives is explained in his
^ Beitrage zur Theorie des Mikroskops.'

The process, in our judgment, requires large experience and
much skill to be of practical service ; but it is based on the following
principle : —

In any combination of lenses of which an objective is composed
the geometrical delineations of the image of any object will be more
or less comj)lete and accurate according as the pencils of light coming
from the object are more or less perfectly focussed on the conjugate
focal plane of the objective. On this depend fine definition and
exact distribution of light and shade. The accuracy of this focussing
function will be best ascertained by analysing the coui-se of isolated
pencils directed upon different jmrts or zones of the aperture, and
observing the union of the several images in the focal plane. Foi-
this purpose it is necessary to bring under view the collective action
of each part of the aperture, centi-al or peripheral, while at the same
time the image which each part singly and separately forms must be
distinguishable and capable of comj)arison with the other images.

1 . The illumination must therefore be so regulated that each zone
of the aperture shall be represented by an image formed in the upper
focal plane of the objective {i.e. close behind or above its back lens),
so that only one narrow track of light be allowed to pass for each
zone, the tracks representing the several zones being kept as far as
possible apart from each other.

Thus, supposing the working surface of the front lens of an
objective to be ^ inch in diameter, the image of the pencil of light
let in should not occupy a lai-ger space than -^^ inch. When
two pencils are employed one of these should fall so as to extend
fi-om the centre of the field to -j\^ inch outside of it, and the other
should fall on the opposite side of the axis in the outer periphery
of the field, leaving thus a space of -j'-,j inch clear between its own
inner margin and the centre of the field. The objective images of
the pencils occupy each a quartei- of the diameter of the whole field.

If three pencils of light be employed, the first should fall so as to
extend from the centre of the field to -^^ inch outside of it ; the
second should occupy a zone on the opposite side of it, between the
J5 and -jV inch (measured from the centre) ; and the third the
f)eripheral zone on the same side as the first in fig. 329.

This arrangement places the pencils of light in their most sensi-
tive position and exposes most vividly any existing defect in correc-
tion, since the course of the rays is such that the pencils meet in

ABBE'S MODE OF TESTING 385

the focal plane of the image at the widest possible angle. As many
distinct images will be perceived as there may be zones or portions of
the front face of the objective put in operation by separate pencils of
light. If the objective be perfect all these images should blend with
one setting of focus into a single cleai", colourless picture. Such a
fusion of images into one is, howevei-, pi-evented by faults of the
image-forming process, which (so far as they arise from spherical
-aberration) do not allow this coincidence of several images from
different parts of the field to take place at the
same time, and (so far as they arise from dis-
persion of colour) produce colouied fringes
on the edges bordering the dark and light
lines of the test-object and the edges of each
separate image, as also of the corresponding Fig. 329. Fig. sso

coincident images in other parts of the field.

It is to be borne in mind that the errors which ai-e apparent with
two or three such pencils of light must necessarily be multiplied
when the whole area of an objective of faulty construction is in
action. This would appear to us to be the strongest reason foi-
utilising the whole area, because what we are seeking is the defects
— the errors of the objective — and to make these as plain as possible
is a sine qua non. Dr. Abbe pi'oceeds, however, to consider —

2. The means by which such isolated pencils can be obtained.

As a special illuminating apparatus, the condenser of Professor
Abbe is recommended, or even a hemispherical lens. But we are
convinced that the illuminating apparatus should be as nearly aj^la-
natic as it can be. This is certainly not true of Abbe's chi-omatic
condenser or a hemispherical lens. The reason is obvious : the
spherical aberration wholly prevents the rays passing through the
holes in the diaphragm from being focussed on the object — the
silvered plate of lines — at the same time. In the loAver focal plane
of the illuminating lens must be fitted diaphragms (easily made of
blackened cai-dboard) pierced with two or three openings of such a.
size that the images, as formed by the objective, may occupy a,
fourth or sixth part of the diameter of the whole apeiture (i.e. of the
field seen when looking down the tube of the instrument, after re-
moving the ocular, upon the objective image). The required size
of these holes, which depends, first, on the focal length of the illumi-
nating lens, and, secondly, on the aperture of the objective, may be
thus foinid. A test-object being first sharply focussed, card dia-
phragms having holes of various sizes (two or three of the same size
in each card) must be tried until one size is found, the image of which
in the posterior foca,l plane of the objective shall be about a fouith
to a sixth part of the diameter of the field of the objective. Holes
having the dimensions thus experimentally found to give the requii-ed
size of image must then be pierced in a card, in such a position as
will produce images situate in the field, as shown by figs. 329 and 330 ;
the card is then fixed in its place below the condenser. We are,
however, strongly inclined to believe, partly from experiment, that
better results would be obtained by putting sections of annular slits
at the back of the objective. If the condenser be fitted so as to

c c

386 OBJECTIVES, EYE-PIECES, THE APERTOMETER

revolve round the axis of the instrument, and also carry with it
the ring or tube to which the card diaphi'agm is fixed, the pencils
of light admitted through the holes will, by simply turning the con-
denser round, sweep the face of the lens in as many zones as there
are holes. Supposing the condenser to be carried on a rotating
sub-stage, no additional arrangement is required besides the
diaphragm-carrier. Thus, for example, if a Collins condenser fitting'
in a rotating sub-stage be used, all that is required is to substitute
for the diaphragm which carries the stops and apertures as arranged
by the maker, a diaphragm pierced with, say, three openings of |-inch
diameter, in which circles of card may be dropped, the card being-
pierced with holes of different sizes according to the directions given
above. We doubt, however, if any sub-stage will revolve with
sufiicient accm^acy for so delicate a test.

Another plan adopted by Dr. Frij)p, and found very convenient
in practice, is to mount a condensing lens (Professor Abbe's in this
case) upon a short piece of tube, which fits in the rotating sub-stage.
On opposite sides of this tube, and at a distance from the lower lens
equal to the focal distance of the combinations, slits are cut out
through which a slip of stout cardboard can be passed across and

Fig, 331.

below the lens. In the cardboard, holes of various sizes, and at
various distances from each other, may be pierced according to
pleasure. By simply passing the slip through the tube, the pencils
of light admitted through the holes (which form images of these
holes in the upper focal plane of the objective) are made to traverse
the field of view, and by rotating the sub-stage the whole face of the
lens is swept, and thus searched in any direction required. But here,
again, the spherical aberration of an uncorrected condenser would,
with an objective of large aperture, cause the oblique pencils
iinder some conditions to pass under the object ; and alteration of
focus will not properly alter this — at least without a disturbance of
the focus of the objective.

When an instrument is not provided with a rotating sub-stage,
it is sufiicient to mount the condeuser on a piece of tubing, which
may slide in the setting always provided for the diaphragm on the
under side of the stage.

Card diaphragms for experiment may be placed upon the top of
a thin piece of tube (open at both ends) made to slide inside that
which carries the condenser, and removal^le at will. By rotating
this inner tube the pencils of light will be made to sweep round in

ABBE'S TEST-PLATE 387

the field, and thus permit each part of the central or pei'ipheral zones
to be brought into play. Against the accurate value of this, again,
the spherical aberration of an uncorrected condensei- would strongly
operate.

Abbe's Test-plate. — This test-plate is intended for the examina-
tion of objectives with reference to their corrections for spherical
and chromatic aberration, and for estimating the thickness of the
cover-glass for which the spherical aberration is best connected.

The test-plate consists of a series of cover-glasses, ranging in
thickness from 0'09 mm. to 0"24 mm., silvered on the under surface
and cemented side by side on a' slide, the
thickness of each being marked on the silver
film. Grou23S of parallel lines are cut through
the films, and these are so coarsely ruled tliat ^
they are easily resolved by the lowest powei s 2h«
yet from the extreme thinness of the sih ei ^
they also form a very delicate test for objecti\ e-- ^
of even the highest power and widest apertiue
The test-plate in its natural size is seen in fig.
331, and one of the circles enlarged is seen in Fig. S32.

fig. 332.

To examine an objective of large apei-ture, the discs must be
focussed in succession, observing in each case the quality of the
image in the centre of the field, and the variation produced by
using alternately central and very oblique illumination.

When the objective is perfectly corrected for spherical aberration
for the particular thickness of cover-glass under examination, the
outlines of the lines in the centre of the field will be perfectly
sharp by oblique illumination, and without .any nebulous doubling
or indistinctness of the minute irregularities of the edges. If, after
exactly adjusting the objective for oblique light, central illumination
is vised, no alteration of the focus should be necessaiy to show the
outlines with equal sharpness.

If an objective fulfils these conditions with any one of the discs
it is free from spherical aberration when used with cover-glasses of
that thickness. On the other hand, if every disc shows nebulous
doubling, or an indistinct ajDpearance of the edges of the lines with
oblique illumination, or if the objective requires a dift'erent focal ad-
justment to get equal sharpness with central as with oblique light,
then the spherical correction of the objective is more or less im-
perfect.

Nebulous doubling with oblique illumination indicates over-
correction of the marginal zone ; indistinctness of the edges without
marked nebulosity indicates under-correction of this zone ; an
alteration of the focus for oblique and central illumination (that is,
a difierence of plane between the image in the peripheral and central
poi'tions of the objective) points to an absence of concui'rent action
of the separate zones, which may be due to either an average under-
or over-correction, or to irregularity in the convergence of the rays.
The test of chromatic correction is based on the chaiacter of the
colour-bands which are visible by oblique illumination. With good

c c 2

388 OBJECTIVES, EYE-PIECES, THE APEETOMETER

correction the edges of the lines in the centre of the field should
show only narrow colour-bands in the complementary colours of the
secondaiy spectrum, namely, on one side yellow-green to apple-green,
and on the other, violet to rose. The more perfect the correction of
the sj)herical abei'ration, the clearer this colour-band appears.

To obtain obliquity of illuniination extending to the mai-ginal
zone of the objective, and a rapid interchange from oblique to
central light, Abbe's illuminating apparatus is manifestly defective
on account of its spherical aberration. We want at least his
achromatic condenser. For the examination of ordinary immersion
objectives, the apertures of which are, as a i-ule, greater than 180°
in arc (1"00 IST.A.), and those homogeneous immersion objectives
which considerably exceed this, it will be necessary to bring the
under surface of the test-plate into contact with the upper lens
of the illuminator by means of cedai- oil, even if water-immersion
objectives are used. We may add, as a matter of experience, that
having once centred the light and the condenser, we hold, with
deference to Dr. Abbe, that the light should on no account be
touched, which, to obtain obliquity, he advises by mirror changes.
We believe that this should be secured solely by the movement of the
diaphragm.

For the examination of objectives of smaller aperture (less than
40° to 50°), we may obtain all the necessar}^ data for the estimation
of the spherical and chromatic collections by placing the concave
mirror so far laterally that its edge is neai-ly in the line of the optic
axis, the incident cone of rays then only filling one-half of the aper-
ture of the objective, by which means the sharpness of the outlines
and the charactei- of the colour-bands can be easily estimated.

It is of fundamental importance, in employing the test-plate, to
have brilliant illumination and to use an eye-piece of high power.
With oblique illumination the light must always be thrown perpen-
dicularly to the direction of the lines.

When from practice the eye has learnt to recognise the finer
differences in the quality of the outlines of the image, this method
of investigation gives very trustworthy results. Differences in the
thickness of cover-glasses of 0"01 or 0"02 mm. can be recognised with
■objectives of 2 or 3 mm. focus. The quality of the image outside
the axis is not dependent on spherical and chromatic correction in
the strict sense of the tei*m.

Indistinctness of the outlines towards the borders of the field of
'view arises, as a rule, from unequal magnification of the different
zones of the objective ; colour-bands in the peripheral portion (with
good colour-correction in the middle) are always caused by unequal
magnification of the different coloured images. Imperfections of this
kind, improperly called ' curvature of the field,' are shown to a
greater or less extent in the best objectives, when their aperture is
considerable.

Testing an objective does not mean seeing the most delicate
points in an object ; it rather menus the manner in which an object
•of some size is defined.

A test for low powers up to | of 80° or N.A. "65 is an object on

OBJECTS FOR LENS-TESTING — APERTOMETEE 389

a dark ground. ISTothing is so sensitive. For the lowest jDowers
one of the smaller and more delicate of the Polycistince, because it
takes light well, is good. For medium j)owers a coarse diatom, a
2\'iceratium fimbriaticm, is excellent ; for unless an objective is well
corrected the image will be fringed and surrounded with scattered
light, and the abei'ration pi'oduced by the cover-glass is plainly
manifest, and by accurate connection can be done away.

Error of centring is one of the special defects of objectives
which the Abbe method of testing does not cover. But if we place
a sensitive object in a certain direction, and when the best adjust-
ments have given the best image, rotate that object through an angle
of 90°, only a well-centred objective will give an unaltered image
throughout. If not well centred it will at certain parts grow
fainter or sharper. The most useful image for this purpose with
medium powei's is a hair of Polyxenus lagurus mounted in balsam
(frontispiece, fig. 6).

For higher powers nothing surpasses a podui'a scale. In this
particular it has always been of great value to opticians. It should
be strongly marked, and must be in optical contact with the cover -
glass ; this may be tested by means of an oil-immersion and the
' vertical illuminator.'

The objectives of widest aperture are now more easily tested,
because homogeneous condensers with much wider aplanatic areas
are now, as we have seen, made by the leading English and
Continental opticians ; and there is little doubt but that there is a
considerable future before homogeneous condensers. The best that
can be done is to take a diatom, such as a Coscinodiscus, in balsam
with strong 'secondaries' (Plate I. figs. 3 and 4), with the largest
aplanatic cone that can be obtained, which at present can be best
accomplished with a semi-apochromatic oil-immersion condenser of
1"3 N.A. It must be a good objective indeed that does not show
signs of breaking down under this strain. An illuminating cone
of N.A. I'O is probably just below the point of overstrain with the
best lenses at present at our disposal.

Testing lenses therefore resolves itself into the following methods,
viz. : —

1 . For low and medium powers : dark ground with a Polycistina
or a diatom, according to the power.

2. Centring for medium powers (an ordeal not needful for very
low powers) should be by means of a hair of Polyxemis lagurus, em-
ploying a I illuminating cone.

3. Centring for high powers : by means of podura scale.

4. Definition : with wide-angled oil immersions, Coscinodiscus
asteromphalus with wide-angled cone obtaining sharp, brilliant,
and clear view of ' secondaries,' or coarse specimen of Navictda
rho7)iboicles, which may be mounted in a dense medium. In
testing a lens it does not so much matter what the object is, because
the real test lies in the ability of the lens to stand a large direct
axial cone. A lens of very great excellence will stand a ^ths cone,
an excellent lens a |ths cone, an indifferent lens only a \ cone, while
a bad lens will not even admit the use of that. A dark s'round is a

590

OBJECTIVES, EYE-PIECES, THE APEETOMETER

vexy severe test, as it is of the nature of a full cone, so to speak, and
only the lower powers will stand it. If a dark ground is required
with the higher objectives it can be obtained by using an oil-immer-
sion condenser, l^ut the apertui-e of the objective will have to be
reduced by a stop.

The apertometer, as its name implies, is an instrument for mea-
suring the aperture of a microsco-pic objective. As correct ideas of

apei'tvu^e have only obtained dur-
ing the past few years, it may be
inferred that apertometers con-
structed before the definition of
aperture was given and accepted
were crude and practically use-
less.

The conti'oversy on the ' apei--
tui'e question,' which was in full
operation some eighteen years
since, is not an altogether satis-
factory page in the history of
the raodern microscope, and for
many reasons it is well to pass
it unobservantly by. It will
suflice to state that dirring its
jjrogress an ajDertometer was de-
vised by R. B. Tolles, of America,
which acciu^ately measured the
true apertru^e of an objective.
About the same time Professor
Abbe gave his attention to the
subject, and with the result, as
we have seen, that he has given
a definite and permanent meaning
to numerical aperture, making
it, as we have seen, the equiva-
lent of the mathematical expres-
sion n sine ^t, n being the refrac-
tive index of the medium, and ii
half the angle of aperture.^

The application of this foi--
mula to, and its general bearing
upon, the difiraction theory of
microscopic vision has been given
in its proper place ; but as the
aim of this manual is thoroughly
practical, we shall be pardoned for even a small measui-e of repeti-
tion in endeavouring to exj)lain the use of this formula in such a
manner that only a knowledge of simjole arithmetic will be requii-ed

^ A knowledge of the meaning of the trigonometrical expression ' sine ' is not
necessary in solving any of the following questions. As the values are all found in
tables, it is only necessary to caution those who are unacquainted with the use of
mathematical tables to see that they have the ' natural sine ' and not the ' log sine.'

Fig.

SIMPLE ILLUSTRATIONS OF THE USE OF N SINE U 391

to enable the student to work out any of the problems which are
likely to arise in his practical work.

We can best accomplish this by illustration.

(i) If a certain dry objective has an angular aperture of 60°, what
is its N.A. {i.e. numerical aperture) ?

All that is needful is to find the value of n sine ic ; in this case
n^=the refractive index of the medium, which is air, is 1 ; and u,
which is half of 60°:= 30° opposite 30° in a table of natural sines,' is
•5 ; sine tt, therefore ="5, which multiplied by 1 gives "5 as the
N.A. of a dry objective having 60° of angular aperture.

(ii) What is the jST.A. of a water-immersion whose angulai-
aperture=44°?

01 here=r33, the refractive index of water ; and u, or half 44°,
is 22°. Sine 22° from tables=-375, which midtiplied by l-33=:-5
(neai-ly), which is the N.A. required.

(iii) What is the N.A. of an oil-immersion objective having 38^°
of angular apei'ture ?

n the refractive index of oil, which is equal to that of crown
glass, is 1"52; ?6=19j and sine to from table= "329, which multi-
plied by l-52=-5.

Thus it is seen that a dry objective of 60°, a water-immersion of
44°, and an oil-immersion of 38^° all have the same N.A. of "5.

It will be well, perhaps, to give the converse of this method.

(iv) If a dry objective is '5 N.A., what is its angular aper-
ture 1

■5

Here because oi sine ■i6="5j sine «=--; the objective being

dry, n-=l, therefore sine u=-5. Opposite "5 in the table of natural

sines is 30° ; hence u=30°. But as u is half the angular ajoerture

of the objective, 2u or 60°=the angular aperture required.

(v) What is the angular aperture of a water-immersion objective

whose N.A.=-5?

•5 -5
Here ■}i=l"33, n sine 26='5 ; sine it= — =:r-^r;:^ = -376 ;

^t^22° (nearly) from tables of sines ; .". 2?6=44°, the angle re-

quii-ed.

(vi) What is the angular ajDerture of an oil-immei'sion objective

of -5 N.A. ?

•5-5
Here n-=l-52, n sine zi=-5 ; sine 26=— =,— ^7r=-329 ;
' ' n 1'52 '

u-=\'d\° (by tables of sines) ; and 2m=38^, the angle required.

We may yet further by a simple illustration explain the use of
n sine ti.

In the accompanying diagram, fig. 333, let n' represent a vessel
of glass ; let the line A be perpendicular to the surface of the water
C D ; suppose now that a pencil of light impinges on the surface of
the water at the point where the pei-pendicular meets it, making an
angle of 30° with the perpendicular. This pencil in penetrating the
water will be refracted or bent towards the perpendicular. The

^ Vide Appendix A to this volume.

392 OBJECTIVES. EYE-PIECES, THE APERTOMETEIx

problem is to find the angle this pencil of light will make with the
pei'pendicular in the water.

To do this we must remember that n sine 'u on the air side is
equal to n' sine t(f on the water side. Thus on the air side ■re^l,
«=30°, and by the tables of sines sine 30° = '5 ; consequently on
the air side we have n sine m='5.

On the water side n'-=.\'?>Z, and u' is to be found. But as

n' sine u' := n sine •?«., therefore sine u' = -^ = = -376 ;

n 1-33

which (as the tables show) is the natural sine of an angle of 22°

(nearly) ; consequently ^i':=22° ; so the pencil of light in passing

out of air into water has been bent 8° from its original direction.

Conversely a pencil in water, making an angle of 22° with the

23erpendicular, would on emerging from the water be bent in air 8°

further away from the perpendicular, and so make an angle of 30°

with it.

Now if we suppose that these pencils of light revolve rottnd the
ferpendicular , cones v:oidd he described, and we can readily see that
a solid cone of 60° in air is the exact equivalent of a solid cone of
44° in water.

If we further suppose that the water in the vessel is replaced by
cedar oil, the pencil in air, remaining the same as before, will, when
it enters the oil, be bent more than it was in the water, because the
oil has a higher refractive index than water ; n in this case is equal
to 1-52.

The exact position of the pencil can be determined in the
same manner as in the previous case. On the air side, as before,
n sine m="5 ; on the oil side n' sine ti' ^=.n sine u\ sine ^<.'=

7 =Y^^ ="329, which (by the tables) is the natural sine of

19J°. It follows that the pencil has been bent in the cedar oil 10|;°
ovit of its original course, and a cone of 60° in air becomes a cone of
38i° in cedar oil or crown glass.

Finally, it is instructive to note the result when an incident pencil
in air m.akes an angle of 90° with the joerpendicular : n sine u becomes
unity, and %h in water 48|°, in oil 41° (nearly) ; consequently a cone
of either 97^° in water, or 82J° in oil or ci-own glass, is the exact
eqtiivalent of the uihole hemispherical radiant in air. In other words,
and to vary the mode in which this great truth has been before
stated, the theoretical maximum aperture for a dry lens is equiva-
lent to a water-immei"sion of 97^° and an oil-immersion of 82|-
angular aperture.

The last problem that need occupy us is to find the angular
aperture of an oil-immersion which shall be equivalent to a water-
immersion of 180° angular aperture.

On the water side n = 1'33, u = 90°, sine 90° = 1, n sine u=

1"33. On the oil side n' = 152 and u' has to be found.

... '' ■ J.1 i- . , n sine u 1'33

As n' sine w = n sine u, therefore sine n' := =

n' 1-52

= -875; u' = 61° (nearly) by the tables; 2u' = 122° (nearly),

the angle required.

THE APERTOMETER 393

It thus appears (1) that dry and immersion objectives having
diflerent angular apertures, if of the same equivalent aperture, are
designated by the same term. Thus objectives of 60° in air, or 44°
in water, or 38^° in oil, have identically the same aperture, and are
known by the same designation of "5 IST.A.

(2) The penetrating power of any objective is proportional to

■^ . ', and its illuminating power to (N.A.)^. Therefore, if we

double the IST.A. we halve the penetrating power, and increase the
illuminating power four times.

In comparing the penetrating and illuminating powers of objec-
tives, however, care must be taken to avoid a popular error, by
making them between objectives of different foci.

It cannot, for example, be said that a |-inch objective of "8 N.A.
has half the penetrating power of a ^-inch of '4 N.A. Neither can
it be said that it has four times the illuminating power. What is
meant is that a ^-inch of "8 N.A. has half the penetrating and four
times the illuminating power of a |-inch objective of '4 N.A.

But because penetrating and illuminating powers diminish as
the square of the foci, a ^-inch objective of '6 ST. A. has four times
the illuminating and nearly four times the penetrating power of a
^-inch of "6 N.A. ; but these conditions only hold when a full
illuminating cone is employed, in other words, when the back lens
of the objective, as seen when the eye-piece is removed, is full of
light. Thus if a small cone of illumination is used with the ^-inch
objective of '6 N.A., its illuminating power would be much
diminished, while its penetrating power would be much increased.

The old nomenclature, in use before numerical aperture was so
happily introduced, did not of course admit of comparisons of pene-
trating and illuminating powers by inspection ; which, however, is
a manifest advantage, contributing to accuracy and precision in
important directions.

(3) It may be well, for the sake of completeness, to repeat ^ here
that the resolving power of an objective is directly proportional to
its numerical aperture. If we double the N.A. we also double the
resolving power ; and this not simply with objectives of the same
foci, as in the case of penetrating and illuminating powers. Thu.s it
is not only true that a ^-inch objectiA'e of "6 N.A. resolves twice as
many lines to the inch as a ^-inch of "3 N.A., but so also does a
^-inch of 1'4 IST.A. resolve twice, and only twice, as many as a J-inch
of -7 N.A.

Within certain limits, then, the advantage lies with long foci of
wide angle, because we thus secure the greatest resolving power
with the greatest penetrating and illuminating powers.

From what has here been shown, then, it becomes evident that
the employment of the microscope as an instrument of precision is
largely due to Abbe's work, and that the introduction of numei-ical
aperture, with its strictly accurate meaning, has been a practical
gain of untold value. But this has been greatly enriched by his
having introduced a thoroughly simple and useful apertometer. This

1 Chapter I.

394

OBJECTIVES, EYE-PIECES, THE APEKTOMETEE

involves the same principle as that of Tolles, but it is carried out in
a simpler manner.

Abbe's instrument is presented in fig. 334. It will be seen that
it consists of a flat cylinder of glass, about three inches in diameter
and half an inch thick, with a large chord cut off so that the portion
left is somewhat more than a semicircle ; the part where the segment
is cut is bevelled from above downwards to an angle of 45°, and it
will be seen that there is a small disc with an aperture in it denoting
the centre of the semicircle. This instrument is used as follows : —

The microscope is placed in a vei-tical position, and the aperto-
meter is placed upon the stage with its circular part to the front
and the chord to the back. Diffused light, either from sun or lamp,
is assumed to be in front and on both sides. Suppose the lens to
be measured is a dry ;^-inch ; then with a 1-inch eye-piece having a
large field, the centre disc with its aperture on the apertometer is
brought into focus. The eye-piece and the draw-tube are now
removed, leaving the focal arrangement undisturbed, and a lens

.1.G15 . ;' - -\?C
A:p e rt om et e n -6.AP ^ ■

Pig. 334. — Abbe's apertometer.

supplied with the apertometer is screwed into the end of the draw-
tube. This lens with the eye-piece in the draw-tube forms a
low-power compound microscope. This is now inserted into the body-
tube, and the back lens of the objective whose aperture we desire
to measure is brought into focus. In the image of the back lens
will be seen stretched across, as it were, the image of the circular
part of the apertometer. It will appear as a bright band, because
the light which enters normally at the surface is reflected by the
bevelled part of the chord in a vertical direction, so that in reality
a fan of 180° in air is formed. There are two sliding screens seen
on either side of the figure of the apertometer ; they slide on the
vertical circular portion of the instrument. The images of these
sci'eens can be seen in the image of the bright band. These screens
should now he tnoved so that their edges just touch the lyeriphery of
the hack lens. They act, as it were, as a diaphragm to cut the fan
and reduce it, so that its angle just equals the aperture of the objec-
tive and no more.

This angle is now determined by the arc of glass between the

THE USE OF THE APEETOMETEE 395

screens ; thus we get an angle in glass the exact equivalent of the
aperture of the objective. As the numerical apei'tui-es of these arcs
ai-e engraved on the apei'tometei- they can be read off by insjjection.
Nevertheless a difficulty is expeiieuced, from the fact that it is not
easy to determine the exact point at which the edge of the screen
touches the periphery of the back lens, or, as we j)i'efer to designa,te
it, the limit of wperture^ for, cui-ious as this expi'ession may appear,
we have found at times that the back lens of an objective is larger
than the aperture of the objective requires. In that case the edges
of the screen refuse to touch the periphery.

On the whole we have found that a far better Avay of emj^loying
this instrument is to use it in connection toith a graduated rotary
stage, the edge of the flame of a paraffin lamjD being the illumi-
nator.

Thus : Set the lamp in a direction at right angles to the chord
of the apertometer, and suppose that the index of the stage is at 0°.
The edge of the flame will be seen in the centre of the bright band.
The sliding screens being dispensed with, rotation of the stage will
cause the image of the flame to travel towards the edge of the
aperture ; rotation is continued until the image of the flame is half
extinguished hy the edge of the aperture, the arc is then read, and
the same thing is repeated on the othei' side, and the mean of the
readings is taken.

If the stage rotates truly, and if the instrument is properly set
up, the reading on the one side ought to be identical \\T.th that on
the other.

Suppose that the sum of the readings on both sides = 60°, the
mean reading is consequently 30°, which is the semi-angle of aperture
of the lens in glass. From this datum we have to determine the N. A.
of the dry J-inch as well as its angular aperture in air.^

(i) As before, N.A. = n sine u,, and n sine u = n' sine u' ;
which means that the aperture on the air side is equal to the aperture
on the glass side ; 72- = 1 for air ; li' = 1*615, the refractive index of
the apertometer ; u,' is the mean angle measm-ed, which in this case
is 30° ; and n sine m has to be found.

ISTow sine 30° = '5 (by the tables) ; «' sine u' = 1-615 x sine 30°
= 1"615 X '5 = '8 = '?^ sine u = the N.A. required.

(ii) Again, to find the angular aperture or lit. As before, n sine u

, . , -, . n' &i\\e u' 1-615 x '5 o eroo

= n snie u and sine ^( = = = -8 ; u, = 5o

n 1

nearly (by the tables) ; 2u = 106°, which is the angle required.

(iii) If it be a toater-im7nerswH we have to deal with, supj)0se
the mean angle = 45° = ttf ; sine 45° := -707 (bv the tables) ; n
= 1-33; -Ai-idn' = 1-615.

n sine it = n' sine vf = 1-615 x '707 = 1-14, the N.A. required.

r \ \ ■ • n' sine It' 1-615 x -707 o^ r-mo

(iv) Again, sine u = := = -86 : u = 594-°

^ ^ ^ ' n 1-33 ' ^

(by the tables) ; and 2t(, = 118^°, the angle required.

(v) In the case of an oil-immersion, suppose the mean angle

1 Vide p. 2 et seq.

396 OBJECTIVES, EYE-PIECES, THE APERTOMETER

= 60° = u ; sine 60° = -866 (by the tables) ; n = 1-52 ; n' = 1-615 ;

n sine u ^=n' sine u' ^ 1"615 x '866 = 1*4, which is the N.A.

required.

, .s . . . n' sine u' 1-615 x -866 ^.o

(vi) As-am, sine u = = = -92.

^ ' ^ ' n 1-52

%i = 67° (by the tables), 2m = 134°, the angle required.

It is manifest that if the refractive index of the apertometer
equals that of the oil of cedar, the mean angle measured is the semi-
angle of aperture of the objective, and its sine multiplied by that
refractive index is the numerical aperture.

This will be found the more accvirate and universally applicable
method of measuring the apertures of objectives, as the extinction
of the light shows precisely when the limit of aperture is reached.

Powell and Lealand's stands lend themselves admirably for use
with the apertometer. The body being removable, the lens can be
placed in the upper part of the nose-piece, and any measurement
can be accurately made. We would advise every microscopist to
master the use of this admirable instrument, and to demonstrate for
himself the aperture capacity of his lenses, that he may know with
precision their true resolving powers. It will facilitate this that
Mr. Nelson has shown (' Journ. R.M.S.' 1896, p. 592) that the use of
the internal lens is not required ; the point of rotation of the stage
when the edge of the flame is eclipsed by the limiting aperture of
the objective can be readily observed by means of a low-power eye-
piece.

When the apparatus is accurately set up in the manner described
above, the exact point is indicated by the dark segments coming
across the field of the eye-piece. One dark segment will be found to
advance slowly from one side, and then when the precise point of
rotation of the stage is reached the other dark segment will come in
from the other side and meet it. For this pm^pose the glass disc
with- its refractive index only engraved upon it is alone required.
Messrs. Zeiss supply this at a much lower cost (25s.) than the
engraved disc and the supplementary lens.

Boucher's circular slide rule is a convenient adjunct to the
apertometer, for the IST.A. can be read off by inspection without the
necessity of looking out sines or making calculations.

397

CHAPTER YI

PBACTICAL MICROSCOPY : MANIPULATION AND PBESEBVATION
OF THE MICBOSCOPE

Without attempting to occujdj space with a discussion of the ques-
tion of the right of ' microscopy ' to be considered a science, we may
venture to affirm that it will be but a I'ecognition of practical facts
if we claim as a definition of microscopi/ that it expresses, and is in-
tended to cai-ry with it, all that belongs to the science and art of the
microscope as a scientific instrument, having regard equally to its
theoretical principles and its practical working. Hence ' practical
microscopy ' will mean a discourse on, or discussion of, the methods
of employing the microscope and all its simplest and more complex
appliances in the most j)erfect manner, based alike and equally upon
theoretical knowledge and practical experience.

On this condition a ' microscoj)ist ' means (or at least implies)
one who, understanding 'microscopy,' applies his theoretical and
practical knowledge eithei- to the further improvement and jDerfec-
tion of the instrument, or to such branches of scientific research as
he may profitably employ his ' microscopy ' in prosecuting. He is,
in fact, a man employing specialised theoretical knowledge and
practical skill to a particular scientific end.

But a ' microsco'pical society ' has a noble raison cVetre, because
it is established, on the one hand, to j)romote — without consideration
of nationality or origin — improvements in the theory and practical
construction of both the optical and mechanical parts of the micro-
scope, and to endeavour to widen its application as a scientific
instrument to every department of human knowledge, recording, in-
vestigating, and discussing every refinement and extension of its
application to every department of science, whether old or new.

In this sense no more practical definition of a ' microscopical
society ' can be given than is contained in the invaluable pages of
the ' Journal of the Royal Microscopical Society ' from the end of
1880 to the present day ; and no better justification for the existence
of such a society can be needed than is aflfovded by the work done
directly or indirectly by it, in inciting to and promoting the theo-
retical and practical progression of the instrument and its ever-
widening applications to the expanding areas of natural knowledge.

In this chapter we propose to discuss the best practical methods
of using the instrument and its appliances, the theory concerning
which has already been discussed, while the mode of applying this

398 MANIPULATION AND PEESERVATION OF THE MICEOSCOPE

knowledge to biological and other investigations is entered upon in
the subsequent chapters of the book.

To begin his work with success — if his object be genuine work —
the student must be jDrovided with some room, or j)ortion of a room,
which he can hold sacred to his jDurpose. Unless special investiga-
tions are undertaken, it is not a large area that is required, but a
space commanding, if possible, a north aspect, and which can be
arranged to readily exclude the daylight and command complete
darkness.

The first requirement wall be a suitable table.

This should be thoroughly Jirm, and it should be rectangular in
shape. A round table, if small especially, is most undesirable, as it
offers no sujjport for the arms on either side of the instrument ; and
with pi'olonged work this is not only a serious, but an absolutely
fatal defect.

In a rectangular table the centre may be kept clear for micro-
scopical work, while there are two corners at the back, one on the
left and the other on the right hand. The former may be used for
the locked case or glass shade for protecting the instrument when
not in use ; and when it is in use, it in no way interferes with the
usefulness of the table. In the same way the right-hand corner may
be used for the cabinet of objects which is being worked, or the
apparatus needful for use.

The most important part of the table — that is, the middle, from
front to back — should be kept quite clear for the purposes of mani-
pulation, and a sufficient space should be kept clear on either side of
the instrument for resting the arms, and no loose pieces of apparatus
should ever be deposited within those spaces. This soon becomes a
habit in practice, for experience teaches — sometimes painfully, by the
unwitting destruction of a more or less valuable appliance.

The spaces to the right, beyond that left for the arm of the
operator, may be used for the work immediately in hand — especially
for a second and simpler microscope. An instrument with only a
coarse adjustment and a 1-inch or a |-inch objective will suffice, or
a good dissecting-stand will answer every purpose. Those who do
much practical work will find such a plan more rapid and more
efficient than the cumbrous method of a I'otaiy nose-piece, especially
where critical woi-k has to be done.

When work is being done in a darkened room there shovild be
on the extreme right a small lamp with a paper shade. (Special
shades for this purpose can be obtained from Baker, of Holborn.)
This light may be kept low or used for general illumination when
required ; it is never obtrusive, and always at hand.

A similar space on the left hand should be reserved for a small
round stand fitted with a flat cylindrical glass shade with a knob on
the top. The stand should be sidtably arranged to hold two eye-
pieces, three objectives, one condenser, a bottle of cedar-oil (fitted
with a suitable pointed dipper), and a box containing the condenser-
stops. This is a most useful arrangement for such a table ; and it
need not have a diameter greater than nine inches.

Jlie size for the top of such a table should be 4^ x 3 feet, and as

MICROSCOPISTS' WORK-TABLES

399

no Avork, such as mounting or dissecting, may be supposed to be done
at this table, it is well to cover the surface with morocco, that
being very pleasant and suitable to work upon.

It should be remembered that for a full-sized microscope a depth
of three feet is required for comfortable work When the micro-
scope is set up for drawing,^ the lamp being used direct, 2 ft. 5 in.
is the narrowest limit in which this can be accomplished.

Another point of much importance is the height of the table.
Ordinary tables, being about 2 ft. 4 in. high, are too low even
for lai'ge microscopes. Two or three inches higher than this will be
found to greatly facilitate all the work to be done. It is best to
have the table made completely, on thoroughly solid square legs, to
the height of 2 ft. 7 in. ; but we may employ the glass blocks
employed underneath piano feet as an expedient. It is further im-
portant to have the table quite open underneath, a nd not with nests
of draAvers on either

/

r^

'5=

=4

\±y

^ y

.9

side, because Avith this
particular table it Avill
be frequently required
that two persons may
sit side by side, which
is only possible with a
clear space beneath.

The accompanying
illustration (fig. 335),
with the appended re-
ferences, will make quite
clear the character- of
the table which we re-
commend, as well as the
mode of using it.

The table aboA'e de-
scribed is sujDposed to
be employed wholly for
genei-al purposes of ob-
servation or research on aa holly or partially mounted objects. Bvit
the mici'oscopist who aims at more than this Avill I'equire an arrange-
ment for dissecting, mounting, and arranging histological and other
preparations, and in some cases a special table for general piirposes
of microscopical biology. These are certainly not essentials, especi-
ally if the work done is a mere occasional occupation ; but Avhei'e
anything like continuity or periodical regularity of occupation Avith
such Avork is intended, these will be of great serAdce.

A dissecting and mounting table is indeed of inestimable value to
those Avho afi'ect complete order and cleanliness in the accomplishment
of such work.

We have found in practice that a table firmly made, with a height
of 2 ft. 6 in., semicircular in form, and a little more than half the
circle in area on the outside, with the arc of another circle cut out
from it to receive the person sitting at work — much after the fashion

1 Chap. IV. p, 287.

Fig. 335. — Microscopist's table.
(Scale, h inch to 1 foot.)

Case for microscope ; 2. Cabinet for objects ;
8. Microscope lamp ; 4. Lamp with shade ;
5. Stand of apparatus ; 6. Book ; 7. Large micro-
scope ; 8. Second microscope; 9. Writing pad;
10. Bull's-eye stand ; 11. Light-modifier.

400 MANIPULATION AND PEESERVATION OF THE MICEOSCOPE

of the jeweller's bench — serves admirably. A rough suggestion of
this is given in fig. 336, which presents the plan of the top of the
table. The whole area beneath should be unoccupied, but at A and
B drawers may be put, not extending moi-e than four inches below
the under surface of the top of the table ; on the side B a couple
of shallow drawers, with everything required in the form of scalpels,
needles, scissors, forcejys, pipettes, life-slides, &c. in the upper one,
and pliers, cutting p)liers, small shears, files of various coarsenesses
and finenesses, &c. in the other ; on the A side a single drawer con-
taining slips, covers of various thicknesses, hone, tin, glass, and other
cells of all (assorted) sizes, loatch-glasses, staining ciips or slabs,
lifters (if used), sato with fine teeth, hones of various shapes, pewter
plate for grinding and polishing glass, &c., platinum capsxde, cam^era
lucida, three ' No. 2 ' sable brushes ( water- coloiu-), &c.

In this way all that is needed for dissection or mounting will be
within reach without moving from the chair ; and if by an arrange-

FiG. 336. — Dissecting and mounting table.

ment which most moderately ingenious manipulators could accom-
plish, each of the articles in the drawers has a fixed place, there will
be no difficulty in finding by touch what is wanted.

The table top may be of pitch pine stained black, or, still better,
some very hard wood finished smoothly, but ' grey.'

The space in the figure immediately in front of the operatoi-
may be cut out to a convenient size and thickness. A thick plate -
glass slab whose edges on the right and left sides shall be slightly
bevelled, so that it may slide fii-mly into a prepared space cut into
the surface of the table, should occupy this space, the surface being
exactly level with the surface of the table. This plate of glass should
be made black on its under side, so as to present a tmiform black
surface. This is often of great value in certain kinds of work.
Equally useful is a piirely ivhite unabsoi-bent surface, and a slab of
lohite jjorcelain may be easily obtained of the same size and be made
to fit exactly into the same place.

APPLIANCES FOE DISSECTING TABLE 40 1

In using this table for dissection the arms ha^-e comjilete rest,
and 1 in the figure would represent the j^osition of the dissecting
microscope.

2 is a suitable position for a small easily managed microtome
for general (chiefly botanical) purposes. We find that of Ryder ^ to
:answei' this pui'pose admirably.

3 is a small vessel of spirit (dilute) for use with the section
knife.

4 is a stand of mounting media, in suitable bottles, as Canada
balsam in paraflin, or xylol, glycerine, &c., as well as small bottles
■of reagents for botanical or zoological histology, &c.

5 is a nest of apertures in which to place partly mounted objects,
to protect them from dust, while the balsam, dammar, &c. may be
hardening on the cover so as to be in a suitable state for final mount-
ing. A slide may go over the sloping front of this and wholly ex-
clude dust.

6 is a stand of cements, varnishes, &c., such as are needful ; and

7 is a turn-table.

For the work of dissection, when the subject requires reflected
light, one of the desiderata is a mode of illumination at once con-
venient and intense. Mr. Frank E. Cheshire, F.L.S., &c., whose work
on ' Bees and Bee-keeping ' is a proof of knowledge and practice of
minute anatomy, adopts an
old plan which we have
always found admirable. It
is illustrated in fig. 337.
Rays of light from a lamp
are parallelised by a bull's-

eye full upon an Abraham's p,^^ 337.-Mode of illumination for

prism and focussed upon the dissection.

object. The prism may be

mounted on a long many -jointed arm, and is of most varied useful-
ness. A Stephenson's binocular is, we believe, einployed by this
gentleman, but it will serve admirably for any form of dissecting
instrument.

For the more general purpose of the private laboratory a jdain,
firm table 4 feet 6 inches X 3 feet in area, of a suitable height for
the worker, should be fitted as follows, viz. : if fig. 338 represent the
rough plan of the table, 1 and 2 are gas fittings attached to the main
to supply blovjpipe, Bunsen^s burner, &c.

4 is a small tube of metal attached to the water main, with a
tap, and bent in the form of an inverted f|, with the attached leg of
the fi the longer. This affords a pleasant stream of water for wash-
ing dissections, &c. ; and if the open end be made with a screio, aiid
have a suitably made piece of tubing fitted to screw on to it, this latter
may be attached to an indiarubber tube, at the other end of luhich v)e
may fasten fine glass 7iozzles, which will act as loash bottles of the

Jinest bore, and serve with the finest dissecting work.

5 is a glass trough for waste, with a perforated aperture, 6, con-

1 Journ. B.M.S. new series, 1887, p. 682.

D D

402 MANIPULATION AND PEESERVATION OF THE MICROSCOPE

Fig. 338. — Laboratory table for microscopical work.

nected with a waste-pipe, through which the waste water, &c. flows
innocuously away.

3 represents the position of a Thoma microtome, and A, B are two
well-framed flat slides, which may be drawn out eighteen inches, or

pushed fully in. They
are found at times to
be of great service,
where the space is some-
what confined.

This table may be
fitted on one side (the
left) at least with a set
of di-awers and shelves
for receiving various ap-
]Daratus and materials,
with lai'ger quantities of
stains and reagents,
hardening, macerating,
and other materials ;:
while if a door covers
the whole, the inner side
of this may be readily
■fitted to receive drop-
bottles ^ containing all the stains, reagents, and similar materials in
constant use. If these be labelled with joaper labels saturated in a

solution of solid paraf-
fin in turpentine, and
after the turpentine
has evaporated firmly
fixed on the bottle,
they are very perma-
nent, and, indeed,
better than anything
we have tried save
where the name of
the contents is en-
amelled or engraved
on the bottle.

It has been al-
ready pointed out
that there are condi-
tions of research in
which the microscope
has to be in a con-
stantly vertical position. This was the case with the researches on
the saprophytic organisms made conjointly by the present Editor
and Dr. J. J. Drysdale.^ It must always be the case where certain
forms of continuous life stages are employed foi- prolonged or con-

FiG. 339.-

-Trixwd for using microscope in an
upright jposition.

1 Chapter VIL

- Monthly Micro. Journ. vols. x. to xviii. ; Journ. B.M.S. vol. iii.
series ii. p. 177 ; vol. vi. p. WS; vol. vii. p. 185 ; toI. viii. p. 177.

1; vol.

SPECIAL aSE OF MICROSCOPE— UPRIGHT

403

tinuous observations on tlie development of the minutei forms of
life.

In such cases the table is quite unsuitable, and special stands
have to be employed that from theii- foi'm give great stability to the
microscope, and afford the body and head of the observei- as much
command and ease in using the insti-ument in this awkward position
as can be obtained.

This is best done by means of a fii'mly made tripod, with a V-
shaped piece at the top made to receive the feet of the microscope.
Fie-. 339 is an outline of the construction. The three legs of the

Fig. 340. — Using the microscope in an upright position for special investigations
necessitating its use in this position.

tripod are well made and firmly braced together Avith metal rods.
A, A is the bed for the ti-ipod feet of Powell and Lealand's large
stand. B is a table which slides to the level of A, A, or down to its
present position. This is mainly to receive the lamjj.

By this ai'rangement the body can so place itself as to command
the instrument fully, and there is an arrangement at the two sides,
A, A, to receive sujDports on which the arms may rest when any
other manipulation than that involved in working the fine adjust-
ment and the milled heads of the stage is I'ecjuii'ed. The manner of

D D 2

404 MANIPULATION _j^XD PRESERVATION OF THE MICROSCOPE

using this ari'angement is seen in fig. 340. In that case, however,
the whole is employed for the making of a camei-a lucida (b-&wing
with a ^V'ii^*^!^ objective ; it is not a desirable position for general
work, but was absolutely needful for the kind of investigation being
pursued ; and the position of the basal tripod, the microscope uj)on
it, the position of the lamp (partly seen in the immediate fore-
ground to the left), and the I'elative ease with which the entire
instrument is at the command of the observer, will be manifest.
In ordei- to use the microscope successfully, we must have an

illumination the inten-
sity of which we can
fully rely on. Daylight
has certain qualities that
involve advantages at
times, and under special
circumstances, in its em-
ployment, hilt this is the
exception rather than the
rule. What is needed
is a well-made lamjD w^th
a flat flame ; this we
should be able to control
with great ease as to
height and distance from
the microscope. No-
thing is eqiial practically
to a ^-inch or a 1-inch
23araifin lamp ; this gives
the whitest light artifi-
cially accessible save the
higher intensities of the
incandescent electric
light. But there is no-
thing of this kind at
present accessible to the
student. The employ-
ment of the edge of the
flame of a well-made
parafiin lamp used with
good ' oil ' has no present,
rival. Its illuminating
power should be about
2^ candles. Gas is much yellower, and not so easy in employment.
To get the best form of microscopical lamp is a matter of some
importance. We call the attention of the reader to the best simple
form of lamp which will accomplish every purj)ose. This is a model
arranged by Mi\ Nelson, the drawing of which is given in fig.
341. The lamp burns parafiin and has an ordinary ^-inch wick
burner. The reservoir is rectangulai' and flat, 5^ x 4 x 1^ ; it
serves three distinct purposes : 1st, it will hold suflicient oil to burn
for a whole day ; 2nd, permits the lamp to be lowered near the

Fig. 341.— Lamp devised by Mr. E. M. Nelson.

NELSON'S LAMP 405

table ; 3rd, I'adiates the heat conducted by the metal chimney, and
prevents the oil boiling. The burner is placed at one angle of the
resei'voir to enable the dame to be placed very near the stage of the
microscope, which is exceedingly useful with some kinds of illumina-
tion, especially with reflected light, with the higher powei's, and for
Powell and Lealand's super-stage condenser.

The hole for filling the reservoir is placed at the diagonal corner
for convenience. The chimney is metal, with an oi-dinary 3x1
glass slip in front ; the diameter of the flame-chamber should not
exceed 1^ inch, and the grooves holding the glass slip should project
J inch from the flame-chamber ; the aperture should be only 1^ inch
long ; length of chimney should be 7 inches. Chimney should be
dead-black inside. This chimney serves four purposes: 1st, image
of flame is not distorted by stride and specks common to ordinary
lamj) chimneys ; 2nd, prevents reflexion from inner surface of
chimney, which causes a double image of flame ; 3rd, prevents
scattered light in. room ; 4th, is not readily broken ; slips can be
easily replaced.^

By rotation of chimney either the edge or flat side of the flame
may be used. The bull's-eye is of Herschel's form, viz. a meniscus
and crossed convex ; it is mounted on an arm which rotates cen-
trally with the lamp flame. Unfortunately, as we have seen
(p. 332), there are errors in Sir J. Herschel's original calcula-
tion, and with these it has been copied by many opticians ; a lens,
it has been demonstrated, can be made on the Herschel formula,
as calculated by Mr. Nelson, having a minimum aberration.
The arm is slotted so that the bull's-eye may be focussed to the
flame ; it can be fixed by a clamping screw. The bull's-eye may
also be elevated or depressed and fixed by a clamping screw, not
shown in the illustration. The bull's-eye, having once been focussed,
is permanently clamped, and it is brought into or taken out of posi-
tion simply by rotation of the arm. There should be a groove in
the pillar with a steadying pin on the lamp to prevent rotation
during elevation or depression.

The form of the clamping screw is important ; it should be at the
upper part of the tube, and not at the lower, as shown in the figure.
This keeps the screw clean from oil, which always, to a greater or less
extent, exudes over parafiin lamps. The screw should be of that
form which closes a pinching ring round the rod, and not merely a
screw which screws on to the rod and bruises it. This lamp, if
made, as it should be, with a japanned tin reservoir and a cast-
iron tripod foot, is quite inexpensive. There is no justification
for a circular foot, except that it can be readily and well finished
in the lathe with better apparent results and less labour than other
forms.

A small lamp is made by Messrs. R. and J. Beck. We illus-
trate it in fig. 342.

The base, A, consists of a heavy ring, into which a square brass

^ It is very important to remove the metal chimney after use, or at least not to
leave it on when not in use, since tlie evajJorating paraffin gathers round it and causes
undesirable scent when the lamp is again lit. The thinnest slips should be used.

406 MANIPULATION AND PRESERVATION OE THE MICROSCOPE

rod, B, is screwed. The square rod carries a socket, C, with an arm,
D, to which the lamp is attached.

On each side of the burner, and attached to the arm, D, is an
upright rod, G, to one of which the chimney is iixed, independent of

Fig. 342.

the reservoir of the lamp, thus enabling the observer to revolve the
burner and reservoir, and obtain either the edge or the flat side of
the flame without altering the position of the chimney. The
chimney, F, is made of thin brass, with two openings opposite to
each other, into which slide 3x1 glass slips of either white, blue, or

LAMP WITH LATERAL MOTION

407

opal glass, the latter serving ns a, reflector ; but we do not consider
the i-eflexion here accomplished as other than an error ; it causes
double reflexion and confuses the condensed image.

A semicircle swings from the two uprights, G, to which it is
attached by the pins, H, placed level with tire middle of the flame ;
to this semicircle is fixed a dovetailed bar, L, carrying a sliding
fitting, O, which bears a Herschel bull's-eye, P. This is complex,
and therefoi'e costly.

The bull's-eye is fixed at any inclination by a milled head working
in a slotted piece of brass, K,
fixed to the arm, D.

For use with the micro-
scope in an upright position,
when prolonged investiga-
tions have to take place, the
lamp becomes even of more
impoi'tance than under ordin-
ary cii'cumstances. The pre-
sent Editor devised a some-
what elaborate apparatus of
this kind, which he always
employs in this kind of ob-
servation. ^ But the essential
part of it is only an ariunge-
xaent by which a milled-head
movement of the entire lamp
may take place to the right
or the left of the observer,
as well as a similar power to
elevate or depress the posi-
tion of the flame. When the
microscope is fixed, and the
rectangular prism for illu-
mination (in place of the
mirror) is fixed at light
angles, the centiing of the
lamp flame upon the object
is more readily done by means
of motion in the lam.p. A
very simple form of this lamp
has been made for the Editor
by Mr. Charles Baker, of
Holborn ; it is seen in fig.

343, being an ordinary lamjD, excejit that the milled head to the
right as we face the flame racks up and down the entire lamp, and
the milled head behind, and at right angles to this, works a rack
and pinion (shown in the engraving) carrying the whole lamp to
the right or left of the middle position. This lamp would be
better, if the student did not object to the cost, to be made with a
metal i-eservoii-, oi- at least to have an arrangement by means of
1 Monthly Micro, Join n. vcl. xv. p. 165.

Fig. 343.

408 MANIPULATION AND PEESERVATION OF THE MICEOSCOPE

which, the bull's-eye (with a catch fixing its focus from the flame)
were so affixed as to be carried up and down and to right and left
with the lamp.

When the microscope is fixed in its upright position, and the
prism is arranged to give direct and not oblique reflexion, the lamp
flame, by means of a card, is arranged as nearly right for the re-
flexion of the image of the flame into the centre of the field as may
be and then a little movement in one or both milled heads will
bring it acciirately into the field.

We may arrange the microscope for ordinary transmitted light,
that is, for light caused to pass through the object into the object-
glass, by placing it upon the table, arranged as already directed ;
the instrument is then sloped to the required position, and a con-
denser, suitable to the power to be employed, ^ is put into the sub-
stage. The lamp is now put into the right position, with a bull's-
eye, on the left of the observer. The condenser is then, as described
below, to be 'centred,' when the objective may be changed
as desired, and the eye-piece altered to suit. But it should be
carefully noted that, if apochromatic powers are being used, there
must be accurate adjustment of the tube length if the best results

are to be obtained ; and
A with any serious increase

of the power of the objec-
tive a condenser of higher
aperture and shorter focus
must be used.

Fig. 344.— Edge of lamp flame in centre and Often, however, aS gOod

focus of bull's-eye. or better results may be

obtained without the em-
ployment of the mirror at all, the light being sent directly through
the condenser from the lamp flame. The mode of arrangement for
this kind of manipulation is presented in Plate Y., where it will be
observed that the microscope is inclined more towards the horizontal
to su.it the observer ; the lamj) is directly in front of the sub-stage,
the mirror is turned aside, and a frame (fixed upon a bull's-eye
stand) carrying a monochromatic screen is placed between the lamp
flame and the condenser (sub-stage).

By this means the light is sent into the condenser and upon the
object, and is then treated as is the case (for centring) when the
mirror is used. The first step in the direction of efficiency in the
vise of the microscope is to understand the jyrinciples of illumination,
and a knowledge of the various effects produced by the bull's-eye
lies on the threshold of this.

Having given details as to the forms of lamj) which are of most
service, we assume that a paraffin lamp with ^-inch wick is used.

If we place the edge of this flame (E, fig. 344) in the centre and
exact focus of the bull's-eye B, A shows the effect of doing so.

If a piece of card were held in the path of the rays proceeding
from B, the picture as shown at A would not be seen — instead of it
an enlarged and inverted image of the flame. The image at A is
.1 Vide Chapter IV. p, 298.

D

THE USE OF THE BULL'S-EYE

409

obtained by placing the eye in the I'ays and by looking dii-ectly at
the bull's-eye.

The light is so intense that it is more pleasant to take the field
lens of a 2 -inch eye-piece and place it in the path of the rays focus-
sing the image of the bull's-eye on a card. It should be noticed
with care that the diameter of the disc A depends upon the diameter
of the bull's-eye B ; but the in-

£)

o

m

K

H

Fig. 345.

-Altered relations between lamp
flame and bull's-eye.

tensity of the light in A depends
on the focal length of B. The
shorter the focus, the more in-
tense will be the light.

We are here assuming
throughout that the field lens is
at a fixed distance fi'om the
bull's-eye B.

But if we move the flame, E
— still central — within the focus
of B, we get the result shown
in D, fig. 345. But by mo\'ing
E without the focus of B we get
the picture H, while K is the picture when E is focussed Init not
centred.

A common error, one repeatedly met with, is that of placing a
concave mirror, C (fig. 346), so that the flame, E, is in its principal
focus. The result of this is that loarallel rays are sent to B. These
rays are brought to a focus at a distance from B about equal to twice
the radius of the cur-
vature of B and then
scattered, a totally
difiei'ent result from
what is aimed at. If
the concave mirror, C,
is to be of any use in " '
illumination, it must
be placed so that E is
not at its jyrincij^al focus., but at its centre of curvature.

The bull's-eye gives an illustration of what is of wider application.
The method of obtaining

a critical image with a " *^'

condenser by means of
transmitted light is
shown in fig. 347. E is
the edge of the flame, S
represents the sub- stage
condenser, and F the
object. F is thus the
focal conjugate of E, and
F and E are in the j)i'in-
cipal axis of S ; that is

to say, these are the relations which exist when a condenser is
focussed on and centred to an object. Let this be understood as

Fig. 346. — Result of placing flame in principal focus
of concave mirror.

Fig. 347.— Mode of obtaining critical image.

4IO MANIPULATION AND PEESERVATION OF THE MICROSCOPE

the law, and there can be but little difficulty remaining in getting
the best results from a condenser.

Fig. 348 illustrates anpther method of getting the same result.
We may illuminate a condenser with light direct from the flame, as

in fig. 347, or w-e may interpose the
mirror as in fig. 348. M is the
plane mirror, and, properly used,
exactly the same result may be
obtained as in the former case. It
is, however, slightly more difficult
to set up, bvit the method shown
in fig. 347 will, on the whole, be
preferable.

Nothing can be of more moment
to the beginner than to understand
the practical use of the condenser.
We must direct the student to what
has been stated concerning it in
Chapter TV. But the following
should be carefully considered.
Fig. 349 shows a sub-stage con-
denser, S, and an objective, 0, both focussed on the same point.
The condenser has an aperture equal to that of the objective. ISTow
if the eye-piece be removed, and we look at the back lens of the
objective, it will be seen to be full of light, as at R. The same
thing, but with the aperture of the condenser cut down by a stop, is
seen in fig. 350. Now only a part of the back of the objective is
filled with light, as at T in the same illustration.

Now it does not follow, because the back lens of the objective is
full of light, as in fig. 349, that therefore the afield ought to be full of
light. The field only shows the bright image of the edge of the flame,

Fig. 348.— Another method of getting
critical image.

It

Fig. 349. — Condenser and object-glass
with the same aperture.

o

FiGi 350. — The same, with the aperture
of the condenser cut down.

and it is in that alone that a critical 2>icti(,7'e can be found. If the
condenser be racked either within or without the focus, the whole
feld tvill become ilhiminated, but at the same time a far smaller por-
tion of the objective will be utilised. On removing the eye-piece and
examining the back lens of the objective, jDietures like D, H, fig. 345,
will be seen — D when within, and H when without the focus.

The condition represented in fig. 349 at R and 0 is the severest
test which can be apjjlied to the mici-oscopic objective ; that is to
say, to fill the whole objective with light and so test the marginal
and central j)ortions at the same time.

Even to obtain the state of illumination known as ' difiused day-

TO USE DIFFUSED DAYLIGHT 4II

light ' with the simple miri'or when no condenser is used is frequently
clone in a most inaccurate manner. The correct method of doing-
this is shown in fig. 351. F is the plane of the object, C is the con-
cave mirror, the mirror being placed at the distance of its principal
focas from the object. But the manner in which it is usually done,
from want of thought or knowledge, or both, is shown in fig. 352,

Pig. 851. — Illumination for ' diffused daylight.'

where it is manifest that there is a total disregard of the true focal
point of the mirror and its incidence on the plane of the object.
From the impracticability of this diagram as a representation of a
working plan of illumination, we may see at once the importance of
having the mirror fixed upon a sliding tube, so that its focal point
may be adjusted

It is also important here to note that in daylight illuminatioyi a

Fig. 352. — Erroneous method of arrangement for ' diffused daylight.'

plane mirror gives a cone of illitmination, as in fig. 353, when there
is ample sky-room ; but a tvindoio acts as a limiting diaphragm.

In regard to the parallelism of the direct solar rays there is of
course no question. But the parallelism of that portion of the solar
light which goes to form the firmament in our own higher atmo-
sphere is so completely broken up by refraction and reflexion
amongst the subtle particles of this higher atmosphere that the rays

412 MANIPULATION AND PEESEEVATION OF THE MICEOSCOPE

which constitute our daylight fall from every point of the Adsible
heavens (though with greatly diminished intensity). That is to say^
we have at disposal a light source extending over 180°, vjhile the sun
itself extends over a visual angle of but half a degree. Being thus
surrounded by an illimitable and self-luminoiis expanse of ether un-
dulations, the question is no longer of parallel rays only, but of light
emanating from an outer circle above the earth upon every point of
the earth's surface ; and a mirror exposed to such a luminous atmo-
sphere must both receive and reflect from all sides and upon all
sides. If, however, it be placed under the stage of a microscope,
all vertical light is intercepted, and there remains nothing but the
oblique incidence as the starting-point of the theory of illumination
by converging light ; for it scarcely needs repetition that obliquity
of incidence gives inevitable rise to obliquity of i-eflexion ; and it

Fig. 353. — Light from the open sky falls upon the muTor in all directions.

becomes equally clear that in order to strike the object the light
miost always fall obliquely on the mirror.

Then it follows from what has been said that the light falling
from the open sky iipon a mirror falls in all conceivable directions.
Thus fig. 353 shows the lines 1 to 7, including an angle of 30°. If
nothing intervene, the light of that sky surface must fall upon the
mirror, a b, and be reflected on 0. The intermediate rays, 2, 3, 4,
5, 6, form the converging ilhominating pencil, with of course an in-
finity of others filling up the spaces between.

In other words, every point of a mirror is a radiant of a whole
hemisphere, and this is eqn,ally true lohether the mirror be plane,
concave, or convex, so long as it is exposed to a boundless sky.
Therefore. a plane, concave, or convex mirror will give a cone of

LIGHT EEFLECTED TO A FOCUS FROM THE OPEN SKY 413

354. — With the open sky, Hght is
focussed at all x^oints.

illumination of which the object is its apex, no matter what the in-
clination or distance of the mii-roi-. The angle of the cone will be
the angle the mirror subtends at the object — subject of course to its
not being cut down by a stop.

As a matter of fact, the boundless sky is an abstraction which is
never obtained in practice ; thei-efore it practically does make a
difference whether the plane or concave mirror is used, and whethei'
the latter is focussed on the object or not.

The dotted lines in fig. 354 show rays falling on six different
points on a plane mirror ; the continuous lines show the reflexions
of these rays on the object.

The heavy lines from either ^^ \ ',

extremity of the mirror to the
object show the maximum angle
of cone that mirror Avill give
in that partictdar position.

The influence of a limita-
tion (as by means of a window)
should therefore be considered.
The extent to which it is ~ _,
limiting, so far as its influence
upon the illuminating cone is
concerned, is shown by an ex- -^^^^
amination of the back of the
lens of the objective when the
eye-piece is removed. Fig. 355 shows the back of the objective when
the plane mirror is used, and fig. 349 R, when the concave mirror
is used, as in fig. 351. The beginner should study these experiments
by repeating them.

Fig. 356 illustrates the method of obtaining dark-ground illumi-
nation when the arrangement shown in fig. 347 or 348 does not give
a sufiiciently illuminated area even when
the flat of the flame is used. Of course
it will be understood that for the dark-
ground result a suitable stop is inserted
beneath the sub-stage condenser.

It has been shown by many illusti'a-
tions on many subjects that certain results
in critical work can be obtained with the
bull's-eye which are not so accessible with-
out its' use. But Mr. T. F. Smith has
made this clear regarding the structure of certain diatoms.

This, there can be no doubt, is due to the fact that the parallel
rays, falling on the sub-stage condenser, shorten its focus and in-
crease the angle of the cone of illumination. It will be noticed that
when the bull's-eye is introduced the condenser will need racking-
up. At the same time we prefer illumination as in fig. 347 or 348,
except in cases where illuminating cones of maximum angles are
required. Thus it will be little needed with transmitted light
except when oil-immersion objectives of large aperture are used,
because illuminating cones up to '9 N.A. can be obtained with good

Fig. 355. — Image at the back
of the objective when day-
light and a plane mirror are
used.

414 MANIPULATION AND PEESERVATION OF THE MTCEOSCOPE

condensers by the method shown in fig. 347. But when the micro-
scope is of necessity used upright the rectangular p)rism or the plane
mirror must be used, fig. 34'8.

The arrangement at fig. 356 is sometimes useful for photo-
microgi'aphy when it is othervnse impossible to illuminate the whole
field. But in ordinary cases it is better to contract the field than
use a bull's-eye, as it invariably impairs the definition.

^

Fig. 356. — Illumination for dark ground (with
stop beneath the condenser).

Fic+. 357. — Same result with concave
mirror.

In regard to this last figure it will be understood that (as before)
E represents the edge of the flame, B the bull's-eye, M the mirror, 8
the condenser under the stage, and F the plane of the obejct.

The same result as the above may be obtained by the concave
mirror (as_ shown in fig. 357) instead of the bull's-eye. But this
is a very difiicult arrangement, yielding the best results only with
gi'eat application and care.

But the supreme folly of using a concave mirror and a hidVs-eye

B

Fig. 358.— Absurdity of using a bull's-eye
and a concave mirror.

Fig. 359. — Absurdity of using a bull's-
eye with the edge of the lamp flame
not in its principal focus.

is shown in fig. 358, where 0 is the concave mirror and (as before) S
the sub-stage condenser ; this secures a result — as will be seen by the
relation of the light to the condenser (S)— which is as far from what
IS sought and desirable as it can well be, while another lesson of
great importance may be learnt from fig. 359, which illustrates
the error of not having the edge of the flame E in the iwincipal focus
of the. huWs-eye B. The rays converge on the condenser S, so that
It will become in all probability impossible to focus it on the

DARK- GROUND ILLUMINATION 4 1 5,

object. This is a lateral lesson on the value of having the bull's-
eye fixed to the lamj), so that both may be moved together ; and
there should be a notch in the slot or arm which cari-ies the
bull's-eye to denote when the flame of the lamp is in its principal
focus.

The above are fundamental principles of illumination, and if the
student is to succeed as a manipulator he must demonstrate and re-
demonstrate them, and become master of their details and what they
collaterally teach.

We may, however, with much advantage gi^'e them a larger and
more detailed application to the practical setting up of a dark-
ground illumination, as in fig. 356.

Let an object such as a triceratium (diatom) be taken, and sup-
pose that the objective employed is a f-inch of "28 N.A. We must
first adjust the lamp and bull's-eye, as in fig. 344, and get the edge
of the lamp flame extended to a disc as at A.

Now let a small aperture be put into the condenser and a tri-
ceratium on the stage and the f objective on the nose-piece.

The microscope being put into position, the lamp should be
placed on the left-hand side of it — a lamp with a fixed bull's-eye is

Fig. 360. Pig. 361. Fig. 362. Fig. 363.

assumed — and it should now be arranged as to height, so that the
rays from the bull's-eye should fall fairly on the plane mirror, this
latter being inclined so as to reflect the beam on the back of the
sub-stage condenser.

Now, with any kind of light, focus, and place in the centre of the
field, the triceratium, as in fig. 360 ; then rack the condenser until
the small aperture in its diaphragm comes into focus ; centre this
to the triceratium, 2is in fig. 361. Rack the condenser closer up
until the bull's-eye is in focus, as in fig. 362.

Here it happens that the bull's-eye is not in the centre, and it is
not uniformly jilled with light, btot has instead two crescents of light.

This is a case which frequently repeats itself, but it is of course
not inevita,ble. The bull's-eye may be more or less filled with lioht,
and may or may not be more nearly centred. In this case we have
next to centre the image of the bull's-eye to the triceratium by
moving the mirror, as in fig. 363.

But it will be noticed that this centring of the image of the
bull's-eye does not rectify the diffusion of the light. This will be at
once done by moving the lamp with attached bull's-eye ; this motion
requires to be a kind of rotation in azimuth round the wick as an
axis. The relative positions of the lamp and bull's-eye must on no

41 6 MANIPULATION AND PRESEEVATION OF THE MICROSCOPE

account he altered, and it is understood that the h^mp was adjusted
to the picture A in fig. 344 by inspection and without the micro-
scope. A very slight movement in azimuth, however, is enough to
efi"ect the desired end (fig. 364), and all that now remains is to open
the full aperture of the condenser and put in the smallest stojj ; if
this does not stop out all the light, a larger one must be tried ; but
it is of the greatest importance that the smallest stop possible be
used, a very little difference in the size of the stop making a remark-
able difiference in the quality of the picture. Hence the need of a
large and varied supply of stops with all condensers.

On account of some residual spherical aberration the condenser
will probably have to be racked up slightly to obtain the greatest
intensity of light.

In fig. 364 the expanded edge of the flame covei's the irzceraimm.
When the whole apertvire of the condenser is opened the size of that
disc vnll not he altered, its intensity only will be
increased. When the stoj) is placed at the back
of the condenser, only in that part of the field
represented by the disc of light will the object
be illuminated on a dark ground. If, therefore,
the disc of light does not cover the object or ob-
jects, bring the lamp nearer the mirror. The
size of the disc of light depends on three things : —
Pig. 364. a. The diameter of the bull's-eye.

/3. The length of the path of the rays from
the bull's-eye to the sub-stage condenser.

y. The magnifying power of the condenser.

If a and y are constants, the only way of varying the size of the
dark field is by /3.

In the same way the intensity of the light in the disc dejDends on
three things.

A. The initial intensity of the illumination.

B. The angular aperture of the bull's-eye.

C. The angular aperture of the sub-stage condenser.

If the student will thoroughly and practically understand the
above series of single demonstrations, and ponder such inevitable
variations as practice will bring in regard to them, the ' difliculties
of illumination ' will have practically passed away.

There are tivo kinds of microscojiical vjork — one, the more usual
and comparatively easy, is the examination of an object to see some-
thing v^hich is known. The other is the examination of an object
in search of the unknown. Thus some blood may be examined
for the purpose of finding a white corpuscle. It matters little
what is the quality of either the lens or the illumination or the
microsco]3e, or whether the room is darkened or not, because the
observer knows that there is such a thing as a white corpuscle. It
is quite immaterial as to whether the observer had ever seen one oi-
' not ; so long as he possesses the knowledge that there is such a thing,
the finding of it, even under unfavoui-able conditions, will be an
easy task.

But if the obsei'vei' lias not that knowledge, he may examine

SEARCH WOEK — LIGHT AND THE EYES 417

"blood many times, under favourable conditions, and yet not notice
the presence of a white corpuscle, and that, too, with one immediately
in the centre of the field ; this, moreover, is a large object. --

It is only those in the habit of seai-ching for new things who can
appreciate the enormous difficulty in first recognising a new point.
Thei'efore, when critical work is undertaken, care should be exercised
to have the conditions as favourable as possible.

Wlien working with artificial light all naked lights in the room
should be avoided.

It is quite unreasonable to expect the retina to remain highly
sensitive if, whenever the eye is removed from the eye-piece, it is
exposed to the glare of a naked gas flame.

At the same time there shou.ld be ample light on the microscope
table, as it is not at all necessary or desirable that the work should
be insufiiciently illuminated. All that is requii-ed is that the lamps
should have shades and be placed at such a height that the direct
rays do not enter the observer's eye.

If these precautions are taken, several hours' continued work
may be carried on without any injurious efiect.

Some observers use only the left eye, some the right, others the
right or left indiscriminately.

It seems immaterial which is used, it being merely a- matter of
habit, as those who are accustomed to use one particulai' eye feel
awkward witli the other. In continuous work, extending over many
months of long daily observation, if the eye has been accustomed to
monocular vision, even with high powers, there is no difiiculty
experienced. The effect of years of work with optical instruments
on those possessed of strong normal sight seems to be an increase
in the defining perception accompanied by a decrease of the
perception of brightness. Those accustomed to use one particular
eye with microscopical work, and who have done much work, would,
if they looked at, say, the moon with that eye, see more detail in it
than if the other eye were used ; at the same time it would not
appear as bright.

If there is too much light, as there often is, when large-angled
illuminating cones are used, it is as well to interpose between the
lamp and the microscope a piece or pieces of signal green glass ; this
softens the light and removes the objectionable yellowness, a featui'e
of illumination not due to the light from the edge of a paraffin lamp,
which, as we have stated, is not particularly yellow. Go'eat yelloio-
ness is a sign of impe^'fect achromatis^n in an objective. We may
with precisely the same conditions find the images yielded by two
objectives of the same powei- and aperture differ, in so much as one
is yellow and dim and the other white and blight ; other things
being equal, the white and bright image is to be preferred. It is
necessary to say ' other things being equal,' because an objective
which gives a blight and a white image may nevertheless be inferior
to the one giving the yellow and dim picture. Thus if the planes
of the lenses of which the objective is composed are not at right
angles to the optic axis thfjre will be serious defects in the image,
although it is bright and white. This fault is known in practice as

E E

41 8 MANIPULATION AND PEESEEVATION OF THE MICE08C0PE

an error of centiing, which also means the error of not placing the
axes of the lenses in the same straight line ; so both faults are
' described by the same term.

It should be undei-stood that signal gi-een glass will not yield
monochromatic illumination ; only the Gifford screen or the filter
screen of Prof. Miethe {q.v.) or the Nelson spectroscopic arrange-
ment {q.v^ can be of I'eal service.

Coloured light derived from a polai'iscope and a selenite is not
monochromatic .

For a'itical vwrk, such as testiny lenses ov foi-cing out the greatest
resolution with the widest-angled oil-immersion lenses, daylight
illumination is inadmissible.

"When daylight illumination is used, a northern aspect, or at least
one away from direct sunlight, is to be preferred.

It is a good plan, where it is possible, to aii-ange the table so that
the window is at the observer's left hand. The microscope should be
placed in a direction parallel to the window, and the light reflected
by the mirroi- through a right angle. A screen may be placed parallel
to the window which just allows the mirror of the microscope to
project beyond it. This cuts off dii'ect light froni the stage and
from the observer's eyes.

A concave mirror with the object in its principal focus is the best
for diffused daylight illumination. The diaphragm should not be
close to the stage. When delicate microscopical work is carried on,
it is important to I'emembei' that the human eye can work best when
the body is in a state of ease. If there is any strain on the muscles
of the body, or if the observer is in a cramped position, vision will
be impaired. Consequently, where permissible, a microscope should
always be inclined^ and the observer seated in such a w^ay that the
eye can be bi-ought to the eye-piece in a perfectly natui-al and com-
fortable manner. The body should also be steadied by resting the
arm.s on the table.

It is advisable to use the bull's-eye as little as possible ; even loith
dark-ground illummation the flat of the flame is preferable, I'eserving
the bull's-eye for those cases where the flat of the flame will not cover
enough of the object. Generally speaking, if the whole field is re-
quired to be illuminated on a dark ground, a bull's-eye will be neces-
sary ; but for an object such as a single diatom the flat side of the
lamp flame will usually be large enough.

In examining diatoms or other objects, such as the karyokinetic
figures in very minute nuclei of microscopic oi'ganisms, or other obscure
and undetermined parts of such forms of life, it is most important,
amongst other means, to resort to the use of large solid cones ; what
they teach and suggest can scai-cely be neglected by the searcher for-
the unknown. Professor Abbe does not advise their employment as
in any way final ; he says that ' the residting image produced by
means of a broad illuminating beam is always a mixture of a multi-
tude of partial images which are more or less diflferent and dissimilar
fi'om the object itself ; ' and he does not conceive that there is any
ground for expectation ' that this mixture should come nearer to a
strictly correct projection of the object . . . than the image which

TO DISPLAY OBJECTS MICROSCOPICALLY 419

is jii'ojected by a naiTow axial illuminating pencil.' This is a weighty
judgment, and should receive full consideration. At the same time
the use of wide and solid cones is so full of suggestive results that
we must employ them with all possible control by othei- means of the
images they present. This is the move a necessity since Mr. Nelson
has been able to obtain the most wonderful results with naiTow cones,
' true ghosts ' and ' false ghosts,' the presence of intercostal markings''
in the image of a fly's eye (!), and many complex and false images
with the coarser diatoms. But with wide cones he has proved that
these false images cannot be produced ; and that when the true image
is reached by a wide cone, the image is not altered by any change of
focus, but simply fades in and out of focus ' as a daisy under a
4-inch objective.'

Mr. Nelson has photographed all these results,-^ and we have seen
them demonstrated. When theory and practice are thus at variance
we must pause for further light.

If it is required to accentuate a known structui'e, such as the per-
forated membrane of a diatom^ it can be done by annulai- illumination,,
which means the same arrangement as for dark ground, but with a
stop insufliciently large to shut out all the light. This method is not
to be recommended when a structui'e is unknown, as it is also liable
to give false images. It must be remarked that diatom and other
delicate structure, when illuminated with a narrow-angled cone, gives
on slight focal alterations a variety of patterns like a kaleidoscope ;
with a wide-angled cone a single structure gives a single focus, i.e
it goes completely out of focus on focal alteration. When a large -
angled and a wide-angled objective are used a change of pattern only
occurs when the structure is fine. This practical obsei-vation has its
value, and must not be forgotten.

To properly dis])lay objects under a microscope is to a cei'tain ex-
tent an art, for it not only demands dexterity in the manijjulation
of the instrument and its appliances, but it also requii-es knowledge
of what sort of illumination is best suited to the particular object.
At this point we think it advisable, especially in the interests of
beginnei's, to cleaily point out the best method of commencing
microscopic work by centring the condenser and arranging the
light for the critical examination of an object.

1st. Place a power of about a f on the nose-piece, and a B or
No. 2 eye-piece in the tube.

2nd. Use as a source of illumination the light from a pai'affin
lamp with a ^-inch wick.

3rd. Place any suitable object on the stage, and, having focussed
it with any kind of illumination, centre it to the field of the eye-
piece.

4th. Place a small diaphi-agm beneath the sub-stage condensei-. oi-
close the iris.

5th. Rack the condenser until the hole in the diaphragm is in
focus (in the plane of the object).

6th. If the hole in the diaphi-agm should not be central to the

1 Joicrn. B. M. S , 1891, p. 90, pi. II.

420 MANIPULATION AND PRESEKVATION OF THE 3IICR0SC0PE

object on the stage, it must be centred by means of the sub-stage ad-
justing screws,

7th. Rack up the condenser until the image of the flame comes
into focus.

8th. Centre the image of the flame to the object on the stage by
moving the position of the lamp, and place the lamp so that the
edge of the flame is presented. In performing tliis adjustment the
sub-stage centring screws must on no account be moved. (If a mirror
is employed, the centring of the image of the flame upon the object
can be effected by moving the miiTor.)

9th. The object to be examined may now be substituted for that
used for centring purposes, and be placed in the image of the edge
of the flame.

10th. The objective by which the object is to be examined is
placed on the nose-piece and the object brought into focus.

11th. The eye-piece is removed and the back lens of the objective
is examined. The diaphragm at the back of the condenser is then
altered so that three-fourths of the back lens of the objective is filled
with an unbroken disc of light.

12th. The eye-piece is replaced and the objective brought into
adjustment either by screw collar or by altering the tube length.

13th. If it is necessary at any time to use a large field for a rough
survey of an object, or to localise any particular portion of an
object, all that is necessary is to rack down the condenser until the
whole field becomes illuminated ; but when any pai't requires critical
■examination the condenser must be racked up again and the image
•of the edge of the flame focussed on the object.

For learning the manipulation of the instrument no class of
objects are as suitable as diatoms ; they are also an excellent means
of training the eye to appreciate critical images. For a general view
of the larger diatoms take a spread slide in balsam ; a ^^^ of 80°, a
good binocular, and a dark-ground illumination will give a fine effect.
This is not merely a pretty object, but it is also a very instructive
one, because we obtain a far clearer idea of the contour of various
diatoms than can be obtained in any other way. The diatoms should
be studied and worked at in this manner most carefully and for a
long time. The same identical specimens should be then viewed with
iiransmitted light. This lesson, if conscientiously learnt, will teach a
.student how to appreciate form by focal alteration. This is a most
important lesson, and, if several days are spent in mastering it, they
will be far from thrown away. Diatoms, especially the larger foiins,
..•are seen very w^ell when mounted dry on cover by means of a |-inch
■ objective and a Lieherkilhn ; the bviU's-eye and the plane mirror should
!be used. Some objects are so transparent, or become so transparent
in the medium in which they are mounted, that they will not bear a
lai'ge illuminating cone, the brightness of the illumination destroying
the contrast. It will illustrate this when we recall that dirt on an
eye-piece which is quite invisible in a strong light becomes im-
mediately apparent in a feeble light. Thus animalcules require a
small cone of illumination when they are being examined, particularly
with a ^-inch objective ; for a general view of ' pond life' a 1^-inch

•CEITICAL' AND UNCEITICAL IMAGES 42 1

objective with a clark-ground illumination, employing a binocular, is
very suitable. Stained bacteria in tissue ai'e best seen with a large
cone, as was pointed out by Dr. Robert Koch, and is directly supjjorted
by Dr. Abbe as suitable in his directions for the use of the Abbe
condenser.^ The brilliancy of the illumination obliterated the thin
tissue which is in a medium whose refractive index is similar to
itself. The bacteria, which are opaque with pigment, then stand
out boldly. A bacterium not in tissue is always better seen by
means of a large cone, provided that the objective is properly
corrected. The very minute hairs on the lining membrane of the
blow-fly's tongue, if examined by a |^ objective and a narrow cone,
appear thickened, shorter, more blunted, and often split into two
parts. This is shown in figs. 2 and 3 in the frontispiece. Fig. 3 is
a critical image magnified 510 diameters. A lens should be used to
examine this.

It will be seen that the hairs, especially the long central one, are
very fine and spinous. They have not the ring socket common to
insect hairs, but grow directly from a delicate membrane.

This photograph was taken with an apochromatic ^ of 'QS N.A.
and No. 3 projection eye-piece ; and it was illuminated by means of
ft large solid cone of 'GS IST.A. from an achromatic condenser.

Fig. 2 is an uncritical image, with all the conditions as above,
save that a cone of small angle, i.e. of 0"1, was used for illumination.

The first alteration which thi'usts itself upon the eye is the
doubling of the hairs which are in the least degree out of focus.
But, fui-ther, it will be noted that there is a bright line with a dark
edge round the hairs which are precisely in focus ; this is a diffraction
efiect, always, in our experience, present in objects illuminated by
cones of insuflicient angle, and it can be easily made to disajjpear by
widening the cone. As the illuminating cone is enlarged they become
sharper and longer, and their edges become more definite. But
nothing is gained, but rather a distinct loss is incurred, by making
the illuminating cone much larger than three-fourths of the objective
cone.

As an example of erroneous interpretation, the representation of
the pygidium of a flea by some leading sources of information of a
few years ago may be instanced. It was a special test of many
authors, and has been carefully figured ; this shows that it is not an
accidental error, which it might have been if it were merely an
ordinary object ; it is an error depending in all probability on a
faulty system of illumination. Moreover, the error cannot be attri-
buted to the object-glasses of the time, as it is a low-power object,
and the low powers of that day were quite as good as those lately in
use. In the descriptions and in the drawings, often beautifully
executed, the hairs proceeding from the centre of the wheel-like
discs are represented as being ' stifi" and longish bristles,' thick at
one end and tapering off to a point. And the small hairs round
are described as ' minute spines ; ' in the drawing they ai-e like the
spinous hairs of an insect, and have the usual socket-joint at the

* Directions for the Use of Abbe's Illuminating Apparatus — a leaflet issued by
Carl Zeiss, 1888.

422 iMANIPULATION AND PEESERVATION OF THE MICROSCOPE

base. In reality the ' stiff and longish bristle ' is an extremely long
and delicate filament, totally unlike a bristle, being not tapered but
of nearly uniform thickness. The ' minute spines ' are in reality
very curious hairs, and, as far as we at present know, unlike any
others. They are delicate, lambent, bulbous hairs. What they
most resemble are the tentacles of a sea-anemone, and there are two
tubes discoverable which ai-e impoi-tant and comparatively large ob-
jects. There appears to be considerable probability that this inte-
resting object upon the last ling of the body of the flea, and known
as its ' pygidium,' acts as an auditory instrument.^ In the examina-
tion of oi'dinary stained histological and pathological sections by
transmitted light, unless some very delicate point is sought, the con-
denser should have a stop, so that when the back of the objective is
examined the stop is seen cutting into the back of the objective by.
about a third. This in some instances may be increased to a half by
diminishing the cone, but it is not advisable to vise anything less
than a half unless it is absolutely necessary. As we have pointed
out above, high-class objectives will stand a | cone perfectly, and very
special objectives will bear even a |- cone ; but for the ordinary run
of objectives § will be found as much as they are able to bear — some
indeed will not stand a ^ cone. Thus, to put it in round numbers,
an illuminating cone "2 N.A. is very suitable for ordinary work with
the apochromatic 1-inch and § objectives, and one of "4 N.A. for the
^ ancl ^, and one of "6 N.A. for the J and ^. It is a good plan to
have one or two stops cut to give special cones, the N.A. of which
should be engraved on them. This subject is one of great import-
ance,, as more than nine-tenths of all microscopic objects are
examined by means of ti-ansmitted light.

Let us now note the effect of large cones on the simplest object.
A microscope is set up having an achromatic condenser with an ii-is
diaphragm ; let three good wide-angled objectives be chosen, say
1-inch, a Vinch, and ^-inch dry. Let the object be the one we have
already studied to some extent in this relation, viz. one of the stiff
hairs on the maxillary palpus of the blow-fly's tongue ; place the
1-inch on the nose-piece, open the full aperture of the condenser and
^■et the instriiment into perfect adjustment. Now close the iris. The
hail- will be surrounded by a luminous border, which will give it
a glazy ajjpearance, and its fine point will be blurred out. Now
■open the iris until the last trace of that glaziness disajopears. The
hair will appear as a difierent object, its outline being perfectly clear
and sharp. If the eye-piece is removed, about two-thirds of the ob-
jective back will be full of light. Now, without disturbing any of
the adjustments, replace the 1-inch by the ^, and it will be found
that the glaziness or false light will have returned. Let the iris
be fui-thei- opened until the last trace of it disappears ; now, on
examination of the back of the objective, two-thirds of it will be found
full of light, and so on with the |-. We call the attention of the
student to these facts as having a direct bearing upon the question
of the comjDai'ative efiects of large and small illuminating cones, and

1 Micros. Journ. April 24, 1885 : ' Pygidium of Flea ' (E. M. Nelson).

VAEIOUS MODES OF ILLUMINATION— LARGE CONES 423

Avith no idea of oftering opposing opinions to those of Professor Abbe ;
Ave have no dii-ect jvidgment, but we recoi-d these facts as factors in
and foi- the ekicidation of the question. It is perhaps better to test
the ^ on some of the more minute hairs which are studded over the
delicate lining membrane. The same results will be obtained. Thus
it would appeal- to suggest itself that this glaziness depends on the
relation of the (vperture of the iUaminating cone to that of the objective
cone. Apochromatic objectives behave pi-ecisely as achromatic ob-
jectives in this respect. Of course, if the hair becomes pale and in-
distinct on the opening of the iris, it shows that there is uncorrected
spherical aberi'ation in the objective ; another objective must there-
fore be used ; that paleness has nothing whatever to do with the
glaze or false light mentioned above.

In photo-micrograj)hs of bacteria one frequently sees a white halo
round them. We have never been able to demonstrate what this is ;
sometimes it denotes the presence of an envelope, and sometimes it
is the result of the use of too small a cone of illumination. Photo-
micrography with a small cone is quite easy, as great contrast can
be secured. With a large cone the difficulties begin — difficulties
of adjustment, difficulties of lens correction, difficulties of exj^osure,
and difficulties of development. If, so far as our experience goes,
<\ good photo -microgi-aph is required, these difficulties must be
mastered.

It is hardly necessary to remind the student that in micrometry
it is essential that the edges of the object should be defined ; conse-
quently a lai'ge cone must then be employed.

For the examination of Poly cystines, Foraminifei'U, &c., a binocular
is useful ; illumination may be by a Lieberkiihn if mounted dry, and
by dark ground by a condenser if mounted in balsam. Parts of
insects should be usually examined with dark-ground illumination ;
whole insects are seen best with the Lieberkiihn, and the binocular
should be used for both.

Some of this class of objects are best seen under double illumina-
tion ; that is, a dark ground vjith a condenser and light thrown from
above loith a silver side-reflector., as the Lieberkiihn cannot be used in
conjunction with an achromatic condenser. It is a good plan with
low-power Lieberkiihn work to interpose between the slip and the
ledge a strip of plain glass Vinch wide ; this prevents the ledge
stopping out light from the Lieberkiihn when it is larger in diametei-
than the slip. Mr. Julius Rheinberg has recently brought to a
high state of perfection a system of colour illumination, and the
special importance of the choice of suitable colours. It is of much
interest, but cannot be condensed in the space at our disposal. The
full paper will be found illustrated in ' Journ. K.M.S.' 1896, p. 373,
and the 'Journ. R. M. S.' for 1899, p. 142.

Polarised light used with a condenser is very useful foi' insect
work. For very low-power woi'k — such as the usual botanical sec-
tions— it is a good plan to give up the cone, and place a piece of fine
ground glass at the back of the condenser ; and with lamplight it is
as well to use a Gilford's screen with it. With objectives of greater
angle than '6 X.A. it is usually difficult to get satisfactory illumina-

424 MANIPULATION AND PKESEEVATION OF THE MICEOSCOPE

tion with a dark ground. The best that can be done is to use an
oil-immersion condenser with a suitable stop ; this will give a good
dark ground up to "65 N.A., but it will fail if the object is dry on
the cover. Generally speaking, the only way of accomplishing this
with objectives of wider aperture is to reduce the aperture of the
objective by a stop placed at the back.

When a condenser is united by a film of oil to a slip, if the slip
is thin, the oil invariably runs cloion ivhen the condenser is focussecl.

The following is a method
by which this may be en-
tirely prevented. A piece
of thick cover-glass about
•02 inch, and 1 inch square^
has a strip of thicker glass,
^ inch broad, cemented by
shellac to one edge. This,
piece of glass is oiled to
the slip, the ledge being-
hooked over the top of the
slide ; this not only pre-
vents its slipping down.
Fig. 865. but also keeps the oil from:

creeping out at the bottom,
edges of the glass coincided.^

Thin slip of glass with
ledge to place glass
slip with oil contact,
so as to vary the
thickness of a slide.

Slide in situ on thin
slip with ledge.

which would be the case if the two
This is illustrated in fig. 365.

In its proper place we have dealt with the suitable relation of
aperture to power, and have j)ointed out the irresistible nature of
the contentions and teachings of Abbe on the subject. Here a
direct practical presentation of the matter may be of service to the
student.

A normal unaided human eye can divide ^\-q inch at ten inches.
Consequently a microscope with a power of 200 should be capable of
showing structure as fine as g-Q^-g-Q inch. Now, as this power can be
made up by ^-inch objective and a 1-inch eye-piece, it follows that
sufficient aperture ought to be given to the f -inch to enable it to
resolve 50,000 lines per inch. This ^ will be '52 N.A. The inch
objective should have half this aperture, and the \ double, and the
^ four times as much, if perfect vision is required ; in other words,
"26 N.A. for every 100 diameters."* These ideals have (as we have
before indicated) been realised, notably by the Zeiss apochromatics,
the 1-inch and the ^-inch "* resolving everything capable of being-
appreciated by the eye when the 12 compensating eye-piece is used.
The J-inch is also a near apjDroach to the ideal, as it has been very
wisely kept a dry lens. The oil-immersion g-in. of 1'4 N.A. witb
a 6 eye-piece also attains the ideal. This relation of aperture to

1 Q. M. C. Journal, November 1885.

^ In reality it will require more, because an axial cone is assumed to be used
instead of an oblique beam.

2 English Mechanic, vol. xxxviii. 1883, No. 979.— E. M. Nelson.

4 This lens, with an 8 compensating eye-piece, will resolve a Fleurosigma
angulatum with an axial cone ; this is the lowest power with which it has ever been
done.

THE QUALITIES OF OBJECTIVES 425

power is veiy significant, and should be carefully pondered by those
who still desire low apertures as the only pei'fect form of objectives.

It is as well to mention that objectives may be arranged in two
series — one the 2, 1, ^, J, and |^, the other 1^, §, ^, ^, ^V. One of
these series will form a complete battery, as it is unnecessary to
have objectives differing from the next in the series by less than
double the j)Ower.

The most usual combination is perhaps the 1 and the J of one
series, or the f- and the ^ of the other. Of these two preference
might rathei- be given to the latter. The only exception would be
the addition of a H-inch for pond life.

Eye-pieces should also double the power thus : 5, 10, and 20
(uncompensated), or 6, 12, and 27 (compensated), the most useful of
the three being the 10 (uncompensated) and the 12 (compensated).
As there is no 6-power compensated eye-piece for the long tube, a 4
for the short tube admirably answers the purpose.

In addition to the explanations already given on the subject of
testing objectives, it may be useful here to note that the qualities of
an objective are seven in number : —

1. Magnifying power (initial).

2. Aperture or jST.A.

3. Resolving power.

4. Penetrating power.

5. Illuminating power.

6. Flatness of field.

7. Defining power.

1 . Magnifying poioer. — No test is required, as the initial magni-
fying power can be directly measured.

2. Aperttijre or N.A. can be directly measured ; no test is there-
fore necessary.

3. Resolving poiver. — A lens illuminated by a large solid axial
cone, when a Gifford's screen is used, should resolve a number of
lines to the inch expressed by its N.A. multiplied by 80,000. '

4. Penetrating jyotoer is the reciprocal of the resolving power of

•-j^r . . No test needed, but penetrating power varies largely with

the combined magnifying power, and also with the magnitude of the
illuminating cone used, as ali'eady intimated.

5. Illuminating potver is the square of the numerical aperture
(N.A.)^. No test is necessary, but the remarks made above in
regard to joenetrating power apply equally here.

6. Flatness of field is, in the strict meaning of the term, an
optical impossibility. The best thing therefore is to contract the
visible field, as is done in the compensating eye-pieces. (Tests : For
low powers a micro-photograph ; for medium and high powers a stage
micrometer.)

7. Defining potoer depends on {a) the reduction of spherical
aberration, (6) the reduction of chromatic aberration, (c) the perfect
centring of the lenses — by which is meant (i.) the alignment of

1 J.B.M.S. 1893, p. 15.— E. M. Nelson.

426 MANIPULATION AND PEESEEVATION OF THE MICROSCOPE

their optic axes, (ii.) the parallelism of their i^lanes, (iii.) the setting
of theii- planes at idght angles to the optic axis.

Defining ji^oifer can only be tested by a critical image. The
following is a list of suitable objects of which a critical image is to
be obtained, using a solid axial cone of illumination equal to at least
thi-ee-foui'ths of the apei-tui-e of the objective.

Very low powers (3-, 2-, and iVinch). — Wing of Agrion pul-
chellum ^ (dragon-fly).

Loio jMioers (1 and §). — Proboscis of l)low-fly. Large diatoms
on dark ground.

Medium powers (^, -j^*j, \, and low-angled J). — Minute hairs on
pi-oboscis of blow-fly ; hair of pencil-tail {Polyxemis lagurus) ;
diatoms on a dark ground. This last is a most sensitive test ; unless
the objective is good there is sure to be false light.

Medium poioers (with wide aperture). — Pleurosigma formosum ;
Navicula lijra in balsam or styrax ; Pleurosigma angulatum dry on
cover ; bacteria and micrococci stained.

' High powers (wide aperture and oil-immersion ^ and ^V)- — The
secondary structure of diatoms, especially the fracture through the
perforations ; Natncula rhomhoides from Cherryfield in balsam or
styi-ax ; bacteria and micrococci stained.

Test with a 10 or 12 eye-piece, and take into account the general
whiteness and bi-illiancy of the picture.

The podui-a scale is not mentioned as a test, as it may be very
misleading in unskilled hands. One great point in testing objectives
is to know youi- object. Care must be exercised to ascertain by
means of vei-tical illuminator if objects such as diatoms dry on the
■cover are in optical contact with the cover-glass. Testing objectives
is an art which can only be acquired in time and with expeiience
gained by seeing large nvimbers of objectives.

In the manipulation of the mici-oscope it is not uncommon to
observe the ojjerator rolling the milled head of ihefine adjustment
instead of firmly grasping it between the finger and thumb and
governing, to the minutest fraction of arc, the amount of alteration
he desires. It is undesirable and an entirely inexpert procedure to
i-oll the milled head, and cannot yield the fine results which a deli-
cate mastery of this part of the instrument necessitates and implies.
To use aright the fine adjustment of a first-class microscope is not
the first and easiest thing mastei-ed by the tyro. We have already
intimated that the fine adjustment should never be resorted to while
the coarse adjustment can be efiiciently employed. The focus should
always be found, even with the highest powers, by means of the
coai'se adjustment. It is only a clumsy microscojoist who bi-ings his
objective by means of the coarse adjustment near the covei'-glass and
looks at the distance he is off it either by the eye or by the aid of a
hand magnifier, and then completes his work with the fine adjust-
ment. In every case the focus ought to be found by the coarse
adjustment, and the working distance should h^ felt by the finger
tilting the slide gently against the front of the objective. Also the
examination of objects for depth of sti-uctui-e with low and medium
powei's up to the diy ^- or ^-inch objective shovxld be performed by

EKRoRS OF INTERPRETATION 427

the coarse adjustment ; only the very finest details, such as the j)odura.
'exclamation' marks, require the fine adjustment.

Beyond the cori-ect and judicious use of the mici-oscojje and all
its appliances, thei-e is the mattei- of the elimination of errors of in-
terpretation to be carefully considered.

The coi'rectness of the conclusions whicli the mici-oscopist will
di-aw i-egai-ding the natui-e of any object fi-om the visual appeai'ances
which it pi-esents to him when examined in the various modes now
specified, will necessarily depend in a great degree upon his previous
experience in microscopic observation and upjon his knowledge of
the class of bodies to which the joai-ticular specimen may belong.
Not only are observations of any kind liable to certain fallacies
arising out of the pi-evious notions which the obsei'ver may entei-tain
in regard to the constitution of the ol^j ects oi- the nature of the actions
to which his attention is dii'ected, but even the most 23i"f>^ctised ob-
server is apt to take no note of such phenomena as his mind is not
prepared to appreciate. Ei'rors and imperfections of this kind can
only be corrected, it is obvious, by general advance in scientific
knowledge ; but the history of them affbixls a useful warning against
hasty conclusions drawn from a too cursory examination. If the
history of almost any scientific investigation were fully made known,
it would generally appear that the stability and completeness of the
conclusions finally ari'ived at had only been attained after many
modifications, or even entire alterations, of doctrine. And it is
therefore of such gi'eat impoi'tance as to be almost essential to the
correctness of our conclusions that they should not be finally formed
and announced until they have been tested in every conceivable
mode. It is due to science that it should be burdened with as few
false facts and false doctiines as possible. It is due to other truth-
seekers that they should not be misled, to the great waste of theii-
time and pains, by our errors. And it is due to ourselves that we
should not commit our reputation to the chance of impairment by
the premature formation and j^^^blication of conclusions which may
be at once reversed by other observei-s better informed than our-
selves, or may be proved to be fallacious at some future time, per-
haps even by our own more extended and careful researches. The
suspension of the judgment whenever there seems room for doubt is
a lesson inculcated by all those philosophers who have gained the
highest repute for practical wisdom ; and it is one which the micro-
scopist cannot too soon leai'n or too constantly practise. Besides these
general warnings, however, cei'tain special cautions should be given
to the young microscopist with i-egard to errors into which he is
liable to be led even when the very best instruments are employed.

Errors of interpretation arising from the imperfection of the
focal adjustment are not at all uncommon amongst microscoj)ists,
and some of the most serious arise fi-om the use of small cones of
illumination. With lenses of high power, and especially with those
of lai'ge numerical apei-ture, it very seldom happens that all the
parts of an object, however minute and flat it may be, can be in
focus togethei- ; and hence, when the focal adjustment is exactly
made for one part, everything that is not in exact focus is not only

428 MANIPULATION AND PEESERVATION OF THE MICROSCOPE

more or less indistinct, but is often wrongly represented. The in-
distinctness of outline will sometimes present the appearance of a
pellucid border, which, like the diffraction-band, may be mistaken
foi- actual substance. But the most common error is that which is
jjroduced by the reversal of the lights and shadows resulting from the
i-efractive powers of the object itself ; thus, the biconcavity of the
blood-discs of human (and other mammalian) blood causes their
centres to appear dark when in the focus of the microscope, through
the divergence of the i-ays which it occasions ; but when they are
brought a little within the focus by a slight approximation of the
object-glass the centres appear brightei- than the peripheral jDarts of
the discs. The student should be warned against supposing that in
all cases the most 'positive and striking appearance is the ti-uest, for
this is often not the case. Mr. Slack's optical illusion, or silica-crack
slide,^ illustrates an error of this desci'iption. A drop of water holding-
colloid silica in solution is allowed to evaporate on a glass slide, and
when quite dry is covei-ed with thin glass to keep it clean. The
silica deposited in this way is curiously cracked, and the finest of
these cracks can be made to pi-esent a very positive and deceptive
appearance of being raised bodies like glass threads. It is also easy
to obtain diffraction-lines at theii' edges, giving an appearance of
duplicity to that which is really single.

A very important and very fretjuent source of error, which
sometimes operates even on experienced microscopists, lies in the
refractive influence exei-ted by certain peculiarities in the internal
structui'C of objects upon the rays of light transmitted through
them, this influence being of a natui'e to give rise to appearances
in the image, which suggest to the observer an idea of their cause
that may be altogether different from the reality. Of this fallacy
we have a ' pregnant instance ' in the misinterpretation of the nature
of the lacunce and canaliculi of bone, which were long supposed
to be solid corpuscles with radiating filaments of peculiar opacity,
instead of being, as is now universally admitted, minute chambers
with diverging passages excavated in the solid osseous substance.
When Canada balsam fills up the excavations, being nearly of the
same refi'active power as the bone itself, it obliterates them
altogether. So, again, if a person who is unaccustomed to the use
of the microscope should have his attention directed to a preparation
mounted in liquid oi- in balsam that might chance to contain air-
bubbles, he will be almost certain to be so much more strongly
impi-essed by the appearances of these than by that of the object,
that his first remark will be upon the number of strange-looking
black rings which he sees, and his first inquiry will be in regard to
their meaning.

Although no experienced microscopist could now be led astray
by such obvious fallacies as those alluded to, it is necessary to
notice them as warnings to those who have still to go through the
same education. The best method of learning to appreciate the
class of appearances in question is the compaiison of the aspect of
globules of oil in water with that of globules of water in oil, or of
1 Monthly Microscojncal Journal, vol. v. 1872, p. 14.

STUDIES IN INTERPKETATION

429

"bubbles of aii- in water oi' Canada balsam. This conipai-ison may-
be very readily made by shaking up some oil with water to which
a little gum has been added, so as to foi'm an emulsion, oi- by
simply placing a drop of oil of tui-pentine (coloui-ed with magenta
or carmine) and a drop of watei- together upon a slide, laying a thin
glass cover ovei- them, and then moving the cover backwards and
forwards sevei-al times on the slide. Equally instructive are the
appearances of an aii--bubble in water and Canada balsam.

The figures which illustrate the appearance at various points

_ JE

Fig. 366. — Air-bubbles in (1) water ; (2) Canada balsam ; (3) fat-globules in water.

of the focus of an air-bubble in water and Canada balsam, and of a
fat-globule in water, may be thus illustrated, viz. a diaphragm of
about 1^ of a mm. being placed at a distance of 5 mm. beneath tlie
stage, and the concave mii-i-oi- exactly centred.

Air-bubbles in ivater. — No. 1 (fig. 366) represents the difierent
appearances of an air-bubble in watei-. On focussing the objective
to the middle of the bubble (B), the centre of the image is seen to be
very bright — brighter than the rest of the field. It is surrounded by
a greyish zone, and a somewhat broad black ring interrupted by one

430 MANIPULATION AND PRESERVATION OF THE MICROSCOPE

or more brighter circles. Round the black ring are again one or
more concentric circles (of diffraction), brighter than the field.

On focussing to the bottom of the bubble (A) the central white
circle diminishes and becomes brighter ; its margin is sharper, and
it is surrounded by a very broad black ring, which has on its
periphery one or more diffraction circles.

When the objective is focussed to the upper surface of the
bubble (C) the central circle increases in size, and is surrounded by
a greater or less number of rings of various shades of grey, around
which is again found a black ring, but narrower than those in the
previous positions of the objective (A and B). The outer circles of
diffraction are also much more numei'ous.

Air-bubbles in Canada balsam. — Canada balsam being of a
higher refractive index than water, the limiting angle, instead of
being 48° 35', is 41° only, so that the rays which are incident much
less obliquely on the surface of separation undergo total reflexion,
and it will be only those rays which face very close to the lower
pole of the bubble that will reach the eye, and the black marginal
zone will therefore be much larger. This is shown in fig. 366, No. 2.

When the objective is focussed to the bottom of the bubble
(A'), we have a small central circle, brighter than the rest of the
field, all the rest of the bubble being black, wdth the exception of
some peripheral diffraction rings. On focussing to the centre (B'')
or upper part (C) of the bubble, we have substantially the same
appearances as in B and 0, with the exception of the smaller size
of the central circle.

Fat-globides in loater (fig. 366, No. 3). — These illustrate the
case of a highly refracting body in a medium of less refractive power.

When the objective is adjusted to the bottom of the globule A''',
it appears as a grey disc a little darker than the field, and separated
from the rest of the field by a darkish ring.

Focussing to the middle of the bubble (B''''), the central disc
becomes somewhat brighter, and is surrounded by a narrow black
ring, bordered within and without by diffi'action circles.

On further removing the objective the dark ring increases in
size, and when the upper part of the bubble is in focus, we have
(fj") a small white central disc, brighter than the rest of the field,
and sharply limited by a broad, dai'k ring which is blacker towards
the centre.

These appearances are the converse; of those presented by the
air-bubble. That, as we saw, has a black ring and a white centre,
which are the sharper as the objective is approached to the lower
pole of the bubble. The fat-globule has, however, a dark ring
which is the broader, and a centre which is the sharper, according
as the objective is brought nearer to the upper pole.

These considerations, apai-t from their enabling us to distinguish
between air-bubbles and fat-giobides, and preventing their being-
confounded with the histological elements, enable two general
principles to be established, viz. bodies which are of greater re-
fi-active power than the surrounding medium have a white centre
which is sharper and smallei-, and a black i-ing which is larger when

'BEOWNIAN' MOVEMENT 43 I

the objective is withdrawn ; whilst those which are of less refractive
power have a centre which is whiter and smaller, and a black ring
which is broader and dai'ker wdaen the objective is lowered.

Monochromatic light. — ^The same phenomena are obsei'ved by
yellow monochi-omatic light, except that the diffi'action fiinges are
more distinct, fui'ther apai-t, and in greater numl)ei-s than with
ordinaiy light.

A fat-globule, indeied, seems to be composed of a series of con-
centric layers like a grain of starch. With blue light these fringes
ai'e also multiplied, but are closer togethei- and finer, so that they
are not so easily visible.

Yellow monochromatic light, therefoi'e, constitutes a good
means for detei'mining whethei- the strife seen on an object ai-e
peculiar to it or are only diffraction lines. In the former case
they are not exaggerated by monochromatic light ; but if, on the
contrary, they are found to be doubled or quadrupled with this
light, we may be certain that they are diffraction fringes.

But there is no source of fallacy, to a certain class of workei'S, so
much to be guaixled against as that arising from errors in the inter-
pretation concerning movements as such, and especially concerning
the movement exhihited by certain very oninute particles of matter in
a state of suspe7ision in fluids. The movement was first observed in
the fine granular particles which exist in great abundance in the
contents of pollen gi-ains of plants known as the fovilla, and which
are set free by crushing the joollen. It was first supposed that they
indicated some special vital movement analogous to tlie motion of
the spermatozoa of animals. But it was discovered in 1827, by Dr.
Robei't Brown, that inorganic substances in a state of fine trituration
would give the same result ; and it is now known that all substances
in a sufficiently fine state of powder are afiected in the same manner,
one of the most remarkable being the movement visible in the con-
tents of the fluid cavities in quartz in the oldest rocks. These have
probably retained their dancing motion for asons. A good illusti'a-
tion is gamboge, which can be easily rubbed from a water-colour
cake upon a glass slip and covered, and will at once show the
characteristic movement ; so will carmine, indigo, and other similarly
light bodies. But the metals which are from seven to twenty times
as heavy as water require to be reduced to a state of minuteness
many times gi'eater ; but, triturated finely enough, these also show
the movement, for a long time known, from the name of its dis-
coverer, as Broio7iian movement, but now more generally called
2iedesis.

The movement is chiefly of an oscillatory nature, but the particles
also i-otate backwards and forwards on their axes, and gradually (if
l^ersistently watched) change their places in the field of view. It is an
extremely characteristic movement, and could not be mistaken for
any vital motion by an observer acquainted with both ; but the
student must familiarise himself with this kind of motion or he will
be utterly unable to distinguish certain kinds of motion in minute
living forms in certain stages of their life fi'om this movement, and
will make erroneous inferences.

432 MANIPULATION AND PKESEEVATION OF THE MICEOSCOPE

The movement of the smallest particles in pedesis is always the
most active, while in the majority of cases particles greater than the
iTToo'th of ^^ inch are wholly inactive. A drop of common ink
which has been exposed to the air foi- some weeks, or a drop of fine
clay (such as the prepared kaolin used by photographers), shaken up
with water, is recommended by Professor Jevons,^ who has recently
studied this subject, as showing the movement (which he designates
2)edesis) extremely well. Biit none of the particles he has examined
is so active as those of pumice-stone that has been ground up in an
agate mortar ; for these are seen under the microscope to leap and
swarm with an incessant quivering movement, so rapid that it is
impossible to follow the course of a particle, which probably changes
its direction of motion fifteen or twenty times in a second. The
distance through which a jDarticle moves at any one bound is usually
less than ^-Q^Q^pth of an inch. This ' Brownian movement ' (as it is
commonly termed) is not due to evaporation of the liquid, for it
continues without the least abatement of energy in a drojj of aqueous
fiuid that is completely surrounded by oil, and is therefore cut ofi"
fi-om all possibility of evaporation ; and it has been known to con-
tinue for many years in a small quantity of fluid enclosed between
two glasses in an air-tight case ; and for the same reason it can
scarcely be connected with the chemical change. But the observa-
tions of Professor Jevons {loc. cit.) show that it is greatly affected
by the admixture of vaiious substances with water, being, for
example, increased by a small admixture of gum, while it is checked
by an extremely minute admixtui'e of sulphuric acid or of various
saline compounds, these (as Professor Jevons j^oints out) being all
such as increase the conducting power of water for electricity. The
rate of subsidence of finely divided clays or other particles suspended
in water thus greatly depends upon the activity of their ' Brownian
movement,' tor when this is brought to a stand the pai'ticles aggre-
gate and sink, so that the liquid clears itself.^

Pedetic motion depends on, that is, is afiected by —

1. The size of the particles.

2. The sjiecific gravity of the j^ft'^'ticles. Metals, or particles of
vermilion, of similar size to particles of silica or gamboge, move much
more slowly and less frequently.

3. The nature of the liquid. Xo liquid stops pedesis, but liquids
which have a chemical action on the substance do hinder it. This
action may be very slow ; still it tends to agglomerate the particles.
For instance, barium sulphate, when precipitated from the cold
solution, takes a long time to settle ; whei-eas, when warm and in
pi-esence of hydrochloiic acid, agglomeration soon occurs. Iron pi-e-
cipitated as hydrate in presence of salts of ammonium, and mud in
salt water, are other instances. The motion does not cease, but the
particles adhere together and move very slowly.

But besides the right appi-eciation of the nature of pedesis,
there is the utmost caution i-equii-ed in the interpretation of the

1 Quarterly Journal of Micro. Science, N.S. vol. viii. 1878, p. 172.

2 See also the Rev. J. Delsaiilx, ' On the Thermo-dynamic Origin of the Brownian
Motions,' in Monthhj Journal of Microsc. Sci. vol. xviii. 1877.

INTEEPRETATION OF MICKOSCOPIC MOVEMENT 433

rapidity of movevient, and kind of movement, which living and motile
forms effect.

The observation of the phenomena of motion under the microscope^
has led to many false views as to the nature of these movements.
If, for instance, swarm-spores are seen to ti'averse the field of view
in one second, it might be thought that they race through the water
at the speed of an ari'ow, whereas they in reality traverse in that
time only a third part of a millimetre, which is somewhat more than
a metre in an hour. It must not, therefore, be forgotten that the
rapidity of motion of microscopical objects is only an apparent one,
and that its accurate estimation is only possible by taking as our
standard the actual ratio between time and space. If we wish, for
the sake of exact comparison, to estimate the magnitude of the mov-
ing bodies, we may always do so ; the ascertainment of the I'eal
rapidity remains, however, with each successive motion, the princi-
pal matter.

If a screw-shaped spiral object, of slight thickness, revolves on
its axis in the focal plane, at the same time moving forward, it
presents the deceptive appearance of a serpentine motion. Thus it
is that the hoi'izontal j^rojections of an object of this kind, corre-
sponding to the successive inoments of time, appear exactly as if the
movement were a true serpentine one. As an example of an appear-
ance of this nature we may mention the alleged serpentine motion
of Spirillum and Yibrio.

Similar illusions are also pi'oduced by swarm-spores and sperma-
tozoa ; they appear to describe serpentine lines, while in reality they
move in a spiral. It was formerly thought that a number of differ-
ent appearances of motion must be distinguished, whereas modern
observers have recognised most of them as consisting of a forward
movement combined with rotation, where the revolution takes place
sometimes round a central, and sometimes round an eccentric, axis.
To this category belong, for instance, the supposed oscillations of
the oscillatorice, whose changes of level, when thus in motion, were
formerly iinnoticed.

In addition to these characteristics of a spiral motion it must, of
course, be ascertained whether it is right- or left-handed. To dis-
tinguish this in spherical or cylindrical bodies, which i-evolve round
a centi-al axis, is by no means easy, and in many cases, if the object
is very small and the contents homogeneous, it is quite impossible.
The slight variations from cylindrical or spherical form, as they
occur in each cell, are therefore just sufiicient to admit of our per-
ceiving whether any rotation does take place. The disco veiy of the
direction of the rotation is only possible when fixed points whose
position to the axis of the spiral is known can be followed in their
motion i^ound the axis. The same holds good Silso, mutatis mictandis,
of spirally wound threads, spiral vessels, &c. ; we must be able
to distinguish clearly which are the sides of the windings tui-ned
towards or turned away from us.

If the course of the windings is very irregular, as in fig. 367, a
little practice and care are needed to distinguish a spiral line as

1 Das Mikroskop, Naegeli and Schwendener, p. 258 (Eng. edit.).

P F

434 MANIPULATION AND PRESERVATION OF THE MICROSCOPE

such in small objects. The mici^oscopical image might easily lead
us to the conclusion that we were examining a cylindrical body
composed of bells or funnels inserted one in another. The spirally
thickened threads, for instance, as they originate from the epidermis
cells of many seeds, were thus interpreted, although here and there
by the side of the irregular spirals quite regular ones are also
observed. In illustration of this a very excellent example is given
in the'Quekett Journal' for 1899 (No. 44), p. 166, where Mr.
Nelson shows that a certain structure in the
remarkable diatom Climacosjyhenia Tnoniligera,
which for a long time has been regarded as inter-
locking teeth, is in reality a spiral pipe.

Moreover, it miist not be foi-gotten that in
the microscopical image a spiral line always ap-
pears wound in the same manner as when seen
with the naked eye, while in a mirror (the inver-
sion being only a half one) a right-handed screw
is obviously repi'esented as left-handed, and con-
versely. If, therefore, the microscopical image
is observed in a mirror, as in drawing with the
Sommering mirror, or if the image-forming pen-
cils are anywhere turned aside by a single reflec-
tion, a similar inversion takes place from right-
handed to left-handed, and this inversion is again
cancelled by a second reflexion in some micro-
scopes. All this is, of course, well known, and to
the practised observei' self-evident ; nevertheless many microscopists
have shown that they are still entirely in the dark about matters of
this kind.

One of Professor Abbe's experiments on diffraction phenomiena
proves that when the diffraction spectra of the first order are stopped
out, while those of the second are admitted, the appearance of the
structure will be doul)le the fineness of the actual structure which is
causing the interference.^

Fig. 367.— a spiral
in motion.

Upon this law there appears to depend a number of possible
fallacies, errors which may arise from either its misapprehension or
misinterpretation. At least these appear to us, from a practical
point of view, to be of sufficient importance to need either caution
or a fuller exposition of the great law of Abbe in regard to them.

If, for example, figs. 368, 369, and 370 may be taken to represent
1 See Chapter II.

INTERPKETATION OF THE N.A. TABLE 435

a square grating having 25,000 holes per Knear inch at the focus of an
objective at P, P D the dioptric beam, P^ P^ difiraction spectra of the
first order, and P^ P^ those of the second order, then if the objective
is aplanatic all those spectra will be brought to an identical focal
conjugate ; and the image of the grating will be a counter jjart of the
structure, characteiistic of such a gi-oup of spectra. Let us suppose
our objective to be over-corrected, as in fig. 369, then when the grat-
ing is focussed at P the spectra of the first order only will be brought
to the focal conjugate ; the image, however, will not be materially
afiected on that account, as the difii-action elements of the first order
are alone suificient to give a truthful representation of the 25,000
per inch grating. If, however, the objective be raised so that the
grating lies at P', the diffraction elements of the second order only are
brought to the focal conjugate ; consequently by the hypothesis the
image will have 50,000 holes per linear inch, or double that of the
original. In other words, placing a grating at the longer focus of an
over-corrected objective is apparently tantamount to cutting out the
diffraction spectra of the first order by a stop at the back of the
objective.

The effect of this is to give an impression that there is a strong-
grating with 25,000 holes per linear inch ; and over it another grat-
ing with 50,000 holes per linear inch. The raising the focus so as
to bring P to P' necessarily gives the idea of the fine sti-ucture being
superimposed on the coarse. Therefore the microscopist should
beware, whenever he notices a structure of double fineness over
another one, lest he has a condition of things similar to fig. 369. The
following is a test which may be apj)lied to confirm the genuineness of
any such structure. First measure by means of the divided head
of the fine-adjustment screw, as accurately as possible, the
movement required to bring P to P' in fig. 369 ; next by means
of the draw-tube increase the distance between the eye-piece and the
objective : this will have the effect of increasing the over-correction
of the objective, and a state of things will be obtained as in fig. 370.
Hence it will require a larger movement of the fine-adjustment
screw to bring P to P'. This will make the distance between the
50,000 grating and the 25,000 grating appear greater than it was
before. If this takes place the 50,000 grating is a mere diffraction
ghost. It is well to note that we have seen a photograph by
Mr. Comber of a diatom surface which is uneven. In those parts
where the focus is correct the structure is single, but in the parts
where the focus is withdrawn it is double.

A precisely similar condition of things exists with an under-
corrected objective, only in that case the false finer gi-ating will
ajDjDear below the original coarse grating, and to increase the distance
between them the draw-tube must be shortened.

It may therefore be of service to give an examj)le of the use of
the numerical aperture table as a check in the interpretation of
structure.

Fig. 371 gives six illustrations of the back of an objective (the
eye-piece being removed) of -83 IST.A., or 112° in air. T) stands for

F F 2

436 MANIPULATION AND PRESERVATION Of THE MICROSCOPE

dioptric beam ; 1 for diffraction spectrum of the first order ; 2 for
diffraction spectrum of the second order.

When the back of an objective of "83 N.A. shows an arrange-
ment as in

No. 1, then, although the structure will be invisible,

it cannot be coarser than . . . 40,000 per inch.

No. 2 „ „ „ 80,000

No. 3, then the structure does not differ greatly from 40,000 „

No. 4 „ „ „ 80,000

No. 5 „ „ „ 20,000

No. 6 „ „ „ 40,000

It will be understood by the student that the 2y''eservation of the
microscope and its apparatus is a matter that must largely depend
ui^on his own action. The stand should be kept from dust, generally
wiped with a soft chamois leather after use, and when needful a
minute quantity of watchmaker's oil may be put to a joint working
stiffly. There is no better way to preserve this stand than to keep

©

o

it either under a bell-glass or in a cabinet which is easily accessible.

All objectives should be examined after use, and all oils or other
fluids carefully wiped away from them with old cambric which has
been thoroughly washed with soda, well rinsed and not ' ironed ' or
finished in any way, but simply dried.

If chemical reagents are employed the cessation of their use
should become the moment for wiping with care the lenses employed ;
and all processes involving the use of the vapours of volatile acids, '
or which develop sulphuretted hydrogen, chlorine, &c., must never
take place in a room in which a microscope of any value is placed.

Dry elder-pith and Japanese paper are by some workers sug-
gested for cleaning the front lenses of homogeneous objectives ; but
while these are excellent, especially the former, we find nothing
better than the simple cambiic we suggest.

Two or three good chamois leathers should be kept by the
worker for specific purposes and not interchanged. Cleanliness,
care, delicacy of touch, and a purpose to be accurate in all that he
4oes or seeks to do are essentials of the successful mici-oscopist.

DUST ON THE EYE-PIECE 437

It may be noted that dust on the eye -piece can be detected in a
dim light, and can be discovered by closing the iiis diaphi'agm. The
lens of the eye-piece on which the dust appears may be localised by
rotation ; and this should be done before wiping. In reference to
dust on the back of the objective, it should be observed that if the
eye-piece be removed, dust sometimes appears to be upon it which
comes really from the focus of the sub-stage condensei', and is, in fact,
not on the back of the objective at all. To find this condition, remove
the light modifier (if in use), for the dust may be on it, and I'otate
the condenser ; else there will be needless and injurious rubbing of
the back lens of the objective.

With oil-immersion objectives dust or air-bubbles in the oil must
be carefully avoided.

If chamois leather be used for cleaning the lenses, it should be
previously well beaten and shaken, and then kept constantly in a
well-made box.

438

OHAPTEE VII

PBEPABATION, MOUNTING, AND COLLECTION OF OBJECTS

Under this head it is intended to give an account of those materials,
instruments, and appliances of various kinds which have been found
most serviceable to microscopists engaged in general biological re-
search, and to describe the most approved methods of employing
them in the prejjaration and mounting of objects for the display of
the minute structures thus brought to our knowledge. Not only is
it of the greatest advantage that the discoveries made by microscopic
research should — as far as possible — be embodied (so to speak) in
' preparations,' which shall enable them to be studied by every one
who may desire to do so, but it is now universally admitted that
such ' preparations ' often show so much more than can be seen in
the fresh organism that no examination of it can be considered as
complete in Avhich the methods most suitable to each particular
case have not been put in practice. It must be obvious that in a
comprehensive treatise like the present such a general treatment of
this subject is all that can be attempted, excepting in a few instances
of peculiar interest ; and as the histological student can find all
the guidance he needs in the numerous manuals now prepared for
his instruction, the Author will not feel it requisite to furnish him
with the special directions that are readily accessible to him else-
where.

Materials, Instruments, and Appliances.

Glass Slides. — The kind of glass best suited for mounting objects
is that which is known as ' patent plate,' and it is now almost in-
variably cut, by the common consent of microscopists in this country,
into slips measuring 3 in. by 1 in. For objects too large to be
mounted on these the size of 3 in. by 1^ in. may be adopted. Such
slips may be purchased, accurately cut to size, and ground at the
edges, for so little more than the cost of the glass that few persons
to whom time is an object would troxible themselves to prepare
them ; it being only when glass slides of some unusual dimensions
are required, or when it is desired to construct ' built-up cells,' that
a facility in cutting glass with a glazier's diamond becomes useful.
The glass slides prepared for use should be free from veins, air-bubbles,
or other flaws, at least in the central part on which the object is
placed ; and any whose defects render them unsuitable for oi'dinary
purposes should be selected and laid aside for uses to which the
working microscopist will find no diflficulty in putting them. As

COVEKING GLASS 439

the slips vary considerably in thickness, it Avill be advantageous to
determine on a gavige for thin, thick, and medium glass. The first
may be employed for mounting delicate objects to be viewed by the
high powers with which the apochromatic and achromatic condensers
are to be used, so as to allow plenty of room foi- the focal point of an
optical combination with great ajoerture to be fixed readily upon the
2)lane of the object ; the second should be set aside for the attach-
ment of objects which are to be ground down, and for which, there-
fore, a stronger mounting than usual is desii'able ; and the third are
to be used for mounting ordinary objects. Great cai-e should be
taken in washing the slides, and in removing fi-om them every trace
of greasiness by the use of a little soda or potass solution. If this
should not sufiice they may be immei-sed in the solution recommended
by Dr. Seiler, composed of 2 oz. of bichromate of potass, 3 fl. oz. of
sulphuric acid, and 25 oz. of water, and afterwards thoroughly rinsed.
(The same solution may be adv^antageously used for cleansing cover-
glasses.) Before they are put away the slides should be wiped
]3erfectly dry, first with an ordinary ' glass cloth,' and afterwards
with an old cambric handkerchief; and before being used each
slide should be washed in methylated sjjiiit to ensure freedom from
greasiness. Where slides that have been already employed for
mounting preparations are again bi'ought into use, great care should
be taken in completely removing all trace of adherent varnish or
cement — first by scraping (care being taken not to scratch the glass),
then by using an appropriate solvent, and then by rubbing the slide
with a mixture of equal parts of alcohol, benzole, and liquor sodae,
finishing with clean water.

Thin Glass. — The older microscopists were obliged to employ thin
lamina? of talc for covering objects to be viewed with lenses of short
focus ; but this material, Avhich was in many respects objectionable,
is entirely superseded by the thin glass manufactured by Messrs.
Chance, of Birmingham, which may be obtained of various degrees of
thickness, down to the 5-g-oth of an inch. This glass, being unannealed,
is very hard and brittle, and much care and some dexterity are re-
quired in cutting it ; hence covers should be purchased, as required,
from the dealers, who usually keep them in several sizes and supply
any others to order. Save the fact that ' cover-glass ' is made by
Messrs. Chance, there is no definite information as to the mode of its
manufacture and the conditions upon which it is most satisfactorily
produced. It would be an advantage to the microscopist to possess
information on this point. The different thicknesses are usually
ranked as 1,2, and 3 ; the first, which should not exceed in thickness
the "006 in., being used for covering objects to be viewed with loio
powers ; the second, which should not exceed '005 in. in thickness,
for objects to be viewed with medium powers ; and the third, which
ought never to exceed "004 in. in thickness, for objects which either
require or may be capable of being used with high powers. It must,
however, be remembered that the achromatic objectives of great
power and great aperture (1"5) will require much thinner covers
than even this. The thinnest glass is of course most difficult to
handle safely, and is most liable to fracture from accidents of various

440 PKEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS

kinds ; and hence it should only be employed for the purpose for
which it is absolutely needed. The thickest pieces, again, may be
most advantageously employed as covers for large cells, in which
objects are mounted in fluid to be viewed by the low powers whose
performance is not sensibly affected by the aberration thus produced.
The working microscopist will find it desirable to provide himself
with some means of measuring the thickness of his cover-glass ; and
this is especially needed if he is in the habit of employing objectives
without adjustment, which are corrected to a particular standard.
A small screw-gauge of steel, made for measuring the thickness of
rolled jolates of brass, and sold at the tool-shops, answers this purpose
very well ; but Ross's lever of contact (fig. 372), devised for this
express purpose, is in many respects preferable. This consists of a
small hoi-izontal table of brass, mounted upon a stand, and having
at one end an arc graduated into twenty divisions, each of which re-
presents the roVo^h of an inch, so that the entire arc measures the
g^Q th of an inch ; at the other end is a pivot on which moves a long and
delicate lever of steel, whose extremity points to the graduated arc,
whilst it has very near its pivot a sort of projecting tooth, which
bears it against a vertical jjl^-te of steel that is screwed to the

Fig. 372. — Ross's lever of contact.

horizontal table. The piece of thin glass to be measured being in-
serted between the vertical plate and the projecting tooth of the
lever, its thickness in thousandths of an inch is given by the number
on the graduated arc to which the extremity of the lever points.
Thus, if the number be 8, the thickness of the glass is "008, or the r^^gth
of an inch. It will be found convenient to sort the covers according
to their thicknesses, and to keep the sortings apart, so that there
may be a suitable thickness of cover for each object. But it is well
to I'emember that, with the exception of objects to which from their
size or nature it is impossible to apply high powers, it is better to
mount the object so that, if it be required or desirable, high powers
may be used upon it.

Another simple and very efficient covei'-glass tester is made by
Zeiss, of Jena, and illustrated in fig. 373. It will be seen that the
measurement is effected by a clip projecting from a box, between the
jaws of which the cover to be measui'ed is placed ; the reading is
given by an indicatoi" moving over a divided circle on the upper face
of the box. The divisions show hundredths of a millimetre, and the
instrument measures to upwards of 5 mm.

MICROMETERS FOR COVERING GLASS

441

One of the continuous aims of the working microscopist is to
save or utilise to its utmost his time. Complicated measurements and
calculations are to be avoided where possible, and a very beautiful
and ingenious instrument, capable of being used as a meter for
cover-glass, has been devised by Mr. J. Ciceri Smith, of 61 Hatton
Garden, London. It is a perfect direct-reading micrometer, and is
constructed to take measurements in thousandths of an inch, and
may be used in gauging the thickness of microscopical glass, metal
and other sheets, balls
for bearings, needles,
wire, &c. Its advan-
tages over the ordinary
micrometer consist in
the measurements
being automatically
and accurately re-
coi'ded in clear figures
on the index, thus
avoiding the strain on
the eyes caused by
reading the fine lines
on the old form of

gauge ; in there being no liability to errors through miscalcula-
tions, and in its being possible to take any number of various
readings with ease, accuracy, and rapidity. We illustrate this appa-
ratus in fig. 374.

As in the ordinary decimal gauge the glass or other article to be
measured is placed between the ' anvil ' (or hexagonal nut) and the
face of the spindle, the thimble being rotated in either direction

Fig. 373. — Zeiss's cover- aiass tester.

Fig. 874. — Mr. J. Ciceri Smith's direct-reading micrometer.

until the required adjustment is obtained, the exact measurement in
decimal pai'ts of an inch being at the same instant automatically and
accurately recorded on the index, these readings responding in either
direction with the most delicate movements of the screw.

To avoid the screw being unduly strained, the spindle is rotated
by friction from the outer spring-tight thimble, the inner thimble
being rigidly fixed to the spindle. Hence it is impossible to strain
the screw, since as soon as the pressure becomes too great the spring

442 PREPAKATION, MOUNTING, AND COLLECTION OF OBJECTS

allows the outer thimble to slip. The connection of the spindle to
the measuring wheels is effected by means of a stop. This takes
into a slot on a sleeve, on wdiich is mounted the thousandths wheel,
which in turn drives the hundredths and tenths wheels through the
intermediate pinions. These latter have a steji-by-step motion, as
in an ordinary counter. The cover of the cage in which the
mechanism is placed is pierced to show the numbers on the dials, but
these openings are covered with glass, with a view to excluding dust
and dirt. It must be understood that gauges of this kind are
expensive, but there is one made by G. Boley, reading to "01, which
answers all purposes and can be jDurchased for five shillings at a
watchmaker's tool shop.

It is well to keep assorted, measured, and cleaned cover-glasses in
small separate wide-stoppered bottles of methylated spirit, each
bottle being labelled with the gauge of thickness of the covers it
contains. What is then required is a simple apparatus for cleaning
the delicate covers with the least risk of breakage. This can be
well accomplished by having two blocks of boxwood, shaped so as to
be easily held one in each hand, turned with perfect trueness on the
faces opposite to the respective handles, so that when the surfaces
so flattened are laid upon and pressed towards each other they are
eveiy where in perfect contact. They should be from two to four
inches in diameter, and these flattened surfaces should each have,
very tightly stretched upon them, a firm, even-textured, moderately
thick piece of chamois leather. If covers be slightly moistened —
even breathed upon — and laid on one of these blocks and pressed
dow^n with the other, breath, or moisture applied by a small camel-
hair brush to the upper surface of the cover, may be applied, and a
few twists of these blocks upon each other when firmly pressed
together will effectually clean without breaking the thinner covers.
It will be often needful to ti-eat both sides of the covers thus, as one
side generally adheres while the other is subject to the friction.

For cleaning slips and covers by hand, finishing should be done
with old fine cambric handkerchiefs. These should not be washed
with soap, but with common soda and hot water, plenty of the latter
being subsequently employed to get rid of every trace of the alkali.
But when diy these cloths must not be ' ironed ' or smoothed in any
way, the ' rough-dry ' surface acting admirably for wiping delicate
glass.

Varnishes and Cements. — There are three very distinct purposes
for which cements which possess the power of holding firmly to glass,
and of resisting not merely water but other preservative liquids,
are required by the microscopist, these being (1) the attachment of
the glass covers to the slides or cells containing the object, (2) the
formation of thin ' cells ' of cement only, and (3) the attachment of
the ' glass plate ' or ' tube-cells ' to the slides. The two former of
these purposes are answered by liquid cements or varnishes, which
may be applied without heat ; the last requires a solid cement of
greater tenacity, which can only be used in the melted state. Among
the many such cements that have been recommended by diflerent
workers, two or three will be selected by the worker for general

VAENISHES AND CEMENTS 443

purposes, and pei'liaps three oi- foui- for special purjDoses, and the i-e-
maiuder will be in pi-actice neglected. We do not hesitate to say
that the two cements on which the most complete ti-ust may be i-e-
posed are japanner's gold size and Bell's cement. This opinion is the
result of over twenty years of special observation.

A good varnish may easily, in a general way, be tested : when it is
thoroughly hai'd and old, if sci-aped off it comes away in shi'eds ; un-
safe varnishes break under the scraper in flakes and dust. To those
who put up valuable preparations and objects of value the risk
should never be run of using a new and unknown varnish oi* cement.
Neither appearance nor facility nor cheapness in use should for one
moment weigh against a varnish or cement of known and tested
worth.

Japan7ier's gold size may be obtained from the colour shops. It
may be used for closing-in mounted objects of almost any description.
It takes a peculiarly firm hold of glass, and when dry it becomes
extremely tough without brittleness. When new it is very liquid
and ' runs ' rather too fi-eely ; so that it is often advantageous to leave
open for a time the bottle containing it until the varnish is some-
what thickened. By keeping it still longer, with occasional exposure
to air, it is rendered much more viscid, and though such ' old ' gold-
size is not fit for ordinary use, yet one or two coats of it may be ad-
vantageously laid over the films of newer varnish, for securing the
thickei' covers of large cells. Whenever any other varnish or cement
is used, either in making a cell or in closing it in, the rings of these
should be covered with one or two layers of gold-size extending
beyond it on either side, so as to form a continuous film extending
from the marginal ring of the cover to the adjacent portion of the
glass slide.

Asphalte Varnish. — This is a black varnish made by dissolving-
half a drachm of caoutchouc in mineral naj^htha, and then adding 4 oz.
of asphaltum, using heat if necessary for its solution. It is very
important that the asphaltum should be genuine, and the other
materials of the best quality. Some use asphalte as a substitute for
gold size ; but the Author's experience leads him to recommend that
it should only be employed either for making shallow ' cement cells '
or for finishing off preparations already secured with gold-size. For
the former purpose it may advantageously be slightly thickened by
evaporation.

BeWs cement is sold by J. Bell and Co., chemists, Oxford Street,
London ; they ai'e the sole makers, and I'etain the secret of its com-
position. It is of great service for glycerin mounts ; but the edge
of the cover should be ringed with glycerin jelly before this cement
is applied. It is an extremely useful and reliable varnish, which is
extremely easy of manipulation. It can be readily dissolved in
either ether or chloroform.

Canada balsam is the oleo-resin fi-om Abies balsamea and Pinus
canadensis ; it is so brittle when hardened by time that it cannot
be safely used as a cement, except for the special purpose of attaching
hard specimens to glass, in order that they may be I'educed by
grinding, &c. Although fresh, soft balsam may be hardened by heating

444 PEEPAEATION, MOUNTING, AND COLLECTFON OF OBJECTS

it on the slide to which the object is to be attached, yet it may be
preferably hardened pai masse by exjjosing it in a shallow vessel to
the prolonged but moderate heat of an oven, until so much of its
volatile- oil has been driven off that it becomes altnost (but not quite)
resinous on cooling. If, when a drop is spread out on a glass and
allowed to become quite cold, it is found to be so hard' as not to be
readily indented by the thumb-nail, and yet not so hard as to ' chip,'
it is in the best condition to be used for cementing. If too soft, it-
will require a little more hardening on the slide, to which it should
be transferred in the liquid state, being brought to it by the heat of
a water -bath ; if too hard it may be dissolved in chloroform or ben-
zole for use as a mounting ' medium ; ' we do not recommend its use
for monnts with glyceiin.

Brunswick black is a very useful cement, obtainable at the op-
tician's as prepared for the use of microscopists. It is one of the best
cements for the purpose of ringing mounts, and it may be recom-
mended for turning cells. We have already stated that we do
not, as a rule, recommend opaque or black-ground mounting ; but if
this is desired or needful no better ' ground ' can be obtained than
by putting on the centre of the slide a disc of Brunswick black the
size of the outside of the cell or cover-glass, and while it is wet
putting a thin cover-glass upon it. The cover-glass becomes quickly
fixed, and a pleasant surface is formed to receive the object which it
is intended to mount. Should it be desirable to have the floor of the
opaque cell dead instead of bright, this can be quickly accomplished
with a little emery-powder and water applied to the surface by a
flattened block of tin fixed in boxwood.

■ Brunswick black is soluble in oil of turpentine, and it dries
quickly.

Glue and honey mixed in equal parts is very valuable for special
purposes, and softens with heat.

Shellac cement is made by keej)ing small pieces of picked shel-
lac in a bottle of rectified spirit, and shaking it from time to time.
It cannot be reconnnended as a substitute for any of the preceding,
but it may be employed to put a thin film upon the edge of all
mounts — however closed and finished — that are to be used with homo-
geneous lenses. It is a sure protection against the otherwise in-
jurious action of the cedar oil. Hollis's liquid glue may also be
employed with confidence for this purpose.

Sealing-ioax varnish, which is made by digesting powdered
sealing-wax at a gentle heat in alcohol, should never be used as a
cement ; it is serviceable only as a varnish, and resists cedar oil.

Venice turpentine is the liquid resinous exudation of Abies larix.
It must be dissolved in enough alcohol to filter readily, and after
filtering must be placed in an evaporating dish, and by means of a
sand-bath must be reduced by evaporation one-fourth.

This cement is used for closing glycerin moiaits. Square coveivs
ai-e used, and we find it best to edge the cover with glycerin jelly.
A piece of copper wire of No. 10 to No. 12 gauge is taken, and one
end of it is bent just the length of one of the sides of the cover at
right angles to the length of the wire. This end is now heated in a

COLOURED VARNISHES— DRY MOUNTING 445

spirit lamp, plunged into the cement, which adheres in fair quantity,
and is instantly bi-ought down upon the slide and the margin of the
cover. The fluid turpentine distributes itself evenly along the cover
and slide and hai-dens at once. We have no long experience of it,
but from some of its charactei-istics we ai'e inclined to believe it will
prove a useful cement for this pui-pose.

Marine glue, which is composed of shellac, caoutchouc, and
naphtha, is distinguished by its extraoi'dinaiy tenacity, and by its
power of resisting solvents of almost every kind. DifFei'ent qualities
of this substance are made for the several purposes to which it is
applied, and the one most suitable to the wants of the microscopist
is knoMar in commerce as G K 4. The special value of this cement,
Avhich can only be applied hot, is in attaching to glass slides the glass
or metal rings which thus form ' cells ' for the reception of objects
to be mounted in fluid, no other cement being compai-able to it
either for tenacity or for durability. The manner of so using it will
be presently described.

Various coloured varnishes are used to give a finish to mounted
preparations, or to mark on the covering glasses of large pi-eparations
the parts containing special kinds of noteworthy sti'ucture. A
very good hlack varnish of this kind is made by woi-king up very
finely powdered lamp-black with gold-size. For red, sealing-wax
varnish may be used ; but it is very liable to chip and leave the glass
when hai'dened by time. The red varnish specially prepared for
microscopic purposes by Messrs. Thompson and Capper, of Liverpool,
seems likely to stand better. For white, ' zinc cement ' answers
well, which is made of benzole, gum dammar, oxide of zinc, and
turpentine. But it is inexpensive, and either in Cole's or Ziegler's
formula may be obtained at the optician's. Blue or green pigments
may be worked up with this if cements of those colours be desired.

For attaching labels to slides either of glass or wood, and for
fixing down small objects to be mounted ' dry ' (^wch&aforammifera,
parts of insects, &c.), the Author has found nothing preferable to a
rather thick mucilage of gum arable, to which enough glycei'in has
been added to prevent it from drying hard, with a few drops of some
essential oil to prevent the development of mould. The following
formula has also been recommended : Dissolve 2 oz. of gum arable
in 2 oz. of water, and then add J oz. of soaked gelatin (for the
solution of which the action of heat will be required), 30 drops of
glycerin, and a lump of camphor. The further advantage is gained
by the addition of a slightly increased proportion of glycerin to
either of the foregoing, that the gum can be very readily softened
by watei-, so that covers may be easily i-emoved (to be cleansed if
necessary) and the arrangement of objects (where many are mounted
together) altered.

Cells for Dry-mounting. — Where the object to be mounted ' dry '
(i.e. not immersed either in fluid or in any ' medium ') is so thin as to
require that the cover should be but little raised above the slide,
a ' cement cell ' answers this purpose very well ; and if the ap-
plication of a gentle warmth be not injurious, the pressing down of
the cover on the softened cement will help both to fix it and to

446 PKEPARATION, MOUNTING, AND COLLECTION OF OBJECTS

prevent the varnisli applied round its border from running in.
Where a somewhat deeper cell is reqviired, Prof. H. L. Smith
(U.S.A.) suggests the following specially for the mounting of
diatoms. A sheet of thin wi-i ting-paper dipped into thick shellac
varnish is hung up to dry ; and rings are then cut out from it by
punches of two different sizes. One of these rings being laid on
a glass slide, and the cover, with the object dried upon it, laid on the
ring, it is to be held in its place by the forceps or spring-cHp, and
the slide gently warmed so as to cause a slight adhesion of the
cover to the ring, and of the ring to the slide ; and this adhesion may
then be rendered complete by laying another glass slide on the cover
and pressing the two slides together, with the aid of a continued
gentle heat. Still deeper cells may be made with rings punched out
of tinfoil of various thicknesses and cemented with shellac varnish
on either side. And if yet deej)er cells are needed, they may be
made of turned rings of vulcanite or ebonite, cemented in the same
manner. There is, however, a tendency in shellac-formed cells to
throw off a cloudiness inside the cell, usually called ' sweating,'
which is very undesirable. It has been found that a ring of solid
paraffin, to which the cover is attached, if first ' ringed ' with the
same material and afterwards with a finishing varnish, makes a
useful and permanently clean dry shallow cell ; or paper may be
saturated with paraffin and treated as described for shellac.

Cement-cells. — Cells for mounting thin objects in any watery
medium may be readily made with asphalte or Brvmswick black
varnish by the use of Mr. Shadbolt's ' turn-table ' or one of its modi-
fications. The glass slide being placed under its spring in such
a manner that its two edges shall be equidistant from the centre (a
guide to which position is afforded by the circles traced on the brass),
and its four comers equally projecting beyond the circular margin
of the plate, a camel's-hair pencil dipped in the varnish is held in the
right hand, so that its point comes into contact with the glass over
whichever of the circles may be selected as the guide to the size
of the ring. The turn-table being made to rotate by the application
of the left forefinger to the milled head beneath, a ring of varnish
of a suitable breadth is made upon the glass ; and if this be set aside
in a horizontal position, it will be found, when hard, to present a very
level surface. If a greater thickness be desired than a single appli-
cation will conveniently make, a second layer may be afterwards
laid on. It will be found convenient to make a considerable number
of such cells at once, and to keep a stock of them ready prepared for
use. If the surface of any ring should not be sufficiently level for a
covering glass to lie flat upon it, a slight rubbing upon a piece of
fine emery paper laid ujoon a flat table (the ring being held down-
wards) will make it so.

Ring-cells. — For mounting objects of greater thickness it is
desii-able to use cells made by cementing rings, either of glass or metal,
to the glass slides, with marine glue. Glass rings of any size, dia-
meter, thickness, and breadth are made by cutting transverse sections
of thick-walled tulles, the surfaces of these sections being ground
flat and parallel. Not only may round cells (fig. 375, A, B) of vari-

MOUNTING IN CELLS

447

ous sizes be made by this simple method, but, by flattening the tube
(when hot) from which they ai'e cut, the sections may be made qua-
di^angular, or square, or oblong (0, D). For intermediate thicknesses
between cement-cells and glass ring-cells, the Editor has found no
kind more convenient thaii the rings stamped out of tin, of various
thicknesses. These, after being cemented to the slides, should have
their surfaces made perfectly flat by rubbing on a piece of fine grit
or a corundum-file, and then smoothed on a Water-of-Ayr stone ;
to such surfaces the glass covers will be found to adhere with great
tenacity. The ebonite and bone cells are cheap, and also easy of
manipulation. They are specially useful foi- dry mounts.

The glass slides and cells which are to be attached to each other
must first be heated on the mounting plate ; and some small cuttings
of marine glue are then to
be placed either upon that
surface of the cell which is
to bo attached, or upon ^
that portion of the slide
on which it is to lie, the
former being perhaps pre-
ferable. When they begin
to melt, they may be b
worked over the surface of
attachment by means of a
needle point ; and in this
manner the melted glue
may be uniformly spread, q
care being taken to pick
out any of the small gritty
particles which this cement
sometimes contains. When
the surface of attachment
is thus completely covered D
with liquefied glue, the cell
is to be taken wp with a
pair of forceps, turned
over, and deposited in its

proper place on the slide ; and it is then to be firmly pressed down
with a stick (such as the handle of the needle), or with a piece of
flat wood, so as to squeeze out any superfluous glue from beneath.
If any air-bubbles should be seen between the cell and the slide,
these should if possible be got rid of by pressure, or by slightly
moving the cell from side to side ; but if their presence results, as is
sometimes the case, from deficiency of cement at that point, the cell
must be lifted off again, and more glue applied at the required
spot. Sometimes, in spite of care, the glue becomes hardened and
blackened by overheating ; and as it will not then stick well to
the glass, it is preferable not to attempt to proceed, but to lift off
the cell from the slide, to let it cool, scrape off" the overheated glue,
and then repeat the process. When the cementing has been satis-
factorily accomplished, the slides should be allowed to cool gradually

Fig. 375. — Glass ring-cells.

448 PEEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS

in ordei' to secure the firm adhesion of the glue ; and this is readily
accomplished, in the first instance, by pushing each, as it is finished,
towards one of the extremities of the plate. If two plates ai-e in
use, the heated plate may then be readily moved away upon the ring
which supports it, the other being brought down in its place ; and as
the heated plate will be some little time in cooling, the firm attach-
ment of the cells will be secured. If, on the other hand, there be
only a single plate, and the ojaerator desire to proceed at once in
mounting more cells, the slides already completed should be carefully
removed from it, and laid uj)on a loooden siirface, the slow conduc-
tion of which will jjrevent them from cooling too fast. Before they
are quite cold, the superfluous glue should be scraped from the glass
with a small chisel or awl, and the surface should then be carefully
cleansed with a solution of potash, which may be rubbed upon it
with a piece of rag covering a stick shaped like a chisel. The cells

should next be washed
with a hard brush and
soap and water, and
may be finally cleansed,
by rubbing with a little
Aveak spirit and a soft
cloth. In cases in which
appearance is not of
much consequence, and
especially in those in
which the cell is to be
used for mounting large
opaque objects, it is de-
cidedly preferable not
to scrape off the glue
too closely round the
edges of attachment, as
the ' hold ' is much
firmer, and the proba-
bility of the penetra-
tion of air or fluid much
less, if the immediate margin of glue be left both outside and
inside the cell. To those to whom time is of value, it is recom-
mended that all cells which require marine glue cementing be
purchased from the dealers in microscopic apparatus, and it is
well to note that all cells cemented with marine glue should be
well ' payed,' as the nautical expression is, or well surrounded
with shellac varnish or gold-size as indicated by the nature of
the enclosed fluid. Many media, saline fluids especially, work their
way between the cell and the slide, and at length destroy the marine
glue.

Plate-glass Cells. — Where large shallow cells with flat bottoms ai-e
required (as for nvouyxtin^ zoophytes, small mechtsoi, &c.), they may be
made by drilling holes in pieces of plate-glass of various sizes,
shapes, and thicknes^ses (fig. 376, A), which are then cemented
to the slide with mai'ine glue. By drilling two holes at a

Pig. 376. — Plate-glass cells.

mouxtinCt cells

449

suitable distance, and cutting out the piece between them, any
required elongation of the cavity may be obtained (B, C, D).

Sunk-cells. — This name is given to round oi- oval hollows, exca-
vated by grinding in the substance of glass slides, which for this
purpose should be
thickei- than oi'dinary.
They are shown in fig.
377,' A, B, C. Such ^
cells have the advan-
tage not only of com-
parative cheapness, but
also of dm'ability, as
they are not liable to
injury by a sudden jai-,
such as sometimes
causes the detachment
of a cemented plate or
ring. For objects whose
shape adapts them to
the foi-m and depth of
the cavity, such cells ^
will be found very con-
venient. It naturally
suggests itself as an
objection to the use of
such cells that the con-
cavity of their bottom
must so deflect the
light-rays as to distort or obscui'e the image ; but as the cavity is
filled either with water or some other liquid of higher refractive
power, the deflection is so slight as to be practically inoperative.
Before mounting objects in such cells the microscopist should see
that their concave surfaces are free from scratches or roughnesses.

Built-up Cells. — When cells are required of forms or ilimensions
not otherwise procurable,
they may be built wp of
sepai-ate pieces of glass
cemented together. Large
shallovj cells, suitaMe for
mounting zoophytes or
similar flat objects, may be
easily constructed after
the following method : A
piece of plate-glass, of a
thickness that shall give
the desired depth to the
cell, is to be cut to the
dimensions of its outside

wall; and a strip is then to be cut ofi' with the diamond from
each of its edges, of such breadth as shall leave the interior piece
equal in its dimensions to the cavity of the cell that is desired.

G G

Fig. 377. — Plate-glass sunk-cells.

Fig. 378. — Built-up cells.

450 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

This j)iece being rejected, the four strips are then to be cemented
upon the glass slide in their original position, so that the diamond-cuts
shall fit together with the most exact precision ; and the upper
surface is then to be ground flat with emery iipon a pewter plate
and left rough. The perfect construction of lai-ge deej) cells of this
kind, as shown in fig. 378, A, B, however, requires a nicety of work-
manship which few amateurs possess, and the exjjenditure of more
time than microscopists generally have to spai-e ; and as it is conse-
quently preferable to obtain them ready-made, dii-ections for making
them need not be here given.

Wooden Slides for Opaque Objects. — Such 'dry' objects as/or«-
minifera, the capsides of mosses, parts of insects, and the like, raay
be conveniently mounted in a very simple form of wooden slide (first
devised by the Author and now come into general use), which also
serves as a protective ' cell.' Let a number of slips of mahogany or
cedar be provided, each of the 3-inch by 1-inch size, and of any
thickness that may be found convenient, with a corresponding
number of slips of card of the same dimensions, and of j)ieces of
dead-h\-Ack paper rather larger than the aperture of the slide. A
piece of this paper being gummed to the middle of the card, and
some stiff gum having been previously spread over one side of the
wooden slide (care being taken that there is no superfiuity of it
immediately around the apei-ture), this is to be laid down upon the
card, and subjected to pressure.^ An extremely neat ' cell ' will thus
be formed for the reception of the object, as we see in fig. 379, the

depth of. which will be detei'-
\ mined by the thickness of the
\ slide, and the diameter by the
\ size of the perforation ; and it
' ' will be found convenient to

Pig. 379. — Slip made of wood. provide slides of various thick-

nesses, with apertvires of diffe-
i-ent sizes. The cell should always be deep enough for its wall to
I'ise above the object ; but, on the other hand, it should not be too
deep for its walls to interfere with the oblique incidence of the light
upon any object that may be neai- its periphei-y. The object, if flat
oi' small, may be attached by gum-mucilage ; if, however, it be large,
and the j)ai't of it to be attached have an iri-egvilar surface, it is
desirable to form a ' l^ed ' to this by gum thickened with stai'ch. If,
on the other hand, it should be desired to mount the object edgeways
(as when the Tnouth oi i\. formmnifer is to be bi'ought into view), the
side of the object may be attached with a little gum to the wall of
the cell. The complete pi-otection thus given to the object is the
great i-ecommendation of this method. But this is by no means
its onl}^ convenience. It allows the slides not only to i-ange in
the oi'dinaiy cabinets, but also to be laid one against or ovei-
another, and to be packed closely in cases, or secured by elastic

' It will be found a very convenient plan to prepare a large number of such slides
at once, and this may be done in a marvellously short time if the slips of card have
been previously cut to the exact size in a bookbinder's press. The slides, when put
together, should be placed in pairs, l^ack to back, and every pair should have each
of its ends embraced by a spring- press (fig. S85) until dry.

TUEN-TABLES— FINISHING

451-

Fiw. 380.— Shadbolt's turn-table.

bands ; which plan is exti'emely convenient not merely foi' the
saving of space, but also for preserving the objects from dust. Should
any moi'e special protection be I'equired, a thin glass cover may be
laid ovei' the top of the cell, and secured there eithei- by a rim of
gum or by a perforated paper cover attached to the slide ; and if
it should be desired to pack these covered slides together, it is only
necessary to interpose guards of card somewhat thicker than the'
glass covers.

Turn-table. — This simple instrument (iig. 380), devised hj
Mr. Shadbolt, is almost indispensable to the microscojDist who desires
to presei've prepai'a-
tions that ai'e

mounted in any
' mediiim ' beneath
circular covers ; since
it not only serves
foi- the making of
those ' cement-cells '
in which thin ti-ans-

parent objects can be best mounted in' any kind of 'medium,' but
also enables him to apply his varnish for the securing of circular
cover-glasses not only with greater neatness and quickness, but also
with greater certainty than he can by the hand alone. The only
special precaution to be observed in the use of this instrument is
that the cover-glass, not the slide, should be ' centred ; ' which can
be readily done, if several concentric circles have been turned on the
rotating-table, by making the cover-glass correspond with the one
having its own diameter. A num-
ber of ingenious modifications have
been devised in this simple instru-
ment with a view to securing exact
centring. The most practicable
and inexpensive of these is an
apphcation of Mr. E. H. Griffith's
device shown in its improved form
in fig. 381.

The centre of the table marked
with circles has a straight spring
attached to it beneath. The slide,
being placed between the two pins
A and B in this centre, is partially
rotated against the spring and
pushed foi'ward, when the spring-
keeps it between the two pins and a third fixed pin, D, at the upper
side of the slide, centring it perfectly for width. The fourth pin,
E, at the left end, 1^ in. from the centre, is for length, and allows
the slide to be always placed in the same relative position. The
recent improvements acid much to the value of the table. One of
them is a countersunk decentring wheel and pin, C, which may be
seen at the upper right-hand side of the slide. The axle of the
wheel passes through the table and is furn'.shed underneath with a

G G 2

Fig. 381.— Griffith's turn-table.

452 PEEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS

short bar with which the decentiiiig wheel may he tui-ned, forcing
the pin against the slide, pushing it as far out of centre as may be
desired. Another improvement is in making the end-pin a screw,
which may be turned down out of the way if desired.

Mounting Plate and Water-Bath. — Whenever heat has to be
apjDlied either in the cementing of cells oi' in the mounting of
objects, it is desirable that the slide should not be exposed direct to
the flame, but that it should be laid iij^on a surface of regulated
temperature. As cementing with marine glue or hardened Canada
balsam requires a heat above that of boiling water, it must be

Fig. 382. — Apparatus for preparing mounting media, paraf&n, &c., for imbedding

by heat.

sujjplied by a plate of metal ; and the Authoi-'s exj^erience leads him
to recommend that this should be a piece of ii-on not less than six
inches square and half an inch thick, and that it should be
supported, not on legs of its own, but on the ring of a retort-stand,
so that by i-aising or lowering the ling any desii-ed amount of heat
may be impai-ted to it by the lamp or gas-flame beneath. The
advantage of a plate of this size and thickness consists in the
(jradationcd temperature which its difterent parts aflibrd, and in the
slowness of its cooling when removed fi'om the lamp. When many
cells are being cemented at once, it is convenient to have two such
plates, that one may be cooling while the other is being heated.

WATER-BATH— SPEING-PRESSES 453

It is also needful to have a snuillei' plate, much thinner, of brass,
having a groove cut in it into which the ordinary 3x1 in. mounting
slip can easih' slide, but so gi-ooved as to leave a space between a
ledge on each side on which the slip i-ests, and the main surface of
the brass under the slip. In this way there is always a film of
heated air bet^v^een the main sui-face of the heated brass and that of
the glass, giving more facility foi- i-apid and delicate heating. This
may be either a separate ' table ' or a plate fitted to a retort-stand.

Beyond this, however, heat of various kinds, dry and moist, of
variable but determinate temperatures, will be recjuired for various
purposes, especially foi' melting the various movmting media, such
as gelatin, agar-agar, &c., and also, as we shall shortly see, for the
preparation of imbedding masses for section cutting and a variety
of other purposes. One of the many pieces of apparatus whicla
have been devised to combine as large a number of the requirements
of the mounter in one construction as can be conveniently done was
devised by Dr. P. Maver and his colleagues. It is illustrated in
fig. 382.

W is the bath ; Z the tube by which it is filled with water ; 1 ,
2, 3, 4 are glass tubes ; a is a pot for melting and clarifying the
paraflan, and' this may be replaced by others for othei- needful
purposes ; b and c are half-cylinders with handles for imbedding ; t
is a thermometer bent at a right angle ; the horizontal leg ends in the
air-bath, and can be closed with a glass plate, which is of service for
biological as well as mounting purposes. The temperature in the
air-bath will be always about 10° less than that in the water-bath. It
serves well for evaporating chloroform, &c. ; t^ is the thermometer for
the water-bath ; R is a Reichert's thermo-regulator. The variation
in temperature is less than 1° 0. ; ?• is the tube in which the gas
and air mix, and / a mica chimney. There is a small independent
and removable water-bath, v, filled with water by means of i-ubber
tubes attached to lateral ojjenings. It is supplied with a thermo-
meter, t.2, is warmed on the platform, F, and is intended chiefly for
fixing objects which are small in the right position in the imbedding
mass, usually known as ' orienting ' objects, under a simjjle lens or
dissecting microscope.

Slide-forceps, Spring-clip, and Spring-press. — For holding
slides to which heat is being applied, especially while cementing
objects to be ground down into thin sections, the wooden slide-
forceps, seen in fig. 383, Avill be found extremely convenient. This,
by its elasticity, affords a secure grasp to a slide of any ordinary
thickness, the wooden blades being separated by pressure upon the
brass studs ; while the lower stud, with the bent piece of brass at
the junction of the blades, affoi-ds a level support to the forceps,
which thus, while resting upon the table, keeps the heated glass from
contact with its surface. For holding down cover-glasses whilst
the balsam or other medium is cooling, if the elasticity of the object
should tend to make them spring up, the wire sjDring-clij) (fig. 384),
sold at a cheap rate by dealers in microscopic apparatus, will be
found extremely" convenient. Or if a stronger pressure be required,
recourse may be had to a simple spring-press made by a slight

454 PKEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS

alteration of the ' American clothes-peg,' which is now in genei-al
use in this countiy for a variety of purposes, all that is necessary
being to rub clown the opposed surfaces of the ' clip ' with a flat file,
so that they shall be parallel to each othei- when an ordinary slide
with its cover is interposed between them (fig. 385). One of these

Fig. 383.— Slide-forceps.

convenient little implements may also be easily made to serve the
purpose of a slide-forceps by cutting back the upper edge of the
clip, and filing the lower to such a plane that when it rests on its
flat side it shall hold the slide parallel to the surface of the table, as
in fig. 383.

Fig. 884. — Spring-clip.

Fig. 385. — Spring-press.

Mounting Instrument. — A simple mode of applying graduated
pressure concui'i-ently with the heat of a lamp, which will be found
very convenient in the mounting of certain classes of objects, is
afforded by the mounting insti-ument devised by Mr. James Smith.
This consists of a plate of brass turned up at its edges, of the proper
size to allow the ordinary glass slide to lie loosely in the bed thus
formed ; this plate has a large perforation in its centre, in order to
allow heat to be directly applied to the slide from beneath ; and it

Fig. 386. — Smith's mounting instrument.

is attached by a stout wire to a handle shown in fig. 386. Close to
this handle there is attached by a joint an upper wii-e, which lies
neai'ly pai-allel to the first, but makes a downward tui-n just above
the centre of the slide-plate, and is terminated by an ivory knob ;
this wire is pressed upwards by a spring beneath it, whilst, on the
other hand, it is made to approximate the lower by a milled head
turning on a screw, so as to bring its ivory knob to bear with greater
or less force on the covei-ing-glass. The special use of this arrange-
ment will be explained hereafter.

ARRANGEMENTS FOE DISSECTING

455

. Dissecting Apparatus. — The mode of making a dissection for
mici'oseopic pmposes mnst be detei'mined by tlie size and character
of the object. Generally speaking, it will be found advantageous to
cai-i-y on the dissection undei- watei-, with which alcohol should be
mingled whei-e the substance has been long immei-sed in spiiit. The
size and depth of the vessel should be proportioned to the dimensions
of the object to be dissected ; since, for the ready access of the hands
and dissecting instruments, it is convenient that the object should

Fig. 387. — Swift's Stephenson binocular dissecting microscope.

neither be fai- from its walls noi- lie under any great depth of water.
Where there is no occasion that the bottom of the vessel should be
ti-ansparent, no kind of dissecting trough is moi-e convenient than
that which every one may readily make for himself, of any dimen-
sion he may desire, by taking a piece of sheet gutta-percha of adequate
size and stoutness, warming it sufficiently to i-ender it flexible, and
then turning up its four sides, drawing out one corner into a sort of
■Spout, which serves to pour away its contents when it needs empty-
ing. The dark coloui- of this substance enables it to furnish a back-

456 PEEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS

gi-ound. which assists the observer in distinguishing delicate mem-
branes, fibres, &c., especially when magnifying lenses are employed ;
and it is hard enough (withoiit being too hai-d) to allow of pins Ijeing
fixed into it, both for securing the object and for keeping apart such
portions as it is useful to put on the stretch. When glass oi- earthen-
ware troughs ai-e employed, a piece of sheet-coi-k loaded with lead
must be provided to answer the same purposes. In carrying on
dissections in such a ti-ough, it is frequently desirable to concentrate
additional light ujDon the part which is being operated on by means
of the smaller condensing lens ; and when a low magnif^TLng power
is wanted it may be supplied either by a single lens, mounted after
the manner of Ross's simple microscope, or by a pair of spectacles
mounted with the ' semi-lenses ' oi-dinarily used for stereoscopes.^
Portions of the body under dissection, being floated oS when detached,
may be conveniently taken up from the trough by placing a slip of
glass beneath them (which is often the only mode in which delicate
membranes can be satisfactorily spread out), and may be then placed
under the microscope for minute examination, being first covered
with thin glass, beneath the edges of which is to be introduced a
little of the liquid wherein the dissection is being carried on.
Where the body under dissection is so transparent that more
advantage is gained by transmitting light through it than by looking
at it as an opaque object, the trough should have a glass bottom ;
and for this purpose, unless the body be of unusual size, some of the
glass cells already described (figs. 376-377) will usually answer very
well. The finest dissections may often be best made upon ordinary
slips of glass, care being taken to keep the object sufficiently sur-
rounded by fiuid. For woi^k of this kind no instrument is more
generally serviceable than the erecting binocular form of stand as
recently modified for dissecting purposes by Swift. It is an instru-
ment which combines conveniences and supplies wants which only
a worker at dissection could have known. It is illusti'ated in fig.
387, and will be thoroughly suitable for all the work in which it will
be required, from diatom mounting to the most delicate dissections.
The supports for the hands on either side of the stage have an ex-
tremely suitable curve, and the instrument lends itself admirably to
the work.

The instrume'iits used in microscopic dissection are for the most
part of the same kind as those which are needed in ordinary minute
anatomical research, such as scalpels, scissors, forceps, &c. ; the fine
instruments used in operations upon the eye, however, will commonly
be found most suitable. A pair of delicate scissors, curved to one
side, is extremely convenient for cutting open tubular parts ;
these should have their points blunted, but other scissors should
have fine points. A j^air of very fine-pointed scissors (fig. 388),
one leg of which is fixed in a light handle, and the other kept

1 These may be recommended as useful in a great variety of manipulations which
are best performed under a low magnifying power, with the conjoint use of both eyes.
Where a high power is needed, recourse may be advantageously had to Messrs.
Beck's 3-inch achromatic binocular magnifier, which is coiistructed on the same
principle, allowing the object to be brought very near the eyes, without requiring any
uncomfortable convergence of their axes.

ANALYSIS OF MOUNTING METHODS 457

apart from it by a sj^ring, so as to close by the pressure of the
finger and to open of itself, will be foiuid (if the blades be well
sharpened) much supeiioi' to any kind of knives foi- cutting through
delicate tissues with as little distui'bance of them as possible. A
pair of small sti-aight forceps with fine points, and another paii- of
curved forceps, will be found useful in addition to the ordinaiy
dissecting forcejjs.

Of all the instruments contrived foi- delicate dissections, however,
few are more sei'viceable than those which the microscopist may
make for himself out of oixlinai-}- needles. These should be fixed in
light wooden handles (the
cedar sticks used foi-
camel-hair pencils, oi* the
handles of steel jDen-

holders, or small porcu- Fig. 388.— Spring scissors.,

pine quills will answer
extremely well) in such a manner that theii' points should not
project fai-, since they will otherwise have too much ' spring ; '
much may be done b}^ their mere tearing action ; but if it be desired
to use them as cutting instruments, all that is necessary is to harden
and temjDei' them, and then give them an edge upon a hone. It will
sometimes be desirable to give a finer j)oint to such needles than
they originally possess ; this also may be done upon a hone. A
needle with its point bent to a right angle, or nearly so, is often use-
ful ; and this may be shaped by simply heating the point in a lamp
or candle, giving to it the required turn with a pair of pliers, and
then hardening the point again by re-heating it and plunging it into
cold water or tallow.

Analysis of Methods of Preparation and Mounting which
follow : —

1. Descriptions of microtomes, and linife-holders and hnife-
position.

2. Mounting objects in general.

3. Prej^ai'ation of soft tissues, under the following subtitles : —

Fixation.
Dehydration.
Clearing.
Staining.
This last is further subdivided as follows : —
Stains for living objects.
Stains for fresh tissues.

Stains for fixed and preserved entire objects.
Nuclear stains for sections.
Plasmatic stains.
Imbedding methods under the following subtitles : —
Imbedding methods in general.
The parafiin method.
This last is further subdivided as follows : —

1. Saturation with a solvent.

2. Saturation with pai-aflin.

458 PKEPARATION, MOUNTING, AND COLLECTION OF OBJECTS

3. Arranging for cutting.

4. Cutting.

5. Flattening sections and mounting, with description of the

best serial section 'methods.
The celloidin method, further subdivided as follows : —
Oelloidin imbedding in general.
Hardening the mass.
Fixing to microtome and cutting.
Staining and mounting, with desci'iption of apjrropriate sej'ial

section methods.

4. Preparation of hard tissues, under the following titles : —

Grinding and polishing sections, with descriptions of lathes.

Decalcification.

Desilicification .

5. tSections dealing with

{a) Vegetable tissues.
(6) Staining bacteria.

(c) Staining flagella.

(d) Chemical testing.

(e) Presei-vative media.

(/) Cleanliness, and labelling.

Microtomes are machines devised for the purpose of obtaining
exti'emely thin and uniform slices, or ' sections ' as they are
technically called, of animal or vegetable tissues, hard or soft.
Some of the purposes to which these are adapted will be found
to be answered by a very simple and inexpensive little instrument,
which may either be held in the hand, or (as is prefei'able) may be
firmly attached by means of a f-shaped piece of wood (fig. 389) to
the end of a table or work-bench, or may be provided with a clamp
for firm attachment to the work-table, as in fig. 390. This instru-
ment essentially consists of an upright hollow cylinder of brass,
with a kind of piston which is pushed from below upwards by a fine-
threaded or ' micrometei' ' screw turned by a large milled head ; at
the upper end the cylinder terminates in a brass table, which is
planed to a flat surface, or (which is preferable) has a piece of plate-
glass cemented to it, to foim its cutting bed. At one side is seen a
small milled head, which acts upon a ' binding sci-ew,' whose ex-
tremity projects into the cavity of the cylinder, and sei-ves to com-
pi-ess and steady anything that it holds. For this is now genei-ally
substituted a pair of screws, working through the side of the
cylinder, instead of one as in fig. 390. A cylindrical stem of wood,
a piece of horn, whalebone, cartilage, &c., is to be fitted to the
interior of the cylinder, so as to project a little above its top, and is
to be steadied by the ' binding screw ; ' it is then to be cut to a level
by means of a sharp knife or razor laid flat upon the table. The
large milled head is next to be moved through such a portion of a
tui-n as may veiy slightly elevate the substance to be cut, so as to
make it project in an almost insensible degree above the table, and
this projecting part is to be sliced off with a knife previously dipped

SECTION CUTTERS

459

Fig. 389. — Simple microtome.

ill water or, prefeiably, iiietliylated spirit and water in equal pai'ts.
An oixlinaiy i-azoi- will answei- for cutting. The motion given to
its edge should be a combination of drawing and pressing. (It will
be genei'ally found that
better sections ai-e made
by working the ]s\\i^e from
the operator than towards
him.) When one slice has
been thus taken off, it
should be removed fi'om
the blade by dipping it into
spirit and watei', oi* by the
use of a camel-hail- bi'ush ;
the milled head should be
again advanced, and an-
other section taken, and so
on. It is advantageous to
have the large milled head
graduated, and furnished
with a fixed index, so that
this amount having been

once determined, the screw shall be so turned as to always produce
the exact elevation required. Wliei'e the substance of which it is
desired to obtain sections by this instrument is of too small a size or
of too soft a texture to be
held firmly in the mannei-
just described, it may be
placed between the two ver-
tical halves of a piece of carrot
of suitable size to be pressed
into the cylinder, and the
carrot with the object it
grasps is then to be sliced
in the manner already de-
scribed, the small section of
the latter being carefully
taken ofi" the knife, or floated
away from it, on each occa-
sion, to prevent it from being
lost among the lamellte of
carrot which ai-e removed at
the same time. Vertical
sections of many leaves may
be successfully made in this
way, and if their texture be
so soft as to be injured by
the pressure of the carrot,
they may be placed between two half-cylindei-s of elder-j)itli, or be
imbedded in any of the ways employed with the more elaborate
microtomes about to be described.

The modern art of section-cutting, as practised by the most

o90. — Microtome.

460 PKEPARATION, MOUNTING, AND COLLECTir)N OF OBJECT

accomplished experts, with the most complete of the many almost
perfect recent microtomes, is one of the most refined and beautiful
with which the scientific mind can concern itself. The combined
cutting, staining, and mounting of the most delicate organic tissues in
almost every conceivable state has thrown a light upon histological
and pathological matters, the present and prosj^ective value of which
we can scarcely estimate too highly ; while some of the profoundest
and most interesting questions of biology are opening themselves to
renewed research by its means.

Throughout this chapter we only seek to give the possessor of a
good microscope a fair outline of the princiioal methods employed,
and clues to the finest processes in detail, for histological, patholo-
gical, and embryological work. For full details we may refer him
to the moi-e or less exhaustive handbooks which the several subjects
have called forth, the fullest account of the subject being that given
in Mr. A. BoUes Lee's ' The Microtomist's Yade-Mecum.' But we
are at the same time convinced that if the student be but rightly
directed as to instruments and the best way of emjjloying them, and
at the same time have the best general j^rocesses concisely indicated
to him, he will soon discover what to him will be the most facile and
satisfactory method of obtaining the best results. In the hands of
an original worker presciiptions are only satisfactory starting-points
to better methods. We shall therefore describe one microtome
which we believe, on the whole, to be the best, and sufiiciently
indicate the character and peculiarities of two or three others, to
enable the student, as we believe, to judge for himself in considera-
tion of his future purpose as to which will best serve him in the
object he has in view.

It will be as well, howevei-, to note that extremely thin sections
are not the supreme purpose of microtomes. Good sections, treated
with success from beginning to end, are the first consideration. The
tenuity of a section must be proportional to the character of the
tissue.

Manifestly a tissue with injected arteries or veins must be thick
enough to contain some of these vessels with their branches entire.
If we require to study the hepatic cells or the renal tubules we must
give depth enough in the sections to include these. But it will be
found that the hardening and imbedding agents contract greatly,
without distorting, the anatomical elements, and sections much
thinner than would be normally required to completely disclose what
is sought may be often successfully made in tissues so prepared.

It is none the less true that a mere race for extreme attenuation
in sections is in every sense undesirable ; and for extremely thin
sections — say the -gi^-oth of an inch in thickness, oi- less — only small
sections should be attemj^ted.

Here it may be advisable to state that the standaixl unit in
microscopy, as accepted by the Council of the Royal Microscopical
Society,^ is the lo^th of a millimetre, which is indicated by the
sign jA, being known as a micron.

^ Journ. Boy. Micro. Soc. ser. ii. vol. vii. pp. 502, 526 ; Nof. xxxviii. p. 221.

THE THOMA 3IICR0T0ME

461

The choice of microtomes, English, Continental, and American, is
very large, and high merit is chai-actei-istic of many. But one of
these, devised by Thoma and made by Jung of Heidelbei-g, entei-ed
the field early, ha\ang fi-om the fii-st been based on thoroughly
sound practical pi'incij)les ; and as a i-esult it has been susceptible
of, and has lent itself to, every improvement suggested by the
advancing refinements of this beautiful ai't of microtomy. In its
latest form we describe and illustrate it, satisfied that it will in
an almost perfect manner meet the general wants of the biologist's
laboratory.

This {the Thoma) microtome is based ivpon the viodel of Rivet ; but
that has been immensely expanded in detail. The body of the
instrument consists of three ]Dlates, the middle plate, M, and the
side plates, S and 0, fig. 391. These are fastened to the bottom
plate by screws. S supports the knife-carriage, M S, which i-ests at

Fig. 391. — Jung's Thoma microtome.

three points on a planed and polished ti-ack ; whilst on the side of
the knife-carriage two other points slide upon the middle plate.
Thus in the angle in which the block carrying the knife slides there
are five points of contact on polished sui'faces, the lilock itself having
weight enough to keep the whole steady, so that at a touch it glides
to and fro with a fii'mness and precision that could scai-cely be
attained in any other way.

The plate 0 is an inclined plane, its highest point being in the
direction of M. The inclination of the angle is 1 : 20 ; it supports
the object-holder, 0 S, which rests in its place exactly as does the
knife-carriage, M S.

This plate also bears the scale TA, which, by means of a vei'nier
on the object-holder, enables'the thickness of the section to be read

off-

The bottom plate is at once a base and a receiver for the dripping

spirit, oil, &c.

462 PKEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS

For fastening the knife a tliunib-screw, C, fig. 391, serves; but
in the modified form of the insti-ument designed by the Zoological
Station, Naples, this is replaced by a single head-screw, C, fig. 392,
which is provided with holes and tightened by means of a lever ;
and to give greater freedom to the use of the knife there are several
holes drilled and tapjjed into which this screw fits.

The knives of the form A, fig. 391, are generally screwed directly
to the knife-carriage, and are used for cutting very large sections, the
oblique position shown in the figure being the one that is generally
indicated for the cutting of very large objects. This knife is now
seldom used except in pathological observations and in studies on the
central nervous system.

Fig. 392. — The Thoma microtome with the usual zoologist's knife.

The knife, hoioever, is also made ujyon another model, E, fig. 392 ;
it then has a special holder a, in which it is secured in a conical slit
by the screws 6, &^, and firmly held.

For deep objects requiring considerable length to cut from, there
are plates j)i"ovided for elevating tlie knives and the knife-holders.

The knife-holder shown in fig. 392 can be rotated round the axis
formed by the screw c. This allows of any degree of slant or
obliquity of direction being given to the knife, from the str ctly
transversal position shown in fig. 392 up to and beyond the slanting
position shown in fig. 391. But it provides no means of altering
the tilt of the blade, that is, of elevating or depressing the back of
the blade relatively to its edge — a point of considerable impoi'tance,
to which we shall return later on. To meet this difficulty, the
maker (R. Jung, 1 2 Landhausstrasse, Heidelberg ; liis instruments,
as well as price lists, maj^ be obtained from Mr. 0. Baker, 244 High
Holborn, London) supplies wedges to be inserted under the knife-

POSITION OF KNIFE IN SECTION CUTTING 463

holder. These (Neumayer's) wedges, are hoi'seshoe-.shaped, so that
they may be slipped round the central screw. They ai'e made in pairs,
one membei- of each pair having the opening of the horseshoe at
the thin end, the other having it at the thick end. The wedge with
the opening at the thin end is slip]3ed 'under the knife-holder
(thin end towards the operator), aiid operates to tilt up the back of
the knife.

The sister wedge is then placed over the slotted stem or handle
of the carrier, thick end towards the operator, in order that the
binding-screw may have a horizontal surface to bear on. The
wedges are sold in sets of three jDairs, of different degrees of bevel.

This simple device is quite sufficient so long as the utmost pre-
cision of section-cutting is not i-equired. For moi-e elaborate work
it is convenient to employ a special knife-holder, which jarovides a
means of elevating or depressing the back of tlie blade by rotating
the blade round its axis. Similai- contrivances have been described
by Dr. Hesse (in the ' Zeitschrift fiir wissenschaftliche Mikroskojjie,'
xiv. 1, 1897, p. 13; see 'Journal of the Royal Microscojjical Soc'
1897, p. 441), and by Prof. Apathy ('Zeitschr.,' xiv. 2, p. 15, and
'Journal,' 1897, p. 582). This last is rather compKcated to work
with, and consequently the Naples Zoological Station has worked
out a new device, made by Jung, which it is hoped will meet all
requirements. This is the ' Model L ' of his price-list, and is
figured in the 'Journal,' 1899, p. 546. That of Hesse is very
simple, and ought to be quite sufficient whei'e no considerable
change of tilt is likely to be required. It is made by Jung.

Before leaving this part of the subject it appears advisable to
consider briefly the question of hnife-position in general — a matter
on which success or failure in section-cutting may often entirely
depend.

' The position of the knife should be varied according to circum-
stances, both accoi-ding as to its slant ov obliquity in relation to the
line of section, and as to its tilt^ or the elevation of its back relatively
to its edge.

As regards slant — the slanting position, fig. 391, is adapted for
cutting soft and watery objects, not imbedded, and tissues imbedded
in celloidin, or the like ; for these cannot be cut with the knife
placed transversely. It is also fi-equently indicated for paraffin
objects ; but on this head no general rule can be laid down. The
transverse position, fig. 392, is indicated for cutting paraffin sections
by the ribbon method (see below, Imbedding Methods, Paraffin), and
also frequently for cutting loose sections by the pai'affin method.

As regards tilt : (1) The knife must alvxtys be tilted enough to
lift the under facet of the edge clear of the tissue as it passes over
it, for if not the tissues will be crushed by it as it passes over
fchem. (2) It must not be too much tilted, or it will not bite, but
will act as a scraper. Prof. Apathy, who has investigated the
subject in an instructive paper in the ' iSitzber. d. med.-naturw.
Section d. Siebenburgischen Museumvereins, Kolozsvar,' xix. 1897,
H. 7, concludes as follows : (1) The knife should always be tiltecl
somewhat more than enough to bring the undei' cuttino-facet of the

464 PKEPARATION, MOUNTINa, AND COLLECTION OF OBJECTS

edge clear of the object. (2) It sliould in general be less tilted for
hard and brittle objects than for soft ones, therefore, cceteris parihas,
less for paraffin than for celloidin. (3) The extent of useful tilt
varies (according to the angle to which the knife is gi'ound, amongst
other factors) between 0° and 16°. (Jung's ordinary knife-holders
have mostly a tilt of about 9°, which is only enough, with the usual
plane-concave knives, for cutting ribbons of sections with hard
paraffin.) (4) Excessive tilt causes paraiSn sections to roll, and may
l^roduce longitudinal rifts in them. It may also set up vibrations in
the blade, which are heard as a humming tone, and which give an
undulatory surface to the sections. Excessive tilt may often be
recognised by the knife giving out a short metallic note just as it
leaves the object. For knives with plane under-surfaces it is seldom
advisable to give less than 10° tilt; whilst knives with concave
under-surfaces on the contrary may i-equii-e to be placed almost
horizontal. A knife with too little tilt will cut a second section,
or a portion of one, without the object having been raised ; showing

Fig. 393. — Object-lioldei- with jaws.

that during the first cut the object was pressed down by the knife
and recovered itself afterwards. This fault is denoted by the ringing
tone given out by the knife on passing hack over the object before it
is raised. Ribbon-cutting requires a relatively hard paraffin and
less tilt. With celloidin it is very important to avoid insuificient
tilt, as the elastic celloidin, with too little tilt, yields before the
knife and is not cut.

The exigencies of section-cutting have given rise to a great variety
of ohject-holclers in this instrument. The simj)lest is seen in 0 S,
fig. 391, which is a pair of jaws clamped by screws and fixed upon
the pivot Si by the milled head a. At n is the vernier, which indi-
cates the position on the mm. scale, TA, and t is an agate highly
polished, upon which the mici-ometer screw on works to drive foi-ward
the object-carrier, 0 S.

The Zoological Station at Naples employs a holder specially de-
signed foi- use with paraflin ; the object is soldered with paraifin on
to the cylinder, h y, fig. 392. This is supported on gimbals and may

THE THOMA MICR0T03IE

465

be shifted vertically and hoiizontally by means of the small screw it,
and is festened by means of the milled head, m. By the pinion n it
may be displaced over 90°, and as great an inclination can he taken
in a plane perpendicular to this by the suppoi-ting metal fi'ames by
means of the pinion j). In this way every desired inclination of the
object to the knife can be readily secured.

Fig. 393 presents the same object-holder, but instead of the
cylindei- a simple paii- of jaws with the sci'ew m to secure ol^jects of
every variety. A cylinder-holder as in fig. 393 can be placed in
these jaws from which the benefits of the JSTeapolitan holder can be
secured. But fig. 396 shows a still greater improvement which can be
applied to both object-holders, viz. a 'peiyetidicular dis'placement hy
means of a coy and pinion govei'ning the height of the mass from
which the sections are to be cut.

The elevator in fig. 393 is supported on one side by the prism P,
and on the other by the rod C ; these are joined l^ythe Ijridge b,

Fig. 394. — Object-holder movable about two horizontal axes at right angles
to each other.

to which a cogged bar is fastened, into which a pinion catches, which
is moved by the lever V, allowing a perpendicular displacement of the
object of 12 mm. At O is the millimetre scale on which the perpen-
dicular displacement can be read off' by means of the index x.

An object-holdei- movable about two horizontal axes situated
perpendicularly to each other is seen in fig. 394. These positions
are fixed by the milled heads b^, b ; e shows the jaws for holding the
object, into which, however, cylinders like fig. 396 may be inti-o-
duced. This object-holder has a perpendicular displacement con-
trolled by a screw. The part, K, which supports the chief axis of the
jaws, is fitted on to the ti-iangular pi-ism S^, the lower pai-t of which is
furnished with hinges ; on the hinge the screw Y moves, which at its
uppei' end lies close to K, and is sustained in this position by the
steel plate g, so that K is carried up and down with it, and this
movement is read off" by a scale under S.

II II

466 PKEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS

Fig. 395 presents an object-holder intended to analyse hy diversified
section objects which are wedged or fan-shaped in form on a fixed
axis, but may be applied to other purposes.

B is a prism-shaped, semicircularly bent bar, moxang in the slot
F F^ ; at 6 and b^ the jaws occupy the position common to those of
the ordinary form.

Fig. 395. — Object-holder for analysis by diversified section.

On the circumference of B a spiral is cut, which becomes slightly
visible at g ; into this spiral a screw passes at H, which is turned by
the milled head S, which can alter the position of the arc to the
horizontal to the extent of 1 mm. ; and the amount of the change of
position can be read oW on the graduated circle K. _

In a fixed position the middle of this section-holder is the plane
of action of the knife. If an object be fixed in the jaws so that the

Pig. 396. — Cylinder for use with jaws.

fixed axis of it lies in this jalane, it will only be required that the
screw S be brought into action to obtain wedge-shaped sections of
whatever thickness is i-equired, which will all be made in this axis.

The set of cylinders which may be used with these and othei-
jaws is represented in fig. 396 : by is the cylinder, G the compressing
screw for it, the block W being held in the jaws.

I'he object-slide with its vernier may be slidden up the incline by

THE THOMA MICEOTOME

467

hand ; but it is much more accurate to coiiti'ol its movement with
the micrometer-screw. The point of this screw in fig. 392, t, works
on the poHshed plane of an agate cone. The clam2i on which the
sci-ew is mounted is held fii-mly in its place by the milled head W in
Sc/i. It may stretch up as far as 0, being I'efastened by W.

The screw m is so cut that a single rotation moves the slide on
.the il^'V mm., Avhich in the inclination of the plane of 1 : 20 gives
an elevation of the object of ixr^jr mm. The bari'el or di-um, K,
situated on the axis of the screw, is divided into fifteen parts ; con-
sequently the interval of each division corresponds to an elevation of

1 oVo mm- .

Thei'e is also an action by means of a spring which gives the ear
as well as the eye cognisance of the amount of elevation which has
taken place, which greatly I'elieves the eye. This, howevei-, can be
brought into action or not at the option of the operatoi'.

Besides these object-holders a freezing apparatus can be added
which is simply placed on the object-slide as shown in fig. 397.

Pig. 397. — Freezing aioparatus for the Tlioma microtome.

The freezing is effected by ether-spray. A specially favourable
effect is obtained if the cylinder g is mica and not glass. A layer of
water freezes in from thirty to thirty-five seconds.

An arrangement of the Thoma foi- cutting large objects has also
been devised which is illustrated in fig. 398.

The knife is to be placed considerably higher in front than
behind, in order to lessen the pressure on the objects. In order to
satisfy all demands, the knife-rest is adjustable.

The knife is so arranged that the whole length of blade can be
used, and then the screw c is fairly tightly screwed down. As strong-
knives, even of a length of 36 cm., easily give, a knife-support has
been constructed ; this is fastened by the screw c' to the carrier.
The support is arranged parallel with the back of the knife M ; if
the extremity n be slightly pressed backwards, so that it touches the

H H 2

THE ROCKINa MTCEOTOME 469

knife, it is then fixed in tliis position by the screw 0 (scarcely evident
in the illusti'ation) .

This done, the spirit-A^essel f^p can be ari'anged in a jjosition
which will not interfere with the free movement of the knife. In
order that a stream of spirit may follow the knife over the object,
the following ai-i-angement is adopted. The spirit-vessel S/j) turns
I'ound an axis on the column h ; to it is joined the arm L, which
carries in front the fine tube r (connected with t f), and also the rod
p ; the latter is movable loei-pendiculaidy, and to its low^er end a
bridge or grip with two small i-ollers i and i' is fastened. The rod
p is so placed that on each side of the metal sti'ij) b, screwed on to
the knife-support, there is one of the rollers. By the adjusting-
screws St, the whole apparatus is so arranged that, when the knife-
carrier is in motion, no other friction occurs than that of the rollers
on the strip h b b.

The vessel is filled by screwing oflf the head Z. As the tube r
acts as a siphon, it is necessary, when the cock is tiirned on, to blow
down the tube. The stream of spirit should be directed at a right
angle to the knife, and about the middle of the object. This done,
the object 06, by means of the screw K, is firmly grasped in the fangs
of the object-carrier; the correct direction for the position of the
knife is given to its surface by the screws at f and _/,, and then
the axes of the fangs are tightened up by the levers q and q\ If
the height of the object is not cpiite correct, adjustment is made by
the screw m. By turning the screws S, S the holder is fixed.

V is a wheel with cranked axle E?y, and this by means of a cat-
gut band moves the knife.

For the rapid ^^I'oduction of ribbons of sections, however, the
instrument j»;ar excelUnce is the Cambridge rocking microtome. It
is illustrated in fig. 399.

The principle is the employment of a rotary instead of a sliding
movement of the parts. Two uprights are cast on the base-plate,
and are provided Avith slots at the top, into which the razor is placed
and clamped by two screws with milled heads. The inner face of
the slot is so made as to give the razor that inclination which has in
practice been found most advantageous. The razor is thus clamped
between a flat surface and a screw acting in the middle of the blade,
and the edge of the razor is consequently in no way injured.

The imbedded object is cemented with paraffin into a brass tube
which fits tightly on to the end of a cast-iron lever. This tube can
be made to slide backwards or forwards, so as to bring the imbedded
object near to the razor ready for adjusting. It is now furnished
with a mechanical arrangement for accurately adjusting the position
of the object. The cast-ii-on lever is pivoted at about 3 in. from the
end of the tube. To the other end of this lever is attached a cord
by which the motion is given, and the object to be cut brought
across the edge of the razor. The bearings of the pivot are
V-shaped grooves, which themselves form part of another pivoted
system.

Immediately under the first pair of V's is another pair of inverted
V's, which rest on a rod fixed to two uprights cast on the base-plate.

470 PREPAEATION, MOUNTING, AND COLLECTION OF OBJECTS

A horizontal arm pi'ojects at right angles to the plane of the two
sets of V's, the whole being parts of the same casting. On the end
of the horizontal arm is a boss with a hole in it, through which a

USINCt the rocking MICR0T03IE 47 1

sci'ew passes fi'eely. The bottom ot the boss is tui'iied out sphei-i-
cally, and into it fits a sphei-ical nut working on the sci-ew. The nut
is pi-evented from tui'ning by a pin passing loosely through a slot
in the boss. The bottom of the sci-ew I'ests on a pin fixed in the
l)ase-plate.

It will be seen that the efleet of tiu'ning the screw is to raise or
lower the end of the horizontal arm, and thei-efoi-e to move liackwards
or forwai-ds the npper paii- of V's, and with them the lever and
object to be cut. The top of the screw is provided with a milled head,
which may be used to adjust the object to the cutting distance. The
distance between the centres of the two jjivoted systems is 1 in. and
the distance of the sci-ew fi'om the fixed rod is 6^ in. The thread
of the screw is 25 to the inch ; thus, if the screw is turned once round,

the obiect to be cut will be moved forward — of - .,, or — in.
-^ 25 61' 156

The tui-ning of the screw is efiected automatically as follows :
A wheel with a milling on the edge is fixed to the bottom of the
screw ; an arm to which a pawl is attached rotates about the pin
which suppoi-ts the screw. This arm is moved backwai-ds and for-
wards by hand or by a cord attached to any convenient motor.
When the arm is moved foi'ward the pawl engages in the milling
and turns the wheel ; when the ai'm is moved back the pawl slips over
the milling without turning the Avheel. A stop acting against the
pawl itself prevents any possibility of the Avheel turning, by its own
momentum, more than the required amount. The arm is always
moved backwards and forwards, between two stops, a definite
amount, but the amount the wheel is turned is vai-ied by an adjustable
sector, which engages a pin fixed to the pawl and prevents the pawl
from engaging the milling of the wheel. By adjusting the position
of this sector, the feed can be varied from nothing to about -g^ of a
turn ; and hence, since the screw has 25 threads to the inch, the
thickness of the sections cut can be varied from a minimum,
depending on the perfection with which the razor is sharpened, to a

5 111

maximum of ^ of — of --, , or — -—- of a tui-n. The pi'actical mini-
32 25 6^: 1000 ^

mum thickness obtainable with a good razor is appi-oximately ^:^jj^,yo

inch. The values of the teeth on the milled wheel ai'e as follows : —

1 tooth of the milled wheel = 57;^ in. = -000625 mm.

2 teeth „ „ =20^05 in. = •001250 mm.
4 „ „ „ =^L_in. = -0025 mm.

16 „ „ ,, =Wcio in. = "01 mm.

The movement of the lever which cai-i-ies the imbedded objec^t is
efiected by a string attached to one end of the lever. This sti-ing
passes under a jjulley and is fastened to the aim cari-ying the pawl.
Attached to the other end of the lever is a spi-ing pulling downwards.
When the arm is moved forward the feed takes place, the string is
pulled, the imbedded object is raised past the razor, and the spring
is stretched. When the arm is allowed to move back, the spi'ing
draws the imbedded object across the edge of the I'azor, and the sec-
tion is cut. The sti-ing is attached to the lever by a screw which

472 PEEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS

allows the j^osition of the imbedded object to be adjusted, so that at
the end of the forward sti-oke it is only just past the edge of the
razor. This is an im2^C)i'tant adjustment, as it causes the razor to
commence the cut when the object is travelling slowly, and produces
the most favourable conditions for the sections to adhere to each
other.

The following are pei-haps the most prominent advantages of this
instrument: (1) The price is low.' (2) Manipulation is simple.
(3) The woi-k is rapid, and extremely accurate. (4) There are no
delicate working parts which can get out of order, and the whole
instrument is easily taken apart for packing, and is very poi'table.

The above description refers to the original form of the instru-
ment. Later, the Cambridge Scientific Insti-ument Company have
l)rought out an impi'oved form, at a higher price. For most
purposes the original form will suifice. The instrument is said by
the makers to cut celloidin obj ects ; but for this purpose a sliding
microtome will certainly be found preferable.

The Minot microtome, of which a desci'iption may be found in
the 'Journal of the Royal Microscoj)ical Society,' 1889, p. 143, is a
neat instrument designed, like the Cambridge rockei-, for cutting
ribbons of parafiin-imbedded objects. It is worked on the sewing-
machine piinciple, and cuts veiy rapidly. But its work is not so
fine as that of the Cambridge instrument, possibly on account of in-
sufiicient comjaensation in the working parts. This defect is said to
have been satisfactorily overcome in the beautiful instrument, con-
structed on the same princij^le, of Reinhold, a description of which
may be foiuid in the journal above quoted, 1893, p. 706. The work
afibrded by this instrument is certainly of the highest order, but
the price is against it, as it costs about 20^. Both of these instru-
ments are said to be able to cut celloidin sections ; but it is self-
evident that they are not so well adaj^ted for that purpose as the
sliding microtome.

It is unnecessary here to do more than allude to the large and
cumbrous instruments specially designed for cutting sections of
brain. Such is the microtome of Strasser, of which a descrij)tion
may be found in the ' Journal of the Royal Microscopical Society,'
1892, p. 703, and that of Gudden and others. They are only
required for certain very special neurological researches, and are
not at all adapted to the wants of the zoologist or histologist in
general. For these, we may here repeat, the all-round instrument
2')ar excellence is Jung's medium-sized Thoma microtome, No. IV., to
which, if lengthy series of jDaraflin sections be frequently required, a
Cambridge rocker may conveniently be added.

But it is needful also to describe one or more of the best instru-
ments designed specially for cutting sections by congelation ov freezing
of the imbedding mass. Dr. R. A. Hayes designed an ether freezing
mici-otome with the object of affording to those who have occasional
need to cut sections of tissues for pathological investigations, &c.,
the means of doing so quickly, conveniently, and accurately. It is
illusti'ated in fig. 400. It is very compact, solidly constructed, and
simple in plan. It freezes rapidly, and permits sections of large

ETHER FREEZING MICROTOMES

473

surface to be made with precision, sections 1 in. x ^ in. having
l)een cut by it without difficulty.

It consists of a soHd cast-iron base, A, 10 in. x 4^ in., which
I'ests upon a mahogany block. Extending the whole length of the
upper surface of the base is a V-shaped gutter, on the planed sides
of which slides a heavy metal block, B, on the flat top of which
the razor is secured (any ordinai'y razoi- can be used), the tang
being grasped between two flat pieces of iron, which are pressed
togethei' by a Avinged nut, 0. The razor by this ai-i'angement can
be secured at any desired angle to the direction of its motion to
and fro.

The freezing-chamber is formed by a short vulcanite cylindei-, D,
its lower end being screw^ed into a brass base, E. To its upper end
is fastened by two bayonet-catches a brass plate, F, on which the
tissue to be cut is placed. Inside the cylinder, D, and rising from
the base, E, is an ordinary spray, the air and ether being supplied
through tubes, g and H, passing outside through the base. There

Fic 400 — Di Ha'^es s ethei fieezmg miciotonie

is also an opening in the floor of the chambei' communicating with
the tube, to allow the overflow of ether in case of any accumulation
inside the cylinder ; any such overflow may be returned by the tube
to the ether supph^ bottle, K. The freezing-chamber is secured to
the top of the micrometer-screw arrangement, Z, which is of the
simjDlest form, but has a perfectly smooth and regular motion. The
nut is divided to indicate a section O'Ol mm. in thickness, but half
this thickness can be cut without difliculty.

The method of using the microtome is very simple. The slide
and block, D, having been carefully rubbed clean and well oiled, the
razor is clamped at any desired angle, the bottle, K, is filled with
ether (good dry methylated ether answei-s perfectly), and the piece
of tissue to be cut, having been previously saturated with thick gum
solution, is placed upon the plate F, and the spray which plays upon
the under surface of the plate, F, set working by the hand-pump,
M ; in a short time the tissue will be frozen quite through, and if a
number of sections are required, an occasional stroke or two of the

474 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

pump will keep the gvim in proper condition for cutting. The
sections are easily cut, as in other microtomes of this class, by
alternate movements of the screw, Z, and strokes of the razor.

The insti'ument may also be used for cutting tissue imbedded in
paraffin or other mass, the object to be cut being secured in position
either by being gently heated at its under surface and pressed on the
plate, F, to which it fiimly adheres on cooling, or by a simjDle clam.p-
ing arrangement, which can be substituted for the freezing-chambei-.
When used in this way large numbei's of sections may be cut in series
by attaching to the razor a light support to receive the sections as
they are cut.

Fig. 401. — Catlicart's freezing microtome.

Another most serviceable and admirable, because inexpensive
and efficient, microtome, especially for freezing purposes, was
devised by Mi-. Cathcart ; and it is now presented in a simplified
and improved condition. The instrument is illustrated in fig. 401.

In this form the clamping arrangements are much more perfect
than in the old foi-m ; the principal sci'ew and its milled head are
largei- and more convenient ; the freezing-jalate is circular, and is
provided Avith an ai'rangement for preventing the ethei-, with which
the freezing is efiected, from reaching the upper side of the plate ;
and the instrument is now so modified that it can be used for oixlinaiy
imbedding as well as freezing.

ETHER FREEZING MICROTOMES

475

The incveased size of the sci-ew gives a more steady movement
than was possessed by the older and smallei- mici'otonie, while the
gi-eater cii-cumference of the sci'ew-head enables an opei-ator to im-
part a finer movement to the screw. The relation between the pitch
of the screw and the circumfei-ence of its head is such that if the
edge be moved forward a quai-ter of an inch, an object will be raised
one-thousandth of an inch ; and if it be moved an eighth of an inch,
the object will be raised a two-thousandth of an inch.

In the oi'iginal instrument the plate was supported on two
pillars, in oi'der that as little heat as possible might be conveyed to
the freezing-plate from the body of the instrument. In the new
instrument the size of the three supporting pillai-s and screws is so
much reduced that the conducting surface is not gi-eatei- than in the
old microtome. The arrangement for cutting imbedded sections
consists of a tube which fits the principal well of the microtome, and
within which fits a hinged part similar to an ordinary vice. With
the insti'ument are provided the means of preparing parafiin blocks
for imbedding sections.

When it is intended to use the microtome for imbedding, the

Fig. 402. — Holder for Cathcart's microtome.

Fig. 403. — Dropping-bottle.

ether spray, spi'ay-bellows, and ethei'-bottle should be removed, and
the freezing-tube, having been raised as far as possible by means of
the principal screw, should then be withdrawn from the well. The
imbedding tube, fig. 402, is now placed in the well, and, having been
pushed down until it rests upon the point of the large screw, it may
be lowered to a convenient height by working the large screw back-
wards.

Mr. Oathcart recommends in freezing with this instrument that
a few drops of mucilage (1 part gum to 3 parts watei-) be jolaced on
the zinc plate, and that a piece of the tissue be cut, of about a quarter
of an inch in thickness, and pressed into the gum ; the ether-bottle,
filled with anhydi'ous methylated ether, is taken and the spi'ay points
pushed into their socket. All spirit must of course have been pre-
viously removed by soaking for a night in water. The tissue should
afterwards be soaked in gum for a like time before being cut. The
operator must now work the spray- bellow^s briskly until the gum
begins to freeze ; after this, Avork more gently. Raise the tissue by
turning the milled head, and cut by sliding the knife along the glass
plates.

476 PKEPARATION, MOUNTING, AND COLLECTION OF OBJECTS

Mounting. — By the term ' mounting ' is meant the arranging of
specimens on slides in such media and in such a manner as are most
favourable for the demonstration of their minute structure by the
microscope. In the case of the most numerous and important class
of objects that it is the function of the microscope to scrutinise,
namely, those derived from the substance of animal or vegetable
organisms, it is found that no methods of mounting will avail to re-
veal their minute structure unless the specimens have first been
submitted to the frequently very elaborate processes of previous
preparation to be hereafter described under the heads of Fixing,
hnhedding , Section-cutting , Staining, and the like. But still there
are many objects of interest and beauty that can be satisfactorily
mounted without the aid of these elaborate processes of previous
jjreparation. And as also the manipulations of mounting sensu
stricto are in principle the same in both cases, it appears advisable
to make the description of the j)rocesses of mounting j)recede that
of the processes of previous preparation ; merely warning the
beginnei- that in the case of the majority of specimens intended to
illustrate the minute structure of the tissues of either animals or
plants, such jjrevious preparation is a sine qua non.

The manipulations of mounting will alone be described here, the
most useful mounting meclia^ heing described later on (' Preserva-
tive and Mounting Media ').

In dealing with the small quantities of fluid media required in
mounting microscoj)ic objects, it is essential for the operator to be
provided with the means of transfei'ring very small quantities fi-om
the vessels containing them to the slide, as well as of taking up from
the slide what may be lying superfluous upon it. Wliei'e some one
fluid, such as glycerin, is in continual use, it will be found very con-
venient to keep it in the small dropping-bottle represented in fig. 403.
The stopper is perforated, and is elongated below into a fine tube,
whilst it expands above into a bulbous funnel, the mouth of which is
covered with a piece of thin vulcanised indiarubber tied firmly
round its lip. If j)ressure be made on this cover with the point of
the finger, and the end of the tube be immersed in the liquid in the
bottle, this will rise into it on the removal of the finger ; if, then,
the funnel be inverted, and the pressure be reapplied, some of the
residual air will be forced out, so that by again immersing the end
of the tube, and removing the pressure, more fluid will enter. This
operation may be repeated as often -as may be necessary, until the
bulb is entirely filled ; and when it is thus charged with fluid, as
much or as little as may be needed is then readily expelled from it
by the pressu^re of the finger on the cover, the bulb being always
refilled if care be taken to immerse the lower end of the tube before
the pressure is withdrawn. We sj)eak from large experience of the
value of this little implement, which is very clean, simple, and use-
ful. But the small pipettes now used so commonly for filling the
stylographic pens, fitted into the centi-e of a cork and placed in any
wide-mouthed bottle, will be found to be, though less elegant,
equally useful and much less costly.

Solutions of Canada balsam and o-um-dammai- in volatile fluids

DROP-BOTTLES— 3101 'NTINCt THIN SECTIONS

477

are best kept in wide-mouthed capped jars, the liquid being taken
out on a pointed glass rod, cut to such a length as will enable it
to stand in the jar when its cap is in place. Clreat cai'e should be
taken to keep the inside of the cap and the part of the neck of the
jar on which it fits quite clean, so as to prevent the fixation of the
neck by the adhesion between these two surfaces. Should such
adhesion take place, the cautious a|3plication of the heat of a spirit-
lamp will usually make the cap i-emovable. In taking out the
liquid care shoxdd be taken not to di'op it prematurely from the rod
— a mischance which may be avoided by not taking up moi'e than it
will proi^erly carry, and by holding it in a horizontal jaosition, after
di-awing it out of the bottle, until its point is just over the slip or
cover on which the liquid is to be dejaosited.

A bottle for use with reagents, enabling the oj)erator to pour out
only the quantity he desires, is invaluable . Small cap j)ed and stoppered
bottles, the stoppers of
which are tubes, and the
well-fitting caj)S of wdiich
pi-es'ent evaporation, are
veiy valuable for aqueous
and thin fliiids. We illus-
trate this bottle in fig. 404.
All that is needfid is to
take the bottle, with the
cap ofi", in the wai'm hand,
and by slight expansion a
drop or more as I'equired
is exuded. These bottles
are easily pi-ocurable.

But we like still better
the small Gei-man bottles,
shown in fig. 405, contain-
ing about oO grammes, in

which two deej) grooves are cut on opposite sides of the stopper, so
arranged that by giving the stopper half a tui-n one groove is
connected with a hole in the neck of the bottle : this will be seen at
a in fig. 405 ; the air travels down this groove, and by inverting the
bottle the fluid enters the other groove of the stopper and finds its
way to a thii'd gi'oove cut in the inside of the neck and extending to
the lip. The figure shows the bottle complete.

Mounting Thin Sections. — ^It is customary to recommend the use
of ' section lifters ' in order to raise delicate sections out of the fluid
in wdiich they finally ai-e placed into the position in which they ai-e
to be mounted. For very large sections they are probably essential ;
but fi-om personal experience, supported by the most accomjjlished
histological mounters of our time, we believe them to be adverse to,
rather than promotive of, good section-mounting. One of the
many patterns recommended is shown in fig. 406, where it wall be
seen that one end of the ' lifter ' is pei-foi-ated, for the pui-pose of
drainage, and the other is plain.

The present writer cannot endorse the i-ecommendation of this

Fig. 404.
Expansion drop-
bottle.

Fig. 405.
German droxi-bottle.

478 PEEPAKATION, MOUNTlNa, AXD COLLECTION OF OBJECTS

instrument, but prefers a smooth glass rod or tube ; the section in
fluid can easily be made to wrap itself round the rod, from which it
may be rolled off into a drop of liquid placed on the slide. It must
be manifest that the less we have to manipulate such delicate sections
as we are now considering, the better ; to get a section on and oflf
the ' lifter ' is a needless process. We should, as stated above,
mount on the cover-glass, and this cover should be the only liftei-
employed.

The cover must be carefully cleaned, and properly selected as to
size and tenuity. By means of a needle or the handle of an ivory
dissecting-knife the clearing fluid in which the section is resting
prior to mounting is gently disturbed, in a good-sized vessel or
saucer, until the section desired is in its proper j^osition on the
cover. Now lay the cover, section upwards, on fresh blotting-paper,
to take off the siiperfiuous liquid from the free side of the cover, and
then hold the edge of the slip at an angle, more or less acute, with
the section towards the blotting-paper, but never suffering the
former to touch the latter ; when this has removed the superfluous

liquid from the section, lay the
cover, section upwards, on a
glass slip, put on (say) the
benzol balsam until it stands in
an evenly diffused mound cover-
ing the section, and lay it aside
absolutely protected from dust
for twenty-four hours in ordei-
that the benzol may evaporate.

Now take it out, place upon
the centre of the section one
small drop of fresh benzol
balsam, and turn the cover over
on to a warm slip, being careful
to have guides to the position on
the slip on which it should be fixed ; and in an hour or so we may
clean off superfluous balsam and finish the slide.

To those who mount much this will prove the quicker plan, as,
for fine results, it is undoubtedly the better.

The above considerations refer only to loose sections in fluid,
or thin membranes, or other thin and isolated objects. It is one of
the advantages of the paraffin process that with paraffin sections no
lifter is required, as these are cut diy, and being stiffened by the
pai'affin may be lifted by means of a flat camel's-haii- brush, or a
scalpel or forceps. The manipulations of mounting series of sections
on one slide are described under ' Imbedding Methods.'

When the preparation has been previously immersed in aqioeoios
liquids, and is to be mounted in glycerin, glycerin jelly, or
Farrants' miedium, the best mode of placing it on the slide is to float
it in a saucer or shallow capsule of watei', to. place the slide or cover
beneath it, and, when the object lies in a suitable position above it,
to raise the slide or cover cautiously, holding the object in place by
a needle, until it is entirely out of the water ; and the small quantity

Fig. 406.

mountinCt 479

of liquid still siuTounding the object is to be carefully drawn ofl" by
lilotting-paper, CfXi-e being taken not to touch the object with it (as
its fibi'es ai-e apt to adhere) or to leave any loose fibi-es on the slide.
Before the object is covered, it should be looked at under a
dissecting or mounting microscojje, for the pui-pose of improving (if
desirable) its disposition on the slide, and of I'emoving any foreign
particles that may be accidentally present. A drop of the medium
(liquefied, if necessary, by a gentle warmth) is then to be placed upon
it, and anothei- di'op placed on the slij) oi' co^-er and allowed to spi-ead
out. The cover being then taken up with a pair of forcejjs must be
inverted over the slide, and brought to touch it at one part of its
margin, the slide being itself inclined in the direction of the place
of contact, so that the medium accumulates there in a little pool.
By gently letting down the covei-, a Httle wave of the medium is
pressed before it, and, if enough of the medium has been deposited,
the whole space beneath the cover will be filled, and the object com-
pletely saturated. If air-bubbles should unfoi'tunately show them-
selves, the covei- must be raised at one margin, and a fui'ther quantity
of the medium deposited.

If, again, there are no air-bubbles, but the medium does not
extend itself to the edge of the covei-, the cover need not be i-aised,
but a little may be deposited at its edge, whence it will soon be drawn
in by capillary attraction, especially if a gentle warmth be applied
to tiie slide. It will then be advantageous again to examine the
preparation undei- the dissecting microscope ; for it will often happen
that an oppoi-tunity may thus be found of spreading it better by the
application of gentle pressure to one pai't oi- anothei' of the covering-
glass, which may be done without injuiious efiect either with a stiff
needle or by a pointed stick ; a uiethod whose jDeculiar value, when
viscid media ai'e employed, was fii'st pointed oiit by Dr. Beale. The
slide should then be set aside for a few days, after which its mount-
ing may be completed. Any excess of the medium must fir»t
be removed. If glycerin has been employed, much of it may be
drawn off by blotting-paper (taking cai'e not to touch the edge of the
cover, as it will be very easily displaced) ; and the remainder may be
washed away with a camel's-hair brush dipped in water, which may
be thus carried to the edge of the covei-. The water having been
drawT:! ofl*, a nai'row ring of liquefied glycerin jelly may be made
around — not on — the margin of the cover (according to the suggestion
of Dr. S. Marsh) for the purjiose of fixing it before the cement is
applied ; and when this has set, the slide may be placed on the turn-
table, and the pi-eparation ' sealed ' by a i-ing either of gold-size oi'
of Bell's cement, which should be carried a little over the edge of the
cover, and outside the margin of the i-ing of glycerin jelly. This
' ringing ' should be repeated two or three times ; and if the pre-
paration is to be viewed with ' oil-immersion ' lenses, it should be
finished oft' with a coat of Hollis's glue or Bell's cement, which are
not attacked by cedar oil. Until the cover has been perfectly secured,
a slide carrying a glycerin preparation should never be placed in an
inclined position, as its cover will be almost sure to slide by its o^m
weight. If glycerin jelly oi- Farrants' medium has been employed,

480 PKEPARATION, 3I0UxNTlNG, AND COLLECTION OF OBJECTS

less caution need be used, as the cover-glass, after a few days' setting,
will adhere with sufficient firmness to resist displacement. The
superfluous medium having been removed by the cautious use of a
knife, the slide and the margin of the cover may be completely
cleansed by a camel's-hair brush dij)ped in warm water ; and, when
quite dried, the slide, placed on the turn-table, may be sealed with
gold-size — any othei- cement being afterwards added, either for
additional security or for ' appeai-ance.'

It is well in mounting in glycerin jelly to soak the object
pi-eviously in dilute glycerin, and we prefer to ' ring ' with benzole
and balsam, which should harden. Then coat the ring with shellac
varnish two or three times and j)ei'manently finish with thin coats of
gold-size.

When, on the other hand, the section or other prejjaration is to
be mounted in a resinous ni?diiom, it must have been j^i'eviously pre-
pared for this in the modes described further on, which Avill present
it to the mounter either in some essential oil, or in xylol or benzol
or the like, or in alcohol. Fi-om eithei-' of these it may be ti'ansferred
to the cover or slide in the manner ali-eady described.

The thin sections cut by the microtome, or membranes obtained
by dissection, do not requii-e to be placed in cells when mounted in
any viscid medium ; since its tenacity Avill sei-ve to keep off injurioiis
pressure by the cover-glass.

Mounting Objects in 'Natural' Balsam. — Although it is pre-
ferable foi' histological pui-poses to employ a solution of hai'dened
balsam, as directed under ' Mounting Media,' yet as there are many
objects foi- mounting for which the use of the ' natural ' balsam is
preferable, it will be well to give some dii'ections for its use. When
sections of hard sub.stances have been gi-ound down on the slides to
which they have been cemented, it is much better that they should
be mounted without being detached, unless they have become clogged
with the abraded particles, and I'equii'e to be cleansed out — as is
sometimes the case with sections of the shells, spines, etc., of echino-
derms, when the balsam by which they have been cemented is too
soft. If the detachment of a specimen be desirable, it may be
loosened by heat, and lifted off with a camel's-hair brush dipped in
oil of turpentine. But, where time is not an object, it is far better
to place the slide to steep in ether or chloi'ofoi-ni in a capped jar
until the object falls off of itself by the solution of its cement. It
may then be thoi-oughly cleansed by boiling it in methylated spirit,
and afterwai-ds laid upon a piece of blotting-paper to dry, after
which it may be mounted in fi-esh balsam on a slide, just as if it
had i-emained attached. The slide having been warmed on the
water-bath lid, a sufficient quantity of balsam should be droj^ped on
the object, and care should be taken that this, if previously loosened,
should be thoroughly penetrated by it. If any air-bubbles arise,
they shoul'd be broken with the needle-jDoint. The cover having
been similaily warmed, a drop of balsam should be placed on it, and
made to spread over its surface ; and the cover should then be
turned ovei- and let down on the object in the manner already de-
scribed. If this operation be performed over the water-bath, instead

MOUNTING— IN BALSAM— IN AQUEOUS LIQUIDS 48 1

of over the spirit-lamp, there will be little risk of the formation of
air-bubbles. However large the section may be, cai^e should be taken
that the balsam is well spread both over its surface and that of
its cover ; and by attending to the pi-ecaution of making it accumu-
late on one side by slopiiig the slide, and letting down the cover
so as to di'ive a wave before it to the opposite side, veiy lai-ge sections
may thus be mounted without a single air-bubble. (The Author has
thus mounted sections of Eozoon three inches square.) In mounting
minute balsam objects, such as diatoms, polycyst'mce, sponge- spicules,-
and the beautiful minute spines of ophiurkla, no better plan can be
adopted than to arrange these objects carefully upon the cover,
either by ' scattering ' or ' arrangement,' and then to drop on to the
whole cover and its arranged objects as much balsam as the cover
will receive without overflow ; this should stand free from dust for
some hours, after which the partly hardened balsam may receive a
small drop of fresh balsam, and being placed upon the slip in proper
position, may by the use of gentle heat be pressed finally into position.
When the chitinous textures of insects are to be th;is mounted, they
must be first softened by steeping in oil of turpentine ; and a large
drop of balsam being placed on a warmed slide, the object taken up
in the forceps is to be plunged in it, and the cover (balsamed as before)
let down upon it. It is with objects of this class that the spring -
clip and the S'pring-j^ress jDrove most useful in holding down the cover
until the balsam has hardened sufficiently to prevent its being lifted
by the elasticity of the object. Various objects (such as the palates
of gasteropods) which have been prepared by dissection in water or
weak sjDirit may be advantageously mounted in balsam ; for which
purpose they must be first dehydrated, and then transferred from
rectified spirit into turpentine or one of the other ' clearing agents '
mentioned below. Sections of horns, hoofs, &c., which afford most
beautiful objects for the ]3olariscope, ai"e best mounted in natural
balsam, which has a remarkable power of increasing their trans-
parence. It is better to set aside in a warm place the slides which
have been thus mounted before attempting to clean off the super-
fluous balsam in order that the covers may be fixed by the gradual
hardening of what lies beneath them.

Mounting Objects in Aqueous Liquids. — By far the greater
number of preparations which are to be preserved in liquid, however,
should be mounted in a cell of some kind, which forms a ivell of
suitable depth, wherein the preservative liquid may be retained.
This is absolutely necessary in the case of all objects whose thickness
is such as to prevent the glass cover from coming into close approxi-
mation with the slide ; and it is desirable whenever that ajDproxima-
tion is not such as to cause the cover to be drawn to the glass slide
by capillary attraction, or whenever the cover is sensibly kept apart
from the slide by the thickness of any portion of the object. Hence
it is only in the case of objects of the most extreme tenuity that
the cell can be advantageously dispensed with ; the danger of not
employing it, in many cases in which there is no difiiculty in.
mounting the object without it, being that after a time the cement
is apt to run in beneath the covei', which pi-ocess is pretty sure to

I I

482 PEEPARATION, MOUNTING, AND COLLECTION OF OBJECTS

continue when it may have once commenced. ^\'lien cement- cells
are employed for this purpose, care must be taken that the sui-face
of the ling is perfectly flat, so that when the cover-glass is laid on
no tilting is produced by pi'essure on any part of its margin. As a
general rule, it is desirable that the object to be mounted should be
steeped for a little time previously in the preservative fluid employed.
A sufiicient quantity of this fluid being deposited to overfill the
cell, the object is to be introduced into it either with the forceps or
the dipping tube ; and the slide should then be examined on the
dissecting microscope that its entire freedom from foreign particles
and from aii'-bubbles may be assured, and that its disposition may
be corrected if necessary. The cover should then be laid on very
cautiously, so as not to displace the object ; which in this case is
best done by keeping the drop highest in the centre, and keeping
the cover parallel to the slide whilst it is being lowered, so as to
expel the superfluous fluid all round. This being taken up by the
syringe, the cement ring and the margin of the cover are to be
dried with blotting-joaper, esjjecial cai'e being taken to avoid dra wing-
off too much liquid, which will cause the gold-size to run in. It is
generally best to apply the first coat of gold-size thin, with a very
small and flexible brush worked with the hand ; this will dry sufii-
ciently in an hour or two to hold the cover whilst being ' ringed '
on the turn-ta,ble. And it is safer to apply a third coat a day or
two afterwards ; old gold-size, which lies thickly, being then applied
so as to raise the ring to the level of the surface of the cover. As
experience shows that pi'eparations thus mounted, which have
remained in pei-fectly good order for several years, may be afterwards
spoiled by leakage, the Author strongly recommends that to prevent
the loss of valuable specimens an additional coating of gold-size be
laid on from time to time. But a device of much greater value in
all fluid mounting is that adopted by Mr. Enoch, ^ who puts a
metallic ring of angular section round the outside of the cell,
slightly overlapjjing the cover-glass and enclosing the rim made
good with cement ; this proves perfect.

Mounting of Objects in Deep Cells. — The objects which require
deep cells are, as a rule, such as are to be viewed by reflected light,
and are usually of sufiicient size and substance to allow of air being-
entangled in their tissues. This is especially liable to occur where
they have undergone the pi'ocess of decalcification, which will veiy
probably leave behind it bubbles of carbonic acid. For the extrac-
tion of such bubbles the use of an air-pump is commonly recommended ;
but the Editor lias seldom found this answer the purpose satisfactoi-ily,
and is much disposed to place confidence in a method lately recom-
mended— steeping the specimen in a stoppered jar filled with freshly
boiled vmter, which has great power of drawing into itself eithei- air
or carbonic acid. Whei'e the structure is one which is not injured
by alcohol, prolonged steeping in this will often have the same efiect.
The next point of importance is to select a cover of a size exactly
suitable to that of the ling, of whose breadth it should cover about
two-thirds, leaving an adequate margin uncovered for the attachment
1 Qiteheti Jonrn. second series, vol. i. p. 40.

MOUNTING IN DEEP CELLS 483

of the cement. And the perfect flatness of that ring should then l^e
cai-efully tested, since on this mainly depends the secuiity of the
mounting. It is to secure this that we prefei- rings of tin or bone,
to those of glass, for cells of modei-ate depth ; for their surface can
be easily made perfectly flat by grinding with water, first on a piece
of grit, and then on a Water-of-Ayr stone, these stones having been
previously reduced to a plane surface, or still better with a good flat
file. If glass rings are not found to be ' true,' they must be ground
do^vn with fine emery on a plate of lead. When the cell has been
thus finished off, it must be carefully cleaned out by dropping into it
some of the mounting fiuid ; and should be then examined under the
dissecting microscope for minute air-bubbles, which often cling to
the bottom or sides. These having been got rid of by the needle,
the cell should be finally filled with the preservative liquid, and the
object immersed in it, care being taken that no air-bubbles are
carried down beneath it. The cell being completely filled so that the
liquid is running over its side, the cover may then be lowered down
upon it as in the preceding case ; or, if the cell be quadrangular,
the cover may be sloped so as to rest one margin on its wall, and
fresh liquid may be thrown in by the syringe, while the other edge
is lowered. Wlien the cover is in place, and the liquid expelled from
it has been taken up by the syringe, it should again be examined
under a lens for air-bubbles ; and if any of these troublesome
inti-uders should present themselves beneath the cover, the slide
should be inclined, so as to cause them to rise towards the highest
pai't of its circumference, and the cover slipped away fi'om that part, so
as to admit of the introduction of a little additional fluid by the pipette
oi" syringe ; and when this has taken the place of the air-bubbles the
cover may be slijjped back into its place. The surface of the ring and
the edge of the cover must then be thoroughly dried with blotting-
paper, care being taken that the fluid be not drawn away from
betAveen the cover and the edge of the cell on which it rests. These
minutiae having been attended to, the closure of the cell may be at
once efiected by carrying a thin layer of gold-size or dammar around
-and upon the edge of the glass cover, taking care that it touches
every point of it, and fills the angular channel which is left along its
margin. The Author has found it advantageous, however, to delay
closing the cell for some little time after the superfluous fluid has
been drawn ofi"; for as soon as evaporation from beneath the edge
of the cover begins to diminish the quantity of fiuid in the cell, air-
bubbles often begin to make their appearance which were previously
hidden in the recesses of the object ; and in the course of half an
hour a considerable number are often collected. The cover should
then be slipped aside, fresh fluid introduced, the air-bubbles removed,
and the cover put on again ; and this operation should be repeated
until it fails to draw forth any more air-bubbles. It will of course
be observed that if the evaporation of fluid should proceed far air-
bubbles will enter beneath the cover ; but these will show themselves
on the surface of the fluid, whereas those which aiise from the
object itself are found in the deeper parts of the cell. When all these

1 1 2

484 PEEPARATION, MOUNTING, AND COLLECTION OF OBJECTS

have been successfully disposed of, the cell may be ' sealed ' and
' ringed ' in the manner already described.

Preparation of Soft Tissues. — It is impossible in the limited
space at disposal hei-e to do more than give a sketch of the very
elaborate art of histological preparation. The i-eader who desires to
pursue the subject fui-ther will find all necessary information in Mr.
A. Bolles Lee's ' The Microtomist's Vade-mecum ' (London : J. & A.
Churchill), from which work the information here given is for the
most part abridged (the passages in quotation marks in the following
pages are taken therefrom verbatim).

Fixation, — ' The first thing to be done with any structure is to
Jix its histological elements. Two things are implied by the word
' fixing : ' first, the rapid killing of the element, so that it may not
have time to change the form it had during life, but is fixed in
death in the attitude it normally had during life ; and second, the
hardening of it to such a degree as may enable it to resist without
fui'thei- change of form the action of the reagents with which it may
.subsequently be treated.' For instance, if you were to take a living
rotifer and throw it into one of the usual staining fluids or preser-
vative liquids, it would at once contract into a shapeless mass, the
■elements of its tissues would be neither properly stained nor properly
preserved, and the result would be an vinrecognisable caricature of
the living organism. But if it be first properly killed and slightly
hardened in the proper manner, it naay be permanently mounted in
.such a way as to show, uninjured and undistorted, even the most
•delicate details of its structure.

Fixation is generally performed by immersing the object to be
fixed ill an appropriate liquid, and leaving it therein until the
•desired degi'ee of hardening has been obtained. After that the
object is well washed to remove all excess of the fixing liquid. The
object may then be fui'ther pi'epared by the wet method, in which
all subsequent operations are performed by means of aqueous media.
It may be mounted at once in an aqueous mounting medium, or it
may be stained (see below), or it may be put away till wanted, with-
out mounting, in some preservative medium.

Or ' the object may be further prepared by the dehydration
method ' (see below), ' which consists in treatment with successive
alcohols of gi-adually inci-easing sti-ength, final dehydration with
•absolute alcohol, clearing ' (see below) ' with an essential oil or other
clearing agent, and lastly either mounting in balsam or imbedding
in j)a,raflin for the purpose of making sections.'

Corrosive sublimate is the fixing agent that is most to be recom-
mended for general work. A good formula consists of a saturated
.solution in watei- containing 1 per cent, of acetic acid. The present
writer adds a little nitric acid, say 1 per cent., which helps to
make the solution keep without precijjitating. Another good solu-
tion is a satui'ated solution in alcohol of 50 per cent., or even 70 per
•cent., also with addition of 1 per cent, of acetic acid.

Whatever solution is taken, the objects should be removed from
it soon aftei- they have become thoroughly peneti-ated by it. For
.sublimate hai-dens very rapidly, and makes tissues brittle if they are

PICEIC AND OSMIC ACIDS 485

allowed to remain too long in it. The objects sliould be well washed
out, after fixing, with alcohol, beginning with alcohol of 50 per cent,
or 70 per cent., and passing gradually to stronger alcohols. In order
to facilitate the i-emoval of the sublimate from the tissues, the
alcohol should have added to it enough tincture of iodine to make it
of a good port- wine colour, and the objects should remain in it till
they themselves have acquired the same coloui-. They may then be
washed with pure alcohol, and further treated as desired.

Solutions of sublimate, or the objects in them, must never be
touched with steel implements, as these produce at once precipitates
that may injure the preparations. To manipulate the objects, wood
or glass implements may be employed ; for dissecting them, hedge-
hog spines, or quill pens, or cactus needles.

Tissues become of an opaque whiteness on fixation with sublimate,
which in the case of small transjoarent objects is a good guide for
conti"olling the duration of the fixing bath. The fixing action is
extremely rapid.

Picric acid is a reagent that gives very fair results for general
work, and is especially to be recomraended where great power of
penetration is required, as is the case in work with chitinous
organisms. A saturated solution in water with the addition of
1 per cent, of acetic acid may be taken, or the picro-nitric acid of
Mayer. This consists of water 100 parts, nitric acid of 25 per cent.
1^20.5, 5 parts, and picric acid to saturation.

Objects should remain in these liquids much longer than in sub-
limate liquids ; for though the penetration is extremely rapid the
hardening power is slight. They may remain for twenty-four hours
without hurt, but in many cases three or four hours will suifice. After
fixation the objects should be brought into alcohol of 70 per cent.
(never water), in which they should remain for a few hours, and
then be transferred to alcohol of 90 per cent., in which they should
remain, the alcohol being frequently changed for fresh, until the
yellow tint of the picric acid has disappeared or at least become
greatly attenuated. Objects prepared in this way are best stained
in alcoholic staining solutions.

Mixtures of picric acid solution with sublimate in various pi'O-
portions have lately been much used, with good results.

Osinic acid is a useful reagent for fixing small objects. It pre-
serves the forms of cells admirably, and at the same time imparts to
tissues a grey stain that is frequently of the greatest value in bring-
ing out delicate structures. This substance is sold in the solid state,
in sealed tubes containing from -^^ grm. to 1 grm. It is extremely
volatile. Care should be taken to avoid exposure to the vapours given
ojff from it, as they are exceedingly irritating to mucous mem-
branes and may easily give rise to serious catarrh, conjunctivitis, &c.
Its solution in pure Avater keeps very badly, as the slightest con-
tamination with any organic dust will cause it to reduce and precij^i-
tate. It is recommended, therefore, that only a small quantity be
kept in stock in the shape of aqueous solution, whilst another
quantity may be preserved in the shape of a 2 per cent, solution in
chromic acid of 1 per cent., or, better, in platinic chloride of the

.1.86 PREPARATION, MOUNTINa, AND COLLECTION OF OBJECTS

.same strength. These solutions do not precipitate so readily, and
may be used for fixation by the vapours.

For it is one of the advantages of osmic acid that it may be
employed for fixation in the form of vapour, and its employment in
this form is indicated in most of the cases in which it is possible to
ex]30se the tissues to be fixed directly to the action of the vaj)our.
For fixation in this way ' the tissues are pinned out on a cork which
must fit well into a wide-mouthed bottle in which is contained a
little solid osmic acid (or a small quantity of 1 per cent, solution will
do). "Very small objects, such as isolated cells, are simply placed on
a slide, which is inverted over the mouth of the bottle. They
remain thei'e until they begin to turn brown (isolated cells will
generally be found to be sufficiently fixed in thirty seconds, whilst
in order to fix the deeper layers of relatively thick objects, such as
retina, an exposure of several hours may be desirable). It is well to
wash the objects Avith water before staining, but a very slight wash-
ing will suffice. For staining, methyl-green may be recommended
for objects destined for study in an aqvieous medium, and, for per-
manent preparations, alum-carmine, picro-carmine, or hfematoxylin.'

' The reasons for preferring the process of fixation by vapour of
osmium, where practicable, are that osmium is more highly penetra-
ting when employed in this shape than when employed in solution,
and produces a more equal fixation, and that the arduous washing
out required by the solutions is here done away with. In many
cases delicate structures are better preserved, all possibility of
deformation through osmosis being here eliminated.' (From Mr. Lee's
'The Microtomist's Yade-mecum.')

For fixation by solutions, strengths of from -^q to \ per cent, may
be taken, which may in general with advantage be acidified with
about 1 per cent, of acetic acid. Small Crustacea, such as the
copepods and the larvae of decapods, may be very well prepared in
this way. After fixation, the osmic acid should be very thoroughly
washed out with water.

If it be desired to intensify the grey stain of the osmium, this
may be easily done by putting the objects into a weak solution of
pyrogallic acid or tannin, which will turn them of a fine black.

Osmic acid stains most fatty substances of an intense black.

Osmic acid is now not so much used in the form of a pure
aqiieous solution as in that of the mixture known as liquid of Flem-
'ming. This consists of 25 parts of 1 per cent, solution of chromic
acid, 10 parts of 1 per cent, osmic acid, 10 parts of 1 per cent, acetic
acid, and 55 of water. This mixtui'e blackens tissues much less than
the pui-e aqueous solution.^

1 Bleaching. —Tissues that have been blackened or browned by osmic or chromic
acid or the like may often with advantage be bleached by Mayer's chlorine method,
and will then be found to stain much more readily. — ' Put into a glass tube a few
crystals of chlorate of potash, add two or three drops of hydrochloric acid, and as
soon as the green colour of the evolving chlorine has begun to show itself, add a few
cubic centimetres of alcohol of 50 to 70 per cent. Now put the objects (which must
have previously been soaked in alcohol of 70 to 90 per cent.) into the tube. They
float at first, but eventually sink. They will be found bleached iu from a
quarter of an hour to one or two days, without the tissues having suffered.
Only in obstinate cases should the liquid be warmed or more acid taken.

CLEAEING 487

Foi' the very numerous othei- fixing reagents and mixtures now
in use, and the mannei' of their employment, the reader must be
referred to Mr. Lee's ' The Microtomist's Vade-mecum.'

After due fixation and washing, objects may be stained and
mounted in an aqueous medium in the manner directed above
(p. 481), if it be desired to prepare them in the wet way. But if
they are • destined to be preserved in balsam, they must first, after
staining if i-equired, be dehydrated and cleared.

Dehydration is performed as follows : — ' The objects are brought
into weak alcohol, and ai'e then passed through successive alcohols
of gradually increased strength, remaining in each the time neces-
sary for complete saturation, and the last bath consisting of absolute
or at least very strong alcohol.' For instance, alcohol first of 30 pei-
cent, or 50 per cent., then 70 per cent., then 95 per cent., or, if the
objects be very delicate, 80 per cent., before the 95 per cent., the
last to be changed at least once.

Clearing. — ' The water having been thus sufiiciently removed, the
alcohol is in its turn removed from the tissues, and its place taken
by some anhydrous substance, generally an essential oil, which is
miscible with the mateiial used for imbedding. This operation is
known as clearhig. It is very important that the passage from the
last alcohol to the clearing agent be made gradual. This is efifected
by placing the clearing medium 'under the alcohol. A sufiicient
quantity of alcohol is placed in a tube (a watch-glass will do, but
tubes are generally better), and then with a pipette a sufiicient
quantity of clearing medium is introduced at the bottom of the
alcohol. Or you may first put the clearing medium into the tube,
and then carefully pour the alcohol on to the top of it. The two
fluids mingle but slowly. The objects to be cleared, being now
quietly put into the supernatant alcohol, float at the surface of
separation of the two fluids, the exchange of fluids takes place
gradually, and the objects slowly sink down into the lower layer.
Wlien they have sunk to the bottom (and the wavy refraction-lines
at first visible I'oiind them have disappeared) the alcohol may be
■drawn oflT with a j^ipette, and the objects will be found to be com-
pletely penetrated by the clearing medium. (It may be noted here
that this method of making the joassage from one fluid to another
applies to all cases in Avhich objects have to be ti'ansfei'red from a
lighter to a densei' fluid — for instance, from alcohol or from water
to glycerine.)' From ' The Microtomist's Yade-mecum.'

Another method of passing the objects from the alcohol to the
clearing agent consists in giving them baths of mixtures of the
alcohol and the clearer, made gradually to contain a higher propor-
tion of the latter.

All clearing agents are liquids of high refraction, having indices
of refraction not greatly inferior to that of the elements of tissues

Sections on slides may be bleached in this way. Instead of hydrochloric acid, nitric
acid may be taken; in which case the active agent is evolved oxygen instead of
chlorine. This method serves also for removing 7iatural pigments, such as those of
the skin, or of the eyes of Arthropods. For bleaching chitin of insects, not alcohol
but water should be added to the chlorate and acid.' (From ' The Microtomist's
Vade-mecum.')

488 PREPAKATION, MOUNTINa, AND COLLECTION OF OBJECTS

in the fixed state. Hence, by penetrating amongst these highly
refractive elements, they I'endei- the tissues transpai-ent and cleai-,
which is the reason of theii- being called * clearing agents.'

The best clearing agent foi- general use is oil of cedar tvoocl. Oil
of cloves is a very good one ; it should be known that it makes
objects brittle, which is sometimes to be desired, sometimes the
reverse. Oil of hergamot is useful ; it will clear from alcohol of no
more than 90 per cent, strength.

It should be noted that the proper stage foi- 2^^'>"fa')'ming Tninute
dissections in is the one at which the objects have now arrived, a
di'op of clearing agent being a most helpful medium for carrying-
out such dissections in. Oil of cedar is very good for this purpose.
But oil of cloves is sometimes to be preferred, not only on account of
its property of making tissues brittle, which is often very helpful,
but also on account of the property it has of forming very convex
drops on the slide.

Staining. — Good histological stains can in general only be
obtained with properly fixed tissues. But it is possible to obtain with
unfixed and even with living tissues a stain which though imperfect
and not ' fast ' may be of considerable utility in research, either as a
means of controlling the results obtained by the examination of fixed
and prepared specimens, or as a means of revealing delicate traits of
structure that may be masked or destroyed by the action of fixing
and preserving reagents, and only visible in the living or perfectly
fresh object.

It goes without saying that staining is performed by immersing'
the tissues in the colouring solution employed. After the tissue has-
become duly stained, all superfluous colour is removed from it by
' washing oiit ' with an ajjpi'opriate liquid.

Stains for Living Objects (Intra Vitam Stains). — The most
widely used of these stains is methylen-hlne (to be obtained from
GrUbler and Hollborn,^ and not to be confounded with methyl-blue,
which is a totally different dye). Small aquatic oi-ganisms (such as
rotifers, infusoria, small annelids, tadj)oles) are stained by adding
a small quantity of the dye (best previously dissolved in distilled
water) to the water in which they are kept, and leaving them till
the stain has taken effect. Enough of the dye should be added to
make the water of a good blue, the proportion required varying-
roughly between 1 part of the dye to 10,000 of the water, and
1 part to 100,000. Most aquatic organisms will live in the
coloured water for many hours, some for days or weeks. They
should be examined as soon as the required intensity of stain has
been attained. For if they are allowed to remain longer the
elements that have taken up the dye will begin to yield it up again
to the watei-, and the objects may become quite pale again even
though they have not been removed fi'om the coloured water. The
stain is an imperfect one, being mostly confined to certain granules
of the protoplasm of cells, and taking effect capriciously now on one
tissue and now on another. It is diificult to. preserve the stain in a.

^ 63 Bayerische Strasse, Leipzig ; or through Mr. C. Baker, 243 High Holborn.

STAINS FOE UNFIXED TISSUES 489'

satisfoctoi'y manner, as it will not beai- mounting in the usual media
without deterioration.

Weak solutions of Bismarck hroion, quinolein-blue, anlUn-hlacl' ,
Congo red, and neutral red {Neutralrotli) may be used in the same
way.

Methylen-blue, used as an intra vitam stain, is an important
reagent for the study of nerve-endings. For the details of this very
difficult branch of technique, as well as for the methods for preserv-
ing the stain obtained with entire living organisms, the reader must
be referred to Mr. A. Bolles Lee's ' The Microtomist's Vade-mecum,'
in which an entire chapter is devoted to the subject.

Stains for Fresh (Unfixed) Tissues or Organisms. — The stains to
be mentioned under this heading resemble the intra vitam stains
described in the last paragraph in that they may be apjolied to living-
tissues or organisms. But they differ from them in that they do
not take effect on the objects without impairing their vitality ; on the
contrary they first kill them, then stain them.

The most important of this class of stains is methyl-green. A
strong solution in water acidified with from ^ to 1 per cent,
of acetic acid is employed. The objects are soaked in the solution
iintil they are penetrated by it, then washed with pure water, or,,
better, acidified water, and either studied therein or mounted. They
may be permanently presei'ved in any of the usual aqueous mount-
ing media, provided that the medium be acid or at most strictly
neutral, and that it contain a little of the dye in solution. Liquid
of Ripart and Petit, or Brun's glucose medium may be recommended
for mounting. It is difficult to mount the stained objects in balsam,,
on account of the great solubility of the dye in alcohol.

The stain is an extremely rapid one ; tissues ai-e stained almost
as soon as they are penetrated by it. It is, generally speaking, a
nuclear stain, nuclei being stained more rapidly than cytoj^lasm,.
though some kinds of cytoplasm and formed material are stained by
it. It j^i'eserves the forms of cells well. It does not overstain^
and requires little washing out. This, if required, is best done witlx
water acidified with acetic acid.

Bismarck hroivn is also a useful stain for fresh tissues. It maj
be used in solution in acidified water, as directed for methyl-green.
But as the dye is not very soluble in water it is not easy to get a
good solution in this way, and the solutions when made keep very
badly. Some jaersons dissolve the dye in dilute glycerin (glycerin
diluted with one or two volumes of water) . This makes a good solu-
tion, but on account of the shrinking action of the glycerin should
only be employed with objects that have been previously well fixed.
Bismarck brown stains quickly, and does not overstain. The stain
is permanent both in aqueous mounting media and in balsam. It is
a nuclear stain in so far as nuclei ai'e stained by it more than proto-
plasm.

The once celebrated mixture known as Ranvier^ s picro-carmine
is irrational in composition, and inconstant and frequently injurious
in its effects, and is now generally abandoned.

490 PEEPAKATION, MOUNTING, AND COLLECTION OF OBJECTS

Stains for Fixed and Preserved Entire Objects or Material to
be Stained in Bulk. — These fall naturally into the two classes of
aqueous stains and alcoholic stains. The aqueous stains are
generally the more precise, and are generally preferable for small
and permeable objects, but the alcoholic stains are absolutely
necessary where great penetration is required, as for instance in the
case of organs or oi-ganisms enclosed in thick chitinovis investments,
as is so generally the case amongst the Arthropoda.

The most precise and the safest of the stains of this class are the
{duTn-carmines — a general term including the divers formulae that
have been recommended under the names of alwni-carmine,
carmalum, alum- cochineal. One of these will suffice.

PartscKs alum-cochineal. — ' Powdered cochineal is boiled for some
time in a 5 per cent, solution of alum, the decoction filtered, and a
little salicylic acid added to preserve it from mould.'

An extremely pi-ecise nuclear stain, and one with which it is hardly
230ssible to overstain. It is permanent in balsam and, it is believed,
in aqueous media if not acid. Objects raay be left in it for several
hours. They should not be very large, as the stain has no great
jDower of penetration. Objects containing calcareous elements that
it is desired to pi-eserve must not be treated with this stain, nor
with any other stain containing alum.

Mayer's carmalum is made with carminic acid 1 gi'm., alum
10 grm., and distilled water 200 c.c. It has the advantage of being
much more penetrating than the other stains of this class.

All the alum-carmine solutions are rather weak stains. If a
more powerful stain be desired, take the following : —

Mayer's hcemalum. — This is made with hfematein, the essential
colouring principle of hpematoxylin (obtainable from Griibler and
Hollborn). One grm. of hsematein is either dissolved with heat
in 50 c.c. of 90 per cent, alcohol, or rubbed up in a mortar with a
little glycerin, and added to a solution of 50 grm. of alum in a litre
of water. This liquid may be used for staining either concentrated
or diluted. Concentrated it stains almost instantaneously. For
ordinary purposes it may be diluted with from ten to twenty
volumes of distilled water, and will then stain through small objects
in an hour or so. Large objects will require an hour or more. The
solution is admirable for staining in bulk. Objects should be well
washed out (for as long a time as they have taken to stain) eithei-
with distilled water oi- tap water. One per cent, alum solution is
also a good medium to wash out in. Overstains may be corrected
by washing-out with O'l to 0"5 ]Der cent, of hydrochloi'ic acid. In
this case the acid should be neutralised afterwards by treatment with
O'l per cent, solution of bicarbonate of soda (or other weak alkali).

Passing now to the alcoholic solutions, Gh'cnacher'' s alcoholic horax-
carmine may be recommended as affording a convenient, safe, and
brilliant stain. Dissolve 2 or 3 per cent, of carmine in a 4 per cent,
solution of borax in water ; boil the solution for half an hour ;
dilute it with an equal volume of 70 per cent, alcohol, allow it to
.stand for twenty-four hours, and filter.

Objects are put into this solution and allowed to remain in it

STAIMNG ENTIEE OBJECTS 49 1

until they are tlioi'oughly penetrated (for days if necessai'y). They
;ii-e then put into alcohol of 70 per cent, acidified with fi-om four to
six di'ops of hydrochloric acid for every 100 c.c. of the alcohol. The
acid alcohol at once begins to i-emove the excess of coloui- from the
objects, which may be seen to give it off in rosy clouds. They
I'emain in it until the coloui- no longer comes away freely and they
have exchanged their piimitive opaque i-ed coloration for a biilliant
transparent coloration. This may require days (the acid alcohol
should be changed frequently).

The staining is now complete, and the objects ai-e washed in pui'e
neutral alcohol, cleai-ed and mounted in balsam or any other desired
medium. The result is a brilliant nuclear stain, quite permanent.
The process must not be used for objects containing calcareous
elements that it is desired to preserve.

For delicate objects, and for very impermeable objects, it may be
well to increase the proportion of 70 per cent, alcohol in the
solution ; the proportion of alcohol may be brought up to about
50 per cent., but should not exceed 60 per cent, in any case.

This process is an example of what is known as regressive oi-
indirect staining ; the objects are first overstained in the carmine
solution, and the excess of stain is then removed to the required
degree in the acid alcohol.

If, as is freqvxently the case, especially in studies on the
Arthropoda, a still more highly alcoholised stain be desired, Mayer's
ulcoholic cochineal may be tried. Cochineal in coarse powder is
macerated for sevei'al days in 70 per cent, alcohol. For each
gramme of the cochineal there is required 8 to 10 c.c. of alcohol.
Stir frequently. Filter, and the solution is ready for staining.

The objects to be stained must previously be well imbibed with
70 per cent, alcohol. They may remain for almost any length of
time in the staining bath. After staining they are washed in
70 per cent, alcohol, which is frequently changed until it takes up no
more colour from the objects. Overstaining seldom happens : it
may be corrected by means of 70 per cent, alcohol containing 1 j)er
cent, of acetic acid or -^^ per cent, of hydrochloric acid.

Small objects or thin sections are stained in a few minutes ; large
objects require hours or days ; a nuclear stain, either red or blue,
according to the chemical composition of the tissues stained. It
does not succeed with all objects. The best stains are obtained with
objects that have been prepared with chromic or picric acid
combinations, or with absolute alcohol. Osmic acid preparations
stain very weakly unless they have been j)i"eviously bleached. All
acids shoiild be carefully washed out of the objects before staining.
The stain is permanent in oil of cloves and balsam.

Kleinenherg 8 Alcoholic Hcematoxylin, once very much xised, is
highly irrational and very inconstant in its composition and its
effects, and is now with reason generally abandoned.

Nuclear Stains for Sections. — Any of the foregoing stains may
of course be used for sections if desired. But in many cases other
stains are indicated, as being more powerful, or moi'e precise, or of
a richer selectivity.

492 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

The solution known as Kernschiaarz may be confidently recom-
mended as a powerful, precise, and very safe stain. It is a black
liquid imported from Russia by Griibler and Hollborn, and coixsists.
of an iron base united to some gallic acid. Sections may be stained
in it, either concentrated or diluted to the required intensity.
Overstaining seldom occurs. If it should occur it may be corrected
by means of any weak acid (solution of liquor ferri sulfurici oxydati,
diluted twentyfold — see the iron-hsematoxylin of Benda, below — is a
very fitting decolorant).

The result is a nuclear stain, sometimes, though by no means
always, also taking effect on protoplasm, of a brownish grey or
black, powerful and precise, and well adapted for photography. It is
permanent in balsam, jjresumably also in aqueous mounting media.
Being a jjrogressive stain, it is possible that it might give good
results for staining in bulk.

The present writer obtains a very similar stain by ' mordanting ''
for a few hours in Benda's liquor ferri, and then bringing the
sections directly for some hours into a 2 per cent, solution of
pyrogallol in water. Similai- results are also obtained by mordanting'
in 2 per cent, solution of tincture of perchloride of iron in 70 pei*
cent, alcohol, and then treating with 2 per cent, solution of pyro-
gallol in spirit : a process which is applicable to staining in bulk.

Benda's iron hcematoxylin is a still more powerful and precise
stain. Sections of mateiial that has been fixed in any way m^ay be
em.ployed. They are ' mordanted ' by soaking for half an hour or
for some hours (as much as twenty-foui-, if a very strong stain be
required) in liquor ferri sulfurici oxydati, P.G., diluted with one or
two volumes of water. ^ They are then well washed, first with
distilled water, then with ta]3 watei-, and are brought into a 1 per
cent, solution of haematoxylin in water, in which they remain till
they have become thoroughly black. They are now overstained,,
and must be ' diffei-entiated.' To this end they are washed and put
either into some of the sulphate solution strongly diluted with water
(say twenty or thirty fold), or into 30 per cent, acetic acid, the
progress of the decoloration being followed in either case under the
microscope. They are then mounted in the usual way.

This gives an extremely powerful blue-black stain, purely nucleai-
if the differentia,tion has been pushed far enough, or nuclear and at
the same time plasmatic if the differentiation is stopped before the
protoplasm has become decoloured. The stain is absolutely
permanent in balsam.

The results obtained by this process are practically identical with
those obtained by the iro7i licematoxylin process of Heidenhain, with
this advantage, that Benda's iron solution is easily made and keeps
indefinitely, whereas Heidenhain's process involves the employment

1 This preparation consists of sulphate of iron 80 parts, water 40, sulphuric acid
15, and nitric acid 18. The ingredients should be mixed, and give at first a black
liquid which gradually acquires a red colour. The operation should be performed
out of doors, or in a chemical laboratory, as during the process of solution voluminous
nitrous vapours are given off, which would be hurtful to lenses and delicate instru-
ments.

eectEESsive staining 493

of ferric alum, which can only be obtained from hirge chemical
Avorks, and does not keep well either in substance or in solution.

Owing to the precision and depth of the stain, pi'epai'ations
made by this process Avill beai- study with higher mici'oscopic
powei's than those made by any othei- means ; that is to say, it is
certainly found in practice that they will beai- notably highei-
eye-piecing.

It will be observed that, as with borax-carmine, this is a
' i-egressive ' stain. The progress of decoloration, being slow, may
be controlled under the microscope, and a little pi-actice with this
process may serve as an inti'oduction to the ai-t of regressive
staining with safi-anin and other tar-colours, with which the
pi'Ogress of decoloration is so I'apid that it cannot be conti-olled
under the microscope.

Safranin is perhaps the most beautiful stain of this class. The
first requisite to success in staining with this coloui- is to obtain a
good sample of the dye. This is absolutely essential. There are at
least a score of brands of safranin on the market, many of which
cannot be made to afford a good stain by any means whatever. The
brand ' Safranin 0 ' supplied by Griibler and Hollborn is an excellent
one.

The dye is employed in the form of a saturated or at least veiy
concentrated solution in water or alcohol. Perhaps the best plan in
general is to make a saturated solution in water, and another
saturated solution in strong alcohol, and then mix the two in equal
parts. Sections are soaked in the solution until thoroughly over-
stained — the longer the better, (jrood stains can often be obtained
after half an hour in the staining bath, but for many objects it is
necessary, in order to ensure good results, to stain for twenty-four
hours, or even for many days.

After the staining comes the ' diflerentiation ' of the stain. The
sections are just rinsed with watei" and brought into strong alcohol,
either in a watch-glass, if they be loose sections, or in a flat-bottomed
tube if they be affixed to a slide. ' The sections in the watch-glass
are seen to give up their colour to the alcohol in clouds, which are
at first very rapidly formed, afterwards more slowly. The sections
on the slide are seen, if the slide be gently lifted above the surface
of the alcohol, to be giving off theii- coloui- in the shape of rivers
running down the glass. In a short time the formation of the clouds
or of the rivers is seen to be on the point of ceasing ; the sections
have become />a.Ze and somewhat trmisparent^ and (in the case of some
objects) have changed colour, owing to the coming into view of the
general ground-colour of the tissues, from which the stain has now
been removed. At this point the differentiation is complete, and
the extraction of the colour must he stojyped instantly!

This may be done if desired by simply putting the sections into
water ; but the more usual practice is to proceed at once to mount
them in balsam. To this end they may be cleared by being put into
clove oil (or by pouring the oil over them on the slide). This will
exti'act slowly a little moi-e coloui-, and may thus sei-ve to complete
the difi'erentiation in a fi-equently vei'y desirable manner. Or you

494 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

may clear or remove the alcohol with an agent that does not remove
any more colour, such as cedar oil, or bergamot oil, or xylol, toluol,
or benzol. This being done, nothing more remains but to add a drop
of xylol-balsam or dammar, and a cover (chloi'oform is best avoided,
either as a clearer or as a menstruum for the mounting medium).

The result is a pure nuclear stain, of exceeding brilliancy, and
perfectly permanent in balsam.

The process is not available for staining in bulk, but besides-
sections such material as is thin enough to behave like a section —
portions of thin membranes, for instance — may be stained in this way.
The process of differentiation takes about a couple of minutes with
most thin sections, but in some cases considerably more is required.

Besides safi'anin, many others of the coal-tar dyes may be used
in the same way : for instance, basic fachsin {rnagenta)^ also a red
stain, or gentian violet or thionin^ both these being blue. Thionin is
peculiarly resistent to alcohol, which is an important quality in some
cases.

Plasma Stains, or Plasmatic Stains. — All the stains we have
hitherto considei'ed (with the exception of the intra vitam stains)
have been nuclear stains — that is, such as stain nuclei either
exclusively, or at least more energetically than protoplasm or formed
material. In veiy many cases they peiform all that the histologist
requires in the way of I'endering structure visible. But still there
are other cases in which it is desirable to obtain a separate stain of
extra-nuclear parts. For this purpose the so-called plasma stains
are employed.

Picric acid is a useful one, especially when employed after a
carmine or hsemfcrtoxylin nuclear stain. The modus operandi is as
simple as possible : it consists merely in adding picric acid to the
alcohol employed for dehydrating the objects, and lea"\dng them
therein until the desired intensity of stain is obtained. ' It has the
gi-eat quality, shared by very few plasma stains, that it can be used
for staining entire objects. And as it is extremely penetrating, it is
very much indicated for the preparation of such objects as small
arthropods or nematodes, mounted whole.'

Lyons blue [Bleu de Lyoii) is a good plasma stain that will work
well after carmine (borax-carmine for instance). It may be used
for staining in bulk, in a very dilute alcoholic solution ; or foi-
staining sections, in a strong aqueous solution. The objects must
not remain too long in alcohol after staining.

The dye known as Wasserblau (loater-blue) gives with sections
a similar but perhaps more delicate stain. It is a good stain to
use in conjunction with safranin, using the Wasserblau first. The
pi'ocess is, first, to stain rather strongly in a concentrated aqueous
solution of the blue, and then foi- from half an hour to four or five
hours in the safranin, as described above.

Either of these stains will pi-obably -be foimd safer than indigo-
carmine, which was once much employed for similar purposes.

A still moi'e ]3i'ecise and delicate plasma stain is Sdurefttchsin
(also known under the synonyms, or names of brands, of acid
fnch&in, Sdureruhin, Fuchsin S, Rubin S, and others). It is

IMBEDDING METHODS 495

important not to confound it with basic fuclisin, as appears to
have been clone by some writers. For staining sections a ^ per
cent, sohition in water may be employed, and allowed to act on
sections for fi'om one to five minutes. A red stain, very resistent to
alcohol and acids, and permanent in balsam. It is an excellent stain
for use after a blue nuclear stain, such as hfematoxylin, thionin,
gentian violet, or the like.

The celebrated mixture known as the Ehrlich-Biondi-Heidenhain
stain involves such complicated and delicate manipulations as to be
totally unsuitable for oi-dinary histological work.

Imbedding Methods, — 'The beautiful processes known as
imbedding methods are employed for a threefold end. Firstly, they
enable us to surround an object, too small or too delicate to be
firmly held by the fingers or by any instrument, with some plastic
substance that will support it on all sides with firmness but without
injurious pressure, so that by cutting sections through the
composite body thus formed, the included object may be cut into
sufiiciently thin slices without distortion. Secondly, they enable us
to fill out with the imbedding mass the natural cavities of the object,
so that their lining membranes or othei' structures contained in
them may be duly cut in situ. And, thirdly, they enable us not
only to surround with the supporting mass each individual organ or
part of any organ that may be present in the interior of the object,
but also to impregnate with it each sepai'ate cell or other anatomical
element, thus giving to the tissiies a consistency they could not other-
wise possess, and ensuring that in the thin slices cut from the mass
all the details of structure mil precisely retain their natural relations
of position.'

' These ends are usually attained in one of two ways. JEither the
object to be imbedded is saturated by soaking with some material
that is liquid while warm and solid when cold, which is the principle
of the paraffin process ; or the ^ object is saturated with some
substance which whilst in solution is sufiiciently fluid to penetrate
the object to be imbedded, whilst at the same time, after the
evaporation or removal by other means of its solvent, it acquires and
imparts to the imbedded object suflicient firmness for the purpose
of cutting,' which is the principle of the celloidin process. (From
Mr. Lee's ' Microtomist's Vade-mecum.')

Any substance used for imbedding is technically termed an
' imbedding mass.'

The older workers were not aware of the importance of
thoroughly saturating the objects to be cut with the imbedding
mass, a point which is very impoi-tant in order to the production
of thin and undistorted sections. They were content with simply
surrounding the objects to be cut with the mass. This primitive
procedure is now rightly abandoned, except in cases in which, on
account of the large size or other peculiarities of the object, it is
impossible to procure due saturation.

Among the numerous methods of imbedding that have been
advocated, only two are in general use at the present day. These
are the imraffin method., and the collodion or celloidin method. And

496 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

■of these, it is the paraffin method that is by fax- the most usually
•em.ployed. It is the most convenient for ordinary work, the
■collodion method only presenting points of superioiity in special
cases, such as the sectioning of extremely large objects, or very
brittle tissues, and other special circumstances.

The Paraffin Method. — The first step in the paraffin method
■consists in saturating the objects with a solvent of paraffin. The
second consists in saturating them with molten paraffin, which
gradually takes the place of the solvent. The thii-d consists in
causing the paraffin to solidify, and arranging the solidified mass in
a suitable form for cutting sections. The fourth consists in cutting
the sections and freeing them from the solid paraffin with which
they are saturated, and if desired affixing them in serial order to a
slide for the purpose of mounting.

1. Saturation loith a solvent. — The solvents employed ai-e either
chloroform, or one of the volatile hydrocarbons, such as benzol,
toluol, or naphtha, or an essential oil, such as oil of cedar or oil of
cloves. None of these are raiscible with water, but all of them are
miscible with alcohol. Therefore the objects to be imbedded are in
the first place thoroughly dehydrated with alcohol, according to the
principles set forth above, p. 487. The alcohol is then removed
from the objects, and the solvent is made to take its place gradually
by one of the substitution methods described above, p. 487, under
' Clearing.' Cedar oil is one of the most convenient solvents ; and
as it is at the same time one of the best of clearing agents, it follows
that any object that has been cleared in it is at once ready for
saturation with paraffin. Other essential oils, such as clove oil, may
also be employed. But the two best saturation agents are certainly
oil of cedar and chloroform. It will be noticed that the best way to
saturate objects with chloroform is to place the chloroform under
the alcohol, and allow the substitution of liquids to take place just
as in clearing with a non- volatile clearing agent, as directed
above, under ' Clearing.'

2. Saturation loith paraffin. — If cedar oil, or other non- volatile
medium, has been employed, proceed as follows : — Melt some
paraffin in a suitable vessel — a watch-glass will do for small objects
— and keep it as nearly as possible at melting-jDoint on a water-bath
or in a stove, taking care to keep it protected from vapour of water.
Remove the object fronr the oil, and put it into the paraffin, and
leave it there till thoroughly saturated. The" length of time
required for this must be found by experience. A piece of
soft tissue of \ inch thickness is generally well saturated in an
hour. If the objects be at all large, the paraffin should be
changed for fresh once or twice, so that none of the oil may remain
to contaminate it and render it soft aftei- cooling.

Some persons prefer to bring the objects gradually from the oil
into the paraffin by passing them through graduated mixtures of oil
and paraffin ; but with cedar oil, at all events, that is not necessary.

If chloroform, or other volatile medium, has been employed,
the procedure may be modified in the following manner, which is
very advantageous for delicate objects : —

ARRANGEMENTS FOR SECTION-CUTTING 497

' The chloroform and the objects in it are gradually warmed up
to the melting-point of the paraffin employed, and during the
warming small pieces of paraffin are by degrees added to the
chlorofoi'm. So soon as it is seen that no more bubbles are given
olf from the objects, the addition of paraffin may cease, for that is a
sign that the pai-affin has entirely displaced the chloroform in the
objects. This displacement having been a gradual one, the i*isk of
shrinkage of the tissues is reduced to a minimum.' After this,
liowever, the whole must be kej)t warm on the water-bath, at the
temperatui'e of the melting-point of the pure paraffin employed,
until all the chloi-oform has been driven off from it, as, if even a
trace of chloi'ofoi'm i-emain in the j^araffin, it will render it soft after
cooling. As this is a A'eiy long process (it may take days for large
objects), it is frequently better to simply transfer the objects from
the paraffin solution to a bath of pui'e paraffin.

3. Arranging for cutting. — After the objects ha^e been duly
saturated, they are arranged in a suitable position for cutting, and
the paraffin is caused to solidify as qtiickly as possible. It imist not
he alloioed to cool slowly., as slow cooling allows the paraffin to
crystallise, and gives a mass less homogeneous and of a consistency
less favourable for cutting than after rapid cooling.

Very small objects may be taken out of the jjfu-affin with a
needle or small spatula, and put to cool on a block of glass, then
imbedded in position for cutting on a cone of jDaraffin already
soldered to the object-carrier of the microtome, or to a cork or
cjdinder of wood fitted into it. This is done as follows : —

' A jDiece of stout wire, or a mounted needle, is heated in the
flame of a spirit-lamp, and with it a hole is melted in the end of the
cone of paraffin ; the specimen is pushed into the melted paraffin,
and placed in any desired position. In the use of the needle or wire
it should be noted that it is important to melt as little paraffin as
2)ossihle at one time, in order that that which is melted may cool
again as rapidly as jjossible. The advantages of the method lie in
the quickness and certainty with which it can be performed.'

If the paraffin bath has been given in a watch-glass, float the
watch-glass with the paraffin and objects on to cold water. Do not
let it sink till all the paraffin has solidified. When cool, warm the
bottom slightly and cut out blocks containing the objects ; do this
with a slightly warmed scalpel. Then fix the blocks to the object-
carrier by means of a heated needle as above described.

Foi' many objects, other methods of arrangement are prefei-able.
These consist chiefly in causing the pai'affin to solidify in a mould of
any desired shape. Fajjer trays are often used as moulds.

To make paper trays, proceed as follows. Take a piece of stout
paper or thin cardboard, of the shape of the annexed figure (fig. 407) ;
thin (foreign) post-cards do very well indeed. Fold it along the
lines a a' and h V , then along c c' and d d' , taking care to fold
always the same way. Then make the folds A A', B B', C C\ D D',
still folding the same way. To do this you apply A c against A a,
and jiinch out the line A A', and so on for the remaining angles.
This done, you have an imperfect tray with dogs' ears at the angles.

K K

498 PEEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS

D'

To finish it, turn the dogs' ears round against the ends of the box,
turn down outside the projecting flaps that remain, and j^inch them
down. A well-made post-card tray will last through several im-

beddings, and will generally
^ work better after having

been used than when new.
(From Mr. Lee's ' Microto-
^ mist's Yade-mecum.')
B To imbed in such a tray,

or similar recej^tacle, some
melted paraffin (or other
I ' mass ') is poured into it ;
Bj at the moment when the

mass has cooled so far as to
have a consistency that will
not allow the object to sink
to the bottom, the object is
placed on its surface, and
10 jjl ^ more melted mass poured on

fl\. .^, J. \(i ^^j^i{\ -tiie object is covered

by it. Or, the paper tray
being placed on cork, the
object may be fixed in posi-
tion in it whilst empty by
means of pins, and the tray
_ filled with melted mass at

6' one pour. (The pins can be

removed from the mass
when cold.)

In either case, when the
mass is cold the paper is removed from it before cutting.

As soon as the tray is filled, and the object in position, cool it on
water, holding it above the surface with only the bottom immersed
until all the paraffin has solidified, as if you let it go to the bottom
at once you will probably get cavities filled with water formed in
your paraffin. Or you may put it to cool on a block of cold metal or
stone.

A better plan is to
employ sets of two
pieces of type-metal,
cast in rectangular form
of various heights and
capable of being placed
together as in fig. 408 ;
in this way a suitable
box is formed, and, the
end of the shorter arm
being triangularly en-
larged outwards, it is closed sufficiently to retain the mass. Placed
in this way, with the short arms nearer to or farther from each
other as a less or greater imbedding mass is required, they are set

Fig. 407.

Fig. luy. - Type-iiictiil cat,e lur imbedding.

CUTTING SECTIONS

499

on a plate of glass which has been wetted with glycerin and gently
warmed. The melted pai'affin is now poni'ed into this mould and
the object is imbedded in it as described for the paper tray.

Still another plan is to take a common fiat medicine-bottle, as in
fig. 409, fitted with a cork through which two tubes pass, or, if the
mouth is small, one tube may be festened into a hole drilled into the
bottle. One of these tubes, A, is connected with hot and cold water ;
the other, B, is a discharge-pipe for the water entering the bottle
by A, and raising or loweiing its temperature as warna or col d water
is allowed to flow in. On the smooth, flat side of the bottle four
pieces of glass rods or strips are cemented fast, so as to inclose
a rectangular space, 0, which foi'ms a receptacle
for the melted paraflin.

As long as the warm water circulates through
the bottle the paraflin remains fluid, and objects
in it may be arranged under the microscope by
light from above or below, and can be oriented
with reference to the sides of the paraflin recep-
tacle or with reference to lines drawn upoia the
surface of the bottle. When the cold water is
allowed to enter in place of the warm, the paraflin
congeals rapidly, and may be easily removed as
one piece. The discharge-pipe should open near
the upper surface of the bottle, to di-aw ofi" any
-air which may accumulate there.

In using any form of microtome where the
object is held in jaws, the imbedding mass must
either be cast a suitable shape, and placed directly
in the jaws, or be cemented to pieces of soft
wood which may be placed in the jaws.

The mould obtained by either of these pro-
cesses is then fixed to the carrier of the micro-
tome, and finally pai-ed into a convenient shape,
■and oriented for cutting.

4. Cutting. — Paraffin sections are always cut
dry — that is, the knife is not wetted with either
•alcohol or any other liquid.

' If the knife be set square — ^that is, with its
axis at right angles to the line of motion (of the
knife for sliding microtomes, and of the object-
carrier for rocking microtomes) — and if the paraffin block be
cut into a rectangle, and also set square — that is, with one edge
parallel to the edge of the knife — sections may be cut in " ribbons."
The sections not being removed from the knife one by one as they
are cut, but allowed to lie undisturbed on the blade, adhere to one
another by the edges so as to foi-m a chain or ribbon, which may be
taken up and transferred to a slide without breaking up, thus greatly
lightening the labour of mounting a series.'

Difficult objects are in general better cut in isolated sections
with an oblique knife. In this case it is best to cut the paraffin into
the shape of a three-sided prism, and arrange it so that the knife-

K K 2

Pig. 409. — Arrange-
ment for the orien-
tation of objects in
paraffin.

500 PKEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS

edge enters it at one angle and leaves it at another angle (in fig. 410,
the knife enters at a and leaves at c). The prism should be so cut
as to leave the imbedded object near to the side which is fiarthest
from the angle a which is first touched by the knife. Then if the
section should roll, at all events the section of the object will come
to lie in the most open spire of the coil, and can thus be more easily
unrolled.

The rolling of sections above referred to is an annoying
phenomenon of very frequent occurrence. Its most usual cause is
over-hardness of the parafiin, but it is favoured by excessive
obliquity of the knife, and other circumstances. With large sections
it is not difiicult to catch them by the edge
as they begin to roll, and hold them down
with a camel' s-hair brush. Or a section-
stretcher may be used.'

If the paraifin be too soft, the sections
will not roll, but will become creased.

Either of these defects may be dimi-
nished, sometimes even totally cured, by
simple means. Firstly, due attention must
be paid to the j^osition of the knife ; not
only to its obliquity, but also to its tilt, as.
explained above.

Secondly, if the parafiin should be too
hard, it may be softened by setting up a
lamp near it, or even by closing the win-
dow, if this should happen to be open, or
by carrying the microtome to a warmer
place, or by any device that will have the
efifect of ex]Dosing the parafiin block to an
increase of temperature. An incredibly
slight increase will sometimes suffice.
Thirdly, if it should be too soft, an opposite
treatment must be tried. The microtome is removed to a cooler
place, or the window is opened, or the like.

If none of these manoeuvres sufiice to obtain sufiiciently good
sections, the object must be re-imbedded in a harder or softer
paraffin. But it will generally be possible to save the sections by
flattening them out by the water method, to be presently described.
The iKiraffin employed for imbedding micst be of a hardness
determined by the temperature of the luorkroom, : hard paraffin for a
warm room, soft jxiraffinfor a cold room,. For the Thoma microtome,
a paraffin melting at 45° C. (oi' 113° F.) gives good results so long as

Fig. 410.

1 ' Section- stretchers are instruments consisting essentially of a little metallic roller
suspended over the object to be cut in such a way as to rest on its free surface with
a pressure that can be delicately regulated so as to be sufficient to keep the section
flat without in any way hindering the knife from gliding beneath it.' They are made in
various forms, the most convenient being that of 'Mayer, Andres and Giesbi edit, of which
a description and figure maybe found \\\i\ie Journal of the Eoi/. Microscopical Soe.
1883, p. 916. Now that the water flattening process (see below, Flattening) has been
perfected, section-stretchers are not so necessary as they were formerly, and for most
work may be dispensed with.

FLATTENING SECTIONS AND MOUNTING 501

the temperature of the laboratory lies between 15° and 17° C. (59°
and 62° F.) ; though many woi-kei'S prefer, even with this insti-ument,
a much harder mass. For microtomes with fixed knives, such as
the Cambi'idge rockei-, hai'der jjai'affins may be used than Avith sliding
microtomes, pai'affins of from 55° to 60° C. (131° to 140° F.) being
used by many workers. For cutting ribbons with these hard
masses it is frequently necessary to coat the face of the block
nearest to the knife with a softer paraffin, in ordei' that the sections
may cohere.

Masses of intermediate consistency may be made by mixing a
hard and a soft paraffin. Two j^arts of paraffin of 50° C. (122° F.)
with one of 36° C. (97° F.) melting-point, give a mass melting at
48° 0. (119° F.).

Mixtures of paraffin with vaseline and with various fatty and
other substances have been recommended. They are now generally
abandoned.

5. Flattening the sections, and mounting. — If the sections have
come off either rolled or creased, they must be flattened before the
paraffin is removed.

If they are large sections, float them on to warm water in a
suitable dish. They will flatten out perfectly in a few seconds, and
they may then be lifted out on a slide or cover-glass slid under
them. The toater must not he warm enough to melt the paraffin, which
must only be warmed, not melted, till the sections have been
securely fixed to the slide or cover. A temperature of about 40° C.
(104° F.) is about right.

Or take a clean slide, free from grease, spread on it with a
brush enough water to float the sections, lay the sections on it, and
warm, either on the water-bath, or on a hot plate, or over a small
flame, taking care not to melt the parafiin.

If the sections are numerous and small, take a perfectly clean
slide, so clean that water will readily spread on it. Breathe on it,
and smear on it with a brush a streak of water as wide as the
sections and of the length of the first intended I'ow. Lay the first
row of sections on this streak. Breathe on the slide again, and
draw on it another streak of water under the first one. Lay a
second row of sections on this ; and so on until the slide is full.
Then warm as before.

The chief difficulty connected with this process lies in the diffi-
culty of getting the water to spread evenly on the slide. The slide
should be well freed from grease, by means of xylol or some good
solvent of fats, and then cleaned with alcohol. The test for suffi-
cient freedom from grease is, that on breathing on the slide the
moisture of the breath should condense on it evenly, and evaporate
evenly. The slide should also be well rubbed with a clean cloth
wetted, or rather moistened, with water, before the water is defi-
nitely spread on it with the brush. Some sorts of slides cannot be
got to spread the water evenly by any means.

The following is said by De Groot (' Zeitschrift f. wiss. Mikro-
skopie,' XV. 1, p. 62) to be infallible. Wrap the corner of a clean
cloth round two fingers and rub it with a piece of chalk. Moisten

502 PEEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS-

it with a drop of water and rub the shde with the chalked part,
then finish with pure water and a clean part of the cloth.

6. The flattening having been accomplished by either of these pro-
cesses, the sections must now he fixed to the slide or cover before the-
paraffin is removed.

The most elegant method of accomplishing this is by what is
known as the water method. It consists simply in drying the sec-
tions on the slide (or cover). After they have been got on the slide
and flattened out by water and warming as above described, the
superfluous water is drained off, and the slide put away to dry. As
soon as the water has entirely evaporated off, the sections will be
found to be so firmly afiixed to the glass that they will bear the^
melting of the paraffin, treatment with solvents, with alcohol or
stains, &c., without moving. A convenient plan is to dry the slides
on the top of the stove or water-bath at a temperature somewhat
under the melting-point of the pai-affin. This will take from half an
hour to three or four hours. When dry the sections will have
assumed a certain horny transparent look. The -paraffin must not he
allowed to melt before the sections are perfectly dry. If they are left
to dry at the temperature of the room, they should be left overnight..

As soon as the sections are quite dry, the paraffin may be melted
by holding the slide for a few seconds over a small flame, after which
it is plunged at once into a tube of xylol or benzol or chloroform or
the like, which in a few seconds or minutes dissolves out all the-
paraffin from the sections.

The water method is very safe for sections that present a sufficient,
uninterrupted surface capable of afibrding adhesion at all points to
the slide. But sections of hollow organs, offering only a relatively
small surface for attachment, adhere very badly. Sections of such
things as tubidar chitinous organs, for instance, will generally not
allow of mounting at all in this way.

In such cases, Mayer's albumen fixative should be employed.
Take 50 c.c. of white of Q%%., 50 c.c. of glycerin, and 1 grm. of
salicylate of soda, shake them up well together, and filter into a clean,
bottle. The filtering may take days. A little, very little of this is
now painted on to the part of the slide destined to receive the sec-
tions, and the layer smoothed by drawing the edge of a slide over it,
(some persons rub off the excess with the ball of a finger). Place a
drop of water on the prepared surface, lay the sections on it and
flatten by warming, drain and evaporate as in the water process,
with this difference, however, that the evaporation need not be-
carried to the point of ^^erfect drying. The slides will be sufficiently
evaporated at a temperature of 40° 0. in ten minutes or a quarter of'
an hour. And if the evajjoration be conducted by waving the slide
to and fro over a flame, from three to five miniites may suffice. The
paraffin is then melted and removed by xylol or other solvent, as
before.. This process has the advantage over the water process of
greater safety and greater i^apidity, but has the disadvantage that
the layer of albumen stains obstinately in some j)lasma stains, thus
producing an inelegant mount.

If the sections be neither rolled nor creased, it is not necessary

CELLOIDIN IMBEDDING 503

to flatten them on water. They may be laid clown on Mayer's
albumen, without water, gently pressed down with a brush, and the
paraffin melted and dissolved at once, the whole process taking only a
few seconds. But for delicate histological work it is well to employ
the water method in any case, as the flattening on wa,ter serves to
somewhat expand the sections, which, unless cut from extremely
hard pai-affin, are generally somewhat compressed by the impact of
the knife.

As soon as the pai-affin has been removed, all that is necessary,
in the pure water process, is to add a drop of balsam and a cover, if
the material has been already stained. If not. the solvent of the
paraffin is removed by alcohol, and the sections are stained in any
manner that may be desired.

But if Mayer's albumen has been employed the sections must be
thoroughly washed with alcohol before the definitive clearing and
mounting. This is necessary in oixler to remove the glycerin, which
would otherwise cause turbidity in the mount.

Tubes for Handling Serial Sections. — The most convenient
vessels for performing the various operations of washing, dehydrating,
clearing, staining, &c., with sections fixed to the slide, are flat-
bottomed corked tubes. They should have an internal diameter
slightly over 1 inch, so as to be able to take two slides placed back
to back ; and they should be nearly 4 inches high, so as not only
to take the slides in an upright position, but to allow room for the
cork. A stand is easily made for them by taking a -piece of inch
deal board, and boring in it with a centrebit holes about ^ inch
deep, large enough to take the bottoms of the tubes, and about 1 inch
apart. A board with three rows of seven holes each does not take
up too much room on the work-table.

The Collodion or Celloidin Imbedding Method. — Celloidin is a
patent collodion, sent out in semi-dry tablets. It may be obtained
through Griibler and Hollborn. To prepare it for use for imbedding
it may either be dissolved at once in a mixtvire of equal parts of
ether and absolute alcohol, or, which is held by some workers to be
preferable, it may be cut up into thin shavings, which are allowed to
dry in the air until they have assumed a horny consistency, and are
then dissolved in the ether and alcohol. It is held that by thus
drying the celloidin all water is removed from it, and a more favour-
able imbedding mass obtained. Either celloidin or common collodion
may be used for imbedding, celloidin having merely the advantage
stated.

A thin celloidin solution is made by dissolving from 4 to
6 per cent, of the dried shavings in the alcohol and ethei- mixture ;
a thick one by dissolving from 10 to 12 per cent, of them.
Thicker solutions than this are not necessary. If common collodion
be taken, a thin solution should be prepared by diluting it with
ether.

The objects to be imbedded must first be thoroughly dehydrated
with absolute alcohol. They are then soaked, till thoroughly pene-
trated, in ether, or, which is better, in a mixture of ether and
absolute alcohol. They are then brought into the collodion.

504 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

They should be soaked first in a thin sokition, until thorouglily
impi-egnated with it, for days, even for small objects ; weeks or
months for large ones. When well saturated with this they should
be brought into a tliich solution, and soaked in it for a long time, the
longer the better.

When it is deemed that they are saturated, they may be imbedded.
In many cases this may be efficiently done by simply gumming the
object by means of a droj) of thick collodion to a cork, or, better, a
piece of soft wood, adapted to be afterwards fitted to the microtome.
But for the purpose of accurate orientation it is preferable to imbed
in a mould. This is done in the manner described for parafiin. A
convenient mould for celloidin is made by taking a cork, and winding
a strip of paper sevei'al times round one end of it, so as to form a
projecting collar, which is fixed with a pin. Before using this, oi-
any paper tray, it should be dressed by having the inside painted
with collodion, which is allowed to dry befoi-e the imbedding mass is
poured into it. The object of this is to prevent bubbles of air
coming in through the bottom or sides of the mould. Watch-glasses,
deep water-colour moulds, and the like, also make convenient
imbedding receptacles. Care should be taken to have them perfectly
dry.

If bubbles should appear after the mass has been poured in, they
should be got rid of before proceeding further by exposing the whole
to the vapour of ether for an hour or two in a closed vessel.

The next step consists in the hardening of the mass. One of the
best ways of doing this is as follows : —

' Put the preparation into a desiccator or other suitable closed
vessel, on the bottom of which a teaspoonful of chloi'oform has been
poured. As soon as the mass has attained sufiicient superficial hard-
ness, it is, of course, well to turn it out of its recipient and turn it
over from time to time, in order that it may be equally exposed on all
sides to the action of the vapour. Small objects may be sufficiently
hardened in from one hour to overnight. When fairly hard (it is not
necessary to wait till the mass has attained all the hardness of which it
is susceptible), throw it into a mixture of one part of chloroform with
one or two parts of cedar oil. From time to time more cedar oil should
be added, so as to bring the mixture up gradually to nearly pure cedai-
oil. As soon as the object is cleared throughout, the mass may be
exposed to the air, and the rest of the chloroform will evaporate
gradually. The block may now be mounted on the holder of the
microtome with a drop of thick collodion (which may be allowed to
dry, or may be hardened by putting back into chloroform vapour),
and may either be cut at once, or may be pi'eserved indefinitely
without change in a stoppered bottle. Cut with a dry knife, the cut
surface will not dry injuriously under seveial hours. The cutting
quality of the mass is often improved by allowing it to evaporate in
the air for some hours.

' The hardening may be done at once in the chloroform and cedai--
wood mixture, instead of the chloroform vapour, but the latter
process is preferable as giving a better hardening. And clearing may
be done in pure cedar oil instead of the mixture, but then it will be

HAEDENING 505

very slow, whereas in the mixture it is extremely rapid.' (From
Mr. Lee's ' Microtomist's Yade-mecum.')

Instead of cedar oil, white oil of thyme may be employed ; and
some workers use glycerin.

The above process is recommended as giving good results with
small objects. For large ones the alcohol process is more generally
employed.

In this the mass is first subjected to a 'preli'minary hardening.
The mass, with the imbedded object, is set under a glass shade or
put into a loosely closed vessel, so as to allow of just enough com-
munication with the air to set up a slow evaporation. It is some-
times a good plan to set it under a bell -jar with a dish containing
alcohol, so that the evaporation is gone through in an atmosphere of
alcohol. As soon as the mass (of which only enough to just cover
the object should have been taken) has so far sunk down that the
object begins to lie dry, fresh thick solution is added, and the whole
is left as before. The process is repeated every few hours for, if
need be, two or three days.

When the mass has attained a consistency such that the ball of
a finger {iiot the nail) no longer leaves an impress on it, it should
be scooped out of the dish or mould, or have the paper removed if
it has been imbedded in paper, and be submitted to the next stage
of the hardening process.

This, the definitive hardening^ consists in putting the preparation
into alcohol, and leaving it till it has attained the right consistency
(one day to several weeks). The strength of alcohol used by
different workers varies between 70 per cent, and 85 per cent., the
latter strength being probably the best. The vessel containing the
alcohol ought not to he tightly closed, hut should he left at least slightly
open.

' To fix the hardened 2'>'>'e2)aration to the microtome, proceed as
follows. Take a piece of soft wood, or, for very small objects, pith,
of a size and shape adapted to fit the holder of the microtome.
Cover it with a layer of collodion, which you allow to dry. Take the
block of collodion, or the impregnated and hardened but not
imbedded object ; cut a slice off the bottom, so as to get a clean
surface ; wet this siirface first with absolute alcohol, then with ether
(or allow it to dry), place one drop of very thick collodion on the
prepared wood or pith, and press down tightly on to it the wetted
or dried surface of the block of collodion. Then throw the whole
into weak (70 per cent.) alcohol for a few hours (or even less), or
into chloroform, or vapour of chlorofoi-m, for a few iniiiutes, in .
order that the joint may harden.' (From Mr. Lee's ' Microtomist's
Yade-mecum.')

Sections of material prepared in this way ai-e cut with a knife
kept abundantly wetted with alcohol (of 50 to 85 or even 95 per
cent.). Some kind of drip arrangement may be found very useful
here. The knife is set in as oblique a position as possible. These
two points are illustrated in fig. 398.

Another method of definitive hardening and cutting is the
freezing method. ' After preliminaiy haixlening by alcohol the mass

5o6 PKEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS

is soaked for a few hours in water in order to get rid of the greater
part of the alcohol (the alcohol should not be removed entirely, or
the mass may freeze too hard). It is then dipped for a few
moments into gum raucilage in order to make it adhere to the
freezing plate, and is frozen. The sections are brought into warm
water. If the mass have frozen too hard, cut with a knife warmed
with warm water.'

Staining and mounting. — The sections are broxight into alcohol
of not more than 95 per cent, as fast as they are cut, and may now
either be stained or mounted at once. It is not in general
necessary nor even desirable to remove the mass from the sections
before staining or mounting. It is no hindrance to staining, and on
being mounted in glycerin or balsam it becomes perfectly invisible.

To mount in glycei-in, nothing more is necessary than to add a
drop of glycerin and a cover.

To m.ount in balsam, dehydrate in alcohol of not more than 95
2)er cent., and clear with an oil that does not dissolve collodion, such
as oil of origanum, bergamot oil, cedar oil, or with chloroform or
xylol.

The foregoing relates to single sections. If it be desired to
mount a series of small sections under one cover, arrange them on
the slide and expose it for a few minutes to the vapours of a
mixture of ether and alcohol in a closed tube. Then treat with 95
per cent, alcohol, clear and mount.

If the sections are to be stained on the slide, care should be
taken when arranging them to let the celloidin of each section over-
lap that of i^s neighbour at the edges, so that the ether vapour may
fuse them all into a continuous sheet. Then on passing the slide
into any aqueous liquid the sheet will be detached, and may then
be treated as a single section.

If the sections shoidd come off the knife creased, they may be
flattened by floating them on to oil of bergamot, after which they
may be got on to the slide and gently pressed on to it with a
cigarette paper or a piece of glossed tissue paper, after which they
may be exposed to the vapour of ether and alcohol as before.

Series may also be aflixed to the slide by means of Mayer's
albumen, as described above for paraflin sections.

For the complicated manipulations involved in the methods of
Weigert, Obregia, and others, wKich are only necessary in very
special cases, the reader must be referred to Mr. A. Bolles Lee's
' The Microtomist's Yade-mecum.'

Grinding and Polishing Sections of Hard Substances. — Sub-
stances which are too hard to be sliced in a mici-otome — such as
bones, teeth, shells, corals, fossils of all kinds, and even some dense
vegetable tissues — can only be reduced to the requisite thinness for
miieroscopical examination by grinding down thick sections until
they become so thin as to be transparent. General directions for
making such preparations will be here given ;^ but those special

'^ The following directions do not apply to siliceous substances, as sections of
these can only be prepared by those who possess a regular lapidary's apj)aratus, and
have been specially instructed in the use of it.

GEINDING AND POLISHING SECTIONS 507

details of management which particular substances may requii-e will
be given when these are respectively described. The first thing to
be done will usually be to procure a section of the substance, as thin
as it can be safely cut. Most substances not siliceous may be divided
by the fine saws used by artisans for cutting brass ; and these may
be best worked eithei- by a mechanical arraiigement such as that
devised by Dr. Matthews,^ or, if by hand, between ' guides,' such as
are attached for this purpose to Hailes's and some other microtomes.
But there are some bodies (such as the enamel of teeth, and porcel-
lanous shells) which, though merely calcareous, are so hard as to
make it very dilficult and tedious to divide them in this mode ; and
it is much the quicker operation to slit them with a disc of soft iron
(resembling that used by the lapidary) charged at its edge with
diamond dust, which disc may be driven in an ordinary lathe. Where
waste of material is of no account, a very expeditious method of
obtaining pieces fit to grind down is to detach them from the mass
with a strong pair of ' cutting pincers,' or, if they be of small
dimensions, with ' cutting pliers ; ' and a flat surface must then be
given to it, either by holding them to the side of an ordinary grind-
stone, or by rubbing on a plate of lead (cast or planed to a perfect
level) charged with emery, or by a strong-toothed file, the former
being the most suitable for the hardest substances, the latter for the
toughest. There are certain substances, especially calcareous fossils
of wood, bone, and teeth, in which the greatest care is required in
the performance of these preliminary operations, on account of their
extreme friability — the vibration produced by the working of the
saw or the file, or by gi-inding on a rough surface, being sufficient to
disintegrate even a thick mass so that it falls to pieces under the
hand ; such specimens, therefore, it is requisite to treat with great
caution, dividing them by the smooth action of the wheel, and then
rubbing them down ujoon nothing I'ougher than a very fine ' grit,' or
on the ' corundum files ' now sold in the tool shops, which are made
by imbedding corundum of various degrees of fineness in a hard,
resinous substance. Where (as often happens) such specimens are
sufficiently porous to admit of the penetration of Canada balsam, it
will be desirable, after soaking them in turpentine for a while, to
lay some liquid balsam upon the parts through which the section is
to pass, and then to place the specimen before the fire or in an oven
for some little time, so as first to cause the balsam to run in, and
then to harden it ; by this means the specimen will be rendered
much more fit for the processes it has afterwards to undergo. It
not unfrequently happens that the small size, awkward shape, or
extreme hardness of the body occasions a difficulty in holding it
either for cutting or grinding ; in such a case it is much better to
attach it to the glass in the first instance by any side that happens
to be flattest, and then to rub it down, by means of the ' hold ' of the
glass upon it, until the projecting portion has been brought to a
plane, and has been prepared for permanent attachment to the glass.
This is the method which it is generally most convenient to pursue
with regard to small bodies ; and there are many which can scarcely
1 Journ. Queliett Microsc. Club, vol. vi. 1880, p. 83.

508 PEEPARATION, MOUNTING, AND COLLECTION OF OBJECTS

be treated in any other way than by attaching a number of them
to the glass at once in such a manner as to make them mutually
support one another.^

The mode in which the operation is then to be proceeded with
depends upon whether the section is to be ultimately set up in Canada
balsam, or is to be mounted ' dry,' or in iiuid. In the former case
the following is the plan to be pursued : — The flattened siu'face is to
be polished by rubbing it with water on a ' Water-of-Ayi- ' stone, oi-
on a hone or ' Turkey ' stone, or on an ' Arkansas ' stone ; the first of
the three is the best for all ordinary purposes, but the two latter,
being much harder, may be employed for substances which resist it.^
Wlien this has been sufiiciently accomplished, the section is to be
attached with hard Canada balsam to a slip of thick, well-annealed
glass ; and as the success of the final result will often depend upon
the completeness of its adhesion to this, the means of most efiectually
securing that adhesion will now be described in detail. The slide
having been placed on the cover of the water-bath, and the previously
hardened balsam having been softened by the immersion of the jar
containing it in the bath itself, a sufficient quantity of this should be
laid on the slide to form, when spread out by liciuefaction, a thick
drop, somewhat larger than the surface of the object to be attached.
The slide should then be allowed to cool in order that the hardness
of the balsam should be tested. If too soft, as indicated by its
ready yielding to the thumbnail, it should be heated a little more,
care being taken not to make it boil so as to form bubbles ; if too
hard, which will be shown by its chipping, it should be remelted
and diluted with more fluid balsam, and then set aside to cool as
before. When it is found to be of the right consistence, the section
should be laid upon its surface with the polished side downwards ;
the slip of glass is next to be gradually warmed until the balsam is
softened, special care being taken to avoid the formation of bubbles ;
and the section is then to be gently pressed down upon the liquefied
balsam, the pressure being at first applied rather on one side than
over its whole area, so as to drive the superfluous balsam in a sort
of wave towards the other side, and an equable pressure being finally

1 Thus, in making horizontal and vertical sections of Foraniinifera, as it would
be impossible to slice them through, they must be laid close together in a bed of
hardened Canada balsam on a slip of glass, in such positions that when rubbed down
the plane of section shall traverse them in the desired directions ; and one flat surface
having been thus obtained for each, this must be turned downwards, and the other
side ground away. The following ingenious plan was suggested by Dr. Wallich (Ann.
of Nat. Hist. July 1861, p. 58) for turning a number of minute objects together, and
thus avoiding the tediousness and difficulty of turning each one separately : — The
specimens are cemented with Canada balsam, in the first instance, to a thin film of
mica, which is then attached to a glass slide by the same means ; when they have
been ground down as far as may be desired, the slide is gradually heated just suffi-
ciently to allow of the detachment of the mica film and the sjDecimens it carries ; and
a clean slide with a thin layer of hardened balsam having been prepared, the mica
film is transferred to it with the ground surface downwards. When its adhesion is
complete, the grinding may be proceeded with ; and as the mica film will yield to the
stone without the least difficulty, the specimens, now reversed in position, may be
reduced to requisite thinness.

^ As the flatness of the polished surface is a matter of the first importance, that
of the stones themselves should be tested from time to time ; and whenever they are
found to have been rubbed down on any one part more than on another, they should
be flattened on a paving-stone with fine sand, or on the lead-]Dlate with emery.

geindinct and polishing 509

made over the whole. If this be cai-efully clone, even a very large
section may be attached to glass without the intervention of any aii--
bubbles. If, however, they should present themselves, and they
cannot be expelled by increasing the pi'essure over the part beneath
wdiich they are, or by slightly shifting the section from side to side,
it is better to take the section entii-ely off, to melt a little fresh
balsam upon the glass, and then to lay the section upon it as before.
When the section has been thus secured to the. glass, and the
attached part thoi-oughly saturated (if it be porous) with hard
Canada balsam, it may be readily i-educed in thickness, either by
grinding or filing, as before, or, if the thickness be excessive, by
taking ofi" the chief part of it at once by the slitting wheel. So
soon, hoAvever, as it approaches the thinness of a piece of ordinary
card, it should be rublied down with water on one of the smooth
stones previously named, the glass slip being held beneath the
fingers with its face downwards, and the pressure being applied
with such equality that the thickness of the section shall be (as
nearly as can be discerned) equal over its entire surface. As soon
as it begins to be translucent, it should be placed under the micro-
scope (pai'ticular regard being had to the method of illumination
so as not to flood the object with light), and note taken of any
inequality ; and then when it is again laid upon the stone, such
inequality may be brought down by making special pressure wath
the forefinger upon the part of the slide above it. When the
thinness of the section is such as to cause the water to spread
around it between the glass and the stone, an excess of thick-
ness on either side may often be detected by noticing the smaller
distance to which the liquid extends. In jaroportion as the sub-
stance attached to the glass is gi-ound away, the superfluous
balsam which may have exuded around it will be brought into con-
tact with the stone ; and this should be i-emoved with a knife,
care being taken, however, that a margin be still left round the edge
of the section. As the section approaches the degree of thinness
which is most suitable for the dis^ilay of its organisation, great care
must be taken that the grinding process be not carried too far ; and
frequent recourse should be had to the microscope, which it is
convenient to have always at hand when work of this kind is being-
carried on. There are many substances whose intimate structure
can only be displayed in its highest perfection when a very little
more reduction would destroy the section altogether ; and every
microscopist who has occupied himself in making such prejDarations
can tell of the number which he has sacrificed in order to attain
this perfection. Hence, if the amount of material be limited, it is
ad\dsable to stop short as soon as a good section has been made, and
to lay it aside — ' letting well alone ' — whilst the attempt is being
made to procure a better one ; if this should fail, another attempt
may be made, and so on, until either success has been attained oi-
the whole of the material has been consumed ; the first section,
however, still remaining, whereas, if the first, like every subsequent
section, be sacrificed in the attempt to obtain perfection, no trace
will be left ' to show what once has been.' In judging of the

5IO PREPARATION, MOUNTINQ, AND COLLECTION OF OBJECTS

appearance of a section in this stage under the microscope, it is to
be remembered that its transparence will subsequently be consider-
ably increased by mounting in Canada balsam : this is particularly
the case with fossils to which a deep hue has been given by the
infiltration of some colouring matter, and with any substances
whose particles have a molecular aggi-egation that is rather amor-
phous than crystalline. When a sufficient thinness has been attained
the section may generally be mounted in Canada balsam ; and the
mode in which this must be managed will be detailed hereafter.

By a slight variation in the foregoing process, sections may be
made of structures in which (as in corals) hard and soft parts are
combined, so as to show both to advantage. »Small pieces of the
substance are first to be stained thoroughly and are then to be
' dehydrated ' by alcohol. A thin solution of copal in chloroform is
to be prepared, in which the pieces are to be immersed ; and this
solution is to be concentrated by slow evaporation, until it can be
drawn out in threads which become brittle on cooling. The pieces
are then to be taken out, and laid aside to harden ; and when the
copal has become so firm that the edge of the finger-nail makes no
impression, they are to be cut into slices and ground down attached
to glass in the manner already described, the sections being finally
mounted in Canada balsam. The sections (attached to glass) may
be partially or completely decalcified, the soft parts remaining in
situ, by first dissolving out the copal with chloroform ; when, after
being well washed in water, they should be again stained, and
mounted either in weak spirit or (after having been dehydrated) in
Canada balsam.^

A difierent mode of j)i"ocedure, however, must be adopted when
it is desired to obtain sections of bone, tooth, or other finely tubular
structures, ^t7?.penetrated by Canada balsam. If tolerably thin sec-
tions of them can be cut in the first instance, or if they are of a size
and shape to be held in the hand whilst they are being roughly ground
down, there will be no occasion to attach them to glass at all ; it is
frequently convenient to do this at first, however, for the purpose of
obtaining a ' hold ' upon the specimen ; but the surface which has
been thus attached must afterwards be completely rubbed away in
order to bring into view a stratum which the Canada balsam shall
not have penetrated. As none but substances possessing considerable
toughness, svich as bones and teeth, can be treated in this manner,
and as these are the substances which are most quickly reduced by
a coarse file, and are least liable to be injured by its action, it will
be genei-ally found possible to reduce the sections nearly to the
required thinness by laying them iipon a piece of cork or soft wood
held in a vice, and operating upon them first with a coarser and then
with a finer file. When this cannot safely be carried farther, the
section must be rubbed down upon that one of the fine stones already
mentioned which is fovind best to suit it ; as long as the section is
tolerably thick, the finger may be iised to press and move it ; but as

"•■ See Koeli in Zoologischer Anzeig. Bd. i. p. 36. The Author, having seen (by
the kindness of Mr. H. N. Moseley) some sections of corals prepared by this process,
can testify to its complete success.

CUTTING HAEU SECTIONS 5 II

soon as the finger itself begins to come into contact with the stone,
it must be guarded by a flat slice of cork, or by a piece of gutta-percha
a little larger than the object. Under either of these, the section
may be rubbed down to the desired thinness ; but even the most
careful working on the finest-grained stone will leave its surface
covered with scratches, which not only deti-act from its appearance,
but prevent the details of its internal structure from being as readily
made out as they can be in a j)olished section. This polish may be
imparted by rubbing the section with jDutty-powder (peroxide of tin)
and watei- upon a leather strap made by covering the surface of a
board with buff leather, having three or four thicknesses of cloth,
flannel, or soft leather beneath it ; this operation must be performed
on both sides of the section, until all the marks of the scratches left
by the stone shall have been rubbed out, when the specimen will be
fit for mounting ' dry,' after having been carefully cleansed from any
adhering particles of putty-powder.

Greater facility in the grinding of hard sections, as well as supe-
riority of result, is attainable by simple mechanical means.

A cutting machine will greatly facilitate the process of preparing

Fig. 411. — Hand machine for cutting hard sections.

rock slices. The thickness of each slice must be mainly regulated
by the nature of the i-ock, the rule being to make it as thin as can
be conveniently cut, so as to save labour in grinding down afterwards.
Perhaps the thickness of a shilling may be taken as a fair average.
This thickness may be still further reduced by cutting and polishing
a face of the specimen, cementing that on glass, and then cutting as
close as possible to the cemented surface. The thin slice thus left
on the glass can then be ground down with comparative ease.

The first (fig. 411) is a hand machine. The specimen is cemented
to the carrier, a, which is movable on the axis, h, and can also be
rotated in two directions. The object is pressed by the weight, c,
against the steel disc, d, which is revolved by the wheel, e, acting on
a smallei'-toothed wheel on the axis of d.

The second (fig. 412) is intended to be worked by the foot. The
parts a, h, c, and d are the same as before. The wheel and treadle at
y and g work the pulley, e, by which the steel disc, cZ, is revolved ; A
is part of the cover for the disc, to prevent the emery flying about.
A box beneath also catches the powder that falls.

(This arrangement is also supplied with fig. 41 1, though not shown
in the woodcut.) A second wheel at i, wnth a cord passing over h.

512 PEEPARATION, MOUNTING, AND COLLECTION OF OBJECTS

actuates a vertical spindle, I, which rotates a horizontal cast-iron
plate at m for polishing.

Decalcification. — Wlien it is clesh-ed to examine the structure of
the organic matiix in which the calcareous salts are deposited that
give hardness to many animal and to a few vegetable structiu^es (such
as the true corallines), these salts must be dissolved away by the
action of some acid, such as nitric or hydrochloric. This should be
employed in a very dilute state, in order that it may make as little
change as possible in the soft tissue it leaves behind. When jthe

Fig. 412. — Treadle macliine for cutting hard sections.

lime is in the state of carbonate (as, for example, in the skeletons of
echhioderms), the body to be decalcified should be placed in a glass
jar or wide-mouthed bottle holding from 4 to 6 oz. of water, and the
acid should be added drop by drop, until the disengagement of air-
bubbles shows that it is taking effect ; and the solvent j^rocess should
be allowed to take place very giudually, more acid being added as
required. When, on the other hand, much of the lime is in the
state of phosphate, as in bones and teeth, the strength of the acid
solvent must be increased ; and for the hardening of the softer parts
of the organic matrix it is desirable that chromic acid should be

DESILICIFICATION 5 i 3

used. Ill the case of small bones, or delicate portions of large
(such as the cochlea of the ear), a ^ per cent, solution of chromic
acid will itself serve as the solvent ; but larger masses require either
nitric or hydrochloric acid in addition, to the extent of 2 per cent,
of the former or 5 per cent, of the latter. By some the chromic and
the nitric or hydi-ochloi-ic acid are mixed-in in the first instance,
while by others it is recommended that the bone should lie first in
the chromic acid solution for a week or ten days, and that the
second acid should be then added. If the softening be not com-
pleted in a month, more acid mu.st be added. When thoroughly
decalcified, the bone should be transferred to rectified spirit ; and
it may then be either sliced in the microtome or torn into shreds
for the demonstration of its lamellae. Acid solvents may also be
employed in removing the outer pai-ts of calcareous skeletons, for the
display of their internal cavities (a plan which the Author has often
found very useful in the study of For aminif era), or forgetting rid of
them entirely, so as to bring into complete view any ' internal cast '
which may have been formed by the silicification of its originally
soft contents. It has been in this mode, even more than by the
cutting of thin sections, that the structui'e of Eozoon canadense has
been elucidated by Professor Dawson and the Author. For the first
of these purposes strong acid should be applied (under the dissecting
microscope) with a fine camel's-hair pencil ; and another such pencil
charged with water should be at hand, to enable the observer to stop
the solvent action whenever he thinks it has been carried far enough.
Foi- the second it is better that the acid should only be sti-ong enough
for the sloio solution of the shelly substance, as the too rapid disen-
gagement of bubbles often produces displacement of delicate parts
of the substituted mineral ; whilst, if the acid be too strong, the
' internal cast ' may be altogether dissolved away.

Busch suggests nitric acid as the best of all agents for decalcifica-
tion, insomuch as it does not cause ' swelling up,' nor injuriously
attack the tissue elements.

One volume of chemically pure nitric acid of specific gravity 1'25
diluted with ten volumes of water may be employed for large and
tough bones ; but it may be diluted to 1 per cent, for young bones.

The method given is that fresh bones should be laid in alcohol
of 95 per cent, for three days ; they must then be placed in the
nitric acid, which must be changed daily for eight days. They must
not remain after the decalcification is complete, or they will become
yellow. On removal the bones must be washed for a couple of hours
in running water and placed again in 95 per cent, alcohol, and in a
few days placed again in fresh alcohol.

Desilicification. — It is desirable to be able to remove siliceous as
well as calcareous elements from objects. To do this a glass vessel
should be cai-efully coated with pai-afiin internally, to prevent the
action of the acid used taking place on the sides of the vessel. The
subject to be cleared of its silica is placed in alcohol in the coated
vessel, and hydrofluoric acid is added drop by drop. As the mucous
membranes ai-e fiei'cely attacked by this acid, gi-eat cai-e must be
exercised in its use ; but small sponges and other similar siliceous

L L

514 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

objects by remaining a few hours or a day in this are wholly deprived
of their silica, while the tissues do not suffer.

Preparation of Vegetable Substances. — Little preparation is

required, beyond steeping for a short time in distilled water to get
rid of saline or other impurities, for mounting in preservative media
specimens of the minuter forms of vegetable life, or portions of the
larger kinds of algce, fungi, or other succulent cryptogams. But
the woody structures of j^hanerogams are often so consolidated by
gummy, resinous, or other deposits that sections of them should not
be cut until they have been softened by being partially or wholly
freed from these. Accordingly, pieces of stems or roots should be
soaked for some days in water, with the aid of a gentle heat if they
are very dense, and shovild then be steejjed for some days in methy-
lated spirit, after which they should again be transferred to water.
The same treatment may be applied to hard-coated seeds, the ' stones '
of fruit, ' vegetable ivory,' and other like substances. Some vegetable
substances, on the other hand, are too soft to be cut sufficiently thin
without previous hardening, either by allowing them to lose some of
their moisture by evaporation, or by drawing it out by steeping them
in spirit. Either treatment answers very well with such substances
as that which forms the tuber of the potato, sections of which
display the starch-grains in situ. Where, on the other hand, it is
desired to preserve colour, spirit must not be used ; and recourse may
be had to gum-imbedding, which is particularly serviceable where
the substance is penetrated by air-cavities, as is the case with the
stem of the rush, the thick leaves of the toater-lily, &c. The tissue
is well soaked in a syrupy solution of gum arable, and this is then
hardened, either by allowing it to slowly evaporate, or by throwing
it into strong alcohol, or by fi'eezing it. But where staining processes
are to be employed, the substance should be previously bleached by
the action of chlorine (preferably by Labarraque's chlorinated soda),
and then ti-eated with alcohol for a few hours.

For the rest, the minute structure of the higher plants is studied
by means of the methods of fixing, staining, and section-cutting
above described for the tissues of animals. Foi- plants, absolute
alcohol is much used as a fixing agent, the other reagents employed
in their preparation being in general the same as those used in
animal histology.

Staining Bacteria. — It is needful to employ somewhat special-
ised methods foi- staining the saprophytic, pathogenic, and other
schizomycetes. Some of these stain admirably, but others, especially
the somewhat larger forms, are much altered, and unless obsei'va-
tions are controlled with accurate and constant observations on the
organisms in a living condition the most egregious errors may arise.

(1) Take half a dozen cases of putrescence in which solid tissues
are decomposing, but which are in different states of decomposition.
From each take out with a pipette a small quantity, and transfer to
a carefully prepared and well-filtered decoction of veal in a small
glass vessel, at the temperature of the respective putre&ctions ; leave
this for half all hour. Then with a fine pipette take out a minute
di'op from each vessel and diffuse each drop upon a cover-glass ; let

STAINING BACTEKIA 515

evaporation go on in a warm room for twenty minutes, then fix the
fihn of saprophytes by means of fairly strong osmic acid vapour ;
float the cover with the surface of bacteiia downwaixls on a vessel
of solution of violet of methyl-anilin for an hour or less, drain the
edge of the cover-glasses on blotting-paper, and mount in glycerin.

(2) Now take drops of the fluid from the several vessels and in
a moist growing cell examine the living foi-ms, and compare these
with your dried and stained preparations.

(3) By another method, which will apply also to the bacillus of
tuberculosis, a layer of sputum or of putrefactive fluid may be spread
as before upon a cover-glass, dried in an air-oven at about 100° F.,
and then passed three times, moderately slowly, thi'ough the flame
of a spirit-lamp, so as to thoroughly ' fix ' the preparation by
coagulating its albumen. Mix 1 c.c. of concentrated solution of
methylen-blue in alcohol, 0'2 c.c. of 10 per cent, solution of potash,
and 200 c.c. of distilled watei-. On to this float the cover with its
surface of bacteria downwards and leave for twenty-four hours ; the
film will be coloured blue ; place a few drops of a solution of vesuvin
all over the film, which drives out the methylen-blue from all but
the bacteria. Finish with alcohol and oil of cloves, and mount in
l:)alsam.

For the same purpose Pi-ofessor Heneage Gibbes gives a method
which has proved of gi-eat value. Take of rosanilin hydrochloride
2 grms., methylen-blue 1 grm. ; rub them up in a glass mortar. Then
dissolve anilin oil, 3 c.c, in rectified spirit, 15 c.c. ; add the spirit
slowly to the stains until all is dissolved, then slowly add distilled
water, 15 c.c. Keep in a stoppered bottle.

In the usual way dry the sputum, &c., on a cover-glass and fix in
a flame as a few drops of the stain are poured into a test-tube and
warmed. As soon as steam rises pour into a watch-glass and float
the covei'-glass on the wai'm stain ; allow it to remain four or five
minutes ; or if we do not heat the stain but use it cold, let it remain
for at least half an hour. Wash in methylated spirit until no
colour comes off ; drain, and then dry in an air-oven, and mount in
balsam.

Staining Bacteria in Tissues (Loflier's solution). — To 100 parts
of solution of caustic potash of 1 : 10,000 add 30 parts of saturated
alcoholic solution of methylen-blue. Filter. Stain section for one
or two hours, wash out with acetic acid of \ per cent., followed by
water. Dehydrate with absolute alcohol, clear with cedar oil, and
mount in balsam.

A process of differential staining of bacillus tuberculosis which
was devised by MM. Pittion and Roux was presented recently (1889)
to the Societe de Medecine de Lyon, and has met with high com-
mendation. It requires three solutions : —

A. Ten parts of fuehsin dissolved in 100 parts of absolute alcohol.

B. Three parts of liquid ammonia dissolved in 100 parts of distilled
water.

0. Alcohol 50 parts, water 30 parts, nitric acid 20 parts, anilin-
green to saturation. In preparing this solution dissolve the green
in the alcohol, add the water, and lastly the acid.

l2

5l6 PREPAKATION, MOUNTING, AND COLLECTION OF OBJECTS

It is used thus, viz. to 10 parts of solution B add one part of
solution A, and heat until vapour shows itself, then immerse the
whole cover- glass prepared as in the ordinary way for staining. One
minute suffices to stain the bacilli. Wash with plenty of water, and
after rinsing with distilled water drop on the film side of the cover-
glass a small quantity of solution C, which is not to remain more
than forty seconds. Wash off with plenty of water, dry, and mount
in xylol balsam.

The bacilli will be found to be stained a fine rose-red upon a pale-
green ground.

Staining Flagella. — The following is the latest form of the cele-
brated method of Loffler. A mordant is made as follows : To 10 c.c.
of a 20 per cent, aqueous solution of tannin are added 5 c.c. of
cold saturated solution of ferrous sulphate and 1 c.c. of (either
aqueous or alcoholic) solution of fuchsin, methyl-violet, or ' WoU-
schwarz.' Cover-glass preparations are made and fixed in a flame
in the manner described above, special care being taken not to over-
heat. Whilst still warm, the preparation is treated with the above
described mordant, and is heated in contact with it for half a
minute, until the liquid begins to vaporise, after which it is washed
in distilled water and then in alcohol. It is then treated in a
similar manner with the stain, which consists of a satiirated solution
of fuchsin in anilin water (water in which a little anilin oil has
been shaken up and filtered), the solution being preferably
neutralised to the point of precipitation by cautious addition of
O'l per cent, soda solution. For some further details concerning this
process, the ' Journal of the Royal Microscopical Society ' for 1 890,
p. 678, may be consulted.

Chemical Testing. — It is often requisite, alike in biological and
in mineralogical investigations, to apply chemical tests in minute
quantity to objects under microscopic examination. Various con-
trivances have been devised for this purpose ; but the Author would
recommend, from his own experience, the small glass syringe already
described, or preferably the drop bottle, pp. 475-477, with a fine-
pointed nozzle, as the most convenient instrument. One of its advan-
tages is the very precise regulation of the quantity of the test to be
deposited which can be obtained by the dexterous use of it ; whilst
another consists in the power of withdrawing any excess. Care must
be taken in using it to avoid the contact of the test-liquid with the
packing of the piston. Whatever method is employed, great care
should be taken to avoid carrying away from the slide to which the
test-liquid is applied any loose particles which may lie upon it, and
which may be thus transferred to some other object, to the great
perplexity of the microscopist. For testing inorganic substances the
ordinaiy chemical reagents are of course to be employed ; but certain
special tests are required in biological investigation, the following
being those most frequently required : —

a. Solution of iodine in water (1 gr. of iodine, 3 grs. of iodide of
potassium, 1 oz. of distilled water) turns starch blue and cellulose
brown ; it also gives an intense brown to albuminous substances.

ft. Chlor-iodide of zinc ('Schultze's solution) is perhaps best made

CHEMICAL TESTING— PRESERVATIVE MEDIA 517

as follows: — Evaporate 100 c.c. of liquor zinci chloridi (B.P.) to
70 c.c. ; dissolve in it 10 grms. of iodide of potassium; then add
0'2 grm. iodine ; shake at intervals till saturated.

This is extremely useful for the detection of pure cellulose. The
zinc chloride convei'ts cellulose into amyloid, which is then turned
blue by fi'ee iodine. Wood-cells, cork-cells, the extine of pollen
grains, and all lignified or corky membranes, are coloured yellow.
Starch colours blue, but is raj)idly disorganised.

A very Aveak solution will instantly detect tannin, the cell con-
tents in which it forms a part becoming reddish or violet.

y. Solution of caustic j^otass or soda (the latter being generally
preferable) has a remarkable solvent effect upon many organic sub-
stances, both animal and vegetable, and is extremely useful in
rendering some structures transparent, whilst others are brought
into view, its special action being upon horny textures, whose
component cells are thus rendered more clearly distinguishable.

8. Dilute sulphuric acid (one of acid to two or three parts of
water) gives to cellulose that has been previously dyed with iodine
a blue or purple hue ; also, when mixed with a solution of sugar, it
gives a rose-red hvie, more or less deep, with nitrogenous substances
and with bile (Pettenkofer's test).

Sulphuric acid causes starch grains to swell and similarly affects
cellulose.

c. Concentrated nitric acid gives to albuminous substances an
intense yellow.

t,. Acid nitrate of mercury (Millon's test) (ten parts of mercury,
ten of fuming nitric acid, and twenty of water) colours albuminous
substances red.

t]. Acetic acid, which should be kept both concentrated and diluted
with from three to five parts of water, is very useful to the animal
histologist from its power of dissolving, or at least of reducing to such a
stage of transparence that they can no longer be distinguished, certain
kinds of membranous and fibrous tissues, so that other parts (especially
nuclei) are brought more strongly into view.

6. Ether dissolves resins, fats, and oils ; but it will not act on
these through membranes penetrated with watery fluid. For the
same purpose chloroform, benzol, oil of turpentine, and carbon bisul-
phide are used.

I. Alcohol dissolves resins and some volatile oils, but it does not
act on ordinary oils and fats. It coagulates albuminous matters, and
consequently renders more opaque such textures as contain them.

K. Osmic acid is a test for fatty matters, which it stains black
in varying degrees ; and in like manner for gallic and tannic acids.

Preservative and Mounting Media. — We have now to consider
the various modes of preserving the preparations that have been
made by the several methods indicated above, and shall first treat
of such as are applicable to those minute animal and vegetable
organisms, and to those sections or dissections of large structures,
which are suitable for being mounted as transparent objects. A
broad distinction may be in the first place laid down between
resinous and aqueous preservative media ; to the former belong

5l8 PEEPARATION, MOUNTING, AND COLLECTION OF OBJECTS

Canada balsam and dammar, while the latter inckide all the mix-
tures of which water is a component ; while partly dehydrating
media, such as glycerin and alcohol, occupy an intermediate position.
The choice between the three kinds of media will partly depend
upon the nature of the processes to which the object may have been
previously subjected and partly upon the degree of transparence
which may be advantageously imparted to it. Sections of substances
which have been not only imbedded in but penetrated by paraffin,
and have been stained (if desired) previously to cutting, are, as a rule,
most conveniently movmted in Canada balsam or dammar ; since
they can be at once transferred to either of these from the menstruura
by which the imbedding material has been dissolved out. The dura-
bility of this method of mounting makes it preferable in all cases to
which it is suitable, the exception being where it renders a very
thin section too transparent. In such cases sections or other
objects may sometimes be more advantageously mounted in some of
those aqueous preparations of glycerin which approach the resinous
media in transparence and permanence. When Canada balsam was
first employed for mounting preparations it was employed in its
natural semi-fluid state, in which it consists of a solution of resin in
volatile oil of turpentine ; and unless a large proportion of the latter
constituent was driven oif by heat in the process of mounting
(bubbles being thus formed of which it was often difficult to get
rid), or the mounted slide was afterwards subjected to a more
moderate heat of long continuance, the balsam would remain soft,
and the cover liable to displacement. This is avoided by the method
now generally adopted of previously getting rid of the turpentine by
protracted exposure of the balsam to a heat not sufficient to boil
it, and dissolving the resin thus obtained either in xylol, benzol, or
chloroform, but far preferably the former, the solution being made
of such viscidity as will allow it to ' run ' freely. Either of these
solvents evaporates so much more quickly than turpentine that the
balsam left behind hardens in a comparatively short time. Xylol-
halsam is now preferred by most mounters. It is made of equal
volumes of xylol and balsam. The natural balsam, however, may be
preferably used (with care to avoid the liberation of bubbles by
overheating) in mounting sections already cemented to the slides
by hardened balsam, and also for mounting the chitinous textures
of insects, which it has a peculiar power of rendering transparent,
and which seem to be penetrated by it more thoroughly than they
are by the artificially prepared solution. The solution of dammar
in xylol is very convenient to woi-k with, and hardens qviickly.

The following are the principal aqueous media whose value has
been best tested by general and protracted experience : —

a. Fresh specimens of minute protophytes can often be vei-y well
preserved in distilled ivater saturated with camphor, the complete
exclusion of air serving both to check their living actions and to
prevent decomposing changes. When the preservation of colour
is not a special object about a tenth part of alcohol may be added, and
this will be found a suitable medium for the preservation of many
delicate animal textures.

PKESERVATIA'E MOUNTING MEDIA 519

/3. Scdt solution, 0"75 per cent, sodium chloride in water. Use-
ful as a medium for temporary examination, but not for permanent
preservation.

y. White of an egg. — Simply filter.

8. Synq^ iii which is dissolved 1 to 5 per cent, of chloi-al
hydrate, or 1 per cent, of carbolic acid.

e. Liquid of Ripart and Petit. — Camphor water (not saturated),
75 grms. ; distilled water, 75 grms. ; glacial acetic acid, 1 grm. ;
acetate of copper, 0"30 grm. ; chloride of copper, 0'30 grm. Maybe
added to preparations stained with methyl-green, which it does not
precipitate, and may be used for preserving either vegetal or animal
tissvies.

t,. Fahre-Domergue' s Glucose Medium. — Glucose syi-up of specific
gravity 1'1968, 1,000 parts; methyl alcohol (wood spirit), 200;
glycerin, 100; camphor to saturation. The glucose to be dissolved
in warm water and the other ingredients added, and the mixture,
which is always acid, neutralised with a little potash or soda.

7}. Chloral Hydrate. — A 5 per cent, solution in water, or 12 grains
chloral hydrate to 1 fluid ounce of camphor water. (Mount in strong
glycerin jelly.)

6. Brion's Glucose Medium. — Distilled water, 140 parts; cam-
phorated spirit, 10 parts; glucose, 40; glycerin, 10. Mix the
water, glvicose, and glycerin, then add the spirit, and filter to remove
the excess of camphor which is precipitated. This medium preserves
the colour of preparations stained with anilin dyes, methyl-green
included.

I. Guin and Byruj). — Gum-mucilage (B.P.) five parts, syiaip three
parts. Add 5 grains of pure carbolic acid to each ounce of the medium.

B.P. gum-mucilage is made by putting 4 oz. of picked gum acacia
in 6 oz. of distilled water until dissolved.

Syi'up is made by dissolving a pound of loaf sugar in a pint of
distilled water and boiling.

K. The glycerin jelly prepared after the manner of Mr. Lawrence
may be strongly recommended as suitable for a great variety of
objects, animal as well as vegetable, subject to the cautions already
given : — ' Take any quantity of Nelson's gelatin, and let it soak for
two or three hours in cold water, pour ofi" the superfluous water, and
heat the soaked gelatin until melted. To each fluid ounce of the
gelatin add one drachm of alcohol and mix well ; then add a fluid
drachm of the white of an &g^. Mix well while the gelatin is fluid,
but cool. Now boil until the albumen coagulates, and the gelatin
is quite clear. Filter through fine flannel, and to each fluid ounce of
the clarified gelatin add six fluid drachms of Price's pure glycerin,
and mix well. For the six fluid drachms of glycerin a mixture of
two parts of glycerin to four of camphor-water may be substituted.
The objects intended to be mounted in this meditun are best prepared
by being immersed for some time in a mixture of one part of glycerin
with one part of diluted alcohol (one of alcohol to six of water).' ^ A
small quantity of absolute phenol may be added to it with advantage.

1 A very pure glycerin jelly, of whicli the Author has made considerable use, is
prepared by Mr. Rimmington, chemist, Bradford, Yorkshire.

520 PEEPARATION, MOUNTING, AND COLLECTION OF OBJECTS

Wlien used, the jelly must be liquefied by gentle warmth, and it is
useful to warm both the slide and the cover-glass previously to
mounting. This takes the place of what was formerly known as
Dean's medium, in which honey was used to prevent the hardening
of the gelatin.

X. For objects which would be injured by the small amount of
heat required to liquefy the last-mentioned medium, the glycerin and
gum medium of Mr. Farrants will be found very useful. This is
made by dissolving four parts (by weight) of picked gum arable in four
parts of cold distilled water, and then adding two parts of glycerin.
The solution must be made without the aid of heat, the mixture being
occasionally stirred, but not shaken, whilst it is proceeding ; after it
has been completed the liquid should be strained (if not perfectly
free from impurity) through fine cambric previously well washed out
by a current of clean cold water ; and it should be kept in a bottle,
closed with a glass stopper or cap (not with cork), containing a small
piece of camphor. The great advantage of this medium is that it
can be used cold, and yet soon viscifies without cracking ; it is well
suited to jDi-eserve delicate animal as well as vegetable tissues, and in
most cases increases their transparence.

Of late years glycerin has been largely used as a preservative,
either alone, according to the method of Dr. Beale, or diluted
with water, or mixed with gelatinous substances. It is much more
favourable to the preservation of colour than most other media, and
is therefore specially useful as a constituent of fluids used for
movinting vegetable objects in their natural aspects. It has also the
property of increasing the transparence of animal structures, though
in a less degree than resinous substances, and may thus be advan-
tageously employed as a component of media for mounting objects
that are rendered too transparent by balsam or dammar. Two
cautions should be given in regard to the emjjloyment of glycerin :
fh'st, that, as it has a solvent power for carbonate of lime, it should
not be used for mounting any object having a calcareous skeleton ;
and second, that, m proportion as it increases the transparence of
organic substances, it diminishes the reflecting j)ower of their
svirfaces, and should never be employed, therefore, in the mounting of
objects to be viewed by reflected light, although many objects
mounted in the media to be presently specified are beautifully
shown by ' dark-ground ' illumination.

1. A mixture of one part of glycerin and two parts of
camphor -water may be used for the preservation of many vegetable
structures.

2. For preserving soft and delicate mai-ine animals which are
shrivelled up, so to speak, by stronger agents, the Author has found
a mixture of one part of glycerin and one of spirit with eight or ten
parts of sea- water the most suitable preservative.

3. For preserving minute vegetable preparations the following
method, devised by Hantsch, is said to be peculiarly efiicient : A mix-
ture is made of three parts of pure alcohol, two parts of distilled water,
and one part of glycerin ; and the object, laid in a cement-cell, is
to be covered with a drop of this liquid, and then putaside under a bell-

PEESEEVATIVE MOUNTING MEDIA 52 1

glass. The alcohol and water soon evaporate, so that the glycerin
alone is left ; and another drop of the liquid is then to be added,
and a second evaporation permitted, the process being repeated, if
necessary, until enough glycerin is left to fill the cell, which is
then to be covered and closed in the usual mode.^

Canada balsam is one of the most universally employed mounting
media ; very old hard balsam should be dissolved in enough pure
xylol or chloroform to make a thin solution, which should be care-
fidly filtered.

Dammar. — Dissolve gum-dammar with heat in a mixture of
equal parts of benzole and turpentine, and evaporate to a syrupy
consistency. This is pleasant to use, but treacherous. Dammar
dissolved in pure xylol in the cold gives a beautiful solution, but
on the score of permanency is not so trustworthy as balsam.

Guvi Styrax. — This is a resin which must be dissolved in benzole,
chloroform, or ether. It should have the consistency of olive oil ;
all the benzole must be evaporated before putting the cover on the
slip; its refractive index is said to be then 1'583. Its value is in
the mounting of diatoms, where a marked difference between the
refractive index of the siliceous fi-ustules and the medium in which
they are mounted facilitates the discovery of obscure details. There is
a marked increase of visibility in pi-opoi'tion as the mounting medium
has a refractive index higher than the object (diatom) mounted.

.Now the refractive index of the silex of diatoms is 1'43. Bu.t
Canada balsam is r52 : hence the ' index of visibility ' in obscure
markings is 9, while styrax by comparison is 15.

Monohromide of naphthalin is another of the media which
may be used with a high refractive index. It is colourless and oil-
like, soluble in alcohol and ether. It has a refractive index of
1"658, and therefore a splendid index of visibility above balsam or
styrax ; but after a laj)se of many months some change takes place
which leaves the preparation as apparently perfect as before, but
having lost all the benefit of great refractive index.

The cover-glass should be run round with a i-ing of wax, then
with a ring of Heller's porcelain cement, and be finally closed with
shellac.

But with the exception of some media of very high i-efractive
index not by any means easy to use, devised by Professor H. L.
Smith, there is no medium of such high value as that suggested and
very successfully employed by Mr. J. W. Stephenson, viz.

PTiospIiorus. — Its refractive index is 2"1, and its consequent
increase of visibility is of immense value in some objects.

Phosphorus, it need hardly be said, is difiicult and somewhat
dangerous to handle on account of its spontaneous combustion in
air, and the severe nature of the bums it inflicts. But it is with
slight practice by no means an unmanageable medium.

To prepare it, take a 2 -drachm bottle with no contraction for the

1 See the Eev. W. W. Spicer's Handy-book to tJie Collection and Preparation
of Freshwater and Marine Algce^ &c.. pp. 57-59. ' Nothing,' says Mr. Spicer, ' can
exceed the beauty of the prej^arations of Des/HicZtaceo? prepared after Herr Hantsch's
method, the form of the plant and the colouring of the endochrome having under-
gone no change whatever.'

522 PEEPARATION, MOUNTING, AND COLLECTION OF OBJECTS

neck. Make a cylinder of wood that will just fit the inside of the
neck. Fold some filtei- paper down and around this cylinder so that
it will just fit tightly into the neck of the bottle, to the bottom of
which it is forced, and the cylinder of wood withdrawn, leaving the
filter in its place. Now moisten the filter carefully with a few drops
of bisulphide of carbon, and a piece of stick phosphorus from a
quarter to three-eighths of an inch long should be placed in the filtei-,
and the bottle corked. The vapour of the bisulphide instantly acts
on the phosphorus, and in about half an hour it will be in a fluid
state remaining in the filter. By releasing the cork and taking
hold of the filter tube with a pair of pliers and slowly drawing it
upwards a partial vacuum is formed beneath it, and the pressure of
the air on the surface of the fluid phosphorus forces it through the
filter, leaving the now brilliant fluid in the bottle.

With care, i-apidity, and firmness withdraw the filter and plunge
instantly into a vessel of water close at hand.

In mounting we assume that the best course as advised above
has been adopted, and that the diatoms to be mounted are either
arranged or diffused upon the cover-glass.

Make a ring upon the slip of glue and honey cement used
wai-m and allowed to cool. It is now a stiff jelly. Lay the cover
in its place, with the diatoms downwards, touching the ring at one
side, but raised by a fine wire on the side next the operator. A
pipette may also be used made of glass tubing an eighth of an inch in
external diameter, drawn to a fine point at one end, and somewhat
enlarged at the other, and to which an indiarubber caj) or nipple is
fastened airtight. This pipette must be passed through the centre
of a cork fitting the bottle of phosphorus solution, and the fine end
should plunge into the fluid and nearly touch the bottom of the bottle.
By squeezing the rubber cap before the insertion of the pipette and
releasing it after the point is well down, a small quantity of phos-
phoi-us rises in the pipette. It is withdrawn and inserted rapidly
beneath the tilted end of the cover ; the slightest pressure on the
cap ejects enough phosj)horus to fill the space between the cover
and the slide ; gently and firmly pi-ess it down and ring it with warm
glue and honey.

In half an hour points of superfluous phosphorus may have
exuded. With a pair of tweezers wet a piece of blotting-paper with
bisulphide and absorb these away, plunging the paper at once into
water. The slides should now be put aside for a day or two, then
they may receive two oi- three ling-coatings of gold-size, and finally
be finished with sealing-wax or shellac varnish.

It often is quite impossible to predicate beforehand what preserva-
tive medium will ajiswer best for a particular kind of preparation ;
and it is consequently desirable, where there is no lack of material,
to mount similar objects in two or three different ways, marking on
each slide the method employed, and comparing the specimens from
time to time, so as to judge the condition of each.

Importance of Cleanliness. — The success of the result of any of
the foregoing oj)ei-ations is greatly detracted from if, in consequence
of the adhesion of foreign substances to the glasses whereon the

LABELLING MOUNTED OBJECTS 523

objects are mounted, or to the implements used in the manipulations,
any extraneous particles are brought into view with the object itself.
Some such will occasionally present themselves, even under careful
management ; especially fibi'es of silk, wool, cotton, or linen, from
the handkerchiefs, ifec, with which the glass slides nvcij have been
wiped ; fibres of the blotting-paper emj)loyed to absoi-b superfluous
fluid ; and grains of starch, which often remain obstinately adherent
to the thin glass covers kept in it. But a careless and uncleanly
manipulator will allow his objects to contract many other impurities
than these ; and especially to be contaminated by particles of dust
floating through the air, the access of which may be readily prevented
by proper precautions. It is desirable to have at hand a well-closed
cupboard furnished with shelves, or a cabinet of well-fitted drawers,
or a number of bell-glasses upon a flat table, for the purpose of
securing glasses, objects, (fcc, from this contamination in the intervals
of the work of preparation ; and the more readily accessible these
receptacles are, the more vise will the microscopist be likely to make
of them. Great care ought, of course, to be taken that the media
employed for mounting should be freed by effectual filtration from all
floating particles, and that they should be kept in well-closed bottles.

Labelling and Keeping Mounted Objects. — The object of labels
on mounted objects is of course to give clear and instant indication
of the nature of the mount. But we must, if our cabinets have any-
thing like scientific pretensions, not only know what the object may
be, but some (perhaps many) other particulars about it. In fact, a
thoroughly scientific cabinet must not rely on the labels on the
mounts for all the information which it is desirable and even
essential to have concerning them. One of the desiderata of every
label should be the pi-esence of a nuonber, and this number should be
at once placed in a book, arranged in columns to suit the requirements
of the student, and most of the details should be placed in this book
in association with the number.

For this to be of permanent service, however, the label on which
the number is placed shoidd be as permanent and immovable as the
slip itself. We know of cabinets in which 07il't/ numbers are
marked on slides, and all details are recorded in ' tlae book.' We
do not advise this ; but all who keep cabinets know how in the course
of years paper labels become displaced and lost, and in many
instances the value of slides is greatly diminished.

Wliat is wanted is a. jyermmie^itly Jixed label, capable of receiving
the chief points of character as well as the name and number of an
object.

The present Editor has found the following plan to be hitherto,
after twenty-three years' trial, quite faultless.

Let the slips which are to be used for mounting have the two
ends of the upper surface finely gi'ound ; at one end the ground
surface may be three quarters of an inch, and at the other end half
an inch. On the ground surface we can write with a hard pencil as
clearly and sharply as with a fine pen on cardboard.

On the broader ground surface let the princijjal facts as to the
nature of the object be written and the number of the slide with a

524 PEEPAEATION, MOUNTINa, AND COLLECTION OF OBJECTS

T'aber pencil mai^ked H H H H. On the nari-ower and opposite
ground surface should be wi-itten what the object is mounted in,
how stained, or whence obtained, the date of mounting, &c.

Now when all this is written take thin covers, cut respectively
1 X f inch and 1 X ^ inch, and by means of benzol balsam, applied
with or without heat, the ground surfaces should have these thin
glasses put on over the writing and the entire ground surfaces ; the
result of course will be that the transparency of what was a ground
and opaque surface will be wholly restored, and the wi-iting will be
clear and ineifaceable. If the bottom of the trays of the cabinets be
whitened it will render still raore easy the instant reading of the
contents of the label.

The grinding of the slips is by no means diilicult, and could not
be costly if there arose a demand for them.

It is easy, however, to do all that is required. A block of wood
to receive the slide in an excavation of its own shape and size, and a
piece of wood half an inch thick, of the exact length (IJ inch)
of the space between the labels, enables a lead ' buff ' to be freely
used with fine emery and the work is speedily done. Of course the
finer the emeiy the finer the surface ; and the finer the surface the
m.ore delicate the writing may be made. The label may in fact be
as ornate and elegant as we please. Nor need we be confined to an
oblong shape. Oval or round spaces could be ground on the slips
and thin covers of corresponding size could be accordingly used.
This method gives a little more trouble and is slightly more
expensive, but in elegance and above all in durability we believe it
has no equal.

For the preservation of objects, the pasteboard boxes now made
at a very reasonable cost, with wooden racks, to contain six, twelve,
or twenty-four slides, will be found extremely useful. For the
management of a large collection the following has proved itself to
be thoroughly practical, and can be universally employed. The
species, genus, and character of the slides may be disregarded. Place
the slides in the cabinet just as they come, numbering each consecu-
tively. The exterior of cahiiiets should show from what number to
what number the cabinet contains : thus, 527 to 842. The porcelain
slab on the drawer may indicate from what number to what number
the drawer contains : thus, 527 to 539. Now a number of note-
books should be procured, so that there may be a separate notebook
for each subject ; the size of the notebook must be regulated to the
importance of the special department the collector has taken up.
Thus a diatomist would have probably a thick ledger for his diatom
collection, whereas an entomologist would have a thin notebook for
his diatoms and a thick ledger for his insects, and so on. The note-
books might be distingviished from one another by a letter of the
alphabet.

In the event of a second notebook being required for the same
subject or class of objects, it might be identified by doubling the
letter — thus, D D. Now a large index notebook will be required in
which one line is ffiven to each slide. This notebook contains

COLLECTION OF OBJECTS 525

merely the numbei- of the slide and the letter and page of the special
notebook wherein all about the slide will be found. Thus : —

'649, F 127.'

This means that in notebook F on page 127 we shall find an
account of slide No. 649.

On turning to notebook F we find (say) that the subject is
geology. The following will be a facsimile of the page : —

Slide No. 649 127

Section of porphyrti from Peterhead, Aug. 1886. — The quartz
crystals in this section have minute cavities containing a liquid, COo.
In each cavity there is a bubble ; some of these bubbles are ex-
tremely minute, and exhibit rapid Brownian movement. A good
example of which is —

No. 2 (referring to a second microscope when used), 46-51.

A large bubble with no Brownian movement.

No. 2 (microscope), 44-47.

Section too thick for oil immersion.

Best seen dry ^ "95 N.A. deep eye-piece ; condenser aperture
•6 N.A.

At the back of each notebook there is an alphabetical index.
In this instance if we look up ' Porphyry' we shall find 127, and if
we look up ' Quartz (cavities in) ' we shall find 127, and if we look up
' Carbonic acid (in quartz)' we shall find 127, and if we look up 'Bubbles
(in quartz) ' we shall find 127.

By this means the collector can find a slide if he know the
subject, and also the subject if he have the slide.

This is the only scientific method we know of dealing with a
microscopical collection ; it is one of the greatest practical mistakes
to make the cabinet its own index. It always ends in supreme
confusion. But for the purposes of the man of science a large
cabinet made with a view to the reception of his own slides is far
preferable. The majority of slides are 3x1 inches ; but all are not —
some geological and mineral ogical sections, sections of coal, &c., are
often much larger. Many objects, again, are in deeper cells than
the ordinary cabinet drawer or slide-box will admit of; all this may
be provided for, and if money be not a special object, a design with
two or three special and smaller cabinets may be made for- the
reception of special series of mounts. '^

Collection op Objects.

A large proportion of the objects with which the microscopist
is concerned is derived from the minute parts of those largei-
organisms, whether vegetable or animal, the collection of which does
not requii'e any other methods than those pursued by the ordinary
naturalist. With regard to such, therefore, no special directions
are required. But there are several most interesting and important
groups, both of plants and animals, which are themselves, on account

^ It will be understood that tliere are many forms of cabinet which space prevents
our describing ; they are made suitable for the pocket, for postal transmission, &c.,
and may be readily seen at the opticians'.

526 PREPAKATION, MOUNTING, AND COLLECTION OF OBJECTS

of their minuteness, essentially microscojnc ; and the collection of
these requires peculiar methods and implements, which are, however,
very simple, the chief element of success lying in the knowledge tvhere
to look and what to look for. In the present place, general direc-
tions only will be given ; the particular details relating to the several
groups being reserved for the account to be hereafter given of each.
Of the microscopic organisms in question, those which inhabit
fresh water must be sought for in pools, ditches, or streams, through
which some of them freely move, whilst others attach themselves
to the stems and leaves of aquatic plants, or even to pieces of stick
or decaying leaves, &c., that may be floating on the surface or sub-
merged beneath it ; while others, again, are to be sought for in the
muddy sediments at the bottom. Of those which have the power of
free motion, some keep near the surface, whilst others swim in the
deeper waters ; but the situation of many depends entirely iipon the
light, since they rise to the surface in sunshine, and svibside again
afterwards. The collector will therefore require a means of obtaining
samples of water at different depths, and of drawing to himself
portions of the larger bodies to which the microscopic organisms may
be attached. For these purposes nothing is so convenient as the j^oncl-
stick, which is made in two lengths, one of them sliding within the
other, so as when closed to serve as a walking-stick. Into the
extremity of this may be fitted, by means of a screw socket, (1) a
cutting-hook or curved knife, for bringing up portions of larger
plants in order to obtain the miriu.te forms of vegetable or animal
life that may be parasitic upon them ; (2) a broad collar, with a
screw in its interior, into which is fitted one of the screw-topped
bottles made by the York Glass Company ; (8) a ring or hoop for a
muslin ring-net. When the bottle is used for collecting at the sur-
face, it shovild be moved sideways with its mouth partly below the
water ; but if it be desii'ed to bring up a sample of the liquid from
below, or to draw into the bottle any bodies that may be loosely
attached to the submerged plants, the bottle is to be plunged into
the water with its mouth downwards, carried into the situation in
which it is desired that it should be filled, and then suddenly turned
with its mouth upwards. By unscrewing the bottle from the collar,
and screwing on its cover, the contents may be securely preserved.
The net should be a bag of fine muslin, which may be simply sewn
to a ring of stout wire. But it is desirable for many purposes that
the muslin should be made removable ; and this may be provided
for by the substitution of a wooden hoop, grooved on its outside, for
the wire ring ; the muslin being strained upon it by a ring of
vulcanised indiarubber, which lies in the groove, and which may be
readily slipped off and on, so as to allow a fresh piece of muslin to be
put in the place of that which has been last used. At the end of the
muslin bag is tied a small rimmed tube-bottle of thin clear glass
three inches long by one inch in diameter. In this, objects can be
fairly seen. The collector should also be furnished with a number
of bottles, into which he may transfer the samples thus obtained,
and none are so convenient as the screw-topped bottles made in all
sizes by the York Glass Company. It is well that the bottles should

COLLECTING 527

be fitted into cases, to avoid the risk of breakage. Wlien animalcules
are being collected, the bottles should not be above two-thirds
filled, so that adequate air-space may be left. Whilst engaged in
the search for microscopic objects, it is desirable for the collector to
possess a means of at once recognising the forms which he may
gather, where this is possible, in order that he may decide whether the
' gathering ' is or is not worth preserving ; and for this purpose we know
of nothing better, unless a small travelling microscope be required, than
a couple of Steinheil loups, magnifying six and ten diameters.

Mr. J. D. Hardy suggests what we have found of great use, viz.
aflat bottle, as a very valuable piece of apparatus for collecting.' It is
made by cutting a (J -shaped piece out of a flat and solid piece of india-
rubber, about 6 inches long by 2| inches broad, and | inch thick ;
against each side is cemented (by means of Miller's caoutchouc
cement) a piece of good thin plate-glass, and the bottle is complete.
A small portion cut from the inner piece makes a natui'ally fitting
cork. One or two more, and smaller, bottles can be made with the
remaining indiai'ubber. It is essential that the material should be
at least \ inch thick in order to make a wide bottle, and allow pond-
weeds to be put inside without difliculty and pressure. A flat bottle is
made by Mr. Stanley, London Bridge, which we have good reason to
write favourably of. It is ground on its outer surfaces, and internal
irregularities almost wholly disappear when filled with water ; an
objective from 3 inches to 1^ inch may be well employed with it.

Even with the best ordinary round dipping bottles it is very
diflicult to see minute animals clearly, whilst with this flat bottle
one can see at a glance almost everything the dip contains, and
every object can be examined with the pocket lens with ease.

For collecting purposes the objects sought in pond or stream are
divisible into free-swimming, and attached or fixed to water-plants, &c.

The free-swimming are to be secui-ed with the net, the bottle
attached to which should be examined after each sweep of the net ;
and the flat bottle may be also filled for examination. The mud at
the bottom of the pond must not be stirred by the net, since of
course it obscures the objects.

The infusoria, rotifera, ifec, are best found with the flat bottle.
Collect a lot of the ' weeds ' growing in pond or stream, and place
these in the bottle ; then, Mr. Rousselet says : ' The tree-like colo-
nies of Yorticellfe ; Epistylis, Zoothamium, and Carchesium, the
trumpet-shaped Stentors, the crown Rotifer Stephanoceros, the
tubes of Melicerta, Lymnias, the various Polyzoa, also Hydra,
and many more, can at once be seen with the naked eye, when
present, and in this way the good branches can be selected. Some
creatures, however, such as the beautiful floscules, cannot be seen
easily, even with the lens, not so much on account of their small
size, as of the perfect transparency of their bodies. Experience
will soon teach one how to see which branches are likely to prove
prolific. As a general rule, old-looking but still sound and green
branches will be the best. The "Water Milfoil (Myriophyllum) is
decidedly the best of water plants to examine and collect, on account

1 Q.M. Jourv. ser. ii. vol. ii. p. 55.

528 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

of the ease with which its leaves can subsequently be placed under the
microscope. Anacharis is much more difficult of manipulation, and
I mostly only take it now to aerate my aquaria.

' In placing a weed in the flat bottle, do not put in more than one
branch at a time, otherwise the branches will only obscure each othei-
and render examination more difficult.

' When searching for Polyzoa, such as Lophopus, Plumatella,
Fredericella, it is advisable to examine the rootlets of trees growing
at the edge of the water, and also to drag up weeds from, the m^iddle
of the pond or canal by means of a loaded hook and line.

' A good collection thus made is transferred to small aquaria 6 to
8 inches high, 5 to 6 inches long, and 1 to 1^ inch wide ; these we
have used for at least ten years and can attest their great value in
making the best possible use of a good day's collecting, and studying
in the most intelligent way the objects collected.

' Rotifers can generally be kept a week or a fortnight, some
species much longer ; tlieh* lives, as well as those of Polyzoa, can be
prolonged by feeding them about twice daily with a green soup
made by crushing some anacharis, or other green weed, in a small
mortar in a little water, which is then filtered through muslin.
They can be seen to feed on this under the microscope, their tiny
stomachs soon becoming filled with little balls of chlorophyll.

' Under favourable conditions Melicerta, Stephanoceros, the Flos-
cules, and also Asplanchna, and other forms, breed and multiply in
the aquarium, and can then be preserved for a considerable time.
A little mud taken from a pond in winter or early spring, and
put in a tank at home, will often produce an unexpected number
and variety of rotifers and infusoria, which are hatched from
winter eggs and dormant germs. ' ^

There must of course be a balance in every tank between the
animal and vegetable life, or aeration must be artificially maintained.
So also food must be obtainable by the organisms, however small.
But experience alone is the perfect teacher in this matter.

The same general method is to be followed in the collection of
such marine forms of vegetable and animal life as inhabit the
neighbourhood of the shore, and can be reached by the pond-stick.
But there are many which need to be brought up from the bottom
by means of the dredge, and many others which swim freely
through the waters of the ocean, and are only to be captured by the
tow-net. As the former is part of the ordinary equipment of eveiy
marine naturalist, whether he concern himself with the microscope
or not, the mode of using it need not be here described ; but the
use of the latter for the purposes of the microscopist requires
special management. The net should be of fine muslin, firmly sewn
to a ring of strong wire about ten or twelve inches in diameter.
This may be either fastened by a pair of strings to the stei'n of a
boat, so as to tow behind it, or it may be fixed to a stick so held in
the hand as to project from the side of the boat. In either case the
net should be taken in from time to time, and held up to allow the

' ' On some Methods of Collecting and Keeping Pond Life for the Microscope,'
from the Trans. Middlesex Nat. Hist. Soc.

COLLECTING 529

water it contiiins to di'ain tln'oiigh it ; and should then be turned
inside out and moved about in a bucket of water carried in the
boat, so that any miniite organisms adhering to it may be washed
off before it is again immersed. It is by this simple method that
mai'ine animalcules, the living foi-ms of Radiolaria., the smaller
Medusoids (with theii- allies Beroe and Cydipjte), Xoctihtca, the
free-swimming lai-vpe of Echinodei'mata, some of the most curious
of the Tanicata, the larv?e of Mollusca, Turhellaria, and Annelida^
some curious adult forms of these classes, Entomostraca, and the
lai'Vfe of higher Crustacea, are obtained by the naturalist ; and
the great increase in our knowledge of these forms which has been
gained within recent years is mainly due to the assiduous use
which has been made of it by qualified observers. It is impoi'tant
to bear in mind that, for the collection of all the more delicate of
the organisms just named (such, for instance, as echinoderm larvce),
it is essential that the boat should be rowed so slowly that the net
may move gently through the water, so as to avoid crushing its soft
contents against its sides. Those of firmer structure (such as the
Entoviostraca), on the other hand, may be obtained by the use of a
tow-net attached to the stern of a sailing-vessel, or even of a
steamer, in much more rapid motion.* When this method is
employed, it will be found advantageous to make the net of conical
form, and to attach to its deepest part a wide-mouthed bottle,
which may be prevented from sinking too deeply by suspending it
from a cork float ; into this bottle many of the minute animals
caught by the net will be carried by the current produced by the
motion of the vessel thi-ough the water, and they will be thus
removed from liability to injury It will also be useful to attach to
the ring an inner net, the cone of which, more obtuse than that of
the outer, is cut off at some little distance from the apex ; this
serves as a kind of valve, to prevent objects once caught from being-
washed out again. The net is to be drawn in from time to time,
and the bottle to be thrust up through the hole in the inner cone ;
and its contents being transferred to a screw-capped bottle for
examination, the net may be again immersed. This form of net,
however, is less suitable for the most delicate objects than the simple
stick-net used in the manner just described. The microscopist on
a visit to the seaside, who prefers a quiet row in tranquil waters
to the trouble (and occasional malaise) of dredging, will find in the
collection of floating animals by the careful use of the stick-net
or tow-net a never-ending source of interesting occupation.

' In the Challenger Expedition tow-uets were almost constantly kept in use,
not only at the surface, but at various dej)ths beneath it, being attached to a line
which was made to liang vertically in the water by the attachment of heavy weights
at its extremity. The collections thus made showed the enormous amount of minute-
animal life pervading the upper waters of the ocean.

M M

530 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

CHAPTER VIII

MICBOSCOPIC FOBMS OF VEGETABLE LIFE— THALLOPHYTES

Those who desire to make themselves familiar with microscopic
appearances, and to acquire dexterity in microscopic manipulation,
cannot do better than educate themselves for more difficult inquiries
by the study of those humblest types of vegetation which present
organic structure under its most elementary aspect. And such as
desire to search out the nature and conditions of living action will
find in the study of its simplest manifestations the best clue to the
analysis of those intricate and diversified combinations under which
it presents itself in the highest animal organisms. For it has now
been put beyond question that the fundamental phenomena of life
are identical in plants and in animals, and that the living substance
which exhibits them is of a nature essentially the same throughout
both kingdoms. The determination of this general fact, which forms
the basis of the science of Biology, is the most important result of
modern microscopic inquiry ; and the illustration of it will be kept
constantly in view in the exposition now to be given of the chief
applications of the microscope to the study of those minute ^jroto-
jihytes (or simplest foims of plant-life) with whose form and structure,
and with whose very existence in many cases, we can only acquaint
ourselves by its aid.

It was formerly supposed that living action could only be
exhibited by organised structure. But we now know that all the
essential functions of life maybe carried on by minute 'jelly-specks,'
in whose apparently homogeneous semi-fluid substance nothing like
' organisation ' can be detected ; and, further, that even in the very
highest organisms, which present us with the greatest variety of
'• differentiated ' structures, the essential part of the life-work is done
by the same material — these structures merely furnishing the
mechanism (so to speak) through which its wonderful properties
exert themselves. Hence this substance,^ known in vegetable
j^hysiology as protoplas')n, but often referred to by zoologists as

1 Attention was drawn in 1835 by Dujardin (the French zoologist to whom we owe
the transfer of the Foraminifera from the highest to the lowest x^lace among inverte-
brate animals) to the fact that the bodies of some of the lowest members of the
animal kingdom consist of a structureless, semi-fluid, contractile substance, to which
he gave the name sarcode (rudimentary flesh). In 1851 the eminent botanist Von
Mohl showed that a similar substance forms the essential constituent of the cells of
plants, and termed it -protoplasm (primitive plastic or organisable material). And in
1863 it was pointed out by Prof. Max Schultze, who had made a special study of the
rhizopod group, that the ' sarcode ' of animals and the ' protoplasm ' of i^lants are
identical. See his memoir TJeber das Protoplasina der Bliizop)oden und Pflansen-
zellen.

SIMPLEST FORMS OF VEGETABLE LIFE 531

sarcode, has heen appropriately designated by Professor Huxley ' the
physical basis of life.' In its typical state (such "as it presents
among ^-/lizojwds) it is a semi-fluid, tenacious, glairy substance,
resembling — alike in aspect and in composition — the albumen (or
uncoagulated ' white '") of an unboiled egg. But it is fundamentally
distinguished from that or any other foi-m of dead matter by two
attributes, which (as being peculiar to living substances) are desig-
nated vital: (!) its power of increase, by assmiilating (that is, con-
verting into the likeness of itself, and endowing with its own pro-
perties) nutrient material obtained from without ; (2) its power of
spontaneous movement, which shows itself in an extraordinary variety
of actions, sometimes slow and progressive, sometimes rapid, some-
times wave-like and continuous, and sometimes I'hythmical with
regular intervals of rest. When examined under a sufficiently high
magnifying power, multitudes of minute granules are usually seen to
be diffused through it, which have been termed ' microsomes.'
Protoplasm, whether living or dead, has a great power of absorbing
water; but the distinction between these two states is singularly
marked by its behaviour in regard to any colouring matter which the
water may contain. Thus, if living protoplasm be treated with a
solution of carmine, it will remain unstained so long as it retains
its vitality. But if the protoplasm be dead, the carmine will at once
pervade its whole siibstance, and stain it throughout with a colour
even more intense than that of the solution ; thus furnishing (as
was first pointed out by Dr. Beale) a ready means of distinguishing
the ' germinal matter,' or protoplasmic component of the tissues of
higher animals, from the ' formed material ' which is the most con-
spicuous part of their structure.

All those minute and simple forms of life with which the micro-
scope brings us into acquaintance consist essentially of particles of
protoplasm, each kind having usually a tolerably definite size and
shape, and showing (at least in some stage of its existence) some-
thing distinctive in its habit of life. And it is rather according to
the manner in which they respectively live, grow, and multiply,
than on account of any strvictural peculiarities, that they are assigned
to the vegetable or to the animal kingdom respectively. It is
impossible, in the present state of our knowledge, to lay down any
definite line of demarcation between the two kingdoms ; since there
is no single character by which the animal or vegetable nature of
any organism can be tested. Probably the one which is most
generally applicable among those that most closely appi-oximate to
one another is not, as formerly supposed, the presence or absence of
spontaneous motion, but, on the one hand, the dependence of the
organism for nutriment upon organic compounds already fornned
which it takes (in some way or other) into the interior of its body,
or, on the other, its possession of the power oi pyvoducing the organic
comjyounds which it applies to the increase of its fabric, at the
expense of the inorganic elements with which it is supplied by air
and water. The former, though perhaps not an absolute, is s^, general
characteristic of the animal kingdom ; the latter, but for the exist-
ence of which animal life would be impossible, is certainly the

532 MICEOSCOPIC FORMS OF VEGETABLE LIFE- THALLOPHYTES

prominent attribvite of the vegetable. We shall find that the ^^rofo.'soa
(or simplest animals) ai'e siipj^orted as exclusively either upon
other protozoa or upon protophytes, as are the highest animals upon
the flesh of other animals or upon the products of the vegetable
kingdom ; whilst many protophytes, in common with the highest
plants, draw tJielr nourishment from the atmosphere or the water in
which they live, and, like them, are distinguished by their power of
decomposing carbonic acid (CO2) under the influence of light —
setting free its oxygen, and combining its carbon with the elements
of water to form the carbohydrates (starch, cellulose, Ac), and with
those of atmospheiic ammonia to form nitrogenous (albuminoid)
compounds. And we shall find, moreover, that even such protozoa
as have neither stomach nor mouth receive their alimentary matter
direct into the very substance of their bodies, in which it under-
goes a kind of digestion ; whilst protop>hytes absorb through their
external surface only, and take in no solid particles of any desci'i].-
tion. With regard to motion, which was formerly' considered the
distinctive attribute of animality, we now know, not merely that
many protojjhytes (perhaps all, at some period or other of their lives)
possess a jjower of spontaneous movement, but also that the instru-
ments of motion (when these can be discovered) are of the very same
character in the plant as in the animal, being little hair-like fila-
ments, termed cilia (from the Latin word cilium, an eyelash), or
longer whip-like Jlagella, by whose rhythmical vibrations the body
of which they form part is propelled in definite directions. The
peculiar contractility of these organs seems to be an intensification
of that of the general protoplasmic substance, of which they are
special extensions.

Thei-e are certain plants, howevei-, which resemble animals in
theii- dependence upon organic compounds prepared by othei-
organisms, being themselves unable to effect that fixation of carbon
by the decomposition of the CO2 of the atmosphere, which is the
first stage in their production. Such is the case, saaong jyJianerogams
(flowering plants), with the leafless ' 23arasites ' which draw theii-
support from the tissues of their ' hosts.' And it is the case also,
among the lower cryptogams, with the entire group of Fungi ;
which, however, in a large number of cases, depend rather for their
nutritive materials upon organic matter in a state of decomposition,
many of them having the power of jjromoting that process by theii-
zymotic (fei-mentative) action. Among animals, again, there are
several in whose tissues are found organic compounds, such as chloro-
phyll, starch, and cellulose, which are characteristically vegetable ;
but it has not yet been proved that they generate these compounds
for themselves Ijy the decomposition of CO,.

The plan of organisation recognisable throughout the vegetable
kingdom presents this i-emarkable feature of uniformity, that the
fabric, alike in the highest and most complicated plants and in the
lowest and simplest fonus . of vegetation, consists of nothing else
than an aggi'egation of the bodies termed cells, every one of which
(save in tlie forms that lie near the bordei'-gi'ound between animal
and vegetal )le life) has its little particle of protoplasm enclosed by a

THE VECtETAELE CELL 533

casing of the substrtuce teiined cellulose — a non-iiitrogeiions substance
identical in chemical composition with starch. The entire mass of
cells of which any vegetable organism is composed has been gene-
rated fi"om one ancestral cell by processes of multiplication to be
presently described ; and the difference between the fabrics of the
lowest and of the highest plants essentially consists in this, that whilst
the cells produced by the repeated multiplication of the ancestral
cell of the protophyte are all mere repetitions of it and of one an-
other each living by and /or itself, those produced by the like multi-
plication of the ancestral cell in the oak or palm not only i-emain in
mutual connection, but go through a progressive ' differentiation,'
the ordinary type of the cell undergoing various modifications to be
described in their proper place. A composite structure is thus
developed, which is made up of a number of distinct ' organs ' (stem,
leaves, roots, liowers, ifec), each of them characterised by specialities
not merely of external form, but of internal structui-e ; and each
performing actions peculiar to itself, which contribute to the life
of the plant as a ivhole. Hence, as was fii'st definitely stated by
Schleiden, it is in the Hfe-hlstory of the incUvidiicd cell that we
find the true basis of the study of vegetable life in general.

We have now to consider in more detail the structure and life-
history of the typical plant-cell, and shall begin by treating of the
cell-vKill. This cell-wall is composed, as long as the cell is in w
living state, chiefly of the substance known as cellulose, one of the
group of compounds called ' carbohydrates,' and bearing the definite
chemical comjiosition OgHjoOg. From a physical point; of view it
consists of particles or micellce of cellulose surrounded by water.
In addition to cellulose, recent observations have shown that pectie
substances enter largely into the composition of the Avail of the
living cell, especially in its early stages. In fungi it is doubtful
whether there is any true cellulose in the cell-walls. With regard
to the mode of growth of the cell-wall, two hypotheses have been
proposed : one, that it is formed by apposition, that is, by the
constant addition of fresh layei'S to the inner surface of the cell-wall ;
the other that it increases by intussusception, or the intercalation of
fresh particles of cellulose between those already in existence. The
results of modern researches tend in the direction of the former
being the more iisual process ; but it is probable that the two co-
operate in producing the total growth of the cell-wall.

The contents of the plant-cell, which may be collectively termed
the endoplasm (answering to the ' endosarc ' of rhizopods), or, when
strongly coloured throughout (as in many algce), the endochrome,
consist in the first place of an outer layer of protoplasmic substance
called the ectoplasm, primordial utricle, or parietal utricle. This is an
extremely thin and delicate layer, so that it escapes attention so long
as it remains in contact with the cell-wall ; and it is only brought
into view when separated from this, either by developmental changes
(fig. 415), or by the influence of reagents which cause it to con-
tract by drawing forth part of its contents (fig. 413, C). It is not
sharply defined on its internal face, but passes gradually into the
inner mass of pi-otoplasm, fi-om which it is chiefly distinguishable by

5 34 MICEOSCOPIC FOEMS OF VEGETABLE LIFE— THALLOPHYTES

the absence of granules ; and it is shown hy the effects of reagents to
have the albuminous composition of protoplasm. It may thus be
regarded as the slightly condensed external film of the protoplasmic
layer with which the inner surface of the cell -wall is in contact ; and
it essentially corresponds to the 'ectosarc' of Amoeba or any other
rhizoiDod. The ' ectoplasm ' and ' cellulose wall ' can be readily dis-
tinguished from each other by chemical tests, and also by the action
of carmine, which stains the protoplasmic substance (when dead)
without affecting the cellulose wall. The further contents of the cell
consist of a watery fluid called cell sap, which holds in solution sugar,
vegetable acids, saline matters, ifcc. ; the peculiar body termed the
nucleus ; and chlorophyll corpuscles (enclosing starch granules),
oil particles, &c. In the young state of the cell the whole cavity
is occupied by the protoj^lasmic substance, which is, however, viscid
and granular near the cell-wall, but more watery towards the interior.
With the enlargement of the cell and the imbibition of water, clear
spaces termed vacuoles, filled with watery cell-sap, are seen in the
protoplasmic substance ; and these progressively increase in size and
nvimber, until they come to occupy a considerable portion of the
cavity, the protoplasm stretching across it as an irregular network
of bands. Each of the vacuoles is enclosed in a very delicate con-
tractile membrane, the tonoplast. When, as usually happens, the
nucleus lies imbedded in the outer protoplasmic layer, these bands
are gradually withdrawn into it, so that the separate vacuoles unite
into one large general vacuole which is filled with watery cell-sap.
Bvit where the nucleus is situated nearer to the centre of the cell,
part of the protoplasm collects around it, and bands or threads of
protoplasm stretch thence to various parts of the parietal layer. It
is by the contractility of the protoplasmic layer that the curious
' cyclosis ' hereafter to be described is carried on within the plant-
cell, which is the most interesting to the microscopist of all its
manifestations of vital activity. The wacleus is a small, body,
usually of lenticular or subglobose form (fig. 413, A, a), and of
albuminous composition, that lies imbedded in protoplasmic sub-
stance, either close to the cell-wall or nearer the centre of the
cavity. Cells containing a number of nuclei, or ' Tntdtinucleated cells^
are not uncommon. They occur, for example, in many alga?, in the
' suspensor ' and ' embryo-sac ' of the ovule of phanerogams, and in
the ' laticiferoiis ' tubes. Within the nucleus are often seen one or
more small distinct particles termed nucleoli (fig. 413, A, 6), which
can be best distinguished by the strong coloration they receive from
a twenty-four hours' immersion in carmine, and subsequent washing
in water slightly acidulated with acetic acid. Though in some points
the precise function of the nucleus is still unknown, there can be no
doubt of its essential relation to the vital activity of the cell, at least
in all the higher plants, although in the cells of some of the lower
cryptogams it has not at present been distinguished with certainty
at any stage of their existence. In the nucleated cells which
exhibit ' cyclosis,' it may be observed that if the nucleus remains
attached to the cell-wall, it constitutes a centre from which the
jtrotoplasmic streams diverge, and to which they return; whilst if

CONTENTS OF THE CELL 535

it retains its freedom to -wander about, the course of the streams
alters in conformity with its position. But it is in the multiplication
of cells by binai'y subdivision, which will be presently described, that
the speciality of the nucleus as the centre of the vital activity of the
cell is most sti-ongly manifested. The chlorophyll corpuscles^ which
are limited to the cells of the parts of plants acted on by light, are
specialised particles of protoplasm through which a green coloiiring
matter is diffused ; and it is by them that the work of decomposing
CO2, and of '^ fixing ' its carbon by union with the oxygen and hydrogen
of water into starch, is effected. The characteristic green of
chlorophyll often gives place to other colours, which seem to be pro-
duced from it by chemical action. Starch grains are always formed
in the first instance in the interior of the chlorophyll corpuscles and
gradually inci'ease in size until they take the places of the corpuscles
that produced them. So long as they continue to grow, they are
always imbedded in the protoplasm of the cell ; and it is only when
fully formed that they lie free within its cavity.

But although these component parts may be made out without
any difficulty in a large proportion of vegetable cells, yet they cannot
be distinguished in some of those humble organisms which are
nearest to the border-line between the two kingdoms. For in them
we find the ' cell-wall ' very imperfectly differentiated from the ' cell-
contents ; ' the former not having by any means the firmness of a
perfect membrane, and the latter not possessing the liquidity which
elsewhere characterises them. And in some instances the cell is
represented only by a mass of endoplasm, so viscid as to retain its
external form without any limiting membrane, though the superficial
layer seems to have a firmer consistence than the interior substances ;
and this may or may not be surrounded by a gelatinous-looking
envelope, which is equally far from possessing a membranous firmness,
and yet is the only representative of the cellulose wall. This viscid
endoplasm consists, as elsewhere, of a colourless protoplasm, through
which minute colouring particles may be diffused, sometimes uni-
formly, sometimes in local aggregations, leaving parts of the proto-
plasm uncoloured. The superficial layer in particular is freqviently
destitute of colour ; and the partial solidification of its surface gives
it the character of an ' ectoplasm.' Such individualised masses of
protoplasm, destitute of a true cell-wall, have sometimes been
termed ' primordial cells.' It is an extremely curious feature in
the cell-life of certain protophytes that they not only move like
animalcules by cilia or flagella, but that they exhibit the rhythmically
contracting vacuoles which are specially characteristic of protozoic
organisms.

So far as we yet know, every vegetable cell derives its existence
from a pre-existing cell ; and this derivation may take place (in the
ordinary process of growth and extension, as distinguished from
' sexual multiplication ') in one of two modes : either (1) binary
subdivision of the parent-cell, or (2) free-cell formation within the
parent-cell. The first stage of the former process consists in the
elongation and transverse constriction of the nucleus ; and this con- •
striction becomes deeper and deeper, luitil the nucleus divides itself

5 36 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPflYTES

into two halves (fig. 413, B, a, a'). These then separating from
each other, the endoplasm of the parent-cell collects round the two
new centres, so as to divide itself into two distinct masses (C, a, a') ;
and by the investment of these two secondary ' endoplasms ' with
cellulose-walls a complete pair of new cells (D, a, a') is formed
within the cavity of the parent-cell. The process oi free-cell forma-
tion is always connected, directly or indirectly, with a process of
reproduction rather than of growth, and takes two difi'erent forms,
the one occurring in the production of the ' zoospores ' or ' swarm-
spores ' of algfe, the other in the formation of pollen-grains, or of

the ' endosperm ' within
the embryo-sac of flowering
plants. In the former
case, the endosperm, in-
stead of dividing itself into
two halves, usually breaks
up into numerous segments
corresponding with one
another in size and form,
each of which, escaping
from the parent - cavity,
becomes an independent
cell, without any investing
cell-wall of cellulose, hence
a ' primordial cell,' en-
dowed with a power of
rapid motion by means of
cilia or flagella. In the
second case the endoplasm
groups itself, more or less
completely, round several
centres, each of which has
its own nucleus, formed by
subdivision of the nucleus
of the parent-cell ; and
these secondary cells, in
various stages of develop-
ment, lie free within the
cavity of the parent-cell,
imbedded in its residvial
endoplasm, each proceeding to complete itself as a cell by the
formation of a limiting wall of cellullose (fig. 414). As a 'new
generation ' in any phanerogamic plant has its origin in the
fertilisation of a highly specialised ' germ-cell ' (contained within
the ovule) by the contents of a 'sperm-cell' (the pollen-gi-ain),
so do we find, among all save the lowest cryptogams, a provision
for the union of the contents of two highly specialised cells,
the ' germ-cells ' being fertilised by the access of motile proto-
plasmic bodies (antherozoids), set free from the cavities of the
' sperm-cells ' within which they were developed. But althovigh
the sexual process can l^e traced downwai'ds under this form into

Fig. 418. — Binary subdivision of cells in endo-
sperm of seed of s<;arlet-runner : A, ordinary
cell, with nucleufs a, and nucleolus b, imbedded
in its protoplasm ; B, cell showing subdivision
of nucleus into two halves, a and a' ; C, cell in
same stage, showing contraction of endoplasm
(produced by addition of water) into two sepa-
rate masses round the two segments of original
nucleus ; D, two complete cells within mother-
cell, divided by a partition.

PROTOPLASM OF THE LIVING CELL

537

the group of thallophytes, we find among the lower types of that
group a- yet simpler mode of bringing it about ; for there is strong
reason to regard the act of ' conjugation ' which takes place in
the Conjugata? and in some 'fungi in the same light, and to look
upon the ' zygospore,' ^ which is its immediate product, as the
originator (like the fertilised embryo-cell of the phanerogamic seed)
of a ' new generation.'
Great attention
has recently been
paid by Strasburger
?ind others to the con-
stitution of the endo-
plasm and to the
processes connected
with cell-division. On
l)oth these subjects it
is impossible here to
give more than the
barest outlines. Stras-
burger distinguishes
between the following
differentiated parts of
the protoplasm of the
living cell : — The
protoplasm outside the
nucleus he terms the

■cytoplas')n ; the portion which constitutes the nucleus is the
nucleojylasm ] that which enters into the composition of the
chlorophyll corpuscles and other allied substances is the chromato-
plasim. Each of these thi-ee portions of protoplasm is composed of a
hyaline matrix or hyalo-plasm and of imbedded granular structures
or microsomes. A distinct substance, known as nuclein,i\h&ent ffoni
the cytoplasm, appears to enter into the composition of the nucleus.
The various substances imbedded in the cytoplasm are known under
the general name of plastids. If colourless, they are leucojjlasts, {u\d

Fig. 414. — Successive stages of free-cell formation
in embryo-sac of seed of scarlet-runner ; a, a, a,
completed cells, each having its prox^er cell-wall,
nucleus, a,nd endoplasm, Ijing in a protoplasmic
mass, through which are dispersed nuclei and cells
in various stages of development.

1 The term ' spore ' has been long used by cryptogamists to designate the minute
rex^roductive particles (such as those set free from the ' fructification ' of ferns, mosses,
&c.) which were supj)osed — in the absence of all knowledge of their sexual relations —
to be the equivalents of the seeds of flowering plants. But it is now known that such
* spores ' have (so to spieak) very different values in different cases, being, in by far the
larger proportion of cryptogams, but the remote descendants of the fertilised cell which
is the immediate product of the sexual act under any of its forms. This cell, which
will be distinguished throughout the present treatise as the oosjjJ/ere, is the real repre-
sentative of the ' germinal cell ' of the ' embryo ' developed within the seed of the
flowering plant. On the other hand, the various kinds of non-sexual spores emitted
by cryptogams, which have received a great variety of designations, are all to be
regarded (as will be presently e.^plained) as equivalents of the leaf -buds of flower-
ing plants.

[The different interpretations placed upon the term ' spore ' and its derivatives by
different writers on cryptogamic botany present a great difficulty to the student. A
different terminology for the one followed here is now employed by some of the best
authorities ; but, in order to avoid the great alteration in the use of terms which
would otherwise be necessary, it has been thought best, in the present edition, to
retain Dr. Carpenter's terminology, at all events until a greater agreement has been
arrived at than is at present the case.— Ed.]

538 MICKOSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

these are the special seat of the formation of the starch gi-ains. If
coloured they are chromoplasts or chromatojyhores, the origin of the
various colouring matters of the cell ; those which give birth to the
chlorophyll corpuscles being distinguished by the special term ddoro-
plasts. Minute bodies termed physodes, endowed with an amoeboid
motion, have been observed within the protoplasm filaments. In
some of the lower plants, at present exclusively in the green algse,
there are found within the chlorophyll corpuscles homogeneous
proteid substances kno'wii as pyrenoids ; they are often surrounded
by starch gi-ains.

The division of the nucleus may take place either directly, when
the process is known ^■ti fragmentation, or indirectly, when it is known
as mitosis or karyokinesis (see fig. 415). In the process of indirect
division, the protoplasm of which the nuclevis is composed undergoes
a great variety of changes, in the course of which it assuraes the
beautiful appearance known as the nuclear spindle, consisting of an
equatorial disc, the nuclear plate, and delicate s-pindle fibres which
converge towards the two poles of the spindle. Apparently con-
nected with the process of cell-division are the peculiar bodies
known as centrospheres, directing spheres, or attracting spheres, corre-
sponding to similar bodies found in animal cells, but at present
detected only in the lower forms of vegetable life. They form two
small homogeneous spheres lying ftear the nucleus, one on each side
of it, and imbedded in the cytoplasm. Each centrosphere has in its
centre a body tei'med the centrosome, composed of one or more small
granules. To follow out all the processes of kai'yokinesis requires
very high magnifying powers of the microscope, great skill in mani-
pulation, and the use of very delicate staining reagents.

The older conception of the vegetable cell regarded it as a com-
pletely closed vesicle, the endoplasm of which is entirely shut off
from contact with that of the adjacent cells. Recent observations
require the modification of this conception. It has been shown that
in many cases the cell-wall is perforated by very minute orifices,
through which excessively fine strings of protoplasm pass from one
cell-cavity to another (fig. 416). This continuity of 2)rotop>lasm has
been observed in some seaweeds and other algae, in the endosperm
of the ovule, in the pulvinus or motile organ of the leaves of the
sensitive plant, and in many other instances, and is regarded by
some authorities as probably a universal phenomenon in living cells.
In the case of the sensitive plant it is undoubtedly connected wdth
the remarkable phenomenon of sensitiveness or irritability displayed
by the leaves.

In the lowest forms of vegetation every single cell is not only
capable of living in a state of isolation from the rest, but even
normally does so ; and thus the plant may be said to be unicellular,
every cell having an independent ' individuality.' There are others,
again, in which amorphous masses are made up by the aggregation
of cells, which, though quite cajjable of living independently, remain
attached to each other by the mutual fusion (so to speak) of their
gelatinous investments ; and there are others, moreover, in which a
definite adhesion exists between the cells, and in which regular

CELL-DIVISION

539

plant-like structures are thus formed, notwitlistanding that every
cell is but a repetition of every other, and is capable of living inde-
pendently if detached, so as still to answer to the designation of a
' unicellular ' or single-celled plant. These difierent conditions we
shall find to arise out of the mode in which each particular species
multiplies by binary subdivision ; foi- where the cells of the new pair
that is produced by division of the previous cell undergo a co'in'plete
separation from one another, they will henceforth live indepen-

FiG. 415. — Division of the pollen-mother-eells of Fritillaria persica. (From Stras-
burger and Hillhouse's 'Practical Botany,' published by Sonnenschein.)

dently ; but if, instead of undergoing this comjjlete fission, they are
held together by the intervening gelatinous envelope, a shapeless
mass results from repeated subdivisions not taking place on any
determinate plan ; and if, moreover, the binary subdivision alwaj^s
takes place in one direction only, a long, narrow filament (fig. 424, D),
or if in two directions only, a broad, fiat, leaf-like expansion (G),
may be generated. To such extended fabrics the term ' unicellular '
plants can scarcely be apjDlied with propriety ; since they may be
built up of many thousands or millions of distinct cells, which have

540 MICROSCOPIC FORMS OF A^EOETABLE LIFE— THALLOPHYTES

no disposition to separate from each other spontaneously. Still
they coi-respond with those which ai-e strictly unicellular, as to the
absence of differentiation, either in structure or in function, between
their component cells, each one of these being a repetition of the
rest, and no relation of mutual dependence existing among them ;
and all such simple oi-ganisms, therefore, may still be included
under the general term of Thallophytes.

Excluding lichens, for the reasons to be stated hereafter, botanists
now rank these thallophytes under two series : — algce, which form
chlorophyll, and can support themselves upon air, water, and mineral
matters; dund fungi, which, not forming chlorophyll for themselves,
depend for their nutriment upon materials di-awn from other organ-
isms. Each series contains a very large variety of forms, which,
when ti-aced fi-om below upwards, pi-esent gradually increasing com-
plexities of structure :
and these gradations
show themselves espe-
cially in the provisions
made for the genera-
tive process. Thus, in
some forms, a ' zygo-
spore ' is produced by
the fusion of the con-
tents of two cells,
which neither present
any apparent sexual
difference the one
from-. the other, nor
can be distinguished
in any way from the
rest. In the next
highest forms, while
the 'conjugating' cells
are still apparently
undifferentiated from
the rest of the structure, a sexual difference shows itself between
them ; the contents of one cell (male) passing over into the cavity
of the other (female), within which the 'zygospore' is formed.
The next stage in the ascent is the resolution of the contents
of the male cell into motile bodies (' antherozoids '), which, escaping
from it, move freely through the watei-, and find their way to
the female cell, whose contents, fertilised by coalescence with the
material they bring, form an 'oospore.' In the lower forms of this
stage, again, the generative cells are not distinguishable from the
rest until the contents begin to show their characteristically sexual
aspect; but in the higher they are developed in special organs,
constituting a true ' fructification.' This must, however, be dis-
tinguished from organs which, though commonly spoken of as the
' fructification,' have no real analogy with the generative apparatus
of flowering plants, their function being merely to give origin to

Fig. 416. — Continuity of j)rotoplasm. (From Vines's
' Physiology of Plants.' Cambridge University Press.)

STRUCTURE OF PALMOGLffiA

541

gonidml ^ cells or groups of cells, which simply mnltiphj the pnrent
stock, in the snnie mannei- that many flowering plants (such as the
potato) can be jii-opagated by the artificial sepai-ation of their leaf-
buds. It frequently hapjiens among cryptogams that this yon idial
fructification is by fai- the more conspicuous, the sexual fructifica-
tion being often so obscui-e that it cannot be detected without
great difficulty ; and we shall presently see that there ai-e some
thallophytes in which the pi-oduction of gonids seems to go on
indefinitely, no form of sexual generation having been detected
in them. These general statements will now be illustrated by
sketches of the life-history of some of those humble thallophytes
which present the phenomena of cell-division, conjugation, and

Fig. 417. — Development of Palmoghea macrococca.

gonidial midtiplication. undei- their simplest and most instructive
aspect.

The first of these lowly forms of life to which we call the
attention of the reader is Palmoglvea viacrococca, Ktz.,^ one of
those humble kinds of vegetation which spi-ead themselves as green
slime over damj) stones, walls, &c. When this slime is examined
with the microscope, it is found to consist of a multitude of green
cells (fig. 417, A), each sui-rounded by a gelatinous envelope ; the cell,
which does not seem to have any distinct membranous wall, is filled
with a granular ' endochi-ome,' consisting of green pai-ticles difi"used
through colourless jjrotoplasm ; and in the midst of this a nucleus

^ The term gonids, originally applied to certain green cells in the lichen-crusts
that are capable, when detached, of reproducing the vegetable portion of the plant,
is used by some writers as a designation of the non-sexual sjJores of cryptogams
generally, which it is very important to discriminate from the genitative ' ciispheres.'
If possessed of motile powers, thej'' are spoken of as • zoospores,' or sometimes (on
account of the appearance they present when a number are set free at once) as
' swarm-spores.' In contradistinction to ' motile ' gonids or ' zoiispores,' those which
show no movement are often termed resting spores, or hgpnusjjoves; but such may
be either sexual oiispheres or non-sexual gonids, the latter, like the former, often
' encysting' themselves in a firm envelope, and then remaining dormant for long periods
of time.

^ [Most of the species of Kiitzing's genus Palmoghea are now regarded as belong-
ing to the Desmidiacece, and are included under the genus Mesotccnium. — ^Ed.]

542 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

may sometiines be distinguished, and can always be brought into
view by tinctvire of iodine, which turns the ' endochvome ' to a
brownish hue, and makes the nucleus (G) dark brown. Other cells
are seen (B), which are considerably elongated, some of them
beginning to present a sort of hour glass contraction across the
middle ; and when cells in this condition are treated with tincture
of iodine, the nucleus is seen to be \indergoing the like elongation
and constriction (H). A more advanced state of the process of
subdivision is seen at C, in which the constriction has proceeded to the
extent of completely cutting off the two halves of the cell, as well as of
the nucleus (I), from each other, though they still i-emain in mutual
contact ; in a yet later stage they are found detached fi-om each
other (D), though still included within the same gelatinous envelope.
Each new cell then begins to secrete its own gelatinous envelope, so
that by its intervention the two are usually soon sepai-ated from
each other (E). Sometimes, however, this is not the case, the
process of subdivision being quickly i-epeated before there is time for
the production of the gelatinous envelope, so that a series of cells
(F) hanging on one to another is produced. There appears to be no
definite limit to this kind of multiplication, and extensive areas
may be quickly covered, in circumstances favourable to the growth
of the plant, by the products of the binary subdivision of one
original cell. This, as already shown, is really an act of (jrowih,
which continues indefinitely so long as moisture is abundant and
the temperature low. But under the influence of heat and dryness
the process of cell-multiplication gives place to that of 'conjugation,'
in which two cells, apparently similar in all respects, fuse together
for the production of a ' zygospore,' which (like the seed of a
flowering plant) can endure being reduced to a quiescent state for
an unlimited time, and may be so completely dried up as to seem
like a particle of dust, yet resumes its vegetative activity w^henever
placed in the conditions favourable to it. The conjugating process
commences by the putting forth of proti'usions fi-om the botindaries
of two adjacent cells, which meet, fuse together (thereby showing
the want of fii-mness of their ' ectoplasms '), and form a connecting
bridge between their cavities (K). The fusion extends before long
through a large part of the contiguous sides of the two cells (L) ;
and at last becomes so complete that the combined mass (M) shows
no trace of its double origin. It soon forms for itself a firm cellulose
envelope, wdiich bursts when the ' zygospoi-e ' is wetted ; and the
contained cell begins life as a new generation, speedily multiplying,
like the former ones, by binary subdivision. It is curious to observe
that during this conjugating process a production of oil particles
takes place in the cells ; these are at first small and distant, but
gradually become larger and approximate more closely to each other,
and at last coalesce so as to form oil-drops of various sizes, the gi-een
granular matter disappeai-ing ; and the colour of the conjiigated
body changes, with the advance of this process, from green to a light
yellowish brown. When the zygospore begins to vegetate, on the
other hand, a convei'se change occurs ; the oil-globules disappear',
and green granular matter takes their place.

STEUCTURE OF PROTOCOCCUS

5.43

If this (as seems probable) constitutes the entire life-cycle of
Palmoglma, it aftbi'ds no example of that curious ' motile stage
which is exhibited by most algal pi-otophytes in some stage of theii-
existence, and which constitutes a large part of the life -history of
the minute unicellular organism now to be described, Protococcus
pluvialis, Ktz. {Chlamydococcus jjhcvialis, A. Br.) (fig. 418), which
is not uncommon in collections of rain-watei'. K^ot only has this
pi'otophyte. in its motile condition, been veiy commonly regarded as
an animalcule, but its different states have been described under
several diftei-ent names. In the first place, the colour of its cells
varies considerably ; since, although they are usually green at the
period of their most active life, they are sometimes reel ; and their
red form has received the distinguishing appellation of Hcemato-

Develoi:inient of Protococcus pluvialis.

coccus. Very commonly the red coloui'ing matter forms only
a central mass of greater or less size, having the appearance of
a nucleus (as shown at E, fig. 418) ; and sometimes it is reduced
to a single granulai- point, which has been described by Professor
Ehrenberg as the eye-spot of these so-called animalcules. It is
quite certain that the i-ed colouring substance is very nearly related
in its chemical character to the green, and that the one may be
converted into the other, though the conditions under which this
conversion takes place are not pi-ecisely known. In the ' still ' form
of the cell, with which we may commence the history of its life, the
endoplasm consists of a colourless protoplasm, through which red oi-
gi'een coloured granules are moi-e or less unifoi-mly diflused ; and the
svxrface of the colourless protoplasm is condensed into an ectoplasm,
which is surrounded by a tolerably firm cell- wall, consisting of cellulose

544 MICEOSCOPIC FORMS OF VEaETABLE LIFE— THALLOPHYTE,'>

or of some modification of it. Outside this (as shown at A), when
the ' still ' cell is formed by a. change in the condition of a cell that
has been previously ' motile,' we find another envelope, which seems
to be of the same nature, but which is separated by the interposition
of aqueous fluid ; this, however, may be altogether wanting. The
multiplication of the ' still ' cells by subdivision takes place as in
Palmogloea^ the endoplasm first undergoing sepai-ation into two
halves (as seen at B), and each of these halves subsequenth' developing
a cellulose envelope around itself, and undergoing the same division
in its turn. Thus two, four, eight, or sixteen new cells are succes-
sively produced ; and these are sometimes set free by the complete
dissolution of the envelope of the original cell ; but they are more
commonly held together by its transformation into a gelatinous
investment, in which they remain imbedded. Sometimes the endo-
plasm subdivides at once into four segments (as at D), of which every
one forthwith acquires the charactei- of an independent cell ; but
this, although an ordinary method of multiplication among the ' mo-
tile ' cells, is comparatively rare in the ' still ' condition. Sometimes,
again, the endoplasm of the ' still ' form subdivides at once inta
eight portions, which, being of small size, and endowed with motile
jjower, may be considered as zobsjyores. As far as the complete life-
history of Frotococcus is at present known, some of these zoospores
retain their motile powers, and develop themselves into the ordinary
' motile ' cells ; others produce a firm cellulose envelope and become
' still ' cells ; and others (pei-haps the majority) perish without any
fui'ther change.

When the ordinary division of the ' still ' cells into two segments
has been repeated four times, so as to produce sixteen cells — and
sometimes at an eailiei* pei'iod — the new cells thus produced assume
the ' motile ' condition, being liberated before the development of the
cellulose envelope, and becoming furnished with two long vibratile
flagella which seem to be extensions of the colourless protoplasm
layer that accumulates at theii- base so as to form a sort of trans-
parent beak (H). In this condition it seems obvious that the colour-
less protoplasm is moi-e develoj^ed i-elatively to the colouring mattei^
than it is in the ' still ' cells ; and it usually contains ' vacuoles '
occupied only by clear aqueous fluid, which are sometimes so
numei'ous as to take in a lai-ge part of the cavity of the cell, so that
the coloured contents seem only like a deposit on its walls. Before
long this ' motile ' cell acquires a peculiai- saccular investment, which
seems to correspond with the cellulose envelope of the ' still ' cells,
but is not so firm in its consistence (I, K, L) ; and between this and
the surface of the ectoplasm a considerable space intervenes, tra-
versed by thread-like extensions of the latter, which are I'endered
more distinct by iodine, and can be made to retract by means of
reagents. The flagella pass through the cellulose envelope, which
invests theii- base with a soi-t of sheath, and in the poi-tion that is
within this sheath no movement is seen. During the active life of
the ' motile ' cell the vibration of these flagella is so rapid that they
can be recognised only by the currents they produce in the watei'
thi'ough which the cells are quickly propelled ; but when the motion

STRUCTUEE OF PROTOCOCCUS 545

becomes slacker the flagella themselves are readily distinguishable,
and they may be made more obvious by the addition of iodine, which,
howevei-, it should be noted, always kills the plant.

The multiplication of these ' motile ' cells may take place in
various modes, giving rise to a great variety of appearances. Some-
times they undergo a regulai- binary subdivision (B), wheieby a pair
of motile cells is produced (C), each resembling its single predecessor
in possessing the cellulose investment, the transparent beak, and the
vibratile flagella, before the dissolution of the original investment.
Sometimes, again, the contents of the original cell undergo a seg-
mentation in the first instance into four divisions (D) ; which may
either become isolated by the dissolution of their envelope, and may
separate from each othei* in the condition of ' free primordial cells '
(H), developing their cellulose investments at a future time, or
may acquire their cellulose investments (as in the preceding case)
before the solution of that of the original cell ; while sometimes,
even after the disappearance of this, and the formation of their own
independent investments, they remain attached to each other at their
beaked extremities, the primordial cells being connected with each
other by peduncular prolongations, and the whole compound body
having the form of a + • This quaternary segmentation appears to
be a more frequent mode of midtiplication among the ' motile ' cells
than the subdivision into two, although, as we have seen, it is less
common in the ' still ' condition. So also a primary segmentation of
the entire endochrome of the ' motile ' cells into eight, sixteen, or even
thirty-two parts, may take place (E, F), thus giving rise to as many
minute gonidial cells. These, when set free, and j)ossessing active
powers of movement, are true zoospores (G) ; they may either develop
a loose cellulose investment or cyst, so as to attain the full dimensions
of the ordinary motile cells (I, K), or they may become clothed with
a dense envelope and lose their flagella, thus jaassing into the ' still '
condition (A) ; and this last transformation may even take place
before they are set free fi-om the envelope within which they wei'e
produced, so that they constitute a mulberry-like mass, which fills
the whole cavity of the original cell, and is kept in motion by its
flagella

To what extent Protococcus is an autonomous organism is still
doubtful, but it appears to be more or less closely connected with
many foi'ms of life which have been described, not merely as dis-
tinct sjyecies, but as distinct genera of animalcules or of protophytes,
such as Ghla'niydomonas, Euglena^ Trachelomonas^ Gyges, Goniwm,
FandoTina^ Botryocystis, Uvella, Syncrypta, Monas. Astasia, Bodo, and
many others. Certain forms, such as the ' motile ' cells I, K, L,
appear in a given infusion, at first exclusively and then principally ;
they gradually diminish, become naore and more rare, and finally
disappear altogether, being replaced by the ' still ' form. After
some time the number of the ' motile ' cells again increases, and
reaches, as before, an extraordinary amount ; and this alternation
may be repeated several times in the course of a few weeks. The
process of segmentation is often accomplished with great rapidity.
If a nvimbei" of ' motile ' cells be transferred from a larger glass into a

N N

546 MICEOSCOPIC FOKMS OF VEGETABLE LIFE— THALLOPHYTEtt

smaller, it will be found, after the lapse of a few hours, that most of
them have subsided to the bottom ; in the coiu-se of the day they
will all be observed to be upon the point of subdivision; on the
following morning the divisional brood will have become quite free ;
and on the next the bottom of the vessel will be found covered with
a new brood of dividing cells, which again proceed to the forma-
tion of a new brood, and so on. The activity of motion and the
activity of multiplication seem to stand, in some degree, in a relation
of reciprocity to each other ; for the dividing process takes place
with greater rapidity in the ' still ' cells than it does in the ' motile.'
Wliat are the precise conditions which determine the transition
between the ' still ' mid the ' motile ' states cannot yet be precisely
defined, but the influences of certain agencies can be predicted with
tolerable certainty. Thus it is only necessary to pour the water
containing these organisms from a smaller and deeper into a larger
and shallower vessel in order at once to determine segmentation in
numerous cells — a phenomenon which is observable also in many other
protophytes. The ' motile ' cells seem to be favourably afiected by
light, for they collect themselves at the surface of the water and at
the edges of the vessel, but when they are about to undergo segmen-
tation or to pass into the ' still ' condition, they sink to the bottom
of the vessel, or retreat to that part of it in which they are least
su.bjected to light. When kept in the dark the ' motile ' cells undergo
a great diminution of their chlorophyll, which becomes very pale,
and is diffused, instead of forming definite granules ; they continue
their movement, however, uninterruptedly without either sinking
to the bottom, or passing into the 'still ' form, or undergoing seg^
mentation. A moderate warmth, jDarticularly that of the vernal sun,
is favourable to the development of the ' motile ' cells ; but a tempe-
rature of excessive elevation prevents it. Rapid evaporation of the
water in which the ' motile ' forms may be contained kills them at
once ; but a more gradual loss, such as takes place in deep glasses,
causes them merely to pass into the ' still ' form ; and in this condi-
tion— especially when they have assumed a red hue — they may be
completely dried up, and may remain in a state of dormant vitality
for many years. It is in this state that they are wafted about in
atmospheric currents, and that, being brought down by rain into
pools, cisterns, &c., they may present themselves where none had
been previously known to exist ; and there under favou liable circum-
stances they may undergo a very rapid multiplication, and may
m.aintain themselves until the water is dried up, or some other
change occurs which is incompatible with the continuance of their
vital activity. They then very commonly become red throughout,
the red colouring substance extending itself from the centre towards
the circumference, and assuming an appearance like that of oil-
drops ; and these red cells, acquiring thick cell-walls and a mucous
envelope, float in flocculent aggregations on the surface of the water.
This state seems to correspond with the ' resting-spores ' of other
protophytes ; and it may continue until wai-mth, air, and moisture
cause the development of the red cells into the ordinary ' still ' cells,
green matter being gradually produced, until the red substance forms

PROTOCOCCUS ; CYANOPHYCE.E 54/

only the central pai-t of the enclochrome. After this the cycle of
changes occurs which has been already described ; and the plant
may pass through a long series of these before it returns to the state
of the red thick- walled cell, in which it may again remain dormant
for an unlimited period. Even this cycle, however, cannot be
regarded as completing the history of Protococcits^ since it does not
include the performance of any true generative act. There can be
little doubt that, in some stage of its existence, a 'conjugation' of
two cells occurs, as in Palmogloea ; and the attention of observers
should be directed to its discovery, as well as to the detection of
other varieties in the condition of this interesting little plant, which
will probably be found to present themselves before and after the
performance of that act.-'

The Cyanophycese or Phycochromaceae constitute another group
of lowly forms of vegetable life, distinguished by their blue-green
colour, differing from the Protococcacese in not containing true
chlorophyll grains, the cell-sap being, on the other hand, coloured by
a soluble blue-green pigment known as ' phycocyanin.' They live
either isolated, or a number congregated together and enclosed in a
more or less dense colourless jelly. They multiply by binary
division, and do not in any case pi-oduce zoospores. To the lowest
family of this group, which strongly resemble the Protococcacefe,
except in the colour of the cells, the Ohroococcacece^ belong the genera
Ghroococcus, Gloeoca-psa. Aj^hanocaiisa, Merismopedia, and many
others, the life-history of which is but very imperfectly known.

The Oscillator iacece constitute a family of Cyanophycese of great
interest to the microscopist, on account both of the extreme sim-
plicity of their structure and of the peculiar animal-like movements
which they exhibit. They consist of fine, usually microscopic
threads, containing a blue-green endochrome, sometimes replaced by
a red or violet, and occur singly or in thick strata in fresh running
or more abundantly in stagnant water. The threads are unbranched
and usvially straight, and either each separate thread or a number
together are, in most of the genera, enclosed in a gelatinous sheath.
Some illustrations of these are seen on Plate VII. The contents of
the sheaths are imperfectly divided into cells by transverse divi-
sion ; small pieces of the threads, consisting of a few cells, occasion-
ally break off, round themselves off at both ends, move about with a
slow undulating motion, and finally develop into new threads ; these
portions are known as hormogones. The most abundant genus, Oscil-
latoria, has been so named from the peculiar oscillating or waving motion
with which the threads are endowed. This consists of a creeping
motion in the direction, of the length of the thread, now backwards,
now forwards, accompanied by a curvature of the thread and rotation
round its own axis. The cause of this motion is still a matter of

1 In the above sketch the Author has presented the facts described by Dr. Cohn
under the relation which they seemed to him naturally to bear, but which differs from
that in which they will be found in the original memoir ; and he is glad to be able
to state, from personal communication with its able author, that Dr. Cohn's later
observations led him to adopt a view of the relationship of the ' still ' and ' motile '
forms which is in essential accordance with his own.

N N 2

548 MICEOSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

controversy. Professor Cohn ^ observed that the oscillating move-
ments take place only when the thread is in contact with a solid
substratum. Zukal ^ compares the motion of SjnruUna to that of a
growing tendril, and asserts that it is intimately connected with the
growth of the filament. Hansgirg,^ on the other hand, considei-s the
twisting and nodding movements to be due, not to the growth of the
thread, but to osmotic changes in the cell-contents. He regards them
as being of the same nature as the movements of the sarcode in the
pseudopodia of rhizoj^ods and other protozoa. Schnetzler "* describes
the movements in Oscillatoiia as of six different kinds : (1) rotation of
the thread or of its segments round its axis ; (2) creeping or gliding
over a solid substrattim ; (3) a free-swimming movement in the water ;
(4) I'otation or flexion of the entire tlxread ; (5) sharp tremblings or

concussions ; and (6) a radiating arrange-
Jll ^,^ ment of the entangled threads. The

raovements are greatly influenced by
temperature and light, being much more
active in warmth and sunshine than in
cold and .shade. There are no zoospores
produced, nor is any sexual mode of
generation known. The Rivulariacece
and Scytoneviacece (PLs. YII and YIII)
are exceedingly common organisms in
stagnant water, resembling the Oscilla-
toriace?e in their blue-green colour, and
in their reproduction by means of
' hormogones.'

Nearly allied to the preceding is the
family of Nostocacece, consisting of
distinctly beaded filaments, which, in
the most familiar genus, Nostoc, lie in
firndy gelatinous envelopes of definite
outline (fig. 419). The filaments are
usually simple, though sometimes densely
interwoven, and are almost always curved
or twisted, often taking a spiral du^ection.
The masses of jelly in which they are imbedded are sometimes
globular or nearly so, and sometimes extend in more or less
regular branches ; they frequently attain a very considerable
size ; and as they occasionally j^resent themselves quite suddenly
(especially in the latter part of autumn on damp garden- walks),
they have received the name of ' fallen stars.' They are not
always so suddenly produced, however, as they appear to be ; for
they shrink up into mere films in dry weather and expand again
with the first shower. Other species are not unfrequent among wet
mqss or on the surface of damp rocks. Species of Anahcena and
Aphanizomenon, genera of Nostocacese, constitute a large portion of

^ Arch. Mikrosh. Anatomie, 1867, p. 48.
^ Oesterreichisclw Bot. Zeitschr. 1880, p. 11.
s See Bot. Centralblatt, vol. xii. 1882, p. 361.
^ Arch. Sci. Plnjs. et Nat. 1885, p. 164.

Fig. 419. — Portion of gelatinous
frond of Nostoc.

CYANOPHYCE^ ; CONJUGAT.E 549

the bluisli-green scum which floats on the surface of stagnant water.
Colonies of species of jYostoc and Anabcena are frequently endophytic
within the cells of Marchantia and other Hepaticfe, the prothallia of
ferns, or other aquatic or moisture-loving plants. Nostoc multiplies,
like the Oscillatoriacefe, by the subdivisioii of its filaments, portions
of which escape from the gelatinous mass wherein they were
imbedded, and move slowly through the water in the direction of their
length. These are ' hormogones,' similar to those of the Oscilla-
toriace*. After a time they cease to move, and a new gelatinous
envelope is formed around each piece, which then begins to increase
in length by the transverse subdivision of its segments. By the
repetition of this process a mass of new filaments is produced, the
parts of which are at first confused, but afterwards become more
distinctly separated by the interposition of the gelatinous substance
developed between them. Besides the ordinary cells of the beaded
filaments, two other kinds are known, both larger than the ordinary
cells, and called respectively heterocysts and resting-spores. The
function of the former is imknown ; the latter develop directly into
new individuals by division in the transverse direction only, with-
out any sexual process. '

Resembling the Protococcacete in the independence of their
individual cells are the two groups Deswtidiacece and Diatomacecu,
forms of such special interest to the microscopist as to require
separate treatment, and a detailed description of which will be found
later on. The Desinidiacece constitute a group of the family
Conjugatse, so called from their mode of reproduction by conjtigation,
a process best exemplified in the higher group, the Zygneinacece, in
which the cells produced by binary subdivision remain attached to
each other, end to end, so as to form long unbranched filaments
(fig. 420), whose length is continually being increased by a repetition
of the same process, which may take place in any part of the filaments,
and not at their ends alone. The plants of this group are not found
so much in running streams as in waters that are perfectly still, such
as those of ponds, of reservoirs, ditches, bogs, or marshy grounds ;
and they are for the most part unattached, floating freely at or near
the surface, especially when buoyed up by the bubbles of gas which
are liberated from the midst of them under the influence of solar
light and heat. In the early stage of their growth, whilst as yet the
cells are undergoing multiplication by division, the endochrome is
frequently diffused pretty uniformly through their cavities (fig. 420,
A) ; but as they advance towards the stage of conjugation, it
ordinarily arranges itself into regular spirals (B, Sjnrogyra)^ a couple
of star-like discs in each cell (Zygnema), or a single plate running-
through it in an axile direction (Mesocarpus). The act of conjugation
usually occurs between the cells of two distinct filaments that happen
to lie in proximity to each other, and all the cells of each filament
generally take part in it at once. The adjacent cells put forth little
protuberances, which come into contact with each other, and then
coalesce by the breaking down of the intervening partitions, so as to
establish a free passage between the cavities of the conjugating cells.
In some genera of this family (such as Mesocarpits) the conjugating

5 50 MICEOSCOPIC FOEMS OF VEGETABLE LIFE— THALLOPHYTES

cells pour their endochromes into a dilatation of the passage that
has been established between them ; and it is there that they com-
mingle so as to form the zygospore. But in the various sjiecies of
Spirogyra (fig. 420, B), which are among the commonest and best
known of Conjugatse, the endochrome of one cell passes over entirely
into the cavity of the other ; and it is within the latter that the
zygospore is formed (C), the two endochromes coalescing into a
simple mass, around which a firm envelope gradually makes its
appearance. Further, it may be generally observed that all the
cells of one filament thus empty themselves, whilst all the cells of
the other filament become the recipients. Here, therefore, we seem
to have a foreshadowing of the sexual distinction of the generative
cells into ' sperm-cells ' and ' germ-cells,' which we shall presently
see in the filamentous Confervacece. Conjugation between^ two
adjacent cells of the same individual also occurs in some species.

Pig. 420. —Various stages of the history of a Spirogi/iri : A, three cells, a, b, c, of a
young filament, of which b is undergoing division ; B, two filaments in the first
stage of conjugation, showing the spiral disposition of their endochromes and the
protuberances from the conjugating cells ; C, completion of the act of conjugation,
the endochromes of the cells of the filament a having entirely passed over to those
of filament b, in which the zygospores are formed.

Although the two conjugating filaments ai'e nearly or quite niorpJio-
logically alike, there must clearly be a physiological difierentiation,
since the conjugation takes place in one direction only. Where
conjugation occurs between cells in the same filament, this sexual
differentiation must be ascribed to the individual cells. Multipli-
cation by zoospores does not take place among the Conjugatse.

From the composite motile forms of Protococcus the transition
is easy to the grovip of Volvocinese, an assemblage of minute plants
of the greatest interest to the microscopist, on account both of the
anim.alcule-like activity of their movements and of the great beauty
and regularity of their forms. The most remarkable example of this
group is the well-known Volvox glohator (Plate VI), which is not
uncommon in fresh-water pools, and which, attaining a diameter of
about -.L or even ^L. of an inch, may be seen with the naked eye
when the drop containing it is held up to the light, swimming through

PLATE VI

Q-A,

Volvox globatoi

West,Ne"wma,ii cliromc-

VOLVOCINE^ 5 5 1

the watei- which it inhabits. Its onwaixl motion is usually of a roll-
ing kind ; but it sometimes glides smoothly along, without turning
on its axis ; whilst sometimes, again, it i-otates like a top, without
changing its position. When examined with a sufficient magnifying
power the Volvox is seen to consist of a hollow sphere, com]30sed of
a very pellucid material, which is studded at regular intei'vals with
minute gi-een sjaots, and which is often (but not constantly) traversed
by green threads connecting these spots. From each of the spots
proceed two long flagella, so that the entire surface is beset with
these lashing filaments, to whose combined action its movements
ai'e due. Within the external sphere may genei'ally be seen from
two to twenty other gjobes, of a darker coloui", and of varying sizes ;
the smaller of these are attached to the inner surface of the investing
sphere, and project into its cavity ; but the larger lie freely within
the cavity, and may often be observed to revolve by the agency of
their own flagella. After a time the original sphere bursts, and the
contained spherules swim forth and speedily develop themselves
into the likeness of that within which they have been evolved,
their coloiired particles, which are at first closely aggregated together,
being sepai'ated from each other by the interposition of the trans-
parent pellicle. It was long supposed that Volvox is a single
animal ; and it was first shown to be a composite fabric, made up of a.
repetition of organisms in all respects similar to each other, by Pro-
fessor Ehrenberg, who, however, considei-ed these organisms as
monads, and described them as each possessing a mouth, several
stomachs, and an eye ! Our present knowledge of their nature,
however, leaves little doubt of their vegetable character ; ^ and the
peculiarity of their history renders it desirable to describe it in some
detail.

Each of the so-called ' monads ' (fig. 421, Xos. 9, 1 1) is a somewhat
flask-shaped plant-cell, about 3q\, ^th of an inch in diameter, consist-
ing, as in the previous instances, of green chlorophyll granules difiused
through a colourless protoplasm, constituting an endochrome (which
commonly includes also a red spot — ' eye-spot ' — of altered chlorophyll),
and bounded by an ectojolasm formed of the condensed and colourless
surface-layer of the protoplasmic mass. It is prolonged outwardly
(or towards the circumference of the sphere) into a sort of colourless
beak or proboscis, from which jDroceed two flagella (fig. 421, No. 11) ;
and it is invested by a pellucid or hyaline envelope i^o. 9, d) of
considerable thickness, the borders of which are flattened against
those of other similar envelopes (No. 5, c, c), but which doesnot appear
to have the tenacity of a true membrane. It is impossible not to
i-ecognise the close similarity between the structure of this body
and that of the motile encysted cell of Protococcits jjhtvicdis (fig.
418, K). There is not, in fact, any perceptible difference between
them, save that which arises from the regular aggregation, in Volvox,

1 Professor Stein, however, in liit; great work on the Infusoria [Organismus der
InfusionstJdere, Abtheilung III., Leipzig, 1878), still ranks the Volvocinece among
the flagellate animalcules, to which they undoubtedly show a remarkable x^arallelism
in structure, the chief evidence of their vegetable nature lying iniheiy: phijsiological
conformity to undoubted thallophytes.

552 MICEOSCOPIC FO"RMS OF VEGETABLE LIFE—THALLOPHYTES

of the cells which normally detach themselves from one another in
Protococcus. The presence of cellulose in the hyaline substance is
not indicated, in the ordinary condition of Volvox glohator, by the
iodine and sulphuric acid test, though the use of ' fSchultz's solution '
gives to it a faint blue tinge ; there can be no doubt of its existence,
however, in the hyaline envelope of Volvox aureus. The flagella
and endoplasm, as in the motile forms of Protococcus, are tinged
a deep brown by iodine, with the exception of one or two starch
particles in each cell, which are turned blue ; and when the contents
of the cell are Kberated, bluish flocculi, apparently indicative of the
presence of cellulose, are brought into view by the action of sulphuric
acid and iodine. All these reactions are characteristically vegetable
in their nature. When the cell is apj^roaching maturity, its endo-
plasm always exhibits one or more vacuoles (fig. 421, JSTo. 9, a, a) of a
spherical foi'm, and usually about one-third of its own diameter ;
and these vacuoles (which are the so -called ' stomachs ' of Ehrenberg)
have been observed to undergo a very curious rhythmical contraction
and dilatation at intervals of about forty seconds ; the contraction
(which seems to amount to complete obliteration of the cavity of the
vacuole) taking place rapidly or suddenly, whilst the dilatation is slow
and gradual. This curious action ceases, however, as the cell
arrives at its full maturity ; ^ a condition which seems to be
marked by the greater consolidation of the ectoplasm, by the
removal or transformation of some of the chlorophyll, and by the
formation of the red spot (6), which obviously consists, as in Proto-
coccus, of a peculiar modification of chlorophyll.

Each cell normally communicates with the cells in nearest
proximity with it by extensions of its own endochrome, which are
sometimes single and sometimes double (fig. 421, No. 5, 6, h) ; and
these connecting processes necessarily cross the lines of division
between their respective hyaline investments. The thickness of these
processes varies very considerably ; for sometimes they are broad
bands, and in other cases mere threads ; whilst they are occasionally
wanting altogether. This difference seems partly to depend upon the
age of the individual, and partly upon the abundance of nutriment
which it obtains ; for, as we shall presently see, the connection is
most intimate at an early period, before the hyaline investments of
the cells have increased so much as to separate the masses of endo-
chrome to a distance from one another (fig. 421, Nos. 2, 3, 4) ; whilst
in a mature individual, in which the separation has taken place to
its full extent and the nutritive processes have become less active, the
masses of endochrome very commonly assume an angular form, and
the connecting processes are drawn out into threads (as seen in No. 5),
or they retain their globular form, and the connecting processes
altogether disappear. The influence of reagents, or the infiltration
of water into the interior of the hyaline investment, will sometimes
cause the connecting processes (as in Protococcus) to be drawn back

1 The existence of rhythmically contracting vacuoles in Volvox (though confirmed
by the observations of Prof. Stein) is denied by Mr. Saville Kent [Manual of the
Infusoria, p. 47) ; but it may be fairly presumed that he has not looked for them at
the stage of development at which their action was witnessed by Mr. Busk.

PLATE YEI,

Osoillciriaceoe and Scytonernaceas.

Woi;t,Nowmen oliromo

YOLYOCINE.^

553

into the central mass of endochrome ; and they will also retreat on
the mere rupture of the hyaline investment. From these circum-
stances it may be inferred that they are not enclosed in any definite
membrane. On the other hand, the connecting threads are some-
times seen as double lines, which seem like tubular prolongations of
a consistent membi-ane, without any protoplasmic granules in their

9 fa%''^^^ ' 10 11

Pig. 421. — Structure of Volvox glohator.

interior. It is obvious, then, that an examination of a considerable
numbei- of specimens, exhibiting various phases of conformation, is
necessary-to demonstrate the nature of these communications ; but
this may -be best made out by attending to the history of their
development, which we shall now describe.

The spherical body of the young Volvox (fig. 421, No. 1) is

5 54 MICEOSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

composed of an aggregation of somewhat angular masses of endochrome
[b), separated by the interposition of hyaline substance ; and the '
whole seems to be enclosed in a distinctly membranous envelope,
which is probably the distended hyaline investment of the original
cell, within which, as will presently apj^ear, the entire aggregation
originated. In the midst of the polygonal masses of endochi'ome,
one mass {a), rather larger than the rest, is seen to present a
circular form. ; and this, as will presently appear, is the originating
cell of what is hereafter to become a new sphere. The growing
Volvox at first increases in size not only by the interposition of new
hyaline substance between its component masses of endochrome,
but also by an increase in these masses themselves (No. 2, a), which
come into continuous connection with each other by the coalescence
of processes (b) which they severally put forth ; at the same time
an increase is observed in the size of the globular cell (c), which is
preliminary to its binary subdivision. A more advanced stage of
the same developmental process is seen in No. 3, in which the con-
necting processes (a, a) have so much increased in size as to establish
a most intimate union between the masses of endochrome, although
the increase of the intervening hyaline substance carries these
masses apart from one another ; whilst the endochrome of the
central globular cell has undergone segmentation into two halves.
In the stage represented in No. 4 the masses of endochrome have
been still more widely separated by the interposition of hyaline
substance ; each has become furnished with its j)aii" of flagella, and
the globular cell has undergone a second segmentation. Finally, in
No. 5, which represents a portion of the spherical wall of a mature
Volvox, the endochrome masses are observed to present a more
scattered aspect, partly on account of their own reduction in size,
and partly through the interjjosition of a greatly increased amount
of hyaline substance, which is secreted from the surface of each mass ;
and that portion which belongs to each cell, standing to the endo-
chrome raass in the relation of the cellulose coat of an ordinary cell
to its ectoplasm, is frequently seen to be mai-ked out from the rest
by delicate lines of hexagonal areolation (c, c), which indicate the
boundaries of each. Of these it is often difficult to obtain a sight,
a nice management of the light being usually requisite with fresh
specimens ; but the prolonged action of water (especially when it
contains a trace of iodine) or of glycerin will often bring them into
clear view. The prolonged action of glycerin, moreover, will often
show that the boundary-lines are double, being formed by the
coalescence of two contiguous cell-walls ; and they sometimes retreat
from each other so far that the hexagonal areolae become rounded.
As the primary sphere approaches maturity, the lai'ge secondary
germ-mass, or zoosporange, whose origin has been traced from the
beginning, also advances in development, its contents undergoing
m.ultiplication by successive segmentations, so that we find it to
consist of eight, sixteen, thirty-two, sixty-four, or still more
numerous divisions, as shown in fig. 421, Nos. 6, 7, 8. Up to this
stage, at which the sphere first appears to become hollow, it is
retained within the hyaline envelope of the cell within which it has

PLATE VIII.

^st,Newm-a.n ckromo

Desmidiacese, Rivulanaceae arid Scytone/naceas.

VOLTOCINEiE 555

Ijeen produced ; a similar envelope can be easily distinguished, as
shown in No. 10, just when the segmentation has been completed,
and at that stage the flagella pass into it, but do not extend beyond
it ; and even in the mature Volvox it continues to foi^m an invest-
ment around the hyaline envelopes of the separate cells, as shown in
the same figure at No. 11. It seems to be by the adhesion of the
hyaline investment of the new sphere to that of the old that the
secondary sphere remains for a time attached to the interior wall of
the primary ; at what exact period, or in what precise manner, the
separation between the two takes place has not yet been determined.
At the time of the separation the developmental process has gene-
rally advanced as far as the stage repi'esented in No. 1, the foundation
of one or more tertiary spheres being usually distinguishable in the
enlargement of certain of its cells.

The development and setting-free of these composite zoosporanges,
which is essentially a process of cell-subdivision or gemtnijmrotis exten-
sion, is the ordinary mode of multiplication in Volvox, taking place
at all times of the year, except when the sexual generation (now to
be described) is in progress. The mode in which this process is
here performed (for our knowledge of which we are indebted to
the persevering investigations of Professor Cohn) shows a great
advance upon the simple conjugation of two similar cells, and
closely resembles that which prevails not only among the higher
algae, but (under some form or other) through a large part of the
cryptogamic series. As autumn advances the Volvox spheres usually
cease to multiply themselves by the formation of zoosporanges, and
certain of their ordinaiy cells begin to undergo changes by which
they are converted, some into male or ' sperm-cells,' others into
female or ' germ-cells,' the greater number, however, remaining
sterile. Each sphere of Volvox glohator (Plate VI, fig. 1) contains
both kinds of sexual cells, so that this species ranks as moncecious ;
but V. aureus is diosciotos, the sperm-cells and germ-cells occurring
in separate spheres. Both kinds of sexual cells are at first dis-
tinguishable from the ordinary sterile cells by their larger size
(fig. 2, a), in this respect resembling zoosporanges in an early
stage ; but their subsequent history is altogether dififerent. The
sper«i-cells begin to undergo subdivision when they attain about
three times the size of the sterile cells ; this, however, takes
place, not on the binary plan, but in such a manner that the
endochrome of the primary cell resolves itself into a cluster of very
peculiar secondary cells (fig. 1, a, «^, fig. 5), each consisting of an
elongated ' body ' containing an orange-coloiu^ed endochrome with a
red corpuscle, and of a long, colourless beak from the base of which
proceeds a pair of long flagella (figs. 6, 7), as in the antherozoids
of the higher ciyptogams. As the sperm-cells approach maturity,
the aggregate clusters may be seen to move within them, at first
slowly, and afterwards more rapidly ; the bundles then separate
into their component antherozoids, which show an active, indepen-
dent movement whilst still within the cavity of the primary cell
(fig. 1, a^), and finally escajDe by the giving-way of its wall (a*),
difiusing themselves through the cavity of the Volvox sphere. The

556 MICROSCOPIC FOEMS OF VEGETABLE LIFE— TKALLOPHYTES

germ-cells (fig. 1, b, b), on the other hand, continue to increase in
size without undergoing subdivision ; at first showing large vacuoles
in their protoplasm (b^, b^), but subsequently becoming filled with
dark-green endochrome. The form of the germ-cell gradually
changes from its original flask-shape to the globular (b^) ; and it
projects into the cavity of. the Volvox sphere, at the same time
acquu'ing a gelatinous envelope. Over this the swarming antherozoids
diffuse themselves (fig. 3), penetrating its substance, so as to find
their way to the interior ; and in this situation they seem to dissolve
away, so as to become incorporated with the oosphere. The product
of this fusion (which is only conjugation under another form) is a
reproductive cell or oospore, which speedily becomes envelojDed by
an internal smooth membrane, and with a thicker external coat,
which is usually beset with conical pointed processes (fig. 4) ; and
the contained chlorophyll gives place, as in Falmoglcea, to starch
and a red or orange coloured oil. As many as forty of such oospores
have been seen by Cohn in a single sphere of Volvox, which thus
acquires the peculiar appearance that has been distinguished by
Ehrenberg by a different specific nam.e, Volvox stellatus. Soon
after the oospores reach maturity, the parent sphere breaks up,
and the oospores fall to the bottom, where they remain during
the winter. Their further history has since been traced out
by Kirchner, who found that their germination commenced in
February with the liberation of the spherical endospore from its
envelope, and with its division into four cells by the formation
of. two partitions at right angles to each other. These partially
separate, holding together only at one end, which becomes one pole
of the globular cluster subsequently formed by cell-multiplication,
the other pole only closing in when a large number of cells have
been formed. The cells are then carried apart from one another by
the hyaline investment formed by each, and the characteristic Volvox
sphere is thus completed.'^

Another phenomenon of a very remarkable nature, namely, the
conversion of the contents of an ordinary vegetable cell into a free
moving mass of protoplasm that bears a strong resemblance to the
animal Amoeba, has been afiirmed by Dr. Hicks ^ to take place in
Volvox, under circumstances that leave no reasonable ground for that
doubt of its reality which has been raised in regard to the accounts
of similar phenomena occurring elsewhere. The endochrome-mass of
one of the ordinary cells increases to nearly double its usual size ;
but, instead of undergoing binary subdivision so as to produce a
zoosporange, it loses its colour and its regularity of form, and

1 The doctrine of the vegetable nature of Volvox, which had been suggested by
Siebold, Braun, and other Grerman naturalists, was first distinctly enunciated by
Prof. Williamson, on the basis of the history of its development, in the Transactions
of the PJiilosojyhical Society of Mancliester, vol. ix.

[The most recent and detailed accounts of the development of the various forms
of Volvox are by Klein (Prmgsheim's Jahrbiicher fur tvissenschaftlicJie Botanilc,
vol. XX. 1889, p. 133) and Overton (Botanisches Gentralhlatt, vol. xxxix. 1889), which
do not differ in any material point from the description given in the text. See also
Bennett and Murray's Handbook of Cryjitogamic Botany, p. 292. — Ed.]

2 Trans, of Micros'c. Society, n.s. vol. viii. 1860, p. 99 ; and Quart. Journ. of
Microsc. Science, n.s vol. ii. 1862, p. 96.

VOLVOCINE.-E ; PALMELLACE^ 557

becomes an irregular mass of coloiirlsss protoplasm, containing a
number of brown or reddish-brown gi'aiiules, and capable of altering
its form by protruding or reti'acting any portion of its membranous
wall, exactly like a true Amoeba. By this self-moving power, each
of these bodies (of which twenty may sometimes be counted within
a single Volvox) glides independently over the inner surface of the
sphere among its unchanged green cells, bending itself round any
one of these with wdiich it may come into contact, precisely after
the manner of an Amoeba. After the 'amoeboid' has begun to
travel, it is always noticed that for every such moving body in the
Volvox there is the empty space of a missing cell ; and this confirms
the belief — founded on obsei-vation of the gradational transition
fi'om the one condition to the other, and on the diiSculty of sup-
posing that any such bodies could have entered the sphere parasiti-
cally from without — ^that the ' amoeboid ' is i-eally the product of the
metamorphosis of a mass of vegetable protoplasm. This meta-
morphosis may take place, accoixling to Dr. Hicks, even after the
process of binary subdivision has commenced. What is the sub-
sequent destination of these amoeboid bodies has not yet been ascer-
tained.'

In other organisms allied to Volvox, and included in the family
Volvocinem, we find a very interesting and instructive transition
between the various modes of multiplication already described. In
Eudorina, a common organism in still water, a sexual process similar
to that in Volvox has been observed. In Pandorina morum the
generative process is performed, according to the observations of
Pringsheim, in a manner curiously intermediate between the lower
and the higher types referred to above. For within each cell of the
original sixteen of which its mulberry-like mass is composed, a brood
of sixteen secondary cells is formed by ordinary binary subdivision ;
and these, when set free by the dissolution of their containing cell-
wall, swim forth as ' swarm-spores,' each being furnished with a
pair of flagella. Among the crowd of these swarm-spores may be
observed some which approach in pairs, as if seeking one another ;
when they meet, their points at first come together, but gradually
their whole bodies coalesce, and a globular zygospore is thus formed
which gei-minates after a period of rest, reproducing by binary
subdivision the original sixteen-celled, mulberry-like Pandorina.
We have here, therefore, a true process of conjugation between
motile protoplasm masses, each of which is in itself indistinguish-
able from a zoospore. A similar process takes place also in
Conferva, Ulothrix, Hydrodictyon, and a number of fresh-water algje
(fig. 422).

Included by many writers under the general term PalmellaceaB
are a number of minute organisms of very simple structure, the
relationship of which to the Protococcacece is not yet fully known.
They all grow either on damp surfaces or in fresh water ; and they
may either form (1) a mere powdery layer, of which the component

1 A similar production of ' amoeboids ' has been observed by Mr. Archer in
Stephanosj)licBra 2>lt(vialis, and is scarcely now to be considered an exceptional
phenomenon.

558 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

particles have little or no adhesion to each other ; or they may pre-
sent themselves (2) in the condition of an indefinite slimy film, or (3)
in that of a tolerably firm and definitely bounded membranous
' frond.' The first of these states we have seen to be characteristic
of Palmoglcea and Protococcas ; the new cells which are originated
by the process of binary subdivision usually separating from each
other after a short time, and, even where they remain in cohesion,
not forming a ' frond ' or membranous expansion. The ' red snow,'
which sometimes colours extensive tracts in Arctic or Alpine regions,
penetrating even to the depth of several feet, and vegetating actively
at a temperattire which reduces most plants to a state of torpor, is
generally considered to be a species of Protococcus ; but as its cells
are connected by a tolerably firm gelatinous investment, it would
rather seem to be a Palmella. The second is the condition of Pal-
mella proper, of which one species, P. cruenta, usually known under
the name of ' gory dew,' is common on damp walls and in shady
places, sometimes extending itself over a
considerable area as a tough gelatinous
mass, of the colour and general appearance
of coagulated blood. A characteristic
illustration of it is also ajSbrded by the
Hcematococcus sang^iineus (fig. 423), which
chiefly differs from Palmella in the partial
persistence of the walls of the parent-cells,
so that the whole mass is subdivided by
partitions, which enclose a larger or smaller
number of cells originating in the sub-
division of their contents. Besides in-
creasing in the ordinary mode of binary
multiplication, the Palmella cells seem
occasionally to rupture and difiuse their
granular contents through the gelatinous
stratum, and thus to give origin to a whole
cluster at once, as seen at e, after the
manner of other simple plants to be pre-
sently described, save that these minute
segments of the endochrome, having no
power of spontaneous motion, cannot be ranked as zoospores.
The gelatinous masses of the Palmella are frequently found to con-
tain parasitic growths formed by the extension of other plants
through their substance ; but numerous branched filaments some-
times present themselves, which, being traceable into absolute
continuity with the cells, must be considered as properly appertaining
to them. Sometimes these filaments radiate in various directions from
a single central cell, and must at first be considered as mere exten-
sions of this ; their extremities dilate, however, into new cells ; and,
when these are fully formed, the tubular connections close up, and the
cells become detached from each other. ^ Of the third condition we

1 This fact, first made public by Mr. Thwaites (Ann. of Nat. Hist. 2nd series,
vol. ii. 1848, p. 313), is one of fundamental importance in the determination of the
real character of this group.

Fig. 422. — A, conjugating
microzoospores of Ulo-
thrix ; B, megazoospore
of TJlothrix, from Vines's
' Physiology of Plants.'

PALMELLACE.IE ; ULVACE^

559

have an example in the curious Pcdmodictyon described by Kiitzing,
the frond of which appears to the naked eye like a delicate network,
consisting of anastomosing branches, each composed of a single or
double row of large vesicles, within every one of which is produced
a pair of elliptical cellules that iiltimately escape as zoospores. The
alternation between the motile form and the still or resting form,
which has been described as occurring in Pi-otococcus, has been ob-
served in several other forms of this group ; and it seems obviously
inteiaded, like the production of zoospores, to secure the dispersion
of the plant and to prevent it from choking itself by overgrowth in
any one locality. It is very commonly by plants of this group that
the algal portions of lichens are formed.^

Notwithstanding the very definite form and large size attained
by the fronds or leafy expansions of the Ulvacese, to which group

Fig. 423. — Hcematococcus sanguineus, in various stages of development; a, single
cells, enclosed in their mucous envelope ; &, c, cluster formed by subdivision of
the parent-cell ; d, more numerous cluster, its component cells in various stages of
division ; e, large mass of young cells, formed by the subdivision of the parent
endochrome, and enclosed within a common mucous envelope.

belong some of the most common grass-green seaweeds (' laver ')
found on every coast, yet their essential structure differs but very
little from that of the preceding group ; and the piincipal advance
is shown in this, that the cells, when multiplied by binary sub-
division, not only remain in firm connection with each other, but
possess a very regular arrangement (in virtue of the determinate
plan on which the subdivision takes place), and form a definite mem-
branous expansion. The mode in which this frond is produced may
be best understood by studying the history of its development, some
of the principal phases of which are seen in fig. 424. The isolated
cells A, in which it originates, resembling in all points those of a

- [The Palmellacece are not now regarded by the best authorities as a distinct
family from the Protococcacece, and the genus Hcematococcus is sunk in Proto-
coccus. — Ed.I

56o MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

Protococcus, give rise, by their successive subdivisions in determinate
directions, to sucli regular clusters as those seen at B and C, or to
such confervoid filaments as that shown at D. A continuation of
the same regular mode of subdivision, taking place alternately in
two directions, may at once extend the clusters B and C into leaf-
like expansions ; or, if the filamentous stage be passed through
(difierent species presenting variations in the history of their develop-
ment), the filament increases in breadth as well as in length (as seen
at E), and finally becomes such a ' frond ' as is shown at F, G. In
the simple memlDranous expansion or thallus thus formed, there is

but little approach to a
difierentiation of parts in
the formation of root, stem,
and leaf, such as the higher
algje present ; every portion
is the exact counterpart of
every other, and every
portion seems to take an
equal share in the opera-
tions of growth and repro-
duction. Each cell is very
commonly found to exhibit
an imperfect partitioning
into four parts preparatory
to multiplication by double
bipartition, and the entire
frond usually shows the
groups of cells arranged in
clusters containing some
multiple of four.

Besides this continuous
increase of the individual
frond, however, we find, in
most species of Ulva, a
provision for extending the
plant by the dispersion of
zoospores. The endochrome
(fig. 425, a) svibdivides into
numerous segments (as at
h and c), which at first are
seen to lie in close contact
within the cell that contains them, then begin to exhibit a kind
of restless motion, and at last escape by the bursting of the
cell-wall, and swim freely through the water as zoospores {d) by
means of their flagella, each zoospore having become endowed
with either two or four flagella during its formation within its
mother-cell. At last, however, they come to rest, attach them-
selves to some fixed point, and begin to grow into clusters or
filaments (e) in the mannei- already described. The walls of the
cells which have thvls discharged their endochrome i-emain as
colourless spots on the frond ; sometimes these are intermingled with

sis,* «••» *'5Si» sy.ss "iJ!

/fiwSS"''''''' ' ' '

'•IJWgf '

I llSHlll 'W^l

!i«SiiiiiiaB B. _

KdS <in»««ii««a«»i«"«^

Vii

.onil'

Ss»»^

«K»>5

Hi**

Fig. 424. — Successive stages of development
of TJlva.

ULVACE^

.561

the poi'tions still vegetating in the usual mode ; but sometimes the
whole endochrome of one portion of the frond may thus escape in
the foi^m of zoospores, leaving behind it nothing but a white flaccid
membrane. If the microscopist who meets with a frond of an Ulva
in this condition examines the line of separation between its green
and its coloured portions, he may not improbably meet with cells in
the very act of discharging their zoospores, which ' swarm ' around
their points of exit very much in the manner that animalcules are
often seen to do around particular spots of the field of view, and
which might easily be taken for true Infusoria ; but on carrying his
observations further, he would see that similar bodies are moving
zoithin cells a little more remote from the dividing line, and that a

Fig. 425. — Formation of zoospores in Viva latissima : a, portion of the ordinary-
frond ; b, cells in which the endochrome is beginning to break up into segments ;
c, cells from the boundary between the coloured and colourless portions, some of
them containing zoospores, others being empty ; d, flagellate zoospores, as in active
motion ; e, subsequent development of the zoospores.

little farther still they are obviously but masses of endochrome in
the act of subdivision.^

More recent observation has brought out the interesting fact
that in ZTlva and its allies there are two kinds of swarm-spore, a larger
kind, ' megazoospores,' with four, and a smaller kind, ' microzoospores,'
with two cilia each (see fig. 422). Of these the megazoospores
germinate directly, as above described, while the microzoospores or
'zoogametes' have been observed to conjugate in pairs, producing
zygospores, by the germination of which a new generation is
produced. The two kinds of zoospore may be produced on the same
or on different individuals.

' Such an observation the Author had the good fortune to make in the year 1842,
when the emission of zoospores from the Ulvacem, although it had been described
lay the Swedish algologist Agardh, had not been seen (he believes) by any British
naturalist.

O O

562 MICEOSCOPIC FOKMS OF VEGETABLE LIFE— THALLOPHYTES'-

Although many of the plants belonging to the family Siphonacese-
attain a considerable size, and resemble the higher seaweeds in their
general mode of growth, yet they retain a simplicity of structure so
extreme as to require them to be ranked among the simpler thallo-

phytes. They are inhabitants
both of fresh water and of the
sea, and consist of very large^
tubular cells, which often ex-
tend themselves into branches,
so as to form an arborescent
frond. These branches, how-
ever, are not separated from
the stem by any intervening
partition, except those parts
where the generative organs
are produced ; but the whole
frond is composed of a simple
continuous tube, the entire
contents of which may be
readily pressed out through an
orifice made by w^ounding any
part of the wall. The genus
Vaucheria may be selected as a
particvilarly good illustration of
this family, its history having
been pretty completely made
out. Most of its species are
inhabitants of fresh water, but
some are marine ; and they
commonly present themselves
in the form of cushion-like
masses, composed of irregularly
branching filaments, which, al-
though they remain distinct,,
are densely tufted together and
variously interwoven. Some-
species form dense green mats
on damp soil in flower-pots, &c.
The formation of motile gonids
or zoosjjores may be readily
observed in these plants, the
whole process usually occupying
but a very short time. The
extremity of one of the filaments
usually swells up in the form of
a club, and the endochrome
accumulates in it so as to give it a darker hue than the rest ;
a separation of this part from the remainder of the filament,
by the interposition of a transparent space, is next seen ; a new
envelope is then formed around the mass thus cut ofi"; and at last-
the membranous wall of the investing tube gives way, and the zoo-

Fig. 426. — Successive phases of generative
process in Vaucheria sessilis : at A are
seen one of the 'horns ' or antherids (a)
and one of the oogones (&), as yet un-
opened ; at B the antherid is seen in
the act of emitting the antherozoids (c),
of which many enter the opening at the
apex of the oogone, whilst others (d)
which do not enter it display their cilia
until they become motionless ; at C the
orifice of the oogone is closed again by
the formation of a cellulose coat around
the oosphere, thus constituting an oospore.

SIPHONACE^ 563

spore escapes, not, liowevei-, until it has undergone marked changes
of form, and exhibited curious movements. Its motions continvxe
for some time after its escape, and are then plainly seen to be due
to the action of the cilia, which form a complete fringe round it.
If it be placed in water in which some carmine or indigo has been
rubbed, the coloured granules are seen to be driven in such a
manner as to show that a powerful current is produced by their
propulsive action, and a long track is left behind it. When it
meets with an obstacle, the ciliary action not being arrested, the
zoospore is flattened against the object ; and it may thus be com-
pressed, even to the extent of causing its endochrome to be dis-
charged. The cilia ai-e best seen when their movements have been
retarded or entirely arrested by meaiis of opium, iodine, or other
chemical reagents. The motion of the spore continues for about
two hours ; bht after the lapse of that time it soon comes to an
end, and the spore begins to develop itself into a new jDlant. It has
been observed by linger that the escape of the zoospores generally
takes place towards 8 a.m. ; to watch this phenomenon, therefore,
the plant should be gathered the day before, and its tufts examined
early in the morning. The same filament may give off two or three
zoospores successively.

In addition to this mode, there exists also in this humble
plant a true process of sexual generation. The branching filaments
are often seen to bear at their sides peculiar globular or oval
capsular protuberances, sometimes separated by the interposition of
a stalk, which are filled with dark endochrome ; and from these,
after a time, new plants arise. In the neighbourhood of these
bodies are found, in most species, cei-tain other projections, which,
from being usually pointed and somewhat curved, have been named
' horns ' (fig. 426, A, a) ; and these have been shown by Pringsheim
to be antherids, which produce antherozoids in their interior ; whilst
the capsule-like bodies (A, h) are oogones or archegones, each con-
taining a mass of endochrome which constitutes an oosphere that is
destined to become, when fertilised, the original cell of a new
generation. The antherozoids (B, c, cZ), when set free from the
antherid «, swarm about the oogone 6, and, attracted by a drop of
mucilage formed at the mouth of the oogone, enter it, one or more
antherozoids becoming absorbed into the substance of the obsphere.
This hitherto naked mass of protoplasm now becomes invested by
an envelope of cellulose (C, &), which increases in thickness and
strength, until it has acquired such a density as enables it to afford
a firm protection to its contents. Wliile in Vaucheria the separate
filaments are so slender as to be scarcely discernible to the naked
eye, the frond of other genera of Siphonacete, mostly natives of
shallow seas in the warmer parts of the globe, attains very large
dimensions. Thus in Godium it is a spongy spherical or cylindrical
floating mass, as much as a foot in length ; in Caiderpa it has the
appearance of a branched leaf springing from a stem, which puts
out roots from its under side ; in Acetahtdaria it takes a mushroom-
like form with a cap or ' jDileus,' a quarter of an inch in diameter,
divided into regvilar chambers, at the summit of a cylindrical stalk,

o o 2

564 MICROSCOPIC FORMS OF VEGETABLE LIFE—THALLOPHYTES

1^ to 3 inches in height. Munier-Oharles ^ believes that many
fossils generally regarded as Foraminifera are in reality the
calcareous skeleton of algse belonging or nearly allied to the
Siphonacese.

The microscopist who wishes to study the development of zoo-
spores, as well as several other phenomena of this low type of vege-
tation, may advantageously have recourse to the little plant termed
Achlya prolifera,^ which grows parasitically upon the bodies of dead
flies lying in water. Its tufts are distinguishable by the naked eye
as clusters of minute colourless filaments ; and these are found,

when examined by the
microscope, to be long
tubes, devoid of all parti-
tions, extending them-
selves in various direc-
tions. The tubes contain
a colourless slightly gra-
nular protoplasm, the
particles of which are
seen to move slowly in
streams along the walls,
as in Chara, the currents
occasionally anastomosing
with each other (fig. 427,
C). Within about thirty-
six hovirs after the first
appearance of the parasite
on any body, the proto-
plasm begins to accumu-
late in the dilated ends
of the filaments, each of
which is then cut oflT from
the remainder by the
formation of a partition ;
and within this dilated
cell the movement of the
protoplasm continues for
a time to be distinguish-
able. Very speedily, how-
ever, its endoplasm shows
the appearance of being broken up into a large number of distinct
masses, which are at first in close contact with each other and
with the walls of the cell (fig. 427, A), but which gradually
become more isolated, each seeming to acquire a proper cell-wall ;
they then begin to move about within the parent-cell ; and, when

1 Comptes Renclus, vol. Ixxx. 1877, p. 814.

2 [This plant, though, as an inhabitant of water, formerly ranked among Algce, is
now generally regarded as belonging to the group of Fungi, on account of its
incapacity for the production of chlorophyll, and its parasitism on the bodies of
animals, from whose juices its cells seem to draw their nourishment. It is very
closely allied to Saprolegnia (see p. 640), a fungus paras-itie on the bodies of living
fish, and causing the very destructive disease to which salmon are liable. — Ed.]

f'l \P^

I'iG. 427. — Development of Achlyaprolifera : A,
dilated extremity of a filament b, separated
from the rest by a partition a, and containing
zoospores in progress of formation ; B, end of
filament after the cell-wall has burst, and
setting free zoospores, a, b, c; C, XDortion of
filament, showing the course of the circulation
of granular protoplasm.

ACHLYA; HYDEODICTYON 565

quite mature, tliey ai-e set free by the ruj)ture of its wall (B), and,
after swarming abovit for a time, develop into tubiform cells resem-
bling those from which they sprang. Each of these zoospores is
possessed of two fiagella ; their movements are not so powerful as
those of the zoospores of Vaucheria, and come to an end sooner.
The generative process in this type is performed in a manner that
may be regarded as an advance upon ordinary conjugation. The
end of one of the long tubiform cells enlarges into a globular dilata-
tion, the cavity of which becomes shut oflf by a transverse partition.
Its contained endoplasm divides into two, three, or four segments,
each of which takes a globular form, aiid is then fertilised by the
penetration of an antheridial tube which comes oflF from the filament
a little below the partition. The oospores thus produced, escaping-
from the globular cavities, acquire firm envelopes, and may remain
unchanged for a long time even in water, when no a|3propriate nidus
exists for them ; but will quickly germinate if a dead insect or other
suitable object be thrown in.

One of the most curious forms of the lower algse is the ' water-
net,' Hydrodictyon reticulatum, which is fovmd in fresh-water pools
in the midland and southern counties of England. Its frond con-
sists of a green open network of filaments, acquiring, when full
gTOwn, a length of from four to six inches, and composed of a vast
number of cylindrical tubular- cells, which attain the length of four
lines or more, and adhere to each other by their rounded extremi-
ties, the points of junction corresponding to the knots or intersections
of the network. Each of these cells may form within itself an
enormous multitude (from 7,000 to 20,000) of zoospores, which at a
certain stage of their development are observed in active motion in
its interior, but come to rest in the course of about half an hour,
and then arrange themselves in such a way that by their elongation
they again form a net of the original kind, which is set free by the
dissokition of the wall of the mother-cell, and attains in the course
of three or four weeks the size of the mother-colony. Besides these
bodies, however, certain cells produce from 30,000 to 100,000
' microzoospores ' of longer shape, each furnished with four long-
fiagella and a red ' eye-spot ; ' these escape from the cell in a swarm,
and move freely in the water for some time. Conjugation between
these smaller zoospores has been observed to take place sometimes
even with the mother-cell. The resulting body or ' zygospore '
retains its green colour, but becomes invested with a firm cell-wall
of cellulose. In this condition these bodies may remain dormant
for a considerable time, and are described as ' hypnospores ' or
' resting-spores ; ' and in this state they are able to endure being
completely dried up without the loss of their vitality, provided that
they are secluded from the action of light, which causes them to
wither and die. In this state they bear a strong resemblance to the
cells of ProtococciLs . The first change that manifests itself in them,
when they begin to germinate, is a simple enlargement ; next the
endochrome divides itself successively into distinct masses, usually
two or four in number ; and these, when set free by the giving wa,y
of the enveloping niembrane, present the characters of ordinary

566 MICEOSCOPIC FOEMS OF VEGETABLE LIFE— THALLOPHYTES

zoospores, each of them possessing two flagella at its anterior semi-
transparent extremity. Their motile condition, however, does not
last long, often giving place to the motionless stage before they have
quite freed themseh'es from the parent-cell ; they then project long
angular processes, so as to assume the form of irregular polyhedra,
at the same time augmenting in size ; and the endochrome contained
within each of these breaks up into a multitude of zoospores, which
are at first quite independent and move actively within the cell-
cavity, but soon unite into a netwoi-k that becomes invested with
a gelatinoiis envelope, and speedily increases so much in size as to
rupture the containing cell-wall, on escaping from which it presents
all the essential characters of a young Hydrodictyon. The rapidity
of the growth of this curious organism is not one of the least
remarkable parts of its history. The individual cells of which the
net is composed, at the time of their emission as zoospores, measure

Fig. 428.— Various phases of development of Pediastrum granulatum.

no more than gsVoth of an inch in length ; but in the course of a
few hours they grow to a length of from jL-th to ^rd of an inch.

The members of the family Pediastrese were formerly included
in the Besmidiacece ; but, though doubtless related to them in
certain particulars, they present too many points of difference to be
properly associated with them. Their chief point of resemblance
consists in the firmness of the outer covering, and in the frequent
interruption of its margin either by the protrusion of ' horns ' (fig.
428, A), or by a notching more or less deep (fig. 429, B) ; but they
differ in these two important particulars — that the cells are not
made up of two symmetrical halves, and that they are always found
in aggregation, which is not, except in such genera as Scenedesmits
which connect this group with the Desmids, in linear series, but in
the form of discoidal fronds. In this tribe we meet with a form of
multiplication by motile ' megazoospores ' which reminds us of the
formation of the motile spheres of Volvox, and which takes place in

PEDIASTEEJ5 5^7

such a manner that the resultant pi-oduct may vary greatly in the
number of its cells, and consequently both in size and in form.
Thus in Fediastrum granidatum (fig. 428) the zoospores formed by
the subdivision of the endochrome of one cell, which may be four,
■eight, sixteen, thirty-two, or sixty-four in number, escape from the
parent -frond still enclosed in the inner layer of the cell- wall ; and it
is within this that they develop themselves into a cluster resembling
that in which they originated, so that the frond may be composed of
•either of the just-mentioned multiples or sub-multiples of 16. At
A is seen an old disc, of irregular shape, nearly emjjtied by the
emission of its zoospores, which had been seen to take place within
a few hours previously from the cells a, b, c, d, e ; most of the
empty cells exhibit the cross slit through which their contents had
been discharged ; and where this does not present itself on the side
next the observer, it is found on the other. Three of the cells still
possess their coloured contents, but in difierent conditions. One of
them exhibits an early stage of the subdivision of the endochrome —
namely, into two halves, one of which already appears halved again.
Two others are filled by sixteen very closely crowded zoospores, only
half of which are visible, as they form a double layer. Besides
these, one cell is in the very act of discharging its zoospores, nine of
which have passed forth from its cavity, though still enveloped in a
vesicle formed by the extension of its innermost membrane ; whilst
seven yet remain in its interior. The new-born family, as it
appears immediately on its complete emission, is shown at B ; the
zoospores are actively moving within the vesicle, and they do not as
yet show any indication either of symmetrical arrangement or of
the peculiar form which they are subsequently to assume. Within
a quarter of an hour, however, the zoospores are observed to settle
down into one plane, and to assume some kind of regular arrange-
ment, most commonly that seen at C, in which there is a single
central body surrounded by a circle of five, and this again by a
circle of ten ; they do not, however, as yet adhere firmly together.
The zoospores now begin to develop themselves into new cells,
increase in size, and come into closer approximation (D) ; and the
edge of each, especially in the marginal row, presents a notch which
foreshadows the production of its characteristic ' horns.' Within
about four or five hours after the escape of the zoospores, the cluster
has come to assume much more of the distinctive aspect of the
species, the marginal cells having grown out into horns (E) ; still,
however, they are not very closely connected with each other, and
between the cells of the inner row considerable spaces yet intervene.
It is in the course of the second day that the cells become closely
applied to each other, and that the growth of the horns is completed,
so as to constitute a perfect disc like that seen at F, in Avhich, how-
ever, the arrangement of the interior cells does not follow the
typical plan.^ The formation of ' microzoospores ' has also been
observed, which have been seen to conjugate.

1 See Prof. Braun on The Phenomenon of Bejuvenescence in Nature, published
by the Ray Society in 1858 ; and its subsequent mexnoiv, Algarum TJnicellularum,
•Genera 7iova aut tninus cognita, 1855.

568 MICEOSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

The varieties which present themselves, indeed, both as to the
number of cells in each cluster and the plan on which they are dis-
posed, are such as to baffle all attempts to base specific distinctions
on such grounds ; and the more attentively the life-history of any
one of these plants is studied, the more evident does it appear that
many reputed ' species ' have no real existence. Some of these,,
indeed, are nothing else than mere transitory forms ; thus it can be
scarcely doubted that the specimen represented in fig. 429, D, under
the name of Pediastrum pertusuon, is in reality nothing else than a
young frond of P. granulatum in the stage represented in fig. 428, E,
but consisting of thirty-two cells. On the other hand, in fig. 429, E,
we see an emptied frond of P. granulatum^ exliibiting the peculiar
surface-marking from which the name of the species is derived, but
composed of no raore than eight cells. And instances every now
and then occur in which the frond consists of only four cells, each of

Fig. 429. — Various species (?) of Pediastrum: A, P. tetras; B, C, P. Elirenlergii]
D, P. ]pertusum; E, empty frond of P. granulatum.

them presenting the two-horned shape. So, again, in fig. 429, B and
C, are shown two varieties of Pediastrum Ehrenhergii, whose frond
is normally composed of sixteen cells ; whilst at A is figured a form
which is designated as P. tetras, but which may be strongly suspected
to be merely a four-celled variety of B and C. Many similar cases
might be cited ; and the Author would strongly urge those micro-
scopists who have the requisite time and opportunities, to apply
themselves to the determination of the real species of these groiips
by stiidying the entire life-history of whatever forms may happen to
lie within their reach, and noting all the varieties which present them-
selves among the ofisets from any one stock. The characters of such
varieties are diffused by the process of binary subdivision amongst
vast multitudes of so-called individuals. Thus it happens that, as
Mr. Balfs has remarked, ' one pool may abound with individuals of
iStaurastrimi dejectitm or Arthrodesiiius incus having the mucro

PEDIASTEE.T2 ; CONFEEVACE^

569

curved outwards ; in a neighbouring pool evei-y specimen may have
it curved inwards ; and in another it may be straight. The cause
of the similarity in each pool no doubt is that all its plants are off-
sets fi'om a few primary fronds.' Hence the universality of any
particular character in all the specimens of one gathering is by no
means sufficient to entitle these to take rank as a distinct species ;
since they are, pi'operly speaking, but repetitions of the same variety
by a process of simple multiplication, really representing in their
entire aggregate the one plant or tree that grows from a single seed.
Almost every pond and ditch contains some members of the
family Confervacese ; but they are especially abundant in moving
water, and they constitute the greater ^

part of those green threads which ., '■'

are to be seen attached to stones, im. . ,-°^^°J' I I'^'l^ """^'1
with their free ends floating in the , H^^^#'/
direction of the current, in every
running stream, and upon almost

every part of the sea-shore, and ip-jVa I II Kb 5 "^S

which are commonly known under llifv-:°l I \§W>^'^^M^

the name of ' silk-weeds,' or ' crow-
silk.' Their form is usually very
regular, each thread being a long-
cylinder made up by the union of a
single filament of short cylindrical
cells united to each other by their
flattened extremities ; sometimes
these threads give off" lateral
branches, which have the same
structure. The endochrome, though
usually green, is occasionally of a
brown or purple hue, and is usually
distribvited unifoi-mly throughout the
cell (as in fig. 430). The plants of
this family are extremely favourable
subjects for the study of the method
of cell-multiplication by binary sub-
division. This process iisually, but
not always, takes place only in the
terminal cell ; and it may be almost
always observed there in some one of
its stages. The first step is seen to be
endochrome, and the inflexion of the

(fig. 430 A, a) ; and thus there is gradually formed a sort of
hour-glass contraction across the cavity of the parent-cell, by
which it is divided into two equal halves (B). The two surfaces
of the infolded utricle produce a doable layer of cellulose mem-
brane between them. (Sometimes, however, as in Cladojjhora
glomerata (a common species), new cells may originate as branches
from any part of the surface by a process of budding, which,
notwithstanding its difi'erence of mode, agrees with that just
described in its essential character, being the result of the sub-

FiG. 430. — Process of cell-multipli-
cation in Clado])liora glomerata:
A, j)ortion of filament with incom-
plete separation at a, and coinplete
partition at & ; B, the separation
completed, a new cellulose parti-
tion,being formed at a. ; C, forma-
tion of additional layers of cellulose
wall, c, beneath the mucous in-
vestment, d, and around the
ectoiDlasin, a, which encloses the
endochrome, h.

the subdivision of the
ectoplasm around it

570 MICROSCOPIC FOEMS OF VEGETABLE LIFE -THALLOPHYTES

division of the original cell. A certain portion of the ectoplasm
seems to undergo increased nutrition, for it is seen to project,
carrying the cellulose envelope before it, so as to form a little
protuberance, and this sometimes attains a considei-able length
before any separation of its cavity from that of the cell which gave
origin to it begins to take place. This separation is gradually
effected, however, by the infolding of the ectoplasm, just as in the
preceding case ; and thus the endochrome of the branch cell becomes
completely severed from that of the stock. The branch then begins
to elongate itself by the subdivision of its first-formed cell ; and this
process may be repeated for a time in all the cells of the filament,
though it usually comes to be restricted at last to the terminal cell.
The very elongated cells of some species of Confervacese are
characterised by the possession of a large number of nuclei. They
are raviltiplied by zoospores, produced apparently indifferently from
any cell of a filament, by free-cell formation. These zoospores are
of two kinds, larger or smaller ; the larger kind have either two or
four cilia, and germinate directly ; the smaller are biciliated, and
conjugation between them has been observed.

Nearly allied to the Confervacefe is a very interesting plant in
which a true sexual mode of reproduction has been observed, Sphaero-
plea annulina, the development and generation of which have been
specially studied by Dr. F. Cohn.^ The oospore, which is the pro-
duct of the sexual process to be presently described, is filled when
matu.re with a red oil, and is enveloped by two membranes, of which
the outer one is furnished with stellate prolongations (fig. 431, No. 1).
When it begins to vegetate, its endochrome breaks up — first into
two halves (No. 2), and then, by successive subdivisions, into numerous
segments (Nos. 3, 4), at the same time becoming green towards its
margin. These segments, set free by the rupture of their containing
envelope, escape in the form of motile zoospores, which are at first
rounded or oval, each having a semi-transparent beak whence proceed
two cilia ; but they gradtially elongate so as to become fusiform
(No. 5), at the same time changing their colour from red to green.
These move actively for a time, and then, losing their motile power,
begin to develop themselves into filaments. The first stage in this
development consists in the elongation of the cell, and the separation
of the endochrome of its two halves by the interposition of a vacuole
(No. 6), and in more advanced stages (Nos. 7, 8) a rejDetition of the
like interposition gives to the endochrome that annular arrange-
ment from which the plant derives its specific name. This is seen
at No. 9, a, as it presents itself in the filaments of the adult plant ;
whilst at &, in the same figure, we see a sort of frothy appearance
which the endochrome comes to possess through the multiplication
of the vacuoles. The next stage in the development of the filaments
that are to produce the ocispheres consists in the aggregation of the
endochrome into definite masses (as seen at No. 10, a), which soon
become star-shaped (as seen at b), each one being contained within a
distinct compartment of the cell. In a somewhat more advanced
stage (as seen at No. 11, a), the masses of endochrome begin to draw
1 Ann. des Sci. Nat. 4eme ser., Bot., torn. v. 1856, p. 187.

SPH^EOPLEA ANNULINA

571

themselves together again ; and they soon assume a globular or
ovoidal shape (b), whilst at the same time definite openings (c) are
formed in their containing cell-wall. Through these openings the
antherozoids developed within other cells gain admission, as
shown at No. 12, d; and they become absorbed into the before-men-

Fig. 431. — Development and rei^roduction of Sphceroijlea.

tioned masses, which soon afterwards become invested with a fiLrm
membranous envelope, as shown in the lower part of ]S"o. 12. These
undergo further changes whilst still contained within their tubular
parent-cells, their colour passing from green to red ; and a second
investment is formed within the fii-st, which extends itself into
stellate prolongations, as seen in No. 13; so that when set free

572 MICEOSCOPIC FOEMS OF VEaETABLE LIFE-THALLOPHYTES

they precisely resemble the mature oospores which we have taken as
the starting-point in this curious history. Certain of the cells (as
in No. 14), instead of giving origin to oospores, have their annular
collections of endochrome converted into antherozoids, which, as
soon as they have disengaged themselves from the mucilaginous
sheath that envelopes them, move about rapidly in the cavity of their
containing cell {a, h) around the large vacuoles which occupy its
interior, and then make their escape through apertures (c, d) which
form themselves in its wall, to find their way through similar aper-
tures into the interior of the oogones, as already described. These
antherozoids are shown in No. 15, as they appear when swimming
actively through the water by means of the two cilia which each
possesses. The peculiar interest of this history consists in the entire
absence of any special organs for the generative process, the ordinary
filamentous cell developing oospheres on the one hand and anthero-
zoids on the other, and in the simplicity of the means by which the
fecundating process is accomplished.

The OEdogoniaceae resemble Confervacece in general aspect and
habit of life, but dijBfer from them in some curious particulars. As
the component cells of the filaments extend themselves longitudi-
nally, new rings of cellulose are formed successively, and are inter-
calated into the cell-wall at its upper end, giving it a ringed appear-
ance. Only a single large zoospore is set free from each cell ; and
its liberation is accomplished by the almost complete fission of the
wall of the cell through one of these rings, a small part only remain-
ing uncleft, which serves as a kind of hinge whereby the two parts
of the filament are prevented from being altogether separated.
Sometimes the zoospore does not completely extricate itself from
the parent-cell ; and it may begin to grow in this situation, the
root-like processes which it puts forth being extended into the
cavity. The zoospores are the largest known in any class of algae ;
each has a nucleus, a red ' eye-spot,' and an anterior hyaline spot to
which is attached a tuft of cilia visible even before its escape from
its mother-cell.

In their generative process, also, the (Edogoniaceoe, show a curious
departure from the ordinary type ; for whilst the oospheres are
formed within certain dilated cells of the ordinary filament (fig. 432,
A, No. 1), which may be termed oogones, and are fertilised by the
penetration of antherozoids (No. 2), these antherozoids are not, in all
the species, the immediate product of the sperm-cells of the same or
of another filament, biit are developed within a body termed an
androsjyore (No. 5), which is set free from within a special cell (No.
4), and which, being furnished with a terminal tuft of cilia, and having
motile powers, very strongly resembles an ordinary zoospore. This
androspore, after its period of activity has come to an end, attaches
itself to the outer surface of an oogone, or of a cell in close proxi-
mity to an oogone, as shown at No. 1, 6 ; it then developes into a
very small male plant, known as a dtoarf-male, consisting of two or
three cells ; the terminal of these cells is an antherid, from the apex
of which a sort of lid drops, as seen in the upper part of No. 1, by
which its contained antherozoids (No. 2) are set free ; and at the

CEDOGONIACE^ ; CHiETOPHORACE>E

.573

same time an apei'tiu-e is foi-med in the wall of the oogone by
which the antherozoirl enters its cavity and fei-tilises its oosphei-e by
becoming absorbed into it. This mass then becomes an oospore (ISTo. 3),
invested with a thick wall of its own, but still retains more or less
of the envelojje derived from the cell within which it was developed.
The oiiices of these different classes of reproductive bodies are only
now beginning to be understood, and the inquiry is one so fraught
with physiological interest, and, fi-om the facility of growing these
plants in aquaria, can be so easily pursued, that it may be hoped

Fig. 432.^A, Sexual generation of CEdogoniuin ciliatimi : 1, filament with two
oogones in process of formation, the lower one having two androspores attached to
its exterior, the contents of the upper oogone in the act of being fertilised by the
entrance of an antherozoid set free from the interior of its androspore ; 2, free
antherozoids ; 3, mature oospore, still invested with the cell-membrane of the
parent-filament ; 4, portions of a filament bearing special cells, from one of which
an androspore is being set free ; 5, liberated androspore.

B, Branches of ChcBtojjhora elegans, in the act of discharging ciliated zoospores,
which are seen as in motion on the right.

that the zeal of microscopists will not long leave any part of it in
obscurity.

The Chsetophoraceae constitute a beautiful and interesting little
gi'oup of confervoid plants, of which some species inhabit the sea,
whilst others ai-e found in fresh and pui-e watei- — rathei- in that of
gently moving streams, however, than in strongly flowing currents.
Generally speaking, their filaments put foi-th lateral branches, and
extend themselves into arborescent fi-onds ; one of the distinc-
tive characters of the group is afibrded by the fact that the
exti-emities of these branches are usually prolonged into bristle-

574 MICEOSCOPIC FORMS OF VEGETABLE LIFE -THALLOPHYTES

shaped processes (fig. 432, B). As in many preceding cases, these
plants multiply themselves by the conversion of the endochrome of
certain of their cells into zoospores, and these, when set free, are
seen to be furnished with either two or four cilia. ' Resting-
spores' have also been seen in ,many species. One of the most
beautiful objects under the microscope is Draparnaldia glomerata,
not uncommon in still water. It consists of an axis composed of a
single row of large transparent cells containing but a small quantity
of chlorophyll. From this proceed at regiilar intervals whorls of
slender branches, the endochrome of which is deep green, and every
branch ends in a delicate hyaKne hair of extraordinary length. The
mode of reproduction of the GhcBtophoracecG closely resembles that of
the GonfervacecB.

The Batrachospermese, whose name is indicative of the strong
resemblance which their beaded filaments bear to frog-spawn, are
now ranked as humble fresh- water forms of a far higher, chiefly
marine, group of algse, the Rhodosjjermece, or red sea-weeds. But
they deserve special notice here on account of the simplicity of their
structure, and the extreme beauty of the objects they afford to the
microscopist (fig. 433). They are chiefly found in water which is
pure and gently flowing. ' They are so extremely flexible,' says Dr.
Hassall, ' that they obey the slightest motion of the fluid which
surrounds them ; and nothing can surpass the ease and grace of
their movements. When removed from the water they lose all
form, and appear like pieces of jelly, without trace of organisation •
on immersion, however, the branches quickly resume their former
disposition.' Their colour is for the most part of a brownish green,
but sometimes they are of a reddish or bluish purple. The central
axis of each plant is at first composed of a single filament of large
cylindrical cells laid end to end ; but this is subsequently invested
by other cells, in the manner to be presently described. It bears
at pretty regular intervals whorls of short radiating branches, each
of which is composed of rounded cells, arranged in a bead-like row
and sometimes subdividing again into two, or themselves giving off
lateral branches. Each of the primary branches originates in a little
protuberance from the primitive cell of the central axis, precisely
after the manner of the lateral cells of Gladophora glomerata ; as this
protuberance increases in size, its cavity is cut off by a septum, so
as to render it an independent cell ; and by the continual repetition
of the process of binary subdivision this single cell becomes con-
verted into a beaded filament. Certain of these branches, however,
instead of radiating from the main axis, grow downwards ^ipon it,
so as to form a closely fitting investment that seems properly to
belong to it. Some of the radiating branches grow out into long
transparent bristles, like those of the Ghmtojohoracece ; and within
those are produced antherozoids, which, though not endowed with
the power of spontaneous movement, find their way to the oospheres
contained in other parts of the filaments ; and by the fertilisation
of the contents of these are produced the somewhat complicated
fructifications known as cystocarps, placed in the axils of the
branches (fig. 433).

BATRACHOSPEEME.E ; COLEOCH.ETACExE ; CHARACE^ 575

A very singular relationship, called by some writers an ' alter-
nation of generations,' exists between Batrachosjjermum and Ghan-
trcmsia, a genus of fresh-water alga? previously placed in a totally
different section. This relationship was first described by Sirodot,"^
and his observations have since been confirmed by others. The
germinating spores of BatrachosjJermum put out, under certain
conditions, a kind of filament, known as a protone'ine^ which develops
into a Chantransia., a non-sexual form of BatracJios'permimi, which
can reproduce itself from generation to generation by simple
budding, or by means of non-sexual spores, without producing
sexual organs. Chantransia is especially found in water where very
little light reaches it. When more exposed to light it vmdergoes
metamorphosis, and then a branch springs up from the protoneme
which is in every respect a Batracliospermum^ bearing true sexual
organs, as above described.
This may tlien go on rej)ro-
ducing itself, or revert to the
Chantransia form.

The Coleochsetaceae are a
small order of fresh - water
Algse, chiefiy represented by
the genus Coleochcete, which
forms minute discs or cushions
attached to submerged plants,
from xo to J inch in diameter,
consisting, in the simplest
forms, of a single layer of cells,
often arranged in rays proceed-
ing from a common centre.
Reproduction takes place non-
sexually, by means of zoospores,
or sexually, by the fertilisation
of an oogone by motile anthero-
zoids, through the agency of a
peculiar tube known as a trichogyne, a forecast of the more com-
plicated process which we shall presently meet with in the Floridese
or Rhodospermeae, the highest class of Alga?.

Among the highest of the Alga? in regard to the complexity of
their generative apparatus, which contrasts strongly with the general
simplicity of their structure, is the family of Characese,^ some
members of which have received a large amount of attention from
microscopists on account of the interesting phenomena they exhibit.
These plants are for the most part inhabitants of fresh waters,
and are found rather in such as are still than in those which
are in motion ; a few species, however, may be met with in
ditches whose waters are rendered salt by communication with the
sea. They may be easily grown for the purposes of observation in

1 Sirodot, Les Batrachospermees, fo. 1884.

- [Many of the best authorities regard the Characece, in consequence of their
mode of reproduction, as a group of primary character, of equal rank with the Algse,
and superior to them in organisation. — Ed.]

Fig. 433.
Ba tra cli osjjerm um moniliforme.

576 MICROSCOPIC FORMS OF VEGETABLE LIFE-THALLOPHYTES

large glass jars exposed to the light, all that is necessary being to
pour off the water occasionally from the upper part of the vessel
(thus carrying away a film that is apt to form on its surface), and
to replace this by fresh water. Each plant is composed of an
assemblage of long tubiform cells placed end to end, with a distinct
central axis, around which the branches are disposed at intervals
with great regularity (fig. 434, A). In Nitdla the stem and
branches are composed of simple cells, which sometimes attain
the length of several inches ; whilst in most species of Char a each
central tube is surrounded by an envelope of smaller ones, which is
formed as in Batrachosj^ermum, save that the investing cells grow
upwards as well as downwards from each node, and meet each other
on the stem halfway between the nodes, their ends dovetailing
into one another. These investing tubes constitute what is termed
the ' cortex ' of Chara. They are of smaller diameter than the central
tube, and are arranged spirally round it, giving the stem a twisted
appearance. Each ' node,' or zone from which the branches spring,
consists of a single jalate or layer of small cells, w^hich, in Chara, are
a continuation of the cortical layer of the ' internode.' The branches
are altogether similar in structure to the primary axis, and
terminate in a large elongated pointed cell, which is not covered by
the cortex. From the lower part of the stem ' rhizoids ' or rooting
filaments are put out, which attach the plant to the soil. Some
species have the power of secreting carbonate of lime from the
water in which they grow, if this be at all impregnated with
calcareous matter ; and by the deposition of it beneath their tegu-
ment they have gained their popular name of ' stoneworts.' The
long tubiform cells of Nitella, and the terminal uncorticated cells of
the branches of Chara, afford a very beautiful and instructive display
of the phenomenon of cyclosis, or rotation of protoplasm in their
interior. Each cell, in the healthy state, is lined by a layer of
chlorophyll grains, which cover every part, except two longitudinal
lines that remain nearly colourless (fig. 434, B) ; and a constant
stream of semi-fluid protoplasm, containing starch grains and
chlorophyll granules, is seen to flow over the green layer, the
current passing up one side, changing its direction at the extremity,
and flowing down the other side, the ascending and descending
spaces being bounded by the transparent lines just mentioned. In
the young cells the rotation may be seen before this granular
lining is formed. The rate of the movement is affected by anything
that influences the vital activity of the plant ; thus it is accelerated
by moderate warmth, whilst it is retarded by cold ; and it may be
at once checked by a slight electric discharge through the plant.
Carried along by the protoplasmic stream are a number of solid
particles, which consist of starchy matter, and are of various sizes,
being sometimes very small and of definite figure, whilst in other
instances they are seen as large irregular masses, which appear to
be formed by the aggregation of the smaller pai'ticles. The produc-
tion of new cells for the extension of the stem or bi-anches, or for
the origination of new whorls, is not here accomplished by the
subdivision of the parent-cell, but takes place by the method of out-

CHAEACE.^"^

577

growth (fig. 434, B, e^f^ (/, A), which, as ah-eady shown, is nothing
but a modification of the usual process of cell-inultiplication ; in
this manner the extension of the individual plant is efiected with
considerable rapidity. When these plants are well supplied with
nutriment, and ai-e actively vegetating under the influence of light,
warmth, etc., they not unfrequently develop ' bulbils,' which are
little clusters of cells, filled with starch, that sprout from the sides
of the central axis, and then, falling oflf, evolve the long tubiform
cells characteristic of the plant from which they were produced.
There are also se^'eral other non-sexual ways in which these plants

Pig. 434. — Nitella flexilis : A, Stem and branches of the natural size : a,b, c, d, our
whorls of branches issuing from the stem; e, f, subdivision of the branches.
B, Portion of the stem and branches enlarged : a, b, joints of stem ; .c, d, whorls ;
e, f, new cells, sprouting from the sides of the branches ; g, h, new cells sprouting
at the extremities of the branches.

ai'e reproduced, but they are peculiar among cryptogams in not
pi-oducing true spores, either stationary or motile. The Oharacew
may be multiplied by artificial subdivision, the separated parts
continuing to grow under favourable circumstances, and gradually
developing themselves into the typical form.

The genei-ative apparatus of Characece consists of two sets of
bodies, both of which grow at the bases of the branches (fig. 435,
A, B), either on the same or on different individuals ; one set,
formerly known as ' globules,' are really antherids ; whilst the
otliei-, known as ' nucules,' contain the oosjiheres, and are true
ocigones or archegones. The globules, which are nearly sphei-ical,

p p

578 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

and often of a bright red colour, have an envelope made up of eight
triangular plates or ' shields ' (B, C), often curiously marked, which
encloses a central portion of a light reddish colour ; this central
poi-tion is principally composed of a mass of filaments rolled tip
compactly together. From the centre of the inner face of each
shield a cylindrical cell termed a manubrium projects inwards nearly
to the centre of the sphere. The antherid is supported on a short

Fig. 435. — Generative organs of Ch.ara fragilis: A, antherid or globule develoiDed
at the base of archegone or nucule ; B, nucule enlarged, and globule laid open by
the separation of its valves ; C, one of the valves, with its group of antheridial
filaments each composed of a linear series of cells, vcithin every one of which an
antlierozoid is formed ; in D, E, and F tlie successive stages of this formation are
seen ; and at G is shown the escax^e of the mature antherozoids, H.

flask-shaped pedicel, which also projects into the interior. At the
apex of each of the eight manubria is a i-oundish hyaline cell, called
a capitulum, and at the apex of each capitulum six smallei' cells or
' secondary ca^jitula.' From the centre of each of these secondary
capitula. grow four long whip-shaped filaments (C), constituting the
mass already refei-red to. The number of these filaments in each
antherid is about 200, and each of these filaments divides by

CHAEACE.1<: ; DESMIDIACE.^-: 579

ti-ansverse septa into from 100 to 200 small disc-shaped cells, wliicli
number, therefore, from 20,000 to 40,000 in each antherid. In
every one of these cells there is formed, by a, gradual change in its
contents (the successive stages of which are seen at D, E, F), an
anthei'ozoid, a sj)iral thread of pi-otoplasm consisting of two or three
coils, which, at first motionless, after a time begins to move and
revolve within the cell, and at last the cell-wall gives way, and the
spiral thread makes its escape (G), partially straightens itself, and
moves actively through the watei- for some time (H) in a, tolerably
determinate direction, by the lashing action of two long and very
delicate cilia with which it is furnished. The exterioi- of the nucule
(A, B) is formed by five or ten spii-ally twisted tubes that give it a
very jjeculiar aspect ; and these enclose a centi-al sac containing
protoplasm, oil, and stai'ch grains. Each of these tubes consists, in
its lower part, of a very long unsegmented cell ; while at its uppei-
part two small cells are segmented off; and these small cells of all
the tubes form together the ' crown ' of the nucule. When ready
for fertilisation the branches of the crown part slightly, forming an
open passage or ' neck ' down to the central germ-cell or oosphere ;
and through this canal the antherozoids make their way down to
perform the act of fertilisation by becoming absorbed into the
substance of the oosphere. Ultimately the nucule, which has now
become a hard black body, falls off, and the fei'tilised germ-cell, or
oospoi-e, gives origin to a new plant after the nucule has i-emained
dormant through the winter.^

Among those simple Alga? whose generative pi-ocess consists in
the ' conjugation ' of two similar cells, there are two gi'oups of such
peculiar interest to the microscopist as to need a special notice ;
these are the Desmidiacece and the Diatomacece. Both of them
wei-e i-anked l^y Ehrenberg and some other naturalists as animal-
cules ; but the fuller knowledge of their life-history and the more
extended acquaintance with the pai-allel histories of other simple
forms of vegetation which have been gained during the last twenty
years, are now genei-ally accepted as decisive of their vegetable
nature.

The Desmidiacese - are minute plants of a bright gi'een colour
growing in fi-esh water ; generally speaking, the cells are inde-
pendent of each other (figs. 436-439) ; but sometimes those which

' A full account of the Characece will be found in Prof. Sachs's Text-Book of
Botany, 2nd English edition, p. 292. Various observers liave asserted that particles
of the protoplasmic contents of the cells of the Characece, when set free by the
rupture of their cells, may continue to live, move, and grow as independent rhizopods.
But tlie writer is disposed to think that the phenomena thus represented are rather
to be regarded as cases of parasitisnr, the decaying cells of Nitella havmg been found
by Cienkowski {Beitrage ziir Kenntniss der Monaden,m Arch. f. Mihr.Anat. Bd. i.
1865, p. 203) to be inhabited by minute, sx^indle-shaped, ciliated bodies, which seem
to correspond with the ' sx^ores ' of the Mijxoinycetes, going through an amoeboid stage,
and then producing a plasmode which, after undergoing a sort of encysting process,
finally breaks uj) into spindle-shaped particles resembling those found in the Nitella
cells.

- Our first accurate knowledge of this group dates from the publication of Mr.
Ralfs's admirable monograpli of the British Desmids in 1848. Later information in
regard to it will be found in the section contributed by Mr. W. Archer to the fourth
edition of Pritchard's Infusoria, and in Cooke's British Desmids, 1887.

p p 2

58o MICROSCOPIC FORMS OF VEGETABLE LIFE — THALLOPHYTES

have been produced by binaiy subdivision from a single parent-
cell remain adherent one to another in linear series, so as to form a
filament (fig. 440 ; Plate IX, fig. 3). They are distinguished by two
peculiar features, one of these being the semblance of a division
of each cell into two spnmetrical halves by a ' sutural Ime,' which
is sometimes so decided as to have led to the belief that the cell
is really double (Plate VIII, figs. 2, 6), though in other cases
it is merely indicated by a slight notch ; the other featvu^e is the
frequency of projections from the surface, which are sometimes
short and inconspicuous, but are often elongated into spines
(Plate YIII, fig. 6), presenting a very symmetrical arrangement.
These projections are generally formed hj the cellulose envelope
alone, which possesses an almost horny consistence, so as to retain
its form after the discharge of its contents (fig. 436, B, D) ; while, in
other instances, they are formed by a notching of the margin of
the cell (Plate IX, fig. 1), which may affect only the outer casmg, or
may extend into the cell-cavity. The outer coat is surrounded by a
very transparent sheet of gelatinous substance, which is sometimes
very distinct (as shown in fig. 440 ; Plate IX, fig. 6) ; but in
other cases its existence is only indicated by its preventing the con-
tact of the cells. Klebs states ^ that in Desmids, as in the other
Oonjugatce, this mucilaginous sheath is composed of two portions — a
homogeneous substance which is but slightly refringent, and a por-
tion which consists of minute rods at right angles to the cell-wall.
He regards the sheath as entirely independent of the substance of
the cell-wall, and as derived from the protoplasmic contents of the
ceil by diffusion through the cell- wall. The true cell- wall encloses
a parietal utricle, which is not always closely adherent to it ; and
this immediately surrounds the endochrome, which occupies nearly
the whole interioi' of the cell, and in certain stages of its growth is
found to contain starch granules. The endochrome and starch
grains are arranged symmetrically in the two halves of the cell,
often in very beavitiful patterns, such as bands or stars.

Many species of desmids have a power of slow movement in the
water, the cause of which is not obvious, these organisms being
entirely destitute of vibratile cilia. Klebs ^ describes this movement
as being of four kinds, viz. : — (1) a forward movement on the
surface, one end of the cell touching the bottom, while the other end
is more or less elevated, and oscillates backwards and forwards ;
(2) an elevation in a direction vertical to the substratum, the fi'ee
end making wide circular movements ; (3) a circular motion,
followed by an alternate sinking of the free end and elevation of
the other end ; and (4) an obliique elev^ation so that both ends
touch the bottom, lateral movements in this position, then an ele-
vation and circular motion of one end, and a: sinking again to an
oblique or horizontal position. Klebs regards all these movements
as due to an exudation of mucilage, and the first two to the forma-
tion during the motions of a filament of mucilage l\y which the
desmid is temporarily attached to the bottom, and which gradually

1 Untersuchiingcn aiis devi Bot. Inst. Tilhingen, 1886, p 33;-!.
-' Biologlsches 'Ccvtralblutt, 1885, p. 353.

PLATE IX,

Desmidiaceae.

"West.Newman chromo.

DESMIDIACEJ3

58i

lengthens. The movements of desmids are especially active when
they are in the process of dividing. Stahl found that, like the move-
ments of zoospores, they are affected by light, and always move
towards the light.

A ' cyclosis ' may be readily observed in many Desmidiacew,
and is pai-ticulai'ly obvious along the convex and concave edges of
the cell of any vigoi-ous specimen of Glosteriiim, with a magnifying
power of 250 or 300 diameters (fig. 436, A, B). By careful focus"
sing the flow may be seen in broad streams over the whole surface
of the endochrome ; and these streams detach and cai-ry with them
from time to time, little oval or globular bodies (A, h) which ai'e put
forth from it, and ai'e cairied by the coui'se of the flow to the trans-
parent spaces at the extremities, where they join a crowd of similai-
bodies. In each of these spaces (B) a protoplasmic flow proceeds
from the somewhat abrupt termination of the endochrome towards
the obtuse end of the cell (as indicated by the interior arrows).

^^.

Fig. 486. — Cyclosis in Closterium lunula : A, cell showing central separation at a,
in which the large particles, 5, are not seen ; B, one extremity enlarged, showing
the movement of particles in the colourless space ; D, cell in a state of division.

and the globules it contains ai-e kept in a sort of twisting movement
on the inner side [a) of the parietal utiicle. Other currents are
seen apparently external to it, which form three or four distinct
courses of pai'ticles, passing towards and away from c (as indicated
by the outer aii-ows). Anothei- curious movement is often to be
witnessed in the interior of the cells of members of this family,
which has been described as ' the swai-ming of the granules,' fi'om
the extraordinary I'esemblance which the mass of particles in active
vibratory motion beai'S to a swarm of bees. It is especially
observable in the hyaline terminal portions of the cells of species of
C'losterittm, as shown in fig. 436, B. This motion continues for
some time after the particles have been expelled by pressure fi-om
the interior of the cell ; and it appeal's to be an active form of the
molecular movement common to other minute particles freely sus-
pended in fluid. This movement of minute particles affords an
instance of the phenomenon known as ' Brownian movement,' and
is probably of a purely mechanical nature.

582 MICEOSCOPIC FORMS OF VEGETABLE LIFE — THALLOPHYTES

"When the single cell has come to its full maturity it commonly
i[ni\lti]Aieii itse\ihj binmy sabdivision; but the plan on which this
takes place is often peculiarly modified, so as to maintain the
symmetry characteristic of the tribe. In a cell of the simple
cylindrical form, of those of Desmidium (fig. 440), little more is
necessary than the separation of the two halves at the sutural line,
and the formation of a partition between them^ by the infolding of
the primordial utricle ; in this manner, out of the lowest cell of the
filament A, a double cell, B, is produced. But it will be observed
that each of the simple cells has a bifid wart-like projection of the
cellulose wall on either side, and that the half of this projection,
which has been appropriated by each of the two new cells, is itself
becoming bifid, though not sjTiimetrically ; in jDrocess of time, how-
ever, the increased development of the sides of the cells which re-
main in contiguity with each other biings up the smaller projections
to the dimensions of the larger, and the symmetry of the cells is
restored. In Closterium (fig. 436 ; Plate IX, fig. 2) the two halves of
the endochrome first retreat from one another at the sutural line, and
a consti'iction takes place round the cellulose wall ; this constriction
deepens until it becomes an hourglass-like contraction, which pro-
ceeds until the cellulose wall entirely closes round the primordial
ntricle of the two segments ; in this state one half commonl}' remains
passive, whilst the other has a motion from side to side, which
gradually becomes more active ; and at last one segment quits the
other with a sort of jerk. At this time a constriction is seen across
the middle of the primordial utricle of each segment, indicating the
formation of the sutural band ; but there is no division of the cell-
cavity, which is that belonging to one of the halves of the original
entire cell. The cyclosis, for some hours previously to subdivision,
and for a few hours afterwards, runs quite round the obtuse end, a,
of the endochrome ; but gTadually a transparent space is formed,
like that at the opposite extremity, by the retreat of the coloured
layer ; whilst at the same time its obtnse form becomes changed to
a more elongated and contracted shape. Thus, in five or six hours
after the separation, the aspect of each extremity becomes the same,
and each half resembles the cell by the division of which it
originated.

The process is seen to be performed after nearly the same method
in Staurastrum, the division taking place across the central con-
striction, and each half gradually acquiring the symmetry of the
original. In snch forms as Gosmariiim, however, in which the cell
consists of two lobes united together by a narrow isthmus, the divi-
sion takes place aftei- a difierent method ; for when the two halves
of the outei' wall separate at the sutural line, a semi-globular protru-
sion of the endochrome is put forth from each half; these proti-u-
sions are separated from each other and from the two halves of the
original cell (which their interposition carries apart) by a narrow
neck ; and they progxessively increase until they assume the appear-
ance of the half-segments of the oiiginal cell. In this state, there-
fore, the plant consists of a i-ow of four segments lying end to end,
the t^\'o old ones forming the exti-emes, and the two new ones (which

DESMIDIACEJ5 5^3

lid not usually acquii-e the full size oi' the characteristic markings of
the original before the division occurs) occupying the intermediate
place. At last the central fission becomes complete, and two bi-
partite fi'onds are formed, each having one old and one young seg-
ment ; the young segment, however, soon acquires the full size and
characteristic aspect of the old one ; and the same process, the
whole of which may take place within twenty-four houi-s, is repeated
ere long. The same general plan is followed in 2ficrasterias ; but
as the small hyaline hemisphei-e, put foi-tli in the first instance from
each half-cell (fig. 437, A), enlarges with the flowing in of the endo-

^J^)^^^

iQ.f/

A-

'm

V.

Pig. 437. — Successive stages of binary subdivision of Micrasterias deiiticulata.

chrome, it undergoes progxessive subdivision at its edges, first into
three lobes (B), then into five (C), then into seven (D). then into
thirteen (E), and finally at the time of its sepai'ation (F) acquires
the characteristic notched outline of its type, being only distinguish-
able from the oldei- half by its smaller size. The whole of this
process may take place within three hours and a half. In
Sphcerozosma the cells thus produced remain connected in rows
within a gelatinous sheath, like those of Desmidiimi (fig. 440) ;
and different stages of the process may commonly be observed in
the different parts of any one of the filaments thus formed. In any

584 MICROSCOPIC FORMS OF VEGETABLE LIFE — THALLOPHYTES

such filament it is obvious that the two oldest segments ai^e found at
its opposite extremities, and that each subdivision of the inter-
mediate cells must carry them farther and farther from each other.
This is a very different mode of increase from that of the Gonfervacecu,
in which commonly the terminal cell alone undergoes subdivision,
and is consequently the one last formed.

The sexual generative process in the Desmidiacece, which occurs
but rarely compared with that of binary division, always consists of
an act of ' conjugation.' It commences with the dehiscence of the
firm external envelope of each of the conjugating cells, so as to
separate it into two valves (fig. 438, C, D; fig. 439, C). The
contents of each cell thus set free without any distinct investment
blend with those of the othei- ; and a zygosjoore is forraed by their
vmion, which soon acquires a truly cellulose envelope.^ This enve-
lope is at first very delicate, and
is filled with green and gi^anular
contents ; by degrees the envelope
acquires increased thickness, and
its contents become bi-own or red.
Ultimately the envelope becomes
differentiated into three layers, of
which the innermost and outer-
most are colourless, while the
middle one is firmer and brown.
The outer surface is sometimes
smooth, as in Closterium and its *
allies (fig. 439 ; Plate IX, fig. 8) ;
but in Cosmarium it becomes
gi-anular, tuberculated, or spinous
(fig. 438, D; Plate VIII, figs. 1,
4), the spines being sometimes
simple and sometimes forked at
their extremities. The mode in
which conjugation takes place in
the filamentous species constitut-
ing the Desniidiece proper is, how-
The filaments first separate into
their component joints, and when two cells approach in conjugation,
the outer cell-wall of each splits or gapes at that part which adjoins
the other cell, and a new growth takes place which forms a sort of con-
necting-tube that unites the cavities of the two cells (fig. 440, D, E).
Through this tube the entire endochrome of one cell passes over
into the cavity of the other (D) ; and the two are commingled so as
to form a single mass (E), as is the case in many of the Conjtigatce,
The joint which contains the zygospore can scarcely be distinguished
at first (after the separation of the empty cell), save by the greater
density of its contents ; but the proper coats of the zygospore
gradually become more distinct, and the enveloping cell-wall disap-
pears.

1 In certain species of Closterium, as in many of the DicdomacecB, the act of
conjugation gives origin to tivo zygospores.

Fig. 438. — Conjugation of Cosmarium
hotrytis : A, mature cell ; B, empty
cell-envelope ; C, transverse view ;
D, zygospore with empty cell enve-
lopes.

ever, in many respects different.

DESMTDIACE.^

585

The subsequent history of the zygospore has been followed out
in the case of Cosmarium botri/tis. After remaining at vest for a
considei-able time, it germinates by the l^ursting of the two outei-
coats, the protoplasmic contents escaping while still enclosed in the
innermost coat. In this body the pi-otoplasm and endochrome are
already divided into two halves, which contract somewhat, and the
whole becomes enveloped in a new cell-wall. A constriction has,
in the meantime, made its appearance between the two halves,
which are of somewhat unequal size, and thus the new desmid is
formed.

The subdivision of this family into genei'a, according to the
method of Mr. Ralfs (' British Desmidie^e '), as modified by Mr.
Archer (Pritchard's ' Infusoria '), is based in the first instance upon
the connection or disconnection of the individual cells, two groups
being thus formed, of which one includes all the geneiu whose cells,
when, multiplied by binary division, remain united into an elongated
filament ; whilst the

other and much larger -'^

one comprehends all those
in which the cells become
separated by the comple-
tion of the fission. The
further division of the
filamentous group, in
which the zygospores are
always globular and
smooth (Plate IX, fig. 8),
is based on the fact that
in one set of genera the
joints are many times
longer than they are
broad, and that they are
neither constricted nor
furnished with lateral
teeth or projections ;
whilst in the other set

(fig. 440; Plate IX, fig. 3) the length and breadth of each
joint are nearly equal, and the joints are more or less con-
stricted, or have lateral teeth or projecting angles, or some other
figure ; and it is for the most part upon the variations in these last
particulars that the generic characters are based. The solitary
group presents a similar basis foi- primary division in the marked
difference in the proportions of its cells, such elongated forms as
Closterium (figs. 436, 439 ; Plate IX, fig. 2), in which the length is
many times the breadth, being thus separated from those in which,
as in Micrasterias (fig. 437 ; Plate IX, fig. 1), Cosmarium (fig. 438 ;
Plate YIII, fig. 2), and Staurastrum (Plate VIII ; figs. 5, 6, 10),
the breadth more nearly equals the length. In the former the
zygospores are smooth, whilst in the latter they are very commonly
spinous (Plate VIII, figs. 1, 4) and are sometimes quadrate. In this
group the chief secondary characters are derived from the degree of

Fig. 439. — Conjugation of Closterium striolatuvi :
A, ordinary cell ; B, empty cell ; C, two cells in
conjugation, witli zygospore.

5 86 MICKOSCOPIC FOEMS OF YEaETABLE LIFE — THALL0PHYTE8

constriction between the two halves of the cell, the division of its
margin into segments by incisions more or less deep, and its exten-
sion into teeth or spines.

The Desmidiacece are not found in running streams, unless the
motion of the water be very slow, but are to be looked for chiefly
in standing waters. Small shallow pools that do not dry up in
summer, especially in open, exposed situations, such as boggy
moors, are most productive. The larger and heavier species
commonly lie at the bottom of the pools, either spread out as a

thin gelatinous stratum,
or collected into finger-
like tufts. By gently
passing the fingers be-
neath these they may be
caused to rise towards the
surface of the water, and
may then be lifted out by
a tin box or scoop. Other
species form a slimy
stratum floating on the
surface of bog-pools, or a
greenish or dirty cloud
upon the stems and leaves
of other aquatic plants ;
and these also are best
detached by passing the
hand beneath them, and
' stripping ' the plant be-
tween the fingers, so as to
carry off upon them what
adhei-ed to it. If, on the
other hand, the bodies of
which we are in seai'ch
should be much diflfused
through the water, there
is no other coui-se than to
take it up in large quanti-
ties by the box or scoop
and to separate them by
straining through a piece
of linen. At first, nothing
appears on the linen but
a mere stain oi- a little dii-t ; but by tne straining of repeated
(quantities a considerable accumulation may be gradually made.
This should then be scraped oft' with a knife, and transferred
into bottles with fresh water. If what has been brought up
by hand be richty charged with these forms, it should be at
once deposited in a bottle ; this at first seems only to contain
foul water ; but by allowing it to remain undisturbed for a
little time, the desmids will sink to the bottom, and most of
the water may then be poui'ed off*, to be rejDlaced by a fresh

Fig. 440. — Binary subdivision and conjugation of
Desmidium cijliiidriciim : A, portion of filament,
surrounded bj- gelatinous envelope : B, dividing
cell ; C, single cell viewed transversely ; D, tvsro
cells in conjugation ; E, formation of zygospore.

DESMIDIACE.T-: : DIATOMACE^ 587

supply. If the bottles be freely exposed to solar liglit, these little
plants will flourish, apparently as well as in theii- native pools ; and
their vai'ious phases of multijjlication and reproduction may be
obsei'ved during successive months or even yeai-s. If the pools be
too deep for the use of the hand and the scoop, a collecting- bottle
attached to a stick may be employed in its stead. The i-ing-net
may also be advantageously emjoloyed, esjiecially if it be so con-
structed as to allow of the read}' substitution of one piece of muslin
for another. For, by using several j)ieces of previously wetted
muslin in succession, a large number of these minute oi'ganisms
may be separated from the water ; the pieces of muslin may be
brought home folded up in wide-mouthed bottles, either separately
or several in one, according as the organisms are obtained fi-om one
or from several waters ; and they are then to be opened out in jars
of filtered river water and exposed to the light, when the desmids
will detach themselves.

The Diatomaceae or Bacillariacese, like the Desmidiacete, are
simple cells, having a firm external coating, within which is included
an endochrome whose superficial layer constitutes a ' parietal
utricle,' but their external coat is consolidated by silex, the pre-
sence of which is one of the most distinctive characters of the
group, and gives rise to the peculiar surface-markings of its members.
It has been thought by some that the solidifying mineial forms a
distinct layer exuded from the exterior of the cellulose wall ; but
there seems good reason for regarding that wall as itself inter-
penetrated by the silex, since a membrane beaiing the characteristic
surface-markings is found to i-emain after its removal by hydro-
fluoric acid. The endochrome of diatoms consists, as in other
plants, of a viscid protojalasm, in which float the granules of
coloimng matter. In the ordinary condition of the cell these
granules are diffiised through it with tolerable uniformity, except
in the central spot, which is occupied by a mccletcs ; round this
nucleus they commonly foi-m a ling, from which i-adiating lines of
granules may be seen to diverge into the cell-cavity. Instead of
being bright green, howevei-, the endochrome is a yellowish brown.
The principal colouring substance ap^^ears to be a modification of
ordinary chloi-ophyll ; it takes a green or greenish-blue tint with
sulphuric acid, and often assumes this hue in drying ; but with it is
combined in greater or less proportion a yellow colouring matter
termed diafomin, which is very unstable in the light and fades in
drying. At certain times, oil-globules are observable in the
protoplasm ; these seem to represent the starch-granules of the
Desmidiacece and the oil-globules of othei- protophytes. A distinct
movement of the granular particles of the endochrome, closely
resembling the cyclosis of the Besmidiacece, has been noticed by
Professor W. Smith in some of the larger species of Diatomacece,
such as Surirella biseriata, Nitzschia scalaris, and Gavipylodiscus
spiralis, and by Professor Max )Schultze in Coscinodiscus, Biddtclphia,
and Rhizosolenia ; but this movement has not the regularity so
remarkable in the preceding grou^D.

The name of the class is derived fi'om the ease with which the

588 MICROSCOPIC FORMS OF VEGETABLE LIFE — THALLOPHYTES

parts separate fi-om each othei-. Tliis is well seen in the genus
Diatonna, formed of rectangular indivirlual frustules, whei-e the
arrangement resulting fi-om the piinciple of lateral union causes
them to develop into filaments or zigzag chains, the frustules remain-
ing perfectly distinct, and united only by a small isthmus or cushion
at the angles. A similar cohesion at the angles is seen in the allied
genus Gra'mmatO'phora (fig. 452), in Isthmia (fig. 457), and in many
other diatoms ; in Biddulphia (fig. 445) there even seems to be a
special organ of attachment at these points. In some diatoms,
however, the fi-ustules pi-oduced by successive acts of binary subdi-
vision habitually remain cohei-ent one to another, and thus are pro-
duced filaments or clusters of various shapes. Thus it is obvious
that when each frustule is a short cylindei-, an aggregation of such
cylinders, end to end, must foi-m a i-ounded filament, as in Jlelosira
(fig. 444) ; and, whatever may l^e the form of the sides of the
frustviles, if they be parallel one to the other a straight filament
will be produced, as in Achnanthes (fig. 461). But if, instead of
being parallel, the sides be somewhat inclined towards each other,
a curved band will be the result ; this may not continue entire,
but may so divide itself as to form fan-shaped expansions, as those
oi Licmophora Jlahellata (fig. 450) ; or the cohesion maybe sufiicient
to occasion the band to wind itself (as it were) round a central axis,
and thus to form, not merely a complete circle, but a spiral of several
turns, as in Meridion clrcidare (fig. 448). Many diatoms, again,
possess a stijje, or stalk-like appendage, by which aggi'egations of
frustules are attached to other plants, or to stones, pieces of wood,
&c.; and this may be a simple foot-like appendage, as in Achnanthes
longi2Jes (fig. 461), or it may be a composite plant-like structure, as
in Licniophora (fig. 450), Go')nphonema (fig. 462), and Alastogloia
(fig. 465). Little is known respecting the nature of this stipe ; it
is, however, quite flexible, and may be conceived to be an extension
of the cellulose coat, unconsolidated by silex, analogous to the
prolongations which have been seen in the Desmidiacece, and to the
filaments which sometimes connect the cells of the Palmellacece.
Some diatoms, again, have a mucous or gelatinous investment, which
may even be so substantial that their frustules lie — as it were — in a
bed of it, as in Mastogloia (figs. 465 B, 466), or may form a sort of
tubular sheath to them, as in Schizonema (fig. 464). In a large
proportion of the group, however, the frustules are always met with
entirely free^ neither remaining in the least degree coherent one to
another after the process, of binary subdivision has once been com-
pleted, nor being in any way connected, either by a stipe, or by a
gelatinous investment. This is the case, for example, with Tricera-
tiuni (fig. 442), Pleurosigma (Plate I, figs. 1, 2), Actinoci/clus,
Actinopti/chits (fig. 467), Arachnoidiscus (Plate XII), Canipi/lodiscus
(fig. A54:),Surirella (fig, 453), Coscinodiscus (Plate I, figs. 3, 4, fig. 455),
Heliopelta, and many others. The solitary discoidal forms, however,
when obtained in their living state, are commonly found cohering
to the surface of aquatic plants.

We have now to examine more minutely into the curious struc-
ture of the silicified casing which encloses every diatom-cell or

DIATOMACE.l': 589

frustule and the presence of which imparts a peculiar interest to
the group ; not merely on account of the elaborately marked pattern
which it often exhibits, but also thi-ough the pei'petuation of the
minutest details of that pattei-n in the specimens obtained from
fossilised deposits. TJiis silicified casing is usually formed of two
perfectly symmetrical valves united to one another by means of two
embracing rings which constitute the connecting zone or girdle, and
thus exactly represent a, minute box which serves for the repi'oduc-
tion of the species. This process is known as the encystment, and
is not uncommon, especially amongst the Xaviculece, frustules being
frequently found amongst them open from the separation of the two
valves, showing the two lings coveiing each othei-, as the lid of a
box may cover a portion of the box itself.

The following definitions of terms used in describing the siliceous
envelope of diatoms have been proposed by the late eminent diato-
mologist, Mr. J. Deby. The radiating lines (called by some ' costfe '
or ' canaliculi ') starting from the outer mai-gin of the valve, and
converging towards the intei-ioi- of the disc, are rays or marginal
rays. They may be simple, which is most usual ; or moniliform,
i.e. composed of a single or double row of ' beads ; ' or infundibuliform,
having the outline of a funnel with a long outlet ; the upper broad
jiortion is the ' funnel,' the slender part the ' stem.' The central
portion of the valve inside the internal tei'mination of the rays is
the area ; it may be smooth and hyaline, or it may be sti-iate, or
simply punctate or dotted, the dots forming regular lines or else
being irregularly scattered. If this area becomes reduced to a
median linear blank space, or to a simj^le elongated line, it is known
as the raphe oi- p>se'udo-raplie.

Dr. O. Miiller proposes the tei'm epitkeca for the overlapping
half-cell of the diatom, the undeiiapping half-cell being the hypo-
theca; for the girdle- bands he proposes the term ^j)/e?wYe.

In describing diatoms, the aspect in which the girdle is turned
towai'ds the observei' is known as the ' front ' oi- ' giixlle ' view ; that
in which the suiface of the valve is turned towards the obsei-ver is
the ' side ' or ' valve ' view.

It is not correct to designate the line shown in the fi-ont view
of the outer ring as the line of ' suture,' since the suture is the line
of meeting bounding two surfaces placed on the same plane. The
form resulting, however, varies widely in different diatoms ; for
sometimes each valve is hemispherical, so that the cavity is globular ;
sometimes it is a smaller segment of a sphei-e i-esembling a watch-
glass, so that the cavity is lenticulai' ; sometimes the centi-al portion
is completely flattened and the sides abi-uptly turned up, so that the
valve i-esembles the cover of a pill-box, in which case the cavity Avill
be cylindi'ical ; and these and othei- varieties may co-exist with any
modifications of the contoui- of the valves, which may be square,
triangular (fig. 442), heart-shaped (fig. 454, A) ,boat-shaped (fig. 453,
A), or very much elongated (fig. 449), and may be furnished (though
this is rare among diatoms) with projecting outgrowths (figs. 458,
459). Hence the shape pi-esented by the frustule differs completely
with the aspect undei- which it is seen. In all instances, the

590 MICROSCOPIC FOIIMS OF VEGETABLE LIFE — THALLOPHYTES

frustule is considered to present its ' fi-ont ' view when its line of
meeting is turned towards the eye, as in fig. 453, B, C ; whilst its
' side ' view is seen when the centre of either valve is directly
beneath the eye (A). Although the two valves meet along the line
of junction in those newly formed frustules which have been just
produced by binary subdivision (as shown in fig. 445, A, e), yet, as
soon as they begin to undergo any inci-ease, the valves sepai-ate from
one another ; and by the silicification of the cell-membrane thus left
exposed a pair of hoojis is formed, each of which is attached by one
edge to the adjacent valve, while the other edge is free.^ As will
be presently explained, one of the valves is always older than the
other ; and the hoop of the oldei- valve pai-tly encloses that of the
younger, just as the cover of a pill-box sui'rounds the upper part of
the box itself.^ As the newly formed cell increases in leng-th,
sepai-ating the valves fi-om one another, both hoojDS inci'ease in
bi-eadth by additions to their free edges, and the outer hoop slides
ofi" the inner one, until there is often but a very small ' overlap.'
As gi-owth and binary division are continually going on when the
frustules are in a healthy vigorous condition, it is rare to find a
specimen in which the valves are not in some degi-ee separated by
the interposition of the hoops.

The impermeability of the silicified casing seems to render neces-
sary the existence of special apertures through which the sui-i-ounding
water may come into communication with the contents of the cell.
Some have believed that they have seen such apertures along the
so-called ' line of suture ' of the disc-shaped diatoms, and at the extre-
mities only of the elongated forms. Ehrenberg, followed by Klitzing,
has intei-preted as apertui-es oi- ostioles the central and tei-minal
nodules of the NaviadeK^ Cyinhellece, and similar forms ; but this
view is moi'e generally regarded as incoi-i-ect. We have, in fact, no
positive demonstration of the existence of sj)ecial apertures communi-
cating between the outside and the inside of the cell ; and we are com-
pelled to have recourse, on this point, to hypothesis. It is, however,
certain that the diatom-cell is always composed of at least two valves
between which the possibility of such a communication must
necessarily be admitted, or at least the existence of endosmotic and
exosmotic curi-ents in the liquids. In the encysted forms we have
ascertained also the existence of an interval between the two rings,
although it may be very minute ; while Navicula has been some-
tiraes seen with the valves actually separated.

1 [This refers to those diatoms in which the process of binary subdivision is
possible ; but this, as will be seen presently, is not the case in many genera. — Ed.]

- This was long since pointed out by Dr. Wallich in his important memoir on the
' Development and Structure of the Diatom-valve' (Transact, of Microsc. Soc. n.s.
vol. viii. 1860, p. 12V)) ; but his observation seems not to have attracted the notice of
diatomists, until in 1877 he called attention to it in a more explicit manner (Monthly
Microsc. ■Toiirn. vol. xvii. p. 61). The correctness of his statement has been con-
firmed by the distinguished American diatomist, Prof. W. Hamilton Smith ; but as it
has been called in question by Mr. J. D. Cox (American Journal of Microscopy,

vol. iii. 1878, p. lOOj, who asserts that in Isthmia there are three hoops two

attached to the two valves, and the third overlapping them both at their line of
junction — the Author has himself made a very careful examination of a large series
of specimens of Isthmia and BidclidpJiia, the result of which has fully satisfied him
of the correctness of Dr. Wallich's original descri]5tion.

DIATOMACE.g^

591

The nature of the delicate mai'kings with which almost eveiy
(Uatom frustule is beset has been one of the most interesting in-
(juiries of the students of these forms since the introduction of the
homogeneous, and especially the apochi-omatic, objectives ; and it
cannot be doubted that certain peculiarities of structure have been
demonstrated which wei-e never before seen. In the present state
of the theory and practice of microscopy it would be extremely
unwise to give absolute adhesion to any present interpretation of
what is now held by some students of diatom structure of no mean
repute and of unrivalled manipulative skill to be the absolute struc-
ture of some of the larger forms.

Thus, concerning the group Coscinodiecece, representing the most
beautiful of the discoid forms of the whole group of Diatomacece, we
represent in Plate I, fig. 3, a photo-micrographic image of Goscino-
discus asteromphalus magnified 110 diameters. But in fig. 441
the areolce of this diatom are
seen under great magnification
with recent powers. It is
contended that the diatom,
although consisting of a single
siliceous membi-ane, has a
double structure, viz. coarse
and fine areolations, the latter
within the former ; and there
appears little reason to doubt
this. The coarse areolations
are for the most part circular
in outline, and the intervening
silex is thick. Inside these
areolations is an exti'emely
delicate perforated membrane,
the outer row of whose perfora-
tions are larger than the rest.
From the very delicacy of this
membrane, and its consequent
easy fracture, it is often want-
ing. In Plate I, fig. 4, we pi-esent a
object magnified 2,000 diameters.

In Isthmia nervosa, a side and front view of which are seen in
fig. 457, a similar construction is discoverable. In this diatom the
coarse areolations are very large and the silex correspondinglv thick ;
but the inner membrane is excessively thin and delicate. The 23er-
forations are large and irregular in shajDe around the margin, but
small and circular in the centre. In fig. 443 the form of areola-
tions is shown, and a broken membrane seen, with the fracture
passing through the perfoi-ations.^

IN'ot less interesting is the beautiful foi-m Aulacodiscus Kittonii ;
a photo-micrograph of this magnified 270 diameters is seen in Plate
I, fig. 5 ; while a small portion of the centre of a kindred form,

1 Note on the finer structure of certain diatoms, E. M. Nelson and G. C. Karop,
Journ. Quekett Club, vol. ii. ser. ii. p. 269.

Fig 441 — M^gnlficatlon of ' ultnnate stiuc
ture ' of Coscinodiscus asteromphalus,
from a drawing by Messrs. Nelson and
Karop {Journ. Quehett Club, vol. ii.
ser. ii. p. 269).

photo-micrograph of the same

592 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

A. Sturtii,magni&ex[ 2,000 times, is shown in fig. 6 in the same

plate.

The ' beaded ' appearance of diatom-valves is so universal in all
those which have beeii examined, that it must be regarded as
common to all diatoms, although this is not yet absolutely proved.
But, while it is admitted that the beading of the valves may be
common to all diatoms, it cannot be regarded as proved that the
siliceous envelope is composed of globular particles of silex arranged

Fig. ii-2. -Ti-iceratinm favus : A, side view ; B, front view.

in regular rows ; while the variety in the size and arrangement of
these particles shows that they are correlated with the vital pro-
cesses of the organisms, and afford characters for the discrimination
of the species. The nature of these granules, their size, and the
mode in which they are arranged have from the earlier days of micro-
scopy rendered diatoms of special value as ' test- objects.' This
appearance has led to the use, in speaking of diatoms, of the incorrect
terms ' transverse,' ' longitudinal,' or ' oblique strife,' these being in
truth simply the intervals which separate
the boundaries of the ' beads,' apertures, or
their equivalents, whatever they may ulti-
mately prove to be ; and this is clearly seen
when they are observed with objectives of
sufiicient numerical aperture and propor-
tional power. Pleurosigma angulatum is
one of the most commonly employed test
objects, and at the same time one of the most
reliable, its remarkable constancy rendering-
it especially valuable for this purpose ;
while, on the conti-ary, Amphipleura pellucida
is extremely variable, and is, as it were, the
toi-ment of microscope-makers and i-ival
diatom-i-esolvei'S, who do not take into account the variability of
this type, forgetting, in fact, that one A. pellucida maybe extremely
fine, and anothei', being in ti-uth a vai-ietal form, may be neai-ly as
coarse as Xavicula rhomboides. The new apochromatic objectives,
;ind the compensating eye-pieces, both for the eye and for projection,
consti'ucted by Zeiss, of Jena, have bi-ought about such progi-ess in
mici'ography that the image of P. anyulatum appears to some minds

Fig. 443. — Areolations in
Isthmia nervosa.

PLATE X.

Pleurosigma Angulatum.

Magnifled 4900 diams.

From a Photo-Micrograph by Dr. R. Zeiss taken with the 2 m/m. Apochromatic Objective
N. A. 1.30 and projection eyepiece 4.

DIATOMACE.^ 593

to leave no doubt as to the details of its structure. If we closely
examine the photographic image of a portion of P. angulatum, pro-
duced under a magnification of 4,300 diameters, and shown in Plate
X, taken from a photograph by Dr. Zeiss, it will, in the majority of
cases, leave perhaps little doubt that the valves are covered by tbe
beads or apertures in a decussate arrangement. We have, in the
judgment of Count Castracane, to do here with ' beads ' and not with
' cavities.' But, from the recent advances of our knowledge, this by no
means follows ; they may with high jDrobability be considered per-
forations in the silex of the fr-ustule. This is, indeed, placed almost
in the form of a demonstration by the interesting fact that Mr. C.
Haughton Gill succeeded in filling up the ' dots ' or ' pearls ' of the
Naviculce and the secondary markings of the discoid and other forms,
so as to give evidence that the filling must be deposited in cavities.
It is done by soaking clean diatoms in a solution of subnitrate of
mercury until their markings are filled with it ; then they are
immersed in sulphide of ammonium ; a double decomposition takes
place, by which black insoluble sulphide of mercury is produced, and
left in the minute cavities in which it certainly apjoears to be formed.
By observing the lines of fr-acture, which always follow the interval
between two rows of ' beads,' there will be much suggestion given to
the observer on this subject. Count Castracane, referring to Plate
X, asked, ' Would it have been possible to have seen these pearl -like
objects isolated, if, instead of beads, we had had apertures or depres-
sions 1 ' We can only reply that misinterpretation on such a subject
is so possible that it is only by employing all the aids to interpretation
which ingenuity can place within our reach, that we can ever be certain
as to our visual interpretation of these minute phenomena. On the
other hand, the areolated valves of Triceratiamfavus (fig. 442) present
a line of fracture which traverses indifierently the hexagonal areolae
and the lines in relief which connect them.

Dr. "Van Heurck has been able to employ the new lens made by
Abbe, having a numerical aperture of r63, upon his special subject,
the Drntomacece. He concludes that diatom valves consist of two
membranes or thin films and of an intermediate layer, the latter
being fierced vnth openings. The outer membrane is delicate, and
may be easily destroyed by acids, friction, and the several processes
of ' cleaning.' Wben the openings or apertures of this interior jDortion
are arranged in alternate rows they assume the hexagonal form ;
when in straight rows then the openings are square or oblong.

It is, how^ever, due to Mr. T. F. Smith, who worked at this
subject for years, to say that he long maintained this view, and
lias presented skilful photo-micrographs in support of his contention,
[n Plate I, fig. 1, we have a photograph of his, showing the inside
of a valve of F. angulatum magnified 1,750 diameters, and ex-
hibiting the ' postage-stamp ' fracture ; while in fig. 2, in the same
plate, we have the outside of P. angulatimi, showing a difierent
structure ; and Mr. Smith has abundant evidence of the existence
of what he has so long maintained.

By using the new lens of the great aperture of 1"63, Dr. Yan
Heurck has produced some i-emarkable photo-micrographs, whicti

Q Q

594 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

rather confirm these genei-al inferences than present any new data
of knowledge concerning the diatoms. By his great courtesy we
have been favoured with a phototype plate prepared by Dr. Yan
Heurck from his own photo -micrographs, and the reader will be
enabled to study these in Plate XI, of which a full description is given
in the earlier part of this treatise, giving descriptions of the plates.
He has further enhanced the plate by giving in fig. 7 a photo-micro-
graph of Nobei't's nineteenth band.

Diatoms, like other oi-ganisms already described, are reproduced
by conjiigation, and multiply by autofission or division. Repro-
duction is necessary to every organism, while multiplication by
fission belongs only to certain organic types. In the early days of
the study of diatoms, it would appear that even that distinguished
observer William Smith had at least not a clear idea of the encvst-

ing of the frustule or

individual diatom, which
implies the existence of
the two valves and of
the double girdle or
zone or connecting ring
pi'ojecting from each
valve in a direction at
right angles to its plane.
Hence, instead of find-
ing, as a result of fission,
a progressive diminu-
tion of the diameter of
the frustules, Mr. Smith
speaks of their increase,
of which he is unable to
ofi'er any explanation.
The fact that in Melosira
suhflexilis (fig. 444, A)
and M. varians (fig.
Melosira varians. 444, B) lai'ge and small
frustules are seen united
in i-ows, ought to be sufficient to show that they ai-e dependent not
only on binary subdivision, but also on the special conditions of
evolution of the new frustule, by which it is able to increase
materially in size. This power of diatoms to expand their siliceous
coatings has therefore been denied by some, who are induced to
maintain this necessary consequence of the division of encysted
frustules, viz. the progressive decrease in size of the young
frustules, which would thus I'each the smallest possible dimensions.
This has led Pfitzei- ' to imagine that when diatoms have reached
their smallest possible dimensions by repeated binary division,
the process of conjugation takes place between them, resulting
in the formation of an auxospore, capable of reproducing two
sporangia! fi-ustules of considerably larger size, which would again
give rise, by fission, to a new series of diminishing frustules,
1 Untersuchungen ilber Ban u. Entwickelung der Bacillarien, 8vo. Bonn, 1871.

A Fig. 444.

Melosira suhflexilis.

PLATE XI.

Fiff. 1

Ficr, 2.

Figf. 5.

-•••

imm^^

Fi

^■1-

1

^.

^

{

■■(

u

1

Fig.

3.

N

I^~l

^

u

r-^^ .

Fie;. 0,

Dr. H. Van Hearck, phot. Collotype Ptg. Co., 2S2 High Holborn, VV.C,

Test Objects for the Microscope.

Objective by C. Zeiss, N.A. I'ljo ; Eyepiece iz. Monochromatic illumination by sunlight.

DIATOMACE.E 595

until these again reach their minimum size. This theoiy has,
in the judgment of Count Castracane, deceived many botanists,
from the idea that it was founded on actual observation, and has
at the same time been in harmony with the natural tendency to
generalisation, in attributing to the whole family of diatoms that
faculty of division which has been I'egaixled as the universal property
of the vegetable cell. The ' auxospore ' theory rests on the supposed
inability of the siliceous walls of diatoms to expand ; and implies,
secondly, the idea that all diatoms are capable of binary sub-
division ; and thii'dly, that there is no mode of repi-oduction except
by auxospores. That the siliceous walls of diatoms are capable of
distension seems to result from the examples ali-eady given of Jlelosira
suhflexilis and M. varians, as also from some othei- species in which
there may often be observed a sudden variation in diameter in frus -
tules united together in a row. But the power of increase in size of
the siliceous diatom-cell is evidently proved by the sporangial frus-
tules of Orthosira JDickiei,^ where, in the chain of cylindrical frus-
tules of the same diameter, the sporangial frustule is dilated in its
equatoi'ial axis, but much more so in its polar axis, pushing back the
base of the next cell and forcing it to fold itself wp so as to occupy
the whole cell-cavity, and sometimes even that of the next frustule.
The exactness and fidelity of the figure given in Smith's ' Synopsis,'
besides being guai'anteed by the authority of the distinguished author
and by the signature of the celebrated artist Tufien West, Count
Castracane was able to confirm by a magnificent prepai'ation
of these diatoms in which are a number of spoi'angial frustules.
The auxosjDoi-e theory supposes the fact that all diatoms ai'e capable
of binary subdivision, since the auxospore is understood, according
to Pfitzer, to pi'ovide foi- the progi'essive deci-ease in size of the
frustules, with the production of larger sporangial frustules, destined
to commence a new descending series. But binary subdivision cannot
take place in genei'a with unequal valves, as it is universally
acknowledged that the two new valves which are formed in the
pi-ocess of binary subdivision must stereotype themselves on the old
valves ; and for this reason the process cannot take place in those
genera in which the axes cross one another, like Camjyylodiscus, or
in those in which the two valves, although equal, yet constantly
unite in such a way that the similai- parts alternate with one
another, as may be seen in Asterolcwipra. That it is impossible for
binary subdivision to take place in these three classes of forms, is
confirmed by the fact that, notwithstanding that there are recorded
not less than seventy-five observations of the process of division in
them, not one afibrds an exception to the rule given above.

Where multiplication by binary subdivision occurs among the
Diatomacece, it takes place on the same general plan as in the Des-
micliacece, but with some modifications incident to peculiarities of
the structure of the former group. The first stage consists in the
elongation of the cell, and the formation of a ' hoop ' adherent to

1 See Castracane, ' The Theory of the EeiDroduction of Diatoms,' Atti delV Accacl.
Pontif. dei Nuovi Lincei, May 81, 1874 ; and ' New Arguments to prove that
Diatoms are reproduced by means of Germs,' ibid. March 19, 1876.

Q Q 2

596 MICEOSCOPIC FOEMS OF VEGETABLE LIFE-THALLOPHYTES

each end-valve, so that the two valves are separated by a band,
which progressively increases in breadth by addition to the free
edges of the hoops, as is well seen in fig. 445, A.. In the newly
formed cell e, the two valves are in immediate apposition ; in d a,
band intervenes ; in a this band has become much wider ; and in b
the increase has gone on until the original form of the cell is com-
pletely changed. At the same time the endochrome separates into
two halves ; the nucleus also subdivides in the manner formerly showTii
(fig. 417, G, H, I); and the parietal utricle folds in, first forming
a mere constriction, then an hour-glass contraction, and finally a
complete double partition, as in other instances. From each of its

adjacent surfaces a new
siliceous valve is formed, as
shown at fig. 445, A, C, just
as a new cellulose wall is
generated in the subdivision
of other cells ; and this valve
is usually the exact counter-
part of the one to which it
is opposed, and forms with
it a complete cell, so that
the original frustule is re-
placed by two frustules, each
of which has one old and one
new valve, just as in Desmi-
diacece. Generally speaking,
the new valves are a little
smaller than their prede-
cessors ; so that, after re-
peated subdivisions (as in
chains of Isthmia), a diminu-
tion of diameter becomes
obvious.^ But sometimes
the new valves are a little
Fig. U^.—Bidclulplna pulcheUa : A, chain of larger than their predeces-

cells in different states : a, full sizo ; b, elon-
gation prejDaratory to subdivision ; c, forma-
tion of two new cells ; d, e, young cells ; B,
end view ; C, side view of a cell more highly
magnified.

sors ; so that, in the fila-
mentous species, there may
be an increase sufficient to
occasion a gradual widening
of the filament, although
not perceptible except when two continuous frustules are com-
pared ; whilst, in the free forms, frustules of difierent sizes may
be met with, of which the larger are more numerous than the
smaller, the increase in number having taken place in geometrical
progression, whilst that of size was uniform. It is not always clear
what becomes of the hoop. In Melosira (fig. 444, A and iB), and
perhaps in the filamentous species generally, the hoops appear to
keep the new frustules united together for some time. This is at
first the case also in Biddulphia and Isthmia (fig. 457), in which the

1 This could not be explained on the hypothesis of the rigidity of the walls within
which fission takes place.

PLATE XII

Akachnoidiscus Japonicus.

DIATOM ACE^ 597

continued connection of tlie two frustules by its means gives rise to
an appearance of two complete frustules having been developed
within the original (fig. 445, A, C) ; svibsequently, however, the two
new frustules slip out of the hoop, which then becomes completely
detached. The same thing happens with many other diatoms, so
that the hoops are to be found in large numbers in the settlings of
water in which these plants have long been growing.

But in some other cases all trace of the hoop is lost, so that it
may be questioned whether it has ever been properly silicified, and
whether it does not become fused (as it were) into the gelatinous
envelope. During the healthy life of the diatom ^ the process of
binary division is continually being repeated ; and a vei-y rapid
multiplication of frustules thus takes place, all of which must be
considered to be repetitions of one and the same individual form.
Hence it may happen that myriads of frustules may be found in one
locality, uniformly distinguished by some peculiarity of form, size,
or marking, which may yet have had the same remote origin as
another collection of frustules found in some different locality, and
alike distinguished by some peculiarity of its own. For there is
strong reason to believe that such differences spring up among the
progeny of any true generative act, and that when that progeny is
dispersed by currents into different localities, each will continue to
multiply its own special type so long as the process of binary division
goes on.

We have seen that division is of the nature of multiplication,
and not of reproduction ; and that, where it does take place, it must
be regarded as the exception, and not as the rule. As respects
reproduction. Count Castracane, who was an observer during
thirty years devoted to the study of diatoms, had the opportunity
of noting in what way the process differs in pai'ticular cases.
He contended that he had been able to see in a Podosphenia the
emission of gonids or spo rules or embryonal foims, in the same way
in which Rabenhorst saw it in Melosira varians, and O'Meara in
Plenrosigraa Spencerii ; and in another case there were seen a
number of oval cysts of a species of Navicula easily recognisable.
The greater number of these were in a quiescent state ; but some
few were seen in motion by means of two flagelliform cilia ; so that
these larger or smaller cysts represented zygospores, and some of
them were shown to be zoozygospores. Castracane had the good
fortune to meet with a number of lai-ge and small oval cysts
imbedded in a gelatinous mass, all of them having in the centre two
similar corpuscles. From the condition of two greenish oblong
indistinct forms, these went on, by an easy transition, to manifest
themselves as naviculoid types, and at length developed into full-
grown frustules of Mastogloia. All this proved, in his judgment,
how reproduction in diatoms may pi'esent itself in different forms
and with different peculiarities ; for which reason one ought to
avoid ai'guing from special cases to genei'al laws. The only thing
which can be asserted of all cases of reproduction, is that it must
be preceded by conjugation, which results in the fertilisation of the
1 This refers to those diatoms in which binary subdivision can take place.

598 MICEOSCOPIC FOEMS OF VEGETABLE LIFE— THALLOPHYTES

sporules or gonids, which, after a period of repose or of incubation
inclosed within a cyst, or within a membranous frond, or within a
frustule, attain a condition for living an independent life and
reproducing in every respect the adult type of the mother-cell ; thus
the cyst, the membranous frond, oi' the frustule, performs the
function of a spoi-ange. Castracane was of opinion that these gonids
or embryonal forms could have no traces of silex in their cell- walls,
scarcely yet formed, until a few years ago,^ among the diatoms
of a marine deposit of the Miocene period, he met with a perfect
frustule of Coscinodiscus jnmctatits, which, between the two planes
of the valves, and therefore within the cell, exhibited some round
marks which admitted of no other interpretation except that of
impressions or traces of the embryonal forms surprised by death
while still attached to the mother-cell. More recently he met with
other cases identical in character, so that he has no longer any
doubt as to the presence of silex in the cell -walls of diatoms which
have not yet emerged to the light.

The formation of ' endocysts ' within the frustule of diatoms has
also been observed by Comber, Murray, and others.

No one apjjears at present to have given attention to a circum-
stance described by Castracane ^ in relation to a specimen of Striatella
imipunctata^ which has passed thousands of times under the eyes of
all, without its significance being recognised. The diatoms which
we have most frequently under our observation do not always
exhibit the same arrangement of their endochrome. The attempt
has, indeed, been made to found the classification of diatoms on the
arrangement of the endochrome, according as it is present in the
form, of plates or of gTanules; thus distinguishing the p/acocAro«2-aiic
and the cocco chromatic forms ; but a difficulty is presented in the
way of this classification by certain types which sometimes belong to
the one, sometimes to the other class. And this cannot be the
result of accident. Such variations might occur in some diatoms
as the result of special biological conditions of the individual.
There may frequently be seen, for example, a specimen of Melosira
varians with its cell-cavity filled with endochrome, not in a condition
of unequal amorphous masses, but of uniform rounded corpuscles ;
and this demands particular attention, or at least gives good ground
for special research. A diligent examination instituted in these
cases has demonsti-ated the existence in them of a special organi-
sation ; and the determination of a narrow and well-defined limit of
outline seems to prove that these were perfectly distinct and
independent of one another. From the perfect resemblance of
these to the gonids and embryonal forms seen to escape from the
mother-cell by Rabenhorst, O'Meara, and Casti^acane, he concludes
that this special arrangement of the endochrome must be interpreted
as a prelude to the process of reproduction.

These observations may possibly attract the attention of some

^ See ' Observations on a Fossil Diatom in relation to the Process of Eeproduc-
tion,' Atti dell' Accad. Pontif. dei Nuovi Lincei, May 17, 1885.

^ See 'The Diatoms of the Coasts of Istria and Dahnatia,' Atti dell' Accad,
Fontif. dei Nuovi Lincei, April 27 and May 25, 1873.

DIATOMACE^ 599

who are applying themselves to the study of diatoms to so important
an argument, on which may depend the possibility of establishing a
really good classification of diatoms which will at length satisfy
diatomists. At present preference is generally accorded to the
classification proposed by H. L. Smith, which establishes the class of
Raphicleoi from the presence of a raphe in the plane of the valves.
If there is, on the valves, in place of the raphe, a simple line of
division, the forms thus characterised are termed Pseudoraphidece ;
while those in which the valves have neither raphe nor its equivalent
are called CryptorapMdece, or, better, Anaraphidece. While, there-
fore, in the present state of our knowledge of diatoms, any classifica-
tion can only be regarded as provisional, we do not propose any
innovation on this point, although we are disposed to accord our
preference to that suggested by H. L. Smith.

Conjugation, so far as is at present known, takes place among
the ordinary DiatoiniaGece almost exactly as among the Desmidiacece,
except that it sometimes results in the production of two ' zygo-
spores ' instead of a single one. Thus in Stirirella (fig. 453), the
valves of two free and adjacent frustules separate from each other,
and the two endochromes (probably included in their parietal
utricles) are discharged ; these coalesce to form a single mass,
which becomes enclosed in a gelatinous envelope, and in due time
this zygospore shapes itself into a frustule resembling that of its
parent, but of larger size. But in Epitliemia (fig. 446, A, B), the
first diatom in which the conjugating process was observed by
Mr. Thwaites,^ the endochrome of each of the conjugating frustules
(C, D) appears to divide at the time of its discharge into two halves ;
each half coalesces with half of the other endochrome ; and thus
tiDO zygospores (E, F) are formed, which, as in the preceding case,
become invested with a gelatinous envelope, and gradually assume
the form and markings of the parent frustules, but grow to a very
much larger size, the sporangia! masses having obviously a power of
self-increase up to the time when their envelopes are consolidated.
It seems to be in this way that the normal size is recovered, after
the progressive diminution which is incident to repeated binary
multiplication. Of the subsequent history of the zygospores much
remains to be learnt ; and it may not be the same in all cases.
Appearances have been seen which make it almost certain that the
contents of each zygospore break up into a brood of gonids, and
that it is from these that the new generation originates. These
gonids, if each be surrounded (as in many other cases) by a distinct
cyst, may remain tmdeveloped for a considerable period ; and they
must augment considerably in size before they obtain the dimensions
of the parent frustule. It is in this stage of the process that the
modifying influence of external agencies is most likely to exert its
effects ; and it may be easily conceived that (as in higher plants and
animals) this influence may give rise to various diversities among
the respective individuals of the same brood ; which diversities, as
we have seen, will be transmitted to all the repetitions of each

1 See Annals of Natural History, vol. xx. ser. i. 1847, pp. 9, 343 and vol. i.
ser. ii. 1848, p. 161.

600 MICEOSCOPIC FOEMS OF VEaETABLE LIFE -THALLOPHYTES

that are pi'oduced by the process of binary division. Hence a veiy
considerable latitude is to be allowed to the limits of species, when the
different forms of Diatomacece are compared ; and here, as in many
other cases, a most important question arises as to what are those
limits — a question which can only be answered by such a careful
study of the entire life-history of every single type as may advan-
tageously occupy the attention of many a microscopist who is at
present devoting himself to the resolution of the markings on
diatom- valves, and to the multiplication of reputed species by the
detection of minute differences.^

This formation of what are termed auxospores — as serving to

augment the size of the
cells which are to give
origin to a new genera-
tion— takes place on a
very different plan in
some of those filamentous
types, such as Melosira
(fig. 444, A, B), in which
a strange inequality
presents itself in the
diameters of the differ-
ent cells of the same
filament, the larger ones
being usually in various
stages of binary sub-
division, by which they
multiply themselves
longitudinally. Accord-
ing to the observations
of Mr. Thwaites {loc.
Fig. 446.— Conjugation of Epithemia turgicla : A, cit.), these also are the
front view of single frustule ; B, side view of the i x f ^ • ^ f

same ; C, two frustules with their concave surfaces Pl^OClUCtS_ Ot a kmcl Ot
in close apposition ; D, front view of one of the conjugation between the
frustules, showing the separation of its valves E, adiacent cells of the or-
F, side and front views after the formation of the A;t,^^^^ diameter takinp-
zvsosDores iiiiiaiv tiianieuei, taKiijg

place before the comple-
tion of their separation. He describes the endochrome of particular
frustules, after separating as if for the formation of a pair of new cells,
as moving back from the extremities towards the centre, rapidly
increasing in quantity and aggregating into a zygospore (fig. 447,
No. 2, a, b, c) : around this a new envelope is developed, which may or
may not resemble that of the ordinary frustules, but which remains
in continuity with them ; and this zygospore soon undei'goes binary

1 See on this subject a valuable paper by Prof. W. Smith ' On the Determination
of Species in the Diatomacece,' in the Quart. Joiirn. of Microsc. Science, vol. iii.
1855, p. 130 ; a memoir by Prof. W. Gregory ' On Shape of Outline as a Specific
Character of DiatomacecB,' in Trans, of Microsc. Soc. 2nd series, vol. iii. 1855,
p. 10 ; and the Author's Presidential Address, in the same volume, pp. 44-50 ; ' On
Navicula crassinervis, Frustulia saxonica, and N. rhoinboides, as Test-objects,' by
W. H. Dalhnger, Monthlij Micro. Journ. 1876, vol. xvii. p. 1 ; also an Additional
note on the identity of these, by the same Author, ihid. p. 173.

DIATOMACE.E

60 1

subdivision (No. 3, a, h, c), the cells of the new series thus developed
presenting the character of those of the original filament (1), but
greatly exceeding them in size. From what has been already- stated,
it seems probable that a gradual i-eversion to the smaller form takes
place in subsequent subdivisions, a further reduction being checked
by a new formation of zygospores. The various modes of formation
of auxospores in the Diatomaceae are classified by Klebahn under
five difierent heads, viz. : — (1) Rejuvenescence of a single cell, accom-
panied by an increase in size ; this is the simplest tyjDe, and one
of the most common. (2) Two daughter- cells are produced from the
protoplasm of a mother-cell, and from these arise two auxospores
[Achnanthes longvpes, Rhabdone'ma arcuatum). (3) Two cells lying-
side by side cast ofi" their old valves, and each grows into an
auxospore, without any previous fusion, or any visible interchange
of contents ; this is the commonest type of all. (4) A true conjuga-
tion takes place ; the protoplasmic contents of the two cells fuse

Fig. 447. — Self- conjugation (?) of Melosira italica {Aulacosira crenulata
Thwaites) : 1, simple filament ; 2, filament developing auxospores ; a, b, c, succes-
sive stages in the formation of auxospores ; auxospore-frustules in suceessiTe stages,
a, b, c, of multixjlicatiou.

together into one, and this mass grows into an auxospore. (5) Before
conjugation, the protoplasm of each of the two cells divides before-
hand into two daughter-cells, and two auxospores ai-e formed by
the fusion of a daughter-cell from each mother-cell wdth the
daughter-cell of the other one lying opposite to it ; this is the most
complicated process (Ar)iphora ovalis, Epithemia Argus, Rliopa-
lodia gibba, &c.).

The most curious phenomenon presented by diatoms is un-
doubtedly their power of movement, which induced Ehrenberg and
the other early observers of these organisms to place them erro-
neously in the animal kingdom, although it affords no evidence of
consciousness. This power of movement, if not common to all
diatoms, is very evident in those species which are normally or
accidentally free, and most conspicuously in oblong foi-ms, such as the
species of Navicula. In those also which are stalked it has been
noticed that if, from any cause, a frustule becomes detached, it is

602 MICPtOSCOPIC FOEMS OF VEGETABLE LIFE—THALLOPHYTES

endowed with a motion similar to that of the species which are
normally free. This circumstance has caused the abandonment
of Mr. W. Smith's proposal to assign a generic value to the condition
in which the frustule is possessed of this property without regard to its
form. Hence those generaai-e not now generally recognised which differ
only in being enclosed in a membranous frond, or in being stalked,
especially since frustules contained in a sheath, for example in Schizo-
nema, ^ have been seen to escape from it, and to be prevented from
returning again to it in company with the sister Navicidcs. Hence
the genera Schizonema, Berkeleya, and Dickiea must be reunited to
Navicula ; GoccoTiema, Endonema, and Colletonema to Cymhella ; and
Homeocladia to Nitzsdiia. The singular phenomenon of movement
which may be observed in many genera of diatoms — am^ong which
the most singular is that presented hy Bacillariaparadoxa (fig. 449),
in which the rod-like frustules are seen to be continually gliding one
along another, in a retrograde direction, before they become detached
— is found to be in general a movement backwards and forwards in a
straight line so far as they meet with no imjaediment, while the
intervention of obstacles determines a passive change of direction.
The backward and forward movements of the Naviculce have been
already described ; in Surirella (fig. 453) and Gamipylodiscus
(fig. 454) the motion never proceeds further than a languid roll from
one side to the other ; and in Gomphonevia (fig. 463), in which a
foramen fulfilling the nutritive oflice is found at the larger extremity
only, the movement (which is only seen when the frustule is separated
from its stipe) is a hardly perceptible advance in intermitted jerks
in the direction of the narrow end. The cause of this movement is
uncertain. It has been referred by different aiithors to the action
of endosraose and exosmose ; to cilia ; to the projection of pseudopode-
like masses of protoplasm through orifices in the raphe, or of a single
elongated protoplasmic thread ; but the most probable interpretation
attributes it to the action of the changes resulting from the nutrition
of the cell, which must necessarily absorb food in a liquid condition.
Taking account, therefore, of the relatively considerable quantity of
silex necessary to the organisation of the diatom cell in proportion
to its minute dimensions, and bearing in mind, at the same time,
the incalculably small traces of silex in solution in the water, it
may be understood how active must be the exchange from the
exterior to the interior of the cell, and vice versa, and hence how
such an exchange mvist determine a continual change of position
backwards and forwards, through the reaction exercised on the
delicate floating frustules.

The principles uj)on which this interesting group should be classi-
fied cannot be properly determined until the history of the genera-
tive process — of which nothing whatevei- is yet known in a large
propoi'tion of diatoms, and but little in any of them — shall have
been thoroughly followed out. The observations of Focke ^ render it

'^ See Castracane, ' Observations on the Genera Homeocladia and Schizonema,'
in Atti dell' Accad. Pontif. dei Nuovi Lincei, May 23, 1880.

^ According to this observer [Ann. of Nat. Hist. 2nd series, vol. xv. 1855, p. 237)
Navicula hifrons forms, by the spontaneous fission of its internal substance, spherical
bodies, which, like gemmules, give rise to S^irirelUij microcora. These by conjuga-

DIATOMACE.*:

603

highly probable that many of the foi-ms at pi-esent considered as dis-
tinct from each other would prove to be but different states of the
same if theii- lohole histoiy wei-e ascertained. On the other hand,
it is by no means impossible that some which appear to be nearly
i-elated in the sti-ucture of their frustules and in theii- mode of
growth may j)rove to have quite different modes of reproduction.
At present, therefore, any classification must be merely provisional ;
and in the notice now to be taken of some of the most interesting
forms of the Diatoinacece, the method of Professor Kiitzing, which
is based upon the characters of the individual frustules, is followed,
in preference to that of Mr. W. Smith, which was founded on
the degree of connection remaining between the several frustules
after binary division.^ In each family the frustules may exist under
four conditions : — (a) free, the binary division being entire, so that the
frustules separate as soon as the process has been completed ; (h)
stipitate, the frustules being implanted upon a common stem (fig.

Fig. 448. — -Mericlion circulare.

Pig. 449.

Fig. 449. — Sacillaria paradoxa.

450), which keeps them in mutual connection after they have them-
selves undergone a complete binary division ; (c) united in a filament,
which will be continuous (fig. 445, A, B) if the cohesion extend to
the entire surfaces of the sides of the frustules, but may be a mere
zigzag chain (fig. 451) if the cohesion be limited to their angles;
{d) aggregated into a frond (fig. 464), which consists of numerous
frustules more or less regularly enclosed in a gelatinous investment.
Commencing with the last-named division (A), the first family

tion produce N. splendida, which gives rise to N. hifrons by the same process. He is
only able to sj)eak positively, however, as to the production of N. bifrons from N.
sjilendida; that of SurireUa microcora from N. bifrons, and that of N. splendida
from SurireUa microcora, being matters of inference from the phenomena witnessed
by him.

1 The method of Kiitzing was the one followed, with some modification, by Mr.
Ralfs in his revision of the group for the fourth edition of Pritchard's Infusoria ;
and to his systematic arrangement the Author would refer such as desire more
detailed information.

604 MICROSCOPIC FORMS OF VEOETABLE LIFE -THALLOPHYTES

is that of Eitnotiece, of which we have already seen a characteristic
example in E'piihe')nia turgida (fig. 446). The essential characters
of this family consist in the more or less Innate form of the frnstules
in the lateral view (fig. 446, B), and in the striae being continuous
across the valves without any interruption by a longitudinal line.
In the genus Eunotia the frustules are free ; in Ejnthemia they are
very commonly adherent by the flat or concave surface of the con-
necting zone ; and in Himantidium they are usually united into
ribbon-like filaments. In the family Meridiece we find a similar
union of the transversely striated individual frustides ; but these are
narrower at one end than at the other, so as to have a cuneate or
wedge-like form, and are regularly disposed with their corresponding
extremities always pointing in the same direction, so that the fila-
ment is cui-ved instead of

straight, as in the beauti-
ful Meridian circidare (fig.
448). Although this plant,
when gathered and placed
under the microscope, pre-
sents the appearance of
circles overlying one an-
other, it really grows in a
helicoid (screw -like) form,
making several continuous
turns. This diatom abounds
in many localities in this
country ; but there is none
in which it presents itself
in such rich luxuriance as
in the mountain-brooks

about West Point in the
United States, the bottoms
of which, according to Pro-
fessor Bailey, ' are literally
covered in the first warm
days of spring with a fer-
ruginous-coloured mucous
matter, about a quarter of
an inch thick, which, on examination by the microscope, proves to
be filled with millions and millions of these exquisitely beautiful
siliceous bodies. Every submerged stone, twig, and spear of grass is
enveloped by them, and the waving plume-like appearance of a fila-
mentotxs body covered in this way is often very elegant.' The frus-
tules of Meridion are attached when young to a gelatinous cushion ;
but this disappears with the advance of age. In the family Licmo-
phorece also the frustules are wedge-shaped ; in some genera they have
transverse markings, whilst in others these are deficient ; but in
most instances there are to be observed two longitudinal suture-like
lines on each valve (which have received the special designation of
vittce) connecting their two extremities. The newly formed part of
the stipe in the genus Licmophora^ instead of itself becoming double

Fig. 450. — LicvtojjJiora flahellata.

DIATOMACE.E

605

with each act of binai'y division of the frustule, increases in breadth,
while the frustnles themselves remain coherent, so that a beautifiil
fan-like arrangement is produced (fig. 450). A splitting away of a
few frustules seems occasionally to take place, from one side or the
other, before the elongation of the stipe ; so that the entire plant
presents us with a more or less complete flahella or fan upon the
summit of the branches, with imperfect flabellte or single frustules
irregularly scattered throughoiit the entire length of the footstalk.
This beautiful plant is marine, and is attached to seaweeds and
zoophytes.

In the next famil}^, that of Fragilariece, the frustules are of the
same breadth at each end, so that if they unite into a filament they
form a straight band. In
some genera they are
smooth, in others trans-
versely striated, with a
central nodule ; when striae
are present, they run across
the valves without inter-
ruption. To this family
belongs the genus Diatoma,
which gives its name to the
entire group, that name
(which means cutting
through) being suggested
by the curious habit of the
genus, in which the frus-
tules, after division, sepa-
rate from each other along
their lines of junction, but
remain connected at their
angles, so as to form zigzag-
chains (fig. 451). The
valves of Diatoma., when
turned sideways (a), are
seen to be strongly marked
by transverse strife, which
extend into the front view.
The proportion between the
length and the breadth of

each valve is found to vary so considerably that, if the extreme forms
only were compared, there would seem adequate ground for regarding
them as belonging to different species. The genus inhabits fresh
water, preferring gently running streams, in which it is sometimes
very abundant. The genus Fragilaria is nearly allied to Diatoma the
difference between them consisting chiefly in the mode of adhesion of
the frustules, which in Fragilaria form long, straight filaments with
parallel sides ; the filaments, however, as the name of the genus
implies, very readily break up into their component frustules, often
separating at the slightest touch. Its various species are very
common in pools and ditches. This family is connected with the

of

PlU. 451.

Fig. 451. — Diatoma vulgare : a, siae view
frustule; b, frustule undergoing division.

Fig. 4^5%.^Grammatoplioraser2}entma: a, front
and side views of single frustule ; h, b, front
and end views of divided frustule ; c, frustule
about to undergo division; d, frustule com-
pletely divided.

6o6 MICEOSCOPIC FOEMR OF VEGETABLE LIFE— THALLOPHYTES

next by the gentis Nitzsclda, which is a somewhat aberrant form, dis-
tinguished by the presence of a prominent keel on each valve, divid-
ing it into two portions which are usually unequal, while the entire
valve is sometimes curved, as in N. sigmoidea, which has been used
as a test-object, but is not suitable for that purpose on account of
the extreme variability of its striation. Nearly allied to this is
the genus Bacillaria^ so naraed from the elongated staff-like form of
its frustules ; its valves have a longitudinal punctated keel, and
their transverse striye are interrupted in the median line. The
principal species of this genus is the B. paradoxa^ whose remarkable
movement has been already described. Owing to this displacement
of the frustules, its filaments seldom present themselves with straight
parallel sides, but nearly always in forms more or less oblique, such
as those represented in fig. 449. This curious object is an inhabitant
of salt or of brackish water. Many of the species formerly ranked
under this genus are now referred to the genus Diatoma. The
genera Nitzschia and Bacillaria have been associated by Mr. Kalfs

with some other genera
which agree with them
in the bacillar or staff-
like form of the frus-
tulesandin the presence
of a longitudinal keel,
in the sub-family jSitz-
schiece, which ranks as
a section of the Suri-
rellece. Another sub-
family, Synedrece, con-
sists of the genus
Synedra and its allies,
in which the bacillar
form is retained, bvit
the keel is wanting,
and the valves are
but little broader than the front of the frustule.

In the Surirellece proper the frustules are no longer bacillar,
and the breadth of the valves is usually (though not always) greater
than the front view. The distinctive character of the genus
Surirella, in addition to the pi'esence of the supposed ' canaliculi,'
is derived from the longitudinal line down the centre of each valve
(fig. 453, A) and the prolongation of the margins into ' al?e.'
Numerous species are known, which are mostly of a somewhat ovate
form, some being broader and others narrower than S. constricta ;
the greater part of them are inhabitants of fresh or brackish water,
though some few are marine ; and several occur in those infusorial
earths which seem to have been deposited at the bottoms of lakes,
such as that of the Mourne Mountains in Ireland (fig. 468, h, c, k).
In the genus Campylodiscus (fig. 454) the valves are so greatly
increased in breadth as to present almost the form of discs (A), and
at the same time have more or less of a j^eculiar twist or saddle-
shaped curvature (B). It is in this genus that the supposed ' cana-

FiG. 453.- — Hurirella constricta : A, side view ;
B, front view; C, binary subdivision.

DIATOMACE.E

607

liculi ' are most developed, and it is consequently here that they may
be best studied ; and of there being here really costce, or internally
projecting ribs, no reasonable doubt can remain aftei- examination
of them under the binocular microscope, especially with the ' black -
grovmd ' illumination. The form of the valves in most of the species
is circular or nearly so ; some are nearly fiat, whilst in others the
twist is greater than in the species here represented. Some of the
species are marine, whilst others occur in fresh water ; a very
beautiful form, the C. clypeus, exists in such abundance in the
infusorial stratum discovered by Ehrenberg at Soos, near Ezer, in
Bohemia, that the earth seems almost entirely composed of it.

The next family, the Striatellece, forms a very distinct group,
differentiated from every other by having longitudinal costas on the
connecting portions of the frustules, these costte being formed by
the inward projection of annular siliceous plates (which do not,
however, reach to the centre), so as to form septa dividing the cavity
of the cell into imperfectly separated chambers. In some instances
these annular septa are only formed during the production of the

Fig. 454. — Camiyylodiscus costatus : A, front view; B, side view.

valves in the act of division, and on each repetition of such produc-
tion, being thus always definite in number ; whilst in other cases
the formation of the septa is continued after the production of the
valves, and is repeated an uncertain number of times before the
recurrence of a new valve-production, so that the annuli are indefinite
in number. In the curious GrammatojyhoQ'a seiyentina (fig. 452)
the septa have several undulations and incurved ends, so as to form
serpentine curves, the number of which seems to vary with the
length of the frustule. The lateral surfaces of the valves in Gram-
matophora are very finely striated, and some species, as G. suhtilissima
and G. marina, are used as test-objects. The frustules in most of
the genera of this family sepai'ate into zigzag chains, as in Diatoma ;
but in a few instances they cohere into a filament, and still more
rarely they are furnished with a stipe. The small ?ia\\\\y Terpsinoece
was separated by Mr. Ralfs from the Striatellece, with which it is
nearly allied in general characters, because its septa, (which in the
latter are longitudinal and divide the central portions into chambers)
are transverse, and are confined to the lateral portions of the

6o8 MICROSCOPIC FOEMS OF VEGETABLE LIFE— THALLOPHYTES

frustules, which appear in the front view as in Biddulphiece. The
typical form of this family is the Terpshioe Tuusica, so named from
the resemblance which the markings of its costse bear to musical
notes.

We next come to two families in which the lateral surfaces of
the frustules are circular ; so that, according to the flatness or con-
vexity of the valves and the breadth of the intervening hooped band,
the frustules may have the form either of thin discs, short cylinders,
biconvex lenses, oblate spheroids, or even of spheres. Looking at
the structure of the individual frustules, the line of demarcation
between these two families, Melosirece and Coscinodiscece, is by no
means distinct, the principal difference between them being that
the valves of the latter are commonly areolated, whilst those of the
former are smooth. Another important difference, however, lies in
this, that the frustules of the Cosci7iodiscece are always free, whilst
those of the Melosirece remain coherent into filaments, which often
so strongly resemble those of the simple Confervacece as to be readily
distinguishable only by the effect of heat. Of these last the most
important genus is Melosira (fig. 444). Some of its species are
marine, others fresh- water ; one of the latter, M. ochracea, seems to
grow best in boggy pools containing a ferruginous impregnation ;
and it is stated by Professor Ehrenberg that it takes up from the
water, and incorporates with its own substance, a considerable
quantity of iron. The filaments of Melosira very commonly fall
apart at the slightest touch, and in the infusorial earths in which
some species abound the fr-ustules are always found detached
(fig. 468, a, a, d, d). The meaning of the remarkable difference in
the sizes and forms of the frustules of the same filaments (fig. 444)
has not yet been fully ascertained. The sides of the valves are
often marked with radiating stripe (fig. 468, d^ d); and in some
species they have toothed or serrated margins, by which the frustules
lock together. To this family belongs the genus Hyalodiscus, of
which H. subtilis was first brought into notice by the late Professor
Bailey as a test-object, its disc being marked, like the engine-turned
back of a watch, with lines of exceeding delicacy, only visible by
good objectives and careful illumination.

The family Coscinodisceoi includes a large proportion of the most
beautiful of those discoidal diatoms of which the valves do not
present any considerable convexity, and are connected by a narrow
zone. The genus Goscinodiscus, which is easily distinguished from
most of the genera of this family by not having its disc divided into
compartments, is of great interest from the vast abundance of its
valves in certain fossil deposits (fig. 467, a, a, a) especially, the
infusorial earth of Richmond in Virginia, of Bei-muda, and of Oran,
as also in guano. Each frustule is of discoidal shape, being com-
posed of two delicately undulating valves united by a hoop ; so that
if the frustules remain in adhesion, they would form a filament
resembling that of Melosira (fig. 444, B). The regularity of the
hexagonal areolation shown by its valves renders them beautiful
microscopic objects ; in some species the areola? are smallest neai'
the centre, and gradually increase in size towards the margin ; in

DIATOMACE^ 609

others a few of the central areolae are the largest, and the rest are
of nearly uniform size ; while in others, again, there are radiating
lines formed by areolae of a size different from the rest. Most of
the species ai-e either marine or are inhabitants of brackish water ;
when living they are most commonly found adhei'ent to seaweeds
or zoophytes ; but when dead the valves fall as a sediment to the
bottom of the water. In both these conditions they were found by
Professor J. Quekett in connection with zoophytes which had been
brought home from Melville Island by -Sir E. Parry ; and the species
seem to be identical with those of the Richmond earth. The in-
vestigations of Mr. J. W. Stephenson^ oxyCoscinodiscits oculus iridis
show that the peculiar ' eye-like ' appearance in the centre of each
of its hexagonal areola? ai-ises from the intermingling of the mark-
ings of two distinct layers, diifering considerably in structure, the
markings of the lower layer being' partially seen through those of
the upper. By fracturing these diatoms Mr. Stephenson succeeded
in separating portions of the two layers, so that each could be
examined singly. He also mounted them in bisulphide of cai-bon,

Fig. 455. — Structure of siliceous valve of Coscinodiscus oculus iriclis : 1, hexagonal
areola of inner or ' eye-spot ' layer ; 2, areola of outer layer.

the refractive index of which is high ; and also in a solution of
phosphoius in bisulphide of carbon, which has a still higher refrac-
tive index. If we suppose a diatom to be marked with convex
depressions, they would act as concave lenses in air, which is less
refractive than their own silex ; but when such lenses are immersed
in bisulphide of carbon, or in the phosphorus solution, they would
be converted into convex lenses of the more refractive substance,
and have their action in air reversed. Analogous but opposite
changes must take place when convex diatom-lenses are viewed first
in air, and then in the more refractive media. Applying these and
other tests to Coscinodiscus ocidus iridis, Mr. Stephenson considered
both layers to be composed of hexagons, represented in fig. 455
from drawings by Mi'. Stewart. The upper layer is much stronger
and thicker than the lower one, and the framework of its hexagons
more readily exhibits its beaded appearance. The lower layer- is
nearly transparent, and but little conspicuous when seen in bisulphide
of carbon, except as shown in the figure, when the framework of
^ Monthly Microscopical Journal, vol. x. 1873, p. 1.

R R

6lO MICROSCOPIC FOEMS OF VEGETABLE LIFE— THALLOPHYTES

the hexagons and the rings in the midst of them appear thickene
and more refractive. In both layers the balance of obsei-vations
tends to the belief that the hexagons have no floors, and are in fact
perforated by foramina like those of minute polycystina. The cells
formed by the hexagons of the uppei- layer are of considerable
depth ; those of the lower layer are shallower. It is very desirable
that living forms of Coscinodisci should be carefully examined ;
since, if they really have foramina, some minute organs may be pi-o-
truded through them.

The genus Actinocyclus ^ closely resembles the jDreceding in form,
but differs in the markings of its valvular discs, which are minutely
and densely punctated or areolated, and are divided radially by
single or double dotted lines, which, however, are not continuous
but interrupted. The discs are generally iridescent ; and, when
mounted in balsam, they present various shades of brown, green,
blue, purple, and red ; blue or purple, however, being the most
frequent. An immense number of species have been erected by
Professor Ehrenberg on minute differences presented by the rays as
to number and distribution ; but since scarcely two specimens can
be found in which thei-e is a perfect identity as to these particulars,
it is evident that such minute differences between organisms other-
wise similar are not of sufficient account to serve for the separation
of species. This form is very common in guano from Ichaboe. Allied
to the preceding are the two genera, Asterolampra and Asteromphalus,
both of which have circular discs of which the mai-ginal portion is
minutely areolated, whilst the central area is smooth and perfectly
hyaline in appearance, but is divided by lines into i-adial compart-
ments which extend from the central umbilicus towards the periphery.
The difference between them simply consists in this, that in Astero-
laifiijira all the compartments are similar and equidistant and the rays
equal, whilst in Asteromphalus (PL I, fig. 3) two of the compartments
are closer together than the rest, and the enclosed hyaline ray (which
is distinguished as the median or basal ray) differs in form from
the others, and is sometimes specially continuous with the umbilicus.
The eccenti'icity thus produced in the other rays has been made the
basis of another generic designation, fipatangidnmn ; but it may be
doubted whether this is founded on a valid distinction.^ These
beautiful discs are for the most part obtainable from guano, and
from soundings in tropical and antarctic seas. From these we pass
on to the genus Act'inoptychus (fig. 456), of w^hich also the frustules
are discoidal in form, but in which each valve, instead of being flat,
has an undulating surface, as is seen in front view (B), giving to the
side view (A) the ap^^earance of being marked by radiating bands.
Owing to this peculiarity of shape, the whole surface cannot
be brought into focus at once except with a low power ; and the

^ The Author concurs with Mr. Ralfs in thinking it preferable to limit the genus
Actinocyclus to the forms originally included in it by Ehrenberg, and to restore the
genus Actinoptychiifi of Ehrenberg, which had been improperly united with Actino-
cyclus by Professors Kiitzing and W. Smith.

- See Greville in Quart. Journ. Microsc. Science, vol. vii. 1859, p. 158; and in
Trans. Microsc. Soc vol. viii. n.s. 18(50, p. 102, and vol. x. 1862, p. 41 ; also Wallich
in the same Transactions, vol. viii. 1860, p. 44.

DIATOMACE.E

6ii

difference of aspect which the difierent I'adial divisions present in
fig. 456 is simply due to the fact that one set is out of focus whilst
the other is in it, since the appearances are reversed by merely
altering the focal adjustment. The number of radial divisions has
been considered a character of sufficient importance to serve for the
distinction of species ; but this is probably subject to variation ;
since we not unfrequently meet with discs, of which one has (say)
eight, and another ten such divisions, but which are precisely alike
in every other particular. The valves of this genus also are very
abundant in the infusorial earth of Richmond, Bermuda, and Oran
(fig. 467, h, h, h), and many of the same species have been found in
guano and in the seas of various parts of the world. The frustules
in their living state appear to be generally attached to seaweeds or
zoophytes.

The Bermuda earth also contains the very beautiful form
which, though scarcely separable from Actinoptychus except by
its marginal spines, has received from Professor Ehrenbei-g the dis-
tinctive appellation of HeUopelta (sun shield). The object is repre-
sented as seen on its internal aspect by the parabolic illuminator,
which brings into view certain features that can scarcely be seen by
ordinary transmitted light.
Five of the radial divisions are
seen to be marked out into
circular areolae ; but in the
five which alternate with them
a minute beaded structure is
observable. This niay be
shown, by careful adjustment
of the focus, to exist over the
whole interior of the valve,
even on the divisions in which
the circular areolation is here displayed ; and it hence appears pro-
bable that this marking belongs to the internal layer, '^ and that the
circular areolation exists in the oiiter layer of the silicified valves.
In the alternating divisions whose surface is here displayed, the
areolation of the outer layer, when brought into view by focussing
down to it, is seen to be formed by equilateral tiiangles ; it is not,
however, neaily so well marked as the circular areolation of the
first-mentioned divisions. The dark spots seen at the end of the
rays, like the dark centre, appear to be solid ai-eolations of silex not
traversed by markings, as in many other diatoms ; they are appa-
rently not orifices, as supposed by Professor Ehrenberg. Of this type,
again, specimens are found presenting six, eight, ten, or twelve radial
divisions, but in other I'espects exactly similar ; on the other hand,
two specimens agreeing in their number of divisions may exhibit
minute differences of other kinds ; in fact, it is rare to find two

Fig. 456. — Actinoptychus unclulatus.
A, side view ; B, front view.

1 It is stated by Mi'. Stodder {Quart. Journ. Microsc. Science, vol. iii. n.s. 1863,
p. 21.5) that not only has he seen, in broken specimens, the inner granulated plate
projecting beyond the outer, but that he has found the inner plate altogether
separated from the outer. The Author is indebted to this gentleman for pointing
out that his figure represents the inner surface of the valve.

K E 2

6l2 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

that are precisely alike. It seems probable, then, that we mvist
allow a considerable latitude of variation in these forms before
attempting to separate any of them as distinct species. Another
very beautiful discoidal diatom, which occurs in guano, and is also
found attached to seaweeds from different parts of the world
(especially to a species employed by the Japanese in making soup),
is the Arachnoidiscus (Plate XII), so named from the resemblance
which the beautiful markings on its disc cause it to bear to a
spider's web. According to Mr. Shadbolt,^ who first carefully
examined its structure, each valve consists of two layers ; the outer
one, a thin flexible horny membrane, indestructible by boiling
in nitric acid ; the inner one siliceous. It is the former which
has upon it the peculiar spider's-web-like markings ; whilst it is
the latter that forms the supporting framework which bears a very
strong resemblance to that of a circular Gothic window. The
two can occasionally be separated entire by first boiling the discs
for a considerable time in nitric acid and then carefully washing
them in distilled water. Even without such separation, however,
the distinctness of the two layers can be made out by focussing for
each separately under a \- or J -inch objective ; or by looking at a
valve as an opaque object (either by the ]3arabolic illuminator,
or by the Lieberkiihn, or by a sitle light) with a ]±j-inch objective,
first from one side and then from the other. But it can be seen to
very best advantage by the use of apochromatic objectives of
suitable power and a suitable diaphragm for dark-ground illumi-
nation.

This family is connected with the succeeding by the small group
Eiqyodiscece, the members of which agree with the Coscinodiscece in
the general character of their discoid frustules, and with the Bid-
dulphiem in having areolar processes on their lateral surfaces. In
the beautiful Aidacodiscus these areolations are situated near the
margin, and are connected with bands radiating from the centre ; the
surface also is frequently inflated in a manner that reminds us of
Actinojoty chits. These forms are for the most part obtained from
guano.

The members of the next family, Bnldidjyhiece, differ greatly in
their general form from the preceding, being remarkable for
the great development of the lateral valves, which, instead of being
nearly flat or discoidal, so as only to present a thin edge in front
view, are so convex or inflated as always to enter largely into the
front view, causing the central zone to appear like a band between
them. This band is very narrow when the new frustules are first
produced by binary division, but it increases gradually in breadth,
until the new frustule is fully formed and is itself undergoing the
same duplicative change. In Biddidjihia (fig. 445) the fi'ustules
have a quadrilateral form, and remain coherent by their alternate
angles (which are elongated into tooth-like projections), so as to form
a zigzag chain. They are marked extei'nally by libbings which seem
to be indicative of internal coske pai'tially subdividing the cavity.
Nearly allied to this is the beautiful genus Isthmia (fig. 457), in
1 Trans. Microsc. Soc. 1st series, vol. iii. p. 49.

DIATOMACE^

613

which the frustules have a trapezoidal form owing to the oblique
prolongation of the valves ; the lower angle of each frustule is
coherent to the middle of the next one beneath, and from the basal
frustule proceeds a stipe by which the filament is attached. Like
the preceding, this genus is marine, and is found attached to the
seaweeds of our own shores. The areolated structure of its surface
is very conspicuous both in the valves and in the connecting ' hoop ; '
and this hoop, being silicified, not only connects the two new frus-
tules (as at h, fig. 457), until they have separated from each other,
but, after such separation, remains for a time round one of the
fi-ustules, so as to give it a truncated appearance {a, c).

The family Anguliferce, distinguished by the angular form of its
valves in their lateral aspect, is in many respects closely allied to
the preceding ; but in the comparative flattening of their valves its
members more resemble the Coscinodiscece
and Eiqyodiscece. Of this family we have
a characteristic example in the genus
Triceratium, of which striking form a con-
siderable number of species are met with
in the Bermuda and other infusorial
earths, while others are inhabitants of the
existing ocean and of tidal rivers. T.favus
(fig. 442), which is one of the largest and
most regularly marked of any of these,
occurs in the mud of the Thames and in
various other estuaries on our own coast ;
it has been found, also, on the surface of
large sea-shells from various parts of the
world, such as those of Hippoinis and
Hcdiotis, before they have been cleaned ;
and it presents itself likewise in the in-
fusorial earth of Petersburg (U.S.A.).
The projections at the angles which are
shown in that species are prolonged in
some other species into ' horns ; ' whilst
in others, again, they are mere tubercular
elevations. Although the triangidar form
of the frustule, when looked at sideways,
is that which is characteristic of the genus, yet in some of the species
there seems a tendency to produce quadrangidar and even pentagonal
forms, these being marked as varieties by their exact correspondence
in sculpture, colour, &c., with the normal triangular forms. ^ This
departure is extremely remarkable, since it breaks down what seems
at first to be the most distinctive character of the genus ; and its
occurrence is an indication of the degree of latitude which we ought
to allow in other cases. It is diflicult, in fact, to distinguish the
square forms of Triceratium from those included in the genus

Fig. 457. — Isthniia nervosa.

1 See Mr. BrigMwell's excellent memoirs ' Oa the genus Triceratium' in
Quart. Journ. Microsc. Science, vol. i. 1853, p. 245 ; vol. iv. 1856, p. 272 ; vol. vi.
1858, p. 153 ; also Wallich in the same Journal, vol. iv. 1858, -p. 242 ; and GreviUe in
Trans. Microsc. Soc. n.s. vol. ix. 1861, pp. 43, 69.

614 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

Amphitetras, which is chiefly characterised by the cubiform shape of
its frustules. In the latter the frvxstules cohere at their angles, so
as to form zigzag filaments, whilst in the former the frustules are
usually free, though they have occasionally been found in chains.

Another group that seems allied to the Bidchilphiece is the curious
assemblage of forms brought together in the family Chcetocerece, som.e
of the filamentous types of which seem also allied to the Melosirece.
The peculiar distinction of this group consists in the presence of
tubular ' awns,' frequently proceeding from the connecting hoop,
sometimes spinous and serrated, and often of great length (fig. 458) ;
by the interlacing of which the frustules are united into filaments
whose continuity, however, is easily broken. In the genus Bacterias-
trum (fig. 459) there are sometimes as many as twelve of these awns,
radiating fi-om each frustule like the spokes of a wheel, and in some

instances regularly bifurcating.
With this group is associated the
genus lihizosolenia, of which several
i%^i^ "y species are distinguished by the ex-

traordinary length of the frustule
(which may be from six to twenty

Fig. 458. — Chcstoceros Wighamil : a, front
view, and 6, side view of frustule ; c, side
view of connecting hoop and awns ;
d. entire filament.

Fig. 459. — Bacteriastrum
furcatum

times its breadth), giving it the aspect of a filament (fig. 460),
by a transverse annulation that imparts to this filament a jointed
appearance, and by the termination of the frustule at each end in a
cone, from the apex of which a straight awn proceeds. It is not a
little remarkable that the greater number of the examples of this
curious family are obtained from the stomachs of Ascidians, Salppe,
Holothurise, and other marine animals.^

The second principal division (B) of the Diatomacece consists, it
will be remembered, of those in which the frustules have a median
longitudinal line and a central nodule. In the first of the families
which it includes, that of Gocconeidecej the central nodvile is obscure
or altogethei" wantine- on one of the valves, which is distinguished as

1 See Bris'htwell in Quart. Joiirn. Microsc. Science, vol. iv. 1856, p. 105 ; vol. vi.
1858, p. 93 ; Wallich in Trans. Microsc. Soc. n.s. vol. viii, 1860, p. 48 ; and West, in
the same, p. 151.

DIATOM AOE.E

615

the inferior. This family consists but of a single genus, Cocconeis,
which includes, however, a great number of species, some or other of
them occurring in every part of the globe. Their form is usually
that of ellipsoidal discs, with surfaces more or less exactly parallel,
plane, or slightly curved ; and they are very commonly found ad-
herent to each othei-. The fiaistules in this genus are frequently in-
vested by a membranous envelope which forms a border to them ; but
this seems to belong to the immature state, subsequently disappear-
ing more or less completely.

i^H

-Bhiso- Fig. 461. — Achnanthes

imhri- longipes: a, b, c, d, e,

frustules in different

stages of binary divi-

FiG. 462. — Gomplionema gemi-
natiim : its frustules connected by
a dicliotomous stipe.

Another family in which there is a dissimilarity in the two
lateral surfaces is that of the Achnanthece, the frustules of which
are remarkable for the bend they show in the direction of their
length, often more conspicuously than in the example here repre-
sented. This family contains free, adherent, and stipitate forms,
one of the most common of the latter being Achnanthes longijjes
(fig. 461), which is often found growing on marine sdgse. The
difference between the markings of the upper and lower valves is
here distinctly seen ; for, while both are traversed by striae, which
are resolvable under a, sufiicient power into rows of dots, as well as

6l6 MICKOSCOPIC FORMS OF VEGETABLE LIFE- THALLOPHYTES

by a longitudinal line which sometimes has a noclnle at each end
(as in JVaviciUa), the lower valve (a) has also a transverse line form-
ing a staiiros, or cross, which is wanting in the upper valve (e). A
persistence of the connecting membrane, so as to form an additional
connection between the cells, may sometimes be observed in this
genus ; thus in fig. 461 it not only holds together the two new
frustules resulting from the subdivision of the lowest cell, a, which
are not yet completely separated the one from the other, but it may
be observed to invest the two frustules h and c, which have not
merely separated, but are themselves beginning to undergo binary
subdivision ; and it may also be perceived to invest the frustule d,
from which the frustule e, being the terminal one, has more com-
pletely freed itself.

In the family Cymhelleae, on the other hand, both valves possess
the longitudinal line with a nodule in the middle of its length ; but
the valves have the general form of those of the Eunotiece, and the
line is so much nearer one margin than the other that the nodule

is sometimes rather mar-
■^ 2 ^ ginal than central, as we

see in Cocconema (fig.
468,/).

The Go'mphonemem,
like the Meridiece, and
Licmo^yhorece, have frus-
tules which are cuneate
or wedge-shaped in their
front view (figs. 462,463),
but are distinguished
from those forms by the
presence of the longi-
tudinal line and central
nodule. Although there
are some free forms in
this family, the greatei-
part of them, included in the genus Gomphonema, have their frus-
tules either afiixed at their bases or attached to a stipe. This stipe
,seem.s to be formed by an exudation from the frustule, which is
secreted only during the process of binary division ; hence, when
this process has been completed, the extension of the single filament
below the frustule ceases ; but when it recommences, a sort of joint
or articulation is formed, from which a new filament begins to sprout
for each of the half-frustules ; and when these separate, they cany
apart the peduncles which support them as far as their divei'gence
can take place. It is in this manner that the dichotomous charactei-
is given to the entire stipe (fig. 462). The species of Gomphonema
are, with few exceptions, inhabitants of fresh water, and are among
the commonest forms of Diatonnaceoi.

Lastly, we come to the large family Navicidece, the members
of which are distinguished by the symmetry of their frustules, as
well in the lateral as in the fi-ont view, and by the presence of a
median longitudinal line and central nodule in both valves. In the

Fig. 463. — Gomphonema geminatum, more liiglily
magnified : A, side view of frustule; B, front view;
C, frustule in the act of division.

DIATOMACE.^i 617

genus Navicida and its allies the tVnstules are free or simply
adherent to each other ; while in another large section they are
included within a gelatinous eiivelope, or are enclosed in a defi-
nite tubular or gelatinous frond. Of the genus N^avicula an
immense number of species have been described, the grounds of
separation being often extremely trivial. Those which have a
lateral sigmoid curvature have been separated by Mr. W. Smith
under the designation Pleurosigma, which is now generally adopted ;
but his separation of another set of species under the name Pinnu-
laria (which had been previously applied by Ehrenberg to designate
the striated species), on the ground that its striae (costfe) are not
resolvable into dots, was not considered valid by Mr. Ralfs,
because in many of the more minute species it is impossible to
distinguish with certainty between stride and costa?. Mr. Slack has
since given an account of the resolution of the so-called costse of
twelve species of Pinnularice into ' beaded ' structures.^ The beauti-
ful genus Stauroneis, which belongs to the same group, differs from
all the preceding forms in having the central nodule of each valve
dilated laterally into a band fi'ee from strife, which forms a cross
with the longitudinal band. The multitudinous species of the genus
Navicula are for the most part inhabitants of fresh water ; and they
constitute a large part of most of the so-called ' infusorial earths '
which were deposited at the bottoms of lakes. Among the most re-
markable of such deposits are the substances largely used in the arts
for the polishing of metals, under the names of Tripoli and rotten -
stone ; these consist in great part of the frustules of Naviculce and
Pinnularice. The Polierschiefer, or ' polishing slate,' of Bilin in
Bohemia, the powder of which is largely used in Germany for the
same purpose, and which also furnishes the fine sand used for the most
delicate castings in iron, occurs in a series of beds averaging fourteen
feet in thickness, and these jjresent appearances which indicate that
they have been at some time exposed to a high temperature. The
well-known ' Turkey-stone,' so generally employed for the sharpening
of edge-tools, seems to be essentially composed of a similar aggrega-
tion of frustules of Naviculce, &c., which have been consolidated by
heat. The species oi Pleurosigma, on the other hand, are for the most
part either marine or are inhabitants of brackish water, and they
comparatively seldom present themselves in a fossilised state. Of
Stauroiieis some species inhabit fresh water, while others are marine ;
and the former present themselves frequently in certain ' infusorial
earths.'

Of the members of the sub-family Schizonemete, consisting of
those Naviculece in which the frustules are united by a gelatinous
envelope, some are remarkable for the great external resemblance
they bear to acknowledged algse. This is especially the case with
the genus tSchizonema, in which the gelatinous envelope forms a
regular tubular frond, more or less branched, and of nearly equal
diameter throughout, within which the frustules lie either in single
file or without any definite arrangement (fig. 464), all these frustules
having arisen from the binary division of one individual. In the
' Monthly Mitcroscopical Journal, vol. vi., 1871, p. 71.

6l8 MICROSCOPIC rOEMS OF VEGETABLE LIFE-THALLOPHYTES

genus Mastogloia, which is specially distinguished by having the
annulus furnished with internal costse projecting into the cavity of
the frustvile, each frustule is separately supported on a gelatinous
cushion (fig. 465, B), which may itself be either borne on a branching
stipe (A), or may be aggregated with others into an indefinite mass
(fig. 466). The careful study of these composite forms is a matter
of great importance, since it enables us to bring into comparison
with each other great numbers of frustules which have unquestionably
a common descent, and which must therefore be accounted as of the
same species, and thus to obtain an idea of the range of variation
prevailing in this group, without a knowledge of which specific defi-
nition is altogether unsafe. Of the very strongly marked varieties
which may occur within the limits of a single species, we have an

B'iG. 464. — Scliizonema Grevillii : A, natural size ; B, portion magnified five
diameters ; C, filament magnified 100 diameters ; D, single frustule.

example in the valves C, D, E, F (fig. 465), which would scarcely
have been supposed to belong to the same specific type did they not
occur upon the same stipe. The careful study of these varieties in
every instance in which any disposition to variation shows itself,
so as to reduce the enormous number of species with which ovir sys-
tematic treatises are loaded, is a pursuit of far greater real value
than the midtiplicatio7i of species by the detection of such minute
differences as may be presented by forms discovered in newly
explored localities ; such difierences as have already been pointed
out being, probably, in a large proportion of cases, the result of the
multiplication of some one form, which, under modifying influences
that we do not yet understand, has departed from the ordinary type.
The more faithfully and comprehensively this study is carried out in

DIATOMACEili:

619

any department of iiatnral history, the more does it prove that tlie
range of vaiiation is far greater than had been previously imagined ;
and this is especially likely' to be the case with such humble
organisms as those we have been considering, since they are obviously
more influenced than those of higher types by the conditions under
which they are developed ; whilst, from the very wide geographical
range through which the same forms are diffused, they are subject
to very great diversities of such conditions.

The general habits of this most interesting group cannot be
better stated than in the words of Mr. W. Smith :— ' The

Pig. 466.

Fig. 465. — Mastogloia Smithii: A, entire .stipe; B, frustule in its gelatinous
envelope; C-F, different forms of frustule as seen in side view; G-, front view
H, frustule undergoing subdivision.

Fig. 466. — Mastogloia lanceolata.

Diatomacece inhabit the sea or fresh water ; but the species peculiar
to the one are never found in a living state in the other locality ;
though there are some which prefer a medium of a mixed nature, and
are only to be met with in water more or less brackish. The latter ai-e
often found in great abundance and variety in districts occasionally
subject to marine influences, such as marshes in the neighbourhood
of the sea, or the deltas of rivers, where, on the occurrence of high
tides, the freshness of the water is affected by percolation from the
adjoining stream, or more directly by the occasional overflow of its
banks. Other favourite habitats of the Diatotnacece are stones of
mountain streams pv watei-falls, and the shallow pools left by the

620 MICKOSCOPIU FORMS OF A^EGETABLE LIFE— THALLOPHYTES

retiring tide at the mouths of our larger rivers. They are not, how-
ever, confined to the localities I have mentioned — they are, in £ict,
most ubiquitous, and thei-e is hardly a roadside ditch, water-trough,
oi- cistern, which will not rewai-d a search and furnish specimens
of the tribe.' Such is theii- abundance in some rivers and estuaries
that their multiplication is affirmed by Professor Ehrenberg to have
exercised an impoi'tant influence in blocking up harbours and
diminishing the depth of channels ! Of their extraordinary abundance
in certain parts of the ocean the best evidence is affiorded by the
observations of Sir J. D. Hooker upon the Diafomacece of the southern
seas ; for within the Antai'ctic Circle they are rendered peculiarly
conspicuous by becoming enclosed in the newly formed ice, and by

Fig. 467. — Fossil Diatomaceee, &c., from Gran: a, a, a, Coscinodiscus; o, h, b,
Actinocyclus ; c, Dictyochya fibula ; d, Lithasferisctcs radiatus ; e, Spongolithis
acictdaris ; f, f, Grammatojpliora parallela (side view) ; g, g, Gravimatophora
angulosa (front view).

being washed up in myriads by the sea on to the ' pack ' and ' bergs,'
every whei-e staining the white ice and snow a pale ochreous brown.
A deposit of mud, chiefly consisting of the siliceous valves of Diato-
viacece, not less than 400 miles long and 120 miles broad, was found
at a depth of between 200 and 400 feet on the flanks of Victoria Land
in 70° south latitude. Of the thickness of this deposit no conjecture
could be formed ; but that it must be continually increasing is evi-
dent, the silex of which it is in a great measure composed being-
indestructible. A fact of peculiar interest in connection with
this deposit is its extension over the submarine flanks of Mount
Erebus, an active volcano of 12,400 feet elevation, since a commu-
nication between the ocean waters and the bowels of a volcano, such

DiATOMACE^

621

;is there are othei- reasons for believing to be occasionally formed,
would account for the presence of Biatoraacem in volcanic ashes
and pumice which was discovered by Professor Ehrenberg. It is
remarked by Sir J. D. Hooker that the universal presence of this
microscopic vegetation throughout the South Polar Ocean is a most
important feature, since there is a marked deficiency in this
i-egion of higher forms of vegetation ; and were it not for them, theie
would neither be food for aquatic animals, nor (if it were possible
for these to maintain themselves by preying on one another) could
the ocean waters be purified of the carbonic acid Avhich animal
respiration and decomposition would be continually imparting to them.

Fig. 468. — Fossil Diatomacese, &c., from Mourne Mountains, Ireland : a, a, a,
Gaillonella (Melosira) procera and G. granvlata ; d, d, d, G. biseriata (side view) ;
b, b, Surirella plicata; c, S. craticula; k, S. caledonica ; e, Gotnphonema
gracile; /, Cocconema fusidiinn ; g, Tabellaria vulgaris; li, Pinnularia
dactylus; i, P. nobilis; I, Sgnedra ulna.

It is interesting to observe that some species of marine diatoms are
found through every degree of latitude between Spitzbergen and
Victoria Land, whilst others seem limited to particular regions. One
of the most singular instances of the preservation of diatomaceous
forms is their existence in guano, into which they must have passed
from the intestinal canals of the birds of whose accumulated excre-
ment that substance is composed, those birds having i-eceived them, it
is probable, from shell-fish, to which these minute organisms serve
as ordinary food.

The indestructible nature of the silicified casings of Diatomacece
has also served to perpetuate their presence in numerous localities
from which theii' living forms have long since disappeai-ed ; foi- the

622 MICKOSCOPIC FOEMS OF VEGETABLE LIFE— THALLOPHYTES

accumulation of sediment formed by their successive production and
death, even on the bed of the ocean or on the bottoms of fresh-
water lakes, gives rise to deposits which may attain considerable
thickness, and which, by subsequent changes of level, may come to
form part of the dry land. Thus very extensive siliceous strata,
consisting almost entirely of marine Diatoriiacece, are found to alter-
nate, in the neighbourhood of the Mediterranean, with calcareous
strata chiefly formed of Foraminifera, the whole series being the
representative of the chalk formation of ISTorthern Eui-ope, in which
the silex that was probably deposited at first in this form has under-
gone conversion into flint, by agencies hereafter to be considered.
Of the diatomaceous composition of these strata we have a character-
istic example in fig. 467, which represents the fossil Diatomacece of
Oran in Algeria. The so-called ' infusorial earth ' of Richmond in
Virginia, and also that of Bermuda, both marine deposits, are very
celebrated among microscopists for the number and beauty of the
forms they have yielded ; the former constitutes a stratum of eighteen
feet in thickness, underlying the whole city, and extending over an
area whose limits are not known. Several deposits of more limited
extent, and apparently of fresh-water origin, have been found in our
own islands ; as, for instance, at Dolgelly in N'orth Wales, at South
Mourne in Ireland (fig. 468), and in the island of Mull in Scotland.
Similar deposits in Sweden and Norway are known under the name
of Bergmehl, or mountain -flour ; and in times of scarcity the inha-
bitants of those countries are accustomed to mix these substances
with their dough in making bread. This has been supposed merely
to have the effect of giving increased bulk to their loaves, so as to
render the really nutritive portion more satisfying ; but as the
Bergmehl has been found to lose from a quarter to a third of its
weight by exposure to a red heat, there seems a strong probability
that it contains organic matter enough to render it nutritious in
itself. When thus occurring in strata of a fossil or sub-fossil
character, the diatomaceous deposits are generally distinguishable as
white or cream-coloured powders of extreme fineness.

For collecting fresh Diatoimacece those genei'al methods are to be
had recourse to which have been ali-eady described. ' Their living-
masses,' says Mr. W. Smith, ' present themselves as coloured fringes
attached to larger plants, or forming a covering to stones or rocks
in cushion-like tufts — or spi'ead over their surface as delicate velvet —
or depositing themselves as a filmy stratum on the mud, or inter-
mixed with the scum of living oi- decayed vegetation floating on the
surface of the water. Their colour is usually a yellowish-brown of a
gi-eater or less intensity, vaiying from a light chestnut in individual
specimens to a shade almost approaching black in the aggregated
masses. Their presence may often be detected, without the aid of a
microscope, by the absence, in many species, of the fibi-ous tenacity
which distinguishes other plants ; when removed from their natural
position, they become distributed thi-ough the water, and are held in
suspension by it, only subsiding after some little time has elapsed.'
Notwithstanding every care, the collected specimens are liable to be
mixed with much foi-eign mattei- ; this may be partly got rid of by

DIATOMACE.'E 623

repeated washings in pui-e water, and by taking advantage, at the
same time, of the difFei-ent specific gi-avities of the diatoms and of
the intermixed substances, to secure their separation. Sand, being
the heaviest, will subside first ; fine particles of mud, on the other
hand, will float after the diatoms have subsided. The tendency of
living diatoms to make their way towards the light will aifoi-d much
assistance in procuring the free forms in a tolerably clean state ; for
if the gathering which contains them be left undisturbed for a suffi-
cient length of time in a shallow vessel exposed to the sunlight, they
may be skimmed from the surface. Maiine foi-ms must be looked
for upon seaweeds, and in the fine mud or sand of soundings or
dredgings ; they are frequently found also, in considerable numbers,
in the stomachs of Holothurite, Ascidians, and Salp?e, in those of the
oyster, scallop, whelk, and other testaceous molluscs, in those of the
crab and lobster, and othei- Ci'ustacea, and even in those of the sole,
turbot, and other flat-fish. In fact, the diatom collector will do
well to examine the digestive cavity of any small aquatic animals
that may fall in his way, rare and beautiful foims having been
obtained from the interior of NoGtUuca. The sepai-ation of the
diatoms from the other contents of these stomachs must be accom-
plished by the same process as that by which they are obtained
from guano or the calcareous ' infusorial earths.' Of this the
following are the most essential particulars : The guano or eai-th is
first to be washed several times in pure watei-, which should be well
stirred, and the sediment then allowed to svibside for some hours
before the water is poured ofi'; since, if it be decanted too soon, it
may carry the lighter forms away with it. )Some kinds of earth
have so little imjaurity tha,t one washing sufiices ; but in any case it
is to be continued so long as the water remains coloured. The
deposit is then to be treated, in a flask or test-tube, with hydro-
chloric acid, and, after the first effervescence is over, a gentle
heat may be applied. As soon as the action has ceased, and time
has been given for the sediment to subside, the acid should be
poured off and another portion added ; and this should be repeated
as often as any efiect is produced. When hydrochloric acid ceases
to act, strong nitric acid should be substituted ; and aftei- the fii'st
effervescence is over, a continued heat of about 200° F. should be
applied for some hours. When sufficient time has been given for
subsidence, the acid may be poured off and the sediment treated with
another portion ; and this is to be i-epeated until no fuiiher action
takes place. The sediment is then to be Avashed until all trace of
the acid is removed ; and, if there have been no admixture of siliceous
sand in the earth or guano, this sediment will consist almost entirely
of Diatoynacece, with the addition, perhaps, of sponge-spicules. The
sepai'ation of siliceous sand and the subdivision of the entire aggre-
gate of diatoms into the larger and the finer kinds, may be accom-
plished by Stirling the sediment in a tall jai' of watei', and then,
while it is still in motion, pouiing off the supernatant fluid as soon
as the coarser particles have subsided ; this fluid should be set aside,
and, as soon as a finer sediment has subsided, it should again be
poiired ofi"; and this process may be repeated three or four times at

624 MICROSCOPIC FOEMS OF VEGETABLE LIFE— THALLOPHYTES

increasing intervals, until no further sediment subsides after the
lapse of half an hour. The first sediment will probably contain all
the sandy particles, with, perhaps, some of the largest diatoms,
which may be picked out from among them ; and the subsequent
sediments will consist almost exclusively of diatoms, the sizes of
which will be so graduated that the earliest sediments may be
examined with the lower powers, the next with medium powers,
while the latest will require the highest powers — a separation which
is attended with great convenience.'^ It sometimes happens that
fossilised diatoms are so strongly united to each other by siliceous
cement as not to be separable by ordinary methods ; in this case,
small lumps of the deposit should be boiled for a short time in a
weak alkaline solution, which will act upon this cement more readily
than on the siliceous frustules ; and as soon as the lump is softened,
so as to crumble to mud, this must be immediately washed in a large
quantity of water, and then treated in the usvial way. If a very
weak alkaline solution does not answer the purpose, a stronger one
may then be tried. This method, devised by Professoi- Bailey, has
been practised by him with much success in various cases. ^

The mode of mounting specimens of Diatoonacece. will depend
upon the piu-pose which they are intended to serve. If they can be
obtained quite fresh, and if it be desired that they should exhibit, as
closely as possible, the appearance presented by the living plants,
they should be put up in aqueous media within cement-cells ; but if
they are not thus mounted within a short time after they have been
gathered, about a tenth part of alcohol should be added to the water.
If it be desired to exhibit the stipitate forms in their natural position
adherent to other aquatic plants, the entire mass may be mounted
in Deane's medium or in glycerin jelly, in a deeper cell ; and such a
preparation is a very beautiful object for the background illumina-
tion. If, on the other hand, the minute structure of the siliceous
envelopes is the feature to be broiight into view, the fresh diatoms
must be boiled in nitric or hydrochloric acid, which must then be
poured off (sufiicient time being allowed for the deposit of the
residue) ; and the sediment, after being washed, should be boiled in
water with a small piece of soap, whereby the diatoms will be cleansed
from the flocculent matter which they often obstinately retain.^
After a further washing in pure water, they are to be either mounted
in balsam in the ordinary raanner, or be set up ' dry ' on a very thin
slide. In order to obtain a satisfactory view of their markings,
objectives of very large aperture are required, and all the improve-

1 A somewhat more complicated method of applying" the same principle is described
by Mr. Okeden in the Quart. Journ. Microsc. Science, vol. iii. 1855, p. 158.
The Author believes, however, that the method above described will answer every
purpose.

- For other methods of cleaning and preparing diatoms, see Quart. Journ. of
Microsc. Science, vol. vii. 1859, p. 167, and vol. i. n.s. 1861, p. 143; and Trans, of
Microsc. Soc. vol. xi. n.s. 1863, p. 4. A little book entitled Practical Directions
for Collecting, Preserving, Transporting, Preparing, and Mounting Diatoms (New
York, 1877), containing papers by Professors A. Mead Edwards, Christopher Johnson,
and Hamilton L. Smith, will be found to contain much useful information.

'' See Prof. H. L. Smith in Amer. Journ. of Microscopy, vol. v. 1880, p. 257.
It is important that the soap should be free from kaolin, silex, or any other insoluble
matter.

DIATOMACE.E ; PH.EOSPOKE.E 625

ments which have recently been inti-oduced in the construction and
mode of using the sub-stage condenser require to be put into prac-
tice. But to those who have the time, the will, and the appliances,
there is a fine field now open foi- working, to a far higher point than
we have touched at present, the true structui-e of such diatoms as
can be made amenable to the powers possessed by our best recent
optical appliances ; and for the leisure of a professional or commercial
man Ave know of no more suitable and attractive employment for the
microscope. It will often be convenient to mount certain particular
forms of Diatomacece separately from the general aggTegate ; but, on
account of their minuteness, they cannot be selected and removed
by the usual means. The larger forms, which may be readily distin-
guished under a simple microscope, may be taken up by a camel's-
hair pencil which has been so trimmed as to leave two or three hairs
projecting beyond the rest. But the smallei' can only be dealt with
by a single fine bristle or stout sable-hair, wdiich may be inserted
into the cleft end of a slender wooden handle ; and if the bristle or
hair should be split at its extremity in a brush-like manner it will
be particularly useful. (Such split hairs may always be found in a
shaving-brush which has been for some time in use ; those should be
selected which have their split portions so closely in contact that
they appear single until touched at their ends.) Wlien the split
extremity of such a hair touches the glass slide, its parts separate
from each other to an amount proportionate to the pressure ; and, on
being brought up to the object, first pushed to the edge of the fluid
on the slide, may generally be made to seize it. A very experienced
American diatomist. Professor Hamilton Smith, strongly recom-
mends a thread of glass drawn out to capillary fineness and flexi-
bility, by which (he says) the most delicate diatom may be safely
taken up, and dejDOsited upon a slide damjDed b}^ the breath. For
the selection and transference of diatoms under the compound
microscope, recourse may be had to some of the forms of ' mechanical
finger ' which have been devised by American diatomists.^

Phseosporeae. — The greater number of the seaweeds exhibit a
higher type of organisation than any that has hitherto been described.
The old classification of seaweeds into Melanos'porece, Rhodosjiorece,
and Ghlorosj)orece, according as their colouring matter is olive-brown,
red, or green, cannot altogether be retained. Under the head of
Phmosporece, are now included a very large number of the brown and
olive-brown seaweeds. In ascending this series we shall have to
notice a gradual differentiation of oi-gans, those set apart for repro-
duction being in the first place separated from those appi'opriated

^ For a description of those of Prof. Hamilton Smith and Dr. Rezner, see Journ.
of Boy. Microsc. Soc. vol. ii. 1879, p. 951, and that of Mr. Veeder, vol. iii. 1880,
p. 700, of the same Journal.

[A very large number of observations have been made during recent years by
Castracane, O. Miiller, Lauterborn, Comber, Murray, Miquel, and others, on the
structure of the diatom-valve, on the various modes of reproduction, and on the
phenomena accompanying their apparently spontaneous powers of motion, and
several schemes of classiiication of the genera have been proposed. On these, too
numerous to mention here, and some of which still require confirmation, the reader
should consult the successive volumes of the Journal of the Royal Microscopical
Society. — Ed.]

S S

626 MICEOSCOPIC FOEMS OF VEGETABLE LIFE- THALLOPHYTES

to nutrition ; while the principal parts of the nutritive apparatus,
which are at first so blended into a uniform expansion or thalhis
that no real distinction exists between root, stem, and leaf, are
progressively evolved on tyi^es more and more pecuHar to each
respectively, and have their functions more and more limited to
themselves alone. Hence we find a ' difierentiation,' not merely in
the external form of organs, but also in their internal structure, its
degree bearing a close correspondence to the degree in which their
functions are respectively sjiecialised or Hmited to particular actions.
But this takes place by very slow gradations, a change of external
form often showing itself before there is any decided difierentiation
either in structui-e or function. Thus in the simple Ulvacece, what-
ever may be the extent of the thallus, every part has exactly the
same structure, and j^ei-forms the same actions, as every other part,
living yb?" and by itself alone. And though, when we pass to the
higher seaweeds, such as the common Fucus and Laminaria, we
observe a certain foreshadowing of the distinction between root,
stem, and leaf, this distinction is very imperfectly carried out, the
root-like and stem-like portions serving for little else than the
mechanical attachment of the leaf-like part of the plant. There is
not yet any departure from the simple cellular type of structui-e,
the only raodification being that the several layei-s of cells, where
many exist, are of dififerent sizes and shapes, the texture being
usually closer on the exterior and looser within, and that the tex-
ture of the stem and roots is denser than that of the leaf- like expan-
sions or fronds. The cells of the Phceosporece contain a svibstance
closely resembling starch, and an olive-brown pigment, which they
share with the Fucacece, known 2.^ phyco-phcein oy fuco-xanthin. The
group of olive-green seaweeds presents us with the lowest type in
the family Ectocmyacece, which, notwithstanding, contains some of the
most elegant structui'es that are anywhere to be found in the group,
the full beauty of which can only be discerned by the microscojje.
Such is the case, for example, with Sphacelaria, a small and delicate
seaweed, which is very commonly found growing upon larger algse,
either near low-water mark or altogether submerged, its general
form being remarkably characterised by a symmetry that extends also
to the individual branches, the ends of which, however, have a
decayed look. The apical cell of each branch is uncorticated, and
frequently develops into a hollow chamber of considerable size,
termed a sphacele, and filled, when young, with a dark mucilaginous
substance which, at a later stage, becomes watery. The Sphacela-
riacece are propagated in a non-sexual manner by peculiai- buds or
gemmae known as propagxdes.

The ordinary mode of propagation of the Phceosjyorece is by non-
sexual zo6s]3ores ; and these are of two kinds, produced respectively in
unilocular and multilocular zocisporanges. The foi-mer are compara-
tively large, nearly spherical, ovoid or pear-shaped cells, the contents
of which break up into a lai'ge number of zoospores. The multi-
locular zoospoi-anges have the appeai-ance of jointed hairs, and ai-e
divided intei'naily into a number of chambers, each of which gives
birth to a single zoospore. The zoospores from the uniloculai-

PH^OSPOKE^ ; FUCACE^

627

sporanges appear in all cases to germinate directly, while those from
the multiloculai- sporanges sometimes coalesce in pairs before ger-
minating. The different families of Phceosjwrece present a most
interesting gradual transition fi-om the conjugation of swarm-cells
to the impregnation of a female ' oosphere ' by male antherozoids.
In Ectocarpus, GiraucUa, and ScytoslpJion, conjugation takes place
between swarm-cells from the multilocular sporanges which appear
to be exactly alike, but a slight differentiation is exhibited in one of
them coming to i-est and partially losing its cilia before conjugation
takes place (fig. 469, II). Male sexual organs also occur in the
S'phacelariacece, but no actual process of conjugation has as yet been
observed. In Cutleria and Zanar-
dinia the diffei-entiation is more
complete. The male and female
swarm-cells are produced either on
the same or on diffei-ent individuals ;
the latter are much lai-ger than the
former, and come perfectly to rest,
entirely losing their cilia before
being impregnated by the former.
In Dictyota the differentiation is
carried still further, and the
female reproductive bodies are true
' oospheres,' being from the first
motionless masses of pi-otoplasm not
provided with cilia, while the an-
therozoids exhibit motility only foi-
a very short time, and each is pro-
vided only with a single cilium of
unusual length. In the family
Laminar iacece, belonging to the
Phceosporeoi, are included many of
the largest of the seaweeds, chiefly
natives of southern seas, the frond
often attaining enormous dimen-
sions, and exliibiting i-udimentary
differentiation into rhizoids or
organs of attachment, stem, and
leaves. Such are Lessonia, which
grows to a great height and re-
sembles a branching tree with pendent leaves two oi- three feet
long ; Macrocystis, where the stalk-like base of each branch of the
leaf is hoUowed out into a large peai'-shaped air-bladdei' ; Nereocystisj
Laminaria, and othei's.

In the Fucacese the generative apparatus is contained in the
globular ' conceptacles,' which are usually sunk in the tissue near the
extremities of the fronds. In some species, as Fucus platycarjiUjS^
the same conceptacles contain both ' antherids ' and ' oogones ; ' in
othei's these two sexual elements are disposed in different conceptacles
on the same plant ; whilst in the commonest of all, F. vesiculosus
(bladder- wrack), they are limited to different individuals. When a

s s 2

Pig. 469. — Process of conjugation
in Ectocaijjtis siliculosus. (Prom
Vines's ' Physiology.') I. a-f, the-
female zoospore coming to rest ;
II., the female zoospore at rest,
surrounded by male zoospores ;
III. a-e, fusion of male and female
zoospores.

628 MICEOSCOPIC EOEMS OF VEGETABLE LIFE— THALLOPHYTES

section is made through one of the flattened conceptacles of F. platy-
carpus, its interior is seen to be a nearly globular cavity (fig. 470),
lined with hairs, some of which are greatly elongated, so as to
project through the pore by which the cavity opens on the surface.
Among these are to be distinguished, towards the period of their
maturity, certain filaments (fig. 471, A),, the anth^rids, whose
granular contents acquire an orange hue, and gradually shape
themselves into oval bodies (B), each with an orange-coloured spot
and two vibratile cilia of unequal length, placed laterally, which,
when discharged by the rupture of the containing cell, have for a

time a rapid, undulatory
motion whei'eby these
anthtrozoids are difiused
through the surround-
ing liquid. Lying amidst
the mass of hairs, near
the walls of the cavity,
are seen (fig. 470)
numerous dark pear-
shaped bodies, which are
the oogones, or parent-
cells of the ods])]ieres.
Each of these oogones
gives origin, by binary
subdivision, to a cluster
of eight ' germ-cells ' or
oospheres ; and these
are liberated from their
envelopes before the act
of fei'tilisation takes
place. This act consists
in the swaiming of the
antherozoids over the
surface of the oospheres,
to which they communi-
cate a rotatory motion
by the vibration of their
own cilia. In the herm-
'PiG. 470. — Vertical section of conceptacle of Fucun aphrodite /^ctci this takes
jpZa^ycfM^^its lined with filaments, among which lie ,-,]Qpp within the cori-

ceptacles, so that the
oospheres do not make
their exit from the cavity until after they have been fecundated ;
but in the monoecious and dioecious species each kind of conceptacle
separately discharges its contents, which come into contact on their
exterior. The antheridial cells are usually ejected entire, but soon
I'upture so as to give exit to the antherozoids ; and the oogones also
discharge their oospheres, which, meeting with antherozoids, are
fecundated by them. The fertilised oospores soon acquire a new and
firm envelope ; and, under fixvourable circumstances, they speedily
begin to develop themselves into new plants. The first change is

the antheridial cells and the oogones containing
oospheres.

FUCACE^:

629

the projection and narrowing of one end into a kind of foot-stalk,
by which the oospore attaclies itself, its form passing from the
globulai' to the pear-shaped ; a partition is speedily observable in its
interior, its single cell being svibdivided into two ; and by a con-
tinuation of a like process of bipartition, first a filament and then
a fi-ondose expansion is produced, which gradually evolves itself
into the likeness of the parent plant.

The whole of this process may be watched without diificulty by
obtaining specimens of F. vesiculosus at the period at which the
fructification is shown to be mature by the recent discharge of the
contents of the conceptacles in little gelatinous masses outside their
orifices ; for if some of the oospheres which have been set free from
the olive-green (female) conceptacles be placed in a drop of sea-
water in a very shallow cell, and a small quantity of the mass of

Fig. 471. — Antherids and antherozoids of Fttcus j)latycarpus : A, branching
articulated hairS; detached from the walls of the conceptacle, bearing antherids in
different stages of development ; B, antherozoids, some of them free, others still
included in their antheridial cells.

antherozoids, set free from the orange-yellow (male) conceptacles,
be mingled with the fluid, they will speedily be observed, with the
aid of a magnifying power of 200 or 250 diameters, to go through
the actions just described ; and the svibsequent processes of germi-
nation may be watched by means of the ' growing slide.' ^ The
winter months, from December to March, are the most favourable
for the observation of these phenomena ; but where Fuci abound,
some individuals will usually be found in fructification at almost
any period of the yeai". This process of fertilisation usually takes
place on fi-onds exposed to the air on the wet beach between high-
and low- water mark ; and, to assist in it, the comparatively heavy
fronds of many Fucacece are buoyed up by air-cavities, which take
the form of the well-known ' bladders ' of the ' bladder- wrack ' and

1 A shallow cell should be used, so as to keep the pressure of the thin glass from
the minute bodies beneath, whose movements it will otherwise impede.

630 MICEOSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

other species of Fucus, imbedded in the frond, and the ' berries ' of
Sargassuni hacciferutn, the ' gulf- weed ' of the Atlantic, which are
elevated on pedicels above the surface of the water. The whole
substance of the Fucacece, including the reproductive organs, is
coloured brown by fuco-xanthin , the same pigment as that which is
found in the Phceosporece.

Among the Floridese, or red seaweeds, also, we find various
simple but most beautiful forms, which connect this gxoup with the
lower algse, especially with the family Coleochcetacece ; such delicate
feathery or leaf-like fronds belong for the most part to the family
Gerainiacece, some members of which are found upon every part of

Fig. 472. — Arrangement of tetraspores in Cmyocanlon mediterraneitm : A, entire
plant; B, longitudinal section of spore-bearing branch. (N.B. — "Where only three
tetraspores are seen, it is merely because the fourth did not hajjpen to be so jilaced
as to be seen at the same view.)

our coasts, attached either to rocks or stones or to larger alg^e, and
often themselves affording an attachment to zoophytes and polyzoa.
They chiefly live in deeper water than the other seaweeds, and
their richest tints are only exhibited when they grow under the
shade of projecting rocks or of larger dark-coloured algee. Hence,
in growing them artificially in aquaria, it is requisite to protect
them from an excess of light, since otherwise they become unhealthy.
Various species of the genera Ceramium, Griffithsia, Callithamnion^
and Ptilota are extremely beautiful objects for low powers when
mounted in glycerin jelly. In many of them the phenomenon to
which we have previously referi-ed under the name of ' continuity of
pi'otoplasm ' is very beautifully exhibited. The colour of the red

FLORIDEiE

631

seaweeds is due to the presence of a pigment known as rhodos'periiiin
or pJiyco-eri/thrin, soluble in fi-esli watei', which may be separated in
the form of beautiful regular crystals.

The only mode of propagation which was until I'ecently known
to exist in this group of seaweeds is the production and liberation
of tetraspores (fig. 472, B), formed by two successive binary
subdivisions of the contents of special cells, which sometimes
form part of the genei'al substance of the fi'ond, but sometimes
congregate in paiticular parts or are restricted to special bi-anches.
If the second binary division takes place in the same direction as
the first, the tetraspores are arranged in linear series ; but if its

Fig. 473. — Nemalion multifidum : I, a branch with a carpogone, c, and pollmoids,
sp ; II, III, coniinencenient of the formation of the fructification ; IV, V, develop-
ment of tlie spore-cluster ; t denotes the trichogyne, c the carpogone and fructifica-
tion. (From Groebel's ' Outline of Classification.' The Clarendon Press.)

direction is transverse to that of the fii-st, the foui- spoi-es cluster
together. These, when separated by the rupture of their envelope,
do not comport themselves as zoospores ; but, being destitute of
propulsive organs, are passively dispersed by the motion of the sea
itself. Their production, however, taking place by simple cell-
division, and not being the result of any form of sexual conjugation,
the tetraspores of the Floridece must be regarded, like the
zoospores of the Ulvacece, as gonids, analogous rather to the htuls
than to the seeds of higher plants. It is now known that a true
sexual process takes place in this group ; but the sexual organs
are not usually found on the plants which produce teti-aspores.
Antheridial cells are found, sometimes on the general surface of the
frond, more commonly at the ends of branches, and occasionally in
special conceptacles. Their contents, however, are not motile

632 MICEOSCOPIC FOKMS OF VEGETABLE LIFE— THALLOPHYTES

antherozoids, but minute rounded particles, known as pollinoids or
' spermatia/ having no power of sjjontaneous movement. Some-
times on the same individuals as the antherids, and sometimes on
different ones, are produced the female oi^gans, which curiously
prefigure the pistil in flowering plants. This organ is known as
the 2)'>'ocar2y, and consists, in its simplest for-m, e.g. in Porphyra^ the
' purple laver,' of a single cell with a lateral hair-like appendage, the
trichogyne. In the higher forms it is composed of one or more
fertile cells constituting the caiyogone, and one or more sterile cells
which make up the trichojyhore, and convey the fertilising substance
from the trichogyne to the carpogone. Fertilisation is efiected by
the attachment of one of the pollinoids to the trichogyne, the walls of
which are absorbed at that spot, so that the fertilising material passes
down its tube to the trichophore, and thence to the carpogone ; one
of the cells of the carpogone contains the oosphere, which, aftei-
fertilisation, breaks uj) into a number of carpospores ; round these
is freqviently formed a hard investment, and this structure is then
known as a cystocarp ; from it the carpospores ultimately escape,
and then germinate. In the true Corallines, which are Floridecp
whose tissue is consolidated by calcareous deposit, not only the
tetraspores, but also both kinds of sexual organ, are produced in
cavities or concejJtacles, imbedded in the thallus or forming wart-like
swellings ; the female conceptacle opens by a terminal orifice or
ostiole ; the pollinoids are furnished with wing-like appendages.
In a considerable number of the red seaweeds, as, for example, in
Dudresnaya, the process of fertilisation is more complex than this,
and consists of two distinct stages. First the trichogyne is impreg-
nated by the pollinoids ; and secondly, the fertilising principle is
then conveyed from the trichophore-cells at the base of the tricho-
gyne to the cells which ultimately produce the carpospores, and which
may be at a considerable distance from the trichogyne, even on a
different branch. This transference is effected by means of long
simple or branched tubes which are known as ' fertilising tubes.'
The late Professor F. Schmitz held that, in the higher Floride^e,
there are two acts of fertilisation, that of the pollinoid with the
trichogyne, and that of the fertilising tube with the cells which produce
the carpospores ; but this view is not accepted by all authorities ;
and it is doubtful whether more than one true act of fertilisation,
i.e. the fusion of male and female nuclei, takes place. The sexual
mode of reproduction has, however, at present been observed in
comparatively few species of seaweed ; and considering the number
of species of Floridece found on our coasts, there is no branch of
microscopical observation which is more likely to reward the young-
investigator with new discoveries.

633

CHAPTER IX

FUNGI

Fungi, as already mentioned, differ essentially from algse in the
absence of chlorophyll, and therefore in the absence of any power of
directly forming starch or other similar substance by the mutual
decomposition of carbonic acid and water, accompanied by evolution
of oxygen. They must therefore, in all cases, be either saprophytes
or parasites, deriving their nourishment from already organised food-
materials, either, in the former case, from decaying animal or vege-
table substances, or, in the latter case, from the living tissues of
other plants or of animals. Fungus-parasites are the cause of most
of the diseases to which plants, and of a large number of those to
which animals, are subject.

The individual fungus always consists of one or more liyplue,
slender filaments containing protoplasm and a nucleus (except possibly
in some of the most simple forms), but no chlorophyll and rarely any
pigment. The cell-wall is composed of a substance differing some-
what in its properties from ordinary cellulose, since it is not coloured
blue by iodine after treatment with sulphuric acid ; it is known as
fungus-cellxdose. These hyphse may be quite distinct or very loosely
attached to one another ; those which penetrate the soil, or the
tissue of the ' host ' on which the fungus is parasitic, constitute the
mycele. In the larger fungi, such as the mushroom, the portion
above the soil is composed of a dense mass of these liyphse, lying
side by side, constituting a so-called p)seiido-parenchyme, but never a
true tissue. In some families the hyphse have a tendency to become
agglomerated into balls of great hardness called sclerotes, which have
the power of maintaining their vitality for very long periods. The
modes of reproduction of fungi, both sexual and "non-sexual, are
very various. Among the latter the most common are by non-
motile s2)ores or gonicls, and by zoospores. The former are very
minute bodies, each composed of a single cell, or less often of several
cells, which are either formed within a spore-case or sjyorange, or
are detached from the extremity of hyphse by a process of pinching
off or abstriction. From their extreme lightness they are wafted
through the air in enormous nurabers, and thus bring about the
extraordinarily rapid spread of many fungi, such as moulds. The
zoospores are, like those of the lower algse, minute naked masses of
protoplasm provided with one or more vibratile cilia, by means of
which they move very rapidly through water, and finally force their
way into the tissue of the host, where the zoospore loses its cilia.

634 FUXGi

invests itself with a cell-wall, and proceeds to germinate. This is
effected, both in the case of the zoospores and in that of the ordinary
spores, by putting out a germinating filament, which ultimately
develops into the new fungus plant. In a lai-ge number of fungi
no process of sexual reproduction is known. The varioiis modes
which do occur will be described under the separate families.

Some families of fungi are characterised by the remarkable pheno-
menon known as alternation of generations. Each species occurs in
two (or sometimes three) perfectly distinct foi^ms, which bear no
resemblance to one another, and were long svipposed to belong to
widely separated families. Each phase or ' generation ' has its own
mode of reproduction, but does not reproduce its own special form,
but the other or one of the other forms ; and two or three generations
are thus required to complete the cycle. Each member of the cycle
is, generally speaking, parasitic on a totally different plant from the
' host ' of the other forms.

The classification of fungi is attended with very great difficulties,
owing to our still imperfect acquaintance with the mode of reproduc-
tion in several of the grovips. The following are the more distinct
and remarkable types : ' —

The Myxomycetes, Myxogastres, or Mycetozoa, are a group of very
singular organisms, on the very confines of the animal and vege-
table kingdoms, doubtfully included among the fungi, and believed
by many to have a closer affinity to the rhizopods. They appear,
indeed, at one period of their life-history to have an animal, at another
period a vegetable mode of existence. Several species are not un-
common on decayed wood, bark, heaps of decaying leaves, &c. The
' plasmode ' of uEthalium sejJticum, known as ' flowers of tan,' forms
yellow flocculent masses in tan-pits. The development of other
species is represented in fig. 474. Commencing with the germina-
tion of the spores, each sjoore is a spherical cell (0) enclosed in a
delicate membranous wall ; and when it falls into water this wall
undergoes rupture (D), and an amoeba-like body (E) escapes from
it, consisting of a little mass of protoplasm, with a round central
nucleus enclosing a nucleole and a contractile vesicle, and having
amoeba -like movements connected with the protrusion and withdrawal
of peculiar processes or pseudopodes. This soon elongates (F), and
becomes pointed at one end, whence a long Jlagelliom is put forth, the
lashing action of which gives motion to the body, which may now be
termed a sivarm-spore. After a time the flagellum disappears and the
active movements of the spore cease ; but it now begins again to put
forth and to withdraw finger-like pseudopodes, by means of which it
creeps about like an Arnceba, and feeds like that rhizopod upon solid
particles which it engulfs within its soft protoplasm. These swarm-
cells may multiply by bipai-tition to an indefinite extent ; but after
a time ' conjugation ' takes place between two of these myxamcebce
(H), their svibstance undergoing a complete fusion into one body (I),

1 [The classification of fungi here adopted is essentially that of De Bary in his
Comi^arative Morphology and 'Biology of the Fungi, Mycetozoa, and Bacteria.
Owing to the very large recent additions to our knowledge of the structure of fungi,
it has been found necessary entirely to rearrange this portion of Dr. Carpenter's
work. — Ed.]

MYXOMYCETES

635

fi'om which extensions ai-e put forth (J) ; and by the tinion of a
number of these bodies are produced the motile protoplasmic bodies
known as jylasmodes, the oixlinary foi-m in which these singular bodies
are known. These continue to gi-ow by the ingestion and assimila-
tion of the solid nutriment which they take into theii- substance ;
and, by the ramification and inosculation of these extensions, a
complete network is formed.

The filaments of this network exhibit active undulatory move-

FiG. 474. — Development of Myxomycetes : A, plasmode of Didymvum serpula ; B,
successive stages, a, a', b, of sporanges of Arcyria flava ; C, ripe spore of
Physarum album ; D, its contents escaping ; E, F, G, the swarm-spore first
becoming flagellated, and then amoeboid ; H, conjugation of two amoeboids, which,
at I, have fused together, and, at J, are beginning to put out extensions and ingest
nutriment, of which two pellets are seen in its interior.

ments, which in the larger ones are visible under an ordinary lens,
or even to the naked eye, but which it i-equii'es microscopic power to
discern in the smaller. "With sufficiently high amplification, a con-
stant movement of granules may be seen flowing along the threads,
and streaming from branch to branch. Hei'e and there ofishoots of
the protoplasm are projected, and again withdi-awn, in the manner of
the pseudopodes of an Amoeba ; while the whole organism may be
occasionally seen to abandon the suppoi-t over which it had gi-own,

636 FUNGI

and to creejD over neighbouring surfaces, thus far resembling in all
respects a colossal ramified Amceba. The plasmodes are often found
to have taken up into them and enclosed a gi-eat variety of foreign
bodies, such as the spores of fungi, parts of plants, &c. They are
curioixsly sensitive to light, and may sometimes be found to have
retreated during the day to the dark side of the leaves, or into the
recesses of the tan over which they had been growing, and again to
creep out on the approach of night. Under certain conditions the
swarm-spores may lose their power of motion and become encysted ;
they are then known as microcysts, and may remain in this resting
condition for a considerable time, especially if desiccated. If again
placed in water, they return to their motile swarming state. The
plasmodes may also enter a resting state, in which they assume a wax-
like consistence, and dry up into a brittle horny mass. They are then
known as sderotes. In a few genera the spores are not contained in
sporanges, but are borne on external supports or sporophores. But
in the gi'eat majority of genera the plasmode becomes ultimately
tiunsformed into sporanges (B, a, a', h) ; either each plasmode
becomes a- single sporange, or it divides into a larger or smaller
number of pieces, each of which undergoes this transformation.
When mature, the cavity of the sporange is either entirely filled
with the very numerous spores^ or in most genera tubes or threads of
different forms occur among the spores, and constitute the cajnllitium.
These capillitium-tubes have often a spiral appearance, owing to
irregular thickenings of the cell- wall, and are very beautiful objects
under the microscope. The growth of many species of Myxomycetes
is exceedingly rapid, going throvigh their whole cycle of development,
with its various phases, in the course of a few days.

The ChytridiaceSB are a group of minute microscopic fungi showing
an afiinity in some respects to the Myxomycetes, and even to the
infusorial animalcules. Their ordinary mode of propagation is by
zo(5spores bearing one or two cilia, which either germinate directly
or conjugate to produce a resting-spore. They are parasitic on fresh-
water organisms, both animal and vegetable ; and their chief interest
to the microscopist is that their zoospores have apparently frequently
been mistaken for antherozoids of the ' host.'

The TJstilaginese are fungi parasitic on flowering plants, attacking
the stem, leaves, and other parts, where they form brown or yellow
spots. They are often exceedingly destructive to vegetation, causing
the diseases of cereal crops known as bunt, smut, &c. The course
of development of these fungi is not yet in all cases accurately
known. The mycele, consisting of slender segmented hyphse, spreads
extensively within the tissues of the host, and bears spores which
either repi-oduce the mycele again directly, or with the intervention
of so-called ' sporids.'

The Uredineae afford the most remarkable illustration among
fungi of the phenomenon already mentioned, that of alternation of
getierations ; forms previously considered to belong to widely separated
groups being now known to be stages in the cycle of development of
the same species. A striking instance of this is furnished by the
well-known and very destructive disease of wheat and other grasses

UEEDINE.E

^1^1

known as ' mildew,' produced by the attacks of the pai-asitic fungus
Puccinia graminis. It -was long ago observed that wheat was
especially liable to this disease in the vicinity of bai'berry bushes ;
and it is now known that a fungus parasitic on bai-berry leaves, foi'-
merly known as .Ecidium herheridis, is the ' fecidiospore ' generation
of the same species of which Puccinia graminis is the ' teleutospore '

Fig. 475. — Puccixia graminis. From De Bary's ' Comparative Morphology and
Biology of the Fungi.' (The Clarendon Press.) a, portion of leaf oiBerheris
with young iscidium; I., section through leaf containing iKcidia ; s^j, sx^ermo-
gones; a, tecidia opened; jj, x^eridiuni ; II., group of ripe teleutospores
bursting through the epiderm e in leaf of Triticum repeals ; t, teleutospores ;
III., teleutospores t, and uredospores ur ; I. slightly magnified ; II. x 190 ;
III. X 390.

generation. The complete cycle of development of the best known
Uredinece, such as the mildew (fig. 475), is this. The form known as
Ptcccinia graminis produces teleutos'jmres, thick- walled spores, boi-ne
usuall}^ in pairs, at the extremity of elongated cells known as basids
or sterigmata. Each of these teleutospores gives rise, on germinating
within the tissue of the grass, to a hypha ov promijcde., the terminal
cells of which develop, on slender basids, each a single spore or

638

FUISGH

sporid. These sporids will germinate only on the leaves of the bar-
berry, where they produce, first of all, a mass of interwoven hyphfe
within the tissue, and then the peculiar reproductive bodies known
as cecidia (fig. 476). The ' eecidium ' is a cup-shaped receptacle of a
bright red or yellow colour, which breaks through the epiderm of
the leaf, and discharges a large number of cecidiosjwres, which are
produced in rows or chains springing from basids at the base of the
receptacle. These are accompanied, often on the other surface of the
leaf, by spermorjones, smaller spherical or flask-shaped receptacles,
which also eventually break through the epiderm, and are filled with
barren hyphae known as paraphyses. Among these are other shorter
hyphfe or ' sterigmata,' from the extremities of which are abstricted
narrow ellipsoidal cells, the sjjermatia. The purpose of these is
unknown ; but they miay be male elements which have lost theii-
function. The ^cidiospores will germinate only on the leaves and
stems of grasses, either producing the teleutospore-form directly, or

Fic 476 — Mcidium iu&silagims A, portion of the plant, magnified ; B, section
of one ot the ' secidia ' with its sjpores.

giving rise to a third ' uredo-form.' This consists of filiform basids,
each of which bears a round oval spore, the uredospore, which ger-
minates veiy rapidly, constantly reproducing the same form. The
same mycele which produces the uredo-form also gives rise subse-
quently to the teleutospore-form. The fungus usually hibernates
and remains in a state of rest in the teleutospore-form.

Of the Peronosporese (fig. 477) some species grow on the dead
bodies of animals and on dead plants, others are parasitic in the
living tissues of flowering plants, causing widespread diseases, such
as the potato -blight. On the mycele, consisting of a number of dis-
tinct septated hyphse, are produced the sexual organs, oogones and
antlierids. Fertilisation is not effected by means of motile anthero-
zoids, as in other classes of fungi and of algae, but the antherid puts
out a cylindrical or conical tube-like pi-ocess, the fertilisation-tuhe.
The antherids and oogones ai'e each single enlai'ged cells produced in
close proximity to one another ; the fertilisation-tube is produced
from the part of the antherid which is in immediate contact with

PERONOSPOREJ^:

639

the oogoiie, and discharges into the lattei- the contents of the
antherid, thus causing its protoplasmic contents or ' oosphere ' to
develop into the impregnated ' oospore.' The further history of the
oospore is singularly different, even in different species of the same
genus. In some it germinates directly into a new mycele ; in others
it bi-eaks up into a number of swarm-spores or zoospores ; each of these
comes to rest, and after a time germinates into a new mycele. In

Fig. 477. — A-G, Cijstopus candiclus : H, Pliytopldhora mfestans. A, branch of
mycele growing at the apex, t, with haustoria, h, between the cells of the pith of
Lepidiitm sativum ; B, branch of mycele bearing gonids ; C, D, E, formation of
swarm-spores from gonids; F, swarm-spores germinating; G-, swarm-si^ores
germinating on a stomate and piercing the epiiderm of the stem of a potato at H.
After De Bary ; magnified about 400 times. From ' Outlines of Classification and
Special Morphology of Plants,' by Dr. K. Goebel.

addition to the sexual organs of reproduction, many species of Perono-
sporepe also produce non-sexual spores or gonids, which are borne on
sjaecial branches springing ei'ect from the mycele, the sjioroj^hores ov
yonidiophores. A similar difference is exhibited in the further
development of these spores. Either they germinate directly in watei-
into a new mycele, or the protoplasmic contents break up into a
number of zoospores which germinate in the same way. In those

640

FUNGI

species which are parasitic on living plants, such as PhytophtJiora
infestans, which produces the potato-disease, and Cystopus candidus,
very common on cress and other cruciferous plants, the rapid spread
of the disease is caused by the great facility with which the spores
are disseminated by the wind ; falling on leaves in moist weathei-,
they there germinate ; the gei'minating tube passes through a

stomate, and the mycele is developed with
great rapidity within the tissvie of the
host. The most favourable condition in
the case of the potato-disease is said by
Professor De Bary to consist in an
undue thinness of the cuticle, accompanied
by excessive humidity, whereby the spores
of the fungus will gei-minate on the sur-
face of the plant, sending out processes
wdiich penetrate to its interior, though
otherwise germinating only on cut sur-
faces.

The Saprolegnieae are saprophytic or
parasitic fungi, nearly allied to the Pero-
nosporeoi, and differing from them chiefly
in two points : although organs are
known in many species closely resem-
bling the antherids of the PeronosporeK,
the act of impregnation has not actually
been observed, the oospore being, at least
in many cases, apparently produced jsar-
thenogenetically, i.e. without impregna-
tion. In some species a single oospore is
produced within each oogone ; but more
often the contents of the latter break up
into a number of oospores, each of which
gives rise to a mycele, or breaks up into
zoospores. In some genera, e.g. Achlya
(fig. 478), zoosjDores are also produced in
very large numbei-s by the breaking-up
of the contents of zoosporcoiges, special
enlai'ged cells of the mycele. The well-
known salmon disease is caiised by the
attacks of the pai-asitic Scqyrolegiiia ferax
on the living flesh of the animal.

The Mucorini are filamentous fungi,
resembling the two last orders in their
vegetative development, but differing in
their mode of i-eproduction. To this family belong some of the most
common moulds which make their appearance on damp or decaying
organic sulistances. The ordinary mode of non- sexual repi-oduction
is by etulogenous spores, produced within a sporange (fig. 479, A).
The sporanges are boi'ne at the ends of sporangiophoi'es, long, erect,
unseptated hyphte, spi-inging directly from the mycele or from
tlie original germinating filament. Several othei- kinds of non-

PiG. 478. — Two zoosporanges of
Achlya. From Goebel's' Out-
lines of Classification and
Specdal Morphology.' a, still
closed; B, open to discharge
the zoiJsi5ores ; a, zoospores
ejected, but still resting ; c,
zoospores which have left
their membrane at b behind
them. Magn. about 300.

MUCOEINI [641

sexual spores occur in the family, including chlamydospores, i-epro-
ductive cells formed within the ordinary cells of the hypha?. Sexual
reproduction takes place by means of zygospores (C), l^ut is ait
present known only in a few species. Either from ordinary hyphaa
or from sporangiophores spring a pair of short branches, the
extremities of which l^ecome firmly attached to one anotlier. These

Fig. 479. — B, mycele (three days old) of Fhy corny ces nitens, grown in a drop of
mucilage with a decoction of phims ; the finest ramifications are omitted ; g, the
conidiophore of If^tco;- ;«i(.ce(:?o in optical longitudinal section; C, a germinating
zygosx^ore of Mueor mucedo ; the germ-tube, k, puts out a lateral conidiophore, g.
In D are conjugating branches, h b, the extremities of which, art, though they have
not yet coalesced, are already cut ofi by transverse walls ; the zygospore is formed
from the coalescence of the cells a a. A, C, D, after Brefeld, greatly magnified ;
B, from nature, slightly magnified. From Goebel's ' Outlines of Classification
and Special Morphology.'

swell out greatly into an obconical form, on account of the passage
into them of a large amount of nutiient material. A larger or
smaller piece is then cut off from each of them by a transverse
wall ; the median cell- wall which sepai-ates them disappears, and the
two terminal portions thus cut off coalesce to form the zygospoi-e,

T T

642 FUNai

which often swells to a considex-able size, and its outer coat be-
comes frequently beautifully covered with warts or other protu-
berances. After a period of i-est the zygospore germinates, its
inner coat of cellulose bursting through the outer warty and
cuticularised einspore, and developing into the first germinating
filament.

Yery nearly alHed to the Mucorini are the Entomophtlioreae,
parasitic fungi, the mycele of which develops within the bodies of
living insects, especially caterpillars and flies, and after death
spreads outside the body as a flocculent felt. An example of this
family of fungi is frequently presented in the destruction of the
common house-fly by Empusa muscce. In its fully developed con-
dition the spore-bearing filaments of this plant stand out from the
body of the fly like the ' pile ' of velvet, and the spores thrown off
from these in all directions form a white circle round it, as it rests
m.otionless on a window-pane. The filaments which show them-
selves externally are the fructification of the fungus which occupies
the interior of the fly's body, and this originates in the spores
which find their way into the circulating fluid from without. A
healthy fly shut up with a diseased one takes the disease from it by
the deposit of a spore on some part of its surface ; for this, beginning
to germinate, sends out a process which finds its way into the
interior, either through the bi-eathing-pores or between the rings
of the body ; and, having reached the interior cavities, it gives oft
the germinating filaments which constitute the earliest stage of the
JUmpusa. Again, it is not at all uncommon in the West Indies to
see individuals of a species of Polistes (the representative of the
wasp of our own country) flying about with plants of their own
length projecting from some pai^t of their surface, the germs of
which have probably been introduced (as in the preceding case)
through the breathing-pores at their sides, and have taken root in
their substance, so as to produce a luxuriant vegetation. In time,
however, this fungus growth spreads through the body and destroys
the life of the insect ; it then seems to grow more rapidly, the
decomposing tissue of the dead body being still more adapted than
the living structure to afibrd it nutriment.

The Ascomycetes include an enormous number of species, most
of which are parasitic on living, or saprophytic on decaying leaves,
many of them microscopic. The mycele always consists of branched
and septated hyphse. In only a comparatively few species is a
sexual raode of reproduction known ; the special character of the
group is the non-sexual reproduction of ascosjyores within elongated
sacs or tubes known as asci. These are commonly collected together
in masses ; the collection of hyphse which give birth to the asci is
known as the hynieniimi, the mass of tissue enclosing or bearing the
hymenia as the receptacle or fructification. Its foi'm and structure
vary greatly in the different sections of the family. The ascospores
are always produced within the ascus by free-cell formation, and
their number is almost always four or a ' power ' of four, most com-
monly eight, occasionally leies than four. The asci are usually
surrounded by enlai-ged club-shaped or sterile hyphse, the para-

ASCOMYCETES

64:

physes. In many Ascomycetes, in addition to the aseospores,
ordinary exogenous spores or conids are pi'oduced at the extremity
of s2)oro2)hores or conidiophores (fig. 480, A). This is the case with
a large number of moulds or mildews, of which the common blue
mould, Penicillium glaucum, may be taken as a type. The familiar-
form of these moulds is that in which they produce these spores in
enormous quantities ; but, under certain conditions, especially when
the supply of nutriment is limited, the sexual mode of I'eproduction

Fig. 480. — Development of Eurotium repens: A, small part of a mycele with, the
conidiophore, c, and yomig ascogones, as ; B, the spiral ascogone, as, with the
antheridial branch, ^j ; C, the same with the filaments beginning to grow round it
to form the wall of the sporocarp ; D, a si^orocarp seen from without ; E, F,
young sporocarp in optical longitudinal section ; w, parietal cells ; /, the filling
tissue (pseudo-parenchymatous) ; as, the ascogone ; Gr, an ascus ; H, an ascospore.
After De Bary. A, magnified 190, the rest 600 times.

sets up (fig. 480, B-H). One of the branches of the mycele
elongates, and coils spirally upon itself into a corkscrew-like body,
the carpogone or ascogone, which constitvites the female organ ;
whilst another branch acts as the male organ or antherid, extending
itself over the spire and impregnating the ascogone by the passage of
its protoplasm into the lattei- organ. The structure thus formed
becomes enclosed in a mass of sterile tissue, and within this are
developed the asci, each containing numerous spores, which

T T 2

644

FUNGI

germinate directly into a new mycele. The enveloping tissue, togethei-
with the asci, is known as the sporocarj). In a large number of
Ascomycetes the asci are, however, formed without any previous
sexual process that has yet been detected. According to the struc-
ture of the mature sporocarp, the Ascomycetes may be an'anged
under three sections : the DiscoTuycetes, in which the sporocarp is
exposed, and is then known as an apothece ; the Pyrenomycetes, in

Fig. 481. — Ihtryfis bassiana : A, the fungus as it first appears at the orifices of the
stigmas : B, tubular filaments bearing short branches, as seen two days after-
wards ; E, magnified view of the same ; C, D, appearance of filaments on the fourth
and sixth days; F, masses of mature spores falling off the branches, with filaments
proceeding from them.

which the perithece is enclosed in a flask-shaped cavity with open
neck ; and a third section, in which the sporocarps are completely
enclo.sed.

In some Ascomycetes a tendency is exhibited to the foi'mation of
sclerotes, dense hai-dened masses of interwoven hyphfe. An example
of this is furnished by the .structure known as ' ergot,' the sclerote
of a fanons of this kind, Claviceps pt(.r2)urea, which attacks the ovary

ASCOMYCETES; SACCHAROMyCETES 645

of rye and other grasses. Many species of Peziza have a pecviliai'
fovni known as the botrytis form, reproduced by conids only, and
long believed to be altogether distinct from the Ascomycetes. Of this
nature is the so-called Botrytis hassiana (fig. 481), a kind of mould, the
growth of which is the real source of the disease termed mitscardine
which foi-merly carried off silkworms in large numbei'S, just when
they were about to enter the chrysalis state, to the great injury of
their breeders. The plant presents itself under a considerable
variety of forms (A-F), all of which, howevei', are of extremely
simple structure, consisting of elongated or rounded cells, connected
in necklace-like filaments, very nearly as in the ordinary ' bead-
moulds.' The spores of this fungus, floating in the air, enter the
breathing-pores which open into the tracheal system of the silk-
worm ; they first develop themselves within the air-tubes, which
are soon blocked up by their growth ; and they then extend them-
selves through the fatty mass beneath the skin, occasioning the
destruction of this tissue, which is very important as a reservoir of
nutriment to the animal when it is about to pass into its chrysalis
condition. The disease invariably occasions the death of the grub
which it attacks ; but it seldom shows itself externally until after-
wards, when it rapidly shoots forth from beneath the skin, especially
at the junction of the rings of the body. Although it spontaneously
attacks only the larva, yet it may be communicated by inoculation,
to the chrysalis and the moth, as well as to the grub ; and it has
also been observed to attack other lepidopterous insects. A careful
investigation of the circumstances which favour the development of
this disease was made by Audouin, who first discovered its real
nature ; and he showed that its spread was favoured by the over-
crowding of the worms in the breeding establishments, and parti-
cularly by the practice of throwing the bodies of such as died into a
heap in the immediate neighbourhood of a living silkworm ; for this
heap speedily became covered with this kind of mould, which
found upon it a most congenial soil ; and it kept up a continual
supply of spores, which, being diffused through the atmosphere of
the neighbourhood, were drawn into the breathing-pores of indi-
viduals previously healthy. The precautions obviously suggested by
the knowledge of the nature of the disease, thus afforded by the
microscope, having been duly put in force, its extension was success-
fully kept down. A similar growth of different species of the genus
Siihceria takes place in the bodies of certain caterpillars, in New
Zealand, Australia, and China ; and being thus completely pervaded
by a dense substance, which, when dried, has almost the solidity of
wood, these caterpillars come to present the appearance of twigs,
with long slender stalks that are formed by the growth of the fungus
itself. The Chinese species is valued as a medicinal drug.

Some forms of Ascomycetes, such as the genus Tuber, to Avliich
the trufile belongs, are formed completely undergi'ound.

The Saccharomycetes are now generally regarded as a degraded
form of the Ascomycetes. They resemble the Schizomycetes in the
simplicity of their character and in their ' zymotic ' action. The most
familiar form of this family is the Saccharomyces {Tortda) cerevisioi,

646 FUNGI

the presence of which in yeast gives to it the power of exciting the
alcoholic fermentation in saccharine liquids. When a small drop of
yeast is placed under a magnifying power of 400 or 500 diameters,
it is seen to consist of a large number of globular or ovoid cells,
averaging about 3^^^^ ^^^th of an inch in diameter, for the most part
isolated, but sometimes connected in short series ; and each cell
is filled with a nearly colourless ' endoplasm,' usually exhibiting
one or more vacuoles. Wlien placed in a fermentable fluid con-
taining some form of nitrogenous matter in addition to sugar, ^
they vegetate in the manner represented in fig. 482. Each cell
puts forth one or two projections, which seem to be young cells
developed as buds or offsets from their predecessors ; these, in
the course of a short time, become complete cells, and again per-
form the same process ; and in this manner the single cells of yeast
develop themselves, in the course of a few hours, into rows of foui-,
five, or six, which remain in connection with each other whilst the
plant is still growing, bvit which separate if the fermenting process
be checked, and return to the isolated condition of those which
originally constituted the yeast. Thus it is that the quantity of
yeast first introduced into the fermentable fluid is multiplied six

O

a

Fig. 482. — Saccharomyces cerevisics, or yeast-plant, as developed during the process
of fermentation : a, b, c, d, successive stages of cell-multiplication.

times or more during the changes in which it takes part. Under
certain conditions not yet determined, the yeast-cells multiply in
another mode — namely, by the breaking up of the endoplasm into
segments, usually four in number, around each of which a new ' cell-
wall ' forms itself ; and these endogenous spores are idtimately set
free by the dissolution of the wall of the parent cell, and soon enlarge
and comport themselves as ordinary yeast-cells. The process of the
formation of these spores resembles in all essential points the
formation of ascospores ; and hence Torula is regarded as a low or
degraded type of that order. Many other fungi of like simplicity
have the power to act as ' ferments ; ' thus in wine-making the
fermentation of the juices of the grapes or other fruit employed is set
going by the development of minute fungi whose germs have settled
on their skins, these germs not being injured by desiccation, and
being readily transported by the atmosphere in the dried-up state.

^ It appears from the researches of Pasteur that, although the presence of albu-
minous matter (such as is contained in a saccharine wort, or in the juices of fruits)
favours the growth and reproduction of yeast, yet that it can live and multiply in a
solution of pure sugar, containing ammonium tartrate with small quantities of mineral
salts, the decomposition of the ammonia salt affording it the nitrogen it requires for
the production of protop:asm, while the sugar and water supply the carbon, oxygen,
and hydrogen.

SACCHAEOMYCETES ; BASIDIOMYCETES

647

There is reason to believe, moreover, that a similar ' zymotic ' action
may be excited by fungi of a higher grade in the earlier stages of
their growth, the alcoholic fermentation being set iip in a suitable
liquid (such as an nqueous solution of cane-sugar, with a little fruit-
juice) by sowing in it the spores of any one df the ordinary moulds,
such as Penicillimn glaucum^ Mttcor, or Aspergilhis, provided the
temperature be kept up to blood-heat ; and this even though the
solution has been pre-
viously heated to 284°
Fahr., a temperature
which must kill any
germs it may itself con-
tain.

The Basidiomycetes
are distinguished by the
entire absence, as far as
is at present known, of
sexual organs, and by
the formation of their
conids or spores at the
apex of special enlarged
cells, the hasids. They
include the largest and
m^ost familiar of our
fungi, such as the genera
Agaricus, Boletus, Poly-
porus, Lycoperdon, Phcd-
hcs, &c. They are sapro-
phytes, obtaining their
nourishment from the
decaying vegetable mat-
ter in the soil, stumps
of trees, &c., &c., among
which the mycele pene-
trates, consisting often
of a dense weft of sep-
tated hyphfe, the ' spawn '
of the mushroom. The
aerial portion, known as
the receptacle or fructifi-
cation, bears either ex-
ternally, as in the case
of the mushroom (fig.
483), or internally, as

in the case of the Lycoperdon, or ' pufi"-ball,' the fertile portion
or hymenium. On this hymenium project the extremities of special
hyphfe, which are swollen into hasids ; the non-sexual conids or
hasidiospores are formed at the extremity of the basids, usually in
fours, from which they are easily detached, and, fi-om their small
size and great lightness, are readily carried through the air in great
quantities. In the Hymenomycetes, of which the common mushroom

Fig. 483. — Agaricus campestris, formation of the
hymenium : A and B, slightly magnified ; C, a part
of B, magnified 550 times. The portion marked
with fine dots is protoplasm. (From Goebel's
' Classification and Morphology of Plants.')

648

FUNGI

(Agaricios campestris) may be taken as a type, the receptacle has the
form of a cap-shaped pileus (fig. 484), raised on a stalk or stipe,
the whole composed of a pseudo-parenchyme consisting of a dense
agglomeration of parallel hyphse, the cortical portion of which is
slightly differentiated itito an epiderm. In the family to which the
mushroom belongs, the hymenium is borne at the edge of narrow
gill-like projections or lamellce radiating from the apex of the stipe
on the under side of the pileus. Am.ong the basids are seen other

cells of similar shape and
usually larger size, also the
extremities of special hyphse,
called cystids, the function of
which is obscure. The basi-
diospores vary gi-eatly in
colour in difierent genera.-
They are always unicellular,'
and the membrane consists
of two coats, the endospore
and exospore, the former of
which consists of fungus-
cellulose, while the latter is
more or less cuticularised.
On germinating the endo-
s]3ore bursts through the
exospore, and grows into a
germinating filament, from
which is developed the my-
cele, and on this ultimately
the receptacles.

Lichens. — The micro-
scopic study of this group
has acquired a new interest
for the botanist, from the
remarkable discovery an-
nounced in its complete
form by Schwendener in
1869 ' (and now accepted
by the highest authorities),
that instead of constituting
a special type of Thallo-

FiG. 484. — Agaricus campestris, natural size
(From Goebel's ' Classification and Morpho
logy of Plants.')

phytes, pai'allel to Al gee (with
which they correspond in
their vegetative characters) and Fungi (to which they are more
allied in fructification), they are really to be regarded as compo-
site structures, having an algal base, on which fungi have sown
themselves and live parasiticaUy. As, however, they do not
furnish objects of interest to the ordinary microscopist (the
peculiai- density of their strvxcture rendering a minute examina-
tion of it more than oi'dinarily difficult), nothing more than a

1 See his memorable work Ueber die Alqentimen der Flechtengonidien (Basel,
1869).

LICHENS

649

general account of their curious organisation will here be attempted.
The algal portion of a lichen belongs to one or other of the lower
groups, and consists of cells termed gonids — usually green, but
sometimes red or bluish-green — interspersed among long cellular
filaments. The proportion between these two components of the
thallus varies in different examples of the tyjje. Thus, in the
simplest wall-lichens the palmella-like parent cell gives origin, by
the ordinary process of cell-division, to a single layer of cells, which
sjireads itself ovei- the stony sui'face in a more or less circular form ;
and the ' thallus,' which increases in thickness by the formation of
new layers upon its free surface, has no veiy defined limit, and, in
consequence of the slight adhesion of its components, is said to be
' pulverulent.' But in the more complex forms of lichens the thallus
is mainly composed of long hyphse, which dip down into the superficial
layers of the bai'k of the ti-ees on which they gi'ow, and form by their

Fig. 485. — Leptofiium scotinum: Vertical section of tlie gelatinous thallus, magnified
550 times. An epidermal layer clothes the inner tissue, which consists for the most
part of formless and colourless jelly, in which the coiled strings of gonids lie ;
single larger cells of the strings (the limiting cells) are of- a higher colour ; between
them run the slender hyphge. (From Goebel's ' Classification.')

interweaving a hard crustaceous ' thallus,' in which the gonids are
imbedded, sometimes irregiilarly, sometimes in definite layers, known
as the gonidial layer (fig. 485), covered by an envelope of interlacing
filaments. It is from this algal portion of the structure that the
soredes of lichens are formed, little projections of the surface, com-
posed of single or aggregate gonids, invested by hyphee, and falling,
Avhen dry, into a powder, of which every particle is capable of
I'eproducing the plant from which it proceeded.

Tlh-G fructification oi lichens, on the other hand, is the production
of their fungal overgrowths, which are nourished by the algal
vegetation. The lichen-forming fungi, in fact, live upon their algal
hosts, like the endophytic fungi (such as the ' blights ' of corn),
which infest the higher forms of vegetation, each of the foi'mer
choosing its own alga, just as the latter mostly attach themselves to
particular victims. The peculiarity in the parasitism of the lichen-
fungi lies in the fact that they are not attached to their host externally
at any one particular spot, and do not penetrate into its cells, but

650

FUNGI

weave themselves round them, and enclose them in their hyphal
tissue. But not only this : the algal constituent of the lichen
appears also to derive benefit from, and to be nourished by, the
ftingus-hyph?e, affording an example of the singular kind of mutual
dependence known as commensalism or symbiosis (fig. 486.) The
formation of sexually produced ' spores ' usually takes place in asci
arranged vertically in the midst of straight elongated sterile cells
termed j'x^rajjhyses, so as to foiin a layer that lies either on the surface

Fig. 486. — Examx^les of various algae which are employed as the gonids of lichens :
h indicates always the hypha of the fungus ; g' the gonid : A, germinating
spore, s, of Physcia parietina, the germ-tube of which adheres closely to Proto-
coccus viridis ; B, a filament of Scijtonema with hyphfe of Stereocaulon rainulosus
twined round it ; C, from the thallus of the lichen Pliysma chalaganuni — a hyphal
branch is entering a cell of the Nostoc filament (gonid) ; D, from the thallus of
the lichen SynaUssa synipliorea — the gonids are the alga Glceoccqosa; E, from
the thallus of the lichen Cladonia furcata ; the gonids, which are being sur-
rounded by the hyphfe, are the cells of Protococciis. After Boruet. A, C, D, E,
magnified 050 ; B, 650 times. (From Goebel's ' Classification and Special Mor-
phology of Plants.')

of ajjotheces, oi- is completely enclosed within peritheces. Each of the
asci contains a definite nvimbei- of ascospores, usually eight, which
are projected from the receptacles with some force ; and theii-
einission, which seems to be due to the difierent efiects of moisture
upon the several layers of the receptacle, is often kept up con-
tinuously for some time. The formation of these asci, as in the
case of the oi'dinary Ascomycetes, is probably the i-esult of a sexual
union which takes place between the male jyollinoids ov ' spermatia '
and the female trichogyne. These pollinoids are pi-oduced within

Plate Xm.

J^iy. Z

Flq 22

-/■<x7 25

Fie z-t

Fig ZS.

F-ig. Ze

• 'i

'^^fl

T^ig,. Z7

Ti^Sl F-ff.3.

FCg-33

\ ^

^

Ti^SO

Fiff 38.

V

\
1

BAC.TERTA , SCHIZOMYCETES, OR FISSION FUNGI. |

LICHENS; BACTERIA 65 I

antlierids which are often specially designated ' spermogones,' formed
within these ca,vities, and, when mature, escaping in great, numbei's
from their orifices. Having no power of spontaneous movement,
they must probably be conveyed by the infiltration of rain-water
to a ti-ichogjTie which lies imbedded in the tissue beneath ; and
when they have imparted their fertilising influence to the contents
of the ascogone at its base, these develop themselves into a spore-
bearing apothece, the whole mass of spores which this contains
being the product of the cell-division of the originally fertilised
' oospore.'

The fungvis-constituent of lichens belongs, in the great majority
of cases, to the Ascomycetes, in a very few to the Basidiomycetes.
The gonids have been referred to a very large number of genera of
algse, among which may be mentioned Protococcus, Chroococcus,
Gloeocapsa., Palmella, Scytonema, JVostoc, and Ohroolejnos.

The Bacteria or ScMzomycetes. — At the close of this chapter
we place the Bacteria, Schizomycetes, or fission-fungi. These micro-
organisms have been defined as minute vegetable cells destitute of
nuclei. In spite of the labour which has been bestow^ed upon this
group, and vast as the literature is to which it has given rise, it is
impossible to assign an exact and clearly definable position to wdiat
is at the same time a remarkable and important gToup ; and we
therefore, as a matter of convenient arrangement, place them as
Protophytes, at the base of the lowest Fungi, for no other, and
therefore for the quite insufficient reason in the main, that they
contain no chlorophyll (Plate XIII).

There can be no doubt that some forms of the Bacteria manifest
affinity with the chlorophyllaceous Algfe ; but the affinity is in the
present state of our knowledge none the less indefinable, even if
our knowledge of the Bacteria as an entire group were complete
enough to admit of a generalisation of their relations. On the
other hand, according to Dallinger, the affinities of the Bacteria as a
complete group are closer with the Flagellata than is generally
admitted ; and whenever the saprophytic Flagellata — which are the
indispensable agents, not in the putrefactive fermentation by which
infusions and gelatine masses are broken up, but by which great
masses of organic tissue are reduced — and at the same time the
Bacteria, as a whole, have been broadly and comprehensively woi-ked
out, it may be found that both their morphological and physiologic.-il
affinities are of the closest order. It is impossible to take, for
example, such a form as B. lineola, which has an easily demonstrated
flagellate character, and reproduces in every fission a flagellum,
common to both dividing forms, which snaps at the moment of
complete division, leaving each form with a flagellum at either end
— perfect as the primal form whence the fission arose — without
observing how completely this coincides with the mode of fission in
half a dozen sapi'ophytic monads. But as an instance Cercomonas
typica (named by Kent) may be given, ^ whei-e the process is
identical. True, the Cercomonas has a conjugating and subsequent
resting stage, after which swarms emerge from spores thus foi-med.
1 Manual of the Infusoria, i. 259.

652 FUNGI

But a fuller knowledge of B. lineola is certainly wanting before we
can deny the further analogy.

No dovibt if such affinity were established, it would lead to much
rearrangement at the base of the organic series.

Since there is an apparent and highly suggesti\'e leaning of the
Bacteria to those forms of Algse which form the group of Nostocaceoe,
these also would be brought nearer the Flagellata ; while the Myce-
tozoa will have singular points of contact with these, one of which
has reference to the mode of s2)oring of one at least of the flagellate
saprophytes,^ while it is suggestive that the same gTOuping, should
the affinity be established, would involve a connection with the Algse
and the Fungi.

It is only definite results leading to a comprehensive view of the
morphology of the Bacteria as a whole that can render generalisation
in this matter safe.

By the word Bacteria we mean, strictly speaking, rod-shaped
micro-organisms, but the term is now commonly iised to indicate
the whole group of fission-fungi, which includes not only rod-forms
varying in leng-th but also spherical and egg-shaped cells. Motile
forms, whether longer or shorter, are possessed, as a rule, of fine
flagella. The mode of multiplication commonly observed is by
fission. The products of successive fissions may remain together in
a single filiform row loosely attached, or attached by the unbroken
filament of the flagella, or they may at once separate from the
jjrimal cell. Multiplication also occurs by processes which may be
considered as representing fructification.

Of the nature of this simplest cell we have hitherto learnt
comparatively little ; the protoplasm is generally homogeneous, but
in some species contains starch granules. Thus Clostridium
butyricum gives the starch reaction with iodine. Sulphur gTanules
are present in species of Beggiatoa which thrive in sulphur springs.
Others again contain pigment. The most remarkable of the
coloured forms uniformly tinged red was found and named by Ray
Lankester ; ^ other forms, coloured green by chlorophyll, have been
described by Van Tieghem and Von Engelmann, but it is quite
possible that these may be Algfe, and further researches are required
before these particular micro-oi'ganisms can be included among
bacteria.

Within the protoplasm of the Bacteria, however, no nuclei have
hitherto been discovered, but there is a delicate investing envelope,
probably a mere thickening of the outmost area of the protoplasm,
which is often also gelatinous in its oviter portions.

Many forms of Bacteria have the power of entirely free move-
ment. Frequently this movement is coincident with a revolution on
the longer axis of the rod, curved or straight, and in the vast
majority of cases this is directly correlated with a vortical action of
a front flagellum — an action which may be seen with the utmost
ease, if the proper means be employed, in the case of Sjnrillum

1 J.B.M.S. vol. V. sei-. ii. pp. 189-90, fig. 16, Plate V.
^ Quart. Joum. Microsc. Sci. new series, xiii. 408.

FOEMS OF BACTERIA 653

volutans, and less easily, but Avith almost equal certainty, in the
majority of other forms, not excluding £. termo.

The simjjlest forms in which Bacteria are found are as isolated
cells of round or ovate sluvpe : these are known as Micrococci, and
must be distinguished from immature and developing monads of the
saprophytic group. The fission by which micrococci multiply may
take place in one direction only, and if the resulting cells remain
attached they form diplococci. If fission again occurs in each of
these cells and is repeated again and again and the resulting cells
remain attached, they give rise to beautiful chains, rosaries, or
streptococci. If fission occurs in single cells in two directions
tetracocci are formed, and if in three directions packets of eight are
formed, or sarcina-cocci.

The rod-like forms are found isolated and free, or in chains.
Formerly short rods were called Bacteria, and long rods Bacilli ;
but as the term bacteria is applied to the whole group of fission-
fungi, it is more usual now to avoid confusion by speaking of all rod
forms, independently, of their length, as bacilli. Some which are
fusifoi-m in appearance are known as Clostridia.

The coiled rods or spiral forms are either (1) closely coiled, when
they are known as Sjnrillum and S'pirochcete (more threadlike) ; or
(2) those more openly coiled are known as Vihriones.

There are also very elongated filiform varieties known as Lepto-
thrix and branched forms as streptothrix. In Beggiatoa the fila-
ments are fixed at one exti-emity and stretch the other free in
the surrounding fluid.

Cohn classified bacteria according to their shape, bvit Ray Lankester,
Zopf, and others have shown that several micro-organisms in theii'
life-cycle exhibit successively the shapes characteristic of the orders
of Cohn. These pleomorphic species may be illustrated by Beggiatoa,
alba.

In the refuse waters discharged from factories, especially the
sulphuretted efiluents of sewage works, is found this form — the
' sewage fungus ' of engineers. It may have a thickness of 5ju, and
it maybe as attenuated as to measure only Iju. It is attached in an
erect manner to objects in the impure water it afiects (fig. 487) ; and
the filaments consist of rows of cells, and in the protoplasm of these
granules of sulphur are enclosed,. The filaments readily break up
into cells about as long as they are broad, and become at length
active but eventually attach themselves to some object and come to
rest, wlien they multiply by fission and accumulate in masses of
zoogloea-.: ' They may develop into rods, and these again into the
filaments after the rods have passed thi-ough the swarming state.'

Spirally twisted forms also arise in this species. These break
into coiled parts, possessed of flagella, and exhibit extremely active
movement. The flagella in these are as strong and easily seen as
in the Spirillum volutans, and these forms were known at an earlier
period as Ophidomonas.

Fig. 487 shows at 1 a group of the attached filaments oi Beggiatoa
alba : 2 to 5 show portions of filaments of differing diameters ; 5
shows a filament in the act of multipartition. The small dark circles

654

FUNGI

throughout represent the granules of sulphur ; 6 to 8 show fragments
rich in sulphur with transverse septation developed by treatment
with methyl-violet solution. In 8 the formation of cocci and spores

Fig. 487. — Beggiatoa alba. (From De Bary's 'Comparative Morphology of Fungi.')

is seen ; 9 shows the result of filaments having broken up into spores ;
10 shows spores in movement. 1 is magnified 540 diameters, the
i-emainder 900 diameters.

Figu.re 488 shows the growth of the curved and spiral forms

BACTERIA

655

of the same : A is a group of attached filaments ; B to H show poi'-
tions of spiral filaments ; 0, D, F, to H represent the act of division
into smaller fragments but without motion ; in H the separate
cells are distinctly sho'^\^l ; E shows the separation of a complete
spirilliun form possessed of flagella and capable of great activity.

Bacteria may be united by some interfusing gelatinous material
in which all action ceases or is of the most limited kind ; and these
living films, which appear on the surface or suspended in the interioi-
of putrescent fluids, are
known as Zooglceif.
They may also be found
on the surfaces of solid
bodies, where the putre-
factive ferment is in
action.

Bacteria have been
divided into two classes,
distinguished by the
formation of endospores
in the one and of arthro-
spores in the other.

I. The endosporous
forms are those whose
multiplication is brought
about by the formation
within a cell of a minute
globular or oval body,
which, while the sur-
rounding protoplasm of
the mother-cellis assimi-
lated, gradually reaches
its mature condition.
What it is that exactly
determines the act of
spoi-e-foi'mation is not
known, but it is probable
that free access to oxygen
constitutes an important
factor.

A chosen illustration
of the endosporous Bac-
teria is Bacillus mega-
therium. It was first

observed on boiled cabbage leaves, and is considered by De Bary as
an ' exceedingly instructive form.' It is 2;5/x in shoi-t diameter and
about four times as long as this. It is illustrated in fig. 489. a re-
presents a motile chain of the Bacilli in active vegetation. This is
magnified 250 diameters, b two active rods magnified 600 diameters.
2) shows the result of treating a form in the condition b with an
alcoholic solution of iodine, c is a rod with five cells preparing to
form spoi'es. d to /" represent successive stages of a pair of rods in

Fig. 488. — Beggiatoa alba, curved and spiral
forms. (From De BaLi'y's ' Comparative Morpho-
logy of Fungi.')

656

FUNGI

the act of foi'ming spores, e an hour later than d, and f an hour
later than e. The cells which did not contain spores disappeared or
perished, r is a quadricellular rod with ripe spores, cj^ is a five-
celled rod with three ripe spores placed in a nutrient solution after

S\%

Pig. 489. — Bacillus
(From De Bary's

iiiegatheriicm.
' Comparative
Morphology of Fungi.')

5/i to 20/i in
length and lyu to 1-25 jj, in width (fig. 490).
Fig. 491 shows two filaments grown on a
microscopic slide (De Bary) in a solution of
meat extract, partly in an advanced state of

several days' desiccation, g^ is the
same an hour after ; g^ is the same
after another two hours and a half.
hi is two spores with the walls of
the mother-cells dried and placed in
a nutrient solution ; 7^.2 is the same
forty-five minutes later ; i, k, I, thi-ee
stages of germination of the spore.

Bacillus anthracis and B. sttbtiUs
ai'e veiy typical examples of endo-
sporous bacteria. B. anthracis has
been proved to be the virus of
anthrax or splenic fever. It is
found in great profusion in the
blood and tissues of animals attacked
by this disease in the form of i-ods
and filaments „

N

Fig. i^Q.— Bacillus anthracis, x 1,200. Blood
corpuscles and bacilli unstained ; from an inocu-
lated mouse. (Friinkel and Pfeiffer.)

Fig. 491.— a, Bacillus
anthracis ; B, B. sub-
tilis. (From De Bary's
' Fungi.')

spore -formation. At the upper pai't of the figure two lipe spores
have escaped. These spores on germination elongate and give rise
to new groups of rods and filaments.

BACTEEIA

657

B. subtilis, which is the Bacilkis common to decomposing hay
infusion, has a life-history extremely similar to B. anthracis. It
spores in precisely the same manner. The outer wall of the spore
is comparatively thick, and the protoplasm elongates in the direction
of the longer axis of the spore and of the mother-cell with which
this coincided. B, fig. 491 , represents the development of B. subtilis :
1 shows fragment of filaments with ripe spores ; at 2 the spore is
beginning to germinate ; 3, the young rod is projecting from the wall
of the spore ; 4 represents germ rods curved in a horseshoe shape
and with the extremities connected, one of them having one extremity
subsequently released ; 5, germ-tubes with the two extremities

Fig. 492. — Leuconostoc. onesenteroides : 1, Spores; 2, Sjjores after germination,
showing gelatinous envelope ; 3, 4, 5, 6, Increase by division ; 7, Glomerular form

of zoogio3a ; 8, Section of an old mass of zoogloea ; 9, Cocci chains with arthro-
spores (Tieghera and Cienkowski).

remaining connected and already greatly increased in size. The whole
represents a magnification of 600 diameters.

II. Arthrosporous forms are rejDroduced by the separation of
single members from their connection with a group, which then
give origin to new generations. These cells, appai-ently not difiering
from the rest, become larger, with toiigher walls and more refrac-
tive, and while the rest of the group die they, having acquired the ,
properties of spores, can produce a new growth in any fresh
nourishing soil.

A sufficiently detailed illustration of the arthrosporous Bacteria
may be seen in Leuconostoc mesenteroides (fig. 492). This micro-
organism occurs occasionally in beetroot juice and the molasses of
sugar-makers, forming large gelatinous masses resembling frog spawn.

u u

6s8

FUNGI

Chains of cocci are found by microscopical examination, and some of
the cells in a chain are enlarged without changing their form and
develop into tj'^pical arthrospores.

The Bacteria behave very variously under the same conditions
of supply or exclusion of oxygen. The aerobic require free oxygen
in quantity, as e.g. B. suhtilis ; while in the anaerobic the vital
activities are promoted by its exclusion. But there can be no doubt
that gradual modification of either condition will bring about adapta-
tions. Naegeli has shown that there are forms which usually depend
on oxygen which continue to vegetate when free oxygen ceases.

Fig. 493. — A, Bacterium termo, each cell furnished with a single flagellum. Magni-
fied 4,000 diameters. B, C, D, Bacterium lineola, each cell when separated
having a flagellum at either end. Magnified 3,000 diameters. (Dallinger.)

Their nutrition is carried on like that of othei- vegetative forms
devoid of chlorophyll. The actual typical group ai-e without doubt
the saprophytic Bacteria. The relation of the 2^c(,'>'ttsitic or 'pathogenic
forms to these is one of the most interesting problems in microscopic
biology. That they are physiological modifications of the saprophytic
forms appears per se a possibility ; but in the light thrown upon
biological change and survival by the hypothesis of the origin of
species, the suggestion incites to practical inquiry and research. If
the parasitic Bacteria are physiological modifications of the saprophytic
forms, to know the path by which they biologically became such may

Fig. 494, — Four individuals of Vibrio rugula, each showing flagellum at one or
both ends ; two other individuals, a and b, se^jarating from each other, and draw-
ing out a protoplasmic filament to form their second flagella. Magnified 2,000
diameters. (Dallinger.)

be to put more into the hands of medicine than could be accomplished
by any othei- means.

Jjacterium termo is the most universally present and abundant of
the saprophytic species. It is Ifx to 1'5/x long, and 0"5 to 0'7/x broad,
usually of dixmbbell form. These Bacteria are usually seen in ' vacil-
lating ' movement in their free state ; each cell bears a flagellum at
each end, as B, D (fig. 493), whilst the double cells bear a flagellum
at each extremity. The formation of the second flagellum takes place
by the di-awing out of a filiiment of protoplasm between two cells
that are separating from each othei- (as in iig. 494, a, b), the i-upture

SPIRILLA

659

of which gives a new flagelluni to each. Their flagella are so minute
as to be among the most ' difficult ' of all microscopic objects, their
diameter being calculated from 200 measuiements by Dallinger
at no more than oTnrcroo^^^ of ^^^ ii:ich.^ Although this species does
not ordinarily multiply in any other way than by transverse sub-
division, yet, under ' cultivation ' at a temperature of 86° Fahr., its
cells have been seen to elongate themselves into motionless rods,
resembling those of Bacilli, whose endoplasm breaks up into separate
particles that are set free as small bright almost spherical spores,
which sometimes congregate so as to form a zodgloeaSlm.. These
germinate into short slender rods, which are at first motionless, but
soon undergo transverse fission, and then acquire flagella. ^

The Vibriones may be i-epresented by V. rugida, seen in fig. 494.
They are slightly curved rods and threads, from 6/i to 16/x long, and
varying in thickness from 0'5^ to 2/.<. They have well-marked flagella,
one at each end. They appear in vegetable infusions, causing fer-
mentation of cellulose.

The S'jnrilla are the largest forms in the group, characterised by

Fig. 495. — A, Spirillum zmclula, showing flagellum at each end. Magnified 3,000
diameters. B, Spirillum voUttans. Magnified 2,000 diameters. (Dallinger.)

their spirally formed cells and their graceful spiral motion. The}'
are fairly represented in fig. 495 by S'pirilhwi itnchda (A) and
Sinrilhtm volutans (B). The threads of the former are from \-\u to
l'4a in thickness, and from 9^ to 12/i in length. They are intensely
active, and possess a flagellum at either end. They are found in
varying decomposing infusions.

Sjnrillum volutans was known to and named by Eln-enberg. It
is from I'S^u to 2"3yu in thickness, and varies from 25^ to 30/x or
more in length. It has distinctly granvilaj- contents, and a very
easily demonstrable flagellum at each end of the spiral ; a fla-
gellum was distinctly suggested by Ehrenberg on account of the voi'-
tical action visible in the fluid before this spirillum as it advanced.

With the beautifully corrected 6mm. power of Zeiss (apochromatic
dry 1S..K. 0"95), all but the most difficult of these can be seen in fresh
specimens with relative ease on a dark ground with a 1 2 or 1 8 eye-
piece, provided they be examined alive tvith the flagella in motion.

Journ. of Boy. Microsc. Soc. vol. i. (1878), p. 175;

- Ewart, loc. cit.
U u 2

66o

FUNGI

Foi" the more cliificult ones {B. termo and B. lineola) move careful
arrangements are required. In dried specimens the fiagella can be
readily demonstrated, and easily photographed, by staining them by
a special method introduced by Loffler (fig. 496).

The germinating power of the spores of Bacteria may be brought
into operation at once on their reaching ripeness, or they may be
desiccated for an indefinite time, and again, on reaching suitable
surroundings, will germinate as before. This power is held in vari-
ous degrees by diflerent forms, but the whole subject needs more
uniform and exhaustive inquiry. The spores of B. suhtilis retain
their vitality for years if kept in a dry air, while those of
B. anthracis are stated by Pasteur to remain alive in absolute
alcohol ; ^ and Brefeld found their power to germinate uninjured
after the lapse of three years in a dry atmosphere. He also found
them proof against the boiling-point of water, and even a higher

temperature, but he
found that fewer and
fewer survived in boil-
ing nutrient fluid until
the end of the third
hour, when all were
destroyed. So Buchner
found that the same
spores wei;e wholly
killed only after three
or four hours' boiling ; -
while Pasteur states
that groups of un-
certain spores can
withstand a tempera-
ture of 130° C. There
is, however, uncer-
tainty, becavxse a want
of uniformity, in the
1,000, stained results from various

Fig. 496.— Fiagella of Typhoid Bacilli,

by Loffler's method. (Frankel and Pfeiffer.)

sources ;
may be

20° to 25° C.
taken as the
average degree of temperature at which these organisms will freely
germinate ; but B. termo, for example, has been known to germinate
from 5-5° C. to 40° C.

Nothing like ' conjugation,' or any other form of sexual genera-
tion, has yet been witnessed in any Bacteria ; and until such shall
have been discovered, no confidence can be felt that we know the
entire life-history of any one type.^ When these fiicts are allowed

1 ' Charbon et Septicemie,' Comjjt. Bend. Ixxxv. p. 99.

- Naegeli, XJnters. iiher nieclere Pilze, 1882, p. 220.

■5 As it seems unquestionable that among the higher Fungi ' conjugation ' often
takes place at a very early stage of growth, it seems a not very improbable surmise
that the ' granular spheres ' observed by Ewart in BaciUiis and S^nrilhnn, which
seem to correspond with the ' microplasts ' observed by Eay Lankester in his
Bacterium riibescens, may be a product of conjugation in the micrococcus stage of
these organisms.

EACILLUS ANTHRACIS

66 1

their due weight, no difficulty can be felt in admitting the action of

Bacteria, &c., in producing decomposition under conditions which

might at first view be fairly supposed to preclude the possibility of

their presence. This action is altogether analogous to that of the

yeast-plant in producing saccharine fermentation ; and the careful

and exact expei-iments of Pasteur, repeated

and verified in a gi-eat variety of modes by

Lister, Tyndall, and others, leave no doubt

on these two points — (1) that putrefactive

fermentation does not take place, even in

liquids which are peculiarly disposed to pass

into it, except in the presence of Bacteria ;

and (2) that neither these germs nor any

others arise in such liquids de novo, but that

they are all conV^eyed into them by the air

when not otherwise introduced. It is thus

also with the parasitic or pathogenic forms

of Bacteria in setting up disease. Thus

^^^^^^^^

* «

• %%*^

Fig. 497.— Spore-bearing threads of Bacillus
anthracis, double-stained withfuchsine and
methylene blue, X 1,200. (Crookshank.)

Fig. 498. — Photograph of a
pure-cultivation of Ba-
cillus anthracis. (Crook-
shank.)

\

' splenic fevei' ' is producible by the inoculation of Bacillus anthracis
(figs. 497 and 498) ; and tetanus or 'lock-jaw' by inoculation with
another species of Bacillus, the microbes having been in both cases
'cultivated,' so as to be free from other contaminating matter.
Similar observations have been made
upon tuberculosis (figs. 499 and 500),
actinomycosis, glanders, so that an
animal sufiering under any of these
diseases may be a focus of infection
to others, for precisely the same
reason that a tub of fei-menting beer
is capable of propagating its fermen-
tation to fresh wort. A most notable
instance of such pi-opagation is
afibrded by the spread of the disease Fig. 499.— Bacilli of tubercle ir
termed ' pebrine ' among the silk- sputum, x 2,500 (from photo
worms of the south of France, which,
according to Pasteur, is caused by a
minute organism named Nosema Bombycis, the mortality caused by
it being estimated to produce a money loss of from three to four
millions sterling annually for several years following 1853, when it

^

graphs)
fiichsine.

tained with carbolised
(Crookshank.)

662

FUNGI

first oroke out with violence. It has heen shown by mici-oscopic
investigation that in silkworms strongly affected with this disease
every tissue and organ in the body is swarming with these minute
cylindrical corpuscles about 4'2^ long, and that these even pass into
the undeveloped eggs of the female moth, so that the disease is
hereditarily transmitted. And it has been further ascertained by
the researches of Pasteiir that these coi-puscles are the active agents
in the production of the disease, which is engendered in healthy
silkworms by their reception into their bodies ; whilst, if due pre-
cautions be taken against their transmission, the malady may be
completely exterminated.

Fig. 500. — Pure-cultivations on glycerine-agar from human tubercular sputum :
a, after six months' growth (fifth sub-culture) ; b, c, after ten months' growth
(fourth sub-culture). (Crookshank.)

Bacteriology is now so distinctly a branch of biological science
that it would be out of place here to present even a summary of its
voluminous details and methods of research. The microscope in its
most perfect form is an indispensable adjunct to the rapidly progres-
sive work of this department of biological research, and the most
delicate and refined employment of the microscope and all its
adjuncts is in the last degree important. Only a skilled microscopist
can be a successful bacteriologist. But for the methods of the
bacteriological laboratory we must refer the reader to treatises on
this branch of science,^ it beinsf enough here to remark that the

1 The English student will find an admirable aid in the Text-hook of Bacterio-
logy and Infective Diseases (4th ed.), by Professor E. Crookshank.

CULTIVATION AND COLONIES

66^

employment of nutrient gelatine, nutrient agar-agai-, and other
similar media on glass plates, and in test-tubes (fig. 501), so as hy

''**W6«l»#»*''

Fig. 501. — Pure-cultivations of Streptococcus 2njogenes : «, on the surface of
nutrient gelatine ; b, in the depth of nutrient gelatine ; c, on the surface of
nutrient agar. (Crookshank.)

^.r- ^"^ r-.

Pig. 502. — Colonies of Bacillus anthiattb, x 80 . a, after 24 hours ;
h, after 48 hours. (Fliigge.)

inoculation to obtain cultures of specific and isolated forms with
their characteristic appearances, is one of the essential methods

664 FUXGI

(Plate XI Y). The inoculated bacteina, instead of moving freely, as
they would in a liquid medium, are fixed to one spot, where they
develop ' colonies ' in a characteristic manner, showing their owti
morphological features (fig. 502). Cleanliness and care, as well as
practice in manipulation, ai-e essential. In the same way we can only
allude to the investigation of the chemical products of bacteria, such
as toxins, and to those antidotal substances or antitoxins which
develop in the blood of suitable animals inoculated with gradually
increasing doses of toxins. Antitoxins and vaccines are now largely
vised in the treatment of tetanus, diphtheria, typhoid fever, plague,
cholera, and septic diseases in the human subject.

The pathological and therapeutic value of these researches is
far beyond our present ability to estimate, and must have an
apparently increasing value. But it is a science with which a woi-k
of this soi't may not deal further than to show the right use of the
microscope and its aj^pliances, by which the work of pathological
bacteriology can alone be successfully done.

Plate XIY.

^0

■oOD

P

CO O

0-

^

s

t^ncrnt Broolc'.Dn.y i Son.Mtlt- ■

665

CHAPTER X

MICBOSCOPIC STBUCTUEE OF THE BIG HE B CBYPTOGAMS

Hepaticse. — Quitting now the algal and fungoid types, and
entering the series of terrestrial cryptogams, Ave have first to notic
the little group of Hepaticce, or liverworts. This group presents
numerous "objects of great interest to the microscopist ; and no
species is richer in these than the very common Marchantia poly-
morplia, which may often be found growing between the paving-
stones of damp courtyards, but
which particularly luxuriates in
the neighbourhood of springs or
waterfalls, where its lobed fronds
are found covering extensive sur-
faces of moist rock or soil, adher-
ing by the radical filaments (rhi-
zoids) which arise from their lower
surface. At the period of fructi-
fication these fronds send up
stalks, which carry at their sum-
mits either round shield-like discs,
or radiating bodies that bear some
resemblance to a wheel without
its tire (fig. 503). The former
carry the male organs or an-
therids ; while the lattei- in the first instance bear the female
organs or archegones, which afterwards give place to the S'jyoranges,
or spore-cases.^

The green surface of the frond of Marchantia is seen, under a low
magnifying power, to be divided into minute diamond-shaped sjDaces
(fig. 504, A, «, a), bounded by raised bands (c, c) ; every one of these
spaces has in its centre a curious brownish- coloured body (6, h), with
an opening in its middle, which allows a few small green cells to be
seen through it. When a thin vertical section is made of the frond
(B), it is seen that each of the lozenge-shaped divisions of its surface
corresponds with an air-chamber in its inteiior, which is bounded
):)elow by a floor (a, a) of closely set cells, from whose under surface
the rhizoids arise ; at the sides by walls (c, c) of similar solid

1 In some species the same shields bear both sets of organs ; and in Marchantia
androgyna we find the upper surface of one half of the shield developing antherids,
whilst the under surface of the other half bears archegones.

Fig. 503. — Ficud d MhiuIki atia j^oly-
morpha, with gemmiparous concep-
tacles, and lobed receptacles bearing
archegones.

666 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

parenchyme, the projection of whose summits forms the raised
bands on the surface ; and above by an epiderm (6, b) formed of a
single layer of cells ; whilst its interior is occupied by a loosely
arranged parenchjmie composed of branching rows of cells {/, f) that
seem to spring fi-om the floor, these cells being what are seen from
above when the observer looks down through the central aperture
^ ]\\iit mentioned. If the vertical

section should happen to traverse
one of the peculiar bodies which
occupy the centres of the divi-
sions, it will bring into view a
sti-ucture of remarkable com-
plexity. Each of these stomates
(as they are termed, from the
Greek a-rofia, mouth) forms a sort
of shaft ((/), composed of four or
five rings (like the ' courses ' of
bricks in a chimney) placed one
upon the other (A), every ring
being made up of four or five
cells ; and the lowest of these
rings (i') appears to regulate the
a]3ei-ture by the contraction or
expansion of the cells which
compose it, and is hence termed
Fig. 504.— Structure of frond of ilf a )-c7ta»- the ' obturator-inng.' In this
tia 2}ohjmorpha : A, portion seen from manner each of the air-chambers

above; « a, lozenge-sliaped divisions; of the frond is brought into COm-
0,0, stomates m the centre of the lozenges; • ■• -^i Tn j_ i

c, c, greenish bands separating the munication With the external
lozenges. B, vertical section of the frond, atmosphere, the degree of that

showing a, a, the dense layer of cellular communication being regulated

by the limitation of the aperture.
We shall hereafter find that the
leaves of the higher plants con-
tain intercellular spaces, which
also communicate with the ex-
terior by stomates, but that the structure of these organs is far less
complex in them than in this humble liverwoi-t.

The frond of Marchantia usually bears upon its surface, as shown
in fig. 503, a number of little ojoen basket-shaped geonmiparous con-
ceptacles (fig. 505), which may often be found in all stages of develop-
ment, and are structures of singular beauty. They contain when
mature a number of little green i-ound or oblong discoidal getiimce,
each comjjosed of two or more layers of cells ; and their wall is sur-
mounted by a glistening fringe of ' teeth,' whose edges are themselves
regularly fringed with minute outgrowths. This fringe is at first
foi-med by the splitting up of the epiderm, as seen at B, at the
time when the conceptacle and its contents are first making their
way above the surface. The little gemma? are at first evolved as
single globular cells, supported upon other cells which form their
footstalks ; these single cells, vmdergoing binaiy subdivision, evolve

tissue forming the floor of the air-
chamber, d, d, the epidermal layer, h, b,
forming its roof ; c, c, its walls ; /,/', loose
cells in its interior ; g, storaate divided per-
pendicularly; //, rings of cells forming its
wall ; i, cells, forming the obturator- ring.

STEUCTUEE OF :VL4KCHANTIA

667

themselves into the gemma^ ; and these gemma?, when mature,
spontaneonsly detach themseh'es fi-om their footstalks, and lie free
within the cavity of the concejjtacle.
Most commonly they are at last
washed out by rain, and are thus
carried to different parts of the
neighbouring soil, on which they
grow very rapidly when Avell sup-
plied with moisture ; sometimes,
however, they may be found gi-ow-
ing whilst still contained within
the conceptacles, forming natui-al
grafts (so to speak) upon the stock
from which they have been de-
veloped or detached ; and many of
the irregular lobes which the frond
of Marcliantia puts forth seem
to have this origin. The very
curious observation was long ago
made by Mirbel, who carefully
watched the development of these
ge'^nnue, that stomates are formed
on the side which hapjDens to be
exposed to the light, and that
rhizoids are put forth from the
lower side, it being apparently a
matter of indifference which side of
the little gemma is at first turned
upwards, since each has the power
of developing either stomates or
rhizoids according to the influence
it receives. After the tendency to
the formation of these organs has
once been given, however, by the
sufficiently prolonged influence of
light upon one side and of darkness and moisture on the other, any
attempt to alter it is found to be vain ; for if the surfaces of the
young fronds be then inverted, a twisting growth soon restores them
to their original aspect.

When Marchantia vegetates in damp shady situations which
are favourable to the nutritive processes, it does not readily produce
the true fructification, which is to be looked for rather in plants
growing in more exj)osed places. Each of the stalked peltate
(shield-like) discs contains a number of flask-shaped cavities opening
upon its upper surface, which are brought into view by a vertical
section ; and in each of these cavities is lodged an antherid Avhich
is comj)osed of a mass of ' sperm-cells,' within which are developed
antherozoids like those of Chara ; the whole being surmounted by a
long neck that projects through tlie mouth of the flask-shaped cavity.
The wheel-like receptacles (fig. 503), on the other hand, bear on their
under surface, at an early stage, concealed between niembT-anes that

Fig. 505. — Gemmiparous conceptacles
oiMarchantiapolymorplia : A, con-
ceptacle fully exxjanded, rising from
the siu-face of the frond, a. a, and
containing gonidial gemmse ah-eady
detached. B, first appearance of
conceptacle on the surface of the
frond, showing the formation of its
fringe by the splitting of the epiderm.

668 MICROSCOPIC STEUCTUEE OF HIGHER CRYPTOGAMS

connect the origins of the lobes with one another, a set of archegones,
shaped like flasks with elongated necks (fig. 507) ; each of these has
in its interior an ' ocisphere ' or ' germ-cell,' to which a canal leads
down from the extremity of the neck, and which is
fertilised by the penetration of the antherozoids
thi'ough this canal until they reach it. Instead,
however, of at once evolving itself into a new plant
resembling its parent, the fertilised oosphere or
' embryo-cell ' develojDS itself into a mass of cells en-
closed within a capsule, which is termed a sporange ;
and thus the mature receptacle, in place of arclie-
gones, bears capsules or sporanges, each of them filled
with an aggregation of cells that constitute the im-
mediate progeny of the fertilised germ-cell. These
cells, discharged by the bursting of the sporange,
are of two kinds : namely, sjoores, each enclosed in
a double sjjore-membrane ; and elaters, which are
very elongated cells, each containing a double spiral
fibre coiled vip in its interior. This fibre is so elastic
that when the surrounding pressure is withdrawn
by the bursting of the sporange, the elaters ex-
tend themselves (fig.
506), tearing apart the
cell - membrane ; and
they do this so suddenly
as to jerk forth the
spores which may be
adherent to their coils,
and thus assist in
their dispersion. The
spores, when subjected
to moisture, with a
moderate amount of
light and warmth, de-
velop themselves into
little collections of cells,
which gradually assume the form of flattened fronds ; and thus the
species is veiy extensively midtiplied, every one of the aggregate of
spores which is the product of a single germ-cell being capable of
giving origin to an independent individual.

Marchantia is the type of the section known as the thalloid
Hepaticte. Another section, the foliose Hepaticse, is represented
by the genus Jimgermannia^ exceedingly common plants, of a moss-
like habit, growing on moist banks and similar situations. While
the structure of the sexual organs, and of the sporanges, resem-
bles in its main features that of Marchantia^ the vegetative
organs are very different, consisting of a slender creeping stem
with small semi-transparent leaves. This distinct differentiation of
stem and leaves indicates a decided advance in organisation, and
marks the passage from the ihallopliytic to the cormophytic type of
structure.

Fig. 506.— Elater
and sx^ores of
Marcliantia.

Fig. 507. — Archegone of Mar-
chantia jpolyviorplia, in succes-
sive stages of development.

STRUCTURE OF MOSSES

669

Musci. — There is not one of the tribe of Mosses whose external
organs do not serve as beautiful objects when viewed with low powei-s
of the microscope ; while their more concealed wonders are ad-
mirably fitted for the detailed scrutiny of the pi-actised observer.
Mosses always possess a distinct axis of growth, commonly more oi-
less erect, on which the minute and delicately formed leaves are
arranged with gi-eat regularity. The stem shows some indication of
the separation of a cortical or external poi-tion from the 'tneduUary
or central, by the intei-vention of a circle of bundles of elongated
cells, which seem to prefigure the woody portion of the stem of

Fig. 508. — Structure of mosses : A, plant of Fiuiaria Jiygroiiietrica, showing, f the
leaves, u the siDoranges supported upon the setas or footstalks s, closed by the opercule
o, and covered by the calyj)ter c. B, sporanges of Encaly]}tra vulgaris, one of them
closed and covered with the calypter, the other open ; u, u, the sporanges ; 0, o, the
opercules ; c, calypter ; p, peristome ; s, s, setae. C, longitudinal section of very
young sporange of Splaclinii m ; a, solid tissue forming the lower jjart of the capsule ;
c, columel ; I, 's\}&.ce around it for the development of the spores ; e, epidermal
layer of cells, thickened at the toj) to form the opercule o ; ji, two intermediate
layers, from which the peristome will be formed ; s, inner layer of cells forming
the wall of the cavity.

higher plants, and from which prolongations pass into the leaves, so
as to afford them a sort of midi-ib. The leaf usually consists of either
a single or a double layer of cells, having flattened sides by which
they adhei-e one to anothei- ; they i-ai-ely present any distinct
epidermal layer ; but such a layer, perforated by stomates of sipmle
structure, is commonly found on the seta or bristle-like footstalk
beaiing the fructification, and sometimes on the midiibs of the leaves.
The rhizoids of mosses, like those of Marchantia, consist of long-
tubular cells of extreme transparency, within which the protoplasm
may frequently be seen to circulate, as in the elongated cells of
Ghara.

670 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

The ' urn,' or ' capsule,' of mosses, filled with spores, and borne
at the top of a long footstalk that springs from the centre of a cluster
of leaves (fig. 508, A), is the ultimate result of an act of fertili-
sation ; for mosses, like liverworts, possess both anther ids and
archegones. These organs are sometimes found in the same envelope.

Fig. 509. — Antherids and antlierozoids of Polytrichum commune: A, grouj) of
antlierids, mingled with hairs and sterile filaments (paraphyses). Of the three
antherids, the central one is in the act of discharging its contents ; that on the
left is not yet mature ; while that on the right has already emptied itself, so that
the cellular structure of its walls becomes apparent B, cellular contents of an
antherid, previously to the development of the antherozoids ; C, the same,
showing the first appearance of the antlierozoids ; D, the same, mature and
discharging the antherozoids.

or perigone, sometimes on difierent parts of the same plant, some
times only on difiei-ent individuals ; but in either case they are
usually situated close to the axis, among the bases of the leaves.
The antherids are globular, oval, or elongated bodies (fig. 509, A),
com.posed of aggregations of cells, of which the interioi- are ' sperm-

FEUCTIFICATION OF MOSSES

671

cells,' each of which, as it conies to matuiity, develops within itself
an antherozoid (B, 0, D) ; and the antherozoids, set free by the
rupture of the cells within which they are foimed, make their
escape by a passage that opens for them at the summit of the antherid.
The antherids are generally surrounded by a cluster of hairlike
filaments (fig. 509, A), which are called paraphyses ; these seem to
be ' sterile ' or undeveloped anthei-ids. In the ' hair-moss,' Poly-
trichu'm commune, one of the largest of our mosses, common on dry
heaths, these antherids are collected into consjaicuous starlike
clusters at the extremities of the branches of the ' male ' plants.
These are to be seen about April, and at the same time the arche-
gones may be detected concealed among the leaves on the ' female '
plant ; while the capsules, or sporanges, in this and most other
mosses, make their appearance late in the summer, and remain
through the winter. The archegones bear a general resemblance to
those of Marchantia (fig. 507), and the fertilisation of their con-
tained oospheres, or germ-cells, is accomplished in the manner
already described. The fertilised embryo-cell becomes gradually
developed by cell-division into a conical body elevated upon a stalk ;
and this at length tears across the walls of the flask-shaped ai-che-
gone by a circular fissure, carrying the higher part upwards on its
summit as a calypter or hood (fig. 508, B, c), while the lower part
remains to form a kind of collar roimd the base of the stalk, known
as the vagine.

The urn, theca, or sporange, which is the immediate product of
the generative act, is closed at its summit by an opercule, or lid
(fig. 508, B, 0, 0), which falls ofi" when the contents of the sporange
are mature, so as to give them free exit ; and the mouth thus laid
open is surrounded, in many mosses, by a beautiful toothed fringe,
which is termed the peristome. This fringe, as seen in its original

FiC4. 510. — Mouth of sporange of Fiinaria,
showing the peristome in situ.

Pig. 511. — Double peristome
of Fontinalis antipyretic a.

undisturbed position (fig. 510), is a beautiful object for the binocular
microscope ; it is very ' hygrometric,' executing, when breathed on,
a curious movement which is probably concerned in the dispersion
of the spores. In figs. 511-513 are show^n three difierent forms of
peristome, spread out and detached, illustrating the varieties which

672 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

it exhibits in different genera of mosses — varieties whose existence
and readiness of recognition render them characters of extreme value
to the systematic botanist, whilst they furnish objects of great
interest and beauty for the microscopist. The peristome seems
always to be originally double, one layer springing from the outer,
and the other from the inner, of two layers of cells which may be
always distinguished in the immature sporange ; but one or other of
these is frequently wanting at the time of maturity, and sometimes
both are obliterated, so that there is no peristome at all. The
number of the teeth is always a 'power' of four, varying from
four to sixty-four ; sometimes they are prolonged into straight or
twisted hairs. The sjjores, or gonidial cells, are contained in the
upper part of the sporange, where they are clustered round a central
pillar which is termed the colmnel. In the young sporange the
whole mass is nearly solid (fig. 508, C), the space (l) in which the

—^i^i^ iLkj UM vu fhi_^ e^ iLy lay jjjHoic^

~^')rir^r)

uROfjaqpRp

Fig. 512. — Double peristome oi:
Bi-yitm intermedium.

Fig. 513. — Double peristome
of CincUdium arcticum.

spores are developed being very small ; but this gradually augments,
the walls becoming more condensed, and at the time of maturity
the interior of the spoi-ange is almost entirely occupied by the spores.
These are formed in groups of four by the binary subdivision of the
m.other-cells which first difFei-entiate themselves from those forming
the capsule itself. The capsule and seta of mosses together consti-
tute the organ known as the spo7^ogone.

The development of the spore into a new plant commences with
the rupture of its firm^ yellowish-brown outer coat or exospore, and
the protrusion of its cell-wall proper, or endospore, from the
pi'ojecting extremity of which new cells ai'e put foi-th by a pi'ocess
of outgrowth, forming a soi-t of confervoid filament known as the
proto7ieme. At certain points of this filament its component cells
multiply by subdivision, so as to form rounded clusters or buds, fi-om
every one of which an independent plant may arise. The Musci,
therefore, present an example of the phenomenon known as alter-
nation of (jenerations. The life-history of each individual may be
divided into two ' genei'ations : ' the sexual generation or ' oophyte,'

SPORANGE OF MOSSES

^7Z

which consists of the leafy plant beai-ing the male and female organs ;
and the non-sexual genei'ation or ' sj)oi'ophyte,' composed of the
sporogone with its spoi-es, these two genei-ations alternating with one
another in the complete cycle of development.

The tribe of Sphagnacece^ or ' bog-mosses,' is now separated by
muscologists from true mosses on account of the marked differences
by which they are distinguished, the three groups, Hepaticce,
Bryacece (or ordinary mosses), and Sphagnacece, being ranked as to-
gether forming the group of Muscinece.
The stem of Sphagncicece is inoi-e dis- b b h

tinctlj^ differentiated than that of
Bnjacece into the central or medullary,
the outer or cortical, and the inter-
mediate or woody portions ; and a very
rapid passage of fluid takes place
thi'ough its elongated cells, especially
in the medullary and cortical layers, so
that if one of the plants be placed dry
in a flask of water, with its i-osette
of leaves bent downwards, the water
will speedily drop from this until the
flask is emptied. The leaf-cells of the
Sjihagnacece exhibit a very curious de-
parture from the oi'dinary type ; foi'
instead of being small and polygonal,
they are large and elongated (fig. 514) ;
they contain no chlorophyll, but have
spiral fibres loosely coiled in their in-
terior ; and theii- membi'anous walls
have large rounded apertui-es, by which
theii- cavities freely communicate with
one another, as is sometimes curiously
evidenced by the passage of wheel-
animalcules that make their habitation in these chambers. Between
these coarsely spiral cells are some thick-walled narrow elongated
cells containing chlorophyll ; these, which give to the leaf its firm-
ness, do not, in the veiy yonng leaf, differ much in appearance from
the others, the peculiarities of both being evolved by a gradual pro-
cess of differentiation. The antherids, or male organs, of Spliagnacece
resemble those of liverworts, rather than those of mosses, in their
form and arrangement ; they are grouped in ' catkins ' at the tips
of lateral branches, each of the imbricated perigonal leaves enclosing
a single globose antherid on a slendei- footstalk, and they are sur-
rounded by very long bi-anched paraphyses of cobweb-like tenuity.
The female organs, or ai-chegones, which do not differ in structure
from those of mosses, are gi'ouped together in a sheath of deep green
leaves at the end of one of the short latei-al branchlets at the side of
the rosette oi- terminal crown of leaves. The two sets of organs
are always distributed on different branches, and in some instances
on different plants. The ' sjjoi-ange ' which is formed as the j^roduct
of the impregnation of the germ-cell is very uniform in all the

X X

Fig. 514.— Portion of the leaf of
Sphagnum, showing the large
empty cells, a, a, a, with spiral
fibres, and communicating aper-
tures ; and the intervening
bands, h, b, b, composed of
small elongated ehloroxjhyllous
cells.

6/4 MICEOSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

species, being almost spherical, with a slightly convex lid, without
beak or point, and showing no trace of a peristome ; and the spores
it contains are produced in gToups of foiu- (as in mosses) around a
hemispherical ' columel.' Besides the ordinary spores, however,
the Sphagnacece, sometimes develop a smaller kind, the ' microspores,'
formed by a further division of the mother-cells ; the significance of
these is unknown.^ The ordinary spores, when germinating, do not
produce the branched confervoid filanient of true mosses, but if
growing on wet peat evolve themselves into a lobed foliaceous ' pro-
thallium,' resembling the frond of liverworts ; whilst if they
develop in water a single long filament is formed, of which the
lower end gives ofi" rhizoids, while the upper enlarges into a bud,
from which the young plant is evolved. In either case the pro-
thallium and its temporary roots wither away as soon as the young-
plant begins to branch. From their extraordinary power of imlDibing
and holding water, the Sj^hagnacece are of great importance in the
economy of ISTature, clothing with vegetation many areas which
would otherwise be sterile, and serving as reservoirs for storing up
moisture for the use of higher forms of vegetation.

Filices. — In the general structure of Ferns we find a much nearer
approximation to flowering plants ; but this does not extend to
their reproductive apparatus, which is formed upon a type essentially
the same as that of mosses, though evolved at a very different period
of life. As the tissues of which their fabrics are composed are

essentially the same as those to be de-
scribed in the next chapter, it will not
be requisite here to dwell uj)on them.
The stem (where it exists) is for the
most part made up of cellular pai--
enchyme, which is separated into a
coi'tical and a medullary poi-tion by the
interposition of a circular series of
fibro-vascular bundles containing true
woody tissue and ducts. These bundles
form a kind of irregular network, from
which pi'olongations are given off that
pass into the leaf-stalks, and thence
into the mididb and its lateral branches ;
and it is their peculiar arrangement in
^'sUlk^'^^ Sr Sr shoSno- *1^^ leaf-stalk of the common brake which
bundle of scalariform ducts. " gives to the transverse section the mark-
ing connnonly known as ' King Charles
in the oak.' A thin section, especially if somewhat oblique (fig. 515),
displays extremely well the peculiar character of the ducts of the
fern, which are termed scalariform from the resemblance of the
regular mai-kings on their walls to the rungs of a ladder. These
bundles of scalariform ducts or ' tracheids ' are usually surrounded by
sheaths of sclerevchyrae, tissue composed of cells the walls of which

1 These so-called ' inicrospores ' are now believed to be spores of a parasitic
fungus. — Ed.

STRUCT LTEE OF FEENS

675

have become very hard and of a deep brown colour. These scleren-
chymatous sheaths are a very conspicuous feature in a transverse
section of the stem or rhizome of most ferns, and are the principal
agent in giving it strength and solidity.

Wliat is usually termed the fructification of the fern affords
a most beautiful and readily prepared class of opaque objects for the
lower powers of the microscope ; nothing more being necessary than
to lay a fragment of the frond that bears it upon the glass stage-
plate or to hold it in the stage-forceps, and to throw an adequate light
upon it by the side-condenser. It usually presents itself in the form
of isolated spots on the under surface of the frond termed sori.
as in the common Polypodium (fig. 516), and in Aspidium (fig.
518); but sometimes these sori are elongated into bands, as in

Fig. 516.— Leaflet of Poly-
podium, with sori.

Fig. 517.

-Portion of frond of Hcemionitis,
with sori.

the common Scolopendrmm (hart's tongue) ; and these may coalesce
with each other, so as almost to cover the surface of the frond with
a network, as in Hcemionitis (fig. 517) ; or they may form merely a
single band along its borders, as in the common Pteris (brake-fern).
The sori are sometimes ' naked ' on the under surface of the fi'onds ;
but they are frequently covered with a delicate membrane termed
the indusiimn, which may either form a sort of cap upon the summit
of each sorus, as in Aspidium (fig. 518), or a long fold, as in Scolo-
pendrimn and Pteris, or a sort of cup, as in Deparia (fig. 519),
Each of these sori, when sufliciently magnified, is found to be made
up of a multitude of sporanges, or spore-capsules (figs. 518, 519).
Avhich are sometimes closely attached to the surface of the fii'ond,
but more commonly spring from it by a pedicel or footstalk. The

X X 2

6^6 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOaAMS

wall of the sporange is composed of flattened cells, applied to each
other by their edges ; but there is generally one row of these thicker
and larger than the rest which springs from the pedicel, and is
continued over the summit of the spoi-ange, so as to form a projecting
ring, which is known as the annulus (fig. 519). This ring has an
elasticity superior to that of all the rest of the wall of the capsule,
causing it to split across when mature, so that the contained spores
may escape ; and in many instances the two halves of the sporange are
carried widely apart from each othei-, the fissure extending to such a
depth as to separate them completely. In Osviunda (the so-called
' flowering fern ' or ' royal fern ' ) and Ophioglossum (adder's tongue)
the sporanges have no annulus, or one greatly modified. It will
frequently happen that specimens of fern-fi-uctifi cation gathered for
the microscope will be found to have all the sporanges burst and
the spores dispersed, whilst in others less advanced the sporanges
may all be closed ; others, however, may often be met with in which
some of the sporanges are closed and others are open ; and if these
be watched with sufficient attentioii the ruptxire of some of the

Fig. 518. — Sorus and indusium of
Asxndium.

Fig. 519. — Sorus and cup-shaped
indusium of Deparia prolifera.

sporanges and the dispersion of the spores may be observed to take
place while the specimen is under observation in the field of the
microscope. In sori whose sporanges have all burst, the an null
connecting their two halves are the most conspicuous objects, look-
ing, when a strong light is thrown upon them, like strongly banded
worms of a bright brown hue. This is particularly the case in
SGolope'}ulriimi, whose elongated sori are remarkably beautiful
objects for the microscope in all their stages ; until quite mature,
however, they need to be brought into view by tuniing back the
two indusial folds that cover them. The commonest ferns, indeed,
which are found in almost every hedge, furnish objects of no less
beauty than those yielded by the rarest exotics ; and it is in every
respect a most valuable training to the young to teach them how
much may be found to interest, when looked for with intelligent
eyes, even in the most familiar, and therefore disregarded, specimens
of Nature's handiwork.

The ' spores ' (fig. 520, A) set free by the bursting of the spo-
ranges, usually have a somewhat angular foi-m, and are invested by a

FEUCTIFICATION OF FEKNS

6J7

yellowish or brownish outer coat, the exospore, which is marked
very mvich in the manner of pollen-grains (fig. 565) with points,
streaks, ridges, or reticulations. When placed upon a damp
surface, and exposed to a sufficiency of light and warmth, the
spore begins to germinate, the first indication of its vegetative
activity being a slight enlargement, which is manifested in the
rounding ofi" of its angles. This is followed by the putting forth
of a tubular prolongation (fig. 520, B, a) of the internal cell- wall or
endospore through an aperture in the outer spore-coat ; and mois-
ture being absorbed through this, the cell becomes so distended as
to burst the external unyielding integument, and soon begins to
elongate itself in a direction opposite to that of the fij.'st rhizoid. A
production of new cells by subdivision then takes place from its grow-

FiG. 520. — Development of ijrothalliuni of Pteris serrulata : A, spore set free from
the sporange ; B, spore begiuning to germinate, putting forth the tubular pro-
longation a, from the principal cell b ; C, first-formed linear series of cells ; D, pro-
thallium taking the form of a leaf-like expansion ; a, first, and b, second rhizoid ;
c, d, the two lobes, and e, the indentation between them ; /, /, first-formed part of
the prothallium ; g, external coat of the original spore ; h, h, antherids.

ing extremity ; this at first proceeds in a single series, so as to form
a kind of confervoid filament (C) ; but the cell-division soon takes
place transversely as well as longitudinally, so that a flattened leaf-
like expansion (D) is produced, so closely resembling that of a young
Marchantia as to be readily mistaken for it. This expansion, which
is termed the 2:)rothallium, varies in its configui'ation in difierent
species, but its essential structure always remains the same. From
its under surface are developed, not merely the rhizoids {a, b), which
sei've at the same time to fix it in the soil and to supply it with
moisture, but also the antherids and archegones, which constitute
the true representatives of the essential parts of the flower of higher
plants. Some of the former may be distinguished at an early period
of the development of the prothallium (A, h) ; and at the time of its
complete evolution these bodies are seen in considerable numbers,

678 MICROSCOPIC STRUCTUEE OF HIGHER CRYPTOGAMS

especially in the neighbourhood of the rhizoids. Each has its oi'igin
in a peculiai' protrusion that takes place from one of the cells of the
prothallium (fig. 521, A, a) ; this is at first entirely filled with
chlorophyll-gTanules, but soon cell-division sets up in it. A central
cell b becomes distinguished from all the rest by its much larger size

and is surrounded by one
or two layers of much
smaller cells known as
the tapetal or mantle-
cells. These take no
part in the formation of
the antherozoids ; but
the protoplasmic con-
tents of the large central
cell divide by free-cell -
Fig. 521.— Development of the antherids and an the- formation into a large
rozoids of Pteris serrulata: A, projection of one number of cells known as
of the cells of the prothallium showing the anthe- ^^^g antherozoid-mother-
ridial cell 0, with its sperm-cells e, within the cavity 77 / \ 1 f +1

of the original cell a. B, antherid completely cews [C) ; each Ot these
developed ; a, wall of antheridial cell ; e, sperm- again breaks Up into
cells, each enclosing an antherozoid. C, anthero-
zoid more highly magnified, showing its l^rge ex-
tremity a, its small extremity h, and its cilia d, d.

B

four cells, not at first
pi'ovided with cell-walls,
the spe7")n-cells. Each
of the sperm-cells (B, e)
is seen, as it approaches
maturity, to contain a
spirally coiled filament ;
and when set free by
the bursting of the
antherid the sperm-
cells themselves burst,
and give exit to theii-
antherozoids (C), which
execute rapid move-

_, ^„„ . , . -r.^ ■ 14 A ments of rotation on

Fig. 522. — Archegone of Fteris serrulata : A, as , . +1 i

seen from above ; a, a, a, cells surrounding the tlieir axes, partly de-
base of the cavity; &, c, d, successive layers of pendent on the long
cells, the highest enclosing a quadrangular orifice, gjj^jg^ with which thev
B, side view, showing a, a, cavity containing the -C • 1 i

germ-cell, a; b, b, walls of the archegone, made ^^^ lurnisnecl.
up of the four layers of cells, h, c, d, e, and having The (ivchegones are

^V-^'T"^'-^'-?" *''%^'''^™V '''•f' '^'^^'TT'^^ fewer in number, and
withm the cavity; g, large extremity; h, vibratile f \ T-f

cilia; i, small extremity in contact with the germ- ^^^ louncl upon a Clli-
cell, and dilated. ferent part of the pro-

thallium. Each of them
originates in a single cell of its superficial layer, which undergoes
subdivision by a horizontal partition. Of the two cells thus produced
the upper gives origin, by successive subdivisions, to the ' neck ' of
the archegone, which, when fully developed (fig. 522), is composed
of twelve or more cells, built up in layers of four cells each, one upon
another, so as to form a kind of chimney or shaft. The lower of the
two first-formed cells becomes the centred cell of the archegone ;

SEXUAL GENERATION OF PERNS 679

and this again undergoing lioiizontal subdivision, the lower half be-
comes the oosphere or (jerm-cell, whilst the upper extends itself into
the neck. By the conversion into mucilage of a central row, an open
passage or canal is formed, through which the antherozoids make
their way to the oosphere lying at its bottom (fig. 522, B, a). The
oosphere, when fertilised by the penetration of the antherozoids,
becomes the ' embryo-cell ' of a new plant, the development of which
speedily commences.^ In the aberrant group of Ophioglossaceoi
(adder's-tongue ferns), the development of the prothallium takes
place underground, in the form of a small roundish tuber, composed
of parenchymatous tissue containing no chlorophyll, and producing
antherids and archegones on its upper surface.

The early development of the embryo-cell takes place according
to the usual method of repeated subdivision, producing a homo-
geneous globular mass of cells. Soon, however, rudiments of special
organs begin to make their appearance ; the embryo gTOws at the
expense of the nutriment prepared for it by the prothallium, and it
bursts forth from the cavity of the archegone, which organ in the
meantime is becoming atrophied. In the very beginning of its
development the tendency is seen in the cells of one extremity to
grow upward so as to evolve the stem and leaves, and in those of the
other extremity to grow downward to form the root ; and when
these organs have been sufiiciently developed to absorb and prepare
the nutriment which the young fern requires, the prothallium decays
away. Thus, then, the ' spore ' of the fern must be considered as a
generative 'gonid ' or detached flower-bud capable of developing
itself into a prothallium that may be likened to a 1 eceptacle bearing
the sexual apparatus. But this pi-othallium serves the further pur-
pose of ' nursing ' the embryos originated by the genei'ative act ;
which embryos finally develop themselves, not, as in mosses, into
mere sporogones, but, as in Phanerogams, into entire plants, com-

1 The study of the development of the spores of ferns, and of the act of fertihsa-
tion and of its products, may be conveniently prosecuted as follows : — Let a frond of
a fern whose fructification is mature be laid upon a piece of fine paper, with its
spore-bearing surface downwards ; in the course of a day or two this paper will be
found to be covered with a very fine brownish dust, which consists of the discharged
spores. This must be carefully collected, and should be spread upon the surface of
a smoothed fragment of porous sandstone, the stone being placed in a saucer, the
bottom of which is covered with water ; and a glass tumble f being inverted over it,
the requisite supply of moisture is ensured, and the spores will germinate luxuriantly.
Some of the prothallia soon advance beyond the rest ; and at the time when the
advanced ones have long ceased to produce antherids, and bear abundance of
archegones, those which have remained behind in their growth are beginning to be
covered with antherids. If the crop be now kept with little moisture for several
weeks, and then suddenly watered, a large number of antherids and archegones
simultaneously open ; and in a few hours afterwards the surface of the larger pro-
thallia will be found almost covered with moving antherozoids. Such prothallia as
exhibit freshly opened archegones are now to be held by one lobe between the forefinger
and thumb of the left hand, so that the upper surface of the prothallium lies upon the
thumb ; and the thinnest xoossible sections are then to be made with a thin narrow-
bladed knife, perpendicularly to its surface. Of these sections, which, after much
practice, may be made no more than one- fifteenth of a line in thickness, some will
probably lay open the canals of the archegones ; and within these, when examined
with a power of 200 or 300 diameters, antherozoids may be occasionally dis-
tinguished. The prothallium of the common Osmunda regalis will be found to
afford peculiar facilities for observation of the development of the antherids, which
are produced at its margin.

68o MICROSCOPIC STRUCTUEE OF HIGHER CRYPTOGAMS

plete in everything but the true generative organs, which evolve
themselves from the detached spores. Here we have, therefore, an
example of alternation of generatio7is differing in one important
respect from that in mosses. In ferns the ' sexual generation ' or
' oophyte ' which results from the germination of the spore consists
of the prothallium only with its archegones and antherids, the leafy
plant which bears the sporanges constituting the ' sporophyte ' or
' non-sexual generation,' the product of the fertilisation of the arche-
gone by an antherozoid. In mosses, on the other hand, the leafy
jDlant lielongs to the sexual generation.

The singular discovery has recently been made by the researches
of De Bary, Farlow, and others, that the ordinary alternation of
generations in ferns may be interrupted by the suppression either of
the sporophyte, the non-sexual or spore-bearing generation, or of the
oophyte or sexual generation which bears the true repi'oductive
organs. These phenomena are called respectively aiyosjjory and
apogamy. The former has been observed especially in varieties of
Athyrium Filix-foemina and Polystichum angidare, and is shown by
the production of prothalloid structures bearing antherids and
archegones on the fronds in the place of ordinary sori. The latter
occurs not unfrequently in Pteris seo'i^ulata, the sporophytic genera-
tion springing directly from the prothallium without the interven-
tion of archegones and antherids.

The little group of Equisetaceae (horse-tails), which seem nearly
allied to the ferns in the type of their generative apparatus, though
that of their vegetative portion is very different, affords certain
objects of considerable interest to the mici^oscopist. The whole of
their structure is penetrated to svich an extraordinary degree
by silex, that even when its organic portion has been destroyed by
prolonged maceration in dilute nitric acid, a consistent skeleton still
remains. This mineral, in fact, constitutes in some species not less
than 13 per cent, of the whole solid matter, and 50 per cent, of the
inorganic ash ; and it especially abounds in the epiderm, which is
used by cabinet-makers for smoothing the surface of wood. Some of
the siliceous particles are distributed in two lines, parallel to the
axis ; others, however, are grouped into oval forms, connected with
each other, like the jewels of a necklace, by a chain of particles
forming a sort of curvilinear quadrangle ; and these (which are, in
fact, the particles occupying the guard-cells of the stomates) are
arranged in pairs. Their form and arrangement are peculiarly well
seen under polarised light, for which the prepared epiderm is an
extremely beautiful object ; and it is asseited by Sii- D. Brewster
(whose authority upon this point has been generally followed) that
each siliceous particle has a regular axis of double refraction. What
is usually designated as the fructification of the Equisetacete forms a
cone or spike at the extremity of certain of the stem-like branches
(the real stem being a horizontal rhizome) and consists of a cluster
of shield-like discs, each of which cai'ries a circle of sporanges or
spore-capsules, that open by longitudinal slits to set free the spores.
In addition to the spoi'es each sporange contains a numbei' of elastic
filaments (fig. 52.S), called elaters. These are at first coiled up around

EQUlSETxlCE.-E : RHIZOCARPE J3 ; LYCOPODIACEJi 68 I

the spore, in the manner represented at A, though more closely
applied to the surface ; but, on the liberation of the sj)ore, they ex-
tend themselves in the manner shown at B, the slightest application
of moisture, however, serving to make them close together (the
assistance which they afford in the dispersion of the spores being no
longer required) when the spores have alighted on a damp surface.
If a number of these spores be spread out on a slip of glass under the
field of view, and, whilst the observer watches them, a bystander
bi'eathes gently upon the glass, all the filaments will be instanta-
neously put in motion, thus presenting an extremely curious spec-
tacle, and will almost as suddenly retur.n to their previous condition
when the efiect of the moisture has passed ofi". If one of the sporanges
which has opened, but has not discharged its spores, be mounted
in a cell with a movable cov6r, this curious action may be exliibited
over and over again. These spores, like those of ferns, develop into a
prothallium ; and this bears antherids and archegones, the former at
theextremitiesof the lobes, and the latter in the angles between them.
Nearly allied to Fei'us, also, is a curious little group of small
aquatic plants, the Rhizocarpeae (or Pepper- worts), which either
float on the surface or creep along shallow bottoms. These difier

Fig. 523. — Si^ores of Equisetum, with their elaters.

from Ferns and Horse-tails in having two kinds of spore, jjroduced
in separate sporanges ; the larger, or ' megaspores,' giving origin to
prothallia which produce archegones only ; and the smaller, or
' microspores,' undergoing progressive subdivision, usually without
the formation of a distinct prothallium, each of the cells thus formed
giving origin to an antherozoid. In this, as we shall presently see,
there is a distinct foreshadowing of the mode in which the genera-
tive process is performed in flowering plants, the ' microspore ' cor-
I'esponding to the pollen-grain, while the ' megaspore ' may be con-
sidered to represent the primitive cell of the ovule.

Another alliance of Ferns is to the Lycopodiacese (Club-mosses),
a gToup which at the present time attains a great development in
warm climates, and which, it would seem, constituted a large part
of the arborescent vegetation of the Carboniferous epoch. In the
Lycopodiew proper the sporanges are all of one kind, and all the
spores are of the same size, each, as in Ophioglossum, giving origin
to a subterraneous prothallium that develops both antherids and
ai'chegones. The j)lant which originates from the fertilised ' germ-
cell ' of the archegone attains in colder climates only a moss-like

682 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

growth, with a creeping stem usually branching dichotomously, and
imbricated leaves ; but is distinguished from the true mosses, not
only by its higher general organisation (which is on a level with that
of ferns), but by the character of its fructification, which is a club-
shaped ' spike,' bearing small imbricated leaves, in the axils of which
lie the sporanges. The spores developed within these are remarkable
for the large quantity of oily matter they contain, giving them an
inflammability that causes their being used in theatres to produce
' artificial lightning.' But in the allied groups of Selaginellece and
Isoetece there are (as in the Bhizocarpece) two kinds of spore pro-
duced in separate sporanges ; one set producing ' megaspores,' from
which archegone-bearing prothallia are developed, and the other
producing ' microspores,' which, by repeated subdivision, give origin
to antherozoids without the formation of prothallia. It is a very
interesting indication of a tendency towards the phanerogamic type
of sexual generation, that the prothallium in this group is chiefly
developed within the sporange, forming a kind of ' endosperm,' only
the small part which projects from the ruptured apex of the spore
producing one or more archegones. The arborescent Lepidodendra
and Sigillarice of the Coal-measures seem to have formed connecting-
links between the Vasctdar Gryjjtogams and the Phanerogams, alike
in the structure of their stems and in their fructification. For the
Lepidostrohi or cone-like ' fruit ' of these trees represent the club-
shaped spikes of the Lycopodiacece, ; and seem to have borne ' mega-
spores ' in the sporanges of their basal portion, and ' microspores '
in those of their upper part. Some of the best seams of coal appear
to have been chiefly formed by the accumulation of these ' mega-
spores.'

Thus, in our ascent from the lower to the higher Cryptogams, Ave
have seen a gradual change in the general plan of structure, bring-
ing their superior types into a close approximation to the flowering-
plant, which is undoubtedly the highest form of vegetation. But
we haA^e everywhere encountered a mode of generation which,
whilst essentially the same throughout the series, is no less essen-
tially distinct from that of the Phanerogam, the fertilising material
of the ' sperm-cells ' being embodied, as it were, in self-moving fila-
ments, the antherozoids, Avhich find their way to the ' germ-cells ' by
their own independent movements, and the ' embryo-cell ' being-
destitute of that store of prepared nutriment Avhich surrounds it in
the true seed, and svipplies the material for its early development.
In the lower Cryptogams we have seen that the fertilised oospoi-e
is thrown at once u]3on the world, so to speak, to get its own living ;
but in ferns and their allies the ' embryo-cell ' is nurtured for a
while by the pi-othallium of the parent plant. Wliile the true
reproduction of the species is effected by the proper generative act,
the multiplication of the i7idividual is accomplished by the production
and dispersion of ' gonidial ' spores ; and this production, as we have
seen, takes place at very different pei-iods of existence in the several

ALTEENATION OF GENEEATIONS 683

groups, dividing the life of each into two separate epochs, in which
it pi'esents itself under two very distinct phases that contrast
remarkably with each other. Thus, the frond of Marchantia,
evolved from the spore and bearing the antherids and archegones,
is that which seems naturally to constitute the plant ; but that which
represents this phase in the ferns is the minute 2Iarchantia-\ike
prothallium. In ferns, on the other hand, the product into which
the fertilised ' embryo-cell ' evolves itself is that which is commonly
i-egarded as the plant ; and this is represented in the liverworts and
mosses by the sporogone alone. ^ We shall encounter a similar
diversity (which has received the inappropiiate designation of ' alter-
nation of generations ' ) in some of the loAver forms of the animal
kingdom.

1 For more detailed information on tlie structure and classification of the Crypto-
gams generally the reader is referred to Goebel's Otitlines of Classification and
Special Morpliology and De Bary's Comparative Anatomy of the Phanerogams
and Ferns, translations of both of which have been published by the Clarendon
Press ; and especially to Bennett and Murray's Handbook of Cryptogamic Botany,
published by Longmans (London, 1889 J.

684 MICROSCOPIC STEUCTUEE OF PHANEROGrAMIC PLANTS

CHAPTER XI

OF THE MICBOSCOPIC STEUCTUEE OF PHANEBOGAMIC PLANTS

Between the two great divisions of the Vegetable Kingdom which
ai-e known as Cryjjtogamia and Phanerogmnia the separation is by
no means so abrupt as it formerly seemed to be. For, as has been
already shown, though the Cryptogamia were formerly regarded as
altogether non-sexual, a true generative process, requiring the
concurrence of male and female elements, is traceable almost through-
out the series. And in the higher types of that series we have seen
a foreshadowing of those provisions for the nurture of the fertilised
embryo which constitute the distinctive characters of the Phanero-
gamia. On the other hand, although we are accustomed to speak of
Phanerogamia as ' flowering plants,' yet not only are the conspicuous
parts of the flower often wanting, but in the important group of
Gymnosperms (including the Coniferce and Cycadece) the essential
parts of the generative apparatvis are reduced to a condition closely
approximating to tliat of the higher Cryptogams. There are, how-
ever, certain fundamental differences between the modes in which
the act of fertilisation is performed in the two groups. For (1)
whilst in all the higher Cryptogams it is in the condition of free-
moving ' antherozoids ' that the contents of the sperm-cell find their
way to the germ-cell, these are conveyed to it, throughout the
phanerogamic series, by an extension of the lining membrane of the
sperm-cell or pollen-grain into a tube, which penetrates to the germ-
cell, contained in the interior of the body called the ovtde} Again
(2), while the ' germ-cell ' or oosphere in the higher Cryptogams is
contained in a structure that originated in a spore detached from the
parent plant, it is not only formed and fertilised in all Phanerogams
whilst still borne on the parent fabric, but continues for some time
to draw from it the nutriment it requires for its development into the
embryo. And at the time of its detachment from the parent the

1 A very remarkable and interesting discover}', for which we are largely indebted
to the brilliant observations of two Japanese botanists, Professors Ikeno and Hirase,
has recently thrown great light on the ap]3roximation referred to by Dr. Carpenter
between the higher Cryptogamia and the lower Phanerogamia. It is now known
that in both the larger groups of Gymnosperms, the Coniferse and the Cycadese, there
are species in which the fertilising body is a motile antherozoid formed within a
pollen-tube, thus combining the distinctive modes of fertilisation characteristic of
the two great sections of the vegetable kingdom. As Dr. Carpenter does aiot include in
his account of the ' Microscopic Structure of Phanerogamic Plants ' a full description
of the mode of impregnation in flowering plants, the reader is referred, for further
details, to the most recent Text-books of Botany, or to the Summary of Current Ee-
searches in Botany in the Journal of the B. Microscopical Society.— 'EiDITO'r.']

STEUCTURE OF PHANEROGAMIA 685

matured ' seed ' contains, not merely an embryo already advanced
a considerable stage, but a store of nuti'iment to serve foi- its further
development during germination. As there is nothing parallel to
this among Cryptogams, it may be said that reproduction by seeds,
not the possession of flowers, is the distinctive character of Phanero-
gams. The ovules, which when fertilised and matured become seeds,
are developed fi-om specially modified leaves, which remain open in
Gymnosperms, but which in all other Phanerogams fold together so
as to enclose the ovules within an ovary. Each ovule consists of a
nucellus surrounded by integuments which remain unclosed at the
apex, leaving open a short canal termed the micropyle or ' foramen.'
One cell of the nucellus undergoes great enlargement, and becomes
the embryo-sac, whose cavity is filled, in the first instance, with a
mucilaginous fluid containing protoplasm. At the end of the
embryo-sac which lies nearest the micropyle a germ-cell or oosphere
is developed ; in Angiosperms by free-cell-formation, but in
Gymnosperms indirectly after the formation of a ' corpuscle,' which
represents the archegone of Selaginella. By a further process of
free-cell-formation the remainder of the embryo-sac comes to be
filled with cells constituting what is termed the endosjjerm,; and
this serves, like the prothallium of ferns, to imbibe and prepare
nutriment which is afterwards appropriated by the embryo. In
many seeds (as those of the Leguminosce) the whole nutritive material
of the endosperm has been absorbed into the cotyledons (or seed-
leaves) of the embryo by the time that the seed is fully matured and
independent of the parent ; but in othei- cases it remains as a ' sepa-
rate endosperm.' In either case it is taken into the substance of the
embryo during its germination.

Elementary Tissues. — No mai-ked change shows itself in general
oi'ganisation as we pass from the cryptogamic to the phanerogamic
series of plants. A large proportion of the fabiic of even the
most elaborately formed tree (including the parts most actively con-
cerned in living action) is made uj) of components of the very same
kind as those which constitute the entii-e organisms of the simplest
ciyptogams. Foi-, although the stems, branches, and i-oots of trees
and shrubs are principally composed of vmody tissue, such as we do
not meet with in any but the highest Cryptogams, yet the special
ofiice of this is to afford mechanical support ; when it is once formed,
it takes no further share in the vital economy than to serve for the
conveyance of fluid from the roots upwards through the stem and
branches to the leaves ; and even in these organs (in Exogens or
Dicotyledons), not only the pith and the cortex, with the ' medullary
rays,' which serve to connect them, but the ' cambium layer ' inter-
vening between the bark and the wood in which the periodical
formation of the new layers both of bark and wood takes place, ai-e
composed of cellidar substance. This tissue is found, in fact,
whei'ever growth is taking place ; as, foi- example, in the growing
points of the root-fibres, in the leaf-buds and leaves, and in the
flower-buds and sexual parts of the flower ; it is only when these
organs attain an advanced stage of development that woody structui'e
is found in them ; its function (as in the stem) being merely to give

686 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

support to their softer textures ; and the small proportion of their
substance which it forms is at once seen in those beautiful ' skeletons '
which, by a little skill and perseverance, may be made of leaves,
flowers, and certain fruits. All the softer and more pulpy tissue

of these organs is com-

,/^:^.'';X^V /-/ posed of ce/Zs, more or less

compactly aggregated to-
gether, and having forms
that approximate more or
less closely to the globu-
lar or ovoidal, which may
be considered as their
original type.

As a general rule, the
rounded shape is pre-
served only when the cells
are but loosely aggre-
gated, as in the parenchy-
matous (or pulpy) sub-
stance of leaves, which
often forms a distinct
layer known as the
' spongy parenchyme '
immediately beneath the
epiderm of the upper sur-
face (fig. 524) ; and it is then only that the distinctness of their
walls becomes evident. When the tissue becomes more solid, the
sides of the vesicles are pressed against each other, so as to flatten

Fig. 524 — Section of leaf of Agave, treated with
dilute nitric acid, showing the i^rotox^lasmic con-
tents contracted in the interior of the cells ; a,
epidermal cells 6, guard-cells of the stomate ;
c, cells of x^arenehyme ; d, their protoplasmic
contents.

Fig. .525. — Sections of cellular i^arenchyme of Aralia, or rice-paper plant
A, transversely to the axis of the stem ; B, in the direction of the axis.

them and to bring them into close apposition, and then the cavities
of adjacent cells are separated by a single partition wall. Fre-
quently it happens that the pressure is exerted more in one direction
than in another, so that the foi'm presented by the outline of the cell

STKFCTUEE OF THE CELL 68/

varies according to the direction in which the section is made. Tliis
is well shown in the pith of the young shoots of elder, lilac, or other
rapidly growing trees, the cells of which, when cut transversely, gene-
I'ally exhibit circular outlines ; whilst, when the section is made vei'ti-
cally, their boixlers are straight, so as to make them ajopear like
cubes or elongated prisms, as in fig. 524. A very good example of
such a cellulai- parenchyme is to be found in the substance known as
' I'ice -paper,' which is made by cutting the herbaceous stem of a
Chinese plant termed Aralia jKqyyrifera vertically round and round
with a long sharp knife, so that its tissues may be (as it were) unrolled
in a sheet. The shape of its cells when thus prepared is irregularly
piismatic, as shown in fig. 525, B ; but if the stem be cut transversely,
their outlines are seen to be circular or nearly so (A). When, as
often happens, the cells have a very elongated form, this elongation
is in the direction of their growth, which is that, of course, wherein
there is least i-esistance. Hence their greatest length is nearly
always in the direction of the axis ; but there is one remarkable
exception, that, namely, which is afibrded by the ' medullary rays '
of exogenous stems, whose cells are greatly elongated in the horizontal
direction (fig. 547, a), tlieir growth being from the centre of the stem
towards its circumference. It is obvious that fluids will be more
readily transmitted in the direction of greatest elongation, being that
in which they will have to pass through the least number of parti-
tions ; and whilst their ordinary course is in the direction of the length
of the roots, stems, or branches, they will be enabled by means of the
medullary rays to find their way in the transverse direction. One
of the most curious varieties of
form which vegetable cells pre-
sent is the stellate cell, rejDre-
sented in fig. 526, forming the
spongy parenchjmiatous substance
in the stems of many aquatic
j)lants, of the rush for example,
which are furnished with air-
spaces. In other instances these
air-spaces are large cavities which
are altogether left void of tissue : ^^g- 526.— Section of stellate

T . J T ■ T^T 1 7 , pareiicnvnie oi rush.

such IS the case m iSuphar Lutea
(the yellow water-lily), the foot-
stalks of whose leaves contain large air-chambers, tho walls of
which are built up of very regular cubical cells, whilst some curiously
formed large stellate cells project into the cavity which they bound
(fig. 527). The dimensions of the component vesicles of cellular tissue
are extremely vai-iable ; for although their diameter is very com-
monly between -j-guth and 5-^th of an inch, they occasionally mea-
sure as much as -g^th of an inch across, whilst in other instances
they are not more than 3-^th.

The cells of a growing tissue are always formed, as we have seen,
by cell-division, that is, by the formation of cellulose walls across
cells previously in existence. The original cell-wall must therefoi-e
always be single. It is only in older thick-walled cells that a line of

688 MICROSCOPIC STRUCTUEE OE PHANEROGAMIC PLANTS

demarcation becomes obvious in the form of an intei'mediate lamella,
at one time called ' intercellular substance,' and supposed to be a
distinct structure, but now shown to be the result merely of a differ-
ence in density or molecular structure of the cell- walls during theii-
thickening. This layer very frequently ultimately assumes a muci-
laginous character. Where cells have a rounded outline, it is
obvious that hitercellular sj)aces must exist between them ; and as
the tissue develops, these spaces often increase greatly in size. They
are called schizogenous if formed simply by the parting of cells from
one another ; lysigenoas if resulting from the disappearance oi-
absorption of cells. Recent observations have shown that the wall
of intercellular spaces is frequently clothed with a lining of proto-
plasm. There are many forms of fully developed cellular paren-
chyme, in which, in consequence of the loose aggregation of their
component cells, these may be readily isolated, so as to be prepared

for separate examination without
the use of reagents which altei-
their condition ; this is the case
with the pulp of ripe fruits,
such as the strawberry or cm-rant
(the snowberry is a particularly
favourable subject for this kind
of examination), and with the
parenchyme of many fleshy leaves,
such as those of the carnation
(^Dianthus caryojihyllus) or the
London pride {Saxifraga win-
brosa). Such cells usually con-
tain evident nuclei which are
turned brownish-yellow by iodine,
whilst theii- membrane is only
turned pale yellow, and in this
way the nucleus may be brought
into view when, as often happens,
it is not previously distinguish-
able. If a drop of the iodised
solution of chloride of zinc be subsequently added, the cell-membrane
becomes of a beautiful blue colour, whilst the nucleus and the granu-
lar protoplasm that surrounds it retain their brownish-yellow tint.
The use of dilute nitric or sulphuric acid, of alcohol, of syrup, or of
several other reagents, serves to bring into view the ' primordial ' or
2X(,rietal utricle, its contents being made to coagulate and shrink, so
that it detaches itself from the cellulose wall with which it is ordi-
narily in contact, and shi-ivels up within its cavity, as shown in
fig. 524. It would be a mistake, however, to regai'd this as a distinct
membrane ; for it is nothing else than the peripheral layer of proto-
plasm, natui-ally somewhat moi-e dense than that which it includes,
but passing into it by insensible gi'adations.

It is probable that all cells, at some stage oi' other of their
growth, exhibit, in a greater or less degree of intensity, that curious
movement of cyclosis which has been already desciibed as occura-ing

Fig. 527. — Cubical j)arenchyme, with
stellate cells, from petiole of Nuphar
lutea.

CYCLOSIS OF PKOTOPLASM 689

in the Characece (see p. 564), and which consists in the steady flow
of one or of several currents of protoplasm over the inner wall of
the cell, this being rendered apparent by the movement of the
particles which the current carries along with it. The best exam-
ples of it are found among submerged plants, in the cells of which it
continues for a much longer period than it usually does elsewhere ;
and aniong these ai-e two, Vallisnericc spiralis and Anacharis alsi-
nastrum (or Elodea canadensis) , which are jjeculiarly fitted for the
exhibition of this interesting phenomenon. Vallisneria is an aquatic
plant that grows abundantly in the rivers of the south of Europe,
but is not a native of this country ; it may, however, be readily
groAvn in a tall glass jar ha\dng at the bottom a couple of inches of
mould, Avhich, after the roots have been inserted into it, should be
closely pressed down, the jar being then filled with water, of which
a portion should be occasionally changed.^ The jar should be freely
exposed to light, and should be kept in as warm but equable a tem-
perature as possible. The long grass-like leaves of this plant are too
thick to allow the transmission of sufficient light through them for
the purpose of this observation, and it is requisite to make a thin
slice or shaving with a sharp knife. If this be taken from the
surface, so that the section chiefly consists of the superficial layer of
cells, these will be found to be small, and the particles of chlorophyll,
though in great abundance, will rarely be seen in motion. This
layer should therefore be sliced off (or perhaps still better, scraped
away) so as to biing into view the deeper layei', which consists of
larger cells, some of them greatly elongated, with particles of chloro
phyll in smaller number, but carried along in active rotation by the
current of protoplasm ; and it will often be noticed that the direc-
tions of the rotation in contiguous cells are opposite. If the move-
ment (as is generally the case) be checked by the shock of the
operation, it will be i-evived again by gentle warmth ; and it may
continue under favourable circumstances, in the separated fragment,
for a period of Aveeks, or even of months. Hence, when it is desired
to exhibit the phenomenon, the preferable method is to prepare the
sections a little time before they are likely to be wanted, and to
carry them in a small vial of water in the waistcoat pocket, so that
they may receive the gentle and continuous warmth of the body.
In summer, when the plant is in its most vigorous state of growth,
the section may be taken from any one of the leaves ; but in wintei-
it is preferable to select those which are a little yellow. An objec-
tive of J-inch focus will serve for the observation of this interesting
phenomenon, and very little more can be seen with a ^-inch ; but
the g^j-inch constructed by Messrs. Powell and Lealand enables the
borders of the protoplasmic current, which carries along the
particles of chlorojjhyll, to be distinctly defined ; and this beautiful

1 Mr. Quekett found it the most convenient method of changing the water in the
jars in which Chara, Vallisneria, &c., are growing, to phice them occasionally under
a water-tap, and allow a very gentle stream to fall into them for some hours ; for by
the prolonged overflow thus occasioned all the impure water, with the Conferva that
is apt to grow on the sides of the vessel, may be readily got rid of.

Y y

690 MICROSCOPIC STRUCTUEE OF PHANEROGAMIC PLANTS

phenomenon may be m.ost luxuiiously watched under their patent
binocular.

Anacharis alsinastrum is a water-weed which, having been acci-
dentally introduced into this country many years ago, has smce
spread itself with such rapidity through our canals and rivers as in
many instances seriously to impede their navigation. It does not
require to root itself in the bottom, but floats in any part of the water
it inhabits ; and it is so tenacious of life that even small fragments
are sufficient for the origination of new plants. The leaves have no
distinct ej)iderm, but are for the most part comj)*^'^®*^ of two layers of
cells, and these are elongated and coloindess in the centre, forming a
kind of midrib ; towards the margins of the leaves, however, there is
but a single layer. Hence no preparation whatever is required for the
exhibition of this interesting phenomenon, all that is necessaiy being
to take a leaf fi-om the stem (one of the older yellowish leaves being-
preferable), and to place it, with a drop of water, either in the aqua-
tic box or on a slip of glass beneath a thin glass cover. A higher
magnifying power is required, however, than that which suffices foi*
the examination of the cyclosis in Chara or in Vallisneria, the g-inch
object-glass being here preferable to the ^-inch, and the assist-
ance of the achromatic condenser being desirable. With this ampli-
fication the phenomenon may be best studied in the single layer of
marginal cells, although, when a lower j)ower is used, it is most evi-
dent in the elongated cells forming the central portion of the leaf.
The number of chlorophyll-granules in each cell varies from three oi-
four to upwards of fifty ; they are somewhat irregular in shape, some
being nearly circular flattened discs, whilst others are oval ; and
they are usually from -gQ^QQ^th to ^-^^-o^h of an inch in diameter.
When the rotation is active the greater number of these granules
travel round the margin of the cells, a few, however, remaining fixed
in the centre ; their rate of movement, though only ^^th of an inch
per minute, being sufficient to carry them several times roiuid the
cell within that period. As in the case of Vallisneria, the motion
may frequently be observed to take place in opposite directions
in contiguous cells. The thickness of the layer of protoplasm in
which the granules ai'e carried round is estimated by Mr. Wenham
at no more than 20000^-^^ ^^ ^^^ inch. When high powers and
careful illumination are employed, delicate ripples may be seen in the
protoplasmic currents. ^

Cyclosis, however, is by no means restricted to submerged plants ;
for it has been witnessed by numerous observers in so great a variety
of other species that it may fairly be presumed to be universal. It is
especially observable in the hairs of the epidermal surface. Such
hairs are furnished by various parts of plants ; and what is chiefly
necessary is that the part from which the hair is gathered should be
in a state of vigorous growth. The hairs should be detached by
tearing off with a pair of fine pointed forceps the portion of the
epiderm from which they spring, care being taken not to grasp the
hair itself, whei-eby such an injury would be done to it as to check
the movement within it. The apochromatic hair should then be
1 Quart. Journ. of Microsc. Science, vol. iii. (1855'l, p. 277.

CYCLOSIS OF PliOTOPLASM

69 (

placed with a drop of water under thin glass ; and it will generally
be found advantageous to use a g^-inch with the 12 or the 18 eye-
piece objective with an achromatic condenser. The nature of
the movement in the hairs of different species is far from being-
uniform. In some instances, the currents pass in single lines
along the entire length of the cells, as in the hairs from the filaments
of Tradescantia virginica, or Virginian spiderwort (fig. 528, A) ; in
others there are several such cur-
rents which i-etain their distinct-
ness, as in the jointed hairs of the
calyx of the same plant (B) ; in
others, again, the streams coalesce
into a network, the reticulations
of which change their position at
short intervals, as in the hairs of
Glaucium luteum ; whilst there
are cases in which the cui'rent
flows in a sluggish uniformly
moving sheet or layer. Where
several distinct currents exist in
one cell, they are all found to
have one common point of depar-
ture and return, namely, the
nucleus (B, a), from which it
seems fairly to be inferred that
this body is the centre of the
vital activity of the cell. In all
cases in which the cyclosis is
seen in the hairs of a plant, the
cells of the epiderm also display
it, provided that their walls ai-e
not so opaque or so strongly
marked as to prevent the move-
ment from being distinguished.
The epiderm may be most readily
torn ofi" from the stalk ov the
midrib of the leaf, and must
then be examined as speedily as
possible, since it loses its vitality
when thus detached much sooner
than do the hairs. Even when
no obvious movement of particles
is to be seen, the existence of

a cyclosis may be concluded from the peculiar arrangement of the
molecules of the protoplasm, which are remarkable for their high
refractive power, and which, when ari-anged in a ' moving train,'
appear as bright lines across the cell ; and these lines, on being'
carefully watched, ai-e seen to alter their relative positions. The
leaf of the common Fkmtago (plantain) furnishes an excellent example
of cyclosis, the movement being distinguishable at the same time
both in the cells and in the hairs of the epiderm torn from, its stalk

■STY 2

Fig. 528.— Rotation of fluid in hairs of
Tradescantia virginica: A, portion of
epiderm with hair attached ; a, b, c,
successive cells of the hair ; d, cells of
the epiderm ; e, stomate. B, joints of a
beaded hair showing several currents ;
a, nucleus.

692 MICEOSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

or midrib. It is a curious circumstance that when a plant whicli
exhibits the cyclosis is kept in a cold dark place for one or two
days, not only is the movement suspended, but the moving particles
collect together in little heaps, which are broken up again by the
separate motion of their particles when the stimulus of light and
warmth occasions a renewal of the activity. It is well to collect the

specimens about midday, that
being the time when the rotation
is most active, and the move-
ment is usually quickened hj
artificial warmth, which, indeed,
is a necessaiy condition in some
instances to its being seen at all.
The most convenient method of
applying this warmth, while the
object is on the stage of the
microscope, is to blow a stream
of air upon the thin glass covei'
through a glass or metal tube
previously heated in a spirit-
lamp.

The walls of the cells of
plants ai'e frequently thickened by deposits, which ai-e first formed
on the inner sui-face, and which may present very difiei-ent appear-
ances according to the manner in which they are arranged. In

i 10- 529 — Jibbue of the testa 01 seed coat
of star amse A, as seen m section ,
B, as seen on the surface.

Fig. 530. — Section of cherry-stone,
cutting the cells transversely.

Fig. 531. — Section of coquilla
nut, in the direction of the
long diameter of the cells.

its simplest condition such a deposit forms a thin uniform layer
over the whole internal surface of the cellulose wall, scarcely detract-
ing at all from its transparency, and chiefly distinguishable by the
* dotted ' appearance which the membrane then presents (fig. 525, A).
These dots, however, are not poi-es, as their aspect might naturally
.suggest, but are merely points at whicli the deposit is wanting, so

TISSUES OF PHANEROGAMIA 693

that the ovigiual cell-wall there I'emains unthickened. A moi'e
complete consolidation of cellular tissue is effected by deposits
of sclerogen (a substance which, when sepaiuted from the resinous
and other matters that ai'e commonly associated with it, is found
to be allied in chemical composition to cellulose) in successive
layers, one within another (fig. 529, A), which present them-
selves as concentric rings when the cells containing them are cut
thi-ough ; and these layers are sometimes so thick and numerous
as almost to obliterate the original cavity of the cell. Such a tissue
is known as sderencliyme or sclerenchymatous tissue. By a con-
tinuance of the same arrangement as that which shows itself in the
single layei' of the dotted cell — each deposit being deficient at certain
points, and these points corresponding with each other in the succes-
sive layers — a series of passages is left, by which the cavity of the
cell is extended at some points to its membranous wall ; and it
commonly happens that the points at which the deposit is wanting
on the walls of the contiguous cells are coincident, so tha,t the
membranous partition is the only obstacle to the communication
between their cavities (figs. 529-531). It is of such tissue that
the ' stones ' of stone-fruit, the giitty substance which surrounds the
seeds and forms little hai-d points in the fleshy substance of the pear,
the shell of the cocoa-nut, and the endosperm of the seed of Phyt-
elephas (known as 'vegetable ivory') are made up; and we see the
use of this very curious arrangement in permitting the cells, even
after they have attained a considerable degree of consolidation,
still to remain permeable to the fluid required for the nutrition of
the parts which such tissue encloses and protects.

The deposit sometimes assumes, however, the form of definite
■fibres, which lie coiled up on the inner
surface of the cells, so as to form a single, . "- ™™~

a double, or even a tiiple or quadruple
spire (fig. 532). Such spiral cells are found
abundantly in the leaves of certain orchi-
daceous plants, immediately beneath the
epidei'm, where they ai'e brought into
view" by vertical sections; and they mtiy
be obtained in an isolated state by mace-
rating the leaf and peeling off .the epiderm
so as to expose the layer beneath, which is
then easily separated into its components. fig, 532.— Spiral cells of leaf
In an orchidaceous plant named Saccola- of Onddiwn.

bhwi guttatum the spiral cells are unusu

ally long, and have spires winding in opposite directions, so that by
their mutual intersection a series of diamond-shaped markings is pro-
duced. Spiral cells ai-e often found upon the surface of the testa or
outer coat of seeds ; and in Gollomia grandiflora, Salvia verhenaca
(wild clary), and some other plants, the membrane of these
cells is so weak, and the elasticity of their fibres so great, that
wdien the membrane is softened by the action of water the fibres
suddenly uncoil and elongate themselves (fig. 533), springing out,
as it were, from the surface of the seed, to which they give a

694 MICKOSCOPIC STRUCTUKE OF PHANEROGAMIC PLANTS

peculiar flocculent appearance. This very cui-iovis phenomenon may
be best observed in the following manner : — A very thin trans-
verse slice of the seed should first be cut, and laid upon the lower

glass of the aquatic box ; the cover
,, B should then be pressed down,

and the box placed upon the
stage, so that the microscope may
be exactly focussed to the object,
the power employed being the
1-inch, f-inch, or Vinch. The
cover of the aquatic box being-
then removed, a small drop of
water should be placed on that
part of its internal surface with
which the slice of the seed had been
in contact ; and the cover being
replaced, the object should be im-
mediately looked at. It is im-
portant that the slice of the seed
should be very thin, for two
reasons : first, that the view of
the spirals may not be confused
by their aggregation in too great numbers ; and second, that the
drop of water should be held in its place by capillary attraction,
instead of i-unning down and leaving the object, as it will do if the
glasses be too widely separated.

In some part or other of most plants we meet with cells contain-
ing granules of starch, which specially abound in the tubers of the
potato and in the seeds of cereals. Starch-grains are originally
formed in the interior of chlorophyll-corpuscles, and therefore within
the protoplasm-layer of the cell ; but as they increase in size, the
protoplasm-layer thins itself out as a mere covering film, and at last
almost entirely disappears. So long as the starch-grains remain
imbedded in the protoplasm-layer, they continue to grow ; but when
they accumulate so as to occupy the cell-cavity, their growth stops.

Fig. 533.-

-Spiral fibres of seed-coat of
Collomia.

Fig. 534.— Cells of peony filled
with starch.

Fig. 535. — Granules of starch as
seen under polarised light.

They ai-e sometimes minute and very numerous, and so closely
packed as to fill the cell-cavity (fig. 534) ; in other instances they
are of much larger dimensions, so that only a comparatively smal
number of them are included in any one cell ; while in other

STARCH-GRAINS 695

cases, again, they ai-e both few and minute, so that they form but
a small proportion of the cell-contents. Their nature is at once
detected by the addition of a solution of iodine, which gives them a
beautiful blue colour. Each giunule when highly magnified exhibits
a peculiar spot, termed the hilmn, round which are seen a set of
circular lines that are foi- the most part concentiic (or neai'ly so)
with it. When viewed by polarised light each grain exhibits a dark
cross, the point of intersection being at the hilum (fig. 535) ; and
when a selenite plate is interposed the cross becomes beautifully
coloured. Opinions have been very much divided regarding the
internal structure of the starch-grain, but the doctrine of ISTageli
that it is composed of successive layers which increase by ' intus-
susception,' that is, by the intercalation of fresh molecules of starch
between those already in existence, is favoured by many authorities,
though the alternative theory of foimation by the ' apposition ' of
successive layers also has many advocates. These layers differ in
their proportion of water, the outermost layer, which is the most
solid, having within it a watery layer, this, again, being succeeded
by a firm layer, which is followed by a watery layer, and so on, the
proportion of water increasing towards the centre in both kinds of
layer, and attaining its maximum in the innermost part of the
grain, where the foiination of new layers takes place, causing the
distension of the older ones. Although the dimensions of the
starch-grains produced by any one species of plant are by no means
constant, yet there is a certain average for each, from which none
of them depart very widely ; and by reference to this average the
starch-grains of different plants that yield this product in abundance
may be microscopically distinguished from one another — a circum-
stance of considerable importance in commerce. The largest starch-
grains in common use are those of the plant (a species of Canna)
known as ' tous-les-mois.' The average diameter of those of the
potato is about the same as the diameter of the smallest of the
' tous-les-mois,' and the size of the ordinary starch-grains of wheat
and of sago is about the same as that of the smallest grains of
potato-starch ; whilst the granules of rice-starch are so very minute
as to be at once distinguishable from any of the preceding.

In certain plants, especially those belonging to particular natural
orders, the stem, leaves, and other parts ai-e permeated by long-
branched tubes, constituting the laticiferous tissue. The elements
of this tissue may be either greatly enlarged prosenchymatous cells
01- true vessels. In either case they contain a copious milky-white
or coloured juice, the latex, which exudes freely when the part con-
taining it is wounded, and dries lupidly on exposure. The chemical
composition of the latex varies ; it may contain in solution powei-ful
alkaloids, as in the case of the opiiun-poppy, or gum-resins. Caou-
tchouc and gutta-percha are the diied latex of tropical trees and
shrubs belonging to several natural orders. Good examples of lati-
ciferous tissue are furnished by the Papaveracese, of which our
common field-poppy is an example, many Compositfe such as the
dandelion and lettuce, Convolvulacese, Euphorbiace8e or spurges,
Apocynaceae, Moracese including the mulbei-ry &c.

696 MICROSCOPIC STRUCT!] EE OF PHANEROGAMIC PLANTS

Deposits of mineral matter in a crystalline condition, known as
raphides, are not unfrequently found in vegetable cells, where they
are at once brought into view by the use of polarised light. Their
designation (derived from pa^U, a needle) is very appropi-iate to one
of the most common states in which these bodies present themselves,
that, namely, of bundles of needle-like crystals, lying side by side in
the cavity of the cells ; such bundles are well seen in the cells lying
immediately beneath the epiderm of the bulb of the medicinal
squill. It does not apply, however, to other forms which are
scarcely less abundant ; thus, instead of bundles of minute needles,
single lai'ge crystals, octahedral or prismatic, are fi-equently met
with, and the prismatic crystals are often aggregated in beautiful
stellate groups. The most common material of these crystals is
oxalate of lime, which is generally found in the stellate form ; and no
plant yields these stellate raphides so abundantly as the common
rhubarb, the best specimens of the dry medicinal root containing as
much as 35 per cent, of them. In the epiderm of the bulb of the
onion the same material occurs in the octahedral or the prismatic
form. In other instances, the calcareous base is combined with
tartaric, citric, or malic acid ; the aciculai- raphides consist almost
iuA^ariably of oxalate of lime. Some raphides are as long as ^r^th of
an inch, while others measure no more than y-^th. They occur in all
parts of plants — the wood, pith, bark, root, leaves, stipules, sepals,
petals, fruit, and even in the pollen. They are always situated in
cells, and not in the intercellular passages ; the cell -membrane, how-
evei', is often so much thiinied away as to be scarcely distinguish-
able. Certain j^lants of the Cactus tribe, when aged, have their
tissues so loaded with raphides as to become quite brittle, so that
when some large specimens of C. senilis^ said to be a thousand years
old, were sent to Kew Gardens from South America, some years
since, it was found necessary for their preservation during ti-ansport
to pack them in cotton like jewellery. Raphides are probably to be
considered as non-essential results of the A^egetative processes, being
for the most part produced by the union of organic acids generated
in the plant with mineral bases imbibed by it from the soil. The
late Mr. E. Quekett succeeded in artificially producing raphides
within the cells of rice-paper, by first filling these with lime-water
by means of the air-pump, and then placing the paper in weak
solutions of phosphoric and oxalic acids. The artificial raphides of
phosphate of lime were rhombohedral ; while those of oxalate of
lime were stellate, exactly resembling the natural raphides of the
rhubai'b. Besides the structures already mentioned as afibrdmg good
illustrations of different kinds of raphides, may be mentioned the
parenchyme of the leaf of Agave, Aloe, Cycas, Encephalartos, &c. ;
the ej)iderm of the bulb of the hyacinth, tulip, and garlic ; the bark of
the apple, Cascarilla, Cinchona, lime, locust, and many other trees; the
pith of Eloiagnus, and the testa of the seeds of Anagallis and the elm.

A large j)ropoi'tion of the denser pai-ts of the fabric of the higher
plants is made up of the substance which is known as woody fibre or
prosenchymatous tissue. This, however, can only be regarded as a
variety of cellular tissue ; foi- it is composed of peculiarly elongated

TISSUES OF PHANEROG-AMIA

697

cells (fig. 551), usually pointed at their two extremities so as to
become spindle-shaped, whose walls have a special tendency to
undergo consolidation by the internal deposit of sclerogen. It is
obvious that a tissue consisting of elongated cells, adherent together
by their entire length, and strengthened by intei-nal deposit, must
possess much gi-eatei' tenacity than any tissue in which the cells
depart but little fi'om the primitive spherical form ; and we accord-
ingly find woody fibre present wherever it is requisite that the fabric
should possess not merely density, but the power of resistance to
tension. In the higher classes of the vegetable kingdom it consti-
tutes the chief part of the stem and branches, where these have a
firm and durable character ; and even in more temporary structures,
such as the herbaceous stems of annual plants, and the leaves and
flowers of almost every tribe, this tissue forms a more or less import-
ant constituent, being especially found in the neighbourhood of the
spiral vessels and ducts, to which it affords protection and support.
Hence the bundles of fasciculi composed of these elements, which
form the ' veins ' of leaves, and which give ' stringiness ' to various
esculent vegetable substances, are commonly known under the
name of fibro-vascidar tissue. In their young and unconsolidated
state the woody cells seem to conduct fluids with great facility in
the direction of their length ; and in the Coniferce, whose stems and
branches are destitute of ducts, they afford the sole channel for the
ascent of the sap. The fihro-vascidm- bundles, which are the chief
strengthening elements of such organs as the stem, l^i'anches, leaf-
stalks, flower-stalks, &c., are, in the highei-
plants, structui'es of considerable com-
plexity ; in Exogens they consist of three
distinct portions, the xyle'in-^oi-tion com-
posed chiefly of the different kinds of
vessels hereafter to be described, ^.j^hloem-
portion composed of prosenchymatous
tissue and ' sieve-tubes,' aiid a formative
cambium-'portion.

A peculiar set of markings seen on
the woody fibres of the Goniferce, and of
some other tribes, is represented in fig.
536 : in each of these spots the innei-
circle appears to mark a deficiency of
the lining deposit, as in the pitted cells
of other plants ; whilst the outer circle
indicates the boundary of a lenticulai-
cavity which intervenes between the ad-
jacent cells at this point. There are
varieties in this arrangement so charac-
teristic of different tribes that it is
sometimes possible to determine, by the

microscopic inspection of a minute fragment, even of a- fossil wood,
the tribe to which it belonged. Markings of this kind, very
characteristic of the wood of Gonifera', though not peculiar to that

Fig. 536. — Section of coniferous
wood in the direction of the
tracheids, showing their
' bordered pits ; ' a, a, a, me-
dullary rays
fibres.

crossing the

698 MICROSCOPIC STEUCTURE OF PHANEROGAMIC PLANTS

order, are known as bordered juts, and the elongated cells in which
they occur as trache'ids.

All the more pei-fect forms of Phanerogams contain, in some
part of their fabric, the peculiar structures which are known as
sjyiral vessels} These have the elongated shape of fibre-cells ; but
the internal deposit, as in the spiral cells, takes the form of a spiral
fibre winding from end to end, and retaining its elasticity ; this
fibre may be single, double, or even quadruple, this last character jDre-
senting itself in the very large elongated fibre-cells of Nepenthes
(pitcher-plant). iSuch vessels are especially found in the delicate
membrane (medullary sheath) surrounding the pith of Exogens,
and in the ' xylem-portion ' of the woody bundles of Exogens and
Endogens ; thence they proceed to the leaf-stalks, through which
they are distributed to the leaves. By careful dissection under the
microscope these fibro-vascular bundles may be separated entire ;
but their structure may be more easily disj)layed by cutting round,
but not through, the leaf-stalk of the strawberry, geranium, &c., and
then di-awing the parts asunder. The membrane composing the
tubes of the vessels will thus be broken across ; but the fibres within,
being elastic, will be drawn out and unrolled. Spiral vessels are
sometimes found to convey fluid, whilst in other cases they contain
air only.

Altlaough fluid generally finds its way with tolei-able facility
through the various forms of cellular tissue, especially in the direction
of the greatest length of the cells, a more direct means of connection
between distant parts is required for its active transmission. This is
aflTorded by the peculiar kind of vessels known as ducts, which consist
of cells laid end to end, the partitions between them being m.ore or
less obliterated. The origin of these ducts is occasionally very evi-
dent, both in the contraction of their diameter at regular intervals,
and in the persistence of remains of their partitions (fig. 551,
h, b) ; but in most cases it can only be ascertained by studying the
history of their development, neither of these indications being trace-
able. Some of these ducts (fig. 537, 2) are indistinguishable from
the spiral vessels already described, save in the want of elasticity in
their spiral fibre, which causes it to break when the attempt is made
to draw it out. This rupture would seem to have taken place, in
some instances, from the natural elongation of the cells by gTOwth,
the fibre being broken up into rings, which lie sometimes close
together, but more commonly at considerable intervals ; such a duct
is said to be annular (fig. 537, i). Intermediate forms between the
spiral and annular ducts, which show the derivation of the latter
from the foi-mer, ai-e very frequently to be met with. The spirals are
sometimes bi-oken wp still more completely, and the fragments of the
fibre extend in various dii-ections, so as to meet and form an irregular
network lining the duct, which is then said to be reticulated. The
continuance of the deposit, however, gi-adually contracts the meshes,

^ So long, however, as they retain their original cellular character, and do not
coalesce with each other, these fusiform spiral cells cannot be regarded as having
any more claim to the designation of vessels, than have the elongated cells of the
woody tissue.

TISSUES OF PHANEKOGAMIA

699

leaving the walls of the duct marked only by pores like those of
porous cells ; and such canals, designated as pitted ducts, ai'e
especially met with in parts of most solid str;icture and least rapid
growth (fig. 537, 3). The sccdariform ducts of ferns may be re-_
garded as a modification of the spiral ; but spiral ducts are fre-
quently to be met with also in the rapidly growing leaf-stalks of
flowering plants, such as the rhubarb. Not unfrequently, howevei-,
we find all forms of ducts in the same bundle, as seen in fig. 537.
The size of these ducts is occasionally so great as to enable their
openings to be distinguished by the unaided eye ; they are usually
largest in stems whose size is small in proportion to the surface of
leaves which they support, such as the common cane or the vine ;
and, generally speaking,
they are larger in woods
of dense texture, such as
oak and mahogany, than
in those of which the
fibres, remaining uncon-
solidated, can serve for the
conveyance of fluid. They
are entirely absent in the
Goniferce.

The vegetable tissues
whose principal forms
have been now described,
but among which an im -
mense variety of detail is
found, may be either
studied as they present
themselves in thin sec-
tions of the various parts
of the plant under exami-
nation, or in the isolated
conditions in which they
are obtained by dissection.
The former process is the
most easy, and yields a
large amount of informa-
tion ; but still it cannot
be considered that the characters of any tissue have been properly
determined until it has been dissected out. Sections of some of the
hardest vegetable substances, such as ' vegetable ivory,' the ' stones '
of fruit, the _' shell ' of the cocoa-nut, &c., can scarcely be obtained
except by slicing and grinding ; and these may be mounted either in
Canada balasm. or in glycerin jelly. In cases, however, in which the
tissues are of only moderate firmness, the section may be miost readily
and efiectually made with the ' microtome ; ' and there are few pai-ts
of the vegetable fabric which may not be advantageously examined
by this means, any very soft oi- thin portions being placed in it
between two pieces of cork, elder-pith, or carrot. In certain cases,
however, in which even this compression would be injurious, the

Fig. 537. — Longitudinal section of stem of Italian
reed : a, cells of the pith ; h, fibro-vascular
bundle, containing 1, annular ducts ; 2, spiral
ducts ; 3, pitted ducts with woody fibre ; c, cells
of the epiderm.

7CO MICKOSCOPIC STEUCTUEE OF PHANEEOaAMIC PLANTS

sections miist be made with a shai'p knife, tLe substance being laid on
the nail or on a slip of glass. In dissecting the vegetable tissues,
scarcely any other instrument will be found really necessary than
a pair of needles (in handles), one of them ground to a cutting edge.
The adhesion between the component cells, fibres, &c., is often
sufficiently weakened by a few hours' maceration to allow of their
readily coming apart, when they are torn asunder by the needle-
points beneath the simple lens of a dissecting microscope. But if
this should not prove to be the case, it is desirable to employ some
other method for the sake of facilitating their isolation. None is so
effectual as the boiling of a thin slice of the substance under exami-
nation either in diUite nitric acid or in a mixture of nitric acid and
chlorate of potassa. This last method (which was devised by
Schultz) is the most rapid and effectual, requiring only a few
minutes for its performance ; but as oxygen is liberated with such
freedom as to give an almost explosive character to the mixture, it
should be put in practice with extreme caution. After being thus
treated, the tissue should be boiled in alcohol, and then in water ;
and it will then be found very easy to tear apart the individual cells,
ducts, efec. of which it may be composed. These may be preserved
by mounting in weak spirit.

Stem and Root. — It is in the stems and roots that we find the
greatest variety of tissues in combination, and the most regular
plans of structure ; and sections of these viewed under a low mag-
nifying power are objects of peculiar beauty, independently of the
scientific information which they afford. The axis (under which
term are included the stem with its branches, and the root with its
ramifications) always has for the basis of its structure a dense cellular
parenchyme ; though in an advanced stage of development this
may constitrite but a small portion of it. In the midst of the
parenchyme we generally find fibro-vascular bundles, consisting of
w^oody fibre, with ducts of various kinds, and (almost always) spiral
vessels. It is in the mode of arrangement of these bundles that the
fundamental difference exists between the stems which are commonly
designated as endogenous (growing from within), and those which
are more correctly termed exogenous (growing on the outside) ; for
in the former the bundles are dispersed throughout the whole
diameter of the axis without any peculiar plan, the intervals between
them being filled up by cellular parenchyme ; whilst in the lattei-
they are ai'ranged side by side in such a manner as to form a cylinder
of wood, which includes within it the portion of the cellular substance
known as 'pith, whilst it is itself enclosed in an envelope of the same
substance that forms the hark. These two plans of axis-formation
respectively charactei'istic of those two great groups into which
Phanerogams are subdivided — namely, the Monocotyledons and the
Dicotyledons — will now be more particularly described.

When a transverse section (fig. 538) of a monocotyledonous stem
is examined microscopically, it is found to exhibit a number of fibro-
vascular bundles, dis^josed without any regularity in the midst of
the mass of cellular tissue, which forais (as it wei'e) the matrix oi-
basis of the fabric. Each bundle contains two, thiee, or more large

STKUCTURE OF STEMS

701

ducts, which are at once distinguished by the size of their openings ;
and these are suiTOunded by woody fibre and spiral vessels, the
ti-ansverse diameter of which is so extremely small that the portion
of the bundles which they form is at once distinguished in transverse

Fig. 538. — Transverse section of stem of young palm.

section by the closeness of its texture (fig. 539). The bundles are
least numerous in the centre of the stem, and become gi'adually more
crowded towards its circumference ; but it frequently happens that the
portion of the area in which they are
most compactly ai'ranged is not abso-
lutelyat its exterior, this portion being
itself surrounded by an investment
composed of cellular tissue only ; and
sometimes we find the central portion
also completely destitute of fibro-vas-
cular bundles ; so that a sort of indica-
tion of the distinction between pith,
wood, and bark is here presented.
This distinction, however, is very im-
perfect ; for we do not find either the
central or the peripheral portions evei'
separable, like pith and bark, from
the intermediate woody layer. In its
young state the centre of the stem is
always filled up with cells ; but these
not unfrequently disappear after a
time, except at the nodes, leaving
the stem hollow, as we see in the

whole tribe of grasses. When a vertical section is made of a w^oody
stem (as that of a palm) of sufiicient length to trace the whole extent
of the fibro- vascular bundles, it is found that, whilst they pass at
their upper extremity into the leaves, they pass at the lower end

Fig. 539. — Portion of transverse
section of stem of Waughie cane.

702 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

towards the surface of the stem, and assist, by their interlacement
with the outer bundles, in forming that extremely tough investment
which the lower ends of these stems present. New fibro-vascular
bundles are being continually formed in the upper part of the stem,
in continuity with the leaves which are successively put forth at its
summit ; but while these take part in the elongation of the stem,

they contribute but little to the increase
of its diameter. For those which are
most recently formed only pass into the
centre of the stem during the higher
part of their course, and usually make
their way again to its exterior at no
great distance below ; and, when once
formed, they receive no further additions.
It was from the idea formerly enter-
tained that these successively formed
bundles descend in the interior of the
stem throtigh its entire length until they
reach the roots, and that the stem is thus
continually receiving additions to its
interior, that the term endogenous was
given to this type of stem-structure ;
but, from the fact just stated regarding
the course of the fibro-vascular bundles,
it is obvious that such a doctrine cannot be any longer admitted.

In the stems of dicotyledonous phanerogams, on the other hand,
we find a method of arrangement of the several pai-ts which must
be regarded as the highest form of the development of the axis,
being that in which the greatest difierentiation exists. A distinct
division is always seen in a transverse section (fig. 540) between three
concentric areas — the 2)ith, the ivood, and the hai'k — the first (a) being

Fig. 540.— Diagram of the first
formation of an exogenous
stem : a, pith ; &, &, bark ; c, c,
plates of cellular tissue (me-
dullary rays) left between the
woody bundles d d.

Fig. 541. — Transverse section of stem of Clematis: a, i^ith; b, h, h, woody bundles
c, c, c, medullary rays.

central, the last (h) peiipheral, and these having the wood interposed
between them, its circle being made up of wedge-shaped bundles
(d d), kept apart by the medullary rays composed of unchanged cel-
lular tissue (c, c) that pass between the pith and the bark. The pith
(fig. 541, a) is almost invariably composed of cellular tissue only,
which usually pi'esents (in transvei'se section) an hexagonal areolation.
When newly foi'med it has a greenish hue, and its cells are filled with

STEUCTURE OF STEMS

70:

fluid ; but it gradually dries up and loses its colour ; and not un-
frequently its component cells are torn apart by the rapid growth
of their envelope, so that irregular cavities are found in it ; or if
the stem should increase with extreme rapidity it becomes hollow,
the pith being reduced to fragments, which are found adhering to
its interior wall. The pith is immediately sarroiurded by a delicate
membrane, consisting almost entirely of spiral vessels, which is
termed the medullary sheath.

The woody portion of the stem (fig. 541, &, &) is made up of w^oody
fibres, usvially with the addition of ducts of various kinds ; these,
however, are absent in one large group, the Goniferce, or fir-tribe
with its allies (figs. 545-548), in which the prosenchymatous cells
or tracheids are of unusually large diameter, and are marked by
the bordered pits already described. In any stem or branch of more
than one year's growth the woody structure presents a more or less
distinct appearance of division into concentric rings, the number of

Fig. 542. — Transverse section of stem
of Hhamnus (buckthorn), showing
concentric layers of wood.

Fig. 543.— Portion of the
same more highly
magnified.

which varies with the age of the tree (fig. 542). The composition of
the several rings, which are the sections of so many cylindrical
layers, is uniformly the same, however different their thickness ; but
the arrangement of the two principal elements — namely, the cellular
and the vascular tissue — varies in difierent species, the vessels being
sometimes almost uniformly difi"used through the whole layer, but in
other instances being confined to its inner part ; while in other
cases, again, they are dispersed with a certain regular irregularity
(if such an expression may be allowed), so as to give a cuiiously
figured appearance to the transverse section (figs. 542, 543). The
general fact, however, is that the vessels predominate towards the
inner side of the ring (which is the part of it first formed), and that
the outer portion of each layer is almost exclusively composed of
cellular tissue. Such an arrangement is shown in fig. 541. This
alternation of vascular and cellular tissue frequently serves to mark
the succession of layers when, as is not uncommon, there is no very
distinct line of separation between them .

704 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

The number of layers is usually considered to coi-respond with
that of the yeai-s during which the stem or branch has been gi-owing ;
and this is, no doubt, generally true in regard to the trees of
temperate climates, which thus ordinarily increase by 'annual layers.'
There can be no doubt, however, that such is not the universal rule ;
and that we should be more correct in stating that each layer indi-
cates an ' epochof vegetation,' which, in temperate climates, is usually
(but not invariably) a year, but which is commonly much less in the
case of trees flourishing in tropical regions. Thus among the latter
it is very common to find the leaves regularly shed and replaced
twice or even thrice in a year, or five times in two years ; and foi-
every crop of leaves there will be a corresponding layer of wood.
It sometimes hapjDens, even in temperate climates, that trees shed
their leaves prematurely in consequence of continued drought, and
that, if rain then follow, a fresh crop of leaves appears in the same
season ; and it cannot be doubted that in such a year there woidd
be two rings of wood produced, which would probably not together
exceed the ordinary single layer in thickness. That such a division
may even occur as a consequence of an inteiTuption to the processes
of vegetation produced by seasonal changes — as by heat and drought

a h c

Fig. 544. — PortioJi of transverse section of stem of hazel, showing, in the j)ortion
a, b, c, six narrow layers of wood

in a tree that flourishes best in a cold, damp atmosphere, or by a fall
of temperature in a tree that requires heat — would appear from the
frequency with which a double or even a multiple succession of rings
is found in transverse sections of wood to occupy the place of a
single one. Thus in a section of hazel stem (in the Author's posses-
sion), of which a portion is represented in fig. 544, between two
layers of the ordinary thickness there intervenes a band whose
breadth is altogether less than that of either of them, and which is
yet composed of no fewer than six layers, four of them (c) being very
narrow, and each of the other two {a, b) being about as wide- as
these four together. The inner rings of wood, being not only the
oldest, bvit the most solidified by resinous matters deposited within
their component cells and vessels, are spoken of collectively under
the designation duramen or ' heart- wood.' On the other hand, it is
through the cells and ducts of the outer and newer layers that the
sap rises from the roots towards the leaves ; and these are conse-
quently designated as alburnum or ' sap-wood.' The line of demar-
cation "between the two is sometimes very distinct, as in lignum vitas
and cocos-wood ; and as a new ring is added every year to the ex-
terior of the alburnum, an additional ring of the innermost part of
the fd>)ui'num is every year consolidated by internal deposit, and is

STEUCTUEE OF STEMS

705

thus added to the exterior of the duramen. More generally, how-
ever, this consolidation is gradually effected, and the alburnum and
duramen are not separated by any abrupt line of division.

The medullary rays which cross the successive rings of wood
connecting the cellular substance of the pith with that of the bark,
and dividing each ring of wood into wedge-shaped segments, are thin

Fig. 545. — Portion'of transverse section of the stem of cedar: a, pith ;
6, h, h, woody layers ; c, bark.

plates of cellular tissue (fig. 541, c, c), not usually extending to any
great depth in the vertical dii-ection. It is not often, however, that
their character can be so clearly seen in a transverse section as in
the diagram just referred to ; for they are usually compressed so
closely as to appear darker than the wedges of woody tissue between
which they intervene (figs. 543, 545), and their real nature is best
understood by a compai-ison of longitudinal sections made in two
different directions — namely,
radial and tangential — with the
transverse. Three such sec-
tions of a fossil coniferous wood
in the Author's possession are
shown in figs. 546-548. The
stem was of such large size that,
in so small a part of the area of
its transverse section as is re-
presented in fig 546, the medul-
lary i-ays seem to run parallel to
each othei-, instead of radiating
from a common centre. They are Fig. 546.— Portion of transverse section of
very narrow ; but are so closelv l^rge stem of coniferous wood (fossil),
, , ,1 J.1 J. 1 X ■ showing part of two annual rings, divided

set together that only two or ^^ ^^ ^ ^^^^ traversed by very thin but
three rows of tracheids (no numerous medullary rays.

ducts being here present) in-
tervene between any pair of them. In the longitudinal section
taken in a radial direction (fig. 547), and consequently passing in the
same course with the meduUaiy rays, these are seen as thin plates
{a, a, a) made vip of superposed cells very much elongated, and
crossing in a horizontal direction the tracheids which lie parallel to
one another vertically. And in the tangential section (fig. 548),

z z

7o6 MICEOSCOPIC STRUCTUKE OF PHANEROGAMIC PLANTS

which is taken in a direction at right angles to that of the medul-
lary rays, and therefore cuts them across, we see that each of the

Fig. 547. — Portion of vertical section of the
same wood, taken in a radial direction,
showing the trachei'ds with ' bordered
pits,' without ducts, crossed by the medul-
lary rays, a, a.

Fig. 548. — Portion of vertical
section of the same wood,
taken in a tangential direc-
tion, so as to cut across the
medullary rays.

plates thus formed has a veiy lim^ited depth from above downwards,
and is composed of no more than one thickness of cells in the
horizontal direction. A section of the stem of mahogany taken in

the same direction as the last (fig. 549)
gives a very good view of the cut ends of
the medullary rays as they pass between
the prosenchymatous cells ; and they are
seen to be here of somewhat greater thick-
ness, being composed of two or three rows
of cells, arranged side by side.

In another fossil wood, whose transverse
section is shown in fig. 550, and its tan-
gential section in fig. 551, the medullary
i-ays ai'e seen to occupy a much larger jiart
of the substance of the stem, being shown
in the transverse section as broad bands
(a a, a a) intervening between the closely
set prosenchymatous cells, among which
some large ducts are scattered ; whilst in
the tangential section they are observed
to be not only deeper than the preceding
from above downwards, but also to have
a much greater thickness. This section
also gives an excellent view of the ducts,
b b, b b, which are here plainly seen to be
formed by the coalescence of lai-ge cylindrical cells lying end to end.
In another fossil wood in the Author's possession the medullary rays

Fig. 549. — Vertical section
mahogany.

of

STRUCTURE OF STEMS

707

•constitute a still larger pi^oportion of the stem ; for in the transverse
section (fig. 552) they are seen as very broad bands {b, b), alternating

H. ^-^ y^^ n'«"^ ■

Fig. 550. — Transverse section of a
fossil wood, showing the medullary
rays, a, a, a, a, a, a, running nearly
parallel to each other, and the
openings of large ducts in the midst
of the prosenchjanatous tissue.

Fi3. 551. — Vertical (tangential) sec-
tion of the same wood, showing the
prosenchymatous cells separated
by the medullary rays, and by the
large ducts, h b, b b.

Avith plates of woody structure {a a), whose thickness is often less
than their own ; whilst in the tangential section (fig. 553) the cut

Figs. 552 and 553.— Transverse and vertical sections of a fossil wood,
showing the separation of the woody plates, a a, a a, by the very
large medullary rays, b b, b b.

■extremities of the medullary rays occupy a very large part of
the area, having apparently determined the sinuous course of the
prosenchymatous cells, instead of looking (as in fig 548) as if they

zz 2

708 MICROSCOPIC STRUCTURE OF PHA^'EROGAMIC PLANTS

had forced their way between these cells, which there hold a nearly
straight and parallel course on either side of them. The medullary
rays maintain a connection between the external and the internal
parts of the cellular tissue or fundamental parenchyme (also called
'ground-tissue') of the stem, which have been separated by the
interposition of the wood.

The harh is usually found to consist of three principal layers :
the external or ejnjMceum, which includes the suberous (or corky)
layer ; the middle, or mesophlceum, also termed the ' cellular envelope ; ^
and the internal, or endophloeum, which is more commordy known
as the liber} The two outer layers ai-e entirely cellular, and are
chiefly distinguished by the form, size, and direction of their cells.
The epijMoeum is generally composed of one or more layers of colour-
less or brownish cells, which usually present a cubical or tabular
form, and are arranged with their long diameters in the horizontal
direction ; it is this which, when developed to an unusual thickness,
forms cork, a substance which is by no m.eans the product of one
Ivind of tree exclusively, but exists in greater or less abundance in
the bark of every exogenous stem. The mesophloeum consists of
cells, usually containing more or less chlorophyll, prismatic in their
form, and disposed -wdth their long diameters parallel to the axis ; it
is more loosely arranged than the preceding, and contains inter-
cellular passages, which often form a network of canals which have
the character of laticiferous vessels ; and, although usually less
developed than the suberous layers, it sometimes constitutes the
chief thickness of the bark. The liher or ' inner bark,' on the other
hand, usually contains woody fibre in addition to the cellular tissue
and laticiferous canals of the preceding ; and thus approaches more
nearly in its character to the woody layers, with which it is in close
proximity on its inner surface. The liber may generally be found to
be made up of a succession of thin layers, equalling in number those
of the wood, the innermost being the last formed ; but no such
succession can be distinctly traced either in the cellular envelope or
in the suberous layer, although it is certain that they, too, augment
in thickness by additions to their interior, whilst their external por-
tions are frequently thrown off in the form of thickish plates, or
detach themselves in smaller and thinner laminfe. The bark is
always sepai'ated from the wood by the cambi'iLm layer, which is the
part wherein all new growth takes jDlace. This layer seems to con-
sist of miacilaginous semi-fluid matter ; but it is really made up of
cells of a very delicate texture, which gradually undergo transfor-
mation, whereby they are for the most part converted into tracheids,
ducts, spiral vessels, &c. These materials are so arranged as to
augment the fibro-vascular bundles of the wood on their external
surface, thus forming a new layer of alburnum, which encloses all
those that preceded it ; whilst they also form a new layer of liber
on the interior of all those which preceded it. They also extend the
medullary rays, which still maintain a continuous connection between
the pith and the bark ; and a portion remains unconverted, so as

1 [Tbe term ' liber ' is also sometimes applied to the ' phloem-portion ' of a fibro-
vascular bundle. — Ed. J

STRUCTUEE OF STEMS 709

always to keep apart the liber and the alburnum. This type of
stem-structure is termed exogenous ; a designation which applies
very correctly to the mode of increase of the woody layers, although
(as just shown) the liber is formed upon a truly endogenous jjlan.

iSTumerous departures froin the normal tyjDe are found in particu-
lar tribes of dicotyledons. Thus in some the wood is not marked by
concentric circles, their gi'owi^h not being interrupted by any seasonal
change. In other cases, again, each woody zone is separated from
the next by the interposition of a thick layer of cellular substance.
Sometimes wood is formed in the bark (as in Gcdycanthiis), so that
several woody columns are produced, which are quite independent of
the principal woody axis, and cluster around it. Occasionally the
woody stem is divided into distinct segments by the peculiar thick-
ness of certain of the medullary rays, and in the stem, of which
fig. 554 represents a transverse section, these cellular plates form

^^

I

SS--''

Fig. 554. — Transverse section of the Fig. 555. — Portion of transverse

stem of a climbing plant {Arisfo- section of Arctium (burdock),

lochia ?) from New Zealand. showing one of the fibro-vascu-

l9.r bundles that lie beneath
the cellular epiderm.

four large segments disposed in the manner of a Maltese cross, and
alternating with the four woody segments, which they equal in size.
The exogenous stem, like the (so-called) endogenous, consists, in
its first-developed state, of cellular tissue only ; but after the leaves
have been actiyely performing their function for a short time, we
find a circle of fibro-vascular bundles, as represented in fig. 540,
interposed between the central (.or vieclidlary) and the peripheral
(or cortical) portions of the fundamental tissue, these fibro-vascular
bundles being themselves separated from each other by plates of
cellular tissue, which still remain to connect the central and the
peripheral portions of that tissue. This first stage in the formation
of the exogenous axis, in which its principal parts — the pith, wood,
bark, and medullary rays — are marked out, is seen even in the
stems of herbaceous plants, which are destined to die down at the
end of the season (fig. 555) ; and sections of these, which are very

7IO MICKOSCOPIC STEUCTUEE OF PHANEEOGAMIC PLAKTS

easily prepared, are most interesting microscopic objects. In such
stems the difference between the endogenous and the exogenous
types is manifested in little else than the disj)osition of the fibro-
vascular layers which are scattered through nearly the whole of
the fundamental tissue (although more abundant tow^ards its
extei'ior) in the former case, but are limited to a cu-cle within the
]3eripheral portion of the cellular tissue in the latter. It is in the
further development which takes place during succeeding years in
the woody stems of perennial exogens that those characters are
displayed which separate them most completely from the ferns and
their allies, whose stems contain a cylindrical layer of jQbro -vascular
bundles, as well as from (so-called) endogens. For whilst the fibro-
vascular layers of the latter, when once formed, undergo no further
increase, those of exogenous stems are progressively augmented on
their outer side by the metamorphosis of the cambium layer; so
that each of the bundles which once lay as a mere series of parallel
cords beneath the cellular ej^iderm of a first-year's stem, may become
in time the small end of a wedge-shaped mass of wood extending
continuously from the centre to the exterior of a trunk of several
feet in diametei-, and becoming progressively thicker as it passes
U23wards. The fibro-vascular bundles of exogens are therefore
spoken of as ' indefinite ' or open^ whilst those of endogens and
vascular cryptogams (ferns, &c.) are said to be ' definite ' or closed.
The open fibro-vascular bundles of exogens and of gymnosperms
may be stated to consist of three distinct parts : the xylem portion,
which consists chiefly of ducts, of the nature of spiral, annular, or
pitted vessels, and which is the portion of the bundle nearest to the
centre of the organ ; the jjhloein or ' bast ' portion, which consists
largely of prosenchymatous cells, among which are almost always
sieve-Uibes with their sieve plates, and which is the peripheral
portion of the bundle ; while between them is the formative cam-
bucm, from which fresh xylem is constantly being formed on one
side, fresh phloem on the other side. The closed bundles of
endogens and of vascular cryptogams consist of xylem and phloem
only. When the xylem and phloem, portions of fibro-vascular
bundle lie side by side, as is usually the case, the bundle is said to
be collateral ; when either portion encloses the other like a cylinder,
it is concentric.

The structure of the roots of endogens and exogens is essentially
the same in plan as that of their respective stems. Generally
speaking, however, the roots of exogens have no pith, although they
have medidlary ra.ys ; and the succession of distinct rings is less
apparent in them than it is in the stems from which they diverge.
In the delicate branches which proceed from the lai'ger root-fibres
a central bundle of vessels will be seen enveloped in a sheath of
cellular substance ; and this investment also covers in the end of
the branch, which is usually somewhat dilated, and is furnished at
its extremity with one or more layers of cells, which are constantly
Ijeing thrown off, known as the jnleorhiza or root-cap. The structure
of the branches of the root may be well studied in the common
buckweed, every floating leaf of which has a single root hanging down

STEUCTUEE 01' STEMS AND EOOTS 711

from its lower surface. The ctsntral fibro-vascular cylinder, which is
characteristic of the finer roots of exogens, as well as of endogens, is
surrounded by a single layer of cells very clearly difierentiated fi-oni
the surrounding fundamental tissue, known as the bundle-sheath.
We have already seen the peculiar form assumed by the bundle-
sheath in the stem of ferns and other vascular cryptogams.

The structure of stems and roots cannot be thoroughly examined
in any other way than by making sections in different directions
with the microtome. The general instructions already given leave
little to be added respecting this special class of obj ects, the chief
points to be attended to being the preparation of the stems, etc. for
slicing, the sharpness of the knife, and the dexterity with which it
is handled, and the method of mounting the sections when made.
The wood, if green, should first be soaked in strong alcohol for a
few days, to get rid of the resinous matter ; and it should then be
macerated in water for some days longer for the removal of its
gum, before being submitted to the cutting process. If the wood
be dry, it should first be softened by soaking for a sufficient length
of time in water, and then treated with spirit, and afterwards with
water, like green wood. Some woods are so little afiected even by
prolonged maceration that boiling in water is necessary to bring
them to the degree of softness requisite for making sections. No
wood that has once been dry, however, yields such good sections as
that which is cuf fresh. When a piece of appropriate length
has been placed in the grasp of the section instrument (wedges of
deal oi- other soft wood being forced in with it, if necessary for its
firm fixation), a few thick slices should first be taken, to reduce its
surface to an exact level ; the surface should then be wetted with
spirit, the micrometer-screw jiaoved through a small part of a revo-
lution, and the slice taken off with the razor, the motion given to
which should partake both of draining and pushing. A little prac-
tice will soon enable the opei-ator to discover in each case how thin
he may venture to cut his sections without a breach of continuity,
and the micrometer-screw should be turned so as to give the required
elevation. If the surface of the wood has been sufficiently wetted,
the section will not curl up in cutting, but will adhere to the sur-
face of the razor, from which it is best detached by dipping the
razor in water so as to float away the slice of wood, a camel-hair
pencil being used to push it off if necessary. All the sections that
may be found sufficiently thin and perfect should be put aside in a
bottle of weak spirit imtil they be mounted. For the minute exami-
nation of their structure, they may be mounted either in weak
spirit or in glycerin -jelly. Where a mere general view only is needed,
dry mounting answers the purpose sufficiently well ; and there are
many stems, such as that of Clematis, of which transverse sections
rather thickei- than ordinary make very beautiful opaque objects
when mounted dry on a black ground. Canada balsam should not
be had recourse to, except in the case of very opaque sections, as it
usually makes the structure too transparent. Transverse sections,
however, when slightly charred by heating between two plates of
glass until they turn brown, may be mounted with advantage in

712 MICEOSCOPIC STEUCTURE OF PHANEEOGAMIC PLAJ^TS

Canada balsam, and are then very sliowy specimens for the gas-
microscope. The number of beautiful and interesting objects which
may be thus obtained from even the commonest trees, shrubs, and
herbaceous plants at the cost of a very small amount of trouble
can scarcely be conceived save by those who have specially attended
to these wonderful structures ; and a careful study of sections
made in different parts of the stem, especially in the neighbourhood
of the ' growing point,' will reveal to the eye of the physiologist
some of the most important phenomena of vegetation. The judi-
cious use of the staining jorocess not only improves the appearance of
such sections, but adds greatly to their scientific value. Fossil
woods, when well preserved, are generally silicified, and can only
be cut and polished by a lapidary's wheel. Should the microscopist
be fortunate enough to meet with a portion of a calcified stem in
Avhich the organic structure is preserved, he should proceed with it

l«l f > k

FiQ-. 556.— Epiderm of leaf of
Yucca, showing stomates.

Pig. 557. — Epiderm of leaf of Indian
corn [Zea Mais), showing stomates.

after the manner of other hard substances which need to be reduced
by grinding.

Epiderm of Leaves. — On all the softer parts of the highei- plants,
save such as grow under water, we find a surface layer differing in
its texture from the par en chyme beneath, and constituting a dis-
tinct membrane, known as the epiderm. This membrane is composed
of cells, the walls of which are flattened above and below, whilst
they adhere closely to each other laterally, so as to form a continuous
stratum (figs. 560, 562, a, a). The shape of these cells is different in
almost every tribe of plants ; thus in the epiderm of the Yiwca (fig.
556), Indian corn (fig. 557), Iris (fig. 561), and most other mono-
cotyledons, they are elongated, and present an approach to a
i-ectangular contour, their margins being straight in the Yucca
and Iris, but minutely sinuous or crenated in the Indian corn.
In most dicotyledons, on the other hand, the cells of the epiderm
depart less from the rounded form, but their margins usually
exhibit large irregular sinuosities, so that they seem to fit together

STEUCTUEE OF LEAVES

71,

like the pieces of a dissected map, as is seen in the epiderm of the apj^le
(fig. 558, h, b). Even here, however, the cells of that portion of the
epidei'm {a, a) which overlies the ' veins ' of the leaf have an elongated
form, approaching that of the wood-cells of which these veins are
chiefly composed ; and it seems likely, therefore, that the elongation
of the ordinaiy epiderm cells of monocotyledons has reference to

Fig. 558. — Portion of ej)iclerm of lower surface of leaf of apple,
with layer of pareiicliynie in immediate contact with it :
a, a, elongated cells overlying the veins of the leaf ; h, b,
ordinary epiderm-cells, overlying the parenchyme ; c, c,
stomates ; d, d, green cells of the spongy parenchyma,
forming a very open network near the lower surface of the
leaf.

that parallel ai'rangement of the veins which their leaves almost
con.stantly exhibit.

The cells of the epiderm are colourless, or nearly so, having no or
but little chlorophyll in their interior ; and their walls are generally

Fig. 559. — Portion of epiderm of upper surface of leaf of
Bochea falcata, as seen at A from its inner side, and at B
from its outer side: a, a, small cells forming inner layer;

■ b, b, large prominent cells of outer layer ; c, c, stomates dis-
posed between the latter.

thickened by secondary deposit, especially on the side nearest the
atmosphere. This outermost hardened continuous wall of the
epidermal layer of cells is known as the cuticle. The deposit (cutin)
is of a nature to render the membrane very impermeable to fluids,
.so as to protect the soft tissue of the leaf from diying up. In most

714 MICEOSCOPIC STEUCTURE OF PHANEEOGAMIC PLANTS

Eui'opean plants the epiderm consists of but a single row of cells,
which, moreover, ai'e usually thin-walled ; whilst in the generality
of ti'opical species there exist two, three, oi- even four layers of
thick-walled cells, this last number being seen in the oleander, the
epiderm of which, when separated, has an almost leatheiy firmness.
This difierence in conformation is obviously adapted to the conditions
of growth under which these plants respectively exist ; since the
epiderm of a plant indigenous to temperate climates would not afibrd
a sufiicient protection to the interior structure against the rays of a
tropical sun ; whilst the less powerful heat of this country would
scarcely overcome the resistance presented by the dense and non-
conducting integument of a species formed to exist in tropical
climates.

A very curious modification of the ejjiderm is presented by
Bochea falcata, which has the surface of its ordinary epiderm (figs.
559, 560, «, a) nearly covered with a layer of large prominent
isolated cells, 6, 5. A somewhat similar structure is found in
Mesembryanthemumcrystallinum, commonly known as the 'ice-plant,'
a designation it owes to the peculiar appearance of its siirface,
which looks as if it were covered with frozen dewdrops. In other
instances the epiderm is partially invested by a. layer of scales,
which are nothing else than flattened hairs, often having a very
peculiar form ; the ' peltate scales ' of Elcjeagnus and other shrubs
and herbs are very beautiful objects under the microscope. In

numerous other cases, again,
we find the surface beset with
true hairs, which occasionally
consist of single elongated
cells, but are more commonly
made up of a linear series,
attached end to end. Some-
times these hairs bear little
glandular bodies at their ex-
tremities, by the secretion of
which a peculiar viscidity is
given to the surface of the leaf,
stem, or flower- stalk, as in
many kinds of rose, gei-anium,
&c. In other instances, the
hair has a glandular body at
its base, containing a peculiar secretion ; when this secretion is of
an irritating quality, as in the nettle, it constitutes a ' sting.' A
gi'eat variety of such organs may be found by a microscopic
examination of the surface of the leaves of plants having any
kind of superficial investment to the epiderm. Many connecting
links present themselves between hairs and scales, such as the
stellate hairs of Deutzia scahra, which a good deal I'esemble those
Avithin the air chambers of the yellow water-lily (fig. 527). The so-
called ' glands ' oi- ' tentacles ' of the sundew [Drosera) are not
really hairs, but outgi-owths of the internal tissue of the leaf, each
l)eing penetrated by a fibro-vascular bundle.

FiCt 560 — Poition of \ eitical section of leaf
of Rochea, showing the small cells, a, a,
of the inner layer of epiderm ; the large
cells, b, b, of the outer layer ; c, one of the
stomates ; d, d, cells of the parenchj'me ;
L, cavity between the x^arenchymatous
cells into which the stomate o]3ens.

STRUCTUKE OF LEAVES

715

The epiderm in many plants, especially those belonging to the
grass tribe, has its cell-walls impregnated with silex^ like that of
Equisetum ; so that, when the organic matter seems to have been
got rid of by heat or by acids, the foi-ms of the epidermal cells, hairs,
stomates, &c., are still marked out in silex, and (unless the dissipa-
tion of the organic matter has been most pei-fectly accomplished)
ai-e most beautifully displayed by polarised light. Such silicified
epiderms are found in the husks of the grains yielded by these plants ;
and there is none in which a larger pi-oportion of mineral matter
exists than that of rice, which contains some curious elongated cells
with toothed margins. The hairs with which t\\Qpalece (chaff-scales)
of most grasses are furnished are strengthened by the like siliceous
deposit ; and in Festaca jjratensis, one of the common meadow-
grasses, the j)aleiB are also beset with longitudinal rows of little cup-
like bodies formed of silex. The epiderm and scaly hairs of Deutzia
scabra also contain a large quantity of silex, and are remarkably
beautiful objects for the polariscope.

In nearly all plants which possess a distinct epiderm, this is
perforated by the minute openings termed stomates (figs. 557, 561),
which are bordered by cells of a peculiar form, the gicard-cells,
differing from those of the epiderm, and more resembling in chai'acter
those of the tissue beneath.
They are further distinguished
by containing a larger number
of chlorophyll-gvains than the
ordinary cells of the epiderm.
These guard-cells are usually
somewhat kidney-shaped, and
lie in pairs (fig. 561, b), with
an oval opening between them ;
but by an alteration in theii-
form, the opening may be con-
tracted or neai'ly closed. In
the epiderm of Yucca, however,
the opening is bounded by two
pairs of cells, and is somewhat
quadrangular (fig. 556) ; and a
like doubling of the guard-
cells, with a nari'ower slit be-
tween them, is seen in the epi-
derm of the Indian corn (fig.
557). In the stomates of no
phanerogam, however, do we

Fig. 561. — Portion of epiderm of leaf of Iris
germanica torn from its surface, and
carrying away with it a portion of the
XDarenchymatous layer in immediate con-
tact with it : ft, a, elongated cells of the
epiderm ; &, h, cells of the stomates ; c, c,
cells of the parenchyme ; d, d, imi^ressions
on the epidermal cells formed by their
contact ; e, cavity in the parenchyme, cor-
responding to the stomate.

meet with any conformation at all to be compai'cd in complexity
with that which has been described in the humble Marchantia.
Stomates are usually found most abundantly (and sometimes exclu-
sively) in the epiderm of the lower surface of leaves, where they open
into the air-chambers that are left in the parenchyme which lies
next the inferior epiderm ; in leaves which float on the surface of
water, however, they are found in the epiderm of the upper surface
only ; whilst in leaves that habitually live entirely submerged, as

7l6 MICEOSCOPIC STEUCTUEE OF PHANEEOGAMIC PLANTS

there is no distinct epiderm, so there are no stomates. In the erect
leaves of grasses, the Iris tribe, &c., they ai-e found equally (or nearly
so) on both surfaces. As a general fact, they are least numerous in
succulent plants, whose moisture, obtained in a scanty supply, is
destined to be I'etained in the system ; whilst they abound most in
those which exhale fluid most readily, and therefore absorl) it most
quickly. It has been estimated that no fewer than 160,000 are con-
tained in every square inch of the under surface of the leaves of
Hydrangea and of several other plants, the greatest number seem-
ing always to be present where the upper surface of the leaves is
entirely destitute of these organs. In Iris germanica each surface
has nearly 12,000 stomates in every square inch ; and in Tucca each
surface has 40,000. In the oleander, Banksia, and some other plants,
the stomates do not open directly upon the lower surface of the
epiderm, but lie in the deepest part of little pits or depressions,
which are excavated in it and lined with hairs ; the mou^ths of these
pits, with the haii-s that line them, are well brought into view by
taking a thin slice from the surface of the epiderm with a sharp
knife ; but the form of the cavities and the position of the stomates
can only be well made out in vertical sections of the leaves.

The internal structure of Leaves is best brought into \'iew by
making vertical sections, traversing the two layers of epiderm and
the intermediate cellular parenchyme ; portions of such sections are
shown in figs. 560, 562, and 563. In close apj)osition with the cells

of the upper epiderm (fig.
562, a, a), which may or may
not be perforated with the
stomates (c, c, d, d), we find a
layer of soft, thin- walled cells,
with their longest diameter
at right angles to the svu'face
of the leaf, and containing
a large quantity of chloro-
phyll ; these generally press
so closely one against an-
other that their sides be-
come mutually flattened, and
no spaces are left, save whei-e
there is a definite air-chamber
into which the stomate opens
(fig. 562, e) ; and the com-
pactness of this superficial layer is well seen when, as often happens, it
adheres so closely to the epiderm as to be carried away with this when
it is torn ofi" (fig. 561, c, c). This layer, usually peculiar to the upper
surface of leaves, is known as the 2jalisade-2yare7ich'i/me. Beneath
this first layer of leaf-cells there are usually several others rather
less comjDactly arranged ; and the tissue gradually becomes more
and more lax, its cells not being in close apposition, and large inter-
cellular passages being left amongst them, until we reach the lower
epiderm, which the pai-enchyme only touches at certain points, its
lowest layei' forming a sort of network, the so-called spongy paren-

PiG. 562. — Vertical section of epiderm and of
portion of subjacent parenchyme of leaf of
Iris gerincmica taken in a transverse direc-
tion : fl, a, cells of epiderm; h, b, cells at the
sides of the stomates; c, c, guard-cells; cl, d,
openings of the stomates ; e, e, cavities in the
parenchyme into which the stomates open ;
/, /, cells of the parenchyme.

STEUCTUEE OF LEAVES

717

chyme (fig. 558, d, d), with large interspaces, into which the stomates
open. It is to this arrangement that the darker shade of green
almost invariably presented by the upper surface of leaves is prin-
cipally due, the colour of the component cells of the parenchyme
not being deeper in one j)art of the leaf than in another. In those
plants, however, whose leaves are erect instead of being horizontal,
so that their two surfaces are equally exposed to light, the paren-
chyme is arranged on both sides in the same manner, and their
epiderms are furnished with an equal number of stomates. This is
the case, for example, with the leaves of the common garden Iris
(fig. 563), in which, moreover, we find a central portion (fZ, d)
formed by thick- walled colourless tissue, very different either from
ordinary leaf-cells or from woody fibre. The explanation of its
presence is to be found in the j)eculiar conformation of the leaves ;
for if we pull one of them from its origin, we shall find that what
apjDears to be the flat expanded blade really exposes but half its
surface, the blade being doubled together longitudinally, so that
what may be considered its luider surface is entirely concealed.

Fig. 563. — Portion of vertical longitudinal section of leaf
of Iris, extending from one of its flattened sides to the
otlier : a, a, elongated cells of epiderm ; b, b, stomata cut
through longitudinally ; c, c, green cells of parenchyme ;
d, d, colourless tissue, occupying interior of leaf.

The two halves are adherent together at their upjDer pai't ; but at
their lower they are commonly separated by a new" leaf which comes
up between them ; and it is from this arrangement, which resembles
the position of the legs of a man on horseback, that the leaves of
the Iris tribe are said to be equitant. 'Now by tracing the middle
layer of colourless cells, d, d, down to that lower jjortion of the leaf
where its two halves diverge from one another, we find that it there
becomes continuous with the epiderm, to the cells of which (fig. 563,
a) these bear a strong resemblance in every respect, save the greater
proportion of their breadth to their length. Another interesting-
variety in leaf-sti-ucture is presented by the water-lily and other
plants w^hose leaves float on the surface ; for here the usual ai'i-ange-
ment is entirely reversed, the closely set layers of green leaf-cells
being found in contact with the lower surface, whilst all the upper
part of the leaf is occupied by a loose spongy parenchyme, containing
a very large nvunbei- of air-spaces that give buoyancy to the leaf ;
and these spaces communicate with the external air through the

7l8 MICROSCOPIC STEUCTURE OF PHANEROGAMIC PLANTS

numerous stomates, which, contrary to the general rule, are here
found in the u^pper epiderra alone.

The examination of the foregoing structures is attended with
veiy little difficulty. Many epiderms may be torn off, by the exer-
cise of a little dexterity, from the surfaces of the leaves they
invest without any preparation ; this is especially the case with
monocotyledons generally, the veins of whose leaves run parallel,
and with such dicotyledons as have veiy little woody structure in
their leaves. In those, on the other hand, whose leaves are furnished
with reticulated veins to which the epiderm adheres (as is the case in
by far the larger proportion), this can only be detached by first
macerating the leaf for a few days in water ; and if their texture
is particularly firm, the addition of a few drops of nitric acid to
the water will render their epiderms more easily separable. Epi-
derms may be advantageously mounted either in weak spirit or in
glycerin-jelly. Very good sections of most leaves m.ay be made by
a sharp knife, handled by a careful manipulator ; but it is generally
preferable to use the microtome, placing the leaf between two pieces
either of very soft cork or of elder-pith or carrot, or imbedding it in
paraffin. In order to study the structure of leaves with the fulness
that is needed for scientific research, numerous sections should be
made in different directions, and slices taken parallel to the surfaces
at different distances from them should also be examined. There is
no known mediiim in which such sections can be preserved altogether
without change ; but some one of the methods formerly described
will generally be found to answer sufficiently well.

Flowers. — Many small flowers, when looked at entire with a low
magnifying power, are very striking microscopic objects ; and the
interest of the yoimg in such observations can scarcely be better

excited than by directing their
attention to the new view they
thus acquire of the ' composite '
nature of the humble down-
trodden daisy, or to the beavity
of the minute blossoms of many
of those umbelliferous plants
which are commonly regarded
only as rank weeds. The
scientific microscopist, how-
ever, looks more to the organi-
sation of the separate parts of
the flowei' ; and among these
he finds abundant sources of
gratification, not merely to his
love of knowledge, but also to his taste for the beautiful. The general
structure of the sepals and petals, which constitute the perianth, or
floral envelope, closely corresponds to that of leaves. The petals
seldom contain unchanged chlorophyll ; but usually eithei- the
chlorophyll in the petals (and sometimes also in the sepals) is
changed into a solid yellow pigment {carotin) ; or the chlorophyll
lias entirely disappeared, and is rejolaced by a pigment, blue, red.

Fig. 564. — Cells from petal of
Pelargonium.

STRUCTURE OF FLOAVEES 7I9

purple, or some other bright colour, anthocyan, erytJiro-pliyll^ cfec,
dissolved in the cell-sap. There are some petals whose cells exhibit
very interesting peculiarities, either of form or marking, in addition
to their distinctive coloration ; such are those of the Pelargonium,
of which a small portion is represented in fig. 564. The different
portions of this petal — when it has been dried after stripping it of
its epiderm, immersed for an hour or two in oil of turpentine, and
then mounted in Canada balsam — exhibit a most beautiful variety
of vivid coloration, which is seen to exist chiefly in the thickened
partitions of the cells ; whilst the surface of each cell presents a
very curious opaque spot with numerous diverging prolongations.
This method of preparation, however, does not give a true idea of
the structure of the cells ; for each of them has a peculiar mammil-
lary protuberance, the base of which is surrounded by hairs ; and
this it is which gives the velvety appearance to the surface of the
petal, and which, when altered by drying and comj^ression, occa-
sions the peculiar spots represented in fig. 564. Their real character
may be brought into view by Dr. Inman's method, which consists
in drying the petal (when stiipped of its epiderm) on a slip of glass,
to which it adheres, and then placing on it a little Canada balsam
diluted with turpentine, which is to be boiled for an instant over
■the spirit lamp, after which it is to be covered with a thin glass.
The boiling ' blisters ' it, but does not remove the colour ; and on
examination many of the cells will be found showing the mammilla
very distinctly, with a score of hairs surrounding its base, each of
these slightly curved, and pointing towards the apex of the mammilla.
The petal of the common scarlet pimpernel (AnagalUs a7-vensis),
that of the common chick weed [Stellaria media), together with many
others of a small and delicate character, are also very beautiful
microscopic objects ; and the two just named are peculiarly favour-
able subjects for the examination of the spiral vessels in their natural
position. For the ' veins ' which traverse these petals are entirely
made up of spiral vessels, none of which individually attain any
great length, but one follows or takes the place of another, the
conical commencement of each somewhat overlapping the like termi-
nation of its predecessor ; and where the ' veins ' seem to branch,
this does not happen by the bifurcation of a spiral vessel but by
the ' splicing on ' (so to speak) of one to the side of another, or of
two new vessels diverging from each other to the end of that which
formed the principal vein.

The Anthers and Pollen-grains also present numerous objects of
great interest, both to the scientific botanist and to the amateur
microscopist. In the first place, they aflx)rd a good opportunity of
studying that form of ' free-cell-formation ' which seems peculiar to
the parts concerned in the reproductive j)rocess, and which consists
in the development of new cell-walls round a number of isolated
masses of protoplasm forming parts of the contents of a parent
cell, so that the new cells lie free within its cavity, instead of being
formed by its subdivision, as in the ordinary method of multiplica-
tion. If the anther be examined by thin sections at an early stage
of its development within the young flower-bud, it will be found to

720 MICROSCOPIC STEUCTUEE OF PHANEEOGAMIC PLANTS

be made up of ordinary cellular parencliyme in which no peculiarity
anywhere shows itself; but a gradual differentiation speedily takes
place, consisting in the development of a set of very large cells in
two vertical rows, which occupy the place of the loculi or ' pollen-
chambers ' that afterwards present themselves ; and these cells give
origin to the pollen-grains, whilst the ordinary parenchyme remains
to form, the walls of the pollen-chambers. The pollen-grains are
formed within ' mother-cells,' the endoplasm of each breaking up
into four segments. These become invested by a double envelope, a
firm extine, and a thin intine ; and they are set free, when mature,
by the bursting of the pollen-chambers. It is not a little curious
that the layer of cells which lines the pollen-chambers should exhibit,
in a considerable proportion of plants, a strong resemblance in struc-
ture, though not in form, to the elaters of Marchantia (fig. 506).
For they have in their interior a fibrous deposit, which sometimes
forms a continuous spiral (like that in fig. 532), as in Narcissus and
Hyoscyamus ; but it is often broken up, as it were, into rings, as in
the Iris and hyacinth ; in many instances it forms an irregular
network, as in the violet and saxifrage ; in other cases again, a
set of interrupted arches, the fibres being deficient on one side, as in
the yellow water-lily, bryony, prirarose, &c. ; whilst a very peculiar
stellate aspect is often given to these cells by the convergence of the
interrupted fibres towards one point of the cell-wall, as in the cactus,
geranium, madder, and many other well-known plants. Various
intermediate modifications exist ; and the particular form presented
often varies in difierent parts of the wall of one and the same anther.
It seems probable that, as in Hepaticas, the elasticity of these spiral
cells may have some share in the opening of the pollen-chambers and
in the dispersion of the pollen-grains.

The form, of the pollen-grains seems to depend in part upon the
mode of division of the cavity of the parent cell into quarters ;
generally speaking, it approaches the spheroidal, but it is very often
elliptical, and sometimes tetrahedral. It varies more, however,
when the pollen is dry than when it is moist ; for the efiect of the
imbibition of fluid, which usually takes place when the pollen is
placed in contact with it, is to soften down angularities, and to
bi'ing the cell neai'ei' to the typical sphere. The extine, or outer
coat of the pollen-grain, often exhibits very curious markings, which
seem due to an increased thickening at some points and a thinning
away at others. Sometimes these markings give to the surface layer
so close a resemblance to a stratum of cells (fig. 565, B, C, D) that
only a very careful examination can detect the difference. The
roughening of the surface by spines or knobby protixberances, as
shown at A, is a very common feature ; and this seems to enable
the pollen-grains more readily to hold to the surface whereon they
may be cast. Besides these and other inequalities of the surface,
most pollen-grains have what appear to be pores or slits in their
extine (varying in number in different species), through which the
intine protrudes itself as a tube, when the bulk of its contents has
been increased by imbibition. It seems probable, however, that the
extine is not absohitely deficient at these points, but is only thinned

POLLEN-GKAINS

721

«k^^'^

away. Sometimes the pores are covered by little disc-like pieces or
lids, which fall off when the pollen-tithe is protruded. This action
takes place natui-ally when the pollen-grains fall upon the surface of
the stigma, which is moistened with a viscid secretion ; and the
pollen-tubes, at first mere protrusions of the inner coat of their cell,
insinuating themselves between the loosely packed cells of the stigma,
grow downwards through the style, sometimes even to the length of
several inches, until they reach the ovary. The first change, namely
the protrusion of the inner membrane through the pores of the exterior,
may be made to take place artificially by moistening the pollen
with water, thin syrup, or dilute acids (different kinds of pollen-
grains requiring different modes of treatment) ; but the subsequent
extension by gi'owth will only take place under the natural con-
ditions. By treating some pollen-grains, as those of Lilium
japonicum, L. ruhruin, or L. auratimn, with the viscid liquid abun-
dantly secreted by the stigma,
not only may the extrusion
and lengthening of the pollen -
tubes be watched, but the
grains with their extruded
tubes may be preserved almost
unchanged by mounting in
this liquid.

The darker kinds of pollen
may be generally rendei'ed
transparent by mounting in
Canada balsam ; or, if it be
desired to avoid the use of
heat, in the benzol solution
of Canada balsam, setting-
aside the slide for a time in
a warm place. For the less
opaque pollens the dammai-
solution is preferable. The
more delicate pollens, how-
ever, become too transparent

in either of these media ; and it is consequently preferable to mount
them either dry, or (if they will bear it without rupturing) in fluid.
The most interesting forms are found, for the most part, in plants
of the orders Amaranthacece, Cichoriacece, Cuctirbitacece, Malvacece,
and Passijlorece ; others are furnished also by Convolvulus, Cam-
panula, (Enothera, Pelargonium {gemnim-n), Poli/gonum, Sedum, and
many other plants. It is frequently preferable to lay down the
entire anther, with its adherent pollen-grains (where these are of
a kind that hold to it), as an opaque object ; this may be done with
great advantage in the case of the common mallow (Jlalva syl-
vestris) or of the hollyhock {Althcm rosea), the anthers being picked
soon after they have opened, whilst a large proportion of their pollen
is yet undischarged, and laid down as flat as possible, before
they have begun to wither, between two pieces of smooth blotting-
paper, then subjected to moderate pressure, and finally mounted

• » A

Fig. 565. — Pollen-grains of — A, Altluea rosea
(hollyhock) ; B, Cohaa scandens ; C, Passi-
flora ccsrulea; D, Ipomcea ijurjpurea.

722 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

upon a black surface. They are then, when properly illuminated,
most beautiful objects for objectives of §-, 1-, 1^-, or 5-in. focus,
especially with the binocular microscope.^

There are, in fact, few more interesting objects for the young
microscopist than pollen-grains, both from the ease with which they
. can always be procured, and the almost infinite variety and beauty
in their forms. Some of the commonest weeds, such as the dandelion
and groundsel, are distinguished by the beauty of their pollen-grains.
The grains are sometimes nearly or quite spherical, as in the hazel,
birch, or poplai' ; or of very irregular ovitline, as in many grasses.
But the most common form is elliptical, with three or five longi-
tudinal furrows, as in the wallflower, hyacinth, and crocus, the
surface being sometimes covered with warts, as in the snowdrop.
In the fuchsia they are triangular. In addition to the mallow and
hollyhock, spiny pollen-grains occur in the groundsel, dandelion.
Cineraria^ and many other plants. Sometimes the grains are united
together by delicate thi-eads, as in the Rhododendron and F'adtsia ;
and this union is much more complete in the Orchidece and Ascle-
piadece., where the whole of the pollen in each anther-lobe is glued
together by a viscid substance into a club-shaped joo^ZwiMWrt, or pollen-
raass. In what are called anemophilous flowers, in which the pollen
is carried throxigh the air by the agency of the wind, the grains are
small, light, dry, and usually spherical ; while in entomophiloids
flowers, the pollen of which is carried from flower to flower by
insects in search of honey, the various forms above described, and
many others, are adapted to cause the grains to adhere to the haii'y
under side of the body of the insect, and thus promote their dis-
persion. The various species of Epilohvum (willow-herb) and
CEnothera (evening primrose) are very favourable objects for ob-
serving the emission of pollen-tubes and their entrance into the
stigma.

The structure and development of the ovules that are produced
within the ovary at the base of the pistil, and the operation in which
their fertilisation essentially consists, are subjects of investigation
which have a peculiar interest for scientific botanists, but which, in
consequ^ence of the special difiiculties that attend the incjuiry, are
not commonly regarded as within the province of ordinary micro-
scopists. Some general instructions, however, may prove useful to
such as would like to inform themselves as to the mode in which the
generative function is performed in phanerogams. In tracing the
origin and early history of the ovule, very thin sections should be made
through the flower-bud, both vertically and transversely ; but when
the ovule is large and distinct enough to be separately examined, it
should be placed on the thumb-nail of the left hand, and very thin

1 It sometimes happens that when the pollen of pines or iirs is set free, large
quantities of it are carried by the wind to a great distance from the woods and
plantations in which it has been j)i'oduced, and are deposited as a fine yellow dust,
so strongly resembling sulphur as to be easily mistaken for it. This (supposed)
general diffusion of sulphur (such as occurred in the neighbourhood of Windsor
in 1879) has frightened ignorant rustics into the belief that the ' end of the world'
was at hand. .Its true nature is at once revealed by placing a few grains of it under
the microscoxje.

FERTILISATION OF THE OVULE

72;

sections made with a shai-p razoi- ; the ovule shoukl not be allowed
to dry up, and the section should be i-emoved from the blade of the
razor by a wetted camel-hair pencil. The tracing downwards of the
pollen-tubes through the tissue of the style may be accomplished by
sections (which, however, will seldom follow one tube continuously
for any great part of its length), or, in some instances, by careful
dissection with needles. Plants of the Orchis ti'ibe ai-e the most
favourable subjects for this kind of investigation, which is best
cai-ried on by artificially applj-ing the pollen to the stigma of several
flowers, and then examining one or more of the styles daily. ' If
the style of a flower of Epijmctis^' says Schacht, ' to which the pollen
has been applied about eight days previously, be examined in the
manner above mentioned, the observer will be surprised at the
extraordinary number of pollen-tubes, and he will easily be able to
trace them in large strings, even as far as the ovules. Viola tricolor
(heartsease) and Rihes nigrum and ruhrum (black and red currant)
are also good plants for the purpose ; in the case of the former plant
withered flowers may be taken and branched pollen-tubes will not
unfrequently be met with.' The entrance of the jjollen-tube into
the micropyle may be most easily observed in orchidaceous plants
and in Eiqihrasia., it being only necessary to tear open with a needle
the ovary of a fiowei' which is just withering, and to detach from the
placenta the ovules, almost every one of which will lie found to have
a pollen-tube sticking in its micropyle. These ovules, however, arj
too small to allow of
sections being made,
whereby the origin of
the embryo may be dis-
cerned ; and for this pur-
pose, (Etiothera (evening
primrose) has been had re-
course to by Hofmeister,
whilst Schacht recom-
mends Lathrcea squam-
aria, Pedicularis pahcs-
tris, and particularly
Pedicidaris sylvatica.

We have now, in
the last place, to notice
the chief points of inter-
est to the microscopist
which are furnished by
mature seeds. Many of
the smaller kinds of
these bodies are very
curious, and some are very beautiful objects when looked at in their
natural state under a low magnifying power. Thus the seed of the
poppy (fig. 566, A) presents a regular reticulation upon its surface,
pits, for the most part hexagonal, being left between projecting walls ;
that of the pink (D)isregularly covered with curiouslyjagged divisions,
every one of Avhich has a small bright black hemispheiical knob in its

.3 A 2

Fig. 566. — Seeds as seen under a low magnifying
power: A, popiDy ; B, Auia ninth us (prince's
feather) ; C, Anth'rhimtm inajus (snapdragon) ;
D, Diaiithus (clove-pink) ; E, Bignonia.

724 MICROSCOPIC STRUCTUEE OF PHANEROGAMIC PLANTS

middle ; that of Amaranthus hy2yochondriacus lias its surface traced'
with extremely delicate markings (B) ; that of Antirrhinum is
strangely irregular in shape (0), and looks almost like a piece of
furnace-slag ; and those of many Bignoniacece are remarkable for the
beautiful radiated structure of the translucent membrane which
surrounds them (E). This structure is extremely well seen in
the seed of Eccremiocarpus scaher, a half-hardy climbing plant
common in our gardens ; and when its membi-anous ' wing ' is
examined tinder a suificient magnifying power, it is found to be
formed by an extraordinary elongation of the cells of the seed-coat
at the margin of the seed ; the side-waUs of which cells (those,
namely, which lie in contact with one another) being thickened so as
to form radiating ribs for the suppoi't of the wing, whilst the front
and back walls (which constitute its membranous surface) retain their
original ti^ansparence, and are marked only with an indication of
spiral deposit in theii' interior. In the seed of Dictyolovia peruviana,
besides the principal ' wing ' prolonged from the edge of the seed-
coat, there is a series of successively smaller wings, whose margins
form concentric rings over either surface of the seed ; and all these
wings are formed of radiating fibres only, composed, as in the pre-
ceding case, of the thickened walls of adjacent cells, the intervening
membrane, originally formed by the front and back walls of these
cells, having disappeared, apparently in consequence of being un-
supported by any secondary deposit. Several other seeds, as those
of hplienogyne speciosa and Lophosjiermuin erubescens, possess wing-
like appendages : bnt the most remarkable development of these
organs is said by Mr. Quekett to exist in a seed of Calosanthes
indica, an East Indian plant, in which the wing extends more than
an inch on either side of the seed. Some seeds are distinguished by
a peculiarity of form which, although readily discernible by the
naked eye, becomes much raore striking when they are viewed under
a very low magnifying power. This is the case, for example, with
the seeds of the carrot, whose long radiating processes make it bear,
under the microscope, no trifling resemblance to some kinds of star-
fish ; and with those of Oyanthus minor, which bear about the same
degree of resemblance to shaving-brushes. In addition to the pre-
ceding, the following may be mentioned as seeds easily to be
obtained and as worth movmting for opaque objects : — AnagalUs,
Anethum graveolens, Begonia, Carimi carui, Coreop)sis tinctoria,
Datura, j)el2yhinium, Digitalis, Elatine, Erica, Gentiana, Gesnera,
Hyoscyamus, Hypericum, Lepidium, Linnnocharis, Linuria, Lychnis^
Mesemhryanthe'mum, Nicotiana, Origanum onites, Orohanche, Petunia^
Reseda, Saxifraga, Scrophidaria, Sedum, Sem2)ervivum, Silene,
SteMaria, Syr^iphytum asperrir)ium, and Verbena. The following-
may be mounted as transparent objects in Canada balsam:
Drosera, Hydrangea, Monotrojia, Orchis, Parnassia, Pyrola, Saxi-
fraga} The seeds of umbelliferous plants generally are remarkable
for the peculiar vittm, or receptacles for essential oil, which are
found in the closely applied pericarp or seed-vessel which encloses

1 A part of these lists have been derived from the Micrographic Dictionary.

STRUCTUEE OF SEEDS 725

iihem. Various points of interest i-especting the structure of the
testa or envelope of seeds, such as the fibre-cells of Oobcca and
Collotnici, the stellate cells of the star-anise, and the densely con-
solidated tissue of the 'shells' of the coqtiilla-nut, cocoa-nut, ttc.
having been already noticed, we cannot here stop to do moi-e than
advert to the peculiarity of the constitution of the husk of coi-n-
grains. In these, as in other grasses, the ovary itself continues to
envelop the seed, giving a covering to it that surrounds the testa,
and closely adheres to it. The ' bran' detached in grinding consists
not only of these two coats, but also (as the microscope i-eveals) of
an outer layer of the grain itself, formed of hexagonal cells disposed
with great regularity. As these are filled with gluten, the removal
of this layer takes away one of the most nutritious parts of the
grain; and it is most desirable, therefore, that only the two outer
indigestible coats should be detached by the ' decorticating ' process
devised for the purpose. The hexagonal cell-layer is so little altered
by a high temperature as still to be readily distinguishable when
the grain has been ground after roasting, thus enabling the
mici'oscopist to detect even a small admixture of roasted com with
coffee or chicory without the least difficulty.^

^ In a case in which the Author was called upon to make sueli an investigation,
he found as many as thirty distinctly recognisable fragments of this cellular enve-
lope in a single grain of a mixture consisting of chicory with only 5 per cent, of
rroa.sted corn.

726

CHAPTER XII

MICBOSCOPIC FOBMS OF ANIMAL LIFE—PBOTOZOA

Passing on, now, to the Animal Kingdom, we begin by directing
our attention to those minute and simple forms which correspond in
the animal series with the Protophyta in the vegetable (Chap. VIII.) ;
and this is the more desirable since the formation of a distinct group
to which the name of Protozoa (first proposed in this sense by
Siebold) may be appropriately given is one of the most interesting
results of microscopic inquiry. This group, which must be placed at
the very base of the animal scale, is characterised by the apparent
simplicity that prevails in the structure of the beings that compose
it, the lowest of them being single protoplasmic particles or 'jelly-
specks,' whilst even among the highest, however numerous their
luiits naay be, these are (as scnxong jwotojyhytes) mere repetitions of one
anothei', each capable of maintaining an independent existence. In
this there is a very curious and significant parallelism to the earliest
embryonic stage of higher animals ; for the fertilised germ of any one
of these first shapes itself as a single cell, and then, by repeated binary
subdivisions, develops itself into a morula or ' mulberry-mass ' of
cells, corresponding to the ' multicellular ' organisms met with
among the higher Protozoa. There is, so far, in neither case any
sign of that ' differentiation' of organs which is characteristic of the
higher animals ; but whilst, in the Protozoon, each cell is not merely
similar to its fellows, but is independent of them, the morula, in
such as go on to a higher stage, becomes the subject of a series of
developmental changes tending to the production of a single whole,
whose parts are mutually dependent. The first of these changes is
its conversion into a gastrula or prinaitive stomach, whose wall is
formed of a double membrane, the outer lam^ella, or ectoderm,}
being derived directly from the external cell-layer of the morula
whilst the inner, or endoderm,, is formed by the ' invagination ' of
that layer into the space left void by the dissolution of the central
cells of the ' morula.' This gastrula-stage} as we shall see hereafter,
remains permanent in the great group of Ccelentera, though the
endoderm and ectoderm are separated from each other in its higher
forms by the development of generative and other organs between

1 The terms epiblast and hypoblast are generally used by English embryologists
in place of the ' ectoderm ' and ' endoderm ' used here.

- The gastrula-stage is in a number of cases brought about by a concentric split-
ting of the walls of the morula into two layers, and by the aj^pearance at one point
of an orifice which leads into the central cavity ; this cavity is tlie original segmenta-
tion cavity of the morula, and not a fresh cavity, as in ' invaginate gastrulse.'

,; PKGTOZOA— PKOTOMYXA 727

them. But in all classes above the. coelenterates the primitive
stomach forms a part, and often only an insignificant part, of the
whole digestive tract. Thiis the whole animal kingdom may be
divided, in the first place, into the Protozoa, which are either single
cells or aggregates of similar cells corresponding to the onorida-
stage of higher types ; and the Metazoa, in which the morula takes
on the condition of an individualised organism, the life of every part
of which contributes to the general hfe of the whole. Putting this
important ti-uth into other words, we may say of the Protozoa that
they ai"e either unicellular or unicellular aggregates, while the
Metazoa are multicellulai', and their constituent cells have dilFerent
functions.

The lowest of the Protozoa, however, like the simj)lest proto-
phj'i^es, do not even attain the rank of a true cell, understanding
by that designation a definite protoplasmic unit [plastid), which is
limited by a cell-wall, and contains a ' nucleus.' For they consist
of particles of protoplasm, termed ' cytodes,' of indefinite extent,
which have neither cell-wall nor nucleus, but which yet take in and
digest food, convert it into the material of their own bodies, cast out
the indigestible portions, and reproduce their kind, with the regu-
larity and completeness that we have been accustomed to regard
as characteristic of higher animals. With regard, however, to this
apparent absence of a nucleus we have to bear in mind that the
progress of i-esearch is continually diminishing the number of foinns
devoid of a nucleus, or, at any rate, of a nuclear material scattered
throughout the substance of the plastid ; in retaining, therefore, the
gi'oup of non-nucleated Pi-otozoa we are acting on the principle of
not going beyond our evidence, and by no means reflecting on the
later systematists who have merged the various types (whether
nucleated or non-nucleated) among other divisions of the Protozoa.
BetAveen some of these Mo7ierozoa (as they have been designated
by Professor Haeckel, who first drew attention to them) and the
Myxoinycetes or the Chlamydomyxa already described, no definite
line of division can be drawn, the only justification for the separa-
tion here adoj)ted being that the affinities of the former seem to
be rather with the lowest forms of vegetation, whilst the whole
life-history of the types now to be described, and the connected
graduation by which they pass into undoubted rhizopods, leave no
doubt of their claim to a place in the animal kingdom.

MONEROZOA.

A characteristic example of this lowest protozoic type is presented
by the Protomyxa aurantiaca (fig. ,567), a marine ' moner ' of an
oi-ange-red colour, found by Professor Haeckel upon the dead shells of
tipirula which are so abundant on the shores of the Canary Islands.
In its active state it has the stellar form shown at F, its arborescent
extensions dividing and inosculating so as to form a constantly
changing network of protoplasmic threads, along which stream in all
directions orange-red granules, obviously belonging to the body

728 MICKOSCOPIC FOKMS OF ANIMAL LIFE— PEOTOZOA

itself, together with foreign organisms (b, c) — such as marine diatoms,
radiolarians, and infusoria — which, having been entrapped in the
pseudopodial netwoi-k, are carried by the pi'otoplasmic stream into
the central mass, where the nutrient matter of their bodies is extracted,
the hard skeletons being cast ovit. Neither nucleus nor contractile
vesicle is to be discerned, but numerous floating and inconstant vacu-
oles (a) are dispersed through the substance of the body. After a time
the currents become slower ; the ramified extensions are gradually

Fig. 567. — Proiomyxa aurantiaca : A, encysted statospore; B, inci-
pient formation of swarm-spores, shown at C escaping from the cyst,
at D swimming freely by their flagellate appendages, and at E creep-
ing in the amoeboid condition ; F, fully developed reticulate organism,
sbowingr numerous vacuoles, «, and captured prey, 6, c.

di-awn inwai'ds ; and, after ejecting any indigestible particles it may
still include, the body takes the form of an orange-red sphere round
which a cyst soon forms itself, as shown in A. After a period of
quiescence the protoplasmic substance retreats from the interior of
the cyst, and breaks up into a number of small sphei-es (B), which, at
first inactive, soon begin to move within the cyst, and change their
shape to that of a pear with the small end drawn out to a point.
The cyst then bursts, and the i-ed pear-shaped bodies issue forth
into the water- (C), moving freely about by the vibrations o^Jlagella

PEOTOZOA — PEOTOilYXA

729

formed by the diawing out of theiv small ends, just as do the
flagellated zoospores of protophytes. These bodies, lieing without
trace of either nucleus, contractile vesicle, or cell-wall, are to be
regarded as particles of simple homogeneous protoplasm, to which
the designation plustklides has been appropriately given. After
abovit a day the motions cease ; the tiagella are drawn in, and the
plastidides take the form and lead the life of Amcebce, putting foi-th
inconstant pseudopodial processes, and engulfing nutrient particles
in their substance (D). Two or more of these amcebifoim bodies
unite to form a ' plasmodium,' as in the Myxomycetes ; its pseudo-
podial extensions send out branches which inosculate to form a net-

FiG. 568. — VampyreUa spirogyrcE, as seen at A, sucking out contents
of S^nrogyra-ceW; at B in encysted condition, the cyst o enclosing
granular protoplasm b\ at C, division of contents of cyst into
tetraspores, of which one is escaping in the amoeboid condition
to develop itself into the adult form shown at D.

work ; and the body grows, by the ingestion of nutriment, to the
size of the original. In this cycle of change there seems no interven-
tion of a generative act, the coalescence of the amcebiform j^lastidules
having none of the characters of a trvie ' conjugation.' But it is by
no means improbable that after a long course of multiplication by
successive subdivisions some kind of conjugation may intervene.

Another very interesting ' moneric ' type is the Vampyrella,
of which one form (fig. 568) has long been known in its enoJ^sted
condition as a minute brick-red sphere attached to the filaments of
the conjugate Spirogyra ; whilst another (fig. 569) similarly
attaches itself to the bi'anches of Gomphonema. The walls of the

730 MICROSCOPIC FORMS OF ANIJMAL LIFE— PROTOZOA

cysts are composed of two lueixihranes, of which the interior givefi
the characteristic reaction of ceUulose, w^hilst the softer external
layer is nitrogenous. After i-emaining some time in the quiescent
condition the encysted pi-otoplasm breaks up into two or four
' teti-aspores ' (fig. 569, b, d) ; these escape by openings in the cyst
{fig. 568, 0), and soon take the spherical form, emitting very slender
pseudopodial filaments (figs. 568, D, 569, e) like those of an Actino-
phrys^ but possessing neither nucleus nor contractile vesicle. In this
condition the}' show great activity, moving about in search of the
special nutriment they requii-e, drawing themselves out in strings
and fine filaments which tear asunder and again unite to send off
bi'anches and form fine fan-like expansions, and these occasionally
contiucting again into minute spheres. When the V. spirogyrce is
watched in water containing some filaments of Spirogyra, it may be
seen to wander until it meets one of these filaments, to which, if it
be healthy and loaded with chlorophyll, it attaches itself. It soon
begins to perforate the wall of the filament ; and when the interior
of this has been reached, its endoplasm, carrying with it the chloro-
phyll-granules it includes, passes slowly into the body of the Vain-
pjyrella. In this mannei- cell after cell is emptied of its contents ;
and the plunderer, satiated with food, resumes its quiescent spherical
foi'm to digest it. The chlorophyll-granules which it has ingested
become diffused through the body, but gradually cease to be distin-
guishable, the protoplasmic mass assuming a brick-red colour. The
first layer it exudes to form its cyst is the outer or nitrogenous invest-
ment, within which the cellulose layer is afterwards formed. The F.
gom2>ko7ieinatis in like mannei- creeps over the stems and branches of
the Gomphonema (fig. 569, e), adapting itself to the form of its sup-
port ; and as soon as it has reached one of the terminal siliceous cells
of the diatom, it extends itself over it so as completely to envelop
the cell in a thin layer of protoplasm. From the surface of this a
number of fine pseudopodia radiate into the surrounding water (/) ;
whilst another portion of the protoplasm finds its way between the
two siliceous valves into the interioi-, and appropriates its contents.
The vah^es, when emptied, break off from their support, and are cast
out of the bod}^ of the Yainpyrella^ which soon proceeds to another
Gomphonema-ceW. and plunders it in the same manner. After thus
ingesting the nutiiment furnished by sevei-al cells, and acquiring its
full size, it passes, like V. spirogyroe, into the encysted condition,
to recommence — after a period of quiescence — the same cycle of
change. Mr. Bolton discovered near Birmingham, and Professor Ray
Lankester described, a foiin allied to VaTiipyrella — Archerina Boltoni
— which is remarkable for being chlorophyllogenous ; this species
presents another interesting peculiarity : — ■'■ Groups of ghost-like
outlines corresponding to chlorophyll-corpviscles and their radiant
filamentous pseudopodia, entirely devoid of any substance,' were
observed, and were compared to the numerous cellulose chambers
wliich are seci-eted and al)andoned by the protoplasm of Ghlamy-
domyxa.

Intermediate between the foi-egoing and the ' reticularian ' i-hizo-
pods to be presently described, is another simple protozoon dis-

LIEBEEKUEHNIA 73 1

covered in ponds in Germany by MM. Claparede and Laclimann,
and named by them Lieherkuehnia Wageneri} The whole sub-
stance of the body of this animal and its psendopodial extensions
(fig. 570) is composed of a homogeneous, semi-fluid, granular proto-
plasm, the particles of which, when the animal is in a state of
activity, are continually pei'forming a circulatory movement, which
may be likened to the rotation of the particles in the protoplasmic

Fig. 569. — Vampyrella gomphonematis : A, colony of
Gomphonema attacked by Vampyrellce ; a, encysted state ;
b, h, cysts with contents breaking up into tetraspores, d, d,
seen escaping at e; at/ is shown a Vampyrella sucking out
contents of Gomplionema-Q.eWs, the emptied frustules of
which, g, 7t, are cast forth. B, isolated Vamjjyrella creeping
about by its extended pseudopodia.

network within, the cell of a Tradescantia. It is a marked peculiarity
of the psendopodial extension of this type that it does not take place
by. radiation from all parts of the body indifferently, but that it

1 Etudes surles Infusoires et les JRhisojiodes, Geneva, 1858-1861. The beautiful
figure of Lieherhiielmia, given by M. Claparede, has been rexsroduced by the Author
in Plate I. of his Introduction to the Study of the Foraminifera.

732 MICKOSCOPIC FORMS OF ANIMAL LIFE— PROTOZOA

pi'oceeds entii^ely from a soi-t of ti-unk that soon divides into branches
which again speedily multiply by further subdivision, until at last
a multitude of finer and yet finer thi-eads ai-e spun out by whose
continual inosculations a complicated network is produced, which
may be likened to an animated spidei-'s web. The protoplasm is
invested in a very delicate and closely applied envelope. Any small
alimentaiy particles that may come into contact with the glutinous
surface of the pseudopodia are i-etained in adhesion by it, and
speedily partake of the general movement going on in their svib-
stance. This movement takes place in two principal directions —
from the body towards the extremities of the pseudopodia, and from

these extremities back to the
body again. In the larger
branches a double current miay
be seen, two streams passing
at the same time in opposite
directions ; but in the finest
filaments the current is single
and a gi'anule may be seen to
move in one of them to its very
extremity, and then to return,
perhaps meeting and carrying
'L^ y"^ W^^^i ! ^ back with it a granule that

,/^'^*'**~/ i ^^^w 1 \ was seen advancing in the

opposite direction. Even in
the broader processes granules
are sometimes observed to come
to a stand, to oscillate for a
time, and then to take a retro-
grade course, as if they had
been entangled in the opposing
current, just as is often to be
seen in Ghara. When a gi^anule
arrives at a point where a fila-
ment bifurcates, it is often arrested for a time, until drawn into one
or the other current ; and when carried across one of the bridge-
like connections into a difierent band, it not unfrequently meets a
current proceeding in the opposite direction, and is thus carried back
to the body without having proceeded very far from it. The
pseudopodia! network along which this ' cyclosis ' takes place is con-
tinually undergoing changes in its own arrangement, new filaments
being put forth in different directions, sometimes from its margin,
sometimes from the midst of its i-amifications, whilst others are
retracted. Not unfrequently it happens that to a spot where two or
more filaments have met, there is an afflux of the protoplasmic sub-
stance that causes it to accumulate there as a sort of secondary centre,
from which a new radiation of filamentous pi-ocesses takes place.
Occasionally the pseudopodia are entirely retracted, and all activity
ceases ; so that the body presents the appearance of an inert lump.
But if watched sufficiently long its activity is resvimed, so that it
may be presumed to have been previously satiated with food, which

Fig. 570.— Lieherkuehnia Wageneri.

RHIZOPODA 733

is undergoing digestion during its stationary pei-iod. No encysting
process has been noticed in Lieberhuehnia ; but Oienkowsky has dis-
covered that in L. jialudosa repi'oduction is effected by a process of
fission, which commences with the formation of a new pseudopodial
stalk at the base of the animal, the envelope being perforated at this
point. As the marine type of it occurs on our own coasts, the fresh-
water type may very likely be found in our ponds, and either may
be recommended as a most worthy object of careful study.

Rhizopoda.

We now arrive at the group of r/«'2:o/j)ocZs, or ' root-footed ' animals
first established by Dujardin foi- the recejDtion of the Amceha and its
allies, which had been included by Professor Ehrenberg among his
infusory animalcules, but which Dujardin separated fi-om them as
being mere particles of sarcode (protoplasm), having neither the defi-
nite body- wall nor the special mouth of the true Infusoria, but put-
ting forth extensions of their sarcodic substance, which he termed
•pseitdopodia (or false feet), serving alike as instruments of locomotion
and as prehensile organs for obtaining food. According to Dujardin's
definition of tliis grou^j, the Monerozoa, already described, would be
included in it ; but it seems on various grounds desirable to limit the
term Rhizopoda to those Protozoa in which the presence of a micleus
the difierentiation of an ectosarc (or firmer superficial layer of proto-
plasm) from the semi-fluid endosarc, together with the more definite
form and restricted size, indicate a distinct approach to the condition
of triie cells. Many diflferent schemes for the classification of the
rhizopods have been proposed, but none of them can be regarded
as entirely satisfactory, our knowledge of the reproductive processes,
and of other important parts of the life-history of these creatures,
being still extremely imperfect ; and as some parts of the scheme
proposed by the Author many years ago,^ based on the characters of
the pseudopodial extensions, have been accepted by more recent
systematists, it seems best still to adhere to it.

I. In the first division, Eeticidaria, the pseudopodia freely
ramify and inosculate, so as to foiin a network, exactly as in Lieber-
huehnia, from which they are distinguished by the jaossession of a
nucleus and by the investment of their sarcodic bodies in a firm
envelope. This is most commonly either a calcareous shell of very
definite shape, or a test built up of sand- grains or other minute
particles more or less firmly united by a calcareous cement exuded
from the sarcodic body. These testaceous forms, which are exclu-
sively marine, constitute the group of Foraminifera, whose special
interest . to the microscopist entitles it to sepai-ate consideration ;
and it is only for convenience that two Reticularia which in-
habit fresh water also, and the envelopes of whose bodies are
usually membranous, are here separated from the Foraminifera (to
which they properly belong) for description as types of the group.
The Reticularia have little locomotive power, and only seem to

^ Natural History Review, 18G1, p. 4.5G; and Introduction to the Studu of the
Foraminifera , 1862, chap. ii.

734 MICROSCOPIC FORMS OF ylxXBIAL LIFE -PROTOZOA

exercise it to find a suitable situation for their attachment, the
capture of their food being effected by their pseudopodial net-
work.

II. The second division, Heliozoa, consists of the rhizopods whose
pseudopodia extend themselves as straight radiating rods, having
little or no tendency to subdivide or ramify, though they are still
sufficiently soft and homogeneous (at least in the lower types) to
coalesce when they come into contact with each other. These have
usually (probably always) a contractile vesicle as well as a nucleus ;
and the higher forms of them are characterised by the enclosure of
symbiotic yellow corpuscles {zoochlorellce) in the substance of their
endosarc. By far the larger number of this group also have skeletons
of mineral matter, which are always siliceous ; and these are some-
times perforated casings of great I'egularity of form, as in the
marine Polycystina, sometimes internal frameworks of marvellous
symmetry, as in the marine Radiolaria. These two groups, also,
will be reserved for special notice', the simple Heliozoa, which
are among the commonest inhabitants of fresh water, furnish-
ing the best illustrations of the essential characters of the type.
They seem, for the most part, to have but little locomotive j)0wer,
capturing their j)i'ey by their extended pseudopodia. The tendency
of modern writers is to sepai-ate the Heliozoa, as here understood, into
the two grotips of Heliozoa (sens, strict.) and Radiolaria, the latter
being distinguished by the presence of a central capsule or mass of
protoplasm siirrounded by a special envelope, the bettei- develop-
ment of the skeleton, the greater tendency of the pseudopodia to
coalesce with one another, and the not unfrequent presence of ' yellow
bodies.'

III. Thfe third group, Lohosa, contains the rhizopods which most
nearly approach the condition of true cells, in the diffei-entiation of
their almost membranous ectosarc and their almost liquid endosarc,
and in the non-coalescence of their pseudopodial extensions, which,
instead of being either thread-like or rod-like, are lohate, that is,
irregular projections of the body, including both ectosarc and endo-
sarc, which ai-e continually undergoing change both in form and
number. The Lohosa are comparatively active in their habits, moving _
freely about in search of food, which is still received into the sub-
stance of their bodies through any part of their surface — unless this
is enclosed in envelopes such as are foi'med by many of them, either
by exudation from the surface of their bodies of some material
(probably chitinous) which hai-dens into a membrane, or by aggre-
gating and uniting grains of sand or other small solid pai-ticles,
which they build up into ' tests.' A large proportion of them are
inhabitants of fresh water, and some are even found in damp
earth.

Reticularia. — This type is very characteristically represented by
the genus Groniia (fig. 571), some of whose species are marine, and
are found, like ordinary Foraminifera, among tufts of corallines,
alga?, &c. ; whilst others inhabit fresh watei-, adhering to Oonfervse
and other plants of running streams. It was in this type that the
presence of a nucleus, formerly supposed to be wanting in Reticularia

/

GEOMIA

735

generally, was first established by Dr. Wallich. The sarcode-body
of this animal is encased in an egg-shaped, brownish-yellow, chitinons
envelope, which may attain a diameter of from i^g^th to Jjjth of an
inch, looking to the naked eye so like the egg of a zoophyte or the
seed of an aquatic plant, that its real nature would not be suspected
so long as it remained quiescent. The 'test' has a single round
oinfice, from which, when
the animal is in a state
of activity, the sarcodic
substance streams forth,
speedily giving ofl'ramify-
ing extensions, which, by
further ramification and
inosculation, form a net-
work like that of Lieber-
kuehnia. But the sarcode
also extends itself so as
to form a continuous
layer over the whole ex-
terior of the '■ test,' and
from any part of this
layer fresh pseudojjodia
may be given off. By
the alternate extension
and contraction of these,
minute protophytes and
protozoa are entrapped
and drawn into the in-
terior of the test, wh.ere
their nutritive material
is extracted and assimi-
lated ; and if the ' test
(as happens in some
species) be sufiiciently
transparent, the indi-
gestible hard pai'ts (such
as the siliceous- valves of
diatoms, shown in fig
571) maybe distinguished
in the midst of the sar-
codic substance. By the
same agency the Gromia
sometimes ci-eeps up the sides of a glass vessel. In the intervals of
quiescence, on the other hand, the whole sarcodic body, except a
film that serves for the attachment of the test, is withdrawn into its
interior.

Another example of the reticularian gTOup is afibrded by the
curious little Jlicrogromia socialis (fig. 572), first discovered by Mr.
Archer, and fui'thei' investigated with great care by Hertwig,^ which

Fig. 571. — Gromia oviforinis, with its
l^seudopodia extended.

1 'Uehev Microgroiui.li.,' in Arcldv fiir Mikr. Anat. bd. x. Sujiplement.

736 MICROSCOPIC FORMS OF ANIMAL LIFE— PROTOZOA

has the curious habit of uniting with neighbouring individuals by the-
fusion of the pseudopodia, into a common ' colony,' the individuals
sometimes i-emaining at a distance from one another as at A, but
sometimes aggregating themselves into compact masses as at B. The
nearly globular thin calcareous shell is prolonged into a short neck
having a circular orifice, from which the sarcode-body extends itself,

Fig. 572. — Microgromia sociaUs : A, colony of individuals in extended state,
some of tlieni undergoing transverse fission ; B, colony of individuals
(some of them separated from the principal mass) in compact state ; C, D,
formation and escape of swarm-spore, seen free at E.

giving ofl:' veiy slender pseudopodia which radiate in all directions.
A distinct nucleus can be seen in the deepest pai-t of the cavity ;
while a contractile vesicle lies imbedded in the sarcodic substance
nearer the mouth. Multiplication by duplicative subdivision has
been distinctly observed in this type ; but with a peculiar departure

HELIOZOA JT^'J

from the usual method. A transverse constriction divides the b;)dy
into two halves — as shown in two individuals of colony A — each half
possessing its own nucleus and conti-actile vesicle ; the posterior seg-
ment, which at first lies free at the bottom of the cell, then presses
forwai'ds towards its oi-ifice, as shown at 0, and finally, by amoeboid
movements, escapes from it, sometimes stretching itself out like a
worm (as seen at D), sometimes contracting itself into a globe, and
sometimes spreading itself out irregulai-ly ovei- the pseudopodia of
the colony. But it finally gathei'S itself togethei- and takes an oval
form ; and either develops a paii- of flagella, and foi-sakes the colony
as a free-swimming vionad, or assumes the form of an Actmophrijs,
moving about by thi-ee or four pointed pseudopodia — probably in
each case coming affcei- a time to I'est, excreting a shell, and laying
the fovindation of a new colony. Tliei-e is i-eason to think that a
multiplication by longitudinal fission also takes place, in which the
escaping segment and the one left behind in the old shell remain
attached by their pseudopodia, and the foi'mer develops a new shell
without undergoing any change of condition.

Heliozoa.^ — The Actinophri/s sol, sometimes termed the ' sun-
animalcule ' (fig. 573), is one of the commonest examples of this group,
being often met with in lakes, ponds, and streams, amongst Confei'Vfe
and other aquatic plants, as a whitish-grey spherical pai-ticle dis-
tinguishable by the naked eye, fi'om which (when it is bi-ought under
sufiicient magnifying power) a number of very pellucid, slender,
pointed I'ods are seen to i-adiate. The centi'al portion of the body is
composed of homogeneous sarcode, inclosing a distinct nucleus ; but
the pei-ipheral part has a ' vesiculai- ' aspect, as in the type next
to be described (fig. 574). This appearance is due to the number
of ' vacuoles ' filled with a watery fiuid, which are inchided in
the sarcodic substance, and which may be artificially made either
to coalesce into larger ones or to subdivide into smaller. A ' con-
tractile vesicle,' pulsating rhythmically with considei-able regu-
larity, is always to be distinguished, either in the midst of the
sarcode body, oi- (moi-e commonly) at or near its surface ; and
it sometimes projects considerably fi-om this, in the foiiii of a
sacculus with a delicate membranous wall, as shown at fig. 573,
A, cv. The cavity of this sacculus is not closed externally, but
communicates with the sui-rounding medium — not, however, by any
distinct and permanent orifice, the membranifoi-m wall giving way
when the vesicle conti-acts, and then closing over again. This alter-
nating action seems to serve a i-espiratoiy pui-pose, the water thus
taken in and expelled being disti-ibuted through a system of channels
and vacuoles excavated in the substance of the body, some of the
vacuoles which ai-e neai-est the surface being observed to undergo
distension when the vesicle conti'acts, and to empty themselves
gradually as it refills. The body of this animal is nearly motionless,^

' A systematic account of this group is to be found in Dr. F. Schaudinn's
' Heliozoa,' the first part of the comprehensive Das Thierreich, edited by the
German Zoological Society, Berlin, 189ti. M. Penard's memoir, ' Etudes sur quelques
Heliozoaires d'Eau Douce,' in vol. ix. of the Arch, cle Biol., should be consulted.

- A swimming Heliozoon has lately been described by M. E. Pennrd, who calls it
Myriophrys paradoxa .

3b

738 MICROSCOPIC FORMS OF ANIMAL LIFE — PROTOZOA

but it is supplied with nouiisliment by the instrunieutaHty of its
pseudopodia, its food being deiived not merely fi'om vegetable pai'-
ticles, but from various small animals, some of which (as the young of
Entomostraca) possess great activity as well as a comparatively high
organisation. When one of these happens to come into contact with
one of the pseudopodia (which have firm axis-filaments (ax) clothed
with a granular sarcode), this usually retains it by adhesion ; but the
mode in which the jiarticle thus taken captive is introduced into the
body dififers according to circumstances. If the pi'ey is large and
vigorous enough to struggle to escape fi'om its entanglement, it may
usually be observed that the neighbouring pseudopodia bend over and

Fig. 573. — Aciinophrijs sol : A, figure showing the wide vacuolated cortical
layer or ectosaxc (e) and the fine gi'anulated endosarc (m) ; n, central
nucleus, ax, axial filaments of pseudopodia ; cv, contractile vacuole ; n, food-
mass inclosed in a large food-vacuole. B, a colony of four individuals, after
treatment with acetic acid; r, m, and N,as before ; v, v, vacuoles. C, a cyst;
z, c, outer and inner envelopes. D, a burst cyst from which the young is
eacaj)ing, though still inclosed by the iamer envelo^De. (From Biitschli,
after Grenacher, Stein, and Cienkowsky.)

apply themselves to it, so as to assist in holding it captive, and that it
is slowly drawn by their joint i-eti'action towai'ds the body of its
captor. Any small particle not capable of ofiei-ing active i-esistance,
on the other hand, may be seen after a little time to glide towards
the central body along the edge of the pseudopodium, without aiw
visiljle movement of the latter, much in the same mannei- as in Gromia.
When in either of these modes the food has been bi-ought to the
surface of the body, this sends over it on either side a prolongation of

HELIOZOA

739

its own 8ai-code-sul)stiUice ; and thus a. marked })r()niinence is formed
(fig. 573, A, n), which gradually subsides as the food is drawn more
completely into the inteiior. The struggles of the larger animals,
and the ciliary action of Infusoria and Rotlfera, may sometimes be
observed to continue even after they have been thus received into
the body ; but these movements at last cease, and the pi-ocess of
digestion begins. The alimentary substance is l■ecei^"ed into one
of the vacuoles, where it lies in the first instance sui'rounded
by liquid ; jjnd its nuti-itive poi'tion is gradually converted into.
an indistinguishable gelatinous mass, which becomes incoi-poi-ated
with the material of the sarcode-body, as may be seen by the
general diflfusion of any colouring particles it may contain. Sevei-al

Fig. 574. — Act inosjihceiiicm Eichornii : m, endosarc :
c, c, contractile vacuoles.

r, ectosarc ;

vacuoles may be thus occupied at one time by alimentaiy particles ;
fi'equently four to eight are thus distinguishable, and occasionally
ten or twelve ; Ehrenberg, in one instance, counted as many as
sixteen, which he described as multiple stomachs. Whilst the
dige.stive pi-ocess, which usually occupies some hours, is going on,
a kind of slow circulation takes place in the entii-e mass of the endo-
sarc with its included vacuoles. If, as often happens, the body
taken in as food possesses some hard indige.stible portion (as the shell
of an entomostracan or rotifer), this, after the digestion of the soft
parts, is gradually pushed towards the sui'face, and is thence extruded
by a process exactly the converse of that by which it was drawn in.
If the particle be lai-ge, it usually escapes at once by an opening which

3 B 2

740 MICROSCOPIC FORMS OF ANIMAL LIFE — PROTOZOA

extemporises itself foi- the occasion ; but if small it sometimes glides
along a pseudopodium from its base to its point, and escapes from
its exti-emity.

The ordinary mode of reproduction in Actlnojjhrys seems to be
by binaiy subdivision, its sphei-ical body showing an annular con-
striction, which gradually deepens so as to separate its two halves by
a soi't of hour-glass constiiction, and the connecting band becoming
more and more slendei-, until the two halves are completely separated.
The segments thus divided are not always equal, and sometimes their
difference in size is very considerable. A junction of two individuals,
-on the other hand, has been seen to take place in Actinophrys, and
has been supposed to correspond to the ' conjugation' of protophytes ;
it is very doubtfid, however, whethei- this junction i-eally involves a
complete fusion of the substance of the bodies which take part in it,

Fig. 575. — Marginal portion of Actinosjiliceriwni Eichoniii
as seen in optical section under a higher magnifying
power: ;«, endosarc ; r, ectosarc ; a, a, a, pseudopodia;
n, n, nuclei with nucleoli ; /, ingested food-mass.

and there is not sufficient evidence that it has any true genei'ative
character. Under these circumstances we must hope that Dr. F.
Schaudinn's preliminaiy notes of his observations * may soon be
followed by a more detailed account. This author claims to have
demonstrated tlie fusion of the nuclei of A . sol, and the resemblance
of the course of events to the matui-ation of the ova of higher
animals is very striking. Certain it is that such a junction or
' zygosis ' may take place, not between two only, but even several
individuals at once, their number being i-ecognised by that of their
contractile vesicles ; and that, aftei- i-emaining thus united for several

1 SB. Akad., Berlin, 1890, p. 49.

HELIOZOA

741

liours as a colony, they may separate again without having
undergone any disco vei-able change. ,,^.

Undei- the geuei'ic name Actinophrys yy&n fovmei-ly I'anked the
larger but less common Heliozotin, now distinguished as Actino-
sphcerhmi Eichomli (fig. 574); the pseudopodia are longer and
more numei'ous ; thei-e are generally a numbei- instead of one con-
tractile vacuole, and there is moi'e than one nucleus. The axis of the
pseudopodia may be seen to be clothed with a layer of soft sarcode
dei'ived fi-om the super-
ficial or cortical zone of
the body. Several nuclei
('/?, 1%) ai'e usually to be
seen imbedded in the
protoplasmic mass. The
genei-al life-histoiy of
this type corresponds
with that of the pre-
ceding, but its mode of
rej^roduction presents
some marked peculiari-
ties. In man}' if not in
all cases it commences,
as first obsei'ved by
KoUikei', with the con-
j ugation of two sepai-ate
individuals. The binary
segmentation is pre-
ceded by a withdrawal
of the pseudopodia, even
their clearly defined
axis becoming indistinct
and finally disappeai-
ing ; the body becomes
enveloped by a cleai-
gelatinous exudation,
which forms a kind of
cyst ; and within this
the process of binary
subdivision is repeatedly
l^erformed, until the
original single mass is
replaced by a sort of

morula, each spherule of which shows the distinction between the
central and cortical i-egions, the former including a single nucleus,
whilst the latter is strengthened by siliceous deposit into a firm
investment. After remaining in this state during the winter the
young Actinosphceria come forth in the spring without this siliceous
investment, and gi-adually grow into the likeness of their parent.^

^ On the results of the artificial division of ActhiosjjJiceriamsee K. Brandt, ZTeber
Actinosphcerium Eirhornii, Halle a/S., 1877; Gruber, Berichte d. Nntiirf. Ges. zu
Freiburg i/B., IHSO ; Nnssbaum, Arch. f. Mikr. Aiiat. xxvi.

Fig. 576. — Clathruliiia elegans: A, complete
organism ; B, swarm-spore showing nucleus, n,
and two contractile vesicles near its opposite end.

742 MICROSCOPIC FORMS OF ANIMAL LIFE — PROTOZOA

,V I L

A large number of new and curious fresh-water forms of this
type are being fi-equently bi'ought under notice, of which the Clathru-
lina elegans (fig. 576) may be specially m.entioned as presenting an
obvious transition to the Polycystine type. This has been found
in various parts of the Continent, and also (by Mr. Archer ^) in
Wales and Ireland, occuriing chiefly in dark ponds shaded by
trees and containing decaying leaves. Its soft sarcode-body, which
is not differentiated into ectosai-c and endosarc, is encased by a
siliceous capsvile of sphei-ical form, regvilarly perforated with oval
apertures, and supported on a long silicified peduncle. The body
•itself and the pseudopodia which it puts forth through the aper-
tures of the capsule seem closely to correspond with those of
Actinophrys. Repi-oduction here takes place not only by binary
fission, but by the formation of ' swarm-spores.' In the first mode,
one of the two segments remains in possession of the siliceous cap-
sule, whilst the other finds its way out through one of the apertures,
lives for some houi-s in a fi-ee condition as an Actinophiys, and
ultimately produces the capsule and stem characteristic of its type.
In the second mode numerous small i-ounded sarcode masses, each
possessing a nucleus, ai-e produced within the capsule, in what
manner cannot be cleai'ly made out ; and every one of these is
'. enveloped in a firm en-

velope, set round with
short spines, probably
siliceous. These cysts
remain for months with-
in the common capsule ;
and when the time ai-rives
for their fui-ther develoj?-
ment the sarcode-cor-
puscles slip out of their
cysts, and escape thi'ough
the orifices of the capsule
as flagellated monads of
oval form (fig. 576, B),
each having a nucleus,
w, near the base of the
flagella, and two con-
tractile vesicles near its
opposite end. After sw^arming foi- some hours in this condition,
they change to the free Actinophrys form, and finally acquire the
siliceous capsule and stem of the Clathrulina.

Lobosa. — No example of the rhizopod type is more common in
streams and ponds, vegetable infusions, &c., than the Amceha
(fig. 577) ; a creature which cannot be described by its form, for
this is as changeable as that of the fabled Pi-oteus, but may yet be
definitely chai-acteiised by peculiai-ities that separate it fi-om the
two groups already desciibed. The distinction between ' ectosarc '
and ' endosarc ' is here clearly marked, so that the body approaches

J See his memoir on Fresh-water Radiolaria in Quart. Journ. of Microsc. Sri.
n.s. vol. ix. 1869, p. 250.

P<'

Fig. 577.— Diagrammatic representation of Amceha
\ proteus : E C, ectosarc ; E N, endosarc ; C V, con-
tractile vesicle ; N, nucleus ; P, pseudopodia ;
I V I L, villous tuft.

LOBOSA 743

much more closely in its cliai-actevs to un oi-<linaiy ' cell ' composed
of cell-wall and cell-contents. It is through the ' endosarc ' alone,
E ]Sr, thiit those coloured and granular particles are diffused, on
which the hue and opacity of the body depend ; its centi-al portion
seems to have an almost watery consistence, the granulai- pai-ticles
being seen to move quite freely upon one anothei* with eveiy change
in the shape of the body ; but its superficial portion is more viscid,
and graduates insensibly into the fii-mer substance of the ' ectosai-c.'
The ectosjxrc, E C, which is perfectly pellucid, forms an almost
membi-anous investment to the endosai-c ; still it is not possessed of
such tenacity as to oppose a solution of its continuity at any point, for
the introduction of alimentary particles, or for the extrusion of effete
matter ; ^ and thus there is no evidence, in ^^losSa and its immediate
allies, of the existence of anymore definite orifice, either oral or anal,
than exists in other rhizopods. The moi'e advanced differentia-
tion of the ectosarc from the endosarc of Amoeba is made evident
by the efiects of reagents. If an Aimeba radiosa be treated with
a, dilute alkaline solution, the granular and molecular endosaic
shrinks together and retreats towards the centi'e, leaving the radia-
ting extensions of the ectosarc in the condition of cfecal tubes, of
which the waUs are not soluble at the ordinary temperature either
in acetic or mineral acids, or in dilute alkaline solutions, thus
agreeing with the envelope noticed by Cohn as possessed by Para-
fiiecium and other ciliated Inftisoria, and with the containing mem-
brane of ordinary animal cells. A ' nucleus,' N, is always distinctly
visible in Amoeba, adhei'ent to the inner portion of the ectosarc, and
projecting from this into the cavity occupied by the endosarc : when
most perfectly seen it pi-esents the aspect of a clear flattened vesicle
surrounding a solid and usually spheiical nucleolus ; it is readily
soluble in alkalies, and first expands and then dissolves when treated
with acetic or sulphuric acid of moderate strength ; but when
treated with dilute acid it is rendei-ed darker and more distinct, in
consequence of the precipitation of a finely granular substance in
the clear vesicular space that surrounds the nucleolus. A ' contrac-
tile vesicle,' 0 Y, seems also to be uniformly present, though it
does not usually make itself so conspicuous by its external prominence
as it does in Actinojihrys ; and the neighbouring part of the body
is often prolonged into a set of villous processes, V I L, the presence
of which has been thought by some to mark a specific distinction,
but which seems too variable and transitory to be so regarded.

The pseudopodia, which are not appendages, but lobate exten-
sions of the body itself, are few in number, short, broad, and i-ounded ;
and their outlines present a sharpness which indicates that the
substance of which their exterior is composed possesses considerable
tenacity. No movement of granules can be seen to take place along
the surface of the pseudopodia ; and when two of these organs come

1 This remarkable character has been stated by Professor Huxley in the following
admirable sentence : ' Physically the ectosarc might be comjjared to the wall of a
soap-bubble, which, though fluid, has a certain viscosity, which not only enables its
' particles to hold together and form a continuous slieet, but permits a rod to be passed
into or through the bubble without bursting it, the walls closing together, and re-
covering their continuity as soon as the rod is drawn away.'

744 MICROSCOPIC FORMS OF ANIMAL LIFE — PROTOZOA

into contact tlie}^ scarcely show any disj^osition eyen to mutual
cohesion, still less to fusion of their substance. Sometimes the
protrusion seems to be foi-med by the ectosarc alone, but more
commonly endosarc also extends into it, and an active cui-i-ent of
granules may be seen to pass from what was previously the centre
of the body into the proti-uded portion, when the latter is undergoing
rapid elongation ; whilst a like current may set towards the centre
of the body from some other protrusion which is being withdrawn
into it. It is in this m.anner that an Amceba moves from place to
place, a protrusion like the finger of a glove being first formed, into
which the substance of the body itself is gradually transferred, and
another protrusion being put forth, either in the same or in some
different direction, so soon as this transfei-ence has been accom-
plished, or even befoi'e it is complete. The kind of pi-ogression thus
executed by an Amoeba is desci-ibed by most observers as a ' rolling '
movement, this being certainly the aspect which it commonly
seems to present ; but it is maintained by MM. Claparede and
Lachmann that the appeai-ance of rolling is an optical illusion,
since the nucleus and conti-actile vesicle always maintain the same
position relatively to the rest of the body, and that ' creeping ' would
be a truer description of the mode of progression. It is in the
course of this movement from place to j)lace that the ATnoeba en-
counters particles which are fitted to afibixl it nourishment ; and it
appeal's to i-eceive such particles into its interior through any part
of the ectosarc, whether of the body itself or of any of its lobose
expansions, insoluble particles which resist the digestive process
being got rid of in the like pi'imitive fashion.

It may often be seen that poi-tions of the sarcode-body of an
Amoeba, detached from the i-est, can maintain an independent exist-
ence ; and it is probable that such separation of fragments is an
ordinary mode of increase in this gi-oup. When a pseudopodial lobe
has been put forth to a considerable length, and has become en-
larged and fixed at its extremit}', the subsequent contraction of the
connecting portion, instead of either di-awing the body towai'ds the
fixed point, or retracting the lobe into the body, causes the connect-
ing band to thin away until it separates ; and the detached portion
speedily shoots out pseudopodial processes of its own, and compoi-ts
itself in all respects as an independent Amceba. Multiplication
also takes place by regular binary subdivision. Various observei's
have seen phenomena which they have supposed to be evidence of the
formation of ' swarm-spores ' ' oi- of the developnaent of cysts, but it
must be borne in mind that a lai-ge number of protozoa pass during
the course of theii- life through amcebiform stages, some of which
may have been taken as ti-ue species of Ama:ba. No sexual act has
})een certainly i-ecognised as pai-t of the life-histoi-y of Amceba, the
union of two or more individuals, which may be occasionally wit-
nessed, having moi-e the charactei- of the ' zygosis ' of Actinophrys.
A sai'codie oi-ganism discovered by (h-eei, and named by him
PeloTnyxci pcdustris (fig. 578), which spreads oA'er the bottom of
stagnant ponds in the condition of slimy masses of indefinite form,
1 Prof. A. M. Edwards (U.S.A.) in Monthly Mirrosc. Joiini. vol. viii.1872, p. 29.

LOBOSA

745

exhibits a fui-ther ad\ance upon the Amoeban type. The substance
of its body, which may be of the size of two millinietres, exhibits a
very clear differentiation between the homogeneous hyaline ectosai-c
(B, ff, d) and the contained endosarc, which contains such a multi-
tude of spherical vacuoles, b. as to have a ' vesiculai- ' or fi-oth}'
aspect. When it feeds upon the decomposing vegetable matter at
the bottom of the pool it iidiabits, its body acquires a blackish hue, •
but in other sitiiations it may be colourless. Besides the vacuoles
thei-e ai'e seen in the endosarc a great numbei- of nucleus-like bodies,

Fig. BTS.— Pelomyxa paJnsti-is: A, as it ai^i^eai-s when in amoeboid
motion; B, portion more highly magnified, showing a, a , the hyaline
ectosarc ; h, one of the vacuoles of the endosarc ; c, rod-like bodies (pro-
bably Bacteria) scattered through the endosarc ; rf, protruded exten-
sion of ectosarc with endosarc passing into it ; e, e, nuclei ; /, /', globular
hyaline bodies.

a, e, and also many hyaline globular brilliant bodies, _/, f, which are
i-egarded by Greef as germs or swarm-spores developed from nucleoli
set fi-ee within the general cavity of the body by the bursting of the
nuclei. Tliis creature during the active period of its life moves like
an amoeba, either by general undulations of its surface, or by special
psevidopodial extensions, d. After a time, howevei', its movements
cease, and it looks as if dead ; but by the giving way of its ecto-
sarc, a multitude of minute amcebiform bodies break forth, each
] laving its nucleus and contractile vesicle. These at first live as
Awcehce., but afterwards pass into a resting state, assuming a spherical

746 MICROSCOPIC fOEMS OF ANIMAL LIFE - PEOTOZOA

or oval shape, and then put forth flagella, by which they swim
actively foi- a time ; latei- on, they probiibly settle down to develop
themselves into the parental foi-m.

The Amoeban like the Actinophiyan type shows itself in the
testaceous as well as in the naked form, the commonest examples
of this being known under the names Arcella and Difflugia. The
body of the former is inclosed in a ' test ' composed of a horny
membrane, apparently resembling in constitution the chitin which
gives solidity to the integuments of insects ; it is usually discoidal
(fig. 579, 0, D) with one face flat and the other arched, the aperture
being in the centre of the flat side ; and its surface is often marked
with a minute and i-egular pattern. The test of Difflugia, on the
other hand, is more or less pitcher-shaped (A, B), and is chiefly
made up of minute particles of gi-avel, shell, &c. cemented together.
In each of these genei'a the sarcode-body resembles that of Amoeba
in every essential particular, the contrast being very marked be-
tween its large, distinct, lobose extensions, and the ramifying and
inosculating pseudopodia of Groniia (fig. 571). In each case a de-
tached poi-tion of the sai-codic body will put forth pseudopodia of

Fig. 579. — Testaceous forms of Amushcm rliizopods : K, Difflugia
proteiformis ; "B, Difflugia obloiiga; C, Arcella acuminata; D,
Arcella clentata.

its own type ; and the separation of a bud or gemmule put forth
from the mouth of the test seems to be an ordinary mode of propa-
gation among the amoebans thus inclosed. In Arcella it has been
observed that the pseudopodia of two or more individuals unite by
bridges of pi'otoplasm, and afterwards separate ; and it seems to be
alm.ost certain that this is a true ' conjugation,' and not a mei-e
' zygosis.' A remarkable method of i-eproduction has been observed
by Gruber in Eugly2)ha alveolata ; in an active form highly refractive
bodies, which, seen from the surface, look like discs, are to be found
beside the nucleus. Reproduction commences with the protrusion
of protoplasm from the orifice of the test, and, later on, the just-
mentioned bodies pass out also, and form a covei-ing for the extruded
protoplasm ; in about an houi- the process is complete, but the new
or daughter-cell is still without a nucleus. This is derived from that
of the mother, which increases in size, elongates greatly, and then
becomes consti-icted ; the anterior poi'tion passes into the daughter-
cell. Here we have the i-emai'kable phenomena of the formation of
the test by the parent-cell and the rare case of division of the
protoplasmic body preceding that of its nucleus.

COCCOLITHS AND COCCOSPHERES

747

Many testaceous anuehans have been recently discovei-ed, which
foi'm tests of remai-kahle regularity and sometimes of singular
beauty ; and it is difficult to determine, in many cases, whether the
minute plates of which they are composed have been foi-med by
exudation from their own bodies or have been picked ixp from the
surface ovei- which the animals crawl. There can be no doubt of
this kind, howevei-, in i-egard to the Quadrula symmetrica repi-e-
sented in fig. 580 ; the sarcode-body is here encased in a pear-shaped
test, of glassy transparence, made up of a great number of squai-e
plates which touch each other by their edges. The sarcode-body
does not usually fill the test, the intervening space being occupied
by a clear liquid, and traversed by bands of pi-otoplasm. In the
posterior part of the body is seen a large cleai' spherical nucleus,
with a distinct dark nucleo-
lus ; and in fi'ont of this ai-e
contractile vesicles, usually
two in number.^

CoGColiths and Cocco-
spheres. — This would seem
the most appropi'iate place
for the description of cei'tain
peculiar little bodies found
very extensively diftused over
the deep-sea bottom, espe-
cially abounding in the Glo-
bigerina-mud, which may be
considered as chalk in process
of formation. It was in the
specimens of this mud
brought up by the ' Cyclops '
soundings in 1857 that Pro-
fessor Huxley first found
the Coccoliths (fig. 581, 1, 2)
which Dr. Wallich in 1860
found aggregated in the
spherical masses which he
designated as ' coccospheres '
(3). Regarding the gelati-
nous matrix in which they

were imbedded as a new type of the Monerozoa described by Haeckel,
having the condition of an indefinitely extended 2ilasmjodmm\ Pi'o-
fessor Huxley proposed to designate it by the name Bathyhius,
indicative of its habitat in the depths of the sea ; and this idea was
accepted by Haeckel, whose representation of a living specimen of
Bathyhius^ with imbedded coccoliths, is given in fig. 581, 3. The
observations made in the ' Challenger ' Expedition, however, have
not confirmed this view ; the supposed Bathyhius being a gelatinous

1 See especially the admirable work of Professor Leidy on the fresh-water
rliizopods of the United States, 1880. It is to be regretted that its able author's time
and opportunities did not permit him to follow out the life-histories of the many
interesting forms which he has described and figured.

Fig.

). — Quadrula symmetrica, with
extended pseudopodia.

748 MICROSCOPIC FOIiMS OF ANIMAL LIFE— PROTOZOA

precipitate, consisting of sulphate of lime, slowly deposited in water
to which strong spirit has been added. Whatever be their nature,^
coccoliths and coccospheres are bodies of great interest ; since their
occuri-ence in chalk and in very early limestones is an additional link
in the evidence of the similarity of the conditions under which they
were formed to those at present prevailing on the sea-bed of the
Atlantic and other oceans. Two distinct types are recognisable among
the coccoliths, which Professor Huxley has designated respectively
discoliths and cyatholiths. The foi-mer are round or oval discs, having
a thick strongly i-efi-acting rim and a thinner internal portion, the
greater part of which is occupied by a slightly opaque, cloud-like
patch lying round a central corpuscle (fig. 518, 5). In general, the
' discoliths ' are slightly convex on one side, slightly concave on the
other, and the rim is raised into a prominent ridge on the more

Fig. 581. — Coccoliths and Coccospheres: 1, 2, 7, cyatholiths seen
obhquely ; 3, coecosphere with imbedded cyathohths ; 4, coccoliths im-
bedded in supposed protoplasmic expansion ; 5, discolith seen in front
view; 6, cyatholith seen in front view, showing (1) central corf)Uscle, (2)
granular zone, (3) transparent outer zone ; 8, 9, discoliths seen edgewise.

con vex. side ; so that when viewed edgewise they pi-esent the appear-
ances shown in figs. S, 9. Their length is ordinarily between 47700^^11
and >, Jy yth of an inch ; but it i-anges from ^to 0*^^ ^^ tto^o^^- '^^^
lai-gest are commonly free, but the smallest ai-e generally found im-
bedded among heaps of gi'anular pai-ticles, of which some are probably
discoliths in an eai'ly stage of development. The ' cyatholiths,' also,
which have the genei-al appeai-ance of a cup and saucei-, have, when
full grown, an oval contoui-, though they are often circular when
immature. They ai-e convex on one face and flat oi* concave on the
other; and when left to themselves they lie on one or other of these
two faces. In either of these aspects they seem to be composed of
two concentric zones (fig. 6, 2, s) sui-i-ounding an oval thick- walled
(central corpuscle (•/), in the ceiitre of which is a clear space some-

' Messrs. Murray and Blaclcman have, in a preliminary notice {Proc. lioij. Soc,
Tjondon, Ixiii. 1898, p. 269), suggested that the Coccospheracese are unicellular Algie.

SPOROZOA 749

times divided into two. The zone (j) iiniuediately suri'oiiiulin^ij
the central corpuscle is usually moi'e or less distinctly gi'anular.
and sometimes has an almost bead-like margin. The nai-rowei-
outer zone (5) is generally cleai', ti-ansparent, and structui'eless.
but sometimes shows radiating strife. When viewed sidewise or
obliquely, however, the ' cyatholiths ' are found to have a form
somewhat resembling that of a shirt-stud (figs. 1, 2, 7). Each con-
sists of a lower j)late, shaped like a deep saucei- or watch-glass ; of
a smaller upper plate, which is sometimes flat, sometimes moi-e' oa-
less concavo-convex ; of the oval, thick-walled, flattened corpuscle,
which connects these two plates together at their centres; and of
an intermediate granular substance which more or less completely
fills up the interval between the two plates. The length of these
cyatholiths ranges fi-om about yyL_.th to n^'^y^th of an inch, those of
TToVo *^f ^^^ inch and under being always circular. It appears
from the action of dilute acids upon the coccolitlis that they must
mainly consist of calcareous matter, as they readily dissolve, leaving
scarcely a trace behind. When the cyatholiths are treated with
very weak acetic acid, the central corpuscle rapidly loses its strongly-
refracting character ; and there remains an extremely delicate,
finely granular membranous framework. When treated with iodine
they are stained, but not very strongly, the intermediate suli-
stance being the most affected. Both discoliths and cyatholiths are
completely destroyed by strong hot solutions of caustic potass oi'
soda. The coccospheres (fig. 3) are made up by the aggregation of
bodies resembling ' cyatholiths ' of the largest size in all but the
absence of the granular zone ; they sometimes attain a diameter of
y|-^th of an inch. What is their I'elation to the coccoliths, and
under what conditions these bodies are formed, ai-e questions whei-eon
no positive judgment can be at present given.

Sporozoa.

The term Sporozoa was applied by Leuckart to a group of
protozoic animals of which the well-known Gregarinida, the Coccidi-
idea, the Ha?mosporidia, the Myxosporidia, and the Sai'cosporidia ^ are
the chief divisions. They are especially characterised by the peculi-
arities of their mode of reproduction, in which a pei-iod of encystation
(which may or may not be preceded by conjugation) is succeeded l^y
the breaking up of the contained protoplasm into a lai-ge number of
small ' spores,' the products of which become intracellular parasites.

The Gregarinida lead a parasitic life, and may often be met with
in the intestinal canal or othei- cavities of earthworm, insects, etc.,
and sometimes in that of higher animals. An individual Grec/arina
essentially consists of a large single cell, usually more or less ovate
in form, and sometimes attaining the extraordinary length of two-
thirds of an inchP' A sort of beak or proboscis frequently projects
from one extremity ; and in some instances this is furnished with a

1 Consult the memoir by Dr. R. Blaucliard in Bull. Soc. Zool. France, x. j). '244.

- See Prof. Ed. Van Beneden on Gregarina gigantea (found in the intestinal
canal of the lobster) in Quart. Journ. Microsc. Sci. n.s. vol. x. 1870, p. ,51, and vol.
xi. p. 242.

750 MICROSCOPIC FORMS OF ANIMAL LIFE — PROTOZOA

circular row of hooklets, closely resembling that whicli is seen on
the head of Taenia. There is here a much more complete differentia-
tion between the cell-membrane and its contents than exists either
in Actinophrys or in Amoeba ; and in this respect we must look upon
Gregarina as representing a decided advance in organisation. Being
nourished upon the juices already prepared for it by the digestive
operations of the animal which it infests, it has no need of any such
ajDparatus for the introduction of solid particles into the interior of
its body, as is provided in the ' pseudopodia ' of the rhizopods and
in the oral cilia of the Infusoria. Within the cavity of the cell,
whose contents are usually milk-white and minutely granular, there
is generally seen a pellucid nucleus ; and when, as often happens,

Fig. 582. — Cyst of Monocystis agilis, the Gi-egannid of the earthworm
(750 dianis.), showing ripe chlamydospores and complete absence of
any residual protoplasm hi the cyst. (After Professor Ray Lankester.)

the cell undergoes duplicative subdivision, the process commences in
a constriction and cleavage of this nucleus. The membrane and its
contents, except the nucleus, are soluble in acetic acid. The move-
ments of the body are of very various kinds ; there is a forward
movement which may be due, as suggested by Lankester, to the
undulations of the body. The cell itself may undergo contraction, and
consequent change in form, which may, or may not, be accompanied
by locomotion ; cii'cular constrictions may extend along the body ;
oi- the cell may Ijend on itself and again stiuighten out. By Van
Beneden the contiuctility of the cell is localised in a layer of the

SPOKOZOA

751

ectoplasm, the so-called ' myocyte ' wliicli he has found to consist of
a layer of contractile fibrils. When the pi-ocess of encystation com-
mences we find that, whatever the original form of the body may be,
it becomes globular, ceases to move, and becomes invested by a
structureless ' cyst,' within which the substance of the body under-
goes a singular change. The nucleus disappears, and the sarcodic
mass breaks up into a series of globular particles, which gradually
resolve themselves (as shown at h, c, cZ, e, fig. 583) into forms very
like those of Naviculci', and a cyst more advanced, and greatly
magnified, is shown in fig. 582. These ' pseudo-navicella? ' or
' spores,' as it is better to call them, are set free in time by the
bursting of the capsule that incloses them ; and they develop them-
selves into a new generation of (iiegaiiUce, fiist pa^snig thi'ough an

Pig. 583. — Gregarina Scennridis, from testis of Tubifex rivulorum,
two adults uniting : ft, succeeding stage ; h, encystation stage ; in c
and d the contents are seen breaking uj) ; in e the characteristic
pseudo-navicellar form has been acquired by the spores. (After
Klilliker.)

amceba-like stage. A sort of ' conjugation ' has been seen to take
place between two individuals whose bodies, coming into contact
with each other by coi'responding points, first became more globular-
in shape, and are then encysted by the formation of a capsule around
them both ; the partition-walls between their cavities disappear ;
and the substance of the two bodies becomes comj)letely fused
together. But as the products of this ' zygosis ' are the same as that
of the ordinary encysting process, there seems no sufiicient reason

b

752 MICROSCOPIC FOEMS OF ANLMAL LIFE — PROTOZOA

for regarding it, like the ' conjugation ' of protophytes, as a true
generative act.

The Coccidia (fig. 584) are Sporozoa which look like minute ova,
and which ai-e found i-esting within the cells of their ho.sts ; the young,
developed from spoi-es, ai-e falciform in shape, and, moving about
actively, are able to penetrate fresh cells. They have been found in
the epithelium of the intestine of various forms, and in the livei- (jf
vertebrates. Some parasites found in the blood (HfemanKebidse),
such as DrejKmidium ranarum. Lankester, are allied to the Coccidia,
but are distinguished by having naked spores. Their chief interest
lies perhaps in their relation to various forms of malai-ia.^ Among

Fig. 584. — Cocckliiiui. oviforme (Leuckavt) from the liver of the rabbit :
«, cyst just formed; i*, condensed contents, the outer envelope has
disappeared ; c, contents divided into four sporoblasts ; d, the sporo-
blasts have become rounded and clearer internally; e and /, formation
of the falciform germ ; g and h, spores more lughly magnified — g from
the side, h from in front.

the Myxosporidia is Gluyea, the cause of the silkworm disease. The
Sarcosporidia are only known from the striped muscuiai' tissue of
some vertebrates.

Of the imperfectly known Myxosporidia it may be said that
their spores are the bodies which are known as ' psorosperms ; '
while the bodies obsei-ved by Rainey and others, and wrongly
regai'ded as the caiise of the cattle plague, are sarcocystids which
live in the muscuiai- fibre of mammals.

1 More and more interest is being taken in this subject, and some of the results of
recent researches are of great interest and importance. Malaria ajipears to be due to a
HiBinamtjebid which develops m gnats of the genus Anopheles ; when they arrive in the
human subject they appear as minute amcebulfe which live in or on the red blood
corpuscles ; they give rise to sporocytes which multiply indefinitely, or to sexual
gametocytes which undertake their sexual functions as soon as they enter the
stomach of gnats. See Ross and Fielding Quid, Quart. Journ. Micr. Sci. xliii.
(1900) p. 571, and a very interesting ' Note on the Morphological Significance of the
Various Phases of Hsemamoebidae,' by E. Ray Lankester, torn. cit. p. 581. The
student should also consult M. A. Labbe's ' Recherches Zoologiques, Cytologiques
et Biologiques sur les Coccidies,' in Arch. Zool. Exper. 1896, p. 517 et seq., and Dr.
Wasielewski's Sporoznerikiuule, Jena, 1896. A detailed bibliography will be found in
Prof. G. Schneideniiihl's Die Protozoen als Kninkheitserreger, Leipzig, 1898. The
various Memoirs of Grafisi, Laveran, and Leger may be profitably studied.

75.

CHAPTER XIII

ANIMALCULES— INFUSOBIA AND BOTIFEBA

Nothing can be more vague or scientifically inappropriate than the
title Animalcules ; since it only expresses the small dimensions of
the beings to which it is ajjplied, and does not indicate any of their
characteristic peculiarities. In the infancy of microscopic know-
ledge, it was natural to associate together all those creatures which
could only be discei'ned at all under a high magnifying power, and
whose internal structure could not be clearly made out with the
instruments then in use ; and thus the most heterogeneous assem-
blage of plants, zoo^^hytes, minute crustaceans, larv?e of worms,
molluscs, etc., came to be aggregated with the true animalcules
under this head. The class was being gradually limited by the
removal of all such forms as could be referred to others ; but still
very little was known of the real nature of those that remained in
it until the study was taken up by Professor Ehrenberg, with the
advantage of instruments which had derived new and vastly im-
proved capabilities fi-om the application of the piinciple of achro-
matism. One of the first and most important results of his study,
and that which has most firmly maintained its ground, notwith-
standing the overthi'ow of Professor Ehrenberg's doctrines on other
points, was the separation of the entire assemblage into two distinct
groups, having scarcely any feature in common except their minute
size, one being of very loio, and the other of comparatively high
organisation. On the lower group he conferred the designation oT
Polygastrica (many-stomached), in consequence of having been led
to form an idea of their organisation which the united voice of the
most trustworthy obsei'vers now pronounces to be ei'i'oneous ; and
as the retention of this term must tend to perpetuate the error, it
is well to fall back on the name I/nfusoria, ov infusory animalcules,
which simply expresses their almost universal prevalence in infusions
of organic matter. To the highei' group Professoi- Ehi'enberg's
name Rotifera or Rotatoria is, on the whole, vei'y appi-opriate, as
significant of that pecidiar arrangement of their cilia upon the
anterior parts of their bodies, which, in some of their most common
forms, gives the appearance (when the cilia are in action) of wheels
in revolution ; the gi'oup, however, includes many members in which
the ciliated lobes ai-e so formed as not to bear the least i-esemblance
to wheels. In theii- general organisation these ' wheel -animalcules '
stand at a much higher level than the unicellulai' Infusoi'ia, but it

3 c

754 MICROSCOPIC FOEMS OF ANIMAL LIFE

is difficult to decide what is their relationship to other groups of
animals. Notwithstanding the wide zoological separation between
these two kinds of animalcules, it seems most suitable to the plan
of the present work to ti'eat of them in connection with one another ;
since the microscopist continually finds them associated together,
and studies them under similar conditions.

Section I. — Infusoria.

This term, as now limited by the sepai-ation of the Bhizojyoda
on the one hand, and of the Rotifera on the otlier, is applied to a
far smaller range of forms than was included by Professor Ehren-
bei'g under the name of ' polygastric ' animalcules. For a large
section of these, including the Besmidiacece, Diatomacece, VolvociQiece,
and many other pi-otophytes, have been ti-ansferi-ed by general
(though not univei'sal) consent to the vegetable kingdom. And
it is not impossible that many of the reputed Infasoria may be but
larval forms of highei- organisms, instead of being themselves com-
plete animals. Still an extensive group remains, of which no other
account can at present be given than that the beings of which it is
composed go through the whole of their lives, so far as we are ac-
quainted with them, in a grade of existence which is essentially
protozoic, each individual aj)j3arently consisting of but a single cell,
though its parts ai'e often so highly differentiated as to represent
(only, however, by way of analogy) the ' organs ' of the highei-
animals after which they ai-e usually named.

Among the ciliate Infusoiia, Avhich foi-m not only by far the
largest, but also the most characteristic division of the group, there
is pi'obably none save such as are degraded by parasitic habits
which has not a mouth, ov permanent orifice for the introduction
of food, which is driven towards it by ciliaiy cui-rents ; while a
distinct anal orifice, for the ejection of the indigestible residue, is
not infrequently present. The mouth is often furnished with a
dental armature, and leads to an cesojjhageal canal, down which
the food passes into the digestive cavity. This cavity is still
occupied, however, as in rhizopods, by the endosarc of the cell ; but
instead of lying in mere vacuoles formed in the midst of this, the
food-particles are usually aggi-egated, dui-ing their passage down
the oesophagus, into minute pellets, each of which receives a special
investment of fii'in pi-otoplasm, constituting it a digestive vesicle
(fig. 589) ; and these go through a sort of circulation within the
cell- cavity.

The ' contractile vesicles,' again, attain a much higher develop-
ment in this group, and are sometimes in connection with a netwoi-k
of canals channelled out in the ' ectosai-c ; ' while theii' rhythmical
action resembles that of the circulatory and resinratory apparatus
of higher animals. Thei-e is ample evidence, also, of the presence
of a specially conti'actile modification of the protoplasmic substance,
having the action (though not the structin^e) of vmiscular fibre ;
and the manner in which tlie movements of the active fi'ee-swimming
Infusoria are directed so us to avoid obstacles and fi'nd ()ut passages

L

INFUSOEIA 755

seems to indicate that another poi'tioii of tlieir pi-otoplasmic sub-
stance must have to a certain degree the special endoAvments whicli
characterise the nervous systems of highei- animals. Altogethei', it
may be said that in the ciliate Infusoria the life of the single cell
H,nds its highest expression.^

Before proceeding to the description of the ciliate Infusoria,
however, it will be of advantage to notice two STnallei- groups — the
flagellate and the sactorial — which, on account of the peculiarities
of their structure and actions, are now ranked as distinct, and of
whose ' unicellulai- ' character there can be no reasonal)le doubt,
since they ai'e. foi' the most part, ' closed' cells, scarcely distinguish-
able mor])hologically fi'om those of protophytes.

Flagellata. — Our knowledge of this tribe has been greatly aug-
mented in recent years, not only by the discovery of a gi-eat variety
of new forms, but still more by the careful study of the life-history
of several among them. The m-onads, pi-operly so called, ^ which are
amongst the smallest living things at present known, are its simplest
representatives ; but it also includes organisms of much greater
complexity ; and some of its composite forms seem, to have a vei-}'
remarkable relation to sponges. The Monas lens, long familiai-
to microscopists as occurring in stagnant Avaters and infusions of
decomposing organic mattei-. is a spheroidal particle of protoplasm,
from -^^^th to - ^yVir'tb of an inch in diameter, enclosed in a delicate
hyaline investment oi- ' ectosai-c,' and moving freely through the
water by the lashing action of its f^ewAev flagellum, whose length
is from three to five times the diametei- of the body. Within the
body may be seen a variable number of vacuoles ; and these ai'e
occasionally occupied by pai'ticles distinguishable by their colour,
which have been introduced as food. These seem to enter the body,
not by any definite mouth (oi- permanent opening in the ectosarc).
but through an aperture that forms itself in some part of the oral
region near the base of the flagelluni. In some true Monadino'
neither nucleus nor contractile vesicle is distinguishable, but in
the majority a nucleus can be clearly seen. The life-history of
several simple Monadince presenting themselves in infusions of
decaying animal mattei- (a cod's head being found the most pro-
dvictive material) has been studied with admirable perseverance

1 The docti'ine of the unicellulai' nature of the Infusoria has been a subject of
keen controversy amongst zoiilogists from the time when it was first definitely put
forward by Von Siebold [Lehrbiieh der vergleich. Anat. Berlin, 1845) in opjDOsition
to the then paramount doctrine of Ehrenberg as to the complexity of their organisa-
tion, which had as yet been eallecl in question only by Dujardin {Hist. Nat. des
Infusoires, Paris, 1841). Of late, however, there has been a decided convergence of
opinion in the direction above indicated ; which has been brought about in great
degree by the contrast between the profo.roic simplicity of the rejiroductive and de-
velopmental jjrocesses in Infusoria, as, for example, shown by Dallinger and Drysdale,
and by the former alone in the life-histories of the Saprophytes, and the complexity
of the like processes as seen in even the lowest of the Metazon, whicli has been
specially and forcibly insisted on by Haeckel (' Zur Morphologie der Infusorien,"
■Jenaische Zeitschr. Bd. vii. lS7:-5). An excellent summary of the whole discussion
was given by Professor Alliiian in his Presidential Address to the Linnean Society in
1875. ■

- The family Monadina of Ehrenberg and Dujardin consists of an aggregate of
forms now known to be of very dissimilar nature, many of them belonging to the
vegetable kingdom

3 c 2

756 MICEOSCOPIC FOEMS OF ANIMAL LIFE

and thoroughness by Messis. Dallinger and Diysdale, of whose im-
portant observations a general summary will now be given. ^

The present Editor adopts the lead of Dr. Carpenter, in
ari^anging the saprophytic monad forms in this place in the organic-
series. They possess features that ally them, as has been already
suggested, to the vegetable series, and indicate affinities with
certain Nostocacese and the Bacteria.

There are some reasons foi' looking at the saprophytic monad
forms as a possibly degiuded but still specialised group. In common
with saprophytic Bacteria, they are specifically related to the setting
up and carrying on of decomposition in dead organic tissues. In
organic infusions and films of gelatine, or tubes of agar-agar, the
bacterial forms are, as a rule, enough to set up and carry on the
destructive ferment. But where gi-eat masses of tissue are decomi-
posing, the presence of the largei- monad forms is certain and in-
evitable ; and by them, accompanied by the Bacteria, the processes
of fermentative I'otting ai'e carried to the end.

- It is their morpholog}' which points to the Flagellata, and we
should incline to considei- them a degenerate, and by degeneration
specialised form of the Flagellata if they — about eight or nine dis-
tinct forms in this latitude — belong' properlv to the Flagellata at
all.

The simplest of these oi-ganisms is represented in fig. l, Plate
XY, A. It has been named by Saville Kent Monas Dallingeri,
and has by comparison a simple life-history. As it is with the
entire grovip, all is subservient to rapidity of multiplication ; and
there are two methods in which this is effected. The first and com-
monest is by fission ; fig. 1, A, represents the normal form of the
oi'ganism. It has a long diameter of about the e oVo^^^ ^^ ^^ inch,
and has great ease and grace, and relative power of movement.
In a certain stage of its history as it swims freely there suddenly
appears a constriction across its body, as in fig. 2. This is at once
accompanied by an appai'ent effort of the opposite flagella to pull
against each other ; the consequence is a very rapid stretching of
a. neck of sarcode between two halves of the body, as at fig. .3. This
becomes longer, as at 4, and attains the length of two flagella as at
.5. when the two dividing halves approach aiid mutually dart from
each other, snapping the connecting fibre of sarcode in the middle,
so that two perfect forms are set free, as in fi and 7.

This, in the course of from two to three minutes, is once more
begun and carried on in each half successively, so that there is an
increase of the form by this means in I'apid geometric ratio.

But this is an exhaustive pi'ocess vitally, for after a period vary-
ing from eight to ten days thei'e always appear in the unaltered
and unchanged field of obsei'vation noi-mal forms, with a I'emarkable
diffluent or amceba-like envelope, as seen in figs. 8 and 9, A. These

' See their successive papers in the Movthhj Microsr. Jonrn. vol. x. 1873,
pp. 53, 245 ; vol. xi. 1874, pp. 7, 69, 97 ; vol. xii. 1874, p. 261 ; and vol. xiii. 1875,
p. 185; and Proceed. Boy. Soe. vol. xxvii. 1878, p. 332. But especially for the latest
results with recent objectives, Journ. Bay. Micro. Soc. vol. v. 1885, p. 177 ; vol. vi.
p. 193; vol. vii. p. 185; vol. viii. p. 177.

Plate jy.

.-€D''

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JCr

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19 1 /

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18 /

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9 -oJ>/'

^ ^ ' '^/7

^ ^6 : >'

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WH.Dalliii^er (lel.aLdna,t.

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iLS.Huth.iithl London.

LIFE HISTORIES OF SAPROPHYTES,

MONAS 757

sometimes swim and sometimes creep, amreba-like, by pseiulopodia ;
bxit directly the dittluent sai'code of one touches tliat of another
they at once melt together, as in tig. 10, A. This leads to the rapid
approach of the oval bodies of the two organisms, as in fig. il, B,
resulting in their fusion, as in figs. 12, 1.3, 14, and a still condition
of the sac (fig. u) for a period of not less than six hours ; when it
biu"sts, as seen in fig. 15, pouring out an immense host of exquisitely
minute sjjores, as shown in fig. 15. These are opaque or semi-opaque,
but by observation upon them at a temperature of 65° to 70° Fahr.,
they in the course of thii-ty minutes become transparent, elongate,
as in figs. 16 and 17, and, continuing to gi'ow, assume the conditions
and sizes represented in figs. 18 and 19 ; and we were able to trace
them through all theii- changes of growth from the spore into the
adult condition, as at fig. 20, until they entered upon and passed
through the self-division into two described and figured in A.

The next form, though even more simple in appearance, has a
much moi'e complex moi'phological history. It is seen in its normal
form in fig. 1, C. It has but one flagellum, and, as we believe, on
that account has a much more restricted power of movement. It
is from the - fi'^^th to the - 1/,, ^th of an inch in long diameter. In
its motion at one stage of its life its oval body becomes uncertain in
form, as seen in 2, 3, 4, C ; but when this has continued for not
more than a minute, the flagellum falls in upon the body, as in 4,
and the organism becomes perfectly still. In this condition, after a
space ranging from ten to twenty minutes, two white bars at right
angles suddenly appear, as in fig. 5 ; this is almost immediately
followed by another and a similar one at right angles to the first, as
in fig. 6. Then the circumference of the flattened sphere twists,
leaving the centre unaffected, so that the body assumes a turbined
appearance as seen in fig. 7. After this the interior substance breaks
up, and becomes a knot of slightly moving but compact forms, as in
fig. 8 ; which remains in this state for from fifteen to twenty
minutes, and then becomes dissociated, as in fig. 9 ; so that we have
here a complex form of multiple partition, giving rise to enormous
numbers, because, altliough much smallei- than the form in which
they arose, they consume and assimilate food all over, and are
simply swimming in their pabulum, and so rapidly reach the
normal size, when they each enter upon and pass through a similar
process.

But here also at certain peiiods there appeared forms that in-
augurate distinctly genetic processes. A form like fig. 10, C, appears,
larger than the normal form, and always mottled in the part neai'-
est the flagellum. These forms rapidly attached themselves to the
normal forms, as seen in fig. 11, which resvilted in a blending of the
two as they swam together, xintil ' either was merited into other ; '
and a still sac, shown in fig. 12, resulted.

This remained from thirty to thirty-six hours absolutely inert ;
but at the expii'ation of that time it burst, as seen in fig. 13, D, and
poured out an enormously diffusive fluid, which as it flowed into the
surrounding water appeared like a denser fluid, diffusing itself through
one of less density ; but no spores were at this stage at all apparent. It

75 H MICEOSCOPIC FOEMS OF ANIMAL LIFE

was only after much eflbi't that we at last, by keeping the finest of our
lenses near the mouth of the empty sac, were able to discover, where
l^efore nothing was visible, the appearance of minute specks, which
became larger and larger, growing as seen in i-i, 15, 16, 17, until
the adult size was reached, as at 18, and by the act of m.ultipartitiou
on the part of one of these, watched from its fiivst disclosure by the
microscope, we were able to re-entei- the cycle of its life-history.

The third form, which we may hei'e consider fully, so as to j^resent
a good group of histoiies typical in their ^presentation of the morpho-
logy of the whole of the monad-sapi-ophytes as we at present know
them, is given in E and F, Plate XY.

The monad has been named by S. Kent JJalUnyeria Drysdali.
The foi-m more recently and completely studied by Mr. Dallinger —
with all the advantages derived from ti-ained experience, and under
objectives of the highest quality and gi-eatest magnifying power — is
seen in its normal shape in fig. i, is a long oval, slightly constricted
in the middle, and having a kind of pointed neck («), from which
proceeds a flagellum about half as long again as the body. From
the shoulder-like projections behind this {h, c,) arise two other long
and fine flagella, which are directed backwards. The sarcode-body
is clear, and apparently structureless, with minute vacuoles dis-
tributed through it ; and in its hinder part a nucleus {d) is dis-
tinguishable. The exti'eme length of the body is seldom more than
the.^(j\>^^th of an inch, and is often the ,;-Qiyyth. This monad swims
with great lapidity, its movements, which are gi-aceful and varied,
being pi-oduced by the action of the flagella, which can not only
impel it in any direction, but can suddenly reverse its course or check
it altogethei-. But Ijesides this free-swimming movement, a very
cui'ious ' spiinging ' action is performed by this monad when the de-
composing organic matter of the infusion is breaking tip, the ^^I'ocess
of disintegration being apparently assisted by it. The two posterior
flagella anchor themselves and coil into a spiral, and the body then
darts forwards and upwards, until the anchored flagella straighten
out again, when the body falls foi-ward to its horizontal position, to
be again drawn back by the spiral coiling of the anchored flagella.
This monad multiplies by longitudinal fission, the first stage of
which is the splitting of the anterior flagelliun into two (fig. 2, «, h),
and a movement of the nucleus (c) towai-ds the centre. In the course
o^from thirty to sixty seconds the fission extends down the neck (fig.
3, rt) ; a line of division is also seen at the posterior end (c), and the
nucleus [h) shows an incipient cleavage. In a few seconds the
cleavage-line i-uns thi-ough the whole length of the body, the separa-
tion being widest posteriorly (fig. 4, a) ; and in from one to four
minutes the cleavage becomes almost complete (fig. 5), the posterior
part of the body, with the two halves {a and h) of the original nucleus,
l)eing now quite disconnected, though the antei-ior pai-ts ai-e still
held togethei- by a ti-ansvei'se band of sarcode, as seen in fig. 6, which
continues to rapidly elongate, as in fig. 7, and becomes the length of
two side flagella, as in fig. s. The forms then appi-oach and rapidly
recede frf)m each other, snapping the cord, as in tigs. 9 and 10. In
tliis way tvo foi-ms exist instead of one ; and each of these, almost im-

. MONADS 759

mediately eiitei's uj)on and passes through the same process of fission,
which from first to last is completed in from four to seven minutes ;
and being repeated at intei'vals of a few minutes, this mode of multi-
plication produces a rapid increase in the number of the monads.

Such fission does not, however, continue indefinitelj'. for after
a successive series of fissions, followed in one of the divided bodies
for eight or nine hours, certain individuals do not again enter upon
the process of fission, but undergo a peculiar change, which shows
itself first in the absorption of the two latei'al flagella and the great
development of the nucleus, and afterwards in the formation of a
transverse granulai- band across the middle of the body (fig. 11, E).
One of these altered forms, swimming into a group in the ' springing '
state, within a few seconds firmly attaches itself to one of them, which
at once unanchors itself, and the two swim freely antl vigorously about,
shown in fig. 12, generally for from thirty-five to foi'ty-five minutes.
Gi'adually, howevei-, a ' fusion ' of the two bodies and of their re-
spective nuclei takes place, the two trailing flagella of the ' springing '
form being drawn in (fig. 1.3, F) ; and in a short time longer the two
anterior flagella also disappeai', and all trace of the separate bodies is
lost, the nuclei vanish, and the resultant is an irregular amoeboid mass
(fig. 14), which gradually acciuires the smooth, distended, and ' still '
condition represented in fig. 14, a. This is a cyst filled with repro-
ductive particles of such extraordinary minuteness that, when
emitted from the ends of the cyst (fig. 15, a) aftei' the lapse of four
or five hours, they can only be distinguished under an amplification
of 5,000 diameters, with perfect central illumination, i.e. the full
cone of a large-angled condenser. Yet these particles, when con-
tinuously watched, are soon observed to enlarge and to undergo
elongation (figs. 16, 17, 18, 19, 2o), and within two hours after their
emission from the sac the axiterior flagellum, and afterwards the two
lateral flagella (fig. 19), can be distinguished. Slight movements then
commence, the neck-like protrusion shows itself, and in about
half an hour more the regular swimming action begins. About
four hours after the escape of its germ from the sac, the monad
acquires its characteristic form (fig. 2l), thovigh still only one-half the
length of its parent ; but this it attains in anothei' hour, and the
process of multiplication by fission, as already described, commences
very soon aftei'wards. There can be no reasonable doubt that the
' conjugation ' of two individuals, followed by the transformation of
their fused bodies into a sac filled with reproductive germs, is to be
legarded (as in protophytes) in the light of a true generaiive process ;
and it is interesting to observe the indication of sexual distinction
here marked by the difierent states of the two conjugating Individ na Is.
There is every I'eason to believe that the entire lije-cycle of this monad
has thus been elucidated ; and it will now be sufiicient to notice the
principal diversities observed by Messrs. Dallinger and Drysdale in
the life-cycles of the other monadine forms which they have studied.
The bi-flagellate or ' acorn ' monad of the same observers (identi-
. fied by Kent with the Poli/toma uvella of Ehrenberg) presents some
remarkable peculiarities in its mode of reproduction. Its binaiy
fission extends only to the protoplasmic substance of its body, leaving

76o MICROSCOPIC FOEMS OF ANI3IAL LIFE

its envelope entire ; and by a repetition of the process, as many as
sixteen segments, each attaining the likeness of the parent, are seen
thus inclosed, their flagella protruding through the general invest-
ment. This compound state being supposed by Ehi'enberg to be the
normal one, he named it accordingly. But the parent-cyst soon
bursts, and sets free the contained 'macro-spores,' which swim about
freely, and soon attain the size of the parent. Again, the posterior
part of the body of certain individuals shows an acciunulation of
granular protoplasm, giving to that region a roughened acorn -cup-
like aspect ; the bursting of the projection, while the creature is
actively swimming thi'ough the water, sets free a mi\ltitude of
indefinitely shaped granular fi-agments, within each of which a
minute bacterium-like corpuscle is developed ; and this, on its
release, acquires in a few hours the size and form of the original
monad. This process seems analogous to the development of ' micro-
spores ' among protophytes by the direct breaking np of the proto-
plasm. It is, like the previous pi'ocess, non-sexual or gonldial, the
true generative process consisting hei'e, as in the preceding cases, in
the • conjugation ' of two individuals, with the usual results.

The hooked inonad [Heteromita uncinata, Kent) is another bi-
flagellate form, usually ovate with one end pointed, and from 3 q^^ ^th
to ^y'^jy^th of an inch in length, being distinguished from the pre-
ceding by the peculiar character of its flagella, of which the one that
projects forwards is not more than half the length of the body, and
is permanently hooked, while the other, whose length is about twice
that of the body, is directed backAvards, flowing in graceful curves.
Its motion consists of a succession of springs or jerks rapidly follow-
ing each other, which seems produced by the action of the hooked
flagellum. Multiplication takes place by transverse fission, and con-
tinues uninterruptedly for several days. A difference then becomes
perceptible between larger and smaller individuals, the former being
further distinguished by the presence of what seems to be a con-
tractile vesicle in the anterior part of the body. Conjugation occurs
between one of the larger and one of the smaller forms, the lattei-
being, as it were, absorbed into the body of the larger ; and the
resulting product is a spherical cyst, which soon begins to exhibit
a cleavage-process in its interior. This continues until the whole
of its sarcodic substance is subdivided into minute oval particles,
which are set free by the rupture of the cyst, and of which each is
usually fiu-nished with a single flagellum, by whose lashing move-
ment it swims freely. These germs speedily attain the size and form
of the parent, and then begin to multi2:)ly by transverse fission, thus
completing the ' genetic ' cycle.

The calycine monad of the same observers {Tetramitus rostratics,
Perty) has a length of from -g^Tyth to yJ^jfth of an inch, and a com-
pressed body tapering backwards to a point. Its four flagella (which
constitute its generic distinction) aiise nearly together from the
flattened front of the body, and its swimming movement is a grace-
ful gliding. Near the base of the flagella ai-e a pair of contractile
vesicles, and fui'ther behind is a lai'ge nucleus. Multiplication takes
place by longitudinal fission, which is preceded by a change to a semi-

MONADS 761

amoeboid state. This gives place to a more regular pear-like form,
the four flagella issuing from the large end ; and the fission commences
at theii- base, two pairs being sepjirated by the cleavage-plane. The
nvicleus also undergoes cleavage, and its two halves are carried apai't
by the backward extension of the cleavage. The two half-bodies
at last remain connected only by their hinder prolongations, which
speedily give way, and set them free. Each, however, has, as yet,
only two flagella ; but these speedily fix themselves by their free
extremities, undergo a rapid vibratoiy movement, Jind in the course
of about two minutes split themselves from end to end. A still
more complete change into the amceboid condition, in which the
creature not only moves, but also feeds, like an Aoiujeba (devouring all
the living and dead Bacteria in its neighbourhood), occurs previously
to ' conjugation ; ' and this takes place between two of the amoeboid
forms, which begin to blend into each other almost immediately
upon coming into contact. The conjugated bodies, however, swim
freely about for a time, the two sets of flagella apparently acting in
concert. But by the end of about eighteen hours the fusion of
the bodies and nuclei is complete, the flagella are lost, and a
spherical distended sac is then formed, which, in a few hours more
without any violent splitting or breaking up, sets free innumei-able
masses of reproductive pai-ticles. These under a magnifying power
of 2,500 diameters can be just recognised as oval granules, which
rapidly develop themselves into the likeness of their parents, and
in their turn multiply by daplicatiA'e fission, thus completing the
' genetic ' cycle.

One of the most important researches thus ably prosecuted l)y
Messrs. Dallinger and Drysdale has reference to the temperatures
respectively endurable by the adult or developed forms of these
monads, and by their reproductive germs. A large number of experi-
ments upon the several foi-ms now desci'ibed indubitably led to the
conclusion that all the adult foi-ms, as well as all those which had
I'eached a stage of development in which they can be distinguished
from the reproductive granules, are utterly desti'oyed by a tempera-
ture of 150° Fahr. But, on the other hand, the reproductive granules
emitted from the cysts that originate in 'conjugation' were found
capable of sustaining a fluid heat of 220°, and a dry heat of about
30° more, those of the Oercomonad surviving exposure to a dry heat of
300° Fahr. This is a fact of the highest interest in its bearing on the
question of ' spontaneous genei'ation,' or abiogenesis ; since it shows
that germs capable of surviving desiccation may be everywhere diflused
through the air, and may, on account of their extreme minuteness
(as they certainly do not exceed 2'oo'oiro''^^^ of ^^^ inch in diameter),
altogether escape the most careful scrutiny and the most thorough
cleansing processes ; while (2) their extraoi'dinary power of resisting
heat will prevent these germs fi-om being killed, either by boiling, or
by dry-heating up to even 300° Fahr.'

Beyond these facts others of some importance, as well as a new

' Descriptions of the special api^aiutus used by Messrs. Dallinger and Drysdale
in their researches will be found in MontJd/j Micros. Joum. vol. xi. 1874, p. 97 ; ibUl.
vol. XV. 1876, p. 165 ; and Proceed. Boy. Soc. vol, xxvii. 1878, p. o43.

762 MICROSCOPIC FORMS OF ANIMAL LIFE

WMprophytic organism ^ of special ehai-actei-, have been discovered
during a recent period. But it will be of more moment here to note
to what an extent in this series of obsei-vations the new homogeneous
objectives, especially in their apochromatic form, have been success-
fully employed in enlarging the ai'ea of knowledge.

The present Editor has gone carefully ovei- the greater part of the
woi-k, revising all the ciitical points with the best apochi-omatic ob-
jectiv'es, and the homogeneous forms of achromatics with an aperture
of 1"50 and with a cleai- demonsti-ation of the immensely greater ease
with which the woi'k could have been done had these lenses been used
in the original investigation.

But the easily accessible proof of this is given iii the work done by
Dr. DaUinger upon the nucleus oiiha nucleated forms of these monads.

Briefly to present the facts, we may recall the part taken in the
act of fission in the foi-m last desciibed [DalUvgeria Drysdali). It
will be seen by I'efei'ence that it appeai-ed to us that the nuclei's fol-
lowed the processes inaugurated by the somatic sarcode. That in fact
it was a passive pai-ticipator in the act of fi.ssion. This is all that
can be made out to-dxiy by the very lenses oi-iginally employed.

But by the employment of a iVth inch and -^i\\ inch homo-
geneous of N.A. 1'50 by Powell and Lealand, and an apochromatic
of -jJgth inch N. A. 1"40 by the same firm; and also by the use of
the beautiful 3 mm. and 2 mm. N.A. 1'40 of Zeiss (apochromatic),
it. can be seen with comparative ease that it is in the nucleus that
all the activities of the body are originated.

This may be followed from a study of Plate XYI. Fig. 1, A,
I'epresents the nucleus of the form drawn at fig. 1, E, Plate
XY. In long diameter it is of an average length of 20000^-^^ ^^ ^^^
inch ; but instead of being a darkly i-efi-active object, as seen with
the objectives used twelve years ago, it is with the present lenses,
freed from chromatic and spherical aberi-ation, a body in the monad
undei'going no process of change, aii oval globule with a complicated
plexus-like involution thi'oughout its substance, as seen in fig. 6, A,
Plate XVI. But directly the process of fission is to be inaugurated,
we need not wait to see its first action in the splitting of the
flagellum, as in fig. 2, E, Plate XV ; for by obsei'ving the nucleus
we discover, before any change has begun in the body-substance,
that the plexus in the nucleus has condensed itself on either side of
the nucleus, as in fig. 1, 5, A, Plate XVI. A clear space is left at c,
and no change has taken place in the body-sai'code, a, a, a. But
shortly an incision takes place in the nucleus, as at cZ, fig. 2, and
this is immediately followed by the incision / in the body-sarcode,
and the process goes on simultaneously in nucleiis and body, as in
fig. .3, until the division of the nucleus is completely efiected, and the
total severance of the body follows.

But as soon as the nucleus is divided, the plexus, which has been
ilui'ing division, as in fig. 3, condensed over part of each dividing
lialf, at once distributes itself evenly again, as in fig. 6, A, and re-
mains so until another change is inaugiu'ated in the form to which
the nucleus belongs.

1 Jonrn. of Royal Micron. Hoc, vol. v. •

Pla.te Xyi

B

/^P

7

ly-*'

\r\>

\V.H.Ds:iliii|ef del. ad n^t

/.S.Hni;J:,Li-di- I-onion,

ic iM qftPROPHYTIC ORGANISMS.

SAPEOPHYTIC LIFE-HISTOEIES 763

Not less I'eniarkable is this in the covjufiatlon of the same foiau.
With the old lenses we could only discover that the end of a
.series of fissions had been i-eached by the change which came over
the entire body of the tei'minal form seen in fig. 11, E, Plate XV.
But now, befoi'e the amoeboid state preceding the assumption of
the condition shown in fig. 1 1 takes place, it can be seen that the
nucleus undei'goes i-emai'kable change, foi' it passes from a highly
I'efractire plexus-like condition into a large milky structureless state,
and in this condition blends with one of the oixlinai'y foi-ms whose
nucleus is of the oi'dinai-y type. The fii-st I'esiilt of fusion is seen in
fig. 4, A, Plate XVI, showing only the gi'eatly magnified blended
nuclei, and where the blending between them is seen to be nearly
complete at a, and a nucleus or nucleolus is manifest ; while
Avhen the blending is moi-e pei-fect thei-e is a difi'usion of this
central or nucleolai- body through the substance of the whole, as in
fig. 5, A.

In B, Plate XVI, the nucleus only, sejDaiate from the body of
the organism known as Tetramiitus rostratus, is shown as we can
I'eveal it with i-ecent German and English apochi'omatic objectives.
This entire organism is i-elatively large, and its nucleus will average
in long diameter the -iTj-gu-o^h of an inch.

Hence it affords a still better means of study. Now this
oi'ganism divides by fission for a veiy considerable time, but at length
many forms become amoeboid — acting pi-ecisely as an ainceba, but
I'etaining traces of theii- primal foi-m. In this state two of them
blend, and as a I'esult a sac of spoi'e is formed fi'om which a new
generation arises.

We coidd with the old objectives detei'mine nothing more than
the fact that the amoeboid foi'm had supeiv^ened ; but now it is easy
to show that the nucleus in the body of a form not yet amoeboid is
undergoing change upon which the amoeboid state is certain to
supervene.

This is even moi-e sticking in the gi'owth of the geiin. It attains
a certain size in gi'owth, and then there is an arrest of all enlarge-
ment. This we liad long obsei'ved in the earlier obsei'vations. But
now with apochi'omatic object-glasses it has been demonstrated that
this arrest of outwaixl growth is only the signal foi' an internal de-
velopment. Fig. 1 , B, Plate XVI, shows the condition of the nucleus
when there is an apparent pause in its gi-owth. Fig. -2 shows the
same nucleus after about forty minutes of external inaction, a plexus-
like formation having filled its substance.

The nucleus remains thus in the matui-e body of the monad
until fission is to he inaugarated, when the change seen in fig. 3,
followed by the changes and deeper division seen in figs. 4, 5, 6, 7,
and 8, ensue, and after the state of the nucleus seen in fig. 4 has
been I'eached, the division of the entire body begins.

It thus appeal's that a foi'in of karyohinesis takes place in the
nucleus of even such lowly forms as these, and that it is the nucleus
that is the seat of tlieii- intensest vitality.

A lai-ge sei-ies of moi'e complex forms of flagellate Infusoria
has been brought to our knowledge by the i-eseai'ches of the late

764

MICEOSCOPIC FOEMS OF ANIMAL LIFE

Professor James-Clark (U.S.A.)/ followed by those of )Stein, Saville
Kentj^and Bevgh. In some of these a sort of collar-like extension of
what appeai-sto be the protoplasmic ectosarc pi-oceeds fi'om the anterior
extremity of the body (fig. 585, d), forming a kind of funnel, from the
bottom of which the flagelhim arises ; and by its vibrations a cur-
rent is produced within the funnel, which bi-ings down food-particles
to the ' oiul disc ' that suiTounds its oiigin while the ectosarc seems
softer than that which envelops the rest of the body. Towards the
base of the collai- a nucleus [n) is seen ; while neai- the posterior
termination of the body is a single oi- double contractile vesicle (cv).
The body is attached by a pedicel pi-oceeding from its posterior
extremity, which also seems to be a prolongation of the ectosarc.
These animalcules multiply by longitudinal fission ; and this, in
some cases (as in the genus Monosiga), proceeds to the extent of a
complete separation of the two bodies, which henceforth, as in the

ordinary Monadina,
live quite independ-
ently of each other.
But in other forms, as
Godosiga, the fission
does not extend thi'ough
the pedicel, and the
twin bodies being thus
held together at their
bases, and themselves
undergoing duplicative
fission, clusters are pro-
duced which spring
from common pedicels
(fig. • 586) ; and by
the extension of the
division down the
pedicels themselves,
composite arborescent
fabi-ics, like those of
zoophytes, are pro-
duced.

In another group a structureless and very transparent horny
calyx, closely i-esembling in miniature the polype-cell of a Gampanu-
laria, forms itself round the body of the monad, which can retract
itself into the bottom of it ; and in the genus SaVpinyoeca both
calyx and collai- ai-e present. In some forms of this gi-oup multi-
plication seems to take place, not by fission, but by gemmation ;
and, as among hydroid polypes, the getnmce may either detach
themselves and live independently, or may remain in connection
with theii' parent-stocks, forming composite fabrics, in some of which
the calyces follow one another in linear series, while in othei's they

Fig. 585. — Single zoiiid. of Codosiga umhellata : cl,
collar ; n, nucleus ; co, double contractile vesicle.

^ See his memoirs in Ann. Nat. Hist. ser. 3, vol. xviii. 1860; cyj. cif. ser. 4,vol. i.
1868 ; vol. vii. 1871 ; and vol. ix. 1872.

- See his Man/ud of the Infunoria, 1880-82, 2 vols, and 1 vol. of> plates.

FLAGELLATA 765

take on a raniifymg aiTangement. While some of these composite
oi'ganisms are sedentaiy, othei's. as Dmohrijon, ai'e free-swimming.

Two solitaiy ilagellate foi-nls, Aniliophysa and Anisonema, may
be specially noticed as presenting sevei'al interesting points of
resemblance to the peculiar type next to be described, the most
noticeable being the pi-esence of a distinct month and the possession
of two different motor organs— one a comparatively stout and stiff
bristle, of uniform diameter thi-oughoxit, which moves by occasional
jerks, and the othei' a very delicate tapeiing flagellum, which is
in constant vibratoi'v motion. If, as appears fi-om the observa-
tions of Biitschli, the well-known Astasia — of which one species has
a blood-red colour, and sometimes multiplies to such an extent as
to tinge the water of the ponds it inhabits — has a true mouth for the

Fig. 586. — Codosiga umhellata : Colony-stock, springing from single
pedicel tripartitely branched.

1-eception of its food, it must be regarded as an animal, and sepa-
i-ated from the Euglena (with which it has been generally associated),
the latter being pretty certainly a plant belonging to the same
group as Vohwx}

There can be no longer any doubt that the well-known Xoctiluca
jmliaris — to which is attributable the diffused luminosity that fre-
quently presents itself in British seas — is to be regarded as a gigantic
type of the ' iinicellular ' Flaciellata. This animal, which is of sphe-
roidal form, and has an average diameter of about ^^t\\ of an inch,
is just large enough to be discerned by the naked eye when the water
in which it may be swinnning is contained in a glass jar held up to

^ See the memoir by Prof. Biitschli in ZeUschrift f. Wissensch. Zool. Bd. xxx.,
of which an abridgment (with plate) is given in Quart. Joiirn. Micros. Sci. vol. xix.
1879, p. 63.

']66

MICROSCOPIC FORMS OF ANIMAL LIFE

the light ; and its tail-like appendage, whose length, about equals
its own diametei', and which sei-ves as an instrument of locomotion,
may be discerned with a hand -magnifier. The foi'm of Xoctiluca is
nearly that of a sphere, so compressed that while on one aspect (fig.
587, A) its outline when projected on a plane is nearly cu-cular, it
is irregularly oval in the aspect (B) at light angles to this. Along
one side of this body is a meridional groove, resembling that of a
peach ; and this leads at one end into a deep depression of the sur-
face a, termed the atrium, from the shallower commencement of
which the tentacle, d^ originates ; whilst it deepens do\^^l at the base
of the tentacle to the mouth, e. Along the opposite meridian there
extends a slightly elevated ridge, c, which commences with the
appearance of a bifurcation at the end of the atrium fai'thest from

Fig. 587. — Noctilitca miliaris as seen at A on the aboral side, and at
B on a jjlane at right angles to it : a, entrance to atrium ; 5, atrium ;
c, superficial ridge ; fZ, tentacle ; e, mouth leading to oesophagus,
within which are seen the flagellum springing from its base, and the
tooth-like process projecting into it from above ; /, broad process from
the central protoplasmic mass proceeding to superficial ridge ; g,
duj)licature of wall; A, nucleus. (Magnified about 90 diameters.)

the tentacle : this is of firmer consistence than the I'est of the body,
and has somewhat the appearance of a rod imbedded in its walls.
The movith opens into a short oesophagus, which leads directly down
to the gi-eat central protoplasmic mass ; on the side of this canal,
farthest from the tentacle, is a firm ridge that forms a tooth-like
projection into its cavity ; whilst fi'om its floor there arises a long

1 The organ here termed ' tentacle ' is commonly designated y/rr^eZ/H/;; ; while
what is here termed the fiagcll/im is spoken of by most of those who have recognised
it as a ciliiun. The Author agrees with M. Robin in considering the former organ,
which has a remarkable resemblance to a single fibrilla of striated muscle, as
one peculiar to Noctiluca, and the latter as the true homologue of the flagellum of
the ordinary Flagellata. It is euriovis that several observers have been unable to dis-
cover the so-called cilium, which was first noticed by Krohn. Professor Huxley sought
for it in at least fifty individuals without success ; and out of the great number which
lie afterwards examined he did not get a clear view of it in more than half a dozen.

NOCTILUCA

7^)

flagellam, Avhich vil)i-;ites freely in its interior. The central proto-
plasmic mass sends off" in all dii/ections branching pi-olongations of
its substance, whose ramifications inosculate ; these become thinnei-
and thinnei- as they appi-oach the periphery, and their ultimate
filaments, coming into contact with the delicate membranous body-
wall, extend themselves ovei- its interior, forming a protoplasmic
network of extreme tenuity (fig. 588). Besides these branching
prolongations, there is sent off from the central pi'otoplasmic mass a
broad, thin, irregulai'ly quadrangular extension (fig. 587, B,/), which
extends to the superficial rod -like ridge, and seems to coalesce with
it ; its lower fi-ee edge has a thickened border ; whilst its uppei-
edge becomes continuous with a plate-like striated structure, y, which
seems to be formed by a peculiai' duplicature of the body-wall. At
one side of the pi-otoplasmic mass is seen a spherical vesicle. A, of

Pig. 588. — Portion of superficial protoplasmic reticulation formed
by ramification of an extension a of central mass. (Magnified
1,000 diameters.)

about 2-(njo*l^^ "^ ^^^ ^^^^^^ ^^^ diameter, having clear coloui'less
contents, among which transparent oval corpuscles may usually be
detected. This, from the changes it undergoes in connection with
the reproductive process, must be regarded as a nucleus.

The particles of food di'awn into the mouth (probably by the
vibrations of the flagellum) seem to be received into the protoplas-
mic mass at the bottom of the esophagus by extensions of its sub-
stance, which inclose them in filmy envelopes that maintain them-
selves as distinct fi-om the sui'rounding pi'otoplasm, and thus consti-
tute extemporised digestive vesicles; These vesicles soon find their
way into the radiating extensions of the central mass (as shown in
fig. 587, B), and are ensheathed by the protoplasmic substance which
goes on to form the peripheral network (fig. 589). Their numl^er
and position are alike variable ; sometimes only one or two are to
be distinguished ; more commonly from four to eight can be seen ;

768 MICKOSCOPIC FOEMS OF ANIMAL LIFE

and even twelve oi- more ai-e occasionally discernible. The place of
each in the body is constantly being changed by the contractions of
the protoplasmic substance, these in the fii'st place cai'iying it fi'om
the centre towaixls the periphery of the body, and then carrying it
back to the central mass, into whose substance it seems to be
fused as soon as it has discharged any indigestible material it may
have contained, which is got rid of thi-ough the mouth. Eveiy part
of the protoplasmic i-eticulation is in a state of incessant change,
which serves to distiibute the nutrient material that finds its way
into it through the walls of the digestive vesicles ; but no i-egular
cyclosis (like that of plants) can be observed in it. Besides the
' digestive vesicles,' vacuoles filled with clear fluid may be distin-
guished, alike in the centi'al jii'otoplasmic mass, and in its extensions
as is shown in the centre of fig. 587. There is no contractile vesicle.
The peculiar- ' tentacle ' of Noctiluca is a flattened whip-like fila-
ment, gi'adually tapeiing from its base to its extremity, the two
flattened faces being dii-ected respectively towards and away from
the oral apertui'e. When either of its flattened faces is examined, it

Fig. 589.— Pair of digestive vesicles of Noctiluca lying in course of exten-
sion of central protoplasmic mass, a, to form peripheral reticulation,
h, and containing remains of Algie. (Magnified 480 diameters.)

shows an alternation of light and dark spaces, in every respect
i-esembling those of sti-iated muscular fibre, except that the clear
spaces are not subdivided. But when looked at in pi-ofile, it is seen
that between the striated band and the aboral surface is a layer of
o-ranular protoplasm. The tentacle slowly bends over towards the
mouth about five times in a minute, and straightens itself still more
slowly, the middle portion i-ising first, while the point approaches
the base, so as to form a sort of loop, which presently straightens.
It seems probable that the contraction of the substance foiming the
dai'k bands produces the bending of the filament ; whilst, when
this relaxes, the filament is sti'aightened again by the elasticity of
the granular layei'.

The extreme transpai'ence of Xoctilaca i-endeivs it a pai-ticularly
favourable subject for the study of the phenomena of phosphoi'escence.
When the surface of the sea is rendei-ed luminous by the general
diflusion of Xoctilucce, they may be obtained by the tow-net in un-
limited quantities ; and when ti-ansfei-red into a jar of sea-water,
they soon rise to the surface, where they form a thick stratum. The
slightest ao'itation of the jar in the daik causes an instant emission of

NOCTILUCA 769

their light, which is of a l)eautiful gi-eenish tint, and is vi^dd enough
to be pei'ceptil:>le by ordinai-y lanip-Hght. This himinosity is but of
an instant's duration, and a short i-est is i-equired for its i-enewal. A
brilhant but short-hyed display of himinosity, to be followed by its
total cessation, may be pi'oduced by electi-ic or chemical stimulation.
Professor Allman found the addition of a drop of alcohol to the water
containing specimens of Noctiluca, on the stage of the microscope,
produced a luminosity strong enough to be yisible undei- a half-inch
objective, lasting with full intensity foi- seyei-al seconds, and then
gradually disappearing. He was thiis able to satisfy himself that
the special seat of the phosphoi-escence is the peripheral protoplasmic
reticulation which lines the external sti'uctureless membrane.

The reproduction in this interesting type is effected in yaiious
ways. According to Cienkowsky, even a small portion of the pi'oto-
plasm of a mutilated J^octiluca will (as among rhizopods) reproduce
the entire animal. IMultiplication by fission or binary subdiyision,
beginning in the enlargement, constriction, and separation of the two
halves of the nucleus, has been frequently observed. Another form
of non-sexual repi-oduction, Avhich seems parallel to the ' swarming '
of many protophytes, commences by a kind of encysting process.
The tentacle and flagellum disappear, and the mouth gradually
narrows, and at last closes up ; the meridional groove also disappears,
so that the animal becomes a closed hollow sphere. The nucleus
elongates, and becomes transversely constricted, and its two halves
separate, each remaining connected with a portion of the protoplasmic
network. This duplicative subdivision is repeated over and over
again, until as many as 512 'gemmules ' are formed, each consisting
of a nuclear particle enveloped by a protoplasmic layer, and each
having its flagellum. The entire aggregate forms a disc-like mass
projecting from the surface of the sphere ; and this mass sometimes
detaches itself as a whole, subsequently bi-eaking up into individuals ;
whilst, more commonly, the gemmules detach themselves one by one,
the separation beginning at the margin of the disc, and proceeding
towards its centre. The gemmules are at first closed monadiform
spheres, each having a nucleus, contractile vesicle, and flagellum ;
the mouth is subsequently formed, and the tentacle and permanent
flagellum afterwai'ds make their appearance. A process of ' conjuga-
tion ' has also been observed, alike in ordinary jVoctihicce and in their
closed or encysted forms, which seems to be sexual in its nature.
Two individuals, applying their oral surfoces to each other, adhere
closely together, and theii- nuclei become connected by a bridge of
protoplasmic substance. The tentacles are thrown oS, the two bodies
gradually coalesce, and the two nuclei fuse into one. The whole
process occupies about five or six houi-s, but its results have not been
followed out.^

^ Nociilnca has been the subject of numerous memoirs, of which the following
are the most recent: Cienkowsky, Arch f. micros. Anat. Bd. vii. 1871, p. 131, and
Bd. ix. 1873, p. 47 ; Allman, Quart. Journ. Microsc. Sci. n.s. vol. xii. 1872, p. 327';
Robin, Jb»7';(. de VAnaf. et cle Physiol, torn. xiv. 1878, p. 586; V \gi\si\, Arch, de
Physiol, aer. ii. torn. v. 1878, j). 415; Stein, i)er Orqanismus dcr Iiifusionsthiere,
iii. 2, 1883; and Biitschh, J/orp/ioZ. Jahrhnch . x. 1885, p. 529. For the group of
which it and the Mediterranean germs. Lej)tod /sens (Hertwig) are the representatives,
Haeckel has suggested the name CtjstofagpUata.

3d

770

MICEOSCOPIC EORMS OF ANIMAL LIFE

The name Cilio-Jlagellata and the definition of the group must
both be altered, now that Klebs and BiitschH have shouii that what
was regarded as ciha in the transverse grooves of their bodies is
really a flagellum ; the name to be used is Dinoflagellata} Al-
though this group does not contain any great diversity of forms, yet it
is specially worthy of notice, not only on account of the occasional
appearance of some of them in extraordinary multitudes, but also for
their power of forming cellulose— a property which is often thought
to be particularly characteristic of plants. The Peridinmin observed
by Professor Allman in 1854 was pi'esent in such quantities that
it imparted a brown colour to the water of some of the large ponds
in Phcenix Park, Dublin, this colour being sometimes unifoi-mly
difl;used, and sometimes showing itself more deeply in dense clouds,
varying in extent from a few square yai'ds to upwards of a hundred.
The animal (fig. 590, A, B) has a form approaching the spherical,
with a diametei- of fi-om ^ g^^^th to j-oVd'tli^ of an inch, and is
partially divided into two hemispheres by a deep equatorial furrow,
a, whilst the flagellum- bearing hemisphere. A, has a deep meridional
gioove on one side, h, extending from the equatorial groove to the
pole, the flagellum taking its origin from the bottom of this vertical

Fig. 590. — Peridinium uherrimnm : A, B, front and back views;-
C, encysted stage ; D, duplicative subdivision.

groove, near its junction with the equatoiial. The membei-s of this
group vary considerably in their mode of taking food ; from the
researches of Bergh it would appear that those which are provided
with chromatophoi-es have a plant-like mode of obtaining food, while
those which are without chromatophores are truly animal in their
method of alimentation. A ' contractile vesicle ' has been rarely
obsei-ved ; but a large nucleus, sometimes oval and sometimes horse-
shoe-shaped, seems always present. The Peridihia multiply by
transvei'se fission (fig. 590, D), which commences in the subdivision
of the nucleus, and then shows itself externally in a constriction of
the ungrooved hemisphei-e, parallel to the equatorial furrow. They
pass into a quiescent condition, subsiding towards the bottom of the
water, and the loiicated forms appear to throw ofi' their envelopes.
There is reason to believe that conjugation obtains in cei-tain cases :
Glenodiniuvi chictum has been olxserved by Professor Askenasy to
copulate, but the development of the zygote, as the product of copu-
lation may be called,, has not yet been worked out. Some of the
Peridinia are found in sea-watei-,^ but the most remarkable marine

1 Or, more correctly, Dinomastigopliora .

2 See F. Schiitt, ' Die Periclineen der Planldon Expedition,' Ergehn. Plankton
Exped. lod5. 170 lip. and 27 pis.

CERATIUM

771

forms of the cilio-flagellate group belong to the genus Ceratkmi (fig.
591), in which the cuirass extends itself into long horny appendages.
In the Ceratium tripos (1) thei-e are three of these appendages ; two
of them curved, proceeding from the anterior portion of the cuirass,
and the third, which is straight or nearly so, from its posterior
portion. They are all more or less jagged or spinous. In Ceratium
furca (2) the two anterior horns are prolonged straight forwards,
one of them being always longer than the other ; whilst the posterior
is prolonged straight backwards. The anterior and posterior halves
of the cuirass are separated by a ciliated furrow, from one point of
which the flagellum arises ; and at the origin of this is a deep

Fig. 591. — 1, Ceratium tripos; 2, Ceratium furca.

depression into which the flagellum may be completely and suddenly
withdrawn. The Author has found the Ceratium tripos extremely
abundant in Lamlash Bay, Arran, where it constitutes a principal
article of the food of the Antedons that inhabit its bottom.^

Ciliata. — As it is in this tribe of animalcules that the action of
the organs termed cilia has the most important connection with
the vital functions, it seems desirable here to introduce a more
particular notice of them. They are always found in connection
with cells, of whose protoplasmic substance they may be considered
as extensions, endowed in a special degree with its characteristic
contractility. The form of the filaments is usually a little flattened,

1 See Allmau in Quart. Microsc. Journ. vol. iii. 1855, p. 24 ; H. James-Clark in
Ann. Nat. Hist. ser. iii. vol. xviii. 1866, p. 429 ; Bergli, Morphol. Jahrbiich. vii. 1881,.
p. 177, and VanhiifEen, Zool. Anzeig. six. 1896, pp. 13o-4.

3d 2

772

MICEOSCOPIC FOEMS OF ANIMAL LIFE

tapering gi-adually from the base to the point. Their size is ex-
tremely variable, the lai-gest that have been observed being about
.^i^th of an inch in length, and the smallest about ^ g-L^th. When
in motion each filament appears to bend fi-om its root to its point,
returning again to its original state, like the stalks of corn when
depressed by the wind ; and when a number are afiected in
.succession with this motion, the appeaiance of progressive waves
following one another is produced, as when a cornfield is agitated
by successive gusts. Wlien the ciliary action is in full activity,
howevei-, little can be distinguished save the whirl of particles in
the surrounding fluid ; but the hack stroke may often be perceived,
when the forvxird stroke is made too quickly to be seen, and the
real direction of the movement is then opposite to the apparent. _ In
this back stroke, when made slowly enough, a sort of ' feathering '
action may be observed, the thin edge being made to cleave the

/ •'•■

Fig. 592. — A, Kerona silurus : a, contractile vesicle ; b,
mouth ; c, c, animalcules swallowed by the Kerona, after
having themselves ingested particles of indigo. B,
Paramecium coMdatum: a, ft, contractile vesicles;
b, mouth. The dotted Inies indicate currents.

liquid which has been struck by the broad surface in the opposite
direction. It is only when the rate of movement has considerably
slackened that the shape and size of the cilia, and the manner in
which their stroke is made, can be cleai-ly seen. Their action has
been observed to continue for many hours, oi- even days, after the
death of the body at large. As cilia are not confined to animal-
cules and zoophytes, but give motion to the zoospores of many
protophytes, and also clothe the free intei-nal sui-faces of the respi-
ratoiy and othei' passages in all the higher animals, including man
(our own experience thus assuring us that theii- action takes place,
not only without any exei-cise of vnll, but even without conscious-
ness), it is clear that to regard animalcules as possessing a ' voluntary '
control ovei- the action of their cilia is altog'ether unscientific.

CILIATA yji

In the ciliated Infusoria, the differentiation of the sarcodic sub-
stance into ' ectosai'c ' or cell-wall, and ' endosarc ' or cell-contents,
becomes very complete, the ectosarc possessing a membranous
firmness which prevents it fi-om readily yielding to pressure, and
liaving a definite internal limit, instead of graduating insensibly
(as in rhizopods) into the protoplasmic layer which lines it. A
■ nucleus ' seems always present, being sometimes ' parietal ' (or
adherent to the intei-ioi- of the ectosarc), in other cases lying in the
midst of the endosai-c. In many Giliata a distinct ' cuticle ' oi-
exudation-layer may be recognised on the surface of the ectosarc ;
and this cuticle, which is studded with regularly arranged markings
like those of Diatomace?e, seems to be the representative of the
carapace of Arcella &c. as of the cellulose coat of protophytes.
It is sometimes hardened, so as to form a ' shield ' that protects
the body on one side only, or a ' loiica, ' that completely invests
it ; and there are other cases in which it is so pi-olonged and
doubled upon itself as to form a sheath i-esembling the ' cell ' of a
zoophyte, within which the body of the animalcule lies loosely, being-
attached only by a stalk at the bottom of the case, and being able
either to project itself from the outlet or to retract itself into the
interior. In the marine forms known as Dictocysta, and in Codonella,
described by Haeckel, the body is enclosed in a silicious lattice-work
shell, usually bell-shaped or helmet-shaped, which bears so strong-
a resemblance to the shells of many Radiolaria as to be easily mis-
taken for them. The form of the body is usually much more
definite than that of the naked rhizopods, each species having its
characteristic shape, which is only departed from, for the most part,
when the animalcule is subjected to pressure from without, or when
its cavity has been distended by the ingestion of any substance
above the ordinary size. The cilia and other mobile appendages of
the body are extensions of the outer layer of the ' ectosarc ' proper ;
and this layer, which retains a high degree of vital activity, is some-
times designated the ' cilia-layer.' Beneath this is a layer in which
(or in certain bands of which) regulai', parallel, fine stripe may be
distinguished, and as this striation is also distinguishable in the
eminently contractile foot-stalk of Vorticella^ (fig. 593, B) there seems
good reason to regard it as indicating a special modification of proto-
plasmic substance, which resembles muscle in its endowments.
Hence this is termed the ' myophan-layer.' Beneath this, in cer-
tain species of Infusoria, there is found a thin stratum of condensed
protoplasm, including minute 'trichocysts,' which resemble in
miniature the ' thread-cells ' of zoophytes ; and this, where it
exists, is known as the ' trichocyst-layer.' The hair-like pro-
cesses of protoplasm may be caused to protrude from the cell
by such irritation as is eflfected by the addition of a little iodine to
the water in which the animalcule is living.

The vibration of ciliary filaments, which are either disposed
along the entire margin of the body, as well as ai'ound the oral

1 On the morphology of the Vorticelhnpe see Biitschh, MorjjlwL Jalirh. xi.
p. 553.

774

MICEOSCOPIC FOEMS OF ANIMAL LIFE

aperture (fig. 593, A, B), or are limited to some one part of it,
which is always in the immediate vicinity of the m.outh, sup-
plies the means in this group of Infusoria both for progi-es-
sion through the water and for drawing alimentary particles into
the interior of their bodiea. In some their vibration is constant,
whilst in others it is only occasional. The modes of movement
which infusory animalcules execute by means of these instru-
ments are extremely varied and remarkable. Soine propel them-
selves directly forwards, with a velocity which appears, when highly
magnified, like that of an arrow, so that the eye can scarcely follow

them ; whilst others drag their
bodies slowly along like a leech.
Some attach themselves by one
of their long filaments to a fixed
point, and revolve around it with
great rapidity, whilst others
move by undulations, leaps, or
successive gyrations : in short,
there is scarcely any kind of
animal movement which they
do not exhibit. But there are
cases in which the locomotive
filaments have a bi-istle-like fii-ni-
ness, and, instead of keeping
themselves in rapid vibration, ai-e
moved (like the spines of Echini)
by the contraction of the integu-
ment from which they arise, in
such a manner that the animal-
cule crawls by their means over
a solid surface, as we see espe-
cially in TricJioda lynceus (fig.
597, P, Q). In Ghilodon and
Nassula, again, the mouth is pro-
vided with a circlet of plications
or folds, looking like bristles,
which, when imperfectly seen, re-
ceived the designation of ' teeth ; '
their function, however, is rather that of laying hold of alimen-
tary particles by their expansion and subsequent drawing together
(somewhat after the fashion of the tentacula of zoophytes) than of
reducing them by any kind of masticatory process. Some, like
Opalina, are entoparasitic, and have no mouth ; a form allied to
Opalina {Anoplophrya cirmdans) lives in the blood of Aselhis
aquaticus; other entoparasites, such as Trichonym'pha in the ' white
ant,' still possess their mouth. The curious contraction of the foot-
stalk of the Vorticella (fig. 593), again, is a movement of a difierent
nature, being due to the contractility of the tissue that occupies
the interior of the tubular pedicle. This stalk serves to attach the
bell-shaped body of the animalcule to some fixed object, such as a
leaf or stem of duck-weed ; and when the animal is in search of

Fig. 593. — Group of Vorticella nehiilifera
showing, A, the ordinary form ; B, the
same with the stalk contracted ; C, the
same with the bell closed ; D, E, F, suc-
cessive stages of fissiparous multijplica-
tion.

CILIATA 775

food, with its cilia in active vibi'ation, the stalk is fully extended.
If, however, the animalcule should have di'awn to its mouth any
particles too large to be received within it, or should be touched by
any other that happens to be swimming neai- it, or should be
'jarred ' by a smai-t tap on the stage of the microscope, the stalk
suddenly contracts into a spiral, from which it shortly aftei'wards
extends itself again into its previous condition. The central cord,
to whose conti'actility this action is due, has been described as
muscular, though not possessing the characteristic structure of either
kind of muscular fibre ; it possesses, however, the sj)ecial irritability
of muscle, being instantly called into contraction (according to the
observations of Kiihne) by electi-ical excitation. The only special
' im.pressionable ' organs ^ for the direction of their actions with the
possession of which Infusoria can be credited are the delicate
bristle-like bodies which project in some of them from the neighbour-
hood of the mouth, and in Stentor from various parts of the surface.
The red spots seen in many Infusoria, which have been designated
as eyes by Professor Ehi-enberg, fi-om their supposed correspondence
with the eye-spots of Rotifera, really beai- a much greater re-
semblance to the red spots which ai'e so frequently seen among
protophytes. R. Hertwig, who seems to have successfully defended
himself against the strictures of Professor Vogt, has described a
vorticellid — Erytliropsls agilis — as having a ]3igment-spot which
cannot but be regarded as a rudimentary eye ; Metschnikoff, Avho
thinks that Eryiliropsis is an Acinetan, found a similar form with a
similar eye near Madeira ; and Hai-ker observed that if light be
allowed to fall on a part only of a colony of Opliridmm versatile all
the members soon congregate to the illuminated portion.^

The interior of the body does not always seem to consist of a
simple undivided cavity occupied by soft protoplasm ; for the tegu-
mentary layer appeai-s in many instances to send prolongations
across it in diflferent directions, so as to divide it into chambers of
ii-regular shape, freely communicating with each other, which may
be occupied either by protoplasm, or by pai-ticles inti-oduced from with-
out. The aliinentary particles which can be distinguished in the
interior of the transparent bodies of Infusoi-ia are usually proto-
phytes of various kinds, either entire or in a fi-agmentary state.
The Diatomacese seem to be the ordinary food of many ; and the
insolubility of their loricce enables the observer to recognise them
unmistakably. Sometimes entii'e Infusoria are observed within the
bodies of othei's not much exceeding them in size (fig. 597, B) ; but
this is only when they have been recently swallowed, since the prey
speedily undergoes digestion. It would seem as if these creatures
do not feed by any means indiscriminately, since pai'ticular kinds of
them are atti-acted by particular kinds of aliment ; the ci-ushed
bodies and eggs of Entomostraca, for example, are so voraciously

1 The term ' organs of sense ' implies a consciousness of impressions, with wliich
it is difficult to conceive that unicellular Infusoria can be endowed. The component
cells of the human body do their work without themselves knowing it.

- These results are confirmed by the observations of E. Franze ; see Zeitschr.
iviss. Zool. Ivi. 1893, pp. 138-G4.

^^6 MICEOSCOPIC FOEMS OF ANIMAL LIFE

consumed by the (Jole2ys that its boch' is sometimes quite altered in
shape by the distension. This circumstance, however, by no means
proves that such creatures possess a sense of taste and a power of
determinate selection ; for many instances might be cited in which
actions of the like apparently conscioiLS nature are performed with-
out any such guidance. The ordinary process of feeding, as well as
the nature and direction of the ciliary currents, may be best studied
by diffusing thi'ough the water containing the animalcules a few
particles of indigo or cai'mine. These may be seen to be cari-ied by
the ciliary vortex into the mouth, and their passage may be traced
for a little distance down a short (usually ciliated) oesophagus.
There they commonly become aggregated together, so as to form a
little pellet of nearly glol^ular form ; and this, when it has attained
the size of the hollow within which it is moulded, seems to receive
an investment of firm sarcodic substance, resembling the ' digestive
vesicles ' of Noctihica, and to be then projected into the softer-
endosarc of the interior of the cell, its place in the cesophagus being-
occupied by other particles subsequently ingested. (This ' moulding,"
howevei', is by no means u.niversal, the aggregations of coloured
particles in the bodies of Infusoria being often destitute of any
regularity of form.) A succession of such pellets being thus intro-
duced into the cell-cavity, a kind of circulation is seen to take place
in its interior, those that first entered making their way out after
a time (first yielding uj) their nutiitive mateiials), generally by a
distinct anal oiifice, but sometimes by the mouth. When the
pellets are thus moving lound the body of the animalcule, two of
them sometimes appeal- to become fused together, so that they
obviously cannot have been separated by any firm membranous in-
vestment. The mode of formation of food vacuoles has been carefully
studied by Miss Greenwood ^ in Carchesium polyinmi'm, which may
be recommended for the study of the j)i-ocesses of protozoan
digestion. When the animalcule has not taken food for some time,
' vacuoles,' or clear spaces, extremely variable both in size and
number, filled only with a very transparent fluid, are often seen in
its proto23lasm ; and their fluid sometimes shows a tinge of colour,
which seems to be due to the solution of some of the vegetable
chlorophyll upon which the animalcule may have fed last.

Contractile vesicles (flg. 592, «, «), usually about the size of the
' vacuoles,' ai-e found, either singly or to the number of from two to
sixteen, in the bodies of most ciliated animalcules ; and may be seen
to execute i-hythmical movements of contraction and dilatation at
tolerably regulai- intervals, being so completely obliterated, when
emptied of their contents, as to be quite undistinguishable, and
coming into view again as they are I'efilled. These ^'esicles do not
change their position in the individual, and they are pretty
constant, both as to size and place, in different individuals of the
same species ; hence they ai-e obviously quite diffei-ent in character
from the ' vacuoles.' In Paramecium thei-e are always to be observed
two globulai vesicles (fig. 592, B, a, a), each of them surrounded by

1 Plril. Trans. 1894, B. pp. S55-83.

CILIATA yyj

several elongated cavities, arranged in a i-adiating manner, so
as to give to the whole somewhat of a stai'-like aspect, and
the liquid contents are seen to be propelled from the foi-mei- into
the lattei-. and vice versa. Further, in tStentor, a complicated net-
work of canals, apparently in connection with the contractile
vesicles, has been detected in the substance of the ' ectosarc,' and
traces of this may be observed in other Infusoria. In some of the
larger animalcules it may be distinctly seen that the conti'actile
vesicles have permanent valvular orifices opening outwards, and that
an expulsion of fluid from the body into the water around it is
effected by their contraction ; in some vorticellids the conti'actile
vesicle is connected by a canal with the ' vestibule ' which lies beneath
the mouth opening, and when the vesicle contracts the water is driven
into the mouth, and so to the exterior. Hence it appeal's likely that
their function is of a respiratory and depuratory nature ; and that they
serve, like the gill-openings of fishes, foi' the expulsion of water
which has been taken in by the mouth, and which has traversed the
interior of the body.

Of the rejaroduction of the ciliated Infusoria our knowledge
though imperfect has advanced. As has been well said by Mr.
Adam Sedgwick, ^ ' the more recent work of Biitschli and Maupas
[has] shown that in their I'eproduction these animals resemble other
Protozoa ; that is to say, that the whole body participates in the
reproductive fission, that the parent disappears in the offspring, and
that special conjugating cells of the natui'e of ova and spei'inatozoa are
not formed. Maupas ^ especially, by following the history of the
individual resulting from conjugation, has definitely established the
fundamental distinction between conjugation and i'eproduction, and
has thrown a flood of light upon the meaning of the whole phenomenon
of conjugation.' The best evidence is that of Gi-uber, which will be
mentioned directly. Binary subdivision would seem to be universal
among them, and has in many instances been observed (as elsewhere)
to commence in the nucleus. The division takes place in some species
longitudinally, that is, in the direction of the greatest length of the body
(fig. 593,D,E,F), in other species transversely (fig. 597, C, D) ; while
in some, as in Ghilodon cucullulus (fig. 595), it has been supposed to
occur in either direction indifferently. But it may fairly be questioned
whethei', in this last case, one set of the apparent ' fissions ' is not
really ' conjugation ' of two individuals. This duplication is per-
formed with such rapidity, under favourable circumstances, that,
according to the calculation of Professor Ehrenberg, no fewer than
268 millions might be produced in a month by the repeated sub-
divisions of a single ParaTiiecium. When this fission occurs in
Vo7'ticella (fig. 593), it extends down the stalk, which thus becomes
double for a greater or less part of its length ; and thus a whole
bunch of these animalcules may spring (by a repetition of the same
process) from one base. In some members of the same family
arborescent structures are produced resembling that of Codosiga

1 Student's Texthooli of Zoology, 1898, p. 26.

^ See particularly his memoirs, in vols. vi. and vii. of the second series of the Arch.
Zool. Exper. 1888-1).

778

MICROSCOPIC FORMS OF ANIMAL LIFE

(fig. 586) by the like process of continuous subdivision. Another
curious I'esult of this mode of multiplication presents itself in
the family OpJirycl'ina, masses of individuals which separately resemble
certain Vort'iGelUna being found imbedded in a gelatinous substance of
a greenish colour, sometimes adherent and sometimes free. These
masses, which may attain the diameter of four or five inches, present
such a strong general resemblance to a mass oiNostoc^ or even of frog's
spawn, as to have been mistaken for such ; but they simply result from
the fact that the multitude of individuals produced by a repetition of
the process of self-division remain connected with each other for a
time by a gelatinous exudation fi-om the surface of their bodies,
instead of at once becoming completely isolated. From a comparison of
the dimensions of the individual Ojihryda, each of which is about ]4o^th
of an inch in length, with those of the composite masses, some estimate

3 may be formed of the

number included in
the latter ; for a
cubic inch would con-
tain nearly eight onil-
lions of them if closely
packed ; and many
times that number
must exist in the
larger masses, even
making allowance for
the fact that the
bodies of the animal-
cules are separated
from each other
by their gelatinous
cushion, and that
the masses have their centi'al poi-tions occupied by water only.
Hence we have, in such clusters, a distinct proof of the extra-
ordinaiy extent to which multiplication by duplicative subdivision
may proceed w^ithout the interposition of any other operation.
These animalcules, however, free themselves at times from theii'
gelatinous bed, and have been observed to vmdergo an 'encysting
process ' corresponding with that of the Vorticellina. The chemical
composition of this jelly or zoocytium has been investigated by
Halliburton, who finds that it resembles vegetable cellulose in its
general properties, but difiei'S fi-om it and agi-ees with the form of
cellulose manufactured by the Tunicata in being less easily converted
into sugar.

Many, perhaps all, ciliated Infusoiia at certain times undei-go an
encysting process, resembling the passage of pi-otophytes into the ' still '
condition, and appai-ently sei-ving like it as a provision for their pre-
sei-vation under cii-cumstances which do not permit the continuance
of their ordinary vital activity. Pi-eviously to the formation of the
cyst, the movements of the animalcule diminish in vigour, and
gi-adually cease altogether ; its fonn becomes more i-ounded : its
oi'al aperture closes ; and its cilia or other filamentous prolonga-

FiG. 594. — Reproduction of Infusoria.

CILIATA

779

tions are eithei- lost oi- retracted, as is well seen in Vorticella
(fig. 596, A). A new wreath of cilia, however, is developed near
the base, and in this condition the animal detaches itself from its

0

Fig. 595. — Fissiparous multiplication of Chilodon ciiniUvlus : A,
B, C, successive stages of longitudinal fission (?) ; D, E, F, succes-
sive stages of transverse fission.

stem and swims fi-eely for a short time, soon passing, however, into
the ' still ' condition. The surface of the body then exudes a gela-
tinous excretion that hardens around it so as to form a complete
coffin-like case, within which little of the original structure of the
animal can be distinguished. Even after the completion of the cyst,
however, the contained
animalcule may often
be observed to move
freely within it, and
may sometimes be
caused to come forth
from its prison by the
mere application of
warmth and moisture.
In the simplest form of
the ' encysting process,'
indeed, the animalcule
seems to remain alto-
gether quiescent through
the whole period of its
torpidity ; so that, how-
ever long may be the
duration of its imprison-
ment, it emerges Avith-
out any essential change
in its form or condition.
But in other cases this
process seems to be sub-
servient either to multi-
plication or to metamorphosis. For in Vorticella the substance
of the encysted body (B) appears to break up (C, D) into eight
or nine segments, which, when set free by the bursting of the
cyst, come forth as spontaneously moving spherules. Each of these
soon increases in size, develops a ciliary wi'eath Avithin Avhich a mouth

Fig. 596. — Encysting process in Vorticella micro-
stoma : A, full-grown incliridual in its encysted
state ; a, retracted oval circlet of cilia ; b, nucleus ;

c, contractile vesicle ; B, a cyst se^aarated from its
stalk ; C, the same more advanced, ihe nucleus
broken up into spore-like globules ; D, the same
more developed, the original body of the Vorticella,

d, having become sacculated, and containing many
clear spaces ; at E, one of the sacculations liaving
burst through the enveloping cyst, a gelatinous
mass, e, containing the gemmules is discharged.

78o MICROSCOPIC FORMS OF ANIMAL LIFE

makes its appearance, and gradiially assumes the form of the Tricho-
dina grandhiella of Ehrenberg. It then develops a posterior wreath
of ciha and multiphes by transverse fission ; each half fijxes itself by
the end on which the mouth is situated, a short stem becomes de-
veloped, and the cilia-wreath disappears. A new mouth and ciha-
wi-eath then form at the free extremity, and the growth of the stem
completes the develoj)ment into the true vorticellan form.^ In
Trichoda lynceus, again, the ' encysting process ' appears subservient
to a hke kind of metamorphosis, the form which emerges from the
cyst differing in many respects from that of the animalcule which
became encysted. According to M. Jules Haime, by whom this
history was very carefully studied,'-^ the form to be considered as the
larval one is that shown in fig. 597, A, E, which has been described
by Professor Ehrenberg under the name of OxytricJia. This possesses
a long, narrow, flattened body, furnished with cilia along the greater
part of both margins, and having also at its two extremities a set of
larger and stronger hair-like filaments ; and its mouth, which is an
oblique slit on the right-hand side of its fore-part, has a fringe of
minute cilia on each lip. Through this mouth large particles are not
unfrequently swallowed, which are seen lying in the m.idst of the
endosarc without any surrounding vesicle ; and sometimes even an
animalcule of the same species, but in a diflTerent stage of its life, is
seen in the interior of one of these voi-acious little devourers (B). In
this phase of its existence the Trichoda undergoes multiphcation by
transverse fission, after the ordinary mode (C, D) ; and it is usually
one of the short-bodied ' doubles ' (E) thus produced that passes
into the next phase. This phase consists in the assumption of the
globular form and the almost entire loss of the locomotive append-
ages (F) ; in the escape of successive portions of the granular proto-
plasm, so that ' vacuoles ' make their appearance (G) ; and in the
formation of a gelatinous envelope or cyst, which, at first soft,
afterwards acquires increased firmness (H). After remaining foi-
some time in this condition, the contents of the cyst become clearly
separated fi-om their envelope ; and a space appears on one side, in
which ciliai-y movement can be distinguished (I). This space
gradually extends all round, and a further dischai-ge of granular
matter takes place from the cyst, by which its form becomes altered
(K) ; and the distinction between the newly formed body to which
the cilia belong and the^efiete residu.e of the old becomes more and
more apparent (L). The former increases in size, whilst the latter
diminishes ; and at last the former makes its escape through an
aperture in the wall of the cyst, a part of the latter still remaining
within its cavity (M). The body thus discharged (IST) does not diflfer
miich in appeai-ance from that of the Oxytricha before its encyst-
ment (F), though of only about two-thirds its diameter ; but it soon
develops itself (0, P, Q) into an animalcule very different from
that in which it originated. First it becomes still smaller by the
discharge of a portion of its substance ; numerous very stiflT bristle-

1 Everts, Untersuclumcjen an Voriicellanebulif era, quoted by Professor Allman,
Joe. cit.

- Annales dcs Sci. Nat. ser. iii. tome xix. 1853, j)- 109.

CILIATA

781

like organs ai-e developed, on which the animalcule creeps, as by
legs, over solid surfaces ; the external integument becomes more
consolidated on its upper sui-face, so as to become a, kind of cara-
pace ; and a mouth is formed by the opening of a slit on one side,
in front of which is a single hair-like flagellum, which turns I'ound
and round with great rapidity, so as to describe a sort of inverted
cone whereby a current is brought towards the mouth. This latter
form had been described by Professor Ehrenberg under the name of
Aspiclisca. It is veiy much smaller than the larva, the difierence
being, in fact, twice as great as that which exists between A and

Fig. 597. — Metamorphoses of Trlcliocla lijnceus : A,lsiVYa (Oxi/tricha) ; B, a
similar larva after swallowing the animalcule represented at M ; C, a very-
large individual on the point of undergoing fission ; D, another in which
the process has advanced further; E, one of the products of such fission;
F, the same body become spherical and motionless ; Gr, aspect of this
sphere fifteen days afterwards ; H, later condition of the same, showing the
formation of the cyst ; I, incipient separation between living substance
and exuvial matter ; K, partial discharge of the latter, with flattening of
the sphere ; L, more distinct formation of the confined animal ; M, its
escape from the cyst ; N, its appearance some days afterwards ; O, more
advanced stage of the same; P, Q, perfect AspidisccB, one as seen side-
ways, moving on its bristles, the other as seen from below (magnified
twice as much as the preceding figures).

P, Q (fig. 597), since the last two figures are drawn under a magni-
fying power double that employed for the j^receding. How the
As'pidisGa-ioYYO. in its turn gives origin to the Oxi/tricha- form
has not yet been made out. A similar ' encysting j)i"Ocess ' has
been observed to take place among several other foi-ms of ciliated
Infusoria ; so that, considering the strong general resemblance
in kind and degree of organisation which prevails throughout the
group, it does not seem vmlikely that it may occur at some stage of
the life of nearly all these animalcules. And it is not improbably
in the ' encysted ' condition that their dispersion chiefly takes place,
since they have been found to endure desiccation in this state,
although in their ordinary condition of activity they cannot be dried

782 MICEOSCOPIC FORMS OF ANIMAL LIFE

up without loss of life. When this circumstance is taken into
account, in conjunction with the extraordinary rapidity of multipli-
cation of these animalcules, there seems no difficulty in accounting
for the universality of their diffusion. It may be stated as a general
fact that wherever decaying organic matter exists in a liquid state,
and is exposed to air and warmth, it speedily becomes peopled with
some or other of these minute inhabitants ; and it may be fairly
presumed that, as in the case of the Fungi, the dried cysts or germs
of Infusoria are everywhere floating about in the air, ready to de-
velop themselves wherever the appropriate conditions are presented ;
but we must remember that but few definite observations have
been made as to the length of time these cysts will survive desiccation ;
at present, the observations of Nussbaum and Maupas make the
limit less than two years.

Gruber has recently reinvestigated the process of conjugation in
the Infusoiia : he finds that the nucleolus of each becomes a striated
spindle, and appi-oaches the nucleolus of the other cell ; the two
touch and finally fuse, thereby effecting an intermixture of the
different germ-plasmas. If this be the correct manner of interpret-
ing the phenomenon, it is clearly comparable to the sexual reproduc-
tion of multicellular animals.

There can be no dovibt as to the occurrence of ' conjugation '
among ciliated Infusoria ; and this not only in the free-swimming,
but also in the attached forms, as Stentor (fig. 594, 3). In
Vorticella, according to several recent observers, what has been
regarded as gemmi parous multiplication — the putting forth of a bud
from the base of the body — is really the conjugation of a small
individual in the free-swimming stage with a fully developed fixed
individual (microgamete) with whose body its own becomes fused.
But it is doubtful whether such conjugation has any reference to the
encysting process. According to Biitschli and Engelmann, the con-
jugating process results in the breaking up of the nucleus and (so-
called) nucleolus of the conjugating individuals ; these individuals
separate again, and aftei- the expulsion of the broken-up nuclear
sti'uctui-es the chai-acteristic nucleus and nucleolus are re-formed.
There is still much uncertainty in regard to the embryonic forms of
ciliate Infusoria, some eminent observers asserting that the
' gemmule ' in the first instance, besides forming a cilia- wreath, puts
forth suctorial appendages (fig. 594, 1, A, B, C), by means of which
it imbibes noui-ishment until the formation of its mouth permits it to
obtain its supplies in the ordinary way ; whilst others maintain these
acinetiform bodies to be parasites, which even imbed themselves in
the substance of the Infusoria they infest.-'

It is obvious that no classification of Infusoria can be of any
permanent value until it shall have been ascertained by the study
of their entii'e life-histoiy what are to be accounted really distinct

1 There can be no doubt that Stein was wrong in his original doctrine that the
fully developed Acinetina are only transition stages in the development of Vorti-
ceUina and other ciliated Infusoria. But the balance of evidence seems to the writer
to be in favour of his later statement, that the bodies figured in fig. 594, 1, are
really infusorian embryos, and not parasitic Acinetee.

SUCTORIA JSS

foi-ms. And the differences between them, consisting chiefly in the
shape of their bodies, the disposition of their ciha, the possession of
other locomotive appendages, the position of the mouth, the presence
of a distinct anal orifice, and the like, are mattei'S of such ti-ivial
importance as compared with those leading features of their structure
and physiology on which we have been dwelling that it does not
seem desirable to attempt in this place to give any detailed account
of them. The life-history of the ciliate Infusoria is a subject
pre-eminently woi'thy of the attention of mici'oscopists, who can
scarcely be bettei- employed than in tracing out the sequence of its
phenomena with similar care and assiduity to that disiolayed by
Messrs. Dallinger and Diysdale in the study of the Monadina. ' In
pursuing our researches,' say these excellent observers, ' we have
become practically convinced of what we have theoretically assvimed
— the absolute necessity for prolonged and patient observation of
the same forms. Competent optical means, careful interpreta-
tion, close observation, and time ai-e alone capable of solving the
problem.'

Suctoria. — The suctorial Infusoria constitute a well-marked
group, all belonging to one family, Acinetina, the nature of which
has been until recently much misunderstood, chiefly on account of
the parasitism of their habit. They may be regarded as a sub-class
of the Infusoria, and be known as the Acinetaria. Like the
typical Monadina, they are closed cells, each having its nucleus and
contractile vesicle ; but instead of freely swimming through the water,
they attach themselves by flexible peduncles, sometimes to the stems of
Vorticellince, but also to filamentous Algse, stems of zoophytes, or to the
bodies of larger animals. Their nutriment is obtained through delicate
tubular extensions of the ectosarc, which act as suctorial tentacles
(fig 598), the free exti-emity of each being dilated into a little
knob, which flattens out into a button-like disc when it is applied
to a food-particle. Free-swimming Infusoria are captured by these
organs, of which several quickly bend over towards the one which
was at fii'st touched, so as firmly to secure the prey ; and when
several have thus attached themselves, the movements of the
imprisoned animal become feebler, and at last cease altogether, its
body being di'awn nearer to that of its captor. Instead, however,
of being received into its interior like the prey of Actinophri/s, the
captured animalcule remains on the outside, but yields up its soft
substance to the suctorial power of its victor. As soon as the suck-
ing disc has worked its way through the envelope of the body to
which it has attached itself, a very raj)id stream, indicated by the
granules it cai-i-ies, sets along the tube, and pours itself into the
interior of the Acineta-body. Solid particles are not received through
these suctoi'ial tentacles, so that the Acinetina cannot be fed with
indigo or cai-mine ; but, so far as can be ascertained by observation
of what goes on within their bodies, there is a general j)rotoplasmic
cyclosis without the formation of any special ' digestive vesicles.'
The better known foi'ms of this group are i-anked undei- the two genera
Acineta ajiid Podofhrya^ which are chiefly distinguished by the
presence of a fii-m envelope or lorica in the formei', wJiile the body

784 MICROSCOPIC FORMS OF ANIMAL LIFE

of the latter is naked. In one curioiLS form, the Opliryodendron^ the
suckers are borne in a bi-ush-hke expansion on a long retractile
proboscis-like organ ; and the rare Dendrosoma, whose size is com-
paratively gigantic, forms by continuous gemmation an arborescent
' colony,' of which the individual members remain in intimate
connection with one another.

Multiplication in this group seems occasionally to take place by
transverse fission, but this is rare in the adult state. Some-
times external gemmce are developed by a sort of pinching off of a
part of the free end of the body, which includes a portion of the
nucleus ; the tentacula of this bud disappear, but its siu-face be-

FiG. 598. — Suctorial Infusoria : 1, Conjugation of Podophrya '^
qua.dripartita ; 2, formation of embryos by enlargement and sub-
division of the nucleus ; 3, ordinary form of the same ; 4, Podo-
]}hv>ja eloiu/ata.

comes clothed with cilia ; and, after a short time, it detaches itself
and swims away— comporting itself subsequently like the internal
embiyos, whose production seems the more ordinary method of
propagation in this type. These originate in the breaking up of
the nucleus into several segments, each of which incloses itself in
a protoplasmic envelope ; and this becomes clothed with cilia, by
the vibrations of which the embiyos ai-e put in motion within the
body of the parent (fig. 598, 2), from which they afterwards escape
by its rupture. In this condition (a) they swim about freely, and
seem identical with what has been described Ijy Ehrenberg as a

^ Now called, after Biitschli, Tokophrija,on account of its, mode of reproduction ;
see his Protozoa, p. 1928.

REPEODUCTION OF INFUSORIA

785

■distinct generic foi'ni, Meyatricha. And, accoixling to the observa-
tions of Mr. Badcock,^ these 3Iegc(tr icha- fovnis nudtiply freely by
self-division. After a short time, however, they settle down npon
filamentous Algte or other suppoi'ts, lose their cilia, put forth suctoi'ial
tentacles (which seem to shoot out suddenly in the first instance
but are afterwai-ds slowly i-etracted and pi-oti'uded with a kind of
spiral movement), and assume a variety of amcebiform shapes (fig.
599, 1, 2, 3), some of them corresponding to that of the genus
Trichopkri/a. In this stage they become quiescent at the approach
■of winter, the suctorial tentacles and the contractile vesicles dis-
appearing; they do not, however, seem to acquire any special
■envelope, remaining as clear, motionless, protoplasmic particles.
iut with the return of warmth their development recommences, a

Fig. 599. — Immature forms of PodophryaquadriiKirtita : 1, Anioe-
boid state {Trichophrya of Claparede and Laohmann) ; 2, the
same more advanced ; 3, incij)ient division into lobes.

footstalk is formed, and they gradually assume the chaiuctei-istic
foi-m of Podophrya quadripartita. A regular ' conjugation ' has been
observed in this type, the body of one individual bending down so
as to apply its fi'ee surface to the corresponding pai't of another,
with which it becomes fused (fig. 598, l) ; l:)ut whether this always
])recedes the pi-oduction of internal embiyos, or is any way pi-epara-
tf)i-y to propagation, has not yet been ascei-tained.^

1 Joiirn. of Roy. Microsc. Sac. vol. iii. 1880, p. .563.

- The Aciiietiiia were described both by Ehrenberg and Dujardin; but the first
full account of their peculiar organisation was given by Stein in his Organisinus der
Infusionsthierrhen. Misled, however, by their parasitic habits, Stein originally sub-
])osed them not to be independent types, but to be merely transitional stages in the
•development of Vorticellhice and other ciliate Infusoria; this doctrine he long
since abandoned. Much information as to this group will also be found in the
Ijeautiful Etudes snr les Infusoires et les BJiizoj^odes of MM. Claxmrede and Lach-
mann, Geneva, 1858-61.

3 E

786

MICROSCOPIC FOEMS OF ANIMAL LIFE

SeCTIOX II. ROTIFERA, OR WhEEL-ANIMALCULES.

We now come to that higher group of animalcules which, in
point of complexity of organisation, is as far removed, from the pre-
ceding as mosses are from the simplest protophytes, the only point
of real resemblance between the two groups, in fact, being the
minuteness of size which is common to both. A few species of the
wheel-animalcules are marine, or the inhabitants of brackish pools
near the seashore. , Dr. E. v. Daday, who has made a study of the

Fig. 600. — Botifer vuhjaris,':-As seen at B, with the wheels drawn m, and
at A with the" wheels expanded : b, eye-spots ; c, wheels ; d, antenna ;
e, jaws and teeth ; /, alimentary canal g, cellnlar mass inclosing it ;
/;, longitudinal muscles ; i, i, tubes 'of water-vascular system ; k,
young animal ; I, cloaca.

Rotifera of the Bay of Naples, stated that in 1891, 50 species were
known from the Baltic, 13 from the Mediterranean, 8 from
elsewhere, but 32 of these occur also in fresh water. The vast
majority known to us belong, therefore, to fresh water, and are to be
found in ditches, ponds, reservoirs, lakes, and slowly running streams
— sometimes attached to the leaves and stems of water-plants, some-
times creeping on Alg?e, on which some are parasitic, ^ sometimes

1 Compare particularly the interesting observations of Prof. W. Rothert in vol. ix.
189C, of the Zoolorj. Jahrbiichcr (Abth. Systemat.), pp. (57'2-71o.

ROTIFEEA 787

swimming freely through the water. They are met with also in
gutters on the house-top, in water-butts, on wet moss, grass, and
liver- worts, in the inteiioi- of Volvox globator and Vaucheria, in vege-
table infusions, on the backs of Entomostraca, in the ^dsceiu of slugs,
earth-worms, and Naiades, and in the body-cavities of Synajytce —
in fact, m almost eveiy place where thei-e are moistui-e and food.
The wheel-like organs from which the class derives its designation
are most characteristically seen in the common Rotifer (fig. 600),
where they consist of two disc-like lobes or pi-ojections of the body
whose margins are fringed with long cilia ; and it is the unintex'rupted
succession of strokes given by these cilia, each row of which nearly
returns (as it were) into itself, that gives rise by an optical illusion
to the notion of ' wheels.' The disj)osition of the cilia varies much
in the different genera, but it may be said broadly that they are ar-
ranged so as to fulfil three different purposes, viz. to bring food to the
mouth, to conduct it through the alimentary canal, and to enable the
animal to swim.

The great transpai-ence of the Rotifei-a permits their general
structure to be easily recognised. They have usually an elongated
form, similar on the two sides ; but this rarely exhibits any traces of
segmental division. The body is covered with an envelope of two
layers. The inner of these is a soft lining to the outer, which may
be soft and flexible, oi- membranous and of very varying degrees of
stifihess, or even of an inflexible substance capable of resisting the
action of caustic potash. In this latter condition it is called a lorica.
The greater number of the Rotifera have an oi-gan of attachment
at the posterior extremity of the body, which is usually prolonged
into a tail or false foot, by which they can affix themselves to any
solid object ; and this is their ordinary position when keeping their
' wheels ' in action for a supply of food or of water ; they have no
difficulty, howevei-, in letting go their hold and moving through the
water in search of a new attachment, and may therefoi-e be con-
sidered as perfectly free. The sessile species, in their adult stage,
on the other hand, remain attached by the posterior extremity to the
spot on which they have at first fixed themselves, and their cilia are
consequently employed for no other purpose than that of creating
currents in the suri'ounding water. In considering the internal struc-
ture of Rotifera we shall take as its type the arrangement which
it presents in Brachionus rubevs (fig. 601), a common large and
handsome animal, and one that bears the temporary captivity of
a compressorium i-emai-kably well.

Its vase-shaped lorica is hard and transparent ; open in front to
allow the protrusion of the head, and closed behind, except Avhere a
small aperture permits the passage of the foot. The anterioi- .
dorsal edge bears six sharp spines, and the ventral edge has a
wavy outline. The head is shaped like a truncated cone, with the
larger end forward, is rounded at each side, and cai-ries on its front
surface three pi-otuberances (s;j>), covered with stout vibi-ating hairs
called styles. All round the rim of the head runs a row of cilia which
on the ventral surface dips down into either side of a ciliated buccal
funnel. At the bottom of the buccal funnel is the niastax (qux), a

3e2

788

MICEOSCOPIC FORMS OF ANIMAL LIFE

luusculai' bulb containing the jaws or trophl (ti). These latter are
haixl, glassy bodies consisting of two hammer-like pieces called
'mctllel (fig. 602) and a third an vil-]3iece called an incus. Each
mnlleus (ms) is in two parts — the manuhriimi {mm), or handle,
and the uncus {us), of five finger-like processes, which unite to

Fig. 601. — Brachioiius ruhens: sp, styligerous prominences cw, coronal
wreath; ts, tactile styles ; a, dorsal ajitenna ; a', a', lateral antennas ; Im,
longitudinal muscles; ce, oesophagus; oy, ovary; oiii, ovum; g, germ;
vt, vibratile tags ; i, intestine ; /, foot ; t, toes ; gn, brain ; e, eye ; mx,
mastax ; ti, trophi ; gg, gastric glands ; s, stomach ; Ic, longitudinal
canals; cy, contractile vesicle ; cl, cloaca; /jc, foot-gland. (After Dr. Hudson.)

form the liammer's head. The incus (is), or anvil, is formed of two
l)rism-shaped bodies, or ra7ni (rs), pointed at tlieir fi-ee ends, and
attached at their broad ends to a thin plate called the f/da-um {fin),
wliich, seen ventrally or dorsally, looks like a, I'od. These various
pai-ts are connected by musculai- fibres, and so acted f)n l>v muscles

EOTIFEEA

789

Mttached to themselves, and to the inteiioi- of the mastax, that the
unci rise and fall at the same time that the rami open and shut.
The food is torn h\ the unci, crushed hj the rami, and then passes
between the lattei- down a shoi't cesophagus (ce) into the stomach (s).
This has thick cellulai' walls, and is lined with cilia, especially at its
lower third, which is often divided by a constiiction from the up^^er
part, and is often so different in its shape and contents as to meiit
the name of an intestine (i). The lower end of the intestine gene-
rally expands into a cloaca (cl), into which open the ducts of the
ovary [oy), and contractile vesicle {cv). Just above the mastax,
and sometimes just below it, on the cesophagus, ai'e what are sup-
posed to be salivary glands ; while ttached to the upper end of
the stomach are two gastric glands'^
(gg), often possessing visible ducts.
There are two further glands (fg)
in the foot, which is itself a j)rolon-
gation of the ventral portion of the
trunk below the aperture of the cloaca.
These foot-glands secrete a viscid sub-
stance which is discharged by ducts
passing to the tips of the two toes (t)
and which serves to attach the animal
to one spot when it is using its frontal
cilia to procure food.

Longitudinal muscles (Im) for with-
drawing the head and foot within the
lorica can be readily seen, and these
parts are driven out again by the
pressure of transverse musculai' fibi-es
body.

On eithei- side of the body is a tortuous tube commencing in a
plexus in the head and running down to open on the contractile
vesicle (cv). These tubes bear little tags (vt), each of which apj^ears
to contain a vibrating cilium. The real structure of these bodies is
uncertain, and the use of the whole apparatus is much disputed ;
but the tags are possibly very minutely ciliated funnels, theii- free
ends open to the body-cavity ; and it seems probable that the fluids
of the body- cavity are conducted through them, along the tortuous
tubes, into the contractile vesicle, and are by it discharged into the
cloaca. The apparatus would therefore be mainly an excretory one. ^

There is a bilobed nervous ganglion (gn) between the buccal
funnel and the doi'sal surface. Above it is the eye (e) — a refracting
sphere on a mass of crimson pigment. From the ganglion pass
nerve-threads to a dorsal antenna {a) and to two lateral antennce (a')
on either side of the dorsal surface. These latter organs are rocket-
headed terminations of the nervous threads, and have each a bvmdle
of fine haii's passing through a hole in the lorica. The dorsal

Fig. 602.— Malleate type of jaw.

111s, malleus

j Its, uncus.

I mm, manubrium.

( rs, ramus.

' (/"'j lulcrum.

acting on the fluids of the

1 But see Dr. Hudson's Presidential Address, Journ. of tJie Boy. Microsc. Soc.
Feb. 1891, p. 13, in which reasons are given for suspecting that the contractile vesicle
may also have a respiratory function, and the vibratile tags and longitudinal canals
an excretory one.

79P

MICKOSCOPIC FORMS OF ANIMAL LIFE

antenna has a similar bundle and lies sheathed in a tube (fig. 605)
which has its base just above the nervous ganghon, and passes
thence between the two central anterior spines of the lorica. It is
furnished with a muscle, by means of which the bunch of setfe at
the free extremity can, by invagination, be drawn within the tube.

The ovary is large and its germs are consj^icuous. The animal is
oviparous and the huge egg is easily discharged through the oviduct
and cloaca owing to the very fluid condition of its contents. It is
retained by a thread till hatched at the bottom of the lorica. There
are three kinds of eggs : the common soft-shelled eggs, which are
large, oval, and produce females ; similar soft eggs, which are
smaller, more spherical, and produce males ; and epliipinal eggs (fig.
603), with thick cellular coverings, often ornamented with spines.
These latter can be dried completely without losing their vitality,
and so, lying buried in the mud of dried-up ponds, preserve the
species for next year.

Fig. 603.
Ephippial egg.

Fig. 604. — Male : e, eye ; Ic, longi-
tudinal canals ; vt, vibratile tag ;
cv, contractile vesicle ; ss^ sperm-
sac ; ^j, penis ; /, foot ; fg, foot-
gland.

The inale (fig. 604) is but a third of the length of the female,
and is unlike it in shape. It has a cylindrical trunk, small foot, and
flat round head, surrounded by a simple ring of long cilia. It has
no lorica nor any alimentary tract of any kind, but it has a
nervous system similar to that of the female, a red eye, and antennae.
Its excretory and muscular systems are also of the female pattern.
The only other internal organ is a lai'ge sperm-sac (ss) ending at its
lower extremity in a protrusile, ciliated, hollow jjenis (/>), whose
outlet holds the position of the anus in the female ; that is, on the
doi-sal surface, at the base of the foot.

The Rotifera have been divided by Dr. Hudson and Mr. P. H. Gosse ^
into four orders, according to their powers of locomotion. These are :
1. Rhizota (the rooted). Fixed when adult.

1 The Hotif era, or Wheel-animalcules. Longmans, 1889. It should be added
t hat Dr. Plate, in 1890 (Zeitschr. f. wiss. Zool. xlxi.), has; suggested a division
a ccording to the jjaired or unpaired character of , the gonads.

OKDERS OF EOTIFERA 79 1

" " 2. Bdelloida {the leech-lihe). That swim with their eiliaiy
wreath, and creep like a leech.

3. Ploima {the sea-ivorthy). That only swim with theii- ciliaiy
wreath.

4. SciRTOPODA {the shipiJers). That swim with their ciliary wreath
and skip with artliropodous limbs.

The ordei' Rhizota contains two femilies, chiefly diflfering from
each other in the position of the mouth, which in the FloscularUdce
(figs. 1 and 2, Plate XYII) is central, lying in the body's longer axis,
but in the Ilelicertidce (fig. 3, Plate XYII) is lateral. Almost all the
species of both families live in gelatinous tubes secreted by themselves,
and often fortified in various ways : by debris gathered fi'om the
water by the action of their ciliary wreaths and showered down at
random. ; by pellets formed in a ciliated cup near the anterior end
of the body, and deposited in regular order on the gela-
tinous tube ; or by large fsecal pellets also regularly
deposited.

The second order, Bdelloida (fig. 7, Plate XYII), while
having many points in common with the Jlelicertidce, have
i\ foot peculiaily their own. It has several false joints
that can be drawn one within the other like those of a
telescope. The corona consists of two neaily circular discs,
each surrounded with a double row of cilia, and both of
these can be withdrawn into an infoldingof the ventral surface at the
anterior end of the body, leaving the animal with a long pointed
conical head. When the discs are so furled the animal fixes the toes of
its foot, elongates the foot and body, catches hold with the furthest
point of the conical head, releases the foot, and then, contracting the
body and foot while the head remains fixed, draws forward the toes
and refixes them, and so da ccqjo. It can swim, however, in the usual
fashion, with its ciliary wreath. All the species of this order can,
under proper cii-cumstances, be dried uj) into balls, which will i-etain
their vitality for even years, though in a state of utter du>stiness.
This is due to their secreting round their bodies (after having drawn in
both head and foot) a gelatinous covering which retains the body-fluids
safe from evaporation. ^ This process takes some time, so that if
an attempt is made to dry them on an oi-dinary glass-slip they simply
disintegrate. In a house gutter or in wet moss or sand, where the
drying up of the water, in which the Rotifei-a are, is slowly accom-
plished, the animals have time to complete their gelatinous coverings
before the water fails them. In this order the males have not as yet
been discovered.

The third order, Ploima^ is divided into a loricate and an illoricate
group, which are not, however, very sharply separated ; as in some
cases the outer layer of the skin is, though horny, yet thin and
flexible. Brachionus ruhens (fig. 601), which has already been fully
described, is a good type of the Loricata and Gopeus cerherus (fig. 6,
Plate XYII) of the Illoricata. Most of the species of this order have

1 See Davis in Monthly Microsc. Journ. vol. ix.!1863, i^. 207 ; Slack, at p. 241 of
eaiiie volume ; and the report of a discussion on the subject at the Eoyal Microsco-
pical Society, Journ. of Royal Microsc. Soc. 1887, p. 179.

792 MICKOSCOPIC FOKMS OF AxXIMAL LIFE

a forked jointed foot, the fork being foi-med of two toes varying-
greatly in size and shape, but ah secreting the viscous fluid already
mentioned. The great majority of the Rotifera belong to the
Pldima.

The foui'th ordei', Scirtopoda, contains but one family, Pedalionidce,
and has only two genera, Fedalion and Hexarthra, and the latter of
these has but one known species, the foi-mer only two. Pedcdion
(figs. 4, 5, 8, Plate XYII) is an extraordinary creature. Its internal
organs are on the usual rotiferous plan, but its body bears no fewer
than six hollow hmbs, ending in plumes like those oithe Arthropod.a,
and woi'ked by pairs of opposing muscles which travei-se their entire
length. These limbs are arranged round the body, some on the
dorsal, some on the ventral suiface, and all lunning parallel to the
body's longei- axis. In Hexartlira, on the contrary, all the limbs
are on the ventral surface, and are arranged radiatingly. There is
no foot in either Rotifei- ; but in Pedalion there are two ciUated
club-like processes at the posteidoi- extremity, rismg above the
dorsal surface and secreting a similar viscous fluid to that secreted
in the toes of othei- Rotifera.

This strange ci-eature was discovered by Dr. C T. Hudson in a pond
near Clifton in 1871 ; Hexarthra was discovered by Dr. Schmarda
in a brackish ditch near the Nile in 1853 ; their arthropodous limbs
give them a strong resemblance to a JSTauplius larva, and make it
]3robable that the nearest relations of the Rotifera are the Aethro-
POi)A ; ^ at any rate, thei-e is more probability in this suggestion than
in that of Professor Hartog that they are allied to the Pilidium-
larva of Nemertine worms. ^

1 The following treatises and memoirs (in addition to those already referred to)
contain valuable information in regard to the life-history of animalcules and their
principal forms: — Ehrenberg, Die Infusionsthierchen, Berlin, 1838; Dujardin,
Histoire naturelle des Zoojjhytes infusoires, Paris, 1841; Pritchard, History of
Infusoria, 4th ed. London, iS61 (a comprehensive repertory of information) ;
Stein, Der Organismus cler Infusionstliiere, Leipzig : Erste Abtheilung, 1859 ; Zweite
Abtheilung, 1867 ; Dritte Abtheilung, Halfte I. 1878. Saville Kent's Manual of the
Infusoria, 1880-1; and Professor Biitschli's Protozoa (1880-1) in the new edition of
Bronn's Thierreichs. For the Ehizopoda and Lifusoria specially see Claparede
and Lachmann, Etudes sur les Infusoires et les Rliizojiodes, Geneva, 1858-61 :
Cohn, in Siebold und KoUiker's Zeitschrift, 1851-4 and 1857; Lieberkiilm, in
Mailer's Arcliiv, 1856, and Ann. of Nat. Hist. 2nd ser. vol. xviii. 1856; Engelmamij
Zur Naturgeschiclite der Infusionstliiere, 1862 ; and Professor Biitschli's Studien
iiber die Conjugation der Infusorien <£-c., 1876. For the Eotifera specially see
Leydig, in /S-ie6oZ(Z und KoUiker's Zeitschrift, Bd. vi. 1854; Gosse on Melicerta
ringens, in Quart. Journ. of Microsc. Sci. vol. i. 1853, p. 1 ; on the Manducatory
Organs of Eotifera, Phil. Trans. 1856 ; Huxley on Lacinularia socialis in Trans.
Microsc. Soc. ser. ii. vol. i. 1853, x^. 1 ; Cohn, in Siebold und KoUiker's Zeitschrift^
Bde. vii. ix. 1856, 1858 ; Dr. Moxon, Trans. Linn. Soc. 1864 ; Karl Eckstein, Siebold
2ind KoUiker's Zeitschrift, 1883; Bourne, Botifera, in the 9th edition of the-BHc/y-
clopcedia Britannica ; Joliet, ' Monographie des Melicertes,' Archiv. zool. exjper. ser.
ii. torn. i. p. 131 ; and Plate, Jenaisclie Zeitschr. xix. p. 1. The Botifera, or Wheel-
animalcules, by Hudson and Gosse, Longmans, 1889. This has been usefully sup-
plemented by Mr. C. P. Eousselet in two papers entitled ' List of New Eotifers since
1889,' in Journ. B. Microsc. Soc. 1893, pp. 450-8, and ' Second List,' &c. in the
same journal for 1897, pp. 10-15. The bibliographical lists appended by Mr. Eousse-
let will be found of much service, as since the publication of the work of Messrs.
Hudson and Gosse there has been a great revival among the students of this group.
Mr. Slack's Marvels of Pond Life, 2nd edit. (London, 1871), contains many interest-
ing observations on the habits of Lafusoria and Eotifera.

^ See his remarks on the relation of the Rotifera to the Trochojjhore, in JBejj. Brit..

West,NewmaTi chromo.

Typical Rotifers

793

APPENDIX TO CHAPTER XIII

The preparation and preservation of Eotifers well extended as in life to
serve as type specimens is now possible, and the following is an outline of
Mr. C. F. Eousselet's method, which consists of three stages: narcotising,
killing and fixing, and preserving. The whole operation is necessarily
performed under a dissecting microscope.

The first step in the preparation of Eotifers is to isolate the animals
by transferring as many as may be available by means of a very
tine pipette to a fresh watchglass full of perfectly clean water until all
particles of foreign matter have been eliminated. This is necessary
because when the animals are dead these particles adhere to the cilia of
the Eotifers, from whence it is very difficult to remove them. In the case
of fixed Eotifers, such as Melicerta, Limnias, Stephanoceros, &c., it is
necessary to cut off and trim a very small piece of the plant to which
they are attached ready for mounting, so as not to have to do this when
the animals are killed and prepared. It is also necessary to separate the
different species, as most of them require a little different, more or less
prolonged, treatment under the narcotic. The great difficulty with
Eotifei's has always been to kill and fix them whilst fully extended as in
life. The most rapid killing agents are too slow to prevent complete re-
traction ; recourse, therefore, has been had to narcotising, and after many
experiments a satisfactory Jiarcotic has been found in the following
mixture :

2 per cent, solution of hydrochlorate of cocaine . . . .3 parts
Methylated spirit . . . . • . . . .1 ,,
Water 6 „

The Eotifers then, separated as to species, and in a watchglass full of
perfectly clean water, are ready for narcotising. One or two drops of the
above solution are added to the water and mixed. The effect of the
narcotic is most varied in different species. Some will not mind it at
all and continue to swim about, others will contract at once but soon
come out again and swim about at a diminishing race until they finally
sink to the bottom with the cilia beating but feebly. Then is the right
time for killing and fixing. In the case of more vigorous species, after
three or four minutes another dose of two or three drops of the narcotic
is added, and then repeated again if necessary imtil it is seen that the
aniroals can move but very slowly. At this moment the animals are
killed quickly and suddenly by adding one drop of very weak (| to ^ per
cent.) solution of osmic acid.

^The different species of Eotifers \avj so much in their behaviour under
the narcotic that it is by no means easy to always hit the exact moment
for killing the animals fully extended ; repeated failures and practice
alone can guide one in this respect. It is very essential that the animals
be still living when the osmic acid is added, as when a Eotifer is quite
dead various post-mortem changes begin immediately to take place in
the tissues, whilst it is desired to fix and preserve the tissues as in life.
The word ' fixing ' implies rapid killing and at the same time hardening
of the tissues to such an extent as to prevent their undergoing any
further change by subsequent treatment with preserving fluids. The
action of osmic acid is very rapid, half a minute being quite enough ; if

Ass. 1896, p. 836, and compare with them the suggestion of Dr. Plate in Zeitschr. f.
lOTSS. ZooZ. xlix. (1889), pp. 1-41.

794 MICROSCOPIC FOEMS OF ANIMAL LIFE

left much longer in this fluid the animals will become more or less
blackened, and it is therefore necessary to remove the Kotifers as soon as
possible, by means of the fine pipette, in three or four changes of clean
water, so as to get rid of every trace of the acid. Finally the animals
are transferred into the preservative fluid, which is a solution of 25- per cent,
formaldehyde (the commercial formalin is a 40 per cent, solution of
formaldehyde).

In this preservative the Rotifers are mounted in ringed or excavated
cells on micro-slides in the usual way.'

^ More detailed jjarticulars in the treatment of the various species and in
mounting in cells will be found in Mr. Rousselet's papers on the subject, jparticularly
those of March 1895 and November 1898, in the Journ. of the Quekett Micr. Club,
vol. vi. pp. 5-13, and vol. vii. pp. 93-97.

795

CHAPTER XIY

FOBAMINIFEBA AND BADIOLABIA

Returning now to the lowest or rhizopod type of animal life
(Chapter XII), we have to direct our attention to two very remarkable
series of forms, almost exclusively marine, under which that type
manifests itself, all of them distinguished by skeletons so consolidated
by mineral deposit as to i-etain their form and intimate structui'e
long after the animals to which they belonged have ceased to live,
even for those undefined periods in which they have been imbedded
as fossils in strata of various geological ages. In the first of these
groups, the Foraminifera, the skeleton usually consists of a calcareous
many-chambered shell, which closely invests the sarcode-body, and
which, in a large proportion of the group, is perforated with numerous
minute apertures ; this shell, however, is sometimes replaced by a
' test,' formed of minute grains of sand cemented together ; and
there are a few cases in which the animal has no other protection
than a membranous envelope. In the second group, the liadiolaria,
the skeleton is always silicious and may either be composed of dis-
connected spicules, or may consist of a symmetiical open framework,
or may have the form of a shell perforated by numerous apertures,
which more or less completely incloses the body. The Foraminifera
probably take, and always have taken, the largest share of any animal
group in the maintenance of the solid calcareous poi'tion of the earth's
crust by separating from its solution in ocean- water the carbonate
of lime continually brought down by rivers from the land. The
Radiolaria do the same, though in far less measure, for the silex.
And both extract from sea- water the organic matter universally dif-
fused through it, converting it into a form that serves for the nutri-
tion of higher marine animals.

Section I. — Foraminifeea.'

The animals of this group belong to that reticularian foim of
the rhizopod type in which^ — with a differentiation between the
containing and the contained protoplasm which is involved
in the formation of a definite investment^ — a distinct nucleus
(sometimes single, in other cases multiple) is probably always

1 For the earlier literature ponsult Mr. C. D. Sherbom's ' Bibliography of the
Foraminifera, recent and fossil, from 1565 to 1888,' London, 1888.

796 MICROSCOPIC FOEMS OF ANIMAL LIFE

present.^ The shells of Foraminifei'a are. for the most pail, ^>oZ,y-
thalamous, or many-chambered (Plates XVIII and XIX), often so
strongly resembling those of i\"awit7?As, Bpirula., and other cephalopod
molluscs, that it is not sui-prising that the older naturahsts, to whom
the striictnre of these animals was entirely miknown, ranked them
luider that class. But independently of the entire difference in the
charactei- of the animal bodies by which the two kinds of shells ai-e
formed, there is a most important distinction l^etween them in i-egard
to the relation of the animal to the shell. For whilst in the
chambered shells of the Nautilus and other cephalopods the animal
is a single individual tenanting only the last formed chamber, and
withdrawing itself from each chamber in succession, as it adds to this
another and lai-gei" one, the animal of a nautiloid foraminifer has a
com2)osite body consisting of a number (sometimes very large) of
' segments,' each repeating the rest, which continues to increase by
gemmation or budding from the last-formed segment. And thus each
of the chambers, howevei' numerous they may be, is not only formed,
but continues to be occupied by its own segment, which is connected
with the segments of earlier and later formation by a continuous
' stolon ' (or creeping stem), that passes through apertures in the
septa or partitions dividing the chambers. From what we know of
the semi-fluid condition of the sarcode-body in the reticularian type,
there can be little doubt that there is an incessant circulatory change
in the actual substance of each segment ; so that the material taken
in as food by the segments nearest the surface or margin is speedily
diffused through the entire mass. The relation between these ' poly-
thalamous ' foi-ms, thei-efore, and the monothalamous or single-
chambei-ed, of which we have already had an example in Gromia.
and of which others will be presently described, is simply that,
whereas any buds produced by the latter detach themselves to form
separate indi"sdduals, those put forth by the former remain in con-
tinuity with the parent stock and with each other, so as to form a
' composite ' animal and a ' polythalamous ' shell.

According to the j)lan on which the gemmation takes place will
be the configuration of the shelly structure produced by the seg-
mented body. Thus, if the bud should be put forth from the
aperture of a Lagena (Plate XIX, fig. 12) in the direction of the
axis of its body, and a second shell should be formed around this
bud in continuity with the first, and this j^rocess should be succes-
sionally repeated, a straight I'od-like shell would be produced, whose
multiple chambei'S communicate with each other by the openings that
originally constituted their mouths, the mouth of the last-formed
chamber being the only aperture through which the protoplasmic
body, thus composed of a number of segments connected by a
pedimcle or ' stolon ' of the same matei-ial, could now project itself
or draw in its food. The successive segments may be all of the
same size, or nearly so, in which case the entire rod will approach
the cylindi'ical foi-m, oi' will resemble a line of beads ; but it often
happens that each segment is somewhat larger than the preceding

1 Dr. Sehaudinn (Zeitschr. f. tviss. Zool. lix. 1895, p. 191) lias traced the
details of nuclear division in Calcituba polymorpha.

798 MICEOSCOPIC FOEMS OF ANIMAL LIFE

to such an extent that they are scarcely, or not at all, ^-isible
externally, as is the case in Cristellaria (fig. 17), Polystomella (fig. 23),
and Nonionina. The turbinoid spire may coil so rapidly round an
elongated axis that the number of chambers in each turn is very
small; thus in Glohigerina (figs. 20, 21, Plate XIX) there are
usually only four ; and in Valvulina the regular number is only
three. Thus we are led to the 6ismfflZarrangement of the chambers,
which is characteristic of the textnlarian group (fig. 8, a, b, and 9,
Plate XYIII), in which we find the chambers arranged in two i-ows,
each chamber communicating with that above and that below it
on the opposite side, without any direct communication with the
chamber of its own side, as will be understood by reference to fig.

Fig. 607. — Discorbina glohularis (Bosalina varians, Schultze),
with its pseudopoclia extended.

622, A, wdiich shows a ' cast ' of the sarcode-body of the animal. On
the other hand, we find in the nautiloid spire a tendency to pass
(by a curious transitional form to be presently described) into the
cyclical mode of growth ; in which the original segment, instead of
budding forth on one side only, developes gemmce all round, so that
a ring of small chambers (or chamberlets) is formed around the
piimordial chamber, and this in its turn surrounds itself after the
like fashion with another ring ; and by successive repetitions of the
same process the shell comes to have the foi-m of a disc made up of
a great number of concentric rings, as we see in OrbitoUtes (fig. 609)
and in Gycloclypeus (fig. 627).

These and other differences in the p/a?i of groioth were made by

Place XIX

AT Hollick lith.

Edwin Wilson, Cumbrido''

A TYPICAL, GROUP OF FOPAMINIFERA 12 bo 26.

FOEAMINIFEEA 799

M. d'Orbigny tlie foundation of his classification of this group,
which, though at one time generally accepted, has now been aban-
doned by most of those who have occupied themselves in the study
of the Foraminifei-a. For it has come to be genei-ally admitted that
' plan of growth ' is a chai-acter of very suboixlinate impoi'tance
among the Foraminifera, so that any classification which is primarily
based upon it must necessarily be altogether unnatural, those
characters being of primary impoi'tance which have an immediate
and direct i-elation to the physiological condition of the animal,
and are thus indicative of the i-eal affinities of the several groups
which they serve to distinguish. The most imjDOiiant of these
characters will now be noticed.^

Two very distinct types of shell structure prevail among ordinary
Foraminifei'a — namely, the forcellanous and the hyaline or vitreous.
The shell of the formei-, when ^dewed by refiected light, presents an
opaque-white aspect which bears a strong resemblance to poi'celain ;
but when thin natural or artificial laminte of it ai'e viewed by trans-
mitted light the opacity gives place to a rich brown or amber
colour, which in a few instances is tinged with crimson. No
structure of any description can be detected in this kind of shell sub-
stance, which is apparently homogeneous throughout. Although
the shells of this ' porcellanous ' tyjDe often present the appearance
of being perforated with foramina, yet this appeai-ance is illusory,
being due to a mere ' pitting ' of the external surface, which, though
often very deep, never extends through the whole thickness of the
shell. Some kind of inequality of that surface, indeed, is extremely
common in the shells of the ' porcellanous ' Foraminifera, one of the
most frequent foims of it being a regular alternation of ridges and
furrows, such as is occasionally seen in Miliola, but which is an
almost constant characteristic of Peneroj^lis (fig. 606). But no
difference of texture accompanies either this or any other kind of
inequality of surface, the raised and depressed portions being alike
homogeneous. In the shells of the vitreous or hyaline type, on the
other hand, the proper shell substance has an almost glassy trans-
parence, which is shown by it alike in thin natural lamellse and in
artificially prepared ■ specimens of such as are thicker and older. It
is usually colourless, even when (as in the case with many RotalUnce)
the substance of the animal is deeply coloured ; but in some few
species, such as Glohigerina rubra, Truncatvlina rosea, and Polytrema
miniaceum, the shell is commonly, like the animal body, of a more
or less deep crimson hue. All the shells of the hyaline type are
beset more or less closely withtubular perforations, which jaass dii'ectly,
and (in general) without any subdivision, from one surface to the
other. These tubuli are in some instances sufficiently coarse for
their orifices to be distinguished with a low magnifying power as
' punctations ' on the surface of the shell, as is shown in fig. 607 ;
whilst in other cases they are so minvite as only to be discernible in
thin sections seen by transmitted light under a higher magnifying

1 This subject^will be found amply discussed in the Author's Introduction to the
Study of the Foraminifera, pubhshed by the Ray Society, to which work he would
refer such of his readers as maj^ desire more detailed information in regard to it.

8oO MICEOSCOPIC FORMS OF ANIMAL LIFE

power, as is shown in figs. 632, 633. When they are veiy numerous
and closely set, the shell deiives from theii- pi-esence that kind of
opacity which is chai-actei-istic of all minutely tubular textui-es
whose tubuli are occupied either by air oi- by any substance having
a i-efi-active powei- diffei-ent from that of the intertubulai- substance,
however perfect may be the ti'anspai-ence of the latter. The straight-
ness, parallelism, and isolation of these tubuli are well seen in verti-
cal sections of the thick shells of the largest examples of the group,
such as Nuimnulites (fig. 631). It often happens, however, that
certain parts of the shell are left unchannelled by these tubuli ; and
such are readily distinguished, even under a low magnifying power,
by the readiness with which they allow transmitted light to pass
through them, and by the j^eculiar viti-eous lustre they exhibit when
light is thrown obliquely on their surface. In shells formed upon
this type we frequently find that the sui-face presents either bands
or spots which ai-e so distinguished, the non-tubular hands visually
marking the position of the septa, and being sometimes raised into
lidges, though in othei- instances they are either level or somewhat
depi-essed ; whilst the non-tubulai- sjjots may occur on any part of
the surface, and are most commonly raised into tubercles, which
sometimes attain a size and number that give a very distinctive
aspect to the shells that bear them.

Between the comparatively coarse pei-forations which are common
in the rotaline type, and the minute tubuli which are characteristic
of the numifnuline, there is such a continuous gradation as indicates
that their mode of formation, and probably their uses, are essen-
tially the same. In the foi-mer, it has been demonsti-ated by actual
observation that they allow the passage of pseudopodial extensions
of the sai'code-body thi-ough evei-y part of the external wall of the
chambers occupied by it (fig. 607) ; and there is nothing to oppose
the idea that they answer the same j)urpose in the latter, since,
minute as they are, their diameter is not too small to enable them
to be travei-sed by the finest of the threads into which the branching
pseudopodia of Foi-aminifera ai'e known to subdivide themselves.
Moreover the close approximation of the tubuli in the most finely
perfoi-ated nummulines makes their collective ai-ea fully equal to
that of the lai-gei' but moi-e scattei-ed poi-es of the most coarsely per-
forated rotalines. Hence it is obvious that the tuhulation or non-
tuhulation of foi-aminifei-al shells is the key to a very important
physiological difierence between the animal inhabitants of the two
kuids i-espectively ; foi- whilst eveiy segment of the sarcode-body in
the foi-mei- case gives oft' pseudopodia, which pass at once into the
suiTOunding medium, and contril)ute by theii- action to the nutrition
of the segment from which they pi-oceed, these pseudopodia ai'e
limited in the latter case to the final segment, issuing forth only
throvigh the apei-tui'e of the last chambei-, so that all the nuti'ient
material which they di-aw in must l^e first i-eceived into the last seg-
ment, and be transmitted thence fi-om one segment to another until
it reaches the earliest. With this difierence in the physiological con-
dition of the animal of these two types is usually associated a further
very impoi-tant diftei-ence in the conformation of the shell — viz.

FOEAMINIFEEA 8oi

that whilst the aperture of communication between the chambers
and between the last chamber and the exterior is visually very small
in the ' vitreous ' shells, sei-ving merely to give passage to a slender
stolon or thread of sarcode from which the succeeding segment may
be budded oflf, it is much wider in the ' porcellanous' shells, so as to
give passage to a ' stolon ' that may not only bud off new segments,
but may serve as the medium for transmitting nutrient material
from the outer to the inner chambers.

Between the highest types of the porcellanous and the vitreous
series respectively, which frequently bear a close resemblance to
each other in Jorm, there are certain other well-mai'ked differences
in structure, which clearly indicate their essential dissimilarity.
Thiis, for example, if we compare Orhitolites (fig. 609) with Cyclo-
clypeus we recognise the same plan of growth in each, the chamber-
lets being arranged in concentric rings around the primordial
chamber ; and to a superficial observer there would appear little
difference between them. But a minuter examination shows that
not only is the textui-e of the shell ' porcellanous ' and non-tubular
in Orhitolites, whilst it is ' vitreous ' and minutely tubular in Cyclo-
clypeus, but that the partitions between the chamberlets are single
in the former, whilst they are double in the latter, each segment of
the sarcode-body hairing its own proper shelly investment. More-
over, betwen these double partitions an additional deposit of cal-
careous substance is very commonly found, constituting what may
be termed the intermediate skeleton ; and this is traversed by a
peculiar system of inosculating canals, which pass around the
chamberlets in interspaces left between the two laminae of their par-
titions, and which seem to convey through its substance extensions
of the sarcode-body whose segments occupy the chamberlets. We
occasionally find this ' intermediate skeleton ' extending itself into
peculiar outgroioths, which have no direct relation to the chambered
shell. Of this we have a very curious example in Calcarina ; and it
is in these that we find the ' canal system ' attaining its greatest
development. Its most regular distribution, however, is seen in
Polystomella and in Operculina ; and an account of it will be given
in the description of those types.

Porcellanea. — Commencing, now, with the porcellanous series,
we shall briefly notice some of its most important forms, which are
so related to each other as to constitute but the one family Miliolida.
Its simplest type is presented by the Cornuspira of our own coasts,
found attached to seaweeds and zoophytes ; this is a minute spiral
shell, of which the interior forms a continuous tube not divided into
chambers ; the latter jjortion of the .spire is often very much flattened
out, as in Peneroplis, so that the form of the mouth is changed from
a circle to a long narrow slit. Among the commonest of the Fora-
minifera, and abounding near the shores of almost every sea, are
some forms of the milioUne type, so named from the resemblance of
some of their minute fossilised forms (of which enormous beds of
limestone in the neighbourhood of Paris are almost entirely com-
posed) to millet-seeds. The peculiar mode of growth by which these
are characterised will be best understood by examinina', in the first

3 F

802 MICROSCOPIC FOEMS OF ANIMAL LIFE

instance, the form whicli has been designated as Sjnroloculina. This
shell is a spiral, elongated in. the direction of one of its diameters,
and having at each turn a contraction at either end of that diameter
which partially divides each convolution into two chambers ; the
separation between the consecutive chambers is often made more
complete by a peculiar projection from the inner side of the cavity,
known as the ' tongue ' or ' valve,' which may be considered as an
imperfect septum. Now it is a very common habit in the milioline
type for the chambers of the later convolutions to extend themselves
over those of the earlier, so as to conceal them more or less com-
pletely ; and this they very commonly do somewhat unequally, so that
more of the earlier chambers are visible on one side than on the
other, ilfi^io^ce thu.s modified (fig. 1, PL XVIII) have received the
names of Quinquelocidina and Triloculina according to the number of
chambers visible externally ; but the extreme inconstancy which is
found to mark such distinctions, when the comparison of specimens
has been sufficiently extended, entirely destroys their value as differ-
ential characters, and the term Miliolina is now more frequently
applied to them collectively. Sometimes, on the other hand, the
earlier convolutions are so completely concealed by the later that only
the two chambers of the last turn are visible externally ; and in this
type, which has been designated Biloculina, there is often such an
increase in the breadth of the chambers as altogether changes the
usual proportions of the shell, which has almost the shape of an egg
when so placed that either the last or the penultimate chamber faces
the observer. It is very common in milioline shells for the external
surface to present a ' pitting,' more or less deep, a ridge-and-furrow
arrangement (fig. .3), or a honeycomb division ; and these diversities
have been used for the characterisation of species. Not only, how
ever, may every intermediate gradation be met with between the
most strongly mai'ked forms, but it is not at all uncommon to find
the surface smooth on some parts, whilst other parts of the surface
' in the same shell are deeply pitted or strongly ribbed or honey-
combed ; so that here, again, the inconstancy of these differences
deprives them of much of their value as distinctive characters.

An interesting ilhistration of the tendency to dimorphism
amongst the Foraminifera has been observed by MM. Munier
Ohalmas and Schlumberger ^ in the structure of the shells of this
group. They find that while two forms, which they distinguish as
form A and form B, are similar externally they difier in internal
structui-e, form B having its initial chamber much smaller than that
of form A, and this ' microsphere ' is followed by a larger numbei- of
chambers than is the ' megasphere ' of form A. What this difference
signifies it is at present impossible to say, but it has been suggested
that it may be one of sexual character, or, better, of a series in a cycle
of generations. The obsei-vations of the French naturalists refei-red
to open out a new field of inquiry, and one which is enjoying the
attention of several woi'kers in this depai-tment of research.^

1 Bulletin Soc. Geol. ser. iii. vol. xiii. p. 273.

- Gf. J. J. Lister in Phil. Trans. 136 B (1895), p. 401, and F. Schaudinn, ' Ueber
den Dimorphismus der Foraminiferen,' S.B. Ges. Naturf. Berlin, 1895, p. 87.

PENEEOPLIS; ORBICULINA 803

Revei'ting again to the primitive type presented in the simple
spiral of Cornuspira., we find the most complete development of
it in Peneroplis (fig. 606), a very beautiful form, which, although
not to be found on our OAvn coasts, is one of the commonest of all
Foraminifera in the shore-sands and shallow-water dredgings of
wanner regions. This is normally a nautiloid shell, of which the
spire flattens itself out as it advances in growth. It is marked
externally by a series of transverse bands, which indicate the posi-
tion of the internal septa that divide the cavity into chambers ; and
these chambers communicate with each other by numerous minute
pores traversing each of the septa, and giving passage to threads
of sarcode that connect the segments of the body. At a is shown
the ' septal plane ' closing in the last-formed chamber, with its single
row of pores thi'ough wdiich the pseudopodial filaments extend them-
selves into the surrounding medium. The surface of the shell,
which has a peculiai-ly ' porcellanous ' aspect, is marked by closely
set strice that cross the spaces between the successive septal bands ;
these markings, however, do not indicate internal divisions, and are
due to a siu^face-furrowing of the shelly walls of the chambers. This
type passes into two very cuiious modifications, one having a spire
which, instead of flattening itself out, remains turgid, like that of a
JVautilios, having only a single apei'ture, which sends out fissured
extensions that subdivide like the branches of a tree, suggesting the
name of Dendritina, the other having its spire continued in a rec-
tilineal direction, so that the shell takes the form of a crosier, this
being distinguished by the name of /Spirolina. A careful examina-
tion of intermediate forms, however, has made it evident that these
modifications, though ranked as of generic value by M. d'Orbigny,
are merely varietal, a continuous gradation being found to exist
from the elongated septal plane of the typical Peneroplis, with its
single row of isolated pores, to the arrow-shaped septal plane of
Dendritina, with all its pores fused together (so to speak) into one
dendritic aperture, and a like gradation being presented between
the ordinary forms and the ' spiroline ' varieties, with oval or even
circulai- septal plane, into which both Peneroplis and Dendritina
tend to elongate themselves.

From the ordinary nautiloid multiloculai- spiral we now pass to
a more complex and highly developed form, which is restricted to
tropical and subtropical regions, but is there very abundant — that,
namely, which has received the designation Orhiculina (fig. 606).
The relation of this to the preceding type Avill be best understood
by an examination of its earlier stage of growth ; for here we
see that the shell resembles that of Peneroplis in its general form,
but that its principal chambers are divided by ' secondary septa '
passing at right angles to the primary into ' chamberlets ' occupied
by sub-segments of the sarcode-body. Each of these secondary
septa is perforated by an aperture, so that a continuous gallery is
formed, through which (as in fig. 609) there passes a stolon that
unites together all the sub-segments of each row. The chambei-lets
of successive rows alternate with one another in position ; and the
pores of the principal septa are so disposed that each chamberlet of

3f2

804 MICROSCOPIC FORMS OF ANIMAL LIFE

any row no,rnially communicates with two chamberlets in each of the
adjacent rows. The later turns of the sjDire very commonly grow com-
pletely over the earher, and thus the central portion or ' umbilicus '
comes to be protuberant, whilst the growing edge is thin. The spire
also opens out at its growing margin, which tends to encircle the first-
formed portion, and thus gives rise to the pecuhar shape represented
in fig. 606, in the illustration on the extreme right, which is the
common acluncal type of this organism. But sometunes even at an
early age the growing margin extends so far round on each side
that its two extremities meet on the opposite side of the original
spire, which is thus completely inclosed by it ; and its subsequent
growth is no longer sjnral but cyclical, a succession of concentric
rings being added, one around the other, as shown in the middle
illustration in the same figure. This change is extremely curious, as
demonstrating the intimate relationship between the spiral and the
cyclical plans of growth, which at first sight appear essentially distinct.
In all but the youngest examples of Orbiculina the septal plane pre-
sents more than a single row of pores, the number of rows increasing
in the thickest specimens to six or eight. This increase is associated
with a change in the form of the sub-segments of sarcode from little
blocks to columns, and with a greater complexity in the genei-al
arrangement, such as wih be more fully described hereafter in
Orhitolites. The largest existing examples of this type are far sur-
passed in size by those which make iip a considerable part of a
Tertiary limestone on the Malabar coast of India, whose diameter
i-eaches seven or eight lines.

A very curious modification of the same general plan is shown in
Alveolina, a genus of which the largest existing forms (fig. 608) are
commonly about one-third of an inch long, while far lai-ger speci-
mens are found in the Tei-tiary limestones of Scinde. Here the
spire turns round a very elongate axis, so that the shell has almost
the form of a cylinder drawn to a point at each extremity. Its
surface shows a series of longitudinal lines which mark the principal
septa ; and the bands that intervene between these are marked trans-
versely by lines which show the subdivision of the principal chambei-s
into ' chamberlets.' The chamberlets of each row are connected with
each other, as in the preceding type, by a continuous gallery ; and
they communicate with those of the next row by a series of multiple
pores in the principal septa, such as constitute the external orifices of
the last-formed series seen on its septal plane at a, a.

The highest development of the cyclical plan of growth which
we have seen to be sometimes taken on by Orbiculina is found in
Orhitolites ; a type which, long known as a veiy abundant fossil in
the earlier Tertiaries of the Paris basin, has lately proved to be
scarcely less abundant in cei-tain parts of the existing ocean. The
largest recent specimens of it, sometimes attaining the size of a
shilling, have hitherto been obtained only from the coast of New
Holland, the Fijian reefs, and various other parts of the Polynesian
Archipelago ; but discs of comparatively minute size and simpler
organisation ai-e to be found in almost all foraminiferal sands and
dredgings from the shores of the warmer regions of the globe, being

ALVEOLINA

805

especially abundant in those of some of the Philippine Islands, of the
Red Sea, of the Mediterranean, andespecially of the^gean. When
such discs are subjected to microscopic examination, they ai-e found
(if uninjured by abi'asion) to present the structure represented in
fig. 609, where we see on the surface (by incident light) a number

of rounded elevations, arranged in concentric zones around a sort of
nucleus (which has been laid open in the figure to show its internal
structui'e) ; whilst at the mai-gin we observe a row of rounded pro-
jections with a single aperture or pore in each of the intervening
depressions. In very thin discs the structure may often be brought
into view by mounting them in Canada balsam and transmitting

8o6

MICKOSCOPIC FOEMS OF ANIMAL LIFE

light through them ; but in those which are too opaque to be thus
seen thi'ough, it is sufficient to rub down one of the surfaces upon a
stone, and then to mount the specim.en in balsam. Each of the
superficial elevations will then be found to be the roof or cover of an
ovate cavity or ' chamberlet,' which communicates by means of a
lateral passage with the chamberlet on either side of it in the same
ring ; so that each circular zone of chamberlets might be described
as a continuous annular passage dilated into cavities at intervals.
On the other hand, each zone communicates with the zones that are
internal and external to it by means of passages in a radiating
direction ; these passages run, however, not from the chamberlets of
the inner zone to those of the outer, but from the connecting pas-
sages of the former to the charaberlets of the latter ; so that the
chamberlets of each zone alternate in position with those of the zones

Fig. 609. — Orhitolites. Ideal representation of a disc of complex type.

internal and extei'nal to it. The radial passages from the outermost
annulus make their way at once to the margin, where they termi-
nate, forming the ' pores ' which (as already mentioned) are to be seen
on its exterior. The central nucleus, when rendered sufficiently
transparent by the means just adverted to, is found to consist of a
' primordial chamber ' (a), usually somewhat pear-shaped, that com-
municates by a narrow passage with a much larger ' circumambient
chambei- ' (6), which nearly surrounds it, and which sends off a vari-
able number of radiating passages towards the chamberlets of the first
zone, which forms a complete ring round the circumambient chamber. '^

1 Although the above maj' be considered the tyj)ical form of the Orbitolite, yet,
in a very large proportion of specimens, the first few zones are not complete circles,
the early growth having taken place from one side only; and there is a very beautiful
variety in which this one-sidedness of increase imparts a distinctly spiral character
to the early growth, which soon, however, gives place to the cyclical. In the Orhito-
lites italica (fig. 611), brought up from depths of 1,500 fathoms or more, the ' nucleus '

OEBITOLITES

807

The idea of the nature of the hving occupant of these cavities
which might be suggested by the foregoing account of their arrange-
ment, is fuUy borne out by the results of the examination of
the sarcode-body, wlaich may be obtained by the maceration in
dilute acid (so as to remove the shelly investment) of specimens of
Orhitolites that have been gathered fresh and preserved in spirit.
For this body is found to be composed (fig. 610) of a multitude of
segments of sarcode, presenting not the least trace of higher organi-
sation in any part, and connected together by ' stolons ' of the like
substance. The ' primordial ' pear-shaped segment, a, is seen to
have budded off its ' circumambient ' segment, h, by a narrow foot-
stalk or stolon ; and this circumambient segment, after passing almost
entirely round the primordial, has budded off three stolons, which
swell into new sub-
segments from which
the first ring is formed.
Scarcely any two speci-
mens are precisely alike
as to the mode in
which the first ring-
originates from the
' circumambient seg-
ment ; ' for sometimes
a score or more of
radial passages extend
themselves from every
part of the margin of
the latter (and this, as
corresponding* with the
plan of growth after-
wards followed, is
probably the typical
arrangement) ; whilst pj^^ qiq_.
in other cases (as in
the example before us)
the number of these
primary offsets is ex-
tremely small. Each zone is seen to consist of an assemblage of
ovate sub-segments, whose height (which could not be shown in
the figure) corresponds with the thickness of the disc ; these sub-
segments, which are all exactly similar and equal to one another,
are connected by annular stolons ; and each zone is connected with
that on its exterior by radial extensions of those stolons passing off
between the sub-segments.

The radial extensions of the outermost zone issue forth as
pseudopodia from the marginal pores, searching for and drawing in
alimentaiy materials in the manner formerly described ; the whole
of the soft body, which has no communication whatever with

is formed by three or four turns of a spiral closely resembling that of a Cornuspira
with an interruption at every half-turn, as in Spirolociilina, the growth after-
wards becoming purely concentric.

-Composite animal of simple type of Orhito-
lites complanata : — a, central mass of sarcode;
b, circumambient segment, giving o& peduncles, in
which originate the concentric zones of sub-segments
connected by annular bands.

8o8

MICROSCOPIC POEMS OF ANIMAL LIFE

the exterior, save through these marginal pores, being nourished
by the transmission of the products of digestion from zone to zone
through similar bands of protoplasmic substance. In all cases in
which the growth of the disc takes place with normal regularity it
is pi'obable that a complete circular zone is added at once. Thus
we find this simple type of organisation giving origin to fabrics of
by no means microscopic dimensions, in which, however, there is no
other difi'erentiation of pai"ts than that concerned in the formation
of the shell, every segment and every stolon (with the exception of
the two forming the ' nucleus ') being, so far as can be ascertained,
a precise repetition of every other, and the segments of the nucleus
differing from the rest in nothing else than their form. The equality
of the endo\^^nents of the segments is shown by the fact — of which
accident has repeatedly furnished proof — that a small portion of a
disc, entirely separated from the remainder, will not only continue

Fig. 611. — Disc of OrbitoUtes italica, Costa, sp. (0. tenuissima, Carp.),
formed round fragment of previous disc.

to live, but will so increase as to form a new disc (fig. 611), the want
of the ' nucleus ' not appearing to be of the slightest consequence,
from the time that active life is established in the outer zones.

One of the most curious features in the histoiy of this type is its
capacity for develoj)ing itself into a foi-m which, whilst funda-
mentally the same as that jareviously described, is very much more
complex. Tn all the larger s]3ecimens of Orhitolites we observe that
the marginal pores, instead of constituting but a single row, form
many rows one above another ; and, besides this, the chamberlets
of tlie two surfaces, instead of being rounded or ovate in form, ai-e
usually oblong and straight-sided, their long diameters lying in a
radial dii-ection, like those of the cyclical type of Orbiculioia. When
a vertical section is made through such a disc, it is found that these
oblong chambers constitute two superficial layei's, between which

OEBITOLITES

809

are interposed columnar chambers of a rounded form ; and these
last are connected together by a complex series of passages, the
arrangement of which will be best understood from the examination
of a part of the sarcode-body that occupies them (fig. 612). For the
oblong superficial chambers are occupied by sub-segments of sarcode,
c c, cl d, lying side by side, so as to form part of an annulus, but
each of them disconnected from its neighbours, and communicating
only by a doiible footstalk with the two annular ' stolons,' a a', h b',
which obviously cori-espond with the single stolon of ' simple ' types
(fig. 610). These indirectly connect together not merely all the
superficial chamberlets of each zone, but also the columnar sub-
segments of the intermediate layer ; for these columns {e e, e' e')
terminate above and below in the annular stolons, sometimes passing
directly from one to the other, but sometimes going out of their
direct course to coalesce with
another column. The columns
of the successive zones (two sets
of which are shown in the
figure) communicate with each
other by threads of sarcode in
such a manner that (as in the
simple type) each column is
thus brought into connection
with two columns of the zone
next interior, to which it alter-
nates in position. Similar
threads, passing off from the
outermost zone through the
multiple ranges of marginal
pores, would doubtless act as
pseudopodia.

Now this plan of growth is
so different from that previously
described that there would at
first seem ample ground for
separating the simple and the
complex types as distinct species.
But the test furnished by the
examination of a large number

of sjyecimens, which ought never to be passed by when it can j)0ssibly
be appealed to, furnishes these very singular results : 1 st, that the
two forms must be considered as specifically identical ; since there is
not only a gradational passage from one to the other, but they are
often combined in the same individual, the inner and first-formed
portion of a large disc frequently presenting the simjale type, whilst
the outer and later-formed pai"t has developed itself upon the complex ;
2nd, that although the last-mentioned circumstance Avould naturally
suggest that the change from the one plan to another may be simply
a feature of advancing age, yet this cannot be the case : since,
although the complex sometimes evolves itself even from the very
first (the ' nucleus,' though resembling that of the simple form, sending

Pig. 612. — Portion of animal of complex
type of Orbitolites complanata:
a a', h &', the upper and lower rings of
two concentric zones ; c c, the upper
layer of superficial sub-segments, and
d cl, the lower layer, connected with the
annular bands of both zones ; e e and
e' e', vertical sub-segments of the two
zones.

8lO MICEOSCOPIC FORMS OF ANIMAL LIFE

out two or more tiers of radiating threads), more frequently the
simple pi-evails for an indefinite number of zones, and then changes
itself in the course of a few zones into the complex. No depart-
ment of natural history could furnish more striking instances than
are afforded by the different forms presented by the foraminiferal
types now described, of the wide range of variation that may occur
within the limits of one and the same species ; and the microscopist
needs to be specially put on his guard as to this point in respect to
the lowei- types of animal as to those of vegetable life, since the
determination of form seems to be far less precise among such than
it is in the higher types. ^

In what manner the reproduction of Orhitolites is accomphshed,
we can as yet do little more than guess ; but from appearances
sometimes presented by the sarcode-body, it seems reasonable to
infer that gemmules, corresponding with the zoospores of proto-
phytes, are occasionally formed by the breaking up of the sarcode
into globular masses, and that these, escaping through the marginal
pores, are sent forth to develop themselves into new fabrics.

Arenacea. — In certain forms of the pi-eceding family, and espe-
cially in the genus Ifiliola, we not unfrequently find the shells en-
crusted Avith pai'ticles of sand, which are imbedded in the proper
shell substance. This incrustation, however, must be looked on as
(so to speak) accidental, since we find shells that are in every
other respect of the same type altogether free from it. A similar
accidental incrustation presents itself among certain ' vitreous ' and
perforate shells ; but there, too, it is usually on a basis of true shell,
and the sandy incrustation is often entirely absent. There is, how-
ever, a group of Foraminifera in which the true shell is constantly
and entirely re-placed by a sandy envelope, which is distinguished as
a ' test,' the arenaceous particles being held together only by a
cement exuded by the animal. It is not a little curious that the
forms of these ai'enaceous ' tests ' should represent those of many
different types among both the ' porcellanous ' and the ' vitreous '
series ; whilst yet they graduate into one another in such a manner
as to indicate that all the members of this ' arenaceous ' group are
closely i-elated to each other, so as to form a series of their own.
And it is further remarkable that while the deep-sea dredgings
recently carried down to depths of from 1,000 to 2,500 fathoms
have brought up few forms of either ' porcellanous ' or ' vitreous '
Foraminifera that were not previously known, they have added
greatly to our knowledge of the ' arenaceous ' types, the number and
vai'iety of which far exceed all previous conception. These have
been systematically described by Mr. H. B. Brady, F.R.S.,^ whose
researches have led him to believe that the long- established division

1 For further information on the subject of Orhitolites see the Author's account
of the genus in the reports of H.M.S. Challenger. Mr. H. B. Brady in his ' Challenger '
Report (p. 224) describes a remarkable allied type from the Southern Ocean —
KeraiHOsphcBra Murrayi — in which the test is spherical, and the chambers are
arranged in concentric layers.

- See his important report on the Foraminifera dredged by H.M.S. Challenger
(1884), illustrated by 116 plates. A large number of deep-sea forms has lately been
described by Dr. A. Goes, from the dredgings of ih.e Albatross ; see Bull.Mus. Comp.
Zool. xxix. (1896).

GLOBIGERINA 8ll

of the Foraminifera into the arenaceous and calcareous groups does
not correspond to any natural arrangement ; for, although the rule ■
is tolerably constant in many groups, there are others, notably certain
sub-families of Textidariidce, in which it is by no means uniform.

In the midst of the sandy mud which formed the bottom where
the warm area of the ' Globigerina mud ' abutted on that over which
a glacial stream flowed, there were found a number of little peUets,
varying in size from a large pin's head to a large pea, formed
of an aggregation of sand-grains, minute foraminifers, &c., held
together by a tenacious protoplasmic substance. On tearing these
open the whole interior was found to have the same composition,
and no trace of any structural arrangement could be discovered in
their mass. Hence they might be supposed to be mere accidental
agglomerations were it not for their conformity to the * monerozoic '
type previously described ; for, just as a simple ' moner,' by a differen-
tiation of its homogeneous sarcode, becomes an Amoeba, so would
one of these uniform blendings of sand and sarcode by a separation
of its two components — the sand forming the investing ' test ' and
the sarcode occupying its interior — become the arenaceous Astro-
rhiza. This type, which abounds on the sea-bed in certain localities
presents remarkable variations of form, being sometimes globular,
sometimes stellate, sometimes cervicorn. But the same general
arrangement prevails throughout, the cavity being occupied by a
dark-green sarcode, while the ' test ' is composed of loosely aggregated
sand-grains not held together by any recognisable cement, and has
no definite orifice, so that the pseudopodia must issue from inter-
stices between the sand-grains, which spaces are probably occupied
during life with living protoplasm that continues to hold together
the sand-grains after death. These are by no means microscopic
forms, the ' stellate ' varieties ranging to 0"3 or even 0"4 inch in
diameter, and the ' cervicorn ' to nearly 0'5 inch in length.^ A much
larger form was found by Mr. Brady among the dredgings made
in the Faroe Channel (see his ' " Challenger " Report,' p. 242) ;
Syringar)imina appears, when complete, to have been a sphere about
an inch and a half in diameter ; owing to its large size the almost com-
plete absence of cement becomes very noticeable, for the fragile form
can scarcely supj)ort its own weight when taken out of the water.

Later on another large and interesting type belonging to
the same group was obtained by Mr. Wood-Mason, late of the
Indian Museum, from the Bay of Bengal. ^ This has received the
generic name Masonella. The test consists of a thin sandy disc,
nearly half an inch in diameter, either flat or saucer-shape, with
a central chamber and simple or branched radiating tubuli open
at the periphery.

The purely arenaceous Foraminifera ai'e ranged by Mr. H. B.
Brady ^ (by whom they have been especially studied) under two

1 See the description and figures of this type given by the Author in Quart.
■Journ. Microsc. Sci. vol. svi. 1876, p. 221.

2 Ann. and Mag. Nat. Hist. 1889, ser. vi. vol. iii. p. 293, woodcuts.

^ See his ' Notes ' in Quart. Journ. of Microcs. Sci. n.s. vol. xix. 1879, \) 20, and
vol. xxi. 1881, p. 31.

8l2

MICKOSCUPIC EOEMS OF ANIMAL LIFE

families, the first of which, Astrorhizida, includes with the preceding
a number of coarse sandy forms, usually of considerable size, and
essentially monothalamous, though sometimes imperfectly chambered
by constrictions at intervals. Some of themore interesting examples
of this family will now be noticed, beginning with the Saccammina '
(Sars), which is a remarkably regular type, composed of coarse sand-
grains firmly cemented together in a globular form, so as to constitute
a wall nearly smooth on the outer, though rough on the inner surface,
with a projecting neck surrounding a circrdar mouth (fig. 613, a, h,
c). This type, which occurs in extraordinary abundance in certam
localities (as the entrance of the Christiania fjord, and still further
north on the shores of Franz Josef Land), is of peculiar interest
from the fact that a closely allied species {Saccamimina Carteri) is,

,..^^||,fc

V

Fig. 613. — Arenaceous Foraniinifera : a, Saccammina sjjJicei-ica ; b, the same
laid open ; c, portion of the test, enlarged to show its component sand-
grains ; cl, PiluUna Jeffreysii ; e, iDortion of the test enlarged, showing
the arrangement of the sponge-spicules.

as Mr. H. B. Brady has shown, one of the chief constituents of
certain beds of the Lower Carboniferous limestone of the north of
England and elsewhere. In striking contrast to the preceding is
another single -chambered type, distinguished by the whiteness of
its ' test,' to which the Author has given the name of Pihdina, from
its resemblance to a homoeopathic 'globule' (fig. 613, cZ, e). The
form of this is a very regular sphere ; and its orifice, instead of
being circular and surrounded by a neck, is a slit or fissure with
slightly raised lips, and having a somewhat S-shaped curvature. It
is by the structure of its ' test,' however, that it is especially dis-
tinguished ; for this is composed of the finest ends of sponge-spicules,
very regularly ' laid ' so as to form a kind of felt, through the sub-

1 For a detailed account of S. sphcBrica consult L. Rliumbler, in vol. Ivii. of Zeitschr-
f. wiss. Zool.

AEENACEOUS FOKAMINIFEKA

813

stance of which very fine sand-grains are dispersed. This ' felt ' is
somewhat flexible, and its components do not seem to be united by
an}^ kind of cement, as it is not affected by being boiled in strong
nitric acid ; its tendency, therefore, seems entirely due to the
wonderful manner in which the separate silicious fibres are ' laid.'
It is not a little curious that these two forms should present them-
selves in the same dredging, and that there should be no perceptible
difference in the character of their sarcode bodies, which, as in the
preceding case, have a dark-green hue. The Marsipella elongata
(fig. 614, d), on the other hand, is somewhat fusiform in shape, and
has its two extremities elongated into tubes, with a circular orifice
at the end of each. The materials of the ' tests ' differ remarkablj^
according to the nature of the bottom whereon they live. When

Fig. 614. — Arenaceous Foraminifera : a, b, upper and lower aspects of Ha/plo-
phragmiumglohigeriniforme; c, Horinosina glohulifera; d, Marsipella
elongata ; e, terminal portion, and /, middle portion of the same, enlarged ;
g, Thurammina impillata ; li, portion of its inner surface, enlarged.

they come up with ' Globigerina mud,' in which sponge-spicules
abound, whilst sand-grains are scarce, they are almost entirely
made up of the former, which are ' laid ' in a sort of lattice-work,
the interspaces of which are filled up by fine sand-grains ; but when
they are brought up from a bottom on which sand predominates,
the larger part of the ' test ' is made up of sand-grains and minute
Foraminifera, with here and there a sponge-spicule (fig. 614, d,f).
In each case, however, the tubular extensions (one of which some-
times forms a sort of proboscis, e, nearly equalling the body itself
in leng-th) are entirely made up of sponge-spicules laid side by side
with extraordinary regularity. The genus Rhahdaimnina (Sars)
resembles Saccammina in the structure of its ' test,' which is com-
posed of sand-grains very firmly cemented together ; but the gi'ains

8 14 MICROSCOPIC FOEMS OF ANIMAL LIFE

are of smallei- size, and they are so disposed as to present a smooth
surface internally, though the exterior is rough. What is most
i-emarkable about this is the geometrical regularity of its foi-m,
which is typically triradiate (fig. 615, c), the rays diverging at equal
angles from the central cavity, and each being a tube (cZ) with an
orifice at its extremity. ISTot unfi-equently, however, it is quadri-
radiate, the rays diverging at right angles ; and occasionally a fifth
ray presents itself, its radiation, however, being generally in a
difierent plane. The three rays are noi-maUy of equal length ; but
one of them is sometimes shorter than the other two ; and when
this is the case the angle between the long rays increases at the
expense of the other two, so that the long rays lie more nearly in a
straight line. Sometimes the place of the third ray is indicated
only by a little knob ; and then the two long rays have very nearly
the same direction. We are thus led to foi-ms in which there is no
vestige of a third I'ay, but merely a single straight tube, -with an
oi-ifice at each end ; and the length of this, which often exceeds
half an inch, taken in connection with the abundance in which it
presents itself in dredgings in which the triradiate forms are rare,
seems to preclude the idea that these long single rods are broken
rays of the latter. It is undoubtedly in this group that we are to
place the genus Haliphysema, which, from consti-ucting its ' test '
entirely of sponge-spicules, and even including these in its pseudo-
podial expansions, has been ranked as a sponge, although observation
of it in its living state leaves no doubt whatever of its rhizopodal
chai-acter.i

Lituolida. — The type of this family, which is named after it, is
a large sandy many-chambered fossil form occurring in the chalk,
to which the name Lituola was given by Lamarck, from its resem-
blance in shape to a crosier. A gi-eat variety of recent forms, mostly
obtained by deep-sea dredging, are now included in it, as bearing
a more or less close resemblance to it and to each other in their
chambered sti-uctui-e, and in the arrangement of the sand-grains of
which their tests are formed. These grains are, for the most part,
finer than those of which the tests of the preceding family are con-
structed, and are set (so to speak) more artistically, and a con-
siderable quantity of a cement exuded by the animal is employed
in uniting them. This is often mixed up with sandy particles of
extreme fineness to form a sort of ' plastei- ' with which the exterior
of the test is smoothed ofi", so as to present quite a polished surface.
It is remarkable that the cement contains a considerable quantity
of oxide of iron, which impai-ts a ferruginous hue to the ' tests ' in
which it is lai-gely employed. The forms of the Lituoline ' tests '
often simulate in a very curious way those of the simpler types of
the vitreous series. Thus, the long spirally coiled undivided sandy
tube of A inmodiscus is the isomorph of Sjnrillina. In the genus Haplo-
phragmium (fig. 614, a, b, and Plate XVIII, fig. 6) we have singular
imitations of the Globigei-ine, Rotaline, and ISTonionine types ; and in

1 See Mr. Saville Kent in Ann. of Nat. Hist. ser. v. vol. ii. 1878 ; Professor Ray
Lankester in Quart. Journ. Microsc. Sci. vol. xix. 1878, p. 476 ; and Professor Mobius's
Foi'aminifera von Mauritius, 1880.

ARENACEOUS FORAMINIFEEA

815

Thurammina pcqnllata (fig. 614, (/)anot less remarkable imitation of
the Orbuline. This last is specially noteworthy for the admii-able
manner in which its component sand-grains are set together, these
being small and very uniform in size, and being disposed in such a
manner as to present a smooth surface both inside and out (fig. 614, h),
whilst there are at intervals nipple-shaped protuberances, in every one
of which thei-e is a rounded orifice. A like perfection of finish is seen
in the test oi Hormosina glohulifera (fig. 614, c), which is composed
of a succession of globular chambers rapidly increasing in size, each
having a narrow tubular neck with a rounded orifice, which is
received into the next segment. In other species of the same genus
there is a nearer approach to the ordinary Nodosarine type, their
tests being sometimes constructed with the regularity characteristic
of the shells of the true Nodosaria^ Plate XIX, 16, whilst in other

Pig. 615. — Arenaceous Foraminifera : «, 5, exterior and sectional views of
BJieophax sabulosa; c,Bhabdam77ii7iaabyssoruvi; cZ, cross section of one
of its arms ; e, Bheojphax scorpiurus ; /, Hormosina Carpenteri.

cases the cham.bers are less regularly disposed (fig. 615, f), having
rather the character of bead-like enlargements of a tube, whilst their
walls show a less exact selection of material, sponge-spicules being-
worked in with the sand-grains, so as to give them a hirsute aspect.
A greater rudeness of structure shows itself in the ISTodosarine forms
of the genus Rheophax^ in which not only are the sand-grains of the
test veiy coarse, but small Foi'aminifera are often worked up with
them (fig. 615, e). A straight, many-chambered form of the same
genus (fig. 615, a, h) is remarkable for the peculiar finish of the neck
of each segment ; for whilst the test generally is composed of sand-
grains, as loosely aggregated as those of which the test of Astrorhiza
is made up, the gxains that form the neck ai-e firmly united by fer-
ruginous cement, forming a very smooth wall to the tubulai- orifice.

The highest development of the ' arenaceous ' type at the j)resent
time is found in the forms that imitate the very regular nautiloid

8i6

MICROSCOPIC FORMS OF ANIMAL LIFE

shells, both of the ' porcellanous ' and the ' vitreous ' series ; and the
most remai-kable of these is the Cyclammina cancellata (fig. 616),
which has been brought up in considerable abundance from depths
ranging downwards to 1,900 fathoms, the largest examples being
found within 700 fathoms. The test (fig. 616, a) is composed of
aggregated sand -grains firmly cemented together and smoothed over
externally with ' plaster,' in which large glistening sand-grains are
sometimes set at regular intervals, as if for ornament. On laying
open the spire it is found to be very regularly diAdded into chambers
by partitions foi-med of cemented sand-grains (6), a communication
between these chambers being left by a fissiire at the inner margin
of the spire, as in Operculina (fig. 628). One of the most curious
features in the structure of this type is the extension of the cavity
of each chamber into passages excavated in its thick external wall,

Pig. 616. — Cyclammina cancellaia, showing at «, its external aspect ;
&, its internal structure ; c, a iDortion of its outer wall in section, more
highly magnified, showing the sand-grains of which it is built up and
the passages excavated in its substance.

each passage being surrounded by a very regular arrangement of
sa,nd-grains, as shown at c. It not unfrequently happens that the
outer layer of the test is worn away, and the ends of the passages
then show themselves as pores upon its surface ; this appearance,
however, is abnormal, the passages simply rvxnning from the chamber-
cavity into the thickness of its wall, and having (so long as this is
complete) no external opening. This ' labyrinthic ' structure is of
great interest, from its relation not only to the similar sti'ucture
of the large fossil examples of the same type, but also to that which
is presented in other gigantic fossil arenaceous forms to be presently
described.

Although some of the nautiloid Lituolm are among the largest
of existing Foraminifera, having a diameter of 0"3 inch, they are
mere dwarfs in comparison with two gigantic fossil forms, whose

Pi^EKERIA

817

strvicture has been elucidated by Mr. H. B. Brady and tlie Author. "^
Geologists who have woi'ked over the Gi-eensand of Cambridgeshire
have long been familiar ^vlth solid spherical bodies which there
present themselves not unfrequently, vaiying in size fi-om that of a
pistol-bullet to that of a small cricket-ball ; and whilst some regarded
them as mineral concretions othei-s were led by certain appearances
pi'esented by their surfaces to suppose them to be fossilised sponges.
A specimen having been fortunately discovered, howevei-, in which
the original structure had remained unconsolidated by mineral in-

FiG. 617. — General view of the internal structure of ParJceria: in the hori-
zontal section, Z', I'', P, I'* mark the four thick layers; in the vertical
sections A marks the internal surface of a layer separated by concentric
fracture ; B, the appearance presented by a similar fracture passing through
the radiating processes ; C, the result of a tangential section passing
through the cancellated substance of a lamella ; D, the appearance pre-
sented by the external surface of a lamella separated by a concentric
fracture which has passed through the radial jjrocesses ; E, the aspect of a
section taken in a radial direction, so as to cross the solid lamellaB and their
intervening spaces ; c^, c'\ c', e*, successive chambers of nucleus.

filti'ation, it was submitted by Professor Morris to the Author, who
was at once led by his examination of it to recognise it as a member
of the arenaceous group of Foraminifera, to which he gave the de-
signation Parkeria, in compliment to his valued friend and coadjutor,
Mr. W K. Parker. A section of the sphere taken through its
centre (fig. 617) presents an aspect very much resembling that of an
Orbitolite, a series of chamberlets being concentiically arranged
round a ' nucleus ; ' and as the same appearance is presented, what-
ever be the direction of the section, it becomes apparent that these

1 See their 'Description of Parkeria and Loftusia' in Philosophical Trans-
actions, 1869, p. 721. Though it appears convenient to allow this description of
Parkeria to remain, it must be noted that some of those most competent to judge
are of opinion that Parkeria is one of the Stromatoporoids, an obscure and difficult
group of fossil Hyclroida isee the memoir by Professor AUeyne Nicholson, published
in 1886 by the Palseontographical Society).

3 G

8l8 MIUEOSCOPIC FOEMS OF ANIMAL LIFE

chamberlets, instead of being arranged in successive rings on a single
plane, so as to form a disc, are grouped in concentric spheres^ each
completely investing that which preceded it in date of formation.
The outer wall of each chamberlet is itself penetrated by extensions
of the cavity into its svibstance, as in the Cyclmnminal&at described ;
. and these passages are separated by partitions very regularly built
up of sancl-grains, which also close in their extremities, as is shown
in fig. 618. The concentric spheres are occasionally separated by
walls of more than ordinary thickness, and such a wall is seen in
fig. 617 to close in the last-formed series of chamberlets. But these

walls have the same ' labyrinthic'
structiu^e as the thinner ones,
and an examination of numerous
specimens shows that they are
not formed at any regular inter-
vals. The ' nucleus ' is always
composed of a single series of
• \^^^^^'^^^ ^'■^^^ ^^ chambers arranged end to end,

sometimes in a straight line, as
in fig. 617, c\ c^, c^, c*, sometimes
forming a spiral, and in one in-
stance returning u^pon itself.
Fig 618.-Poi-tion of one of the lamellEe g^^^ ^^le outermost chamber en-
01 l^arAerzfl, snowuig the sand-grams 01 , . t •, ij? .i

which it is built up, and the passages iarges, and extends itselt over the
extending into its substance. whole ' nucleus,' very much as the

' circumambient ' chamber of the
Orbitolite extends itself round the primordial chamber ; and radial
prolongations given ofi" from this in every direction form the first
investing sphere, round which the entire series of concentric
spheres are successively formed. Of the sand of which this remark-
able fabric is constructed about 60 per cent, consists of phosphate of
lime, and nearly the whole remainder of carbonate of lime. Another
large fossil arenaceous type, constructed upon the same general plan,
but growing spirally round an elongated axis, after the manner of
Alveoliria (fig. 608), and attaining a length of three inches, has been
described by Mr. H. B. Brady {loc. cit.) under the name Loftusia, after
its discoverer, the late Mr. W. K. Loftus, who brought it from the
Turko-Persian frontier, where specimens were found in considerable
numbers imbedded in ' a blue marly limestone,' probably of early
Tertiary age.

There is nothing, it seems to the Author, more wonderful in
Nature than the building up of these elaborate and symmetrical
structures by mere 'jelly-specks,' presenting no trace whatever of
that definite ' organisation ' which we are accustomed to regard as
necessary to the manifestations of conscious life. Suppose a human
mason to be put down by the side of a pile of stones of various shapes
and sizes, and to be told to build a dome of these, smooth on both
surfaces, without using more than the least possible quantity of a
very tenacious but very costly cement in holding the stones together.
If he accomplished this well, he would receive credit for great in-
telligence and skill. Yet this is exactly what these little 'jelly-specks '

VITEEOUS FORAMINIFEEA 819

do on a most minute scale, the ' tests ' they construct, when highly
magnified, bearing comparison with the most skilful masonry of man.
From the smne sandy bottom one species picks up the coarser quartz-
grains, unites them together with a fei-ruginous cement secreted from
its own substance, and thus constructs a flask-shaped ' test,' having
a short neck and a single large orifice. Another picks up the finer
grains and puts them togethei- with the same cement into pei'fectly
spherical ' tests ' of the most exti'aordinary finish, perforated with
numerous small pores disposed at pretty regular intei-vals. Another
selects the minutest sand-grains and the terminal portions of sponge-
spicviles and works these up together — apparently with no cement
at all, bvit by the mere ' laying ' of the spicules — into perfect white
spheres, like homojopathic globules, each having a single fissured
orifice. And another, which makes a straight many-chambered 'test,'
the conical mouth of each chamber projecting into the cavity of the
next, while forming the walls of its chambers of ordinary sand-grains
rather loosely held together, shapes the conical mouths of the suc-
cessive chambers by firmly cementing to each other the quartz-grains
which border it. To give these actions the vague designation ' in-
stinctive ' does not in the least help us to account for them ; since
what we want is to discover the mechanism by which they are worked
out ; and it is most difficult to conceive how so artificial a selection
can be made by creatures so simple.

Vitrea. — Returning now to the Foraminifera which form true
shells by the calcification of the superficial layer of their sarcode-
bodies, we shall take a similar general survey of the vitreous series,
whose shells are perforated by multitudes of minute foramina (fig.
607). Thus, SjyiTillina has a minute, spirally convoluted, undivided
tiibe, resembling that of Coi'nuspira, but having its wall somewhat
coarsely perforated by niunerous apertures for the emission of pseudo-
podia. The ' monothalamous ' forms of this growth mostly belong to
the family Lagenida, which also contains a series of transition forms
leading up gradationally to the ' polythalamous ' nautiloid type. In
Lagena (Plate XIX, figs. 12, 13, li, 15) the mouth is narrowed and
prolonged into a tubular neck, giving to the shell the form of a micro-
scopic flask ; this neck terminates in an everted lip, which is marked
with radiating furrows. A mouth of this kind is a distinctive
character of a large gi'oup of many-chambered shells, of which each
single chamber bears a more or less close resemblance to the simple
Lagena, and of which, like it, the external surface generally presents
some kind of ornamentation, which may have the form either of
longitudinal ribs or of pointed tubercles. Thus the shell of JVodo-
saria (Plate XIX, fig. 16) is obviously made up of a succession
of lageniform chambers, the neck of each being received into the
cavity of that which succeeds it ; whilst in Cristellaria (fig. 17)
we have a similar succession of chambers, presenting the characteristic
radiate aperture, and often longitudinally ribbed, disposed in a
nautiloid spiral. Between JSfodosaria and Cristellaria, moreover,
there is such a gradational series of connecting forms as shows that
no essential difierence exists between these two types, and it is a fact
of no little interest that some of the simpler of these varietal forms,

3 G 2

820 MICEOSCOPIC FOEMS OF ANIMAL LIFE

of which many are to be met with on our own shores, but which are
more abundant on those of the Mediterranean and especially of the
Adriatic, can be traced backwards in geological time at least as far
as the Pei-mian epoch. In another genus, Folymorphina^ we find
the shell to be made up of lageniform chambers arranged in a double
series, alternating with each other on the two or more sides of a
rectilinear axis ; here, again, the forms of the individual chambers,
and the mode in which they are set one upon another, vary in such
a manner as to give rise to very marked differences in the general
configuration of the shell, which are indicated by the name it bears.
Globigerinida. — Returning once again to the simple ' monothala-
mous ' condition, we have in Orhtdina — a minute spherical shell that
presents itself in greater or less abundance in deep-sea di'edgings,
from almost every region of the world — a globular chambei- with
porous walls, but destitute of any general aperture, the ofiice of which
is served by a series of larger pores scattered throughout the wall of
the sphere. It has been maintained by some that Orhulina is reaUy
a detached generative segment of Glohigerina, with which it is
generally found associated. The shell of Glohigerina consists of an
assemblage of nearly spherical chambers (fig. 619), having coarsely

Fig. 619. — Glohigerina hulloides as seen in three positions.

porous walls, and cohering extei-nally into a more or less regular
turbinoid spire, each turn of which consists of four chambers pro-
gressively increasing in size. These chambers, whose total number
seldom exceeds sixteen, may not communicate directly with each
other, but open separately into a common ' vestibule ' which occupies
the centre of the under side of the spire. This type has attracted
great attention, from the extraoi'dinary abundance in which it occurs
at great dej)ths over large areas of the ocean bottom. Thus its minute
shells have been found to constitute no less than 97 per cent, of
the 'ooze' brought up from depths of from 1,260 to 2,000 fathoms
in the middle of the northern parts of the Atlantic Ocean. The
younger shells, consisting of from eight to twelve chambers, are
thin and smooth, but the older shells are thicker, their surface is
raised into ridges that form an hexagonal areolation round the pores
(fig. 620) ; and this thickening is shown by examination of thin
sections of the shell to be produced by an exogenous deposit around
the original chamber wall (corresj)onding with the ' intermediate
skeleton ' of the more complex types), which sometimes contains
little flask-shaped cavities filled with sarcode — as was first pointer!
out by the late Dr. Wallich. But the sweeping of the upper waters

GLOBIGEEINA

821

of the ocean by the ' tow net,' which was systematically carried on
dvii-ing the voyage of the ' Challenger,' brought into prominence the
fact that these waters in all but the coldest seas are inhabited by
floating Glohigerince, whose shells are beset with multitvides of de-
licate calcareous spines, which extend themselves radially from the
angles at which the ridges meet to a length equal to four or five
times the diameter of the shell (fig. 621). Among the bases of these
spines the sai'codic substance of the body exvides through the pores
of the shell, forming a ilocculent fringe around it ; and this extends

Fig. 620.-

-Glohigerina conglohata (Brady) : a, h, c, bottom specimens ;
d, section of shell.

itself on each of the spines, creejiing up one side to its extremity,
and passing down the other with the peculiar flowing movement
already described. The whole of this sarcodic extension is at once
retracted if the cell which holds the Globigerina receives a sudden
shock, or a drop of any irritating fluid is added to the water it con-
tains. It was maintained by Sir Wyville Thomson that the bottom
deposit is formed by the continual ' raining down ' of the Globigerinte
of the upper waters, which (he afiirmed) only live at or near the sur-
face, and which, when they die, lose their spines and subside. The

822

MICROSCOPIC FOEMS OF ANIMAL LIFE

Author, however, from his own examination of the Globigerina ooze,
is of opinion that the shells forming its surface-iayer must live on the
bottom, being incapable of floating in consequence of their weight ;
and that if they have passed the earlier part of their lives in the
upper waters they drop down as soon as the calcareous deposit con-
tinually exuding from the body of each animal, instead of being em-
ployed in the formation of new chambers, is applied to the thicken-
ing of those previously formed. That many types of Foraminifera

jmss their v^hole lives at
depths of at least 2,000
fathoms is proved, in regard
to those forming calcareous
shells, by their attachment
to stones, corals, &c. ; and
in the case of the arena-
ceous types by the fact that
they can only procure on the
bottom the sand of which
their ' tests ' are m.ade up.

A very remai-kable type
has recently been discovered
adherent to shells and corals
brought from trojaical seas,
to which the name Carpen-
ter ia has been given. This
may be regarded as a highly
developed form of Globi-
gerina, its first formed por-

FiG. ^"hX.—GloUge.rina, as captured by tow-net ^^^^^ havnig all the essential
floating at or near surface. characters of that genus.

It grows attached by the
apex of its spire, and its later chambers increase rapidly in size,
and are piled on the earlier in such a manner as to form a depressed
cone with an irregular spreading base. The essential character of
Globigerina — the separate orifice of each of its chambers — is here re-
tained with a curious modification ; for the central vestibule into
which they all open forras a sort of vent whose oi-ifice is at the apex
of the cone, and is sometimes prolonged into a tube that proceeds
from it ; and the extei'nal wall of this cone is so marked out by
septal bands that it comes to bear a sti'ong resemblance to a minute
Balanus (acorn-shell), for which this type was at first mistaken. The
jK'incipal chambers are partly divided into chamberlets by incomplete
partitions, as we shall find them to be in Eozoon. The presence of
sponge-spicules in large quantity in the chambers of many of the
best jJreserved examples of this type was for some time a source of
perplexity ; but this was explained by the late Professor Max
8chultze,^ who showed how the pseudo podia of this rhizopod have
the habit, like those oi Haliphysema, of taking into themselves sponge-
spicules, which they draw into the chambers, so that they become
incorporated with the sarcode-body. It should be added that Pro-
^ Arcliiv f. Natiorgesch. xxix. 1863, p. 81.

TEXTULARIA 823

fessor Schultze, with whom Mr. H. J. Cai^ter/ Mr. H. B. Brady,- and
Dr. Goes ^ are in agreement, regard C arpenteria as allied to Polytrema.
Some interesting observations have been made by Professor Mobius •*
on a large branching and spreading form of Carjjenteria which he
recently met with on a reef near Mauritius, and to which he has given
the name of C. rhajyhidodendron.

A less aberrant modification of the Globigerine type, however, is
presented in the two great series which may be designated (after the
leading forms of each) as the Textularian and the Rotalian. For,
notwithstanding the marked difference in their respective plans of
growth, the characters of the individual chambers are the same,
their walls being coarsely porous, and their apertures being oval,
semi-oval, or crescent-shaped, sometimes merely fissured. In Textu-
laria (Plate XVIII, fig. 9) the chambers are arranged biserially
along a straight axis, the jjosition of those on the two sides of it being
alternate, and each chamber opening into those above and below it
on the opposite side by a narrow fissure, as is well shown in such

Fig. 622.— Internal silicious casts representing the forms of the segments of
the annuals of, A, Textularia; B, Botalia. .

' internal casts ' (fig. 622, A) as exhibit the forms and connections of
the segments of sarcode by which the chambers were occupied during
life. In the genus Bidimina the chambers are so arranged as to form
;i spire like that of a Bulimus, and the aperture is a curved fissure
whose direction is nearly transverse to that of the fissure of Textu-
laria ; bvit in this, as in the preceding type, there is an extraoi-dinary
variety in the disposition of the chambers. In both, moreover, the
shell is often covered by a sandy incrustation, so that its perforations
are completely hidden, and can only be made visible by the removal of
the adherent crust. And so many cases are now known in which
the shell of Textularinice is entirely replaced by a sandy test, that
some systematists prefer to range this group among the Arenacea.

In the Rotalian series the chambers are disposed in a turbinoid
spire, opening one into another by an apei'ture situated on the lower

1 Annals and Mag. Nat. Hist. ser. iv. vols. xvii. xix. xx.

^ ' Challenger' Report.

5 K. SvensJca Vet. Handlingar, xix. No. 4, p. 94.

* See his Foraminifera von Mauritius, 1880, plates v. vi.

A^A

u

824 MICEOSOOPIC FORMS OF ANEVIAL LIFE

and inner side of the spire, as shown in Plate XIX, fig. 22, the forms
and connections of the segments of their sarcode-bodies being shown
in such ' internal casts ' as are represented in fig. 622, B. One of the
lowest and simplest forms of this type is that very common one now-
distinguished as Discorhina. The early form of Planorhulina is a
Rotaline spire, very much resembling that of Discorhina ; but this
afterwards gives place to a cyclical plan of growth, and in those
most developed forms of this type which occur in warmer seas the
earlier chambers are completely overgrown by the latter, which are
often piled up in an irregular ' acervuline ' manner, spreading over
the surfaces of shells or clustering round the stems of zoophytes.

In the genus Tinoporus there is a
more regul&,r growth of this kind, the
chambers being piled successively on
the two sides of the original median
plane, and those of adjacent piles com-
municating with each other obliquely
(like those of Textularia) by large
^^^^^^ apertures, whilst they communicate
with those directly above and below
by the ordinary pores of the shell.
The simple or smooth varieties of this
genus forming the sub-genus Gypsina
present great diversities of shape,with
Fig. 623.— TwioiJ077ts haculatus. great constancy in theii- internal struc-
ture, being sometimes spherical, some-
times resembling a minute sugar-loaf, and sometimes being irregu-
larly flattened out. The typical form (fig. 623), in which the walls
of the piles are thickened at their meeting angles into solid columns
that appear on the surface as tubercles, and are sometimes pro-
longed into spinous outgrowths that radiate from the central mass,
is of very common occurrence in shore-sands and shallow.-watei-
dredgings on some parts of the Aiistralian coasts and among the
Polynesian islands. To the simple form of this genus we are
probably to refer many of the fossils of the Cretaceous and
early Tertiary period that have been described under the name
Orbitolina, some of which attain a very large size. Globular Orhito-
lince, which appear to have been artificially perforated and strung
as beads, are not unfrequently found associated with the ' flint-imple-
ments ' of gravel-beds. Another very curious modification of the
Rotaline type is presented by Polytrema, which so much resembles
a zoophyte as to have been taken for a minute millepore, but which
is made up of an aggregation of ' Globigerine ' chambers communi-
cating with each other like those of Tinoporus^ and differs from that
genus j)rimarily in its erect and usually branching manner of growth
and the freei- communication between its chambei-s. This, again, is
of special intei'est in relation to Eozoon, showing that an indefinite
zijophytic mode of growth is perfectly compatible with trvily fora-
miniferal structure.

In Rotalia, properly so called, we find a marked advance towards
the highest type of foraminiferal structui-e, the partitions th^t

EOTALIA

825

divide the chambers being in the best developed examples composed
of two laminae, and spaces being left between them which give
passage to a system of canals whose general distribution is shown
in fig. 624. The proper walls of the chambers, moreover, ai'e
thickened by an extraneous deposit or ' intermediate skeleton,' which
sometimes forms radiating outgroAvths. This peculiaidty of conforma-
tion, however, is cai-i-ied much further in the genus Calcarina, which
has been so designated
from its resemblance to a
spur-rowel (fig. 629). The
solid club-shaped append-
ages with which this shell
is provided entirely be-
long to the ' intermediate
skeleton ' h, which is quite
independent of the cham-
bei-ed structure a ; and this
is nourished by a set of
canals containing prolonga-
tions of the sarcode-body
which not only furrow the
sui'face of these appendages,
but are seen to traverse
their interior when this is
laid open by section, as
shown at c. In no other
recent foraminifer does the
' canal system ' attain a like
development ; and its dis-
tribution in this minute

shell, which has been made out by careful microscopic study, afibrds
a valuable clue to its meaning in the gigantic fossil organism
Eozbon canadense. The resemblance which Calcarina bears to the
radiate forms of Tinoporus (fig. 623), which are often found with
them in the same dredgings, is frequently extremely striking ; and
in their early growth the two can scarcely be distinguished, since
both commence in a '■ Rotaline ' spire with radiating appendages ;
but whilst the successive chambers of Calcarina continue to be
added on the same plane, those of Tinoporus are heaped up in less
regular jailes.

Certain beds of Carboniferous limestone in Russia are entirely
made up, like the more modern Nummiditic limestone, of an aggre-
gation of the remains of a peculiar type of Foraminifera, to which
the name Fusulina (indicative of its fusiform or spindle-like shape)
has been given (fig. 625). In general aspect and plan of growth it
so much resembles Alveolina that its I'elationship to that type would
scarcely be questioned by the superficial observer. But when its
mouth is examined it is found to consist of a single slit in the
middle of the lip ; and the interior, instead of being minutely
divided into chamberlets, is found to consist of a regular series of
simple chambers ; while from each of these proceeds a pair of

Fig 624 — Section of Botaha Schi oetei lana near
its base and parallel to it, showing, a, a, the
radiating interseptal canals ; b, their internal
bifurcations ; c, a transverse branch ; d, tubulated
wall of the chambers.

826 MICEOSCOPIC POEMS OF ANIMAL LIFE

elongated extensions, whic4i correspond to the ' alar prolongations '
of othei- spirally growing Foraminifera, but which, instead of wrapping
round the preceding whorls, are prolonged in the direction of the
axis of the spire, those of each whorl projecting beyond those of the
preceding, so that the shell is elongated with every increase in its
diameter. Thus it appears that in its general plan of growth
Fusulina bears much the same relation to a symmetrical Rotaline or
Nummidine shell that Alveolina bears to Orhiculina ; and this view
of its affinities is fidly confirmed by the Author's microscopic exami-
nation of the structure of its shell. For although the Fusulina
limestone of Russia has undergone a degree of metamorphism,
which so far obscures the tubulation of its component shells as to
prevent him from confidently affirming it, yet the appearances he
could distinguish were decidedly in its favour. And ha\dng since
received from Dr. C. A. White specimens from the Upper Coal
Measures of Iowa, U.S.A., which are in a much more perfect state of

Fig. 625. — Section of Fusulina limestone.

preservation, he is able to state with certainty, not only that Fusulina
is tubular, but that its tubulation is of the large coarse nature that
marks its affinity rather to the Rotaline than to the Numinidine
series. This type is of peculiar interest as having long been regarded
as the oldest form of Foraminifera which was known to have occurred
in sufficient abundance to form rocks by the aggregation of its in-
dividuals. It will be presently shown, however, that in point both
of antiquity and of importance it is far surpassed by another.

Nummulinidse. — All the most elaborately constructed, and the
gTeater part of the largest, of the ' vitreous ' Foraminifera belong to
the group of which the well-known Nummulite may be taken as the
representative. Various plans of growth prevail in the family ;
but its distinguishing characters consist in the completeness of the
wall that surrounds each segment of the body (the septa being
generally double instead of single), the density and fine porosity of
the shell-substance, and the presence of an ' intermediate skeleton,'

POLYSTOMELLA 82/

with a ' canal system ' for its nutrition. It is true that these cha-
racters ai'e also exhibited in the highest of the Rotaline series, whilst
they are deficient in the genus Amphistegina, which connects the
Nummuline series with the Rotaline ; but the occurrence of such
modifications in their border forms is common to other truly natural
gt'oups. With the exception of Ainjyhistegina, all the genera of this
family are sjTnmetrical in form, the spire being nautiloid in such
as follow that plan of growth, whilst in those which follow the
cyclical plan there is a constant equality on the two sides of the
median plane ; but in Am2)hi8tegina there is a reversion to the
Rotalian type in the turbinoid form of its spire, as in the characters
already specified, althoiigh its general conformity to the ISTummuline
type is such as to leave no reasonable doubt as to its title to be
placed in this family. JSTotwithstanding the want of symmetry of
its spire, it accords with Oiyerculina and Nummiulites in having its
chambers extended by ' alar prolongations ' over each surface of
the previous whorl ; but on the under side these prolongations are
almost entirely cut off from the principal chambers, and are so dis-
placed as apparently to alternate with them in position, so that M.
d'Orbigny, supposing them to constitute a distinct series of chambers,
described its plan of growth as a biserial spire, and made this the
character of a separate order. ^

The existing Nuni'mulinidce are almost entirely restricted to
tropical climates ; but a beautiful little form, Polystomella crispa,
the representative of a genus that presents the most regular and
complete develoj)ment of the ' canal system ' anywhei-e to be
met with, is comm.on on our own coasts. The peculiar surface-
marking shown in the figure consists in a strongly marked
ridge-and-furrow plication of the shelly wall of each segment along
its posterior margin, the furrows being sometimes so deep as to
resemble fissures opening into the cavity of the chamber beneath.
ISTo such openings, however, exist, the only communication which
the sarcocle-body of any segment has with the exterior being
either through the fine tubuli of its shelly walls or through the
I'ow of pores that are seen in front view along the inner margin
of the septal plane, collectively representing a fissured aperture
divided by minute bridges of shell. The meaning of the plication of
the shelly wall comes to be understood when we examine the con-
formation of the segments of the sarcode-body, which may be seen
in the common Polystomella crispa by dissolving away the shell of
fresh specimens by the action of dilute acid, but which may be better
studied in such internal casts (fig. 626) of the sarcode-body and
canal system of the large P. craticidata of the Australian coast as
may sometimes be obtained by the same means from dead shells
which have undergone infiltration with ferruginous silicates.^ Here

1 For an account of this curious modification of the Nummuline plan of growth,
the real nature of which was first elucidated by Messrs. Parker and Rupert Jones,
see the Author's Introduction to the Study of the Foraminifera (published by the
Ray Society).

^ It was by Professor Ehrenberg that the existence of such ' casts ' in the Green-
sands of various geological periods (from the Silurian to the Tertiary) was first
pointed out, in his memoir ' Ueber den Grihasand und seine Erlauterung des

828

MICEOSCOPIC FORMS OF ANIMAL LIFE

we see that the segments of the sarcode-body are smooth along their
anterior edge b, b^, but that along their posterior edge, a, they are
prolonged backwards into a set of ' retral pi-ocesses ; ' and these pro-
cesses lie under the ridges of the shell, whilst the shelly wall dips
down into the spaces between them, so as to form the furrows seen
on the surface. The connections of the segments by stolons, c, c\
passing throiigh the pores at the inner margin of each septum, are
also admirably displayed in such ' casts.' But what they serve most
beautifully to demonstrate is the canal system, of which the distri-
bution is here most remarkably complete and symm^etrical. At d,
d^, c^^ are seen three turns of a spiral canal which passes along one
end of all the segments of the like number of convolutions, whilst a
corresponding canal is found on the side which in the figure is under-
most ; these two spires are connected by a set of meridional canals,
e, e^, e^, which pass down between the two layers of the septa that

Pig. 626. — Internal cast of Polystomella craticulata : «., retral processes
proceeding from the posterior margin of one of the segments ; h, b^, smooth
anterior margin of the same segment; c, c^, stolons connecting successive
segments, and uniting themselves with the diverging branches of the meri-
dional canals; d, d^, d?, three turns of one of the spiral canals; e, e^, e^,
three of the meridional canals; f,f^,f'^, their diverging branches.

divide the segments ; whilst from each of these there passes oft'
towards the surface a set of pairs of diverging branches,/', y\/^, which
open upon the surface along the two sides of each septal band, the
external openings of those on its anterior margin being in the fui--
rows between the retral processes of the next segment. These canals
appear to be occixpied in the living state by prolongations of the
sarcode-body ; and the diverging branches of those of each convolu-
tion unite themselves, when this is inclosed by another convolution,

organischen Lebens,' in Ahhandlungen der Jconigl. AJcad. der Wissenschafte)i,
Berlin, 1855. It was soon afterwards shown by the late Prof essor Bailey {Quart. Joitrn.
Microsc. Sci. vol. v. 1857, p. 83) that the like infiltration occasionally takes place in
recent Foraminifera, enabling similar ' casts ' to be obtained from theni by the solu-
tion of their shells in dilute acid ; the Author, as well as Messrs. Parker and Rupert
Jones, soon afterwards obtained most beautiful and complete internal casts from
recent Foraminifera brought from various localities. A number of Greensands yield-
ing similar casts were collected on the ' Challenger ' Expedition, the most notable from
the coast of Australia.

POLYSTOMELLA

829

with the stolon processes connecting the successive segments of the
latter, as seen at c'. There can be little doubt that this remarkable
development of the canal system has reference to the unusual amount
of shell -substance which is deposited as an ' intermediate skeleton '
upon the layer that forms the proper walls of the chambers, and

Fig, 627. — Cycloclyjieus— external surface and vertical and horizontal sections.

which fills up with a solid ' boss ' what would otherwise be the de-
pression at the umbilicus of the spire. The substance of this 'boss'
is traversed by a set of straight canals, which pass directly from the
spiral canal beneath, towards the A:ternal surface, where they open
in little pits, as is shown in Plate XIX, 23, the umbilical boss
in P. crisj)a, however, being miich smaller in proportion than it

Fig. 628. — OjJercKlina laid open to show its internal structure : a, marginal
cord seen in cross-section at a'; b, b, external walls of the chambers'
c, c, cavities of the chambers ; c', c, their alar prolongations ; d, d, sejjta
divided at d' d' and at d" so as to lay open the interseptal canals the
general distribution of which is seen in the septa e, e ; the lines radiatino-
from e, e point to the secondary pores ; g, g, non- tubular columns.

is in P. craticulata. There is a group of Foraminifera to which the
term Nonionina is properly applicable, that is probably to be con-
sidered as a sub-genus oi PolystompJla, agreeing with it in its general
conformation, and especially in the distribution of its canal system,
but difiering in its aperture, which is here a single fissure at the
inner edge of the septal plane, and in the absence of the ' retral

830 MICROSCOPIC FORMS OF ANIMAL LIFE

processes ' of the segments of the sarcode-body, the external walls of
the chambers being smooth. This form constitutes a transition to
the ordinary ISTnmmuline type, of which Polystomella is a more aber-
rant modification.

The ISTummxiline type is most characteristically represented at the
present time by the genns Operculma^ which is so intimately united
to the true Nunnimidite by intermediate forms that it is not easy to
separate the two, notwithstanding that their typical examples are
widely dissimilar. The former genus (fig. 628) is represented on our
own coast and in northern seas by very small and feeble forms, but
it attains a much higher development in the tropics, where its
diameter sometimes reaches one-fourth of an inch. The shell is a
flattened nautiloid spire, the breadth of whose earlier convolutions
increases in a regular progression, but of which the last convolution
(in full-grown specimens) usually flattens itself out like that of
PenerojMs, so as to be very much broader than the preceding. The
external walls of the chambers, arching over the spaces between the
septa, are seen at 5, 5 ; and these are bounded at the outer edge of

Fig. 629. — CaZcnrwiftlaid open to show its internal structure : o, chambered
portion ; &, intermediate skeleton ; c, one of the radiating prolongations
proceeding from it, with extensions of the canal system.

each convolution by a peculiar band, a, termed the ' marginal cord.'
This cord, instead of being perforated by minute tubuli like those
which pass from the innei' to the outer surface of the chamber-walls
without division or inosculation (fig. 632), is traversed by a system
of comparatively large inoscidating passages seen in cross-section at
a', and these form part of the canal system to be presently de-
scribed. The principal cavities of the chambers are seen at c, c;
while the ' alar prolongations ' of those cavities over the surface of
the preceding whorl are shown at c', c' . The chambers are separated
by the septa d, d, d, formed of two laminae of shell, one belonging
to each chamber, and having spaces between them in which lie the
' interseptal canals,' whose general distribution is seen in the septa
marked e, e, and whose smaller branches are seen ii-i-egularly divided
in the septa d' , d' , whilst in the septvim d" one of the principal
trunks is laid open through its whole length. At the approach of
each septum to the marginal cord of the preceding is seen the
narrow fissure which constitutes the principal aperture of communi-

NUMMULITES 83 1

cation between the chambers ; in most of the septa, however, there
are also some isolated pores (to which the lines point that radiate
from e, e) varying both in number and position. The interseptal
canals of each septum take their departure at its inner extremity
fi-om a pair of spiral canals, of which one passes along each side of
the marginal cord ; and they communicate at their outer extremity
with the canal system of the ' marginal cord,' as shown in fig. 634.
The external walls of the chambers are composed of the same finely
tubular shell-substance that forms them in the N'ummulite ; but, as
in that genus, not only are the septa themselves composed of vitreous
non-tubular substance, but that which lies over them, continuing
them to the surface of the shell, has the same character, showing
itself externally in the form sometimes of continuous ridges, some-
times of rows of tubercles, which mark the position of the septa
beneath. These non-tubular plates or columns are often traversed
by branches of the canal system, as seen at g, g . )Similar columns
of non-tubular substance, of which the raimmits show themselves as
tubercles on the surface, are not unfrequently seen between the
septal bands, giving a variation to the surface-marking which, taken
in conjunction with variations in general conformation, might be
fairly held sufiicient to characterise distinct species, were it not that
on a comjyarison of a great number of specimens these variations
are found to be so gradational that no distinct line of demarcation
can be drawn between the individuals which present them.

The genus Nummulites, though repi-esented at the present time
by small and comparatively infreqiient examples, was formerly de-
veloped to a vast extent, the Kununulitic limestone, chiefly made up
by the aggregation of its remains (the material of which the P}T:"amids
are built), forming a band, often 1 ,800 miles in breadth and frequently
of enormous thickness, that may be traced from the Atlantic shores
of Europe and Africa, through Western Asia to Northern India and
China, and likewise over vast areas of North America (fig. 630).
The diameter of a large proportion of fossil Nummulites ranges
between half an inch and an inch ; but there are some whose
diameter does not exceed y^^th of an inch, whilst others attain the
gigantic diameter of 4^ inches. Their typical form is that of a
double-convex lens ; bu^t sometimes it much more nearly approaches
the globular shape, whilst in other cases it is very much flattened ;
and great diflferences exist in this respect among individuals of what
must be accounted one and the same species. Although there are
some Nummulites which closely approximate Operculince in theii'
mode of growth, yet the ty^Dical forms of this genus present certain
well-maiked distipictive peculiarities. Each convolution is so com-
pletely invested by that which succeeds it, and the external wall oi-
spiral lamina of the new convolution is so comj)letely separated.from
that of the convolution it incloses by the ' alar prolongations ' of its
own chambers (the peculiar arrangement of which will be presently
described), that the spire is scarcely if at all visible on the external
surface. It is brought into view, however, by splitting the Num-
mulite through the median plane, which may often be accom-
plished simply by striking it on one edge with a hammer, the opposite

832

MICEOSCOPIC FORMS OF ANIMAL LIFE

edge being placed on a firm support ; or, if this method shovild not
succeed, by heating it in the flame of a spirit-lamp, and then tin-ow-
ing it into cold water or striking it edgeways. Nummulites usually
show many more turns, and a more gradual rate of inci-ease in the
l^readth of the spire, than Foraminifera generally : this will be appa-
rent from an examination of the vertical section shown in fig. 631,
which is taken from one of the commonest and most chai-acteristic

Pig. 630. — A, piece of Nummiilitic limestone from Pyrenees,
showing Nummnlites laid open by fracture through median
plane ; B, Orbitoides.

fossil examples 'of the genus, and which shows no fewer than ten convo-
lutions in a fragment that does not nearly extend to the centre of the
spire. This section also shows the complete inclosure of the older
convolutions by the newer, and the interposition of the alar prolonga-
tions of the chambers between the successive layers of the spiral
lamina. These prolongations are variously arranged in different

Fig. 631. — Vertical section of portion of NwnmuUteslcBvigata: n, margin
of external whorl ; h, one of the outer row of chanibers ; c, c, whorl invested
by a ; d, one of the chambers of the fourth whorl from the margin ; e, e' ,
marginal portions of the inclosed whorls ; /, investing portions of outer
whorl ; g, g, spaces left between the investing portion of successive whorls ;
h, h, sections of the partitions dividing these.

examples of the genus ; thus in some, as JSf. distans, they' keep their
own separate course, all tending i-adially towards the centre ; in
others, as iV. Icevigata, their pai'titions inosculate with each other, so
as to divide the space intervening between each layer and the next
into an iri-egular network, presenting in vei'tical section the appear-
ance shown in fig. 631 ; whilst in i\^. garansensis they are broken

NUMMULITES

^33

up into a number of chamberlets having little or no direct communi-
cation with each other.

Notwithstanding that the inner chambers are thus so deeply
buried in the mass of investing Avhorls. yet there is evidence that

■ '!ia<:^""""'

Pig. 632. — Portion of a thin section of Nummulites Icevigata. taken in the
direction of the preceding, highly magnified to show the minute structure
of the shell : a, a, portions of the ordinary shell-substaiice traversed by
parallel tubuli ; b, b, portions forming the marginal cord, traversed by
diverging and larger tubuli ; c, one of the chambers laid open ; d, d, d,
pillars of solid substance not perforated by tubuli.

the segments of sarcode which they contained were not cut off from
communication with the exterior, but that they may have retained
their vitality to the last. The shell itself is almost every-
where minutely porous, being penetrated by parallel tubuli, which
pass directly from one surface to the other. These tubes are shown,
as divided lengthwise by a vertical section, in fig. 632, a, a ; whilst
the appearance they present when cut across in a horizontal section
is shown in fig. 633, the
transparent shell-substance
a, a, a being closely dotted
with minute punctations
which mark their orifices.
In that portion of the shell,
however, which forms the
margin of each whorl (fig.
632, b, b), the tubes are larger,
and diverge from each other
at greater intervals ; and it
is shown by horizontal sections
that they communicate freely
with each other laterally, so
as to form a network such as

Fig. 633. — Portion of horizontal section of
Nummidites showing the structure of the
walls and of the sej^ta of the chambers :
a, o, a, portion of the wall covering three
chambers, the punctations of vsrhich are the
orifices of tubuli ; 6, b septa between these
chambers containing canals which send out
lateral branches, c, c, entering the chambers
by larger orifices, one of which is seen at d.

is seen at h, b, fig. 634. At
certain other points, d, d, d,
fig. 632, the shell -substance
is not ]Derforated by tubes, but

is peculiarly dense in its texture, forming solid pillars which seem
to strengthen the other parts ; and in Nummulites whose surfaces
have been much exposed to attrition, it commonly happens that the
pillai's of the superficial layei', being harder than the ordinary shell-
substance, and being consequently less worn down, are left as

3h

834

MICEOSCOPIC FORMS OF ANIMAL LIFE

Fig. 634, — Internal cast of two of the cham-
bers of Nummulites striata, with the
network of canals, h, in the marginal
cord communicating with canals j^assing
between the chambers.

prominences, the presence of which has often been accounted (but
erroneously) as a specific character. The successive chambers of the
same whorl communicate with each other by a passage left between

the inner edge of the partition
that separates them and the
' marginal cord ' of the pre-
ceding whorl ; this passage is
sometimes a single large broad
apertvire, but is more com-
monly formed by the more or
less complete coalescence of
several separate pei'forations,
as is seen in fig. 631, h. There
is also, as in Operculina, a
variable number of isolated
pores in most of the septa,
forming a secondary means of
communication between the
charabers. The canal system
of Nummulites seems to be ar-
ranged upon essentially the
same plan as that of Oper-
culina ; its passages, however,
are usually more or less obscured by fossilising material. A careful
examination will generally disclose traces of them in the middle of
the partitions that divide the chambers (fig. 633, 6, 6), while from
these may be seen to proceed the lateral branches (c, c), which, aftei'
burrowing (so to speak) in the walls of the chambers, enter them
by large orifices (d). These ' interseptal ' canals, and their communi-
cation with the inosciilating system of passages excavated in the

marginal cord, are extremely
well seen in the ' internal cast '
represented in fig. 634.

A very interesting modifi-
cation of the Num-muline type
is presented in the genus
Heterostegina (fig. 635), which
bears a very strong resemblance
to Orhiculina in its plan of
growth, whilst in every other
respect it is essentially dif-
ferent. If the principal cham-
bers of an Operculina were
divided into chamberlets by
secondary partitions in a direc-
tion transverse to that of the
principal septa, it would be
converted into a Heterostegina,
just as a Pe'tieroplis would be converted by the like subdivision into
an Orhiculina. Moreover, we see in Heterostegina, as in Orhiculina,
a great tendency to the opening out of the spire with the advance of

Fig. 685. — Heterostegiiia.

NUMMULITES

835

age ; so that the apertm-al margin extends round a large part of the
shell, which thus tends to become discoidal. And it is not a little
curious that we have in this series another form, Cycloclypeus, which
bears exactly the same relation to Heterostegina that Orbitolites does
to Orbiciilma, in being constructed upon the cyclical plan from the
commencement, its chamberlets being arranged in rings around a
central chamber. This remarkable genus, at present only known in
the recent condition by specimens dredged at considerable depths
from the coast of Borneo and at one or two points in the Western
Pacific, is perhaps the largest of existing Foraminifera, some speci-
mens of its discs in the British Museum having a diameter of two
and a quarter inches. jSTotwithstanding the difference of its plan
of growth, it so precisely accords with
the Nummuline type in every cha-
racter which essentially distinguishes
the genus that there cannot be a
doubt of the intimacy of their rela-
tionship. It will be seen from the
examination of that portion of the
figure which shows Cycloclypeus in
vertical section that the solid layers
of shell by which the chambered por-
tion is inclosed are so much thicker,
and consist of so many more lamellas
in the central portion of the disc
than they do nearer its edge, that
new lamellfe must be progressively
added to the surfaces of the disc
concurrently with the addition of new
rings of chamberlets to its margin.
These lamellfe, however, are closely
applied one to the other without any
intervening spaces ; and they are all -pm. 636.— Section of Orhitoides
traversed by columns of non-tubular Fortisii, parallel to the surface,
substance, which spring from the traversing at «« the superficial
, , ' 1 11 • layer, and at 0, 0, the median

septal bands, and gradually increase layer.

in diametei- with their approach to

the surface, from which they project in the central portion of the

disc as glistening tubercles. -"^

The Nummulitic lirtiestone of certain localities (as the south-west
of France, Southern Germany, North-Eastern India, &c.) contains a
vast abundance of discoidal bodies termed Orhitoides (fig. 630, B),
which are so similar to Nummulites as to have been taken for them,
but which beai- a mueh closer resemblance to Cycloclypeus. These
are only known in the fossil state ; and their structure can only be
ascertained by the examination of sections thin enough to be trans-
lucent. When one of these discs (which vary in size, in different
species, from that of a fourpenny-piece to that of half a crown or
even larger) is rubbed down so as to display its internal organisation,

1 Dr. L. Rhumbler's ' Entwurf eines natiirlichen Systems der Thalamophoren '
(Naclir, Ges. Gottingen. 1891, p. 51) is chiefly based on palaeontological considerations.

3 H 2

836

MICROSCOPIC FORMS OF AI^IMAL LIFE

two different kinds of structure are visually seen in it, one being
composed of chamberlets of very definite form, quadrangular in some
species, circular in othei'S, arranged with a general but not constant
regularity in concentric cii^cles (figs. 636, 637, h, h) ; the other, less

Pig. 637. — Portions of the section of Orhitoides Fortisti, shown in fig. 636,
more highly niagnifiecl : a, superficial layer; b, median layer.

ti-ansparent, being formed of minuter chamberlets which have no
such constancy of form, but which might almost be taken for the
pieces of a dissected map {a, a). In the upper and lower walls of
these last, minute punctations may be observed, which seem to be

Fjg. 638. — Vertical section of Orhitoides Fortisii, showing the large
central chamber at a, and the median layer surrounding it,
covered above and below by the superficial layers.

the orifices of connecting tubes whereby they are perforated. The
relations of these two kinds of structure to each other are made
evident by the examination of a vertical section (fig. 638), which
shows that the portion b, figs. 636, 637, forms the median plane,
its concentric circles of chamberlets being arranged round a large
central chamber, as in Cydoclypeus ; whilst the chamberlets of the

portion a are irregularly superposed one
upon the other, so as to form several
layers which are most numerous towards
the centre of the disc, and thin away
gradually towards its margin. The dis-
position and connections of the cham-
bei'lets of the median layer in Orhitoides
seem to correspond very closely with
those which have been already described
as prevailing in Gycloclypeus, the most
satisfactory indications to this efiect
being furnished by the silicious ' internal
casts' to be met with in certain Green-
sands, which afford a model of the sar-
code-body of the animal. In such a
fragment (fig. 639) we recognise the chamberlets of three successive
zones, a, a', a", each of which seems normally to communicate by
one or two passages with the chamberlets of the zone internal and
external to its own ; whilst between the chamberlets of the same

Pig. 639. — Internal cast of por-
tion of median plane of Orhi-
toides Fortisii, showing, at
a a, a' a', a", a", six chambers
of each of three zones, with
their mutual communications ;
and at 6 h, h' V, h" b", portions
of three annular canals.

EOZOON

837

zone there seems to be no direct connection . They are brought into
relation, however, by means of annular canals, which seem to repre-
sent the spii'al canals of the Nummulite, and of which the ' internal
casts ' are seen at h b, h' b', b" b" .

A most remarkable fossil, referable to the foraminiferal type,
was discovered in strata much older than the very earliest
that were previously knowni to contain organic remains ; and the
determination of its real chai'acter may be regarded as one of
the most interesting resvilts of microscopic research. This fossil,
which has received the name Eozoon canadense (fig. 640), is found
in beds of Serpentine limestone that occur near the base of the

Fig. 640. — Vertical section of Eozoon canadense, showing alternation of
calcareous (light) and serpentinous (dark) laniellse.

Laurentian formation of Canada, which has its parallel in Europe in
the ' fundamental gneiss ' of Bohemia and Bavaria, and in the very
earliest stratified rocks of Scandinavia and Scotland. These beds
are found in many parts to contain masses of considei'able size, but
usually of indeterminate form, disposed after the manner of an
ancient coral reef, and consisting of alternating layers — frequently
numbering from 50 to 100 — of carbonate of lime and serpentine
(silicate of magnesia). The regularity of this alternation and the
fact that it presents itself also between other calcareous and silicious
minerals having led to a suspicion that it had its origin in organic
structure, thin sections of well-preserved specimens were submitted
to microscopic examination by the late Sir W. Dawson, of Montreal,

838 MICROSCOPIC FOEMS OF ANIMAL LIFE

who at once recognised its foraminiferal nature/ the calcareous
layers presenting the characteristic appearances of true shell, so dis-
posed as to form an irregularly chambered structure, and frequently
traversed by systems of ramifying canals corresponding to those of
Calcarina ; whilst the serjientinous or other silicious layers were
regarded by him as having been formed by the infiltration of sili-
cates in solution into the cavities originally occupied by the sarcode-
body of the animal — a process of whose occurrence at various geo-
logical periods, and also at the present time, abundant evidence has
already been adduced. Having himself taken up the investigation
(at the instance of Sir William Logan), the Author was not only able
to confirm Dr. Dawson's conclusions, but to adduce new and im-
portant evidence in support of them. ^ Although this determination
has been called in question, on the ground that some resemblance to
the supposed organic structure of Eozoon is presented by bodies of
purely mineral origin,-^ yet, as it has been accepted not only by most
of those whose knowledge of foraminiferal structure gives weight to
their judgment (among whom the late Professor Max Schultze may
be specially named), but also by geologists who have specially
studied the micro-mineralogical structure of the older Metamorphic
rocks,* the Aiithor feels justified in here describing Eozoon as
he believes it to have existed when it originally extended itself as
an animal growth over vast ai'eas of the sea-bottom in the Laiu^entian
epoch.

Whilst essentially belonging to the Ntimmuline group, in virtue
of the fine tubulation of the shelly layers forming the ' proper wall '
of its chambers, Eozoon is related to various types of recent Foi-a-
minifera in its other characters. For in its indeterminate zoophytic
mode of growth it agrees with Polytrona in the incomplete separa-
tion of its chambers ; it has its parallel in Carpenteria ; whilst in the
high development of its ' intermediate skeleton ' and of the ' canal
system ' by which this is formed and nourished, it finds its nearest
representative in Calcarina. Its calcareous layers were so super-
posed one upon another as to include between them a siiccession

1 This recognition was due, as Dr. Dawson has expHcitly stated in his original
memoir [Qtiart. Journ. of Geol. Soc. vol. xxi. p. 54), to his acquaintance, not merely
with the Author's previous researches on the minute structure of the Foraminifera,
but with the special characters presented by thin sections of Calcarina which had
been transmitted to him by the Author. Dr. Dawson has given an account of the
geological and mineralogical relations of Eozoon, as well as of its organic structure, in
a small book entitled The J)aiv)i of Life.

2 For a fuller account of the results of the Author's own study of Eozoon, and of the
basis on which the above reconstruction is founded, see his papers in Quart. Journ.
of Geol. Soc. vol. xxi. p. 59, and vol. xxii. j)- 219, and in the Intellectual Observer,
vol. vii. 1865, p. 278 ; and his ' Further Eesearches ' in Ann. of Nat. Hist. June 1874.

^ See the memoirs of Professors King and Rowney in Quart. Journ. of Geol. Soc.
vol. xxii. p. 185, and Ann. of Nat. Hist. May 1874.

* Among these the Author is jpermitted to mention Professor Geikie, of Edinburgh,
who has thus studied the older rocks of Scotland, and Professor Bonney of London, who
has made a like study of the Cornish and other Serpentines. By both these eminent
authorities he is assured that they have met with no purely mineral structure in the
least resembling Eozoon, either in its regular alternation of calcareous and serpen-
tinous lamellfe, or in the dendritic extensions of the latter into the former ; and while
they accept as entirely satisfactory the doctrine of its organic origin maintained by
the Author, they find themselves unable to conceive of any inorganic agency by which
such a structure could have been produced.

EOZOON

839

of 'storeys' of chambers (fig. 641, A^, A', A^, A^), the chambers
of each ' storey ' usually opening one into another, as at a, a, like
apartments en suite, but being occasionally divided by complete septa,
as at h, h. These septa are traversed by passages of communication
between the chambei'S which they separate, resembling those which,
in existing types, are occupied by stolons connecting together the
segments of the sarcode-bocly. Each layer of shell consists of two
finely tubulated or ' Nummuline ' lamellfe, B, B, which form the
boundaries of the chambers beneath and above, serving (so to speak)
as the ceiling of the former, and as the floor of the latter ; and of
an intervening deposit of homogeneous shell-substance C, C, which
constitutes the ' inter-
mediate skeleton.' The
tubuli of this ' ISTum-
muline' layer (fig. 643)
are usually filled up (as
in the JSTummulites
of the ' Nummulitic
limestone ') by mineral
infiltration, so as in
transparent sections to
present a fibrous ap-
pearance ; but it for-
tvmately happens that
through their having
in some cases escaped
infiltration the tubu-
lation is as distinct
as it is even in recent
Nummuline shells (fig.
643), bearing a singu-
lar resemblance in its
occasional waviness to
that of the crab's claw.
The thickness of this
interposed layer varies
considerably in differ-
ent parts of the same
base and progressively

Fig. 641. — Portion of the calcareous shell of Eozoon
canadense as it would appear if the serpentine
that fills its chambers were dissolved away : A', A^,
chambers of lower storey opening into each other
at a, a, but occasionally separated by a septum,
h, b ; A^, A^, chambers of upper storey ; B, B,
proper walls of the chambers, formed of a finely
tubular or Nummuline substance ; C, C, inter-
mediate skeleton, occasionally traversed by large
stolon-passages, I), connecting the chambers of
different storeys, and penetrated by the arbores-
cent systems of canals, E, E, E.

mass, being in general greatest near its
diminishing towards its upper surface.
The ' intermediate skeleton ' is occasionally traversed by large
passages (D), which seem to establish a connection between the
successive layers of chambers ; and it is penetrated by arborescent
systems of canals (E, E), which are often distributed both so
extensively and so minutely through its substance as to leave
very little of it without a branch. Tliese canals take their origin,
not directly from the chambers, but from irregular lacunar or
interspaces between the outside of the proper chamber-walls and
the ' intermediate skeleton,' exactly as in Calcarina, the exten-
sions of the sarcode-body which occupied them having apparently
been foi-med by the coalescence of the pseudopodial filaments that
passed through the tubulated lamellse.

840 MICKOSCOPIC FOKXS OF ANIMAL LIFE

In the fossilised condition in which Eozoon is most commonly
found, not only the cavities of the chambers, but the canal systems
to their smallest ramifications are filled vip by the silicions infiltra-
tion which has taken the place of the original sarcode-body, as in the
cases already cited, and thus when a piece of this fossil is siibjected
to the action of dilute acid, by which its calcareous portion is dis-
solved away, we obtain an internal cast of its chambers and canal
system (fig. 642), which, though altogether dissimilar in arrangement,
is essentially analogous in character to the ' internal casts ' repre-
sented in figs. 622, 626. This cast presents us, therefore, with a
model in hard serpentine of the soft sarcode-body which originally
occupied the chambers, and extended itself into the ramifjdng canals,
of the. calcareous shell ; and, like that of Polystomella, it affords an
even more satisfactory elucidation of the relations of these parts
than we could have gained from the study of the living organism.

-n^

Fig. 642. — Decalcified portion of Eozoon canadense shell, showing the ser-
pentinous internal cast of the chambers, canals, and tubuli of the original,
presenting an exact model of the animal substance which originally filled
them.

We see that each of the layers of serpentine, forming the lower part
of such a specimen, is made up of a number of coherent segments,
which have only undergone a partial separation ; these appear to
have extended themselves horizontally without any definite limit,
but have here and there developed new segments in a vertical dii^ec-
tion, so as to give origin to new layers. In the spaces between these
successive layers, which were originally occupied by the calcareous
shell, we see the • internal casts ' of the branching canal system,
which give us the exact models of the extensions of the sarcode-body
that originally passed into them. But this is not all. In specimens
in which the.Nummuline layer constituting the ' proper wall ' of the
chambers was originally well preserved, and in which the decalcifying
process has been carefully managed (so as not, by too rapid an evolu-
tion of carbonic acid gas, to disturb the arrangement of the serjjen
tinous residuum), that layer is represented by a thin white film
covering the exposed surfaces of the segments ; the superficial asj)ect

EOZOON

841

of Avhich, as well as its sectional view, is shown in fig. 642. And
when this layer is examined with a sufficient magnifying powei" it is
found to consist of extremely minute needle-like fibres of serpentine,
which sometimes stand upright, parallel, and almost in contact with
each other, like the fibres of asbestos (so that the film which they
form has been termed the ' asbestiform layer '), but which are fre-
quently grouped in converging brush-like bundles, so as to be very
close to each other in certain spots at the surface of the film, whilst
widely separated in others. Now these fibres, which are less than
1000 0^^^ °f ^^^ inch in diameter, are the ' internal casts ' of the tubuli
of the ISTummuline layer (a precise parallel to them being presented in
the ' internal cast ' of a recent Amjjhistegina which Avas in the Author's
possession) ; and their arrangement presents all the varieties which
have been mentioned as existing in the shells of OpercuUna. Thus

t \^"

Fig. 643. — Vertical section of a portion of one of the calcareous lamellse of
Eozoon canadense : a a, Nummuline layer perforated by parallel tubuli,
which show a flexure along the line «' a' ; beneath this is seen the inter-
mediate skeleton, c, c, traversed by the large canals, b, b, and by oblique

■ cleavage planes, which extend also into the Nummuline layer.

these delicate and beautiful silicious fibres represent those jiseudo-
podial threads of sarcode which originally traversed the minutely
tubular walls of the chambers ; and a j^recise model of the most
ancient animal of which we have any knowledge, notwithstanding
the extreme softness and tenuity of its substance, is thus presented
to us with a completeness that is scarcely even approached in any
later fossil.

In the ujDper part of the ' decalcified ' specimen shown in fig. 642
it is to be observed that the segments are confusedly heaped together
instead of being regulai'ly arranged in layers, the lamellated mode
of growth having given place to the acervuline. This change is by
no means uncommon among Foraminifera, an irregular piling-
together of the chambers being frequently met with in the latei-
growth of types whose earlier increase takes place upon some much

842 MICKOSCOPIC FOEMS OF ANIMAL LIFE

more definite plan. After what fashion the earliest development of
Eozoon took place, we have at present no knowledge whatever ; but
in a young specimen which has been recently discovered it is obvious
that each successive ' storey ' of chambers was limited by the closing
in of the shelly layer at its edges, so as to give to the entire fabric a
definite form closely resembling that of a straightened Peneroplis.
Thus it is obvious that the chief peculiarity of Eozoon lay in its
capacity for indefinite extension, so that the product of a single germ
might attain a size comparable to that of a massive coral. Now this,
it will be observed, is simply due to the fact that its increase by
gemmation takes place continuously, the new segments successively
budded off remaining in connection with the original stock, instead
of detaching themselves from it as in Foraminifera generally. Thus
the little Globigerina forms a shell of which the number of chambers
does not usually seem to increase beyond sixteen, any additional
segments detaching themselves so as to form separate shells ; but by
the repetition of this multiplication the sea-bottom of large areas of
the Atlantic Ocean at the present time has come to be covered with
accumulations of Globigerince, which, if fossilised, would form beds of
limestone not less massive than those which have had their origin in
the growth of Eozoon. The difference between the two modes of
increase may be compared to the difference between a herb and a
tree. For in the herb the individual organism never attains any
considerable size, its extension by gemmation being limited ; though
the aggregation of individuals produced by the detachment of its buds
(as in a potato-field) may give rise to a mass of vegetation as great
as that formed in the largest tree by the continuous putting forth of
new buds.

It has been hitherto only in the Laurentian serpentine lime-
stone of Canada that Eozoon has presented itself in such a state of
preservation as fully to justify the assumption of its organic nature.
But from the greater or less resemblance which is presented to this
by serpentine-limestones occurring in various localities among strata
that seem the geological equivalents of the Canadian Laiirentians, it
seems a justifiable conclusion that this type was very generally dif-
fused in the earlier ages of the earth's history, and that it had a
large (and probably the chief) share in the production of the most
ancient calcareous strata, separating carbonate of lime fi-om its solu-
tion in ocean water, in the same manner as do the polypes by whose
growth coi-al reefs and islands are being upraised at the present time.

An elaborate work, ' Der Bau des Eozoon Canadense ' (1878),
has been recently j)ublished by Professor Mobius, of Kiel, in which
the structure of Eozoon is compared with that of various types of
Foraminifera, and, as it differs from that of every one of them, is
aflirmed not to be organic at all, but purely mineral. Upon this the
Author would remark, that if the validity of this mode of reasoning-
be admitted, any fossil whose structure does not correspond with that
of some existing type is to be similarly rejected. Thus the Stroma-
topora of Silurian and Devonian rocks, which some palaeontologists
I'egard as a coral, others as polyzoary, others as a calcareous sponge,
and others as a foraminifer, would not be a fossil at all, because it

EOZOON 843

(liffei'S from every known living form. Yet the suggestion that it is
of mineral origin wo aid be scouted as absurd by every palasontologist.
Again it is lu-ged by Professor Mobius that as the supposed canal
system of Eozoon has not the constancy and regularity of distribu-
tion which it presents in existing Foraminifera, it must be accounted
a mineral infiltration. To this the Author would rej^ly — (1) that
a prolonged and careful study of this ' canal system,' in a gTeat
variety of modes, with an amount of material at his disposal many
times greater than Professor Mobius could command, has satisfied
him that in well-preserved specimens the canal system, so far from
being vague and indefinite, has a very regular plan of distribution ;
(2) that this plan does not differ more from the arrangements
characteristic of the several types of existing Foraminifera than
these diflfer from each other, its general conformity to them being-
such as to satisf}^ Professor Max Schultze (one of the ablest students
of the gi'oup) of its foraminiferal character ; and (3) that not only
does the distribution of the canal system of Eozoon differ in certain
essential features from every form of mineral infilti'ation hitherto
brought to light, but that canal systems in no respect differing from
each other in distribiition are occupied by different minerals ; a fact
which seems conclusively to point to their p>re- existence in the cal-
careous layers, and the subsequent penetration of these minerals into
the passages previously occupied by sarcode — precisely as has
happened in those ' internal casts ' of existing Foraminifera which
Professor Mobius altogether ignores.

The argument for the foraminiferal nature of Eozoon is essentially
a cumulative one, resting on a number of independent probabilities,
no one of which, taken separately, has the cogency of a proof; yet
the accordance of them all with that hypothesis has an almost
demonstrative value, no othei- hypothesis accounting at once for the
whole assemblage of facts. ^

Collection and Selection of Foraminifera. — Many of the Fora-
minifera attach themselves in the living state to seaweeds, zoophytes,
etc. ; and they should therefore be carefully looked for on such
bodies, especially when it is desired to observe their internal organi-
sation and their habits of life. They are often to be collected in

^ The above account of Eozoon is allowed to stand as Dr. Carpenter's name has
become so intimately connected with the view, now not commonly held, that the body
has an animal origin. It may be noted that Prof. J. W. Gregory, wlio has had an
opportunity of examining the so-called Tudor specimen of Eozoon, communicated
to the Geological Society, on March 11, 1891, a paper, of which the following is an
abstract : —

After careful examination of all the slides and figures, and after consideration of
Sir W. Dawson's interpretation, the author is absolutely unable to recognise in the
specimen any trace of the ' proper wall,' ' canals,' or ' stolon passages,' which are
claimed to occur in Eozoon., or any reasons for regarding the calcite bands as the
' mtermediate skeleton ' of a foraminifer. There are ^joints in Sir W. Dawson's
figure which might pass as ' stolon passages,' but they appear very different in a
photograph, and the siaecimen agrees with the latter. The author, however, gives
reasons for concluding that the case against the organic origin of the Tudor specimen
does not rest on negative evidence alone ; for, though the rock is much contorted, the
twin lamellas and cleavage-planes of the calcite are not bent ; and the fact that the
crystalline bands cut across the bedding-planes further shows their secondary origin.
The rock in which the specimen was found is not ' Lower Laurentian,' and is included
by Messrs. Selwyu and Vennor in the Huronian.

844 MICEOSCOPIC FOEMS OF ANIMAL LIFE

much larger numbers, however, from the sand or mud dredged vip
from^ the sea-bottom, or even from that taken from between the tide-
marks. In a paper containing some valuable hints on this subject ^
Mr. Legg mentions that, in walking over the Small-mouth Sand,
which is situated on the north side of Portland Bay, he observed
the sand to be distinctly raarked with white ridges, many yards
in length, running parallel with the edge of the water ; and upon
examining portions of these, he found Foraminifera in considerable
abundance. One of the most fertile sources of supply that om- own
coasts afford is the ooze of the oyster-beds, in which large numbers
of living specimens will be found ; the variety of specific forms, how-
ever, is usually not very great. In separating these bodies from the
particles of sand, mud, ifcc, with which they are mixed, various
methods may be adopted in order to shorten the tedious labour of
picking them out one by one under the simple microscope ; and the
choice to be made among these will mainly depend upon the condi-
tion of the Foraminifera, the importance (or otherwise) of obtaining
them alive, and the nature of the substances with which they are
mingled. Thus, if it be desired to obtain living specimens from the
oyster-ooze for the examination of theu" soft parts, or for preservation
in an aquarium, much time will be saved by stirring the mud (which
should be taken from the surface only of the deposit) in a jar with
water, and then allowing it to stand for a few moments ; for the
finer particles will remain difiVised through the liquid, while the
coai-ser will subside ; and, as the Foraminifera (in the present case)
will be among the heavier, they will be found at the bottom of the
vessel with comparatively little extraneous matter, after this opera-
tion has been repeated two or three times. It would always be well
to examine the first deposit let fall by the water that has been
poured away, as this may contain the smaller and lighter forms of
Foraminifera. But supposing that it be only desired to obtain the
dead shells from a mass of sand brought up by the dredge, a very
different method should be adopted. The whole mass should be
exposed for some hours to the heat of an oven, and be turned ovei-
several times, until it is found to have been thoroughly dried
throughout ; and then, after being allowed to cool, it should be
stirred in a large vessel of water. The chambers of their shells
being now occupied by air alone (for the bodies of such as were
alive will have shrunk up almost to nothing), the Foraminifera will
be the lightest portion of the mass ; and they will be found floating
on the water, while the particles of sand &c. subside. Another
method, devised by Mr. Legg, consists in taking advantage of the
relative sizes of different kinds of Foraminifera and of the substances
that accompany them. This, which is especially applicable to the sand
and rubbish obtainable fi-om sponges (which may be got in large quan-
tity from the sponge-merchants), consists in sifting the whole aggre-
gate through successive sieves of wire-gauze, commencing with one
of ten wires to the inch, which will separate large extraneous particles,
and proceeding to those of twenty, forty, seventy, and a hundred
wires to the inch, each (especially that of seventy) retaining a much
^ Trans, of Microsc. Soc. ser. ii. vol. ii. 1854, p. 19,

collectinct foeaminifera 845

larger proportion of foraminifei'al shells than of the accompanying
particles ; so that, a large portion of the extraneous matter being thus
got rid of, the final selection becomes comparatively easy. Certain
forms of Foraminifera are found attached to shells, especially bivalves
(such as the Chainiclce) with foliated surfaces ; and a careful exami-
nation of those of tropical seas, when brought home ' in the rough,'
is almost sure to yield most valuable results. The final selection of
specimens for mounting should always be made undei' some appropriate
form of single microscope, a fine camel-hair pencil, with the point
wetted between the lips, being the instrument which may be most con-
venientl}^ and safely employed, even for the most delicate specimens.
In mounting Foraminifera as microscopic objects the method to be
adopted must entirely depend upon whether they are to be viewed
by transmitted or by reflected light. In the former case they should
be mounted in Canada balsam, the various precautions to prevent
the retention of air-bubbles, which have been already described, being
carefully observed. In the latter no plan is so simple, easy, and .
effectual as attaching them with a little gum to wooden slides.
They should be fixed in various positions, so as to present all
the different aspects of the shell, particular care being taken
that its mouth is clearly displayed ; and this may often be most
readily managed by attaching the specimen sideivays to the wall of
the circular depression of the slide. Or the specimens may be
attached to discs fitted for being held in a disc-holder ; whilst for
the examination of specimens in every variety of position Mr. R.
Beck's disc-holder will be found extremely convenient. Where, as
will often happen, the several individuals differ considerably from,
one another, special care should be taken to arrange them in series
illustrative of theii- range of variation and of the mutual connections
of even the most diverse forms. For the display of the internal
structure of Foraminifera it will often be necessaiy to make exti'emely
thin sections, in the manner already described ; and much time will
be saved by attaching a number of specimens to the glass slide at
once and by grinding them down together. For the preparation of
sections, however, of the extreme thinness that is often required,
those which have been thus reduced should be transferred to
separate slides and finished off each one by itself.

For the collection and examination of fossil Foraminifera, which
are of great interest and importance, the following .suggestions will
be of use ; they are the result of the lipe experience of Mr. F.
Chapman :

Perhaps the foraminiferous clays are the most satisfactoiy for
those who desire to collect foraminifera. Ordinary clays require to
be slowly and thoroughly dried, broken into small j)ieces of about a
cubic inch or so, and placed in a vessel of water with steep sides.
After some little time the material will be found to have become
disintegrated. The vessel should then be shaken round, and after
the coarser particles have subsided the fine muddy portion may be
poured off. The materials should again be shaken with ver}^ little
water, and more water should then be added so as to cleanse the
mud, and the decanting process afterwards repeated. If this be done

846 MICROSCOPIC FORMS OF ANIMAL LIFE

several times a fine sand with foraminiferal and other shells will be
obtained. This can be then dried and sifted in the manner already
described for the sands from modern deposits. To insure obtaining
the minutest shells, the water which is poured off should be passed
through a fine cambric or silken sieve.

The following are some of the more productive of thefossiliferous
deposits :

Weathered surfaces of carboniferous limestone and seams of clay
in the joints of it.

Clay from the lias formation.

Gault clay especially from the upper zones.

The softer beds of the upper chalk and especially the phosphatic
chalk of Taplow, which washes down easily.

Foraminifera may be fixed by gum arabic with three drops of
glycerine added to the ounce, or with gum tragacanth, which has the
advantage of drying with a dead surface.

SECTIOJiT II. RaDIOLARIA.

It has been shown that one series of foi-ms belonging to the
rhizopod type is characterised by the radiating arrangement of theii'
rod-like pseudopodia, suggesting the designation Heliozoa or ' sun-
animalcules ; ' and that even among those fi-esh-water forms that do
not depart widely from the common Actinophrys there are some
whose bodies are inclosed in a complete silicious skeleton. Now
just as the reticularian type of rhizopod life culminates in the marine
calcareous-shelled Foraminifera, so does the heliozoic type seem to
culminate in the marine Radiolaria ; which, living for the most
part near the surface of the ocean, form silicious skeletons (often of
marvellous symmetry and beauty) that fall to the bottom on the
death of the animals that produced them, and may remain unchanged,
like those of the diatoms, through unlimited periods of time. Some
of these skeletons, mingled with those of diatoms, had been detected
by Professor Ehrenberg in the midst of various deposits of foramini-
feral origin, su^ch as the calcareous Tertiaries of Sicily and Greece,
and of Oran in Africa ; and he established for them the group of
Polycystina, to which he was able also to refer a beautiful series of
forms making up nearly the whole of a silicious sandstone prevail-
ing through an extensive district in the island of Barbadoes (fig. 644).
iSTothing, however, was known of the nature of the animals that
formed them until they were discovered and studied in the living state
by Professor J. MUller,^ who established the group of Radiolaria,
including therein, with the Polycystina of Ehrenberg, the AcantJio-
metrina first recognised by himself, and the Thalassicolla which had
been discovered by Professor Huxley. Not long afterwards appeared
tlie magnificent and ' epoch-making ' work of Professor . Haeckel ; '■^

1 ' Ueber die Thalassicollen, Polycystinen, und Acanthometren des Mittel-
meeres,' in Ahhandlungen der konigl. Ahad. der Wisseiisch. zu Berlin, 1858, and
separately published ; also ' Ueber die im Hafen von Messina beobachteten Pohj-
cystinen,' in the MonatshericJite of the Berlin Academy for 1855, pp. 671-676.

^ Die Radiolarien (Rhizopoda Eadiaria), Berlin, 1862. This great work has
lately been followed by a gigantic monograph published in the ' Challenger ' Beports,

EADIOLAEIA

847

and since that time much has been added by various observers to
our knowledge of this group, which still remains, however, very
imperfect.

Each individual radiolarian consists of two portions of coloured
or colourless sarcode — one portion nucleated and central, the other
portion peripheral, and almost always containing certain yellow
corpviscles. These two portions are separated by a membrane called
the capsule ; but this is so porous as to allow of their free communi-
cation with each other. The innei- central capsule is also the special

Fig. 644. — Fossil Hadiolaria from Barbacloes : n, Podocyrtis mitra ; h,
RhahdolitJms sceptruvi; c,Lychiiocanium falciferum \ d, Eucyrtidium
tuhulus; e, Flustrella concentrica\f,Lychnocaninm lucerna; g, Eucyr-
tidium elegans; h, Dictyospyris clathrus; i, Eucyrtidium Mongolfieri ;
Jc, Stephanolithis spinescens; I, S. nodosa; m, Lithocyclia ocellus; n,
Cephalolithis sylvina; 0, Podocyrtis cotliurnata; pi, Wiahdolithus ptipia.

organ of reproduction, for it is the intracapsular jjroto^jlasm, with
the nuclei imbedded in it, which serves for the formation of flagellate
spores ; the outer capsule has the special ofiice of protecting and
pi-oviding nouri.shment for the cell.^ The pseudopodi a radiate in all
directions (fig. 645) from the deeper portion of the extracapsular
sarcode ; they have generally much pei'sistency of direction and very

which extends over 1,800 pages, and is illustrated by 1 40 j)lates. Li it are described
4,318 species, of which 3,508 are new to science.

^ The structure of the central capsule of ^4 »/flcan^7ia has been carefully worked
out by "W. Karawaiew, in Zool. Anzeig. xviii. (1895), p. 286 and p. 293.

848

MICKOSCOPIC FOEMS OF ANIMAL LIFE

little flexibility ; in some species (but not ordinarily) they branch
and anastomose, while in others they are inclosed in hollow
rods that form part of the silicious skeleton, and issue forth from
the extremities of these. A flow of granules takes place among
them ; and the mode in which they obtain food-particles (consisting
of diatoms and other minute algse, marine infusoria, &c.), and draw
them into the sarcode-bodies of the radiolarians, appears to corre-
spond entirely with their action in Actinophrys and other Heliozoa.
The yellow cells, or Zdoxanthellce, as K. Brandt has proposed to
call them, so often seen in these cells, are not confined to Radiolaria,

Fig. 645.— Polycystina : A, Halioinma hijstrix ; B, Pterocaniiun, with animal.

for they are found also in Actiniae and various other invertebrates ;
nor are they always present in examples studied ; they are now com-
pletely recognised ^ as algag which form a ' symbiotic ' relation with
their host, the animal profiting by the removal of its waste products
by its messmate, by the oxygen which its guest evolves in sunlight,
and. by the food-material it provides after death, while the plant
feeds on the waste of the animal.

In most Eadiola/)-ia skeletal structures are developed in the
sarcode-body, either inside or outside the capsule, or m both positions ;
sometimes in the form of investing networks having more or less of
a spheroidal form (fig. 647, 1, 2), or of radiating spines, 3, or of
combinations of these, 4, 5. But in many cases the skeleton consists
only of a few scattered spicules ; and this is especially the case in
certain large composite forms or ' colonies ' (fig. 652), which may

• See especially K. Brandt, Verhandl. Physiol. Gesellsch. Berlin, 1881-82, p. 22 ;
Mitth. Zool. Stat. Neapel, iv. p. 191 ; P. Geddes, Nature, xxv. p. 303.

POLYCYSTINA

849

consist of as many as a thousand zooicls aggregated together in various
forms, discoidal, cylindrical, spheroidal, chain-like, or even necklace-
like. The ' colonies ' seem to l^e produced, like the multij^le segments
of the bodies of Foraminifera, by the non-sexual multiplication of a
primoi'dial zooid ; but whether this multiplication takes place by
fission, or by the budding off' of portions of the sarcode-body, has
not yet been clearly made out. The emission of flagellated zoospores,
provided with a very large nucleus, and in some cases with a rod-
like crystal, has been observed in many radiolarians ;' but of the mode
in which they are pi'oduced, and of their subsequent history, very
little is at present knoAvn. Until the structure and life history of
the aiiimals of this very interesting type shall have been more fully
elucidated, no satisfactory classification of them can be framed ; and
nothing more wiW be here attempted than to indicate some of the
principal forms under which the radiolarian type presents itself.^

Discida. — Among the
beautiful silicious struc-
tiu-es which are met with
in the radiolarian sand-
stone of Barbadoes (fig.
644) there is none more
interesting than the ske-
leton of Astr ovinia (fig.
648), in which we have a
remarkable example of
the range of variation that
is compatible with con-
foi-mity to a general plan
of structure. As in other
foi-ms of Haeckel's group
of Discida, there is in
this skeleton a combina-
tion of radial and of cir-
cumferential parts, the
former consisting of solid
spoke-like rods, whilst the
latter is composed of a silicious network more or less completely
fiUiug up the spaces between the rays. The radial part of the skele-
ton predominates in the beautiful four-rayed example represented at
D, having the form of a cross with equal arms ; whilst in F and G it
still shows itself very conspicuously, though the spaces between the
rays are in great part filled up by the circumferential network. In the
five-rayed specimens A and B, on the other hand, the radial portion
is much less developed, while the circumferential becomes more dis-
coidal. And in C and E, while the circumferential network forms a
pentagonal disc, the radial portion is represented only by solid projec-
tions at its angles. The transition between the extreme forms is
found to be so gradual when a number of specimens are compared
that no lines of specific distinction can be drawn between them ; and

1 Considerate attention has been given to the question of the classification of
the Eadiolaria by Haeckel and by R. Hertwig, Jenaische Denkscltr. ii. 1879, p. 129.

Fig. 646. — Polycystina : A, Poclocyrtis Sclwui-
hurgkii', B, Bhopalocaniwn ornatiun.

850

MICROSCOPIC FORMS OF ANIMAL LIFE

the difi'erence in the number of rays is pi'obably of no move account in
these low forms of animal life than it is in the discoidal diatoms.
Other discoidal forms, showing a like combination of radial and
circumferential parts, are represented in figs. 649 and 650, and also
in fiff. 644, e, in.

Fig. 647. — Various forms of Radiolaria (after Haeckel) : 1, Etlimospluera
sii^limuiphora; 2, Actinomma inerme \ 3, Acantliomeira xijMcantlui ;
4, Amrhno8})licera oligacantha ; 5, Cladococcvs viniinalis.

Entosphserida. — In this grouj) the silicious shell is spheroidal,
and is formed ivlthln the capsule; and it is not traversed by radii,
althcjugh jii'olongations of the shell often extend themselves radially

POLYCYSTINA

851

outwards, as in Cladococcus (fig. 647, 5). Sometimes the central
sphere is inclosed in two, three, or even more concentric spheres
connected by radii, as in the beautiful Actinomma (fig. 647, 2), re-
minding us of the wonderful concentric spheres carved in ivory by

Fig. 648. — Varietal modifications of Astromma.

the Chinese. One of the most common examples of this group is
the Haliomma HumholdtU (fig. 651), in which the shell is double.

Polycystina. — This name, which originally included the preceding
group, is now restricted to those which have the shell formed outside

Fig. 649. — Periclilomydmm prcetexi am.

Fig. 650. — Stylochjctya gracilis.

the capsule. This shell may, as in the preceding, be a simple sphere
composed of an open silicious network, as in Ethmosphcera (fig. 647, l) ;
or it may consist of two or three concentric spheres connected by
radii ; or, again, it may put forth radial outgrowths, Avhich sometimes

3 i2

852

MICEOSCOPIC FOEMS OF ANIMAL LIFE

extend themselves to several times the diameter of the shell, and
ramify more or less minutely, as in Araclinospluera (fig. 647, 4). But
more frequently the shell opens out at one pole into a form more or
less bell-like, as in Podocyrtis (fig. 646, A, and fig. 644, «, o), Rliopalo-
canium (fig. 646, B), and Fterocanium (fig 645, B) ; or it may
be elongated into a somewhat cylindrical form, one pole remaining
closed, while the other is more or less contracted, as in Eucyrtidium
(fig. 644, d, g, i). The transition between these forms, again, proves
to be as gradational, when many specimens are compared,' as it is
among Foraminifera.

Acanthometrina. — In this group the animal is not inclosed within
a shell, but is furnished with a very regular skeleton, composed
of elongated spines, which i-adiate in all directions from a common
centre (fig. 64.5, A). The soft sarcode-body is spherical in form, and
occupies the spaces left betw^een the bases of these spines, which
are sometimes partly inclosed (as in the species represented) by
transverse projections. The ' capsule ' is jDierced by the pseudo-
podia, whose convergence may be traced from without inwards,

afterwards passing through it ; and it
is itself enveloped in a layer of less
tenacious protoplasm, resembling that
of which the pseudopodia are composed.
One species, the Acanthometra echin-
oides, which presents itself to the naked
eye as a crimson-red point, the dia-
meter of the central part of its body
being about y^jL_ths of an inch, is very
common on some parts of the coast of
Korway, especially during the preva-
lence of westerly winds ; and the
Author has himself met with it abun-
dantly near Shetland, in the floating
brown masses termed madre by the fishermen (who believe them
to furnish food to the herring), which consists mainly of this
Acanthometra mingled with Entomosti-aca.

Phseodaria. — -Among the most important of the Radiolaria
collected by the ' Challenger ' are the comparatively lai-ge (as much as
1 mm. in diameter) single-celled forms which are remarkable for the
constant presence of large dark brown gi-anules, which are scattered
irregularly round the centiul capsule and cover the greater part of
its outer surface. The nucleus is large, the capsular membrane is
always double, and is piei-ced by one or more lai-ge openings ; the
whole cell is inclosed in a thick gelatinous covering, and there is
nearly always a well-developed extracapsular silicious skeleton,
according to the structvxre of which the group is subdivided.^

GoUozoa. — To this group belong those remarkable composite
forms which, exhibiting the characteristic radiolarian type in their

' The general plan of structure of the Polycijstiua, and the signification of their
immense variety of forms, were ably discussed by Dr. Wallich in the Trans, of the
Microsc. Soc. n.s. vol. xiii. 1865, p. 75.

^ On reproduction in this group, cf. A. Borgert, Zool. Aiizeig. xix. (1896), p. 307.

n^vs

Fig. 651. — Haliomma Sumboldti

RADIOLAEIA

853

>nV.

individvial zooicls, are aggregated into masses in which the skeleton
is represented onlj^ by scattered spicules, as in ^phcerozoum (fig. 652)
and Thalassicolla. These 'sea-jellies,' which so abound in the seas of
warm latitudes as to be among the commonest objects collected by
the tow-net, are small gelatinous i-ounded bodies, of very variable
size and shape, but usually either globular oi- discoidal. Externally
they are invested b}^ a layei- of coiadensed sarcode, which sends forth
psevidopodial extensions that commonly stand out like rays, but
sometimes inosculate with each other so as to form a network. To-
wards the inner surface of this coat are scattered a great number of
oval bodies resembling cells having a tolerably distinct membraniform
wall and a consj)icuous round central nucleus. Each of these bodies
appears to be without any direct connection with the rest, but it
serves as a centre round which a number of minute yellowish-green
vesicles are disposed. , __

Each of these groups is
protected by a silicious
skeleton, which some-
times consists of separate
spicules (as in fig. 652),
but which may be a thin
perforated sphere, like
that of certain Poly-
cystina, sometimes ex-
tending itself into radial
prolongations. The in-
tei'nal portion of each
mass is composed of an
aggregation of large
vesicle-like bodies im-
bedded in a softer sar-
codic substance.'

From the researches
made during the ' Chal-
lenger ' Expedition, it
appears that the Radiolaria are very widely difiiised through the
waters of the ocean, some forms being more abundant in tropical
and others in temperate seas ; and that they live not only at or near
the surface, but also at considerable depths. Their silicious skeletons
accumulate in some localities (in which the calcareous remains of
Foraminifera are wanting) to such an extent as to form a ' radio-
larian ooze ; ' and it is obvious that the elevation of such a deposit
into dry land would form a bed of silicious sandstone resembling the
well-known Barbadoes I'ock, which is said to attain a thickness of
1,100 feet, 01- a similar rock of yet greater thickness in the Nicobar

1 See Professor Huxley (to whom we owe our first knowledge of these forms) in Ann.
Nat. Hist. ser. ii. vol. viii. 1851, p. 433 ; also Professor Miiller, of Berlin, in Quart. Joiirn.
Microsc. Sci. vol. iv. 1856, p. 72, and in his treatise Ueher die ThalassicoUeii, Pohj-
rijstinen, unci Acanthometren des Mittelmeeres, the magniiicent work of Professor
Ha.eckel, Die Radiolarien, and the monograph by K. Brandt, published in the Faitna
luul Flora des Golfes von Neajjel, 1885, 'Die koloniebildenden Eadiolarien
(Sphserozoeen) des Golfes von Neapel.'

^>

-ns

Fig. 652. — Sphcerozoum ovodimare.

854 MICEOSCOPIC FOEMS OF ANIMAL LIFE

Tslands. Few microscopic objects are more beautiful than an
assemblage of the most remarkable forms of the Barbadian Poly-
cystina (fig. 644), especially when seen brightly illuminated upon
a black ground ; since (for the reason formerly explained) theii-
solid forms then become much more apparent than they are when
these objects are examined by light transmitted through them. And
when they are mounted in Canada balsam the black-ground illu-
mination is much to be preferred for the purpose of display,
although minute details of structure can be better made out when
they are viewed as transparent objects with higher powers. Many
of the more solid forms when exjDosed to a high temperature on a
slip of platinum foil undergo a change in aspect which renders them
peculiarly beautiful as opaque objects, their glassy transparence
giving place to an enamel-like opacity. They xnay then be moiuited
on a black ground and illuminated either with a side condenser or
with the parabolic speculum. Xo class of object is more suitable
than these to the binocular microscope, its stereoscopic projection
causing them to be presented to the mind's eye in complete relief,
so as to bring out with the most marvellous and beautiful effect all
their delicate sculpture.^

1 For a fuller description of the fossil forms of this group see Professor Ehrenberg's

memoirs in the Monatsberichte of the Berlin Academy for 1846, 1847, and 1850 ; also

his Microgeologie, 1854; and Ann. of Nat. Hist. vol. xx. 1847. The best method of

separating the Polycystina from the Barbadoes sandstone is described by Mr. Fnr-

' long in the Quart. Journ. of Microsc. Sci. n.s. vol. i. 1861, p. 64.

855

CHAPTER XY

SPONGES AND ZOOPHYTES

I. Sponges

We now leave the Protozoa and commence the study of the Metazoa,
or those forms in which the egg-cell undergoes subdivision, the result-
ing elements of which do not separate or lead an independent
existence, but combine to form an organic whole, various pai'ts
undertaking various functions. Of these Metazoa the simplest ex-
amples are to be found among Sponges. The deteimination of the
I'eal character of the animals of this class has been entirely effected by
the microscopic examination of their minute structure ; for until this
came to be ]3i-opei-ly understood, not only was the general nature of
these organisms entirely misapprehended, but they were regarded
by many naturalists as having no certain claim to a place in the
animal kingdom. What that place is, is, to some extent, still an
open question,^ but it may now be unhesitatingly aiiirmed that a
sponge is an aggregate of protozoic units only in the sense in which
all Metazoa ai-e composed of cells ; some of these cells have a striking
resemblance to the collared Flagellata (fig. 585), whilst others re-
semble Amcehce (fig. 577). These units are held together by a con-
tinuous connective-tissue-like substance which clothes the skeletal
framework that represents our usual idea of a sponge, and is itself
made up of distinct cellular elements. In the simpler foi-ms of
sponges, however, this framework is altogether absent ; in others it
is represented only by calcareous or silicious ' spicules,' which are
dispersed through the sarcodic substance (fig. 654, B) ; in othei's,
again, the skeleton is a keratose (horny) network, which may be
entirely destitute (as in our ordinary sponge) of any mineral support,
but which is often strengthened by calcareous or silicious spicules
(fig. 654) ; whilst in Avhat may be regarded as the highest types of
the group, the silicious component of the skeleton increases, and the
keratose diminishes until the skeleton consists of a beautiful silicious
network i-esembling spun glass. But whatever may be the condi-
tion of the skeleton, that of the body that clothes it remains

1 For an instructive discussion on this point, consult Prof. E. A. Minchin's essay-
on ' The Position of Sponges in the Animal Kingdom' in Scioice Paj^ierSji. {ji.s.)
(1897), to which is appended a useful list of works on the subject. Some authors
demur to the association of sponges with other Metazoa, and Professor Sollas has sug-
gested the use of the group-nanae Parazoa. See also Treatise on Zoology, vol. ii.
London, 1900.

856

SPONGES AND ZOOPHYTES

excurrent canals.

a sjs3ssrr=5^

essentially the same ; and the peculiarity that chiefly distinguishes
the sponge-colony from the plant-like colonies of the flagellate
Infusoria is that whilst the latter extend themselves oaticards by
repeated ramification, sending their zooid-bearing branches to
meet the water they inhabit, the surface of the former extends
itself inuiards, forming a system of passages and cavities lined by
these and the amoeboid cells, through which a current of water is
drawn in to meet them by the action of the flagella. The minute
pores (fig. 653, h, h) with which the surface «, a of the living sponge
is beset lead to incuri-ent passages that open into chambers lying
beneath it (c. c), and open into the ' ampullaceous sacs,' or, as they
are now called, ' flagellate chambers,' from the presence round
their walls of the flagellate or collared cells. The water drawn
in by their agency is driven outwards through a system of
which, uniting into larger trunks, pi'oceed to the

osGula or projecting
vents, cZ, from each of
which, during the
active life of the
sponge, a stream of
water, carrying out ex-
crementitious matter,
is continually issuing.
The in-current brings
into the chambers
both food-material and
oxygen ; and from the
manner in which
coloured j)articles ex-
perimentally diffused
through the water wherein a sponge is living are received into its
protoplasmic substance, it seems clear that the nutrition of the entire
fabric is the resultant of the feeding action of the flagellate units,
each of which takes in, after its kind, the food-particles brought by
the current of water, and imparts the jivoduct of its digestion of them
to the general sarcodic mass. P '

The continuous substance that clothes the skeleton of the
sponge and constitutes the chief part of its living body includes
great numbers of stellate granular cells. Their loaig slencler pseudo-
podia, radiating towards those of their neighbours, often unite
together to form a complex network ; on the chief parts of the
course of the water-way they become fusiform in shape and con-
tractile in function, and it is by their agency that the continual
contractions and expansions of the oscula are produced, which are
very characteristic of the living sjDonge. As was first shown by
Professor C. Stewart, sensory organs, formed of groups of cells
with long projecting filaments, are to be seen on the surface of
many sponges. Any one of these amoeboids, again, detached from
the mass, may lay the foundation of a new ' colony.' In the
aggregate mass produced by its continuous segmentation certain
globular clusters are distinguishable, each having, a cavity in

sponge :

Fig. 653. — Diagrammatic section of

superficial layer ; &, inhalant apertures or pores ;
flagellated chambers ; r7, exhalant oscule ; e, deeper
substance of the sponge.

SPONGES 857.

its interior ; and the amceljoids that form the wall of this cavity
become metamorphosed into collaied flagellate cells whose flagella
project into it. Thns is formed one of the characteristic ' ampul-
laceous sacs,' Avhich, <it first closed, afterwards communicates with
the exterior, on the one hand by an incnrrent passage, and on the
other with the excviri'ent canal-system leading to the oscnla. Be-
sides this reproduction by ' microspores,' thei-e is another form
of non-sexual rejsroduction by macrosjjores, which are clusters of
amceboids encysted in firm capsules, frequently strengthened on
their exterior by a layer of spicules of very peculiar form. These
' seed-like bodies,' which answer to the encysted states of many
protoph}'tes, are met with in the substance of the sponge, chiefly in
winter ; and after being set free through the oscula they give exit
to their contained amceboids, each of which may found a new colony.
A true process of sexual generation, moreover, is known to take place
in sponges, certain of the amceboids, like certain cells of Volvox,
becoming ' sperm-cells,' and developing spermatozoa by the meta-
morphosis of theii- nuclei ; while others become ' germ-cells,'
developing themselves by segmentation (wdien fertilised) into the
bodies known as ' ciliated gemmules,' which are set free from the
walls of the canals, swim forth from the vents, and for a time move
actively through the water. In a word, there is true sexual repro-
duction by ova and spermatozoa, as in all animals that are not
Protozoa.^

The arrangement of the keratose reticulation in the sponges with
which we are most familiar may be best made out by cutting thin
slices of a piece of sponge submitted to firm compression, and view-
ing these slices, mounted upon a dark ground, with alow magnifying
power under incident light. Such sections, thus illuminated, are
not merely striking objects, but serve to show very characteristically
the general disposition of the larger canals and of the smaller pores
with which they communicate. In the ordinary sponge the fibrous
skeleton is almost entirely destitute of spicules, the absence of
which, in fact, is one important condition of that flexibility and
compressibility on which its uses depend. When spicules exist in
connection with such a skeleton, they are usually either altogethei-
imbedded in the fibres, or are implanted into them at their bases ;
but smaller and simpler sponges, such as Grantia, have no horny
skeleton, and their calcareous spicules are imbedded in the general
substance of the body. Sponge-spicules are ranch more fre-
quently silicious than calcareous ; and the variety of forms pre-
sented by the silicious spicules is much greater than that which
we find in the compai-atively small division in which they are
composed of carbonate of lime. The long needle-like spicules, which
are extremely abundant in several sponges, lying close together
in bundles, are sometimes straight, sometimes slightly curved ;
they ai'e sometimes pointed at both ends, sometimes at one only ;
one or both ends may be furnished with a head like that of a

' See Chapter V. of Mr. Saville Kent's Manual of the Infusoria, and Chapter V. of
vol. i. of Mr. Balfour's Co)i)2Xirative Emhryology, as well as Professor Haeckel's im-
portant work on the Calcareous Sponges.

858

SPONGES AND ZOOPHYTES

pin or may cai'iy three or more diverging points which sometimes
curve back so as to form hooks. When the spicules project from the
horny framework they are usually somewhat conical in form, and
their surface is often beset with little spines arranged at regular inter-

FiG. 654. — A, section through PliakelUa ventUahrum, var. connexida, taken at right
angles to the surface, to show the arrangement of the parts of a sponge : pt pores
on the surface leading to ic, the inhalant canals, then to the flagellated chambers,
fc, and thence to the exhalant canals, ec, to o, the oscula in the dermal membrane,
dm. B, more highly magnified view of the internal portion (choanosome) of
Axinella jyaradoxax 290 : 7hc, so-called mesodermal cells. Other letters as in A.
(After Ridley and Dendy.)

SPONGE-SPICULES

859

vals, giving them a jointed appearance. ' The more i-ecent aiithoi'ities
on Sponges, such as Professor Sollas and Messrs. Ridley and Dendy,
have recognised tliat in the present state of our knowledge the spicules
which are ordinaiily found in silicious Sponges belong to one of two
groups, which, as they differ considerably in size, may be called
megascleres (oi-, more correctly, megalosclei-es) and microscleres. It is
to the definite arrangement of the foi-mer that, with or without the
addition of spongin, the sponge owes its definite skeleton ; the micro-
scleres give consistency to the
tissue of the sponge, and are ir-
regularly scattered throughout
its substance. If we desire to
give them physiological names
we may call the megaloscleres
skeletal spicules, and the micro-
scleres flesh-spicules. If we
bear in mind that in the
opinion of the most competent
spongiologists the polyaxial
spicules are the most primi-
tive, there is no practical
objection to our noticing them
in the reverse order, a method
which will be found to conduce
to simplicity of description.
In the examination of spicules,
it is necessary, first of all, to
distinguish between axes and
rays ; thus in the Monaxonida
the megaloscleres have but a
single axis, but the gi'owth
from the point of origin may
be on either side, when we
have two-rayed or diactinal
megaloscleres, or it may extend
in one direction only, when the
scleres are said to be monactinal. In the Calcispongise thei-e are
three axes and three rays ; but in some sponges, such as Yenus's
flower-basket, the growth is along both directions of the axes, so
that while there ai-e three axes there are six rays, or the spicules
are hexactinellid. In othei-s, such as Geodia and the Lithistid
Sponges, there are foiir axes, whence such forms are called tetraxonid.

Fig. 655.— Structure of the chelas of Mo-
naxonid Sponges : 1, tridentate anisochela
from in front; la, from the side; 2, 2a,
front and side views of a pahnate isochela ;
t, t', tubercle; at, at', anterior tooth or
pahn ; It, It', lateral tooth or palm ; s, shaft ;
f, fimlDria. (After Eidley and Dendy.)

1 A minute account of the various forms of spicules contained in Sponges is given
by Mr. Bowerbank in his first memoir ' On the Anatomy and Physiology of the
Spongiadse ' in Plill. Trans. 1858, pp. 279-332; and in his Monograjjli of the
British SjJoiifjiaclce, published by the Bay Society. The Calcareous Sponges have
been made by Professor Haeckel the subject of an elaborate monograph, Die Kalk-
schwdmme, Hevlin, 1872. For enumerations and classifications of the various kinds
of spicules, see Professor Sollas, art. ' Sj^onges,' in the 9th edition of the Encijcl.
Britannica, a.\i(\. M.es&v'a. Bidley and Dendy, Report on tlie '■ Challenger' Monax-
onida, pp. xv-xxi.

86o SPONGES AND ZOOPHYTES

Lastly there are the least regular megaloscleres, which may be multi-
radiate or spherical.

As is well known, the flesh-spicules are of the most varied forms,
and it is a matter of some difficulty so to group them as to render
them more easy of comprehension by the student. Messrs. Ridley and
Dendy suggest that, provisionally at any rate, we should regard them
as (1) simple linear, (2) hooked, (3) stellate. The first may be pointed
at either end, and these are often spinous, or they may be long and
hair -like, and be or not be arranged in bundles (dragtnata) ; a common
form is that of a bow, and these, again, are sometimes arranged in
bundles, which have been formed within one and the same cell.
The hooked forms may be simple sigmiform microscleres, or the
shape may be complicated by the inner margin of the shaft or hook
thinning out to a fine knife-edge. The most complex forms of this
group are the microscleres which the just-quoted authors denominate
chelce. They describe these scleres (fig. 655) as having a more or
less curved shaft (s), which bears at each end a variable number of

sharj)ly recurved processes (ai, «i5', It^ It').
which they call the ' teeth,' or if broad
and expanded the ' palms ; ' these are con-
nected with the shaft by a buttress-like
projection, which is generally so trans-
parent as to be with difficulty made out.
The shaft itself is frequently di^awn out
at the side into wing-like processes or

„^, , fimbriae (/"). If the two ends of the

FiG.656.— AnisoclielEeof CZ«f7o- . , ^'' ' , , • r 7 -j?

rUza inversa, showing n, the spicule are equal, we have isochelm ; if un-

nucleus of the mother-cell of eqvial, anisochelce. The stellate micro-

the spicule, from in front, «, scleres may be spiral, have a shaft with

and from the side, b. x bOU. . , '' , ^ i- i • i i j?j. -j-i

(After Ridley and Dendy.) spmose whorls, or a cylindrical shaft with
a toothed whorl at either end. The
spicules of sjjonges cannot be considered, like the 7'aphides of plants,
as mere deposits of mineral matter in a crystalline state ; for, like
all other parts of the organism, they are of cellular origin (fig. 656),
and the special cells which produce them are distinguished as silico-
blasts ; in this there is first develojied a central organic thi'ead
around which concentric layers of silica or chalk are laid down.^

There is an extremely interesting group of Sponges in which the
horny skeleton is entirely replaced by a silicious framework of great
firmness and of singular beauty of construction. This framework
may be regarded as fundamentally consisting of an arrangement of
six-rayed spicules, the extensions of which come to be, as it were,
soldered to one another ; and hence the group is distinguished as
sexradiate. Of this type the beautiful Evjdectella of the Manila
seas — which was for a long time one of the greatest of zoological
rarities, but which now, under the name of ' Venus's flower-basket,'
is a common ornament of our drawing-rooms — is one of the most
characteristic examples.'^ Another example is presented by the

^ For a compendious statement of the characters of sponge-spicules see pp. 82-4
of Mr. A. Sedgwick's Student's Text-book of Zoology, London, 1891S.

^ The structure and arrangement of the soft i^arts of JEwplectella aspergillinit have

SPONGES 86 1

Holtenia Carpeiiteri, of which four specimens, dredged up from a
depth of 530 fixthoms between the Faroe Islands and the north of
Scotland, were among the most valuable of the ' ti-easures of the
deep ' obtained during the first deep-sea exploration (1868) carried
out by Sir Wyville Thomson and the Author. This is a turnip-shaped
body, with a cavity in its interior, the circular- mouth of which is
surrounded with a fringe of elongated silicious spicules ; whilst from
its base there hangs a sort of beard of silicious threads that extend
themselves, sometimes to a length of several feet, into the Atlantic
mud on which these bodies are found. The framework is much
more massive than that of Eaplectella, but it is not so exclusively
mineral ; for if it be boiled in nitric acid it is resolved into separate
spicules, these being not soldered together by silicious continuity,
but held together by animal matter. Besides the I'egular sex-
radiate spicules, there is a i-emarkable variety of other forms, which
have been fully described and figured by Sir Wyville Thomson. ^
One of the greatest features of interest in this Holtenia is its
singular resemblance to the Ventriculites of the Cretaceous forniation.
Subsequent investigations have shown that it is very widely diffused,
and that it is only one of several deep-sea forms, including some
of sin'gulai'ly beautiful structure, which are the existing repre-
sentatives of the old ventriculite type. One of these was previously
known from, being occasionally cast up on the shore of Bai-badoes
after a storm. This Dictyocalyx jnimiceus has the shape of a mush-
room, the diameter of its disc sometimes ranging to a foot. A small
portion of its reticvdated skeleton is a singularl}^ beautiful object
when viewed with incident light under a low magnifying power.

With the exception of the genus S-pongilla and its allies, all
known sponges are marine, but they diflei- very much in habit of
growth. For whilst some can only be obtained by dredging at con-
siderable dej)ths, others live near the surface, Avhilst others attach
themselves to the surfaces of rocks, shells, ifec. between the tide-
marks. The various species of Grantia in which, of all the marine
sponges, the flagellate cells can most readily be observed, belong to
this last category. They have a peculiarly simple structure, each
being a sort of bag whose wall is so thin that no system of canals is
required, the Avater absorbed by the outer surface passing directly
towards the inner, and being expelled by the mouth of the bag. The
flagella may ^be plainly distinguished with a ^-inch objective on some
of the cells of the gelatinous substance sciuped from the interior of
the bag ; or they may be seen in situ by making very thin trans-
verse sections of the substance of the sponge. It is by such sections
alone that the internal structure of sponges, and the i-elation of
their spicular and hoi'ny skeletons to their fleshy substance, can be
demonsti'ated. They are best made by the imbedding jii-ocess. In
order to obtain the sj^imdes in an isolated condition, the animal
matter must be got rid of either by incineration or by chemical

been investigated by Prof. F. E. Schulze, Trans. Hoijal Soc. of Eduihnrgli, xxix.
p. 661.

- See his elaborate memoir in Phil. Trans. 1870, and his Depths of tlie Sea,
1872 p. 71.

862

SPONGES AND ZOOPHYTES

i-eagents. The latter method is prefei-able, as it is difficult to free
the miiiei*al residue from carbonaceous particles by heat alone. If
(as is commonly the case) the spicules are silicioas, the sjjonge may
be treated with strong niti-ic or nit ro -muriatic acid, until its animal
substance is dissolved away ; if, on the other hand, they are cal-
careoios, a strong solution of potass may be employed instead of the
acid. The operation is more rapidly accomplished by the aid of
heat ; but if the saving of time be not of importance, it is pi-efeiuble
on several accounts to dispense with it. The spicules, when obtained
in a separate state, should be mounted in Canada balsam. Sponge
tissue may often be distinctly recognised in sections of agate,
chalcedony, and other silicious concretions, as will be more fully
stated hereafter.^

11. Zoophytes (Ccelentera)

Under the general designation Zoojihytes it will be still con-
venient to group those animals which form composite skeletons or

' polyparies ' of a more or less
plant-like character, associating
with them the Acalej)hs, which
are now known to be the '^ sexual
zooids' of polypes, but excluding
the Polyzoa on account of their
very difl'erent structure, not-
withstanding their zoophytic
forms and habits of life. The
animals belonging to this group
may be considered as formed
upon the primiti"\'e gastrula
type, theii- gasti-ic cavity
(though sometimes extending
itself almost indefinitely) being-
lined by the original endoderm,
and their surface being covered
by the original ectoderm, and
these two lamellae not being
sepai-ated by the interposition
of any body-cavity or ccelom.
It is a fact of great interest
that although the product of
the development of a morula is
here a distinctly individualised
polype, in which several mutu-
ally dependent pai-ts make up
a single organic whole, yet these parts still i-etain much of their
independent protozoie life ; which is manifested in two very re-
markable modes. In the first place, the digestive sac is observed
to be lined by a layer of amceboid cells, which send out pseudopodial

1 A complete and valuable handbook to the Sponges has been published by
Dr. G. C. Vosmaer as vol. ii. of Bronn's Klassen unci Ordnungen des Thierreichs,
Leipzig, 1887. Compare also the article by Professor Sollas in the ninth edition of the

i

' 0,

Fig. 657. — Longitudinal section of the body
of a hydra killed in fu.ll digestion : ec,
ectoderm ; en, endoderin ; nq), muscular
processes ; d, a diatom ; /', food. (After
T. J. Parker.)

Ca]LENTERA 863

prolongiitions into its ca\'ity (fig. 657) by wliose agency (it may be
pretty certainly affirmed) tlie nuti-ient material is first introdnced
into the body-substance. This pr-ocess of ' intracellular digestion '
was first noticed b}' Professor Allman in the beautiful hydi-oid polype
Myriotliela ; ^ the like has been since shown by Mr. Jeflfery Parker
to be true of the ordinai-y Hydra ; - and Professoi- E. Ray Lankestei'
has made the same observation upon the cui'ious little Medusa [Limno-
codium), which lives in fresh-ioater tanks in this country, whithei-
it has undoubtedly been introduced ; Avhile the observations of
Krukenlierg have shown that a similar process obtains among the sea-
anemones.^ (It may be mentioned in this connection, that Metschni-
kofl' has seen the cells which line the alimentary canal of the lower
planarian worms gorging themselves with coloured food-particles,
exactlv in the manner of Amcebce and the liver-fluke, and that a
number of lai-vaj are known to obtain theii- nourishment in the same
way.*) The second ' survival ' of jDi'otozoic independence is shown
in the extraordinary power possessed by Hydra, Actinia, &c. of
reproducing the entire organism fi-oni a mere ft-agment. This great
division includes the two principal groups the Hydrozoa and the
AcTixozoA, the former comprehending the Polypes, and the latter
the Anemones. In the Hydrozoa the mouth is placed on a projecting
oral cone, while in the Anthozoa it is sunk below the level of the oral
circlet of tentacles, and the cavity developed from and connected
with the digestive cavity separates its wall from the body-wall and
is traversed by a series of vertical partitions or septa. As most of
the hydroid polypes are essentially microscopic animals, they need
to be described with some minuteness ; Avhilst in regard to the
Actinozoa those points only will be dwelt on which are of special
interest to the microscopist.

Hydrozoa. — The type of this group is the Hydra, or fresh-water
polype, a very common inhabitant of pools and ditches, whei-e it is
most commonly to be found attached to the leaves or stems of aquatic
plants, floating pieces of stick, etc. Two species ai-e common in this
country, the //. v'vridis or green polype, and the H. vidgaris, which
is usually oraiige-brown, but sometimes yellowish or red (its coloui-
being liable to some variation according to the nature of the food
on Avhich it has been subsisting) ; a third less common species, the
H.fasca, is distinguished from both the preceding by the length of
its tentacles, which in the formei- are scarcely as long as the body,
whilst in the latter they are, when fully extended, many times longer

Eiirijclopcedia Byitaniiica ; the ' Challenger' lieports by Professor Scliulze, Messrs.
Ridley and Dendy, Polejaeff, and Sollas ; and the numerous memoirs of Professors
O. Schmidt and Schulze. More recently imjiortant additions to our knowledge of
Sponges have been made by Prof. Yves Delage and Monsieur E. Topsent in the
Arclt. Zijol. Ex'per. ei Gen. 1892-5, and by Dr. O. Maas in the Mittli. Ziiol. Stat.
Neapel, x. and elsewhere.

I Fhil. Trans. 1875, p. 552. It should be noted that the late Professor Glaus
called attention to the ingestion of foreign bodies by amoeboid cells of Mono'phycs
in 1874. See his Schriffen ZcioL Inhalts (Wien, 18741, p. oO.

I Proc. of Boij. Son. vol. xxx. 1880, p. Gl.

''■ Quart. Jniirn. Microsc. Sci. n.s. vol. xx. 1880, p. 371.

"* Consult an interesting article on 'Intercellular Digestion,' by Metschnikoff, in
Sevue Scientifqiie, ser. iii. vol. xi. p. 683.

864

SPONGES AND ZOOPHYTES

(fig. 658).' The body of the Hydra consists of a shnple bag or sac,
which may be regarded as a stomach, and is capable of varying its
shape and dimensions in a vei'y remai-kable degree, sometimes ex-
tending itself in a straight line so as to form a long narrow cylinder,
at other times being seen (when empty) as a minute contracted
globe, whilst, if distended with food, it may present the form of an
inverted flask or bottle, or even of a button. At the upper end of
this sac is a central opening, the mouth ; and this is surrounded by

a circle of tentacles or ' arms,'
usually from six to ten in number,
which are ari-a'nged with great
regularity around the orifice.
The body is prolonged at its lowei-
end into a narrow base, which
is furnished with a suctorial disc,
and the Hydra usually attaches
itself by this, while it allows its
tendril-like tentacles to float
freely in the water. The wall
of the body is composed of two
layers of cells ; and between
these, which are the ectoderm
and endoderm, there is a deli-
cate intermediate layer, which
forms the supporting lamella. -
The arms are made up of the
same materials as the body ;
but their surface is beset with
little wart-like prominences,
which, when carefully examined,
are found to be composed of
clusters of ' thread-cells,' having
a single large cell with a long
spiculum in the centre of each.
The structure of these thread-
cells or ' urticating organs ' will
be described hereafter ; at pre-
FiG. 658.— ir^rf;a/«sca, with a young bud sent it will be enough to point
at h, and a more advanced bud at c. out that this apparatus, repeated

many times on each tentacle, is
douVjtless intended to give to the organ a, great pi-ehensile powei',
the minute filaments forming a i-ough svirface adapted to pi-event
the object from readily slipping out of the grasp of the arm, whilst
the central sj^icule or ' dart ' is projected into its substance, probably
conveying into it a poisonous fluid secreted hy a vesicle at its base.

1 On the specific characters of Hijdra consult Haacke, Jenainclie Zeitschr. xiv.
p. 133 ; and Jickeli, Zool. Anxeig. v. -^. 491.

2 To this intermediate layer, Mr. G. C. Bourne applies the term inesogloea. For an
account of its variations and structure among the Coelentera, and a discussion of its
homology with the mesoderm of higher Metazoa, see his essay on Fungia in vol.
xxvii. of the Quart. Jourii. Microsc. Sci. n.s.

HYDEOZOA

865

The latter inference is founded upon the oft-repeated observation
that if the living pi-ey seized by the tentacles have a body destitute
of hard integument, as is the case with the minute aquatic worms
Avhich constitute a large part of its aliment, this speedily dies,
even though, instead of being swallowed, it escapes from their grasp ;
whilst, on the other hand, minute Entomostraca, insects, and other
animals or ova, with hard envelopes, may escape without injury, even
after having been detained for some time in the polyj)e's embrace.
The contractility of the
tentacles (the interioi'
of which is traversed by
a canal that communi-
cates with the cavity
of the stomach) is very
remarkable, especially
in the Hydra fusca,
whose arms, when ex-
tended in search of
prey, are not less than
seven or eight inches in
length ; whilst they are
sometimes so contract-
ed, when the stomach
is filled with food, as
to appear only like little
tubercles around its en-
trance. By means of
these instruments the
Hydra is enabled to
draw its suppoi't from
animals whose activity,
as compared with its
own slight powers of
locomotion, might have
been supposed to re-
move them altogether
from its reach ; for
when, in its movements
through the water, a
minute worm or a water-
flea happens to touch
one of the tentacles of

the polype, spread out as these are in readiness for prey, it is
immediately seized by this ; other arms are soon coiled around it,,
and the unfortunate victim is speedily conveyed to the stomach,
within which it may frequently be seen to continue moving for
some little time. Soon, however, its struggles cease, and its outline
is obscured by a turbid film, which gradually thickens, so that at
last its form is wholly lost. The soft parts are soon completely dis-
solved, and the harder indigestible portions are rejected through the
mouth. A second orifice has been observed at the lower extremity

3 K

Fig. 659. — Gamj)anularia gelatinosa.

866

SPONGES AND ZOOPHYTES

of the stomach ; but this would not seem to be properly i-egarcled as
anal, since it is not vised for the discharge of such exuviae ; it is
pi'obably i-athei' to be considered as representing, in the Hydra, the
entrance to that ramifying cavity which, in the compound Hydrozoa,
brings into mutual connection the lower extremities of the stomachs
of all the individual polypes.

The ordinary mode of reproduction in this animal is by a ' gemma-
tion ' resembling that of plants. Little bud-like processes (fig. 658,
h, c) developed from its external surface gradually come to resemble
the parent in character, and to possess a digestive sac, mouth, and
tentacles ; for a long time, however, their cavity is connected with
that of the parent, but at last the communication is cut off by the

closure of the canal of the foot-
stalk, and the young polype quits
its attachment and goes in quest
of its own maintenance. A
second generation of buds is
sometimes observed on the young
polype befoi-e quitting its parent ;
and as many as nineteen young
Hydrce in difiei'ent stages of
development have been seen thus
connected with a single original
stock (fig. 660). This process
takes place most rapidly under
the influence of warmth and
abundant food ; it is usually sus-
pended in winter, but may be
made to continue by keeping the
polypes in a warm situation and
well supplied with food. Another
very curious endowment seems to
depend on the same condition —
the exti'aordinary power which
one portion j)ossesses of repro-
ducing the rest. Into whatever
number of parts a Hydra may
be divided, each may retain its
vitality, and give origin to a
new and entire fabric ; so that thirty or forty individuals may
be formed by the section of one. The Hydra also propagates itself,
however, by a truly sexual pi-ocess, the fecundating apjDaratus, or
vesicle producing ' sperm-cells,' and the ovum (containing the ' germ-
cell,' imbedded in a store of nutriment adapted for its early develop-
ment), being both evolved in the substance of the walls of the
stomach — the male apparatvis forming a conical pi-qjection just
beneath the arms, while the female ovary, or portion of the body-
substance in which the ovum is genei-ated, has the form of a
knob protruding from the middle of its length. It would appear
that sometimes one individual Hydiu develojDS only the male cysts
or sperm-cells, while another develops only the female cysts or ovi-

-Hydra fusca in gemmation :
b, base ; c, origin of one of the

HYDEOZOA 867

sacs ; but the general laile seems to be that the same individual
foi'ms both oi'gans. The fertilisation of the ova, however, cannot
take place until after the I'uptui'e of the spermatic cyst and of the
ovisac, by which the contents of both are set free. The autumn is
the chief time for the development of the sexual organs, but they
also present themselves in the earlier part of the year, chiefly be-
tween Api'il and July. According to Ecker, the eggs of H. viridis
produced early in the season run their course in the summer of
the same year ; while those pi-oduced in the autumn pass the winter
without change. When the ovum is nearly i-ipe for fecundation
the ovary bursts its ectodei'mal covering, and I'emains attached by
a kind of pedicle. It seems to be at this stage that the act of
fecundation occui'S ; a veiy strong elastic shell or capsule then
forms round the ovum, the surface of which is in some cases studded
with spine-like points, in others tuberculated, the divisions between
the tubercles being polygonal. The ovum finally drops from its
pedicle, and attaches itself by means of a mucous secretion, till the
hatching of the young Hydra, which comes forth provided with four
rudimentary tentacles like buds. The Hydra possesses the power of
free locomotion, being able to remove from the spot to which it has
attached itself to any other tha.t may be inore suitable to its wants ;
its changes of place, however, seem rather to be performed under the
influence of light, towai'ds which the Hydra seeks to move itself, than
with reference to the search after food.^

The compound Hydroids may be likened to a Hydra whose
gemma?, instead of becoming detached, remain permanently connected
with the parent ; and as these in their tui-n may develop gemmae
from theii- own bodies, a structure of more or less arborescent
character, termed a 'polypary, may be produced. The form which
this will present, and the relation of the component polypes to each
other, will depend upon the mode in which the gemmation takes
place ; in all instances, however, the entire cluster is produced by
continuous growth from a single individual ; and the stomachs of the
several polypes are united by tubes, which pi'oceed fi-om the base of
each, along the stalk and branches, to communicate witli the cavity
of the central stem. Whatever may be the form taken by the stem
and branches constituting the polypary of a hydroid colony, they will
be found to be, or to contain, fleshy tubes having two distinct layers,
the inner (endoderm) having nutritive functions ; the outer (ecto-
derm) usually secreting a hard coi'tical layer, and thus giving rise
to fabrics of vaiious forms. Between these a muscular coat is some-
times noticed. The fleshy tube, whether single or compound, is called
a ccenosm-G, and thi'ough it the nutrient mattei- cii-culates. The
' ztioids,' or individual members of the colony, are of two kinds : one
the polyjyite, or alimentary zooid, resembling the Hydra in essential

1 A very full account of the structure and develoi3ment of Hijclra lias been
published by Kleinenberg, of whose admirable monograph a summary is given by
Professor Allman, with valuable remarks of his own, in Quart. Jov rii. Microsc. Sci. n.s.
vol. xiv. 1874, p. 1. See also the important paper by the late Mr. Jeifery Parker
already cited. On the chlorophyll corpuscles of H. viridis consult Brandt, Mitth.
Ziiol. Stat. Neapel,iv. -p. 191; Hamann, Zool. Anzeig. vi. p. o67 ; and Lankester
Quart. Journ. Microsc. Sci. n.s. xxii. ]}. 229.

3 K 2

868 SPONGES AND ZOOPHYTES

structure, and more or less in aspect ; the other, the gonozooid, or
sexual zooid, developed at certain seasons only, in lauds of particular
shape. ^ ■

The simplest di\dsion of the Hydroida is that adopted by Mr.
Hincks,^ who groups them under the sub-order Athecata and Thecata,
the latter being- again divided into the Tlip^caphora and the Gynino-
chroa. In the first, neither the ' polypites ' nor the sexual zooids
bear true protective cases ; in the second the polypites are lodged in
cells, or, as Mr. Hincks prefers to call them, calycles, many of which
i-esemble exquisitely forraed crystal cups, variously ornamented, and
sometimes furnished with lids or opercula ; in the third, which con-
tains the Hydi-as, there is no polypary, and the repi-oductive zooids
(gonozooids) are always fixed and developed in the body -walls. Ac-
cording to Mr. Hincks, the two sexes are sometimes borne on the
same colony, hwt more commonly the zoophyte is dicecious. The
cases, however, are much less rare than has been supposed in which
both male and female are mingled on the same shoots. The sexual
zooids either remain attached, and discharge their contents at
maturity, or become free and enter upon an independent existence.
The free forms nearly always take the shape of Sledasce (jelly-fish),
swimming by rhythmical contractions of their bell or umbrella. The
digestive cavity is in the handle (manubrium) of the bell ; and the
generative elements (sperm-cells or ova) are developed either between
the membranes of the manubrium or in special sacs in the canals
radiating from it. The ova, when fertilised by the spermatozoa,
undergo ' segmentation ' according to the ordinary type, the whole
yolk-mass subdividing successively into two, four, eight, sixteen,
thirty -two or more parts, until a ' mulberry mass ' is formed ; this
then begins to elongate itself, its surface being at first smooth and
showing a transparent margin, but afterwards becoming clothed with
cilia, by whose agency these little iiilamdce^ closely resembling ciliated
Infusoria, first move about within the capsule, and then swim forth
fi'eely when liberated by the opening of its mouth. At this period
the embryo can be made out to consist of an outer and an inner
layer of cells, with a hollow interior ; after some little time the cilia
disappear, and one extremity becomes expanded into a kind of disc
by which it attaches itself to some fixed object ; a mouth is formed,
and tentacles sprout foi'th around it ; and the body increases in length
and thickness, so as gradually to acquire the likeness of one of the
parent polypes, after which the ' polypary ' characteristic of the genus
is gradually evolved by the successive development of polype-buds
from the first-formed polype and its subsequent offsets. The Medusae
of these polypes (fig. 663) belong to the division called ' naked-eyed,'
on account of the eye-spots usually seen sui-rounding the margin of
the bell at the base of the tentacles.

A charactei-istic example of this production of medusa-like
' gonozooids ' is presented by the foi'm termed Syncoryne Satsii (fig.

' A useful list of the principal terms used in describing hj'droids, -with definitions,
will be found on pp. 16 and 17 of Professor Allman's Report on the Hydroida [Flu-
inulariidce) of the Challenger.

'' History of British Hydroid Zoophytes, iSOb.

DEVELOPMENT OE HYDEOZOA

869

661) belonging to the sub-oi-dei- Athecata. At A is sr.ov/n the ali-
mentary zooid, or polypite, Avith its tentacles, and at B the sncces-
sive stages a, h, c, of the sexual ziioids, or medusa-buds. When
sufficiently developed the Medusa swims away, and as it grows to
maturity enlarges its manubrium, so that it hangs below the bell.
The Medusfe of the genus Syncoryne (as now restricted) have the
form named Sarsia in honour of the Swedish naturalist Sars. Theii
normal character is that of free swimmers ; but Agassiz ascertained
that in some cases towards the
end of the breeding season the
sexual zooids remiain fixed, and
mature their products while at-
tached to the zoophyte.^ This
latter condition of the sexual
zooids is very common amongst
the Hydroida ; and various inter-
mediate stages may be traced in
different genera between the
mode in wdiich the gonozooids
are produced in the common
Hydra, as already described, and
that of Syncoryne. In Tuhii,-
laria the gonozooids, though
permanently attached, are fui--
nished with swimming bells,
having four tubercles repre-
senting marginal tentacles. A
common and interesting species,
Tuhularia indivisa, receives its
specific name from the infre-
quency with which branches are
given off from the stems, these for
the most part standing ei-ect and
parallel, like the stalks of corn,
upon the base to which they are
attached. This beautiful zoo-
phyte, which sometimes grows
between the tide-marks, but is
more abundantly obtained by
dredging in deep water, often
attains a size which renders it
scai'cely a microscoj^ic object, its
stems being sometimes no less

than a foot in height and a line in diameter. Several curious
phenomena, however, are brought into view by microscopic examina-
tion. The polype-stomach is connected with the cavity of the
stem by a circular opening, which is surrounded by a sphincter ;
and an alternate movement of dilatation and contraction takes
place in it, fluid being apparently forced up from below, and then
expelled again, after which the sphincter closes in preparation foi-
1 Hincks, op. cit. p. 49.

Fig. 661. — Development of Medusa-buds
ill Syncoryne Sarsii : A, an ordinary
polype, with its club-shaped body covered
with tentacles ; B, a jjolype putting forth
medusoid geiumse ; a, a very young bud ;
h, a bud more advanced, the quadran-
gular form of which, with the four
nuclei whence the cirrhi afterwards
spring, is shown at d; c, a bud still
more advanced.

870 SPONGES AND ZOOPHYTES

a recurrence of the operation, this, as observed by Mr. Listei',
being repeated at intei-^^als of eighty seconds. Besides the foregoing-
movement, a regvilar flow of fluid, cariying with it solid particles of
various sizes, may be observed along the whole leng-th of the stem,
passing in a somewhat spiral direction. It is worthy of mention
here that when a Tubularia is kept in confinement the polype-heads
almost always drop off after a few days, but are soon renewed
by a new growth from the stem beneath ; and this exu^aation and
regeneration may take place many times in the same individual.^

It is in the families Camjxinulariida and Sertvlariida (whose
polyparies are commonly known as ' corallines ') that the horny
branching fabric attains its completest development, not only afford-
ing an investment to the stem, but forming cups or cells for the
protection of the polypites, as well as cajDSules for the reproductive
gonozooids. Both these families thus belong to the sub-order Thecata.
In the Caonpanulariida the polype-cells are campanulate or bell-
shaped, and are borne at the extremities of ringed stalks (fig. 659, c) ;
in the Sertulariida, on the other hand, the polype-cells lie along the
stem and branches, attached either to one side only, or to both sides
(fig. 662). In both the general structure of the individual polypes
(fig. 659, B, d) closely corresponds with that of the Hydjva ; and the
mode in which they obtain their food is essentially the same. Of
the pi'oducts of digestion, however, a portion finds its way down into
the tubular stem, for the nourishment of the general fabric ; and
very much the same kind of circulatory movement can be seen in
G a'm'panularia as in Tubvilaria, the circulation being most vigorous
in the neighbourhood of growing parts. It is from the ' coenosarc '
(fig. 659, y) contained in the stem and branches that new polype-
buds (6) are evolved ; these carry before them (so to speak) a portion
of the horny integument, which at first completely invests the bud ;
but as the latter acquires the organisation of a polype, the case'
thins away at its most prominent part, and an opening is formed
through which the young polype protrudes itself.

The origin of the repi'oductive capsules or ' gonothecfe ' (e) is
exactly similar, but their destination is very difierent. Within
them are evolved, by a budding process, the genei-ative organs of
the zoophyte ; and these in the Ccmipmiularikla may either develop
themselves into the form of independent medusoids, which com-
pletely detach themselves from the stock that bore them, make their
way out of the capsule, and swim forth freely, to mature their
sexual products (some developing sperm-cells, and others ova), and
give origin to a new generation of polyjjes ; or, in cases in which
the meclusoid structure is less distinctly pronounced, may not com-
pletely detach themselves, but (like the flower-buds of a plant) exj)and
one after another at the mouth of the capsule, withering and drop-
ping ofi" after they have matured their generative products. In the
Sertidariida, on the othei- hand, the medusan conformation is wanting,
as the gonozooids are always fixed ; the reproductive cells (fig. 662, «),
which were shown by Professor Edward Forbes to be really meta-

1 The Britiish Tubiclariida form the subject of a most complete and beautiful
monograph by the late Professor Allman, published by the Ray Society.

COLLECTING ZOOPHYTES

871

morphosed branches, developing in their intei-ior certain bodies which
were formerly supposed to be ova, but which are now known to be
' medusoids ' reduced to their most rudimentary condition. Witliin
these are developed — in separate gonotheca?, sometimes perhaps on
distinct polyparies — spermatozoa and ova ; and the latter are ferti-
lised by the entrance of the former whilst still contained within
their capsules. The fertilised ova, whether produced in free or in
attached medusoids, develop themselves in the &st instance into
ciliated ' gemmules,' or planula?, which .soon evolve themselves into
true polypes, from every one of which a new composite polypary
may spring.

There ai'e few parts of our coast which A\-ill not supply some or
other of the beautiful and
interesting forms of zoo-
phytic life which have been
thus briefly noticed, with-
out any more trouble in
searching for them than
that of examining the sur-
faces of rocks, stones, sea-
weeds, and dead shells
between the tide-marks.
Many of them habitually
live in that situation ; and
others are frequently cast
uj? by the waves from the
deeper waters, especially
after a storm . Many kinds,
however, can only be ob-
tained by means of the
dredge. Of the remarkable
forms dredged by the 'Chal-
lenger ' mention can only
be made here of the gigantic
Tubularian — Mojiocaulus —
the stem of which measured
seven feet four inches,
while there was a spread
of nine inches from tip to tip of the extended tentacles, and of the
elegant S'treptoccndus pulcJierrmius, in which by the twisting of
the stem the ultimate ramules are thrown into ' a gTaceful and
beautiful spiral.' For observing them during their li^-ing state, no
means is so convenient as the zoophyte -trough. In mounting com-
pound Hydrozoa, as well as Polyzoa, it will be foimd of gi'eat
advantage to place the specimens ahve in the cells they are per-
manently to occupy, and to then add osmic acid drop by drop to
the sea-water ; this has the effect of causing the protrusion of the
animals, and of rendering their tentacles rigid. The licjuid may be
withdrawn, and replaced by Goadby's solution, Deane's gelatine,
glycerin jelly, weak spirit, diluted glycerin, a mixture of spirit and
glycerin with sea- water, or any other menstruum, by means of

Fig. 662.-

-Sertularia cupressina : A, natural
size ; B, i^ortion magnified.

872 SPONGES AND ZOOPHYTES

the syringe ; and it is well to mount specimens in several dif-
ferent menstrua, marking the nature and strength of each, as
some forms are better preserved by one and some by another.^
An excellent method of preservation has been discovered by
M. Foettinger ^ in the use of chloral hydrate: when all the
polypes in a vessel containing 100 c.c. of water are fiilly expanded
some crystals of chloral hydrate are to be dropped into the
vessel ; these dissolve rapidly and gradually diffuse through the
water. About ten minutes later a little more chloral should be
added, and in three-quarters of an hour the whole colony will be
found to have become insensible ; the advantage of this method
lies in the fact that the action is merely narcotic, and that the tissues
are not affected. Wlien the influence is so complete that irritation
fails to produce retraction of the polypes the colony may be put into
alcohol. The size of the cell must of course be proportioned to that
of the object ; and if it be desired to mount such a specimen as may
serve for a characteristic illustration of the mode of growth of the
species it represents, the large shallow cells, whose walls are made
by cementing four strips of glass to the plate that foi-ms the bottom,
will generally be found preferable. The horny polyparies of the
Sertulariida^ when mounted in Canada balsam, are beautiful objects
for the polariscope ; but in order to prepare them successfully some
nicety of management is required. The following are the outlines
of the method recommended by Dr. Golding Bird, who very success-
fully practised it. The specimens selected, which should not exceed
two inches in length, are first to be submitted, while immersed in
water of 120°, to the vacuum of an air-pump. The ebullition
which will take place within the cavities will have the effect of free-
ing the polyparies from dead polypes and other animal matter ; and
this cleansing process should be repeated several times. The
specimens are then to be dried, by first draining them for a few
seconds on bibulous paper, and then by submitting them to the
vacuum of an air-pump, within a thick earthenware ointment-pot
fitted with a cover, which has been previously heated to about 200° ;
by this means the specimens are very quickly and completely dried,
the water being evaporated so quickly that the cells and tubes
hardly collapse or wrinkle. The specimens are then placed in
camphine, and again subjected to the exhausting process for the
displacement of the air by that liquid ; and when they have been
thoroughly saturated, they should be mounted in Canada balsam in
the usual mode. When thus prepared they become very beautiful
transparent objects for low magnifying powers ; and they present a
gorgeous display of colours when examined by polai'ised light, with
the interposition of a plate of selenite, the effect being much en-
hanced by the use of black-ground illumination.

No result of microscopic research was more unexpected than
the discovery of the close relationship subsisting between the
hydroid ZbojihyUs and the medusoid Acale2yhce (or 'jelly-fish '). We
now know that the small free-swimming medusoids belonging to

1 See Mr. J. W. Morris in Quart. Journ. of Microsc. Sci. n.s. vol. ii. 1862, ]}. 116.
- Archives cle Bioloffie, vi. p. 115.

JELLY-FISHES

8/3

the ' naked-eye ' group, of which Thcmmantias (fig. 663) may be
taken as a representative, are really to be considered as the detached
sexual apparatus of the zoophytes from which they have been

Pig. 663. — A, Thaumantias pilosella, one of the ' naked-eye ' Medusae : a, a,
oral tentacles ; h, stomach ; c, gastro- vascular canals, having the ovaries,
d d, on either side, and terminating in the marginal canal, e e. B, TTiau-
mantias Eschscholtzii, Haeckel.

budded off, endowed with independent organs of nutrition and
locomotion, whereby they become cajiable of maintaining their own
existence and of developing their sexual products. The general con-
formation of these organs will be understood from the accompany-
ing figure. Many of this group are very beautiful objects for

8/4 SPONGES AND ZOOPHYTES

microscopic examination, being small enough to be viewed entire in
the zoophyte-trough. There are few parts of the coast on which they
may not l3e found, especially on a calm warm day, by skimming the
surface of the sea with the tow-net ; and they are capable of being
stained and preserved in cells after being hardened by osmic acid.
The history of the large and highly developed Medusce^ or AcA-
LEPH^ which are commonly known as "jelly-fish ' is essentially simi-
lar ; for their progeny have lieen ascertained to develop themselves
in the first instance under the polype form, and to lead a life which
in all essential respects is zoophytic ; their development into Meduspe
taking place only in the closing phase of their existence, and then
rather by gemmation from the original polype than by a metamoi--
phosis of its own fabric. The huge Rhizostoma found commonly
swimming round our coasts, and the beautiful Clirysaora remarkable
for its long ' furbelows ' which act as organs of prehension, are oceanic
acalephs develojaed from very small polypites, which fix themselves
by a basal cup or disc. The embryo emerges from the cavity of its
parent, within which the first stages of its development have taken
place, in the condition of a ciliated ' planula,' of rather oblong form,
very closely resembling an infusory animalcule, but destitute of a
mouth. One end soon conti-acts and attaches itself, however, so as
to form a foot ; the othei- enlarges and oj)ens to form a mouth,
four tubercles spi'outing around it which grow into tentacles ; whilst
a slit in the midst of the central cells gives rise to the cavity of the
stomach. Thus a hydra-like polype is formed, which soon acquires
many additional tentacles ; and this, according to the observations
of Sir J. G. Dalyell on the Hydra-tuha, which is the polype stage of
the Clirysaora and other jelly-fish, leads in every important particular
the life of a Hydra ; propagates like it by repeated gemmation, so
that whole colonies are formed as offsets from a single stock ; and
can be multiplied like it by artificial division, each segment develop-
ing itself into a perfect Hydra. There seems to be no definite limit
to its continuance in this state, or to its power of giving origin to
new polype-buds ; but when the time comes for the development of
its sexual gonozooids, the polype quits its original condition of a
minute bell with slender tentacles (fig. 664), assumes a cylin-
drical form, and elongates itself considerably ; a constriction or
indentation is then seen around it, just below the ring which encircles
the mouth and gives origin to the tentacles ; and similar constrictions
are soon repeated round the lower parts of the cylinder, so as to give
to the whole body somewhat the appearance of a rouleaii of coins ;
a sort of fleshy bulb, a (fig. 664, II), somewhat of the form of the
original polype, being still left at the attached extremity. The
number of circles is indefinite, and all are not formed at once, new
constrictions appearing below, after the upper portions have been de-
tached ; as many as thii'ty oi' even forty have thus been produced in
one specimen. The constrictions then gradually deepen, so as to divide
the cylinder into a pile of saucer-like bodies, the division being

1 See Professor Claus, Untersuchimgenuher die Organisation unci Entwic'kclwnq
(ler Meclusen, Prague and Leipzig, 1883, and Miss Ida H. Hyde, ' Entwickelungs-
geschiclite einiger Scyphomednsen,' in Zeitsclir. f. luiss. Zool. Iviii. p. 531.

KEPEODUGTION OF ACALEPHS

875

most complete above, and the ui)pei- discs usually pi-esenting some
increase in diameter ; and whilst this is taking place the edges of
the discs become divided into lobes, each lobe soon presenting the
cleft with the supjjosed i-udimentary eye at the bottom of it, which
is to be plainly seen in the detached Medusa? (fig. 665, C). Up to
this period, the tentacles of the original polype surmount the highest
of the discs ; but befoi-e the detachment of the topmost disc, this
cii'cle disapjiears, and a new one is developed at the summit of the
biilb which remains at the base of the pile. At last the topmost
and largest disc begins to exhibit a sort of convulsive struggle ; it

^ n

Pig. 664. — I, two Hijclrce tuha- ( Sci/jihistonia-steige) of Cyanea
cajjiUata, with two («, h) undergoing fission {StrobUa-st&ge).
II, a and b of fig. I three days later. In a the tentacles are
develoj^ed beneath the lowest of the EphyrcE, from the stalk
of the Sfrubihi, which will persist as a Hydra tube. (After
Van Beneden.)

becomes detached, and swims freely away ; and the same series of
changes takes place from above downwards, until the whole pile of
discs is detached and converted into free-swimming Medusa^. But
the original polypoid body still remains, and may return to its
original polype-like mode of gemmation, becoming the progenitor of
a new colony, every membei- of ■\^■hich may in its turn bud off a pile
of Medusa discs.

The bodies thus detached have all the essential characters of the
adult Meclusce. Each consists of an umbrella-like disc divided at
its edge into a variable number of lobes, usually eight ; and of a

876

SPONGES AND ZOOPHYTES

stomach, which occupies a considerable proportion of the disc, and
projects downwards in the form of a proboscis, in the centi-e of which
is the quadrangular mouth (fig. 665, A, B). As the animal advances
towards matui-ity the intervals between the segments of the border
of the disc gradually fill up, so that the divisions are obliterated ;
tubular prolongations of the stomach extend themselves over the
disc ; and from its borders there sprout forth tendril-like filaments
which hang down like a fringe around its margin. From the four
angles of the mouth, which, even in the youngest detached animal,
admits of being greatly extended and protruded, prolongations are
put forth, which form the four large tentacles of the adult. The
young Medus?e are very voi-acious, and grow rapidly, so as to attain

Fig. 665. — Development of Chrtjsaora from Hydra tuba : A,
detached individual viewed sideways, and enlarged, showing
the x^roboscis a, and b the bifid lobes ; B, individual seen from
above, showing the bifid lobes of the margin, and the quadri-
lateral mouth ; C, one of the bifid lobes still more enlarged,
showing the rudimentary eye (?) at the bottom of the cleft ;
D, group of young Medusas, as seen swimming in the water,
of the natural size.

a very large size. The Cyanece emd Chrysaorce, which are common all
round our coasts, often have a diameter of from six to fifteen inches ;
while Rhizostoma sometimes reaches a diameter of from two to three
feet. The quantity of solid matter, however, which their fabrics con-
tain is extremely small. It is not until advilt age has been attained
that the generative organs make their appearance, in four chambei's
disposed around the stomach, which are occupied by plaited mem-
branous ribbons containing sperm-cells in the male and ova in the
female ; and the embryos evolved from the latter, when they have
been fertilised by the agency of the former, repeat the exti'aoi'dinaiy
cycle of phenomena which has been now described, develoj^ing them-
selves in the first instance into hydi-oid polypes, fi'om which medusoids
are subsequently budded off.

ACTINOZOA 877

This cycle of phenomena, is one of those to which the term ' alter-
nation of generations ' was applied by Steenstrup/ who brought
together under this designation a number of cases in which genera-
tion A does not produce a form resembling itself, but a different form,
B ; whilst generation B gives oi-igin to a form which does not re-
semble itself, but i-eturns to the form A, from which B itself sprang.
It was early pointed out, however, by the Author ^ that the term
' alternation of generations ' does not appropriately represent the
facts either of this case or of any of the other cases grouped luider
the same category, the real fact being that the two organisms, A
and B, constitute two stages in the life-history of one generation,
and the production of one form from the other being in only one
instance by a truly generative or sexual act, whilst in the other it is
by a process of gemmation or budding. Thus the Meclusce of both
orders (the ' naked-eyed ' and the ' covered-eyed ' of Forbes) are de-
tached flower-buds, so to speak, of the hydroid zoophytes which bud
them off, the zciophytic phase of life being the most conspicuous in
such Thecata as Cannpanulariida and Sertulariida, whose Medusa-
buds are of small size and simple conformation, and not unfrequently
do not detach themselves as independent organisms ; whilst the
Medusan phase of life is the most conspicuous in the ordinary Acalephs,
their zoophytic stage being passed in such obscurity as only to be
detected by careful research. The Author's views on this subject,
which were at first strongly contested by Professor E. Forbes and
other eminent zoologists, have now come to be generally adopted.*^

Actinozoa. — Of this group the common sea-anemones may be
taken as types, constituting, with their allies, the order Zoantharia,
or helianthoid polypes, which have numerous tentacles disposed in
several rows. Next to them come the Alcyonaria, consisting of
those whose polypes, having always eight broad short tentacles,
present a star-like aspect when expanded ; as is the case with various
composite sponge-like bodies, unpossessed of any hard skeleton, which
inhabit our own shores, and also with the red coral and the Tuhipora
of wanner seas, which have a stony skeleton that is internal in the
first case and external in the second, as also with the sea pens and
the Gorgonice or sea-fans. A. third ordei-, jRugosa, consists of fossil
corals, whose stony polyparies are intermediate in character between
those of the two preceding. And lastly, the Ctenojjhora., free-swim-
ming gelatinous animals, many of which are beautiful objects foi-
the microscope, are by some zoologists ranked with the Actinozoa.^

Of the Zoantharia the common Actinia or ' sea-anemone ' may
be taken as the type, the individual polypites of all the composite
fabrics included in the groujD being constructed upon the same model.''
In by far the larger proportion of these zoophytes, the bases of the

^ See his treatise on The AUernation of Generations, a translation of which has
been pubhshed by the Ray Society.

- Brit, and For. Med. Chir. Beview, vol. i. 1848, jd. 192 et seq.

^ Compare Huxley, Anatomy of Invertebrated Animals, p. 133; and Balfour,
Comparative Emhrijologtj, i. p. 151.

^ Professor Haeckel, led by the study of Ctenaria ctenopliora, associates the
Ctenophora with the Hydrozoa [Sitzungsber. Jenaisclie GeseUsrhaft, May 16, 1879).

•^ On the anatomy of Actinia and its allies, see O. and R. Hertwig's monograph
in vols. xiii. and xiv. of the Jenaische Zeitschrift.

8yS SPONGES AJND ZOOPHYTES

polypites, as well as the soft flesh that connects together the members
of aggregate masses, are consolidated by calcareous deposit into stony
corals ; and the surfaces of these are beset with ' cells,' usually of a
nearly circular form, each having numerous vertical plates or lamellcn
radiating from its centre towards its circumference, which are
formed by the consolidation of the lower portions of the radiating
partitions that divide the space intervening between the stomach and
the general integument of the animal into separate chambers. This
arrangement is seen on a large scale in the Fungia or ' mtishroom-
coi-al ' of tropical seas, which is the stony base of a solitary anemone-
like animal ; on a far smaller scale, it is seen in the little Caryo-
pliyllia, a like solitary anemone of oui- own coasts, which is scarcely
distinguishable from an Actinia by any other character than the
presence of this disc, and also on the sui-face of many of those stony
corals known as ' madrepores ; ' whilst in some of these the indivi-
dual polype-cells are so small that the lamellated arrangement can
only be made out when they are considerably magnified. Portions
of the surface of such corals, or sections taken at a small depth, are
veiy beautiful objects for low powers, the formei- being viewed by
reflected and the latter by transmitted light. And thin sections of
various fossil corals of this group are very striking objects for the
lower powers of the oxy-hydrogen microscope. An exceedingly use-
ful method of preparing sections of corals has been devised by Dr. G.
von Koch ; the corals with all their soft parts in plate are hardened
in absolute alcohol, and then placed in a solution of copal in chloro-
form. After thorough pei-meation they are taken out and dried
slowly until the masses become qaite hard. These masses may now
be cut into sections with a fine saw and rubbed down on a whetstone
in the ordinary manner ; after staining, the sections maybe mounted
in Canada balsam. The great value of this method lies in the fact
that by it the soft and hard parts are retained in their proper rela-
tions with each other. ^

The chief point of interest to the microscopist, however, in the
structure of these animals lies in the extraordinary abundance and
high development of those ' filiferous capsules,' or ' thread-cells,' the
presence of which on the tentacles of the hydroid polypes has been
already noticed, and which are also to be found, sometimes sparingly
sometimes very abundantly, in the tentacles surrounding the mouth
of the Medusa?, as well as on other parts of their bodies. If a
tentacle of any of the sea-anemones so abundant on our coasts (the
smaller and more transparent kinds being selected in preference) be
cut ofli", and be subjected to gentle pressure between the two glasses
of the aquatic box or the compressorium, multitudes of little dart-
like oi'gans will be seen to project themselves from its surface near
its tip ; and if the pressure be gradually augmented, many additional
darts will every moment come into view. Not only do these organs
present difierent forms in diff'ei-ent species, but even in one and the
same individual very strongly marked diversities are shown, of
which a few examples are given in fig. 666. At A, B, C, D is
shown the appearance of the ' filiferous capsules,' whilst as yet the

' See ZoohKj'iHclier Anzeiger, i. p. 30; and Proi-. Ziiol. Hue. London, 1880, p. 24.

ALCYONAEIA

879

thread lies coiled up in their interior ; and at E F G H are seen
a few of the most striking forms whicli they exhibit when the thread
or dart has started forth. These thread-cells are found not merely in
the tentacles and other parts of
the external integument of Ac-
tinozoa, but also in the long fila-
ments which lie in coils within
the chambei'S that sui'round the
stomach, in contact with the
sexual organs which are attached
to the lamella? dividing the cham-
bers. The latter sometimes con-
tain ' sperm-cells ' and sometimes
ova, the two sexes being here
divided, not united in the same
individual. What can be the
office of the filiferous filaments
thus contained in the interior of
the body it is difficult to guess
at. They are often found to pro-
trude from rents in the external
tegument, when any violence has
been used in detaching the animal
fi'om its base ; and when thei-e is
no external rupture they are often
forced through the wall of the
stomach into its cavity, and may
be seen hanging out of the mouth.
The largest of these cajisules, in
their unprotected state, are about
3-g^Q^th of an inch in length ; while
the thread or dart, in Corynactis
Allmanni, when fully extended
is not less than |-th of an inch,
or thirty-seven times the length
of its capsule. '^

Of the Alci/onaria -A character-
istic example is found in the Alcy-
onium dkjitatum of our coasts ; Fig. 066.— Filiferous capsules of Acti-
a lobed sponge-like mass, covered nozoa: A, B, Cori/nactis Allmanni;
with a tough skin, which is com- ^' f.^ ^' Car!,oph;jlUaSmithu-, D, G,

. . * ' , ^ Actinia crassicornis \ H., Actinia cnn-

monly known under the name or did a.

' dead-man's toes,' oi- by the

more elegant name of ' mermaid's fingers.' When a specimen of

this is first torn from the rock to which it has attached itself, it

conti'acts into an unshapely mass, whose surface presents nothing

' See Mr. Gosse's Naturalist's Bamhles on the Devonshire Coast, and Professor
Mijbius, ' Ueber den Bau u.s.w. der Nesselkapseln einiger Polypeii und Quallen,' in
Abhandl. Naturw. Vereins zu Hanihitrg, Band v. 1866. On the relations of stinging
cells to the nervous system, see Dr. v. Lendenfeld, Quart. Joiirn. of Microsc. Sci. n.s.
xxvii. p. 393. On the stinging cells of Coelentera generally, see N. Iwanzoff in Bull .
Sor. Moscow, 1896, pp. 95 and 323.

88o

SPONGES AND ZOOPHYTES

but a series of slight dej)ressions arranged with a certain regularity.
But after being immersed for a little time in a jar of sea-water the
mass swells out again, and from every one of these depressions an
eight-armed polype is protruded, ' which resembles a flower of ex-
quisite beauty and perfect symmetry. In specimens recently taken,
each of the petal-like tentacula is seen with a hand-glass to be fur-
nished with a row of delicately slender irhiiKje or filaments, fringing
each margin, and arching onwards ; and with a higher power these
pinnae are seen to be roughened throughout their whole length with
numerous prickly rings. After a day's captivity, however, the petals
shrink up into short, thick, imshapely masses, rudely notched at their
edges.' (Gosse.) When a mass of this sort is cut into it is found
to be channelled out somewhat like a sponge by ramifying canals ; the
vents of which open into the stomachal cavities of the polypes, which
ai-e thus brought into fi-ee communication with each other, a cha-
ractei" that especially distinguishes this order. A movement of fluid
is kept up within these canals (as may be distinctly seen through

Fig. 667. — Spicules of Alcyonium
and Gorgonia.

A-V

Fig. 668. — A, spicules of Gorgonia guttata
B, spicules of Muricea elongata.

their transparent bodies) by means of cilia lining the internal surfaces
of the polypes ; but no cilia can be discerned on their external sur-
faces. The tissue of this spongy polypidom is strengthened through-
oiit, like that of sponges, with mineral spicviles (always, however, cal-
careous), which are i-emarkable for the elegance of their forms ; these
ai-e disposed with great regularity around the bases of the polypes,
and even extend part of their length upwards on their bodies. In
the Gorgonia or sea-fan, Avhilst the central part of the polypidom is
consolidated into a horny axis, the soft flesh which clothes this axis
is so full of tuberculated spicules, especially in its outer layer, that,
when this dries up, they form a thick yellowish or reddish incrusta-
tion upon the horny stem. This crust is, however, so friable that it
may be easily rubbed down between the fingers, and when examined
with the microscope it is found to consist of spicules of different
shapes and sizes, more or less resembling those shown in figs. 667, 668,
sometimes colourless, but sometimes of a beautiful ci-imson, yellow.

CTENOPHOEA

88l

or purple. These spicules are best seen by black-ground, illumination,
especially when viewed by the binoculai- microscope. They are, of

course, to be sepa-
rated fi'om the
animal substance in
the same manner as
the calcareous spi-
cules of sponges ;
and they should be
mounted, like them,
in Canada balsam.
The spicules always
possess an organic
basis, as is proved
by the fact that
when their lime is
dissolved by dilute
acid a gelatinous-
looking residuum is
left which preserves
the form of the
spicule.

The Gteno'pliora,
or ' comb-bearers,'
are so named from
the comb-like ar-
rangement of the
rows of tiny

Fig. 669. — 1. 'Ewplokaviia stationis, with its tentacles
extended, about twice the natural size: m, mouth; e,
ctenophoral plate ; t, tentacular apparatus. (After Chun.)
2. Diagrammatic view of Hormijohora pluinosa, seen
from the aboral pole : c, as before ; tv, tentacular vessel ;
PIU polar plates. (After Chun.)

Pig. 670. — BeroU
Forskalii, show-
ing the - tubular
prolongations of
the stomach.

' paddles ' by the movement of which the liodies of these animals aie
propelled. A very beautiful and not uncommon representative of

3l

882 SPONGES AND ZOOPHYTES

this order is furnished by the Cydipiye^ pileus (compare fig. 669), very
commonly known as the Beroe, which designation, however, properly
appertains to another animal (fig. 670) of the same grade of organisa-
tion. The body of Cydijij^e is a nearly globular mass of soft jelly,
usually about |ths of an inch in diameter, and it may be observed,
even with the naked eye, to be marked by eight bright bands,
which proceed from j)ole to pole like meridian lines. These bands
are seen with the microscope to be formed of rows of flattened
filaments, far larger than ordinary cilia, but lashing the water in
the same manner ; they sometimes act quite independently of one
another, so as to give to the body every variety of motion, but
sometimes work all together. If the sunlight should fall upon them
when they are in activity, they display very beautiful ii-idescent
colours. In addition to these ' paddles ' the Cydippp. is furnished
with a pair of long tendril-like filaments, rising from the bottom of
a pair of cavities in the posterior part of the body, and furnished
with lateral branches ; within these cavities they may lie doubled up,
.so as not to be visible externally; and when they are ejected, which
often happens quite suddenly, the main filaments first come forth, and
the lateral tendrils subsequently uncoil themselves, to be drawn in
again and packed up within the cavities with almost equal sudden-
ness. The mouth of the animal situated at one of the poles leads
first to a quadrifid cavity bounded by four folds which seem to repre-
sent the oral proboscis of the ordinary Meduste (fig. 664) ; and this
leads to the true stomach, which passes towards the opposite pole,
near to which it bifurcates, its branches passing towarcls the polai-
surface on either side of a little body which has every appearance of
being a nervous ganglion, and which is sui-mounted externally by a
fringe-like apparatus that seems essentially to consist of sensory
tentacles.^ From the cavity of the stomach tubular prolongations
pass ofi" beneath the ciliated bands, very much as in the true Beroe.
These may easily be injected with coloured liquids by the intro-
duction of the extremity of a fine-pointed glass syringe into the
movith. The liveliness of this little creature, which may sometimes
be collected in large quantities at once by the stick-net, renders it a
most beautiful subject for observation when due scope is given to its
movements ; but for the sake of microscopic examination, it is of
course necessary to confine these. Various species of true Beroe,^
some of them even attaining the size of a small lemon, are occasionally
to be met with on our coasts, in all of which the movements of the

1 More correctly Hormiphora.

- It is commonly stated that the two branches of the alimentary canal open on
the surface by two jiores situated in the hollow of the fringe, one on either side of the
nervous ganglion. The Author, however, has not been able to satisfy himself of the
existence of sucli excretory pores in the ordinary Cy(li2)2}e or Beroe, although he has
repeatedly injected their whole alimentary canal and its extensions, and has atten-
tively watched the currents produced by ciliary action in the interior of the bifurcat-
ing prolongations, which currents always appear to him to return as from Ciecal
extremities. He is himself inclined to believe that this arrangement has reference
solely to the nutrition of the nervous ganglion and tentacular apparatus, which lies
imbedded (so to speak) in the bifurcation of the alimentary canal, so as to be able
to draw its sui^ply of nutriment direct from that cavity.

^ On the anatomy of Beroe, see Eimer, Zoologische Stiullen auf Ccqyri. I- Ueher
Beroii 0 vat us, Leipzig, 1873.

CCELENTEEA 883

body are eflected by the like agency of paddles arranged in meridional
bands. These are splendidly luminous in the dark, and the lum.i-
nosity is i-etained even by fragments of their bodies, being augmented
by agitation of the water containing them. All the CUnopliora are
reproduced from eggs, and are already quite advanced in theii- deve-
lopment by the time they are hatched. Long before they escape,
indeed, they swim about with great activity within the walls of their
diminutive prison, their rows of locomotive paddles early attaining
a large size, although the long flexile tentacles of Cijdijype are then
only short stumpy protuberances. By Cceloplana and Gtenoplana
the Ctenophora appear to be allied to the Planarian Worms. ^

Those who may desire to acquire a more systematic and detailed acquaintance
with the zoophyte group may be especially referred to the following treatises and
memoirs, in addition to those already cited, and to the various recent systematic
treatises on zoology :^Dr. Johnston's JUstory of British Zoojjhytes ; Professor Milne-
Edwards's ' Recherches sur les Polypes,' and his 'Histoire des Corallaires' (in the
Suites a, Buffon), Paris, 1857 ; Professor Van Beneden, ' Sur les Tubulaires ' and ' Sur les
Campauulaires,' in Mem. de I' Acad. Boy. de Brnxelles, torn, xvii., and his ' Recherches
sur I'Hist. Nat. des Polypes qui frequentent les Cotes de Belgique,' ojj. cit. torn,
xxxvi. ; Sir J. G. Dalyell's Bare and Bemarkable Animals of Scotland, vol. i. ;
Trembley's Jlfe/^. pour servir a Vhistoire d'un genre de Polype d'eait, douce; M.
Hollard's ' Monograx^hie du Genre Actinia' in Ann. des Sci. Nat. ser. iii. tom. xv. ;
Professor Max Schultze, ' On the Male Reproductive Organs of Campanularia cjenicu-
lata' in Quart. Journ. Micr. Sci. vol. iii. 185.5, p. 59; Professor P. E. Schulze's memoirs
on Cordylopliora lacustris, Leii^zig, 1871, and on Syncoryne, 1873 ; Professor Agassiz's
beautiful monograph on American Medusas, forming the third volume of his Contri-
butions to the Natural History of the United States of America; Mr. Hincks's
British Hydroid Zoophytes ; Professor AUman's admirable memoirs on Cordylophora
and Myriothela in the Phil. Trans, for 1853 and 1875 ; Professor Lacaze-Dutliiers's
Hist. Nat. du Corail, Paris, 1864, and his essays on the Development of Corals, in
vols. i. and ii. of the Archives de Zool. expervmentale; Professor J. R. Greene's
Manucd of the Sub-kingdom Ccelenterata, which contains abibliography very com-
plete to the date of its i^ublication, and the articles ' Actinozoa,' ' Ctenophora,' and
' Hydrozoa ' in the supplement to the Natural History Division of the English Cyclo-
pcedia. The Ctenophora are specially treated of in vol. iii. of Professor Agassiz's
Contributions to the Natural History of the United States. See also Professor
Alex. Agassiz's Seaside Studies in Natural History and his Illustrated Catalogue
of the Museum of Comparative Zoology at Harvard College; Professor James
Clark in American Journal of Science, ser. ii. vol. xxxv. p. 348; Dr. D. Macdonald in
Trans. Boy. Soc Edinb. vol. xxiii. p. 515 ; Mr. H. N. Moseley, ' On the Structure of
a Species of Millepora,' in Phil. Trans. 1877, p. 117, and ' On the Structure of the
Stylasteridce,' ibid. 1878, p. 125 ; and on the Acalephce, Professor Haeckel's Beit rage
zwr Naturgeschichte cler Hydromedusen ; the masterly work of the brothers Hertwig,
Bas Nervensystem and die Sinnesorgane der Medusen, 1878 ; and the memoir of
Professor Schiifer, ' On the Nervous Sj'stem of Aurelia aurita,' in Phil. Trans. 1878,
p. 568. Of later treatises Professor Ray Lankester's article on Hydrozoa, in the 9th
edition of the Encyclopcedia Britannica; the ^Challenger'' Eeports of Professor
Alhnan on the Hydroida (Plumulariidse only), Professor Haeckel on the Medusae,
Professor Moseley on Deep-sea Corals, Dr. R. Hertwig on the Actiniaria, Professors
E. P. Wright and Studer on the Alcyonaria, and Mr. George Brook on the Antipatharia ;
the monograijhs by Dr. A. Andres on Actinias and by Dr. C. Chun on Ctenophora,
published in the Fauna und Flora, des Golfes von Neapel, should be consulted.
Dr. Chun has made some progress with a general account of the Coelentera in Bronn's
' Thierreich,' Bd. ii. Abth. 2. On fresh-water Medusee, see Mr. R. T. Giinther
in Quart. Journ. Micr. Sci. xxxvi. j>. 284.

1 See Korotneff, Zeitschr. f. wiss. Zool. xliii. p. 242, and Dr. A. Willey Quart.
Journ. Micr. Sci. xxxix. p. 323.

3 L 2

884

CHAPTER XYI

ECHINODEBMA

As we ascend the scale of animal life, we meet with such a rapid
advance in complexity of structure that it is no longei- possible to
acquaint oneself with any organism by microscopic examination
of it as a whole ; and the dissection or analysis which becomes
necessary, in order that each separate part may be studied in detail,
belongs rather to the comparative anatomist than to the ordinary
microscopist. This is especially the case with the Echinus (sea-
urchin), Aster ias (star-fish), and other members of the class Echino-
derma, of whose complex organisation even a general account
would be quite foreign to the purpose of this work. Yet there are
certain parts of theii- structure which furnish microscopic objects of
svich beauty and interest that they cannot by any means be passed
by ; while the study of theii' embryonic forras, which can be pro-
secuted by any seaside observer, brings into view an order of facts
of the highest scientific interest.

It is in the structure of that calcareous skeleton which exists
under some form in neai-ly every member of this class that the ordi-
nary microscopist finds most to interest him. This attains its highest
development in the Bchinoidea, in which it forms a box-like shell oi-
'test,' composed of numerous polygonal plates jointed to each other
with great exactness, and beset on its external surface with ' spines,'
which may have the form of prickles of no great length, or may be
stout club-shaped bodies, or, again, may be very long and slender
rods. The intimate stiaicture of the shell is everywhere the same ;
for it is composed of a network, which consists of carbonate of lime
with a very small quantity of animal matter as a basis, and which
extends in every direction (i.e. in thickness as well as in length and
breadth), its areolce or interspaces freely communicating with each
other (figs. 671, 672). These ' areolae,' and the solid structui'e which
suiTounds them, may bear an extremely variable propoi'tion one to
the othei- ; so that in two masses of equal size the one or the other
may gi-eatly pi'edominate ; and the texture may have eithei' a re-
markable lightness and porosity, if the netwoi-k be a very open one,
like that of fig. 671, or may possess a considerable degree of com-
pactness, if the solid portion be strengthened. Generally speaking,
the difierent layers of this network, which are connected together
by pillars that pass from one to the othei- in a direction perpendicu-

STEUCTUKE OF ECHINOIDS

885

l;ir to their plane, are so arrangetl that the perfoi'ations in one shall
correspond to the intermediate solid structure in the next ; and their
transparence is such that when we are examining a section thin
enough to contain only two or three such layers, it is easy, by
properly focussing the microscope, to bring either one of them into
distinct view. From this very simple but very beautiful ai-range-
ment, it comes to pass that the plates of which the entire ' test ' is
made up possess a veiy considerable degree of strength, notwith-
standing that their porousness is such that if a portion of a fractured
edge, or any other part from which the investing membrane has
been removed, be laid upon fluid of almost any description, this will
be rapidly sucked up into its substance. A very beautiful example
of the same kind of calcareous skeleton, having a more regular con-
formation, is furnished by the disc or ' rosette ' which is contained
in the tip of every one of the tubular suckers put forth by the living
Echinus from the ' ambulacral pores ' that are seen in the rows of

Fig. 671.— Section of shell of Echinus
showing the calcareovTS network of
which it is composed: a a, portions
of a deeper layer.

Pig. 672. — Transverse section of cen-
tral portion of spine of Seterocen-
trotus, showing its more ox)en net-
worlr.

smaller plates interposed between the larger spine-bearing plates of
its box-like shell. If the entire disc be cut off, and be mounted
when dry in Canada balsam, the calcareous rosette may be seen
sufiiciently well ; but its beautiful structure is better made out when
the animal membrane that incloses it has been got rid of by boiling
in a solution of caustic potass ; and the appearance of one of the
five segments of which it is composed, when thus prepared, is sho^vn
in fig. 674.

The most beautiful disj^lay of this reticulated structiu'e, however,
is shown in the conformation of the ' spines ' of Echinus, Cidaris, &c.,
in which it is combined with solid ribs or pillars, disposed in such a
manner as to increase the strength of these organs, a regular and
elaborate pattern being formed by their intermixture, which shows
considerable variety in different species. When we make a thin
transverse section of almost any spine belonging to the genus
Echimis (the small spines of our British species, however, being
exceptional in this respect) or its immediate allies, we see it to be

ECHI^'ODEKMA

made up of a number of concentric layers, arranged in a manner that
strongly reminds us of the concentric rings of an exogenous tree

m^mii^

Fig. 673. — Transverse section of spine of Echinometra,

(fig. 673). The number of these layers is extremely variable, de-
pending not merely upon the age of the spine, but (as will presently

ajjpear) upon the part of
its length from which the
section happens to be
taken. The centre is
usually occupied by a
very open network (fig.
672) ; and this is bounded
by a row of transparent
spaces (like those at a a',
b V, c c', &c., fig, 675),
which on a cursory in-
spection might be sup-
posed to be void, but are
found on closer examins)-
tion to be the sections of
solid ribs or pillars, which
run in the direction of the length of the spine, and form the exterior
of every layer. Their solidity l^ecomes very obvious when we

Fig. 674. — One of the segments of the calcareous
skeleton of an ambulacral disc of Echinus.

SPINES OF ECHINOIDS

887

either examine a section of a spine whose substance is jDervaded (as
often happens) with a colouring matter of some depth, oi- when we
look at a very thin section by black-ground illumination. Around
the innermost circle of these solid pillars there is another layei- of
the calcareous network, which again is surrounded by another circle
of solid pillars ; and this arrangement may be i-epeated many times,
as shown in fig. 675, the outermost row of pillars forming the
pi-ojecting ribs that are commonly to be distinguished on the surface
of the spine. Around the cup-shaped base of the spine is a membrane
which is continuous with that covering the surface of the shell, and
sei'ves not merely to hold down the cup upon the tubercle over which
it works, but also by its conti-actility to move the spine in any required
direction. The increase in size of the S]3ine appears to be due to the
protoplasmic substance which fills up the spaces in the open network
of the spine and othei- skeletal structures. Each new formation
completely ensheathes the old, not merely surrounding the part pre-
viously formed, but also projecting considerably beyond it ; and thus
it happens that the number of layers shown in a ti-ansverse section

Fig. 675. — Portion of transverse section of spine of Heferocevtrofus
mamniillatus.

Avill depend in part u]3on the place of that section. For if it cross
near the base, it will traverse every one of the successive layers from
the very commencement ; whilst if it cross near the apex, it will
traverse only the single layer of the last gi-owth, notwithstanding
that, in the club-shaped spines, this terminal portion may be of con-
siderably larger diameter than the basal ; and in any intermediate
part of the spine, so many layers will be traversed as have been
formed since the spine first attained that length. The basal portion
of the spine is enveloped in a reticulation of a very close texture
without concentric layers, forming the cup oi- socket which works
over the tubercle of the shell.

Their combination of elegance of pattern with richness of coloui'-
ing renders well-prepared specimens of these spines among the most
beautiful objects that the microscopist can anywhere meet with.
The large spines of the various species of the genus Heterocentrotus
furnish sections most remarkable for size and elaborateness, as well
as for depth of colour (in which last point, however, the deep purple
spines of Echinus livichts are pre-eminent) ; but for exquisite

ECHINODEEMA

neatness of pattern there are no spines that can approach those
of Echinoinetra (fig. 673). The spines of Stomopnetistes variola/ris
are also remarkable for their beauty. No succession of concentric
layers is seen in the spines of the British Echim., probably be-
cause (according to the opinion of the late Sir J. G. Dalyell) these
spines are cast off and renewed every year, each new formation
thus going to make an entire spine, instead of making an addition
to that previously existing. Most curious indications are some-
times afforded by sections of Echinus-spines of an extraordinary
power of reparation inherent in these bodies. For irregtilarities
are often seen in the transverse sections which can be accounted
for in no other way than by supposing the spines to have received
an injury when the irregular part was at the exterior, and to
have had its loss of substance supplied by the growth of new

Fig. 676. — Transverse section of a spine of Goniocidaris florigera,
which shows that the prickles on the spine are formed, not by the
crust only, but also by the inner reticular tissue. (From Bell.)

tissue, over which the subsequent layers have been formed as usual.
And sometimes a peculiar ring may be seen upon the surface of a
spine, which indicates the place of a complete fracture, all beyond
it being a new growth, whose unconformableness to the older or
basal portion is cleai-ly shown by a longitudinal section.^ The spines
of Cidm-is present a marked departure from the plan of structure
exhibited in Echinus ; for not only are they destitute of concentric
layers, but the calcareous netwoi-k which forms their princij)al
substance is incased in a solid calcareous sheath perforated with
tubules, which seems to take the place of the separate pillars of the
Echini. This is usually found to close in the spine at its tip also ;

1 See tlie Author's description of such reparations in the Monthly Microsco'pical
Journal, vol. iii. 1870, p. 22.5.

SPINES ; PEDICELLAKIJ5

889

and thus it would appear that the entire spine must be formed at
once, since no addition could be made either to its length or to its
diameter, save on the outside of the sheath, whei-e it is never to be
found. The sheath itself often rises up in prominent points or
ridges on the surface of these spines ; but, as is shown in fig. 676,
the reticular portion may have a share in the formation of the rings.
This view of the mode of formation of the Cidarid spine is con-
tested by Professor Jeflrey Bell, who has brought foi-ward/ evidence
to show that if two spines of different sizes be taken from two
examples of Cidaris metidaria, also differing in size, the quantity of
solid calcareous sheath seen in transverse section is proportionately
less in the larger than in the smaller spine ; from this he concludes
that the growth is due to the internal reticulated portion rather
than to the outer crust. The slender, almost filamentary spines

Fig. 677. — Spine of Sjjatangus.

of S'patangtis (fig. 677) and the innumerable minute hair-like pro-
cesses attached to the shell of Glypeaster are composed of the like
regularly reticulated substance ; - and these are very beautiful objects
for the lower powers of the microscope, when laid upon a black
ground and examined by reflected light without any further prepara-
tion. It is interesting also to find that the same structure presents
itself in the curious Pedicellarice (forceps-like bodies often mounted on
long stalks), which are found on the surface of many Echinida and
Asterida, and the nature of which was formerly a source of much
perplexity to naturalists, some having maintained that they were
parasites, whilst others considered them as proper appendages of the
Echinus itself. The complete conformity which exists between the
structure of their skeleton and that of the animal to which they are
attached removes all doubt of their being truly appendages to it, as
observation of their actions in the living state would indicate.'^

1 Journ. Boy. Microsc. Soc. 1884, p. 845.

^ A number of rare spines are described and figured by Prof. H. W. Mackintosh
in vols. xxvi. (p. 475) and xxviii. (pi3. 241 and 259) of the Trans. Boy. Irish Academy.

^ Prof. Alex. Agassiz has shown the relations of the Pedicellarise to the spines.
Much information regarding the various forras of these curious bodies will be found
in Professor Perrier's memoir in the .4toi. Sc. Nat, (5), vols.xii.andxiii. ; Mr Sladen's

890

ECHINODEEMA

Another example of the same- structure is found in the peculiar
framework of plates which surrounds the interior of the oral orifice
of the shell, and which includes the five teeth that may often be seen
projecting externally through that orifice, the whole forming what
is known as the ' lantern of Aristotle.' The texture of the plates
or jaws resembles that of the shell in every respect, save that the
network is more open ; but that of the teeth difiiers from it so widely
as to have been likened to that of the bone and dentine of vertebrate
animals. The careful investigations of Mr. James Salter, ^ however,
have fully demonstrated that the appearances which have suggested
this comparison are to be otherwise explained, the plan of structm-e
of the tooth being essentially the same as that of the shell, although
greatly modified in its working out. The complete tooth has some-

FiG. 678. — Structure of the tooth of Echinus : A, vertical section, showing
the form of the apex of the tooth as produced by wear, and retained bj'
the relative hardness of its elementarj^ parts ; a, the clear condensed axis ;
h, the body formed of x^lates ; c, the so-called enamel ; d, the keel. B,
commencing growth of the tooth, as seen at its base, showing its two sys-
tems of plates ; the dark appearance in the central portion of the upper
p)art is produced by the incipient reticulations of the flabelliform processes.
C, transverse section of the tooth, showing at a the ridge of the keel ; at b
its lateral xJortion, resembling the shell in texture ; at c c, the enamel.

what the form of that of the fi-ont tooth of a rodent, save that its
concave side is strengthened by a projecting ' keel,' so that a trans-
verse section of the tooth presents the form of a j_. This keel is
composed of cylindrical rods of carbonate of lime, having club-shaped
extremities lying obliquely to the axis of the tooth (fig. 678, A, d) ;
these rods do not adhere very firmly together, so that it is difficult
to keep them in their places in making sections of the part. The

essay in Ann. and Mag. Nat. Hist. (5), vi. p. 101 ; and M. Foettinger's paper in vol. ii.
J). 455 of the Archives de Biologic.

1 See his memoir, ' On the Structure and Growth of the Tooth of Ecliinus,' in
Phil. Trans, for 1861, p. 387. See also Giesbrecht, ' Der feinere Bau der Seeigel-
ziihne,' Morph. Jahrbuch, vi. p. 79.

CALCAEEOUS TISSUE 89 1

convex sui'&ce of tlie tooth (c, c, c) is covered with a firmer layer,
which has I'eceived the name of ' enameh' This is composed of
shorter rods, also obliquely arranged, but having a much more
intimate mutual adhesion than we find among the rods of the keel.
The principal part of the svibstance of the tooth (A, b) is made up of
what may be called the ' primary plates.' These are triangular plates
of calcareous shell-substance, arranged in two series (as shown at
B), and constituting a sort of framework with which the other parts
•to be presently described become connected. These plates may be
seen by examining the growing base of an adult tooth that has
been presei-ved with its attached soft jDarts in alcohol, oi- (which is
preferable) bj" examining the base of the tooth of a fresh specimen,
the minuter the better. The lengthening of a tooth below, as it
is worn away above, is mainly efi'ected by the successive addition of
new ' primary plates.' To the outer edge of the primary plates at
some little distance from the base we find attached a set of lappet -
like appendages, which are formed of similar plates of calcareous
shell-substance, and are denominated by Mr. Salter ' secondary
plates.' Another set of appendages termed ' flabellifoim processes '
is added at some little distance from the growing base ; these consist
of elaborate reticulations of calcareous fibres, ending in fan-shaped
extremities. And at a point still fui-ther from the base we find the
different components of the tooth connected together by ' soldering
particles,' which are minute calcareous discs interposed between the
previously formed structures ; and it is by the increased develop-
ment of this connective substance that the intervening spaces are
narrowed into the semblance of tubuli like those of bone or dentine.
Thus a vertical section of the tooth comes to present an appearance
very like that of the bone of a vertebrate animal, with its lacunae,
canaliculi, and lamella ; but in a transverse section the body of the
tooth bears a stronger resemblance to dentine ; whilst the keel and
enamel layer more resemble an oblique section of Pinna than any
other form of shell-structure.

The calcareous plates which form the less compact skeletons of
the Asteroidea (' star-fish ' and their allies) and of the Ophlaroidea
('sand-stars' and 'brittle stars') have the same texture as those of
the shell of Echinus. And this
presents itself, too, in the sj)ines or
prickles of their surface when
these (as in the great Goniaster
pquestris or ' knotty cushion-star ')
are large enough to be furnished
with a calcareous framework. An
example of this kind, furnished by
the Astrojyhyton, is represented in

fig. 679. The spines with which p.g. 679.-Calcareous plate and claw

the arms of the species of OphiotJirix of Astrowlnjto)i.

(' brittle star ') are beset are often

I'emarkable for their beauty of conformation ; those of 0. innta-

'phyllimi, one of the most common kinds, might serve (as Professor

E. Forbes justly remarked), in point of lightness and beauty, as

892 ECHINODERMA

models foi* the spire of a cathedral. These are seen to the greatest
advantage when mounted in Canada balsam, and viewed by the
binocular microscope with black-ground illumination. It is inter-
esting to remark that the minute tooth of OpMoilirix cleaiiy exhibits,
with scarcely any preparation, that gradational transition between
the ordinary reticular structure of the shell and the peculiar sub-
stance of the tooth which in the adult tooth of the Echinus can
only be traced by making sections of it near its base. The tooth of
OpMoilirix may be mounted in balsam as a transparent object with
scarcely any grinding down ; and it is then seen that the basal poi--
tion of the tooth is formed upon the open reticular plan characteristic
of the ' shell,' whilst this is so modified in the older poi-tion by sub-
sequent addition that the upper part of the tooth has a bone-like
character.

The calcareous skeleton is very highly developed in the Crinoidea,
their stems and branches being made \ip of a calcareous network
closely resembling that of the shell of the Echinus. This is extremely
well seen, not only in the recent Pentacrinus asterius^ a somewhat rare
animal of the West Indian seas, but also in a large proportion of
the fossil ciinoids, whose remains are so abundant in many of the
older geological formations ; for, notwithstanding that these bodies
have been penetrated in the act of fossilisation by a mineral infiltra-
tion, which seems to have substituted itself for the original fabric
(a regularly crystalline cleavage being commonly found to exist in
the fossil stems of Encrinites, &c., as in the fossil sj)ines of Echinida),
yet their organic structure is often most perfectly preserved.^ In
the circular stems of Encrinites the texture of the calcareous net-
work is uniform, or nearly so, throughout ; but in the pentangular
Pentacrini a certain figure or pattern is formed by variations of
texture in different parts of the transverse section. ^

The minute structure of the shells, spines, and other solid parts
of the skeleton of Echinodei'ma can only be displayed by thin
sections made upon the general plan already described in Chapter YII.
But their peculiar texture i-equii'es that certain precautions should
be taken : in the first place, in order to prevent the section from
breaking whilst being reduced to the desirable thickness ; and in
the second, to prevent the interspaces of the network from being
clogged by the particles abraded in the reducing process. An illus-
tration of a section cut from a spine of Echinometra is given in
fig. 673. A section of the shell, spine, or other portion of the
skeleton should first be cut with a fine saw, and be rubbed on a. flat
file until it is about as thin as ordinary card, after which it shoiild
be smoothed on one side by friction with water on a Water-of-Ayr

1 The calcareous skeleton even of living Ecliinoderms has a crystalline aggregation,
as is very obvious in the more solid spines of Ecldnovietrce, &c. ; for it is difficult, in
sawing these across, to avoid their tendency to cleavage in the oblique plane of
calcite. And the Author is informed by Mr. Sorby that the calcareous deposit which
fills up the areolfe of the fossilised skeleton has always the same crystalline system
with the skeleton itself, as is shown not merely by the uniformity of their cleavage,
liut by their similar action on jjolarised light.

- See figs. 74-76 of the Author's memoir on ' Shell Structure ' in the Beqjort of
the British Association, 1847.

PEEPARING SPINES 893

stone. It should then, after cai'eful washing, be dried, first on white
blotting-paper, afterwards by exposui'e for some time to a gentle
heat, so that no water may be i-etained in the interstices of the net-
work which would oppose the complete penetration of the Canada
balsam. Next, it is to be attached to a glass slip by balsam hardened
in the usual manner ; but particular care should be taken, first, that
the balsam be brought to exactly the right degree of hardness, and
second, that there be enough not merely to attach the specimen to
the glass, but also to satui-ate its substance throughout. The right
degree of haixlness is that at Avhich the balsam can be with difliculty
indented by the thumb-nail ; if it be made harder than this, it is
apt to chip oft' the glass in grinding, so that the specimen also breaks
away ; and if it be softer, it holds the abraded particles, so that
the openings of the netwoi-k become clogged with them. If, when
rubbed down nearly to the I'equired thinness, the section appears to
be uniform and satisfactory throughout, the reduction may be com-
pleted without displacing it ; but if (as often happens) some inequality
in thickness should be observable, oi- some minute air-bubbles should
show themselves between the glass and the under surface, it is de-
sirable to loosen the specimen by the a|jplication of just enough heat
to melt the balsam (special care being taken to avoid the production
of fresh aii'-bubbles) and to turn it over so as to attach the side
last polished to the glass, taking care to remove oi- to break with
the needle point any air-bubbles that there may be in the balsam
covering the part of the glass on which it is laid. The surface now
brought uppermost is then to be very carefully ground clown,
special care being taken to keep its thickness uniform through every
part (which may be even battel* j uclged of by the touch than by the
eye), and to cany the I'educing pi-ocess far enough, without cai-rying
it too far. Until practice shall have enabled the operator to judge
of this by passing his finger ovei' the specimen, he must have con-
tinual recourse to the microscope during the latter stages of his
work ; and he should bear constantly in mind that, as the specimen
will become much more translucent when mounted in balsam and
covered with glass than it is when the ground surface is exposed, he
need not carry his i^educing process so fai- as to produce at once the
entire translucence he aims at, the attempt to accomplish which
would involve the risk of the destruction of the specimen. In
' mounting ' the specimen liquid balsam should be employed, and
only a very gentle heat (not sufiicient to produce air-bubbles or to
loosen the specimen fi-om the glass) should be applied ; and if, after
it has been mounted, the section should be found too thick, it will
be easy to remove the glass cover and to I'educe it further, cai'e being
taken to harden to the joi'oper degree the balsam which has been
newly laid on.

If a number of sections ai'e to be prepared at once (which it is
often useful to do for the sake of economy of time, or in ordei- to
compare sections taken from difierent parts of the same spine), this
may be most readily accomplished by laying them down, when cut
off by the saw, without any preliminary prepai-ation save the blow-
ing of the calcareous dust fi-om theii' surfaces, upon a thick slip of

894 ECHINODEEMA

glass well covered with hai-dened balsam ; a large proj^oi-tion of its
surface may thus be occupied by the sections attached to it, the
chief pi'ecaution required being that all the sections come into
equally close contact with it. Their surfaces may then be brought
to an exact level by rubbing them down, first ujDon a flat piece of
grit (which is veiy suitable for the rough grinding of such sections)
and then upon a large Water-of-Ayr stone whose surface is ' true.'
When this level has been attained the ground surface is to be well
washed and dried, and some balsam previously hardened is to be
spread over it, so as to be sucked in by the sections, a moderate heat
being at the same time applied to the glass slide ; and when this
has been increased sufiiciently to loosen the sections without over-
heating the balsam, the sections are to be turned over, one by one,
so that the ground surfaces are now to be attached to the glass sliji,
special care being taken to press them all into close contact with it.
They are then to be very carefully rubbed down, until they ai-e
nearly reduced to the required thinness ; and if, on examining them
from time to time, their thinness should be fovmd to be uniform
throughout, the reduction of the entii-e set may be completed at once ;
and when it has been carried sufficiently far, the sections, loosened by
warmth, are to be taken up on a camel-hair bi-ush dipped in turpen-
tine and transferred to separate slips of glass whereon some liquid
balsam has been previously laid, in which they are to be mounted in
the usvial mannei-. It more frequently happens, however, that, not-
withstanding every care, the sections, when ground in a number
together, are not of uniform thickness, owing to some of them being
underlain by a thicker stratum of balsam than others ; and it is
then necessary to transfer them to separate slips before the reducing
process is completed, attaching them with hai'dened balsam, and
finishing each section separately.

A very curious internal skeleton, formed of detached plates or
spicules, is found in many members of this class, often foi-ming an
investment like a coat of mail to some of the viscera, especially the
ovaries. The forms of these plates and spicules are generally so
diverse, even in closely allied species, as to afford very good differ-
ential characters. This subject is one that has been as yet but veiy
little studied, Mr. Stewart being the only microscopist who has given
much attention to it,^ but it is well worthy of much more extended
research.

It now remains for us to notice the curious and often veiy beau-
tiful structui-es wldch i-epresent, in the class Holothurioidea, the solid
calcareous skeleton of the classes already noticed. The greater
number of the animals belonging to this order ai-e distinguished by
the flexibility and absence of firmness of their envelopes ; and ex-
cepting in the case of the vai'ious species which have a set of cal-
cai'eous plates, disposed ai-ound the wall of the pharynx, we do not find
among them any representation, that is apparent to the unassisted
eye, of that skeleton which constitutes so distinctive a feature of the

* See his memoir in the Linnean Traiisactions, xxv. p. :J(;5 ; see also BelJ,
Joum. Jioij. MicroHc. Hoc. 1882, p. 227.

HOLOTHURIAN SPICULES

895

class generally.^ But a microscopic examination of their integument
at once brings to view the existence of great numbers of minute
isolated plates, every one of them presenting the characteristic i-e-
ticulated structure, which are set with greater or less closeness in

Fig. 680. — Holothurioidea : I, Sticliojjus Kefersteinii; «, calcareous
plate of same ; &, c, calcareous plates of Holothuria vagabunda ;
d, the same of H. inliabilis; e, th.e same of H. botelliis; f, of H.
pardalis: cj, ol H. ediilis.

the substance of the skin. Various forms of the plates which thus
present themselves in Holotlitiria are shown in fig. 680.'^ In the
■Synapta, one of the long-bodied forms of this order, which abounds
in the Meditei-ranean Sea, and of which two species (the S. digitata

Bf'

^505/

Pig. 681. — Calcareous skeleton of Synapta : A, plate imbedded in
skin ; B, the same, with its anchor-like spine attached ; C, anchor-
like spine sexDarated.

■and ;S'. inhcerens) occasionally occur upon our own coasts,'' the cal-
careous plates of the integument have the regular form shown at A,
fig. 681 ; and each of these carries the curious anchor-like appendage,
"C, which is articulated to it by the notched piece at the foot, in the

' For an account of a very remarkable form see Moseley ' On the Pharynx of an
unknown Holothurian, of the family Dendrochirotifi, in which the calcareous skeleton
is remarkably developed,' Quart. Juurn. Microsc. Sci. n.s. xxiv. p. 255.

- For figures of the spicules of British Holothurians, see Bell, Catalogue of the
British Ecliuioderins, London, 1892, pis. i.-vi.

■' ' On the spicules of Stjnajita, together with some general remarks on the archi-
tecture of Echinoderm spicules,' consult E. Senion, Mitth. Zool. Stat. Neapel, vii.
p. 272. An excellent summary of our knowledge of the spicules of Holothurians is
given by Prof. Ludwig in his volume in Bronn's Tliierreicti, jjp. 35-61.

896

ECHINODEEMA

manner shown (in side view) at B. The anchoi'-like appendages
project from the surface of the skin, and may be considered as re-
presenting the spines of Echinida. Nearly allied to the Synapta is
the Ghiridota, the integument of which is entirely destitute of
' anchors,' but is fui-nished with very remarkable wheel-like plates :
those represented in fig. 682 are found in the skin of Chiridota
violacea, a species inhabiting the western parts of the Indian Ocean.
These ' wheels ' are objects of singular beauty and dehcacy, being
especially remarkable for the, very minute notching (scarcely to be
discerned in the figure without the aid of a magnifying glass) which
is traceable round the innei- margin of their ' tires.' There can be
scarcely any reasonable doubt that almost every member of this class
has some kind of calcareous skeleton disposed in a manner conform-
able to the examples now cited ; and it is now generally acknow-
ledged that the marked peculiarities by wliich they are respectively
distinguished are most useful in the determination of genera and
species.^ The plates may be obtained separately by the usual

method of treating the skin
with a solution of potass, and
they should be mounted in
Canada balsam. But their posi-
tion in the skin can only be
ascertained by making sections
of the integument both vertical
and parallel to its surface ; and
these sections, when dry, are
most advantageously mounted
in the same medium, by which
their transparence is greatly increased. All the objects of this class
are most beautifully displayed by the black-ground illumination, and
their solid forms are seen with increased efiect under the binocular.
The black-ground illumination applied to very thin sections of Echinus
spines brings out some effects of marvellous beauty ; and even in these
the solid form of the network connecting the pillars is better seen
with the binoculai- than it can be with the ordinary microscope.^

Echinoderm Larvse. — We have now to notice that most remark-
able set of objects furnished to the microscopic inquirer by the larval
states of this class ; foi- our first knowledge of which we were in-
debted to the painstaking and widely extended investigations of
Professor J. Mliller."^ All that our limits permit is a notice of two of
the most cvirious forms of these larvse by way of sample of the won-

1 No systematic account of a species of Holothurian can be regarded as complete
which does not contain an account of the form of its spicules, when these are present.
Figures of various forms will be found in Professor Samper's Reisen im Archipel tier
Philippinen : Holothurien, Dr. Theel's ' C/iaKew^e?- ' iipyjor^s, and the memoirs of
Professors Bell, Ludwig, and Selenka.

2 It may be here pointed out that the reticulated appearance is sometimes de-
ceptive, what seems to be solid network being in many instances a hollow network
of passages channelled out in a solid calcareous substance. Between these two con-
ditions, in which the relation between the solid framework and the intervening space
is completely reversed, there is every intermediate gradation.

3 Of later works consult especially the ' Selections from Embryological Mono-
graphs, ii. Echinodermata,' edited by Mr. A. Agassiz, in vol. ix. of the Memoirs of the
Museum of Comi^draiive Zoology,

PiGi 682. — Wheel-like plates from skin of
Chiridota violacecii

LARVAL ECHINODEEMS

897

derful phenomena which his researches brought to light, and to which
the attention of niici-oscopists who have the oppoi-tiinity of studying
them shoukl be the more assiduously dii-ected, as even the most deli-
cate of these organisms have beeii found capable of such perfect
preservation as to admit of being studied, when movnited as pre-
parations, even bettei- than when alive. The lai-val zijoids have, by
secondary adaptations to their mode of life, acquired a type ciuite
different from that which characterises the adults ; for instead of a
radial symmetry they exhibit a bilateral, the two sides being pi'e-
eisely alike, and each having a ciliated fringe along the greater pai-t
or the whole of its length. The
two fringes are united by a
superior and an inferior trans-
verse ciliated band, and be-
tween these two the mouth of
the zooid is always situated.
The external forms of these
larvse, however, vary in a most
remarkable degree, owing to the
unequal evolution of their dif-
ferent parts ; and there is also
a considerable diversity in the
several orders as to the propor-
tion of the fabric of the larva
which enters into the compo-
sition of the adult form. When
the young begins to acquire the
characters of the fully developed
star-fish and sea-urchin, the
parts which are not retained
shrivel up, and their substance
goes to feed the young form.

One of the most remarkable
forms of Echinoderm lai'va? is
that which has received the
namie of Bipinnaria (fig. 683),
from the symmetrical arrange-
ment of its natatory organs. The mouth («), which opens in the
middle of a transvei'se furrow, leads through an cesophagus, a', to a
large stomach, around which the body of a star-fish is developing
itself; and on one side of this mouth are observed the intestinal
tube and anus (b). On either side of the anterior portion of the
body are six or more narrow fin-like appendages, which ai-e fringed
with cilia ; and the posterior pai-t of the body is prolonged into
a sort of pedicle, bilobed towards its extremity, which also is
covered with cilia. The organisation of this larva seems completed,
and its movements through the water become very active, before
the mass at its anterior extremity presents anything of the aspect of
the star-fish, in this respect corresponding with the movements of
the Pluteus of the Echinoidea. The temporary mouth of the larva
does not remain as the permanent movith of the star-fish ; for the

3 M

Fig. 683. — Bi]jinnaria asterigera, or larva
of star-fish : a, mouth ; a', cesophagus ; b,
intestinal tube and anal orifice ; c, furrow
in which the mouth is situated ; d cV, bi-
lobed peduncle ; 1, 2, 3, 4, 5, 0, 7, ciliated
arms.

898 ECHINODEEMA

oesophagus of the latter enters on what is to l^ecome the dorsal side of
its body, and the true mouth is subsequently formed by the thinning
away of the integument on its ventral surface. The young star-fish
is separated from the Bipinnarian larva by the forcible contractions
of the connecting stalk, as soon as the calcareous consolidation of its
integument has taken place and its true mouth has been formed, but
long before it has attained the adult condition ; and as its ulterior
development has not hitherto been observed in any instance, it is not
yet known what are the sj)ecies in which this mode of evolution
prevails. The larval zooid continues active for several days after its
detachment ; and it is possible, though perhaps scarcely probable,
that it may develop another asteroid by a repetition of this process
of gemmation.

In the Bipinnaria, as in other larval zooids of the Asteroidea,
there is no internal calcareous framework ; such a framework, how-
ever, is found in the larvae of the Echinoidea and Ojyhiuroidea, of
which the form delineated in fig. 684 is an example. The embryo
issues from the ovum, as soon as it has attained, by repeated ' seg-
mentation ' of the yolk, the condition of the ' mulberi-y-mass,' and
the superficial cells of this are covered with cilia by whose agency
it swims freely through the water. So rapid are the early processes
of development that no more than from twelve to twenty-foui-
hours intervene between fecundation and the emersion of the embryo,
the division into two, four, or even eight segments taking place
within three hours after impregnation. Within a few hours after
its emersion the embryo changes from the spherical into a sub-
pyramidal form with a flattened base ; and in the centre of this
base is a depression, which gradually deepens, so as to form a mouth
that communicates with a cavity in the interior of the body which
is surrounded by a portion of the yolk-mass that has returned to the
liquid granular state. Subsequently a short intestinal tube is found,
with an anal orifice opening on one side of the body. The pyramid
is at first triangular, but it afterwards becomes quadrangular ; and
the angles are greatly prolonged round the mouth (or base), whilst
the apex of the pyramid is sometimes much extended in the opposite
direction, but is sometimes rounded off into a kind of dome (fig.
684, A). All parts of this curious body, and especially its most
projecting portions, are strengthened by a framework of thread-like
calcareous rods (e). In this condition the embryo swims freely
through the water, being proj^elled by the action of the cilia, which
clothe the foxir angles of the pyramid and its projecting arms, and
which are sometimes thickly set upon two or four projecting lobes
(y) ; and it has received the designation of Pluteus. The mouth is
usually surrounded by a sort of proboscis, the angles of which are
prolonged into four slender processes {g, g, g, g), shorter than the four
outer legs, but furnished with a similar calcareous framework.

The first indication of the production of the young Echinus from
its ' pluteus ' is given by the formation of a circular disc (fig. 684,
A, c) on one side of the central stomach (b) ; and this disc soon
presents five prominent tubercles (B), which subsequently become
elongated into tubular processes, which will form the 'sucking-

LARVAL ECHINI

899

feet ' of the adult. The disc gradually extends itself over the stomach,
and between its tu.bules the rudiments of spines are seen to protrude
(D) ; these, with the tubules, increase in length, so as to project against
the envelope of the pluteus, and to push themselves through it ; whilst,
at the same time, the original angular appendages of the pluteus
diminish in size, the
ciliary movement be-
comes less active, being
superseded by the action
of the suckers and spines,
and the mouth of the
pluteus closes up. By
the time that the disc
has grown over half of
the gastric sphere, very
little of the pluteus re-
mains, except some of
the slender calcareous
rods, and the number
of suckei's and spines
rapidly increases. The
calcareous fi-amework of
the shell at first consists,
like that of the star-
fishes, of a series of
isolated networks de-
veloped between the
cirrhi, and upon these
rest the first formed
spines (D). But they
gradually become more
consolidated, and extend
themselves over the
granular mass, so as to
form the series of plates
constituting the shell.
The mouth of the Echi-
nus (which is altogether
distinct from that of the
pluteus) is formed at
that side of the granular
mass over which the
shell is last extended ;
and the first indication
of it consists in the ap-

PiG. 684. — Embryonic development of Ecliinns : A
Fluteus larva at the time of the first appearance
of the disc ; a, mouth, in the midst of the four-
pronged proboscis ; h, stomach ; c, Echinoid disc ;
d, d, d, d, four arms of the pluteus-body ; e, cal-
careous framework ; /, ciliated lobes ; g^ g, g, g,
ciliated processes of the proboscis ; B, disc with
the first indication of the sucking-feet ; C, disc,
with the origin of the spines between the tubular
sucking-feet ; D, more advanced disc, with the feet,
g, and spines, x, projecting considerably from the
surface. (N.B. — In B, C, and D, the Pluteus is not
rejoresented, its parts having undergone no change,
save in becoming relatively smaller.)

pearance of the five cal-
careous concretions, which are the summits of the five portions of
the framework of jaws and teeth that surround it. All traces of
the original pluteus are now lost ; and the larva, which now
presents the general asjaect of an Echinoid animal, gradually
augments in size, multiplies the number of its plates, cirrhi, and

3 m2

900 ECHINODERMA

spines, evolves itself into its particular generic and specific type,
and undergoes various changes of internal structure tending to
the development of the complete organism.^

An excellent summary of the developmental history of the
several Echinoderm types, with references to the principal memoirs
which treat of it, will be found in Chapter XX. of Mr. Balfour's
' Comparative Embryology,' and in Professor A. Lang's ' Jahrbuch
der vergleichenden Anatomie,' which has been translated into English. ^
In collecting the free-swimming larvfe of Echinoderma the stick-
net should be carefully employed in the manner already described,
and the search for them is of course most likely to be successful in
those localities in which the adult forms of the respective species
abound, and on warm calm days, in which they seem to come to the
surface in the greatest numbers. The following mode of preparing
and mounting them has been kindly communicated to the Author
by Mr. Percy Sladen : — ' For killing and preserving echinodei-m zooids,
I have come to prefer either osmic acid or the picro-sulphuric mix-
ture of Kleinenberg of one-third strength. The latter, of course,
destroys all calcareous structures ; but the soft parts are preserved
in a wonderful manner. If the diluted Kleinenberg's mixture is
used, let the zooids remain in it for one or two hours; then wash
them thoroughly in 70 per cent, spirit, itntil all trace of acid is re-
moved ; then stain ; then again wash in 70 per cent, spirit, transfer
them to 90 per cent, spirit for some hours, and lastly to absolute
alcohol. Transfer them from this to oil of cloves ; and finally mount
in Canada balsam in the usual manner. If osmic acid be used, place
three or four of the liying zooids in a watch-glass of sea- water, and
add a drop of the 1 per cent, solution. They should not remain even
in this weak solution for more than a minute, and should then be
thoroughly washed in a superabundance of 35 per cent, spirit, to pre-
vent the deposit of crystals of salt consequent on the action of the
osmic acid. Then transfer the specimens to 70 per cent, spirit, and
proceed as in the other case.'

One of the most interesting to the microscopist of all Echino-
derma is the Antedon ^ (more generally known as Coonatula), or
' feather-star' (fig. 685), which is the commonest existing representa-
tive of the great fossil series of Grinoidea^ or ' lily-stars,' that were
among the most abundant types of this class in the earlier epochs of
the world's history. Like these, the young of Antedon is attached
by a stalk to a fixed base, part of which is shown in fig. 686 ; but
when it has arrived at a certain stage of development it drops ofi'from
this like a fruit from its stalk, and the animal is thenceforth free to
move through the ocean water it inhabits. It can swim with con-

' Abbreviated development, in which there is no free-swimming larva, is now
known to be more common than was once supposed : among Holothurians Cucu
■maria crocea, among Ophiuroids Ophiacaniha vivijjara, and among Echinoids
Hemiaster cavernosus may be cited as examples.

^ Those who wish to carry their study further must consult the recent memoirs
of Mr. Bury, Prof. MacBride, and Dr. Willey, and that of Dr. T. Mortensen, Die
Echinodermenlarven cltr Plankton Expedition (Kiel and Leipzig, 1898), in which
there is a systematic revision of the Echinoderm larvae already known.

■' See the Authors ' Researches on the Structure, Physiology, and Development
of Antedon rosacens,' Part I., in Phil. Trans. 1866, p. 671.

ANTEDON

901

siderable activity, but it exerts this power cliiefly to gain a suitable
place for attaching itself by means of the jointed prehensile cirrhi
put forth from the aboral (under) side of the central disc (fig. 685) ;
so that, notwithstanding its locomotive power, it is nearly as station-
ary in its free adult condition as it is in its earlier pentacrinoid
stage. The pentacrinoid larva ^ — first discovered by Mr. J. V.
Thompson, of Cork, in 1823, but originally supposed by him to be a
permanently attached Ci'inoid — forms a most beautiful object for the
lower powers of the microscope, when well preserved in fluid, and
viewed by a strong incident light (fig. 686,3) ; and a series of specimens
in different stages of development shows most cui-ious modifications
in the form and arrangement of the various component pieces
of its calcareous skeleton. In its earlier stage (fig. 686, 1) the
body is inclosed in a
calyx composed of
two circles of plates,
namely, five basals,
forming a sort of
pyramid whose apex
points downwards, and
is attached to the
highest joint of the
stem ; and five orals
superposed on these,
forming when closed
a like pyramid whose
apex points upwards,
but usually sepai'ating
to give passage to the
tentacles, of which a
circlet suri^ounds the
mouth. In this con-
dition there is no
rudiment of arms. In
the more advanced
stage shown at 2,

the arms have begun to make their appearance, and the skeleton
when carefully examined is found to consist of the following pieces,
as shown in fig. 686, l, b, b, the circlet of basals su2Dported on the
top of the stem ; »•', the circle oi first raclials, now interposed between
the basals and the orals, and alternating with both ; between two
of these is interposed the single anal plate a ; whilst they support
the second and the third radials (r^, 9-'''), from the latter of which
the bifurcating arms spring ; finally, between the second radials we
see the five orals lifted from the basals on which they originally
rested by the interposition of the first radials. In the more advanced
stage shown in fig. 686, .3, we find the highest joint of the stem

FiG. G85. — Anfedon (Comatula), or feather-star,
seen from its aboral side.

' The peutacriiioid larvse of Antedon have been found abundantly (attached to
seaweeds and zoophytes) at Millport, on the Clyde, and in Lanilash Bay, Arran; in
Kirkwall Bay, Orki;ey ; in Lough Strangford, near Belfast, and in the Bay of Cork ;
and at Ilfracombe and in Salccmbe Bay, Devon.

902 ECHINODERMA

beginning to enlarge, to form the centro-dorsal plate (2, c d), from
whicli are beginning to spring the dorsal cirrhi (cir) that serve to

Fig. 686. — Pentacrinoid larva of Aidedon. 1. Skeleton of early pentacrinoid,
under black-ground illumination, showing its component plates : h, h,
basals, articulated below to the laighest point of the stem ; r', r^, first
radials, between two of which is seen the single anal ]3late, a ; •?■-, second
radials ; ?."', third radials, giving off the bifurcating arms at their summit ;
o, o, orals. 2, 3. Back and front views of a more advanced pentacrinoid,
as seen by incident light, one of the pair of arms being cut away in fig. '6
in order to bring the mouth and its surrounding parts into view : h, h,
basals ; r^, r'^, 1^, first, second, and third radials ; a, anal, now carried
upwards by the projection of the vent, v\ o, o, orals ; cir, dorsal cirrhi,
developed from the highest joint of the stem.

ANTEDON 903

anchor the animal when it drops from the stem ; this suppoi'ts the
basccls, on which rest the Jirst radials {r^); whilst the anal plate is
now lifted nearly to the level of the second radials (r^) by the
development of the anal funnel or vent to which it is attached. The
oral plates ai-e not at first appai'ent, as they no longer occupy their
first position ; but on being carefully looked for they are found still
to form a circlet around the mouth (3, 0, o), not having undergone
any increase in size, whilst the visceral disc and the calyx in which
it is lodged have greatly extended. These oral plates finally dis-
appear by absorption ; while the hasals are at first concealed by the
great enlargement of the centro-dorsal (which finally extends so far
as to conceal the first I'adials also) ; and at last undergo metamor-
phosis into a beautiful ' rosette,' which lies between the cavity of the
centro-dorsal and that of the calyx. In common with other members
of its class, the Antedon is represented in its earliest phase of develojD-
ment by a free-swimming ' larval zijoid ' or pseudembryo, which was
first observed by Buisch, and has been since carefully studied by
Professors Wyville Thomson ' and Goette.^ This zooid has an
elongated egg-like form, and is furnished with transverse bands of
cilia and with a mouth and anus of its own. After a time, how-
ever, rudiments of the calcareous plates forming the stem and calyx
begin to show themselves in its interior ; a disc is then formed at the
posterior extremity by which it attaches itself to a seaweed (very
commonly Laniinaria) , zoophyte, or polyzoary ; the calyx containing
the true stomach, with its central mouth surrounded by tentacles, is
gradually evolved ; and the sarcodic substance of the pseudembryo,
by which this calyx and the rudimentary stem were originally in-
vested, gradually shrinks, until the young pentacrinoid presents
itself in its characteristic form and proportions.^

1 ' On the Development of Antedon rosaceus ' in Phil. Trans, for 1865, p. 513.

2 Archiv f. mikrosk. Anat. Bd. xii. p. 583.

^ The general results of the Author's own later studies of this most interesting
type (the key to the life-history of the entire geological, succession of Crinoidea) are
embodied in a notice communicated to the Proceedings of the Royal Society for
1876, p. 211, and in a subsequent note, p. 451. Of the further contributions recently
made to our knowledge of it the memoir of Dr. H. Ludwig ' Zur Anatomie der
Crinoideen ' (Leipzig, 1877), forming part of his Morpliolocjische Studienan Echino-
dernien, is the most imxDortant. Those who wish to carry further their study of the
Crinoidea should consult the two monographs by Dr. P. Herbert Carpenter in the
' Cliallencjer ' Reports.

904

CHAPTER XVII

POLYZOA AND TUNIC AT A

As in previous editions of this work the Author followed the once
prevalent habit of regarding the Polyzoa and Tunicata as structurally
allied, and as it would be necessary to entirely recast the work wei'e
the two groups to be now otherwise dealt with, and as, finally, there
is no real inconvenience or improjmety in discussing them in one
chapter, it is proposed to continue, with this word of warning, the
original arrangement of the Author. Some membei-s of both these
groups are found on almost every coast, and are most interesting-
objects for anatomical examination, as well as for observation in the
living state. ^

Polyzoa. — The group Avhich is known under this name to many
British naturalists (corresponding with that which by Continental
zoologists is designated Bryozoa) was formerly i-anked as an order of
zoophytes, and it has been entirely by microscopic study that its com-
paratively high organisation has been ascertained. The animals of
the Polyzoa, in consequence of their universal tendency to multipli-
cation by gemmation, are seldom or never found solitary, but form
clusters or colonies of various kinds ; and as each is inclosed in either
a horny or a calcareous sheath or ' cell,' a composite structure is
formed, closely corresponding with the 'polypidom' of a zoophyte,
which has been appropriately designated the polyzoary. The indi-
vidual cells of the polyzoary are sometimes only connected with each
other by their common relation to a creeping stem or stolon, as in
Laguncula (fig. 687) ; but more frequently they bud forth directly,
one from another, and extend themselves in different directions over
plane surfaces, as is the case with Flustrce, Lepralice, &c. (fig. 688) ;
whilst not unfrequently the polyzoary develops itself into an arbores-
cent structure (fig. 689), which may even present somewhat of the
density and massiveness of the stony corals. Each individual is com-
posed externally of a sort of sac, of which the outer or tegumentary
layer is either simply membranous, or is horny, or in some instances
calcified, so as to form the cell ; this investing sac is lined by a more
delicate membrane, which closes its orifice, and which then becomes
continuous with the wall of the alimentary canal ; this lies freely in
the visceral sac, floating (as it were) in the liquid which it contains.

The principal features in the structure of this group will be best
iinderstood from the examination of a characteristic example, such as
the Laguncula repens, which is shown in the state of expansion at A,
fig. 687, and in the state of contraction at B and C. The mouth is

1 For a good general account see Dr. Harmer invol.ii. of the Cambridge Natural
History, 1896.

POLYZOA

905

surrounded Ijy a circle of tubular tentacles, which are clothed by
vibratile cilia ; these tentacles, in the species we are considering,
vary from ten to twelve in niuiibei-, but in some other instances they
aremorenumerous. By
the ciliary investment
of the tentacles the
Polyzoa are at once dis-
tinguishable from those
hydroid polypes to
Avhich they l^ear a
superficial resemblance,
and -with which they
were at one time con-
foimded ; and accord-
ingly, while still ranked
among zoophytes, they
wei-e characterised as
cHiohrachiate. The ten-
tacula are seated upon
an annular disc, which
is termed the lopho-
phore^ and which foi-ms
the roof of the viscei'al
or perigastric cavity ;
and this cavity extends
itself into the interior of
the tentacula,^ through
pei'forations in the lo-
phophore, as is shown at
D, fig. 687, representing
a portion of the ten-
tacular circle on a
larger scale, a a being
the tentacula, b b their
internal canals, c the
muscles of the tentacula,
d the lophophore, and e
its retractile muscles.
The mouth situated in
the centre of the lopho-
phore, as shown at A,
leads to a funnel-shaped
cavity or pharynx, J,
which is separated from
the CBSophagus, d, by a
valve at c ; and this oeso-
phagus opens into the

e\nnnr.

Pig. 687. — Structure of Lagunciila repens (Van Bene-
den). A, xjolypide expanded ; B, j)olypide retracted;
C, another view of the same, with the visceral
apparatus in outline, that the manner in which it
is doubled on itself, with the tentacular crown and
muscular system, may be more distinctly seen :
a a, tentacula ; 6, pharynx ; c, pharyngeal valve :
d, cesophagus ; e, stomach ; /', its pyloric orifice ;
(/, cilia on its inner surface ; /;, biliary follicles lodged
in its wall ; ;', intestine ; A-, particles of excremen-
titious matter ; Z, anal orifice; ?h, testis; », ovary;
o, ova lying loose in the perivisceral cavity; p, out-
let for their discharge ; q, spermatozoa in the peri-
visceral cavity ; r, s, t, ii, v, w, x, muscles. D, por-
tion of the lophophore more enlarged : a a, tenta-
cula ; b b, their internal canals ; c, their muscles ;
(1, lophophore ; e, its retractor muscles.

stomach, e, which occu

pies a considerable part of the visceial cavity. (In the Boirprbankia

^ This communication between the tentacular and visceral cavities is denied by
Dr. Yigelius, who has recently made a careful search for it.

9o6

POLYZOA AND TUNICATA

and some other Polyzoa a muscular stomach or gizzard foi- the tri- '
turation of the food intervenes between the oesophagus and ttie
true digestive stomach.) The walls of the stomach, h, have consider-
able thickness, and the epithelial cells which line them seem to have
the character of a rudimentaiy digestive gland. This, however, is
more obvious in some other members of the group. The stomach is
hned, especially at its upper part, with vibratile cilia, as seen at c, y ;
and by the action of these the food is kept in a state of constant
agitation during the digestive process. From the upper part of the
stomach, which is (as it were) doubled upon itself, the intestine (^)
opens, by a pyloric orifice, /', which is furnished with a regular valve ;
within the intestine are seen at k particles of excrementitious matter
which are discharged by the anal orifice at I. No special cii-culating
apparatus here exists ; bvit the liquid which fills the caAity that sur-
rounds the \'iscera con-
tains the nutritive
matter which has been
prepared by the diges-
tive operation, and
which has transuded
through the walls of
the alimentary canal ;
a few corpuscles of ir-
regular size are seen to
float in it. No other
respiratory organs exist
than the tentacula, into
whose cavity the 'nutri-
tive fluid is probably
sent from the peri-
visceral cavity for aera-
tion by the current of
water that is continu-
Cells of PoZ;//0oa: A ilfos^J£,oi;7wraiIv/7ifZ- ^^ flowing over them.
Cnbriiina nqitians; C, Umhonula ''„, ^ -, . ■ r.

The production of

gemmce or buds may
take place either from the bodies of the polypides themselves, which
is what always happens when the cells are in mutual apposition,
or from the connecting stem or ' stolon,' where the cells are distinct
one from the other, as in Laguncula. In the lattei- case thei'e is
first seen a bud-like protuberance of the horny external integu-
ment, into which the soft membranous lining prolongs itself ; the
cavity thus formed, however, is not to become (as in Hydra and its
allies) the stomach of the new zooid, but it constitutes the chamber
sui-rounding the digestive viscera, which organs have their oi-igin
in a thickening of the lining membrane that projects from one side
of the cavity into its interior, and gradually shapes itself into the
alimentary canal with its tentacular appendages. Of the produc-
tion of gemmae from the polypides themselves the best examples ai'e
furnished by the Flustrui and their allies. Fi-om a single cell of the
Flustrse five such buds may be sent off", which develop themselves

Pig. 688.-
■manni ; B,
verrucosa.

POLYZOA 907

into new polypides around it ; and these in theii- turn pi'oduce buds
from their unattached margins, so as rapidly to augment the number
of cells. To this extension there seems no definite limit, and it often
happens that the cells in the central portion of the leaf-like expan-
sion of a Flustra are devoid of contents and have lost theii- vitality,
whilst the edges ai-e in a state of active gi-owth.' Independently of
their propagation by gemmation, the Polyzoa have a true sexual
generation, the sexes, however, being usually, if not invariably, united
in the same polypides. The sperm-cells are developed in a glandular
body, the testis, m, which lies beneath the base of the stomach, or
they are developed from large portions of the inner surface of the
body- wall ; when inature they rupture, and set free the spermatozoa,
q q, which swim freely in the liquid of the visceral cavity. The ova,
on the other hand, ai'e formed in an ovarium, n, which is lodged in
the membrane lining the tegumentary sheath near its outlet or is
placed near the end of the csecal process of the stomach ; the ova,
having escaped from this into the visceral cavity, as at o, are fer-
tilised by the spermatozoa which they there meet with, and are
finally discharged by an outlet at j», beneath the tentacular circle.

These creatures possess a considerable number of muscles, by
which their bodies may be projected from their sheaths, oi- drawn
within them ; of these miiscles, r, s, t, tt,^ v, iv, x, the direction and
points of attachment sufiiciently indicate the uses ; they are for the
most part retractors^ serving to draw in and double up the body, to
fold together the circle of tentacula, and to close the aperture of the
sheath, when the animal has been completely withdrawn into its
interior. The proyxtion and expansion of the animal, on the con-
trary, appeal- to be chiefly accomplished by a general pressure upon
the sheath, which will tend to force out all that can be expelled from
it. The tentacles themselves are furnished with distinct muscular
fibres, by which their sepai'ate movements seem to be produced. At
the base of the tentacular circle, just above the anal orifice, is a small
body (seen at A, a), which is a nervous ganglion ; as yet no branches
have been distinctly seen to be connected with it in this species ; but
its character is less doubtful in some other Polyzoa. Besides the
independent movements of the individual polypides, other movements
may be observed, which ai-e performed by so many of them simulta-
neovisly as to indicate the existence of some connecting agency ; and
such connecting agency, it is aifirmed by Dr. Fritz Mliller,- is fur-
nished by what he tei-ms a ' colonial nervous system.' In a Seria-
laria having a branching polyzoary that spreads itself on seaweeds
over a space of three or four inches, he states that a nervous
ganglion may be distinguished at the origin of each branch, and
another ganglion at the origin of each polypide-bud, all these
ganglia being connected together, not merely by principal trunks,

1 For further details consult Haddon ' On Budding in Polyzoa,' Quart. Journ.
Microsc. Sci. xxiii. p. 516. Embryonic fission has been observed by Harnierin Crista
and LichenoiJora.

- See his memoir in Wiegmann's Archiv, 1860, p. 311, translated in Quart. Journ.
of Microsc. Sci. n.s. vol. i. 1861, p. 300 ; Rev. T. Hincks's ' Note on the Movements of
the Vibracula in Caberea horyi, and on the supposed common Nervous System in
the Polyzoa,' Quart. Journ. Microsc. Sci. xviii. p. 7.

908 POLYZOA AND TUNICATA

but also by plexuses of nerve-fibres, which may be distinctly made
out with the aid of chromic acid in the cylindrical joints of the poly-
zoary. His views, however, are not now accepted, observers of
great histological experience maintaining that what he regards as
nerve-fibres are only connective tissue.

Of all the Polyzoa of our own coasts the MembraniporidoE, or
' sea-mats ' {Flustra, Memhranipora), are the most common ; these
present fiat expanded surfaces resembling in form those of many sea-
weeds (for which they are often mistaken), but exhibiting, when
viewed with even a low magnifying power, a most beautiful network,
which at once indicates their real character. The cells are generally
arranged on both sides, and it was calculated by Dr. Grant that as
a single square inch of an ordinary Flustra contains 1,800 such cells,
and as an average specimen presents about ten square inches of
surface, it will consist of no fewer than 18,000 polypides. The want
of transparence in the cell-wall, howeA^er, and the infrequency with
which the animal jDrojects its body far beyond the mouth of the cell,
render the sj)ecies of this genus less favourable subjects for micro-
scopic examination than are those of the Boioerhankia, a polyzoon
with a trailing stem and separated cells like those of Laguncula, which
is very commonly found clustering around the base of masses of
Flustra?. It was in this that many of the details of the organisation
of the interesting group we are considering were first studied by Dr.
A. Fai-re, who discovered it in 1837, and subjected it to a far more
minute examination than any polyzoon had previously received ; ^
and it is one of the best adapted of all the marine forms yet known
for the display of the beauties and wonders of this type of organisa-
tion. The Alcyonidium, however, is one of the most remarkable of
all the marine forms for the comparatively large size of the tentacular
crowns, these, when expanded, being very distinctly visible to the
naked eye, and presenting a spectacle of the greatest beauty when
viewed under a sufiicient magnifying power. The polyzoary of this
genus has a spongy aspect and texture, very much resembling that of
certain Alcyonian zoophytes, for which it might readily be mistaken
when its contained animals are all v/ithdrawn into their cells ; when
these are expanded, however, the aspect of the two is altogether
different, as the minute plumose tufts which then issue from the
surface of the Alcyonidium, making it look as if it were covered with
the most delicate downy film, are in striking contrast with the larger
solid-looking polypes of the Alcyonium. The opacity of the polyzoary
of the Alcyonidium renders it quite unsuitable for the examination of
anything more than the tentacular ciown and the oesophagus which
it surmounts, the stomach and the remainder of the visceral appa-
ratus being always retained within the cell. It furnishes, however,
a most beautiful object for the binocular microscope, when mounted
with all its polypides expanded. ^ Several of the fresh- water Polyzoa
are peculiarly interesting subjects for microscopic examination, alike

1 See his memoir ' On the Minute Structure of some of the Higher Forms of
Polypi,' in the Phil. Trans, for 1837, p. 387.

- Mr. J. Lomas has detected calcareous spicules in Alcyonidium gelatinosum,
and finds that they are more abundant in older than in younger colonies. See Proceed-
ings of the Llverpcol Geological Society, v. y>- 241.

GROUPS OF POLYZOA 909

on account of the I'emarkable distinctness with which the various
pai'ts of their organisation may be seen and the very beautiful man-
ner in which their ciHated tentacula are ai'ranged upon a deeply
crescentic or horseshoe-shaped loiohophore. By this j)eculiarity the
fresh-watei' Polyzoa are distingviished from the mai-ine ; and they,
with the marine Rhahdopleura, may be further distinguished by the
possession of an epistome, or moveable process above the mouth,
whence Professor Allman calls them the Pliylactolcnnata. as coan-
pared with the others, which are Gymnolcvmata., or have no epistome.
The cells of the Phylactokemata ai'e for the most part lodged in a
sort of gelatinous substratum which spreads over the leaves of
aquatic plants, sometimes forming masses of considerable size ; but
in the very curious and beautiful GristateUa the polyzoaiy is un-
attached, so as to be capable of moving freely through the watei'.^

In the marine Polyzoa, constituting by far the most numerous
division of the class, the anus opens either outside (Ectoprocta) or
within (Entoprocta) the circlet of tentacles ; the former comprise
three groups : — I. Gheilostomata, in which the mouth of the cell is
sub-terminal, or not quite at its extremity (fig. 688), is somewhat
crescentic in form, and is furnished with a movable (generally mem-
branous) lip, which closes it when the animal retreats. This includes
a large part of the species that most abound on our own coast, not-
withstanding their wide differences in form and habit. Thus the
polyzoaries of some (as Fhistra) are horny and flexible, whilst those
of others (as Eschara and Retepora) are so penetrated with calcareous
matter as to be quite rigid ; some grow as independent plant-like
structures (as Bugida and Geniellarid), whilst othei's, having a, like
arborescent form, creep over the surfaces of rocks or stones (as
Hippothoa) ; and others, again, have their cells in close apposition,
and form crusts which possess no definite figure (as is the case with
Lepralia and Memhranipora). II. The second order, Gydostoonqta,
consists of those Polyzoa which have the mouth at the termination oi
tubular calcareous cells, without any movable appendage oi- lip (fig.
689). This includes a comparatively small numbei' of geneia, of which
Grisia and Tuhulipora contain the largest proportion of the species
that occur on our own coasts. III. The distinguishing character of
the third order, Gtenostomata, is derived from the pi-esence of a comb-
like circular fringe of bristles, connected by a delicate membrane,
around the mouth of the cell, when the animal is projected from it,
this fringe being drawn in when the animal is retracted. The poly-
zoaries of this group are very various in character, the cells being
sometimes horny and separate (as in Farrella and JJovjerhaoikia),
sometimes fleshy and coalescent (as in Alcyonidium). TV . In the
Entojjrocta, which are represented by Loxosoma and Pedicellina,
and are doubtless the most archaic of the true Polyzoa. the lopho-
phore is produced upwards on the back of the tentacles, uniting
them at their base in a sort of muscular calyx, and giving to the
animal when expanded somewhat the form of an invei'ted bell, like

1 See Professor Allman'sbeautifulilfonog'rapT!- of the British FTesh-trater Pol ijzoa,
published by the Ray Society, 18.57 ; and J. Jullien, ' Monograiihie des Bryozoaires
d'eau douce,' Bull. Soc. Zool. de France, x. p. 01.

9IO

POLYZOA AND TUNICATA

that of Vorticella (fig. 593). As the Polyzoa altogether resemble
hydroid zoophytes in theii' habits, and are found in the same localities,
it is not requisite to add anything to what has already been said
respecting the collection, examination, and mounting of this very
interesting class of objects.^

A large proportioai of the Gheilostomata are furnished with very
peculiar motile appendages, which are of two kinds, avicularia and
vibracula. The avicularia or ' bird's head processes,' so named from
the striking resemblance they present to the head and jaws of a bird

(fig. 689, B), are generally,
when highly differentiated,
' sessile ' upon the angles
or margins of the cells,
that is, are attached at
once to them without the
intervention of a stalk, as
at A, being either ' pro-
jecting' or ' im.mersed ; ' but
in the genera Bugula and
Bicellaria, where they are
present at all, they are
' pedunculate,' or mounted
on foot-stalks (B). Under
one form or the other, they
are wanting in but few of
the genera belonging to
this order ; and their pre-
sence or absence furnishes
valuable characters for the
discrimination of species.
Each avicularium has two
' mandibles,' of which one
is fixed, like the upper jaw
of a bird, the other mov-
able, like its lower jaw ; the
latter is opened and closed
by two sets of muscles
which are seen in the interior of the ' head,' and between them is a
peculiar body, furnished with a pencil of bristles, which is probably a

1 For a more detailed account of the structure and classification of the marine
Polyzoa see Professor Van Beneden's ' Recherches sur les Bryozoaires de la cote
d'Ostende ' in Mem. de I' Acad. Boy. de Bruxelles, torn. xvii. ; Mr. Gr. Busk's
Catalogue of the Marine Polyzoa in the Collection of the British. Museum ; Mr.
Hincks's British Marine Polyzoa, 1880 ; and Nitsche, ' Beitrage zur Kenntniss der
Bryozoen,' in Zeitschrift f. wiss. Zool. Bde. xx. xxi. xxiv. Of the more important
recent publications we may note Mr. Busk's Reports on the Polyzoa of the Challenger
voyage ; Mr. Harmer, ' On the Structure and Development of Loxosoma ' and ' On
the Life-history of Pedicellina,' in vols. xxv. and xxvi. of the Quart. Journ. of
Microsc. Sci. ; J. Barrois, ' Recherches sur I'Embryologie des Bryozoaires,' Lille, 1877,
and other memoirs ; W. J. Vigelius, ' Morphologische Untersucliungen iiber Flustra
Membranaceo-truncata,' Biolog. Centralblatt, iii. p. 705, and Bijdragen tot de
Die7'kunde, xi. For a general account see Professor Ray Lankester's article ' Polyzoa,'
in the 9th edition of the EncyclojJCBdia Britannica, and Dr. Harmer's work already
referred to.

Fig. 689. — A, portion of Bicellaria ciliata, en-
larged ; B, one of the ' bird's head ' processes of
Bugula avicularia, more highly magnified, and
seen in the act of grasjiing another.

AYICULAKIA AND VIBRACULA 911

tactile oi'gau. being brought forwards wlien the mouth is open, so
that the bristles project beyond it, and being drawn back when the
mandible closes. The avicularia keep up a continual snapping action
during the life of the polyzoary ; and they may often be observed to
lay hold of minute woi-ms or other bodies, sometimes even closing
upon the beaks of adjacent organs of the same kind, as shown at B.
In the pedunculate forms, besides the snapping action, there is a
continual, rhythmical nodding of the head upon the stalk ; and few
spectacles are more curious than a portion of the polyzoary of
Bugtda avicularia (a very common British species) in a state of
active vitality, when viewed under a power sufficiently low to allow
a number of these bodies to be in sight at once. It is still very
doubtful what is their precise function in the economy of the animal —
whether it is to retain within the reach of the ciliary current bodies
that may serve as food, or whether it is, like the Pedicellarite of
Echini, to remove extraneous pai-ticles that may be in contact with
the surface of the polyzoary. The latter would seem to be the func-
tion of the inhracula, which are long bristle-shaped organs (fig. 688,
A), each one springing at its base out of a soi-t of cup that contains
muscles by which it is kept in almost constant motion, sweeping
slowly and carefully over the sui-face of the polyzoary, and removing
what might be injurious to the delicate inhabitants of the cells when
their tentacles are protruded.^

Tunicata. — The zoological position of the Tunicata, which has
long been a subject of great discussion, appears to be now approxi-
mately settled ; the study of their development has shown that
they are provided with a notochord, and that their nervous system
follows the course which is characteristic of what are often called
Vertebrata, but should better be called Ghordata. As the noto-
chord is always restricted to the hinder part of the body, the
Tunicata may be called Urochordata. In all (except, perhaps,
Apjjendicularia) there are distinct signs of degeneration. They have
been named Tunicata from the inclosure of their bodies in a ' tunic,'
which is sometimes leathery or even cartilaginous in its texture, and
which sometimes includes calcareous spicules, whose forms are often
very beautiful. They are often found to resemble the Polyzoa in
their tendency to produce composite structures by gemmation ; but in
their habits they are for the most part very inactive, exhibiting
scarcely anything comparable to those rapid movements of expansion
and retraction which it is so interesting to watch among the Polyzoa ;
whilst, with the exception of the Salpidce and other floating species
which ai-e chiefly found in seas warmer than those that surround our
coast, and the curious Ajypendicidaria to be presently noticed, they
are rooted to one spot during all but the earliest period of their lives.
The larger forms of the Ascidian group, which constitutes the bulk
of the class, are always solitary ; not propagating by gemmation,
except in the case of the Clavelinida?. Although of special importance

1 See Mr. G. Busk's ' Remarks on the Structure and Function of the Avicularian
and Vibracular Organs of Polyzoa ' in Trans. Microsc. Soc. ser. ii. vol. ii. 1854,
Tp. 26; and Mr. A. W. Waters, ' On the use of the Avicularian Mandible in the Deter-
mination of the Cheilostomatous Bryozoa,' Joiirn. Boy. Mic7'osc. Soc. (2), v. p. 774.

912 POLYZOA AND TUNICATA

to the comparative anatomist and the zoologist, this gi'oup does not
afford much to interest the ordinary microscopist, except in the pecu-
Har actions of its respiratory and circulatory apparatus. In common
with the composite forms of the gr-oup, the solitary Ascidians have
a large branchial sac, with fissured walls, resembling that shown in
figs. 690, B, and 692 ; into this sac water is admitted by the oral
oi-ifice, and a large proportion of it is caused to pass througli the
fissures, by the agency of the cilia with which they are fringed, into
a suri-ounding chambei-, whence it is expelled through the atriopoi-e,
or opening of the mantle. This action may be distinctly watched
through the external walls in the smaller and more transparent
species ; and not even the ciliary action of the tentacles of the Polyzoa
aifords a more beautiful spectacle. It is peculiarly remarkable in one
species that occurs on our own coasts, the Corella parallelogrmnma^
in which the wall of the branchial sac is divided into a number of
areol?e, each of them shaped into a shallow funnel ; and round one
of these funnels each branchial fissure makes two or three turns of a
spiral. When the cilia of all these spiral fissures are in active move-
ment at once, the effect is most singular. Another most remarkable
phenomenon presented throughout the group, and well seen in the
solitary Ascidian just referred to, is the alternation in the dii-ection
of the circulation. The heart, which lies at the bottom of the
l^ranchial sac, has its one end connected with the principal trunk
leading to the bod}^, and the other with that leading to the branchial
sac. At one time it will be seen that the blood flows from the
respiratory apparatus to the end of the heart in which its trunk
terminates, which then contracts so as to drive it through the sys-
temic trunk to the body at lai-ge ; but after this course has been main-
tained for a time the heart ceases to pulsate for a moment or two,
and the course is reversed, the blood flowing into the heart frotn
the body generally, and being propelled to the branchial sac. After
this reversed course has continued for some time another pause
occurs, and the first course is resumed. The length of time inter-
vening between the changes does not seem by any means constant.
It is usually stated at from half a minute to two minutes in the com-
posite forms ; bvit in the solitary Corella 2^ci'''cdlelogramma (a species
very common in Lamlash Bay, Arran), the Author has repeatedly
observed an interval of from five to fifteen minutes, and in some
instances he has seen the circulation go on for half an hour, or even
longer, without change — always, however, reversing at last.

The compoitnd Ascidians are very commonly found adherent to
seaweeds, zoophytes, and stones between the tide-marks ; and they
present objects of great interest to the microscopist, since the small
size and transparence of their bodies when they are detached fi-om
the mass in which they are imbedded not only enable their structure
to be clearly discerned without dissection, but allow many of their
living actions to be watched. Of these w^e have a characteristic
example in Amarot^ciicm proUferum, of which the form of the com-

1 See Alder in Ann. of Nat. Hist. ser. iii. vol. xi. 1863, p. 157; and Hancock in
Journ. Linn. Soc. ix. p. 333.

TUNICATA

913

posite mass and the anatomy of a single individual ai-e displayed in
fig. 690. Its clusters appear almost completely inanimate, exhibiting
no very obvious movements when
ii'iitated ; but if they be placed
when fiesh in sea-watei- a slight
pouting of the orifices will soon be
perceptible, and a constant and
energetic series of cvirrents will be
found to enter by one set and to be
ejected by the other, indicating that
ail the machinery of active life is
going on within these apathetic
bodies. In the family Polyclinidce
to which this genus belongs the
body is elongated, and may be
divided into thi-ee regions : the thorax
(A), which is chiefly occupied by the
respiratory sac ; the abdomen (B),
which contains the digestive appa-
ratus ; and the post-abdomen (C), in
which the heart and generative
organs ai-e lodged. At the summit _^_^

of the thorax is seen the oral orifice, ( f^}f\)\

c, which leads to the branchial sac e ;
this is perforated by an immense
number of slits, which allow part of
the water to pass into the space
between the branchial sac and the
muscular mantle. At h is seen the

Fig. 690. — Compound mass of AmarouciuiiiiJroliferuni with the anatomy of a
single zooid : A, thorax ; B, abdomen ; C, post-abdomen ; c, oral orifice ;
e, branchial sac; /, thoracic blood-vessel; i, atriopore; ?', projection over-
hanging it ; j, nervous ganglion ; A", oesophagus ; I, stomach surrounded by
digestive tubuli ; m, intestine ; n, anus opening into the cloaca formed by
the mantle ; o, heart ; o', pericardium ; p, ovarium ; p' , egg ready to escape ;
q, testis ; ;•, spermatic canal ; i'', termination of this canal in the cloaca.

oesophagus, which is continuous with the lower part of the pha-
ryngeal cavity ; this leads to the stomach, Z, which is suri-ounded
by glandular follicles ; and from this passes off" the intestine, m, which
terminates at n in the A"ent A current of water is continually

914 POLYZOA AND TUNICATA

(liuwn in through the mouth by the action of the ciHa of the bran-
chial sac and of the alimentary canal ; a part of this cun-ent passes
through the fissures of the branchial sac into the peribranchial
cavity, and thence into the cloaca ; whilst another portion, entering
the stomach by an aperture at the bottom of the pharyngeal sac,
passes through the alimentary canal, giving up any nutritive
materials it may contain, and carrying away with it any excre-
mentitious matter to be discharged ; and this having met the
respiratory current in the cloaca, the two mingled currents pass foi-th
togethei- by the atiial orifice, i. The long post-abdomen is principally
occupied by the large ovarium, p, which contains ova in various stages
of development. These, when matured and set free, find their way
into the cloaca, where two large ova are seen (one marked p and
the other immediately below it) waiting for expulsion. In this posi-
tion they receive the fertilising material from the testis, q. which
discharges its products by the long spermatic canal, r, that opens into
the cloaca, ?■ . At the very bottom of the post-abdomen we find the
heart, o, inclosed in its pericardium, o\ In the group we are now
considei'ing a number of such animals are imbedded together in a
sort of gelatinous mass, and covered with an integument common to
them all ; the composition of this gelatinous substance is remarkable
as including celkilose, which generally ranks as a vegetable product.
The mode in which new individuals are develojDed in this mass is by
the extension of stolons oi- creeping stems from the bases of those
previously existing ; and from each of these stolons several buds may
be put forth, eveiy one of which may evolve itself into the likeness
of the stock from which it proceeded, and may in its turn increase
and multiply after the same fashion.

In the family of Didemnians the post-abdomen is absent, the heart
and generative apparatus being placed by the side of the intestine in the
abdominal portion of the body. The zooids are frequently arranged
in star-shaped clusters, their anal orifices being all directed towards
a common vent which occujDies the centre. This shortening is still
more remarkable, however, in the family of Botryllians, whose
beautiful stellate gelatinous inci-ustations are extremely common upon
seaweeds and submerged rocks (fig. 691). The anatomy of these
animals is very similar to that of the Amaroucium already described ;
with this exception, that the body exhibits no distinction of cavities,
all the organs being brought together in one, which must be con-
sidered as thoracic. In this respect there is an evident approximation
towards the solitary species.^

This approximation is still closer, however, in the ' social ' Asci-
dians, or Glavellinidce, in which the general plan of strixcture is
nearly the same, but the zooids are simply connected by their stolons
instead of being included in a common investment ; so that their
i-elation to each other is very nearly the same as that of the poly-

1 For more special information respecting the compound Ascidians see espe-
cially the admirable monograph of Professor Milne-Edwards on that group ; Mr. Lister's
memoir, ' On the Structure and Functions of Tubular and Cellular Polypi, and of
Ascidiae,' in the Phil. Trans. 1834 ; and the article ' Tunicata,' by Professor T. Rupert
Jones, in the Gyclopcedia of Anatomy and Physiology. More recent authorities
are cited on p. 918.

TUNICaTA

915

picles of Laguncida, the chief difference being that a i-egulai- cir-
culation takes place thi-ough the stolon in the one case, such as has
no existence in the other. A better opportunity of studying the
living actions of the Ascidians can scarcely be found than that which
is afforded by the genus Pero})hora, first discovered by Mr. Lister,
which occurs not unfrequently on the south coast of England and in
the Ii-ish Sea, living attached to seaweeds, and looking like an assem-
blage of minute globules of jelly, dotted with orange and brown, and
linked by a silvery winding thread. The isolation of the body of
each zooid from that of its fellows, and the extreme transpai'ence of
its tunics, not only enable the movements of flviid within the body to
be distinctly discerned, but also allow the action of the cilia that
border the slits of the i-espiratory sac to be clearly made out. This
sac is perfoi'ated with foui- rows of narrow oval openings, through
which a portion of the water that enters its oral orifice escapes

Fig. 691. — Botryllus violaceus: A, cluster on the surface of a Fueus ;
B, portion of the same enlarged.

into the space between the sac and the mantle, and is thus dis-
charged immediately by the atrial funnel. Whatever little particles,
animate or inanimate, the current of water brings flow into the
sac unless stopped at its entrance by the tentacles, which do not
appear fastidious. The particles which are admitted usually lodge
somewhere on the sides of the sac, and then travel horizontally until
they arrive at that part of it down which the cui-rent proceeds to the
entrance of the stomach, which is situated at the bottom of the
sac. Minute animals are often swallowed alive, and have been
observed darting about in the cavity for some days, without any ap-
parent injury either to themselves or to the creature which incloses
them. In general, however, particles which are unsuited for reception
into the stomach are rejected by the sudden contraction of the mantle
(or muscular tunic), the atriopore being at the same time closed, so
that they are forced out by a powerful current through the oral
orifice. The curious alternation of the circulation that is character-
istic of the class generally may be particularly well studied in
Perophora. The creeping stalk that connects the individuals of

3n 2

gi6

POLYZOA AND TUNICATA

any group contains two distinct canals, which send off branches
into each peduncle. One of these branches terminates in the heart,
which is nothing more than a contractile dilatation of the principal
trunk ; this trunk subdivides into vessels (or rather sinuses, which are
mere channels not having proper walls of their own), of which some
ramify over the respiratory sac, branching off at each of the passages
between the oval slits, whilst others are first distributed to the
stomach and intestine, and to the soft surface of the mantle. All

these reimite so as to form
a trunk, which passes to the
peduncle and constitutes the
returning branch. Although
the circulation in the dif-
ferent bodies is brought
into connection by the com-
mon stem, yet that of each
is independent of the rest,
continuing when the current
through its own foot-stalk is
interrupted by a ligature ;
and the stream which re-
turns from the branchial
sac and the viscera is then
poured into the posterioi-
part of the heart instead
of entering the peduncle.

The development of the
Ascidians, the early stages
of which are observable
whilst the ova are still
within the cloaca of the

1?IG. 692.— Diagrammatic longitudinal section of parent, presents SOme phe-

Ascidia showing the heart, the blood-vessels, nomena of much interest

the branchial sac, the alimentary canal &c. ^ ^j microSCOpist which

from the left side : br.si., branchial siphon : ^ ".

at.si., atrial siphon; t., test; m., mantle; alone can be noticed here.
br.s., branchial sac; ]).br., peribranchial ' After the ordinary repeated

cavity; cl., cloaca; n.g , nerve ganglion; cjAo-mpntflfinn nf +bp ^-nlk

tn., tentacle; gl., neural gland; ce.a., oeso- Segmentation Ot tne } Oik,

phageal aperture ; st., stomach ; i., intestine ; whereby a ' mulberry mass '

r., rectum; a., anus; o.v., genital organs; is produced, a SOrt of ring

q.cl., genital ducts; h., heart; c.sn.. cardio- ■ • t -x j_ ^

fplanchnic vessel ; ^;.^., vessel to the test; ^^ ^^en encircling its central

portion ; but this soon
shows itself as a tapering
tail-like prolongation from
one side of the yolk, which
gradually becomes more
and more detached from
it, save at the part from
which it springs. Either
whilst the egg is still within
the cloaca, or soon after it has escaped from the vent, its envelope
bursts, and the larva escapes, and in this condition it presents veiy

t.k., terminal knob on vessel in test ; v.t'.,
vessel from the test ; v.st., vessel to the
stomach &c. ; v.m,., vessel to the mantle;
v.m'., vessel from the mantle ; d.v., dorsal
vessel ; tr., transverse vessel of branchial
sac ; I.V., fine longitudinal vessel of branchial
sac; sg., stigmata of branchial sac; v.v.,
ventral vessel ; br.c, branchio-cardiac
vessel; sp.br., splanchno-branchial vessel.
(After Prof. Herdman.)

TUNICATA 917

much the appeai-auce of a tadpole, tlie tail being straightened
out, and propelling the body freely thi-ough the watei' by its latei-al
strokes. The centre of the body is occupied by a mass of licpiid yolk,
and this is continued into the interior of three prolongations which
extend themselves from the opposite extremity, each terminating in a
sort of Slicker. After swimming about for some hours with an active
wriggling movement, the larva attaches itself to some solid body by
means of one of these suckers ; if disturbed from its position, it at
first swims about as before ; but it soon completely loses its activity,
and becomes permanently attached ; and iinportant changes manifest
themselves in its interior. The organs and tissues which constitute
the chief part of the future animal are gradually drawn back, so that
the whole of it is concentrated into one mass ; and the tail, now con-
sisting only of the gelatinous envelope, is either detached enrtire from
the body by the contraction of the connecting portion, or withers,
and is thrown off gradually in shreds. The shaping of the internal
organs out of the yolk mass takes place veiy rapidly, so that by the
end of the second day of the sedentary state the outlines of the
branchial sac and of the stomach and intestine may be traced, no
external orifices, however, being as yet visible. The pulsation of the
heart is first seen on the third day, and the foi'mation of the branchial
and anal orifices takes place on the fourth, after which the ciliary
currents ai-e immediately established through the branchial sac and
alimentary canal. The embryonic development of other Ascidians,
solitary as well as composite, takes place on a plan essentially the
same as the foregoing, a free tadpole-like larva being always produced
in the first instance with the curious exception of some species of
2£olgula}

This larval condition is represented in a very curious adult free-
swimming form, termed A'pjiendicularia, which is frequently to be
taken with the tow-net on oui- own coasts. This animal has an oval
or flask-like body, which in large specimens attains the length of
one-fifth of an inch, but which is often not more than one-fourth or
one-fifth of that size. It is furnished with a tail-like appendage
three or four times its own length, broad, flattened, and rounded at
its extremity ; and by the powerful vibrations of this appendage it is
propelled rapidly through the water. The structure of the body dif-
fers greatly from that of the Ascidians, its plan being much simpler ;
in particular, the pharyngeal sac is entii'ely destitute of ciliated
branchial fissures opening into a surrounding cavity ; but two canals,

1 The study of the development of Ascidians derived a new interest and im-
portance from the discovery, made by Kowalevsky in 1867, that their free-swimming
larvi=e present a most striking parallelism to vertebrate embryos, in exhibiting the
beginnings of a spinal marrow and a notochord ; thns bridging over the gulf that was
supposed to separate them from Invertebrata, and (when taken in connection with
the curious Ascidian affinities of Amphioxus, the lowest vertebrate at present known)
affording strong reason for belief in the derivation of the vertebrate and tunicate
types from a common original. See his memoir ' Entwickelungsgeschichte der
einfachen Ascidien ' in Mem. St. Petersh. Acad. Sci. tom. x. 1867, and the abstract
of it in Quart. Jouru. Microsc. Sci. x. n.s. 1870, p. 59 ; also Professor Haeckel's Historij
of Creation, ii. -p-p- ^52, 200. Further information will be found in cliaia. ii. of vol.
ii. of the late Professor Balfour's Comparative Embryology, and an application of the
facts of development to the philosophy of the subject in Professor Ray Lankester's
Beyeneration (London, 1880).

9l8 POLYZOA AND TUNICATA

one on either side of the entrance to the stomach, are prolonged from
it to the external surface ; and by the action of the long cilia with
which these ai-e furnished, in conjunction with the cilia of the
branchial sac, a current of water is maintained through its cavity.
From the observations of Huxley, however, it appears that the
direction of this current is by no means constant ; since, although it
usually enters by the mouth and passes out by the ciliated canals,
it sometimes enters by the latter and passes out by the former. The
caudal appendage has a central axis (notochord), above and below
which is a ribbon-like layer of muscular fibres ; a nervous cord,
studded at intervals with minute ganglia, may be traced along its
whole length. By Mertens, one of the early observers of this animal,
it. was said to be furnished with a peculiar gelatinous envelope oi-
Ilaus (house), very easily detached from the body, and capable of
being re-formed after having been lost. Notwithstanding the great
numbers of specimens which have been studied by Miiller, Huxley,
Leuckart, and Gegenbaur, none of these excellent observers has
met with this appendage ; but it has been since seen by Allman,
who describes it as an egg-shaped gelatinous mass, in which
the body is imbedded, the tail alone being free ; whilst from either
side of the central plane there radiates a kind of double fan, which
seems to be formed by a semicircular membranous lamina folded
upon itself. It was surmised by Allman, with much probability, that
this curious appendage is ' nidamental,' having reference to the
development and protection of the young ; but on this point further
observations are much needed ; and any microscopist who may meet
with Appendicularia furnished with its ' house ' should do all he can
to determine its structure and its relations to the body of the
animal.^

1 For details in respect to the structure of Appendicularia, see Huxley in Phil.
Trans, for 1851, and in Qua?-t. Journ. of Microsc. Sci. vol. iv. 1856, p. 181 ; also
Allman in the same journal, vol. vii. 1859, p. 86 ; Gegenbaur in Siebold U7id Kdlliker's
Zeitschrift, Bd. vi. 1855, p. 406 ; Leuckart's Zoologische TJntersuchungen, Heft ii.
1854; Foi's' Etudes sur les Appendiculaires ' m Arcliiv. Zool. ea;^er. tom. i. 1872,
p. 57 ; the three memoirs by H. Lohmann published in 1896. For the Tunicata
generally, see Professor T. Rupert Jones in vol. iv. of the Cyclop, of Anatomy
and- Physiology ; Professor Herdman's article, ' Tunicata,' in the 9th edition
of the Encyclopcsdia Britannica ; Mr. Alder's ' Observations on the British
Tunicata ' in Ann. of Nat. Hist. ser. iv. vol. xi. 1863, p. 153 ; and Mr. Hancock's
memoir ' On the Anatomy and Physiology of the Tunicata ' in the Journal of the
Linnean Society, vol. ix. p. 309. Great additions to our knowledge have been
made by Professor Herdman, whose rej)orts on the forms collected by H.M.S.
Challenger should be consulted, and by Professors Van Beneden and Julin (see espe-
cially their memoirs in the Archives de Biologic). See also Eoule, ' Recherches sur les
Ascidies simples des cotes de Provence,' Ann. Museum Marseilles, ii. ; Seeliger,
' Die Entwickelungsgeschichte der Socialen Ascidien,' Jenaische Zeitschr. xviii.
p. 528 ; Salensky, ' Neue Untersuchungen liber die embryonale Entwickelung der
Salpen,' Mitth. Zool. Stat. Nea2)el, iv. pp. 90, 327 ; and Ulianin, ' Die Arten des
Gattung Doliolum im Golfe von Neapel,' in the Fauna und Flora des Golfes von
Neapel, x. The above titles by no means exhaust the list of recent important memoirs
on Tunicata, but the researches of Caullery, Metcalf, Pizon, and Seeliger are beyond
the scope of this work. The last-named has commenced a systematic accomit of the
group in Bronn's Thierreich.

919

CHAPTER XVIII

MOLLUSC A AND BRACHTOPODA

The various forms of ' shell-fish,' with their ' naked ' or shell-less
allies, furnish a great abundance of objects of interest to the micro-
scopist, of which, however, the greater part may be grouped under
three heads — namely (1) the structure of the shell, which is most
interesting in the Conchifera (or Lamellibranchiata) and Brachio-
PODA, in both of which classes the shells are ' bivalve,' while the animals
difier from each other essentially in general plan of structure ; (2)
the structure of the tongiie or palate of the Gastropoda, most of which
have ' univalve ' shells, others, however, being ' naked ; ' (3) the
developmental history of the embryo, for the study of which certain
of the Gastropods present the greatest facilities. These three subjects,
therefore, will be first treated of systematically, and a few miscella-
neous facts of interest will be subjoined.

Shells of MoUusca. — These investments were formerly regarded
as mere inorganic exudations, composed of calcareous particles,
cemented together by animal glue ; microscopic examination, how-
ever, has shown that they possess a definite structure, and that this
structure pi-esents certain very remarkable variations in some of the
groups of which the molluscous series is composed. We shall first
describe that which may be regarded as the characteristic structure
.of the oi'dinaiy bivalves, taking as a type the group of Margaritacece,
which includes the Meleagrina ov ' pearl oyster ' and its allies, the
common Pinna ranking amongst the latter. In all these shells we
readily distinguish the existence of two distinct layers : an external,
of a brownish-yellow colour; and an internal, which has a pearly
or ' nacreous ' aspect, and is commonly of a lighter hue.

The structui-e of the outer layei- may be conveniently studied in
the shell of Pinna, in which it commonly pi-oj ects beyond the inner,
and there often foi'ms laminae sufiiciently thin and transparent to
exhibit its genei-al chai-acters without any ai-tificial reduction. If a
small portion of such a lamina be examined with a low magnifying
power by ti-ansmitted light, each of its surfaces will present very
much the appearance of a honeycomb ; whilst its broken edge exhibits
an aspect which is evidently fibrous to the eye, but which, when
examined under the microscope with reflected light, resembles that
of an assemblage of segments of basaltic columns (fig. 696). This
outer layer is thus seen to be composed of a vast number ot prisms,
having a tolei-ably unifoi-m size, and usually presenting an approach

920

MOLLUSCA AND BEACHIOPODA

Fig. 693. — Section of shell of Pinna, taken
transversely to the direction of its prism.

to the hexagonal shape. These ai-e arranged perpendicularly (or
nearly so) to the surface of the lamina of the shell ; so that its thick-
ness is foi^med by their length, and its two surfaces by their extremi-
ties. A more satisfactory view of these prisms is obtained by grinding,
down a lamina imtil it possesses a high degree of transparence, the

prisms being then seen (fig.
693) to be themselves com-
posed of a veiy homogeneous
substance, but to be sepa-
i-ated by definite and
strongly marked lines of
division. When such a
lamina is submitted to the
action of dilute acid, so as
to dissolve away the car-
bonate of lime, a tolerably
firm and consistent mem-
brane is left, which exhibits
the prismatic structure j ust
as perfectly as did the
original shell (fig. 694), its
hexagonal divisions bearing a strong resemblance to the walls of
the cells of the pith or bark of a plant. By making a section of the
shell perpendicularly to its surface, we obtain a view of the prisms
cut in the direction of their length (fig. 695) ; these ai-e frequently
seen to be marked by delicate transverse stri* (fig. 696) closely re-
sembling those obsei'vable on the prisms of the enamel of teeth, to
which this kind of shell-sti-ucture may be considered as bearing a
very close i-esemblance, except as regards the mineralising ingredient.

If a similar section be de-
calcified by dilute acid, the
membranous residuum will
exhibit the same resem-
blance to the walls of pris-
matic cells viewed longitu-
dinally, and will be seen to
be more or less regulai'ly
marked by the ti-ansverse
strise just alluded to. It
sometimes happens in re-
cent but still more com-
monly in fossil shells, that
the decay of the animal
membrane leaves the con-
tained prisms without any connecting medium ; as they are then
quite isolated, they can be readily detached one from another ; and
each one may be observed to be marked by the hke striations,
which, when a sufficiently high magnifying power is used, are seen
to be minute grooves, apparently resulting from a thickening of the
intermediate wall in those situations. These appearances seem best
accounted for by supposing that each is lengthened by successive

Fig. 694. — Membranous basis of the same.

STKUCTUEE OF SHELLS

921

additions at its base, the lines of junction of which coi-respond with
the transvei'se stiiation ; and this view coi'responds well with the
fact that the shell-menibi'ane not unfrequently shows a tendency to
split into thin lamina? along the lines of striation, whilst we occa-
sionally meet with an excessively thin natural lamin;i lying l»etween
the thicker prismatic layers, with
one of which it would have
probably coalesced but for some
accidental cause which preserved
its distinctness. That the prisms
are not formed in their entii'e
length at once, but that they are
progressively lengthened and
consolidated at their lower ex-
tremities, would appear also
from the fact that where the
shell presents a deep colour (as
in Pinna nigrina) this colour
is usually disposed in distinct
strata, the outer portion of each layer being the part most deeply
tinged, whilst the inner extremities of the prisms are almost colour-
less.

This ' prismatic ' arrangement of the cai-bonate of lime in the
shells of Pinna and its allies has been long familiar to concholo-
gists, and regarded by them as the result of crystallisation. When

Fig. 695. — Section of the shell of Pinna
hi the direction of its prisms.

Fig. 696. — Oblique section of prismatic shell-substance.

it was first more minutely investigated by Mr. Bowei'bank' and the
Author,^ and was shown to be connected with a similar ari'angement
in the membranous residuum left after the decalcification of the shell -
substance by acid, microscopists genei-ally ^ agreed to i-egai'd it as a
' calcified epidermis,' the long prismatic cells being supposed to be
formed by the coalescence of the epidermic cells in piles, and gi\'ing

' ' On the Structure of the Shells of Molluscous and Conchiferous Animals,' in
Trans. Microsc. Soc. ser. i. vol. i. 1844, j). 123.

- ' On the Microscopic Structure of Shells ' in Reports of British Association for
1844 and 1847.

■' See Mr. Quekett's Histological Catalogue of the College of Surgeons' Museum
and his Lectures on Histology, vol. ii.

922 MOLLUSC A AND BRACHIOPODA

their shape to the deposit of carbonate of lime formed within them.
The progress of inquiry, however, has led to an important modifica-
tion of this intei-pretation, the Author being now disposed to agree
with Huxley ^ in the belief that the entire thickness of the shell
is formed as an excretion from the surface of the epidermis, and
that the horny layer which in ordinary shells forms their external
envelope or ' periostracum,' ^ being here thrown out at the same time
with the calcifying material, is converted into the likeness of a
cellular membrane by the pressure of the prisms that are formed by
crystallisation at regular distances in the midst of it. The pecu-
liar conditions under which calcareous concretions form themselves
in an organic matrix have been carefully studied by Mr. Rainey
and Dr. W. M. Ord, of whose researches some account will be given
hereafter.

The internal layer of the shells of the Margaritacece and some
other families has a ' nacreous ' or iridescent lustre, which depends
(as Sir D. Brewster has shown '^) upon the striation of its surface
with a series of grooved lines, which usually run nearly parallel to
each other (fig. 697). As these lines are not obliterated by any
amount of polishing, it is obvious that their presence depends upon
something peculiar in the texture of this substance, and not upon
any mere superficial arrangement. When a piece of the nacre (com-
monly known as ' mother of-pearl ') of the Meleagrina or ' pearl-oyster '
is carefully examined, it becomes evident that the lines are produced
by the cropping out of laminae of shell situated more or less obliquely
to the plane of the surface. The greater the clip of these laminfe, the
closer will theii- edges be ; whilst the less the angle which they make
with the surface, the wider will be the interval between the lines.
When the section passes for any distance in the plane of a lamina, no
lines will pi-esent themselves on that space. And thus the appearance
of a section of nacre is such as to have been aptly compared by Sir J.
Herschel to the surface of a smoothed deal board, in which the woody
layers are cut perpendicularly to their surface in one part, and nearly
in their plane in another. Sir D. Brewster (loc. cit.) appears to have
supposed that nacre consists of a multitude of layers of carbonate of
lime alternating with animal membrane, and that the presence of
the grooved lines on the most highly polished surface is due to the
weai-ing away of the edges of the animal laminae, whilst those of the
hard calcareous laminte stand out. If each line upon the nacreous
surface, however, indicates a distinct layer of shell-substance, a very
thin section of ' mother-of-pearl ' ought to contain many hundred
lamina?, in accordance with the number of lines upon its surface,
these being frequently no more than ^Q-^th of an inch apart. But
when the nacre is treated with dilute acid, so as to dissolve its cal-

1 See his article, ' Tegumentary Organs,' in Cyclopcedia of Anatomy and
Physiology, supplementary volume, p-p. 489-492.

- The periostracum is the yellowish-brown membrane covering the surface of
many shells, which is often (but erroneously) termed their epidermis.

5 Phil. Trans. 1814, p. 397.— The late Mr. Barton (of the Mint) succeeded in
producing an artificial iridescence on metallic buttons by drawing closely approxi-
mating lines with a diamond point upon the surface of the steel die by which they
were struck.

STEUCTURE OF SHELLS

923

careous poi'tion, no such repetition of membranous layers is to be
found ; on the contrary, if the j)iece of nacre be the pi-oduct of one
act of shell formation, there is but a single layer of membrane. This
layer, however, is found to present a more or less folded or plaited
arrangement, and the lineation of the nacreous surface may perhaps
be thus accounted for. A similar arrangement is found in pearls,
which are rounded concretions projecting from the inner surface of
the shell of Meleagrina, and possessing a nacreous structure corre-
sponding to that of ' mother-of-pearl.' Such concretions are found in
many other shells, especially the fresh-water miissels, Unio and Ano-
don ; but these are usually less remarkable for their pearly lustre ;
and, when formed at the edge of the valves, they may be partly oi-
even entirely made up of the prismatic substance of the external
layer, and may be consequently altogether destitute of the pearly
character.

In all the genei'a of the Margaritacecc, we find the external layer

Pig. 697. — Section of nacreous lining of shell of Meleagrina
margaritifera (pearl-oyster) .

of the shell prismatic, and of considerable thickness, the internal
layer being nacreous. But it is only in the shells of a few families
of bivalves that the combination of organic with mineral components
is seen in the same distinct form ; and these families are for the most
part nearly allied to Pinna. In the Unionidce (or ' fresh-water
mussels ') nearly the whole thickness of the shell is made up of the
internal or ' nacreous ' layer ; but a uniform stratum of jDrismatic
substance is always found between the nacre and the periostracum,
really constituting the inner layer of the latter, the outer being
simply horny. In the Ostreacea! (or oyster tribe), also, the greater
part of the thickness of the shell is composed of a ' sub-nacreous '
substance, representing the inner lajei- of the shells of Margaritacese,
its successively formed lamime, however, having very little adhesion
to each other ; and every one of these laminae is bordered at its free
edge by a layer of the prismatirc substance distinguished by its

924

MOLLUSCA AND BRACHIOPODA

fe^Xwil/ ^^j^x> 1 l///^^>^^^^^\ ll/l/'f,\^

bi'ownish-yellow colour. In these and some other cases a distinct
membranous residuum is left after the decalcification of the prismatic
layer by dilute acid ; and this is most tenacious and substantial
where (as in the JIargaritacece) there is no proper periostracum.
Generally speaking, a thin prismatic layer may be detected upon the
external surface of bivalve shells, where this has been protected
by a periostracum, or has been prevented in any other manner
from iindergoing abrasion ; thus it is found pretty generally in
Chama, Trigonia, and Soleii, and occasionally in Anoniia and Pecten.
In many other instances, however, nothing like a cellular struc-
ture can be distinctly seen in the delicate membrane left after decal-
cification ; and in such cases the animal basis bears but a very small
proportion to the calcareous substance, and the shell is usvially ex-
tremely hard. This hardness ap-
pears to depend upon the mineral
arrangement of the cai-bonate of
lime ; for whilst in the prismatic
and ordinai-y nacreous layer this
has the crystalline condition of
calcite, it can be shown in the hard
shell of Pholas to have the arrange-
ment of arragonite. the difierence
between the two being made evi-
dent by polarised light. A very
curious appearance is pi-esented by
a section of the large hinge-tooth
of Mya arenaria (fig. 698), in
which the carbonate of lime seems
to be deposited in nodules that
possess a crystalline structure re-
sembling that of the mineral
termed ivavellite. Apjjroaches to
this curious ari-angement are seen in many other shells.

There are several bivalve shells which almost entirely consist of
what may be termed a suh-nacreous substance, their polished
surfaces being marked by lines, but these lines being destitute of
that regularity of arrangement which is necessary to produce the
iridescent lustre. This is the case, for example, with most of the
Pectiniclce (or scallop tribe), also with some of the Mytilacece (or
mussel tribe), and with the common Oyster. In the internal layer
of by far the greater number of bivalve shells, howevei-, there is not
the least approach to the nacreous aspect ; nor is there anything
that caii be described as definite structure ; and the residuum
left after its decalcification is usuall}^ a sti-uctureless ' l:)asement
membrane.'

The ordinary account of the mode of growth of the shells of
bivalve Mollusca — that they ai-e pi'ogressively enlarged by the depo-
sition of new laminae, each of which is in contact with the internal
surface of the preceding, and extends beyond it — does not express
the whole truth ; for it takes no account of the fact that most shells
are composed of two layers of very difi:erent textiii-e, and does not

Fig. 698.— Section of hinge-tootli of
Mya are7iaria.

SHELLS OF LAMELLIBRANCHS

925

specify whether both these layei's ai'e thus formed by the entii'e
.surface of the ' mantle ' whenever the shell has to be extended, or
whethei- only one is produced. An examination of fig. 699 will
clearly show the mode in which the operation is eifected. This figui'e
represents a section of one of the valves of Unio occidens, taken per-
pendicularly to its surface, and passing from the margin or lip (at
the left hand of the figure) towards the hinge (which would be at
some distance beyond the right). This section brings into view the
two substances of which the shell is composed, travei'sing the outer
or prismatic layer in the direction of the length of its prisms, and
passing through the nacreous lining in .such a manner as to bring
into view its numerous laminte, separated by the lines a a', h h\ c c',
&c. These lines evidently indicate the successive formations of this
layer, and it may be easily shown by tracing them towards the
hinge on the one side and towards the margin on the other, that at
every enlargement of the .shell its whole interior is lined by a new
nacreous lamina in immediate contact with that which preceded it.

Fig. 699. — Vertical section of the lip of one of the valves of the
shell of Unio : a, b, c, successive formations of the outer
prismatic layer ; «', &', c', the same of the inner nacreous layer.

The number of such laminte, therefore, in the oldest part of the shell
indicates the number of enlargements which it has imdergone. The
outer or prismatic layei- of the growing shell, on the other hand, is
only formed where the new structure projects beyond the margin of
the old ; and thus we do not find one layer of it overlajjping another
except at the lines of junction of two distinct formations. When the
shell has attained its full dimensions, however, new laminae of both
layers still continue to be added, and thus the lip becomes thickened
by successive formations of prismatic structure, each being applied
to the inner surface of the preceding, instead of to its free margin.
A like arrangement may be well seen m the Oi/sier, with this diffei--
ence, that the successive layers have but a comparatively slight
adhesion to each other. ^

The shells of Terehrahdch and of most other BracMopods ai-e
distinguished by peculiarities of structure which differentiate them
from those of the Mollusca. When thin sections of them are
microscopically examined, they exhibit the appearance of long flat-
tened prisms (fig. 700, A, 6), which are arranged with such obliquity

1 The most important recent work on the shells of Lamellibranchs is that of
the lately deceased P. Bernard ; see Bull. Soc. Geol. France, vols, xxiii. and xxiv.

926

MOLLUSCA AND BE,ACHI0P0DA

that their rounded extremities crop out upon the inner surface of the
shell in an imbricated (tile-like) manner [a). All true Tpyrehratulidie,
both recent and fossil, exhibit another very remarkable peculiarity ;
namely, the -perforation of the shell by a lai-ge number of canals.

•V^:^ h

^

-^

Fig. 700. — A, internal surface, «, and oblique section, &, of shell of Waldheimia
azistralis ; B, external surface of the same.

which generally pass nearly perpendiculai-ly from one surface to the
other (as is shown in vertical sections, fig. 701), and terminate inter-
nally by open oiifices (fig. 700, A), whilst externally the}^ are covered

by the periostracum (B). Their
diameter is greatest towards
the external surface, where
they sometimes expand sud-
denly, so as to become ti'um-
23et-shaped ; and it is usually
narrowed rather suddenly
when, as sometimes happens,
a new internal layer is formed
as a lining to the preceding
(fig. 701, A, d d). Hence the
diameter of these canals, as
shown in difiierent transverse
sections of one and the same
shell, w^ill vai-y according to
heimia australis, showing at'T^he cLnak ^he part of its thickness which
opening by large trumpet-shaped orifices the section happens to tra-
on the outer surface, and contracting at verse. The shells of different
d, d into narrow tubes ; and showing at B • f ,.pTfnrntprl ni-rtrhin

a bifurcation of the canals. species Ol peiloiatecl m aclllO

pods, however, present very
striking diversities in the size and closeness of their canals, as shown
by sections taken in corresjaonding parts ; three examples of this
kind are given for the sake of comparison in figs. 702-704. These
canals are occupied in the living state by tubular prolongations of
the mantle, whose inteiior is filled with a fluid containing minute
cells and granules, which, from its corresponding in appeai-ance with
the fluid contained in the great sinuses of the mantle, may perhaps

SHELLS OF BEACHIOPODA

927

be considered to be the animars blood. Of their special function in
the economy of the animal it is difficult to form any probalile idea ;
but it is intei-esting to remark (in connection with the hypothesis of
a relationship between Brachiopods and Polyzoa) that they seem to
have their pai'allel in extensions of the perivisceral cavity of many
species of Fhistra, Eschara, Lepralia, &c., into passages excavated in
the walls of the cells of the polyzoary. Professor SoUas ' finds in
the centre of these prolongations an axial fibre which can be traced
backwards to the nerve-cells of the mantle ; at the distal end is a
terminal cell which is connected by a fibril with the axial fibre, and
is covered externally by a transparent chitinous layer ; save for the
absence (or the unproved presence) of pigment cells we should be
justified in regai-ding the processes as organs which are sensitive to
luminous impressions.

In the family Rhynchonellidce, which is represented by only
six recent species, but which contains a very large jDroportion of

Fig. 702. Fig. 703.

Pig. 702. — Horizontal section of shell of Terebratula bullata (fossil, Oolite).
Fig. 703. „ „ Megerlia lima (fossil, Chalk).

Fig. 704. „ ,, Spij-iferina rostrata (Triassic).

fossil Brachiopods, these canals ai-e almost entirely absent ; so
that the uniformity of their presence in the Terehratulidce, and their
general absence in the Rhynchonellidce, supply a chai-acter of
great value in the discrimination of the fossil shells belonging
to these two groups respectively. Great caution is necessary,
however, in applying this test ; mere surface 'markings cannot he
relied on ; and no statement on this point is worthy of reliance
which is not based on a microscopic examination of thin sections of
the shell. In the families Spiriferidm and Strophonnenidce, on the
other hand, some species possess the perforations, whilst others are
destitute of them ; so that their pi-esence or absence there serves only
to mark out subordinate groups. This, however, is what holds good
in regard to characters of almost every description in other depai-t-
ments of natural history ; a character which is of fundamental
importance from its close relation to the general plan of organisation
in one group being, from its want of constancy, of far less account
in another. 2

1 Froc. Boy. Dublin Soc. v. 318.

2 For a particular account of the Author's researches on this group see his memoir
on the subject, forming part of the introduction of Mr. Davidson's Monograph of the

928 MOLLUSCA AND BEACHIOPODA

There is not by any means the same amount of diversity in the
structure of the shell in the class of Gastropods^ a certain typical
plan of construction being common to by far the greatei' numbei- of
them. The small proportion of animal matter contained in most of
these shells is a very marked feature in their character, and it
serves to rendei- other features indistinct, since the residuum left
after the removal of the calcareous matter is usually so imperfect as
to give no clue whatever to the explanation of the aj^pearances shown
by sections. Nevertheless, the structure of these shells is by no
means homogeneous, but always exhibits indications, more or less
clear, of a definite arrangement. The ' porcellanous ' shells are com-
posed of three layers, all presenting the same kind of structure", but
each difiering from the others in the mode in which this is disposed.
For each layer is made up of an assemblage of thin laminae placed
side by side, which separate one from another, apparently in the
planes of rhomboidal cleavage, when the shell is fractured ; and, as
w^as first pointed out by Mr. Bowerbank, each of these laminae con-
sists of a series of elongated spicules (considered by him as prismatic
cells filled with carbonate of lime) lying side by side in close apposi-
tion ; and these series are disposed alternately in contrary directions,
so as to intei'sect each other nearly at right angles, though still
lying in parallel planes. The direction of the planes is different,
however, in the three layers of the shell, bearing the same relation
to each other as have those three sides of a cube which meet each
other at the same angle ; and by this arrangement, which is better
seen in the fractured edge of the Cyjjrcea or any similar shell than
in thin sections, the strength of the shell is greatly augmented. A
similar arrangement, obviously answering the same purpose, has
been shown by the late Sir John Tomes to exist in the enamel
of the teeth of Rodentia, and by Professor Rolleston in that of the
elephant.

The principal departures from this plan of structure are seen in
Patella^ Chiton, Haliotis, Turho and its allies, and in the ' naked '
Gastropods, many of which last, both terrestrial and marine, have
some rudiment of a shell. Thus in the common slug, Limax rvfus,
a thin oval plate of calcareous texture is found imbedded in the
shield-like fold of the mantle covering the fore part of its back ; and
if this be examined in an early stage of its growth it is found to
consist of an aggregation of minute calcareous nodules, generally
somewhat hexagonal in form, and sometimes quite transparent,
whilst in othei' instances it presents an appearance closely resembling
that delineated in fig. 698. In the epidermis of the mantle of some
species of Boris, on the other hand, we find long calcareous spicules,
generally lying in parallel directions, but not in contact with each
other, giving firmness to the whole of its dorsal portion ; and these
are sometimes covered with small tubercles, like the spicules of

British Fossil BracMopoda, published by the Palseontographical Society. A very
remarkable example of the importance of the presence or absence of the perforations
in distinguishing shells whose internal structure shows them to be generically dif-
ferent, whilst from their external conformation they would be supposed to be not
only qenerlcaUij but sjyecifically identical, will be found in the An7i. Nat. Hist.
ser. ii'i. vol. xx. i867, p. 68.

SHELLS OF MOLLUSCA 929

Gorgonia. They may be separated from the soft tissue in which
they are imbedded by means of caustic potash ; and when treated
with dihite acid, whereby the calcareous matter is dissolved away,
an organic basis is left, retaining in some degree the form of the
original spicule. This basis seems to be a cell in the earliest stage of
its formation, being an isolated particle of protoplasm without wall
or cavity, and the close correspondence between the appearance pre-
sented by thin sections of various univalve shells, a,nd the forms of
the spicules of Doris, seems to justify the conclusion that even the
most compact shells of this group are constructed out of the like
elements, in a state of closer aggregation and more definite arrange-
ment, with the occasional occurrence of a layer of more spheroidal
bodies of the same kind, like those forming the vestigial shell of
Limax.

The structure of shells generally is best examined by making
sections in different planes as nearly parallel as may be possible to
the surfaces of the shell, and other sections at right angles to these ;
the former may be designated as horizontal, the latter as vertical.
Nothing need here be added to the full directions for making such
sections which have already been given. Many of them are beautiful
and irLteresting objects for the polariscope. Much valuable informa-
tion may also be derived from the examination of the surfaces pre-
sented iyj fracture. The membranous residua left after the decalci-
fication of the shell by dilute acid may be mounted in weak spirit or
in Goadby's solution.

The animals composing the class of Cephalopoda (cuttle-fish and
nautilus tribe) are for the most part without shells ; and the
structure of the few that we meet with in the genera Nautilus, Argo-
nauta {' paper nautilus '), and Bpirida does not present any joeculi-
arities that need here detain us. The rudimentary shell or sepiostaire
of the common cuttle-fish, however, which is frequently spoken of
as the ' cuttle-fish bone,' exhibits a very beautiful and remarkable
structure, such as causes sections of it to be very interesting micro-
scopic objects. The outer shelly portion of this body consists of
horny layers, alternating with calcified layers, in which last may be
seen an hexagonal arrangement somewhat corresponding with that
shown in fig. 698. The soft friable substance that occupies the hollow
of this boat-shaped shell is formed of a number of delicate calcareous
plates running across it from one side to the other in parallel
directions, but separated by intervals several times wider than the
thickness of the plates ; and these intervals are in great part filled
up by what appear to be fibres or slender pillars passing from one
plate or floor to another. A more careful examination shows,
however, that, instead of a large number of detached pillars, there
exists a comparatively small number of very thin sinuous laminae,
which pass from one surface to the other, winding and doubling upon
themselves, so that each lamina occupies a considerable space. Their
precise arrangement is best seen by examining the parallel plates,
after the sinuous laminse have been detached from them, the lines
of junction being distinctly indicated upon these. By this arrange-
ment each layer is most efiectually supported by those with which

3o

930

MOLLUSCA AND BEACHIOPODA

Palate of Cephalophorous

sometimes referred to under

it is connected above and below, and the sinuosity of the thin
intervening laminse, answering exactly the same purpose as the
' corrugation ' given to iron plates for the sake of diminishing their
flexibility, adds greatly to the strength of this cvirious texture,
which is at the same time lightened by the large amount of open
space between the parallel plates that intervenes among the sinu-
osities of the laminae. The best method of examining this structure
is to make sections of it with a sharp knife in various directions,
taking care that the sections are no thicker than is requisite foi-
holding together ; these may be mounted on a black ground as
opaque objects, or in Canada balsam as transparent objects, under
which last aspect they furnish very beautiful objects for the polari-
scope.

Molluscs. — The organ which is
this designation, and sometimes
as the 'tongue,' is one of a
very singular nature, and
cannot be likened to either
the tongue or the palate of
higher animals ; it is best to
call it by its distinctive name
' odontophore.' For it is a
tube that passes backwards
and downwards beneath the
mouth, closed at its hinder
end, whilst in front it opens
obliquely upon the floor of

the mouth, being (as it were)
S^ie; 705.-Portaon of the left half of the palate I -^ ^^^| ^^^j O^,^ gO ^g

of Heiix iiortensts, the rows of teeth near n i n j. j?

the edge separated from each other to show to lorm a nearly flat surtace.

their form. On the interior of the tube,

as well as on the flat expan-
sion of it, we find numerous transverse rows of minute teeth, which
are set upon flattened plates, each principal tooth sometimes
having a basal plate of its own, whilst in other instances one plate
carries several teeth. Of the former arrangement we have an
example in the palate of many terrestrial Gastropods, such as the
snail [Helix) and slug (Lionax), in which the number of plates in
each row is very considerable (figs. 705, 706), amounting to 180
in the large garden slug [Limax maximus) ; whilst the latter prevails
in many marine Gastropods, such as the common whelk [Buccimmi
undatum), the palate of which has only three plates in each row, one
bearing the small central teeth, and the two others the large lateral
teeth (fig. 709), The length of the palatal tube and the number of
rows of teeth vary greatly in diflferent species. Generally speaking,
the tube of the terrestrial Gastropods is short, and is contained
entirely within the nearly globular head ; but the rows of teeth
being closely set togethei- are usually very numerous, there being
frequently more than 100, and in some species as many as 160 or
170 ; so that the total number of teeth may mount up, as in Helix
pomatia, to 21,000, and in Limax maximus to 26,800. The trans-

PALATES OF GASTEOPODA

931

Fig. 706. — Palate of Syalinia cellaria.

verse rows are usually more or less curved, as shown in fig. 706,
whilst the longitudinal rows are quite straight, and the curvature
takes its departure on each side from a central longitudinal row, the
teeth of which are symmetrical, whilst those of the lateral portions
of each transverse row present
a modification of that symmetry,
the prominences on the inner
side of each tooth being sup-
pressed, whilst those on the outer
side are increased ; this modifica-
tion may be observed to augment
in degree as we pass from the
central line towards the edges.

The palatal tube of the
marine Gastropods is generally
longer, and its teeth larger,
and in many instances it extends

far beyond the head, which may, indeed, contain but a small
part of it. Thus in a common limpet {Patella) we find the principal
part of the tube to lie folded up, but perfectly free, in the abdominal
cavity, between the greatly elongated intestine and the muscular
foot ; and in some species its length is twice or even three times as
great as that of the entire animal. In a large proportion of cases
these palates exhibit a very marked separation between the central
and the lateral portions (figs.
707, 708), the teeth of the cen-
tral band being frequently small
and smooth at their edges,
whilst those of the lateral are
large and serrated. The palate
of Trochus zizyjyhinus, repre-
sented in fig. 707, is one of the
most beautiful examples of this
form, not only the large teeth
of the lateral bands, but the
delicate leaf-like teeth of the
central portion having their
edges minutely serrated. A yet
more complex type, however, is
found in the palate of Haliotis,
in which there is a central band
of teeth having nearly straight E'ig. 707.— Palate of Trochus zizyphinus.
edges instead of points ; then, on

each side, a lateral band consisting of large teeth shaped like those
of the shark ; and beyond this, again, another lateral band on either
side, composed of several rows of smaller teeth. Very curious
difierences also present themselves among the different species of
the same genus. Thus in Doris jnlosa the central band is almost
entirely wanting, and each lateral band is formed of a single row
of very large hooked teeth, set obliquely like those of the lateral
band in fig. 707 ; whilst in Doris tuberculata the centi'al band is the

3o2

932

MOLLUSCA AND BEACHIOPODA

part most developed, and contains a number of rows of conical teeth,
standing almost perpendicularly, like those of a harrow (fig. 708).

Many other varieties might be described did space permit ; but
we must be content with adding that the form and arrangement of
the teeth of these ' palates ' afford characters of great value in classi-
fication, as was first pointed out by Professor Loven (of Stockholm)
in 1847, and has been since very strongly urged by Dr. J. E. Gray,
who considers that the structure of these organs is one of the best
guides to the natural affinities of the species, genera, and families of
tliis group, since any important alteration in the form or position of
the teeth must be accompanied by some corresponding peculiarity in
the habits and food of the animal.^ Hence a systematic examination
and delineation of the structure and arrangement of these organs, by
the aid of the microscope and camera lucida, would be of the greatest
service to this department of natural history. The short thick tube

of Limax and other terrestrial
Gastropods appears adapted for
the trituration of the food pre-
viously to its passing into the
oesophagus ; for in these animals
we find the roof of the mouth
furnished with a large strong
horny plate, against which the
flat end of the tongue can work.
On the other hand, the flattened
portion of the palate of Bucci-
mlm (whelk) and its allies is
used by these animals as a file,
with which they bore holes
through the shells of the molluscs
that serve as their prey ; this
they are enabled to efiect by everting that part of the proboscis-
shaped mouth whose floor is formed by the flattened part of the
tube, which is thus brought to the exterior, and by giving a kind of
sawing motion to the organ by means of the alternate action of
two pairs of muscles — a protractor and a retractor — which put
forth and draw back a pair of cartilages whereon the tongue is
supported, and also elevate and depress its teeth. The use of the
long blind tubular part of the palate in these Gastropods is that
of a ' cavity of reserve,' from which a new toothed surface may be
continually supplied as the old one is worn away- — somewhat as the
front teeth of the rodents are constantly being regenerated from the
surface of the pulps which occupy their hollow conical bases — as fast
as they are rubbed down at their edges, or as a nail is constantly
being worn away at its free end, and fashioned anew in its
' bed.'

The preparation of these palates for the microscope can, of course,
be only accomplished by carefully dissecting them from their attach-
ments within the head ; and it will be also necessary to remove the
membrane that forms the sheath of the tube, when this is thick
1 Ann. Nat. Hist, ser. ii, vol, x. 1852, p. 413.

Fig. 708. — Palate of Doris tuberciolata.

DEVELOPMENT OF MOLLUSC A

933

Fig. 709.— Palate of Bucci-
num undatum as seen iinder
polarised light.

enough to interfere with its transparence. The tube itself should be
slit up with a pair of fine scissors through its entire length, and
should be so opened out that its expanded
surface may be a continuation of that
which forms the floor of the mouth. The
mode of mounting it will depend upon the
manner in which it is to be viewed. For
the ordinary purposes of microscopic ex-
amination no method is so good as mount-
ing in fluid, either weak spirit or Goadby's
solution answering very well. But many
of these palates, especially those of the
marine Gastropods, become most beautiful
objects for the polariscope when they are
mounted in Canada balsam, the form
and arrangement of the teeth being very
strongly brought out by it (fig. 709), and
a gorgeous play of colours being exhibited
when a selenite plate is placed behind the
object, and the analysing prism is made to
rotate.^

Development of Molluscs. — Leaving to
the scientific embryologist the large field of study that lies open to
him in this direction,^ the ordinary microscopist will find much to
interest him in the observation of certain special phenomena of
which a general account will be here given. Attached to the gills of
fresh- water mussels {Unio and Anoclon) there are often found in the
spring or early summer minute bodies which, when first observed,
were described as parasites, under the name of Glochidia, but are
now known to be their own progeny in an early phase of develop-
ment. When they are expelled from between the valves of their
parent, they attach themselves in a peculiar manner to the fins and
gills of fresh- water fish. In this stage of the existence of the young
Anoclon, its valves are provided with curious barbed or serrated
hooks (fig. 710, A), and are continually snapping together, until
they have inserted their hooks into the skin of the fish, which seems
so to retain the barbs as to prevent the reopening of the valves. I4
this stage of its existence no internal organ is definitely formed,
except the strong ' adductor ' muscle (aacl) which draws the valves
together, and the long, slender byssus-filainent {hy) which makes
its appearance while the embryo is still within the egg-mem-
brane, lying coiled up between the lateral lobes. The hollow of
each valve is filled with a soft granular-looking mass, in which
are to be distinguished what are perhaps the rudiments of the

^ For additional details on the organisation of the palate and teeth of the
Gastropod molluscs, see Mr. W. Thomson in Cyclop. Anat, and Physiol, vol. iv.
pp. 1142, 1143, and in Ann. Nat. Hist. ser. ii. Tol. vii. p. 86; Professor Troschel, Das
Gebiss der SclinecheJi, Berlin, 1856-79; A. Riicker, ' Ueber die Bildung der Eadula
bei Helix pomatia,' JBericht oherliess. Gesellsch. Giessen, xxii. p. 209 ; P. Geddes, ' On
the Mechanism of the Odontophore in certain Molluscs,' Trans. Zool. Soc. x. p. 485.

2 See Balfour's Com2Kbrative Embryology, vol. i. chaiD. ix. More recent text-
boobs of embryology, such as that of Professor Korschelt and Heider, need not here
be specifically cited.

934

MOLLUSCA AND BEACHIOPODA

branchiae and of oral tentacles ; hnt their nature can only be cer-
tainly determined by further observation, which is rendered difficult
by the opacity of the valves. By keeping a supply of fish, however,
with these embryos attached, the entire history of the development
of the fresh-water mussel may be worked out.^

In certain members of the class Gastrojjoda the history of em-
bryonic development presents numerous phenomena of great interest.
The eggs (save among the terrestrial species) are usually deposited in
aggregate masses, each inclosed in a common protective envelope or
nidamentum. The nature of this envelope, however, varies greatly ;
thus, in the common Limnceus stagnalis, or ' water-snail,' of our ponds
and ditches it is nothing else than a mass of soft j elly , about the size
of a sixpence, in which from fifty to sixty eggs are imbedded, and
which is attached to the leaves or stems of aquatic plants ; in the
Bucchium undatum, or comm.on whelk, it is a membranous case,

Fig. 710. — A, Glochidium immediately after it is hatched : ad, ad-
ductor ; sli, shell ; by, byssus-cord ; s, sense-organs. B, the same
after it has been on the fish for some weeks : 6r, branchise ; auv,
auditory sac; /, food; a.ad and jp.ad, anterior and posterior
adductors ; al, mesenteron ; mt, mantle.

connected with a considerable number of similar cases by short stalks,
so as to form large globular masses which may often be picked up on
our shores, especially between April and June ; in the Piirjjura
lapillus, or ' rock- whelk,' it is a little flask-shaped capsule, having
a firm horny wall, which is attached by a short stem to the surface
of rocks between tide marks, great numbers being often found
standing erect side by side ; whilst in the Nudibranchiate order
generally (consisting of the Doris, Eolis, and other 'sea-slugs') it
forms a long tube with a membranous wall, in which immense
numbers of eggs (even half a million or more) are packed closely
together in the midst of a jelly-like substance, this tube being disposed
in coils of vaiious forms, which are usually attached to seaweeds or
zoophytes. The course of development, in the first and last of these
instances, may be readily obsei-ved fi-om the very eai'liest period down

1 See the Rev. W. Houghton, ' On the Parasitic Nature of the Fry of the Ano-
donta cygnea,' in Quart. Journ. Microsc. Sci. n.s. vol. ii. 1861, p. 162, and especially
Balfour, op. cit. pp. 220-223. On the embryonal byssus-gland of Anodonta, see
J. Carriers, Zliolog. Anzeig. vii. p. 41.

DEVELOPMENT OF DORIS

935

to that of the emersion of the embryo, owing to the extreme trans-
parence of the nidamentum and of the egg-membranes themselves.
The first change which will be noticed by the ordinary observer is
the ' segmentation ' of the yolk-mass, which divides itself (after the
manner of a cell undergoing binary subdivision) into two parts, each
of these two into two othei's, and so on until a monda, ov mulberry-
like mass of minute yolk-segments, is produced (fig. 711, A-F),
which is converted by ' invagination ' into a ' gastrula,' whose form

/

Fig. 711. — Embryonic development of Doris bilameUata : A, ovum, consist-
ing of enveloping membrane, a, and yolk, ft ; B, C, D, E, F, successive
stages of segmentation of yolk ; Gr, first marking out of the shape of the
embryo ; H, embryo on the eighth day ; I, the same on the ninth day ; K, the
same on the twelfth day, seen on the left side at L ; M, still more advanced
embryo, seen at N as retracted within its shell ; a, position of shell-gland ;
c, c, ciliated lobes ; d, foot ; g, hard plate or operculum attached to it ;
h, stomach ; i, intestine ; di, n, masses (glandular ?) at the sides of the
oesophagus; o, heart (?) ; s, retractor muscle (?) ; t, situation of funnel;
V, membrane enveloping the body ; x, auditory vesicles ; y, mouth.

936 ■ MOLLUSCA AND BEACHIOPODA

is stiown at G. This 'gastrula' soon begins to exhibit a very curious
alternating rotation within the egg, two or three turns being made
in one direction, and the same number in a reverse dii-ection : this
movement is due to the ciHa fringing a sort of fold of the ecto-
derm termed the velum, which afterwards usually gives origin to a
pair of large ciliated lobes (H-L, c) resembling those of Rotifers.
The velumi is so little developed in Limnceus, however, that its
existence was commonly overlooked until i-ecognised by Professor
Ray Lankester,^ who also has been able to distinguish its fringe of
minute cilia. This, howevei-, has only a transitory existence ; and
the later rotation of the embryo, which presents a very curious
spectacle when a number of ova are viewed at once under a low
magnifying power, is due to the action of the cilia fringing the head
and foot.

A separation is usually seen at an early period between the
anterior or 'cephalic' portion, and the posterior or 'visceral' portion,
of the embryonic mass, and the development of the former advances
with the greater activity. One of the first changes which are seen in
it consists in its extension into a sort of fin-like membrane on either
side, the edges of which are fringed with long ciha (fig. 711, H-L, c),
whose movements may be clearly distinguished whilst the embryo is
still shut up within the egg ; at a very early period may also be dis-
cerned the ' auditory vesicles' (K,cc) or rudimentary organs of hearing,
which scarcely attain any higher development in these creatures
during the whole of life ; and from the immediate neighbourhood of
these is put forth a projection, which is aftei-wards to be evolved into
the ' foot ' or muscular disc of the animal. While these organs are
making their appearance, the shell is being formed on the surface of
the posterior portion, appearing first as a thin covering over its hindei-
part and gradually extending itself until it becomes large enough to
inclose the embiyo completely, when this contracts itself. The
cihated lobes are best seen in the embryos of Nudibranchs ; and the
fact of the universal presence of a shell in the embryos of that gTOup
is of peculiar interest, as it is destined to be cast off very soon after
they enter upon active life. These embryos may be seen to move
about, as freely as the narrowness of their prison permits, for some
time previous to their emersion ; and when set free by the rupture
of the egg-cases they swim forth with great activity by the action
of their ciliated lobes — these, like the 'wheels' of Rotifera, serving also
to bring food to the mouth, which is at that time unprovided -svith
the reducing apparatus subsequently found in it. The same is true
of the embryo of Lymncetis, save that its swimming movements are
less active, in consequence of the non-development of the ciliated
lobes ; and the cui-rents produced by the cilia that fringe the head
and the orifice of the respiratory sac seem to have reference chiefly
to the provision of supplies of food and of aerated water for respira-

'^ See his valuable ' Observations on the Development of LimncBus stagnalis and
on the early stages of other Mollusca ' in Quart. Journ. -Microsc. Sci. October 1874 ;
and ' On the Developmental History of the Mollusca,' Phil. Trans. 1875. See also
Lereboullet, ' Recherches sur le Developpement du Limn^e,' in Ann. des Sci. Nat.
Zool. 4" serie, torn, xviii. p. 47.

DEVELOPMENT OF PUEPUEA

937

tion. The disappearance of the ciHa has been observed by Mr. Hogg
to be coincident with the development of the teeth to a degree suf-
ficient to enable the young water-snail to crop its vegetable food ;
and he has further ascertained that if the growing animal be kept in
fi-esh water alone for some time, without vegetable matter of any
kind, the gastric teeth are very imperfectly developed, and the cilia
are still retained. ^

A very curious modification of the oi'dinary plan of development
is presented in Purpura lapillus, and it is probable that something
of the same kind exists also in Buccinum, as well as in other Gas-
tropods of the same extensive order {Pectinibranchiata). Each of
the capsules already described contains from 500 to 600 egg-like
bodies (fig. 712, A) imbedded in a viscid gelatinous substance ; but
only from twelve to thirty embryos usually attain complete develop-
ment, and it is obvious, from the large comparative size Avhich these
attain (fig. 713, B), that each of
them must include an amount of
substance equal to that of a great
number of the bodies originally
found within the capsule. The
explanation of this fact (long-
since noticed by Dr. J. E. Gray
in regard to Bticcinum) seems to
be as follows. Of those 500 or
600 egg-like bodies, only a small
part are fertile ova, the remaindei-
being unfertilised eggs, the yolk
material of which serves for the
nutrition of the embryos in the Pig. 712.— Early stages of embryonic
later stages of their intracapsular development of Furpura Icqnllus: A,
life Thp distinction bptwppn egg-like spherule ; B, C, E, F, G, suc-
nre. ±ne CllStmctlon Detween cessive stages of segmentation of yolk-

them manifests itself at a very spherules; D, H, I, J, K, successive

early period, even in the first stages of development of early embryos.

segmentation ; for, while the latter

divide into two equal hemispheres (fig. 712, B), the fertilised ova
divide into a larger and a smaller segment (D) ; in the cleft between
these are seen the minute ' directive vesicles,' which appear to be
always double, although from being seen ' end on,' only one may
be visible ; and near these is generally to be seen a clear space
in each segment. The difference is still more strongly marked in
the subsequent divisions ; for whilst the cleavage of the infertile
eggs goes on irregularly, so as to divide each into from fourteen to
twenty segments, having no definiteness of arrangement (C, E, F, G),
that of the fertile ova takes place in svich a manner as to mark out
the distinction already alluded to between the ' cephalic ' and the
' visceral ' portions of the mass (H), and the evolution of the
former into distinct organs very speedily commences. In the first
instance a narrow transparent border is seen around the whole
embryonic mass, which is broader at the cephalic portion (I) ; next,

1 See Trans, Microsc, Soc. ser, ii. vol. ii, 1854, p. 93.

938

MOLLUSCA AND BEACHIOPODA

this boi'dei- is fringed with short cilia, and the cephalic extension
into two lobes begins to show itself; and then between the lobes a
large mouth is formed, opening through a short wide oesophagus,
the interior of which is ciliated, into the visceral ca^dty, occu-
pied as yet only by the yolk-particles originally belonging to the
ovum (K).

Whilst these developmental changes are taking place in the embryo,
the whole aggregate of segments formed by the yolk-cleavage of the
infertile eggs coalesces into one mass, as shown at A, fig. 713 ; and
the embryos are often, in the first instance, so completely buiied
within this as only to be discovei-able by tearing its portions asunder ;
but some of them may commonly be found upon its exterior, and
those contained in one capsule very commonly exhibit the difierent

Fig. 713. — Later stages of embryonic development of Puiyura lapillus.
A, conglomerate mass of vitelline segments, to which were attached the
embryos a, h, c, d, e. B, full-sized embryo in more advanced stage of
develojpment.

stages of development represented in fig. 712, H-K. After a short
time, however, it becomes apparent that the most advanced embryos
are beginning to swcdloio the yolk segments of the conglomerate mass,
and capsules will not unfrequently be met with in which embryos
of various sizes, as a, h, c, cl, e (fig. 713, A), are projecting from its
surface, their difierence of size not being accompanied by advance in
development, but merely depending ujDon the amount of this ' supple-
mental' yolk which the embryos have respectively gulped down.
For during the time in which they are engaged in appropriating this
additional supply of nutriment, although they increase in size, yet
they scarcely exhibit any othei- change ; so that the large embryo,
fig. 713, e, is not apparently more advanced, as regards the formation
of its organs, than the small embryo, fig. 712, K. So soon as this
operation has been completed, however, and the embryo has attained
its full bulk, the evolution of its organs takes place very rapidly ; the

DEVELOPMENT OF PUKPUEA 939

ciliated lobes are much more highly developed, being extended in a
long sinuous margin, so as almost to remind the observer of the
' wheels ' of Rotifera, and being furnished with very long cilia (fig.
713, B) ; the auditory vesicles, the tentacula, the eyes, and the foot
successively make their appearance ; a curious rhythmically contractile
vesicle is seen, just beneath the edge of the shell in the region of the
neck, which may, perhaps, serve as a temporary heart ; a little later
the real heart may be seen pulsating beneath the dorsal j^art of the
shell ; and the mass of yolk-segments of which the body is made up
gradually shapes itself into the various organs of digestion, respira-
tion, etc., during the evolution of which (and while they areas yet far
from complete) the capsule thins away at its summit and the embryos
make their escape from it.^

It happens not unfrequently that one of the embryos which a
capsule contains does not acquire its ' supplemental ' yolk in the
manner now described, and can only proceed in its development as far
as its original yolk will afford it material ; and thus, at the time when
the other embryos have attained their full size and maturity, a strange-
looking creature, consisting of two large ciKated lobes with scarcely
the riidiment of a body, may be seen in active motion among them.
This may happen, indeed, not only to one, but to several embryos
within the same capsule, especially if their number should be con-
siderable ; for it sometimes appears as if there were not food enough
for all, so that, whilst some attain their full dimensions and complete
develojDment, others i-emain of unusually small size, without being-
deficient in any of their organs ; and others, again, are more or less
comjjletely abortive — the supply of supjDlemental yolk which they
have obtained having been too small for the development of their
viscera, although it may have afibrded what was needed for that of
the ciliated lobes, eyes, tentacles, auditory vesicles, and even the
foot — or, on the other hand, no additional supply whatever having
been acquired by them, so that their development has been arrested
at a still earlier stage. These j^henomena are of so remarkable a
character that they furnish an abundant source of interest to any
microscopist who may happen to be spending the months of August
and September in a locality in which the Purjnira abounds ; since,
by opening a sufiicient number of capsules, no difiiculty need be
experienced in arriving at all the facts which have been noticed in
this brief summary.^ It is much to be desired that such microscopists

1 The Author thinks it worth while to mention the method which he has found
most convenient for examining the contents of the egg-capsules of Purpura, as he
believes that it may be advantageously adopted in many other cases. This consists
in cutting off the two ends of the capsule (taking care not to cut far into its cavity),
and in then forcing a jet of water through it by inserting the end of a fine-pointed
syringe into one of the orifices thus made, so as to drive the contents of the capsule
before it through the other. These should be received into a shallow cell and first
examined under the simple microscope. For some further observations on the de-
velopment of Purpura, see Professor Haddon, ' Notes on the Development of the
Mollusca,' Quart. Journ. Microsc. Sci. xxii. p. 367.

^ Fuller details on this subject will be found in the Author's account of his re-
searches in Trans. Microsc. Soc. ser. ii. vol. iii. 1855, p. 17. His account of the
process was called in question by MM. Koreu and Danielssen, who had previously
given an entirely different version of it, but was fully confirmed by the observations
of Dr. Dyster. See A7171. Nat. Hist. ser. ii. vol. xx. 1857, p. 16. The independent

940 MOLLUSCA AND EEACHIOPODA

as possess the requisite opportunity would apply themselves to the
study of the corresponding history in other Pectinibranchiate Gastro-
pods, with a view of determining how far the plan now described
prevails tlu?ough the order. And now that these molluscs have been
brought not only to live, but to breed, in artificial aquaria, it may be
anticipated that a great addition to our knowledge of this part of
their life-history will ere long be made.

Ciliary Motion on Gills. — There is no object that is better
suited to exhibit the genei-al phenomena of ciliary m.otion than a
portion of the gill of some bivalve mollusc. The Oyster will answer
the purpose sufficiently well ; but the cilia are much larger on the
gills of the Mussel (Mt/tilus),^ as they are also on those of the Anodon
or common ' fresh-water mussel ' of our ponds and streams. Kothing
more is necessary than to detach a small portion of one of the ribbon-
like bands which will be seen running parallel with the edge of each
of the valves when the shell is opened, and to place this, "with a
little of the liquor contained within the shell, iipon a slip of glass —
taking care to spread it out sufficiently with needles to separate the
bars of which it is composed, since it is on the edges of these, and
round their knobbed extremities, that the ciliary movement presents
itself — and then covering it with a thin glass disc. Or it will be
convenient to place the object in the aquatic box, which will enable
the observer to subject it to any degree of pressure that he may find
convenient. A magnifying power of about 120 diameters is amply
sufficient to affiord a general view of this spectacle ; but a much
greater amphfication is needed to bring into view the pecuhar mode in
.which the stroke of each cilium is made. Few spectacles are more
striking to the unprepared mind than the exhibition of such won-
derful activity as will then become apparent in a body which to all
ordinary observation is so inert. This activity serves a double pur-
pose ; for it not only drives a continual current of water over the
surface of the gills themselves, so as to efiect the aeration of the
blood, but also directs a portion of this current to the mouth, so
as to supply the digestive apparatus with the aliment afibrded by
the Diato77iaceie, Infusoria, &c. which it carries in with it.

Organs of Sense of Molluscs. — Some of the minuter and moi-e
rudimentary forms of the special organs of sight, hearing, and touch
which the molluscous series presents are very interesting objects of
microscopic examination. Thus, just within the margin of each valve
of Pecten, we see (when we observe the animal in its living state
under water) a row of minute circular points of great brilliancy, each
surrounded by a dark ling ; these are the eyes with which this
creature is provided, and by which its peculiarly active movements
are directed. Each of them, when their structure is carefully exa-
mined, is found to be protected by a sclerotic coat with a transparent

observations of M. Claparede on the development of Neritina fluviatilis (Miiller's
Arcliiv, 1857, p. 109, and abstract in Ann. of Nat. Hist. ser. ii. vol. xx. 1857, p. 196)
showed the mode of development in that species to be the same in all essential par-
ticulars as that of Finyura. The subject has again been recently studied with great
minuteness by Selenka, Niedei'lanclisches Archiv fiir Zoologie, Bd. i. July 1862.

1 This shellfish may be obtained, not merely at the seaside, but likewise at the
shops of the fishmongers who supply the humbler classes, even in Midland towns.

SENSE-OEGANS OF MOLLUSCA 94 1

cornea in front, and to possess a coloured iris (having a pupil) that
is continuous with a layer of pigment lining the sclerotic, a crystalline
lens and vitreous body, and a retinal expansion proceeding from an
optic nerve which passes to each eye from the trunk that runs along
the margin of the mantle.' Professor H. N. Moseley made the
interesting discovery that many of the Ghito7iidce are provided with
a large number of minute eyes on the exposed areas of the outer
surfaces of their shells ; as the fibres of the optic nerve are directed
to the rods from behind these eyes are of the ordinary invertebrate
type, and differ therein from the just mentioned eyes of Pecten, or
those which are found on the back of Onchidium, which resemble
the vertebrate retina in having the optic fibres inserted into the front
aspect of the layer of rods.^ Eyes of still higher organisation are
borne upon the head of most Gastropod molluscs, generally at the
base of one of the pairs of tentacles, but sometimes, as in the Snail
and Slug, at the points of these organs. In the latter case the ten-
tacles are furnished with a very peculiar provision for the protection
of the eyes ; for when the extremity of either of them is toviched it
is drawn back into the basal part of the organ, much as the finger of
a glove may be pushed back into the palm. The retraction of the
tentacle is accomplished by a strong muscular band, which arises
within the head and proceeds to the extremity of the tentacles ;
whilst its protrusion is effected by the agency of the circular bands
with which the tubular wall of the tentacle is itself furnished, the
inverted portion being (as it were) squeezed out by the contraction
of the lower j)art into which it has been drawn back. The structure
of the eyes and the curious provision just described may easily be
examined by snipping ofi" one of the eye-bearing tentacles with a pair
of scissors. None but the Cephalopod molluscs have distinct organs
of hearing ; but rudiments of such organs may be found in most
Gastropods (fig. 711, K, cc), attached to some part of the nervous
collar that surrounds the oesophagus, and even in many bivalves, in
connection with the nervous ganglion imbedded in the base of the
foot. These ' auditory vesicles,' as they are termed, are minute sac-
culi, each of which contains a fluid, wherein are suspended a number
of minute calcareous particles (named otoliths, or ear-stones), which
are kept in a state of continual movement by the action of cilia
lining the vesicles. This ' wonderful spectacle,' as it was truly
designated by its discoverer Siebold, may be brought into view
without any dissection by submitting the head of any smaU and not
very thick-skinned Gastropod, or the young of the larger forms, to
gentle compression under the microscope and transmitting a strong
light through it. The very early appearance of the auditory vesicles
in the embryo Gastropod has been already alluded to. Those who
have the opportunity of examining young specimens of the common
Pecten will find it extremely intei-esting to watch the action of the

1 See Mr. S. J. Hickson on ' The Eye of Pecten ' in Quart. Journ. Microsc. Sci.
vol. XX. n.s. 1880, p. 443, and K. E. Schreiner, ' Die Augen bei Pecten und Lima,'
Bergens Miis. Aarhog, 1896, no. 1.

2 See Professor Moseley' On the Presence of Eyes in the Shells of certain Chitonidse
and on the Structure of these Organs,' in Quart. Jonr-n. Microsc. Sci. xxv. p. 37.

942 MOLLUSCA AND BRACHIOPODA

very delicate tentacles which they have the power of putting forth
from the margin of their mantle, the animal being confined in a
shallow cell, or in the zoophyte trough ; and if the observer should
be fortunate enough to obtain a specimen so young that the valves
are quite transparent, he will find the spectacle presented by the
ciliary movement of the gills, as well as the active play of the foot
(of which the adult can make no such use), to be worthy of more
than a cursory glance.^

Chromatophores of Cephalopods. — Almost any species of cuttle-
fish [6'epia) or squid (Loligo) will afibrd the opportvinity of examining
the very curious provision which their skin contains for changing its
hue. This consists in the presence of numerous large ' pigment-cells,'
containing colouring matter of various tints, the prevailing colour,
however, being that of the fluid of the ink-bag. These pigment-cells
may present very difi"erent forms, being sometimes nearly globular,
whilst at other times they are flattened and extended into radiating
prolongations ; and, by the peculiar contractility with which they are
endowed, they can pass from one to the other of these conditions, so
as to spread their coloured contents over a comparatively large
surface, or to limit them within a comparatively small area. "Very
commonly there are different layers of these pigment-cells, their con-
tents having different hues in each layer ; and thus a great variety of
coloration may be given by the alteration in the form of the cells of
which one or another layer is made up. It is curious that the
changes in the hue of the skin appear to be influenced, as in the case
of the chameleon, by the colour of the surface with which it may be
in proximity. The alternate contractions and extensions of these
pigment-cells, or chromatojihores, may be easily observed in a piece of
skin detached from the living animal and viewed as a transparent
object, since they will continue for some time if the skin be placed
in sea- water. And they may also be well seen in the embryo cuttle-
fish, which will sometimes be found in a state of sufficient advance-
ment in the grape-like eggs of these animals attached to sea- weeds,
zoophytes, &c. The eggs of the small cuttle-fish termed the Sepiola,
which is very common on our southern coasts, are imbedded, like those
of the Doris, in gelatinous masses which are attached to seaweeds,
zoophytes, &c. ; and their embryos, when near maturity, are ex-
tremely beautiful and interesting objects, being sufiiciently trans-
parent to allow the action of the heart to be distinguished, as well as
to show most advantageously the changes incessantly occurring in
the form and hue of the ' chromatophores.' ^

1 Much valuable information concerning the sensory organs of molluscs will be
found in Dr. H. Simroth's memoir, ' Ueber die Sinneswerkzeuge unserer einheimi-
schen'Weichthie're,' Zeitschr. fiir iviss. Zool. xxvi. p. 227-

- For further information regarding the chromatophores see an essay by Dr.
Klemensiewicz in the Sitzungsherichte of the Vienna Academy, vol. Ixxviii. p. 7,
and Krukenberg, Vergl. pliysiol. Studien, 1880.

The following works and memoirs on the Mollusca generally may be consulted by
the student : S. P. Woodward, A Manual of the Mollusca, 3rd ed. London, 1875 ;
Keferstein, in Bronn's Klassen und Ordmmgen des ThierreicJis ; the article ' Mollusca,'
by Professor Ray Lankester, in the 9th edition of the Encyclopcedia Britannica ;
M. P. Fischer's Manuel de Conchyliologie, Paris, 1881-87; and the Rev. A. H.
Cooke's volume in the Camhridge Natural History ; as well as the numerous reports
on the Mollusca collected by H.M.S. Challenger.

943

CHAPTER XIX

WOBMS

Under the general designation of Worms many naturalists still
group a number of Metazoa, which differ considerably among them-
selves, and exhibit on the one hand very simple, and on the other
somewhat complex plans of organisation ; the assemblage is, indeed,
hardly anything else than a zoological lumber-room, from which,
with the progress of research, group after grouj) may be expected to
be removed. Among others there are included in it the Entozoa or
intestinal worms, the Rotifera or wheel-animalcules, Ttirhellaria, and
Annulata, each of which furnishes many objects for microscopic
examination that are of the highest scientific interest. As our
business, however, is less with the professed morphologist than with
the general inquirer into the minute wonders and beauties of Nature,
we shall pass over these classes (the Rotifera having been ali-eady
treated of in detail, Chapter XIII) with only a notice of such points as
are likely to be specially deserving the attention of observers of the
latter order.

Entozoa. — This term is one which has been applied to such worms
as are parasitic within the bodies of other animals, and which obtain
their nutriment by the absorjation of the juices of these, thus
bearing a striking analogy to the pai'asitic Fungi. ^ The most re-
markable feature in their structure consists in the entire absence or
the extremely low development of their nutritive system, and the
extraordinary development of their reproductive apjjaratus. Thus
in the common Tcenia (' tape-worm '), which may be taken as the type
of the Cestoid group, there is neither mouth nor stomach, the so-called
' head ' being merely an organ for attachment, whilst the segments of
the ' body ' contain repetitions of a complex generatis^e ajaj^fii'atus,
the male and female sexual organs being so united in each as to
enable it to fertilise and biing to maturity its own very numerous
eggs ; and the chief connection between these segments is established
by two pairs of longitudinal canals, which appear to represent the
' water- vascular system,' whose simplest condition has been noticed
in the wheel-animalcule. Few among the striking results of micro-
scopic inquiry have been more curious than the elucidation of the
real nature of the bodies formerly denominated cystic Entozoa, which

1 The most important work on human entozoic parasites is that by Professor
Leuckart, Die menschlicJien Parasiten, of which a second edition is now in course
of pubhcation ; of this the first portion has been translated into English by
Mr. W. E. Hoyle.

944 WOEMS

had been previously ranked as a distinct group. These are not
found, hke the preceding, in the cavity of the alimentary canal of
the animals they infest, but always occur in the substance of soUd
organs, such as the glands, muscles, &c. They present themselves to
the eye as bags or vesicles of various sizes, sometimes occurring
singly, sometimes in groups ; but xipon careful examination each
vesicle is found to bear upon some part a ' head ' furnished with
booklets and suckers ; and this may be either single, as in Cysticercus
(the entozoon whose presence gives to pork what is known as the
' measly' disorder), or multiple, as in CrewiWMs, which is developed in
the brain, chiefly of sheep, where it gives rise to the disorder kno^vn
as ' the staggers.' Now, in none of these cystic forms has any
generative apparattis ever been discovered, and hence they are ob-
viously to be considered as imperfect animals. The close resemblance
between the ' heads ' of certain Gysticerci and that of certain Tcenice
first suggested that the two might be diflferent states of the same
animal ; and experiments made by those who have devoted them-
selves to the working out of this curious subject have led to the
assured conclusion that the cystic Entozoa are nothing else than
cestoid worms, whose development has been modified by the
peculiarity of their position, the large bag being formed by a sort
of dropsical accumulation of fluid when the young are evolved in the
midst of solid tissues ; whilst the very same bodies, conveyed into the
alimentary canal of some carnivorous animal which has fed upon the
flesh infested with them, begin to bud forth the generative segments,
the long succession of which, united end to end, gives to the entire
series a band-like aspect.

Other forms of Entozoa belong to the Nematoid or thread-Hke
order" — of which the common Ascaris may be taken as a type ; one
species of this (the A . hmihricoides or ' round worm ') is a common
parasite in the small intestine of man, while, another (the Oxyi(,ris
vermicularis or ' thread- worm ') is found i-ather in the lower bowel —
and they are much less profoundly degraded in their organisation ;
they have a distinct alimentary canal, which commences with a mouth
at the anterior extremity of the body, and which terminates by an anal
orifice near the other extremity ; and they also possess a regular
arrangement of circular and longitudinal muscular fibres by which
the body can be shortened, elongated, or bent in any direction. The
smaller Nematode worms, by some or other of which almost every
vertebrated animal is infested, are so transparent that every part of
their internal organisation may be made out, especially with the
assistance of the compressor, without any dissection ; and the study
of the structure and actions of their generative apparatus has yielded
many very interesting results, especially in regard to the first forma-
tion of the ova, the mode of their fertilisation, and the history of
their subsequent development. ' Some of the worms belonging to
this group are not parasitic in the bodies of other animals, but live
in the midst of dead or decomposing vegetable matter. Others, such
as Gordiiis or the ' hair-worm,' are parasitic for the greater part of

' See particularly the various recent memoirs of Van Beneden and of Boveri, based
on a study of Ascaris megalocephala.

NEMATODES AND TEEMATODES 945

their existence, but leave their host for the purpose of maturing
their generative prockicts ; in these later stages the Gordius is fre-
quently found in 1 large knot-like masses (whence its name) in the
water or mud of the pools inhabited by the insects in which the
earlier stages were passed. The Anguilhdce are little eel-like worms,
of which one species, A.fluviatilis, is very often found in fresh water
amongst Desmidice, Confervce, (fee, also ijj wet moss and moist earth,
and sometimes also in the alimentary canals of snails, frogs, fishes,
insects, and larger worms ; whilst an allied species, Tylenchus tritici,
is met with in the ears of wheat aifected with the blight termed the
'cockle;' another, the A. glutinis {A. aceti), is found in sour paste,
and was often found in stale vinegar, until the more complete
removal of mucilage and the addition of sulphuric acid, in the
course of the manufacture, rendered this liquid a less favourable
' habitat ' for these little creatures. A writhing mass of any of these
species of ' eels ' is one of the most curious spectacles which the
microscopist can exhibit to the unscientific observer ; and the
capability which they all possess (in common with Rotifers and
Tardigrades) of revival after desiccation, at a very remote interval,
enables him to command the spectacle at any time. A grain of
wheat within which these worms (often erroneously called Vihriones)
are being developed gradually assumes the appearance of a black
peppercorn ; and if it be divided the interior will be fovmd almost
completely filled with a dense white cottony mass, occupying the
place of the flour, and leaving merely a small place for a little
glutinous matter. The cottoriy substance seems to the eye to consist
of bundles of fine fibres closely packed together ; but on taking out
a small jportion, and putting it under the microscope with a little
water under a thin glass cover, it will be found after a short time (if
not immediately) to be a wriggling mass of life, the apparent fibres
being really Anguillulce or ' eels ' of the microscopist. If the seeds
be soaked in water for a couple of hours before they are laid open,
the eels will be found in a state of activity from the first ; their
movements, however, are by no means so energetic as those of the
A. glutinis, or ' paste eel.' This last frequently makes its appearance
spontaneously in the midst of paste that is turning sour ; but the
best means of securing a supply for any occasion consists in allowing
a portion of any mass of paste in which they may present themselves
to dry up, and then, laying this by so long as it may not be wanted,
to introduce it into a mass of fresh paste, which if it be kept warm
and moist will be found after a few days to swarm with these curious
little creatures.

Besides the foi'egoing orders of Entozoa, the Trematode group,
which is more closely allied to the Gestoda than to the Nematodes,
must be named ; of this the Distoma hepaticitm, or ' fluke,' found
in the livers of sheep afiected with the ' rot,' is a typical example.
Into the details of the structure of this animal, which has the
general form of a sole, there is no occasion for us here to enter ;
it is remarkable, however, for the branching form of its diges-
tive cavity, which extends throughout almost the entire body, very
much as in the allied Plcmarice (fig. 714) ; and also for the curious

3p

946 WOKMS

phenomena of its development, several distinct forms being passed
through between one sexual generation and another. These have
been especially studied in the Distoma, which infests Paludina,
the ova of which are not developed into the likeness of their
parents, but into minute worm-like bodies, which seem to be little
else than masses of cells inclosed in a contractile integument, no
formed organs being found in them ; these cells, in their turn, are
developed into independent larvse, which escape from their contain-
ing cyst in the condition of free ciliated animalcules ; in this con-
dition they remain for some time, and then imbed themselves in
the mucus that covers the tail of the mollusc, in which they undergo
a gradual development into true Distomata ; and ha-\dng thus ac-
quired their perfect form, they penetrate the soft integument, and
take up their habitation in the interior of the body. Thus a con-
siderable number of Distomata may be produced from a single ovum
by a process of cell-multiplication in an early stage of its develop-
ment. In some instances the free ciliated larvae are provided with
pigment-spots or rudimentary optic organs, although these organs are
wanting in the fully developed Distoma, the peculiar ' habitat ' of
which would render them useless. -"^

Turbellaria. — This grouj) of animals, which is distinguished by
the presence of cilia over the entire surface of the body, contains
forms which are among the simplest of those in which the Metazoic
organisation obtains. It deserves special notice here chiefly on ac-
count of the frequency with which the worms of the Planarian
tribe present themselves among collections both of marine and of
fresh-water animals (particular species inhabiting either locality)
and on account of the curious organisation which many of these
possess. Most of the members of this tribe have elongated, flattened
bodies, and move by a sort of gliding or crawling action over the
surfaces of aquatic plants and animals. Some of the smaller kinds
are sufiiciently transparent to allow of their internal structure being
seen by transmitted light, especially when they are slightly com-
pressed ; and the opposite figure (fig. 714) displays the general
conformation of their principal organs as thus shown. The body
has the flattened sole-like shape of the Trematode Entozoa ; its
mouth, which is situated at a considerable distance from the anterior
extremity of the body, is surrounded by a circular sucker that is
applied to the living surface from which the animal draws its nutri-
ment ; and the buccal cavity (b) opens into a short oesophagus (c)
which leads at once to the cavity of the stomach. This cavity does
not give origin to any intestinal tube, nor is it provided with any
second orifice ; but a large number of ramifying canals are prolonged
from it, which carry its contents into every part of the body. This
seems to render unnecessary any system of vessels for the circulation
of nutritive fluid ; and the two principal trunks, with connecting
and ramifying branches, which may be observed in them may be

1 On the development and life-history of the ' Liver-fluke ' see Professor A. P.
Thomas, Quay-t. Journ. Microsc. Sci. xxiii. p. 1 ; and R. Leuckart, Archiv fiir Natur-
gescli. xlviii. p. 80. On its anatomy, see Dr. F. Sommer, Zeitschr. fiir wiss. Zool.
xxxiv.

PLANAEIA

947

regarded in the light of a gastro-vascular system, the function of
which is not only digestive, but also circulatory. Both sets of sexual
organs are combined in the same individuals, though the congress
of two, each impi-egnating the ova of the other, seems to be gene-
rally necessary. The ovaria, as
in the Entozoa, extend through
a large part of the body, their
i^amifications proceeding from
the two oviducts (k, k), which
have a dilatation (I) at their
point of junction. The Pla-
narice ^ do not multiply by eggs
alone ; for they occasionally un-
dergo spontaneous fission in a
transverse direction, each seg-
ment becoming a perfect animal ;
and an artificial division into
two or even more parts may be
practised with a like result. In
fact, the power of the Planarice
to reproduce portions which
have been removed seems but
little inferior to that of the
Hydra ; a circumstance which
is peculiarly remarkable when
the much higher character of
their organisation is borne in
mind. They possess a distinct
pair of nervous ganglia (/,/),
from which branches proceed to
various parts of the body ; and
in the neighbourhood of these
are usually to be observed a
number (varying from two to
forty) of ocelli or rudimentary
eyes, each having its refracting
body or crystalline lens, its pig-
ment-layer, its nerve-bulb, and
its cornea-like bulging of the
skin. The integument of many
of these animals is furnished

with cells containing rods or spindles which are very possibly
comparable to the ' thread-cells ' of zoophytes.^

Annulata. — This class includes all the higher kinds of worm-like
animals, the greater part of which are marine, though there is one
well-marked group the membei's of which inhabit fresh water or live

1 See Balfour's Comparative Enihrijoloqy, vol. i. pp. 159-162.

2 For further information regarding the Titrhellaria consult Dr. L. Graff's article
on Planarians in the 9th edition of the Encijclopcedia Britannica, and his magnifi-
cent Monographie der Turbellariden, Leipzig, 1882; A. Lang, Die Polycladen,
Leipzig, 1884 ; P. Hallez, Contributions a Vhistoire naturelle des Turbellaries,
Lille, 1879. On transverse fission, see Bell, Journ. Boij. Microsc. Soc. (2),vi. j). 1107.

3 p 2

Fig. 714. — Structure of Polycelis levi-
gatus (a Planarian worm) : a, mouth,
surrounded by its circular sucker ; b,
buccal cavity ; c, oesophageal orifice ;
d, stomach ; e, ramifications of gastric
canals ; /, cephalic ganglia and their
nervous filaments ; g, g, testes ; h,
vesicula seminalis ; i, male genital
canal ; k, k, oviducts ; I, dilatation at
their point of junction; m, female
genital orifice.

948,

WORMS

on land. The body in this class is usually elongated and nearly
always presents a well-marked segmental division, the segments
being for the most part similar and equal to each other, except at
the two extremities; though in some, as the leech and its allies,

the segmental division is very in-
distinctly seen, on account of the
general softness of the integument.
A large portion of the marine An-
nelids have special respiratory ap-
pendages, into which the fluids of
the body are sent for aeration, and
these are situated upon the head
(fig. 715) in those species which
(like the Serpida, Terehella, Sahel-
laria, ifcc.) have their bodies inclosed
by tubes, either formed of a shelly
substance produced from their o^\n
surface, or built up by the agglutina-
tion of grains of sand, fragments of
shell, &c. ; ^ whilst they are distri-
buted along the two sides of the body
in such as swim freely through the
water, or crawl over the surfaces of
rocks, as is the case with the Nereiclce,
or simply bury themselves in the
sand, as the Arenicola or ' lob- worm.'
In these resjDiratory appendages the
circulation of the fluids may be dis-
tinctly seen by microscopic exami-
nation ; and these fluids are of two
kinds : first, a colourless fluid, con-
taining numerous cell-like cor-
puscles, which can be seen in the
smaller and more transparent
species to occupy the space that
intervenes between the outer sur-
face of the alimentary canal and
the inner wall of the body, and to
pass from this into canals which
often ramify extensively in the
respiratory organs, but are never
furnished with a returning series
of passages ; and second, a fluid
which is usually red, contains few
floating particles, and is inclosed in
a system of proper vessels that communicates with a central pro-
pelling organ, and not only carries the fluid away from this, but also
brings it back again. In Terehella we find a distinct provision for the

Fig. 715. — Circulating apparatus of
TereheUa concliilega : a, labial ring ;
I, b, tentacles; c, first segment of
the trunk ; d, skin of the back ; e,
pharynx ; /, intestine ; g, longitudinal
muscles of the inferior surface of the
body; h, glandular organ; i, organs
of generation ; j, feet ; k, k, branchiae ;
I, dorsal vessel acting as a respiratory
heart ; to, dorso-intestinal vessel ;
n, venous sinus surrounding oesopha-
gus ; n', inferior intestinal vessel ;
0, o, ventra'' trunk ; p, lateral vascular
branches. •

1 For an interesting account of the formation of these tubes see Mr. A. T. Watson's
paper in Journ. Boy. Micr. Soc. 1890, p. 685.

DEVELOPMENT OF WOEMS 949

aeration of both fluids ; for tlie first is transmitted to the tendril -
like tentacles which surrotxnd the mouth (fig. 715, 6, b), whilst the
second circulates through the beautiful arborescent gill-tufts [k, k)
situated just behind the head. The former are covered with cilia, the
action of which continually renews the stratum of water in contact
with them, whilst the latter are destitute of these organs ; and this
seems to be the general fact as to the several appendages to which
these two fluids are respectively sent for aeration, the nature of their
distribution varying greatly in the difierent members of the class. In
the observation of the beautiful spectacle presented by the respiratory
circvdation of the various kinds of Annulates which swarm on most
of our shores, and in the examination of what is going on in the
interior of their bodies (where this is rendered possible by their
transparence), the microscopist will find a most fertile source of
interesting occupation ; and he may easily, with care and patience,
make many valuable additions to our present stock of knowledge on
these points. There are many of these marine worms in which
the appendages of various kinds put forth from the sides of their
bodies furnish very beautiful microscopic objects ; as do also the
difierent forms of teeth, jaws, &c. with which the mouth is com-
monly armed in the free or non-tubicolar species, which are
eminently carnivorous.

The early history of their development is extremely curious ;
for many come forth from the egg in a condition very little
more advanced than the ciliated gemmules of polypes, consist-
ing of a globular mass of untransformed cells, certain parts of
whose surface are covered with cilia, which ordinarily become
arranged in one or more definite rings ; in a few hours, however,
this embryonic mass elongates, and the indications of a segmental
division become apparent, the head being (as it were) marked ofi
in front, whilst behind this is a large segment thickly covered with
cilia, then a narrower and non-ciliated segment, and lastly the
caudal or tail segment, which is furnished with cilia. A little
later a new segment is seen to be interposed in front of the
caudal, and the dark internal granular mass shapes itself into the
outline of an alimentary canal. ^ The number of segments pro
gressively increases by the interposition of new ones between the
caudal and its preceding segments ; the various internal oi-gans
become more and more distinct, eye-spots make their appearance,
little bristly appendages are put forth from the segments, and
the animal gradually assumes the likeness of its parent ; a few
days being passed by the tubicolar kinds, however, in the actively

1 A most curious transformation once occurred within the Author's experience
in the larva of an Annehd, which was furnished with a broad collar or disc fringed
with very long cilia, and showed merely an appearance of segmentation in its hinder
part ; for in the course of a few minutes, during which it was not under observation,
this lai-va assumed the ordinary form of a marine worm three or four times its pre-
vious length, and the ciliated disc entirely disappeared. An accident unfortunately
prevented the more minute examination of this worm, which the Author would have
otherwise made ; but he may state that he is certain that there was no fallacy as to
the fact above stated, this larva haraig been placed by itself in a cell, on purpose
that it might be carefully studied, and having been only laid aside for a short time
wliilst other selections were being made from the same gathering of the tow-net.

950

WORMS

moving condition, before they settle down to the formation of a
tube.^

To carry out any systematic observations on the embryonic
development of Annulata the eggs should be searched for in the
situations which these animals haunt ; but in places where Annu-
lata abound free-swimming larvag are often to be obtained at the
same time and in the same manner as small Medusae ; and there is
probably no part of our coasts off which some very curious forms
may not be met with. The following may be specially mentioned
as departing widely from the ordinary type, and as in themselves
extremely beautiful objects : The Actinotrocha, which is now known

to be the young stage of the Gephyrean
worm Phoronis (fig. 716), bears a
strong resemblance in many particulars
to the ' bipinnarian ' larva of a star-
fish, having an elongated body, with
a series of ciliated tentacles {d) sym-
metrically arranged ; these tentacles,
however, proceed from a sort of disc
which somewhat resembles the ' lopho-
phore ' of certain Polyzoa. The mouth
(e) is concealed by a broad but pointed
hood or 'epistome' (a), which some-
times closes down upon the tentacular
disc, but is sometimes raised and ex-
tended forwards. The nearly cylin-
drical body terminates abruptly at the
other extremity, where the anal orifice
of the intestine (h) is surrounded by a
circlet of very large cilia. This animal
swims with great activity, sometimes
by the tentacular cilia, sometimes by
the anal circlet, sometimes by both
combined ; and besides its movement
of progression it frequently doubles
Fig ll&.-AcUnotrocha hrancU- itself together, so as to bring the anal
ata: a, epistome or hood; b, , P ,' ,, . , *^ , • ,

auus ; c, stomach ; cl, cihated extremity and the epistome almost into
tentacles ; e, mouth. contact. It is SO transparent that the

whole of its alimentary canal may be
as distinctly seen as that of Laguncula ; and, as in that polyzoon,
the alimentary masses often to be seen within the stomaph (c) are
kept in a continual whirling movement by the agency of cilia, with
which its waUs are clothed.^ An even more extraordinary departure
from the ordinary type is presented by the larva which has received
the name Pilidvam (fig. 717), its shape being that of a helmet, the

i_For further mformation on this subject see Balfour's Comparative Embryology,
vol. i. chap. xii. and the memoirs there cited.

2 'Ueber Pilidium und Actinotrocha' in Midler's Archiv, 1858, p. 293. For
more recent observations upon the latter creature, see Balfour's ComjiaraMve
Embryology, vol. i. pp. 299-302 ; and a paper on ' The Origin and Significance of the
Metamorphosis of Actinotrocha,' by Mr. E. B. Wilson (of Baltimore), in Quart.
Journ. Microsc. Sci. April 1881.

LAEV^ OF WORMS

951

plume of which is replaced by a single long bristle-like appendage
that is in continual motion, its point moving round and round in a
circle. This curious organism, first noticed by Johannes Mliller, has
been since ascertained to be the larva of some species of the Nemer-
tine worms, which belong to the division Annpla, a group in which
there are no stylets to the proboscis.^

Among the animals captured by the tow-net the marine
zoologist will not be unlikely to meet with a worm which,

Pig. 717. —Pilidmm gyrans . A, young, showing at a the ahmentary
canal, and at h the rudiment of the Nemertid ; B, more advanced
stage of the same ; C, newly freed Nemertid.

although by no means microscoj)ic in its dimensions, is an admirable
subject for microscopic observation, owing to the extreme trans-
parence of its entire body, which is such as to render it difficult to
be distinguished when swimming in a glass jar except by a very
favourable light. This is the Toinojiteris, so named from the
division of the lateral portions of its body into a succession of wing-
like segments (fig. 718, B), each of them carrying at its extremity a
pair of pinnules, by the movements of which it is rapidly propelled
through the water. The full-grown animal, which measures nearly

1 See especially Leuckart and Pagenstecher's ' Untersuchungen liber niedere
Seethiere ' in Midler's Archiv, 1853, p. 569 ; and Balfour, op. cit.'p. 165. The Author
has frequently met with Pilidium in Lamlash Bay.

952

WOEMS

an incti in length, has first a curious pair of ' frontal horns ' pro-
jecting laterally from the head, so as to give the animal the appear -

Fig. 718. — Structure and developnieut of Tomopteris onisciforniis : A, xDortion
of caudal prolongations, containing the spermatic sacs, a a ; B, adult male
specimen ; C, hinder part of adult female specimen, more enlarged, showing
ova, lying freely in the iDerivisceral cavity and its caudal prolongation ; D,
ciliated canal, commencing externally in the larger and smaller rosette-like
discs, a, h ; B, one of the jjinnulated segments, showing the position of the
ciliated canal, c, and its rosette-like discs, a, b ; showing also the incipient
development of the ova, d, at the extremity of the segment; F, cephalic gan-
glion, with its pair of auditory (?) vesicles, a a, and its two ocelli, b b ; G, very
young Tomo2}teris, showing at a a the larval antennae ; b b, the incijDient
long antennae of the adult ; c, d, e, f, four pairs of succeeding pinnulated
segments, followed by bifid tail.

TOMOPTEEIS 953

ance of a ' hammer-headed ' shark ; behind these there is a paii- of
very long antennte, in each of which we distinguish a rigid bristle-
like stem or seta, inclosed in a soft sheath, and moved at its base
by a set of muscles contained within the lateral protuberances at
the head. Behind these are about sixteen pairs of the ordinary
pinnulated segments, of which the hinder ones are much smaller
than those in front, gradually lessening in size until they become
almost rudimentary ; and where these cease the body is continued
onwards into a tail-like prolongation, the length of which varies
greatly according as it is contracted or extended. This prolongation,
however, bears four or five pairs of very minute appendages, and
the intestine is continued to its very extremity, so that it is really
to be regarded as a continuation of the body. In the head we find,
between the origins of the antennfe, a ganglionic mass, the comjDonent
cells of which may be clearly distinguished under a suificient mag-
nifying power, as shown at F ; seated upon this are two pigment-
spots (6, h), each bearing a double pellucid lens-like body, which are
obviously rudimentary eyes ; whilst imbedded in its anterior por-
tion are two peculiar nucleated vesicles, a, a, which are probably
the rudiments of some other sensory organs. On the under side of
the head is sitiiated the mouth, which, like that of many other
Annelids, is furnished with a sort of proboscis that can be either
projected or drawn in ; a short cesophagus leads to an elongated
stomach, which, when distended with fluid, occupies the whole
cavity of the central portion of the body, as shown in fig. B, but
which is sometimes so empty and contracted as to be like a mere
cord, as shown in fig. 0. In the caudal appendage, however, it is
always narrowed into an intestinal canal ; this, when the appendage
is in an extended state, as at C, is nearly straight ; but when the
appendage is contracted, as seen at B, it is thrown into convolutions.
The perivisceral cavity is occupied by fluid, in which some minute
corpuscles may be distinguished ; and these are kept in motion by
cilia which clothe some parts of the outer surface of the alimentary
canal and line some part of the wall of the body. No other more
special apparatus, either for the circulation or for the aeration of
the nutrient fluid, exists in this curious worm, unless we are to
regard as subservient to the respiratory function the ciliated canal
which may be observed in each of the lateral appendages except
the five anterior pairs. This canal commences by two orifices at
the base of the segment, as shown at tig. E, 6, and on a larger scale
at fig. D ; each of these orifices (D, «, h) is siirrounded by a sort of
rosette, and the rosette of the larger one [a) is furnished with
radiating ciliated ridges. The two branches incline towards each
other, and unite into a single canal that runs along for some dis-
tance in the wall of the body, and then terminates in the perivisceral
cavity, and the direction of the motion of the cilia which line it is
from without inwards.

The reproduction and developmental history of this Annelid
present many points of great interest. The sexes appear to be
distinct, ova being found in some individuals and spermatozoa in
others. The development of the ova commences in certain ' germ-

954 WOEMS -

cells ' situated within the extremities of the pinnulated segments,
where they project inwards from the wall of the body ; these, when
set free, float in the fluid of the perivisceral cavity and multiply
themselves by self-division ; and it is only after their number has
thus been considerably augmented that they begin to increase in
size and to assume the characteristic appearance of ova. In this
stage they usually fill the perivisceral cavity, not only of the body,
but of its caudal extension, as shown at C ; and they escape from
it through transverse fissures which form in the oviter wall of the
body at the third aird fourth seginents. The male reproductive
organs, on the other hand, are limited to the caudal prolongation,
where the sperm-cells are develojDed within the pinnulated append-
ages, as the germ-cells of the female are within the appendages of
the body. Instead of being set free, however, into the perivisceral
cavity, they are retained within a saccular envelope forming a testis
(A, a, a) which fills up the whole cavity of each appendage ; and
within this the spermatozoa may be observed, when mature, in
active movement. They make their escape externally by a passage
that seems to communicate with the smaller of the two just men-
tioned rosettes ; but they also appear to escape into the perivisceral
cavity by an aj)erture that forms itself when the spermatozoa are
mature. Whether the ova are fertilised while yet within the body
of the female by the entrance of spermatozoa tln^ough the ciliated
canals, or after they have made their escape from it, has not yet
been ascertained. Of the earliest stages of embryonic development
nothing whatever is yet known ; but it has been ascertained that
the animal passes through a larval form, which difiers from the
adult not merely in the number of the segments of the body (which
successively augment by additions at the posterior extremity), but
also in that of the antennae. At G is represented the earliest larva
hitherto met with, enlarged as much as ten times in proportion to
the adult at B ; and here we see that the head is destitute of the
frontal horns, but carries a pair of setigerous antennse, a, a, behind
which there are five pairs of bifid appendages, b, c, d, e,f, in the
first of which, b, one of the pinnules is furnished with a seta. In
more advanced larvse having eight or ten segments this is developed
into a second pair of antennae resembling the first ; and the animal
in this stage has been described as a distinct species, T. quaclricornis.
At a more advanced age, however, the second pair attains the
enormous development shown at B, and the first or larval antennae
disappear, the setigerous portions separating at a sort of joint (G, a,
a), whilst the basal pi'ojections are absorbed into the general wall
of the body. This beautiful creature has been met with on so many
parts of our coast that it cannot be considered at all ixncommon,
and the microscopist can scarcely have a more pleasing object for
study. ^ Its elegant form, its crystal clearness, and its sprightly,
graceful movements render it attractive even to the unscientific

1 See the memoirs of the Author and M. Claparede in vol. sxii. of the Linnea7i
Transactions and the authorities there referred to ; also a memoir by Dr. F.
Vejdovsky in Zeitschrift f. Wiss. Zool. Bd. xxxi. 1878.

NAis 955

observer ; Avhilst it is of special interest to the morphologist as one
of tlie simplest examples yet known of the Annelid type.

To one jDhenomenon of the greatest interest presented by various
small marine Annelids the attention of the microscopist should be
specially directed ; this is their luminosity^ which is not a steady
glow like that of the glow-worm or fire-fly, but a series of vivid
scintillations (strongly resembling those produced by an electric
discharge through a tube spotted with tinfoil), that pass along a
considerable number of segments, lasting for an instant only, but
capable of being repeatedly excited by any irritation applied to the
body of the animal. These scintillations may be discerned under
the microscope, even in separate segments, when they are subjected
to the irritation of a needle-point or a gentle pressure ; and it has been
ascertained by the careful observations of M. de Quatrefages that
they are given out by the muscular fibres in the act of contraction.^

Among the fresh-water Annelids those most interesting to the
microscopist are the worms of the Nais tribe, which are common in
our rivers and ponds, living chiefly amidst the mud at the bottom,
and especially among the roots of aquatic plants. Being blood-red
in colour, they give to the surface of the mud, when they protrude
themselves from it in large numbers and keep the protruded portion
of their bodies in constant undulation, a very peculiar appearance ;
but if disturbed they withdraw themselves suddenly and completely.
These worms, from the extreme transparency of their bodies, present
peculiar facilities for microscopic examination, and especially for the
study of the internal circulation of the red liquid commonly con-
sidered as blood. There are here no external respiratoiy organs, and
the thinness of the general integument appears to supply all needful
facility for the aei'ation of the fluids. One large vascular trunk (dorsal)
may be seen lying above the intestinal canal, and another (ventral) be-
neath it, and each of these enters a contractile dilatation, or heart-
like organ, situated just behind the head. The fluid moves forwards
in the dorsal trunk as far as the heart, which it enters and dilates ;
and when this contracts it propels the fluid partly to the head and
partly to the ventral heart, which is distended by it. The ventral
heart, contracting in its turn, sends the blood backwards along the
ventral trunk to the tail, whence it passes towards the head as
before. In this circulation the stream branches ofi:' from each of
the principal trunks into numerous vessels proceeding to different
parts of the body, which then return into the other trunk ; and
there is a jDeculiar set of vascular coils, hanging down in the peri-
visceral cavity that contains the corpusculated liquid representing
the true blood, which seem specially destined to convey to it the
aerating influence received by the red fluid in its circuit, thus
acting (so to speak) like internal gills. The Naiad worms have
been observed to undergo spontaneous division during the summer
months, a new head and its organs being formed for the posterioi'
segment behind the line of constriction before its separation from

1 See his memoirs on the Annelida of La Manche in Ann. des Sci. Nat. ser. ii.
Zool. torn. xix. and ser. iii. Zool. tom. xiv. ; and Professor Mcintosh in Nature,
xxxii. p. 478.

956 WOEMS

the anterior.^ In the Leech tribe the dental apparatus with which
the mouth is furnished is one of the most curious among their
points of minute structure, and the common ' medicinal ' leech
affords one of the most interesting examples of it. What is
commonly termed the ' bite ' of the leech is really a saw-cut, or
rather a combination of three saw-cuts, radiating from a common
centre. If the mouth of the leech be examined with a hand-
magnifiei', or even with the naked eye, it will be seen to be a
triangular aperture in the midst of a sucking disc, and on turning
back the lips of that apei-tui-e three little white ridges are brought
into view. Each of these is the convex edge of a horny semicircle,
strengthened by a deposit of carbonate of lime which is bordered by
a I'ow of eighty or ninety minute hard and sharp teeth ; whilst
the straight border of the semicircle is imbedded in the muscular
substance of the disc, by the action of which it is made to move
backwards and forwards in a saw-like manner, so that the teeth are
enabled to cut into the skin to which the suctorial disc has affixed
itself. 2

1 See Professor A. G. Bourne, ' On Budding in the Oligochfeta,' Heport Brit,
Assoc. 1885, p. 1096.

2 Among the various sources of information as to the anatomy and iDhysiology of
the Annelids the following may be specially mentioned : the ' Histoire Naturelle des
Anneles Marins et d'Eau douce ' of M. de Quatrefages, forming part of the Suites d
Buffon ; the successive admirable monographs of the late Professor Ed. Claparede,
Becherches Anatomiques sur les Annilides, Turbellaries, Opalines et Gregarines,
observes dans les Hebrides, Geneva, 1861 ; Becherches Anatomiques sur les Oligo-
cheies, Geneva, 1862 ; Beobachtungen uber Anatomie und Entivickelungsgeschichte
wirbelloser Thiere an der Kiiste von Normandie, Leipzig, 1863 ; and Les Annelides
Chetopodes du Golfe de Naples, Geneva, 1868-70 ; the monograph of Dr. Ehlers, Die
Borstenivilrmer {Annelida. ChcBtopoda), 1864-68. With the exception of Professor
Mcintosh's article in the Encyclopcsdia Britannica, and the various articles on
' Worms ' in the Cainbridge Natural Sistory, vrhich can be warmly commended to
the student, most of the recent ]papers on Annelids have dealt with small groups only,
but of these a very large number has appeared. For the descriptions of new forms
the memoirs of Grube, Mcintosh, and St. Joseph are especially to be consulted;
Hatschek, Kleinenberg, and Salensky have written the most important contributions
to our knowledge of development ; Benham, Bergh, Bourne, Eisig, Meyer, Perrier,
and Wliitman have, among others, added to our knowledge of their anatomy and
morphology.

957

CHAPTER XX

CBUSTACEA

Passing to the division of Arthropods, in which the body is
furnished with distinctly articulated or jointed limbs, some of which
are always modified to serve as mouth-organs, we come first to the
class of Crustacea^ which ordinarily includes (when used in its
most comprehensive sense) all those animals belonging to this group
which are fitted for aquatic respiration, though the king-crab
(Limulus) seems to have closer relations to the scorpions, and the
Pycnogonids to the spiders. It thus comprehends a very extensive
range of forms ; for although we are acciistomed to think of the crab,
lobster, cray-fish, and other well-known species of the order Decapoda
(ten-footed) as its typical examples, yet all these belong to the highest
of its many orders ; and among the lower are many of a far simpler
structure, not a few which would not be recognised as belonging to
the class at all were it not for the information given by the
study of their development as to their real nature, which is far more
apparent in their early than it is in their adult condition. Many
of the inferior kinds of Crustacea are so minute and transj)arent
that their whole structure may be made out by the aid of the
microscope without any preparation ; this is the case, indeed, with
nearly the whole group of Entomostraca, and with the larval forms
even of the crab, and its allies ; and we shall give our first atten-
tion to these, afterwards noticing such points in the structure of the
larger kinds as are likely to be of general interest.

A curious example of the reduction of an elevated type to a
very simple foi'm is presented by the group of Pycnogonida, or no-
body crabs, some of the members of which may be found by atten-
tive search in almost every locality where seaweeds abound, it
being their habit to crawl (or rather to sprawl) over the surfaces of
these, and probably to imbibe as food the gelatinous substance with
which they are invested. ^ The general form of their bodies (fig.
719) usually reminds us of that of some of the long-legged crabs,
the abdomen being almost or altogether deficient, whilst the head is
very small, and fused (as it were) into the thorax ; so that the last-
named region, with the members attached to it, constitutes nearly
the whole bulk of the animal. The head is extended in front into

1 It is remarkable that very large forms of this group, sometimes extending to
more than twelve inches across, have been brought up from great depths of the sea.

958

CEUSTACEA

a proboscis-like projection, at the extremity of which is the narrow
orifice of the mouth, which draws in the semi-fluid aliment. Instead
of being furnished (as in the higher crustaceans) with two pairs of
antennae and numerous pairs of ' foot-jaws,' it has but a single pair
of either ; it also bears four minute ocelli, or rudimentary eyes, set
at a little distance from each other on a sort of tubercle. From
the thorax proceed four pairs of legs, each composed of several joints,
and terminated by a hooked claw ; and by these members the
animal drags itself slowly along, instead of walking actively upon
them like a crab. The mouth leads to a very narrow oesophagus
(a), which passes back to the central stomach {h) situated in the

Fig. 719. — Ammothea pycnogonoides : a, narrow oesophagus ;
b, stomach; c, intestine ; d, digestive ceeca of the foot-jaws ;
e, e, digestive caeca of the legs.

midst of the thorax, fi-om the hinder end of which a narrow intes-
tine (c) passes off, to terminate at the posterior extremity of the
body. From the central stomach five pairs of c?ecal prolongations
radiate, one pair {d) entering the foot-jaws, the other four (e, e)
penetrating the legs, and passing along them as far as the last joint
but one ; and those extensions are covered with a layer of brownish-
yellow granules, which are probably to be regarded as a digestive
gland. The stomach and its caecal prolongations are continually
executing peristaltic movements of a very curiovis kind ; for they
contract and dilate with an irregular altei-nation, so that a flux and
reflux of their contents is constantly taking place between the
central portion and its radiating extensions. The perivisceral space
between the widely extended stomach and the walls of the body and

PYCNOGONIDA; ENTOMOSTEACA 959

limbs is occupied by a transparent liquid, in which are seen floating
a number of minute transparent coi'jjuscles of irregular size ; and
this fluid, which represents the blood, is kept in continual motion,
not only by the general movements of the animal, but also by the
actions of the digestive apparatus ; since, Avhenever the cjecum of
any one of the legs undergoes dilatation, a part of the cii'cum-
ambient liquid will be pi-essed out from the cavity of that limb,
eithei- into the thorax or into some othei- limb whose stomach is
contracting. The fluid must obtain its aeration through the general
surface of the body, as there are no special organs of respiration.
The nervous system consists of a single ganglion in the head (formed
by the coalescence of a pair), and of another in the thorax (formed
by the coalescence of four pairs), with which the cephalic ganglion
is connected in the usual mode, namely, by two nervous cords which
diverge from each other to embrace the oesophagus. In the study
of the very curious phenomena exhibited by the digestive apparatus,
as well as of the various points of internal conformation which have
been described, the achromatic condenser will be found useful, even
with the 1-inch, |-inch, or ^-inch objectives ; for the imperfect
transparence of the bodies of these animals renders it of importance
to drive a large cjuantity of light through them, and to give to this
light such a quantity as shall sharply define the internal organs.^

Entomostraca. — This group of crustaceans, many of the existing
members of which are of such minute size as to be only just visible to
the naked eye, is distinguished by the fact that they never have more
than three pairs of their appendages converted into moiith-organs,
nor possess any appendage on such segments as may lie behind the
generative orifices. The segments into which the body is divided
are frequently very numerous, and are for the most part similar to
each other ; but there is a mai'ked diflference in regard to the
appendages which they bear, and to the mode in which these
minister to the locomotion of the animals. For in what have been
called the Lojjhyropoda, or 'bristly-footed' tribe, a small number of
legs not exceeding five pairs have their function limited to locomotion,
the respiratory oi'gans being attached to the parts in the neighbour-
hood of the mouth ; whilst in the Branchiojjoda, or ' gill- footed ' tribe,
the members (known as 'fin-feet') serve both for locomotion and for
respiration, and the number of these is commonly large, being in Ajnis
as many as sixty pairs. The character of their movements differs
accordingly ; for whilst all the members of the first-named tribe dart
through the water in a succession of jerks, so as to have acquired the com-
mon name of 'water-fleas,' those among the latter which possess a great

1 Certain points of resemblance borne by Fycnogonida to spiders make the
careful study of their development a matter of special interest and importance, as
there is some reason to regard them rather as Arachnida adapted to a marine
habitat than as Crustacea. See Balfour's Comparative Emhryolocjy, pp. 448, 449,
and the authorities there referred to. The most recent additions to the literature
of the Pycnogonids are Dr. A. Dohrn's Die Pantopoden des Golfes von Neapel
&c., Leipzig, 1881 ; Dr. P. P. C. Hoek's ' Report on the Pycnogonida of the Challenger,'
1881, and his ' Nouvelle Etude sur les Pyenogonides,' in Archives de Zool. Exper. ix.
p. 445 ; and Professor G. 0. Sars's report in the Zoology of the Nonvegian North Sea
Expedition.

960 CEUSTACEA

number of ' fin-feet ' swim with an easy gliding movement, sometimes
on their back alone (as is the case with Branchipus) and sometimes
with equal facility on the back, belly, or sides (as is done by Artemla
salina, the ' brine-shrimp '). Some of the most common forms of
both tribes will now be briefly noticed.

The first group contains two orders, of which the first, Ostracoda,
is distinguished by the complete inclosure of the body in a bivalve
shell, by the small number of legs, and by the absence of an external
egg-sac. One of the best known examples is the little C'l/jn'is, which
is a common inhabitant of pools and streams ; this may be recognised
by its j)ossession of two pairs of antennfe, the first having numerous
joints with a pencil-like tuft of filaments, and projecting forwards
from the front of the head, whilst the second has more the shape of
legs, and is directed downwards, and by the limitation of its legs to
two pairs, of which the posterior does not make its appearance outside
the shell, being bent upwards to give sujoport to the ovaries. The
valves are generally opened widely enough to allow the greater part
of both pairs of antennae and of the front pair of legs to jDass out
between them ; but when the animals are alarmed, they draw these
members within the shell, and close the valves firmly. They are
very lively creatures, being almost constantly seen in motion, either
swimming by the united action of their foot-like antennae and legs,
or walking upon jjlants and other solid bodies floating in the water.
Nearly allied to the preceding is Cythere, whose body is furnished
with three pairs of legs, all projecting out of the shell, and whose
superior antennae are destitute of the filamentous brush ; this genus
is almost entirely marine, and some species of it may almost in-
variably be met with in little pools among the rocks between the
tide-marks, creeping about (but not summing) amongst Confervas
and Corallines. There is abundant evidence of the former existence
of Crustacea of larger size than any now existing, for in certain
fresh- water strata, both of the Secondary and Tertiary series, we find
layers, sometimes of great extent and thickness, which are almost
entirely composed of the fossilised shells of Cyprides ; whilst in
certain parts of the chalk, which was a marine deposit, the remains
of bivalve shells resembling those of Cythere present themselves
in such abundance as to form a considerable part of its substance.^

In the order Copepoda there is a jointed shell forming a kind
of buckler or carapace that almost entirely incloses the head and
thorax, an opening being left beneath, through which the appendages
project ; and there are five pairs of legs, mostly adapted for swim-
ming, the fifth pair, however, being rudimentary in the genus Cyclops^
the commonest example of the group. This genus receives its name
from possessing only a single eye, or rather a single cluster of ocelli ;
which character, however, it has in common with the two genera
already named, as well as with Daphnia, and with many other
Entomostraca. It contains numerous species, some of which belong

1 On the recent British Ostracoda see the monograph by G. S. Brady in vol. xxvi.
of the Transactions of the Linnean Society of London ; compare also Zenker,
' Monographic der Ostracoden,' Archiv fur Naturg. xx. 1854. Claus has an essay on
the development of CyjJris, Marburg, 1868 ; see also Dr. Brady's' CJiaUenger 'Re-port.'

ENTOMOSTEACA

961

,/#^^

to the fresh watei-, whilst others are marine. The fresh-water
species often abound in the muddiest and most stagnant pools,
as well as in the clearest springs. Of the marine species some
are to be found in the localities in which the Gythere is most
abundant, whilst others inhabit the open ocean, and must be col-
lected by the tow- net. The body of the Cyclops is soft and gela-
tinous, and it is composed of two distinct jmrts, a thorax (fig. 720, «)
and an abdomen (6), of which the latter, being comparatively slender,
is commonly considered as a tail, though traversed by the intestine,
which terminates near its
extremity. The head, which
coalesces with the thorax,
bears one very large pair
of antennse (c), possessing
numerous articulations and
furnished with bristly ap-
pendages, and another small
pair (cZ) ; it is also furnished
with a pair of mandibles or
true jaws and with two
pairs of ' maxillfe,' of which
the hinder pair is the longer
and more abundantly sup-
plied with bristles. The
legs (e) are all beset with
plumose tufts, as is also the
tail (/, f) which is borne at
the extremity of the ab-
domen. On either side of
the abdomen of the female,
there is often to be seen an
Q^^ - capsule (B) ; within
which the ova, after be-
ing fertilised, undergo the
earlier stages of their de-
velopment. The Cyclops is
a very active creature, and

strikes the watei- in swimming, not merely with its legs and tail
but also with its antennae. The rapidly repeated movements of its
feet-jaws serve to create a whirlpool in the surrounding water, by
which minute animals of various kinds, and even its own yoiing, are
brought to its mouth to be devoured.'

The tribe of Brancliiopoda is divided also into two groups, of
which the Cladocera present the nearest approach to the preceding,
having a bivalve carapace, no more than from four to six pairs of
legs, two pairs of antennae, of which one is large and branched and
adapted for swimming, and a single eye. The commonest form of

1 See for British forms Professor Gr. S. Brady's Monograph of the free and
semi-parasitic Copepocla. of the British Islands, published by the Ray Society,
1878-80, and Mr. I. C. Thompson's accounts of those collected near the Isle of Man,
published by the Liverpool Biological Society.

3 Q

Fig. 720. — A, female of Cyclops quadricornis :
a, body ; b, tail ; c, antenna ; d, antennule ; e,
feet ; /, plumose setae of tail. B, tail, with
external egg-sacs. C, D, E, F, Gr, successive
stages of development of young.

962 CKUSTACEA

this is the Daplmia pulex, which is sometimes called the ' arborescent
water-flea,' from^ the branching form of its antennfe. It is very
abundant in many ponds and ditches, coming to the surface in the
mornings and evenings and in cloudy weather, but seeking the
depths of the water during the heat of the day. It swims by
taking short springs ; and feeds on minute particles of vegetable sub-
stances, but does not, however, reject animal matter when offered.
Some of the peculiar phenomena of its reproduction will be presently
described.

The other group, Phyllojwda, includes those Branchiopoda whose
body is divided into a great number of segments, nearly all of which
are furnished with leaflike appendages, or ' fin-feet.' The two
families which this group includes, however, differ considerably in
their conformation ; for in that of which the genei-a Apus and Nehalia ^
are representatives, the body is inclosed in a shell, either shield-like
or bivalve, and the feet are generally very numerous ; whilst in that
which contains Branchipus and Artemia, the body is entirely unpro-
tected, and the number of pairs of feet does not exceed eleven. The
Ajnis cancriformis.^ which is an animal of comparatively large size, its
entire length being about 2^ inches, is an inhabitant of stagnant
waters ; but although occasionally very abundant in particular pools,
or ditches, it is not to be met with nearly so commonly as the Ento-
mostraca already noticed ; in this country, indeed, it is exceedingly
rare. It is recognised by its large oval carapace, which covers the
head and body like a shield ; by the nearly cylindrical form of its
body, which is composed of thirty articulations, and by the large
number of its appendages, which amount to about sixty pairs. The
number of joints in these is so great that in a single individual they
may be safely estimated at not less than two millions. These organs,
however, ai'e for the most part small ; and the instruments chiefly
used by the animal for locomotion are the first pair of feet, which are
very much elongated (bearing such a resemblance to the principal
antennae of other Entomostraca as to be commonly ranked in the
same light), and are distinguished as rami or oars. With these they
can swim freely in any position ; but when the rami are at rest, and
the animal floats idly on the water, its fin-feet may be seen in in-
cessant motion, causing a sort of whirlpool in the water, and bringing
to the mouth the minute animals (chiefly the smaller Entomostraca
inhabiting the same localities) that serve for its food. The Branchipus
stagnalis has a slender, cylindriform, and very transparent body, of
nearly an inch in length, furnished with eleven pairs of fin-feet, but
is destitute of any protecting envelope ; its head is furnished with a
pair of very curious prehensile organs, which are really modified
antennse, whence it has received the name of Cheirocephalxis ; but

1 Professor Glaus has pointed out the relations of Nehalia to the Malacostraca, or
higher division of the Crustacea, and has suggested for the group which they re-
present the name of Leptostraca. See the Zeitsclir. fiir wiss. Zool. 1872, p. 823 ;
Claus, JJntersuchiingen zur Erforscliung der genealogischen Grundlage des
Crustaceen- Systems, Wien, 1876, as well as ' Ueber den Organismus der Nebaliiden
und die systematisclie Stellung der Leptostraken,' in Arh. Zool. Inst. Wien. viii.
(1889), pp. 1-148, 15 pis. ; but a different view is taken by Professor G. O. Sars in his
Report on the Challenger Phyllocarida.

ENTOMOSTRACA 963

these are not used by it for the seizure of prey, as the food of this
animal is vegetable, but to clasp the female in the act of copulation.
The Branchipics or Cheiroc&phalus is certainly the most beautiful and
elegant of all the Entomostraca, being rendei'ed extremely attractive
to the view by ' the uninterrupted undulatory wavy motion of its
graceful branchial feet, slightly tinged as they are with a light red-
dish hue, the brilliant mixture of transparent bluish-gTeen and bright
red of its prehensile antenna?, and its bright red tail with the beauti-
ful plumose setfe springing from it.' Unfortunately, however, it is
a very rare animal in this country. The Artemia scdina, or ' bi'ine-
shrimp,' is an animal of very similar organisation, and almost
equally beautiful in its appearance and movements, but of smaller
size, its body being about half an inch in length. Its ' habitat ' is
very peculiar, for it is only found in the salt-pans or brine-pits in
which sea- water is undergoing concentration (as at Lymington) ; and
in these situations it is sometimes so abundant as to communicate a
red tinge to the liquid.

Some of the most interesting points in the history of the Ento-
mostraca lie in the pecuHar mode in which their generative function
is performed, and in their tenacity of life when desiccated, in which
last respect they correspond with many B-otifers. By this pro-
vision they escape being completely exterminated, as they might
otherwise soon be, by the drying up of the pools, ditches, and other
small collections of water which constitute their usual habitats.
We do not, of course, imply that the adult animals can bear a com-
jdete desiccation, although they will preserve their vitality in mud
that holds the smallest quantity of moistui'e ; but their eggs are
more tenacious of life, and there is ample evidence that these will
become fertile on being moistened, after having remained for a long
time in the condition of fine dust. Most Entomostraca, too, are
killed by severe cold, and thus the whole race of adults perishes
every winter ; but their eggs seem unafiected by the lowest temjDera-
ture, and thus continue the species, which would be otherwise ex-
terminated. Again, we frequently meet in this group with that
agamic reproduction which we have seen to 23i"evail so extensively
among the lower forms. In many species there is a double
mode of multiplication, the sexual and the non-sexual. The
former takes place at certain seasons only, the males (which are
often so different in conformation from the females that they would
not be supposed to belong to the same sj^ecies if they were not seen
in actual congress) disappearing entirely at other times. The latter,
on the other hand, continues at all periods of the year, so long as
warmth and food are supplied, and is repeated many times so as to
give origin to as many successive ' broods.' Further, a single act of
impregnation may serve to fertilise, not merely the ova which are
then mature or nearly so, but all those subsequently produced by
the same female, which are deposited at considerable inteiwals. In
these two modes the multiplication of these little creatures is carried
on with great rapidity, the young animal speedily coming to maturity
and beginning to propagate, so that, according to the computation
of Jurine. founded ,upon data ascertained by actual observation, a

3 Q 2

964 CKUSTACEA

single fertilised female of the common Cyclox>s quadricornis may be
the progenitor in one year of 4,442,189,120 yomig.^

The eggs of some Entomostraca are deposited freely in the w^ater,
or are carefully attached in ckisters to aquatic plants ; but they are
m.ore frequently cariied for some time by the parent in special
receptacles developed fi'om the posterior part of the body ; and in
many cases they are retained there imtii the young are ready to
come forth, so that these animals may be said to be ovo- viviparous.
In Baphnia the eggs are received into a large ca-sdty between the
back of the animal and its shell, and there the young undergo almost
their whole development, so as to come forth in a form nearly
resembling that of their parent. Soon after their birth a moult or
exuviation of the shell takes place, and the egg-coverings are cast
off with it. In a very short time afterwards another brood of eggs
is seen in the cavity and the same process is repeated, the shell
being again exuviated after the young have been brought to maturity.
At certain times, however, the Daphnia may be seen with a dark
opaque substance within the back of the shell, which has been called
the ephijjjnum, from its resemblance to a saddle. This, when care-
fully examined, is found to be of dense texture, and to be composed
of a mass of hexagonal cells ; and it contains two oval bodies, each
consisting of an ovum covered with a horny casing, enveloped in a
capsule which opens like a bivalve shell. From the observations of
Sir J. Lubbock,^ it appears that the ephippium is really only an
altered portion of the carapace, its outer valve being a part of the
outer layer of the epidermis, and its inner valve the corresponding-
part of the inner layer. The development of the ephipjaial eggs takes
place at the posterior part of the ovaries, and is accompanied by the
formation of a greenish-brown mass of granules ; and from this
situation the eggs pass into the receptacle formed by the new cara-
pace, where they become included between the two layei-s of the
ephippium. This is cast off, in process of time, with the rest of the
skin, from which, however, it soon becomes detached ; and it con-
tinues to envelope the eggs, generally floating on the surface of
the water until they are hatched with the returning warmth of
spring. This curious provision obviously affords protection to
the eggs which are to endure the severity of winter cold ; and an
appi'oach to it may be seen in the remarkable firmness of the
envelopes of the ' winter eggs ' of some Rotifera. There seems a
strong probability, from the observations of Sir J. Lubbock (now
Loixl Avebiiry), that the ' ephippial ' eggs are true sexual products,
since males are to be found at the time when the ephippia are de-
veloped ; whilst it is certain that the ordinary eggs can be produced
non-sexually, and that the young which spring from them can multi-
ply the race in like manner. The young which are produced from
the ephippial eggs seem to have the same power of continuing the

1 For an interesting account of the parthenogenetic development of Apiis and its
allies see the sixth of Von Siebold's Beitrdge zur Parthenogenesis der Arthrox>oclen
(Leipzig, 1871).

- ' An account of the two Methods of Reproduction in Dajjhnia., and of the
Structure of the Ephippium,' in Phil. Trans. 1857, p. 79. On the ' summer-egg ' of
Daphnia see Lebedinsky, Zool. Anzeig. xiv. p. 149.

ENTOMOSTEACA 965

race by non-sexnal reproduction as the young developed under
ordinary circumstances.

In most Entomostraca the young at the time of their emersion
from the egg differ considerably from the parent, especially in having
only the thoracic portion of the body as yet developed, and in pos-
sessing but a small number of locomotor appendages (see fig. 720,
C-G) ; the visual organs, too, are frequently wanting at first. The
process of development, however, takes place with great rapidity,
the animal at each successive moult (which pi'ocess is very commonly
repeated at intervals of a day or two) presenting some new parts,
and becoming miore and more like its parent, which it very early
resembles in its power of multiplication, the female laying eggs
before she has attained her own full size. Even when the Entomo-
straca have attained their full growth, they continue to exuviate
their shell at short intervals during the whole of life ; and this
repeated moulting seems to prevent the animal from being injured,
or its movements obstructed, by the overgrowth of parasitic animal-
cules and confervee, weak and sickly individuals being frequently
seen to be so covered with such parasites that their motion and life
are soon arrested, apparently because they have not strength to cast
ofi" and renew their envelopes. The process of development appears
to depend in some degree upon the influence of light, being retarded
when the animals are secluded from it ; but its rate is still more
influenced by heat ; and this appears also to be the chief agent that
regulates the time Avhich elapses between the moultings of the adult,
these, in Bajy/mia, taking place at intervals of two days in warm,
summer weather, whilst several days intervene between them when
the weather is colder. The cast shell carries with it the sheaths not
only of the limbs and plumes, but of the most delicate hairs and
setje which are attached to them.. If the animal have previously
sustained the loss of a limb, it is generally renewed at the next moult,
as in higher Crustacea. '^

Forming part of the entomostracous group is the tribe of
suctorial Crustacea,^ which for the most part live as parasites upon
the extei-ior of other animals (especially fish), whose juices they
imbibe by means of the peculiar proboscis-like organ which takes
in them the place of the jaws of other crustaceans ; whilst other
appendages, representing the foot-jaws, are furnished with hooks,
by which these parasites attach themselves to the animals from
whose juices they derive their nutriment. Many of the suctorial
Crustacea bear a strong resemblance, even in their adult condition,
to other Entomostraca; but more commonly it is between the
earlier forms of the two that the resemblance is the closest,
most of the Sibctoria undergoing such extraordinary changes in their

1 For a systematic and detailed account of this group Dr. Baird's Natural
History of tlie British Entomostraca, jDublished by the Ray Society in 1849, must
still be recommended. The numerous essays by Professor Claus should also be
consulted.

- It is now generally recognised that these should be placed with the Copepoda,
which may be divided into the Eucopiej^oda and the Brancliiura ; the former are
divisible into the Gnat]iosto?nata, most of which are non-parasitic, and have been
already described under Copepoda, and the Sipihonostomata, of which Lerncsa is an
example.

966 CRUSTACEA

progress towards the adult condition that, if their complete forms
were alone attended to, they might be excluded from the class
altogether, as was (in fact) done by many earlier zoologists. Of the
suctorial Crustacea which form the group Branchiura may be
specially mentioned the Arguhis foliaceus, which attaches itself to
the siu-face of the bodies of fresh-water fish, such as the stickleback,
and is commonly knowai under the name of the ' fish-louse.' This
animal has its iDody covered Avith a large firm oval shield, which
does not extend, however, over the posterior part of the abdomen.
The mouth is armed with a pair of styliform mandibles ; and on
each side of the proboscis there is a large, short, cylindincal ap-
pendage, terminated by a curious sort of sucldng-disc, with another
pair of longer jointed members, terminated by prehensile hooks.
These two pairs of appendages, which are probably to be considered
as representing the foot-jaws, are followed by four pairs of legs,
which, like those of the branchiopods, are chiefly adapted for
swimming ; and the tail, also, is a kind of swimmeret. This little
animal can leave the fish upon which it feeds, and then swims freely in
the water, usually in a straight line, but frequently and suddenly
changing its direction, and sometimes turning over and over several
times in succession. The stomach is remarkable for the large caecal
prolongations which it sends out on either side, immediately beneath
the shell ; for these subdivide and ramify in such a manner that they
are distributed almost as minutely as the caecal jDrolongations of the
stomach of the Planaria (fig. 714). The proper alimentary canal, how-
ever, is continued backwards from the central cavity of the stomach, as
an intestinal tube, which terminates in an anal orifice at the extremity
of the abdomen. A far more remarkable departure from the typical
form of the class is shown in the Lerncea, which is found attached
to the gills of fishes. This creature has a long suctorial proboscis ;
a short thorax, to which is attached a single pair of legs, which meet
at their extremities, where they bear a sucker which helps to give
attachment to the parasite ; a large abdomen ; and a pair of pendent
egg-sacs. In its adult condition it buries its anterior portion in the
soft tissue of the animal it infests, and appeai-s to have little or no
power of changing its place. But the young, when they come forth
from the egg., are as active as the young of Cyclops (fig. 720, 0, D),
which they much resemble ; and only attain the adult form after a
series of metamorphoses, in which they cast ofi" their locomotive
members and eyes. It is curious that the original form is retained
with comparatively slight change by the males, which increase but
little in size, and are so unlike the females that no one would suppose
the two to belong to the same family, much less to the same species,
but for the study of their development.^

From the parasitic suctorial Crustacea the transition is not

^ As the group of suctorial Crustacea is interesting rather to the larofessed
naturalist than to the amateur microscopist, even an outline view of it would be un-
suitable to the present work ; and the Author would refer such of his readers as may-
desire to study it to the excellent treatise by Dr. Baird already referred to. Of the
numerous recent essays and memoirs those of Professor Claus should by all means
be consulted. Mr. P. W. Bassett-Smith, Staff-surgeon K.N., has in the last few years
published several interesting papers.

CIKEIPEDIA

967

really so abrupt as it might at first sight appear to the group of
Cirri2)edia, consisting of the barnacles and their allies ; for these,
like many of the Suctoria, are fixed to one spot during the adult
portion of their lives, but come into the world in a condition that
bears a strong resemblance to the early state of many other
Crustacea. The dejDarture from the ordinary crustacean type in
the adults is, in fact, so great that it is not surprising that zoolo-
gists in general should have ranked them in a distinct class, their
superficial resemblance to the Mollusca, indeed, having caused most
systematists to place them in that series, until due weight was
given to those structural features which mark their ' articulated '
character. We must limit ourselves, in our notice of this group,
to that very remarkable part of their history, the microscopic
study of which has contributed most essentially to the elucidation

Fig. 721. — Development of Balanus balanoides : A, earliest
form ; B, larva after second moult ; C, side view of the same ;
D, stage immediately preceding the loss of activity; a,
stomach (?) ; b, nucleus of future attachment (?).

of their real nature. The observations of Mr. J. Y. Thompson,^ with
the extensions and rectifications which they have subsequently
received from others (especially Mr. Spence Bate^ and Mr. Dar-
win ^), show that there is no essential difierence between the early
forms of the sessile Cirripeds [Balanidce or 'acorn-shells') and of the
joednjUGidated (Lepadidce or ' barnacles ') ; for both are active little
animals (fig. 721, A), possessing three pairs of legs and a pair of
compound eyes, and having the body covered with an expanded
carapace, like that of many entomostracous crustaceans, so as in no

1 Zoological Besearches, No. IV. 1830, and Pliil. Trans. 1835, p. 855.
^ ' On the Development of the Cirripedia' h\ Ann. Nat. Hist. ser. ii. vol. viii.
1851, p. 324.

^ Monograph of the Sub-Class Cirripedia, x^ublished by the Ray Society.

968 CEUSTACEA

essential particular to differ from the larva of Cyclops (fig. 720, C).
After going through a series of metamorphoses, one stage of which
is represented in fig. 721, B, C, these larvae come to present a form,
D, which reminds us strongly of that of Daphnia, the body being
inclosed in a shell composed of two valves, which are united along
the back, whilst they are free along their lower margin, where they
separate for the proti'usion of a large and strong anterior pair of
prehensile limbs, provided with an adhesive sucker and hooks, and
of six pairs of posterior legs adapted for swimming. This bivalve
shell, with the members of both kinds, is subsequently thrown off;
the animal then attaches itself by its head, a portion of which, in
the barnacle, becomes excessively elongated into the ' peduncle ' of
attachment, whilst in Balanus it expands into a broad disc of
adhesion ; the first thoracic segment sends backwards a prolongation
which arches over the rest of the body, so as completely to inclose
it, and of which the exterior layer is consolidated into the ' multi-
valve ' shell ; whilst from the other thoracic segments are evolved
the six pairs of cirri, from whose peculiar character the name of
the group is derived. These are long, slender, many -jointed, tendril-
like appendages, fringed with delicate filaments covered with
cilia, whose action serves both to bring food to the mouth and to
maintain aerating currents in the water. The balani are peculiarly
interesting objects in the aquarium on account of the pumping
action of their beautiful feathery appendages, which may be watched
through a tank microscope ; and theii- cast skins, often collected by
the tow-net, are well worth mounting.^

Malacostraca. — The chief points of interest to the microscopist
in the more highly organised forms of Crustacea are furnished by
the structure of the exoskeleton, and by the phenomena of meta-
morphosis, both which may be best studied in the commonest kinds.
The exoskeleton of the Decap)ods in its most complete form consists
of three sti"ata, viz. 1, a horny structureless layer covering the
exterior ; 2, an areolated stratum ; and 3, a laminated tubular sub-
stance. The innermost and even the middle layers, however, may be
altogether wanting ; thus, in the larval forms known as Fhyllosomata
or ' glass-crabs,' the envelope is formed by the transparent horny
layer alone ; and in many of the small crabs belonging to the genus Por-
tunus the whole substance of the carapace beneath the horny invest-
ment presents the areolated structure. It is in the large thick-shelled
crabs that we find the three layers most differentiated. Thus in
the common Cancer ptaguriis we may easily separate the structure-
less horny covering after a short maceration in dilute acid ; the
areolated layer, in which the pigmentary matter of the coloured
parts of the shell is chiefly contained, may be easily brought into
view by grinding away from the inner side as flat a piece as can be
selected, having first cemented the outer surface to the glass slide,
and by examining this with a magnifying power of 250 diameters,
driving a strong light through it with the achromatic condenser ;

^ Valuable details as to the structure of this grouiJ will be found in Dr. P. P. C.
Hoek's report on the Cirripeds collected by H.M.S. Challenger. Compare, also,
M. Nussbaum, Anatomische Stuclien, Bonn, 1890.

MALACOSTEACA 969

whilst the tubular structure of the thick inner layer may be readily
demonstrated by means of sections parallel and perpendicular to its
surface. This structure, which resembles that of dentine, save that
the tubuli do not branch, but remain of the same size through their
whole course, may be particularly well seen in the black extremity
of the claw, which (apparently from some peculiarity in the mole-
cular arrangement of its mineral particles) is much denser than the
rest of the shell, the former having almost the semi-transparence
of ivory, whilst the latter has a chalky opacity. In a transverse
section of the claw the tubuli may be seen to radiate from the central
cavity towards the surface, so as very sti'ongly to resemble their
arrangement in a tooth ; and the resemblance is still further increased
by the presence, at tolerably regular intervals, of minute sinuosities
corresponding with the laminations of the shell, which seem, like
the ' secondary curvatures ' of the dentinal tubuli, to indicate suc-
cessive stages in the calcification of the animal basis. In thin
sections of the areolated layer it may be seen that the ajDparent
walls of the areolae are merely translucent spaces from which the
tubuli are absent, their orifices being abundant in the intervening
spaces.^ The tubular layer rises up through the pigmentary layer
of the crab's shell in little papillary elevations, which seem to be
concretionary nodules ; and it is from the deficiency of the pig-
mentary layer at these parts that the coloured poi-tion of the shell
derives its minutely speckled appearance. Many departures from
this type are presented by the different species of decapods ; thus
in the prawns there are large stellate pigment-spots resembling
those of frogs, the colours of which are often in remarkable con-
formity with those of the bottom of the rock-pools frequented by
these creatures ; whilst in the shrimps there is seldom any distinct
trace of the areolated layer, and the calcareous portion of the skele-
ton is disposed in the form of concentric rings, which seem to be the
result of the concretionary aggregation of the calcifying deposit. ^

It is a very curious circumstance that a strongly marked dif-
ference exists between crustaceans that are otherwise very closely
allied in regard to the degree of change to which their young are
subject in their progress towards the adult condition. For, whilst
the common crab, lobster, spiny lobster, prawn, and shrimp
undergo a regular metamorphosis, the young of the crayfish and
some land-crabs come forth from the Qgg in a form which corre-
sponds in all essential particulars with that of their parents.
Generally speaking, a strong resemblance exists among the young
of all the species of decapods which undergo a metamorphosis, whether
they are afterwards to belong to the macrurous (long-tailed) or to
the hrachyurous (short-tailed) division of the group ; and the forms

^ Tlie Author is now quite satisfied of the correctness of the interpretation put by
Professor Huxley (see his article, ' Tegumentary Organs,' in the Cyclop. Anat. and
Phys. vol. V. -p- 487), and by Professor W. C. Williamson ( ' On some Histological
Features in the Shells of Crustacea' in Quart. Journ. Microsc. Sci. vol. viii. 1860,
p. 38) upon the appearances which he formerly described [Report of British Asso-
ciation for 1847, p. 128) as indicating a cellular structure in this layer.

^ Consult Braun, ' Ueber die histologischen Vorgiinge bei der Hiiutung von
Astacics fluviatilis,' Arbeit. Zool. Inst. Wiirzburg, ii. p. 121.

970

CEUSTACEA

of these larvae are so peculiar, and so entirely different from any of
those into which they are ultimately to be developed, that they were
considei-ed as belonging to a distinct genus, Zoea., until their real
nature was first ascertained by Mr. J. V. Thompson. Thus, in the
earliest state of Carci7iics 7ncenas (small edible crab), we see the head
and thorax, which form the principal bulk of the body, included
within a large carapace or shield (fig. 722, A) furnished with a long
projecting spine, beneath which the fin-feet are put forth ; whilst
the abdominal segments, narrowed and prolonged, carry at the end
a flattened tail-fin, by the strokes of which upon the water the pro-
pulsion of the animal is chiefly effected. Its condition is hence
comparable, in almost all essential particulars, to that of Cyclops.
In the case of the lobster, prawn, and other ' macrurous ' species,
the metamorjihosis chiefly consists in the separation of the loco-
motor and respiratory organs, true legs being developed from the
thoracic segments for the former, and true gills (concealed within a
special chamber formed by an extension of the carapace beneath the

Fig. 722. — Metamorphosis of Carcinus mcenas : A, first or Zoea
stage ; B, second or Megalopa stage ; C, tliird stage, in which
it begins to assume the adult form ; D, perfect form.

body) for the latter ; while the abdominal segments increase in size
and become furnished with appendages (false feet) of their own. In
the crabs, or ' brachyurous ' species, on the other hand, the altera-
tion is much greater ; for, besides the change first noticed in the
thoracic members and respiratory organs, the thoracic region becomes
much mor'e developed at the expense of the abdominal, as seen at
B, in which stage the larva is remarkable for the large size of its
eyes, and hence received the name of MegalojKi, when it was sup-
posed to be a distinct type. In the next stage, 0, we find the
abdominal portion reduced to an almost rudimentary condition, and
bent under the body ; the thoracic limbs are more completely adapted
for walking, save the first pair, which are developed into chelce or
pincers ; and the little creature entirely loses the active swimming
habits which it originally possessed, and takes on the mode of life
peculiar to the adult. ^

In collecting minute Crustacea the ring-net should be used for

1 On the metamorphoses of Crustacea and Cirri^Dedia, see especially the TJnter-
sucTiungen iiber Crustaceen of Professor Claus, Vienna, 1876. A number of

COLLECTING CRUSTACEA 97 1

the fresh-water species, and the tow-net for the marine. In loeaKties
favourable for the latter the feame ' gathering ' will often contain
multitudes of varioiis species of Entomostraca, accompanied perhaps
by the larvfe of higher Ci'ustacea, echinoderm larvte, annelid larvse,
and the smaller Medusce. The water containing these should be put
into a large glass jar, freely exposed to the light ; and, after a little
practice, the eye will become so far habituated to the genei'al appear-
ance and modes of movement of these different forms of animal life
as to be able to distinguish them one from the other. In selecting
any specimen for microscopic examination the dipping-tube will be
found invaluable. The collector will frequently find Megalopa larvae,
recognisable by the brightness of their two black eye-spots, on the sur-
face of floating leaves of Zostera. The study of the metamorphosis
will be best prosecuted, however, by obtaining the fertilised eggs,
which are carried about by the females, and watching the history of
their products. For preserving specimens, whether of Entomostraca
or of larvse of the higher Crustacea, the Author would recommend
sterilised glycerin-jelly as the best medium.

interesting facts and speculations on the Crustacea will be found in P. Miiller's Facts
and Arguments for Darivin (London, 1869). The work of Eeichenbach on the
Development of the Crayfish is contained in vol. xsix. of the Zeitschr. f. wiss. Zool.
p. 123, 1877, and vol. xiv. of the Abhancll. Senckenberg. Naturf. Gesells. 1886. See
also the essay, by W. K. Brooks, On the Development of Lucifer, in Phil. Trans.
1882, p. 57. Mr. P. H. Herrick's memoir on the American Lobster [Bull. U.S. Fish.
Comin. XV. [1895] ) contains matter of much interest. Professor Sars's fully illustrated
monograph of the Crustacea of Norway is being steadily and rapidly published.

972

CHAPTER XXI

INSECTS AND ABACHNIDA

There is no class in the whole animal kingdom which affords to the
microscopist such a wonderful variety of interesting objects, and
such facilities for obtaining an almost endless succession of novelties,
as that of insects. For in the first place, the number of difi'erent
kinds that may be brought together (at the proper time) with ex-
tremely little trouble far surpasses that which any other group of
animals can supply to the most painstaking collector ; then, again,
each specimen will afford to him who knows how to employ his
materials a considerable number of microscopic objects of very
different kinds ; and thirdly, although some of these objects require
much care and dexterity in their preparation, a large propoi^tion
may be got out, examined, and mounted Avith A^ery little skill or
trouble. Take, for example, the common house-fly; its e^/es may
be easily mounted, one as a transparent, the other as an opaque
object; its antennae, although not such beautiful objects as those of
many other Diptera, are still well worth examination ; its tongtce or
' proboscis ' is a peculiai-ly interesting object, though requiring some
care in its preparation ; its spiracles, which may be easily cut out
from the sides of its body, have a very curious structure ; its
alimentary canal affords a very good example of the minute distri-
bution of the trachece ; its tving, examined in a living sjDecimen
newly come forth from the pupa state, exhibits the circulation of
the blood in the ' nervures,' and when dead shows a most beautiful
play of iridescent colours, and a remarkable areolation of surface,
when examined by light reflected from its surface at a particular
angle ; its foot has a very peculiar conformation, which is doubtless
connected with its singular power of walking over smooth surfaces
in direct opposition to the force of gravity, while the structiire
and physiology of its sexual apparatus, with the history of its develop-
ment and metamorphoses, would of itself suffice to occupy the whole
time of an observer who should desire thoroughly to work it out, not
only for months, but for years. ^ Hence, in treating of this dej)artment
in such a work as the present, the Author labours under the emharras
cles richesses ; for, to enter into such a description of the parts of the
structure of insects most interesting to the microscopist as should

^ See Mr. Lowne's valuable treatise on The Anatomy and Physiology of the
Blow-fly, 1870 ; second edition 1891-4.

MOUNTING INSECTS 973

be at all comparable in fiilness with the accounts which it has been
thought desirable to give of other classes would swell out the
volume to an inconvenient bulk ; and no course seems open but to
limit the treatment of the subject to a notice of the kinds of
objects which are likely to prove most generally interesting, with a
few illustrations that may serve to make the descriptions more clear,
and with an enumeration of some of the sources whence a variety
of specimens of each class may be most readily obtained. And this
limitation is the less to be regretted, since there already exist in
our language numerous elementary treatises on entomology, wherein
the general structure of insects is fully explained, and the conforma-
tion of their minute parts as seen with the microscope is adequately
illustrated.^

A considerable number of the smallei' insects — especially those
belonging to the orders Goleoptera (beetles), Neuroptera (dragon-fly,
May-fly, &c.), Hymenoptera (bee, wasp, &c.), and Diptera (two-winged
flies) — may be mounted entire as opaque objects for low magnifying
powers, care being taken to spread out their legs, wings, &c., so as
adequately to display them, which may be accomplished, even after
they have dried in other positions, by softening them by steeping
them in hot water, or, where this is objectionable, by exposing
them to steam. Directions on this point, ajoplicable to small and
lai-ge insects alike, may be found in various text -books of ento-
mology. There are some, however, whose translucence allows them
to be viewed as transparent objects, and these are either to
be mounted in Canada balsam or in Dean's medium, glycerin
jelly, or Farrant's gum, according to the degree in which the horny
opacity of their integument requires the assistance of the balsam to
facilitate the transmission of light through it, or the softness and
delicacy of their textures render an aqueous medium more desirable.
Thus an ordinary flea or hug will best be mounted in balsam ; but
the various parasites of the louse kind, with some or other of which
almost every kind of animal is affected, should be set up in some of
the ' media.' Some of the aquatic larvfe of the Diptera and Neuro-
ptera, which are so transparent that their whole internal organisa-
tion can be made out without dissection, are very beautiful and
interesting objects when examined in the living state, especially
because they allow the circulation of the blood and the action of the
dorsal vessel to be discerned. Among these there is none prefer-
able to the larva of the Ephemera marginata (day-fly), which is dis-
tinguished by the possession of a number of beautiful appendages
on its body and tail, and is, moreover, an extremely common
inhabitant of our ponds and streams. This insect passes two or
even three years in its larval state, and during this time it
repeatedly throws off its skin ; the cast skin, when perfect, is an
object of extreme beaiity, since, as it formed a complete sheath to
the various appendages of the body and tail, it continues to exhibit
their outlines with the utmost delicacy ; and by keeping these larvae

1 An excellent introduction to the study of insects will be found in Tlie Structure
and, Life-history of the Cockroach, by L. C. Miall and A. Denny (London, 1886).
See also Dr. D. Sharp in the Cambridge Natural History.

974 INSECTS AND AEACHNIDA

in an aquarium, and by mounting the entire series of their cast
skins, a record is preserved of the successive changes they undergo.
Much care is necessary, however, to extend them upon slides in con-
sequence of their extreme fragility ; and the best plan is to place
the slip of glass under the skin whilst it is floating on water, and to
lift the object out upon the slide. Thin sections of insects, cater-
pillars, (fee, which bring the internal parts into view in their normal
relations, may be cut with the microtome by first soaking the body
(as suggested by Dr. Halifax) in thick gum-mucilage, which passes
into its substance, and gives support to its tissues, and then inclos-
ing it in a casing of melted paraffin made to fit the cavity of the
section-instrument.

Structure of the Integument. — In treating of these separate parts
of the organisation of insects which furnish the most interesting
objects of microscopic study we may most aj)propriately commence
with their integument and its appendages (scales, hairs, (fee). The
body and members are closely invested by a hardened skin, which
acts as their skeleton, and afibrds points of attachment to the muscles
by which their several parts are moved, being soft and flexible, how-
ever, at the joints. This skin is usually more or less horny in its
texture, and is consolidated by the aniraal substance termed chitine,
as well as in some cases by a small quantity of mineral matter. It
is in the Ooleoptera that it attains its greatest development, the
^ dermo-skeleton ' of many beetles being so firm as not only to confer
upon them an extraordinary power of passive resistance, but also to
enable them to put forth enormous force by the action of the power-
ful muscles which are attached to it. The outer layer of this dermo-
skeleton is continuous, the cells which secrete it lying beneath the
parallel laminae of which it is made up ; on the surface the chitinous
cuticle may be seen to be marked out into a number of polygonal
(usually hexagonal) areas which correspond to the subjacent secret-
ing cells. Of this we have a very good example in the superficial
layers (fig. 737, B) of the thin horny lamellse or blades which
constitute the terminal portion of the antenna of the cockchafer,
this layer being easily distinguished from the intermediate portion
(A) of the lamina by careful focussing. In many beetles the hexa-
gonal areolation of the surface is distinguishable when the light is
reflected from it at a particu.lar angle, even when not discernible in
transparent sections. The integument of the common reel ant
exhibits the hexagonal cellular arrangement very distinctly through-
out ; and the broad flat expansion of the leg of the Crahro (' sand-
wasp') affords another beautiful example of a distinctly cellulai'
arrangement of the outer layer of the integument. The inner layer,
however, which constitutes the principal jjart of the thickness of the
horny casing of the beetle tribe, seldom exhibits any distinct organi-
sation, though it may be usually separated into several lamellae,
which are sometimes ti'aversed by tubes that pass into them from
the inner surface, and extend towards the outer without reach-
ing it.

Tegumentary Appendages. — The surface of the insects is often
beset, and is sometimes completely covered, with appendages having

INTEGUMENT 975

either the form of broad flat scales oi- that of hairs moi-e or less
approaching the cylindrical shape, or some form intermediate be-
tween the two. The scaly investment is most complete among the
Lepidojytera (butterfly and moth tribe), the distinguishing character
of the insects of this oi'der being derived fi'om the presence of a
regular layer of scales ujDon each side of their large membranous
Avings. It is to the peculiar coloration of the scales that the various
hues and figures are due, by which these wings are so commonly
distinguished, all the scales on one patch (for example) being green,
those of another red, and so on ; for the subjacent membrane
remains perfectly transparent and colourless when the scales have
been brushed oflf from its surface. Each scale seems to be composed
of two or more membranous lamellas, often with an intervening
deposit of pigment, on which, especially in Lepidoptera, their colour
depends. Certain scales, however, especially in the beetle tribe,
have a metallic lustre, and exhibit brilliant colours that vary with
the mode in which the light glances from them; and this 'irides-
cence,' which is specially noteworthy in the scales of the Curcidio
imjMrialis ('diamond beetle'), seems to be a purely oj)tical efiect,
depending either (like the prismatic hues of a soap-bubble) on the
extreme thinness of the membranous lamellee, oi- (like those of
' mother-of-pearl ') on a lineation of surface produced by their corru-
gation. Each scale is furnished at one end with a sort of handle or
' pedicle ' (figs. 723, 724), by which it is fitted into a minute socket
attached to the surface of the insect ; and on the wings of Lepido-
ptera these sockets are so arranged that the scales lie in very regular
rows, each row overlapping a portion of the next, so as to give to
their surface, when sufftcientlj' magnified, veiy much the appearance
of being tiled like the roof of a house. Such an arrangement is said
to be ' imbricated.' The forms of these scales are often very curious,
and fi-equently difi"er a good deal on the several jDai-ts of the wings
and of the body of the same individual, being usually more expanded
on the former and narrower and more hairlike on the latter. A
peculiar type of scale, which has been distinguished by the designa-
tion 2ylwimde, is met with among the Fieridce, one of the principal
families of the diurnal Lepidoptei^a. The ' plumules ' are not flat,
but cylindrical or bellows-shaped, and are hollow ; they are attached
to the wing by a bulb at the end of a thin elastic peduncle that
difiers in length in difierent species, and proceeds from the broader,
not from the narrower end of the scale ; whilst the free extremity
usually tapers oS and ends in a kind of brush, though sometimes it
is broad and has its edge fringed with minute filaments. These
' plumules ,' which are peculiar to the males, are found on the upper
surface of the wings, partly between and partly under the ordinary
scales. They seem to be represented among the Lyccenidce by the
' battledore' scales to be presently described. ^

The peculiar markings exhibited by many of the scales very early
attracted the attention of opticians engaged in the application of

1 See Mr. Watson's memoirs ' On the Scales of Battledore Butterflies,' in Monthly
Microscopical Journal, ii. pp. 73, 314.

976

INSECTS AND ARACHNIDA

achromatism to the microscope ; for, as the clearness and strength
with which they could be shown were found to depend on the
deoree to which the angular apertui-e of an objective could be opened
without sacrifice of perfect correction for spherical and chromatic
aberration, such scales proved very serviceable as 'tests.' The
Author can well remember the time when those of the Mooyho Mene-
laus (fig. 723), the ordinary and ' battledore ' scales of the Polyom-
matus Argtos (figs. 724, 725), and the scales of the Lejnsma saccharina
(fig. 726), which are now only used for testing objects of lotv or
medium power, were the recognised tests for objects of high power ;
while the exhibition of alternating light and dark bands on a
Podura scale was regarded as a first-rate performance. It is easy
for anyone possessed of a good apochromatic objective of 6 mm.
(i- inch) to obtain all the characteristic features of the scale ; but

the determination of the method of
construction of the scale and the proper
interpretation of the ' markings ' is a
matter that the wise microscopist will
prefer to relegate to the days when the
apertures of our best present lenses will
be looked upon comparatively as we now
look upon the earliest achromatic ob-
jectives. No one can give a fairly
comprehensive and satisfactory sugges-
tion of the true nature of the Poduo'a
scale, and yet on no one object has
microscopy lavished so much labour for
so many years.

The easier test scales are furnished
by the Lepidoptera (butterflies and
moths), and among the most beautiful
of these, both for colour and for regu-
larity of marking, are those of the
Morpho Menelcms\&g. 723). These are
of a rich blue tint, and exhibit strong
longitudinal stria?, which seem due to
ribbed elevations of one of the superficial layers. There is also an
appearance of transverse striation, which cannot be seen at all with
an inferior objective, but becomes very decided with a good objective
of medium focus ; and this is found, when submitted to the test of a
high power and good illumination, to depend upon the presence of
transverse thickenings or corrugations (fig. 723), probably on the in-
ternal surface of one of the membranes. The large scales of the Poly-
ommattis Argus ('azure blue ' butterfly) resemble those of the Menelaus
in form and structure, but are more delicately marked (fig. 724).
Their ribs are more nearly parallel than those of the Menelaus scale,
and do not show the same transverse striation. When one of these
scales lies partly over another, the efibct of the optical intersection
of the two sets of ribs at an oblique angle is to produce a set of
interrupted striations (5), very much resembling those o{ the Podura
scale. The same butterfly furnishes smaller scales, which are com-

FiG. 723.— Scale of Morplio
Me7ielai(,s.

SCALES

977

monly termed the ' battledore ' scales, fi'om their resemblance in
form to that object (fig. 724, a). These scales, which occur in the
males of several genera of the family Li/cceiiidce, and present a
considerable variety of shape, ' are marked by narroAv longitudinal
ribbings, which at intervals seem to expand into i-ounded or oval
elevations that give to the scales a dotted appearance (fig. 725) ; at
the lower part of the scale, however, these dots are wanting.
Dr. Anthonj^ describes and figures them as elevated bodies, some-
what resembling dumb-bells or shirt-studs, ranged along the ribs,
and standing out from the general surface.^ Other good observers,
however, whilst recognising the stud-like bodies described by Dr.
Anthony, regard them as not projecting from the external surface
of the scale, but as interposed between its two lamella? ; ^ and this
view seems to the Author to be more conformable than Dr. Anthony's
to general probability.

The more difficult ' test scales ' are furnished by little wingless
insects ranked together by Latreille in the order Thi/scmura, but

Fig. 724. — Scales of Polyommatus Argus
(azure blue) : a, battledore scale ; h,
interference striae.

Fig. 725. — Battledore scale of
Polyommatus Argus (azure
blue).

now separated by Sir John Lubbock,* on account of important
differences in internal structure, into the two groups Collemhola and
true TJiysanitra. Of the former of these the Lejyismiclce constitute
the typical family ; and the scale of the common Lejnsma saccha-
rina, or ' sugar-louse,' ^ very early atti-acted the attention of

1 See Watson, loc. cit.

- ' The Markings on the Battledore Scales of some of the LejnclojJtera ' in Monthly
Microscojncal Journal, vol. vii. 1872, pp. 1, 250.

s See ' Proceedings of the Microscopical Society,' ojj. cit. p. 278.

* See his Monograph of the Collemhola and Thysanura, pubHshed by the Eay
Society, 1872.

■'' This insect may be found in most old houses, frequenting damp warm cupboards,
and especially such as contain sweets ; it may be readily caught in a small pill-box,
which should have a few pinholes in the lid; and if a drop of chloroform be put
over the holes the inmate will soon become insensible, and may be then turned out
upon a piece of clean paper, and some of its scales transferred to a slip of glass by
simply piressing this gently on its body.

3 R

978

INSECTS AND AEACHNIDA

mici'oscopists on account of its beautiful shell-like sculpture. When
viewed under a low magnifying power it presents a beautiful
' watered-silk ' appearance, which, with higher amplification, is found
to depend (as Mr. R. Beck first pointed out)^ upon the intersection
of two sets of sti'ite, representing the different structural arrange-
ments of its two superficial membranes. One of its sui-faces (since
ascertained by Mr. Joseph Beck ^ to be the ttnder or attached
surface of the scale) is raised, either by corrugation or thickening,
into a series of strongly marked longitudinal i-ibs, which run nearly
parallel from one end of the scale to the other, and are particularly
distinct at its margins and at its free extremity ; whilst the other

surface (the free oi- outer, according
to Mr. J. Beck) presents a set of less
definite corrugations, radiating from
the pedicle, where they are strongest,
towards the sides and free extremity
of the scale, and therefore crossing
the parallel ribs at angles more or
less acute (fig. 726). It was further
pointed out by Mr. R. Beck that the
intersection of these two sets of cor-
rugations at difierent angles produces
most 'curious efl'ects upon the appear-
ances which optically represent them.
For where the diverging ribs cross
the longitudinal ribs very obliquely,
as they do near the free extremity of
the scale, the longitudinal ribs seem
broken up into a series of ' excla-
mation markings,' like those of the
Poclura ; but where the crossing is
ti"ansverse or nearly so, as at the
sides of the scale, an appearance is
presented as of successions of large
bright beads. The conclusion drawn
by the Messrs. Beck, that these in-
terrupted appearances are ' produced
by two sets of uninterrupted lines on difierent surfaces,' has been
confirmed by the cai-eful investigations of Mr. Moorhouse.^ The
minute beaded structure observed by Dr. Royston-Pigott * alike in
the ribs and in the intervening spaces may now be certainly re-
garded as an optical eflect of difii-action. In the scale of a type
nearly allied to Lejnsma, the Machilis poh/poda, the very distinct
ribbing (fig. 727) is produced by the corrugation of the imder mem-
branous lamina alone, the upper or exposed lamina being smooth,
with the exception of slight undulations near the pedicle, and the
cross-markings being due to structure between the superposed

1 The Achromatic Microscoi^e, p. 50.

-_ See his appendix to Sir Ooliu Lubbock's Monograph.

•' Monthly Microscojncal Journal, vol. xi. 1874, p. 13, and vol. xviii. 1877, p. 31.

^ Ibid. vol. ix. 1873, p. 63.

Fig. 726.— Scale of Lej}isma
saccharina.

SCALES

979

membranes, pi-obably a deposit on the intei'ior surface of one oi- both
of them.'^

Although the Poduridoi and Lepismidce now i-ank as distinct
families, yet they approximate sufficiently in general organisation,
as well as in habits, to justify the expectation that their scales
would be framed upon the same plan. The Poduridm are found
amidst the sawdust of wine-cellars, in garden tool-houses, or near
decaying wood, and derive their populai- name of ' spring-tails '
from the possession by many of them of a curious caudal appen-
dage by which they can leap like fleas. This is particulai-ly
well developed in the species now designated Lepidocyrtus curvi-
collis, wdiich furnishes what are oi-dinai'ily known as ' Podura ' scales.
' When full grown and unrubbed,' says Sir
John Lubbock, ' this species is very beauti-
ful, and reflects the most gorgeous metallic
tints.' Its scales are of different sizes and
of different degrees of strength of marking
(fig. 728, A, B), and are therefore by no
means of uniform value as tests. The
general appearance of their sui-face, under
a power not sufficient to resolve their mark-
ings, is that of watered silk, light and dark
bands passing across it with wavy irregu-
larity; but a well-corrected objective of
very moderate aperture now suffices to re-
solve every dark band into a row of dis-
tinct ' exclamation marks.' A certain
longitudinal continuity may be traced be-
tween the ' exclamation marks ' in the
ordinary test scale ; but this is much more
apparent in other scales from the same
species (fig. 729), as well as in the
scales of various allied types, which were
carefully studied by the late Mr. R. Beck.^
In certain other types, indeed, the scales
have very distinct longitudinal parallel
ribs, sometimes with regularly disposed
cross-bars ; these ribs, being confined to one
surface only (that which is in contact with
the body), are not subject to any such interference with their optical
continuity as has been shown to occur in Lepisma ; but more oi- less
distinct indications of I'adiating corrugations often pi-esent them-
selves. The appearance of the interrupted ' exclamation marks '
Mr. J. Beck considers to be due ' to irregvilar coi-rugations of the
outer surface of the under membrane, to slight undulations on the
outer surface of the upper membrane, and to structui'e between the
superposed membranes.' It has, indeed, been stated by Mr. Joseph

Pig,

727.— Scale of Much His
]}ohjpoda.

1 See Mr. Joseph Beck in Sir J. Lubbock's Monograjili, p. 255.

- Trans. Microsc. Soc. n.s. vol. x. 1862, p. 83. See also Mr. Joseph Beck, in the
axJijendix to Sir John Liibbock's Monograph^ and in Monthhj Microsco2ncal Journal
iv. p. 253.

3 K 2

98o

INSECTS AND ARACHNIDA

Beck that the scales of a lepidopterous insect belonging to the genus
Mortno, which under a low power present the watered-silk appear-
ance seen in the Podura scale, under a J in. obj . show the ' exclama-
tion markings,' whilst under a -j-^ in. obj. they exhibit distinct ribs
from pedicle to apex, thus showing in one scale how the appearances
run from one into the other. ^

The hairs of many insects, and still more of their larvae, are
very interesting objects for the microscope on account of their
branched or tufted conformation, this being pai'ticularly remarkable
in those with which the common hairy caterpillars are so abundantly
beset. Some of these afford very good tests for the perfect correction
of objectives. Thus the hair of the bee is pretty sure to exhibit

willlCi;!!''
'0

■»

iiili

Pig. 728. — Test scales of LepidocyrtUs curvi-
collis : A, large strongly marked scale ; B,
small scale, more faintly marked.

Fig. 729. — Ordinary scale
of Lepidocyrtus curvi-
coUis.

sti'ong prismatic colours if the chromatic aberration should not have
been exactly neutralised ; and that of the lai'va of a Dermestes
(commonly, bvit erroneously, termed the ' bacon-beetle ') was once
thought a very good test of defining power, and is still useful foi-
this purpose. It has a cylindrical shaft (fig. 730, B) with closely set
whorls of spiny protuberances, four oi- five on each whorl ; the
highest of these whorls is composed of mere knobby spines ; and the
hair is surmounted by a curious circle of six or seven large filament's,
attached by theii- pointed ends to its shaft, whilst at their free ex-
tremities they dilate into knobs. An approach to this structure is
seen in the 'hairs of certain Myriopods (centipedes, galley-worms, &c.),
of which an example is shown in fig. 730, A ; but a beautiful photo-

1 Journ. Boy. Microsc. Soc. vol. ii. 1879, p. 810. On the subject generally
Dr. A. Spuler's 'Beitrag zur Kenutniss des feinerenBaues . . . der Fliigelbedeckung
der Schmetterlinge,' in Zool. Jahrh. Anat. viii. should be consulted.

r

HAIES

981

mici'ogi'aph of the hair of Polyxenus lagurtis, of the family Poly-
desmiclce (order Chilognatha), is given in fig. 6 of the frontispiece.
This is one of the finest test objects for medium poweivs. Other
minute forms of this ck\ss ai'e most beautiful objects inider the
binocular microscope on account of the remai-kable structui'e and
i-egular ai'rangement of theii- haii's.

In examining the integument of insects and its appendages
parts of the surface may be vieAved either by I'eflected or ti-ansmitted
light, according to their degi-ee of transpai'ence and the nature of
their covering. The beetle and butterfly tribes furnish the greater
number of the specimens suitable to be viewed as ojjaqite objects ;
and nothing is easier than to mount portions of the elytra of the
former (usually the most showy pai'ts of theii-
bodies), or of the wings of the latter, in the ^ b

manner described in Chapter YII. The tribe
of Curculionidce, in which the surface is beset
with scales having the most varied and lustrous
hues, is distinguished among Coleoptera for the
brilliancy of the objects it afibrds, the most
remarkable in this respect being the well-known
Curculio imjMrialis, or ' diamond beetle ' of South
America, parts of whose el;yiira, when properly
illuminated and looked at with a low jjower,
show like clusters of jewels flashing against a
dark velvet ground. In many of the British
Curcidiooiidce, which are smaller and far
less brilliant, the scales lie at the bottom of
little depressions of the surface ; and if the
elytra of the diamond beetle be carefully
examined, it will be found that each of the
clusters of scales which are arranged upon it
in rows seems to rise out of a deep joit which
sinks in by its side. The transition from scales
to hairs is extremely well seen by comparing
the different parts of the surface of the diamond
beetle with each other. The beauty and bril-
liancy of many obj ects of this kind are increased
by mounting them in cells in Canada balsam,
even though they are to be viewed with reflected

light ; other objects, however, ai-e rendered less attractive by this
treatment ; and in order to ascertain whether it is likely to improve
or to deteriorate the specimen, it is a good plan fii'st to test some
other portion of the body having scales of the same kind by touching
it with turpentine, and then to mount the part selected as an object,
either in balsam oi' dry, accoi-ding as the turpentine increases or
diminishes the brilliancy of the scales on the spot to which it was
applied. Portions of the Avings of Lepidoptera are best mounted as
opaque objects without any other preparation than gumming them
flat down to the disc of the Avooden slide, care being taken to avoid
disturbing the arrangement of the scales and to keep the objects,
when mounted, as secluded as possible from dust. In selecting such

Fig. 730. — A, hair of
Myriopod; B, hair of
Derniestes.

982

INSECTS AND AEACHNIDA

portions it is well to choose those which have the brightest and
the most contrasted colours, exotic butterflies being in this respect
usually pi'eferable to British ; and before attaching them to their
slides cai'e should be taken to ascertain in what position, with the
arrangement of light ordinarily used, they ai-e seen to the best ad-
vantage, and to fix them there accordingly. Wlienever portions of the
integument of insects are to be viewed as transparent objects, for the
display of their intimate structure, they should be mounted in
Canada balsam, after soaking for some time in turpentine, since this
substance has a peculiar effect in increasing their translucence. Not
only the hoi-ny cases of perfect insects of various orders, but also of
those of theii' pupse, are worthy of this kind of study ; and objects
of great beauty (such as the chrysalis case of the emperor moth), as
well as of scientific interest, are sure to reward such as may prose-
cute it with any assiduity. Further information may often be gained
by softening such parts in potash and viewing them in fluid. The

scales of the wings of Lepido-

jDtera etc. are best transferred

to the slide by simply pressing a

portion of the wing either upon

the slip of glass or upon the

cover ; if none should adhere

the glass may first be gently

breathed on. Some of them

are best seen when examined

' dry,' whilst others are more

clear when mounted in fluid ;

and for the determination of

their exact structure it is well

to have recourse to both these

Fig. 731.— Head and compound eyes of the methods. Hairs, on the other

bee, showing the ocellites in situ on one i i i x monn+prl in

side, A, and displaced on the other, B ; J^f ^^' ^^® "^®^^ mounted m

a, a, a, stemmata ; h, b, antennse. balsam.

Parts of the Head.— The

eyes of insects, situated upon the upper and outer part of the head,
are usually very conspicuous oi'gans, and are frequently so large as
to touch each other in front (fig. 731). We find in their structure
a remarkable example of that multiplication of similar parts which
seems to be the predominating ' idea ' in the conformation of arti-
culated animals ; for each of the large protuberant bodies which we
designate as an eye is really a ' compound ' eye, made up of many
hundred or even many thousand minute conical ocelli (B). Ap-
proaches to this structure are seen in Entomostraca ; but the
number of ' ocellites ' thus grouped together is usually small.
In the higher Crustacea, however, the ' ocelli ' are very numerous ;
and their compound eyes are constructed upon the same general
plan as those of insects, though their shape and position are often
very peculiar. The individual ocelli are at once i-ecognised when
the 'compound eyes' are examined under even a low magnifying
power by the 'faceted' appearance of the surface (fig. 731, A),
which is marked out by very regular divisions either into hexagons

EYES

983

cornea ; h, transparent pyramids
surrounded with pigment ; c, fibres
of the optic nerve ; d, trunk of the
optic nerve.

or squares ; each facet is the ' corneule ' of a separate ocellite, and

has a convexity of its own; hence, by counting the facets, we

can ascei'taiu the number of ocelli

in each ' compound eye.' In the two ^

eyes of the common fly there are

as many as 4,000 ; in those of the

cabbage-buttei'fly thei-e are about

17,000; in the dragon-fly 24,000;

and in the Mordella beetle 25,000.

The structure of the arthropod eye

is best explained by a comparative

account of the various stages of

complication which it presents.

In various larva? the cuticular

layer is modified to form a single

lens, behind which are simple, sepa- ^m. 732.-Diagram of a section of the

rate, elongated hypodermic cells, composite eye of Melolontha vul-

SOme of which ai^e continuous with 5'«^'*s (cockchafer) : a, facets of the

fine branches of the optic nerve ;
these may be called retinal cells.
The next stage in complication is
seen when these last combine to foi'ui
groups, ' retinula? ; ' the sensitive
cells, may become divided into two
regions, an outer one, which is
' vitreous ' and refractive in function,
while the inner part remains sensi-
tive ; the corneal surface may be-
com.e broken up into a number of
facets, each of which corresponds to
one of the ' pyramids ' so formed,
and within the retinula there may
be dififerentiated a rhabdom (see fig.
733) formed by the nerve-rod.

After traversing the pyramids
the rays reach the extremities of
the fibres of the optic nerve, which
are surrounded, like the pyramid,
by pigmentary substance. Thus the
rays which have passed through the
several ' corneules ' are prevented
from mixing with each other ; and
no rays, save those which pass in
the axes of the pyramids, can reach
the fibres of the optic nerve. Hence,
it is evident that, as no two ocelli
on the same side (fig. 731) have
exactly the same axis, no two can
receive their rays from the same point of an object ; and thus,
as each compound eye is immovably fixed ujDon the head, the
combined action of the entire aggregate will probably afford but

Fig. 733. — Part of the compound eye
of Phryganea ; tlie retinal cells are
seen to be united into a retinula {r)
which is differentiated into a rhab-
dom [in) posteriorly ; cc, crystalline
cone ; /, facet of compound eye ;
pg, pigment. (After Grenadier.)

984

INSECTS AND ARACHNIDA

a single image, i^esembling that which we obtain by means of our
single eyes. This judgment has received a confirmation as unex-
pected as it is complete and beautiful. The subject of the real
nature of compound vision can be considered no longer a matter of
doubt. We have as complete evidence of its character as we have
of that of vision by vertebi-ate eyes. It is to Professor S. Exner,
of Vienna, that we are indebted for the striking though simple
resvilts. He has been engaged foi- years on cognate researches,
and has at length succeeded in taking a photo-micrograph of the
image presented at the back of a compound insect eye in precisely
the same manner as a similar photogi-aph might be taken with
the retina removed at the back of the eye of one of the higher
vertebrates.

The demonstration was satisfactorily made, and the present Editor
is indebted for a knowledge of the following details to the courtesy
of a private communication from Professor Exner.

The general result of the researches on the subject is presented
in fig. 734, which is the image at the back of the compound eye

of Lampyris splendklula
(fire-fly), in the position
in which it would be por-
trayed upon the retina, but
magnified 1 20 diameters.
On to the window pane a
letter R cut out in black
was fixed ; the distance of
the window from the eye
was 225 cm., while the dis-
tance of the church from the
window through which it is
seen in the magnified image
was 1.35 paces.

The result is unmistak-
able ; there may appear to be
some matters of interest still
needing interpretation, but
these are explained in the
monograph by Exner, giving
the complete details of the
method he adopted and the
mathematical explanation of
the results he obtained. The
rectitude of the image and
the reversion of the R are
certainly noteworthy ; and
as a contribution to our
knowledge of the physiology
of sight in insects and other animals with compound eyes, the im-
portance of the result obtained by the ingenuity and skill of Professor
Exner is great, giving us a new start on solid ground. The mathe-'
matics of the question are fully discussed by Exner in a memoir, to

Pig. 734. — Image of a window with the
letter R on one of its panes, and a church
beyond, taken through the compound eye
of Lampyris s^lendidula, and magnified
120 diams.

EYES

985

which the student must be referred for comj^lete inforinatiou.^ The
kind of image formed by the compound eye has long been a matter
of discussion amongst physiologists.'^

The process of taking the photo-micrograph coj^ied in fig. 734
was this : The eye of the Laimpyris was carefully dissected out from
the head, the retina and pigment i-emoved with a fine camel-hair
pencil, and the back of the eye immersed in a raixture of glycerin
and water, possessing a refractive index of 1 "SiG ; this was already
known to be the refractive index of the blood of the Lampyris. The
whole was placed upon an ordinary cover-glass, this being fixed by
its edges to a slide or object-carrier with a circulai- aperture cut in
it, as in fig. 735, 0 ; a is the slide Avith an aperture less in diameter

Fig. 735. — Diagrammatic illustration of the method by wliich
the image in fig. 734 was photo-micrographed.

than the cover-glass h cut through it ; c is the fluid-medium of
7i= 1-346 in which the back parts of the eye are immersed, thus
fulfilling the conditions of living sight, while the cornea with its
lenses is shown at cZ, being, as in the normal state, in air. If the eye

1 Sitzungsher. Akad. Wissensch. Wien, Bd. xcviii. (18S9), pp. 13, 143 ; also Die
Physiologie der facettirten Aitgen von Krebsen imd Insecten (Levgzig und Wien,
1891).

- A critical history of the discussion will be found in Chapter VII. of Sir J.
Lubbock's Senses of Anhnals (London, 1888), and in Dr. D. Sharp's Annual Address
to the Entomological Society of London, 1888 (1889). See also Mr. A. JVIallock in Proc.
Roij. Soc. Land. vol. Iv. p. 85. The question of the physiology of the compound eye
of Arthropods has given rise to much discussion. For further details as to its
structure consult Grenadier's great work, TJntersuchungen ilber das Sehorgan der
Artliropoden &c. (Gljttingen, 1879) ; Carriere, Die Sehorgaiie der Thiere &c. (Munich
and Leipzig, 1885) ; Hickson, ' The Eye and Optic Tract of Insects,' Quart. Journ.
Microsc. Sci. xxv. p. 215 ; Lankester and Bourne, ' The Minute Structure of the
Lateral and Central Eyes of ScorjDio and Limulus,' Quart. Journ. Microsc. Sci.
xxiii. p. 177 ; Lowne, ' On the Compound Vision and the Morphology of the Eye in
Insects,' Trans. Linn. Soc. (2), ii. p. 389 ; Patten, ' Eyes of Molluscs and Arthropods,'
Mitth. Ziiol. Stat. Neajiel, vi.

986 INSECTS AND ARACHNIDA

be noAv examined with a microscope (the C of Zeiss was employed),
the ' lenses ' will be distinctly seen, but if the focus be readjusted
to the focal plane of the image in the eye this image will be seen
and magnified. This will be understood from D (fig. 735), where
e,y represent the image, h the cornea with its ' lenses ' gr, e'-f being
the image of the object thrown upon the position from which the
retina has been removed, and which is now made the focal plane of
the objective employed.

It was this image {e.'-/') which was photographed in the ordinary
manner with a Zeiss photo-micrographic apparatus and the C object-
glass. The manner in which this was done is seen diagrammatically
at E (fig. 735), where i indicates the cornea of the eye exposed
to air, k the image thrown though the ' lenses ' as a unified
picture at the focal point of the microscope, and I is the sensitised
plate on which the image was photographed. This piece of admi-
rable research and its clear results have a value not only physio-
logical but philosophical.

Although the structure already described may be considered as
typical of the eyes of insects, yet there are various departures fi-om
it (most of them slight) in the dififerent members of the class.
Thus in some cases the posterior surface of each ' corneule ' is
concave ; and a space is left between it and the iris-like dia-
phragm, which seems to be occupied by a watery fluid or ' aqueovis
humour.' In other instances, again, this space is occupied by a
double-convex body, which seems to represent the ' crystalline
lens,' and this body is sometimes found behind the iris, the num-
ber of ocelli being reduced, and each one being larger, so that the
cluster presents more resemblance to that of spiders, ifec. Besides
their ' compound ' eyes, insects usually possess a small number of
' simple ' eyes (termed ocelli or stemmata) seated upon the top of the
head (fig. 731, a, a, a). Each of these consists of a single very con-
vex corneule, to the back of which proceeds a bundle of rods that
are in connection with fibrils of the ojatic nerve. Such ocelli are
the only visual organs of the larvse of insects that undergo complete
metamorphosis, the ' compound ' eyes being only developed towards
the end of the pupa stage.

Various modes of preparing and mounting the eyes of insects
may be adopted, according to the manner wherein they are to be
viewed. For the observation of their external faceted surface by
reflected light it is better to lay down the entire head, so as to
present a front face or a side face, according to the position of the
eyes, the former giving a view of both eyes when they approach
each other so as nearly or quite to meet (as in fig. 731), whilst the
latter will best display one when the eyes are situated more at the
sides of the head. For the minuter examination of the ' corneules,'
however, these must be separated from the hemispheroidal m.ass
whose exterior they form by pi-olonged maceration, and the pig-
ment must be carefully washed away by means of a fine camel-hair
brush from the inner or posterior surface. In flattening them out
upon the glass slide one of two things must necessarily happen :
either the mai-gin must tear when the central portion is pressed

ANTENNA

987

down to a level, or, the margin remaining entire, the central por-
tion must be thrown into plaits, so that its corneules overlap one
another. As the latter condition interferes with the examination
of the strnctm'e much more than the former does, it should be
avoided by making a numbei- of slits in the margin of the convex
membi-ane befoi'e it is flattened out. Vertical sections, adapted to
demonstrate the structure of the ocelli and their relations to the
optic nerve, can be only made when the insect is fresh or has been
preserved in strong spirit. Mr. Lowne recommends that the head
should be hardened in a 2 per cent, solution of chromic acid, and
then imbedded in cacao butter ; the sections must be cut very thin,
and should be mounted in Canada balsam. The following ai'e some
of the insects whose eyes are best adapted for microscopic pre-
parations ; Coleoptera^ Cicindela, Dytiscus, Melolontha (cockchafer),
Lucanus (stag-beetle) ; Orthoj)tera, Acheta (house and field crickets),
Locusta ; Hemiptera, Notonecta (boat-fly) ; Netiroj^tera, Libellula
(dragon-fly), Agrion ; Hymenojjtera, Yespida? (wasps) and Apid?e
(bees) of all kinds ; Lepidoptera^ Vanessa (various species of), Sphinx
ligusti-i (jDrivet hawk-moth), Bombyx (silkworm moth and its allies) ;
Diptera, Tabanus (gad-fly), Asilus, Eristalis (drone-fly), Tipula (crane-
fly), Musca (house-fly), and
many others.

The antennce. which are
the two jointed appendages
arising from the upper part
of the head of insects (fig.
731, h b), present a most
wonderful vai-iety of confor-
mation in the several tribes
of insects, often differing
considerably in the several
species of one genus, and
even in the two sexes of the
same species. Hence the
characters which they afibi-d
are extremely useful in classi-
fication, especially since their
structure must almost neces-
sarily be in some way related
to the habits and general
economy of the creatures to
which they belong, although
our imperfect acquaintance
with their function niay pi-e-
vent us from clearly discerning this relation. Thus among the
Coleoptera we find one large family, including the glow-worm, fire-
fly, skip-jackj &c., distinguished by the toothed or serrated form of
the antennte, and hence called Serricornia ; in another, of which the
burying-beetle is the type, the antennae are tei-minated by a club-
shaped enlargement, so that these beetles ai-e termed Glavicornia ;
in anothei', again, of which the Hydrophihis, or large water-beetle,

Fig. 736.— Antenna of Melolontlia
(cockchafer).

988

INSECTS AND ARACHNIDA

is an example, the antennfe ai-e never longer, and are commonly
shorter, than one of the pairs of palpi, whence the name of Paljji-
cornia is given to this group ; in the very large family that includes
the Lucani, or stag-beetles, with the Scarabcei, of which the cockchafei-
is the commonest example, the antennae terminate in a set of leallike
appendages, which are sometimes ai-ranged like a fan or the leaves
of an open book {fig. 736), are sometimes parallel to each other like
the teeth of a comb, and sometimes fold one over the other, thence
giving the name Lamellicornia ; whilst another large family is
distinguished by the appellation Longicornia, from the great length
of the antennfe, which are at least as long as the body, and often
longer. Among the Lepidoptera, again, the conformation of the
antennpe frequently enables us at once to distinguish the group to
which any sjDecimen belongs. As every treatise on entomology con-
tains figures and descriptions of the principal types of conformation
of these organs, there is no occasion here to dwell upon them longei-
than to specify such as are most interesting to the microscopist :
Coleoptera, Brachinus, Calathus, Harjoalus, Dytiscus, Staphylinus,
Philonthus, Elater, Lampyris, Silpha, Hydrophilus, Aphodius,
Melolontha, Cetonia, Curculio, Necrophorus ; Orihoptera, Forficula
(earwig), Blatta (cockroach) ; Lejndojjtera, Sphingidae (hawk-moths),
and Noctuina (moths) of vai-ious kinds, the large ' plumed ' antenna?
of the latter being peculiaidy beautiful objects under a low magni-
fying power ; DijJtera^ Culicida? (gnats of various kinds), Tipulidfe
(crane-flies and midges), Tabanus, Eristalis, and Muscida; (flies of

various kinds). All the
V . T! larger antennae, when not

mounted ' dry ' as opaque
objects, should be put up in
balsam, after being soaked
for some time in tur-
pentine ; but the small
feathery antennte of gnats
and midges are so liable
to distortion when thus
mounted that it is better
to set them up in fluid, the
head with its pair of an-
tennas being thus preserved
together when not too
large. A curious set of organs is to be discovered in the
antennae of many insects, which have been supposed to constitute
collectively an apparatus for hearing. Each consists of a cavity
hollowed out in the horny integument, sometimes nearly spherical,
sometimes flask-shaped, and sometimes prolonged into numerous
extensions formed by the folding of its lining membrane ; the mouth
of the cavity seems to be normally closed in by a continuation of this
membrane, though its presence cannot always be satisfactorily detei'-
mined ; whilst to its deepest part a nerve-fibre may be traced. The
expanded lamellpe of the antenna? of Melolontha present a great dis-
play of these cavities, which are indicated in fig. 737, A, by the

|ffc;?%'

^^

nOTOOOL

Fig. 737. — Miimte structure of leaflike expan-
sions of antenna of Melolontha : A, their in-
ternal layer ; B, their superficial layer.

THE MOUTH OF INSECTS 989

small circles that beset almost their entire area ; their form, which
is very peculiar, can here be only marie out by vertical sections ; but
in many of the smaller antenna?, such as those of the bee, the
cavities can be seen sidewise without any other ti'ouble than that
of bleaching the specimen to render it more transparent.^

The next point in the organisation of insects to which the atten-
tion of the microscopist may be directed is the structure of the
mouth. Here, again, we find almost infinite varieties in the details
of conformation ; but these may be for the most pai't reduced to a
small number of types or plans, which are characteristic of the dif-
ferent orders of insects. It is among the Coleojitera, or beetles, that
we find the several parts of which the mouth is composed in their
most distinct form ; for, although some of these parts are much more
highly developed in other insects, other parts may be so much altered
or so little developed as to be scarcely recognisable. The Coleoptera
present the typical conformation of the mandihidate mouth, which is
. adapted for the prehension and division of solid substances ; and this
consists of the following pai'ts : 1, a pair of jaws, termed mandibles,
frequently furnished with powerful teeth, opening laterally on either
side of the mouth, and serving as the chief instruments of mandaca-
tion ; 2, a second pair of jaws, teimed m.axiUce, smaller and weaker
than the preceding, beneath which they are placed, and serving to
hold the food, and to convey it to the back of the mouth ; 3, an
upper lip, or lahrum ; 4, a lower lip or labium ; 5, one or two pairs
of small jointed appendages, termed palpi, attached to the maxillfe,
and hence called maxillary palpi ; 6, a pair of labial jicdpi. The
labium ^ is often composed of several distinct parts, its basal portion
being distinguished as the menUim or chin, and its anterior portion
being sometimes considerably prolonged forwards, so as to form an
organ which is propei-ly designated the ligtda, but which is more
commonly known as the ' tongue,' though not really entitled to that
designation, the real tongue being a soft and j)rojecting oigan which
forms the floor of the mouth, and which is only found as a distinct
]3art in a comparatively small numbei- of insects, as the cricket. This
ligula is extremely developed in the /??/ kind, in which it forms the
chief part of what is commonly called the ' proboscis ' (fig. 739) ; ^

1 See the memoir of Dr. Hichs, ' On a new Structure in the Antennas of Insects,'
in Trans. Linn. Soc. xxii. p. 147 ; and his ' Further Eeraarks ' at p. 383 of the
same volume. See also the memoir of M. Lespes, ' Siir I'Appaveil auditif des Insectes,'
in Ann. des Sci. Nat. s^r. iv. Zool. torn. ix. p. 258 ; and that of M. Claparede, ' Sur les
pretendus Organes auditifs des Coleopteres lamellicornes et autres Insectes,' in Ami.
rJes Sci. Nat. s€r. iv. Zool. torn. x. p. 236. Dr. Hicks lays great stress on the 'bleach-
ing process ' as essential to success iai this investigation, and he gives the following
directions for iDerforming it : Take of chlorate of potass a drachm, and of water a
drachm and a half ; mix these in a small wide bottle containing about an ounce ; wait
five minutes and then add about a drachm and a half of strong hydrochloric acid.
Clilorine is thus slowly developed, and the mixture will retain its bleaching power
for some time. For an account of Herr F. Ruland's observations see Journ. Boy.
Micr Soc. 1888, p. 723.

^ The labium and the labial palps are, morphologically, a second pair of maxillse
whicli have undergone more or less fusion of the basal parts along the median line,

s The representation given in the figure is taken from one of the ordinary pre-
parations of the fly's proboscis, which is made by slitting it open, flattening it out,
and mounting it in balsam. For representations of the true relative positions of
the different i^arts of this wonderful organ, and for minute descriptions of them, the

990

INSECTS AND AKACHNIDA

and it also forms the ' tongue ' of the hee and its alhes (fig. 738).
The ligula of the common fly presents a curious modification of the
ordinary ti^acheal structure, the purpose of which is not apparent ;
for instead of its trachese being kept pervious, after the usual
fashion, by the winding of a continuous spiral fibre through their
interior, the fibre is broken into rings, and these rings do not sur-
round the whole tube, l)ut are tei-minated by a set of arches that pass
from one to another (fig. 739, B).i In the Diptera, or two-winged
flies generally, the labrum, maxillae, mandibles, and the internal
tongue (where it exists) are converted into delicate lancet-shaped
organs termed setoe, which, when closed together, are received into

a hollow on the uppei- side of the
labium (fig. 739), but which are
capable of being used to make
punctures in the skin of animals or
the epidermis of plants, whence
the juices may be drawn forth by
the j)roboscis. Frequently, how-
evei-, two or more of these organs
may be w^anting, so that their
number is reduced from, six to
fovir, three, or two. In the
Hymenojitera (bee and wasp ti-ibe)
the labrum and the mandibles
(fig. 738, h) much resemble those
of mandibulate insects, and are
used for cori-esponding purposes ;
the maxillfe (c) are greatly elon-
gated, and form, when closed, a
tubulai- sheath for the ligula, or
' tongue,' through which the
honey is drawn up ; the labial
Fig. 738.— Parts of the mouth of Apis palpi {d) also are greatly de-

mellifica (honey-bee) : a, mentuin ; h,
mandibles ; f, maxillae ; d, labial palpi ;
c, ligula, or prolonged labium, com-
monly termed the 'tongue.'

veloped, and fold together, like
the maxillae, so as to form an
inner sheath for the ' tongue ; '
while the ' ligula ' itself (e) is a
long tapering muscular oi-gan, marked by an immense number of
short annular divisions, and densely covered over its own length
with long hairs. It is not tubular, as some have stated, but is
solid ; when actively employed in taking food it is extended to a

reader is referred to Mr. Suffolk's memoir, ' On the Proboscis of the Blow-fly,' in
Monthly Microsc. Journ. i. p. 331, and to Mr. Lowne's treatise on The Anatorny
and Physiology of the Bloiv-fly.

1 According to Dr. Anthony {Monthly Microsco-pical Journ. vol. xi. p. 242), these
' pseudo-tracheffi ' are suctorial organs, which can take in liquid alike at their ex-
tremities and through the whole length of the fissure caused by the interruption of
the rings, the edges of this fissure being formed by the alternating series of ' ear-like
appendages ' connected with the terminal ' arches,' the closing together of which
converts the pseudo-trachea; into a complete tube. Dr. Anthony considers each of
these ear-like appendages to be a minute sucker, ' either for the adhesion of the fleshy
tongue, or for the imbibition of fluids, or perhaps for both purposes.' The point is
well worthy of further investigation.

MOUTH-PARTS OF INSECTS

991

Fig. 739. — A, tongue of common fly : a, lobes of lignla ; b, portion inclosing
the lancets, formed by the nietamor23hosis of the maxillue ; c, maxillary
palpi. B, a portion of some of the pseudo-tracheaa more highly magnified.

992

INSECTS AND AEACHNIDA

great distance beyond the other parts of the mouth ; but when at
rest it is closely packed up and concealed between the maxillfe. ' The
manner,' says Mr. Newport, ' in which the honey is obtained when
the organ is plunged into it at the bottom of a flower is by " lapping,"
or a constant succession of short and quick extensions and contrac-
tions of the organ, which occasion the fluid t© accumulate upon it
and to ascend along its upper surface, until it reaches the orifice of
the tube formed by the approximation of the maxillae above, and of
the labial palpi and this part of the ligula below.'

By the plan of conformation just described we are led to that
which prevails among the LejndopUra, or butterfly tribe, which,
being pre-eminently adapted for suction, is termed the haustellate
mouth. In these insects the labium and mandibles are reduced to
three minute triangular- plates ; whilst the maxillfe are immensely
elongated, and are united together along the median line to form

the haustellura, or
true ' proboscis,'
which contains a
tube formed by the
junction of the two
grooves that are
channelled out
along their mutu-
ally applied sur-
faces, and which
serves to pump
up the juices of
deep cup-shaped
flowers, into which
the size of their
wings prevents
these insects from
entering. The

length of this haustellum varies greatly : thus in such Lepidoptera as
take no food in their perfect state it is a very insignificant organ ; in
some of the white hawk-moths, which hover ovei' blossoms without
alighting, it is nearly two inches in length, and in most butterflies and
moths it is about as long as the body itself; in Amphonyx, one of the
tSphingidai^ it is more than nine inches long, or about three times the
length of the body. This haustellum, which, when not in use, is
coiled up in a spiral beneath the mouth, is an extremely beautiful
microscopic object, owing to the peculiar banded arrangement it ex-
hibits (fig. 740), which is probably due to the disposition of its muscles.
In many instances the two halves may be seen to be locked together
by a set of hooked teeth, which are inserted into little depressions
between the teeth of the ojjposite side. Each half, moreover, may
be ascertained to contain a trachea or air-tube, and it is jDrobable,
from the observations of Mr. Newport, that the sucking up of the
juices of a flower through the proboscis (which is accomplished with
great rapidity) is effected by theagency of the respiratory apparatus.
The proboscis of many butterflies is furnished, for some distance from

Fig. 740. — Haustellum (iDi-oboscis) of Vanessa.

PAETS OF THE BODY 993

its extremity, with a double row of small projecting bari-el-shaped
bodies (shown in fig. 740), which are surmised by Mr. Newport
(whose opinion is confirmed by the kindred inquiries of Dr. Hicks)
to be organs of taste. Numerous othei- modifications of the structure
of the mouth, existing in the different tribes of insects, are well
worthy of the careful study of the microscopist ; but as detailed
descriptions of most of these will be found in every systematic trea-
tise on entomology, the foregoing general account of the principal
types must sufiice.

Parts of the Body. — The conformation of the several divisions of
the alimentary canal presents such a multitude of diversities, not
only in different tribes of insects, but in different states of the same
individual, that it would be utterly vain to attempt here to give
even a general idea of it, more especially as it is a subject of far
less interest to the ordinary microscopist than to the professed
anatomist. Hence we shall only stop to mention that the 'musctilar
gizzard,' in which the oesophagus very commonly terminates, is often
lined by several rows of strong horny teeth for the reduction of the
food, which furnish very beautiful objects, especially for the bino-
cular. These are particularly developed among the gi'asshoppei-s,
crickets, and locusts, the nature of whose food causes them to require
powerful instruments for its reduction.^

The circulation of blood may be distinctly watched in many of
the more transparent larvse, and may sometimes be observed in the
perfect insect. It is kept up by a ' dorsal vessel ' (so named from
the position it always occupies along the middle of the back in the
thoi'acic and abdominal regions), which really consists of a succession
of muscular contractile cavities, one for each segment, opening one
into another from behind forwards, so as to form a continuous trunk
divided by valvular partitions. In many larvae, however, these
partitions are very indistinct ; and the walls of the ' dorsal -vessel '
are so thin and transparent that it can with difficulty be made out,
a limitation of the light by the diaphragm being often necessary.
The blood which moves through this trunk, and which is distributed
by it to the body, is a transparent and nearly colourless fluid, carry-
ing with it a number of ' oat-shaped ' corpuscles, by the motion of
which its flow can be f^,!^owed.2 The current enters the 'dorsal
vessel ' at its posterior exti'emity, and is propelled forwards by the
conti'actions of the successive chambers, being prevented from moving
in the opposite direction by the valves between the chambers, which
only open forwards. Arrived at the anterior extremity of the
' dorsal vessel,' the blood is distributed in three principal channels :
a central one, namely, passing to the head, and a lateral one to eithei-
side, descending so as to approach the lower surface of the body.
It is from the two lateral currents that the secondary streams
diverge, which pass into the legs and wings, and then return back
to the main stream ; and it is from these also that in the larva

^ The student who desires to carry further the stufly of the digestive apparatus
should consult Professor Plateau's memoir, ' Eecherches sur les Phenomenes de
la Digestion chez les Insectes,' Mem. Acad. Soy. de Belgique., xli.

2 On the blood-tissue of insects consult Mr. W. M. Wheeler in vol. vi. of the
American journal Psyche.

3,s

994 INSECTS AND ARACHNIDA

of the E'phemera marginata (day-fly), the exti-eme ti-ansparence of
which i-enclers it one of the best of all subjects for the observation
of insect circulation, the smaller currents diverge into the gill-like
appendages with which the body is furnished. The blood-cm-rents
seem r-ather to pass through channels excavated among the tissues
than through vessels with distinct walls. In many aquatic larvae,
especially those of the CuUcidce (gnat tribe), the body is almost
entirely occupied by the visceral cavity ; and the blood may be seen
to move backwards in the space that surrounds the alimentary
canal, which here serves the purpose of the channels usually exca-
vated through the solid tissues, and which fi'eely communicates at
each end with the dorsal vessel. This condition strongly resembles
that found in many Annulata.^

The circulation may be easily seen in the wings of many insects
in their jyujxi state, especially in those of the Neuroptera (such as
dragon-flies and day-flies), which pass this part of their lives under
water in a condition of acti^dty, the pupa of Agrion puella, one of
the smaller dragon-flies, being a particularly favourable subject for
such observations. Each of the ' nervures ' of the wings contains a
' trachea ' or air-tube, which branches off ft-om the tracheal system
of the body ; and it is in a space around the trachea that the blood
may be seen to move when the hard framework of the nervure itself
is not too opaque. The same may be seen, however, in the wings of
pupse of bees, butterflies, &c., which remain shut up motionless in
their cases ; foi- this condition of apparent torpor is one of great
activity of their nutritive system, those organs, especially, which
are peculiar to the perfect insect being then in a state of rapid
growth, and having a vigorous circulation of blood through them.
In certain insects of nearly every order a movement of fluid may
be seen in the wings for some little time after their last meta-
morphosis ; but this movement soon ceases and the wings dry up.
The common fly is as good a subject for this observation as can
be easily found ; it must be caught within a few hours or days of its
first appearance ; and the circulation may be most conveniently
Ijrought into view by inclosing it (without water) in the aquatic
box. and pressing down the cover suffici-ently to keep the body at 'rest
without doing it any injury.

The res'piratory apparatus of insects affords a very interest-
ing series of microscopic objects ; for, with great uniformity in its
general plan, there is almost infinite variety in its details. The
aeration of the blood in this class is provided for, not by the trans-
mission of the fluid to any special organ representing the lung of a
vertebrated animal or the gill of a mollusc, but by the introduction
of air into every part of the body, through a system of minutely
distributed tracheoi, or air-tubes, which penetrate even the smallest
and most delicate organs. Thus, as we have seen, they pass into
the haustell'um, or ' proboscis,' of the butterfly, and they are aninutely
1 Seethe memoirs on Coretln-ajjlumicornis, by Professor Eymer Jones, in Trans.
Microsc. Sor. n.s. vol. xv. 1867, p. 99 ; by Professor E. Ray Lankester, in the Popular
Science Beview for October 1865 ; and by Dr. A. Weismann, in Zeitsclir. f. tuiss. ZiJol.
Bd. xvi. p. 45. On the circulatory system of insects consult Graber, ' Ueber den pro-
pulsatorischen Apparat der Insecten,' Arch.fiir mikr. Anat. ix. p. 129.

EESPIEATOEY APPAKATUS

995

distributed in tlie elongated labkmi or ' tongue ' of the % (fig. 739)
Their general distribution is shown in fig. 741, where we see two
long trunks (/) passing from one end of the body to the other, and
connected with each other by a transverse canal in every segment •
these trunks communicate, on the one hand, by short wide passa<^es
with the 'stigmata,' "spiracles,' or 'breathing pores' {g), throuo-h
which the air enters and is discharged ; whilst they give ofi" branches
to the different segments,

which divide again and / a ^

again into I'amifications of
extreme minuteness. They
usually communicate also
with a pair of air-sacs (h)
Avhich is situated in the
thorax ; but the size of
these (which are only found
in the perfect insect, no
ti-ace of them existing in
the larvae) varies greatly
in diflerent tribes, being
usually greatest in those
insects which (like the bee)
can sustain the longest and
most powerful flight, and
least in such as habitually
live upon the ground or
upon the surface of the
water. The structure of
the air-tubes reminds us
of that of the ' spiral
vessels ' of plants, which
seemed destined (in part
at least) to perform a
similar office : for within
the membrane that forms
their outer wall an elastic
fibre winds round and
i-ound, so as to form a
spiral closely resembling
in its position and func-
tions the spiral wire spring
of flexible gas pipes : with-
in this, again, howevei',

there is another membranous wall to the air -tubes, so that the spii-e
winds between their inner and outer coats. When a j^oi'tion of one
of the great trunks with some of the principal branches of the
tracheal system has been dissected out, and so pressed in mounting
that the sides of the tubes are flattened against each other (as has
happened in the specimen represented in fig. 742), the spire forms
two layers which are brought into close apposition, and a'' veiy
beautiful appearance, resembling that of watered silk, is produced

3s2

Pig. 741. — Tracheal system of NeiJcc (water-
scorpion) : a, head ; 6, first pair of legs ; c, first
segment of thorax ; d, second pair of wings ; e,
second pair of legs ; /, tracheal trunk ; g, one
of the stigmata ; 7^, air-sac.

996

INSECTS AND AEACHNIDA

by the ci'ossing of the two sets of fibres, of which one overlies the
other. That this appearance, however, is altogether an optical illu-
sion may be easily demonstrated by carefully following the course
of any one of the fibres, which will be found to be perfectly regular.
The ' stigmata ' or ' spiracles ' through which the air enters the
ti-acheal system are generally visible on the exterior of the

body of the insect (espe-
cially on the abdomi-
nal segments) as a series
of pores along each
margin of the under sur-
face. In most larvae,
neai-ly every segment is
provided with a pair, but
in the perfect insect
several of them remain
closed, especially in the
thoracic region, so that
their number is often con-
siderably reduced. The
structure of the spiracles
varies greatly in regard to
complexity in difierent in-
sects ; and even where the
general plan is the same
the details of conforma-
tion are peculiar, so that
perhaps in scarcely any two species are they alike. Generally speak-
ing, they are furnished with some kind of sieve at their entrance by
which particles of dust, soot, &;c., which would otherwise enter the
air-passages, are filtered out ; and this sieve may be formed by

the interlacement of the
/ / --. branches of minute arbo-

rescent growths from the
border of the spu'acle, as
in the common fly (fig.
743), or in the Dytiscus ;
or it may be a membrane
perforated with minute
holes, and supported upon
a framework of bars that
is prolonged in like manner
from the thickened margin
of the aperture (fig. 744),
as in the larvae of the
Melolontha (cockchafer). Not unfrequently the centre of the aper-
ture is occupied by an impervious disc, from which radii proceed
to its margin, as is well seen in the spiracle of T'lpula (crane-
fly).' In those aquatic larvse which breathe air we often find one

1 Consult Landois and Thiele, ' Der Traclieenverschluss bei den Insecten,' Zeit-
schrift f. wiss. Zool. xvii. p. 187.

Pig. 742.— Portion of a large trachea of Dijttsciis,
with some of its x^rincipal branches.

Fig. 743. — Spiracle of common fly.

EESPIEATOEY APPAEATUS 997

of the spiracles of the last segment of the abdomen prolonged into a
tube, the mouth of which remains at the surface while the body is
immersed ; the larvse of the gnat tribe may frequently be observed
in this position.

There are many aquatic larvse, however, which have an entirely
different provision for respiration, being furnished with external leaf-
like or brush-like appendages into which the tracheae are prolonged, so
that by absoi'bing air from the water that bathes them they may con-
vey this into the interioi- of the body. We cannot have a better example
of this than is afforded by the larva of the common Ephemera (day-
fly), the body of which is furnished with a set of branchial appendages
resembling the ' fin-feet ' of branchiopods, whilst the three-pronged
tail also is fringed with clusters of delicate hairs which appear to
minister to the same function. In the larva of the Lihellula
(dragon-fly) the extension of the surface for aquatic respiration
takes place within the termination of the intestine, the lining
membrane of which is folded into an immense number of plaits,
each containing a minutely ramified system of tracheae ; the water
slowly drawn in through the anus
for bathing this surface is ejected
with such violence that the body
is impelled in the opposite direc-
tion ; and the air taken up by its
tracheae is carried through the
system of air-tvibes of which they
form part into the remotest organs.
This apparatus is a peculiarly in-
teresting object for the microscope
on account of the extraordinarily
rich distribution of the trachefe in
the intestinal folds. -pm. 744.— Spiracle of larva of

The main trunks of the tracheal cochchafer.

system, with their principal ramifi-
cations, may geneially be got out with little difficulty by laying
open the body of an insect or larva under water in a dissecting
trough, and removing the whole visceral mass, taking care to leave
as many as possible of the branches, which will be seen pro-
ceeding to this from the two great longitudinal tracheae, to whose
position these branches will serve as a guide. Mr. Quekett recom-
mended the following as the most simple method of obtaining a
perfect system of tracheal tubes from a larva. A small opening
having been made in its body, this is to be placed in strong acetic
acid, which will soften or decompose all the viscera ; and the tracheae
may then be well washed with the syringe, and removed fi-om the
body with the greatest facility, by cutting away the connections of
the main tubes with the spiracles by means of fine-pointed scissors.
In order to mount them they should be floated upon the slide, on
which they should then be laid ovit in the position best adapted foi'
displaying them. If they are to be mounted in Canada balsam they
should be allowed to dry upon the slide, and should then be treated
in the usual way ; but their natural appearance is best preserved

998 INSECTS AND ARACHNJDA

by mounting them in fluid (weak spirit oi- (Toadby's solution), using
a shallow cell to prevent pressure. The finer ramifications of the
tracheal system may generally be seen j)articulai'ly well in the mem-
branous wall of the stomach or intestine ; and this, having been laid out
and dried upon the glass, may be mounted in balsam so as to keep the
tracheae full of air (whereby they are much better displayed), if care
be taken to use balsam that has been pi-eviously thickened, to drop
this on the object without liquefying it more than is absolutely
necessary, and to heat the slide and the cover (the heat may be
advantageously applied directly to the cover after it has been put
on by turning over the slide so that its upper face shall look down-
ward) only to such a degree as to allow the balsam to spread and
the cover to be pressed down. The spiracles are easily dissected out
by means of a pointed knife or a jjair of fine scissors ; they should
be mounted in glycerin jelly when their texture is soft, and in
balsam when the integument is hard and hoi'ny.

Wings. — These organs are essentially composed of an extension
of the external membranous layer of the integument over a frame-
work foi-med by prolongations of the inner horny layer, within
which prolongations tracheae are nearly always to be found, whilst
they also include channels through which blood circulates during
the growth of the wing and for a short time after its completion.
This is the simple structure presented to us in the wings of Netiro-
ptera (dragon-flies, &c.), Hytnenoj^tera (bees and wasps), Diptera
(two- winged flies), and also of many Homoptera {Gicadce and A2)hicles) ;
and the principal interest of these wings as microscopic objects* lies
in the distribution of their ' veins ' or ' nervures ' (for by both names
are the ramifications of their skeleton known) and in certain points
of accessory structure. The venation of the wdngs is most beautiful
in the smaller Neuroptera, since it is the distinguishing feature of
this order that the veins, after subdividing, reunite again, so as to
form a close network ; whilst in the Hymenoptera and Diptera such
i-eunions are rare, especially towards the margins of the wings, and
the areolae are much larger. Although the membrane of which
these wings are composed appears perfectly homogeneous when
\dewed by transmitted light, even with a high magnifying j)ower,
yet when viewed by light reflected obliquely from their surfaces
an appearance of cellular areolation is ofteia discernible ; this is well
seen in the common fly, in which each of these areolae has a hair in
its centre. In order to make this observation, as well as to bring
out the very beautiful iridescent hues which the wings of many
minute insects (as the Aphides) exhibit when thus viewed, it is con-
venient to hold the wing in the stage-forceps for the sake of giving
it every variety of inclination ; and when that position has been
found which best displays its most interesting features, it should be
set up as nearly as possible in the same. For this purpose it should
be mounted on an opaque slide, but instead of being laid down
upon its surface the wing should be raised a little above it, its
' stalk ' being held in the proper position by a little cone of soft wax,
in the apex of which it may be imbedded. The wings of most
Hymenoptera are remarkable for the peculiar apparatus by which

WINGS; SOUND-ORGANS 999

those of the same side are connected togethei', so as to constitute in
flight but one large wing ; this consists of a I'ow of cui'ved hooklets
on the anterior margin of the posterior wing, which lay hold of the
thickened and doubled down posteiioi- edge of the anterior wing.
These hooklets are sufficiently apparent in the wings of the common
bee, when examined with even a low magnifying power ; but thev
are seen better in the wasp, and better still in the hoi'net. The
peculiar scaly covering of the wings of the Lepidoptera has already
been noticed ; but it may here be added that the entire wings of
many of the smaller and commoner insects of this order, such as the
Tineidce or ' clothes-moths,' form very beautiful opaque objects for
low powers, the most beautiful of all being the divided wings of
the Fissipennia or 'plum.ed moths,' especially those of the genus
Pterophorus} 1>^,J

There are many insects, howevei', in which the wings are more or
less consolidated by the interposition of a layer of horny substance
between the two layers of membrane. This plan of structure is
most fully carried out in the Coleoptera (beetles), whose anterior
wings are metamorphosed into ■ elytra or ' wing-cases ; ' and it is
upon these that the brilliant hues by which the integument of many
of these insects is distinguished are most strikingly displayed. In
the anterior wings of the Forficulidce^ or earwig tribe, the cellular
structure may often be readily distinguished when they are viewed
by transmitted light, especially after having been mounted in Canada
balsam. The anterior wings of the Ortlioptera (grasshoppers,
crickets, etc.), although not by any means so solidified as those of
Coleoptera, contain a good deal of horny matter ; they ai'e usually
rendered sufficiently transparent, howevei', by Canada balsam to be
viewed with transmitted light ; and many of them are so coloured
as to be very showy objects (as are also the posterior fan-like wings)
for the electric or gas microscope, although their large size and the
absence of any minute structure prevent them from affording much
interest to the ordinary microscopist. We must not omit to men-
tion, however, the curious sound-producing apparatus which is
possessed by most insects of this order, and especially by the common
house-cricket. This consists of the ' tymjDanum,' or drum, which is
a space on each of the upper wings, scai-cely crossed by veins, but
bounded extei'nally by a large dark vein provided with thi-ee or four
longitudinal ridges ; and of the ' file ' or ' bow,' which is a transverse
horny ridge in front of the tympanum, furnished with numerous
teeth ; and it is believed that the sound is produced by the rubbing
of the two bows across each other, while its intensity is increased
by the sound-board action of the tympanum. The wings of the
Fulgoridce (lantern-flies) have much the same textvire as those of the
Orthoptera, and possess about the same value as microscopic objects,
differing considerably from the purely membranous wings of the
Gicadce and Aj^hides, which are associated with them in the order
Homoptera. In the order Hemiptera, to which belong various kinds

1 Compare the recently published memoir by M. Baer, ' Ueber Bau und Farben
der Fliigelsohuppen bei Tagfaltern,' in Zeitsclir. f. luiss. Zool. Ixv. (1898), pp. 50-6.5,
as also M. von Linden on the development of the markings, pp. 1-50 of the same volume.

lOOO INSECTS AND ARACHNIDA

of land and water insects that have a suctorial mouth resembling
that of the common bug, the wings of the anterior pair are usually
of parchmenty consistence, though membranous near their tips, and
are often so lichly coloured as to become very beautiful objects
when mounted in balsam and vieAved by transmitted light ; this is
the case especially with the terrestrial vegetable-feeding kinds, such
as the Pentatoma and its allies, some of the tropical forms of which
rival the most brilliant of the beetles. The British sj^ecies are by
no means so interesting, and the aquatic kinds, which, next to the
bed-bugs, ai-e the most common, always have a dull brown or almost
black hue ; even among these last, however, of wliich the Notonecta
(water -boatman) and the Nepa (water-scorpion) are well-known
examples, the wings are beautifully variegated by differences in the
depth of that hue. The halteres of the Diptera, which are the re-
presentatives of the posterior wings, have been shown by Dr. J. B.
Hicks to present a very curious structure, which is found also in
the elytra of Coleoptera and in many other situations, consisting in
a multitude of vesicular projections of the superficial membrane, to
each of which there proceeds a nervous filament, that comes to it
through an aperture in the tegumentary wall on which it is seated.
Various considerations are stated by Dr. Hicks which lead him to
the belief that this apparatus, when developed in the neighbourhood
of the spiracles or breathing pores, essentially ministers to the sense
of sTuell, whilst, when developed upon the palpi and other organs in
the neighbourhood of the mouth, it ministers to the sense of taste}

Feet. — Although the feet of insects are formed pretty much on
one general plan, yet that plan is subject to considerable modifica-
tions in accordance with the habits of life of different species. The
entire limb usually consists of five divisions, namely, the coxa or hip,
the trochanter, the femur or thigh, the tibia or shank, and the tarsus
or foot ; and this last part is made up of several successive joints.
The typical number of these joints seems to be Jive,^ but that
number is subject to reduction ; and the vast order Coleoptera is
subdivided into primary groups, according as the tarsus consists of
five, four, or t/i7^ee segments. The last joint of the tarsus is usually
furnished with a pair of strong hooks or claws (figs. 745, 746) ; and
these are often serrated (that is, furnished with saw-like teeth),
especially near the base. The under surface of the other joints is
frequently beset with tufts of hairs, which are arranged in various
modes, sometimes forming a complete ' sole ; ' this is especially the
case in the family Curculionidce ; a pair of the feet of the ' diamond
beetle ' mounted so that one shows the upper surface made resplendent
by its jewel-like scales, and the other the hairy cushion beneath, is
a very interesting object. In many insects, especially of the fly
kind, the foot is furnished with a pair of membranous expansions

1 See his memoir, 'On a new Organ in Insects,' in Journ. Linn. Soc. vol. i. 1856,
p. 136 ; his ' Further Remarks on the Organs found on the Bases of the Halteres
and Wings of Insects,' in Trans. Linn. Soc. xxii. p. 141 ; and his memoir, ' On
certain Sensory Organs in Insects hitherbo undescribed,' in Trans. Linn. Soc.
xxiii. p. 189. Compare also the interesting memoir of Weinland, in Zeitsclir. f.
wiss. Zool. li. (18S01, pp. .85-160, 5 pis.

^ See, however, Professor Huxley (Anat. of Invertebrate Animals, j). 348), who,
regarding the ' pulvillus ' of the cockroach as a joint, finds the number to be six.

FEET

lOOI

termed pulvilU (fig. 745) ; and these are beset with numerous hairs,
each of which has a minvxte disc at its extremity. This structure is
evidently connected with the power which these insects possess of
walking ovei- smooth surfaces in opposition to the force of gravity ;
yet thei'e is still considerable uncertainty as to the precise mode in
which it ministei's to this faculty. Some believe that the discs act
as suckers, the insect being held up by the pressure of the air against
their upper surface when a vacuum is formed beneath ; whilst others
maintain that the adhesion is the result of the secretion of a viscid
liquid from the under side of the foot. The careful observations, of
Mr. Hepworth have led him to a conclusion which seems in harmony
with all the facts of the case — namely, that each hair is a tube con-
veying a liquid from a glandular sacculus situated in the tarsus,
and that when the disc is applied to a surface the pouring forth of
this liquid serves to make its adhesion perfect. That this adhesion
is not produced by atmospheric pressure alone is joroved by the
fact that the feet of flies continue to hold on to the interior of an
exhausted receiver ; whilst,
on the other hand, that the
feet pour forth a secreted
fluid is evidenced by the
marks left by their attach-
ment on a clean surface of
glass. Although, when all
the hairs have the strain
put upon them equally, the
adhesion of their discs suf-
fices to support the insect,
yet each row may be de-
tached separately by the
gradual raising of the tarsus
and pul villi, as when we
remove a piece of adhesive
jjlaster by lifting it from
the edge or cornei- . Flies are
often found adherent to window-panes in the autumn, their reduced
strength not being sufiicient to enable them to detach their tarsi. ^
A similar appaiutus on a far larger scale presents itself on the foot
of the Dytiscus (fig. 746, A). The first joints of the tarsus of this
insect are widely expanded, so as to form a nearly circular plate,
and this is provided with a very remarkable apparatus of suckers,
of which one disc («) is extremely large, and is furnished with strong
i-adiating fibi-es ; a second (6) is a smaller one formed on the same
plan (a third, of the like kind, being often present) ; whilst the
greater number are comparatively small tubular club-shaped bodies,
each having a very delicate membranous sucker at its extremity, as
shown on a larger scale at B. These all have essentially the same

1 See Mr. Hepworth's commuuieatious to the Quart. Journ. Microsc. Sci. vol. ii.
1854, p. 158, and vol. iii. 1855, p. 312. See also Mr. Tuffen West's memoir ' On the
Foot of the Fly,' in Trans. Linn. Soc. xxii. p. 393; Mr. Lowiie's Anatomy of
the Blow-fly ; H. Dewitz in Zoologisclier Anzeiger, vi. p. 273 ; and G. Sinimer-
macher in Zeitsclir. f. wiss. Zool. xl. p. 481.

Fig. 745.— Foot of fly.

I0O2

INSECTS AND ARACHNIDA

sti-ucture, the large suckers being fumislied, like the hairs of the
fly's foot, with secreting sacculi, which pour forth fluid thi-ough the
tubular footstalks that cai-ry the discs, whose adhesion is thus
secui'ed ; whilst the small suckers form the connecting link between
the larger suckers and the hairs of many beetles, especially Curcu-
lionidce.^ ' The leg and foot of the Dytiscus, if mounted without
compression, furnish a peculiarly beautiful object for the binocular
microscope. The feet of caterpillars differ considerably from those
of perfect insects. Those of the first three segments, which are
afterwards to be replaced by true legs, are furnished with strong
horny claws ; but each of those of the other segments, which are
termed ' pro-legs,' is composed of a circular series of comparativ^ely
slender curved booklets, by which the caterpillar is enabled to cling-
to the minute roughness of the surface of the leaves, &c., on which
it feeds. This structiu^e is well seen in the pro-legs of the common
silkworm.

Stings and Ovipositors. — The insects of the order Hymenoptera
are all distinguished by the prolongation of the antepenultimate and

Fig. 746. — A, foot of Dytiscus, showing its apparatus of suckers : a, h, large
suckers ; c, ordinary suckers. B, one of the ordinary suckers more highly
magnified.

penviltimate segments of the abdomen (the eighth and ninth ab-
dominal segments of the larva) into a peculiar organ, which in one
division of the order is a ' sting,' and in the other is an ' ovipositor '
or instrument for the deposition of the eggs, which is usually also
provided with the means of boring a hole for their reception. The
former group consists of the bees, wasps, ants, &c. ; the latter of the
saw-flies, gall-flies, ichneumon-flies, &c. These two sets of instru-
ments are not so unlike in structui-e as they ai-e in function.^ The

' See Mr. Lowne, ' On the so-called Suckers of Dijtisciis and the Pulvilli of Insects,'
in Montlily Microsc. Journ. v. p. 267.

- See Kraepelin, ' Untersuchungen iiber den Bau, Mechanismus und Entwicke-
lungsgeschichte der bienenartigen Thiere,' in Zeitsclu: f. Wiss. Zool. xxiii. p. 289 ;
Dewitz, ' Ueber Bau und Entwickelung des Stachels und der Legescheide,' op- cit.

STINGOS AND OVIPOSITORS IOO3

' sting ' is usually foi'ined of a pair of dai'ts, beset with barbed teeth
at their points, and furnished at their roots with jjowerful muscles,
Avhereby they can be caused to project fi'oni their sheath, which is a
horny case formed by the prolongation of the integument of the last
segment, slit into two halves, which separate to allow the protrusion
of the sting ; whilst the peculiar ' venom ' of the sting is due to the
ejection, by the same muscular action, of a poisonous liquid, from a
bag situated neai- the root of the sting, which passes down a canal
excavated between the darts, so as to be inserted into the puncture
which they make. The stings of the common bee, wasp, and hornet
may all be made to display this structure without much difficulty in
the dissection. The ' ovipositor ' of such insects as deposit their
eggs in holes ready-made, or in soft animal or vegetable substances
(as is the case with the Ichneumonidce), is simply a long tube, which
is inclosed, like the sting, in a cleft sheath. In the gall-flies
(Ci/nipidce) the extremity of the ovipositor has a toothed edge, so
as to act as a kind of saw whereby harder substances may be pene-
trated ; and thus an aperture is made in the leaf, stalk, or bud of
the plant or tree infested by the particular species, in which the egg
' is deposited, together with a drop of fluid that has a peculiarly
irritating eflect upon the vegetable tissues, occasioning the production
of the ' galls,' which are new growths that serve not only to protect the
larvae, but also to afford them nvitriment. The oak is infested by
several species of these insects, which deposit their eggs in different
parts of its fabric ; and some of the small ' galls ' which ai'e often
found upon the surface of oak-leaves are extremely beautiful objects
for the lower powers of the microscope. In the TenthredinidcB, ov
' saw-flies,' and in their allies, the Siricidce, the ovipositor is furnished
with a still more powerful apparatus foi- penetration, by means of
which some of these insects can bore into hard timber. This consists
of a pair of ' saws ' which are not unlike the ' stings ' of bees, ifec,
but are broader and toothed for a greater length, and are made to
slide along a firm piece that supports each blade, like the ' back ' of
a carpenter's ' tenon-saw ; ' they are worked alternately (one being
protruded while the other is drawn back) with great rapidity ; but
when not in use they lie in a fissure beneath a sort of ai'ch formed
by the tei'minal segment of the body. When a slit has been made
by the working of the saws they are withdrawn into this sheath ;
the ovipositor is then protruded from the end of the abdomen (the
body of the insect being curved downwards), and, being guided into
the slit by a pair of small hairy feelers, there deposits an egg}
Many other insects, especially of the order Diptera, have very pro-
longed ovipositors, by means of which they can insert their eggs
into the integuments of animals or into other situations in which
the larvfe will obtain appropriate nutriment. A remarkable example

XXV. p. 174 ; and ' Ueber Ban und Entwickelung des Stachels der Ameisen,' of. cit.
xxviii. p. 527.

1 The above is the account of the process given by Mr. J. W. Goocli, who has
informed the Author that he has repeatedly verified the statement formerly inade by
him (Science Gossip, Feb. 1, 1873,i, that the eggs are deposited, not, as originally
stated by Beaumur, by means of a tube formed by the coaptation of the saws, but
through a separate ovipositor, protruded when the saws have been withdrawn.

I004

INSECTS AND ARACHNIDA

of this is furnished by the gad-fly {Tabanus), whose ovipositor is
composed of several joints, capable of being drawn together or
extended like those of a telescope, and is terminated by boring
instruments ; and the egg being conveyed by its means, not only
into but through the integument of the ox, so as to be imbedded in
the tissue beneath, a peculiar kiiad of inflammation is set up theie,
which (as in the analogous case of the gall-fly) forms a nidus appro-
priate both to the protection and to the nutrition of the larva. Other
insects which deposit their eggs in the ground, such as the locusts,
have their ovipositors so shaped as to answer for digging holes for
their reception. The preparations which serve to display the fore-

Fia. 747. — Various eggs, chiefly of the Mallophaga (Anoplura

going parts ai-e best seen when mounted in balsam, save in the case
of the muscles and poison-apparatus of the sting, which are better
preserved in fluid or in glycerin jelly.

The sexual organs of insects furnish numerous objects of extreme
interest to the anatomist and physiologist ; but as an account of
them would be unsuitable to the present work, a reference to a
copious soui'ce of information respecting one of their most curious
features, and to a list of the species that afford good illustrations,
must here suffice.^ The eggs of not only the class Insecta, but of

1 See the memoirs of M. Lacaze-Duthiers, ' Sur I'Armure G^nitale des Insectes,' in
Ann. des Sci. Nat. s6v. iii. Zool. tomes xii. xiv. xvii. xviii. xix. ; and M. Ch. Robin's

EGGS

1005

many of the minuter forms of the class Arachnicla, as for example
the Acarina, or mites and ticks, present to those who are in search of
objects of beauty a wide and most interesting field. In fig. 747 we
give a group of eggs, all but the central form being eggs or organisms
of this order. It is thus with the eggs of many insects ; they ai-e
objects of great beauty, on account of the regularity of their foi-m
and the symmetry of the markings on their surface (fig. 748). The
most interesting belong for the most part to the order Lepidoptera ;
and there are few among these that are not worth examination,
some of the commonest (such as those of the cabbage buttei-fly,

Fig. 748. — Eggs of butterflies and moths

which are found covering large patches of the leaves of that plant)
being as remarkable as any. Those of the puss-moth [Cerura
vvmda), the privet hawk-moth {Sj)hinx ligustri), the small toi'toise-
shell butterfly (^Vmiessa urticce), the meadow-brown butterfly (Hip-
imrchia janh^a), the brimstone-moth [Rumia cratcegata), and the
silkworm {Bomhyx 7nori) may be particularly sjjecified ; and, from
other orders, those of the cockroach (Blatta orientalis), field-cricket
{Acheta campestris), water-scorpion (Nepa ranatra), bug (Cimsx
lecfularius), cow-dung fly (Scatophaga stercoraria), and blow-fly

Memoire aur les Objets qui ■peitvent itre conserves en Preparations microscoijiques
(Paris, 1856), which is peculiarly full iu the enumeration of the objects of interest
afforded bv the class of Insects.

I006 INSECTS AND AEACHNIDA

i^Musca vomitorid)} In order to preserve these eggs they should
be mounted in fluid in a cell, since they will otherwise dry up, and
may lose their shape. They are very good objects for securing some
of the best binocular effects.

The remarkable mode of reproduction that exists among the
Aphides must not pass unnoticed here, from its curious connection
with the non-sexual reproduction of Entomostraca and Rotifera, as
also of Hydra and Zoophytes generally, all of which fall specially,
most of them exclusively, under the observation of the microscopist.
The Aphides, which may be seen in the spring and early summei-,
and which are commonly, but not always, wingless, are all of one
sex, and give birth to a brood of similar Aphides, which come into
the world alive, and before long go through a like process of multi-
plication. As many as from seven to ten successive broods may thus
be produced in the course of a single season ; so that from a single
Aphis it has been calculated that no fewer than ten thousand million
millions may be evolved within that j^eiiod. In the latter part of
the year, however, some of these viviparous Aphides attain their full
development into males and females ; and these perform the ti-ue
generative process, whose products are eggs, which, when hatched in
the succeeding spring, give origin to a new viviparous brood that
repeat the curious life-history of their predecessors. It appears from
the observations of Huxley- that the broods of viviparous Aphides
originate in ova which are not to be distinguished from those
deposited by the perfect winged female. Nevertheless, this non-
sexual or agamic reproduction must be considered analogous rathei-
to the ' gemmation ' of other animals and jjlants than to their sexual
' generation ; ' foi- it is favoured, like the gemmation of Hydra, by
warmth and copious sustenance, so that by appropriate treatment the
viviparous reproduction may be caused to continue (as it would
seem) indefinitely, without any recurrence to the sexual process.
Further, it seems now certain that this mode of reproduction is not
at all peculiar to the Aphides, but that many other insects ordinarily
multiply by ' agamic ' propagation, the production of males and the
perfoi'mance of the true generative act being only an occasional
phenomenon ; and the researches of Professor Siebold have led him to
conclude that even in the ordinary economy of the hive-bee the same
double mode of reproduction occurs. The queen, who is the only
perfect female iai the hive, after impregnation by one of the drones
(or males) deposits eggs in the ' royal ' cells, which are in due time
developed into young queens ; others in the drone cells, which become
di'ones ; and othei-s in the ordinary cells, which become workers or
neuters. It has long been known that these last are really un-
developed females, which, under certain conditions, might become
queens ; and it has been observed by bee-keepers that worker-bees,
in common with virgin or unimpregnated queens, occasionally lay

1 Compare R. Leuckart in Arcliiv f. Anat. 1853, p. 90, ' Ueber die Micropyle unci
den feinern Ban der Schalenhaut bei den Insecteneiern,' and A. Brandt, Ueber das
Ei und seine Bllduiigstdtte, Leipzig, 1878.

2 'On the Agamic Reproduction and Morphology of AjjMs' in Trans. Linn.
Hoc. xxii. p. 193. For observations on American Aphides see various papers by
Mr. C. M. Weed in Pijsrlie and other American journals.

DEVELOPMENT OF INSECTS lOO/

eggs from which eggs none but dronesai^e ever produced. From careful
microscopic examination of the drone eggs laid even by impregnated
queens, Siebold di-ew the conclusion that they have not received the
fertilising influence of the male fluid, which is communicated to the
queen-eggs and worker-eggs alone ; so that the products of sexual
generation are always female, the males being develoj)ed from these
by a process which is essentially one of gemmation.^

The embryonic development of insects is a study of peculiar
interest from the fact that it may be considered as divided (at
least in such as undergo a ' complete metamorphosis ') into two
stages- that are separated by the whole active life of the larva — that,
namely, by which the lai-va is j)roducecl Avithin the egg, and that by
which the imago or perfect insect is produced within the body of
the pupa. Various circumstances combine, however, to render the
study a very difficult one ; so that it is not one to be taken up by
the inexperienced microscopist. The following summary of the
history of the process in the common blow-fly, however, will pro-
bably be acceptable. A gastrula with two membranous lanaellaj
having been evolved in the first instance, the outer lamella very
rapidly shapes itself into the form of the larva, and shows a well-
marked segmental division. The alimentary canal, in like manner,
shapes itself from the inner lamella, at first being straight and
very capacious, including the whole yolk, but gradually becoming
narrow and tortuous as additional layers of cells are developed
between the two primitive lamellae, from which the other internal
organs are evolved. When the larva comes forth from the egg it
still contains the remains of the yolk ; it soon begins, however, to
feed voraciously ; and in no long period it grows to many thousand
times its original weight, without making any essential progress in
development, but simply accumulating material for future use. An
adequate store of nutriment (analogous to the ' supplemental yolk '
of Purpura) having thus been laid up within the body of the
larva, it resumes (so to speak) its embryonic development, its passage
into the pupa state, from which the imago is to come forth, involving
a degeneration of all the larval tissues ; whilst the tissues and
organs of the imago 'are redeveloped from cells which originate
from the disintegrated parts of the larva, under conditions similar
to those appertaining to the formation of the embryonic tissues from
the yolk.' The development of the segments of the head and body
in insects generally proceeds from the corresponding larval segments ;
but; according to Dr. Weismann, there is a marked exception in the
case of the Diptera and other insects whose larva? are unfurnished
with legs, their head and thorax being newly formed from ' imaginal
discs,' which adhere to the nerves and trachefe of the anterior
extremity of the larva ; - and, strange as this assertion may seem,

^ See Professor Siebold's memoir, On True Parthenogenesis in Moths and Bees,
translated by W. S. Dallas (London, 1857) ; ajid his Beltrage zur Parthenogenesis
der Arthropoden (Leipzig, 1871'.

- See his ' Entwickelung der Dipteren ' in Zeitschrift f. Wiss. Zool. xiii. and xiv. ;
Mr. Lowne's Anatomy of the Blow-fly (1st ed.), pp. 6-9, 113-121 ; and A. Kowalevsky,
' Beitrage zur Kenntnis der Nacliembryonalen-Entwickelung der Musciden,' Zeitschr.
f. Wiss. Zool. xliv. p. 542.

I008 INSECTS AND AEACHNIDA

it has been confirmed by the subsequent investigations of Mi-.
Lowne.^

The Arachnida, or scorpions and pseudo-scorpions, and the Ara-
neida or spiders, present much that is of interest even to the unscien-
tific who use the microscope only for pleasure. The general remarks
which have been made in regard to insects are equally aj)plicable
to these, but have special application in that gToup known as the
Acarina, consisting of the 77iites and ticks. Some of these are
parasitic, and are popularly associated with the wingless parasitic
insects, to which they bear a strong general resemblance, save in
having eight legs instead of six. The Acarina are the true ' mites ; '
they generally have the legs adapted for walking, and some of them
are of active habits. The common cheese-mite, as seen by the naked
eye, is familiar to eveiy one ; yet few who have not seen it under a
microscope have any idea of its real conformation and movements ;
and a cluster of them, cut out of the cheese they infest, and j^laced
under a magnifying power sufiiciently low to enable a large numbei-
to be seen at once,, is one of the most amusing objects that can be
shown to the young. There are many other species, which closely re-
semble the cheese-mite in structure and habits, but which feed upon
diiferent substances ; and some of these are extremely destructive.

The Acarina are the smallest of the Arthropoda^ and are sj)ecially
well fitted for microscopical examination ; indeed, with the exception
of the Ixodidce (including the Argosince), which attain a substantial
size, particvilaily in tropical countries, but little can be learnt
respecting them without such aid ; as far as is at present known,
other mites are not larger iri hot countries than in Europe. Many
species make beautiful objects for the microscope, and may be well
preserved, the hard-bodied specimens in balsam without heat oi-
pressure, the soft-bodied in glycerin or glycerin jelly ; e.g. the
nymphs of Leiosotna palviacincttiin, Tegeocranus cejiheiformis, T.
dentatus, and the adults of Glycipliagiis plumigcr and G. jmhnifer
are admirable. They are all British, and are found respectively on
lichen at the Land's End, on the fallen bark and needles of fir-trees,
on fallen oak-wood, in the fodder in stables, and on cellar-walls.
Many of the Trombidiidce and Hydrachnidce also are very beautiful ;
and the Der^naleichi, especially the males, and such creatures as
Myohia, Listropliorus, &c., are extremely curious. With the excep-
tion of the Phyto'ptidce, all Acarina in the adult stage have eight
legs and the constriction between cephalo-thorax and abdomen is
far less marked than in insects and spiders — in many genera it is
wholly lost. The sexes are distinct and often very diflerent from
each other ; the reproduction is oviparous or ovo-vivipai'ous — pos-
sibly in rare and exceptional instances viviparous. The ova are
usually elliptical or oval ; in those which have a hard shell a
curious stage known as the ' deutovium ' exists ; as the egg increases
in size the shell splits into two symmetrical halves, which remain
attached to the lining membrane, but are widely separated, the

1 Reference should be made to Professor Biitschli's observations in Morphol.
Jahrbnch, xiv. p. 170, and Dr. Voeltzkow's paper in Arheit. Zool. Zool. Inst. Wilrz-
burg, ix. p. 1.

Pi ATE XX,

Acarina ,

West.Newrman chromo

MITES 1009

membrane becoming the external covering in the space left. The
eggs of the so-called stone-niite {Fetrohia lajyidum) are discoidal and
sculptured ; they occasionally appear in countless numbers over a
large space of ground in a single night, making the place look
whitewashed ; they have been mistaken for fungi and called Grate-
ritim pyrifor'me ; they are good microscopical objects. The larvae
of all Acarina, except Phytojjtus and possibly Dermanyssus, are
hexapod ; the fourth pair of legs is absent. The nymphal stage is
usually the principal period of growth ; occasionally, however, it is
wanting. The nymph is an active chrysalis, as in the Orthoptera ; it
usually undergoes several ecdyses. In many species of the Orihatidce
the whole skin is not cast, but splits round the edge of the body,
and the dorso-abdominal portion remains attached to the new skin ;
often it has a row of elegant spines or hairs round its edge ; thus
after two or three ecdyses these spines form concentric rings on the
notogaster (Plate XXI, fig. 2). In the Trombidiidce^ Tyroglyphi^ kc.
the nymphs usually greatly resemble the adults ; in the Orihatidce
they are often totally different, and every intermediate stage occurs.
The change from nymph to adult is usually preceded by an inert
period.

The number and variety of the families, and the differences in
the external form and internal anatomy, are so great and so endless
that it is impossible here to do more than indicate a few leading
features and refer to a few examples of interest. The caput is, of
course, fused with the thorax, but sometimes a constriction at the
base of the rostrum gives a false appearance of there being a distinct
head. The trophi are extremely different in the respective families,
or even genera. In the more highly organised of the Gamasidce
almost all the parts which exist in the most elaborate insect-mouths
except the labial palpi may be found ; they are well described by
M. Megnin.^ A large oral tube is formed by the ankylosed
maxillae and probably upper lip and lingua. Up the centre of this
tube the mandibles pass freely ; they are very long and chelate ;
the fh-st joint is simply cylindrical ; the second similar, but having
the fixed chela at its distal end ; the third is the movable chela.
They are capable of being projected far beyond the body, or of being
withdrawn wholly within it, the muscles which withdraw them
often arising from quite the posterior end of the body. These man-
dibles are different in the two sexes, and those of the male often
have most remarkable appendages. One of the best examples is
that of Gamasus terribilis, a species found in moles' nests by Mr.
Michael. Professor Canestrini, of Padua, also has figured some very
singular forms. In the Orihatidce, TetranychiLS, the Sarco-ptidce,
&c. the mandibles are also chelate, but of two joints only, shorter,
more powerful, and not capable of such great protrusion. In the
Hydrachnidoi, Tromhidiince, &c. the mandible is not chelate, but
the terminal joint shuts back like a clasp-knife, as in the poison-
fangs of spiders. Other forms of mandible are found. The maxillse
are large toothed crushing organs in the Orihatidce ; they are very

1 Jourii. cle VAnat. et cle la Physiol. Eobin, May 1876.

3 T

lOIO INSECTS AND ARACHNID A

strongly developed in Hoplophora, which is a wood-boring creature.
In other families they are more commonly joined, forming a maxil-
lary lip with a flexible edge for sucking purposes. The maxillary
palpi vary greatly ; in the Sarcoptidce, Myohia, &c. they are anky-
losed to the lip ; in the Pliytopti Nalepa is of opinion that they are
needle-like piercing organs, but these may well be the maxillee. In
some predatory forms, as Gheyletus, Trombidiimi, &c., they assum.e
great importance, being the raptorial organs ; in the first they
are extremely large and powerful and work horizontally ; they are
provided with a number of long chitinous spines and comb-like
appendages of a very singular character. In Trombidium the
ultimate joint is articulated at the base of, or part of the way down,
the penultimate, forming a sjjecies of chela. In Bdella the palpi
are long thin organs, carried upward and backward, and have the
appearance of antennae. The joints of the legs are from three
{pemodex) to seven (some Tromhidiidce and Gcmiasidoe) ; five is the
most usual number. They are terminated by a sucker as in the Sar-
coptidce, where it is often very large ; or by a claw or claws, or both
together. In some parasitic species the claws are developed in a
special manner for holding the hairs of the host ; thus Myohia has
the claw of the first leg flattened out so as to form a broad lamina,
which curls round the hair and 23resses it against a chitinous peg on
the tarsus ; Myocoptes has a similar arrangement on the third leg.
Both these generar contain species which are parasites of the m.ouse,
and easily obtained. In the Oribatidce, TyroglypM., &c. the legs
are all strictly walking organs ; but in Gheyletus, most Gamasidce,
&c. the first pair are tactile, and not used in locomotion. The legs
generally correspond on the two sides of the body, but in Freyana
lieterojyus, an extraordinary parasite of the cormorant discovered by
Mr. Michael (Plate XXII, fig. 3), the second leg of the male is developed
to a much greater extent on one side than on the other, and is
supported by a difi'erent sternal skeleton on the tM^o sides ; the
strangest fact is that it is not always the same side that is thus
develo]3ed ; it is usually the left, but occasionally the right. The
integument of the Acarina is almost always soft in the immature
forms ; in the advilts it is hard and chitinised in the Oribatidce and
most GmnasidcE ; partly so in the Ixodidcs ; and usually soft in most
other families, and often minutely striated. The hairs and other
appendages of the integument of a similar nature are often very
characteristic and extraordinary. In the nymph of Leiosoma pahna-
cinctum they are large scale-like processes of a Japanese- fan shape,
which entirely cover up and conceal the body of the creature ; a
leaf-like form is also connnon. In Glyciphagiis plimiiger they are
elegant plumes ; in some Sarcoptidce, e.g. Synibiotes tripilis, some of
the simple setiform hairs are three times the length of the body ;
in the Tro7nbidiidce the body-hairs are often extremely fanciful. The
setiform hairs are the principal organs of touch, those on the front
legs being specially important. So sensitive are they that Gheyletios
and some Gamasids, which are predatory and capture such active
creatures as Thysanaridce, are entirely eyeless, and trust to the
tactile sense only. Haller was of opinion that certain specialised

PLATE XXI.

"West-Newma-n diromo.

Acapina.

MITES lOII

hairs had an auditory function. In the Ixodidce a singular dru^m-
like structure in the first leg has been considered by Haller and
others to be the hearing organ ; while in the OribaticUe that organ
appears to be located in the pseudo-stigmata, two paired organs at
the side of the cephalo-thorax which were long taken for true stig-
mata. The Gamasiclce, Oribaticke. Tyroghjpliiclce, Sarcojjtidce, etc.
are entirely without special organs of vision. The Hydrachnidce
have two pairs of simple eyes, each pair being so close together as
to look like a single eye. The Troonbidiidce mostly have simple
eyes, the number and position of which vary with the species.
As to internal anatomy it should be noted that there is almost
endless variety. The alimentary canal most commonly consists of a
long thin oesophagus, provided with distensor muscles on each side,
so as to make it a sucking organ ; it usually passes right through or
close under the great ganglion known as the brain ; in some species,
as Damceus genictdatus, the cesophagus is followed by a large pro-
ventriculus, but this is not usual ; it more commonly leads directly
into the ventriculus, which generally is a principal viscus, and in
most families furnished with more or less glandular caecal ap-
pendages, not numerous, but often very large, occasionally larger
than the organ itself. A valve in many cases separates the ven-
triculus from the hind-gut, which is commonly divided into what
may be called colon and rectum. In the Gamasidce a single very
large Malpighian vessel on each side of the body enters between
the two last-named divisions of the alimentary canal. These vessels
run right along the side of the body, and strong pulsation may be
seen in them. In the Orihatidce they are absent, their function
being apparently performed by supercoxal glands. The Tyro-
glyphidce^ Sarco'piidce, Phytoptidce^ &c. are without special respira-
tory organs ; the Orihatidce and some Uropoda have simple un-
branched tracheae, much in the same condition as those of Perijxctios.
The other Gcwiasidce, the TromhidAidce, Cheyletidce, Ixodidce, <fec.
usually have branched tracheae, like insects ; air-sacs are occasion-
ally found, but not anything like the tracheal lungs or gills (so
called) of spiders and scorpions. The principal nerve-centre is
much concentrated, and consists usually of either a large supra-
oesophageal and smaller subcesophageal ganglion joined by com-
missures ; or, more frequently, the whole forms one mass which is
pierced by the oesophagus, which may be pulled out, leaving a neat
roimd hole ; the nerves, of course, radiate from this mass, but there
is not space here to describe their course. A pulsating organ of the
nature of the dorsal vessel of insects, but much shorter, and with
only one or two pairs of ostia, has been detected in some Gamasidce,
and in Ixodes, first by Kramer and afterwards by Winkler and
Glaus ; it has a median aorta running forward ; it is best seen in
life in young specimens still transparent ; it lies at the rear of the
ventriculus, near the dorsal surface. Nothing of the nature of a
heart has yet been discovered in other Acarina. The reproductive
organs are, perhaps, most frequently of the ' ring ' type, well known
in the Arcochnida ; thus in female Oribatidce, they consist of a central
ovary, with an oviduct springing from near each end, in Avhich the

3 T 2

IOI2 INSECTS AND AEACHNIDA

eggs are matured ; the oviducts botli terminate in an unpaired
vagina, whence the eggs pass into a long, membranous, extensible
ovipositor, often wrinkled or striated with singular fineness and
beauty. The external aperture is closed by chitinous folding doors.
A more or less similar arrangement may be found in most Gamasidce,
Hydrachnidce, &c., biit withou.t the ovipositor. Spermathecfe are
often found in the Gamasidcs, Tyroglyphidce^ &c., and accessory
glands frequently accompany the vagina in almost all families. The
male system varies greatly, but is frequently constructed on similar
lines, preserving somewhat of the ' ring ' form.

The jjrincipal families into which the Acarina are di\"ided are as
follows : —

The Gamasidce, which in the adult stage are mostly pro-
vided with a hard chitinous cuticle in all parts of the body. They
are mostly predatory, but the females and young are often parasitic.
Ptero'ptus and Dermanyssus, however, are more leathery in texture,
and are parasitic during their whole lives, the former on bats, the
latter on birds. This family have the true stigmata, one on each
side of the ventral surface, usually between the second and third
pairs of legs ; these do not communicate directly witli the external
air, but have a long tubular peritreme in the cliitin of the ventral
surface, often very elaborate in form, and emerging to the air usually
between the first and second legs. This is highly characteristic of
the family.

The Ixodidce, or ticks, most of which are probably primarily
vegetable feeders, but will, when opjDortunity ofi'ers, attach them-
selves to animals by sinking their long serrated rostral projection
into the skin, have a single ventral stigma on each side, com-
m.unicating directly with the air by a, large cullender-plate, which
is an interesting microscopical object. The males have the dorsal
surface of the abdomen almost entirely covered by a chitinous plate,
which is much smaller in the females ; but the leathery portion of
the abdomen in that sex is capable of great distension for the pur-
pose of permitting the suction of animal juices. The Argasidce
must be included in this group ; their tenacity of life and power of
existing without food are marvellous ; their bite is severe, but the
terrible stories told of the results of the bite of the Persian Argas
have not been supported on investigation.

The Oribatidce are mostly wholly chitinised, the chitin being-
very hard and brittle. The stigmata are in the ace tabula of the
legs. The pseudo-stigmata (hearing organs) of this family have
been before referred to. Oribatidce are vegetable feeders, living in
moss, lichen, fungus, dead wood, under bark of trees, &c., and some
few species on aquatic j)lants. They are Avidely distributed from
the arctic regions to the equatorial. Ho2)lo})hora has the power of
withdrawing the legs wholly within the carapace, and then shutting
down the cephalo-thorax against the abdomen, so as to close the
opening, when it appears like a chitinous ball ; from this power it
has been called the ' box-mite.' The sexes have not any external
diflei-ence.

The Tromhidiidoi are a large and varied group, mostly predatory

PLATE XXII

Wesl,NevvmaD lilh.

.Ac

ariiicL.

MITES IOI3

and with soft, often velvety skins, frequently of scarlet and otlier
brilliant colours. The large Trombidium holosericimn is a well-known
microscopical object. The Tetranychi are usually included in this
family ; they are, however, rather doubtful members ; they are the ' red-
spiders ' of our greenhouses, much dreaded by horticulturists. Each
foot is provided with about four singular hairs with round knobs at
the end. Bryohia is an allied geniis found in great numbers on ivy
&c. in gardens and is a beautiful object. The hexapod larvas of several
species of Tromhidium often attach themselves temporarily to the skin
of animals, including man, and produce intolerable itching. They were
supposed by the earlier Acarologists to be all one species, and to
be adult, and to form a distinct family ; they were called Lejjtus
wutuinnalis, and are known in England as the ' harvest-bug,' and
in France as the rotcget. The Bdellidce are also included in this
family ; some authors also include the Gheyleti, which, however, seem
to need a separate family, having many curious characters, including
the dorsal position of the male organs.

The Hydrachnidce, or water-mites, as well as the Trombidiidce,
have the two stigmata in the rostrum ; the legs are swimming
organs, the sexes often very different ; they live in fresh water and
are often parasitic in their immature, but not in the adult stages.
They are mostly soft-bodied and often of brilliant colours.

The Limnocaridce are sometimes treated as^ a sub -family of the
Hydrachnidce, but are crawling, not swimming creatures, and are
found in fresh water ; but the Hcdicaridce, which either constitute
a sub-family of, or are closely associated with them, are marine,
and are much found among Hydrozoa, on which they ]3robably
prey.

The parasitic Myobiidoe are by some included in the Cheyletidce ;
the differences, however, are very considerable. They are the last
tracheate family.

The Tyroglyphidce are the cheese-mite family ; they are far the
most destructive of all Acarhia, swarming in countless numbers and
devouring hay, cheese, drugs, growing plants and roots, &c. ; the
genus Glyciphagus contains many singular and interesting forms, as
G. flatygaster and G. Krameri, found in moles' nests. It is in this
family that the curious hypopial stage exists ; some of the indi-
viduals of some species, instead of following the ordinary life-history,
are changed at one ecdysis into a totally different-looking creature,
with a highly chitinised cuticle and rudimentary mouth-organs,
which can endure draught and other unfavourable circumstances
which would kill the ordinary form. They attain the same adult
stage as other individuals. The HypopuH is provided with adhesive
suckers whereby it attaches itself temporarily to other creatures, and
this serves for the distribution of the species.

The Tarsonemidce are minute creatures, some leaf-miners, some
parasitic on bees &c.

The Barcoptidce are divided into two great sub-families, the Sar-
cojjtince, or itch-mites, of which the well-known Sarcoptes scabiei of man
(Plate XXII, fig. 4) is the type, and the A ncdgesi7ice, or bird-parasite
mites ; all have soft bodies with finely striated cuticles. Sarcoptes

IOI4 INSECTS AND AEACHNLDA

scabiei is a minute creature of almost cii^cular form, the fem.ale of
which burrows under the epidermis, causing the disease. The m.ite is
found at the end of the burrow, not in the pustule at its commence-
ment. The first two pairs of legs and the third leg of the male
are terminated by suckers, the other legs by long bristles. The
male is smaller than the female. The Analgesince (Derinaleichi) are
a very large and curious group ; the males often differ greatly from
the females, and the skin is often greatly strengthened by chitinous
plates and str Pictures. The species are not always parasitic on one
bird only ; often the same species may be found on numerous birds,
while sevei-al species frequently live on the same bird ; they are not
usually supposed to be injurious to the birds ; they are found on the
feathers.

The Phytojotidce are extremely minute creatures living in galls
which they form on the leaves and twigs of numerous trees and
plants ; they are elongated in form, with the two hind pairs of legs
abortive ; there is but little variety among them. Slightly resem-
bling them in general form, but very difierent in other respects, is
Demodex follicidorum^ which is found in the sebaceous follicles of
the hiiman skin, particularly the nose. Those follicles, which are
enlarged and whitish with a terminal exterior black spot, may be
forced out by pressure, and the Acarus will often be found within.
Similar parasites exist on the dog and pig.

There are numerous other curious and interesting forms which
cannot be included in any of the families mentioned above.

The number of objects furnished to the microscopist by the
spider ti-ibe is very large from a biological point of view, although
mere objects of microscopical interest popularly are not so numerous
as in insects. Their eyes exhibit a condition intermediate between
that of insects and ci'ustaceans and that of vertebrata, for they are
simple like the '■ stemmata ' of the former, usually number from six
to eight, are sometimes clustered together in one mass, but more
frequently disposed separately ; while they present a decided ap-
proach in internal structure to the type characteristic of the visual
organs of the latter.

The structure of the mouth is always mandibulate, and is less
complicated than that of the mandibulate insects. The respiratory
apparatus is not tracheal, as in insects and some Acarina, but is
constructed upon a very different plan, for the ' stigmata,' which
are usually four in number on each side, open upon a like number
of respiratory sacculi, each of which contains a series of leaf-like
folds of its lining membrane upon which the blood is distributed so
as to afford a large surface to the air.

In the structure of the limbs, the principal point worthy of
notice is the peculiar appendage with which they usually terminate,
for the strong claws, with a pair of which the last joint of the
foot is furnished, have their edges cut into comb-like teeth, which
appear to be used by the animal as cleansing instruments, and in
many cases for the manipulation of the silk of their snares. But a
feature deserving study by the microscopist is the physical cause of
the exquisite sensitiveness of these ' feet.' By resting these upon a

SPIDEES

IOI5

Fig. 749.— Foot, with comb-like claws, of the
common spider (EjJeira).

trap-line of silk carried to her den, she can, by a veritable telegraphy,
discover instantly, not only the fact that there is prey npon her
snare, bnt the exact spot in the web of the snare in which that
prey is entangled. In the same way, by seizing certain tightly
stretched threads commnnicating with the main lines of the snare,
she can discover in an instant the presence and position of her i^i-ey,
though far beyond the reach of vision.

The most characteristic and interesting part in the special
organisation of the spider is the ' spinning apparatus,' by means of
which its often elaborately
constructed webs are pro-
duced. These consist of
' spinnerets ' on the ex-
terior of the body and
glandular organs lying
within the abdomen ; it is
by them that the silk from
which all the elements of
the snare are produced is
secreted.

Of these glands there
are two pairs which are
sac-like in form, with a
coiled tube opening di-
rectly on the spinnerets :

there are three pairs, of a convoluted appearance, opening on the
hinder spinnerets ; and there are three of a sinuous tubular form
opening on the hinder and middle spinnerets. Beyond these there
are respectively 200 and 400 smaller glands, which ojoen on the
front, middle, and hinder spinnerets. They all terminate in tubes
of great delicacy, through which the silk is drawn at the will of the
spinster ; and, while the scaffolding or framework of the web of
Epeira is double and hardens rapidly in air (fig. 750, A), those which
lie across the polygons of

the scafiblding are stud- A

ded at regular intervals
with viscid globules, as
seen in fig. 750, B ; and
it is to these viscid glo-
bules that the peculiarly
adhesive character of the
web is due.

The usual number of the spinnerets is six. They are little teat-
like processes crowiied with silk tubes. They are movable at the
will of the spider, and can be erected or depressed, and one, many,
or all of the tubes crowning a spinneret may be caused to exude
and have drawn from it or them the silk as the spider determines.
There can be no doubt that there is a difierence in the silk secreted
by different glands, and its appropriate employment is a part of
the skill of the spider.

It is certain that the silken threads of a snare are of two kinds ;

Fig. 750. — Ordinary thread (A) and viscid
^ thread (B) of the common spider.

IOl6 INSECTS AND AEACHNIDA

(1) that wliicli i^apidly hardens on contact with the air, and which
is employed in the construction of the framework of the snare ; and

(2) a viscid silk with which the entangling meshes by which prey is
caught are put in. The latter present beautiful objects for popular
observation, because the thread has strung upon it, as it were,
innumerable pearl-like globules in which the viscidity remains.
These beads are produced after the thread is drawn out by a
special vibratory action set up in the thread by the spider.

The eggs of spiders are not objects of special optical interest,
but they afford opportunities for good embryological work,' and the
habits of spiders offer a good scope for industrious study in the field.^

1 See the work of Kishinouyi in Joitrn. Coll. Sci. Imp. Univ. Japan, vol. iv.

2 See pariiicularly McCook's American Spiders and their Spinning Work,
Philadelphia, 1889 and 1890, and the various papers of Mr. and Mrs. Peckham in the
American journals.

lOl/

CHAPTER XXII

VEBTEBBATED ANIMALS

We are now arrived at the highest division of the animal kingdom,
in which the bodily fabric attains its greatest development, not only
as to completeness, but also as to size ; and it is in most striking
contrast with the class we have been last consideiing. Since not
only the entire bodies of vertebrated animals, but, generally speak-
ing, the smallest of their integral parts, are far too large to be viewed
as microscopic objects, we can study their structure only by a
separate examination of their component elements ; and it seems,
therefore, to be a most apj)ropriate course to give under this head a
sketch of the microscopic characters of those jjrimai'p tissues of
which their fabric is made up, and which, although they may be
traced with more or less distinctness in the lower tribes of animals,
attain their most com2:)lete development in this group. ^

Although there would at first sight appear but little in common
between the simple bodies of those humble Protozoa which con-
stitute the lowest types of the animal series, and the complex
fabric of man or other vertebrates, yet it seems certain that in
the latter, as in the former, the process of ' formation ' is essentially
carried on by the instrumentality of jjrotoplasmic stibstance, univer-
sally difiiised through it in such a manner as to bear a close resem-
blance to the pseudopodial network of the rhizopod ; whilst the
tissues produced by its agency lie, as it were, on the outside of
this, bearing the same kind of relation to it as the foraminiferal
shell does to the sarcodic substance which fills its cavities and
extends itself over its surface. For, as was first pointed out by
Dr. Beale,^ the smallest living ' elementary part ' of every organised

^ This sketch is intencled, not for the professional student, but oaily for the
amateur microseopist who wishes to gain some general idea of the elementary struc-
ture of his own body and of that of vertebrate animals generally. Those who wish
to go more deeply into the inquiry are referred to the following. The translation of
Strieker's Manual of Histology, published by the New Sydenham Society ; the
translation of the 4th edition of Professor Frey's Histology and Histo-Chemistry of
Man ; the ' General Anatomy ' of the 10th edition of Quain's Anatomy, 1893. by
Professor Schafer; and the Atlas of Histology, by Dr. Klein and Mr. Noble Smith.

^ Professor Beale's views are most systematically expounded in his lectures On the
Structure of the Simple Tissues of the Human Body, 1861 ; in his Hotv to work
with the Microscope, 5th edition, 1880 ; and in the introductory portion of his new

IOl8 VEETEBEATED ANIMALS

fabric is composed of organic matter in two states : the protoplasmic
(which he termed gerviinal matter) , possessing the power of selecting
pabulum from the blood, and of transforming this either into the
material of its own extension or into some product which it
elaborates ; whilst the other, which may be termed formed material,
may present every gradation of character from a mere inorganic
deposit to a highly organised structure, but is in every case altogether
incapable of self-increase. A very definite line of demarcation can
be generally drawn between these two substances by the careful use
of the staining process ; but there are many instances in which there
is the same gradation between the one and the other as we have
formerly noticed between the ' endosarc ' and the ' ectosarc ' of the
Amoeba. Thus it is on the protoplasmic component that the exist-
ence of every form of animal organisation essentially depends ;
since it serves as the instrument by which the nutrient material
furnished by the blood is converted into the several forms of tissue.
Like the sarcodic substance of the rhizopods, it seems capable of in-
definite extension ; and it may divide and subdivide into independ-
ent portions, each of which may act as the instrument of formation
of an ' elementajy part.' Two principal forms of such elementary
parts present themselves in the fabric of the higher animals,
viz. cells and fibres (which are modified cells) ; and it will be
desirable to give a brief notice, of these before proceeding to describe
those more complex tissues which are the products of a higher
elaboration.

The cells of which a few animal tissues are essentially composed
consist, in some cases, of the same parts as the typical cell of the
plant, viz. a definite ' cell- wall,' inclosing ' cell-contents ' and a
' nucleus,' which is the seat of its formative activity. It is of such
cells, retaining more or less of their characteristic spheroidal shape,
that every mass of fat, whether large or small, is chiefly made up.
In a large number of cases the cell shows itself in a somewhat
difierent form, the ' elementary part ' being a corpuscle of proto-
plasm of which the exterior has undergone a slight consolidation,
like that which constitutes the ' primordial utricle ' of the vegetable
cell or the ' ectosarc ' of the Amoeba, but in which there is no proper
distinction between ' cell-wair and 'cell-contents.' This condition,
which is characteristically exhibited by the nearly globular colourless
coryuscles of the blood, appears to be common to all cells in the in-
cipient stage of their formation, and the progress of their develop-
ment consists in the gradual differentiation of their parts, the ' cell-
wall ' becoming distinctly separated from the ' cell-contents,' and
these from the ' nucleus,' and the original protoplasm being very

edition of Todd and Bowman's Physiological Anatomy, 1867. The princixjal results
of the inquiries of G-erman histologists on this point are well stated in a pa]per by
Dr. DufSn on ' Protoplasm, and the Part it plays in the Actions of Living Beings ' in
Quart. Journ. Microsr. Sci. n.s. vol. iii. 1863, p. 251. The Author feels it necessary,
however, to express his dissent from Professor Beale's views in one important particular,
viz. his denial of ' vital ' endowments to the ' formed material ' of any of the tissues ;
since it seems to him illogical to designate contractile muscular fibre (for example)
as ' dead,' merely because it has not the power of self-reparation.

CELLS I 019

commonly replaced more or less completely by some special product
(sucli as fat in the cells of adipose tissue, or hfemogiobin in the red
corpuscles of the blood), in which cases the nucleus often disappears
altogether. In the earlier stages of cell-development multiplication
takes place with great activity by a duplicative subdivision that
corresponds in all essential particulars with that of the plant-cell,
as is well seen in cartilage, a section of which will often exliibit in
one vieAv the successive stages of the process. ■■ Whether ' free ' cell-
multiplication ever takes place in the higher animals is at present
uncei-tain.

A large part of the fabric of the higher animals is made up of
fibrous tissues, which serve to bind together the other components,
and which, when consolidated by calcareous deposit, constitute the
substance of the skeleton. In these the relation of the ' germinal
matter ' and the ' formed material ' presents itself under an aspect
which seems at first sight very difierent from that just described.
A careful examination, however, of those ' connective tissue cor-
puscles ' that have long been distinguished in the midst of the fibres
of which these tissues are made ujj, shows that they are the equi-
valents of the corpuscles of ' germinal matter,' which in the previous
instance came to constitute cell-nuclei, and that the fibres hold the
same relation to them that the ' walls ' and ' contents ' of cells do to
their germinal corpuscles. The transition from the one type to the
other is well seen in fibro-cartilage, in which the so-called ' inter-
cellular substance ' is often as fibrous as tendon. The difierence
between the two types, in fact, seems essentially to consist in this,
that, whilst the segments of ' germinal matter ' Avhich form the cell-
nuclei in cartilage and in other cellular tissues are completely
isolated from each other, each being completely surrounded by the
product of its own elaborating action, those which form the ' con-
nective-tissue corpuscles ' are connected together by radiating pro-
longations that pass between the fibres, so as to form a con-
tinuous network closely resembling that formed by the pseudo-
podia of the rhizopod. Of this we have a most beautiful example
in bone ; for whilst its solid substance may be considered as
connective tissue solidified by calcareous deposit, the ' lacunse ' and
' canaliculi ' which are excavated in it (fig. 752) give lodgment to a
set of radiating corpuscles closely resembling those just described ;
and these are centres of ' germinal matter,' which appear to have an
active share in the formation and subsequent nutrition of the osseous
texture. In dentine (or tooth-substance) we seem to have another

1 Great attention has lately been given by many able observers to the changes
which take place in the nucleus before and during its cleavage. A full account of
these is contained in Professor Strassburger's Zellhildung und Zelltlieilung, 1880.
See also Dr. Klein's ' Observations on the Structure of Cells and Nuclei ' in Quart.
Journ. Microsc. Sci. n.s. vol. xviii. 1878, p. 315, and vol. xix. 1879, pp. 125, 404 ; and
chaiJ. xliv. of his Atlas of Histology. The numerovis essays of Plemming, in the
Archivf. mikr. Anat. from 1875 to 1890 ; Gruber, on the Nucleus of Protozoa, in vol. xl.
of the Zeitschr. f. Wiss. Zool. ; and Carnoy, in La Cellule, may be studied by those
who desire to carry further the history of the cell. A remarkable series of observa-
tions have followed the publication of Professor E. van Bcneden's work on the egg
of Ascaris megalocephala in Bull. Acad. Boy. Sci. Belg. xiv. pp. 21.5-95.

I020 VERTEBRATED ANIMALS

form of the same thing, the walls of its ' tubuli ' and the ' inter-
tubular substance ' being the ' formed material ' that is produced
from thread-like prolongations of ' germinal matter ' issuing from
its pnlp, and continuing during the life of the tooth to occupy its
tubes ; just as in the Foraminifera we have seen a minutely tubular
structure to be formed around the individual threads of sarcode
which proceeded from the body of the contained animal. It may
now be asserted, indeed, that the bodies of even the highest animals
are everywhere penetrated by that protoplasmic substance of which
those of the lowest and simplest are entirely composed ; and that
this substance, which forms a continuous network through almost
every portion of the fabric, is the main instrument of the formation,
nutrition, and reparation of the more specialised or differentiated
tissues. As it is the purpose of this work, not to instruct the
professional student in histology (or the science of the tissues),
but to supply scientific information of general interest to the
ordinary microscopist, no attempt will here be made to do more
than describe the most important of those distinctive characters
which the principal tissues present when subjected to microscopic
examination ; and as it is of no essential consequence what order is
adopted, we may conveniently begin with the structure of the
skeleton,^ which gives support and protection to the softer parts of
the fabric.

Bone. — The microscopic characters of osseous tissue may some-
times be seen in a very thin natural plate of bone, such as in that
forming the scapula (shoulder-blade) of a mouse ; but they are dis-
played more perfectly by artificial sections, the details of the arrange-
ment being dependent upon the nature of the specimen selected and
the direction in which the section is made. Thus when the shaft of
a ' long ' bone of a bird or mammal is cut across in the middle of its
length, we find it to consist of a hollow cylinder of dense bone,
surrounding a cavity which is occupied by an oily marrow ; but if
the section be made nearer its extremity we find the outside wall
gradually becoming thinner, whilst the interior, instead of forming
one large cavity, is divided into a vast number of small chambers,
partially divided by a sort of ' lattice work ' of osseous fibres, but
communicating with each other and with the cavity of the shaft,
and filled like it with marrow. In the bones of reptiles and fishes,
on the other hand, this ' cancellated ' structure usually extends
throughout the shaft, which is not so comjdetely difierentiated into
solid bone and medullary cavity as it is in the higher Yertebrata.
In the most developed kinds of ' flat ' bones, again, such as those of
the head, we find the two surfaces to be composed of dense plates of
bone, with a ' cancellated ' structure between them ; whilst in the less
perfect type pi-esented to us in the lower Vertebrata, the whole
thickness is usually more or less ' cancellated,' that is, divided up
into minute medullary cavities. When we examine, under a low
magnifying power, a longitudinal section of a long bone, or a section

1 This term is used in its most general sense, as including not only the proper
internal skeleton, but also the hard parts protecting the exterior of the body, which
form the dermal skeleton.

STEUCTUKE OF BONE

I02r

of a flat bone parallel to its surface, we find it traversed by numerous
canals, termed Haversian after their discoverer Havers, which are in
connection witli the central cavity, and are filled like it with marrow.
In the shafts of ' long ' bones these canals usually run in the direction
of their length, but are connected here and there by cross-branches ;
whilst in the flat bones they form an irregular network. On api^ly-
ing a higher magnifying power to a thin transverse section of a long
bone we observe that each of the canals whose orifices present them-
selves in the field of view (fig. 751) is the centre of a rod of bony
tissue (1), usually more or less circular in its form, which is arranged
around it in concentric rings, resembling those of an exogenous
stem. These rings are marked out and divided by circles of little
dark spots, which, when closely examined (2), are seen to be minute
flattened cavities excavated in the solid substance of the bone, from
the two flat sides of which
pass forth a number of
extremely minute tubules,
one set extending inwards,
or in the direction of the
centre of the system of
rings, and the other out-
wards, or in the direction
of its circumference ; and
by the inosculation of the
tubules (or canaliculi) of
the different rings with
each other a continuous
commimication is esta-
blished between the cen-
tral Haversian canal and
the outermost part of the
bony rod that surrounds
it, which doubtless minis-
ters to the nutrition of

the texture. Blood-vessels are traceable into the Haversian canals,
but the ' canaliculi ' are far too minute to carry blood-corpuscles ; they
are occupied, however, in the living bone by threads of protoplasmic
substance, which bring the segments of ' germinal matter ' contained
in the lacunse into communication with the walls of the blood-
vessels.

The minute cavities of lacunce from Avhich the canaliculi proceed
(fig. 752) are highly characteristic of true osseous tissue, being never
deficient in the minutest parts of the bones of the higher Vertebrata
although those of fishes are occasionally destitute of them. The dark
appearance which they present in sections of a dried bone is not due
to opacity, but is simply an optical efiect, dejaendent (like the black-
ness of air-bubbles in liquids) ujDon the dispersion of the rays by the
highly refracting substance that surrounds them. The size and
form of the lacunar differ considerably in the several classes of Ver-
tebrata, and even in some instances in the orders, so that it is often
possible to determine the group to which a bone belonged by the

Fig. 751. — Minute structure of bone as seen in
transverse section : 1, a rod surrounding an
Haversian canal, 3. showing the concentric
arrangement of the lamellBe ; 2, the same, with
the lacunse and canaUculi ; 4, portion of the
lamellas ijarallel with the external surface.

1022

YEKTEBKATED ANIMALS

microscopic examination of even a minute fragment of it. The
following are the average dimensions of the lacunae, in characteristic
examples drawn from four principal divisions, expressed in fractions
of an inch : —

Long Diameter

Short Diameter

Man .

1-1440 to 1-2400

1-4000 to 1-8000

Ostrich

1-1333 „ 1-2250

1-5425 „ 1-9650

Turtle .

1-375 „ 1-1150

1-4500 „ 1-5840

Conger-eel .

1-550 „ 1-1135

1-4500 „ 1-8000

^-■-

The lacunse of birds are thus distinguished from those of mam-
mals by their somewhat greater length and smaller breadth, but

they differ still more in the
remarkable tortuosity of theii-
canaliculi, which wind back-
wards and forwards in a very
irregular manner. There is an
extraordinary increase in length
in the lacunae of reptiles, with-
out a corresponding increase in
breadth ; and this is also seen
in some fishes, though in ge-
neral the lacunae of the latter
are remarkable for their angularity of form and the fewness of their
radiations, as shown in fig. 753, which represents the lacunae and
canaliculi in the bony scale of the Lepidosteus (' bony pike ' of the
North American lakes and rivers), with which the bones of its in-
ternal skeleton perfectly agree in structure. The dimensions of the
lacun'a3 in any bone do not bear any relation to the size of the animal

Fig. 752. — Lacunge of osseous substance
a, central cavity ; h, its ramifications.

Fig. 753. — Section of the bony scale of Lepidosteus : a, show-
ing the regular distribution of the lacunse and of the connecting
canaliculi ; &, small portion more highly magnified.

to which it belonged ; thus there is little or no perceptible difference
between their size in the enormous extinct Iguanodon and in the
smallest lizard now inhabiting the earth. But they bear a close rela-
tion to the size of the blood-corpuscles in the several classes ; and
this relation is particularly obvious in the ' perennibranchiate '
Batrachia, the extraordinarily large size of whose blood-corpuscles
will be presently noticed.

TEETH

Long Diameter

Short Diameter

Proteus .

1-570 to 1-980

1-885 to 1-1200

Siren

1-290 „ 1-480

1-540 „ 1-975

Menopoma

1-450 „ 1-700

1-1300 „ 1-2100

Lepidosireii

1-375 „ 1-494

1-980 „ 1-2200

Pterodactyle

1-445 „ 1-1185

1-4000 „ 1-5225

I02

In preparing sections of bone it is important to avoid the pene-
tration of the Canada balsam into the interior of the lacunae and
canaliculi, since when these are filled by it they become almost
invisible. Hence it is preferable not to employ this cement at all,
except it may be in the first instance, but to rub down the section
beneath the finger, guarding its surface with a slice of cork or a slip
of gutta-percha, and to give it such a polish that it may be seen to
advantage even when mounted dry. As the polishing, however,
occupies much time, the benefit which is derived from covering the
surfaces of the specimen with Canada balsam may be obtained
without the injury resulting from the penetration of the balsam into
its interior, by adopting the following method. A quantity of
balsam proportioned to the size of the specimen is to be spread upon
a glass slip, and to be rendered stifFer by boiling, until it becomes
nearly solid when cold ; the same is to be done to the thin glass
cover ; next, the specimen being jolaced on the balsamed surface of
the slide, and being overlain by the balsamed cover, siich a degree of
warmth is to be applied as will suffice to liquefy the balsam without
causing it to flow freely, and the glass cover is then to be quickly
pressed down, and the slide to be rapidly cooled, so as to give as
little time as possible for the penetration of the liquefied balsam into
the lacunar system. The same method may be employed in making
sections of teeth. ^ The study of the ossein or organic basis of bone
should be pursued by macerating a fresh bone in dilute nitro-hydro-
chloric acid, then steeping it for some time in pure water, and
tearing thin shreds from the residual substance, which will be
found to consist of an imperfectly fibrillated material , allied in its
essential constitution to the ' white fibrous ' tissue.

Teeth. — The intimate structure of the teeth in the several classes
and orders of Vertebrata presents diflferences which are no less
remarkable than those of their external form, arrangement, and suc-
cession. It will obviously be impossible here to do more than sketch
some of the most important of these varieties. The principal part of
the substance of all teeth is made up of a solid tissue that has been
appropriately termed dentine. In sharks as in many other fishes
the general sti-ucture of this dentine is extremely similar to
that of bone, the tooth being traversed by numerous canals, which
are continuous with the Haversian canals of the subjacent bone, and
receive blood-vessels from them (fig. 754), while each of these canals

^ See Professor J. Quekett's memoir on this subject in the Trans. Microsc. Sac.
ser. i. vol. ii. ; and his more ample illustration of it in the Illustrated Catalogue of
the Histological Collection in the Museum of the Boyal College of Surgeons,
vol. ii.

^ Some useful hints on the mode of making these preparations will be found in
the Quart. Journ Microsc. Sci. vol. vii. 1859, p. 258.

I024

VEETEBEATED ANIMALS

is surrounded by a system of tubuli (fig. 755), which radiate into
the surroimding solid substance. These tubuli, however, do not enter
lacunae, nor is there any concentric annular arrangement around the
medullary canals ; but each system of tubuli is contintied onwards,
through its own division of the tooth, the individual tubes sometimes
giving off lateral branches, whilst in other instances their trunks
bifurcate. This arrangement is peculiarly well displayed, when
sections of teeth constructed- upon this type are viewed as opaque
objects (fig. 756). In the teeth of the higher Yertebrata, however,
we usually find the centre excavated into a single cavity (fig. 757),
and the remainder destitute of vascular canals ; but there are inter-
mediate cases (as in the teeth of the great fossil sloths) in which the
inner portion of the dentine is traversed by prolongations of this
cavity, conveying blood-vessels, which do not pass into the exterior

Fig. 754. — Perpendicular section of
tooth of Lanina, moderately en-
larged, showing network of me-
dullary canals.

Fig. 755. — Transverse section of por-
tion of tooth oiPristis, more highly
magnified, showing orifices of me-
dullary canals, with systems of
radiating and inosculating tubuli.

layers. The tubuli of the 'non-vascular' dentine, which exists by
itself in the teeth of nearly all mammalia, and which in the elephant
is known as ' ivory,' all radiate from the central cavity, and pass
towards the surface of the tooth in a nearly parallel course. Their
diamieter at their largest part averages ^ jj-g-g-Qth of an inch ; their
smallest branches are immeasurably fine. The tubuli in their course
present greater and lesser undulations ; the former are few in number,
but the latter are numerous ; and as they occur at the same part of
the course of several contiguous tubes they give rise to the appeai-ance
of lines concentric with the centre of radiation. These 'secondary
curvatures ' probably indicate in dentine, as in the crab's shell, suc-
cessive stages of calcification. The tubuli are occupied, during the
life of the tooth, by delicate threads of protoplasmic substance, ex-
tending into them from the central pulp.

Two other substances, one of them harder and the other softei'

TEETH

1025

than dentine, are frequently found associated with it ; the former is
termed enamel, and the latter cementum or crusta pefrosa. The enamel
is composed of long prisms, closely resembling those of the ' prismatic '
shell-substance formerly described, but on a far more minute scale, the
diameter of the prisms not being more in man than ^jJjj^th of an
inch. The length of the prisms corresponds with the thickness of
the layer of enamel ; and the
two surfaces of this layer pre-
sent the ends of the
the form of which usually
proaches the hexagonal,
course of the enamel
more or less wavy,

are marked by numerous trtins- p.-^. v !i>^s"^/" '^ ' ^/'J// \
verse stripe, resembling
of the prismatic shell-
stance, and probably ori
ting in the same cause — the
coalescence of a series of shorter

nif^m'^m^

Fig. 756.— Transverse section of tooth of
Myliohates (eagle ray), viewed as an
opaque object.

prisms to form the lengthened
prism. In man and in car-
nivorous animals the enamel
covers the crown of the tooth
only, with a simple cap or
superficial layer of tolerably
uniform thickness (fig. 757, «),
which follows the surface of
the dentine in all its inequali-
ties ; and its component prisms
are directed at right angles to
that surface, their inner ex-
tremities resting in slight but
regular depressions on the ex-
terior of the dentine. In the
teeth of many herbivorous
animals, however, the enamel
forms (with the cementum) a
series of vertical plates which
dip down into the substance
of the dentine, and present
their edges alternately with it
at the grinding surface of the
tooth ; and there is in such
teeth no continuous layer of
enamel over the crown. This
arrangement provides by the unequal wear of these three sub-
stances (of which the enamel is the hardest, and the cementum the
softest) for the constant maintenance of a rough surface, adapted to
triturate the tough vegetable substances on which these animals feed.
Though the enamel is not always present, it has been shown by Mr.
Charles Tomes that the germ from which it is formed always appears-

3u

Fig. 757.— Vertical section of human molar
tooth: ft, enamel; h, cementum or crusta
X3etrosa; c, dentine or ivory; d, osseous
excrescence arising from hypertrophy of
cementum ; e, pulp-cavity ; /, osseous lacunae
at outer part of dentine.

I026

VEETEBEATED ANIMALS

in tlie embi'yonic tooth ; and he has furthei- shown that it is much
more frequently present than used to be supposed. The cementum,
or crufita petrosa, has the characters of true bone, possessing its dis-
tinctive stellate lacunae and radiating canaliculi. Where it exists
in small amount we do not find it traversed by medullary canals ;
but, like dentine, it is occasionally furnished with them, and thus
I'Bsembles bone in every particular. These medullary canals enter
its substance from the exterior of the tooth, and consequently pass
towards those which radiate from the central cavity in the direction
of the surface of the dentine, where this possesses a similar vascu-
larity, as was remarkably the case in the teeth of the great extinct
Megatherium. In the human tooth, however, the cementum has no
such vascularity, but forms a thin layer (fig. 757, 6), which envelopes
the root of the tooth commencing near the termination of the cap
of enamel. In the teeth of many herbivorous mammals it dips
down with the enamel to form the vertical plates of the interior of
the tooth ; and in the teeth of the Edentata, as well as of many
reptiles and fishes, it forms a thick continuous envelope over the
whole surfece, until worn away at the crowTi.^

Dermal Skeleton. — The skin of fishes, of a few amphibians, of
most reptiles, and of few mammals, is strengthened by plates of a
horny, cartilaginous, bony, or even enamel-like textu.re, which are
sometimes fitted together at their edges, so as to form a continuous
box-like envelope ; whilst more commonly they are so arranged as
jiartially to overlie one another, like the tiles on a roof; and it is
in this latter case that they are' usually known as scales. Although
we are accustomed to associate in our minds the ' scales ' of fishes
with those of reptiles, yet essentially difierent structures have been

included under this name,
those of the former and of
many of the latter being-
developed in the substance
of the true skin (with a
layer of which, in addition
to the epidermis, they are
always covered), and bear-
ing a resemblance to car-
tilage and bone in their
texture and composition ;
whilst others, such as the
scales of snakes or the tor-
toise-shell, are formed upon the surface of the true skin, and are
to be considered as analogous to nails, hoofs, &c. and other 'epi-
dermic appendages.' In neai-ly all the existing fishes the scales are
flexible, being but little consolidated by calcareous deposit ; and in
some species they are so' thin and transparent that, as they do not
project obliquely from the. surface of the skin, they can only be
detected by raising the superficial layer of the skin and searching

Fig. 758. — Portion of skin of sole, viewed as an
opaque object.

1 The student is recommended to consult Mr. C. S. Tomes's Manual of Dental
Anatomy, Human and Comjiarative.

SCALES OF FISHES

102:

beneath it, or by tearing off the entire thickness of the skin and
looking for them near its under surface. This is the case, foi-
example, with the common eel, and with the viviparous hlenny ; of
eithei' of which fish the skin is a very interesting object when dried
and mounted in Canada balsam, the scales being seen imbedded in
its substance, whilst its outer surface is studded with pigment-cells.
Generally speaking, howevei', the posterior extremity of each scale
projects oblicpiely from the genei-al surface, carrying before it the
thin membrane that incloses it, which is studded with pigment-
ceils ; and a portion of the skin of almost any fish, but especially of
such as have scales of the ctenoid kind (that is, furnished at theii-
posterior extremities with comb-like teeth, fig. 759), when dried
with its scales in situ, is a very beautiful opaque object for the low
powers of the microscope (fig. 758), especially with the binoculai-
arrangement. Care must be taken, howevei', that the light is made
to glance upon it in the most advan-
tageous manner, since the brilliance with
which it is reflected from the comb-like
projections entirely depends upon the
angle at which it falls upon them. The
only appearance of structure exhibited by
the thin flat scale of the eel, when ex-
amined microscopically, is the presence of
a layer of isolated spheroidal transparent
bodies, imbedded in a plate of like trans-
parence ; these, from the researches of
Professor W. C. Williamson ^ upon other
scales, appeal- not to be cells (as they
might readily be supposed to be), but con-
cretions of carbonate of lime. When the
scale of the eel is examined by polarised
light its surface exhibits a beautiful St.
Andrew's cross ; and if a plate of selenite
is placed behind it, and the analysing
prism be made to revolve, a remarkable
play of colours is presented.

In studying the structure of the more highly developed scales,
we may take as an illustration that of the cmy, in which two very
distinct layers can be made out by a vertical section, with a third
but incomplete layer interposed between them. The outer layer is
composed of several concentric lamina? of a structureless trans-
parent substance like that of cartilage ; the outermost of these
laminae is the smallest, and the size of the plates increases pro-
gressively from without inwards, so that their mai'gins appear- on the
surface as a series of concentric lines ; and their surfaces are thrown
into ridges and furrows, which commonly have a radiating direction.
The inner layer is composed of numei'ous laminte of a fibrous

Fig. 759. — Scale of sole, viewed
as a transparent object.

1 See his elaborate memoirs, ' On the Microscopic Structure of the Scales and
Dermal Teeth of some Ganoid and Placoid Fish,' in Fliil. To-ans. 1849 ; and ' Investi-
srations into the Structure and Development of the Scales and Bones of Fishes,' in
Phil. Trans. 1851.

3u2

I028 VEETEBKATED ANIMALS

structure, the fibres of each lamina Vjeing inclined at various angles
to those of the lamina above and below it. Between these two layers
is interposed a stratum of calcareous concretions, resembling those
of the scale of the eel ; these are sometimes globular or spheroidal,
but more commonly ' lenticular,' that is, having the form of a double
convex lens. The scales which resemble those of the carp in having
a form more or less circular, and in being destitute of comb-like
prolongations, are called cycloid ; and such are the characters of
those of the salmon, herring, roach, &c. The structure of the ctenoid
scales (fig. 759), which we find in the sole, perch, pike, &c., does not
difier essentially from that of the cycloid, save as to the projection
of the comb-like teeth from the posterior margin ; and it does not
appear that the strongly marked division which Professor Agassiz
has attempted to establish between the ' cycloid ' and the ' ctenoid '
orders of fishes, on the basis of this difierence, is in harmony
with their general organisation. Scales of every kind may become
consolidated to a considerable extent by the calcification of their
soft substance ; but they never present any approach to the true
bony structure, such as is shown in the two orders to be next ad-
verted to.

In the ganoid scales, on the other hand, the whole substance of
the scale is composed of a material which is essentially bony in its
nature, its intimate structure being always comparable to that of one
or other of the varieties which present themselves in the bones of the
vertebrate skeleton, and being very frequently identical with that
of the bones of the same fish, as is the case with the Lepidosteus (fig.
753), one of the few existing representatives of this order, which, in
former ages of the earth's history, comprehended a large number of
important families. Their name (from yaroc, splendour) is bestowed
on account of the smoothness, hardness, and high polish of the outer
surface of the scales, which are due to the presence of a peculiar layer
that has been likened to the enamel of teeth. The scales of this
order are for the most part angular in their form, and are arranged
in regvilar rows, the posterior edges of each slightly overlapping the
anterior ones of the next, so as to form a very complete defensive
armour to the body. The scales of the jdacoid type, which charac-
terise the existing sharks and rays, with their fossil allies, are
irregular in their shape, and very commonly do not come into mutual
contact, but are separately imbedded in the skin, projecting from its
surface utider variovis forms. In the rays each scale usually consists
of a flattened plate of a rounded shape, with a hard spine projecting
from its centre ; in the sharks (to which tribe belongs the ' dog-fish '
of our own coast) the scales have more of the shape of teeth. This
resemblance is not confined to external form ; for their intimate
structure strongly resembles that of dentine, their dense substance
being traversed by tubuli, which extend from their centre to their
circumference in minute ramifications, without any trace of osseous
lacun?e. These tooth-like scales are often so small as to be invisible
to the naked eye ; but they are well seen by drying a piece of the skin
to which they are attached, and mounting it in Canada balsam ; and
they are most brilliantly shown by the assistance of polarised light.

HAIR 1029

A like structure is found to exist in the ' spiny rays ' of the dorsal fin,
which, also, are parts of the dermal skeleton ; and these rays usually
have a central cavity filled with medulla, from which the tubuli
radiate towards the circumference. This structure is very well seen
in thin sections of the fossil ' spiny rays,' which, with the teeth and
scales, are often the sole relics of the vast multitudes of shai-ks that
must have swarmed in the ancient seas, their cartilaginous internal
skeletons having entirely decayed away. In making sections of bony
scales, spiny rays, &c. the method must be followed which has been
already detailed under the head of bone.^

The scales of reptiles, the feathers of birds, and the hairs, hoofs,
nails, claws, and horns (when not bony) of mammals are all e^i-
dermic appendages ; that is, they are produced upon the surface, not
within the substance of the true skin, and are allied in structure to
the epidermis, being essentially composed of aggregations of cells
filled with horny matter and frequently much altered in form. This
structure may generally be made out in horns, nails, &c. with little
difiiculty by treating thin sections of them with a dilute solution of
soda, which after a short time causes the cells that had been
flattened into scales to resume their globiilar form. The most
interesting modifications of this structure are presented to us in
hairs and in feathers ; which forms of clothing are very similar to
each other in their essential nature, and are developed in the same
mannei" — viz. by an increased production of epidermic cells at the
bottom of a flask-shaped follicle, which is formed in the substance
of the true skin, and which is supplied with abundance of blood
by a special distribution of vessels to its walls. When a hair is
pulled out ' by its root,' its base exhibits a bulbous enlargement,
of which the exterior is tolerably firm, whilst its interior is occu-
pied by a softer substance, which is known as the ' pulp ; ' and it
is to the continual augmentation of this pulp in the deeper part
of the follicle, and to its conversion into the peculiar substance of
the hair when it has been pushed upwards to its narrow neck, that
the growth of the hair is due. The same is true of feathers, the stems
of which are but hairs on a larger scale ; for the ' quill ' is the part
contained within the follicle answering to the ' bulb ' of the hair ;
and whilst the outer part of this is converted into the peculiarly solid
horny substance forming the ' barrel ' of the quill, its interior is
occupied, during the whole peiiod of the growth of the feather, with
the soft pulp, only the shrivelled remains of which, however, are
found wdthin it after the quill has ceased to groAV.

Although the hairs of difierent mammals difier greatly in the
appearances they present, we may generally distinguish in them
two elementary parts — viz. a cortical or investing substance, of a
dense horny texture, and a medullary or pith -like substance, usually
of a much softer texture, occupying the interior. The former can

1 For further information regarding the scales of fishes, see the papers by O
Hertwig in vol. viii. of the Jenaische Zeitschrift, and vols. ii. and v. of the
Morpholog. Jahrbuch. A condensed summary of our knowledge, from the more
recent standpoint, will be found in Dean's Fishes, Living and Fossil (New York,
1895), pp. 23-6.

I030

VERTEBRATED ANIMALS

sometimes be distinctly made out to consist of flattened scales
arranged in an imbricated manner, as in some of the hairs of the
sable (fig. 760) ; whilst in the same hairs, the medullary substance
is composed of large spheroidal cells. In the musk-deer, on the
othei- hand, the cortical substance is nearly undistinguishable, and

Fig. 760. — Hair of sable, showing large
rounded cells in its interior, covered
by imbricated scales or flattened cells.

Fig. 761. — Hair of musk-deer, consist-
ing almost entirely of polygonal cells.

almost the entire hair seems made up of thin -walled polygonal cells
(fig. 761). The hair of the reindeer, though much larger, has a very
similar structure ; and its cells, except near the root, are occupied
with hair alone, so as to seem black by transmitted light, except
when penetrated by the fluid in which they are mounted. In the
hair of the mouse, squirrel, and other small rodents (fig. 762, A, B),

the cortical substance forms a
tube, which we see crossed at
intervals by partitions that
are sometimes complete,
sometimes only partial ;
these are the walls of the single
or double line of cells, of which
the medvxllary substance is made
vip. The hairs of the bat tribe
are commonly distinguished by
the projections on their surface,
f I^Si W/^H^ which are foi-med by extensions

" '^^' ..MM/// Qf ^\^Q component scales of the

cortical substance : these ai-e
particularly well seen in the
hairs of one of the Indian
species, which has a set of
whorls of long narrow leaflets
(so to speak) arranged at
regular intervals on its stem (C). In the hair of the peccary (fig. 763)
the cortical envelope sends inwards a set of radial prolongations, the
interspaces of which are occupied by the polygonal cells of the medul-
lary substance ; and this, on a larger scale, is the structiare of the
' quills ' of the porcupine, the radiating partitions of which, when seen
through the more transparent parts of the coi-tical sheath, give to

I

Fig. 762. — A, small hair of squirrel ; B, large
hair of squirrel ; C, hair of Indian bat.

HAIR

lOSI

tlie surface of the latter a fluted appeai-ance. The hair of the ornitho-
rhynchus is a very curious object ; for whilst the lower pai't of it
resembles the fine hair of the mouse or squirrel, this thins away and
then dilates again into a very thick fibi-e, having a central portion
composed of polygonal cells, inclosed in
a flattened sheath of a l3rown fibrous
substance.

The structure of the human hair is
in certain respects peculiar. When its
oviter surface is examined, it is seen to
be traversed by irregular lines (fig. 764,
A), which are most strongly marked in
foetal hairs ; and these are the indications
of the imbricated arrangement of the

flattened cells or scales which form the cuticular layer. This
layer, as is shown by transverse sections (0, D), is a very thin
and transparent cylinder ; and it incloses the peculiar fibrous sub-
stance that constitutes the principal part of the shaft of the haii-.
The constituent fibres of the substance, which are marked out by
the delicate strife that may be traced in longitudinal sections of the
hair (B), may be separated from each other by crushing the hair,
especially after it has been macerated for some time in sulphuric
acid ; and each of them, when completely isolated from its fellows,
is found to be a long spindle-shaped cell. In the axis of this fibrous
cylinder there is very commonly a band which is formed of spheroidal

Fig. 7Go. — Tiausverse section
of hair of peccary.

B

ii'il'S'.,^,.,,. ,^

yill''l'lit^-^■''•^'''-^-•*istf^

Fig. 764. — Structure of liuman hair : A, external surface of the shaft, show-
ing the transverse strite and jagged boundary caused by the imbrications of
the cuticular layer ; B, longitudinal section of the shaft, showing the
fibrous character of the cortical substance, and the arrangement of the
pigmentary matter ; C, transverse section, showing the distinction be-
tween the cuticular envelope, the cylinder of cortical substance, and the
medullary centre ; D, another transverse section, showing deficiency of
the central cellular substance.

cells ; but this ' meduUaiy ' substance is usually deficient in the fine
hairs scattered over the general surface of the body, and is not
always present in those of the head. The hue of the hair is due
partly to the presence of pigmentary granules, either collected into
patches or difliiised through its substance, but partly also to the
existence of a multitude of minute air-spaces, which cause it to

I032 VERTEBRATED ANIMALS

appear dark by transmitted and white by reflected light. The cells
of the medullary axis in particular are very commonly found to
contain air, giving it the black appearance shown at C. The
difference between the blackness of pigment and that of air-spaces
may be readily determined by attending to the characters of the
latter as already laid down, and by watching the effects of the
penetration of oil of turpentine or other liquids, which do not alter
the appearance of pigment spots, but obliterate all the markings
produced by air-spaces, these returning again as the hair dries. In
mounting hairs as microscopic preparations they should in the first
instance be cleansed of all their fatty matter by maceration in
ether, and they may then be put up either in weak spirit or in
Canada balsam, as may be thought preferable, the former menstruum
being well adapted to display the characters of the finer and more
transparent hairs, while the latter allow the light to penetrate more
readily through the coarser and more opaque. Transverse sections
of hairs are best made by glueing or gumming sevei-al together and
then putting them into the microtome ;* those of human hair may
be easily obtained, however, by shaving a second time, very closely,
a part of the surface over which the razor has already passed more
lightly, and by picking out from the lather, and carefully washing,
the sections thus taken off.^

The stems oi feathers exhibit the same kind of structure as hairs,
their cortical portion being the horny sheath that envelopes the
shaft, and their medullary portion being the jDith-like substance
which that sheath includes. In small feathers this may usually be
made veiy plain by mounting them in Canada balsam ; in large
feathers, however, the texture is sometimes so altered by the drying
up of the pith (the cells of which are always found to be occupied
by air alone) that the cellular structure cannot be demonstrated
save by boiling thin slices in a dilute solution of potass, and not
always even then. In small feathers, especially such as have a
downy character, the cellular structure is very distinctly seen in the
lateral harhs, which are sometimes found to be composed of single
files of j^ear-shaped cells, laid end to end ; but in larger feathers
it is usuall}' necessary to increase the transparence of the barbs,
esjoecially when these are thick and but little pei-vious to light,
either by soaking them in turpentine, mounting them in Canada
balsam, or boiling them in a weak solution of potass. In feathers
which are destined to strike the air with great force in the act of
flight, we find each barb fringed on either side with slender flattened
filaments or ' barbules ; ' the barbules of the distal side of each barb
are furnished on their attached half with curved hooks, whilst those
of the jDroximal side have thick turned-up edges in their median
portion ; as the two sets of barbules that spring from two adjacent
barbs cross each other at an angle, and as each hooked barbule of
one locks into the thickened edge of several barbules of the other,
the barbs are connected very firmly, in a mode very similar to that

1 On the minute structure of hair, consult Grimm's Atlas der menscliUclien %md
tierischen Haare (Lahr, 1884, 4to, with a preface by W. Waldeyer).

flOENS, HOOFvS, CLAAVS IO33

in which the antei-ior and posterior wings of certain hymenopterous
insects are locked together. Feathers or portions of feathers of
biixls distingnished by the splendour of their plumage ai'e very good
objects for low magnifying powers when illuminated on an opaque
ground ; but care must be taken that the light falls upon them at
the angle necessary to produce their most brilliant reflection into
the axis of the microscope ; since feathers which exhibit the most
splendid metallic lustre to an observer at one point may seem very
dull to the eye of another in a different position. The small feathers
of humming-birds, portions of the feathers of the peacock, and
others of a like kind, are well worthy of examination ; and the
scientific microscopist, who is but little attracted by mere gorgeous-
ness, may well apply himself to the discovery of the peculiar
structure which imparts to these objects their most remarkable
character.^

Sections of horns, hoofs, claws, and other like modifications of
epidermic structure — which can be easily made by the microtome,
the substance to be cut having been softened, if necessary, by soaking
in warm water — do not in general afford any very interesting
features when viewed in the ordinary mode ; but there are no objects
on which polarised light produces more reinai-kable effects, or which
display a more beautiful variety of colours when a plate of selenite is
placed behind them and the analysing prism is made to rotate. A
curious modification of the

ordinary structure of horn is ,aA r i^^^' /

presented in the appendage ^..s, . m^,.^/ 11 /-, /

borne by the rlimoceros upon '^,-- - - ^ '

its snout, which in many T
points resembles a bundle of
hairs, its substance being .• J
arranged in minute cylinders ^^& V

around a number of separate ~'^^ 4,

centres, which have probably _^ S

been formed by independ- \
ent papillae (fig. 765). When ^-

transverse sections of these ^^^ ^ ;,,^.

cylinders are viewed by polar- ' ^ f .I'

ised light, each of them is ^' ^ 1

seen to be marked by a cross. Fig. 765. — Transverse section of horn of

somewhat resembling that of rhinoceros viewed by polarised Ught.

starch-grains ; and the light

and shadow of this ci-oss are replaced by contrasted colours when
the selenite plate is interposed. The substance commonly but erro-
neously termed whalebone, which is formed from the surface of the
membrane that lines the mouth of the whale, and has no relation
to its true bony skeleton, is almost identical in structure with
rhinoceros-horn, and is similarly affected by polarised light. The
central portion of each of its component threads, like the medullary

1 See E. S. Wray, ' On the Structure of the Barbs, Barbules, and Barbicels of a
typical Pennaceous Feather,' in the Ihis for 1887, j). 420.

I034

VERTEBRATED ANIMALS

substance of hairs, contains cells that have been so little altered as
to be easily recognised ; and the outer or cortical portion also may-
be shown to have a like structure by macerating it in a solution of
potass and then in water. Sections of any of the horny tissues are
best mounted in Canada balsam.

Slood. — Carrying our microscopic survey, now, to the elementary
parts of which those softer tissues are made up that are subservient
to the active life of the body rather than to its merely mechanical
requirements, we shall in the first place notice the isolated floating

cells contained in the blood,

,^^>. which are known as blood-cor-

T^^/N pviscles. These are of two

~~~-"-'' kinds: the 'red' and the

' white ' or ' colourless.' The

red present, in every instance,

ip the form of a flattened disc,

which is circular in man and

^^^^^ most mammalia (fig. 767), but

6 is oval in birds, reptiles (fig.

Pig. 766.— Red corpuscles of frog's blood: 766), and fishes, as also in a

aa, their flattened face ; &, particle turned f^^ mammals (all belonging to

nearly edgeways; c, colourless corpuscle; , i +,,•!,, \ t J?

d, red corpuscles altered by diluted acetic the camel tribe). In the one

acid.

J

ibe).
form as in the other, these
corpuscles seem to be flattened
cells, the walls of which, how-
ever, are not distinctly dif-
ferentiated from the ground
substance they contain, as
appears from the changes of
form which they spontaneously
undergo when kept by means
of a ' warm stage ' at a tem-
perature of about 100° F., and
Fig. 767.-Red corpuscles of human blood, f^^^^ ^^iQ effects of pressure in
represented at a, as they are seen when ■,■,.,■, mi i

rather within the focus of the microscope ; breakuig them up. ihe red
and at 6, as they appear when precisely in corpuscles in the blood of
the focus. oviparous Vertebrata are dis-

tinguished by the presence of a
centi-al spot or nucleus ; this is most distinctly brought into view
by treating the blood-discs with acetic acid, which causes the nuclevis
to shrink and become more opaque, whilst rendering the remaining
portion extremely transparent (fig. 766, d). By examining un-
altered red corpuscles of thQ frog or newt under a sufiiciently high
magnifying power the nucleus is seen to be traversed by a network
of filaments, which extends from it throughout the ground sub-
stance of the corpuscle, constituting an intx-acellular reticulation.
The red corpuscles of the blood of mammals, however, possess no
distinguishable nucleus, the dark spot which is seen in their centre
(fig. 767, h) being merely an effect of refraction, consequent upon
the double concave form of the disc. When these corpuscles are
treated with water, so that their form becomes first flat and then

BLOOD-CORPUSCLES

1035

double convex, the dark spot disappeaivs ; whilst, on the other hand,
it is made more evident when the concavity is increased by the
partial shrinkage of the corpuscles, which may be brought about
by treating them with fluids of greater density than their own sub-
stance. When floating in a suiiiciently thick stratum of blood
drawn fi'om the body, and placed under a cover-glass, the red
corpuscles show a marked tendency to approach one another, adher-
ing by their discoidal surfaces so as to present the aspect of a pile
of coins ; or, if the stratum be too thin to admit of this, partially
overlapping, or simply adhering by their edges, which then become
polygonal instead of circular-. The size of the red coi-piiscles is not
altogether uniform in the same blood ; thus it varies in that of man
from about the ^.yyth to the ^^i^th of an inch. But we generally find
that there is an avei'age size, which is pi-etty constantly maintained
among the diflferent individuals of the same species ; that of man may
be stated at about 3 oVoth of an inch. The following table ^ exhibits

MAMMALS

Man .

. 1-3200

Camel .

1-3254, 1-5921

Dog .

. 1-3542

Llama .

1-3361, 1-6294

Whale

. 1-3099

Javan chevrotain . 1-12325

Elephant .

. 1-2745

Caucasian goal

1-7045

Mouse

. 1-3814
BIB

Two-toed sloth
lDS

. 1-2865

Golden eagle

. 1-1812, 1-3882

Ostrich .

1-1649, 1-3000

Owl

. 1-1830, 1-3400

Cassowary

1-1455, 1-2800

Crow

. 1-1961, 1-4000

Heron

1-1913, 1-3491

Blue-tit .

. 1-2313, 1-4128

Fowl

1-2102 1-3466

Parrot

. 1-1898, 1-4000

Gull

. 1-2097,1-4000

REPTILES AN]

D BATRACHIA

Turtle .

. 1-1231, 1-1882

Frog

1-1108, 1-1821

Crocodile

. 1-1231, 1-2286

Water-newt

1-8014, 1-1246

Green lizard

. 1-1555, 1-2743

Siren

1-420, 1-760

Slow-worm

. 1-1178, 1-2666

Proteus .

1-400, 1-727

Viper

. 1-1274, 1-1800
FIS

Amphiuma
HES

1-345, 1-561

Perch .

. 1-2099, 1-2824

Pike

1-2000, 1-3555

Carp

. 1-2142, 1-3429

Eel.

1-1745, 1-2842

Gold-fish

. 1-1777, 1-2824

Gymnotus

1-1745, 1-2599

the average dimensions of some of the most interesting examples of
the red corpuscles in the four classes of vertebrated animals, expressed
in fractions of an inch. Where two measurements are given they
are the long and the short diameters of the same corpuscles. (See also
fig. 768.) Thus it appears that the smallest red corpuscles known
are those of the Javan chevrotain {Tragtdus javanicus), whilst the
largest are those of that curious group of Batrachia (frog tribe) which

1 These measurements are chiefly selected from those given by Mr. C4ulliver in
his edition of Hewson's Works, p. 236 et seq.

1036

VERTEBEATED ANIMALS

.O

retain the gills through the whole of life ; one of the oval blood-discs
of the Po'oteus, being more than thirty times as long and seventeen
times as broad as those of the musk-deer, would cover no
fewer than 510 of them. Those of the Amphiuma are still larger. i
According to the estimate of Vierordt, a cubic inch of human
blood contains upwards of eighty millions of red corpuscles and
nearly a quarter of a million of the colourless.

The tvhite or ' colourless ' corpuscles are more readily distinguished

in the blood of batrachians
than in that of man,
being in the former case
of much smaller size, as
well as having a circular
outline (fig. 766, c) ; whilst
in the latter their size and
contour are so nearly the
same that, as the red cor-
puscles themselves, when
seen in a single layer, have
but a very pale hue, the
deficiency of colour does
not sensibly mark ■ their
diiference of nature. The
proportion of white to red
corpuscles being scarcely
even greater (in a healthy
man) than 1 to 250, and
often as low as from one
half to one quarter of that
ratio, there are seldom
many of them to be seen
in the field at once ; and
these may be recognised
rather by their isolation
than their colour, espe-

FiG. 768. — Comparative sizes of red blood cor- cially if the glass COVer be
puscles: 1, man; 2, elephant; 3 musk-deer; j^^Q^ed a little on the slide,
4, dromedary ; o, ostrich ; b, pigeon ; 7, nummnig- j-i i

bird ; 8, crocodile ; 9, python ; 10, proteus ; 11, SO as to cause tlie red COr-
perch ; 12, pike ; 13, shark. puscles to become aggrega-

ted into rows and irregular
masses. It is remarkable that, notwithstanding the great variations
in the sizes of the red corpuscles in difierent species of vertebrated ani-
mals, the size of the white is extremely constant throughout, their dia-
meter being seldom much greater or less than ^ „\, (^th of an inch in the
warm-blooded classes and T.gVo'tli i^i reptiles. Their ordinary form
is globular, but their aspect is subject to considerable variations,
which seem to depend in great part upon their phase of development.

1 A very interesting account of the ' Structure of the Red Corpuscles of the
A')rvphiuma triclactylum ' has been given by Dr. H. D. Schmidt, of New Orleans, in
the Journ. Boy. Microsc. Soc. vol. i. 1879, pp. 57, 97.

ELOOD-CORPUSCLES

103;

Thus, in their eai-ly state, in ^\'hich they seem to be identical with
the corpuscles found floating in chyle and lymph, they seem to be
nearly homogeneous particles of protoplasmic substance, but in
their more advanced condition, according to Dr. Klein, their sub-
stance consists of a reticulation of very fine contractile proto-
plasmic fibres, termed the ' intracellular network,' in the meshes of
which a hyaline interstitial material is included, and which is con-
tinuoiTS with a similar network that can be discerned in the substance
of the single or double nucleus when this comes into view after the
withdrawal of these corpuscles from the body. In their living state,
however, whilst circulating in the vessels, the white corpuscles,
although clearl}^ distinguishable in the slow-moving stratum in
contact with their walls (the red corpuscles rushing rapidly
through the centre of the tube), do not usually show a distinct
nucleus. This may be readily brought into view by treating
the corpuscles with water, which causes them to swell up,
become granular, and at last
disintegrate, with emission of
grantiles which may have been
previously seen in active mole-
cular movement within the
corpuscle. When the white
corpuscles in a drop of freshly
drawn blood are carefully
watched for a short time, they
may be observed to undergo
changes of form, and even
to move from place to place,
after the manner of Amoebce.
When thus moving they
engulf particles which lie
in their course — such as
granules of vermilion that

have been injected into the blood-vessels of the living animal — and
afterwards eject these in the like fashion.^ Such movements will
continue for some time in the colourless corpuscles of cold-blooded
animals, but still longer if they are kept in a temperature of about
75^. The movement will speedily come to an end, however,
in the white corpuscles of man or other warm-blooded animals,

1 Metschnikoff has made the highly interesting and imiDortant observation that the
immunity of certain animals to certain diseases apjjears to be due to the power that the
white corpuscles jjossess of acting as ' phagocytes,' or eating the germs of the disease.
Metschnikoff found that the virulent rods of the Bacillus of anthrax ' when intro-
duced by inoculation into an animal liable to take the fever, such as a rodent, were
absorbed by the blood-cells only in exceptional instances. They were readily absorbed
by the blood-cells of animals not liable to the disease, as frogs and lizards, when the
temperature was not artificially raised (fig. 770), and then disappeared inside the
cells. . . . From all these data we must assume with Metschnikoff that the Bacillus
is harmless because it is absorbed and destroyed by the blood-cells, and injurious
because this does not happen ; or at least that it becomes harmless if the destruction
by the blood-cells takes i^lace more rapidly, and to a greater extent than the growth
and multiplication of the Bacillus, the converse being also true ' (see A. de Bary,
0;i iJ«c<e7-irt, English edition, p. 136). The importance of phagocytes is becoming
more and more recognised by the pathologist.

Fig. 769. — Altered white corpuscle of blood
an hour after having been drawn from the
finger.

1038 VEKTEBRATED ANIMALS

vmless the slide is kept on a warm stage at the temperature
of about 100° F. A remarkable example of an extreme change of
form in a white corpuscle of human blood is represented in fig. 769.
(Similar changes have been observed also in the corpuscles floating in
the circulating fluid of the higher invertebrata, as the crab, wliich
resemble the ' white ' corpuscles of vertebrated blood, rather than
its ' red ' corpuscles — these last, in fact, being altogether peculiar to
the circulating fluid of vertebrated animals.

In examining the blood microscopically it is, of course, impor-
• tant to obtain as thin a stratum of it as possible, so that the cor-
puscles may not overlie one another.
This is best accomplished by selecting
a piece of thin glass of perfect flat-
ness, and then, having received a
small drop of blood upon a glass
slide, to lay the thin glass cover not
iil^n this, but with its edge just
touching the edge of the drop : for
the blood will then be drawn in by
Pig. 770.-«, blood-cell of a~kog m capillary attraction, so as to spread
the act of engulfing a rod of m a uniformly thin layer between
£aciZ?ws a«^^7T(c»s, observed in the the two glasses. Such thin films
living state in a drop of aqueous 1 .^ypopv-ved in the lirmid ^tafp

humour; 6, the same a few minutes ^^J 06 piesei\ea in Tiie liquid State

later. (After MetschnikofE ; highly by applying a cover glass and ce-
magnified.) menting it with gold-size before

evaporation has taken place ; but it
is preferable first to expose the drop to the vapour of osmic
acid, and then to apply a drop of a weak solution of acetate of
potass ; after which a cover glass may be put on, and secured
with gold-size in the usual way. It is far simpler, however,
to allow such films to dry without any cover, and then merely to
cover them for protection ; and in this condition the general
characters of the corpuscles can be very well made out, notwith-
standing that they have in some degree been shrivelled by the
desiccation they have undergone. This method is particularly ser-
viceable as affording a fair means of comparison, when the assist-
ance of the microscopist is sought in determining, for medico-legal
purposes, the source of suspicious blood-stains, the average dimen-
sions of the dried blood-corpuscle of the several domestic animals
being sufiiciently diffei-ent from each other, and from those of man,
to allow the nature of any specimen to be pronounced upon with a
high degree of probability.^

Simple Fibrous Tissues. — A very beautiful example of a tissue of
this kind is furnished by the membrane of the common fowl's egg ;
which (as may be seen by examining an Q^g whose shell remains
soft for want of consolidation by calcareous particles) consists of two
principal layers, one serving as a basis of the shell itself, and the
other forming that lining to it which is known as the memhrana

' This is a matter which has given rise to much discussion among experts. See
Froc. Amer. Micr. Soc. xiv. (1893), pp. 91-120^

FIBROUS TISSUE

1039

jiutcaninis. The latter may be sepai-ated by careful tearing with
needles and forceps, after prolonged maceration in water, into several
matted lamella? i-esemblmg that I'epresented in fig. 771 ; and similar
lamellae may be readily obtained from the shell itself by dissolving

Fig, 771. — Fibrous membrane
from ea'Sf-sliell.

Fig. 772. — White fibrous tissue
from lieament.

away its lime by dilute acid. The simjjly fibrous structures of the
body generally, however, belong to one of two very definite kinds of
tissue, the ' white ' and the ' yellow,' whose appearance, comjDOsition,
and properties are very difi'erent. The tohite
fibrous tissue, though sometimes apparently
composed of distinct fibres, more commonly
presents the aspect of bands, usually of a flat-
tened form, and attaining the breadth of 75-g-oth
of an inch, which ai'e marked by numerous
longitudinal streaks, but can seldom be torn up
into minute fibres of determinate size. The
fibres and bands are occasionally somewhat
wavy in theii' dii-ection ; and they have a pecu-
liar tendency to fall into undulations, when it is
attempted to teai' them apai't from each other
(fig. 772). This tissue is easily distinguished
from the other by the effect of acetic acid,
which swells it up and renders it transparent,
at the same time biinging into view certain
oval nuclear particles of ' germinal matter,'
which are known as ' connective tissue cor-
puscles.' These are i-elatively much lai-ger, and
their connections more distinct, in the earlier
stages of the foi'ination of this tissue (fig. 773).
It is perfectly inelastic ; and we find it in such
parts as tendons, oi-dinary ligaments, fibrous

capsules, &c. whose funcision it is to resist tension without yielding
to it. It constitutes, also, the organic basis or matrix of bone ; for
although thesubstance wliich is left when a bone has been macerated
sufiiciently long in dilute acid foi- all its minei-al components to be

Fig 773 — Poition of
young tendon, show
ing the corpuscles
of ' germinal matter,'
with their stellate
prolongations, inter-
posed among its fibres.

I040

VEETEBRATED ANIMALS

removed is commonly designated as cartilage, this is shown by
careful microscopic analysis not to be a correct description of it,
since it does not show any of the characteristic structure of car-
tilage, but is capable of being torn into lamellfe, in which, if suf-
ficiently thin, the ordinary structure of a fibrous membrane can be
distinguished. The yelloiv fibrous tissue exists in the form of long,
single, elastic, branching filaments, with a dark decided border ;
which ai-e disposed to curl when not put on the stretch (fig. 774),
and frequently anastomose, so as to form a network. They are for
the most part between g-oVo't'^ ^^^ iwoT^th of an inch in diameter ;
but they are often met with both larger and smaller. This tissue
does not undergo any change when treated with acetic acid. It
exists alone (that is, without any mixture of the white) in parts
which require a peculiar elasticity, such as the middle coat of the
arteries, the ' vocal cords,' the ' ligamentum nuchas ' of quadru23eds,

the elastic ligament which
holds together the valves of
a bivalve shell, and that by
which the claws of the feline
tribe are retracted when
not in use ; and it enters
largely into the composition
of areolar or connective
tissue.

The tissue formerly
known to anatomists as
' cellular,' but now more
properly designated connec-
tive or areolar tissue, con-
sists of a network of minute
fibres and bands which are
interwoven in every direction, so as to leave innumerable areolce or
little spaces that communicate freely with one another. Of these
fibres some are of the ' yellow ' or elastic kind, but the majority are
composed of the ' white ' fibrous tissue ; and, as in that form of ele-
mentary structure, they frequently present the condition of broad
flattened bands or membranous shreds in wdiich no distinct fibrous
arrangement is visible. The proportion of the two forms varies,
according to the amount of elasticity, or of simple resisting power,
which the endowments of the part may require. We find this tissue
in a very large proportion of the bodies of higher animals ; thus it
binds together the ultimate muscular fibres into minute fasciculi,
unites these fasciculi into lai-ger ones, these again into still larger
ones which are obvious to the eye, and these into the entire muscle ;
whilst it also forms the membranous divisions between distinct
muscles. In like manner it unites the elements of nerves, glands,
&c., binds together the fat-cells into minute masses (fig. 780), these
into large ones, and so on ; and in this way penetrates and forms
part of all the softer organs of the body. But whilst the fibrous
structures of which the '■ formed tissue ' is composed have a purely
miechanical function, there is good reason to regard the ' connective

Fig. 774. — Yellow fibrous tissue from liga-
mentum nuchse of calf.

SKIN

IO4I

tissue corpuscles ' which are everywhere dispersed among them, as
having a most important function in the first production and sub-
sequent maintenance of the more definitely organised portions of
the fabric. In these corpuscles distinct movements, analogous -to
those of the sarcodic extensions of rhizopods, have been recognised
in transparent parts, such as the cornea of the eye and the tail of
the young tadpole, by observations made on these parts whilst living.
For the "display of the characters of the fibrous tissues small and
thin threads may be cut with the curved
scissors from any part that afibrds
them ; and these must be torn asunder
with needles under the simple micro-
scope, until the fibres are separated to
a degree suflicient to enable them to
be examined to advantage under a
higher magnifying power. The differ-
ence between the ' white ' and the
' yellow ' components of connective tissue
is at once made apparent by the efiect
of acetic acid ; whilst the ' connec-
tive tissue corpuscles ' are best dis-
tinguished by the staining process,
especially in the early stage of the
formation of these tissues (fig. 773).

Skin ; Mucous and Serous Mem-
branes.— The skin, which forms the ex-
ternal envelope of the body, is divisible Fig. 77£
into two princi]3al layers : the cutis vera
or ' true skin,' which usually makes up
by far the larger j)art of its thickness,
and the ' cuticle,' ' scarfskin,' or epi-
dermis, which covers it. At the mouth,
nostrils, and the other orifices of the
open cavities and canals of the body,
the skin passes into the membrane that
lines these, which is distinguished as
the mucous membrane, from the j^ecu-
liar glairy secretion of mucus by which
its surface is protected. But those great
closed cavities of the body which surround the heart, lungs, intes-
tines, &c. are lined by membranes of a difierent kind ; which, as
they secrete only a thin serous fiuid from their surfaces, are known
as serous membranes. Both mucous and serous membranes consist,
like the skin, of a cellular membranous basis, and of a thin cuticular
layer, which, as it difiers in many points from the epidermis, is dis-
tinguished as the epithelium. The substance of the ' true skin ' and
of the ' mucous ' and ' serous ' membranes is principally composed of
the fibrous tissues last described ; but the skin and the mucous mem-
branes are very copiously supplied with blood-vessels and with glan-
dula3 of various kinds ; and in the skin we also find abundance of
nerves and lymphatic vessels, as well as, in some parts, of hair-

3x

-Vertical section of skiu
of finger : A, epidermis, the
surface of which shows dejpres-
sions a a, between the emi-
nences h b, on which open the
persx^iratory ducts s ; at m is
seen the deeper layer of the
epidermis, or stratum Malpighii.
B, cutis vera, in which are im-
bedded the sweat-glands d,
with their ducts e, and aggre-
gations of fat-cells/; g, arterial
twig suj)plying the vascular
jjapillse 23 ; t, one of the tactile
papillae with its nerve.

1042

VEETEBRATED ANIMALS

follicles. The general appearance ordinarily presented by a thin
vertical section of the skin of a part furnished with numerous sensory
papillce is shown in fig. 775 ; where we see in the deeper layers
of the cutis vera little clumps of fat-cells, /", and the sweat-
glands, d d, whose ducts, e e, pass upwards : whilst on its surface
we distinguish the vascular papillae, p, svipplied with loops of blood-
vessels from the trvmk, g, and a tactile papilla, t, with its nerve
twig. The spaces between the papillae are filled up by the soft
' Malpighian layer,' m, of the epidermis, A, in which its colouring
matter is chiefly contained, whilst this is covered by the horny layer,
h, which is traversed by the spirally twisted continuations of the
perspiratory ducts, opening at s ixpon the surface, which presents
alternating depressions, a, and elevations, h. The distribution of
the blood-vessels in the skin and mucous membranes, whieh is one
of the most interesting features in then- structure, and which is in-
timately connected with their several functions, will come under
our notice hereafter. In serous membranes, on the other hand,
whose function is simply j)rotective, the supply of blood-vessels is
more scanty.

Epidermic and Epithelial Cell-layers. — The epidermis or ' cviticle '
covers the whole exterior of the body as a thin semitransparent
pellicle, which is shown by microscopic examination to consist of
a series of layers of cells that ai'e continvially wearing ofi" at the
external surface, and being renewed at the surface of the true skin ;
so that the newest and deepest layers gradually become the oldest
and most superficial, and are at last thrown ofi' by slow desquamation.
In their progress from the internal to the external surface of the
epidermis the cells undergo a series of well-
marked changes. Wlien we examine the
innermost layer, we find it soft and granu-
lar, consisting of nucleated cells which are
flatter in the upper than the lower strata,
which make up the layer. This was for-
merly considered as a distinct tissue, and
was supposed to be the peculiar seat of the
colour of the skin ; it received the desig-
nation of Malpighian layer or rete inucosum.
The change in form is accompanied by a
change in the chemical composition of the
tissue, which seems to be due to the metamoi--
phosis of the contents of the cells into a.
horny substance identical with that of which
hair, horn, nails, hoofs, &c. are composed. Mingled with the epi-
dermic cells we find others which secrete colouring matter instead
of horn ; these, which are termed ' pigment-cells,' are especially to
be noticed in the epidermis of the negi-o and other dark races, and
ai-e most distinguishable in the Malpighian layer, their colour ap-
pearing to fade as they pass towards the sui-face. The most remai-k-
able development of pigment-cells in the higher animals, however,
is on the inner surface of the choroid coat of the eye, where they
have a very i-egular arr-angement, and form several layei-s, known as

Fig. 776. — Cells from the pig-
mentum nigrum of the eye :
a, pigmentary granules con-
cealing the nucleus ; h, the
nucleus.

EPIDEEMIS

1043

the jngmentum nigrum. When examined sepai'ately these cells ai-e
found to have a polygonal form (fig. 776, a), and to have a distinct
nucleus (b) in their interior. The black colour is due to the accu-
mulation, within each cell, of a number of flat rounded or oval
granules, of extreme minuteness, which exhibit an active movement
when set free from the cell, and even whilst inclosed within it.
The pigment-cells are not always, however, of this simply rounded or
polygonal form ; they sometimes present remarkable stellate pro-
longations, under which form they are well seen in the skin of the
frog (fig. 791, c c). The gradual formation of these prolongations
may be traced in the pigment-cells of the tadpole during its meta-
morphosis (fig. 777). Similar varieties of
form are to be met with in the pigmentary
cells of fishes and small Crustacea, which
also present a great variety of hues ; and
these seem to take the colour of the bottom
over which the animal may live, so as to
serve the better for its concealment.

The structure of the epidei-mis may be
examined in a vaiiety of ways. If it be
removed by maceiution from the true skin,
the cellular nature of its under surface is at
once recognised, when it is subjected to a
magnifying power of 200 or 300 diameters,
by light transmitted through it, with this
surface uppermost ; and if the epidermis be
that of a negro or any other dark-skinned
r^ce, the pigment cells will be very distinctly
seen. This under-surface of the epidei'mis
is not flat but is excavated into pits and

channels for the reception of the papillary Fig. 777. —Pigment -cells
elevations of the true skin ; an ari'angement fi'om tail of tadpole : a a.
which is shown on a large scale in the thick
cuticular covering of the dog's foot, the sub-
jacent papillas being large enough to be dis-
tinctly seen (when injected) with the naked

eye. The cellular nature of the newly forming layei-s is best seen
by examining a little of the soft film that is found upon the surface
of the true skin, after the more consistent layers of the cuticle have
been raised by a blister. The alteration which the cells of the
external layers have undergone tends to obscure their character ;
but if any fragment of epidermis be macerated for a little time in a
weak solution of soda or potass, its dry scales become softened, and
are filled out by imbibition into rounded or polygonal cells. The
same mode of treatment enables us to make out the cellular struc-
ture in warts and corns, which are epidei-mic growths from the
surface of papillse enlarged by hypertrophy.

The epithelium may be designated as a delicate cuticle, covering
all the free internal surfaces of the body, and thus lining all its
cavities, canals, &c. Save in the mouth and other parts in which
it approximates to the ordinary cuticle, both in locality and in

3x2

simple forms of recent

origin ; h b, more complex

forms subsequently as-
sumed.

1044

VEETEBEATED AXIMALS

nature, its cells (fig. 778) usually form but a single layer ; and are
so deficient in tenacity of mutual adhesion that they cannot be de-
tached in the form of a continuous membi-ane. Their shape varies
greatly. Sometimes they are broad, flat, and scale-like, and their
edges appi-oximate closely to each other, so as to form what is
termed a ' pavement ' or ' tessellated ' epithelium : such cells are
observable on the web of a frog's foot or on the tail of a tadpole ;
for, though covering an external surface, the soft moist cuticle of
these pai-ts has all the characters of an epithelium. In other cases
the cells have more of the form of cylindei-s, standing erect side by
side, one extremity of each cylinder forming part of the free surface,
whilst the other rests upon the membrane to which it serves as a
covering. If the cylinders be closely pressed together, their form is
changed into prisms ; and such epithelium is often known as
' prismatic' On the other hand, if the surface on which it rests be
convex, the bases or lower ends of the cylinders become smaller than

Fi<j. 778. — Detached exDithelium-cells :
a, with nuclei 6, and nucleoli c,
from mucous membrane of the
mouth.

Fig. 779. — Ciliated epithelium :
«, nucleated cells resting on
their smaller extremities; b,
cilia.

their free extremities ; and thus each has the form of a truncated
cone rather than of a cylinder, and such epithelium (of which
that covering the villi of the intestine is a peculiarly good ex-
ample) is termed ' conical.' But between these primary forms of
epithelial cells there are several intermediate gradations ; and one
often passes almost insensibly into the other. Any of these forms
of epithelium may be furnished with cilia ; but these appendages are
more commonly found attached to the elongated than to the
flattened forms of epithelial cells (fig. 779). Ciliated ej)ithelium is
found upon the lining membrane of the air-passages in all air-
breathing Yertebrata ; and it also presents itself in many other
situations, in which a propulsive power is needed to prevent an ac-
cumulation of mucous oi- other secretions. Owing to the very slight
attachment that usually exists between the epithelium and the
membranous surface whereon it lies, there is usually no difiiculty
whatever in examining it, nothing more being necessary than to
scrape the surface of the membrane with a knife and to add a little
water to what has been thus removed. The ciliary action will
generally be found to persist for some hours or even days after
death if the animal has been previously in full vigoui- ; and the
cells that bear the cilia, when detached from each other, will

FAT 1045

swim freely about in water. If the thin fluid that is copiously dis-
charged from the nose in the first stage of an ordinary ' cold in the
head ' be subjected to microscopic exammation, it will commonly
be found to contain a great number of ciliated epithelium-cells,
which have been thrown off from the lining membrane of the nasal
passages.

Fat. — One of the best examples which the bodies of higher
animals afibrd, of a tissue composed of an aggregation of cells, is
presented by fat, the cells of which are distinguished by their power
of drawing into themselves oleaginous matter from the blood. Fat-
cells are sometimes dispersed in the interspaces of areolar tissue ;
whilst in other cases they are aggregated in distinct masses, con-
stituting the proper adipose substance. The individual fat-cells
always present a nearly spherical or spheroidal form ; sometimes,
however, when they are closely pressed together, they become some-
what polyhedral, from the flattening of their
walls against each other (fig. 780). Their
intervals are traversed by a minute network
of blood-vessels (fig. 795), from which they
derive their secretion ; and it is probably
by the constant moistening of their walls
with a loatery fluid, that their contents are
retained without the least ti-ansudation,
although these are quite fluid at the tem-
perature of the living body. Fat-cells, when
filled with their characteristic contents, have
the peculiar appearance which has been
already described as appertaining to oil-
globules, being very bright in their centre,
and very dark towards their margin, in Fig. 780.— Areolar and adi-
consequence of their hiffh refractive power ; ^°^Z ^^^I^^ ' "' '^l *^*"°®\^^ '■>

0 0 IlDr6S 01 ctl'GOlclir

but if, as often happens in preparations that tissue.
have been long mounted, the oily contents

should have escaped, they then look like any other cells of the same
form. Although the fatty matter which fills these cells (consisting
of a solution of stearine or margarine in oleine) is liquid at the
ordinary temperature of the body of a warm-blooded animal, yet its
harder portion sometimes crystallises on cooling, the ciystals shoot-
ing from a centre, so as to form a star-shaped cluster. Osmic acid
has been found by Dr. B. Solger to separate a more fluid central
portion from a firmer peripheral part. In examining the structure
of adipose tissue it is desirable, where practicable, to have recourse
to some specimen in which the fat-cells lie in single layers, and in
which they can be observed without disturbing or laying them
open ; such a condition is found, for example, in the mesentery of
the mouse ; and it is also occasionally met with in the fat-deposits
which present themselves at intervals in the connective tissues of the
muscles, joints, &c. Small collections of fat-cells exist in the deeper
layers of the true skin, and are bi-ought into view by vertical
sections of it (fig. 775, /). And the structure of large masses of fat
may be examined by thin sections, these being placed under water

1046

YEHTEBEATED ANIMALS

Fit

781. — Cellular cartilage of
mouse's ear.

in thin cells, so as to take off the pressure of the glass cover from
their surface, which would cause the escape of the oil-particles. No
method of movmting (so far as the Author is aware) is sviccessful in
causing these cells permanently to retain their contents.

Cartilage. — In the ordinary forms of cartilage, also, we have an
example of a tissue obviously composed of cells ; but these are com-
monly separated from each other by
K)~'" ■': >•■'--' y ^'- an ' intercellular suKstance,' which is

so closely adherent to the outer walls
of the cells as not to be separable
from them. The thickness of this
substance differs greatly in different
kinds of cartilage, and even in dif-
V- ferent stages of the growth of any

one. Thus in the cartilage of the
external ear of a bat or mouse (fig.
781), the cells ai-e packed as closely
together as are those of an ordinary
vegetable parenchyma ; and this seems to be the early condition
of most cartilages that are afterwards to present a different
aspect. In the ordinary cartilages, however, that cover the ex-
tremities of the bones, so as to form smooth surfaces for the work-
ing of the joints, the amount of intercellular substance is usually
considerable ; and the cartilage-cells are commonly found imbedded
there in clusters of two, three, or four (fig. 782), which are evidentl}^
formed by a process of ' binary subdivision.' The substance of these

cellular cartilages is entirely
.ii'L;te~sF"Bpr-™^««ss.. destitute of blood-vessels,

being nourished solely by
imbibition from the blood
brought to the membrane
covering their surface. Hence
they may be compared, in
regard to their grade of or-
ganisation, with the larger
algse, which consist, like
them, of aggregations of cells
held together by intercellular
substance, without vessels of
Fig. 782.— Section of the branchial cartilage of any kind, and are nourished

tadpole : «, group of four cells, sex^arating
from each other ; &, pair of cells in apposi-
tion ; c c, nuclei of cartilage-cells ; d, cavity
containing three cells (the fourth probably
behind).

by imbibition through theii-
whole surface. There are
many cases, however, in
which the structureless intei'-
cellular substance is replaced
by bundles of fibres, sometimes elastic, but more commonly non-
elastic ; such combinations, which ai-e termed ^j^ro-cartilages, are
interposed in certain joints, wherein tension as well as pressure has
to be resisted ; as, for example, between the vertebrae of the spinal
column and the bones of the pelvis. In examining the structure
of cartilage nothing moi-e is necessary than to make very thin

GLANDS 1047

•sections, preferably with the microtome. These sections may be
mounted in weak spii-it, Goadby's solution, or glycerin-jelly ; but
in whatever way they ai'C mounted, they undergo a gradual change
by lapse of time, which renders them less fit to disj)lay the cha-
racteristic features of their structure.

Structure of the Glands. — The various seci^etions of the body (as
saliva, bile, urine, etc.) are formed by the instrumentality of organs
termed glands ; which are, for the most part, constructed on one
fundamental type, whatever be the nature of their product. The
simplest idea of a gland is that which we gain from an examination
of the ' follicles ' or little bags imbedded in the wall of the stomach,
some of which secrete mucus for the protection of its surface and
other gastric juice. These little bags are filled with cells of a
spheroidal form, which may be considered as constituting their
epithelial lining ; these cells, in the progress of their development,
draw into themselves from the blood the constituents of the par-
ticular product thej^ are to secrete ; and they then seem to deliver
it up, either by the bui'sting or by the melting away of their walls,
so that this product may be poured forth fi-om the mouth of the bag
into the cavity in which it is wanted. The organ which is generally,
though by no means accurately, called the liver pi-esents this con
dition in the lowest animals wherein.it is found. In many Polyzoa,
compound Tunicata, and Annulata the cells of this organ can be seen
to occupy follicles in the walls of the stomach ; in insects these
follicles are few in number, but are immensely elongated, so as to
form tubes which lie loosely within the abdominal cavity, frequently
making many convolutions within it, and discharge their contents
into the commencement of the intestinal canal ; whilst in the
higher Mollusca, and in Crustacea, the follicles are vastly multiplied
in number, and are connected with the ramifications of gland-ducts,
like grapes upon the stalks of their bunch, so as to form a distinct
mass which now becomes known as the liver. The examination of
the tubes of this organ in the insect, or of
the follicles of the crab, which may be
accomplished with the utmost facility, is
well adapted to give an idea of the
essential nature of glandular structure.
Among vertebrated animals the salivary
glands, the pancreas (sweetbreads), and
the mammary glands are well adapted to
display the follicular structure (fig. 783),
nothing more being necessary than to ^i^; 783.— Ultimate follicles

T ° , . r- , 1 , 1 • 1 of mammary gland, with

make sections of these organs thm enough ti-^^ij. secreting cells a a,
to be viewed as transparent objects. The containing nuclei b b.
kidneys of vertebrated animals are made

up of elongated tubes, which ai-e straight, and are lined with a
pavement-epithelium in the inner or 'medullary' portion of the
kidney, whilst they are convoluted and filled with a spheroidal
epithelium in the outer or ' cortical.' Certain flask-shaped dilata-
tions of these tubes include curious little knots of blood-vessels,
which are known as the ' Malpighian bodies ' of the kidney ; these

1048

VERTEBRATE D ANIMALS

are well displayed in injected preparations. For such a full and
complete investigation of the structvire of these organs as the
anatomist and physiologist require, various methods must be put
in practice which this is not the place to detail. It is perfectly
easy to demonstrate the cellular nature of the substance of the
liver by simply scraping a portion of its cut surface, since a number
of its cells will then be detached. The general arrangement of the
cells in the lobules may be disjDlayed by means of sections thin
enough to be transparent ; whilst the arrangement of the blood-
vessels can only be shown by means of injections. Fragments of
the tubules of the kidney, sometimes having the Malpighian cap-
sules in connection with them, may also be detached by scraping its
cut surface ; but the true relations of these parts can only be shown
by thin transparent sections, and by injections of the blood-vessels
and tubuli. The simple follicles contained in the walls of the
stomach are brought into view by vertical sections ; but they may
be still better examined by leaving small portions of the lining
membrane for a few days in dilute nitric acid (one part to four of
water), whereby the fibrous tissue will be so softened that the
clusters of glandular epithelium lining the follicles (which are but
very little altered) will be readily separated.

Muscular Tissue. — Although we are accustomed to speak of this
tissue as consisting of ' fibres,' yet the ultimate structiu"e of the
' muscular fibre ' is very diflferent from that of the ' simple fibrous
tissues' already described. When we examine an ordinary
muscle (or piece of ' flesh ') with the naked eye, we observe that it
is made up of a number of fasciculi or bundles
of fibres (fig. 784), which are arranged side by
side with great regularity, in the direction in
which the muscle is to act, and are united by
connective tissue. These fasciculi may be
separated into smaller parts, which appear like
simple fibres ; but when these are examined by
the microscope, they are found to be themselves
fasciculi, comjDosed of minuter fibres bound
together by delicate filaments of connective
tissue. By carefully separating these we may
obtain the ultimate muscular fibre. This fibre
exists under two forms, the striated and the

Y84. Fasciculus '''t'On- striated. The former is chiefly distinguished

of striated muscular by the transversely striated appearance which
fibre, showing at a the it presents (fig. 785), and which is due to an

transverse striae, and -..'- ,. n ^■ i j_ i i i ^ •!_

at b its iunction with alternation 01 light and dark spaces along its
the tendon. whole extent ; the breadth and distance of

these stride vary, however, in different fibres,
and even in difierent parts of the same fibre, according to theii-
state of contraction or relaxation. Longitudinal stripe are also
frequently visible, which are due to a partial separation between
the component fibrillfe into which the fibre may be broken up.
When a fibre of this kind is more closely examined, it is seen to be
inclosed within a delicate tubular sheath, which is quite distinct on

Fig.

MUSCLE

1049

Fig. 785. — Striated muscular fibre, separating
into fibrillee.

the one hand from the connective tissue that binds the fibres into
fasciculi, and equally distinct from the internal substance of the
fibre. This membranous tube, which is termed the sarcolemma, is
not perforated by capillaiy vessels, which therefore lie outside the
ultimate elements of the muscular substance ; whether it is pene-
trated by the ultimate
fibres of nerves is a point
not yet certainly ascer-
tained. The diameter of
the fibres varies greatly
in different kinds of verte-
brated animals. Its ave-
rage is greater in reptiles
and fishes than in birds
and mammals, and its ex-
tremes also are wider ; thus
its dimensions vary in the
frog from -j-^th to -y-^^th.

of an inch, and in the skate from -(jl-th to 3-5-oth ; whilst in the
human subject the average is about ^^y^th of an inch, and the
extremes about 2-0-0 'th and -y-i-jjth.

The substance of the fibre, when broken up by ' teasing ' with
needles, is found to consist of very minute fibrillse, which, when
examined under a magnifying power of from 250 to 400 diameters,
are seen to present a slightly beaded form, and to show the same
alternation of light and dark spaces as when
the fibrillae are united into fibres or into
small bundles (fig. 785). The dark and light
spaces are usually of nearly equal length ;
each light space is divided by a transverse
line, called ' Dobie's line,' while each dark
space is crossed by a lighter band, known as
' Hensen's stripe.' It has been generally
supposed that these markings indicate dif-
ferences in the convposition of the fibre ; but
Professor J. B. Haycraft has revived an
idea, which originated with Mr. Bowman,
that they are the optical expressions of its
shape. The borders of the striated fibre
(he truly states) present wavy margins, in-
dicative of a transverse ridging and furrow-
ing, the whole fibre (or a single fibril) thus
consisting of a succession of convex bead-
like projections with intermediate concave
depressions. Wlien the axis of the fibre is in
true focus, Dobie's line, D (fig. 786), crosses

the deepest part of the concavity, while Hensen's stiipe, H, crosses
the most projecting part of the convexity, and it can be shown, both
theoretically and experimentally, that this alternation of lights and
shades will be produced by the passage of light through a similarly
shaped homogeneous rod of any transparent substance. If, on the

Fig. 786.— Diagram'of
striated fibrilla.

I050 YERTEBRATED ANIMALS

other hand, the surface of the fibre be brought into focus, the convex
ribbings appear light and intervening depressions dark, which is the
aspect originally represented by Bowman. The appearances are the
same in the extended and contracted states of the fibre ; with the
exception that the alternation of light and dark striae is closer in the
contracted state, while the breadth (representing the thickness) of
the fibre is correspondingly increased.^ It is well none the less in
the present state of our knowledge to refrain from conclusions as to
the absolute structure of the striated fibrillse. It ranges itself, from
the modern microscojDist's point of view, with other striated objects,
and will require the possession of lenses of a N.A. twice or thrice that
of those which are now within our reach. There is no immediate pro-
spect of these, it is true ; but they cannot be considered impossible
by the student of the past history of microscopy.

In the examination of muscular tissue a small portion may be
cut out with the curved scissors ; this should be torn up into its
component fibres ; and these, if possible, should be separated into
their fibrillse by dissection with a pair of needles under the simple
microscope. The general characters of the striated fibre are admi-
rably shown in the large fibres of the fi'og ; and by selecting a
portion in which these fibres spread themselves out to unite with a
broad tendinous expansion, they may often be found so well dis-
played in a single layer as not only to exhibit all their characters
without any dissection, but also to show their mode of connection
with the ' simple fibrous ' tissue of which that expansion is formed.
As the ordinary characters of the fibre are but little altered by
boiling, recourse may be had to this process for their more ready
separation, especially in the case of the tongue. Dr. Beale recom-
mends glycerin for the preparation, and glycerin media for the
preservation, of objects of this class ; and states that the alternation
of light and dark spaces in the fibrillae is rendered more distinct by
such treatment. The fibrillae are often more readily separable when
the muscle has been macerated in a weak solution of chromic acid.
The shape of the fibres can only be properly seen in cross-sections ; -
and these are best made by the freezing microtome. Striated fibres,
separable with gi^eat facility into their component fibrillae, are
readily obtainable from the limbs of Crustacea and of insects ; and
their presence is also readily distinguishable in the bodies of worms,
even of very low organisation ; so that it may be regarded as charac-
teristic of the articulated series generally. On the other hand, the
molluscous classes are, for the most part, distinguished by the non-
striation of their fibre ; there are, however, some exceptions, such as
the muscles of the odontophore in the snail and the powerful adductor
muscle of Pecten. Its presence seems related to energy and rapidity
of movement, the non-striated presenting itself where the move-
ments are slower and feebler in their character.

The ' smooth ' or non-striated form of muscular fibre, which is

1 Quart. Journ. Microsc. Sci. n.s. xxi. p. 307. More recent views will be found in
Mr. C. F. Marshall's paper in vol. xxviii. of the same journal, and in the memoirs
cited by him. The subject is one which will doubtless long occupy the attention of
the histologist.

MUSCLE ; NERVE

1051

especially found in the walls of the stomach, intestines, bladder, and
other similar parts, is composed of flattened bands whose diameter is
usnally between o-jjVoth and ■g-o'du'tlT^ of aii inch ; and these bands are
collected into fasciculi, which do not lie parallel with each other, but
cross and interlace. By macerating a portion of such muscular sub-
stance, however, in dilute nitric acid (about one part of ordinary
acid to three parts of water) for two or three days, it is found that
the bands just mentioned may be easily separated into elongated fusi-
form cells, not vmlike ' woody fibre ' in shape (fig. 787, a a) ; each
distinguished, for the most part, by the presence of a long stafi"-
shaped nucleus, 5, brought into view by the action of acetic acid, c.
These cells, in which the distinction between cell- wall and cell-con-
tents can by no means be clearly seen, are composed of a soft yellow
substance often containing small pale granules, and sometimes yellow
globules of fatty matter. In the coats of the blood-vessels are found

Pig. 787. — ^ixnctwveoinon-striated
muscular fibre : A, portion of
tissue showing fusiform cells a a,
with elongated nuclei h b ; B, a
single cell isolated and more
highly magnified ; C, a similar
cell treated with acetic acid.

Fig. 788. — Ganglion-cells and nerve-
fibres from a ganglion of lamprey.

cells having the same general characters, but shorter and wider in
form ; and although some of these approach very closely in their
general appearance to epithelium-cells, yet they seem to have quite
a different nature, being distinguished by their elongated nuclei, as
well as by theii- contractile endowments.

Nerve-substance. — Wherever a distinct nervous system can be
made out, it is found to consist of two very difierent forms of tissue,
namely, the cellular, which are the essential components of the
ganglionic centres, and i\\e fibrous, of which the connecting trunks
consist. The typical form of the nerve-cells or ' ganglion-globules
may be regarded as globular ; but they often present an extension
into one or more long processes, which give them a ' caudate ' or
' stellate ' aspect. These processes have been traced into continuity,
in some instances, with the axis-cylinders of nerve-tubes (fig, 788) ;
whilst in other cases they seem to inosculate with those of other

1052

VERTEBEATED ANIMALS

vesicles. The cells, which do not seem to possess a definite cell-wall,
are, for the most part, composed of a finely graniilar substance, which
extends into their prolongations ; and in the midst of this is usually
to be seen a large well-defined nucleus. They also generally contain
pigment-granules, which give them a reddish or yellowish-brown
colour, and thus impart to collections of ganglionic cells in the
warm-blooded Vertebrata that peculiar hue which causes them to be
known as the cineritious or grei/ matter, but which is commonly
absent among the lower animals. Each of the tubular nerve-fibres,
on the other hand, of which the trunks are made up, consists, in its
fully developed form, of a delicate membranous sheath, within which
is a hollow cylinder of a material known as the ' white substance of
Schwann,' whose outer and inner boundaries are marked out by two
distinct lines, giving to each margin of the nerve-tube what is de-
scribed as a ' double contour.' The contents of the membranous
envelope are very soft, yielding to slight pressiire ; and they are so
quickly altered by the contact of water or of any liquids which are
foreign to their nature that their characters can only be properly
judged of when they are quite fresh. The
centre or axis of the tu.be is then found to
be occupied by a transparent substance
which is known as the ' axis cyhnder ; '
and there is reason to believe that this last,
which is a protoplasmic substance, is the
essenticd component of the nerve-fibre,
while the function of the hollow cylinder
that surrounds it, which is composed of a
combination of fat and albuminous matter,
is simply protective. The diameter of the
nerve-tubes differs in difierent nerves, being
sometimes as great as y-^g-gth of an inch,

and as small in other instances as

rth.

Fig. 789.— Gelatinous nerve-
fibres, from olfactory nerve.

In many of the lower invertebrata, such as
Meclusce and Comatulce, we seem fully
justified by physiological evidence in re-
garding as nerves certain protoplasmic
fibres which do not possess the characteristic structure of ' nerve-
tubes,' and fibres destitute of the ' double contour ' are found also
in certain parts of the body of even the highest vertebrates. These
fibres, which are known as ' gelatinous,' are considerably smaller
than the preceding, and do not exhibit any differentiation of parts
(fig. 789). They are flattened, soft, and homogeneous in their ap-
pearance, and contain numerous nuclear particles which are brought
into view by acetic acid. They can sometimes be seen to be
continuous with the axis-cylinders of the ordinary fibres, and also
with the radiating prolongations of the ganglion-cells ; so that their
nei'vous character, which has been questioned by some anatomists,
seems established beyond doubt.

The ultimate distribution of the nerve-fibres is a subject on
which there has been great divergence of opinion, and one which can
only be successfully investigated by observers of great experience.

NEEVE-FIBRES

1053

The Author believes that it may be stated as a general fact, that in
both the motor and the sensory nerve-tubes, as they apjDroach their
terminations in the muscles and in the skin respectively, the
protoplasmic axis-cylinder is continued beyond its envelopes,
often then breaking up into very minute fibrillse, which inosculate
with each other, so as to form a network closely resembling that
formed by the pseudopodial threads of Rhizopods. Recent observers
have described the fibrillar of motor nerves as terminating in
' motorial end-plates ' seated upon or in the muscular fibres ; and
these seem analogous to the little ' islets ' of sarcodic substance into
which those threads often dilate. Where the skin is specially
endowed with tactile sensibility we find a special pajnllary
apparatus, which in the skin may be readily made out in thin
vertical sections treated with solution of soda (fig. 790). It was
formei'ly supposed that all the cutaneous papillse are furnished with
nerve-fibres, and minister to sensation ; but it is now known that a
large proportion (at any rate) of those that are furnished with loops
of blood-vessels (figs. 775, p, 798), being destitute of nerve-fibres,
must have for their special

ofl&ce the production of
epidermis ; whilst those
which, possessing nerve-
fibres, have sensory func-
tions, are usually destitute
of blood-vessels. The

greater part of the interior
of each sensory papilla
(fig. 790, cc) of' the skin
is occupied hj a peculiar
' axile body,' which seems
to be merely a bundle of
ordinary connective tissue,
whereon the nei-ve-fibre
appears to terminate. The
nerve - fibres are more
readily seen, however, in
the ' fungiform ' papillae of

the tongue, to each of which several of them proceed ; these bodies,
which are very transparent, may be well seen by snipping ofi" minute
portions of the tongue of the frog ; or by snipping off the papillse
themselves from the surface of the living human tongue, which can
be readily done by a dexterous use of the cui-ved scissors, with no
more pain than the prick of a pin would give. The transparence
of these papillse also is increased by treating them with a weak
solution of soda. Nerve-fibres have also been found to terminate
on sensory surfaces in minute ' end-bulbs ' of spheroidal shape and
about -g-g-oth of an inch in diameter, each of them being composed
of a simple outer capsule of connective tissue, filled with clear
soft matter, in the midst of which the nerve-fibre, after losing its
dark border, ends in a knob. The ' Pacinian corpuscles,' which are
best seen in the mesentery of the cat, and are from -r^ath to i^th of

Fig. 790. — Vertical section of skin of finger, show-
ing the branches of the cutaneous nerves, a, h,
inosculating to form a plexus, of which the ulti-
mate fibres pass into the cutaneous papillse, c c.

IOS4 VERTEERATED ANIM^iLS

an inch long, seem to he more develojDed forms of these ' end-
biilbs.'

For the sake of obtaining a general acquaintance with the
microscopic characters of these principal forms of nerve-substance,
it is best to have recourse to minute nerves and ganglia. The small
nerves which are found between the skin and the muscles of the back
of the frog, and which become apparent when the former is being
stripped off, are extremely suitable for this purpose ; but they are Ijest
seen in the ffi/la or ' tree-frog,' which is recommended by Dr. Beale
as being much superior to the common frog for the general purposes
of minute histological investigation. If it be wished to examine the
natural appearance of the nerve-fibres, no other fluid should be used
than a little blood-serum ; but if they be treated with strong acetic
acid, a contraction of their tubes takes place, by which the axis-
cylinders are forced out from their cut extremities, so as to be made
more apparent than they can be in any other way. On the other
hand, by immersion of the tissue in a dilute solution of chromic acid
(about one part of the solid crystals to two hundred of water), the
nerve-fibres are rendered firmer and more distinct. Again, the axis-
cylinders are brought into distinct view by the staining process,
being dyed mvich moi-e quickly than their envelopes ; and they
may thus be readily made out by reflected light in transverse
sections of nerves that have been thus treated. The gelatinous
fibres are found in the greatest abundance in the sympathetic nei'ves ;
and their chaiucters may be best studied in the smaller branches of
that system. So for the examination of the ganglionic cells, and of
their relation to the nerve-tubes, it is better to take some minute
ganglion as a whole (such as one of the sympathetic ganglia of the
frog, mouse, or other small animal) than to dissect the larger
ganglionic masses, whose structure can only be successfully studied
by such as are proficient in this kind of investigation. The nerves
of the oi-bit of the eyes of fishes, with the ophthalmic ganglion and
its bi-anches, which may be very readily got at in the skate, and of
which the components may be separated without much difiiculty,
form one of the most convenient objects for the demonstration of the
princij)al forms of nerve-tissue, and especially for the connection of
nerve-fibres and ganglion-cells. For minute inquiries, however, into
the ultimate distribution of the nerve-fibres in muscles and sense-
organs, certain special methods miist be followed, and very high
magnifying powers must be employed. Those who desire to follow
out this inquiry should acquaint themselves with the methods which
have been found most successful in the hands of the able histologists
who have devoted themselves to it.^

Circulation of the Blood. — One of the most interesting spectacles
that the microscopist can enjoy is that which is furnished by the

1 For further information regarding the nervous system the memoir of F. Nansen
on ' The Structure and Combination of the Histological Elements of the Central
Nervous System' in Bergen's Museums Aarsheretning for 1886 (18S7), p. 29, should
be consulted. An excellent summary of the more valuable modern methods of
staining nerve-fibres and cells was given in 1892 to the Royal Microscopical Society
by Dr. C. E. Beevor. See their Journal, 1892, p. 897.

CIECULATION OF BLOOD IO55

circulation of the blood in the ccvpillctry blood-vessels which dis-
tribute the fluid through the tissues it nourishes. This, of course,
can only be observed in such parts of animal bodies as are sufficiently
thin and transparent to allow of the transmission of light thi-ough
them, without any disturbance of their ordinary structure ; and the
number of these is very limited. The web of the frog's foot is per-
haps the most suitable for ordinary purposes, more especially since
this animal is to be easily obtained in almost every locality ; and the
following is the simple arrangement preferred by the Author : A
piece of thin cork is to be obtained, about nine inches long and three
inches wide (such pieces are prepared by cork-cutters, as soles), and a
hole about |ths of an inch in diameter is to be cut at about the middle
of its length, in such a position that, when the cork is secured upon the
stage, this aperture may correspond with the axis of the microscope.
The body of the frog is then to be folded in a piece of wet calico,
one leg being left free, in such a manner as to confine its move-
ments, but not to press too tightly upon its body ; and being then
laid down near one end of the cork-plate, the free leg is to be ex-
tended, so that the foot caii be laid over the central aperture. The
spreading out of the foot over the aperture is to be accomplished
either by j)assing pins through the edge of the web into the cork be-
neath, or by tying the ends of the toes with threads to pins stuck
into the cork at a small distance from the aperture ; the former
method is by far the least troiiblesome, and it may be doubted
whether it is really the source of more suffering to the animal than
the latter, the confinement being obviously that which is most felt.
A few turns of tape, carried loosely around the calico bag, the pro-
jecting leg, and the cork, serve to prevent any sudden start ; and
when all is secure, the cork-plate is to be laid down upon the stage
of the microscope, where a few more turns of the tape will serve to
keep it in place. The web being moistened with water (a precaution
which should be repeated as often as the membrane exhibits the
least appearance of dryness) and an adequate light being reflected
through the web from the mirror, this wonderful spectacle ig brought
into view on the adjustment of the focus (a power of from 75 to 100
diameters being the most suitable for ordinary purposes), j)i-ovided
that no obstacle to the movement of the blood be produced by
undue pressure upon the body or leg of the animal. It will not un-
frequently be found, however, that the current of blood is nearly or
altogether stagnant for a time ; this seems occasionally due to the
animal's alarm at its new position, which weakens or suspends the
action of its heart, the movement recommencing again after the
lapse of a few minutes, although no change has been made in
any of the external conditions. But if the movement should not
renew itself, the tape which passes over the body should be slackened ;
and if this does not produce the desired effect, the calico envelope
also must be loosened. Wlien everything has once been properly
adjusted, the animal will often lie for hours without moving, or
will only give an occasional twitch ; and even this may be avoided by
previously subjecting it to the influence of ether or chloroform, which
may be renewed from time to time whilst it is under observation.

1056

VEETEBRATED ANIMALS

The movement of the blood will be distinctly seen by that of its
corpuscles (fig. 791), which course after one another through the
network of capillaries that intervenes between the smallest arteries
and the smallest veins ; in those tubes which pass most directly
from the veins to the arteries the current is always in the same
direction ; but in those which joass across between these it may not
unfrequently be seen that the direction of the movement changes
from time to time. The larger vessels with which the capillaries are
seen to be connected are almost always veins, as may be known
from the direction of the flow of blood in them from the branches
{b b) towards their trunks (a) ; the arteries, whose ultimate sub-
divisions discharge themselves into the capillary network, are for
the most part restricted to the immediate borders of the toes. When
a power of 200 or 250 diametei's is employed, the visible area is of
course greatly reduced ; but the individual vessels and their contents
6 h

%

Pig. 791 ■ — Capillary circulation in a portion of the web of a frog's foot :
a, trunk of vein ; h, b, its branches ; c, c, pigment-cells.

are much more plainly seen : and it may then be observed that whilst
the ' red ' corpuscles flow at a veiy rapid i-ate along the centre of each
tube, the ' white ' corpuscles, which are occasionally discernible, move
slowly in the clear sti-eam near its margin.

The circulation may also be displayed in the tongue of the frog
by laying the animal (previously chloroformed) on its back, with its
head close to the hole in the cork-plate, and, after securing the body
in this position, drawing out the tongue with the forcejDS and fixing
it on the other side of the hole with pins. So, again, the circula-
tion may be examined in the hmgs — where it affords a spectacle of
singular beauty — or in the mesentery of the living frog by laying-
open its body and drawing forth either organ, the animal having
previously been made insensible by chloroform. The tadpole of the
frog, when sufficiently young, furnishes a good display of the capillai-y
circulation in its tail ; and the difficulty of keeping it quiet during

CIKCULATION OF BLOOD IO57

the observation may be overcome by gradually mixing some wai'ni
Avater with that in which it is swimming until it becomes motion-
less ; this usually happens when it has been raised to a temperature of
between 100° and 110° Fahr. ; and, notwithstanding that the muscles
of the body are thrown into a state of spasmodic rigidity by this
treatment, the heart continues to pulsate, and the circulation is
maintained.^ The larva of the vrnter-nev^t, when it can be obtained,
furnishes a most beautiful display of the circulation, both in its
external gills and in its delicate feet. It may be inclosed in a large
aquatic box or in a shallow cell, gentle pressure being made upon
its body, so as to confine its movements without stopping the heart's
action. The circulation may also be seen in the tails of small fish,
such as the minnow or the sticklehack, by confining these animals in
tubes, 01- in shallow cells, or in a large aquatic box ; but although
the extreme transparence of these parts adapts them, well for this
purpose in one respect, yet the comparative scantiness of their
blood-vessels prevents them from being as suitable as the frog's web
in another not less important particular. One of the most beautiful
of all displays of the circulation, however, is that which may be seen
upon the yolk-hag of young fish (such as the salmon or trout) soon
after they have been hatched ; and as it is their habit to remain
almost entirely motionless at this stage of their existence, the obser-
vation can be made with the greatest facility by means of the
zooph}i;e-trough. The store of yolk which the yolk-bag supplies
for the nutrition of the embryo not being exhausted in the fish (as
it is in the bird) pi'eviously to the hatching of the egg, this bag-
hangs down from the belly of the little creature on its emersion,
and continues to do so until its contents have been absorbed into
the body, which does not take place for some little time after-
wards. And the blood is distribvited over it in copious streams,
partly that it may draw into itself fresh nutiitive material, and
partly that it may be subjected to the aerating influence of the
surrounding water.

The tadpole serves, moreover, for the display, under jaroper
management, not only of the capillary, but of the general circulation ;
and if this be studied under the binocular microscope, the observei-
not only enjoys the gratification of witnessing a most wonderful
spectacle, but may also obtain a more accurate notion of the rela-
tions of the different parts of the circulating system than is other-
wise possible. The tadpole, as every naturalist is aware, is
essentially a fish in the early period of its existence, breathing by
gills alone, and having its circulating apparatus arranged accord-
ingly ; but as its limbs are developed, and its tail becomes relatively
shortened, its lungs are gradually evolved in preparation for its
terrestrial life, and the course of the blood is considerably changed.
In the tadpole as it comes forth from the egg the gills are external,
forming a pair of fringes hanging at the sides of the head (fig. 792, 1)
and at the bases of these, concealed by opercula or gill-flaps

1 A special form of live-box for the observation of living tadpoles &c., contrived
by Prof. *F. E. Schulze, is described and figured in the Qioart. Journ. Microsc. Sci.
U.S. vol. vii. 1867, p. 261.

3 T

IOS8

VERTEBEATED ANIMALS

resembling those of fishes, are seen the rudiments of the internal
gills, which soon b3gin to be developed in the stead of the preceding.

Fig. 792. — Circulation in the tadpole.

1. Anterior x^ortion of young tadpole, showing the external gills, with the incipient
tufts of the internal gills, and the pair of minute tubes between the heart and the
spirally coiled intestine, which are the rudiments of the future lungs.

2. More advanced tadpole, in which the external gills have almost disappeared :
a, remnant of external gills on the left side ; h, operculum ; c, remnant of external o-iH
on the right side, turned in.

3. Advanced tadpole, showing the course of the general circulation : a, heart ;
h, branchial arteries ; c, pericardium ; d, internal gill ; e, first or cephalic trunk ;
/, branch to lip; {/, branches to head; h, second or branchial trunk; f, third trunk
uniting with its fellow to form the abdominal aorta, which is continued as the caudal
artery, k, to the extremity of i the tail; Z, caudal vein; m, kidney; n, vena cava •
o, liver ; p, vena portfe ; q, sinus venosus, receiving the jugular vein, r, and the ab-
dominal veins, t, u, as also the branchial vein, v.

i The branchial circulation on a larger scale : A, B, C, three primary branches of
the branchial artery ; a, cartilaginous arches ; b, additional framework ; c, e, twigs of
branchial artery ; d, /, rootlets of branchial vein.

5. Origin of the vessels of the internal gills, g, from the roots of those of the
external.

6. The heart, systemic arteries, joulmonary arteries and veins, and lungs, in the
adult frog, the heart being turned U]} in the right-hand figure, to show the junction
of the p)ulmonary veins and their entrance into the left auricle.

CIRCULATION IN TADPOLE IO59

'The external gills reach their highest development on the fourth or
fifth day after emersion ; and the}" then wither so rapidly (whilst
"being at the same time drawn in by the growth of the animnl) that
by the end of the first week onlj^ n. remnant of the right gill can be
seen under the edge of the operculum (2, c), though the left gill
{h) is somewhat later in its disappeai-ance. Concurrently^ with this
change the internal gills are undergoing rapid development ; and
the beautiful arraiigement of their vascular tufts, which originate
from the roots of the arteries of the external gills, as seen at g, 5, is
shown in 4. It is requisite that the tadpole subjected to obser-
vation should not be so far advanced as to have lost its early trans-
pai-ence of skin ; and it is further essential to the tracing out of the
course of the abdominal vessels that the creature should have been
kept without food for some days, so that the intestine may empty
itself. This starving process reduces the quantity of red corpuscles,
and thus renders the blood palei- ; but this, although it makes the
smaller branches less obvious, brings the circulation in the larger
trunks into more distinct view. ' Placing the tadpole on his back,'
says Mr. Whitney, ' we look, as through a pane of glass, into the
chamber of the chest. Before us is the beating heart, a bulbous-
looking cavity, formed of the most delicate transparent tissues,
through which are seen the globules of the blood, perpetually, but
alternately, entering by one orifice and leaving it by another. The
heart (fig. 792, 3, a) appears to be slung, as it were, between two
arms or branches, extending right and left. From these trunks (ft)
the main arteries arise. The heart is inclosed within an envelope or
pericardium (c), which is, perhaps, the most delicate, and is, certainly,
the most elegant structure in the creature's organism. Its extreme
fineness makes it often elude the eye under the single microscope,
but under the binocular its form is distinctly revealed. Then it is
seen as a canopy or tent, inclosing the heart, but of such extreme
tenuity that its folds are really the means by which its existence is
recognised. Passing along the course of the great vessels to the
right and left of the heart, the eye is arrested by a large oval body
(d) of a more complicated structure and dazzling appearance. This
is the internal gill, which in the tadpole is a cavity formed of most
delicate transparent tissue, traversed by certain arteries, and lined
by a crimson network of blood-vessels, the interlacing of which, with
their rapid currents and dancing globules, forms one of the most
beautiful and dazzling exhibitions of vascularity.' Of the three
rarterial trunks which arise on each side from the truncus arteriosus,
h, the first, or ceplial'ic^ e, is distributed entirely to the head, running
first along the upper edge of the gill, and giving off a branch,/, to
the thick fringed lip which surrounds the mouth ; after which it
suddenly curves upwards and backwards, so as to reach the upper
surface of the head, where it dips between the eye and the brain.
The second main trunk, h, seems to be chiefly distributed to the gill,
although it freely communicates by a network of vessels both with
the first or cephalic and with the third or abdominal tiunk. The
latter also enters the gill and gives off branches ; but it continues
its course as a large trunk, bending downwards and curving towards

3\-2

Io6o VERTEBRATED ANIMALS

the spine, where it meets its fellow to form the abdominal aorta, i,
which, after giving off branches to the abdominal viscera, is con-
tinned as the caxidal artery, k, to the extremity of the tail. The
blood is returned from the tail by the caudal vein, I, which is
gradually increased in size by its successive tributaries as it passes
towards the abdominal cavity ; here it approaches the kidney, m^
and sends off a branch which incloses that organ on one side, while
the main trunk continues its course on the other, receiving tributaries
from the kidney as it passes. The venous blood returned from the
abdominal viscera, on the other hand, is collected into a trunk, ^;,
known as the j^ortal vein, which distributes it through the substance
of the liver, o, as in man ; and after traversing that organ it is dis-
charged by numerous fine channels, which converge towards the
great abdominal trunk, or vena cava, n, as it passes in close proximity
to the liver, onwards to the sinus venosus, q, or rudimentary auricle-
of the heart. This also receives the jugidar vein, r, from the head,,
which first, however, passes downwards in front of the gill close to
its inner edge, and meets a vein t, coming up from the abdomen,
after which it turns abruptly in the direction of the heart. Two
other abdominal veins, u, meet arid pour their blood direct into the
sinus venosus ; and into this cavity is also poured the aerated blood
returned from the gill by the hranchial vein, v, of which only the
one on the right side can be distinguished. The lungs may be de-
tected in a rudimentary state, even in the very young tadpole,
being in that stage a pair of minute tubular sacs, united at the upper
extremities, and lying behind the intestine and close to the spine.
They may be best brought into view by immersing the tadpole for a
few days in a weak solution of chromic acid, wdiich renders the
tissue friable, so that the parts that conceal them may be more
readily peeled away. Their gradual enlargement may be traced
during the period of the tadpole's transparence ; but they can only
be brought into view by dissection when the metamorphosis has
been completed. The following are Mr. "Whitney's directions for
displaying the cii'ciTlation in these organs : ' Put the young frog into
a wineglass and drop on him a single drop of chloroform. This
suffices to extinguish sensibility. Then lay him on the back on a
piece of cork and fix him with small pins passed through the web
of each foot. Remove the skin of the abdomen with a fine pair of
sharp scissors and forceps. Turn aside the intestines from the left
side, and thus expose the left lung, which may now be seen as a
glistening transparent sac containing air-bubbles. With a fine
camel-hair pencil the lung may now be turned out, so as to enable
the operator to see a large part of it by transmitted light. Unpin
the frog and place him on a slip of glass, and then transmit the
light through the everted j)ortion of lung. Remember that the lung-
is very elastic, and is emptied and collapsed by very slight pressure.
Therefore, to succeed with this experiment, the lung should be
touched as little as possible, and in the lightest manner, with the
brush. If the heart is acting feebly you will see simply a trans-
parent sac, shaped according to the quantity of air-bubbles it may
happen to contain, but void of red vascularity and circulation. But

INJECTED PREPAEATIONS

IO61

sliould the operator succeed in getting the lung well placed, full of
air, and have the heart still beating ^dgorously, he will see before him
a brilliant picture of ci-inison network, alive with the dance and
-dazzle of blood-globules, in rapid chase of one another through the
delicate and living lace-woi'k which lines the chamber of the lung.'
The position of the lungs in relation to the heart and the great
vascular trunks is shown in fig. 792, 6.

Injected Preparations. — Next to the circulation of the blood in
the li^ang body, the vai-ied distribution of the capillaries in its
several organs, as shower by
means of ' injections ' of colour-
ing matter thrown into their
principal vessels, is one of the
most interesting subjects of
microscopic examination. The
•art of making successful pre-
parations of this kind is one
in which perfection can usually
be attained only by long prac-
tice and by attention to a
great number of minute par-
ticulars ; and better specimens
may be obtained, therefore,
from those who have made it
a business to proditce them

than are likely to be prepared by amatevirs for themselves. For
this reason no account of the process will be here offered, the minute
-details which need to be attended to, in order to attain successful

Fig. 793. — Transverse section of small intes-
tine of rat, showing the villi in situ.

Tig. 794. — Section of tlie toe of a mouse : a, a, a, tarsal bones ; h, digital artery ;
c, vascular loops in the papilla? forming the thick epidermic cushion on the under
surface ; d, distribution of vessels in the matrix of the claw.

results, being readily accessible elsewhere to such as desire to put it
in practice.^

1 See especially the article 'Injection' in the Mici'ograjiJdc Dictionarij; M,

I062

VERTEBKATED AXIMAL.s

Many anatomical parts, when well injected and mounted, become
objects of both interest and instruction. This is the case with the-
villi of tlie intestine, seen in fig. 793, which presents a transverse
section, in which they are seen in situ. A thin section of the toe
of a mouse (fig. 794) is another illustration of the eflFectiveness of
this mode of preparation.

A relation may generally be traced between the disposition of
the capillary vessels and the functions they subserve ; but that
relation is obviously, so to speak, of a mechanical kind, the arrange-

FiG. 795. — Capillary network
around fat-cells.

Fig. 796. — Capillary network of
muscle.

ment of the vessels not in any way determining the function, but
merely administering to it, like the arrangement of water or gas
pipes in a manufactory. Thus, in fig. 795, we see that the capil-
laries of adipose substance are disposed in a network with rounded
meshes, so as to distribute the blood among the fat-cells ; whilst in
fig. 796 we see the meshes enormously elongated, so as to permit
the muscular fibres to lie in them. Again, in fig. 797, we observe
the disposition of the capillaries around the orifices of the follicles

Pig. 797. — Distribution of capil-
laries in mucous membrane.

Fig. 798. — Distribution of capil-
laries in skin of finger.

of a mucous membrane ; whilst in fig. 798 we see the looped
arrangement which exists in the papillary surface of the skin, and
which is subservient to the nutrition of the epidermis and to the
activity of the sensory nerves.

In no part of the circidating apparatus, however, does the
disposition of the capillaries present more points of interest than it
does in the respiratory organs. In bony fishes the respiratory surface

Eobin's work, Du Microscope et des Injections ; Prof. H. Fray's treatise, Das Mikro-
skoj} rind die mikrosJcoinsche TeclmiJc; Dr. Beale's How w ■work with the Micro-
scope; the Handbook to the Phiisiiilogical Laboratory; and Rutherford's and'
Schiifer's treatises on Pra,ctical Histologij.

EESPIEATORY ORGANS

1063

is foi'mecl by an outwai'd extension into fringes of gills, each of which
consists of an arch Avith straight lamin;>3 hanging down from it, and
every one of these hamina? (fig. 799) is furnished with a double row
of leaflets, which is most minutely supplied with blood-vessels,
theii- network (as seen at A)
being so close that its meshes
(indicated by the dots in the
figure) cover less space than the
vessels themselves. The gills of
fish are not ciliated on theii-
surface, like those of molluscs
and of the lai-va of the water-
newt, the necessity for such a
mode of renewdng the fluid in
contact with them being super-
seded by the musculai' apparatus
with which their gill- chamber is
furnished. But in batrachians
and reptiles the respiratory sur-
face is formed by the walls of
an internal cavity, that of the
lungs : these organs, however,
are constructed on a plan very
different from that which they
present in higher Vertebrata,
the great extension of surface
which is effected in the latter ^^^- 799- — Two branchial processes of the-
1)V the minute subdivision of ^'^^ °^ *^^ ^^1' showing the branchial

lamellae : A, portion of one of these pro-
cesses enlarged, showing the capillary-
network of the lamellEG.

the cavity not being here neces-
sary. In the frog (for example)
the cavity of each lung is un-
divided ; its walls, which are
thin and membranous at the
lower part, there present a
simple smooth expanse ; and it
is only at the upper part, where
the extensions of the tracheal
cartilage form a network ovei"
the intei'ior, that its sui-face is
depressed into sacculi wdiose
lining is crowded with blood-
vessels (fig. 800). In this
manner a set of air-cells is
formed in the thickness of the
upper wall of the lung, which
communicate with the general
cavity, and very much increase

the surface over whicli the blood comes into relation with the air ;
but each air-cell has a capillary network of its own, which lies
on one side against its wall, so as only to be exposed to the air
on its free surface. In the elongated lung of the snake the samie

Fig. 800.

-Interior of upper part of
lung of frog.

1064

VERTEBEATED ANIMALS

general arrangement prevails ; but the cartilaginous i/eticulation
of its upper part projects mucli farther into the cavity, and incloses
in its meshes (which are usually square, or neai4y so) several layers
of air-cells, which communicate, one through another, with the
genei-al cavity. The structure of the lungs of birds presents us with
an arrangement of a very different kind, the purpose of which is to
expose a very laig'e amount of capillary surface to the influence of the
air. The entire mass of each lung may be considered as subdivided
into an immense number of lobules ' or 'kinglets' (fig. 801, B), each of

Pig. 801. — Interior structure of lung of fowl, as displayed by a section, A
passing in the direction of a bronchial tube, and by another section B
cutting it across.

Fig. 802.

-Arrangement of the capillaries on the walls of the air-cells of
the human lung.

"which has its own bronchial tube (or subdivision of the windpipe)
and its own system of blood-vessels, which have very little com-
munication with those of other lobules. Each lobvile has a central
cavity, which closely i-esembles that of a frog's lung in miniature,
having its walls strengthened by a network of caitilage derived from
the bronchial tube. A, in the intersj)aces of which are openings lead-
ing to sacculi in their substance. But each of these cavities is sur-
rounded by a solid plexus of blood-vessels, which does not seem to be
covered by any limiting membrane, but which admits air from the

LUNGS 1065

central cavity freely between its meshes ; and thus its capillaries are
in immediate relation with air on all sides — a provision that is ob-
viously veiy favoui-able to the complete and rapid aeration of the blood
they contain.^ In the lung of man and mammals, again, the plan of
structure difl'ers from the foi-egoing, though the general effect of it is
the same. For its Avhole interior is divided up into minute air-cells,
which fi-eely communicate with each other, and with the ultimate
ramifications of the aii'-tubes into which the trachea subdivides ; and
the network of blood-vessels (fig. 802) is so disposed in the partitions
between these cavities that the blood is exposed to the air on both
sides. It has been calculated that the number of these air-cells
grouped around the termination of each air-tube in man is not less
than eighteen thousand, and that the total number in the entire
lung is six hundred millions.

1 On the respiratory organs of birds, see Campana, La Hespiration des Oiseaux,
Paris, 1875.

io66

CHAPTER XXIII

APPLICATION OF THE MICROSCOPE TO GEOLOGICAL
INVESTIGATION

The utility of the microscope is by no means limited to the deter-
mination of the structure and actions of the organised beings at
present living on the surface of the ea,rth ; for a vast amount of
information is afforded by its means to the geological inquirer, not
only with regard to the essential nature and composition of the rock-
masses of which its crust is composed, but also with regard to the
minute characters of the many vegetable and animal remains that
are intombed therein.

The systematic employment of the instrument in petrographical
research dates from 1858, when Dr. H. C. Sorby, F.R.S., published his
classical paper ' On the Microscopical Structure of Crystals, indicating
the Origin of Minerals and Rocks.' ^ The observations in this paper
were based upon the microscopical examination of thin sections of
rocks and minerals ; still, although Dr. Soi-by was the first to ajjply
this manner of investigation to such objects, the first to suggest and
arrange the method of preparing thin sections appears to have been
William Nicol. A description of his method is given by H. Witham
(1831).^ Previous to 1858 only those minerals could be examined
microscopically which possessed the necessary degree of transparency,
whilst rocks were largely closed secrets. Nevertheless Oordier (in
1815) w^as able to determine the constituent minerals of many rocks
by the study of the powder under the microscope ; a procedure which
Fleurian de Bellevue had previously recommended in 1800, and
which is still found valuable for certain purposes. Seven years before
Dr. Soi'by's paper appeared, the German scholar Oschatz exhibited
a series of thin sections of minerals and rocks and drew attention
to their important beai'ing upon structural studies, but the collection
was regarded moi'e as a curiosity than as a scientific achievement.^

That paj)er, however, gave an enormous impetus to geological
research, and this, in the hands of English and German students,
led to the growth of a ' micro-petrology.'

In order to examine minerals and rocks, sections must be pre-
pared thin enough to permit of the use of transmitted light ; foi'

^ Quart. Journ. Geol. Soc. vol. xiv. 1858, j)p. 453-500.

- Observations on Fossil Vegetables, Edinburgh and London, 1831.

•^ The history of the application of the microsco]De to geology has been sketched
by P. Zirkel in his paper Die Einfilhrung des Mikvoshops in das mineralogisch-
geologiscJie Studium, Iieipzig, 1881.

MICKOSCOPIC SECTIONS OF KOCKS I067

this purpose they should be from about xuir'th to j^^th of an inch
thick.

A chip about an inch squai'e is struck oi- cut off the specimen to
be studied. One surface of this is then ground down on a flat cast-
iron jilate with emerj- and water. This grinding may be done either by
hand or by means of a machine specially constructed for this purpose
(Chap. YII).i The former method will be described here. When
a smooth surface is at last obtained the specimen is well washed with
water and then polished upon a slab of plate glass with the finest
flour emery and water. When all inequalities are thus removed the
fragment is again well cleansed from all adhering emery.

The next process is to cement it with Canada balsam upon a slab
of glass about two inches square and about an eighth of an inch in
thickness. The Canada balsam is first heated over a spirit lamp in
an iron spoon, care being taken not to allow it to burn. This is the
most diflieult j^art of the whole process, and only experience can teach
how long the balsam must be heated in order to possess, on cooling,
the necessary hardness. If it be heated too long it will crack upon
cooling. The right ]3oint appears to be that in which large air-bubbles
force themselves through the viscous mass.

A small quantity of the warm balsam is poui-ed upon the slab of
glass, and the smooth sui-face of the rock-fragment, being pressed into
the balsam, is held doAvn upon the glass till the balsam hardens. The
slab is then examined fi'om its under side to see that no aii'-bubbles
have been included between the glass and the stone. Should they be
present in any quantity, the whole process must be repeated. When
the balsam has quite hardened, the other side of the fi-agment is
ground down with coarse emery and water on the iron plate. Uj)on
the section commencing to become transparent, the grinding with the
coarse emery must cease. The stone is then thoroughly cleansed
with water, and the final grinding is conducted upon the plate-glass
slab with flour emery and water.

The slide is then placed under a stream of water in order to
remove all traces of the emery powder from the minute pores of the
rock. This is now the time to employ chemical tests to the com-
ponent minerals, if such a course be deemed advisable. If the rock
is of a fragile nature, it is well to mount the section as it is ; but in
most cases it is possible by delicate manipulation- to remove it to a
mounting more suited to 023tical work. This transference is effected

1 F. G. Cuttell (61 Camden Road, N.W.), T. Riley (18 Burnfoot Avenue,
Fulham, S.W.), and J. Rhodes, Museum of Geology, Jermyn Street, S.W., prepare
good sections ; and the principal petrologioal opticians can generally recommend
efficient operators. Voigt and Hochgesang (Gottingen, Rothe Str. 13) and R. Fuess
(Berlin, S.W., 108 Alte Jacob Str.) do also most excellent work. German, craftsmen
are more skilful in overcoming difficulties [e.g. with soft rocks) than English, and
can make thinner slices. Hence, it is better to send specimens to Germany when
thinness is desired ; but when the size of the slice is important, to have the work done
in England. In a very thin slice the colour phenomena are less conspicuous, so
that reduction in thickness bej'ond a certain limit is not all gain ; but in rocks of
an opaque character, or in the study of very minute structures, it is hardly x^ossible
to err on the side of thinness, and slices ' made in Germany ' are much the better.
If a student is purchasing ready-made specimens from a dealer, he will find the
following rough test useful. Look through the slice at a window with a clear sky
beyond ; it is too thick when the bar cannot be distinctly seen.

I068 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

by the application of a gentle heat to the slab until the balsam
becomes liquefied, when the section can be pushed with a piece of
wii-e on to a suitable slide of glass. Obviously a drop of balsam
should be poured upon the latter before the section is transferred.
The slide is then warmed until the balsam becomes liquid, when the
superfluous quantity is drawn over the upper surface of the section.
When the section is completely covered with the balsam, a thin
clean cover-glass is held foi- a moment over the spirit flame and laid
upon the section. Gentle pressure is then aj^plied to the surface to
bring it close down to the section and to remove all air-bubbles.
The slide is then allowed to become quite hard, when it may be
cleansed with turpentine or alcohol and ether.

Very porous rocks must first be treated with Canada balsam, in
order to give them the consistency necessary for the preparation of
thin sections. Isolated mineral grains and sands can be mounted
by means of Canada balsam dissolved in chloroform. The slide must
not be heated, but evaporation allowed to take place. Another
method is described by Thoulet ; ^ whilst very soft or decomposed
rocks should be mounted according to Wichmann's proposal. ^

In the application of the microscope to petrological and minera-
logical reseai'ch the employment of polarised light is constantly re-
quired, and various means and appliances are needful for its most
advantageous apjalication, which are not required by the ordinary
microscopist. Considerable pains have been bestowed by both
English and Continental makers to fulfil the requirements, and good
instruments are now plentiful.^

An instrument designed by Mr. Allan Dick has been brought
out by Messrs. J. Swift and Son. As this combines all that experi-
ence has led petrologists to consider desirable for mineralogical
and petrological investigation, a brief account of it is subjoined. It
is specially adapted to the study of the optical properties of minerals
generally, and particularly to that of the thin plates of minerals seen
in ordinary sections of rocks prepared for microscopical examination.
The microscope is shown in fig. 803, but since the engraving was
made one or two improvements as to matters of detail have been
introduced.*

The eyepiece tube is slotted at E to receive the micrometer scale
(shown detached at F), and to the tube is hinged the analyser B',
which is capable of independent rotation in the usual manner.
Upon the eyepiece tube is mounted a toothed wheel, which gears
into another toothed wheel mounted on one end of a rod formed of
pinion wire. The stage, in the newest forms, is fitted with a scale
of rectangular divisions inserted to act as a finder, and with a roller
object-clip (patented by the makers) in place of the usual sliding bar.
Below the stage, which has neither sliding nor rotatory movements,

1 Annales de Ckimie et cle Physique (5), xx. pp. 362-432.

2 Tsehermak's Mineralogische und Petrogr. Mitt. Bd. v. 1882, p. 33.

^ Mr. J. Swift, of Tottenham Court Road, Mr. Watsou, of Holborii, London, and
Messrs. Henry Crouch, Limited, make suitable instruments. Those constructed by
Zeiss, of Jena ; Nachet, of Paris ; Voigt and Hochgesang, of Guttingen ; Puess, of
Berlin ; and Hartnack, of Potsdam, can also be recommended.

■* The instrument is protected by letters patent.

PETEOLOGICAL MICKOSCOPE

1069

is mounted the polariser, B. capable of independent I'otation like the
analyser, and upon the tube of the polaviser is mounted a toothed

Fig. 803. — Swift's petrological microscope.

wheel of the same size as that upon the analyser ; this wheel o-ears
into a wheel carried by a tube which forms a telescopic extension of

I070 THE MICROSCOPE IN GEOLOGICAL INVESTIGATIOX

the pinion wire, the object being to allow of the raising or lowering
of the body of the microscope for focussing. The analyser and the
polariser may thus be rotated synchronously without disconnecting
their toothed wheels. The polariser, in the latest form of the
instrument, is mounted on a crank arm, so that, if not required, it
may be thrown out of the axis of the stand. New, in the microscopes
usually constructed for peti-ological work the rotation of a small
crystal on the stage between the polarising and the analysing prisms
is liable to put it out of position in regard to the cross-threads in
the eyepiece, as the centring of the objective is scarcely ever so
perfect, as not to j)roduce some displacement ; and, if the centring
be adjusted so as to be perfect for one objective, it is likelv to be
faulty for another. (By a small crystal is meant a crystal under the
-J Jjj-y th of an inch in diameter, and of such thickness as one finds at
the edges of petrological sections.) Hence, by the arrangement
described above, centring is dispensed with, and the object is made to
rotate between the two prisms of the polarising apparatus without
changing its position beneath the objective. To a petrologist who
is accustomed to a rotating stage and fixed cross-wires, a familiar
section appears strange when first looked at on a fixed stage with
movable cross-wires,-but after a few hours' work with the instrument
the feeling of strangeness passes and that of the solid advantage of
a perfect centring remains.

On the polariser tube, above the toothed wheel and below the
stage, is fitted a goniometer, D, which, in combination with crossed
lines in the eyepiece, will permit of the measurement of the angles of
crystals without necessitating the shifting of the object when once
adjusted in the field. C is a set screw by wdiich the polarising
apparatus and goniometer may be fixed in any desired position.
Both the analysing and polarising prisms are divided to every 45°, a
spring catch maiking the extinction point. The opening between
the upper lens of the eyepiece and the analysing prism B' (fig. 803)
is for the purpose of placing such plates as the ^-undulation plate K
in position.

The great value of the instrument is in the facility with which
studies in convergent light can be performed. G is a slide fitted
with a double convex lens which may be used for showing the
optical figures of ciystals, and H is a similar slide carrying a lens
and a diaphragm of small aperture used for showing optical pictures
in minute crystals. The polai-iser is fitted with two convergent lenses,
which work in conjunction with the lens A. on the slide of the stage,
when great convergence is required. This slide may be pushed in
without disturbing the object iipon the stage. The achromatic con-
denser, A, shown at the foot of the figure, also works in conjunction
Avith the slidinglens, A, when the highest angularaperture is required.^

1 111 the latest made instruments a new achromatic convergent system is intro-
duced over the polariser. It gives a N.A. of 1-00, and an aplanatic cone 0-92. When
used as an immersion condenser, these are increased respectively to 1-12 and 1-05.
It is fitted with an iris diaxAragm placed above the polarising prism. A milled
collar actuates the focussing of the lower portion of the condenser. The fine adjust-
ment is the differential-screw form, which is sufficiently delicate and accurate to
determine the refractive index of minerals by the difference between the focus taken

COERODED CRYSTALS IO71

When convergent light is requii'ed the slide on the stage and
either G or H are j)ushed in, and the eyepiece covei-ed with the
analysei' B'. The. optical figures of the crystal then appear with
almost ideal cleai-ness. If this simple method is compared with that
previously in use, the superiority of the instrument will be im-
mediately recognised. It is in fact the most pei'fect petrological
microscope yet issued, and is one which will suit equally the minera-
logical and petrological student.

The microscopical investigation of rock sections has almost re-
volutionised petrology. Although the geologist has no difficulty in
detei-mining by his unaided eye with the use of simple chemical tests
the mineral components of rocks of coarse texture, the case is
difierent with those of extremely fine grain ; still more with such as
present an appai'ently homogeneous, compact, or glassy character.
The study reveals facts of the most striking significance, and wel-
come light has been thi-own upon the question of the order and
method of foxmation of rock constituents.^

The material which issues from a volcano during an eruption
is rarely in a state of complete fusion. In most cases it contains
crystals and parts of crystals which have formed befoi-e the ari-ival
of the fluid mass at the surface of the earth. Such ciystals are
iTSually of large size and can generally be recognised with the naked
eye. But sometimes these have undergone other changes before the
final consolidation of the rock. They may have been foimed under high
pressure, for the pressure lowers the melting-point of most substances.
Accordingly, as the jaressure is relieved uj^on the lava getting at or
near the svirface, the crystals which are floating iu the fused mass
at the time are liable to become corroded or redissolved. Again, some
subterranean change may produce a distinct rise in the temjoeratui-e
of the mass, or an access of heated water may increase the solvent
power of the ruolten portion. Instances of cori'osion fi-om one oi-
more of these causes are numerous. The quartzes of the quartz-
through the substance ancT its outside measure, the milled head being divided to 50,
and each division equalling one thousandth of a millimetre. A wheel of small aper-
tures is fitted to the upper Bertrand lens of the microscope for the purjiose of show-
ing optical pictures in minute crystals of various sizes.

1 Thereader is referred to the following works treating of the microscopical charac-
ters of minerals androcks : — F. Fouque et Michel L^vy, MineJxilogiemicrof/rapJiique,
Paris, 1878 ; E. Hussak, Anleitung ziim Bestimmen der gesteinsbildenden Mineralien,
Leipzig, 1885; E. Kalkowsky, Elemente der LitJiologie, Heidelberg, 1886; A. V.
Lasaulx, Elemente der Petrographie, Bonn, 1875, and Einfilhriong in die Gesteins-
lehre, Breslau, 1886 (also edition in French) ; Levy et Lacroix, Les Mineranx des
BocheSj'Pa.vis, 1888 ; F. H. Hosenhnsch, Mikroskopische Physiographie, 2nd edition,
vol. i. ' Die Mineralien ' (translated into English by Iddings), vol. ii. ' Die massigen
Gesteine ; ' Hulfstabellen zur mikroskopischen Mineralhestimniung in Gesteinen
(translated into English by F. H. Hatch) ; and Elemente der GesteiiUehre, 1898 ;
P. Rutley, The Study of Bocks, 3rd edition, 1884, and Bock-forming Minerals, 1888 ;
J. J. H. Teall, British Petrograp)hy, 1888; F. Zirkel, Lehrhuch der Petrographie,
2 vols. 2nd edition, 1893 ; Basaltgesteine, Bonn, 1870 ; Die inikroskopische Beschaf-
fenheit der Mineralien und Gesteine, Leipzig, 1873 ; Microscopical Petrography
"(U.S. Geol. Exploration of 40th parallel), Washington, 1876 ; A. Harker, Petrology
for Stiulents, 1895 (1st edition). The English student will find much valuable infor-
mation and useful directions in G. A. J. Cole's Aids to Practical Geology. But the
literature is now so vokiminous that it is practically imx^ossible to give anything like
a complete list ; for important papers will be found in almost every periodical deal-
ing with geology, among which those published in the United States must not be
forgotten.

I072 THE MICEOSCOPE IN GEOLOGICAL INVESTIGATION

porphyries have this corroded appearance ; whilst the porjihyritic
constituents of the basic rocks (hornblende, olivine, &c.) not in-
frequently show the same alteration (vide fig. 804 ; the dotted line
marks the original outline). In the case of the hornblende the
dissolved portions usually give rise to the formation of small grains
of augite and magnetite, which are then found encircling the
' mother-crystal.' Biotite is somewhat similarly affected, and some-
times the whole crystal in either mineral may be rendered almost
opaque by the separation of minute grains of magnetite.

The movement of the igneous mass may cause fracture of the
crystals owing to strain or to mutual pressure. The pieces of such
broken crystals may often be found in one and the same section,
sometimes at no great distance from each other. As the magma
solidifies, a further development of crystals occurs. The products of
this period constitute the ' ground-mass ' of the rock and are usuall}-
small in size, the microscope being frequently required for their
detection and determination. I

A glass is sometimes produced in the last stage of consolidation,

!i

Fig. 804. — Corroded olivine in basalt
of Kilimanjaro, East Africa.

Fig. 805.— Microlites. (After Zirkel.)

and appears as a base or ' setting' to the previously formed minerals.
This, however, is usually studded by minute mineral products endea-
vouring to crystallise under unfavourable circumstances. Generally
speaking, these 23roducts are present in two stages of develojDment.
The less perfectly develojDed forms of these are known as crystallites.
They occur in a variety of forms — hair-like, spherical, &c. — and the
smaller forms appear to be optically inactive. In some instances,
such as those termed ' globulites,' they may be minute segrega-
tions of a glassy nature ; in others crystalline aggregates, in which
from the extreme minuteness of the constituents and their mutual
interference the usual tests fail ; in other cases they may be desig-
nated embryonic crystals.

The bodies beloirging to the higher stage of develoj)ment are called
iiiicrolites or microliths (fig. 805). They differ from the crystallites
in possessing the internal sti-ucture of true ciystals and in acting on
polarised light. The position of the microlites Avith reference to
each other or to the large crystals is frequently an indicator of the
movements of the original fluid mass. When streams of microlites
are seen lying with their long axes in one direction, this direction is

STEUCTUEES OF CEYSTALS

1073

equivalent to that of the flow, and where such streams encounter
large crystals they sweep round them in graceful curves : this
appearance in a rock is known as fluxion-structure.

In certain glassy i-ocks microlites are collected into more oi-
less spherical masses, exhibiting a I'adial structvire, called spheru-
lites ; commonly these are not bigger than a pea, but sometimes
they are one or two inches in diameter ; they are then less regular
in shape and structure and are often named for distinction pyro-
merides. Chemical analysis often shows that they differ slightly
in composition fi'om the base. Crystalline rocks also sometimes
exhibit a similar structure, e.g. the orbicular diorite of Corsica.
A spherulitic structure can be produced in a compact rock by subse-
quent heating, short of melting, and many glassy rocks in lapse of
time become ' devitrified ' by setting up an obscure confused crys-
talline structure.^

Masses of molten material may, however, consolidate at a con-
siderable depth beneath the surface of the earth ; in such cases the
distinction between the first and second periods
of crystallisation is not generally so well
marked.

A crystal is, in one respect, like an
organism — it is afiected by its environment.
The crystal modifies its surroundings, and
is in turn modified by them ; there is action
and reaction between it and its environment.
This remarkable property of all crystalline
bodies is well shown by the microscope.
Crystals are constantly found built up of
different layers or zones of material slightly
unlike in their optical characters, and thus
dissimilar in chemical constitution. This is

the so-called zonal structure, and is common in the felspars and
augites — in short, in nearly all minerals which admit of isomor-
phic replacement in their constituents (fig. 806). Its presence in the
case of the augites is often indicated by a difference in colour. This
structure may be experimentally produced by placing an artificial
crystal in a solution of a substance isomorphic with that of the
crystal.

The microscope has I'endered another great service, inasmuch as
it has enabled the petrologist to draw conclusions as to the physical
condition of the fused mass or magrtia at the time crystallisation
commenced. All chemists are aware that when crystals are deposited
from solutions at ordinary temperatures they usually contain small
cavities full of the mother -liquor. Now, the growth of crystals in
igneous rocks is exactly analogous to that in a supersaturated saline
solution. Portions of the fiised mass become entangled, which on
cooling remain in a glassy condition, oi- ' become stony, so as to
produce what may be called glass- or stone-cavities.' ^ When formed

1 This subject is discussed iu Quart. Journ. Geol. Soc. 1885 (Presidential
address).

2 Sorby, Quart. Journ. Geol. Soc. 1858, p. 242.

8 z

Fig. 806. — Augite showing-
zonal structure. (After
Zirkel.)

1074 THE MICEOSCOPE IN GEOLOGICAL INVESTIGATION

under great pressure by the combined influence of liquid water and
fused mineral matter the crystals will contain glass-cavities and also
fluid inclusions.

Glass-inclusions are very abundant in the porphyritic crystals
of volcanic rocks, and represent to some extent the composition of
the fused mass at the period of inclosure. The glass composing the
inclusions is often darker in colour than the glass forming the base
of the rock. This is perhaps due to the presence in the glass of
the inclusions of a greater amount of iron and the bases usually
associated with it. The glass often contains crystallites and micro-
lites, due sometimes to inclosure at the same time, sometimes to a
subsequent crystallising action set up by the glass. Gas bubbles
are also inclosed.

The existence of fluid inclusions in crystals has long been known ;
but not until Dr. Sorby directed his attention to the subject was
their universal distribution in rock-constituents imagined, or their
bearing upon geological problems recognised. They are often very
minute, being frequently less than rpooo'^^ o^ ^'^ i^ch in diameter.
They are rare or absent in rocks of the volcanic group, biit are
especially characteristic of the plutonic rocks, such as granite,
gabbro, diorite, &c. Where glass-inclusions are common, fluid
inclusions are rare or wanting.

The forms of such inclusions vary, but sometimes they are
bounded by planes corresponding to the external faces of the crystals,
in which case they are termed ' negative ' crystals.

Sometimes the fluid inclusions are so numerous in the quartzes

. of the granites as to be, according to Dr. Sorby,^ ' not above the

j^^th of an inch apart. This agrees with the proportion of a

thousand millions to a cubic inch, and in some cases they must be

more than ten times as many.'

An intimate relation usually exists - between the number of
cavities in a crystal and the rate at which it was formed.
Generally speaking, it may be said that the more rapid the growth,
the more numerous the inclusions.

Not infrequently the cavities contain bubbles varying from
j--g-i^_^th to -573-0-0 ir'tli of an inch in size. These bubbles sometimes
possess an apparently spontaneous movement, at other times heat
must be applied to produce a change of position.

According to Dr. Sorby's experiments, the bubbles arise in con-
sequence of the contraction of the liquid on cooling from the higli
temperature at which the cavities were filled.

The nature of the inclosed fluid has been determined with some
accuracy. Generally the liquid is a solution of water charged with
salts ; bvit occasionally it is sufficiently concentrated to cause the de-
position in the cavities of little cubes of salt. The presence has also
been established of liquid carbonic dioxide, the bubble of which dis-
appeared at about 32° C, the critical point for this gas.^

The discovery in the mineral components of plutonic rocks of

• 1 Sorby, Quart. Journ. Geoh Soc. 1858, id. 486.

2 The application of the burning end of a cigar to the section is usually sufficient
to cause the bubble to disappear. . •

INCLUSIONS IN MINERALS lO/S

these fluid inclusions is manifestly of high importance. Daubree's
experiments have shown the enormous mineral-foi'ming powers
possessed by greatly heated water, while the presence of liquid carbonic
dioxide testiiies to the enormous pressure under which pkitonic
rocks, such as granite and diorite, have consolidated.

Inclusions of gaseous matter are also common ; and it is self-
evident that the occurrence of one mineral in another- is no rarity ;
the included mineral being generally the older. To such microscopic
inclusions of crystalline bodies is clue the remai-kable colour of some
minerals. In fact, so numerous and so minute are the inclusions in
some minerals that even with high powers the minerals appear to
be charged with the finest dust. Leucite sometimes affords a good
instance of this (fig. 807). Not unfrequently, as with it, the included
microlites are so arranged as to outline a ci^ystal of the mineral.

The foregoing allows us to conclude that an absolutely pure
mineral is exceptional. All such mineral bodies contain inclosures
of foreign matter which have become entangled during their forma-
tion ; when they contain glass-inclusions they have been precipitated
out of a mass in the condition of igneous fusion. It follows, therefore,
that the presence of amorphous glass, either as
a glassy residue or as glass-inclusions, is a
frequent characteristic of igneous rocks. Still,
the absence of such material does not always
demonstrate a non-igneous origin, for plutonic
rocks, such as granite, do not possess this feature,
having become solid under circumstances which -,-,,,, q,,„ t ■+ t

Y • 1 IT • r> 1 I' IG. 807. — L/Sucite irom

brought about complete crystallisation of the Kilimanjaro, East
materials. Glass-inclusions are certainly re- Africa.
ported by Sigmund ^ to be present in the quartzes

of the granites of the Monte Mulatto, near Predazzo, in South
Tyrol, but V. Ohrustschoff considers them products of contact-
metamorphism .

"We have dealt hitherto more especially with igneous masses, but
the sedimentary rocks demand some attention.

The microscope enables us to recognise to some extent the sources
whence the materials composing clastic ^ rocks were derived. For
instance, the presence of quartzes containing numerous fluid inclusions
(especially those of carbonic dioxide) and hair-like crystals of r utile
leads us to conclude they are derived from granites or similar rocks.
The cemented material can also be studied and its nature determined.
In certain loose sands and sandstones thei'e has sometimes occurred a
curious process which the microscope first bi'ought under notice.
This is the precipitation on the outer surface of rounded quartz-
grains of a greater or less amount of silica, which has been deposited
in crystalline continuity with that of the original nuclei (fig. 808).
The phenomenon is like that which happens wdien an irregular fi'ag-
ment of a crystal is placed in a concentrated solution of the same

1 ' Petrograpliisehe Studien am Granit von Predazzo,' Jahrh. k. k. geol. Beichs-
anstalt, Bd. xxix. 1879, pp. 305-316.

- Greek K:A.a(rTbs = broken. See on this subject T. G. Bonney, Presidential ad-
dress to Section C, Brit. Assoc. Reports (Birmingham), 1886.

3 z 2

1076 THE MICEOSCOPE IN GEOLOGICAL INVESTIGATION

salt slowly evapoi^ating. Restoration of the broken angles first takes
place ; then deposition goes on over the whole exposed surface, in
perfect optical and crystalline continuity, so as to change a broken
fragment into a definite crystal. A similar process frequently takes
place in limestones which are not absokitely pure.^ Sometimes this
secondary deposit is carried so far on the grains of a clean sandstone
that the interstices are completely fiUed up and the rock is converted
into a quai-tzite.

By the microscopical examination of volcanic dust or ashes it is
possible to determine the constitution of the igneous mass whose
eruption gave rise to such material. Thus the ashes and dust which
fell at various places after the great Krakatoa eruption in 1883 were
found to belong to an acid lava, a pyroxene andesite.^

Further, glacial boulders can be satisfactorily identified with rocks

in situ by a microscopical examination of their thin sections. Thus

Norwegian rocks have been shown to occur as boulders in the

Eastern Counties, while Swedish and Finnish

rocks are common in the drift of North

Germany and Saxony.

,We now come to the discussion of the
metamorphism to which all rock-masses are
liable. The m.etamorphism cau^sed by atmo-
spheric agencies results in decomposition and
disintegration. The constituents are, of
course, very differently affected, biit rapidity
of disintegration demands the decomposition
of one of the principal constituents. Such a
^ with°l7h qulf"?^) cfe^- constituent is felspar, which decomposes under
posited on the surface the influence of water charged with cai-bonic
(After Dr. Sorby.) acid into kaolin ; while the products of the

decomposition of non-aluminous minerals are
carbonates, ferric oxide, and quartz. The minute accessory con-
stituents, such as the titanium oxides, are not afiected by these
agencies, and hence are to be found in all clays and sands. ^ At
greater depths from the surface disintegration is replaced by the
formation of new, especially hydrous, minerals. Thus serpentine
is formed from olivine, and sometimes from suitable varieties of
augite or hoi-nl^lende ; chlorite from biotite ; epidote from suitable
minerals, and so on.

Thermal waters charged with various substances are common in
all volcanic districts and play their part in the metamorphosis of
rocks. In this way a volcanic rock may become silicified through
the percolation of such solutions ; and microscopical examination has

1 E. "Wethered, Quart. Journ. Geol. Soc. xlviii. (1892), p. 377.

- See J. Murray and A. Renard on ' Volcanic Ashes and Cosmic Dust ' iia Nature,
1884, vol. xxix. p. 585 ; also J. W. Judd, Krakatoa Eeport, published by the Eoyal
Society.

^ W. M. Hutchins, however, is of opinion that rutile is produced as a
secondary mineral in certain slates, though he would not dispute its occurrence as
stated above {Geol. Mag. 1890, p. 264). A series of paj)ers bearing on the subject
which he has published since that date in the same periodical are all worthy of
careful study.

METAMOEPHISM OF EOCKS IO77

shown that in portions of the Roche Castle I'ock, in Pembrokeshire,
the porphyritic felspars have been replaced by quai'tz. The
tourmaline, gilbei'tite, and other minerals often found at »r near
the junction of granite and sedimentaries (e.g. in parts of Cornwall
and Devon) are probably results of hydrothermal metamorphism, and
in this way many metallic ores may be deposited ; while the conver-
sion of peridotites into serpentines, sandstones into quartzites (not
to mention other instances), are results of the action of water,
probably with some slight increase of pressure and temperature.

The intrusion of an igneous rock generally has an important
influence on the structure and mineralogical composition of the
surrounding mass, portions of which it can include and partially
dissolve (contact-metamorphism). Sections from the junction of an
igneous rock with one of sedimentary origin are highly interesting.
The metamorphism is found to consist largely in the development of
new minerals, such as chiastolite, andalusite, brown and white mica,
garnets, staurolite, etc. ; the first and third of these appear to form most
readily, andalusite after a time replacing chiastolite ; while the last
three require high temperatures. Gradually the original sedi-
mentary structure disappears from a rock affected by contact-
metamorphism, and one truly crystalline is set up, which, howevei",
has characters of its own.^ Limestone becomes ciystalline, fossils
disappearing, and minerals such as wollastonite, idocrase, &c., are
formed from impurities. Occasionally the heat is so intense as to
fuse at least the matrix of sandstones into a brownish glass.

The microscojae has also proved most useful in studying questions
relating to dynamic metamorphism, or that due to ' earth-stresses.'
The deformation by movement has sometimes been so great as to
obliterate, partially or even wholly, the original structure of a rock.'^
The intense pressiires must produce some elevation of tempera-
ture and increase the solvent action of water, so that the original
constituents of the rock are destroyed, partially, if not wholly, and
at a later stage new minerals are produced. It has been shown that
many gneisses and schists (though not all) have been formed by
crushing or shearing from igneous rock, e.g. gneiss from granite,
hornblende schist from dolerite. In the former case, the crushing
of the felspar, the formation of white mica and free quartz from
its dust,^ the effects produced on the other minerals, can all be
studied under the microscope ; and in the latter the conversion
of augite into hornblende. This, however, may be brought about by
more than one cause, and each probably produces effects which can be
distinguished. These questions, however, on which many experienced
petrologists have been engaged for at least fifteen years, are much
too difficult and technical to be discussed in a book of this character ;
enough to say that heat, pressure, and water, singly and conjointly,
produce important changes in rocks, many of which can now be
identified.

1 Bonney, Quart. Journ. Geol. Soc. xliv. (1888), -p. 11.

I Tresca, ' How of Solids,' Pron. Inst. Mech. Enrj. 1878, p. 301.

^ A minute hydrous mica, often called sericite, seems to form readily in an
argillaceous rock under pressure. The silky-looking slates (to which the name
phyllite is restricted by some authors) are largely composed of it.

1078 THE MICROSCOPE IN GEOLOGICAL INVESTIGATIOX

The optical methods now in nse enable the petrologist to determine
the constituents of rock-masses with great success. The colour
of the mineral in transmitted light, the crystallographic ovitlines,
the direction of the cleavage planes, the polarisation tints, the posi-
tion of the axes of elasticity, as also of the optical axes, all these,
with other minor properties, render his determinations of real value.
In certain cases pleochroism is a valuable test ; this is well deve-
loped in such minerals as hornblende, biotite, tourmaline, (fee.

"Very important service has been rendered by the microscope in
the study of the phenomena known as ojDtical anomalies. There
exist a large number of minerals which show in thin sections optical
pi'operties which do not agree with those of the crystal system to
which they belong. Experiment has proved that compression,
strain, or other mechanical distortion, may c^use araorphous bodies,
like glass, and crystals belonging to the regular system to become
double-refracting, and a uniaxial crystal becomes biaxial by the appli-
cation of pressure at right angles to its optical axis.

Mention may well be made here of
the anomalies presented by the mineral
leucite, which is a most important con-
stituent of the lavas of Vesuvius and
the neighbourhood of Rome. It ciystal-
lises apparently in icositetrahedra (fig.
809), and thus to belong to the regular
system it should remain dark under
crossed nicols, that is, be isotropic. The
small crystals certainly behave in this
manner, but the large ones display more
or less double refraction with decided
traces of twin-lamellae (fig. 809). This
anomaly was for a long time inexplicable,
till Klein showed ^ that such crystals
revert when heated to 500° C. to a condition of perfect isotropy,
which property they again lose upon becoming cool. The conclusion
to be drawn from his classical investigation is that the leucite
originally crystallised in the regular system and that its present
optical condition is owing to molecular change due to strains set up
as the temperature falls during and after solidification. It is
worthy of notice that MM. Fouque and Michel Levy have syn-
thetically produced a leucite rock, the leucites of which possessed
the optical anomalies described above.

The relation between optical characters and chemical constitu-
tion has received some degree of attention, and in the case of the
felspar group has been accurately determined. Only the ' quantitative '
jjortion of the subject can be dealt with here, and we must abstain
from the discussion of those minerals whose microscopical appearance
leads the trained petrologist to draw qtialitative conclusions. By
employing convergent light, a slice of a mineral, cut in the right
direction, can be examined and an 'optical picture' obtained.

1 For a description of the so-called ' Erhitzungs-Mikroskop,' see Groth's Physi-
IriMsche Krysiallographie, Leipzig, 1885, p. 631.

Fig. 809. — Leucite sliowing twin
striation under crossed nicols
(After Zirkel.)

OPTICAL EXAMINATION OF MINERALS IO79

Inferences may be di'awn from the pi'esence or absence of this on the
surface of easiest cleavage in a flake. In a slice fi'om a rock the
minerals may be cut in any dii'ection, and are often too small for
proper study ; nevertheless imj)ortant inferences may be drawn from
the shadows seen to sweep over them as the stage is rotated between
crossed nicols.^ Even if only pai-allel rays be used, with the ordinary
apparatus, minerals often may be identified with pi'actical certainty
from their optical characters. Minerals of the regular system, like
colloids, being isotropic,^ produce no effect on the jjolarised rays, and
thus remain dark between crossed nicols. So do all slices cut from
a uniaxial mineral j)erpendicular to the principal axis (that of
symmetry), for they are isotropic to light passing in that direction.
The same projoerty exists in all biaxial minerals in two directions
(called the optic axes). But in passing tlu-ough slices cut in any
other directions from doubly refracting minerals, the polarised ray is
divided into two rays, vibrating in directions perpendicular to each
other and coincident with three lines called the axes of elasticity,
i.e. the directions of greatest, least, and mean elasticity. When the
slice is turned into such a jDosition that two of these correspond
with the vibration planes of the crossed nicols, it becomes dark. If
extinction (of light) occurs parallel with the trace of a pinacoid or
prism face (or with a corresponding cleavage plane) in a section
through the vertical axis, or with the trace of the former in a section
perpendicular to it, this is called ' straight extinction,' but if not, it
is said to be oblique. Thus in a uniaxial crystal every slice cut
parallel with the principal axis gives straight extinction. In the
orthorhombic system, the axes of elasticity correspond with the
crystallographic axes, so minerals belonging to it also extinguish
straight. In the monoclinic system the orthodiagonal axis is an axis
of elasticity, hence the extinction angle is at a maximum in clino-
diagonal sections, and is zero in the zone containing the ortho- and
basal pinacoids. In the triclinic system there is no relation between
the two sets of axes. Of this system, however, oscillatory twinning,
producing alternately banded colours, is a frequent characteristic.
Measurements of the extinction angle are of much value for dis-
tinctive purposes. Thus a rhombic pyroxene can at once be dis-
tinguished from a monoclinic by its straight extinction.^ Again the
maximum extinction angle in a hornblende falls short of 20° ; in an
augite it may exceed 40°. The magnitude of this angle is affected
by changes in the chemical composition of a mineral : for instance, it
is very small in soda-hornblendes, such as glaucophane and riebeckite.
It varies in the felspar group, and is very useful in distinguishing
the several species.'' But as the minerals in a rock-section seldom
chance to lie in the right positions for accurate measurement, better

1 See for a full account of this, with illustratious, F. Fouque and M. Levy, Minera-
logie Micrograjjhique, 1879, pp. 101-3. Also F. 'Rutley, Bock-forming Minerals, p. 84.

^ That is, having the ether equally elastic in all directions.

^ Obviously, more than one observation is needed, because, as intimated above, a
monoclinic mineral, if cut in certain directions, also gives straight extinction.

* Levy, Determination des Felsjjaths (1894) p. 31. Summaries of results will be
found in Rutley, Bock-forming Minerals, pp. 204, 221, and Cole, Aids in Practical
Geology (see ' Felspar' for the references).

I080 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

results are generally obtained by crushing up a small fragment of
the rock itself and mounting a few selected flakes, which can readily
be arranged for examination. Indeed, the study of a little powdered
rock is often valuable as an adjunct to that of a section, and when we
have some special purpose in view, or specimens do not promise to be
interesting, it may even obviate the necessity of cutting slices.

The researches of the late Max Schuster have established the im-
portant fact that in the normal plagioclase felspars, which may be
considered as isoniorphous mixtures of albite (]S'a2(Al2)SigOjg) and
anorthite (Ca(Al2)Si208), the optical and chemical characters stand
in the closest possible relations to each other. Hence, given the
extinction angle on a known surface, the chemical constitution is
known and, roughly speaking, the specific gravity.

Another optical test of importance is the refractive index of a
mineral. The methods of measuring this are described in most of
the larger text-books, but much — often enough for all practical pur-
poses— can be done in a rough and ready way. For instance,
minerals with a high refractive index, such as diamond, garnet,
zircon, appear to stand out conspicuously on the slide. Wlien they
occur in sand or the powder of a rock this is even more marked, and
internal reflection due to the large critical angle gives to the grain
a strong dark outline. Again, if a mineral with a high refractive
index be in apposition (as in a slice from a rock) with another having
a lower one, or with Canada balsam, and a quarter-inch objective be
used (with a plane reflector) and focussed on the top of the first
mineral, a thin bright line is seen just within its edge ; but when
the focus is changed to the bottom, this appears without the edge.

The importance of pleochroism has been already mentioned. It
is not seen in colotirless minerals, or in slices so cut as to be isotropic
in the plane at right angles to the path of the transraitted beam.
In augite it is generally weak, though visible in some green
varieties ; but in hornblende strong, especially in certain varieties.
Glaucophane exhibits a violet blue and a reddish purple ; riebeckite
turns almost black ; biotite, chlorite, amblystegite, and tourmaline
show it well, biit in iolite it can be seen only in thick slices. The
student should note the results as the polarised beam vibrates parallel
with each axis of elasticity ; these facts, however, as a rule, are more
important to the petrographer than to the petrologist, and the latter
will not find it worth his while to spend time in determining them.

The polarisation tints of a mineral, i.e. those seen with crossed
nicols, depend to some extent on the thickness of the slices, as has
been already stated, biit they are often variable even in the same mineral .
Hence, though, as a rule, the student will find each species gives a
certain group of tints in the order of the chromatic scale, he must
be prepared for abnormalities. For instance, quartz, when it occurs
in a granite, usually gives high tints, but in a trachyte they are
rather low. At first the student must be cautious in drawing
inferences from polarisation tints, but after a certain amount of
practice he may do this with more confidence, though he will rely more
on the ' quality ' than on the ' quantity ' of the colour. For in-
stance, though both augite and olivine usually afibrd rich colours, an

EXAMINATION OF MINERALS IO81

experienced eye can genei-ally tell the diflei-ence, foi- the latter
appears more diaphanous than the former. In petrology, as in
medicine, a cautious empiricism, which signifies experience concen-
trated and regulated by common sense, is sometimes even more
valuable than any amount of printed rules.

On this account the student may be glad to have a few general
directions as to the best method of studying a rock slice. First,
look at it with a rather strong pocket lens, especially if it be crystal-
line or fragmental, so as to get a good idea of its general structure,
which is sometimes less easily seen under the microscope, because the
field of view at any one time is small, and high magnification may
make it ' hard to see the wood for the trees.' Then place it on the
stage and examine first with transmitted, next with reflected light.
The former shows what minerals are colourless, and the natural
tints of the coloured, bringing out well slight differences of structure,
especially any due to incipient decomposition.^ The latter enables
him to distinguish the opaque minerals, e.g. pyrite from magnetite,
sometimes the latter from other iron oxides ; to identify native iron,
awaruite, and gold ; perhaps also graphite, but it is better to verify
the last by powdering a little of the rock, when the streak is easily ob-
tained. Sometimes we are helped in distinguishing even transparent
minerals by the different way in which they i-eflect light. Next,
put on the polariser and examine pleochroism ; and lastly, insei-t the
analyser, for the general study of the tints produced and especially
of the extinction angles of certain of the minerals. When a mineral
gives very low polarisation tints, especially in the case of certain
aggregates, or we are searching for a glassy base in a slice crowded
with microliths, we may be helped by inserting a selenite or quartz
plate (better just below the slide) to obtain a coloured field, ^ for the
eye can be more sure of a difference of tint than of a very faint
glimmer of light.

In dealing with rocks apparently clastic we have to determine
whether the stru cture is original, or has been superinduced (by crushing
or shearing) ; also what amount of mineral change has subsequently
occurred, and of what this is significant — investigations which,
though of the highest interest, are often by no means easy, so that
the most experienced worker may occasionally be baffled. One final
piece of advice : before adopting a conclusion, look at it all round,
to see how it fits in with previously acquired knowledge and the
probabilities in the particular case.

The micro-spectroscope has not at present been so much used
by petrologists as it might have been. It has been employed
by Professor Orville Derby in the determination of the pre-
sence of monazite in Brazilian sands. ^ This mineral contains a
large percentage of didymium, and accordingly gives the bands

1 Holes in the slice and bubbles in the balsam, which often perplex beginners,
are now most readily detected. Also a mineral of easy cleavage is sometimes slightly
ruptured in the grinding, producing diffraction tints (as in calcite). These, between
crossed nicols, might be mistaken for oscillatory twinning ; but at the present stage
their true nature is obvious.

2 This method can also be used to enhance a weak pleochroism.
^ American Journal of Science, vol. xxxvii. 1889, p. 109.

I082 THE MICROSCOPE IN GEOLOGICAL INVESTIGATIOX

characteristic of that element. Tlie test afforded by studpng the
colour of the flame when a small fragment is acted npon by the blow-
pipe is often valuable — but this, of course, hardly forms part of
microscopy.^

The discover)^ of the presence of foreign inclusions in all minerals
has led to a i-emarkable revolution in mineral-chemistry. In earlier
days it was customary to analyse a mineral without questioning its
purity. Hence the early analyses and the formulae developed there-
from express the actual constitution j^^us the inclusions. Methods
have now been invented by which the foreign matter can be removed.
Advantage is taken of the difference that is usual between the
specific gravity of the minei-al and that of its inclusions, the so-called
'heavy solutions' being employed for the separation.^ Most satis-
factoiy results have been obtained by such means. In cases where
the greatest accuracy is necessary, the apparatus designed by Dr.
P. Mann had better be employed.^ It is well microscopically to
examine the isolated substance before executing the analysis, for
the optical test with polarised light is so sensitive as to detect the
smallest impurities. Also, in the case of ordinary bulk analyses of
rocks, it is advisable to follow the same course, as by doing so one
is often enabled to make a qualitative analysis with the microscope
alone.

A valuable adjiinct to petrology is to be found in micro-
chemistry.* Instances sometimes occur where a mineral cannot
be satisfactorily determined by its optical characters, and in such
cases micro-chemical methods are resorted to. Let us suppose it is
desirable to see whether any of the rock-components are silicates
containing soda and soluble in acids. The cover-glass is accordingly
removed and the balsam dissolved in alcohol. A weak solution of
hydrochloric acid is then poured over the surface, when, if soluble
silicates are present, gelatinisation will take place. Upon allowing
the gelatinous mass to evajoorate little squares of salt will form if
such a silicate is present. Sometimes colouring substances may be
used for the same purpose. By the treatment of a slide with nitric
acid a silicate like nepheline becomes porous and j)ermeable to
anilin blue, fuchsin, ifec. In the case of nej)heline the colouring
matter cannot be washed out, and hence ' staining ' proves a delicate
test.

Where such a course is possible, minute pieces of the question-
able minerals should be isolated and treated singly. There are two

1 It was suggested by Professor Szabu and is well described in G. A. J. Cole, Aids
in Practical Geology, Part ii. ch. viii.

^ For their iiiode of prejjaration see Rosenbusch, Mikroskoinsche Physiograpliie,
p. 206 et seq. (English edition by Iddings.)

5 Neues Jahrbioch fur Mineralogie, &c. Bd. ii. 1884, p. 172.

* The following works can be consulted on this subject:— E. Boricky, Elemente
einerneuenchemiscJi-mikroskopisclien Mineral- unci Gesteinsanalyse , Prague, 1877;
T. H. Behrens, Mikrochemische Methoden zur Mineralanalyse, Amsterdam, 1881 ;
Haushofer, MikroskopiscJie Beactionen, Braunschweig, 1885 ; Klement et Eenard,
Reactions microcliimiques d cristaux, etc., Bruxelles, 1886 ; Eosenbusch, Mikro-
skopiscJie Physiograplne, vol. i. 1885, pp. 195-238 (English edition by Iddings);
F. Eutley, Hock-forming Minerals, London, 1888. A useful summary of a number
of microchemical investigations is given by C. A. McMahon, Mineralog. Magazine,
vol. X. p. 79.

PALAEONTOLOGY IO83

methods in use for testing such particles micro-chemically. The
first is that proposed by Boricky, who employed pure hydro-fluo-
silicic acid (HaSiFg), which attacks almost all rock-forming minerals.
The mineral particle is j^laced upon a glass object-holder j)i"otected
from the action of the acid by a covering of Canada balsam, and the
acid allowed to attack the mineral. After evaporation an examina-
tion under the microscope reveals the presence of delicate ci-ystals
of the sili co-fluorides of the metals present in the mineral. The
nature of the crystals may then be determined mici-oscopically.

The second method is that proposed by Behi-ens, and mostly
follows the usual method of chemical analysis. The isolated particle
is heated in a small platinum crucible with ammonium fluoride, the
mass then evaporated with sulphuric acid and dissolved in hot water.
A small quantity of the solution is then evaporated and examined.
If calcium is present in the mineral small crystals of gypsum will
form. Other quantities are treated with the ordinary reagents.
The crystalline products, which are the result, can be identified by
optical methods. It is possible by Behrens's tests to detect the
presence of O'OOOS mgr. CaO in a grain.

In all cases it is advisable to protect the objective during the
microscopical examination with a thin sheet of white mica.

The microscope has always played an important part in the
science of Palaeontology. The great work on ' Micro-geology,'
published in 1855 by Professor Ehrenberg, testifies to the influence
it had, even at that period, upon research of this nature.

The result of the microscopic examination of lignite or fossilised
wood and of ordinary coal is a good example of the value of the
instrument in this interesting department. Specimens of fossil
wood in a state of more or less complete preservation are found in
numerous strata of very different ages. Genei-ally speaking, it is
only when the wood is found to have been penetrated by silica that
its organic structure is well preserved ; but instances occur eveiy now
and then in which penetration by carbonate of lime has proved equally
favourable. In either case transparent sections are needed for the
full display of the organisation. Occasionally, however, it has hap-
pened that the infiltration has filled the cavities of the cells and
vessels, without consolidating their walls ; and as the latter have
undergone decay without being replaced by any cementing material,
the lignite, thus composed of the internal ' casts ' of the woody tissues,
is very friable, its fibres separating from each other like those of
asbestos ; and laminae split asunder with a knife, or isolated fibres
separated by rubbing down between the fingers, exhibit the characters
of the woody structure extremely well when mounted in Canada
balsam. Generally speaking, the lignites of the Tertiary strata
present a tolerably close resemblance to the woods of the existing
period : thus the ordinary structure of dicotyledonous and monocotyle-
donous stems may be discovered in such lignites in the utmost
perfection ; and the ]Deculiar modification presented by coniferous
wood is also most distinctly exhibited. As we go back, however,
through the strata to the Secondary period, we more and more rai'ely
meet with the ordinary dicotyledonous structure ; and the lignites of

1084 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

the earliest deposits of these series are, ahuost universally, either
gymnosperms ^ or palms.

Descending into the palaeozoic series, we are presented in the
vast coal formations of our own and other countries with an extra-
ordinary proof of the prevalence of a most luxuriant vegetation in a
comparatively early period of the world's history. The determina-
tion of the characters of the Ferns, Sigillarice, Lepidodendra, Cala-
mites, and other kinds of vegetation whose forms are preserved in
the shales or sandstones that are interposed between the strata of
coal, has been hitherto chiefly based on their external characters ;
since it is seldom that these specimens present any such traces
of minute internal structure as can be subjected to microscopic
elucidation. But persevering search has brought to light numerous
examples of coal-plants whose internal structure is sufficiently well
preserved to allow of its being studied microscopically ; and the
careful researches of Professor W. 0. Williamson have shown that
they formed a series of connecting links between Cryptogamia
and flowering plants, being obviously allied to Equiseiacece, Lyco-
podiacece, &c., in the character of their fructification, whilst their
stem-structure foreshadowed both the ' endogenous ' and ' exogenous '
types of the latter.^ Notwithstanding the general absence of any
definite form in the masses of decomposed vegetable matter of which
coal itself consists, the traces of structure revealed by the microscope
are often sufficient — especially in the ordinary ' bituminous ' coal —
not only to determine its vegetable origin, but in some cases to
justify the botanist in assigning the character of the vegetation
from which it must have been derived ; and even where the stems
and leaves are represented by nothing else than a structureless mass
of black carbonaceous matter, there are found diffused through this
a multitude of minute resinoid yellowish-brown granules, which are
sometimes aggregated in clusters and inclosed in sacculi ; and these
may now be pretty certainly affirmed to represent the spores, while
the sacculi represent the sporangia, of gigantic Lycopodiacece, of the
Carboniferous flora. ^

Lime-secreting algae are now known to have often played an
important part in the formation of calcareous rocks. Those
organisms called coccoliths and rhabdoliths, which though so
minute are important constituents in chalk and some other lime-
stones, are referred to these plants (? to the class Floridece), and a
tiny tubular organism named Girvanella which occurs in various
palaeozoic and later limestones is now generally regarded as an
alga. According to Mr. E. Wethered * it plays an important jDart
in the formation of pisolitic and oolitic grains. Moreover
calcareous algae, such as Lithothamnion, are sometimes important
constituents in Tertiary limestones, as for instance in the Leitha-

1 Under this head are included the Cijcadece, along with the ordinary Goniferce,
or pine and iir tribe.

^ See his memoirs on the coal-plants published in the volumes of the Phil. Trans.,
which are now being continued by Dr. D. H. Scott.

•^ For notes upon methods to be emx^loyed in making preparations of coal, see
Rutley, Study of Bocks, 1884, p. 71.

-i Qiuirt. Journ. Geol. Sac. xlvi. (1890), p. 270, xlviii. p. 377, xHx. p. 236.

MINUTE OEGANISMS AS EOCK-MAKERS I085

kalk of Europe. They have also been identified in rocks of iSecondary
and even of Palfeozoic age. It is an admitted rule in geological
science that the past history of the eai-th is to be interpreted, so far
as may be found possible, by the study of the changes which are
still going on. Thus, when we meet with an extensive stratum of
fossilised Diatomacece in what is now di-y land, we can entertain no
doubt that this silicious deposit originally accumulated either at the
bottom of a fresh-watei" lake oi- beneath the waters of the ocean ;
just as such deposits are formed at the present time by the produc-
tion and death of successive generations of these bodies, whose
indestructible casings accumulate in the lajDse of ages, so as to form
layers whose thickness is only limited by the time during which
this process has been in action. In like manner, when we meet
with a limestone rock entirely composed of the calcareous shells of
Foraminifera, some of them entire, othei'S broken up into minute
particles (as in the case of the Fusulina limestone of the Carboni-
ferous period, and the Nummulitic limestone of the Eocene), we
interpret the phenomenon by the fact that the dredgings obtained
from certain parts of the ocean-bottom consist almost entirely of
I'emains of existing Foraminifera, in which entire shells, the animals
of which may be yet alive, are mingled with the debris of others
that have been reduced to a fragmentary state. Such a deposit,
consisting chiefly of Orbitolites, is at present in process of formation
on certain pai'ts of the shores of Australia, as Dr. Carpenter was
informed by Mr. J. Beete Jukes, thus affording the exact pai'allel to
the stratum of Orbitolites (belonging, as his own investigations have
led him to believe, to the very same species) that forms part of the
' calcaire grossier ' of the Paris basin. So in the fine white mud
which is brought up from almost every part of the sea-bottom of the
Levant, where it forms a stratum that is continually undergoing a
slow but steady increase in thickness, the microscopic researches of
Professor W. C. Williamson ^ have shown, not only that it contains
multitudes of minute remains of living organisms, both animal and
vegetable, but that it is entirely or almost wholly composed of such
remains. Amongst these are about twenty-six species of Dia-
tomacese (silicious), eight species of Foraminifera (calcareous), and a
miscellaneous group of objects (fig. 810), consisting of calcareous and
silicious spicules of sponges and Gorgonice, and fragments of the
calcareous skeletons of echinoderms and molluscs. A collection of
forms strongly resembling that of the Levant mud, with the exception
of the silicious Diatomacete, is found in many parts of the ' calcaire
grossier ' of the Paris basin, as well as in other extensive deposits of
the same early Tertiary period.

It is, however, in regard to the great chalk formation that the
information afforded by the microscope has been most valuable.
Mention has already been made of the fact that a large proportion
of the North Atlantic sea-bed has been found to be covered with an
' ooze ' chiefly formed of the shells of Qlobigerince ; and this fact, first
determined by the examination of the small quantities bi'ought up
by the sounding apparatus, has been fully confirmed by the results of

1 Memoirs of the Mancliester Literarij and PliilosopMcal Society, vol. vii.

I086 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

the more recent explorations of the deep-sea with the dredge ; which,
bringing up half a ton of this deposit at once, has shown that it is
not a mere surface-film, but an enormous mass Avhose thickness cannot
be even guessed at. ' Under the microscope,' says Professor Wp^ille
Thomson ^ of a sample of 1^ cwt. obtained by the dredge from a depth
of nearly three miles, ' the surface-layer was found to consist chiefly

Fig. 810. — Microscopic organisms in Levant mud: A, C, D, silicious
spicules of Tethya ; B, H, spicules of Geodia ; E, calcareous spicule of
Grantia ; F, G, M, O, portions of calcareous skeleton of Echinodermata ;
I, calcareous spicule of Gorgonia ; K, L, N, silicious spicules of sponges ;
P, portion of prismatic layer of shell of Pinna.

of entire shells of Glohigerina buMoides, large and small, and of frag-
ments of such .shells mixed with a quantity of amorphous calcareous
matter in fine particles, a little fine sand, and many spicules, portions
of spicules, and .shells of Radiolaria., a few spicules of sponges, and a
few frustules of diatoms. Below the surface-layer the sediment be-
comes gradually more compact, and a slight grey colour, due probably

1 The Depths of the Sea, p. 410. See also Voyage of Challenger, ch. iii., and
Challenger Beports, especially Deep Sea. Deposits (Murray and Renard.)

MINUTE OKGANISMS AS EOCK-MAKERS

1087

to the decomposing oi-ganic matter, becomes more pi'onounced, while
perfect shells of Glohigerina almost disappear, fragments become
smaller, and calcareous mud, stiaictui'eless, and in a fine state of
division, is in greatly preponderating proportion. One can have no
doubt, on examining this sediment, that it is formed in the main by
the accumulation and disintegration of the shells of Glohigerina ; the
shells fresh, whole, and living in the surface-layer of the deposit ;
and in the lower layers dead, and gradually crumbling down by the
decomposition of their organic cement, and by the pressure of the
layers above.' This white calcareous mud also contains in large
amount the ' coccoliths ' and ' coccospheres ' formerly mentioned.
jS'ow the resemblance which this Globigerlna-raud, when di-ied, bears

Fig. 811. — Microscopic organisms in chalk from Gravesend : a, b, c, d,
Textidaria glohulosa; e, e, e, Botalia aspera; f, Textularia aculeata;
g, Planularia liexas; h, Navicula.

to chalk is so close as at once to suggest the similar origin of the
latter ; and this is fully confirmed by microscopic examination. For
many samples of it consist in great part of the minuter kinds of
Foraminifera, especially Globigerince, whose shells ai'e imbedded in a
mass of apparently amorphous particles, many of which, nevertheless,
present indications of being the disintegrated fi'agments of similar
shells, or of lai'ger calcareous organisms. In the chalk of some
localities the disintegrated prisms of Pinna, or of other large shells
of the like sti-uctui-e (as Inoceramus), form the gi-eat bulk of the
recognisable components ; whilst in other cases, again, the chief part
is made up of the shells of Cytherina, a marine form of entomo-
sti-acous crustacean. Different specimens of chalk vary greatly in

I088 THE MICEOSCOPE IX GEOLOGICAL IN\rESTIGATION

the proportion which not onl}' the distinctly oi-ganic remains bear to
the amorphous residuum, but also the diflferent kinds of the former
bear to each other ; and this is quite what might be anticipated when
we remember how one or another tribe of animals predominates
in the several parts of a large area ; but it may be fairly concluded,
from what has been already stated of the amorphous component of
the Globigerina-xnxid, that the amorphous constituent of chalk like-
wise is the disintegrated residuum of foraminiferal shells, or at any
rate of some small calcai-eoiis organism. But, fm-ther, the Glohigerina:
mud now in process of formation is in some places literally crowded
with sponges having a complete silicious skeleton ; and some of them
bear such an exti-aordinarily close resemblance, alike in structure

Fig. 812. — MicroscoiJic organisms (chi&^y for aminif era) in chalk from Meudon,
seen partly as opaque, and partly as transparent objects.

and in external form, to the Ventriculites which are well known as
chalk fossils, as to leave no i-easonable doubt that these also were
silicious sponges living on the bottom of the ci'etaceous sea. Finally
(as was first pointed out by Dr. Soi-by) the coccoliths and cocco-
spheres at present found on the sea-bottom ai-e often to be discovered
by the m.icroscopic examination of chalk. ^ All these correspondences
show that the formation of chalk took place under conditions
essentially similar to those under which the deposit of Glohigerina-
mud is being formed over the Atlantic sea-bed at the present time.
In examining chalk or other similai- mixed aggi-egations, whose

^ 'On the Organic Origin of the so-called " Crystalloids" of Chalk' in Ann. Nat.
Hist. ser. iii. vol. viii. 1861, pj). 193-200. Murray and Renard, Deep Sea Deposits
[Challenger Eeports), p. 257.

. CHALK, FLINT, AND OHEKT I089

■component particles are easily separable from each other, it is de-
.sii-able to sepai-ate, with as little trouble as jtossible, the lai-ger and
more definitely organised bodies from the minnteamoi'phous particles ;
^nd the mode of doing this will depend upon whether we ai-e operat-
ing upon the laige or upon the small scale. If the foimer, a quantity
•of soft chalk should be i-ubbed to powdei- with water by means of a
soft brush ; and this water should then be proceeded with according
to the method of levigation already directed for separating the
Diatoniacece. It will usually be found that the first deposits contain
the larger Foraminifera, fragments of shell, etc., and that the smaller
Foraminifera and sponge-spicules fall next, the fine amorphous pai--
ticles i-emaining dilfused through the water aftei- it has been standing
for some time, so that they may be poured away. The organisms
thus separated should be dried and mounted in Canada balsam. If
the smaller scale of prejDaration be preferi'ed, as much chalk scraped
fine as will lie on the point of a knife is to be laid on a drop of water
■on the glass slide, and allowed to remain there for a few seconds ;
-the water, with any particles still floating on it, should then be re-
moved ; and the sediment left on the glass should be dried and
mounted in balsam. For examining the sti'uctui'e of flints such
•chips as may be obtained with a hammer will commonly serve very
Avell, a clear translucent flint being first selected, and the chips that
are obtained being soaked for a short time in turpentine (which in-
creases their transparence) ; those which show organic structure,
whether sponge-tissue or xanthidia, are to be selected and mounted
in Canada balsam. The most pei-fect specimens of sponge-structure,
liowever, are only to be obtained by slicing and polishing.

The study of thin slices of fiint and chert during late years
has thrown much light on their origin and on the structm^e of
fi3ssil sponges. >Spicules are often found to be extremely abundant
as in the chert (Upper Greensand) from the quarry by Ventnor
.station (Isle of Wight), where they can be detected by the naked eye.
The radiolaria from the Tertiary mail of Bai'badoes have long been
known to mici'oscopists, but these organisms moi'C recently have been
■detected in cherts. In Britain such cherts have been described
from the Ordovician rocks of Mullion Island, Cornwall, and of south
tScotland, and the Carboniferous of south-west England. ^

There are vai-ious other deposits, of less extent and importance
than the great chalk-formation, which are, like it, composed in great
part of microscopic organisms, chiefly minute Foraminifera ; ^ and the
presence of these may be largely recognised, by the assistance of the
microscope, in sections of calcai-eous rocks of various dates, whose
other materials were fragments of corals, ci-inoid-stems, or the shells
of molluscs. In the formation of the Coralline Crag (Tei'tiaiy) of the
eastei-n coast of England, polyzoaries had the greatest share ; but

1 On the former subject see G. J. Hinde, British Museum Catalogue of Fossil
Sponges; on the latter, the same, Quart. Journ. Geol. Soc. vols. xlvi. xlix. li.

^ For illustrations of fossil foraminifera, see Carpenter, Introduction to Study of
Foraminifera (Ray Society 1. and the publications of the Palajontographical
Society; Crag Foraniiiiifera (T. 'Rxi])ert Jones, &c.); Carboniferous and Permian
Foraminifera (H. B. Brady). The series also contains voluires upon the Crag
Polyzoa and various small Entomostraca of different ages.

4a

I090 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

tlie Tertiary limestone of which Paris is chiefly built cousLsts almost
exclusively of the shells of Miliolida, and is thus known as miliolite
(nrillet-seed) limestone. In the vast stratum of nummulitic lime-
stone which was formed in the earlier part of the Tertiary period
the m.icroscope enables us to see that the matrix in which the large
entire nummulites are imbedded is itself composed of comminuted
fragments and young shells of the same, together with minuter
Foraminifera. Similar organisms, with fragments of crinoids,
mollusca, coral, &c., are abundantly present in the Jvn-assic limie-
stones in this country, in those of Secondary age generally in
Europe, as well as in the Carboniferous and other Palaeozoic lime-
stones ; in fact, wherever subsequent changes have not rendered the
structure of the original constituents indistinguishable. Thus in
the great plains of Kussia there are certain bands of limestone of
this epoch, varying in thickness from fifteen inches to five feet, and
frequently repeated through a vertical depth of two hundred feet
over very wide areas, which are almost entirely composed of the
extinct genus Fusulma. Again, those pai'ts of the Carboniferous
limestone of Ireland which have undergone least disturbance can be
plainly shown, by the examination of microscopic sections, to consist
of the remains of Foraminifera, Polyzoa, fragments of corals, ifcc.
And where, as not unfrequently happens, beds of this limestone are
separated by clay seams, these are found to be loaded with ' microzoa '
of various kinds, particularly Foraminifera (of which the Saccamina
has come down to the present time), and the beautiful polyzoaries
known as ' lace-corals.'

Mention has been already made of Professor Ehrenberg's very
remarkable discovery that a large proportion (to say the least) of the
green sands which present themselves in various stratified deposits,
from the Silurian period to the Tertiary era, and in that called
the Upper Greensand, is composed of the casts of the interior of
minute shells of Foraminifera and Mollusca, the shells themselves
having entirely disappeai-ed. The mineral material of these casts
has not merely filled the chambei-s and their communicating
passages, but has also penetrated, even to its minutest ramifications,
the canal-system of the intermediate skeleton. The precise parallel
to these deposits presents itself in certain spots of the existing sea-
bottom, such as the Agulhas bank, near the Cape of Good Hope,
where the dredge comes up laden with a green sand, which on
microscopic examination proves to consist almost entirely of
'internal casts' of existing Foraminifera. ^

It is, however, in the case of the teeth, the bones, and the dermal
skeleton of vertebrate animals that the value of microscopic inquiry
becomes most apparent ; since their structure presents so many
characteristics which are subject to well-marked variations in their
several classes, orders, and families that a knowledge of these
characters frequently enables the microscopist to determine the

1 See Challenger Beiwrts ; Deep Sea Deposits (Murray and Renard), p. 378,
&c. The same volume describes and figures the microscopic structure of remarkable
manganese concretions, dredged at great depths in the ocean, and often associated
with organisms.

DETEKMINATION OF FOSSIL TEETH AND BONES I09I

nature of even the most fragmentary specimens. It was in regai'd
to teeth that the possibility of such determinations was first made
clear by the laborious researches of Professor Owen ; ^ and the
following may be given as examj)les of their value : — A rock-
formation extends over many parts of Russia whose mineral
characters might justify its being likened either to the Old or to the
New Red Sandstone of this country, and whose position relatively to
other strata is such that thei-e is great difficulty in obtaining
evidence from the usual sources as to its place in the series. Hence
the only hope of settling this question (which was one of great
practical importance, since, if the formation were Npav Red, coal
might be expected to underlie it, whilst if Old Red, no reasonable
hope of coal could be entertained) lay in the determination of the
organic remains which this stratum might yield ; but unfortunately
these were few and fragmentary, consisting chiefly of teeth, which
are seldom perfectly pre-
served. From the gigan-
tic size of these teeth,
together wif.h their form,
it was at first inferred
that they belonged to sau.-
rian reptiles, in which
case the sandstone would
have been considered as
New Red ; but micro-
scopic examination of
their intimate structure
unmistakably proved

them to belong to a
genus of fishes {Dendro-
dus) which is exclusively
l^ala^ozoic, and thus de-
cided that the formation
must be Old Red. kSo,
again, the microscopic

examination of certain fragments of teeth found in a sandstone of
Warwickshire disclosed a most remarkable type of tooth-structure
(shown in fig. 813), which was also ascertained to exist in certain teeth
that had been discovered in the ' Keupersandstein ' of Wiirtemberg ;
and the identity or close resemblance of the animals to which these
teeth belonged having been thus established, it became almost
certain that the Warwickshire and Wiirtemberg sandstones were
equivalent formations. The next question arising out of this discovery
was the nature of the animal (provisionally termed Lahyrinthodon,
a name expressive of the most peculiar feature in its dental structure)
to which these teeth belonged. They had been referred, from external
characters merely, to the order of saurian reptiles ; but it is now
clear that they were gigantic salamandroid Amphibia, having many
points of relationship to Ceratodics (the Australian 'mud-fish'),
which shows a similar, though simpler, dental organisation.

1 See his Odontography.

4 A 2

Fig. 813. — Section of tooth of LabyrintJiodon.

1092 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

The researches of Professoi^ Quekett on the minute structure of
bone ^ have shown that from the average size and form of the hxcun*,
their disposition in regai-d to each othei- and to the Haversian
canals, and the number and course of the canaliculi, the nature of
even a minute fragment of bone may often be determined with a
considerable approach to certainty, as in the following examples,
among many which might be cited : — Dr. Falconer, the distinguished
investigator of the fossil remains of the Himalayan region, and the
discoverer of the gigantic fossil tortoise of the Sivalik hills, having
met with certain small bones about which he was doubtful, placed
them for minute examination in the hands of Professoi- Quekett,
who informed him, on microscopic evidence, that they might certainly
be pronounced i-ej)tilian, and j)robably belonged to an animal of the
toi'toise tribe ; and this determination was fully borne out by other
evidence, which led Dr. Falconer to conclude that they were toe-
bones of his great tortoise. Some fragments of bone were found,
many years since, in a chalk-pit, which were considered by Professor
Owen to have formed part of the wing-bones of a long-winged sea-
bird allied to the albatross. This determination, founded solely on
considerations derived from the very imperfectly preserved external
forms of these fragments, was called in question by sorae other
palfeontologists, who thought it more probable that these bones
belonged to a large species of the extinct genus Pterodactylus, a flying-
lizard whose wing was extended upon a single immensely prolonged
digit. No species of pterodactyle, however, at all comparable to
this in dimensions, was at that time known ; and the chai'acters
furnished by the configuration of the bones not being in any degree
decisive, the question would have long remained unsettled had not
an appeal been made to the microscopic test. This appeal was so
decisive, by showing that the minute structure of the bone in ques-
tion corresponded exactly with that of pterodactyle bone, and differed
essentially fi'om that of every known bii'd, that no one who placed
much reliance upon that evidence could entertain the slightest doubt
o\\ the matter. By Professor Owen, however, the validity of that
determination was questioned, and the bone was still maintained to
be that of a bird, until the question was finally set at rest, and the
vakie of the microscopic test triumphantly confirmed, by the discovery
of undoubted pterodactyle bones of corresponding and even of greatei-
dimensions in the same and other chalk quarries.

The microscopic examination of the sediments now in course of
deposition on various pai-ts of the great oceanic ai-ea, and especiall}*
of the large number of samples brought up in the ' Challenger ' sound-
ings, has led to this very remarkable conclusion — that the detritus
resulting fi-om the degradation of continental .land -masses is not
cai-ried fai- from their shores, being entii'ely absent from the bottom
of the ocean-basins. The sediments there found Avere not of
organic origin, but mainly consist of volcanic debris and of clay that
seems to have been produced by the disintegration of masses of very

1 See his memoir on the ' Comparative Structure of Bone ' iuthe Trans. Microsc.
Sac. ser. i. vol. ii. ; and the Catalogue of the Histological Museum of the Boij. Coll.
of Surgeons, vol. ii.

ORIGIN OF OCEANIC AREAS IO93

vesicular lava, which, aftei* long floating and dispersion by surface-
drift oi' ocean-currents, have become water-logged and have sunk to
the bottom. As no ordinary silicious sand is found anywhere save
in the neighbourhood of continents and continental islands, and as
almost all oceanic islands are either of volcanic origin oi- coral atolls,
this almost univei'sal absence of any ti-ace of submerged continental
land over the great oceanic area affords strong confii-mation to the
belief that the sedimentary rocks which form the existing land were
deposited in the neighbourhood of ^^re-existing land, whose degrada-
tion fui-nished their matei-ials ; and suggests that the original
disposition of the great continental and oceanic areas was not very
different from what it uoav is.^ Further, the microscopic examination
of these oceanic sediments reveals the presence of extremely minute
particles, which seem to correspond in composition to meteorites, and
which there is strong reason for regarding as ' cosmic dust ' pervading
the interplanetary spaces. Thus the application of the microscope
to the study of these deposits brings us in contact with the greatest
questions not only of terrestrial, but also of cosmical physics, and
furnishes evidence of the highest value for their solution.

1 See Sir A. Geikie on ' Geographical Evolution,' Proc. Boy. Geog. Soc. July 1879 ;
and for detailed results 'Preliminary Report of Cruise of "Challenger"' (Wyville
Thomson), P7-oc. Boy. Soc. vol. xxiv. (1876) p. 463, aiid ' Challenger ' Bejwrfs (Mvirray
and Eenard), Deep Sea Deposits, p. 327.

I094

CHAPTER XXIY

MICBOCBYSTALLISATION. OPTICAL PBOFEETIES OF CRYSTALS.
MOLECULAB COALESCENCE. MICBO-CHEMICAL ANALYSIS.

Although by fai^ the most numerous and most important applica-
tions of the microscope were formerly those by which the structure
and actions of organised beings are made known to us, yet the in-
creased attention which has been paid during i-ecent years to the
use of the microscope in elucidating the internal structure of
crystalline substances, whethei' of natural or artificial origin, has
made this instrument as indispensable to the crystallographer and
the mineralogist as it formerly was to the physiologist. Solid sub-
stances are almost invariably found in nature or obtained as labora-
tory products in the form of individual fragments, each bounded by
plane surfaces which are inclined at such angles that the whole
figure is possessed of a greater or lesser degree of geometrical
symmetry. Such solid bodies are termed crystals, and, although
formerly the regularity of external shape constituted the only avail-
able means of recognising them, it is now demonstrated that the
external form is only the result of the so-called homogeneous
intei-nal structure of the crystal. This homogeneity of structure
consists m the arrangement of the smallest characteristic particles
or units of the structure being the same about every unit of the
structure. The different kinds of possible homogeneous arrange-
ments of points in space have been investigated by Bravais, Sohncke,
and others, '^ and on classifying them according to their symmetry
they fall into thirty-two classes identical with the thirty-two known
crystalline systems. These thirty-two types of structure differ in
their symmetry, and this difference is expressed in the symmetry of
the external form ; the external form, however, is very liable to
distortion, in consequence of a lack of uniformity in the conditions
23revailing during the gi-owth of the crystal, and so is at best but an
untrustworthy guide to the symmetry of the internal structure. The
optical properties of the solid structure, also themselves expressions
of the symmetry, and consequently of the crystalline system, are
not disturbed by casual influences to nearly so great an extent as is
the regular external form ; the symmetrical variation of the optical
properties of crystalline structui-es in accordance with the symmetiy

^ See A. Sclioenflies, Krijst alls ijst erne unci Erystallstructur, Leipzig, 1891.

FORMATION OF CRYSTALS

1095

of arrangement of the stvuctui-al units gives vise to the phenomena
of double refraction, circuhii- polarisation, pleochroism, &c., observed
with crystalline bodies. The important i-esults to be anticipated
fi'om the mici-oscopic examination of crystalline pi-eparations such as
rock sections, etc., was pointed out by H. C. Sorby in 1858 ; the micro-
scopic methods as at present applied to pure ciystallogi-aphy have
been fully described by P. Groth ^ and by Th. Liebisch,^ whilst their
applicability to the identification of the crystalline constituents of
i-ocks has been exhaustively ti-eated by H. Rosenbusch.^

The study of crystalline materials in such minute crystals as are
appropriate subjects for observation by the microscope is not only
a very interesting application of its powers, but is capable of
affoi'ding some valuable hints to the designer. This is j)ai'ticu-
larly the case with crystals of snoio, which belong to one of the
' hexagonal systems,' the basis of every figvire being a hexagon of
six rays ; for these I'ays ' become incrusted Avith an endless variety
of secondary formations of the same
kind, some consisting of thin lamina?
alone, others of solid but translucent
prisms heaped one upon another, and
others gorgeously combining lamina?
and prisms in the lichest profu-
sion,' "* the angles by which these
figures are bounded being invari-
ably 60° or 120°. Beautiful ar-
borescent forms are not unfrequently
produced by the peculiar mode of
aggregation of individual crystals ;
of this we have often an example on
a large scale on a frosted window ;
but microscopic crystallisations some-
times present the same curious phe-
nomenon (fig. 814). Avanturine,
lapis lazuli, crystallised silver, &c.
make very good specimens ; whilst

thin sections of granite, gabbro, and othei- crj^stalline rocks, also of
agate, aragonite, piedmontite, the zeolites, and other minerals, are
very beautiful objects for the polariscope.

The actual jjrocess of the formation of crystcds may be watched
under the microscope Avith the greatest facility, all that is necessary
being to lay on a slip of glass, previously wai-med, a saturated solu-
tion of the substance, and to incline the stage in a slight degree, so that
the drop shall be thicker at its lower than at its upper edge. The
crystallisation will speedily begin at the uppei' edge, where the pi-o-
portion of liquid to solid is most quickly reduced by evaporation, and
will gradually extend downwards. If it should go on too slowly,

Fifi. 814.— Crystallised silver.

1 Phijsikalische Krijstallographie, Leii^zig, 1893.
- Grundriss der 2}hysikalischeu KrystaUograjjhie, Leipzig, 1896.
3 Microscojncal Physiographij of the Bock-malcing Minerals, London, 1895.
■* Glaislier on ' Snow-crystals in 1855,' Quart. Joimi. Microsc. Sci. vol. iii. 1855,
p. 179. See also C. A. Hering, Zeits. f. Krijst. Bd. xiv. 1888, p. 250.

1096 MlCKOCEYSTALLlSATIOX, ETC.

or should cease altogetliei-, whilst a large propoi-tion of the liquif?
still remains, the slide may be again warmed, so as to re-dissolve-
the j)ai"t already solidified, after Avdiich the process will recom-
mence with increased rapidity. This interesting spectacle may
be watched under any microscope, but the instrument specially
designed by O. Lehmann ^ is particularly adapted to studies of this
kind. The degree of heat can be varied at will. The phenomena
become far more striking, however, when the crystals, as they come
into being, are made to stand out bright upon a dai-k ground, by
the use of the spot lens, the paraboloid, or any other foi-m of black-
gTOund illumination ; still more beautiful is the spectacle when the
polarising apparatus is employed, so as to invest the crystals with
the most gorgeous variety of hues.

By chemically precipitating crystalline products under the micro-
scope we can obtain a still deeper insight into the crystallisation
process. One of the earliest workers at this subject was Link,^
who observed that precipitates first separate in the form of veiy
minute liquid globules, and that these subsequently coagulate to
form an undoubtedly ci-ystalline precijDitate. Later investigation
of the subject by Frankenheim, and then by Vogelsang,^ led to the
conclusion that during the passage of a substance from the dissolved
to the crystalline state it passes through a whole series of inter-
mediate stages. On allowing sulphur to crystallise A'ery slowly from
a carbon bisulphide solution thickened with Canada balsam, the
liquid globules, which first separate gradually, solidify to small
isotropic spheres termed globulites ; these embryonic forms then
coalesce, yielding regular aggregates known as crystcdlites. The
latter subsequently arrange themselves in rows as margarites,
several of which then amalgamate, forming longulites, and the
process of aggregation proceeds until at last the crystalloicls^t\iQ
first product in which the structure of the crystal itself is traceable
— are obtained. The separate existence of so many transition foi-ms
has been disputed, notably by Behrens ; ^ but their mention serves
the purpose of indicating that the formation of crystalline bodies is
really an operation of considerable complexity.

Upon the temperatiire maintained during crystallisation depends
the size and arrangement of the crystals. Thus santonin, when
crystallising rapidly on a very hot plate, forms large crystals
radiating from centimes without any undulations ; when the heat is
less considerable the crystals are smaller, and show concentric
waves of very decided form (fig. 815); but when the slip of glass
is cool the crystals are exceedingly minute. In the case of cupric
sulphate, Mr. R. Thomas '^ succeeded, by keeping the slide at a
temperature of from 80° to 90°, in obtaining most singular and
beautiful forms of spiral crystallisation, such as that represented in

1 Molekularjphysil-, 2 vols. Leipzig, 1888 and 1889.

2 Fogg. Ann. Bd. xlvi. 1839, p. 258. = Die Krystalliten, Bonn, 1875.
^ Die Krystalliten, Kiel, 1874.

^ See his paper ' On the Crystallisation at various temjaeratures of the T)ouble
Salt, Sulphate of Magnesia and Sulphate of Zinc,' in Quart. Journ. Microsc. Sci. u.s.
vi. pp. 137, 177. See also H. N. Draper on ' Crystals for the Micro-polariscope,''
in Intellectual Observe?; vol. vi. 1865, p. 437.

OPTICAL PEOPEKTIES OF CEYSTALS IO97

fig. 816. Ml-. Slack has shown that a great vai-iety of spiral and
curved forms can be obtained by dissolving metallic salts, or salicin,
santonin, &c., in water containing 3 or 4 per cent, of colloid
silica. The nature of the action that takes place may be under-
stood by allowing a drop of the silica solution to dry upon a slide ;
the result of which will be the production of a complicated series of
cracks, many of them cur\dlinear. When a grouj) of crystals in for-
mation tend to radiate from a centi-e, the conti-actions of the silica
will often give them a tangential pull. Another action of the
silica is to introduce a very slight curling with just enough eleva-
tion above the slide to exhibit fragments of Newton's rine's, when it
IS illuminated with Powell and Lealand's modification of Professor

Fig. 815. — Eadiating crystallisation of santonin.

Smith's dark-ground illuminator for high powers, and viewed with
a g-th objective. With crystalline substances these actions add to
the variety of colours to be obtained with the polariscope, the
best slides exhibiting a series of tertiary tints.' Yery interesting
results may often be obtained from a mixture of two or more salts,
and some of the double salts give forms of peculiar beauty. 0. Leh-
mann has done excellent work in this department ; but reference
must be had to his previously mentioned work on ' Molekularphysik '
for a description of the phenomena such mixtures exhibit. The
foUowing list specifies the salts and other substances whose crystalline
forms are most interesting. When these are viewed with polarised
light some of them exhibit a beautiful variety of colours of their
own, whilst others require the interposition of the selenite plate for

1 ' On the Employment of Colloid Silica in tlie preparation of Crystals for the
Polariscope,' in Mo?ithly Microsc. Journ. v. p. 50.

1098

MICKOCRYSTALLISATION, ETC.

the development of colour. The substances mai-ked d are distin-
guished by possessing the curious property termed ■pleochroism,
which was first noticed by Dr. WoUaston and carefully investigated
by Sir D. Brewster. This j^roperty, to which was previously applied
the misnomer dichroism, consists in the exhibition by these crystals
of colours varying with the direction in which they ai-e examined ;
thus, the cube-shaped crystals of magnesivim platinocyanide reflect
light of a deep red colour from two parallel faces, whilst light of a
vivid beetle-green is reflected from the other four faces. Pleochroism
is only exhibited by doubly refracting substances, and is caused by
the fact that the two plane polarised rays into which a ray j)assing
into the crystal is decomposed, are absorbed selectively — that is to
say, the crystalline medium absorbs light of ceitain colours from the
one polarised ray, whilst absorbing quite differently coloured coru-
ponents from the second ray. Pleochroic substances are most easily

Fig. 816. — Spiral ci-ystallisation of copper sulphate.

recognised by the fact that they change in colour when rotated on
the microscope stage in plane polarised light — namely, when only one
JSTicol prism is interposed between the eye and the lamp. It not
nnfrequently hapjaens that a remarkably beautiful specimen of
crystallisation develops itself wdiich the observer desires to keep
for display. In ordei' to do this successfully, it is necessary to
exclude the air ; and Mr. Warrington recommends castoi- oil as the
best preservative. A small quantity of this should be j)oured on
the ci-ystallised sui'face, a gentle warmth applied, and a thin glass
cover then laid npon the droj) and gradually pressed down ; and
after the supei-fluous oil has been removed fi-om the margin a coat
of gold-size or other varnish is to be applied. Although most of the
objects furnished by vegetable and animal structures, which are
advantageously shown by polaiised light, have been already noticed
in their appropriate places, it will be useful here to recapitulate the
princij^al, with some additions.

SALTS FOE CEYSTALLISATION

1099

Alum

Ammonium Borate
„ Chloride

„ Hydrogen Tartrate

,, Nitrate

„ Oxalate

,, Oxalurate

„ PhosiJhate

,, Platinocyanide, d

„ Sulphate

„ Urate

Asparagine
Aspartic Acid
Barium Chloride

„ Nitrate
Bismuth ,,
Boracic Acid
Cadmium Sulphate
Calcium Carbonate (from urine of

horse)
■Calcium Hydrogen Tartrate

„ Oxalate
■Cholesterin

Chromic Ammonium Oxalate, d
„ Oxalate

„ Potassium Oxalate, d
,, ,, Binoxalate

Ciuehonidine
Citric Acid
Cobalt Chloride
■Cupric Acetate, d

,, Ammonium Chloride
,, „ Sulphate

„ Magnesium „
„ Potassium ,,

„ Nitrate
„ Sulphate
Ferrous Cobalt Sulphate

„ Sulphate
Hippuric Acid
Lead Phosphate, d
Magnesium Ammonium Phosphate

(from urine)
Magnesium Sulphate
Manganese Acetate
Mannitol
Margarine
Mercuric Chloride
,, Cyanide
Murexide
Nickel Sulphate
Oxalic Acid
Potassium Arsenate
Carbonate
Chlorate
Chromate
Bichromate
Ferricyanide
Ferrocyanide

Potassium Hydrogen Carbonate
,, ,, Tartrate

,, Iodide

,, Nitrate

„ Oxalate

„ Permanganate

„ Sulphate

Quinidine

Quinine Hydriodide
Salicin
Saligenin
Santonin
Sodium Acetate

,, Borate (borax)
„ Carbonate
„ Chloride
,, Nitrate
,, Oxalate
,, Phosphate
„ Sulphate
,, Tartrate
,, Urate
Stearin

Strontium Nitrate
Sugar

Tartaric Acid
Thallium Platinichloride
Uranium Nitrate
LTric Acid
Zinc Acetate
,, Sulphate

Vegetable
Cuticles, Hairs, and Scales, from

Leaves
Fibres of Cotton and Flax
Eaphides

Spiral cells and vessels
Starch-grains
Wood, longitudinal sections of,

mounted in balsam

Animal
Fibres and Spicules of Sponges
Polypidoms of Hydrozoa
Spicules of Gorgoniag
Polyzoaries
Tongues (Palates) of Gasteropods

mounted in balsam
Cuttle-fish bone
Scales of Fishes
Sections of Egg-shells
„ Hairs

Quills
,, Horns

of Shells
„ Skin

Teeth
„ Tendon, longitudinal

Molecular Coalescence. — Remarkable modifications are shown

IIOO

MICEOCKYSTALLISATIOy, ETC.

in the oi-dinaiy forms of crystallisable substances, when the aggre-
gation of the inorganic particles takes place in the presence of certain
kinds of organic matter ; and a class of facts of great interest in
their bearing upon the mode of formation of various calcified struc-
tures in the bodies of animals was brought to light by the ingenious
researches of Mr. Rainey/ whose method of experimenting essentially
consisted in bringing about a slow decomposition of the calcium salts
contained in gum-arabic by the agency of potassium hydrogen car-
bonate. The result is the formation of spheroidal concretions of calcium
cai-bonate, which progressively increase in diameter at the expense of
an amorphous dejoosit which at first intervenes betw^een them, two
such sphei'ules sometimes coalescing to jjroduce ' dumb-bells,' whilst
the coalescence of a largei- number gives rise to the mulberry-like
body shown in fig. 817, h. The j^articles of such composite spherules
aj)pear subsequently to undergo I'earrangement according to a definite
plan of which the stages are shown at c and d ; and it is upon this
plan that the further increase takes place, by which such larger con-
cretions as are shown at «, a
ct. f^ are gradually produced. The

structure of these, especially
Avhen examined by polarised
light, is found to correspond
very closely with that of the
small calculous concretions
which are common in the
urine of the horse, and
which were at one time
supposed to have a matrix
of cellular structure. The
small calcareous concretions
termed otoliths, or ear-
stones, found in the audi-
tory sacs of fishes, present an
arrangement of their par-
ticles essentially the same.
Similar concretionaiy spheroids have already been mentioned as
occurring in the skin of the shrimp and other imperfectly calcified
shells of Crustacea ; they occur also in certain imperfect layers of
the shells of Mollusca ; and we have a very good example of them
in the outer layer of the envelope of what is commonly known as a
' soft egg,,' or an ' egg without shell,' the calcareous deposit in the
fibrous matting ali-eady described being here insufficient to solidify
it. In the external layer of an ordinary egg-shell, on the other
hand, the concretions have enlarged themselves by the progres-
sive accretion of calcareous particles, so as to form a continuous
layer, which consists of a series of polygonal plates resembling those
of a tessellated ptavement. In the solid ' shells ' of the eggs of the

Fig. 817. — Artificial concretions of
carbonate of lime.

^ See his treatise ' On the Mode of Formation of the Shells of Animals, of Bone,
and of several other structures, by a process of Molecular Coalescence, demonstrable-
iu certain artificially formed products,' 1858; and his 'Further Experiments and
Observations ' in Quart. Journ. Microsc. Sci. n.s. vol. i. 1801, j)- 23.

HARTING'S CALCO-GLOEULINE I 10 1

osti'ich and cassowaiy this concretionaiy layei- is of considei'able
thickness ; and vertical as well as horizontal sections of it are very
interesting objects, showing also beautiful eiiects of colour under polai--
ised light. And from the researches of Professor W. C. Williamson
on the scales of fishes, there can be no doubt that much of the
calcareous deposit which they contain is formed upon the same plan.
This line of inquiry has been contempoi'aneously pursued by
Professoi- Hai'ting, of Utrecht, who, working on a j^lan funda-
mentally the same as that of Mr. Rainey (viz. the slow precipitation
of insoluble calcium salts in the presence of an organic 'colloid'),
has not only confii'med but greatly extended his i-esults, showing
that Avith animal colloids (such as egg-albumen, blood-serum, or a
.solution of gelatine) a much greater variety of forms may be thus
produced, many of them having a sti-ong I'esemblance to calcareous
structures hitherto known only as occui'i-ing in the bodies of animals
of various classes. The mode of exj^eiimenting usually followed by
Pi'ofessor Harting was to cover the hollow of an oi-dinary poi-celain
jjlate with a layer of the organic liquid to the depth of from 0'4 to
O'G of an inch, and then to immerse in the border of the liquid,
but at diametrically opposite points, the solid salts intended to act
on one another by double decomposition, such as calcium chloride,
nitrate, or acetate, and potassium or sodium carbonate ; so that,
being veiy gradually dissolved, the two substances may come slowly
to act upon each other, and may throw down their precijiitate in the
midst of the ' colloid.' The whole is then coA'ered with a plate of glass,
and left for some days in a state of pei-fect tranquillity ; when there
begins to appear at various spots on the surface minute points re-
flecting light, which gradually increase and coalesce, so as to form a
ci'ust that comes to adhere to the boixler of the jjlate ; whilst another
portion of the pi'ecipitate subsides, and covers the bottom of the
plate. Round the two spots where the salts are placed in the first
instance the calcareous deposits have a diSerent character : so that
in the same experiment sevei-al very distinct pi'oducts are generally
obtained, each in some particulai- spot. The length of time requisite
is found to vary with the temperatui'e, being generally from two to
eight weeks. By the introduction of such a colouring matter as
madder, logwood, or carmine, the concretions take the hue of the
one employed. When these concretions are treated with dilute
acid, so that their calcareous particles are wholly dissolved out,
there is found to i-emain a basis substance which preserves the form
of each ; this, which consists of the ' colloid ' somewhat modified, is
termed by Hai'ting calco-globuline. Besides the globular concretions
with the peculiar concentric and I'adiating ai-rangement obtained
by Mr. Rainey (fig. 817), Professor Harting obtained a great
variety of forms bearing some resemblance to the following : 1. The
'discoliths' and ' cyatholiths ' of Huxley. 2. The tuberculated
' spicules ' of Alcyonaria, and the very similar spicules in the
mantle of some species of Doris. 3. Lamella? of ' prismatic shell-
substance,' which are very closely imitated l)y ci'vists formed of
flattened polyhedra, found on the surface of the ' colloid.' 4. The
sphei'oidal concretions which foim a sort of ru<limentary shell within

I 102 MICRO CEYSTALLISATIOX, ETC.

the body ofLimax. 5. The sinuous lamellae which intei-vene between
the pai-allel plates of the ' sepiostaii-e ' of the cuttle-fish, the imitation
of this being singularly exact. 6. The calcareous concretions that
give solidity to the ' shell ' of the bird's egg, the semblance of which
Professor Harting was able to produce in situ by dissolving awav
the calcareous component of the egg-shell by dilute acid, then im-
mersing the entire egg in a concentrated solution of calcium chloride,
and transferring it thence to a concentrated solution of potassium
carbonate, with which, in some cases, a little sodium phosphate
was mixed. ^ Other forms of remarkable regularity and definite-
ness, differing entirely from anything that ordinary crystallisation
would produce, but not known to have their parallels in living bodies,
have been obtained by Professor Harting. Looking to the relations
between the calcareous deposits in the scales of fishes and those by
which bones and teeth are solidified, it can scarcely be doubted that
the principle of ' molecular coalescence ' is applicable to the latter,
as well as to the former ; and that an extension and variation of this
method of experimenting would thi'ow ranch light on the process of
ossification and tooth formation. The connection of these results
with the work of Yogelsang (p. 1096) on globulites and other
embryonic crystalline forms is obvious. The inquiry has been
further prosecuted by Dr. W. M. Ord, with express reference to the
formation of urinary and other calculi.^

Micro-chemical Analysis. — The methods which serve for the
qualitative analysis of chemical substances, and which are based
upon the reactions shown by such substances when treated with
solutions of vai'ious reagents, have been applied by number.s of
workers to the identification of the constituents of a matei-ial by the
aid of chemical reactions, the results of which ai'e traced upon the
microscope stage. Thus a very complete scheme has been worked
out by H. Behrens for the detection of the constituents of inorganic
compounds,^ and a somewhat similar, although naturally less com-
prehensive, scheme has been given by the same author for the
identification of organic compounds.'* The analj'tical methods are
intended primarily to serve foi- identifying the components of a
matei'ial available only in small quantities ; but in many cases the
micro-chemical method is moi-e rapidly applied, and is more
accurate in its results, than the ordinary processes of qualitative
analysis. In applying the microscope for this purpose the substance
to be examined is placed upon a watch-glass or glass slide, either in
the solid state or in the form of a solution ; the various crystalline
forms which make their appeaiance as a result of the addition of
dilFerent reagents are then noted, and from the information thus
obtained a knowledge of the constituents of the original substance is
deduced. A very important application of mici-o-chemical analysis

1 See Prof. Harting's Becherches cle Morphologie sipitlietique sur la 23^0(111^^071
artificielle cle quelques Formations GaU-alresInorganiques,pubUeesparVAcad6inie
Boyale Neerlandaise des Sciences, Amsteidara, 1872 ; and Quart. Journ. Microsc.
Sci. xii. p. 118.

2 See his treatise On the Influence of Colloids upon Crystalline Form and
Cohesion, London, 1879.

■^ Anleitung zur mihrochemischen Analyse, Hamburg, 1895.

'* Mikrocliemisclten Analyse der organischen Verbindungen, Hamburg, 1895.

MICRO-CHEMICAL ANALYSIS I IO3

has been made in connection with the detection of poisons, and by
a judicious combination of microscopical witli chemical research,
the application of I'eagents may be made effectual for the de-
tection of poisonous or othei* substances in quantities far more-
minute than have been previously sujjposed to be recognisable.
Thus it is stated by Di-. Wormley ^ that micro-chemical analysis
enables us by a very few miniites' labour to recognise with un-
erring certainty the reaction of the looiroot-'^ j)art of a grain of
either hydrocyanic acid, mercury, or arsenic ; and that in many
other instances we can easily detect by its means the presence of
very minute c[uantities of substances, the true nature of which
could only be otherwise determined in comparatively lai-ge quantitj'^,
and by considerable labour. This inquiry may be prosecuted, how-
ever, not only by the application of ordinary chemical tests under
the microscope, but also by the use of other means of recognition
which the use of the microscope affords. Thus it has been shown that
by the careful sublimation of ai'senic and arsenious acid, the subli-
mates being deposited upon small discs of thin glass, these are dis-
tinctly recognisable by the forms they present under the microscope
(especially the binocular) in extremely minute quantities ; and that
the same method of procedure may be applied to the volatile elements,,
mercury, cadmium, selenium, tellurium, and some of their compounds,
and to some other volatile bodies, as sal-ammoniac, camphor, and
sulphur. The method of sublimation was afterwards extended to the
vegetable alkaloids, such as morphine, strychnine, veratrine,^ &c.
And subsequently it was shoAvn that the same method could be furthei-
extended to such animal products as the constituents of the blood
and of urine, and to volatile and decomposable organic substances
generally By the careful prosecution of micro-chemical inquiry,
especially with the aid of the spectroscope (where possible), the
detection of poisons and othei- substances in very minute quantity
can be accomplished with a facility and certainty such as were
formerly scarcely conceivable. 1

1 Micro-chemistry of Poisons, New York, 1857.

- See Wynter Blyth, Poisons, tlieir Effects and Detection, London, 1895.

APPENDICES AND TABLES

USEFUL TO THE MICEOSCOPIST

4b

1 107
APPENDIX A

TABLE OF NATURAL SINES

°

0'

15'= 1°

30' = i°

45'=f°

°

0'

15' = i°
•7224

30' = i°

45'=r

0

•0000

•0044

•0087

•0131

46

•7193

•7254

•7284

!•

•0175

•0218

•0262

•0305

47

•7314

•7343

•7373

•7402

2

•0349

•0393

•0436

•0480

48

•7431

•7461

•7490

•7518

3

•0523

•0567

•0610

•0654

49

•7547

•7576

•7604

•7632

4

•0698

•0741

•0785

•0828

50

•7660

•7688

•7716

•7744

5

•0872

•0915

•0958

•1002

51

•7771

•7799

•7826

•7853

6

•1045

•1089

•1132

•1175

52

•7880

•7907

■7934

•7960

7

•1219

•1262

•1305

•1349

53

•7986

•8013

■8039

•8064

8

•1392

•1435

•1478

•1521

54

•8090

■8116

•8141

•8166

9

•1564

•1607

•1650

•1693

55

•8192

•8216

■8241

•8266

10

•1736

•1779

•1822

•1865

56

•8290

•8315

■8339

•8363

11

•1908

•1951

•1994

•2036

57

•8387

•8410

•8434

•8457

12

•2079

•2122

•2164

•2207

58

•8480

•8504

•8526

•8549

13

•2250

•2292

•2334

•2377

59

•8572

•8594

■8616

•8638

14

•2419

•2462

•2504

•2546

60

•8660

■8682

•8704

•8725

15

•2588

•2630

•2672

•2714

61

•8746

■8767

•8788

•8809

16

•2756

•2798

•2840

•2882

62

•8829

•8850

•8870

■8890

17

•2924

•2965

•3007

•3049

63

•8910

•8930

•8949

•8969

18

•3090

•3132

•3173

•3214

64

•8988

•9007

•9026

•9045

19

•3256

•3297

•3338

•3379

65

•9063

•9081

•9100

•9118

20

•3420

•3461

•3502

•3543

66

•9135

•9153

•9171

•9188

21

•3584

•3624

•3665

•3706

67

•9205

•9222

•9239

•9255

22

•3746

•3786

•3827

•3867

68

•9272

•9288

•9304

•9320

23

•3907

•3947

•3987

•4027

69

•9336

•9351

•9367

•9382

24

•4067

•4107

•4147

•4187

70

•9397

•9412

•9426

•9441

25

•4226

•4266

•4305

•4344

71

•9455

•9469

•9483

•9497

26

•4384

•4423

•4462

•4501

72

•9511

•9524

•9537

•9550

27

•4540

•4579

•4617

•4656

73

•9563

•9576

•9588

•9600

28

•4695

•4733

•4772

•4810

74

•9613

•9625

•9636

•9648

29

•4848

•4886

•4924

•4962

75

•9659

•9670

•9681

•9692

30

•5000

•5038

•5075

•5113

76

•9703

•9713

•9724

•9734

31

•5150

•5188

•5225

•5262

77

•9744

■9753

•9763

•9772

32

•5299

•5336

•5373

•5410

78

•9781

•9790

•9799

•9808

33

■5446

•5483

•5519

•5556

79

•9816

•9825

■9833

•9840

34

•5592

•5628

•5664

•5700

80

•9848

•9856

■9863

•9870

35

•5736

•5771

•5807

•5842

81

•9877

•9884

■9890

•9897

36

•5878

•5913

•5948

•5983

82

•9903

■9909

•9914

•9920

37

•6018

•6053

•6088

•6122

83

■9925

•9931

•9936

•9941

38

•6157

•6191

•6225

•6259

84

•9945

•9950

•9954

•9958

39

•6293

•6327

•6361

•6394

85

•9962

•9966

•9969

•9973

40

•6428

•6461

•6494

•6528

86

•9976

•9979

•9981

•9984

41

•6561

•6593

•6626

•6659

87

•9986

•9988

•9990

•9992

42

•6691

•6724

•6756

•6788

88

•9994

•9995

•9997

•9998

43

•6820

•6852

•6884

•6915

89

•9998

•9999

1^0000

1^0000

44

•6947

•6978

•7009

•7040

90

[■0000

45

•7071

•7102

•7133

•7163

Xote. — The sine of any given angle is the length of the perpendicular opposite the given angle in a
right-angled triangle which contains the given angle divided by the length of the hypotenuse. The
above table is constructed on the principle that the hypotenuse is always equal to unity, by which
means the fraction is got rid of, as the denominator may be left out. Thus,

cin 9no_pei'Pendicular J

hypotenuse "~i~ '^

4b2

II08

APPENDICES AND TABLES

APPENDIX B

TABLE OP BEFBACTIVE INDICES

Substance

/«,-l

Water . .

Saliva .

Sea-water

Human blood

Alum (sat. sol.) .

Ether (60° Fahr.) .

Albumen

Absolute Alcohol .

Oil of Ambergris .

Salt (sat. sol.)

Eluor Spar .

Diatom Silex

Spermaceti .

Bees-wax

•Oil of Olives (sp. gr. 0-913)

Borax .

Naphtha

Oil of Turpentine (sp. gr. -885)

Oil of Linseed (sp. gr. -932)

Castor Oil .

Chloride of Tin .

Oil of Cinnamon .

€il of Cedar .

Gum Arabic .

Dammar

Oil of Cloves

Sugar .

Felspar

■Cedrene

■Canada Balsam ,

Oil of Fennel

JRock Crystal

Eock Salt (sp. gr. 2-143)

Nitro-benzene

Styrax .

Meta-cinnamene .

•Quinidine

Benzylaniline

Methyldiphenylamine

Balsam of Tolu .

Bisulphide of Carbon

Oil of Cassia

■Quinoline

Tourmalin (ordinary ray)

Ereasote

Petroleum .

Phenyl-thiocarbimide

Iceland Spar (ordinary ray)

n,eiid,i.

wve iuuex

S^

tlT>

1-334

54-7

tl-E

1-339

—

flE

1-343

iJl E

1-354

—

^D

1-457

—

fiiy

1-357

84-9

IJ.D

1-350

—

MD

1-364

58-6

IJ. E

1-368

—

^E

1-375

—

flD

1-4338

97-3

/XD

1-434

—

fJ.J>

1-503

—

IJ.-D

1-553

—

MD

1-476

54-7

jUD

1-515

60-6

fM E

1-475

—

^D

1-474

46-5

jUD

1-485

—

MD

1-490

—

MD

1-503

—

flB

1-619

14-3

/ID

1-510

—

^E

1-512

—

fJ.D

1-520

—

flT>

1-533

—

fi D

1-535

—

p. D

1-764

—

IX D

1-539

—

^D

1-526

41-5

MD

1-544

—

MD

1-545

70-0

MD

1-555

—

MD

1-558

—

jUD

1-582

—

/XD

1-597

29-8

jUD

1-602

24-1

MD

1-611

—

jUD

1-616

—

IX-E.

1-618

—

IXX,

1-630

18-3

fXD

1-578

17-0

fi.J)

1-633

—

/XT)

1-668

—

/XD

1-538

29-9

/XD

1-457

15-3

fJ. D

1-654

18-7

IXV

1-657

49-0

USEFUL TO THE MICEOSCOPIST

no9

Substance Refrai

Monobromonaphthalene fiD

Pipeline and Balsam fi d

Naphthyl-phenyl-ketone /j-d

Bromide of Antimony . . (approximately) /x d

Pipeline fj. d

Methylene di-iodide |U d

Sulphur in methylene di-iodide . . . . /u d
Zircon . . . . . . . . . /xd

Carbonate of Lead fx d

Borate of Lead fi a

Phosphorus in methylene di-iodide (equal weights) fi r>

Sulphur (melted) /u e

Phosphorus . . . . . . . . ^ d

Diamond (sp. gr. 3-4) /u d

Chromate of Lead ' . /j. v

Eealgar (artificial) /u e

tive Index
1-658

19-9

1-657

—

1-G69

17-&

1-680

—

1-681

9-88

1-743

21-2

1-778

—

1-950

—

1-81 to 2-08

—

1-866

—

1-944

17-1

2-148

—

2-224

—

2-47

—

2-50 to 2-97

—

2-549

—

Glass

Substance

Refractive Index

ft.

Crown

AID

1-51 to 1-56

59-0 to 46-0

Plate

flB

1-516

—

Extra Light Flint

IXD

1-541

49-2

Light Flint ....

IJ.D

1-574

41-0

Dense Flint ....

fl-D

1-622

36-5

Extra Dense Fluid

(ijy

1-650

34-2

Double Extra Dense Flint .

(ID

1-710

30-0

Boro-silicate Crown

M D

1-51

64-0

re

Phosphate Crown

IMB

1-51 to 1-56

70-0 to 67-0

§

Barium Silicate Crown

ft D

1-54 „ 1-60

59-0 „ 55-0

S-

Boro-silicate Flint

/XD

1-55 „ 1-57

49-0 „ 47-0

Borate Flint

flD

1-55 „ 1-68

55-0 ,. 33-0

1-3

Barium Phosphate Crown

MD

1-58

65-2

Wery heavy Silicate Flint

flV

1-963

19-7

Gla

ss of Antimony

MD

2-216

-

The extraordinary dispersion of the alkaloid Piperine will be noticed. Its
refractive index is less than that of Chance's Double Extra Dense Flint, yet
Piperine has thi'ee times its dispersion.

mo APPENDICES AND TABLES

APPENDIX C

TABLE OF ENGLISH MEASURES AND WEIGHTS, WITH THEIB
METRICAL EQUIVALENTS

The following are calculated from, the values of the metre, determined
in 1896, and the kilogramme in 1883, by the order of the Board of Trade.

Length

Inch . ; = 2-539998 Centimetres.

Foot = 12 inches - 3-047997 Decimetres.

Yard = 3 feet = -914399 Metre.

Fathom = 2 yards =1-828798 „

Pole = 5A yards = 5-029196 Metres.

Chain = 4 poles = 2-011678 Decametres.

Furlong = 10 chains . . . . . . = 2-011678 Hectometres.

Statute Mile = 8 fm-longs = 5,280 feet = 1-609343 Kilometre.

Geographical Mile = 6,087-23 feet . = 1-855386

Knot = 6,080 feet =1-853182

Superficies

Square Inch = 6-45159 Square Centimetres.

-00645 MUliare.
Foot = 144 Sq. Inches = -92903
„ Yard = 9 „ Feet = 8-36126 MiUiares.

-83613 Centiare.
Perch = 30i „ Yards = 2-52928 Declares.
Eood = 40 Perches . = 10-11712 Ares.
Acre = 4 Pioods . . = 40-46849 „
Square Mile = 258-99836 Hectares.

Volume

Cubic Inch = 16-387 Cubic Centimetres.

„ Foot . . = 1728 Cubiclnches = 2-83168 Centisteres.
„ Yard. . =27 „ Feet = 7-64553 Decisteres.

Capacity
Apothecaries'
Minim, TT|^ . . . = -05919 Cubic Centimetre or Millilitre.
Drachm, f 5 = 60 1]\ = 3-5515 ,, Centimetres or MilliUtres.
Ounce, f 5 =8f5 = 28-4123 „ „ = 2-84123 Centilitres.

Pint, 0 . = 20 f 3 = 568-245 „ „ = 5-68245 Decilitres.

Gallon, C =80= 4-54596 ,, Decimetres, Millisteres, or Litres.

Imperial

Gill =142-061 Cubic Centimetres = 1-42061 Decilitre.

Pint . = 4 gills = 568-245 „ „ = 5-68245 Decilitres.

Quart . =2 pints = 1-13649 ,, Decimetre, Millistere, or Litre.

Gallon. =4 quarts = 4-54596 ,, Decimetres, Millisteres, or Litres.

Peck , = 2 gallons = 9-09193 „ „ „ „

Bushel = 4 pecks = 3-63677 Decahtres.

Quarter = 8 bushels = 2 90942 Hectolitres.

USEFUL TO THE MICROSCOPIST

nil

Weight

Apothecaries^

Grain, gr. ,
Scruple, 3
Draclim, 5
Ounce, 5

. . . . = 6-479892 Centigrammes.

. = 20 gr. = 1-29598 Gramme.
33= 60 gr. = 3-88794 Grammes.
8 5 = 480 gr. = 8-11035 Decagrammes.

Avoirdupois

Grain, gr =

Draclim, dr = 27-34375 gr. =

Ounce, oz. . . . =16 dr. = 437-5 gr. =
Pound, lb. . . . =16 oz. = 7000 gr. =

Stone, St. ... = 14 lb =

Quarter, qr. . . = 28 lb =

6-479892 Centigrammes.
1-77185 Gramme.
2-83495 Decagrammes.
4-5359243 Hectogrammes.
6-35029 Kilogrammes.
12-70059

Hundredweight, cwt. = 4 qr = 50-80235 = -50802 Quintal.

Ton =20 cwt = 1-01605 Tonne.

1 lb. Avoirdupois = -822857 lb. Troy or Apothecaries'.
1 lb. Troy or Apothecaries = 1-21527 lb. Avoirdupois.

TABLE OF METBIG MEASUBES AND WEIGHTS, WITH THEIB
ENGLISH EQUIVALENTS

The metre was originally intended to be the TTytruWTnjth part of the
distance from the pole of the earth to the equator, measured along a
certain meridian, but owing to an error its length is too short. The
metre is therefore the lens;th of a definite standard in Paris.

Length

Micron, i.e. /x
Millimetre .
Centimetre .
Decimetre .
Metre . .

Decametre .
Hectometre
Kilometre .

Milliare .
Centiare .
Declare .
Are = Unit
Hectare .

■ ToVo- Millimetre .
= iV Centimetre
= iV Decimetre .
: ^^ Metre . .
: Unit ....

10 Metres . .
10 Decametres .
■■ 10 Hectometres ,

= -00003937 Inch.

= -03937

= -39370

= 3-93701 Inches.

= 3-28084 Feet.

= 1-093614 Yard.

= 1-98839 Pole.

= 4-97097 Chains.

= 4-97097 Furlongs.

= -6213716 Statute Mile.

= -5389714 Geographical Mile.

= -5396124 Knot.

Superficies

= 10 Sq. Decimetres = 1-07639 Sq. Ft. = 155-0006 Sq. In.

= 1 „ Metre = 1-19599 Square Yard.

= 10 „ Metres = 11-95992 „ Yards.

= 1 „ Decametre =119-59921

= 1 ,, Hectometre = 2-47106 Acres.

Millistere .
Centistere .
Decistere
Stere = Unit
Decastere .
Hectostere .

Volume

= 1 Cubic Decimetre = 61-0239 Cubic Inches.
= 10 „ Decimetres = 610-239
= 100 „ „ = 3-531476 „ Feet.

= 1 „ Metre = 1-30795 „ Yard.
= 10 „ Metres = 13-07954 ,, Yards.

10 Decasteres

130-7954

II 12

APPENDICES AND TABLES

Capacity

Millilitre = Cubic Centimetre ....... = -007039 Impr. Gill.

Centilitre = 10 Cubic Centimetres = -07039 ,.

Decilitre =100 „ „ = -7089 ,',

Litre . =Millistere =1-7698 „ Pint.

Decalitre =10 Litres =2-19975 „ Gals.

Hectolitre = 10 Decalitres = 2*74969 „ Bush.

Kilolitre = 10 Hectolitres = 1 Stere = 1 Cubic Metre = 3-43712 „ Qrs.

Milligramme

Centigramme

Decigramme

Gramme . .

Decagramme

Hectogramme

Kilogramme

Myriagramme

Quintal . .

Tonneau

Weight

= x5 Centigramme
= ^ Decigramme
■■■^^ Gramme
:Unit

10 Grammes
= 10 Decagrammes
• 10 Hectogrammes
= 10 Kilogrammes

10 Myriagrammes

10 Quintals

Avoirdupois
= -01548 Grain.
= -15432 „
= 1-54324 „
= 15-432356 Grains.
= 5-64383 dr.
= 3-5274 oz.
= 2-204622 lb.
= 22-04622 „
= 1-96841 cwt.
= -98421 ton.

The legal equivalent of the metre is 39-37079 inches, and of the kilo-
gramme 15432-34874 grains. In the above tables the values obtained in
1883 and 1896 by the order of the Board of Trade have been adopted as
being the more accurate. In 1893 the metre was measm-ed by Eogers,
who found it equal to 89-370155 inches.

Weights can be more accurately compared than either lengths or
capacities. The actual weight of the standard kilogramme in Paris is
15432-35639 grains, and the English avoirdupois pound is equal to
453-5924277 grammes.

USEFUL TO THE MICROSGOPIST

III3

CONV^BSION OF BBITISH AND METBIC MEASUBE8

Computed by Mr. E. M. Nelson from the New Coefficient obtained by Order of
the Board of Trade in 1896.

Lineal.

Meti

ic into British.

ins.

mm.

ius.

mm.

ins.

000039

X

•039370

51

2-007876

000079

2

■078740

52

2-047246

000H8

3

•118110

53

2-086616

000157

4

•157480

54

2-125986

000197

5

•196851

55

2-165356

000236

6

•236221

56

2^204726

000276

7

•275591

57

2^244096

000315

8

•314961

58

2^283467

000354

9

•354331

59

2^322837

000394

10

•393701

60

2-362207

000433

11

•433071

61

2-401577

000472

12

•472441

62

2-440947

000512

13

•511811

63

2-480317

000551

14

•551182

64

2-519687

000591

15

•590552

65

2-559057

000630

16

•629922

66

2-598427

000669

17

•669292

67

2-637798

000709

18

•708662

68

2-677168

000748

19

•748032

69

2-716538

000787

20

•787402

70

2-765903

000827

21

•826772

71

2-795278

000866

22

•866142

72

2-834648

000906

23

•905513

73

2-874018

000945

24

•944883

74

2-913388

000984

23

•984253

75

2-952758

001024

26

1^023623

76

2-992129

001063

27

1-062993

77

3-031499

001102

28

1-102363

78

3-070869

001142

29

1^141733

79

3-110239

001181

30

1-181103

80

3-149609

001220

31

1-220473

81

3-188979

001260

32

1-259844

82

3-228849

001299

33

1-299214 •

83

3-267719

001339

34

1-338584

84

3-307089

001378

35

1-377954

85

3-346460

001417

36

1-417324

86

3-385830

001457

37

1-456694

87

3-425200

001496

38

1-496064

88

3-464570

001535

39

1-535434

89

■3-503940

001575

40

1-574805

90

3-543310

001614

41

1-614175

91

3-582680

001654

42

1-653545

92

3-622050

001693

43

1-692915

93

3-661420

001782

44

1-732285

94

3-700791

001772

45

1-771655

95

3-740161

001811

46

1-811025

96

3-779531

001850

47

1-850395

97

3-818901

001890

48

1-889765

98

3-858271

001929

49

1-929136

99

3-897641

001969

50

1-968506

002362

002756

deoim.

ins.

003150

1

3-9370113

003543

2

7-8740226

003937

3

11-8110339

007874

4

15^7480452

011811

5

19-6850565

015748

6

23-6220678

019685

7

27-5590791

023622

8

31-4960904

027559

9

35-4331017

031496

035433

1 metre

B^2808428 ft.

( =

1 mm.)

1^09361425 yd.

1 1 14

APPENDICES AND TABLES

British into Metric.

in.
1

mm.
25-399978

2

50-799956

3

76-199934

4

101-599912

5

126-999890

6

152-399868

7

177-799846

8

203-199824

9

228-599802

10

253-999780

11

279-399758

1ft.

304-799736

1yd.

914-399208

in.

mm.

1
3

12-699989

±
3

8-466659

3

16-933319

4

6-349994

3

4

19-049983

1
5

5-079996

10-159991

3

15-239987

4
5

20-319982

1

4-233330

5

21-166648

1

7

3-628568

1

3-174997

8

9-524992

5

8

15-874986

7

22-224980

1
9

2-822220

i

2-539998

3
10

7-619993

7

10

17-779985

9

TO

22-859980

li

2-

2-

10-

14-

23-

1-

1-

1-

1-

4'

7'

11

14

17

20

23

1'

1

1

1

1

1'

1

1

1

mm.
309089
116665
583324
816654
283313
953844
814284
693332
587499
762496
937493
112490
'287487
■462485
■637482
■812479
494116
411110
336841
269999
209523
154544
104347
058332
015999
■846666
■725714
634999
■564444
•508000
•461818
■423333
•390769
•362857
-338666
•317500

in.

mm.

1

85

-298823

1
90

-282222

1
95

-267368

1
100

•254000

1
15 0

-169333

200

•127000

_1
250

-101600

1
300

■084667

_1
350

•072571

400

•063500

_1

450

•056444

■ 1

500

•050800

1
550

•046182

1
600

•042333

1
650

•039077

700

•036286

1
75 0

•033867

1
800

•031750

1
850

•029882

900

•028222

950

•026737

in.

iJ-

1
1000

25^399978

0000

12^699989

" 1

8^466659

4000

6^349994

1
5000

5^079996

]

4^233330

6000

1
7000

3-628568

8000

3^174997

1

2-822220

1
10000

2-539998

1

1-693332

1

20000

1-269999

1
i>5000

1-015999

USEFUL TO THE MICROSCOPIST

III5

TABLE FOB THE CONVEBSION OF FRACTIONAL PABTS OF AN
ENGLISH INCH INTO METBICAL LINEAR MEASURE.

1 +

mm.

1 +

1

Micra.

1 -r

Micra.

1 ■¥

Micra.

2

12-70

33

770

66

385

99

256

3

8-47

34

747

67

379

100

254

4

6-35

35

726

68

374

105

242

5

5-08

36

706

69

368

110

231

6

4-23

37

686

70

363

115

221

7

3-63

38

668

71

358

120

212

8

3-17

39

651

72

353

125

203

9

2-82

40

635

73

348

130

195

10

2-54

41

619

74

343

135

188

11

2-31

42

605

75

339

140

181

12

2-12

43

591

76

334

145

175

13

1-95

44

577

77

330

150

169

14

1-81

45

564

78

326

155

164

15

1-69

46

552

79

321

160

159

16

1-59

47

540

80

317

165

154

17

1-49

48

529

81

314

170

149

18

1-41

49

518

82

310

175

145

19

1-34

50

508

83

306

180

141

20

1-27

51

498

84

302

185

137

21

1-21

52

488

85

299

190

134

22

1-15

53

479

86

295

195

130

23

1-10

54

470

87

292

200

127

24

1-06

55

462

88

289

205

124

25

1-02

56

454

89

285

210

121

57

445

90

282

215

118

Micra.

58

438

91

279

220

115

26

977

59

430

92

276

225

113

27

941

60

423

93

273

230

110

28

907

61

416

94

270

235

108

29

876

62

410

95

267

240

106

30

847

63

403

96

265

245

104

31

819

64

397

97

262

250

102

32

794

65

391

98

259

iii6

APPENDICES AND TABLES

Lines

Lines

Lines

Lines

Fractions

per inch

in mm.

per inch

in mm.

of an inch

^

5,000

197

200,000

7,874

l-5,000th

5-08

10,000

394

210,000

8,268

10,000

2-54

15,000

591

220,000

8,661

20,000

1-27

20,000

787

230,000

9,055

30,000

•847

25,000

984

240,000

9,449

40,000

•635

30,000

1,181

250,000

9,843

50,000

•508

35,000

1,378

260,000

10,236

60,000

•423

40,000

1,575

270,000

10,630

70,000

•363

45,000

1,772

280,000

11,024

80,000

•317

50,000

1,968

290,000

11,417

90,000

•282

55,000

2,165

300,000

11,811

100,000

•254

60,000

2,362

350,000

13,780

110,000

•231

65,000

2,559

400,000

15,748

120,000

•212

70,000

2,756

450,000

17,717

130,000

•195

75,000

2,953

500,000

19,685

140,000

•181

80,000

3,150

25,399-98

50,800

76,200
101,600
127,000

Lines in ju,
1
2
3
4
5

150,000

•169

85,000

3,346

160,000

•159

90,000

3,543

170,000

•149

95,000

3,740

180,000

•141

100,000

3,937

190,000

•134

110,000

4,331

152,400
177,800
203,200
228,600
254,000

6

200,000

•127

120,000

4,724

250,000

•1016

130,000

5,118

i

8

9

10

300,000

■0847

140,000

5,512

350,000

•0726

150,000

5,906

400,000

•0635

160,000

6,299

450,000

•0564

170,000

6,693

500,000

•0508

180,000

7,087

190,000

7,480

USEFUL TO THE MICEOSCOPIST III7

It is often necessary in the examination of a photo-micrograph of
diatomic or other periodic structures to determine at what rate per inch
or per mm. the structure is in the original object, the ampHfication of the
photo-micrograph being known.

Example : In a photo-micrograph of a diatom amplified 735 diams.
12 dots can be counted in "3 of an inch. At what rate per inch is the
structure in the diatom ?

,,s magnifying power x number counted .

space counted

I^^Jl^ = 29,400 per inch.
•3 inch '■

(2) If the answer is required in rate per mm., the space in which
the number is counted being in inches as before, then, because 1 inch
= 25'4 mm.

735x12 iirrr-

= 1157*0 per mm.

•3 inch X 25*4

(3) Suppose a rule divided in mm. is used to determine the space in
which the number on the photo-micrograph is counted, and the rate per
inch is required ; if twelve dots can be counted in 7 mm., then, because
1 inch = 25'4 mm.

735x12x25-4 00 nn. -1
^ = 32,004 per men.

IIl8 APPENDICES AND TABLES

APPENDIX D

GOMPABISON OF THE SCALES OF FAHRENHEIT'S, THE
GENTIGBADE, AND BEAUMUB'S THEBMOMETEBS

These three thermometers are graduated so that the range of temperature
between the freezing and boiling points of water is divided bv Fahrenheit's
scale into 180° (from 32° to 212°), by the Centigrade into 100° (from 0° to
100°), and by that of Eeaumur into 80° (from 0° to 80°) portions or degrees.
Hence we derive the following equivalents : —

A degree of Fahrenheit is equal to '5 of the Centigrade, or to "4 of
Eeaumiu''s ; a degree of the Centigrade is equal to 1*8 of Fahrenheit's, or
to "8 of Eeaumur's; and a degree of Eeaumur's is equal to 2*25 of
Fahrenheit's, or to 1'25 of the Centigrade.

To convert degrees of Fahrenheit into the Centigrade or Eeaumiu-'s,
subtract 32 and multiply the remainder by |- for the Centigrade, or f for
Eeaumur's.

To convert degrees of the Centigrade or Reaumur's into Fahrenheit's,
multiply the Centigrade by f , or Eeaumur's by f , as the case may be,
and add 32 to the product.

Example

Let F, C, and E = the number of degrees Fahrenheit, Centigrade, and
Eeaumur respectively. Then

F = ^— + 32; F=— + 32;

5 4

^^_5 (F-32). c=~

9 ' 4 '

P 4 (F-32). p 4C.

E = g— , E=_.

F = C + E + 32.

This last formula is of use, because in England thermometers are
usually graduated in Fahrenheit and Centigrade. Eeaumur may be found
by inspection by subtracting the Centigrade from the Fahrenheit and
taking 32 from the remainder. On the Continent thermometers are
generally graduated in Eeaumur and Centigrade. Fahrenheit can be found
by adding Eeaumur and Centigrade and 32. — Example : If the thermometer
reads 8 Eeaumur and 10 Centigrade, the Fahrenheit will be
8 + 10 + 32 = 50 F.

USEFUL TO THE JMICEOSCOPIST I I 19

APPENDIX E

OPTICAL FORMULA

To find C, the optical centre of a lens : Let A and B be the vertices, let
the radius of the cui"ve A = r, and that of B = s, i = thickness of the lens
and jtx the refractive index. Then

AC=-^; BC = ~ (i)

r—s r—s

Example explaining the method of treating the signs : First, it should
be particularly noticed that all curves which are convex to the left hand
have positive radii, and those turned the other way negative radii.

In a biconvex let r = 2, s = — 3, and t = l; then by (i)

AC= -?Ai-= — =?• BO- -3x1 _j-B _ 3
2-(-3) 2T3 5' 2-(-3) 2 + 3 5"

The point 0 is measured, therefore, to the right hand from A, and to
the left from B. In a plano-concave let r= — 2, s = 00 , and t = l; then

AC = ^i^^=0; BC=-5^^ =^^-= -1 . . (i)

-2- 00 -2-QO -00 ^^

c is therefore coincident with A.

The principal points D and E may be found thus :

AD = 1. — ; B:E = -. — (ii)

/x r — s fji. r — s

1 3

Example : In a meniscus r= — 3, s = — 2, i = -, and u = - ; concavities

facing the left hand.

-3 ^ _? -?

1 J ^ 2 ~4 ^2 4^23^1 ..

^^ - 3'_3_(_2) 3 • ^^3T2 3'"^ 3 '4 2 ^"^
2
D is measured f inch to the right from A.

-2.1 -1

^.^ _ 1 4__ _ 2 2 ^211 .

-t^^ - 3 • -3-(-2) - 3 ■ ^3T2 3-2-3 ' ' ^"'

2
E is measured ^ inch to the right from B.

If the meniscus is turned round so that its convexities face the left
hand, r = 2, s = 3, i = -, w = - ;

2 1

14 2 1 , 1 ,..,

^^ = S-TTs = 3-2--^ = -3 • • • • ^">
2

Similarly B E = — -. Both are therefore measured to the left. The

II20 APPENDICES AND TABLES

formulae (ii) are approximations, sufficiently accurate for general practical
purposes, but in cases of importance the following, longer but more
accurate, formulae should be used :

-f,{r-s)-t{i,-l)' ~f^(r-s)-i(^-l) ^'"^

Plano-convex Lens. — Let /=the jprincipal focal point and y = th.e
semi-aperture ; then if parallel rays are incident on A, the plane side of
the lens, r = co, and by (ii) B E = 0. The principal point is therefore at
the vertex B, and the focal length

B/^JUl; E/=B/ . . (iv)

The spherical aberration

'^--li^J'i «

Thus when m =5>

S/= -4-5^1 (v)

If the parallel rays are incident on the convex side A, s = oo ,
BE= —- (ii), and the focal length

B/=-Ar--. .... .(vi). E/=-!L (vii)

fi — lfi M — 1

The sphericaP aberration
"When fi = 1"516 (plate glass)

When iJL = 1-62 (flint glass)

gy^ _^>-2)+2 ^^ ,
•^ 2^(;x-l)^ / • • • • ^^

S/= -1-1^ . . . . . . . . (viii)

g/= --8042^ (viii)

To find the radius of a plano-convex lens, the ref. index and focus
.E/ being given :

'•=/(m-1) ■ • . • (vii)

To find the radius of a plano-convex lens, the ref. index, the thickness,
and the focus B/ being given :

^_i^-l)i^f^t) _ ^ ^^.j

A plano-concave lens follows a plano-convex ; / will be negative,
which shows that the focus is virtual. Concaves being thin, t is usually
aieglected.

Equi-convex and equi-concave generally :

^/-at^ • • • • •• ■ ■ • '=^'

Equi-convex more accurately :

* Heath's Geometrical Ojptics, 1887.

USEFUL TO THE MICROSCOPIST 1 121

Equi-conves more accurately :

B/=.T^)-47 W

Spherical ^ aberration

'•^ 8/x(^.-ir ■ / ^' ^

In an eqni-convex lens when /^ = 1 516

8f = -1-618'^ (xi)

To find the radius of either an equi-convex or equi-concave lens,
generally, the ref. index and the focus B/ being given :

r = 20x-l)/ (ix)

To find the radius of an equi-convex lens, the ref. index, the thickness,
and the focus B / being given :

^_0x-1)(4m/+^) (^^

2/x

Bi-conves and bi-concave, generally :

E/=_\ . ^^ (xii>

ju, — 1 s -r

Correction for thickness ;

^ '^^ — (xiii)

Bi-concave t may be neglected B/= E/ practically.

Bi-convex more accurately, and converging and diverging menisci :

s ((;x-l)^-r|
B/= ^- 1 L^ (xiv)

When the light is travelling frora right to left

A/= L 1 ^ (xiv).

Spherical aberration :

Example : Let r = 2, s = - 3, « = 1, and /j. = '^ ; then by (xiv)
' Heath's Geometrical Optics, 1887.

4 c

1 1 22 APPENDICES AND TABLES

B/

2

q 5

-OX _ ,

3 5 ol / . ^

= -r-T^ = 7^'7- ^--)

2 ■ 3 3

Similarly A f = - 2^ (xiv)

7

1 144

By (xii) and (xiii) B/ = ^^ -il£jl _ 1?_ A _ . A

•^5 3 , " 5 "25 ~ -^ 25

- -^ 4
2

This is larger by J-^ inch than the result obtained by (xiv).

The following is an example worthy of note. Suppose

r — s<t and > (/^^ — !)-•

Thus let r = 5 -. s = 5, i^ = ] , u = -.

2 ^2

Then by (xiv) B f = _^ ll = A = _310.

2\2'3/ 12

It will be observed that, although this meniscus is thickest in the
middle, it has, however, a large negative focus.

The principal points of a sphere are at its centre.
The focus of a sphere, measured from the centre :

•^ 2(^-1) ^

The focus of a sphere measured from its surface :

B/ = ^1^'^^ (xvi)

•^ 2(^.-1) ^ '

The focus of a hemisphere measured from the plane surface, the light
being incident on the convex surface :

B/= ^^ (vi)

But when the light is incident on the plane surface, the lens being
turned round :

B/ = -^ (iv)

When /x = 1-5 the focus of a sphere measured from the surface = ^ the
radius.

The focus of a hemispliere measured from the plane side - I5 the
radius, and when measured from the convex side the focus - 2 radii.

In a cylindrical lens the principal points cross over.

To find the radii r and s of a crossed lens of minimum aberration for
parallel rays :

USEFUL TO THE MICEOSCOPIST I I 23

2(^-1) (^ + 2) ,^2(^-1) (m + 2) _ _ (^^ii)

For boro-silicate glass /Li = 1-51 ; r = -5898/; and s = -3-769/; (xvii)

8f= -1-042 ^ (xv)

For flint glass ^ = 1-62 ; r = -053/; and s = - 12-OG/; (xvii)

S/=--798^ (XV)

Critical angle. — Let 6 be the critical angle for a ray passing out of a
denser naedinm into a rarer one.

Then sin ^ = - (xviii)

When /x = 1-333, ^ = 48°36i'; m = J , ^ = 41°48i ; ^ = 1-52 ^ = 41° 8f ;

^ = 1-62, e = 38° 7'.

Let /be the principal focus, and j^ = the distance from the object to
the optical centre of the lens, ^' = the distance from, the optical centre of
the lens to the conjujjate image.

Then P-^/, P=4A; / = -^ (xix)

p-f P --/ P^P

Let V be the distance from the object to /, and w be the distance from
/ on the other side of the lens to the conjugate image.
Then

v = p—f; iv=^'ip'-f\ ■]) = v+f\ p'^iv+f; and viv=f~\ v = -'-

w

/"-
10 = -'- .... (xx)

If o be the size of the object and i the size of its conjugate image
otv _of _op' _ of _o(p'-f).

"'T

V p p-f f

■IV

•"7-

Jf^ip^ if ^i(p-f).
w p' p'-f f

op'
p = —.—
i

f{i + o), , ip f{i^o)
i ' 00

of

V = —^ ;

i

0 i+ 0 i + o

.... (xxi)

Examples : With an objective of §-inch focus it is required to project
an image of a diatom -03 long, so that it may be 1-5 inch on the screen,
what must be the distance of the screen from the optical centre of the lens ?

,y JJti^ = -SCl-S ^ -03) ^ 25.5

Therefore ^j' = 25- inches, the distance required (^xi)

Conversely, if the image of a diatom projected by a -^-inch objective
measures 2 inches on the screen at 40J inches from the optic centre
what is the size of the diatom ?

0 = --^^^ =.——_.= ^-- = -0120 .... (xxi)

P-f 40^-1 ^0
4 4
the size of the diatom required.

4 c 2

I I 24 APPENDICES AND TABLES

The last formula of (xxi) is very convenient for finding the focus of
an objective ; w must, of course, be large in proportion to the focus ;
o may be a stage micrometer.

As the posterior focus, /, is in ordinary microscope objectives of
1-inch focus and upwards, near the back lens, the distance w may be
measured from there.

Example : The image of "01 inch on a stage micrometer projected by
an objective is 2*4 inches on a screen, distant 5 feet from the back lens ;
required the focus of the objective.

J. 0 w '01 X 60 "6 1 / -N

•^=-=-2^ = 2^4 = 4 ^^^'^

To find F, the equivalent focus of two lenses in contact :

F=/^ (xxii)

where / is the focus of one lens and f that of the second.

Example : It is required to make a combination of two plano-convex
lenses, the focus of one lens, /, being twice /', that of the other, and whose

3

combined focus F = '6, /^ = x ; find their radii (see figs. 4, 6, 8, and 9).

Then/ =2/'.

2//_2/^_2/.
2/+/ 3/ 3 '

/ = ^ = L8 = .9; and/=2/ = l-8.

. (xxii)

r = (^ = 1) /= /^ - l\ 1-8 = -9 ; similarly r'
\2 /

= •45 .

• (vil)

The focus for three lenses follows that for two, thus :

F = JlH Cxxii)

which may be written — = 2 -.

F /

To find F, the equivalent focus of two lenses, not in contact, generally,
F to be measured from the last principal point (E') of the second lens ;
Let d = the distance between the lenses :

f^f-d ^^^"'^

More accurately, let D E be the principal points of the first lens and
D'E' those of the second, AB and A' B' being the respective vertices,
d = the distance from E to D' ; then G and G', the principal points of
the combination, are :

•^="*/4^«! <'^^">

and F= -J-j-, — - (xxvi)

F is measm-ed from one of the principal points of the combination. An
example will be of interest. Let parallel rays fall on the convex face of
the field lens of a Huyghenian eyepiece ; find their focus.

Let /, tlie focus of the field lens = 3, and that of the eye lens /■' = !;

USEFUL TO THE MICEOSCOPIST I I 25

fi = -, and the distance between the surfaces, that is B A', = 1-8 ; t the
A

3 3

thickness of the field lens = - ; and t' that of the eve lens = '^ ; A D = 0

10 '^ 20

(ii); BE= -^=--2 (ii). Similarly A' D ' = 0 ; B' E'= -1= - ^ (ii) ;

/Li /i 10

^ = EB + BA' = -2 + l-8 = 2. Now

^-3-1^2 = 1 ■ ■ ■ <-^"

We see, therefore, that the equivalent focus is 1^ inch, but the
principal point G', from which the focus is measured, is 1 inch to the left
from E' ; therefore the focal point is h inch to the right from E'. Now as
E' is -^ inch to the left of B, the plane surface of the eye lens, it follows
that F, the focal point, is -^^ inch to the right of the plane surface of the
■eye lens. If this problem is worked by the simpler formula (xxiii), the
answer will be '44 from the plane surface of the eye lens ; this is only an
error of "04 in excess.

This explains ' the microscope objective of 10-ft. focus.'
The equivalent focus of the objective was 10 ft., but the principal point
O' from which that focus was measured was 9 ft. 11| inches from the
objective, which would give ^ inch as the working distance of the lens.
The objective in question has a double convex back lens and a plano-
concave front ; a small decrease in the distance between the lenses, such
as a jij- inch, has the effect of causing the principal point G' to recede
many feet, and of causing a great increase in the equivalent focus.

With regard to the tube length, which is equal to d in (xxvi), the
position of the principal points of a combination plays an important part.
Suppose the Huyghenian eyepiece, in the preceding example, were
mounted as an objective ; the tube length would have to be measured
from the first principal point of the eyepiece, wherever that might be, to the
second principal point of the objective, which in the example before us is

a = D+ -J^-L =D + 3 (xxiv)

o + J. — A

Gr is therefore measured 3 inches to the right from the point D ; D is,
as we have seen, coincident with A, the convex vertex of the field lens.
So anyone measuring the tube length from the field lens, which is the
posterior lens of our supposed objective, or from the middle of the
combination, would be 1| or 3 inches in error. The correct point from
which the measurement should be made lies one inch in front of the eye
lens, which is the front lens of our supposed objective.

The importance of this cannot be over-estimated, as the optical tube
length has a direct bearing on the power. If Q = the distance of vision
(say 10 inches), M = the magnifying power, F = the equivalent focus of the
eyepiece, F' = the equivalent focus of the objective, O = Prof. Abbe's
' Optical Tube length,' viz. the denominator in the fraction in formula
(xxvi) ; then

M=§|, (xxvii)

If 0 = the focal length of the entire microscope, N.A. = the numerical
aperture, and e = the diameter of the eye-spot, then

<f) = -t = (XXVlll) '

^ M O ^ '

1 Journal R.M.S.

1126 APPENDICES AND TABLES

6 = 2 (N.A.) (^ ; N.A. = -^ (xxix) '

2 (p

If \ -the number of waves per inch of h'ght of a given colour, L the

limit of resolving power of any objective with an illuminating beam of

maximum obliquity is

L = 2A(N.A.) (xxx;

But with a solid =f axial cone and white light the resolving limit is
equal to the N.A. multiplied by 70,000. When Gifford's screen is used, or
photography employed, the limit is raised to the N.A. multiplied by 80,000.

The aberration for non-joarallel rays. — It is a little more troublesome
to find the aberration of raj^s other than parallel, but if the following
instructions are carefully attended to the problem merely becomes the
simi^lification of a vulgar fraction. Let P and P' be the distances of the
point and its image from the lens. First find a, by either (xxxi) or
(xxxii) :

«■ = -^ — 1 (xxxi) ; a = 1 — ^ (xxxii)

Next find x by (xxxiii) or (xxxiv) :

^^2(/i-l)/_^_ . . (xxxiii); x = l^ ^(m-3)/^ ; _ (^^^i^A
, r s

Then find w by (xxxv) :

o, = ^^ \ If^x'^ + ^^.-,l)ax+ (3m + 2) (m - 1) a-+ J^\ (xxxv)

8/^(^ -1)/ l^-l /x-lj

1 1 1 1, xo ^ o / -X

-=^--+-{x-ay—^+coy~. . . (xxxvi)

The aberration 5 P' = — mP'-?/- . . . (xxxvii)

To find the aberration of two lenses in contact. Let Q and Q' be the
object and its conjugate at the second lens,/' be the focus of the second
lens, and F the focus of the combination ; then P' = — Q.

Ill . . . Ill /.s

p.-7-p- • • ■ <-''>'^ Q'=/'-Q • • ■ (-)

for the first lens, —,= -^ — - + co y- . . . (xxxvi)
for the second lens, ^ = + + a>' y- . . . (xxxvi)

m / '■^

for both lenses, /^ = ^ + /•, ~ p + ('^ "^ ^') V" ■ (^xxviii)
^ J J f

Therefore, for n lenses, -^„ = 2-- — +'2coy'' . (xxxviii)

The aberration S Q' = — 2 w Q'~y~ and S F = — 2 w F-y- . (xxxix)
Example : Two plano-convex lenses of equal foci have their convex
surfaces in contact (fig. 7) ; find the aberration for parallel rays. Then

M = |;/-.f- F=/. ...... . (xxii)

For the first Iens7- = Q0 ; therefore x= —1 (xxxiii) ; P = oo ; therefore
g

a= -l(xxxi); and <u -- -- (xxxv)

^/

For the second lens s = co ; therefore x = l (xxxiv) ; -^ = — - ; there-

H J

1 Journal B.M.S.

USEFUL TO THE IVnCROSCOPIST 112/

13 20

fore a= -3 (xxxi) ; co'= -- (xxxv) ; tu + a>' = 2 6) = — — ;

on
/= 2 F (xxii) ; therefore 2 co = ^ . ;

^^ 20JV_ 5 r .

This is half the aberration of an equi-convex lens (fig. 1) of the same
focal length as the combination where

5/= -|-^ (xi)

If the front lens of the combination be turned round so that its convex
siufface faces the incident light the aberration is

^F= -J2 • 1^ ■ (ixxix)

or half what it was before (fig. 5).

This is nearly a third of the aberration of a plano-convex in the best
position (fig. 2), which is

8/ = -Z .r. (viii)

The following figures pictorially illustrate spherical aberration in
single lenses and in various combinations of two plano-convex lenses, all
having the same focus F, the same aperture, and the same refractive
index |. The dot nearer the lens is the focal point for the marginal, and
that farther away the focal point for the central rays ; the distance
between the dots is the spherical aberration S F.

I i I 11

Pig. 1. Fig. 2.

Fig. 1. An equi-convex, r = F ;

8F= -1-6|' =
F

V.

Fig. 3.

•173

(xi)

Fig. 2. A plano-convex, '' = ^1

SF= - 1-1(3 1^" =
F

•121

7 7

Fig. 3. A crossed convex, r = _— F ; s = — -F

1^ ^

8F= -1-07|-
F

-•111

(viii)
(xvii)

(XV)

Fig. 7. A combination of two pianos with their convex faces in con-
tact, the focus / of the first lens being equal to /', that of the second.

The focus of the combination F = ^ (xxii)

8F-

•833 1^ = - -087
F

Fig. 4. The same, only 1f=f ;

5F= -1-611 1^ = --168 .
F

(xxxix)
(xxxix)

II28

APPENDICES AND TABLES

Fig. 6. The same, only/=2/';

8F= --5 ^= --052
F

Fig. 5. The first lens inverted, /=/'' ;

SF= --416^^= --043
F

Fig. 8. The same, only 2/=/' ;

SF= -•623|'= --065
F

Fig. 9. The same, only/=2/';

5F= -•3761-''= --039
F

(xxxix)

(xxxix)
(xxxix)
(xxxix)

Fig. 4.

Pig. 5.

Fig. 6.

Fig. 7.

I I

Fig. 8.

Fig. 9.

We see, therefore, that with the same focal length F the aberration of
fig. 1 is the greatest, and that of fig. 9 the least. We also see in the com-
binations that by decreasing up to a certain point the focus of the first
lens the aberration is increased, and vice versa. The best form of a
combination of plate glass, fx = 1"516, for parallel rays similar in arrange-

ment to fig. 9 is when /= -^-.

The Aplanatic Meniscus.— K spherical refracting surface has two
aplanatic foci, such that if converging rays, which have their focus at P',
meet a convex spherical refracting surface, whose centre of curvature
is r, and if the distance between the points P' and r = /x r, then those
rays will be refracted aplanatically to some other point, say P, which will
lie on the same side of the surface as P'. This fact is of great service,
because it enables an aplanatic meniscus to be constructed ; thus, if we
make r the radius of the curve A, we can make s, the radius of the curve
B, a radius from the point P. If, then, P is a radiant, the light travelling
from left to right will pass through the curve B without refraction, because
P is the centre of the curve B. The light will then pass on unchanged
to the curve A, and will by it be refracted aplanatically, as if it had come
trom P'. P will be negative and P' positive.

The formulae for finding r and P' when P is given are :

^^ - "P'---/xP (xl)

/x + 1

P'=

and those for finding r and P when P' is given are :

P'

r =

M + J

P'
P=--

(Xli)

USEFUL TO THE MICROSCOPIST I I 29

An excellent combination, suitable for a bull's-eye, can be made of
boro-silicate glass, refractive index 1-51, i/ = 64-0

1st lens ' crossed r = + 2-359 , ^i^^^ter 2-1
s= +15'078*

2nd lens ^ meniscus r = + 1-280 , ^^^^^^^ ^.g

S= —0'4:04:'

Distance between lenses, -05 ; equivalent focus, 2-0 ; back focus, 1-55 ;
total aberration, - -103 ; clear aperture, 2-0 ; angle, 62°.
This combination is eminently suitable for photo-micrography, and for
those cases where a bull's-eye is necessary.

A simpler form of bull's-eye can be made of two pianos, using the
same glass ; see tig. 9, p. 1128.

1st lens, radius + 3-0, diameter 2-1
2nd „ „ +1-8 „ 1-9

Distance between lenses, -05 ; equivalent focus, 2|.

To find the radii r and s of a lens which will refract light from a point
j; to point p' with minimum aberration.

y=JLP;; ^,'=Z/ . . . (xix); K^Jil^, . . . (xlii)

r= ^LSt±^LP ^. . (xhii); s= -P^^ . . . (xhv)

^(2/. + l)K-4(/x-hl) p-rK "■

Let |3 be the coefficient of ^ in formulae v, viii, xi, and xv, then for
parallel rays in each particular case the lateral aberration = ^- 13 . . (xlv)

Diameter of least cu'cle of aberration = 0/2^ (xlvi)

3 y"^

Distance of least circle of aberration from focus = — - ^ /3 . (xlvii)

"When the rays are not parallel

1 3

(xlv) = a)_p'2/^ (xlvi) = — a)jp'3/^ (xlvu) = -■g'^p'^y'

It is interesting to note tliat-^ =2(/it — l)i (xlviii)

Therefore, when m = ^, 1 ^^'

To find m, the magnifying power of simple lenses or magnifying
glasses. Let d be the least distance of distinct vision apart from the lens,
and / be the principal or solar focus of the lens. Then, when the eye is
lield close to the lens,

w = 1 + -— (xlix)

When the eye is held at the back principal focus of the lens, subtract
one from this quantity. For real images projected upon a screen, the
distance of the screen being d, subtract two.

It may be of interest to note that formula (xix) on this page may be used
to determine the focus of spectacles required to bring the abnormal focus

1 In this formula the convention used with regard to the signs is that of manu-
facturing opticians, and not that employed in the rest of the appendix.

I I 30 APPENDICES AND TABLES

of either a presbyopic or myopic person to a normal focus. Make p the
abnormal, and j?' the normal focus ; then/ will be the focus of the spectacles
required.

In both cases 2^ is a negative quantity, because it is on the same side
of the lens as p>' ', it is usual to make p' 10 or 12 inches.

Achromatism

Let fx be the refractive index of a mean ray (D line nearly) for a
certain material, /li„ that for a blue ray, and ii^ that for a red ray ; the dis-
persive power of the material is 1^ — ^ ; this is usually written — ^ , or xb-.
The formula for achromatism is

^/^ 1 + Ail -'- =0-

that is, ^ + ^ = 0 (1)

The foci of the two lenses are therefore directly as their dispersive
powers, and the focus of one will be negative.

An achromatic effect, which is not achromatism in the strict mean-
ing of the term, can be obtained with two lenses of the same kind of glass
by making d the distance between the lenses :

^ = 4^^ (li)

If^ is large,/ in the denoininator may be neglected; this will make
d half the sum of the foci, which is the formula for both the Huyghenian
and Eamsden eyepieces ; but when ]) = f, d is the sum of the foci.

Fortnulcs relating to Spherical Mirrors

Let^ = one focus, p' = its conjugate,/ = principal focus, and r
= radius of curvature ; then in concave mirrors

• f=~'
p+p' 2'

- ..2/; ^, = ?^/ (xix)

p+p' P f

To find p interchange p and p '.

If o is the size of an object, and i the size of its image, and v the dis-
tance of the image from the principal focal point, then

. 0 7)' ■ 0 f , ..

p V

In convex mirrors prefix a negative sign, thus: r= —2/, and so on
with the other formulae.

The formulae for mirrors may be derived from those of lenses by sub-
stituting—1 for /x ; thusr= — 2/(vii).

Let y = the semi-aperture ; then the spherical aberration

^f= -\-% (^) °^ (^"^>

A mirror to be aplanatic for parallel rays must have a parabolic curve.

A mirror to reflect rays diverging from a point p, so that they may
converge aplanatically to another point p', must be elliptical, having i)
and f' for its foci.

USEi< UL TO THE MICROSCOPIST I I 3 I

Formulce relating to Prisms

L(jt I =■■ tlie refracting angle of the prism, (jj the angle of incidence on
the first surface, 0' the angle of refraction at the first surface, yj/ the angle
pf incidence m the prism at the second surface, and >//■' the angle of re-
fraction on emergence ; then the total deviation

D = 0 + \//-'-t; (f)' + ^ = i (Hi)

\Mien the ray passes through the prism symmetrically the deviation
is at a minimum : (p = ^', (p^ = \l^ = ~, and

. i + D

/^ = dm)

t

sin -

2

by which formula the refractive indices of media can he found, because
both t and D are capable of accurate measurement.

Formulce relating to Conic Sections
Ellipse. — Let A = major axis; a. = minor axis. Then

Focus = (hv)

Parabola. — Let A = height ; a= - base. Then

Focus = -— - (Iv)

4 A

Hyperbola. — Let A = major axis ; a = minor axis. Then

Va^ + o-^-A ,, .>

rocus = (Ivi)

Works consulted:- — Coddington, Camb. 1830; Parkinson, Camb.; ' Ency-
clopaedia Britannica' ; ' Jom-nal E.M.S.'; Heath, Camb. 1887, &c. It will be
seen that several of the formulae have been entirely reset, while some
appear now for the first time.

I 132 APPENDICES AND TABLES

APPENDIX F

EXAMPLES USEFUL TO THE MICBOSCOPIST

Square ^ inch . .

. =

10-08061

square

millimetres

)) 1^ j> • •

=

6-45159

jj

» XV 5) ■ •

. =

4-48027

55

>! liu )>

=

-06452

,,

55

=

64515-9

/*

" lo'U'U ))

. =

645-159

55

Square centimetre = 15-50006 square ^ inch.

„ miUimetre = 15-50006 „ -^^-^ „

„ 100 /x =15-50006 „ tbW 55

„ 10 yu, = -15500 „ „ „

„ /x = -00155 „ „ „

Multiples of the above may be found by multiplying the values given
by the square of the multiplier.

Thus, square -^-^ inch = ^^^ x 4 ; the square of 4 = 4 x 4 = 16, and 6-45159
X 16 ^ 103-2254 square milhmetres, the answer required.

Cubic i inch

10 55
J.

12" 55
_1

ITTTT 5>

1

1017(5" 55

= 32*00589 cubic millimetres.

16-38702 „ „

9-48323 „ „

•01639 „ „

= 16387-02 „ n

Cubic centimetre = 61-0239 cubic ^jj inch.

,, millimetre = 61-0239 ., xhs r,

„ 100 ^ = 61-0239 „ T^^ „

„ 10 /x = -061023 „ „ „

„ fx = -000061023 „ „ „

Multiples of the above may be found by multiplying the values given
by the cube of the multiplier.

Thus, 2 cubic milhmetres : 2 cubed = 2 x 2 x 2 = 8, and 61-0239 x 8
= 488-1912 cubic xJo iiich, the answer required.

Areas of Circles

^ inch diameter = 1-22718 sq. xV i^^ch = 7-91726 sq. millimetres
tV 5. .. = -78539816 „ „ „ = 5-06706 „

T^ „ „ = -545415 „ „ „ = 3-51879 „

T*TT ,5 ,5 = -78540 „ rU >, = "05067 „

= 50670-6 „ iJL

x^T>„ 55 = -78540 „x^ViT5. = 506-7

1 millimetre diam. = -78539816 sq. mm. = 12-17372 sq. -^ij^ inch.

100 ^ ... . = 7854-0 „ M = 12-17372 „ TxrW v

10 /x ..... = 78-54 „ „ = -12174 „ „

/x = -7854 „ „ = -0012174 „ „

USEFUL TO THE MICEOSCOPIST 1 1 33

Multiples of any of the above may be obtained in the same manner as
in the preceding example.

Thus, if the diameter of the circle = ^iu inch, then the square of 3 being
9 and '7854 x 9 = 7*0686 sq. x^^ inch and -05067 x 9 = -45603 sq. millimetre,
is the area required.

Volumes of Spheres

= 1-02266 cubic -^jj inch= 16-75835 cubic millimetres.
= -52360 „ „ = 8-58024 „

= -30301 „ „ = 4-96543 „

= -52360 „ i^o „ = -00858 „
= -52360 „ x<jVo ,, =8580-24 „ fx

. = -52360 cubic mm. = 31-952 cubic xijo inch.

. =523600-0 „ ij. =31-952 „ TTfVo ..

. = 523-60 „ „ = -03195,, „ „

. = -52360 „ „ = -00003195 „ „

Multiples of any of the above follow the preceding example of cubic
measures. Thus, if the diameter of the sphere = 30 fi, then the cube of 3
being 27 and 523-6 x 27 = 14137-2 cubic /x and -03195 x 27 = -86265 cubicx^Vu
inch, is the volume required.

F i"'

. diameter

_i_

100 ')

1 mm.

diam. . .

100 ;x

10 JU

„ • .

^

17

1134 APPENDICES AND TABLES

APPENDIX G

USEFUL NUMBEES AND FOBMUL^

Paris line = -088813783 inch.

Cubic foot of water weighs 62-2786 lb. avoirdupois at 62° Fahr.

Cubic inch of water weighs 252*286 grs. at 62° Fahr.

Gallon of water weighs 10 lb. avoirdupois at 62° Fahr.

1 gallon = 277-27384 cubic inches.

Cubic foot of sea water weighs 63-96 lb.

"Weight of sea water = 1-027 weight of fresh water.

1 inch of rainfall = 100 tons per acre.

Pressure of water in lb. per sq. inch = '433 head of water.

Expansion of water between 32° Fahr. and 212° Fahr. = -04775.

Dip of horizon (including refraction) in nautical miles = 1-16 Vheight.

Marks on hand lead-line for sea soundings 1, 2, and 3 fathoms, 1, 2,
and 3 tags of leather respectively ; 5 and 15 fathoms white rag ; 7 and
17 fathoms red rag ; 10 fatlionis leather with hole in it ; 13 fathoms blue
rag ; 20 fathoms 2 knots : 30 fathoms 3 knots, &c. A small knot is placed
at intermediate 5 fathoms after 20 fathoms — viz. at 25, 35, 45, &c.

Pressure of wind in lb. per sq. foot = 0-002288 (velocity in feet per
second)*.

Areas and Volumes

Area of triangle = base x i perpendicular.

Volume of wedge = area of base x i perpendicular height.

Volume of cone or pyramid = area of base x ^ perpendicular height.

Surface of side of cone = circumference of base x ^ length of side.

Area of parabola = base x I height.

Velocity of light =• 186,377 statute miles per second.^

Wave-length of yellow light = - inch.

° -^ ^ 43100

Number of vibrations per second 508,961,293,000,000.

Falling Bodies

S, space fallen in feet ; V, velocity in feet per second ; g = 32-2 ; t, time
in seconds.

S = -^i! ; V = V2~g . ^/S = 8-025 ^ S.

Arithmetical Progression

A, first term ; B, last term ; S, sum ; d, difference between terms ; n,
number of terms.

' Latest determination by Prof. Newconib, of Washington.

USEFUL TO THE MICROSCOPIST

A = B-cZ(7i-l). B = A + (Z(»-1). S = {A + B)

Geometrical Fro;/ reus ion.
m, multiplier or divisor.

II35

B

B = A»i("-». S =

B ?K — A

m — 1

Properties of Circles aiicl Spheres d~c
TV = 3-141592653589798 +

log 77 = 0-4971498727
1

77-' = 9-8696

^/7^ = 1-77245

- = -31831

77

1= -10132

77"

^ = -56419

VTT

- = -785396
4

^ = -5236
6

-v/2 = 1-41421

^/27/ = 8-02496

Circumference, C. Ai-ea, A. liadius, r. Diameter, d. Volume, V.
Surface, S.

6

degrees in arc x area of circle

C = 2 77 r = 77 <f. A = ^

S^

d'. V = '

^,=^.

x\rea of sector of circle = ="

360

Length of arc = number of degrees x -017453 r.

Unit of circular measure = 57°-29578.

Side of square of equal area to a circle = /■ a/77.

Side of inscribed square = r \/2.

Area of ellipse = ^ major axis x ^ minor axis x 77.

Volume of spheroid = polar axis x (equatorial axis)- x !r.

6

Number of Threads per inch in Whitworth^s Standard Screws

Sizes I . . No. of threads 20

18
16
14
12
11

Convenient Approximations for rapid Calculations

0 knots = 7 miles, more correctly 13 knots = 15 miles.
8 kilometres =5 ,, „ „ 50 kilometres = 31 ,,

1 metre = 3 ft. 3^ in. „ „ 32 metres = 35 yards.
5 centimetres = 2 inches ,, ,, 33 centimetres = 13 inches.
3 millimetres = ^ inch „ ,, 5 millimetres = 1 inch.

1 pole = 5 metres ; 1 furlong = 2 hectometres.

5m = oi/^TT inch ; ^^-^ inch = \ mm. ; xw(jVo if inch = \ n-

2 are = 239 sq. yds ; 1 rood = 10 are ; 2 acres = 81 are ; 100 hectare
= 247 acres ; 3 cubic yards = 23 decisteres ; 1 decastere = 13 cubic yards ;
2 decilitres = 7/3 (ounces) ; 4 litres = 7 pints ; 2 grammes = 31 grains ;

zes 3L

. No. of threads 150

)? Hi

80

. ^\

60

1
» 8

40

5

32

„ f'o

24

I I S6 APPENDICES AND TABLES USEFUL TO THE MICROSCOPIST

4 grammes = 15 (drachm) (apothecaries') ; 7 grammes = 4 dr. (drachms)
(avoirdupois).

5 kilogrammes = 11 lb. (avoirdupois).

50 kilogrammes = 1 cwt.

Nobert's 19 Band Test Plate
Band Lines per inch Band Lines per inch

1 11259-5

5 33778-5

10 61927-3

Difference between each band = 5629*75

15 90076-1

19 112595-1

Nobert's last 20 Band Test Plate

Band Lines per inch Band Lines per inch

1 11259-5 I 15 168892-7

5 56297-6 ( 20 225190-8

10 1125951 I

Difference between each band = 11259'5.

Convenient Formula for Lantern Projection or Enlargement and
Reduction,

Let D be the distance of the screen, and d the distance of the object
from the optical centre of the lens, F the equivalent focus of the lens, M
the magnifying power or ' number of times ' for enlargement or reduction,
then —

D = F(M + 1); d = F + ^=^-, F=_5_^-; M = ^-;
^ ' MM M + 1 F*

Example : It is required to project by a lens of 6 inches equivalent
focus a slide having a 3-inch mask so that it may give a 10-ft. disc, what
must be the distance of the screen ? Here M the magnification will be
40 times. D = F (M + 1) = 6 (40 + 1) = 246 inches = 20^ feet.

Note, in a double combination the optical centre may be assumed to
be half way between the lenses. To reduce, interchange the object and the
screen.

Royal Microscoxncal Society's Gauges

Substage 1-527 inch = 38-786 mm.

Eyepieces, No. 1 -9173 „ = 23-300 „
„ 2 1-04 „ =26-416 „
„ 3 1-27 „ =32-258 „
» „ 4 1-41 „ =35-814 ,.

For the screw of the objective and nose-piece the Society supply, at
cost price, a sizing die and plug, also screw chasers. They do not, how-
ever, supply standard screw gauges. Full particulars regarding the
screw will be found in the ' E.M.S. Journal,' 1890, p. 389.

INDB

IT

-/.:

ABB

A

Abbe (Prof.), on amplifying power of lens,
26 ; on homogeneous immersion, 28 ;
on improvement of optical glass, 32 ;
on classification of eye-pieces, 34 ; on
principle of microscox^ic vision, 43, 44,
45 ; on definition of aperture, 44 ; on
aperture, 48 note ; on radiation, 57 ;
on angle of aperture, 60, 61, 62 ; on
diffraction, 63-75 ; on ' intercostal
points,' 78; on 'penetration,' 82; on
over-amplification, 90; on stereoscopic
vision, 90, 94 ; on ' aplanatic system,'
94 ; on ortlioscopic efllect, 95 ; on Rams-
den's circles, 106 ; on solid cones of
light, 418

— his stereoscopic eye-piece, 102 ; camera
lucida, 281-284 ; apertometer, 807,
390-396 ; chromatic condenser, 308-
309 ; achromatic condenser, 311-312,
385 ; condenser, iris-diaphragm fitted
to, 312 ; diffraction theory and homo-
geneous immersion, 364 ; compensa-
tion eye-piece, 378 ; method of testing
object-glasses, 384-387; test plate,
387-390 : experiments in diffraction
phenomena, 434

Aberration, 19; positive, 21, 360 note;
negative, 21, 27, 360 7iote; chromatic,
31 ; spherical, 31, 301, 806, 388 ; errors
of spherical and chromatic, corrected
by Ross, 357

Abies balsamea, 443

Abiogenesis, 761

Abraham's prism, 401

Absorption or dioptrical image, 64

— and diffraction images due to diffrac-
tion, 65 note

— of light rays, Angstrom's law, 323

— bands, 323-327
Abstriction of spores, 633
Acaleplice, sexual zooids of polypes, 862 ;

relationship to hydroids, 872 ; develop-
ment of, 874 ; medusan phase of, 877

Acanthometra xipliicantha , 850 ; ecM-
noides, 852

Aca7ithoinetrina, 846, 852 ; central cap-
sule of, 852

Acari7ia, eggs of, 1004-1006 ; anatomy of.

1009-1012 ; larvee of, 1009 ; nymph of,
1009; integument of, 1010; legs of,
1010 ; eyes of, 1011 ; classification of,
1012
Accommodation, of the eye, 88 ; depth, 89
Acetabularia, 563 ; pileus of, 563
Acetic acid, as a test for nuclei, 517
Acheta, 987

— campestris, eggs of, 1005

Achlya, zoospores of, 564 ; oospores of,

565 ; zoosporanges of, 640
— ■ prolifera, 563 and note, 563
AcJmanthecB, characters of, 615
Achnanthes, frustules of, 588, 615 ; stipe
of, 588, 615 ; ' stauros ' of, 616 ; struc-
ture of frustule, 615
Achnanthes longijjes, 615
Achromatic, comparison of, with chro-
matic and apochromatic lenses, 368

— condenser, Abbe's, 260, 308-312, 314 ;
Powell and Lealand's, 301, 311 ; for ob-
servation of pycnogonids, 959

— doublet, Fraunhofer's, 148 ; meniscus,
376

— lenses, Charles's, 148 ; Marzoli's, 353 ;
Selligue's, 354

— objectives, 19, 32 ; Tully's, 354 ; Wen-
ham's, 361, 862 ; cover slips for use
with, 439

— oil condenser, Powell and Lealand's,
310, 811

Achromatism, 17, 19, 150 ; in j)hoto-
micrography, 34 ; rise of, 147 ; in-
augurated, 365 ; imperfect, causing
yellowness, 417

Acineta, 783

' Acinetiform young ' of Ciliata, 782 note

Acinetina, 783; food of, 783

'Acorn' monad, 759

' Acorn-shells,' 967

Actinia, reproduction from fragments,
863

— Candida, thread-cells of, 879

— c7~assicornis, thread-cells of, 879
Actinocyclus, 588, 610, 620
Actinonima inerme, 850, 851
Actinophrys, 846

— form of Microgromia, 786

— sol, 737-742
Actinoptychus, 588, 610, 611

-1 D

II38

INDEX

ACT

Actinosphcerium Eicliornii, 741

Actiiiotrocha, 950

AcTiNOZOA, 863, 877-883

Actius, on myopy, 118

Actuarius, on myopy, 118

Adams' variable microscope, 142; non-

acliromatic microscope, 148
'Adder's tongue ' fern, 679 ; sporanges of,

675
Adipose substance, 1045
Adjusting objectives, Ross's, 357, 360
Adjustment, coarse, 159-162 ; Swift's

diagonal rack, 161 ; Nelson's stepped

rack, 161 ; Wale's, 224-226

— fine, 162-175

Ross's, 153, 155, 175 ; to Pritchard s

microscope, 153 ; Watson's long
lever, 162, 172, 175 ; ' Continental '
type, 162 ; Swift's vertical side
lever, 16?, 173, 174; Campbell's
differential screw, 164, 165, 175,
230 ; Zeiss's, 166, 175 ; Reicliert's
new lever, 171, 175 ; Powell's, 174 ;
short side lever, 174 ; speeds of,
175 ; to the sub-stage. Nelson's,
185 ; for Powell and Lealand's sub-
stage, 186

— screw collar, 360

jEcidiospore generation of Puccinia,
637

^cidium berberidis, relation to Puc-
cinia, 637

• — tussilaginis, 638

^thaliuin septicum, plasmode of, 634

Agaricus, 647

— -campestris, 648
Agate, 1095

Agave, leaf of, 686 ; raphides of, 696
Agrion, 987

— piUchellum, wing of, as test for defi-
nition, 426

— puella, pupa of, 994 ; wing of, 994
Air-angle, 50, 78

Air -bubbles, microscopic appearance of,

429, 430
Air-chamber of leaves, 716
Alas of Surirella, 606
' Alar prolongations ' in Fusulina, 826 ;

in Nitmmnlites, 827, 831 ; of Galca-

rina, 830
Albite, 1080
Albuminous substances, tests for, 516,

517
Alburnum, 704, 708
Alcohol, as solvent for resins, &c., 517;

as hardening agent, 484
Alcyonaria, 877, 879; spines of, imi-
tated, 1101
Alcyonian, resembled by polyzoan, 908
Alcyonidiimi, 908 ; polyzoary of, 909
— gelatinosum, calcareous spicules in,

908 note
Alcyonium digitatum, 879 ; spicules of,

880
Alder, on branchial sac of Co7'ella, 912

note
Alexander, on myopy and presbyopy, 118

ANA

AjjGM, preparation of, 514; included
under general term of ' thallophytes,'
540 ; symbiotic in radiolarians, 848

— Ume secreting, 1084

Algal constituents of lichens, 650

Alkaloids, micro-chemical examination of,
1103

Allman's experiments on luminosity of
Noctiluca, 769

Alhnan, on Polyzoa, 909 ; on the ' Haus '
of AjjpeMdicularia, 918 ; on Myrio-
thela, 863 note

Aloe, raphides of, 696

Alternation of generations in Batracho-
spermuin, 575; \\\ Fungi, &'d'^; in ferns,
680; in MeduscB, 877

AlthcBa rosea, pollen-grains, 721

Alveolina,^Qi.\ resembled hy Loftusia,
818 ; resembled by Fusulina, 825

AmaranthacecB, pollen-grains, 721

Aniaranthus Inipocliondriacus, seeds of,
724

Amarouciuni proliferum, as example of
compound ascidian, 912

Amici suggests oil for immersion lenses,
29

Amici's invention of immersion system,
27 ; horizontal microscope, 148 ; camera
lucida, • 279 ; objectives, 355 ; triple-
back objectives, 361 ; water-immersion
objectives, 362 ; oil-immersion objec-

■ tives, 364

Ani7nodiscus, 814

Ammothea pycnogonoides, 958

Amoeba, 733, 742-747, 1018

A?7iceba-]yh.a,se of Manas, 756

Amoeba protects, 742

— radiosa, experiments on, 743
Amcebce, cells of sponges resembling,

855
Amceboid phase of Tetramitus, 761
Amphibians, plates in skin of, 1026
Amphioxus, affinities with ascidians, 917

note
Ampliipleura pellucida, with oblique

illumination, 59, 75 ; resolution of, 85 ;

markings measured, 274 ; markings

on, 592
Amphistegina, 827 ; internal cast of,

841
Ampliitetras, 614

Amphiuma, red blood-corpuscle of, 1036
Amphonyx, haustelUum of, 992
Amplification, 88-90

— linear, 26, 39 ; of images, 45
Ampullaceous sacs of sponges, 856, 857
Anabmna, 548

Anacharis, 528

— alsinastrum, cyclosis in, 689 ; habitat,
690

Anagallis, rapliides of, 696 ; seeds of,
724

— arvensis, ]}etals of, 719
Anal plate of Antedon, 903
Analgesina, 1013, 1014
Analyser, 318
Analysing nose-piece, 294

INDEX

1 1 39

-Analysis, micro-chemical, 1102 ; method

of, 1102
Aiiarapliidece , 599
Anchor-like jjlates of Synajjta, 895
Andalusite, 1077
Androapore of CEdogoyiiuni, 572
Anemones, 8(53. See Actinozoa
Anemophilous flowers, 722
Anetltum graveolens, seeds of, 724
Angle, extinction, 1079

— of incidence, 3 ; of refraction, 8 ;
of ai^erture, 61

Angles of aperture, air, balsam, oil,

water, 88-87
Angstrijm's law for the absorption of

light rays, 823
Anguillula aceti, 945

— Jiuviatilis, 945

— glutinis, 945
Anguillulce, 945
Angular aperture, 895

of dry objective, 891 ; of oil immer-
sion, 891

of aperture, resolution dependent

on, 44

of water immersion, 391

Angular distribution of rays, 56 ; grip,
61 ; semi-aperture, 77

AnguUferce, characters of, 618

Animal kingdom, two divisions of, 727

Animalcule cage, 846

Animalcules, 753. See Eotifeba, Infu-
soria, Rhizopoda, &c.

Animals and plants, differences between,
530

AnisocheliB of sjponges, 860

Anisonema, 765

Annual layers in trees, 704

Annular cell, Weber's, 850

— ducts of Phanerogams, 698

— illumination and false images, 419

— illumination for examining perforated
membrane of diatom, 419

Annulata (Annelids), larvtB of, collecting,
529 ; marine, 947 ; appendages of, 949 ;
jaws of, 949 ; development of, 949 ;
eggs of, 950 ; fresh-water, 955 ; lumi-
nosity of, 955 ; bibliography, 956 ;
' liver ' of, 1047

Annulus of sporange of fern, 676

Anodon, pearls in, 923 ; glochidia of,
933 ; for observation of ciliary motion,
940

Aiiomia, prismatic layer in, 924

Anopla (Nemertiues), 951

Anoplophrya circulans, 774

Anorthite, 1080

Antedon, food of, 771 ; pentacrinoid
larva of, 900-902 ; pseudembryo of, 903

Antennae of insects, 987 ; i^reparation of,
988, 989 note

Antherid of Vaucheria, 563 ; of Chnra,
511, 578; of Fucacece, 627, 628; of
FloridecB, 631 ; of Feronosporece, 638 ;
of Marchantia, 665, 667 ; of mosses,
670; of SphagnacecB, 673; of ferns,
677 ; tapetal cells in, 678

Antherozoids, 536, 540 ; of Volvox, 555 ;
of Vaucheria, 563 ; of Spliceroplea,
572; of (Edogonium, 572; of Batra-
chospermum, 574; of Chara, 579; of
PkcsosporecB, 627; of Fucacece, 628;
of ferns, 678 ; of Bhizocarpece, 681

Anthers, 719

Anthony (Dr.), on pseudo-trachese of fly's
proboscis, 990 note

Anthojphysa, 765

Antirrhinum majus, seed of, 724

Apertometer, 195,' 890 ; Abbe's, 307, 394 ;
Tolles', 390 ; use of, 894

Aperture, in microscopic objectives, 88,
42-50, 60 ; how obtained, 45 ; Abbe on
definition of, 45, 48 note

— angular, 49 note, 53, 395

— numerical, theoretical maximum of,
80

— numerical, 49 note, 53, 76, 390 ; for
dry objective, 50 ; for oil immersion,
50 ; for water immersion, 50

— numerical, of Zeiss's apochromatic
series of objectives, 371

— of objective, 890

— relation of, to power, 82, 83, 363 ; as-
certained by vertical illumination, 338

— table, Royston-Pigott's, 30

— numerical, table of, 84-87
Apertures, relative, 49
Aphanizojnenon, 548
Aphanocapsa, 547

Aphides, wings of, 998, 999 ; agamic re-
production in, 1006
Ap)hodius, antennte of, 988
Apidce, 987

Apis mellifica, mouth-parts of, 990
Aplanatic system, 20,28

— objective use of, 21

— cone, 307

— aperture, 309, 315

— foci. Lister's discovery, 355
Aj)ochromatic objectives, 19, 30, 33,

85, 80, 258; advantages of, 33, 34;
objective, Zeiss's, 365-371 ; dry, 368 ;
comparison of, with chromatic and
achromatic lenses, 368 ; homogeneous
objectives, value of, in study of
monads, 762; objective, use with
various test scales, 976

— condenser, Powell and Lealand's, 302
Apochromatism, 866, 369
ApocynacecB, laticiferous tissue of,

695
Apoganiy in ferns, 680
Apospory in ferns, 680
Axiotheces of lichens, 650, 651
Apx^areut creation of structure, 68
Ap2]endicularia, 911, 917; i^haryngeal

sac of, 917 ; tail of, 917 ; notochord, 918 ;

' Haus ' of, 918
Apple, raphides in bark of, 696
Apposition, growth by, 583

— mode of growth of starch, 695
Apus, 959, 962 ; parthenogenesis of, 964

note

— cancriformis, carapace of, 9G2

4 D 2

II40

INDEX

Aair

Aquarium microscopes, 266-269 ; J. W.

Stepiienson's, 267-268 ; Eousselet's,

268-269
Aquatic microscope, 147
Abachnida, 1008

— eggs of, 1005 ; related to Pycnogonicla,
959 note ; reproductive organs of, 1011

Arachnoicliscus, 588, 612
Arachnospliara obligacantha, 850, 852
Aragonite, 1095

— in shell of Pholas, 924

Aralia jjapyrifera, parenchyme of, 687

Araneida, 1008

Arcella, 746

Archegones of Vaucheria, 563 ; of Chara,
577, 578 ; of Marchantia, 665, 668 ; of
mosses, 670, 671 ; of Sphagnacecs,
673; of ferns, 677, 678; of Lyco-
podiecs, 681 ; of BhizocarpiecB, 681

Archer, on amosbiform phase of Stepha-
nosphcB7-a, 557 note ; on desmids, 579
note ; classification of, 585 ; on
Clathrulina, 742 note

Archerina Boltoni, 730

Arctium, stem of, 709

Arcyria flava, sporanges of, 635

Arenacea, 810-814

Arenaceous character of Textiolarinim,
823

— Foraminifera, varying size of particles
in test of, 818

— test of Foraminifera, 810
Arenicola, 948

Areola of frustule of Coscinodiscus, 591

Areolar connective tissue, 1040, 1045

Argas, bite of, 1012

ArgasidcB, 1012

Argonauta, 929

Argosincs, 1008

Argulus foliaceus, 966

Aristolochia, stem of, 709

' Aristotle's lantern ' of ecliinids, 890

Aristotle on myoj)y and presbyopy, 118

Arsenic, micro-chemical analysis of, 1103

Artemia, 962

— salina, movement of, 960 ; habitat of,
963

Arteries, 1056
Arthrodesmus incus, 568
Artheopoda, 957-1016 ; smallest of, 1008 ;
eye of, 983

— limbs of Pedalion compared with
those of, 792

Arthrosporous Bacteria, 657

Artificial light, 417

' Artificial lightning,' 682

Ascaris luinhricoides, 944

Asci of Ascomycetes, 642 ; of lichens, 650

Ascidians, diatoms in stomach of, 614,

623; solitary, 911; branchial sac of,

912, 913, 915 ; circulation in, 912, 915 ;

compound, 912 ; cloaca of, 913 ; stolons

of, 914 ; bibliography of, 914 note ;

social, 914 ; general structure of, 916 ;

development of, 916 ; tadpole of, 917 ;

affinities with Ampltioxus, 917 note
AsclepiiadecE, p)ollinium of, 722

BAG

ABcogoneoi Ascomycetes, M^; of lichens,
651

Ascomycetes, 642-645 ; as fungus-con-
stituents of lichens, 651

Ascopores of Ascomycetes, 642; of
lichens, 650

Asellus aquaticus, ciliated parasite in
blood of, 774

Asilus, eye of, 987

Asjjergillus, fermentation by, 647

Asphalte for cells, 446

— varnish, 443

Aspidisca, a phase in development of

Trichoda, 781
Aspidium, indusium of, 675 ; sori of, 675
Asplanchna, in confinement, 528
Astasia, 545 ; mouth in, 765
Asteroidea, skeleton of, 891 ; spines of,,

891 ; larva of, 897
Asterolampra, 595, 610
Asteromphalus, 610
Astromma, 849
Astrophyto7i, spines of, 891
Astrorhiza, 811, 815
Astrorhizicla, 812
Athecata, 868
Athyrium Filix-fcemina, apospory in,.

680
Atrium of Noctiluca, 766
Auditory vesicles of Mollusca, 941
Audouin, on ' muscardine,' 645
Augite, 1072 ; zonal structure in, 1073
Aiolaco discus, 612

— Kittonii, markings on, 591

— Sturtii, markings on, 592
Autofission of diatoms, 594
Auxospore, 594-601
Avanturine, 1095
Avicularia of Polyzoa, 910, 911
' Awns ' of ClicetocerecB, 614
Axile body of tactile papilla, 1053
Axinella piaradoxa, 858

Axis cylinders of nerve-tube, 1051, 1052'

BacillariacecB of Kiitzing, 587
Bacillaria varadoxa, movements of^

602, 606
Bacilli, form of, 653
Bacillus, ' granular spheres ' of, 660 note

— anthracis, 656 ; spores live in abso-
lute alcohol, 660

— megaterium, 655

— of anthrax, 1037 note

— of tuberculosis, modes of staining,
515, 516

— subtilis, 656, 657, 658 ; spores of, 660
' Bacon-beetle,' 980

Bacon (Roger), inventor of simple micro-
scope, 126

Bacteria, use of large and small cones in
examining, 421 ; j)hoto-mierographs,.
423 ; as test for definition, 426 ; staining,
514-516 ; (see Schizomycetes), 651 ;,
affinities to Algce, 651; to Flagellataf

INDEX

1 141

BAG

'651; to Nosfocnce(s, 652; movements

of, 652 ; mode of multiplication, 652 ;

forms of, 653 ; classification of, 655 ;

nutrition of, 658 ; fiagella of, 659 ;

germinating power of, 660 ; spores of,

660
JBacteriastrum fnrcattim, 614
Bacteriology, 662
Sacterium lineola, compared with

Cercomonas, 651

— lineola, 658

— termo, flagellum of, 72, 658; move-
ment of, 653 ; zoclgloea of, 659 ; germi-
nation of, 660

Bailey, on internal casts of Foramini-
fera, 828 note

Bailey's method of isolating diatoms,
624

Baker, on Cuff's microscope, 142

Baker's microscopes, 202, 218-221, 280,
246, 251 ; mechanical stage, 181 ; sub-
stage, 188, 189 ; achromatic condenser,
306 ; fitting for condenser, 312

— lamp, 407
BalanidcB, 967

Balanus balanoides, 967 ; disc of, 968
Balsam angle, 50, 78

— refractive index of, 77
BaiiTtsia, stomates of, 716
Barbadoes earth, 846, 849
Bark, 700, 702, 708

Barker's Gregorian telescope, 145

Bar movement, 262

' Barlow lens ' applied to a microscope,
149

Barnacles, 967. See Cirripedia

Basals of Antedon, 901

Basidlomycetes, 647 ; as fungus consti-
tuent of lichens, 651

Basidiospores of Basidiornycetes, 647 ;
of Hymenomycetes, 647

Basids of Puccinia, 637 ; of Basidiorny-
cetes, 647

Bast, 710

Bat, parasite of, 1012 ; hair of, 1030 ;
cartilage in ear of, 1046

'' Bathyhius,' 747

BatracJiia, red blood-corpuscles, 1035 ;
lungs of, 1063

Batrachospermece, 574

Batrachospermum moniliforme, 575

— protoneme of, 575

' Battledore scale ' of LyccsnidcB, 975

Bausch and Lomb's microscopes, 212-214,
222, 239, 247, 252; mechanical stage,
183, 184; sub-stage, 212; chemical
microscoiDe, 263 ; camera lucida, 285 ;
objectives, 375

Bdella, maxillary palps of, 1010

Bdellidce, 1013

Bdelloida, 791

Bead-moulds, 645

Beale's camera, 279, 288 ; glycerin method
of preserving, 520

Beale, on organic structure, 1017

Beck's microscopes, 228, 233 ; mechani-
cal stage, 184 ; rotatory nose-piece, 291 ;

condensers, 304, 305 ; side reflector,
333 ; vertical illuminator, 337 ; disc-
holder, 339 ; compressor, 347 ; rings
for locking coarse adjustmeait, 352 ;
objectives, 375 ; lamp, 405-407 ; achro-
matic binocular magnifier, 456 note ;
disc-holder for examination of Fora-
minifera, 845

Beck (R.), on markings of Podura scale,
978

Bee, hairs of, 980 ; head of, 982 ; wing of,
994, 998 ; sting of, 1003

Beeldsnyder's achromatic objective, 147

Beetles. See Coleopitera

Beggiatoa, form of, 652, 653

— alba, 653-655
Begonia, seeds of, 724

Behrens' method of analysing minerals,

1083 ; of micro-chemical analysis, 1102
Bell (Jeffrey), on the spines of Cidaris,

889
Bell's cements, 443, 479
Beneden (Ed. Van), on Gregarina gi-

gantea, 749 note ; on movement of

gregarines, 750
Benzol, uses of, 517
Bergh, on Flagellata, 764
' Bergmehl,' 622
BerJceleya, 602
Bermuda earth, 608, 611
Beroe, collecting, 529

— ForskaUi, 881, 882

— ovatus, Eimer on, 882 note
Bicellaria ciliata, 910

Biconvex lens, formulse relating to, 21
Biddulpliia, 612

— cyclosis in, 587 ; chains of, 588, 596 ;
structure of frustule, 590 note

Biddulphiece, character of, 612

Biflagellate monad, 759

Bignonia, seed of, 724

Bignoniacece, winged seeds, 724

Biloculina, 802

Binary subdivision of cell, 535, 536

Binocular eye-isieee, Tolles', 101 ; Abbe's,
102

Binocular magnifier, Beck's achromatic,
456 note

Binocular microscope, 61, 97

Riddell's, 96 ; Nachet's, 98 ; stereo-
scopic, Wenham's, 98 ; Stephenson's,
100 ; Stephenson's erecting, 100 ;
stereoscopic, for study of opaque ob-
jects, 103 ; non-stereoscopic, 105 ;
Powell and Lealand's high-power, 105,
106 ; Eousselet's jportable, 245 ; Ste-
phenson's for dissection, 248, 401, 456 ;
spectrum microscope, 327

Biology, 530

' Bipinnaria,' resemblance of Actino-
trocha to, 950

Bipinnaria asterigera, 897

Birch, pollen-grains of, 722

Bird, x^arasite of, 1012, 1014 ; lacunae in
bone of, 1022 ; epidermic appendages
of, 1029 ; red blood-corpuscles of, 1034,
1035 ; lungs of, 1064

1 142

INDEX

BIK

Bird's egg, concretions on shell imitated,

1102
* Bird's head processes,' see Avicularia,

910
Bismarck brown, 489
Bivalves, structure of ligament in, 1010
' Black dot,' Nelson's, 277
' Bladderwrack,' 627
Blanchard (R.), on Sporozoa, 719 7iote
Blatta, antennse of, 988

— orientalis, eggs of, 1005
Blenny, scales of, 1027

Blood, colourless corpuscles, 101 8; method
of mounting, 1038 ; circulation of, 1054 ;
flow of, 1055 ; micro-chemical examina-
tion, 1103

— of insects, circulation of, 993, 994 ;
of Vertebrata, 1034

Blood-corpviscle, relation of size to that

of bone lacunee, 1022
Blood-corpuscles of Vertebrata, 1034
Blowfly's maxillary palpus, hairs on,

examination of, 422
Blowfly, proboscis of, examination with

apochromatic, 371 ; hairs on, as test

for definition, 426

— development of, 1007 ; ' imaginal discs '
of, 1007

' Blue mould,' 643

Bodo, 545

Body of the microscope, 157

' Bog -mosses,' 673

Boletus, 647

Bombyx, 987

— ■ mori, eggs of, 1005

Bonannus's microscope, 132 ; his hori-
zontal inicroscoxje, 133, 134 ; his com-
pound condensers, 298, 299

Bone, 1020 ; structure of, 1020-1023 ;
preparation of, 1023 ; matrix of, 1039 ;
decalcification of, 512

Bones, fossihsed, 1090-1092

' Bony pike,' scale of, 1022

Borax carmine, 490

Bordered pits in the trachei'des of coni-
fers, 698, 703

Boscovich, on chromatic dispersion, 42

Botryllians, 914

Botryllus violaceus, 915

Botryocystis, 545

Botrytis bassiana, 645

Botterill's growing slides, 340 ; his
zoophyte trough, 348

Bouguet, on uniform radiation, 51

Bowerbank, on sponge spicules, 859 note ;
on structure of molluscan shells, 921,
928

Boweriankia, gizzard of, 905 ; stem of,
908 ; polyzoaries of, 909

' Box-mite,' 1012

Brachimis, antennse of, 988

BracMonus rubens, 787-790, 791 ; male
of, 790

BRAcraopoDA, shells of, 919, 925-927;
relation of shell to mantle, 926 ; affini-
ties to Polyzoa, 927

Brachyurous decajjods, young of, 969

Brady (H. B.), on Foraminifera, 810 ; on
arenaceous Foramiiiifera, 811 ; on
affinity of Carpenteria, 823

Brady and Carpenter, on fossil Lituolce,.
817

' Brake-fern,' 675. See Aspidiuni

Bran, 725

Branchiae of annelids, 948, 949

Branchiopoda, 959 ; divisions of, 961

Branchipus, movement of, 960

— stagnalis, 962, 963
Branchiura, 965 note, 966

Brandt (K.), on artificial division of
Actinosplicerium, lil note ; on
zooxanthellee, 848

Braun, on Pediastriim, 567

Brewster, his hand magnifier, 37 ; on
modification of stereoscope, 91 ; on
' lens ' from Sargon's palace, 119 ; his.
' Treatise on the Microscope,' 120 ; on
achromatic condensers, 299, 300

Bright-line spectro-micrometer, 325

Brightwell, on Triceratiiivi, 613 note ;
on ChcBtocerecE, 614 note

Brilliancy of image, 382

' Brimstone moth,' eggs of, 1005

Brine shrimp, 960, 963

'Brittle stars,' 891. See Opihiuroidea

Brooke's nose-piece, 291

Brownian movement, experiments, 431,.
432

Browning's bright-line spectro-micro-
meter, 325

Briicke lens, 38

Brunswick black as a black ' ground,'^
444 ; for cells, 446

BryacecB, 673

Bryobia, 1013

Bryony, cells of ]Dollen-chambers, 720

Bryozoa, 904. See Polyzoa

Bryiom interinedium, peristome of,.
672

Bubbles in cavities of crystals, 1074

Buccinum, 937

— tmdatum, palate of, 930, 932, 933 ;
nidamentum of, 934

Bucliner's experiment on spores of

Bacteria, 660
Buckthorn, stem of, 703
Bug, mounting mediuin for, 973
Bugula, polyzoary of, 909
— • avicularia, 910, 911
Built-up ' cells,' 449
Bulbils of Nitella, 577
Bulloch's modification of Zentmayer's

microscope, 204
Bull's-eye, 300; use of, 329-333, 407-

420 ; with high power, 331; for use in

study of saprophytic organisms, 333 ;

Powell and Lealand's, 333
Bull's-eye stand, 248
Bundle-sheath, 711
Burdock, stem of, 709
Biitschli, on mouth of Astasia, 765 ; on

VorticellcB, 773 note

— and Engelmann, on conjugating vorti-
ceUids, 782

INDEX

1 143

Butterflies, wing of, 994
Butterfly. See Lepicloptera

Cabbage-butterfly, eye of, 983 ; number

of facets in, 983 ; eggs of, 1005
Caberea Boryi, vibracula of, 907 note
Cabinet for slides, 523 ; arrangement of,

523
Cactus, cells of pollen-chambers, 720
Cactus senilis, raphides of, 696 ; brittle-

ness of, 696
Cacumaria crocea, development of, 900

note
Calamites, 1084
Calathus, antennse of, 988
Calcarina, 825, 830 ; compared with

Eozoon, 838
Calcispongice, spicules of, 859
Calcite in shells, 924
Calco-globuline, 1101
Callitliamnion, 630

Calosanthes indica, winged seed of, 724
Calotte diaphragms, 297
Calycanthiis, bark of, 709
Calycine monad, 760
Calycles of hydroids, 868
Calypter of mosses, 671
Calyx of Flagellata, 764
Cambium, 710

— in Exogens, 697
'— layer, 708

Cambridge rockmg microtome, 469
Camera lucida, 277 ; Soemmering's, 278 ;
Wollaston's, 278; Amici's, 279; Nel-
son's, 279, 280; Beale's, 279, 288
Cooke's, 280, 281; Abbe's, 281-284
Swift's modification of Abbe's, 284
Bausch and Lomb's modification of
Abbe's, 285 ; Schroder's, 285, 286
Campani's microscope, 128 ; eye-piece, 376
Campanula, pollen-grain of, 721
Cainjiaiiidaria, 870

— gelatinosa, 865
Campanulariida, 870 ; zociphytic stage

of, 877

Campbell's differential screw, 162, 164,
165, 174, 202, 230

Campylodisciis, 587, 588, 595 ; move-
ments of, 602 ; structure of frustule,
606

— clypeus, 607

— • spiralis, cyclosis in, 587

Canada balsam, 443

capped jars for, 477 ; as mounting

medium, 480, 521 ; as a preserv-
ative medium, 518 ; mode of pre-
paration, 518 ; refractive index, 521 ;
for mounting insects, 973

Canal system of Calcarina, 825 ; of
Polystomella, 827 ; of Nummulites,
827

CanaHculi of bone, 1019, 1021

Cancellated structure of bone, 1020

Cancer pagurus, skeleton of, 968

Canna, starch-grains of, 695
Cannocehiale, 125
Capacity of object-glass, 382
Capillaries, 1056, 1062
Capillitium of Myxoniycetes, 636
Capsule, central, of Badiolaria, 847

— of mosses, 670 ; of Purpura, 934

— silicious, of Clathrulina, 742
Carapace of Cofepoda, 960 ; of Clado-

cera, 961
Carbon bisulphide as a solvent for oils,

&c., 517
Carboniferous epoch, vegetation of, 681

— limestone, 1090
Carchesiuvi, collecting, 527
Carcinus mcenas, metamorphosis of^

970
Carnation, parenchyme of, 688
Car7iivora, arrangement of enamel in,

1025
Carp, scales of, 1027
Carpenter (H. P.), on crinoids, 903 note
Carpenter (W. B.), on stereoscopic vision^
90-93 ; on classification of Foramini-
fera, 799 ; on Eozoon, 838 ; on alter-
nation of generation in Medusce, 877 ;
on the so-called excretory pores of
Ctenophora, 882 note ; on development
of Antedon, 903 note ; on structure of
molluscan shells, 921
Carpenteria, 822 ; mode of growth com-
pared with Eozoon, 838

— rhaphidodendron, 823
Carpogone of Floridece, 632 ; of Ascomy-

cetes, 643
Carpospores of Floridece, 632
Carrot, seeds of, 724
Carter (H. J.), on affinity of Carpenteria,

823
Cartilage, 1046 ; mounting. 1047
Garum carui, seeds of, 724
Caryopihyllia, lamellse of, 878

— Smithii, thread-cell of, 879

Cascarilla, raphides of, 696

Cassowary, egg-shell of, 1101

Castracane, on beaded structure of di-
atoms, 593 ; on Pfitzer's auxospores,
595 ; on sporangial frustules of
diatoms, 595 ; on reproduction of di-
atoms, 597 ; on diatoans, 598

Cat, Pacinian corpuscles of, 1053

Caterpillars, ' pro-legs ' of, 1002 ; feet of,
1002

Cathcart's freezing microtome, 474, 475

Catoptric form of microscope, 145, 146

Caulerpa, 563

Cauterisation by focussing the sun's rays
(PHny), 117

Cedar, stem of, 705

Cell, contents of, 533-535 ; binary sub-
division of, 535

Cell-division and nucleus, 1018, 1019 and
note

' Cell ' of Polyzoa, 904

Celloidin imbedding method, 503-506

— staining and mounting, sections, 506
Cell-sap, 584

1 144

INDEX

CEL

* Cells ' for examining Infusoria, &c.,
349 ; for dry mounting, 445 ; of cement,
446 ; paraffin, 446 ; paper, 446 ; ring-
cells, 446 ; of plate-glass, for zoo-
phytes, &c., 448 ; built up, 449 ; sunk,
449 ; mounting in, 482-484 ; of bone,
483 ; of tin, 483

Cells of plants, 532 ; multi-nucleated, 534 ;
primordial, 536; of vertebrates, 1018

Cell-structure, Strasburger on, 537

Cellular cartilage, 1046

— parenchyme, 688
Cellulose, 533

— tests for, 516, 517 ; envelope of des-
mids, 580; in Dinoflagellata, 770; in
zoocytium of Oplirydium, 778

Cell-wall, 533 ; mode of growth of, 533 ;
apposition, 533 ; intussusception, 533

Cell-wall of Phanerogams, 692

Cement-cells, 446

Cements, 442 ; liquid, 442 ; Bell's, 443,
479 ; japanner's gold size, 443 ; Bruns-
wick black, 444 ; glue and honey, 444 ;
shellac, 444 ; Hollis's liquid glue, 444,
479 ; Venice turpentine, 444 ; marine
glue, 445 ; Heller's porcelain, 521

■Cementura of teeth, 1025, 1026

Centixjedes. See Myriopoda

Central capsule of Badiolaria, 734

Centring, 382, 389

Centring nose-piece, 293 ; as sub-stage,
230

Centro-dorsal plate of Antedon, 902

Geijlialolithis sylvina, 847

Cephalophorous moUusca, palates of,
930-933

Cephalopoda, 929

— organs of hearing in, 941 ; chromato-
phores of, 942

CeramiacecB, 630
'Ceramiiom, 630
Geratiuni, 111

— furca, 111

— tripos, 111
Ceratodios, 1091

Cercomonas typica, compared with Bac-
teria, 651
Cereals, seeds of, starch in, 694
Cemra vimda, eggs of, 1005
Cestoid, 943

Cetonia, antennae of, 988
ChcBtocerecB, affinities of, 614

— ' awns ' of, 614

— - occurrence in marine animals, 614

Ghcetoceros WighamU, 614 *

Chatophoracece, 573

■' Chaif-scales,' silex in, 715

Chalk, microscopic constituents of, 1085,

1087 ; resemblance to Glohigerina

ooze, 1087 ; mode of preparation for

examination, 1088
■Chania, prismatic layer in, 924
Chamberlets in Foraminifera, 798, 803,

804 ; of Parkeria, 817 ; in Fusulina,

825 ; of Gycloclypeus, 835
GhamidcB, Foranmiifera, attached to

shells of, 845

Changes of form of white corpuscles, 1037

Ghantransia and Batrachospermum, 575

Ghara, 576, 669 ; antherozoids of, 667

CharacecE, 575-579

Charles's achromatic lenses, 148

Cheese-mite, 1008, 1013

Gheilostomata, characters of, 909 ; ex-
amples of, 910

Gheirocephalus, 962, 963

Chemical tests for biolpgical work, 516,
517

Cherry-stone, section of, 693

Chert, 1089

Cherubin d'Orleans, his binocular micro-
scope, 130, 131 ; his compound micro-
scope, 130

Chevalier, on Charles's achromatic
lenses, 148

Chevalier's combination of lenses, 38 ;
achromatic microscope, 148, 150 ; mo-
difications of Selligue's lenses, 354 ;
objectives, Lister's note on, 354, 355

Cheyleti, 1013

CheyletidcB, tracheae of, 1011

Gheyletus, hairs of, 1010; legs of, 1010;
mouth parts of, 1010

Chickweed, petals of, 719

Chicory, adulteration of, 725 note

Chilodon, mouth of, 774

— cuculliilus, binary division of, 777, 779
Chilognatha, 981

Chirodota violacea, ' wheels ' of, 896
Chitin, in test of Arcella, 746 ; of insects'

skin, 974
Chitinous substances, mounting, 481
Chiton, shell structure, 928 ; eyes on

shells of, 941
Chlamydotnonas, 545
Ghlamydomyxa, affinity with Moiiero-

zoa, 121
Ghlamydospores, of Mucorini, 641 ; of

gregarines, 750
Chloral hydrate as a preservative me-

diuni, 519
Chloroform, uses of, 517
Chlorophyll corpuscles, 534, 535
Ghlorospiorece, 625
Choedata, 911. See Vertebbata
Choroid coat of eye, pigment-cells in,

1042
Chromatic, comparison of, vs^ith achro-
matic and apochromatic lenses, 368

— aberration, 16, 17, 31

— condenser, Abbe's, 308, 309 ; 311, 312 ;
385

— Powell and Lealand's, 301-303

— correction, test for, 388

— dispersion, diminished by Huyghens'
objective, 42

Chrumatophores of Peridinium, 770 ; of

Cephalopods, 942
Chromatoplasm, 537
GhroococcacecB, characters of, 547
Chroococcus, 547 ; as gonid of lichen, 651
Chroolepus, as gonid of lichen, 651
Ghrijsaora, 874, 876; development of,

876

INDEX

I 145

Chyle, corpuscles in, 1037

Chytridiacece, 636

CicadcB, wings of, 998, 999

OiclioriacecB, pollen-grains of, 721

Cicindela, 987

■Cidaris, spine of, 885, 888

— ■ metularia,niodie of formation of spines

in, 889
■Cienkowski, on decaying cells of Nitella,

579 note ; on parasitic plasmode in

Nitella, 579 note ; on reproduction of

Noctiluca, 769
•Cilia, 532, 1044 ; of Infusoria, 771 ; use

of, in Ciliata, 773 ; of Turbellaria, 946
Ciliary action, 772

— motion on gills of Mollnsca, 940

— movement in protophyes, 535
Ciliata, 771-783 ; ciliary action of, 772,

774; 'shield' of, 773; loriea of, 773;
myophan-layer, 773 ; trichocysts of,
773 ; ento-parasitic forms, 774 ; mouth
of, 774; foot-stalk in, 774 ; impression-
able organs of, 775 ; ' eye-spots ' of,
775 ; food of, 775 ; artificial feeding,
776 ; contractile vesicles of, 776 ; mul-
tiplication of, 777 ; conjugation of, 777,
782 ; encystment of, 778-782 ; disper-
sion of, 781 ; desiccation of, 781 ; Stein
on acinetiform young of, 782 note

Ciliate Infusoria, general structure of,
754

Ciliated epithelium, 1044

Ciliobrachiate zoophytes, 905

Cilio-Jiagellata, 770

Cilium of Noctiluca, 766 note

Cimex lectularius, eggs of, 1005

Cinchona, raphides of, 696

Cinclidiuni arcticum, peristome of, 672

Cineraria, pollen-grains of, 722

Cmeritious matter, 1052

Circulation in ascidians, 912, 915

— of blood, 1054

Circumambient chamber in Orhitolites,

806
Cirrhi of Cirripedia, 968
Cirripedia, 967
Cladocera, 961
Cladococciis viminalis, 851
Cladonia furcata, 650
Cladophora glomerata, 574 ; cell division

of, 569, 574
Cladorhiza inversa, 860
Claparede and Lachiuann, on Lieber-

kuehnia, 731 ; on ' rolling ' movement

of Amoeba, lii
Clark (James), on Flagellata, 764
Clastic rocks, 1075
Clathrulina elegans, 742
Clausius on emission of light, 54
Clavelinidce, gemmation of, 911 ; stolons

of, 914
Claviceps jpurpurea, 644
Clavicornia, antennse of, 987
Claws, 1029, 1033
Clay, 1092

Cleanliness, importance of, 522
Clematis, stem of, 702

COL

' Closed ' bundles, 710

Closteriiim, cyclosis in, 581 ; ' swarming
of granules ' in, 581 ; binary division
in, 582 ; two zygospores in, 584 note ;
zygospore of, 584 ; form of cell, 585

Clostridia, form of, 653

' Clothes-moth,' 999

Clove-pink, seed of, 723

' Club-mosses,' 681

Clypeaster, spines of, 889

Coal, ' bituminous,' 1084

Coal-plants, 1083

Coarse adjustment, 159 ; ' stepped ' rack-
work for, 161 ; arrangements for ' lock-
ing,' 352

GobcBa, testa of seeds of, 725

— scandens, j)ollen-grains of, 721
Coccidia, 752

CoccidiicicB, 749

Coccidium oviforine, 752

Coccoliths, 747-749 ; in chalk, 1084, 1088

Cocconeidece, characters of, 614

Cocconeis, 615

Cocconema, 602, 616

— fusidiwin, 621

Coccospheres, 747-749 ; in chalk, 1088
Cockchafer, antennse of, 974. See Melo-

lontha
' Cockle ' in wheat, 945
Cockroach. See Blatta
Cocoa-nut, 725

— shell of, 693
Cocos-wood, 704 -
Coddington lens, 37
Codiicm, 563

Codonella, silicious shell of, 773
Codosiga tivibellata, iission of, 764 ;

arborescent colonies of, 765
CtELENTEEATA, 862-883 ; bibliography of,

883; permanent gastrula- stage of, 726

— See Zoophytes
Cceloplana, 883

Coenosarc, of hydroids, 867, 870

Ccenurus, 944

Cohn, on sexual generation of Volvox,
555 ; on movements in Oscillatoria, 548 ;
on reproduction of Sphceroplea, 570

ColeoclicBtacece , 575 ; zoospores of, 575 ;
trichogyne of, 575

ColeochcBte, 575

Coleoptera, 973 ; dermo-skeleton of, 974 ;
scales of, 975 ; elytra of, 981 ; eyes of
983, 987 ; antennas of, 987, 988 ; mouth-
parts of, 989 ; winss of, 999 ; leg of,
1000

Cqleps, food of, 776

Collar correction, 358

Collared cells of sponges, 856

'Collars' of Flagellata, 764

' Collateral ' bundles, 710

Collection of microscopic objects, appara-
tus for, 526-529

Collembola, 977

Colletonema, 602

Collins's condenser with rotating sub-
stage, 386

Collomia testa of seeds of, 725

1 146

INDEX

COL

Collomia grandifiora, s^Diral fibres in
seeds of, 693

CoUozoa, 852

Colonial Acinetina, 784

Colonies, in Codosiga, 764 ; of Eadiola-
rians, 849 ; of Polyzoa, 924

Columel of Sphagnacece, 674

Comatula, 900, 901 ; nerves of, 1052

' Comb-bearers,' 881. See Ctenophora

Commensalisni, in lichens, 650

Compensating eye-pieces, 34, 273, 378

Compositce, laticiferous tissue of, 695

Comi30und condenser, sub-stage, 134
— microscope, construction of, 39; in-
vention of, Grovi on, 120

ComiDression of light rays, 57

Compressor, Rousselet's, 346 ; Davis's,
347 ; Beck's, 347

Compressorium, 346

' Concentric ' bundles, 710

Conceptacles of Fiicacece, 627 ; of Mar-
chantia, 666

Conchifera, shell of, 919

Concretionary spheroids, 1100

Condensers, 190, 298-316

— Kellner eye-piece used as, 196 ;
Gillett's, 204, SOO ; Hartsoeker's, 298 ;
Bonannus's compound, 298, 299;
Powell and Lealand's, 301, 302, 310;
apochromatic, 302; Swift's, 302, 305;
immersion, 303, 305 ; Watson's, 303,
304; Beck's, 304, 305; Zeiss's, 305,
308, 309; Baker's, 306; Webster's,
308; Abbe's, 308, 309; fittings for,
312-314; Swift's, for use with polari-
scope, 314

— total aperture of, 307

— tabular list of, 315

— achromatic, 300, 304, 305, 306, 311 ;
Brewster on, 299

— chromatic, 308, 309, 311

— compound, 134

— cone of light with, 190
Conferva, 557

Co7ifervacece, 569, 570 ; binary division
of, 569 ; zoospores of, 570 ; resemblance
of Melosirea to, 608

Conferva, 945, 960

Conical epithelium, 1044

Conids, of Asconiycetes, 648 ; of Basidio-
mycetes, 647

Coniferce, 684 ; woody cells of, 697

Coniferous wood fossilised, 705, 1083

ConjitgatcB, affinities of, 549

Conjugate foci, 13 ; focus, 24 ; image, 24

Conjugating cells, 540

Conjugation, a sexual act, 537

Conjugation of Mesocarpus, 549 ; of
Spiirogyra, 549 ; of Ulothrix, 557 ; of
Sydrodictyon, 565 ; of DesmidiacecB,
584 ; of diatoms, 599 ; of Phceospiorece,
627 ; of Myxomycetes, 634 ; of Ajxella,
746 ; (zygosis) of Gregarince, 751 ; of
Heteromita, '760 ; of Tetramitus, 761 ;
of Noctiluca, 769 ; of Glenodinium,
770; of Podophrya, 785; of Giliata,
782 ; of Vorticelia, 782

COS

Connective tissue, 1019 ; fibrous, 1038 •;
corpuscles of, 1039, 1040 ; areolar, 1040'
Contact metamorphism, 1077
Continental correctional collar, 359

— microscopes, objections to, 162

— model, 254-261 ; criticism on, 259
Continuity of protoplasm, 538 ; in Flori-

decE, 630
Contractile vacuole in Volvox, 552

— vesicle, of Actinoplirys, 737 ; of
Microgromia, 737 ; of Amceha, 743 ;
of Infusoria, function of, 754 ; in
Flagellata, 764 ; of Paramecium, 776 ;
of Ciliata, 776; of Stentor, 777; of
Botifera, 789

Convergence of light, 18

Convergent light in petrology, 1070, 1078

Conversion of relief in spectroscope, 92

Convolvulacece, laiticileicous tissue of, 695'

Convolvulus, pollen-grains, 721'

Copejyoda, 960; classification of , 965 oiote

Copeus cerberics, 791

Copper sulphate, crystallisation of, 1096

Coquilla-nut, 725

— section of, 692

Coralline crag, microscopic constituents

of, 1089
Coralhnes, 960

— conceptacles of, 632 ; ostiole of, 632

— (sertularids), 870

Corals, section of hard and soft parts^
510

— red, 877 ; stony, 878 ; mushroom, 878
Corella parallelogramma, branchial sae

of, 912
Coreopisis tinctoria, seeds of, 724
Cork, 708

Corky layer of bark, 708
Cormophytic tj'pe, 668
Cormorant, parasite of, 1010
Comeules of arthropod eyes, 983
Corn-grains, husk of, 725
Gornuspira, 801, 803
' Corpuscle ' of gymnosperms, 685
Corpuscles, white, 1037 ; change of form

of, 1038 ; of connective tissue, 1039,,

1041 ; of blood, flow of, 1056
Corrected lenses, 382
Correction collar, 21, 30, 50, 274 ; Enghsh,,

357 ; Continental, 359
Corroded crystals, 1071
Corrosive sublimate, as a fixative, 484
Gorynactis Allmanni, thread-cell of, 879
CoscinodiscecB, characters of, 608
Coscinodiscus, 588, 620

— cyclosis in, 587 ; frustules of, 589, 590 ;
markings on frustule of, 591 ; areolae
of, 591

— asteromphalus, 591 ; for testing lenses,.
389

— oculus iridis, 609

— punctatus, fossil, with embryonal
form, 598

Gosmarium, division of, 582 ; form of
cell, 585

— botrytis, zygospore of, 584
Cosmic dust, 1093

INDEX

I 147

cos

Costse of Caminjlodiscus, 607
Cotyledons, 685
Cover-glass, 439

— consequence of using, 19 ; as section
lifter, 478

— tester, 440; Zeiss' s, 440; Ross's, 440;
Smith's (J. Ciceri), 441

— varying thicknesses of, 439 ; with
achromatic objectives, 439 ; cleaning
them, 442

Cox (J. D.), on structure of frustule in
Istlimia, 590 note.

Crab, 957 ; metamorphosis, 969 ; blood-
corpuscles of, 1038 ; ' liver ' of, 1047

Crabro, leg of, 974

Crane-fly. See Tipula

Craterium 'pyriforme, 1009

Crayfish, 957'; young of, 969

Creation of structure by diaphragms, 68

Cribrilina figularis, 906

Cricket, gizzard of, 993 ; wings of, 999 ;
sound-producing apparatus, 999. See
Aclieta

Cnnoidea, skeleton of, 892; larva of,
898

Crisia, 909

Crisp (F.), on 'aperture,' 44; on radia-
tion, 57 ; on collection of microscopes,
117

Cristatella, 909

Cristellaria, shell of, 798, 819

Critical angle, 6, 7; image, 30, 299;
images, 287 ; mode of obtaining, 409,
410

Crocus, pollen-grains of, 722

Crouch's adapter for parabolic speculum,
338

' Crow silk,' 569

Crown glass, refractive index of, 5 ; com-
]3osition of, 32

Crusta petrosa of teeth, 1025, 1026

Crustacea, 957-971

— larvte of, collecting, 529
Crustacea, suctorial, 965

— collecting, 970 ; preserving, 971 ; com-
pound eyes of, 982 ; pigment-cells of,
1043 ; ' liver ' of, 1047 ; concretionary,
spheroids in shells of, 1100

Ceyptogamia, 530-683

— preparation of, 514 ; structure of, 532-
535 ; reproduction of, 535-549 ; litera-
ture, 683 ; passage to Phanerogamia,
684 and note

Cryptoraijliidece, 599

Crystalline forms, list of, for microscope,

1099
Crystallisation, microscopic examination

of, 1095-1098

— effect of temperature on, 1096

— preservation of specimens of, 1098
Crystallisation, process of, 1096
Crystalhtes, 1072, 1096

— in glass cavities, 1074
Crystalloids, 1096

Crystals, corroded, 1071 ; in lava, 1071 ;
zonal markings in, 1073 ; cavities in,
1073 ; inclusions in, 1074, 1075 ; micro-

CYP

scopical structure of, 1075 ; optical pro-
perties and chemical constitution,
1078, 1079 ; as microscoi^ic objects,
1094 ; of snow, 1095 ; as objects for
polariscope, 1097
Crystals, their homogeneous structure,
1094

types of structure 1094

optical properties, 1094

• variations in symmetry, 1094

Ctenaria ctenoplwra, 877 note

Ctenoid scales, 1028

Ctenophora, 811, 881, 883; excretory

pores of, 882 note
Ctenostomata, characters of, 909
CucurbitacecE , pollen-grains of, 721
Cuff's micrometer, 142 ; microscope, 142
CulicidcB, antennte of, 988 ; larvffi, blood

of, 994
Curcidio, antennae of, 988

— iinperialis, scales of, 975 ; elytra of,,
981

Cur cuUonidcc, 981; foot of, 1000; suckers
on foot of, 1002

Currant, parenchyme of fruit, 685 ; pollen-
tubes of, 723

Curvature of the field, 388

' Cushion-star,' 891. See Goniaster

Cuticle, 1041, 1042

— of leaves, 713; of Clliata, IIB
Cutin, 713

Cutis vera, 1041

Cutleria, conjugation of, 627

Cuttle-fish, 929, 942, See Sepia

— ' sepiostaire ' of, structure, 929 ; imi-
tated, 1102

' Cuttle-fish bone,' structure of, 929

CrjancEa capillata, ephyrae of, 874,875;
scyphistoma of, 875 ; strobila, 875

Cyanthus minor, seed of, 724

Cyatholiths, 748, 749; artificially pro-
duced, 1101

Cycadece, 684

Cycas, raphides of, 696

Cyclammina cancellata, 816, 818

Cyclical mode of growth in shell of
Foraminifera, 798

Cycloclypeus, 829 ; shell of, 798

— comiDared with OrbitoUtes, 801, 835
Cycloid scales, 1028

C'ycJojDS, eye of, 960 ; larva, 968

— quadricornis, 961 ; number of off-
spring of, 964

Cyclosis, 534 ; in Chara, 576 ; in des-
mids, 581 ; in DiatoinacecB, 587 ; in
Phanerogam cells, 688 ; in plant hairs^
690; in LieberkueJmia, 732; in Acine-
tina, 783

Cyclostomata (Polyzoa), characters of>
909

Cydippie, collecting, 529

— pileus, 882
CijmbeUa, 602
CijmheUece, affinities of, 616
Cynipidce, ovipositor of, 1008
Cypircea, shell of, 928
Cypiris, 960

1 148

INDEX

Cyst, of Protococcits, 544, 551 ; of Froto-
myxa, 728 ; of Clathrulina, 742 ; of
gregarines, 751 ; of Dallingeria, 759 ;
of Polytoma, 760
Cystic Entozoa, relation to cestoids, 944
Gysticercus, relation to cestoids, 944
Cystids of Hymenomycetes, 648
Cystocarp of FloridetB, 632 ; of Batra-

chospermum, 574
Cystoious candichts, 640
Gijthere, 960, 961

Cytlierina, shells of, in chalk, 1087
Cytodes, contrasted with plastid, 727
Cytoplasm, 537

D

Dallinger and Drysdale's moist stage,

341 ; tripod, 402 ; on life-history of

monads, 756-763 ; on effects of tempe-
rature on monads, 761
Dallinger (W. H.), on Navicida, &c., as

test objects, 600 note ; on nucleus of
• monads, 762

Dallinger's thernio-static stage, 344-346
Dallingeria Drysdali, life-history and

structure of, 758 ; nucleus of, 762
Dalyell (J. G-.), on Hydra tuba, 874
DamcBUS geniculatus, proventriculus of,

1011
Dammar, as a lareservative medium, 518

as a mounting medium, 521
Dandelion, laticiferous tissue of, 695
' pollen-grains of, 721
Dajjhnia, eye of, 960 ; eggs of, 964

ephippial eggs of, 964
Daphnia jjulex, 962
Darwin (Charleston Cirrij^edia, 967
Datura, seeds of, 724
Davis, on desiccation of Botifera, 791

note
Dawson (W.), on foraminiferal nature of

Eozoon, 837
' Day-fly.' See Ephemera.
' Dead-man's toes,' 879. ^ee Alcyoniuni
Dean's medium for mounting insects,

973
De Bary, on fungi, &c., 634 note ; on

potato-disease, 640 ; on alternation of

generations in ferns, 680
Decalcification, 512 ; of echinoderms,

512; of bones, 512; of teeth, 512; of

Foraminifera, 513 ; of Eozoon, 513
Decapoda, 957; exoskeleton of, 968;

macrurous, 969 ; brachyurous, 969
Decomposition, produced by Bacteria,

661

— of rock-masses, 1076
Defining power, 425 ; tests for, 426
Definition of image, 382
Degeneration in Tunicata, 911
Dehydration, 487
Dellebarre's microscope, 144
Delphinium, seeds of, 724
Deinodex, legs of^ 1010

— folliculorum, 1014

DIA

De Monconys, his compound microsco]De,

128
Dendritina, a varietal form of Peneroplis,

803
Dendrodus, teeth of, 1091
Dendrosoma, 784
Dentine, 1019, 1023-1026

— resemblance of cuticle of crabs to,
969 ; in placoid scales, 1028

Deparia, indusium of, 675

— prolifera, 676

Depth of focus, 83, 89 ; of vision, 88, 89,

90 ; perception of, 94, 95
Dermal skeleton of Vertehrata, 1026
Dermaleichi, 1008, 1014
Dermanyssus, 1012

— larva of, 1009
Dermestes, hair of larva, 980
Descartes' simple microscope with reflec-
tion, 126

Desiccation of rotifers, 791

Desiderata in a microscope, 261-263

Desilicification, 513

DESMiDiACEiE, 549, 579-587 I connection
with PediastrecE, 566 ; sutural hne of,
580 ; cellulose envelope, 580 ; mucila-
ginous sheath, 580 ; primordial utricle,
580 ; endochrorre, 580 ; movements of,
580 ; cyclosis in, 581 ; binary division of,
582 ; sexual reproduction, 584 ; classi-
fication of, 585 ; habitat of, 586 ; mode
of collecting, 586

— Hantzsch's glycerin method of pre-
serving, 520

DesmidiecE, 945

— conjugation of, 584 ; zygospore of, 584
Desjnidium, binary division, 582 ; fila-
ments of, 583

Desmids. See Desmidiacece

Deutovium of Acarina, 1008

Deutzia scahra, stellate hairs of, 714 ;
epiderm of, 715

Development of Hydra, 866, 867 ; of hy-
droids, 868 ; of embryo in Gastropoda,
919 ; of molluscs, 933 ; of Annelida,
949 ; of Tomopteris, 953 ; of insects,
1007

Deviation, 9

' Diamond Beetle,' 975

Dianthus, seed of, 723

— caryophyllcBus, parenchyme of, 688
Diaphragm, 261, 297, 306, 308, 310, 312,

313, 314

— with two openings for double illumina-
tion, 104

— Zeiss's iris, 297 ; calotte, 297 ; in eye-
pieces, 376-379, 381 ; for use in test-
ing object-glasses, 385, 386

— in Tully's microscope, 149
Diatoma, 588 ; frustules of, 588, 605

— vulgare, chains of, 605
DiATOMACE^, 549, 587-625

— perforated membrane of, examined
with annular illumination, 419 ; mode
of examination of, 419 ; mounting, 481
silicious coat, refractive index of, 521
stipes of, 588 ; beaded appearance, 592

INDEX

1 149

DIA

markings of, 593 ; binary division
of, 594-597 ; reproduction of, 594-601 ;
j)lacocliromatic, 598 ; coccochromatic,
598 ; conjugation of, 599 ; zj'gospores
of, 599 ; gonids of, 599 ; movements of,
601 ; classification of, 602 ; habits of,
619 ; habitats of, 620 ; distribution of,
621; fossil forms of, 622; used as
food, 622 ; collecting, 622 ; cleaning,
623, 624 note ; mounting, 624 ; as food
of Ciliata, 775 ; in mud of Levant,
1085

Diatom.-frustules in ooze, 1086

Diatomin, 587

Diatoms in stomach of ascidians, Holo-
tJmricB, &c., 614, 623

Diatoms. See Diatomace^

Dichroism, 1098. See Pleocheoism

DicUea, 602

Dicotyledonous stems, fossilised, 1083

Dicotyledons, 700 ; stem of medullary
rays of, 702 ; epiderm of, 712

Dictyocalyx piimiceus, 861

Dictyochija fibula, 620

Dictyocysta, silicious shell of, 773

Dictyoloma peruviana, winged seed,
724

Dictyospyris clathrus, 847

Dictyota, oospheres of, 627

Didemnians, 914

Dkhjmium serpula, plasmode of, 635

Differential screw, Campbell's fine ad-
justment, 162, 164, 165, 174, 202, 230

Differential staining, 493

Differentiation of cell, 533

Difflugia, 746; test of, 746

Diffraction, 62

— Abbe's theory of, and homogeneous
immersion, 363

— Fraunhofer's law, 57

— rays are image-forming, 59

— spectra, 28, 67; phenomena, 62, 64;
image, 64, 72 ; experiments, 66-70 ; fan
of isolated corpuscles, 72 ; problem, 73 ;
pencil, 74, 75 ; hypothesis of Abbe, 74 ;
fan, 75 ; theory, application of, 76, 78 ;
bands, 277 ; phenomena, Abbe's experi-
ments, 434 ; ghost, 435

Digestive vesicles of Ciliata, 776

Digitalis, seeds of, 724

Dimorphism in Forami7ufe?rc, 802

Dinohryon, 765

Dinofiagellata, 770

Dinoniastigoplwra, 770 note

Dioptric investigations by Gauss, 106-

110
Dioptrical image, 30, 72
"Diorite, fluid inclusions in, 1074
Dipping tubes, 350
Diptera, 973 ; eyes of, 987 ; antennae of,

988 ; mouth-parts of, 990 ; wings of,

998 ; ovipositor of, 1008 ; imaginal discs

of, 1007
Direct division of nucleus, 538
' Directive vesicles ' of egg of Purpura,

937
Disc-holder, Beck's, 339

DYT

Discida, 849

Discoliths, 748, 749 ; artificially produced,

1101
Discorbina, 824

— globularis, 798
Disintegration of rock-masses, 1076
Dispersion, 9, 17 ; in glass, 31

— and desiccation of encysted Ciliata^
781

Dispersive power, 2, 9, 18 ; of flint glass,.

10
Dissecting ajDparatus, 455

— microscope, GJ-reenough's binocular,
248 ; Stephenson's binocular, 248 ;.
Huxley's, 251 ; Zeiss' s, 251, 253
Bausch and Lonib's, 252

Distance of i^rojection of image, 26, 27
Distinct vision, 26
Distoma, life-history of, 946

— hepaticum, 945
Divergence of light, 18

Divini's compound microscope, 129
Division, binary, of cells, 535 ; of desmids,,
582

— artificial, of Actinospliceriimi, 741 note

— of naiads, 955
Dobie's line, 1049
Dog-fish, scales of, 1028

D'Orbigny, on plan of growth of Fora-

minifera, 799
Doris, spicules in mantle, 928, 929 ; nida-

mentum of, 934 ; eggs of, 942 ; spines

of, imitated, 1101

— hilarnellata, development of, 935-
937

— pilosa, palate of, 931

— tuberculata, palate of, 931

Double illumination, Stejjhenson's me-
thod, 105
Doublet, Wollaston's, 36, 153
Dragmata, of sponges, 860
Dragon-flies, wings of, 998
Dragon-fly, facets in eyes of, 983

— See Libellula
Draparnaldia glomerata, 574
Draw-tube of microscope, 157
Drebbel's modification of Ke^jlerian

telescope, 121

Dredge, 528

Drepanidium ixmarum, 752

Drone-fly. See Eristalis.

Dropping-bottle, 476 ; German, 477 ; ex-
pansion, 477

Drosera, glands of, 714 ; seeds of, 724

Dry-mounting, Smith's ' cells ' for, 446

Ducts of Phanerogams, 698

Dudresnaya, fertilisation in, 632 ; ferti-
lising tubes, 632

Dujardin, on ' sarcode,' 530 note

— separates Amceha from Difusoria,.
733

Dunning's zoophyte trough, 348
Duramen, 704

Dwarf-male of GSdogonium, 572
Dytiscus, eye of, 987 ; antenna of, 988 ;,

spiracle of, 996 ; trachea of, 996 ; foot

of, 1001, 1002

1 150

INDEX

EAR
E

Earth-stresses, 1077
Earwig. See Forficula
Eccreviocarpus scaber, winged seeds of,

724
Echinoderm larvEe, collecting, 900 ;

preparing, 900 ; mounting, 900

— skeletons in mud of Levant, 1085
EcHiNODERMATA, larvse of, collecting,

529

— 884-903 ; skeleton of, 884, 891, 892,
894; spines of, 885-889, 891; pedi-
cellarifB of, 889; teeth in, 890, 892;
preparation of skeleton spines, &c.,
892 ; internal skeleton, 894 ; larvse of,
896

Echinoderms, decalcification of, 512
Echinoidea, skeleton of, 884 ; spines of,

885 ; pedicellarise of, 889 ; larva of,

898 ; direct development in, 900 note
JEcJmiometra, spine of, 886, 892 ; colour

of spines, 888
.Echinus, shell of, 885, 886 ; spines of,

885 ; teeth of, 890

— lividus, coloured spines of, 887
EctocarpacecB, 626

Ectocarpus siliculosus, conjugation of,
627

Ectoderm, 726

Ectoplasm, 535

Ectoprocta, 909

Ectosarc, 534 ; in Bhizopoda, 733 ;
exj)eriments on, 743 ; of Giliata, 773

Edentata, cement in teeth of, 1026

Edible crab, metamorphosis of, 970

Edwards (A. M.), on supposed 'swarm-
spores ' of Amoeba, 744

Eel, scales of, 1027

' Egg without shell,' concretionary sphe-
roids in, 1100

Egg-capsule of Cyclops, 961

Egg-sacs of Lerncea, 966

Egg-shell membrane, 1038

Eggs of Sejnola, Doris, 942 ; of Acarina,
1005 ; of insects, 1005

Ehrenberg, on eye-spot in Protococcus,
543 ; on Volvox, 551 ; on structure of
frustules, 590 ; on rapidity of repro-
duction of Paramecium, 111 ; on
internal casts of Foraminifera, 827
note ; on fossil Badiolaria, 854 note

ElcBagnus, raphides in pith of, 696 ;
peltate scales of, 714

Elastic ligament of bivalves, structure of,
1040

Elater, antennae of, 988

Elaters of Marchantia, 668 ; of Equi-
setacecB, 680

Elatine, seeds of, 724

Elder, pith of, 687

Ellis's aquatic microscope, 147

Elm, raphides of, 696

Elodea canadensis, cyclosis in, 689

Elytra of Coleop)tera, 981, 999

Embryo of Phanerogams, 723

— cell of fern, development of, 679

Embryo-sac, 685

— of ovule in Phanerogams, 534 ; free-
cell formation in, 536

Emission of light, power of, 51, 54 ;

unequal, 52
Emitted light, unequal intensity of, 51
Empusa muscce, 642
Enamel of teeth, 1025

— of teeth of Echinus, 891

— on ganoid scales, 1028
Encephalartos, raphides of, 696
Encrinites, 892

End-bulbs, 1053

Endochrome, 533 ; of Palmoglcea, oil ;

of Spirogyra, 550 ; of Volvox, 551,

552, 554 ; of desmids, 580
Endoderm, 726
Endogenous spores of Mucorini, 040

— stems, 700-712
Endogens, spiral vessels of, 698
Endonema, 602
EndoiDhloeum, 708
Endoplasm, 533

Endosarc, 533 ; in Bhizopoda, 733 ; of
Ciliata, 113

Endosperm, 685

Endospores of mosses, 672 ; in ferns, 677 ;
of Volvox, 556 ; of Symenomycetes,
648

Endosporous Bacteria, 655

Enock's metallic ring for mounting, 482

Entomophilous flowers, 722

Entomop)hthorecB, 642

Entomostraca, 957, 959-965 ; desicca-
tion of, 963 ; agamic reproduction of,
963 ; eggs of, 964 ; development of,
965 ; eye of, 982 ; non-sexual repro-
duction, 1006

— collecting, 529

— Botifera upon, 787
Entomostracan eggs as food of Ciliata,

775

Entoprocta, 909

Entosp>hcerida, 850

Entozoa, 943

Eolis, nidanientum of, 934

Eozoon, 837 ; mounting, 481 ; mode of
growth of, compared with that of
Polytrema, 824 ; canal system com-
pared with Calcarina, 825 ; affinities
of, 838 ; intermediate skeleton, 839 ;
nummuline layer, 839 ; internal cast
of, 840 ; asbestiform layer, 841 ; pseu-
dopodia of, 841 ; young of, 842

— canadense, 837

— decalcification, 513

Epe'ira, foot of, 1015 ; silk threads of,

1015
Ephemera, branchiae of larva, 997

— marginata, larva of, 973 ; circulation
of blood in larva of, 994

Ephippial eggs of Botifera, 790
Ephyrse of Cyancea, 875 ; of Chrysaora,

876
Epiblast, 726 note
Epiderm of leaves, 712
Epidermic appendages, 1029

INDEX

IISI

Epidermis, 1041, 1042 ; method of ex-
amining, 1043

Epidote, 1076

Epilohiiun, emission of pollen-tubes, 722

EjnjJCiciis, pollen-tubes of, 723

Epipliloeum, 708

Epispore of Mucorini, 642

Epistome of Polyzoa, 909; of Actino-
trocha, 950

Ejnstylis, collecting, 527

Epithelium, 1043, 1044

Epithemia, conjugation of, 599 ; zygo-
spores of, 599 ,

— turgida, 604
Equiconcave lens, 22
Equilucent zones of light, 868
EqidsetacecB, 680 ; in coal, 1084
Equisetum,' spores and elaters of, 681 ;

epiderm of, 715 ; silex in, 715
Equitant leaves of Iris, &c., 717
Erecting binocular, Stephenson's, 100

— prism, Stephenson's, 101
Ergot, 644

Erica, seeds of, 724

Eristalis, eye of, 987 ; antenna of, 988

Error of centring, 889

Erythropsis agilis, eye-spot of, 775

Eschara, calcareous polyzoaries of, 909 ;

extension of perivisceral cavity, 927
Ether as a solvent, 517
Ether-freezing microtome, Hayes's, 472;

Cathcart's, 474
Ethmosphmra siphonophora, 850, 851
Eiicalyp)tra vulgaris, 669
Eucopepoda, 965 note
Eucyrtidium elegans, 847, 852

— Mongolfieri, 847

— tubulus, 847

Eudorina, sexual process of, 557

Euglena, 545, 765

Euglypha alveolata, reproduction of,
746

Euler's microscope, 148

Euler on achromatic microscopes, 147

Eunotia, 604

EunotiecB, characters of, 604

EuphorhiacecB, laticiferous tissue of, 695

Euphrasia, micropyle of, 723

Euplectella asxiergillum, 8Q0 note

EupodiscecB, characters of, 612

Eurotium rejoens, 643

Evening primrose, emission of pollen-
tubes, 722

' Exclamation markings ' on scales, 978

Excretory organ of Botifera, 789, 790

Exner (S.), on the image in eye of
Lanvpyris, 984

Exogenous stems, 700

— stem, structure of, 708

— and endogenous stems contrasted, 709,
710

Exogens, fibro-vascular bundles, 697,
698 ; medullary sheath of, 698 ; spiral
vessels in, 698

Exoskeleton of decapods, 968

Exospores of mosses, 672 ; of ferns, 677 ;
of Hymenomycetes, 648

Extinction, straight, 1079

angle, measurement of, 1079

Extine of pollen-grains, 720 ; markings

on, 720
Eye, accommodation of, 88

— of Pecten, 940 ; of OncJiidium, 941 ;
of slug, 941 ; of snail, 941 ; of arthro-
pod, structure of, 983

Eye-glass of compound microscope, 36,
39

Eye-lens, 376

Eye-piece, 375-381 ; Abbe's compensa-
tion, 40, 378 ; Huyghenian, 40 ; Kell-
ner's, 42, 376 ; Ramsden's, 48, 378 ;
Campani's, 376 ; Huyghens', 376 ; Nel-
son's new Huyghenian, 377 ; Watson's
Holoscopic, 379

— binocular, Tolles', 101 ; Abbe's, 102

— Kellner's, as condenser, 196

— micrometer, 271-277, 380 ; orthoscopic,
376 ; projection, 380, 381 ; index, 381 ;
pointer in, 381 ; diaphragms in, 381

— stereoscopic. Abbe's, 102
Eye-pieces, classification of, by Abbe, 34 ;

compensating, 34, 35, 878 ; negative,
376, 377; positive, 877; solid, 878;
searcher, vs^orking, projection, 378
Eyes on Chiton shells, 941

— compound, of insects, 982, 983

— compound, 982-987 ; simple, 982, 986 ;
preparing, 986 ; mounting, 986

Faber, inventor of the name microscope

124, 125
Falciform young of Coccidia, 752
False images, 419

Farrants's medium, 478, 520 ; for mount-
ing insects, 978
Farre (A.), on structure of Polyzoa, 90S

iwte
FarreUa, polyzoaries of, 909
Fat, 1045

Fat-cells, 1018, 1040, 1042, 1045 ; capil-
lary network around, 1062
Fats, solvents for, 517
Feathers, 1029, 1032
' Feather-star,' 900. See Antedon
Feeding, mode of, in Actinojjhrys, 738 ;

in sponges, 856
Feet of insects, 1000-1002; of spiders,

1014
Felspar, decomxDosition of, 1076, 1077
Felspar rock, effect of dynamic meta-

morphism on, 1077
FelsxDars, zonal structure in, 1078
'Female' plants of Polytrichum, 671
Fermentation of alcohol by yeast, 646 ;

by Penicillium, Mucor, &c., 647
— putrefactive, 661
Fermentative action of Fungi, 532
Ferns (see Filires), 674 ; in coal, 1084
Fertilisation of Phanerogams, 722
Fertilisation-tubes of Peronosporece, 638
FertiUsing tube of Dudresnaya, 632

1 152

INDEX

FES

Festuca pratensis, pale® of, 715
Fibres and cells of Vertebrates, 1018,

1019
Fibro-cartilage, 1019, 1046
Fibro-vascular bundles, 697, 708, 710

— of ferns, 674 ; in the ' veins ' of leaves,
697 ; of Exogens, 697, 698 ; of Phane-
rogams, 700

Fibrous tissues of Vertebrates, 1019

— tissue, 1038 ; white, 1039, 1040 ; yellow,
1040

Field of eye-pieces, 379

Field-glass, 40

Field-lens, 376 ; applied to eye-lens by
de Monconys, 128, 376 ; by Hooke, 128,
876

FiLiCES, 674-680 ; stem, structure of,
674 ; fructification of, 675 ; prothallium
of, 677 ; antherids of, 677 ; archegones
of, 677 ; development of, 679 ; apospory
in, 680 ; apogamy in, 680 ; alternation
of generations in, 680

' Filiferous capsules.' See Thread-cells

Finder, 295 ; Maltwood's, 296

Fine adjustment, 162-175

applied to the stage by Powell,

155 ; by moving the whole body, 162 ;
by simply moving the nose-piece, 162,
173 ; continental, 162-164 ; Campbell's
differential screw, 164 ; Zeiss's, 166 ;
Reichert's, 171 ; Watson's lever, 172 ;
Swift's vertical side-lever, 173 ;
Powell's, 174

Fire-fly, antennse of, 987

' Fire-fly,' 984, 988. See Lamjnjris

Fish, circulation in tail of, 1057 ; on
yolk-sac, 1057

' Fish-louse,' 966

Fish-scales, concretions in, 1101

Fishes, lacunae in bone of, 1022 ; dentine
of, 1023; cement of teeth in, 1026;
plates in skin of, 1026 ; red blood-
corpuscles of, 1084, 1035; j)igment-
cells of, 1043 ; muscle fibre of, 1049 ;
gills of, 1063

Fission in Lieherkuelinia, 733; of
Monas, 756 ; of Monosiga, 764 ; of
Codosiga, 764 ; of planarians, 947

Fissijjennes, wings of, 999

Fixation, 484-487

Fixing agents : alcohol, 484 ; corrosive
sublimate, 484 ; osmic acid, 485 ; picric
acid, 485

Flabella of Licinophoiri, 605

Flagella, 532 ; of Bacteria, 652, 658, 659

Flagellata, 755-771

— experiments on, 761 ; nucleus in, 762 ;
karyokinesis in, 768 ; colonial forms,
764

— collared, resembling cells of sponges,
855

Flagellate chambers of sponges, 856,

857
Flagellum of Noctiluca, 766 oiote
Flat bottle for collecting, 527
Flatness of field, 425
Flea, presumed auditory organ of, 422;

FOR

hairs on pygidium of, as a test, 421

mounting medium for, 973
' Flesh,' 1048
Flint, derivation of, 622

— glass, refractive index of, 5 ; disper-
sive power of, 10 ; composition of, 32

— implements found with OrhitolincBf
824

Flints, preparation of, 1089

Floral envelope, 718

Floridece, 630-682 ; affinities of, 630

FlosculariadcB, 791

Floscules in confinement, 528

' Flowering fern,' sporanges of, 676

' Flowering plants,' 684. See Phaneeo-

GAMIA

Flowers, 718-723; Liman's method of
Ijreparation, 719

' Flowers of tan,' 634

Fluid inclusions in crystals, 1074

' Fluke,' 945

Fluorite lenses for apochromatic objec-
tive, 85, 366

Fliistra, mode of growth in, 904 ; gem-
mation in, 906 ; number of ]polypides,
908 ; polyzoaries of, 909 ; extensions of
perivisceral cavity in, 927

Flustrella concentrica, 847

Fly, various instructive organs to be ob-
tained from, 972 ; eye of, facets in,
983 ; proboscis of, 989 ; circulation in
whig of, 994 ; si^iracle of, 996 ; areolse
on wings of, 998 ; foot of, 1000

Focal alteration and form of objects, 421

— depth, 38

— distances, by feeling, 177

— length of a plano-convex lens, 15
Focke on Navicula and Surirella, 602

note
Focus, virtual conjugate, 14, 25 ; princi-
pal, 16 ; mean, 17 ; virtual, 22 ; conju-
gate, 24 ; depth of, 83, 89

— of lenses, 13, 21, 22 ; chromatic, 16
Focussing arrangements, 159-175
Fontinalis anti]}yretica, 671

Food of Hydra, 685
Foraminifera, 733, 795-846

— study of, by means of Beck's disc-
holder, 339 ; examination of, 423 ;
wooden slides for moimting, 450 ;
method for sectionising, 508 note ; de-
calcification of, 513 ; structure of, 795 ;
chamberlets in, 798, 808, 804, 806 ;
cyclical mode of growth in, 798 ; plans
of growth, 798, 804 ; porcellanous
shells, 799 ; vitreous shells, 799 ; tubu-
lation of shell in, 799, 800 ; rotaHne
type, 800 ; nummuline type, 800 ; in-
termediate skeleton of, 801 ; canal sys-
tem of, 801; Porcellanea, 801; fos-
sihsed forms of, 801, 804, 812, 824, 837 ;
dimorphism in, 802 ; secondary septa
in, 803; Arenacea, 810; sandy iso-
morphs, 814 ; nodosarine type, 815 ;
Vitrea, 819; internal casts, 828, 827
note ; nummuline series, 826 ; alar
prolongations, 830, 831 ; interseptal

INDEX

II53

I'OK

GEE

canals, 830 ; marginal cord in, 830, 834 ;
collecting, 843 ; method of separating
from sand .&c., 844 ; mounting, 845 ;
tubuli of, compared with those of den-
tine, 1020; in mud of Levant, 1085;
in rocks, 1085 ; internal casts of, 1090

Forbes, on reproduction of Sertulariida,
870

Forceps, 351

— slide, 453

— stage, 339

Forficitla, antennai of, 988
Fo7]ficiUidcB, wings of, 999

Form of objects and focal alteration,

421
Formation of microscopic images, 43
' Formed material,' 1018 ; of fibrous

tissue, 1019 ; of dentine, 1020
Fossil coniferous wood, 705, 1083

— crinoids, 892 ; echinids, 892

— GypridcB, 960

— Foraminifera, 801, 824-826

— LituolcB, 816

— JRadiolaria, 846, 854 note

— Saccam7nina, 812

— sponges, 1089

— wood, 705, 700

Fossilised Foraminifera (Eozoon), 837

— wood, sections of, 712
Fragilaria, 605
FragilariecB, characters of, 605
Fragmentation of nucleus, 538
Fraunhofer's law of diffraction, 57
— ■ achromatic doublet, 148

— lines, 323-326
Fredericella, collecting, 528
Free-cell formation, 535, 719

■ in embryo-sac, 534, 536

Freezing apparatus for Thoma's (Jung's)

microtome, 467
— ■ niicrotome, Hayes's, 472 ; Cathcart's,
474

— imbedding by, 505

Fresnel, on Selligue and Adams's micro-
scope, 148 ; on range of magnification,
149

Freyana heterojjus, legs of, 1010

Fripp's method of testing object-glasses,
386

Frog, blood- corpuscles of, 1034, 1035;
muscle fibre of, 1049 ; papilla3 on tongue
of, 1053 ; circulation in mesentery of,
1056 ; circulation in tongue of, 1056 ;
lung of, 1063

Frog's bladder, histology of, as seen with
apochromatic, 372

— foot, epithelium of web of, 1044 ; cir-
culation in web of, 1055

Frond of PhceospcrecB, 626
Fructification, gonidial, 541 ; sexual, 541

— of thallophytes, 540 ; of Ascomycetes,
642 ; of hchens, 649 ; of mosses, 670 ;
of ferns, 675 ; of Equisetacece, 680

Frustules of JDiatoniacece, 588; shapes
of, 588, 589; structure of, 589, 590
note ; girdle, 589 ; ostioles in, 590 ;
markings on, 591 ; character of, as basis

of classification, 602 ; of Coscinodisciis,

609
FucacecB, 627 ; conceptacles of, 627
Fuchsia, pollen-grains of, 722
Fuciis, 626
Fucus platycarjyiis, 627, 628

— vesiculosiis, 629
Fidgoridm, wings of, 999
Funaria hy go-onietrica, 669

— sporange of, 671
Fungi, 540, 633-664

— preparation of, 514 ; zymotic action of,
532 ; alternation of generations in
classification of, 634 ; parasitic on in-
sects, 642

Ficngia, lamellse of, 878

Fungiform papilliB, 1053

Fungus-cellulose, 633

Fusion in Dallingeria, 759

Fuss's description of a microscope, 147

Fusulina, 825, 826, 1090

Fus'ioUna-liiaestone, 825, 1085

G
Gabbro, 1095

T-^ fluid inclusions in, 1074
Gad-fly, ovipositor of, 1004
— ■ See Tabanus
Gaillonella -procera, 621.

— granidata, 621

— biseriata, 621

Galileo, inventor of the compound micro-
scope, 120-125 ; Viviani's life of, 120;
his invention of compound microscope,
Wodderborn on, 121 ; his occhialino,
121, 124; his occUale, 122, 124; his
microscope, 127

' Gall-flies,' ovijpositor of, 1003

Galley-worms. See Myrioj^oda

Gamasidce, legs of, 1010; integimient of,
1010; Malpighian vessel of, 1011;
heart of, 1011 ; trachea of, 1011 ; cha-
racters of, 1012 ; reproductive organs

' of, 1012

Ga'inasus terribilis, mandibles of, 1009

:Ganglion-globules (cells), 1051

Ganglionic cells, 1054

Ganoid scales, 1028

Garlic, raphides of, 696

Garnets, 1077

Gas. bubbles in glass cavities, 1074

Gaseous inclusions in crystals, 1075

Gastropoda, palates of, mounting, 481 ;
palate of, 919 ; development of, 919 ;
shell structure of, 928 ; embryonic
development of, 934-940 ; organs of
hearing in, 941

Gastrula, 726 ; -stage in Coslenterata,
726 ; formation of, 726 note ; of zoo-
phytes, 862 ; of Gastropoda, 935; of
blowfly, 1007

Gauss's oj)tical investigations, 106-110 ;
his dioptric investigations, 106-110 ; his
system, practical example of, 111-1 J 6

Gelatinous nerve-fibres, 1052

in sympathetic, 1054

4 E

1 1 54

INDEX

'G-emellaria, polyzoary of, 909
"G-emmse of Marchantia, 666, 667; of
Salpingceca, 764 ; of Suctoria, 784 ; in
Foraminifera, 798 ; of Polyzoa, 906
Gemmation and shape of shell in Fora-
minifera, 796
Gemmules of Noctiluca, 769 ; of sponges,

857
Gentiana, seeds of, 724
Geoclia, spicules of, 859, 1086
Gephyrean worm, 950
Geranium, glandular hairs of, 714 ; cells
of pollen-chambers, 720 ; j)ollen-grains,
720
Germ-cells of Volvox, 555 ; of Marchan-
tia, 668 ; of mosses, 671 ; of ferns, 679 ;
of Phanerogams, 685 ; of sponges, 857 ;
of Hych-a, 866
'Germinal matter,' 1018; of fibrous tis-
sue, 1019 ; of dentine, 1020
Gesneria, seeds of, 724
Ghostly diffraction image. Nelson on, 72

note
Gibbes {Heneage , on st&ining Bacteria,

515
Gifford's screen, 321
Gill (C. Haughton), on the 'dots' of

Navicula, 593
Gillett's condenser, 204, 300
Gills of tadpole, 1057, 1059
Giraudia, conjugation of, 627
Girvanella, 1084
' Gizzard ' of insects, 993
Glanders, 661
Glands, structure of, 1047
— .of Drosera, 714
Glass-cavities in crystals, 1074 ; gas

bubbles in, 1074
' Glass-crabs,' 968
Glass inclusions in crystals, 1074
— rings for cells, 446-448
Glaucium luteum, cyclosis in, 691
Glenodinium cinctum, conjugation of,

770
Glohigerina, shell of, 798 ; mud, 811 ;
pseudopodia of, 821 ; mode of life of,
821 ; Wyville Thomson's views on, 821 ;
Carpenter's views on, 822
Glohigerina hidloules, 820 ; in the ' ooze,'
1086

— conglohata, 821

— ooze, 820, 1085 ; resemblance to chalk,
1087

— rubra, colour of, 799

Globigerine shell, sandy isomorph of,

814
Globigerinida, 820
Globule of Charcc, 577, 578
Globulites, 1096
Glochidia of Anodon, 933
Glceocajjsa, 547 ; as gonid of lichen, 651
Glow-worm, 984 ; antennae of, 988
Glue and honey cement, 444
Gluten of grass seeds, 725
Glycerin, as preservative medium, 518,

520 ; Hantzsch's method, 520 ; Beale's

method, 520

Glycerin-jelly, Lawrence's mounting in,
480,519; solvent for CaCOg, 520 ; for
mounting insects, 973 ; for mounting
cartilage, 1047

Glyciphagus Krameri, 1013

— nalmifer, 1008

— platygaster, 1013

— • plumiger, 1008 ; hairs of, 1010
Gnathostom,ata (Crustacean), 965 note
Goadby's solution for mounting cartilage,

1047
Goes (Dr.\ on affinity of Carpenteria, 823
Goette, on development of Antedon, 903
Gold size, 443

Gomphonenia, stipe of, 588, 616 ; move-
ments of, 602; attackedby Vainpijrella,
730

— geniinatum, 616 ; stipe of, 616

— gracile, 621

Gomp)lioneniece, characters of, 616
Goniaster equestris, spines of, 891
Gonidial cells, 541

— fructification, 541

— layer of lichens, 649
Gonidiophores of Peronospliorece, 639
Gonids, or non-sexual spores of Crypto-
gams, 541 note; of Vaucheria, 562;
of Podos2}he7iia, 597 ; of FloridecB,631 ;
of Fungi, 633 ; of PeronosporecB, 639

Goniocidaris florigera, spine of, 888
Gonium, 545

Gonothecffi of Campanulariida, 870
Gonozoid of hydroids, 868 ; of Syncoryne,

869; of Tuhularia, 869
Gonozoids of Sertidarlida, 870
Gordius, 944, 945
Gorgonia, spicules in, 929

— guttata, spicules of, 880
Gorgonice, 877 ; spicules of, in mud of

Levant, 1085
Goring (Dr.), on magnification of objects,

43
' Gory dew,' due to Palmella cruenta,

558
Govi, on invention of microscope by

Gahleo, 120
Graduated rotary stage, 395
Gra7nmatopho7'a, chains of, 588, 607

— angulosa, 620

— marina, 607
Grammatop)lwra parallela, 620

— serpentina, 607

— subtilissima, 607
Granite, 1095

— fluid inclusions in, 1074
Grantia, 857, 861 ; spicule of, 1086
Grasses, nodes of, 701 ; silex in epiderm

of, 715 ; palere of, 715 ; seed of, 725
Grasshopper, gizzard of, 993 ; wings of,

999
Green glass for softening light, 321, 417
Greensands, microscopic constituents of,

1090
Gregarina, characters of, 749 ; move-
ment of, 750

— gigantea, in lobster, 749 note

— Scenuridis, 751

INDEX

1 155

GEE

Gregarinida, 749

Gregory (J. W.), on Eozoon, 843 note

Gregory (W.), on species of diatoms, 600

note
Greville, on Sj^atangidium, 610; on

Triceratiitni, 613 note
Grey matter, 1052
Griffith's turn-table, 451
Griffithsia, 630
Grinding sections of hard substances,

506
Grindl's microscope, 132
Gromia, 734-736, 796

— and Arcella, pseudopodia of, con-
trasted, 746

Ground-mass of rocks, 1072
Groundsel, pollen-grains of, 722
Growing slides, Botterill's, 340 ; Mad-

dox's, 341; Lewis's, 841
Guard-cells, 715
'Gulf- weed,' 630
Gum and glycerin, 520 ; and syrup, as a

preservative medium, 519

— imbedding for vegetable substances,
514

— arable, formula, 445 ; for freezing, 505

— resins, latex of, 695

— styrax, as a mounting medium, 521 ;
index of refraction, 521

Gyges, 545
G^jmnochroa, 868
Gymnolcemata, 909
Gymnosxierms, fossilised, 1084

— generative apparatus in, compared
with Cryptogams, 684

Gypsina, 824

H

Haddon, on budding in Polyzoa, 907 note
Haeckel (E.), on Madiolaria, 846 ; on

Hydrozocin affinity of Ctenophora, 877

note

— and Hertwig, on classification of
radiolarians, 849 note

HcemamcebidcB, 752 and note
Scematococcus, red phase of Proto-
coccus, 543

— sanguineus, 558
Hsematoxylin, solutions, 491, 492
Hcemionitis, sori of, 675
Hcemosporidia, 749

Haime (Jules), on development of Tri-

choda, 780
' Hair-moss,' 671
■ Hair-worm,' 944
Hairs of leaves, 714 ; of insects, 980 ; of

Acarina, 1010 ; of mammals, 1029
HalicaridcB, 1013
Haliomnia Bhmiboldtii, 851

— hystrix, 848
Haliotis (diatom), 613

— (mollusc), shell structure of, 928 ;
palate of, 931

Haliphysema, 814 ; sponge-spicules in.

HET

Haller, on auditory organs of Acarina,

1010
Halteres of Dip>tera, 1000
Hand-magnifier, Brewster's, 37
Hansgirg, on movement of Oscillato-

riacece, 548
Hiintzsch's glycerin method for desmids,

520
Haptlophragmiwm, 814

— glohigeriniforme, 813

Hardening agents, 484-487 ; corrosive
sublimate, 484 ; alcohol, 484 ; osmic
acid, 485 ; picric acid, 485

Hardy's flat bottle for collecting, 527

Sarpalus, antennae of, 988

Harting, on Janssen's microscope, 120 ;
his experiments on formation of con-
cretions, 1101

Hartnack, on immersion system, 27

Hartnack's model, 256

Hartsoeker's simple microscope, 134 ; his
condenser, 134, 298

' Hart's-tongue,' 675. See Scolopen-
drium

' Harvest-bug,' 1013

' Haus ' of ApijJendicularia, 918

Haustellate mouth, 992

Haustelhum, 992

Haversian canals in bone, 1021

Haycraft (J. B.), on structure of striated
muscle fibre, 1049

Hayes's ether freezing microtome, 472 ;
minimum thickness of sections there-
with, 473

Hazel, peculiar stem of, 704 ; pollen-
grains of, 722

Hearing, organs of, in Gastropioda, 941 ;
in Cepihalopoda, 941

Heart of ascidians, 912 ; of Acarina, 1011

Heartsease, pollen-tubes of, 723

' Heart-wood,' 704

Heating-bath, Mayer's, 453

Heliopelta, 588, 611

Seliozoa, characters of, 734 ; examples
of, 737-742

Helix pomatia, teeth of, 930

— Iwrtensis, palate of, 930
Heller's porcelain cement, 521
Helmholtz on aperture, 47
Hemiaster cavernosas, development of,

900 note
Hemijitera, eyes of, 987 ; wings of, 999 ;

suctorial mouth of, 1000
Hensen's stripe, 1049
Hepaticce, 665; thalloid, 668; foliose,

668 ; elaters of, compared with sisiral

cells, &c., of pollen-chamber, 720
Herhivora, arrangement of enamel in

teeth of, 1025 ; cement in teeth of, 1026
Herring, scales of, 1028
Herschelan doublet, 309
Hertel's compound microscope, 137, 139
Hertwig's research on Microgroynia, 735

note ; on Actinia, 877 note
Heterocentrotus, spine of, 885

— mammillatus, spine of, 887
Heterocysts of Nostoc, 549

4 e2

1 1 56

INDEX

HET

Meteroviita uoicinata, life-history of, 760

Heterostegina, 834

Heurck (Van\ on markings of diatoms,

593
Hexarthra, 792
Hicks, on amoebiform j)liase of Volvox,

556 ; on preparation of insect antennae,

989 note ; on structure of halteres and

elytra, 1000
Simantidiuni, 604
Sipparchia janira, eggs of, 1005
Hi2]2}Ofus, 613
Hi2)pot]ioa, 909
Holland's triplet, 37
Hollis's liquid glue, 444
Hollyhock, pollen-grains of, 721, 722
Holotlniria boteUus, plates of, 895
— • eclulis, i^lates of, 895

— inhabilis, \Aa.tes of, 895

— vagahunda, plates of, 895
Holothiirice, diatoms in stomach of, 614,

628

IIolothurioidea,&\e\eiaQ.qi, 894; pharyn-
geal skeleton of, 895 note ; plates in
skin of, 895 ; prex^aration of calcareous
plates, 896 ; abbreviated development
in, 900 note

Soltenia Carpenieri, 861

IIo7tieocladia, 602

Homogeneous, word first applied to
lenses, 30

— immersion, 864 ; Abbe's combination,^
365

— immersion lenses of Powell and Lea-
land, 80; of Zeiss, 29

— objectives, value of, in study of monads,
: 762

• — system, 28

IIomop)tera, wings of, 998, 999 ■

Hood of mosses, 671

Hoofs, 1029, 1033

— sections of,. mounting, 481; for polari-
scope, 481

Hooke's adoption of field-lens to eye-
lens, 128, 376

— comx^ound microscope, 128 .
Hooked monad, 760

Hooker (J. D.), on diatoms of antarctic

circle, 621
Hooklets on wings of HyvienojJtera, 999
Hojylothora, 1012
-^ maxillas of, 1010
Hormogones of OscillatariacecB, 547 ; of

Bivulariacece, 548; of Scytonemacece,

548 ; of Nostoc, 549
Hormosina globulifera, 813. 815

— Carpenteri, 815 :
Hornblende, 1077

— corroded crj stals of, 1072 ; pleochroism
in, 1078

Hornet, wing of, 999 ; sting of, 1003

Horns, 1029, 1088

Hornv substances, chemical treatment

of, 517 ;

' Horse-tails,' 680. See Equisetacece
Hosts of parasitic plants, 532
House-fly. See Musca

Hudson, on the functions of contractile

vesicle of rotifers, 789 note
Hudson and Gosse, on classification of

rotifers, 790
Human blood-corpuscles, 1084

— hair, 1031

Husk of corn-grains, 725

Huxley, on the ectosarc of Amceha, 743
note; on coccoliths, 747 ; on Bathybius,
747 ; on CoUozoa, 853 note; on struc-
ture of mollusean shells, 922 ; on pul-
villus of cockroach, 1000 note ; on agamic
reproduction of Aphis, 1006

Huxley's simple dissecting microscope,
251, 252

Huyghenian eye-piece and spherical
aberration, 42

Hyacinth, raphides of, 696 ; cells of
pollen-chambers, 720 ; pollen-grains of,
722

Hyaline shells of Foraminifera, 799

HyaUnia cellaria, i^alate of, 981

Hyalodiscus subtilis, 608

HyaloiJlasm, 537

Hydra, collecting, 527 ; intracellular
digestion in, 868 ; thread-cells of, 864-;-
structure of, 864 ; reproduction of, 866 ;
gemmation of, 866

— fusca, 863, 865

— viridis, 863, 867

— vulgaris, 868

'Hydra tuba ' of Clirysaora, 874, 876
HydracJinidce, 1008 ; mandible of, 1009 ;

eyes of, 1011 ; reproductive organs

of, 1012 ; characters of, 1013
Hydrangea, iiumhev of stomates in, .716;

seeds of, 724
Hydrodictyon, 557, 566

— 7'eticulatuvi, 565
Hydroida, classification of, 868
Hydroids, compound, 867 ; structure of,

867 et seq. ; Medusce of, 868 ; planulse
of, 868, 871; habitats of, 871; ex-
amination of, 871 ; mounting, 871 ;
polariscope with, 872 ; preservation of,
872

HydropJiihis, antennse of, 987, 988

Hydrozoa, 863-877

Hydrozoa and marine mites, 1018

Hyla, nerves of, 1054

Hymenium of Ascomycetes, 642 ; oiBasi-
diomycetes, 647; of Hymenomycetes,
648

Hymenomycetes, 647 ; pileus of, 648 ;
stipe of, 648

Hymenojytera, 973; eyes of, 987; mouth-
parts of, 990 ; wings of, 998 ; sting of,
1002, 1003; ovipositor of, 1002, 1003

Hyoscyamus, spiral cells of pollen-
chambers of, 720 ; seeds of, 724

Hypericum, seeds of, 724

Hyphre ol fungi, 683

Hypnospoi-e of Hydrodictyon, 565

Hypnpsj)ores, meaning of, 541 note

Hypoblast, 726 note

Hypopial stage of Tijroglypihidce, 1013

Hyp)op)us, 1013

INDEX

II57

' Ice-plant,' epiderm of, 714
Ichneumonidce, ovipositor of, 1003
Illuminating power, 425

— power of objectives, 54; compared
with x^enetrating power, 393

Illumination for dissection, 401

— for opaque objects, 149

— oblique, 190, 191, 388

— of objects, Ross on, 300 ; monochro-
matic, 321-323 ; GifEord's screen for,
321 ; Meithe's filter for, 322; Nelson's
apj)aratus for, 323 ; by reflection, 329-
338 ; ojpaque, 329 ; from the open sky,
412; by diffused daylight, 412; for
dark ground, 413 ; experiments in, 414 ;
monochromatic, means of obtaining,
417, 418 ; annular, 419 ; colour, 423 ;
double, objects for study with, 423 ;
with small cones, as causs of errors in
interpretation, 427

Illuminator, oblique, 190 ; white cloud,
194 ; parabolic, 316-317 ; Swift's sub-
stage, 319; Smith's vertical, 336;
Powell and Lealand's, 337; IBeck's,
337 ; for exainination of metals, 337

Image, real, 14 note ; virtual, 14 note,37& ;
conjugate, 24 ; inverted conjugate, 24 ;
absorption or dioptrical, 64 ; diffrac-
tion, 64; negative, 64; positive, 64;
soHd, 95 ; real object, 375 ; definition
of, 382 ; formed by compound eye, 984,
985

Images, by diffraction, dioptric and
interference, 72

Imaginal discs in larva of blowfly, 1007

Imbedding processes, 495-506 ; paper
trays for, 497 ; in paraffin, metal case
for, 498 ; orienting bottle for, 499

paraffin method, 409-503 ; in gum,

475, 505, 506; celloidin method, 503-
505

— by coagulation or freezing, 505, 506
Immersion lenses and vertical illumina-
tors, 337, 338

homogeneous, outcome of Abbe's

theory of diffraction, 364

water, Zeiss's, 370

Amici's, 362 ; Powell and Lea-
land's, 362, 364 ; Prazmowski and Hart-
nack's,362; Tolles', 362

— objectives, 28 ; examination of, 387

— system, 27-29 ; invented by Amici,
27

Imperfect achromatism, cause of yellow-
ness, 417
' Impressionable organs ' in Ciliata, 775
Incidence, angle of, 3
Incident ray, 2
Incus of Botifera, 788
Index eye-piece, 381

— of visibility, 521

Indian corn, epiderm of, 712 ; stomates

of, 715
Indirect division of nucleus, 538
Indusium in ferns, 675

Inflection of diverging rays, 62
Infusoria, 754-785 ; as food of Actino-

j)Ji7-ys, 739 ; Ehrenberg's work on, 753 ;

ciliate, 754, 772 ; unicellular nature of,

755 note ; character of, 772
Infusorial earth, 607, 608, 611, 613, 617,

620-622 ; from Barbadoes, 846, 849
Injected preparations, 1061
Inocerarnus, portions of shell of, in chalk,

1087
Insects, 972-1007
— ■ mounting media for, 973 ; integument

of, 974 ; tegunientary appendages of,

974 ; scales'' of, 975-980 ; hairs of, 980 ;

parts of head, 982; eyes, 982-987;

antennse of, 987 ; mouth-parts of, 989 ;

circulation of blood, 993 ; alimentary

canal, 993; wings of, 994, 998-1000;

trachete of, 994 ; stigmata of, 995 ;

sound-producing apparatus, 999 ; organ

of smell, 1000; organ of taste, 1000;

feet of, 1000-1002; stings of, 1002,

1003 ; ovipositors of, 1002, 1003 ; eggs

of, 1004 ; agamic rej)roduction of, 1006 ;

embryonic development of, 1007 ;

' hver ' of, 1047

— parasitic fungi in, 642, 645

— parts of, wooden slides for mountmg,
450

Insect work, dark-ground illumination
for, 423 ; polarised light for, 423

Integument of insects, 974 ; of Acarina,
1010

Integuments of ovule, 685

Intensity of light, necessaries for, 417

Intercellular substance, 1019; in carti-
lage, 1046

Intercostal points, Stephenson on, 73 ;
not revelation of real structure, 73

Interference, 62

— image, 72

Intermediate skeleton in Foraminifera,
801; of Glohigerinicla, 820; of Calca-

^ rina, 825; oiBotalia, 825 ; of Nwmmii-.
lites, 826 ; of Eosoon, 839

Internal casts of Textularia, 823; of
BotaUa,8M; of Eozoon, 840 ; of wood,
1083 ; of shells in greensand, 1090

Interpretation, errors of, 427

' Interseptal canals ' of Calcariiia, 830

Intestine, cells of villi in, 1044

Intine of pollen-grains, 720

Intracellular digestion in zooijhytes, 863

Intussusception, 533

— mode of growth of starch, 694
Invagination, 726

Invertebrata, blood corpuscles of, 1038

Inverted conjugate image, 24

Iodine, as a test for starch, &c., 516

Ipomoea purpurea, x3ollen-grains of, 721

Iridescent scales of insects, 975

Iris, epiderm of, 712 ; leaves of, 717 ; cells

of pollen-chambers, 720
Iris-diaphragm, 297, 313 ; fitted to Abbe's

condenser, 312
Iris <jermanica, epiderm and stomates of,

715, 716

II58

INDEX

Irrationality of spectrum, 19, 365

Isochelse of sx^onges, 860

Isoetece, 682

Isotropism, 1079

Isthviia, chains aud frustules of, 588, 612 ;
structure of frustules, 590 note ; divi-
sion of, 596

- — nervosa, 613

— areolatious in, 592

Italian reed, stem of, 699

' Itch-mites,' 1013

Ivory, 1024

Ixodes, heart of, 1011

IxodidcB, 1008 ; integument of, 1010 ;
auditory organ, 1011 ; tracheae of, 1011 ;
characters of, 1012

Jackson's modification of Ross model,
199 ; his eye-piece micrometer, 276

Janssen (H. and J.), inventors of first
microscope, 120 ; their compound
microscope, 120

Jars, capped, for Canada balsam, 477

Jelly-fish. Qee Acale2}hcB and Medusa

Jones's compound microscope, 144, 145

Jungermannia, 668

Jung's (Thoma's) microtome, 461-469

K

Kaolin, 1076

Karop, his fine adjustment to sub-stage,
.187

Karop and Nelson on fine structure of
diatoms, 591 note

Karyokinesis in monads, 763

Kelhaer's eye-piece, 42, 376 ; as a con-
denser, 196

Kent (Saville), on contractile vacuoles of
VoJvox, 552 note; an. Flag ell at a, 764

Kepleriau telescope, Drebbel's modifica-
tion as a microscope, 121

Keramosplicera MmTayi, 810 7iote

Keratose network of sponges, 855 ; pre-
paration of, 857

Kidneys of Vertehrata, 1047

King-crab, 957

Kirchner, on the oospores of Volvox,
556

Klebahn, on formation of auxospores of
diatoms, 601

Klebs, on mucilaginous sheath of des-
mids, 580 ; on movement of desmids,
580

— and Biitschli, on the ' cilia ' of Dino-
flagellata, 770

Klein, on Volvox, 556 note

Knife, s^Decial, for microtome, 462

Koch's method of sectionising corals,
878

Kowalevsky, on development of ascidians,
917 note

Krukeuberg.on digestion in sea-anemones,
863

Kiitzing, on Palmodictyon, 559 ; on struc-
ture of frustules of diatoms, 590 ; his
classification of diatoms, 603

Labarraque's fluid for bleaching vege-
table substance, 514

Labels, permanent, 528

Labyrinthic structure of Cyclammina,
816 ; of Parkeria, 818

Laliyrinthodon, tooth of, 1091

Lacunae and canaliculi of bone, misinter-
pretation of, 428

— of bone, 1019-1022 ; dimensions of, in
various animals, 1022

— relation of size to that of blood cor-
puscle, 1022

Lagena, 796, 819
Lagenicla, 819
Laguncula, 906, 908, 950

— stolon of, 904 ; polypides of, compared
with Clavellinidce, 914

— repens, anatomy of, 904, 905
' Lamellae ' of corals, 878

— of Hymenomycetes, 648
LameUlhranchiata, shell of, 919
LaineUicornia, antenuee of, 988
Laminaria, 626, 627
Laminariacecs, 627

Lamna, tooth of, 1024

Lamp, Nelson's, 404 ; Beck's 406 ;

Baker's, 407
Lampyris, antennae of, 988

— splendidula, photograph through eye
of, 984

Land-crab, young of, 969

Lankester (E. Bay), on Bacteria, 652 ; on
movement of gregarines, 750 ; on
HcBmamcehidcE, 752 note ; on intra-
cellular digestion in Liinnocodium,
863

Lantern-flies, wings of, 999

Lapis lazuli, 1095

Larva of Ecliinode7-mata, 896 ; of As-
teroidea, 898 ; of Echinoidea, 898 ; of
OpJiiuroidea, 898 ; of Crinoidea, 900 ;
of ascidians, 916; of fly, 1007; of
Acarina, 1009

Latex of Phanerogams, 695

Latlircea squamaria, embryo of, 723

Laticiferous tubes, free-cell formation in,
534

— tissue of Phanerogams, 695
Laurentian rocks, S'dl, 842

' Laver,' or green seaweed, 559

Lawrence's glycerin jelly, 519

Leaves, epiderm of, 712 ; internal struc-
ture of, 716; mode of preparation for
examination of, 718

Leech, 956

Leeuwenhoek's simple microscope, 132

Legg's method of selecting Foraminifera,
844

Legs of insects, 1000, 1002; of Acarina,
1008, 1010

INDEX

■I 159

LegicminoscB, seeds of, 685

Leiosoma pahnacinctum, 1008 ; hairs of,

1010
Leitz's microscopes, 206, 227

— bull's eye, 330

— objectives, 374

Lens, spherical, 12 ; biconvex, 12, 13 ;
j)lano-concave, 13 ; diverging meniscus,
13 ; j)lano-convex, 13, 15, 22, 37 ; con-
verging meniscus, 13 ; biconcave, 13 ;
plano-convex, focal length of, 15 ;
crossed biconcave, 16; crossed bicon-
vex, 16 ; equiconvex, 16, 22 ; Stanhope,
37 ; Coddington, 37 ; Briicke, 38

— from Sargon's j)alace, 119

— invention of, 119, 120

— achromatic, Charles's, 148; Barlow's,
149

Lenses, refraction by, 10, 25

— homogeneous immersion, of Powell
and Lealand, 30 ; of Zeiss, 29

— fluorite ; for apochromatic objectives,
35

— combination of, 37

— resolving power of, 64, 382 ; amplify-
ing power of, 25, 26

— testing by Diatoms, 389
Lepadidce, 967
Lepidiurn, seeds of, 724
Lepidocyrtus curvicolUs,aciiAes of, 979
Lepidodendra, 682, 1084
Lepidoptera, scales of, 975, 976 ; wings

of, 981, 999 ; scales of, mounting, 981,

982 ; eyes of, 987 ; antennas of, 988 ;

mouth-parts, 992; eggs of, 1005
Lepiidosteus, bony scale of, 1022, 1028
Lepidostrohl, 682

Lejnsjiia saccharina, scales of, 976, 977
Lepisynidce, 979
Lepralia, 909 ; mode of growth in, 904 ;

extension of perivisceral cavity of,

927
Leptodiscus (ally of Noctiluca), 769 note
Leptogonium scotinum, 649
Leptothrix, form of, 653
Leptus autumncdis, 1013
Lerncea, 965 note, 966
Lessonia, 627

Lettuce, laticiferous tissue, 695
Leucite, mineral inclusions in, 1075 ;

anomalies in, 1078
Lever of contact, floss's, for testing

covers, 440
Libellula, eye of, 983, 987; respiratory

apparatus of larva, 997 ; wings of,

998
Liber, or inner bark, 708
Lichens, 648-651 ; fungus-constituents

of, 651
Licmo23hora, stipe of, 588, 604 ; flabella

of, 605

— flabellata, 588, 604
LicmophorecB, 616

— characters of, 604 ; vittse of, 604
Lieherhuelinia, movement of, 732

— pahidosa, 733

— Wagneri, 731

Lieberkiihn's microscope, 139 ; his specu-
lum, 334-336
'Ligamentum nuchse,' structure of, 1040
Light ; refraction of, 2 ; recomijosition
of, by prisms, 18 ; convergence of, 18 ;
path of, through compound microscope,
40 ; quantity of, 50, 51, 54 ; emission
of, 51, 54 ; quantity of, and aperture,
54 oiote ; cone of, 190 ; monochromatic,
321, 417, 418 ; intensity of, necessaries
for, 416

— convergent, in petrology, 1070, 1078
Lignified tissue, test for, 517
Lignites, 1083

Lignum vita, wood of, 704

Lilac, pith of, 687

Lilium, experiments with jDollen-grains

of, 721
' Lily-stars,' 900. See Crinoidea
Limax maxinius, palate of, 930

— shell of, imitated, 1102

— rufus, shell structure of, 928
Lime, raphides of, 696

— secreting Algse, 1084
Limestone, metamorphism of, 1077

— rocks, 1084, 1085

LimncBus stagnalis, nidamentum of,
934 ; velum of, 936

LimnocaridcB, characters of, 1013

Limnocharis, seeds of, 724

Limnocodium, intracellular digestion in,
863

Limpet. See Patella

Lirnulits, 957

Linaria, seeds of, 724

Lister's struts for support of body, 149 ;
his influence on improvement of Eng-
lish achromatic object-glasses, 150 ;
his zoophyte trough, 348; his discovery
of two aplanatic foci, 355 ; his note on
Chevalier's objectives, 355 ; his influ-
ence on microscopical optics, 356 ; his
triple-front combination, 360

Listropliorus, 1008

Lithasteriscus radiatus, 620

Lithistid sponges, spicules of, 859

Lithocyclia ocellus, 847

Litliptliamnion, 1084

Lituola, 814

Lituolce, large fossil forms of, 816

Lituolida, 814

Live-box, 346

Liver, 1047

Liver-cells, 1048

' Liverworts,' 665. See Hepaticm

Lobosa, characters of, 734 ; examples of,
742-747

Lobster, 957 ; metamorphosis of, 969

' Lobworin,' 948

Loculi, of anthers, 720

Locust, gizzard of, 993 ; ovipositors of,
1004

Locusta, eye of, 987

Loftusia,^lS

Loligo, pigment-cells of, 942

Lomas (J.), on calcareous spicules in
Alcyonidiuni, 908 note

ii6o

INDEX

' London Pride,' parenchyme of, 688
Longicomia, antennae of, 988
Longulites, 1096
Lophopliore of Polyzoa, 905, 950 ; of

fresh-water Polyzoa, 909
Lophojpus, collecting, 528
Lo2oliospermurn erubescens, winged seed

of, 724
Lojiliyropoda, 959
Lorica of Ciliata, 773 ; of Acineta, 783 ;

of Botifera, 787
Loup-holders, 248

— for tank work, Steinlieil's, 268
Loups, Reichert's, 38 ; Steinlieirs,-38, 378 ;

Steinlieil's aplanatic, 248 ; Zeiss's, 268
Louse, mounting media for, 973
Loven, on classificatory value of palates

in Gastropoda, 932
Loxosoma, loijliophore of, 909
Lubbock, on Thysanura, 977 ; on Podicra

scale, 979
Lucanics, eye of, 987 ; antennae of, 988
Luminosity of Noctiluca, 765 ; of Gteno-

pliora, 883 ; of annelids, 955
Lungs, circulation in, 1056, 1062-1065
Lychnis, seeds of, 724
Lychnocanium falciferuvi, 847

— lucerna, 847

Lycoperdon, 647 ; hymenium of, 647

Lycopodiacece, 681 ; in coal, 1084

LycopodiecE-, 681

Lyniinas, collecting, 527

Lymph, corpuscles, 1037

Lysigenous spaces in Phanerogams, 688

M

Maceration of vegetable tissues, 700 ;

Schultz's method, 700
Machilis p)olyp>oda, scale of, 978
Machines for cutting hard sections, 511,

512
Macrocystis, 627
Macrospores of Polytoma, 760; of

sponges, 857
Macrurous Decapoda, young of, 969, 970
Madder, cells of pollen-chambers, 720
' '^&dite,' Acantlioinetra, occurring in, 852
Madrepores, 878
Magma, 1073
Magnetite, 1072
Magnification, range of, of Selligue's

microscope, 149
Magnifying power, testing of objectives,

425 ; determination of, 288
Mahogany, size of ducts of, 699 ; stem of,

706
Malacostraca, 968
' Male ' plants of Pohjtrichimi, 671
Mallei of Botifera, 788
Mallow, pollen-grains of, 721, 722
Malpighian vessel of GamasidcB, 1011

— layer of skin in mammals, 1042

— bodies in vertebrate kidney, 1047
Malt wood's finder, 296

Malva sylvestris, pollen-grains of, 721

MEC

Malvaceae, pollen-grains of, 721

Mammalia : lacunae in bones of, 1022 ;
plates in skin of, 1026 ; epidermic ap-
pendages of, 1029 ; red blood-corpuscles
of, 1034, 1035 ; epidermis of, 1042 ;
muscle fibre of, 1049 ; lungs of, 1065

Mammary glands, 1047

Man, arrangement of enamel in teeth of,
1025 ; cement in teeth of, 1026 ; hair of,
1031 ; muscle fibre of, 1049 ; lung of,
1065

Mandibulate mouth, 989

Manganese concretions, 1090

' Mantle ' and growth of shell in Mollusca,
925

Marchantia, 665-668 ; archegones of, 665,
668 ; stomates of, 666 ; elaters of, 668

— androgyna, 665 note

— polymorpha, 665-668
Margaritacece, 919 ; nacreous layer of,

922 ; prismatic layer of, 923
Margarites, 1096
' Marginal cord ' of Operculina, 830

— of Nummulites, 834
Marine forms, collecting, 528

— glue for forming ' cells,' 445

— mites, 1013

— work, tow-net for, 528 ; dredge for, 528 ;
stick-net for, 529

Marshall's compound microscope, 135,

136, 138, 139
Marsipella elongata, 813
Martin's ' pocket ' reflecting microscope,
140 ; his large microscope, 140 ; his
improvements in optical and mechani-
cal arrangements, 142 ; his achromatic
microscope, 147 ; his reflecting micro-
scope, 147; his achromatic objective, 147
Marzoli's achromatic lenses, 353
Masonella, 811
Mastax of Botifera, 787
Mastigophora Hyndmanni, 906
Mastogloia, stipe of, 588, 619 ; gelatinous
sheath of, 588, 619 ; development of,
597 ; range of variation in, 618
■ — lanceolata, 619
— Smitliii, 619
Matthews' s method of sectionising hard

substances, 507
Mayall, on history of microscope, 117 ;

on Divini's microscope, 130
Mayall's removable mechanical stage, 183
Mayer's heating bath, 453
' Meadow-brown,' eggs of, 1005
' Measly i^ork,' due to Cysticercics, 944
' Mechanical finger ' for selecting di-
atoms, 625

— movements of the stage in Lister's
(Tully's) microscope, 149

— stage, 175

— — Turrell's, 176; Watson's, 177;
Nelson's, 179, 181; Zeiss's, 179, 183;
Swift's, 180 ; Allen's, 180 ; Mayall's re-
movable, 183 ; Reichert's, 183 ; Bausch
and Lomb's, 183, 184 ; Beck's, 184 '

Continental, 179

— tube-length of microscope, 158

INDEX

I l6l

MED

Medullary rays, 705

in dicotyledons, 702

' Medullary sheath ' of Exogens, 698 ; of

dicotyledons. 703
Medusa of fresh-water, 863
MeduscB, mounting, 448 ; of Hydroids,

868 ; naked-eyed, 868 ; development

of, 874 ; alternation of generations in,

877 ; nerves of, 1052
Medusoids, collecting, 529
MegaJopa, 970 ^

Megaloscleres, 859
Megasphere of certain Foraminifera,

802
Megasijores of lilnzocarpecB, 681 ; of

carboniferous trees, 682 ; of Isoetece,

682 ; of SelagineUece, 682
Megatherium, teeth of, 1026
Megatriclia of Ehrenberg, a phase in

development of Suctoria, 785 ; Badcock

on, 785
Megazoospores of Ulothrix, 557; of TJlva,

561 ; of Scenedesmus, 566
Megerlia lima, shell of, 927
Melanosporece, 625
Meleagrina, 919, 922
— • margaritifera, 923
Melicerta, collecting, 527 ; in confine-
ment, 528
Melicertidce, 791
Melolontha, eye of, 987 ; antennse of,

988 ; spiracle of larva, 996

— vulgaris, eye of, 983

Melosira, frustules of, 588, 594 ; auxo-
spores of, 595, 600 ; sporules of, 597 ;
zygospore of, 600

— ochracea, 608

— suhflexilis, 594, 595

— varians, 594, 595 ; endochrome of,
598 ■

MelosirecB, characters of, 608 ; resem-
blance to ConfervacecB, 608

Membrana putaminis, 1032

Membranipora, 908, 909

MembraniporiclcE, 908

Mercury nitrate as a test for albuminous
substances, 517

MeridiecB, 604, 616

— characters of, 604
Meridian circulare, 588, 604
Merismopedia, 547

' Mermaid's fingers,' 879. See Alcyo-

nium
MesembryantJtetnufn, seeds of, 724

— crystallinum, epiderm of, 714
ikfesocarjows, conjugation of, 549; zygo-
spore of, 550

Mesogloea of Hydra, &c., 864 note
Mesophloeum, 708
Metal case for imbedding, 498
Metamorphism, dynamic, 1077
Metamorphism of rock-masses, 1076,

1077 ; of hmestones, 1090
Metamorphosis of LerncBa, 966 ; of

Cirripedia, 967 ; of Malacostraca,

969
Metazoa, 727, 855

MIC

I Meteorites in oceanic sediments, 1093
Metschnikoff, on acinetan character of
Erythropsis, 775 ; on intracellular di-
gestion, 863 ; on phagocytes, 1037 note
Mica, 1077
I Michael's (A.) opalescent mirror, 194
I Micrasterias denticulata, binary divi-
sion of, 583 ; form of cell of, 585
Micro-chemical analysis, 1102

— method of, 1102
Micro-chemistry in petrology, 1082, 1083 ;

of poisons, 1103
Micrococci, form of, 653 ' '

Microcysts of Myxomycetes, 636
Microgromia socialis, 735
Microlites, 1072 ; in glass-cavities, 1074
Micrometer, Cuff's, 142

— use of, 274 ■

— eye-piece, 271

Nelson's new, 271, 272, 273 ; Zeiss's,

272; Jackson's, 276

Micrometers, 270-277

Micrometry by photo-micrography, 277

Micron, a, 82 note, 460

Micro-petrology, 1066

' Microplasts ' of Bacterium rubescens,
660 7iote

Micropyle in ovule, 685 ; of Euphrasia,
723 ; in orchids, &c., 723

Microscleres, 859, 860

Microscope, Mayall on the, 117 ; history
and evolution of the, 117-269 ; inven-
tion of, 120 ; inventor of the name, 124 ;
essentials in, 157-194 ; adjustments in,
159-175 ; stage of, 175-184 ; sub-stage
of, 184-191 ; mirror of, 191-194 ; desi-
derata in, 261-263 ; x^i'eservation of,
436

— Galileo's, 127 ; Campani's, 128 ; Prit-
ehard's, with Continental fine adjust-
ment, 153; Ross's 'Lister' model,
153 ; Powell's (H.), 155 ; James
Smith's, 155

— aoliromatic, Euler on, 147; Martin's,
147; Chevalier's, 148, 150; Selligue's,
148; Tully's, 149; Ross's early form
of, 152

— aquarium, 266-269

— binocular, Riddell's, 97 ; Nachet's,
98; Wenham's stereoscopic, 98; Ste-
phenson's, 100, 248, 455 ; G-reenough's,
102,250; Powell and Lealand's, 105;
Cherubin d'Orleans', 180 ; Ross's, 196 ;
Ross-Zentmayer's, 199 ; Rousselet's,
245 ; Sorby's spectrum, 327

— chemical, Bausch and Lomb's, 263, 264

— compound, 36, 39-42, 120, 125 ; con-
struction of, 39 ; i^ath of light through,
40; Rezzi on invention of, 125 ; Jans-

■ sen's, 120 ; Hooke's, 128 ; de Mon-
cony's, 128; Divini's, 129; Marshall's,
135; Hertel's, 139; Joblot's, 139; Cui-
peper and Scarlet's, 140; Martin's,
140; Adams's variable, 142, 148;
Jones's, 144, 148

■ — comparison of English and Conti-
nental models, 254-261

1 1 62

INDEX

Microscope, concentric, 191, 199

— dissecting, Greenough's, 102, 250 ;
Stephenson's binocular, 248; Baker's
(Huxley's), 251; Bausch and Lomb's
(Barnes), 252 ; Zeiss's, 253

— horizontal, Bonannus's, 134; Amici's,
148

— petrological, 1068

— photographic, 257, 258

— radial, 191, 199; Ross-Wenham's, 199

— reflecting, Newton's, 132 ; Martin's,
140, 147 ; Smith's, 145

— simple, 86, 126, 248; path of light
through, 25; inventor of, 126 ; Bacon's,
126; Descartes', 126 ; Bonannus's, 132 ;
Muschenbroei's,132 ; Leeuwenhoek's,
132 ; Hartsoeker's, 134 ; Wilson's, 140

— spectrum binocular, 327

— three great types of, 174, 199
Microscopes, for chemical purposes, 263,

264

— for examination of metals, 264-266

— modern, 194-269 ; Powell and Lea-
land's, 194, 218, 237 ; Ross's, 196, 230 ;
Watson's, 199, 218, 224, 234, 237;
Baker's, 202, 218, 230; Swift's, 203,
224,228,233, 1068; Leitz's, 206, 237;
Reichert's, 206, 224, 241, 242, 264;
Zeiss's, 206, 237, 250; Bausch and
Lomb's, 212, 222, 239, 252, 263; Spen-
cer Lens Company's, 214 ; Beck's, 228,
233

— portable, 245-247; Powell and Lea-
land's, 245; Swift's, 245; Rousselet's,
245; Baker's, 246; Bausch and Lomb's,
247

Microscoijic and macroscopic vision, 62

— determination of geological formations,
1090

— dissection, single lenses for, 38

— investigation of rocks, &c., 1066

— vision, jjrinciples of, 43
Microsriopical optics, principles of, 1
Microscopiist's work-table, 398-403
Microscopy, definition of, 397
Microsomes, 531, 537

Micro - spectroscope, Sorby - Browning,
323-327; Swift's, 325 note; Hilger's,
325 note

- — method of using, 328 ; in petrology,
1083

Microsphere of cevt&m For aviinif era, 802

Microspores of SphagnacecB, 674 ; of
RhizocarpecB, 681 ; in carboniferous
trees, 682 ; of IsoetecB, 682 ; of Selagi-
nellecB, 682; of Polytoma, 760; of
sponges, 857

Microtome, 458-475 ; Ryder's, 401 ; sim-
ple, 458-460 ; Thoma's (Jung's), 461-
469 ; freezing apparatus for, 467 ; Mi-
not's, 472; Strasser's, 472 ; Gudden's,

; 472

— Cambridge rocking, 469-472 ; advan-
tages of, 472

— freezing, Hayes's, 472 ; minimum'
thickness of sections with, 478 ; Cath-
cart's, 474

MON

Microzocispores of TJlothrix, 557 ; of

Ulva, 561 ; of Hydrodictyon, 565
' Mildew,' 637. See UredinecB
Miliola, shell of, 799 ; encrusted with

sand, 810
MiliolcB, 802

Miliolida, 801 ; in limestone, 1090
Miliolina, 802

Miholine Foraminifera, fossils of, 801
Mihohte hmestone, 1090
Millepore, resemblance of Polytrema to,

824
Millon's test for albuminous substances,

517
Mineral nature of Eozoon, 843

— sections, where to get made, 1067
Minerals and rocks, bibliography of, 1071

note

— optic axes of, 1079

— refractive index of, 1080

— chemical, spectroscopic and micro-
scopic testing of, 1078-1083

Minnow, circulation in tail of, 1057
Mirror, 191-194

— opalescent, as a substitute for polaris-
ing prism, 194

— replaced by rectangular prism, 192
Mites, 1008. See Acarina

Mobius, on mineral nature of Eozoon, 848
Mohl (Von), on protoplasm, 530 note
Moist-stage, Dallinger and Drysdale's,

341-344
Molecular coalescence, 1099-1102
Molgula, development of, 917
MoLLUSCA, larvae of, collecting, 529

— shells of, 919 ; shell-structure of, 919-
925 ; colour of shell, 921 ; mantle and
shell-growth, 925 ; palate of, 930 ; de-
velopment of, 933 ; cihation of gills,
940 ; organs of sense in, 940 ; biblio-
graphy, 942 ; resemblance of barnacles
to, 967 ; ' liver ' of, 1047 ; muscle fibre of,
1050 ; internal casts of, 1090 ; concre-
tionary spheroids in shells of, 1100

Molluscan shells in mud of Levant, 1085
Monad-form of Microgromia, 737
Monadincs, life-histories of, 755-763 ;
saprophytic, affinities of, 756 ; effect
of temperature on, 761 ; nucleus in, 762
Monads, 755. See MonadincB
Manas, 575
• — Dallingeri, life-history of, 756

— lens, 755

Monaxonida, spicules of, 859
Monazite, 1081

Monconys (De) devises microscope with

field-lens, 128
Monerozoa, 727-738
Monocaulus, 871
Monochromatic light, 321, 417, 481

— illumination, means of obtaining, 417,
418

MoNOcOTYLEDOKS, 700 ; stem of, 700 ;

nodes of, 701 ; epiderm of, 712
Monocotyledonous stem, fossilised, 1083
Monocular, Powell aaid Lealand's, 194,

195

INDEX

1 165

MON

Monocystis agilis, cyst of, 750
Monopliyes, digestion iu, 863 note
Monosiga, fission of, 764
Monothalamous Foraminifera, 796
MonotrojJn, seeds of, 724
MoracecB, laticiferous tissue of, 695
Mordella beetle, eye of, facets in, 988
Mormo, scales of, 980
Morplio Menelaus, scales of, 976
Morula of higher animals compared with

' multicellular ' Protozoa, 726
Morula of Gastropoda,, 935
Moseley (H. NJ, on skeleton of pharynx

of holothurian, 895 note; on Chiton's

eyes, 941
Mosses, 669-674

— capsules of, wooden slides for mount-
ing, 450

' Mother-of-pearl,' 922

Moths. See Lepidoptera

Motion, spiral, 433, 434

Motor nerves, 1058

Motorial end-plates, 1053

' Moulds,' 640, 643

Moults of Entomostraca, 964, 965

' Mountain-flour,' 622

Mounted objects, keeping, 523 ; labelling,

523 ; arrangement of, 524
Mounting plate, 452

— instrument, James Smith's, 454

— thin sections, 477

— in natural balsam, 480 ; in aqueous
liquids, 481 ; in deep cells, 482

— diatoms, 481, 624 ; Ophiurida, 481 ;
Folycystince, 481 ; sponge-spicules,
481 ; chitinous substances, 481 ; palates
of gastropods, 481 ; sections of horns,
&c., 481 ; Lepidop)tera scales, 982 ;
hairs of insects, 982 ; eyes of insects,
986; blood, 1038

— media, 517-522 ; camphor water, 518;
salt solution, 519 ; white of egg, 519 ;
syrup, 519 ; Eipart and Petit' s fluid, 519 ;
glucose media, 519 ; chloral hydrate,
519 ; gum and syrup, 519 ; glycerin
jelly, 519 ; Farrant's medium, 520 ;
glycerin and mixtures of, 520 ; Canada
balsam, 521 ; Dammar, 521 ; Styrax,
521 ; monobromide of naphthalin, 521 ;
phosphorus, 521

Mouse, hair of, 1080-1081 ; cartilage in

ear of, 1046
Mouse's intestine, villi of, 1062
Mouth, suctorial, of HeniijJtera, 999

— of Acarina, 1009
Mouth-parts of insects, 989
Movement, interj)retation of, 481-434

— of Lieberkuehnia, 732; of Amceha,
744 ; of Dallingeria, 758 ; of plana-
rians, 946 ; of Artemia, 960; of Bran-
chipus, 960 ; of fly on smooth surface,
1001 ; of white corpuscles, 1037 ; of con-
nective tissue corpuscles, 1041 ; of
Oscillatoriacece, 547 ; of desmids, 580 ;
of diatoms, 601 ; of Bacteria, 652 ; of
Ciliata, 774

Mucilaginous sheath of desmids, 580

MXX

Mucor, f ermentatioji; by, 647

— mucedo, 641

Mucorini, 640 ; spores of, 640 ; epispores
of, 642

Mucous membrane, 1041 ; capillaries in,
1062

Mud of Levant, microscopic constituents
of, 1085

MullDerry, laticiferous tissue of, 695

Mulberry-mass, 720

Miiller (J.), on the Radiolaria, 846 ; on
larva of Nemertines, 951

Miiller' s (Fr.) ' Common Nervous Sys-
tem' in Polyzoa, 907 and note

Multicellular organisms, 726

Multiplication of Palmoglcea, 541 ; of
Protococcus, 543 ; of Volvox, 555 ; of
Pahnella, 55.8 ; of Bacteria, 652 ; of
Microgromia, 736 ; of Amceha, 744 ;
of Dallingeria, 758 ; of Heteromita
760; of Tetramitus, 760; of Noctihica,
769 ; of Peridinium, 770 ; of Suctoria
784 ; of Ciliata, 111

Multiplying power of eye-]nece, 290

Munier Chalmas and Schlumberger, on
dimorphism of Foraminifera, 802

Munier-Charles, on certain fossil Fora-
minifera, 564

Mioricea elongata, spicules of, 880

Musca, eye of, 987 ; antennas of, 988

— vouiitoria, eggs of, 1006
' Muscardine,' 645
Musci, 670-674
MuscinecB, 673
Muscle-cells, 1051

Muscular fibre, 1048 ; structure of, 1049 ;

capillary network in, 1062
Muscular tissue, preparation of, 1050
Mushroom, 647 ;

— spawn of, 647
Musk-deer, hair of, 1080
Musschenbroek's simple microscope, 182;
Mussels. See TJnionidce and MytilacecB
Mya arenaria, hinge tooth of, 924
Mycele of Fungi, 633 ; of V stilaginece , 636
Mycetozoa, 634

Myliohates, tooth of, 1025

Myohia, 1008; legs of, 1010; maxillre of,
1010

Myobiidce, 1013

Myocoptes, legs of, 1010

' Myophan-layer ' of Vorticella, 778

Myopy, 118

Myriophyllum a good weed to collect, 527

Mybiopoda, hairs of, 980

Myriothela, intracellular digestion in,
863

Mytilacece, sub-nacreous layer in, 924

Mytilus, for observation of ciliary motion,
940

Myxamwbcs, 634

Myxogastres, 684

Myxomycetes, 579 note, 684; develop-
ment of, 634, 636; spores of, 634, 686 ;
swarm-spores of, 634; affinity with
Monerozoa, 121

Myxosporidia, 749, 752

1164

INDEX

NAC

N

Nachet, on ' immersion system,' 27 ; his
binocular, 97, 98, 99; his changing
nose-piece, 293

Nacreous layer in moUuscan shells, 919,
922, 924

Naegeli and Schwendener, on microscopi-
cal optics, 67

Nageli's theory of formation of starch,
695

Nails, 1029, 1033

Nais, 955

Naphthalin, monobromide of, as a mount-
ing medium, 521 ; refractive index of,
521

JVarciss its, spiral cells of pollen-chambers
in, 720

Nassida, mouth of, 774

Nauplius, compared with FedalionidcB,
792

Nautiloid shell of Foraminifera, 797

Nautilus, 929

Navicula, 590, 597, 617; markings on,
593 ; cysts of, 597 ; zygospores of, 597 ;
zobzygospores of, 597

' — bifrons, presumed relation to Sztri-
rella microcora, 602 note

— in chalk, 1087

— lyra, as test for definition, 426

— r7e,o«i6oif?es, markings on, 592 ; as test
for definition, 426

■ NavicidecE, frustule of, 589 ; ostioles in,
590

— .characters of, 616
Nebalia, carapace of, 962

Needles for dissection, their mode of use,

457
Negative aberration, 27, 360 note

— crystals, 1074

— eye-pieces, 376, 377, 378

Nelson, on the sub-stage condenser, 72
note ; on ghostly diffraction images, 72
note; his model, with Swift's fine-
adjustment screw, 172 ; his horse-shoe
stage, 179, 228 ; his fine adjustment to
the sub-stage, 185 ; his screw micro-
meter eye-piece, 271 ; his new micro-
meter eye-piece, 272 ; his ' black dot,'
277 ; his plan for estimating edges of
minute objects, 277 ; his changing nose-
piece, 294 ; his revolving nose-piece,
295 ; on rings and brushes, 319, 320 ;
his means of obtaining monochromatic
illumination, 323 ; his lamp, 404

Nelson and Karop, on fine structure of
diatoms, 591 note

Nemalion multifidum, 631

Nematodes, desiccation of, 945

Nematoid worms, 944

Nemertine larva, 951

Nepa, tracheal system, 995 ; wings of,
1000

— ranatra, eggs of, 1005
Nepenthes, spiral fibre-cells of, 698
Nereidw, 948

Nereocystis, 627

NTTC

Nerve-cells, 1051 •

Nerve-fibres, 1052

Nerve-substance, 1051 : mode of prepara-
tion, 1054

Nerve-tubes, 1051

Nervures of wing of Agrion, 994

Nettle, hairs of, 714

Neuroptera, 973 ; eyes of, 987 ; circula-
tion in wings of pupa, 994 ; wings of,
998

Newt, red blood-corpuscles of, 1034 ; cir-
culation in gills of larva, 1057

Newton's reflecting microscope, 132

— suggestion of reflecting microscope,
145

— rings, 1097
Nicol prisms, 318

Nicol's analysing prism, 294 ; for resolv-
ing striae, 381

Nicotiana, seeds of, 724

' Nidamentum ' of Gastropoda, 934

Nitella, 576

Nitric acid as a test for albuminous sub-
stances, 517

Nitrogenous substances, test for, 517

Nitzschia, 602

: — scalaris, cyclosis in, 587

— sigmoidea, 606
NitzscJiiecB, 606

Noctiluca, collecting, 529 ; tentacle
(flagellum) of, 766, 768 ; cihum of, 766
note ; protoplasmic network of, 767 ;
reproduction of, 769

^- 7niliaris, 765-769

Noctuina, antennae of, 988

Nodes of monocotyledons, 701

Nodosaria, 819

NodosarincB, shell of, 797

Nodosarine shell, sandy isomorphs of,
815

Nonionina, 829

— shell of, 797, 798

Nonionine shell, sandy isomorph of, 814
Non-stereoscopic binoculars, 105
Non-striated muscle, 1048, 1050
Nose-pieces, 291-295 ; centring, used as
sub-stage, 228; Brooke's, 291; Beck's
rotating, 291 ; Powell and Lealand's,
. 291; Watson's dustproof, 292 ; Zeiss's
calotte, 292; centring, 293; Nachet's

— changing, 293 ; analysing, 294 ; Vogan's,
294 ; Nelson's revolving, 295

Nosenia hombycis, cause of pebrine, 661
Nostoc, 548, 549 ; as • gonid of lichen,

651 ; resemblance of Ophrydium to,

778
Nostocacece, 548 ; affinities with Bacteria

and Myxomycetes, 652
Notochord in Tunicata, 911 ; of Ap)pen-
■ dicularia, 918 -

Notonecta, 987 ; wings of; 1000
■Nucellus, 685
Nuclear stains, 491-494
— spindle, 538 ; plate, 538
Nuclein, 537
NucleoH, 534
Nucleoplasm, 537

INDEX

1 165

wuc

Nucleus, 534'

— action of acetic acid on, 517 ; its im-
portance to cell, 535 ; division of, 538 ;
fragmentation of, 538 ; presumed ab-
sence of, in some forms, 727 ; imtiative
action in monads, 762

— and cell division, 1019 note
Nucule of Chara, 577, 579
Nudibranclis, nidamentum of, 934 ; em-
bryos of, 936

Numerical aperture, 29, 53, 60, 390,
425 ; formula for, 390 ; problems on,
391

of dry objective, 391 ; of water-
immersion, 391 ; of oil-immersion, 391

and resolving power of objective,

393

— apertures, table of, 84-87
Nummuline layer of JEozoon, 840

— plan of growth, Parker and Rupert
Jones on, 827, note

NuvimulinidcB , 826 _ ^

NwmmuUtes, 826, 827, 831

— (Ustans, 832

— garansensis, 832

— Imvigata, 832 i

— striata, internal cast of, 834

— tubuli in shell of, 800
NuraanuUtic limestone, 831, 835, 1085,

1090
Nujphar lutea, parenchyma, 687 ; stellate

cells of, 687
Nyraph of Acarina, 1009 ; of Oribatidm,

1009

O

Oak, size of ducts in, 699

— galls, 1003 . .
Oberhauser's spiral fine adjustment, 153
Object-glass of compound microscope,

36, 39 ; of long focus, 40 ; of shorfi
fo.cus, 40 ; caijacity of, 382;

Object-glasses, power of, 44 :

testing, 381 ; Abbe's method of

testing, 384-387 ; diaphragms for use
in testing, 385 ; Fripp's method of
testing, 386

Object-holder for Thoma's (Jung's) mi-
crotome, 464,465, 466

— changer, Zeiss's, 293

Objectives, achromatic, 19, 32 ; aplanatic,
19 ; apochromatic, 19, 30, 34, 80 ; cor-'
rected, 20, 21 ; immersion, 28, 34, 5:8 ;
aperture of, 43, 65, 390 ; maximum
aperture of, 44 ; comparison of, 46 ;
illuminating power of, 54 note ; im-
mersion V. dry, 54, 79 ; dry, with balsam
mounted objects, 55 ; dry, 58 ; dry, for
study of Ufe-histories, 81 ; penetrating
power of, 83, 393 ; sliding plate with,
290 ; rotating disc with, 290 ; of wide
aperture, 369 ; of small aperture, ex-
amination of, 388 ; tests for, 388, 394',
resolving power of, and numerical aper-
ture, 393

Objectives, triple-back, 361 ; Wenham's
single front, 361; duplex front, 362;
Leitz's, 374; Reichert's, 374; adjust-
ing, 357, 360

— achromatic, Martin's, 147 ; Marzoli's,
353; Tully's, 354; Selhgue's, 354;
Amici's, 355 ; Ross's, 356, 360 ; Powell's,
356, 361 ; Smith's, 356, 360 ; Wenham's,
361 ; covers for use with, 439

— apochromatic, 366, 370, 371-375

— homogeneous immersion, introduction
of, 364

— ' semi-aj)ochromatie,' 35, 374, 375

— oil-immersion, Powell and Lealand's,
30; Amici's, 364; Tolles', 364;
Zeiss's, 370 ; Leitz's, 374 ; Reichert's,
374; Swift's, 375; Beck's, 375;
Bausch and Lomb's, 375 ; Watson's,
375

— water-immersion, Powell and Lea-
land's, 362, 365 ; Prazmowski and Hart-
nack's, 362 ; Zeiss's, 370

Obhque illumination, 190, 191, 387

—- illuminator, 190

Obliteration of structure by diaphragms,
68

Occhiale, Galileo's, 122, 123

Occhialino, Gi-alileo's, 121, 124

Oceanic sediments, microscopic examina-
tion of, 1092

Ocelli of planai'ians, 947 ; of insects, 982,
986

Ocellites of compound eye, 982

Ocular, 40, 375 ; spectral, 327

CEdogoniacecB, 572

CEdogonium ciliatuiii, 573

CEnothera, pollen-grain, 721; emission
of pollen-tubes of, 722 ; embryo of, 723

Oil for immersion lenses, suggested by
Amici, 29

— of cedar-wood, for immersion objec-
tives, 29

Oil-globule«, 429-431

Oil-immersion, 29

objectives. See Objectives, oil-
immersion

Oils, solvents for, 517

Okeden, on isolation of diatoms, 624
note

Oleander, epiderm of, 714 ; stomates of,
716

Olivine, corroded crystals of, 1072

Onchidium, eyes of, 941

Oncidium, spiral cells of, 693

Onion, raphides of, 696

Oogones of Vauclieria, 563 ; of Sjplicero-
plea, 572 ; of CEdogonium, 572 ; of
Gliara, 577 ; of Fucacece, 627, 628 ; of
Peronos]}orecB, 638

Oolitic grains, 1084

OoiDhyte in ferns, 680

Oospheres, use of the term, 537 note; of
Volvox, 556; of Vaucheria, 563; of
Sphceroplea, 570 ; of CEdogoniutn, 572
of Chara, 577 ; of PhceosjmrecB, 627
of Fucacece, 628 ; of Marcliantia, 668
of ferns, 679

ii66

INDEX

Oospores, 540 ; of Volvox, 556 ; of Vau-
cheria, 563; of Aclilya, 565; of
Splicer oplea, 572 ; of (Edogonium, 573 ;
of CJiara, 579 ; of Fucacea, 628

Ooze, Glohigerina, organisms in, 811,
813, 820 ; compared with chalk, 1085

Opalescent mirror as a substitute for
polarising prism, 194

Opalina, 774

Opaque illumination by side reflector, 333

— mounts, 336

' Open ' bundles, 710

Ojperculina, 830 ; and Nicmmidites com-
pared, 834

Opercuie of mosses, 671

Ophiacantlia vivipara, development of,
900 note

Opihioglossacecs, development of pro-
thallium of, 679

Ophioglossimi, sporanges of, 676

Ophiothrix pentaphyllum, spines of,
891 ; teeth of, 892

Ophiuricla, mounting, 481

Ophiuroidea, skeleton of, 891 ; spines of,
891; teeth of, 892; larva of, 898;
direct development in, 900 note

Ojjhrydia, quantities of, 777

Ophrydium, cellulose in zoooytium of, 778

— versatile, effect of light on, 775
Ophryodendron, 784

Opium poppy, latex of, 695

Optic axis of Powell and Lealand's No. 1,

194
Optical anomalies in petrology, 1078

— centre, 24

— tube-length of microscope, 158, 159
Orals of Antedon, 901
OrhicuUna, 803, 804, 808

— comjjared with Heterostegina, 834
Orhitoides, 835

— and Gycloclypeus compared, 835

— Fortisii, 836
Orhitolina, 824

OrbitolincB, occurring with flint instru-
ments, 824
OrhitoUtes, 804-810

— shell of, 798 ; range of variation in,
810 ; structure of Parkeria resembling,
817 ; deposits of, 1085

— and Gycloclypeus compared, 801

— cojnplanata, animal of, 807-809

— italiaca, 806 note, 808

— tenuissima, 808
Orbulina, 820

Orbuline shell, sandy isomorph of, 815

Orchid.ece, polliniura of, 722

Orchids, mycropyle of, 723

Orchis, pollen- tubes of, 723; seeds of,
724

Organised structure and living action,
530

Organs, 533

' Organs of sense ' in Ciliata, lib note

Oribatidce, nymph of, 1009 ; mouth-parts
of, 1009 ; legs of, 1010 ; integument of,
1010 ; auditory organ, 1011 ; rei^roduc-
tive organs, 1011 ; suj)ercoxal glands

PAL

of, 1011 ; traeheas of, 1011 ; characters

of, 1012
Orienting small objects for sectionising,

499
Origanum onites, seeds of, 724
Ornitliorliynclms, hair of, 1081
Orohanche, seeds of, 724
Ortlioptera, eyes of, 987 ; antennae of,

988; wings of, 999 ; nymph of, 1009
Orthoscopic effect, 95 ; with Ramsden's

circles, 106

— eye-piece, 376

Orthosira Dickiei, sporangial frustule

of, 595
Oscillatoria, movement of, 547
Oscillatoriacece, 547

— movements of, 433
Oscula of sponges, 856

Osmic acid and fatty structures, 517
Osmunda, sporanges of, 676

— regalis, prothallium of, 679 note
Ossein, of bone, 1023

Ostiole of conceptacle of coralHnes, 632

Ostioles of Naviculacece, 590 ; of Gym-
hellecB, 590

Ostracoda, 960

Ostreacece, shell of, 923

Ostrich, egg-shell of, 1101

Otoliths compared with artificial concre-
tions, 1100

— of MoUusca, 941
Ovarium of Polyzoa, 907
Over-amplification, 88
Over-corrected objective, 20
Over-correction, 358-360
Overton, on Volvox, 556 note
Ovipositor of Oribatidce, 1012
Ovipositors of insects, 1002-1004
Ovule of Phanerogams, 684

— suspensor of, 534

— structure of, 684-685 ; development of ,
722

Ovum of Hydra, 866

Oxytriclia, a phase in development of

Trichoda, 780
Oxijuris veryiiicularis, 944
Oysters, shell of, 923

Pacinian corpuscles, 1053
Palaeontology, use of microscope in, 1083
' Palate ' of Gastropoda, 919, 930 ; classi-

ficatory value of, 932 ; preparation of,

932 ; viewed with polariscope, 933 ;

bibliography, 938
Paleae of grasses, silex in, 715
Palisade-parenchyma of leaves, 716
Fcdni, stem of, 701
PcdiiieUa, as gonid of lichen, 651
— cruenta, 558

Palmellacece, 557 ; frond of, 558
Palmodictyon, 559 ; zoospores of, 559
Pcdnioglcea macrococca, life-history of,

541, 542
Pcdpicornia, antennae of, 988

INDEX

1167

Paludina, infested by Distoma, 946
Pancreas, 1047
Pandorina, 545

— ynorum, generative process of, 557 ;
swarm-spores of, 557

Papaveracece, laticiferous tissue of, 695

Paper-cells, 446

Parabolic illuminator, 316; speculum,

333; reflector (Sorby's), 334
Paraboloid illuminator, 316
Parafiin, solvents for, 496

— imbedding ]uetliod, 496-503

— for imbedding, melting point of, 500

— mounting, sections, 501

— cells, 446

Paramedian, Colm's experiments on,
743 ; contractile vesicles of, 776

Paraphyses of Puccinia, 638 ; of lichens,
650 ; of mosses, 671

Parasites, nourishment of, 532

Parasitic Crustacea, 965

— Fungi, 633
Parietal utricle, 533
Parker (T. J.), on Hydra, 863
Parkeria, 817; a possible Stromato-

poroid, 817 7iote
Parnassia, seeds of, 724
Parthenogenesis, 1007 note

— in Sajirolegnice, 640

Passiflora cwrulea, pollen-grains of, 721
Passifloi-ece, pollen-grains of, 721
Paste-worm, 945
Pasteur's solution for growing yeast, 646

note ; his experiments with Bacteria,

660, 661
Patella, shell structure, 928 ; palate of,

931
Path of ray of light through a compound

microscope, 40
Pathogenic bacteria, 658
Pavement epithelium, 1044
Pear, constitution of fruit, 693
' Pearl oyster.' See Meleagrina
Pearls, 923

' P6brine ' in silkworms, 661
Peccary, hair of, 1030
Pecten, prismatic layer in, 924 ; pallial

eyes of, 940 ; fibres of adductor muscle,

1050
PectinibrancMata, 937
PectinidcB, sub-nacreous layer in, 924
Pedalion, 792
Pedalionidce, 792
Pedesis, 431 ; experiments in, 482
Pediastrece, 566 ; afflnities of, 566
Pediastrum, zoospores, 567 ; micro-

zoospores, 567

— Ehrenbergii, 568

^ granulatuyn, 566-568

— pertusum, 568

— tetras, 568

Pedicellarise of ecliinids and asterids,

889
PedicelUna, lophophore of, 909
Pedicularis 2^ahistris, 723

— sylvatica, embryo of, 723
Pedunculated cirripeds, 967

PHA

Pelargonium, petal of, 719 ; pollen-grain,

721
Pelomyxa palustris, 744
Penerojjlis, 801

— variation in shape of shell in, 797;
shell of, 799 ; varietal forms of, 803

Penetrating jjower, 425
in objectives 83 ; of objective, com-
pared with illuminating power, 393
Penetration, 38, 82, 83
Penicilliuin, fermentation by, 647

— glaucum, 643

Pentacrimts asterius, skeleton of, 892

Peiitatoma, wings of, 1000

Peony, starch in cells of, 694

' Pepperworts,' 681

Perception of depth, 94

Perch, scales of, 1028

Perforated shells of Brachiopoda, 926

Perforation of shell in Foraminifera, 799,
800

Perianth, 718

PericManiydium prcetextum, 851

Peridiiiia,110. Ill

Peridiniuin iiberrimum, 770

Perigone of mosses, 670

Periodic structures, 74

Periostracum of molluscan shells, 922 ;
of brachiopod shells, 926

Peripatus, trachese of , 1011

Peritheces of lichens, 650

PeronosporecE, 638-640

Perophora, respiratory sac of, 915 ; cir-
culation of, 915

' Perspicillum,' Wodderborn's, 125

Petals, 718

Petrobia lapidum, eggs of, 1009

Petrological microscope. Swift's, 1068

Petrology : micro-spectroscope in, 1081 ;
micro-chemistry in, 1082

Pettenkofer's test, 517

Petunia, seeds of, 724

Pesiea, botrytis-iorni of, 645

Pfitzer, on reproduction of diatoms, 594

PhcBodaria, 852

PkceosporecB, 625-627

Phagocytes, 1037 note

Phakellia ventilabrum, 858

Phallus, 647

Phaneeogamia, woody structures, pre-
paration of, 514

— embryo-sac of, free-cell formation in,
534-536

— relation of, to Cryptogams, 682, 684
and note ; structure of stems, &c., 685,
700 ; structure of cells, 686-688 ; inter-
mediate lamella, 688 ; intercellular
spaces, 688 ; cell-wall of, 692 ; sclerogen,
693 ; spiral cells in, 693 ; laticiferous
tissue of, 695 ; mineral deposits in cells
of, 696 ; woody fibre in, 696 et seq.; fibro-
vascular bundles, 097 ; root, structure
of, 700 ; epiderm of leaves, 712-718
flowers of, 718 ; pollen-grains of, 719
fertihsation of, 722; ovules of, 722
seeds of, 723

Phanerogams. See Phaneeogamia

1 1 68

INDEX

PHI

Philoiithus, anteniiEe of, 988- '
Phloem, 710

— of Exogens, 697
Pholas, shell of, 924
Phoronis, 950

Phosphorescence of sea, due to Noctiluca,

765
Phosphorus, as a mounting medium, 521
Photographic microscope, Zeiss's, 257,

258
Photometrical equivalent of different

apertures, 50
Photo-micrograph through eye of Lam-

pyris, 984
Photo-micrography for micrometry, 277 ;

projection eye-pieces for, 380

— Campbell's differential screw used in.
165

Phryganea, eye of, 983

Phycocyanin in Ghroococcacece , 547

Phyco-erythrin, 631

Phycomyces nitens, 641

Phycopheein, 626

Pliylactolcemata, 909

Phyllite, 1077 note

Phyllopoda, 962

Phyllosomata, skeleton of, 968

Pliysarum alhutn, development of, 635

Physcia parietina, 650

Physma chalaganuin, 650

PJiyt('lr/)Jias, endosperm of seed of, 693

Phytophtliora infestans, 639, 640

Phytopiti, mouth-parts of, 1010

PhytoptidcB, 1008; characters of, 1014

Phytoptus, larva of, 1009

Picric acid, 485

Picro-carmine, 489

Piedmontite, 1095

Pieridce, scales of, 975

Pigment-cells of cuttles, 942 ; of ver-
tebrate skin, 1042; of fishes, 1048; of
Crustacea, 1043

Pigmentum nigrum, of eye, 1043

Pik;e, scales of, 1028

Pileorhiza, 710

Pileus of Acetabularia, 563

Pilidiiun gyrans, 950

Pilulina Jeffreysii, 812

Pimpernel, petals of, 719

Pines, jpoUen-grains, showers of, 722 note

Pinna, structure of shell of, 919-922 ;

• prisms of shell of, in Glohigerina ooze,
1086 ; prisms of, in chalk, 1087
^- nigrina, colour of shell of, 921
Pinnularia, 617

— dactylus, 621

— nohilis, 621
Pinus ccmw.densis, 443
Pipette, 351, 476
Pisolithic grains, 1084
Pistil, 722

Pitcher-j)lant, spiral fibre-cells of, 698
Pith, arrangement of, 700, 762
Pitted ducts of Phanerogams, 699
Placoid scales, 1028
Plagioclase felsxsar, 1080
Planaria, stomach of, 946

POL

Planarice,:^i&'] movement of, 946; fis-
sion of, 947 ; ocelh of, 947 ; intracellular
digestion in, 863

Planarians. See Planarice

— allied to Ctenopliora, 883
Plano-concave lens, 13
Plano-convex lenses, 13, 15, 22, 37
Planorhidina, 824.

Plantago, 'Plantain,' cyclosis in, 691
Plants and animals, differences between,

531
Planulffi, 868

Planularia hexas, in chalk, 1087
Plasmode in cells of Nitella, 579 note ;

of yEthaliuvi, 634 ; of Myxoinycetes,

635
Plasmodium of Protomyxa aurantiaca,

729
Plastid, contrasted with cytode, 727
Plastidules, flagellated, of Protomyxa,

729
Plates, calcareous, of HolotJiurioidea,

895
Pleochroism, 1078, 1098
Pleochroismj variations of, 1080
Pleurosigma, 588, 617

— diffraction image of, 71

— angulatum, 69-71 ; as test for defi-
nition, 426 ; markings on, 592, 593

— formosum, as test for definition, 426

— Spiencerii, sporules of, 597

PHny, on cauterisation by focussing sun's

rays, 117 ; on sight, 118
Ploima, 791, 792
Plumatella, collecting, 528
'Plumed-moth,' wings of, 999
Plumule of Pieridce, 975
Plutarch, on myopy, 118
Pluteus larva of echinoids, 897-899
Podocyrtis cotlmirnqta, 847

— mitra, 847, 852

— ■ Schomhiprghii, 849, 852

Podophrgcb qiiadripartita, 784; imma-
ture form, 785

— ■ elongatajflSS ■ .

Podosphenia, sporules of,, 597

Podura scale as test for high powers,
389

' Podura scales,' 976, 979

PoduridtE, 979

Pointer in eye-piece, 381

Poisons, micro-chemistry of, 1103

Polarisation tints, 1080 ■

Polariscope, condensers for use with,
314 ; for examination of gastropod

. palates, 933 ; crystals for use with,
1097; list of objects for, 1099

Polarised light, rings and brushes of mine-
rals under, 319, 320 ; for insect work,
423 ; use of, in micro-petrology, 1068

Polariser, 318, 319 ; achromatic conver-
gent for, 1070'7io^e

Polarising apj)aratus, 317-319 ; condenser
for, 314 ; Swift's illuminating and, 319

Polarising prism, substitution of opales-
cent mirror for, 194

' Polierschiefer,' 617

INDEX

ii6q

POL

Polishing ground sections, 511

— - sections of hard substances, 506

— ■ -slate, 617

— -stones, 508, 617

Polistes (wasp), with attached mould,
642

Pollen- chambers of anthers, 720

grain and tube, 684

grains, 719 ; form of, 720 ; experi-
ments with, 721

— mass, of orchids, 722

— tube, 721

— tubes, traced through the style, 723
Pollinium of orchids and asclepiads, 722
Pollinoids of Floriclece, 632 ; of lichens,

650
Polyasial spicules, 859
Polycelis levigatus, 947
PolyclinidcB, 913
Polycystina, 846, 851
PolycystincB, as test for low powers,

389 ; mounting, 481
Polydesmidce, 981
' Polygastrica,' Ehrenberg's erroneous

views on, 753
Polygonum, pollen-grains of, 721
Polymorpliina, 820
Polyommatus Argus, scales of, 976
Polyparies of zoophytes, 862
Polypary of hydroids, 867
Polypes, 863. See Hyclrozoa
Polypide, of Polyzoa, 906 ; formation of

buds from, 907
Polypidom of zoophyte, 904
Polypite, of hydroids, 867
Polypodium, sori of, 675
Polyporus, 647

Polystichuvi angulare, apospory in, 680
Polystomella, shell of, 797

— craticulata, 827, 829

— crispa, 827, 829
Polythalamous Foraminifera, 796
Polytonia uvella, life-history of, 759
Polytrenia, 824; mode of growth com-
pared with Eozoon, 838

— ininiaceum, colour of, 799
Polytrichum commicne, 670, 671
Polyxenus lagurus, hair of, 981

hair of, as test for objectives, 389 ;

as test for definition, 426

Polyzoa, collecting, 527, 528; keeping
alive, 528 ; ' cell ' of, 904 ; structure
of, 904 ; gemmse of, 906 ; muscular
system, 907 ; sexual reproduction of,
907 ; ' colonial nervous system,' 907
and note ; fresh-water, lophophore of,
909 ; epistome of, 909 ; classification
of the group, 909; bibliography of,
910 ; relation to Birichiopoda, 927 ;
' liver ' of, 1047

Polyzoaries in coralline crag, 1090

Polyzoary, 904

Pond-stick, 526

Poplar, pollen-grains of, 722

Poppy, laticiferous tissue, 695 ; seed of,
723

Porcellanea, 801-810

PEI

Porcellanous shells of Foraminifera,
799 ; of Gastropoda, 928

— and vitreous Foraminifera, difference
in, 799-801

Porcupine, hair of, 1030

Pores of sponges, 856

Porphyra, trychogyne of, 632

Porphyritic crystals, glass inclusions in,
1074

' Portable ' microscope, 245-247 ; Powell
and Lealand's, 245 ; Rousselet's bino-
cular, 245 ; Swift's, 245 ; Baker's, 246 ;
Bauscli and Lomb's, 247

Portunus, skeleton of, 968

Positive aberration, 360 note

— eye-piece, 43

— eye-pieces, 377, 378

Potash, caustic, action on horny sub-
stances, 517
Potato-disease, 640

— starch-grains of, 695

— tubers, starch in, 694

Powell (T.), formula for objective, 34
Powell and Lealand's homogeneous im-
mersion objective, SO ; fiuorite lenses,
35 ; high-j)ower binocular, 105 ; sub-
stage, 186, 195, 196 ; their microscopes,
194, 218, 237 ; portable microscope,
245 ; rotating nose-pieces, 291 ; achro-
matic condenser, 301 ; achromatic oil
condenser, 302 ; apochromatic con-
denser, 302 ; dry achromatic condenser,
809 ; chromatic oil condenser, 310 ;
condenser for polariscope, 314 ; bull's-
eye, 333 ; vertical illuminator, 337 ;
j)rotecting ring for coarse adjustment,
352 ; water-immersion objectives, 362,
364 ; TTs-inch objective, for observation
of cyclosis, 689 ; objectives for study
of monads, 762
Powell's (H.) microscope, 155 ; fine ad-
justment applied to the stage, 155

— lenses, 361

— fine adjustment, 174
Prasmowski and Hartnack's water-im-
mersion objectives, 362

Prawn, skeleton of, pigment of, 969
Preparation of vegetable tissues, 514
Presbyopy, 118
Preservative media, 517-522
Primary tissues of Vertehrata, 1017
Primordial cells, 535, 536

— utricle, 533 ; of desmids, 580 ; of Pha-
nerogam cells, 688

— chamber in Foraminifera, 798 ; of
Orhitolites, 806

Primrose, cells of pollen-chambers, 720
' Prince's feather,' seed of, 723
Principle of microscopic vision, 43
Principles of microscopical optics, 1
Pringsheim, on generative process of

Pandorina, 557 ; on Vaucheria, 563
Prism, refraction by, 8, 9 ; Wenham's,

98 ; Stephenson's erecting, 100
— • polarising, substitution of opalescent

mirror for, 194
— • rectangular, in place of mirror, 192
4 P

1 I/O

INDEX

PRI

Prism, Nicol's, 318 ; Nicol's analysing,
for resolving striae, 381 ; Abraham's,
401

— refracting angle of, 9, 18
Prisnaatic epithelium, 1044

— layer in molluscari shells, 919-925

— layer of shells compared with enamel,
920, 1025

— shell-substances imitated, 1102
Prisms, recomposition of light by, 18
Pristis, tooth of, 1024
Pritchard's doublets, 298

— microscope with Continental fine ad-
justment, 153

Privet hawk-moth, eggs of, 1005
Problems on refractive index, 5
Procarp, of FloridecB, 632
Projection eye-piece, 380
Promycele of Puccinia, 637
Prosenchymatous tissue, 696
Proteus, red blood-corpuscle of, 1036
Prothallium of Sphagnacece, 674 ; of
ferns, 677 ; of EquisetacecB, 681 ; of
Ithizocarpece, 681; of Lijcopodiacece,
681
Protococcus, as gonid of lichens, 651

— pluvialis, 543-547 ; life-history of,
543 ; multiplication of, 544 ; zoospores
of, 544 ; mobile and still forms of, 545-
547 ; encysted, 551

Protomyxa auraiitiaca, 727-729
Protoneme of Batraclwspermiim, 575
Protophytes, 530, 651, 726

— mounting, 518 ; mode of nourishment
of, 532 ; movement by cilia and con-
tracting vacuoles of, 535

Protoplasm, 580 ; vital attributes of, 531 ;
continuity of, 538, 630 ; of Mhizopoda,

733 ; of Noctiluca, 767
Protoplasmic substance in Vertebrata,

1017
Pbotozoa, 726-785

— mode of nourishment of, 532

' Pseudembryo ' of Antedon, 903

Pseudo-navicellee, 751

Pseudo-parenchyme of Fungi, 633

Pseudopodia of Protomyxa, 728 ; of
Vampiyrella, 730 ; of Lieherkuehnia,
731 ; of Ithizopoda, 733 ; of Heticu-
laria, 734 ; of Heliozoa, 734 ; of Lobosa,

734 ; of Gromia, 735 ; of Microgromia,
736 ; of Actinoplirys, 738 ; of Amoeba,
743 ; of Arcella, &c., 746 ; in Amwba-
phase of monad, 757 ; of Eozoon, 841 ;
of Globigerina, 822; of Hadiolcuia,
847 ; of endoderm cells in zoophytes,
862

Pseudorapliidece, 599
PseudoscojDe, Wheatstone's, 92
Pseudosco]3ic effects, 95

— effect with Eamsden's circles, 106

— vision, 92
Pseudo-scorpions, 1008
Pseudo-stigmata of Oribatidcs, 1011,

1012
Pseudo-trachea3, on fly's proboscis, 990
note

' Psorosperms,' 752

Pteris, sori of, 675 ; indusium of, 675

— serrulata, apogamy in, 680
Pterocanium, 852
Pterodactylus, bones of, 1092
Pterophorics, wings of, 999
Pteroptus, 1012

Ptilota, 630

Puccinia graminis, 637

Puff-ball, 647

Pulvilliof insects, 1001 ; cockroach, 1000

note
Pupa of Neuroptera, circulation in, 994

— stage of fly, 1007
' Purple laver,' 632

Purpura, method of examination of egg-
capsules of, 939 ; supplemental yolk of,
938, 1007

— lapillus, nidameutum of, 934 ; develop-
ment of yolk-segments of, 937

' Puss-moth,' eggs of, 1005
Pycnogonida, 957; related to Arachnida,

959 note
Pyj'ola, seeds of, 724
Pyroxene, audesite, 1076

Quadr-ida symmetrica, 747

Quartz-porphyries, 1072

Quartzite, 1077

Quekett (E.), on Martin's microscope,

140 ; on production of raphides, 696 ;

on lareparation of trachea of insects,

997 ; on minute structure of bone,

1092
' Quills ' of porcujpine, 1080
Quinqueloculina, 802

R

Radials of Antedon, 90l
Radiating crystallisation, 1097
Radiation of light in different media, 53-

58 ; in air and balsam, 55-57
Badiolaria, collecting, 529 ; fossilised

forms of, 846, 854 note ; central cajj-

sule of, 847 ; skeleton of, 848-854 ; zoci-

xanthellffi in, 848 ; bibliography of, 853
— colonies- of, 848 ; distribution of, 853-

854 ; mounting, 854
Radiolarian, shells in ' ooze,' 1086
Rainey, on presumed cause of cattle

plague, 752 ; on moleciilar coalescence,

1100
Ralfs, on British desmids, 579 note ;

classification, 585; on Nitzschia and

Bacillaria, 606
Ramsden circles, 106
Ramsden's ' screw micrometer eye-piece,'

272 ; positive eye-piece, 42, 378, 380
Baphidea, 599
Raphides of Phanerogams, 696 ; of plants

and sponge-spicules compared, 860
Rays, scales of, 1028

INDEX

I 171

Eeagents, mode of labelling bottles, 402
Real image, U note; formation of, 23, 24

— object image, 375
Eecomposition of light by prisms, 18
Red ant, integument of, 974

— blood-corpuscles of Vetebrata, 1034 ;
size of, in various Verfebrata, 1035 ;
relative sizes of, in various Vertebrata,
1086

— coral, 877

— coriDUScles, flow of, 1056

' Red snow,' due to Palmella cruenta,

558
' Red spider,' 1013
Red spots in Infusoria, 775
Reflector, Sorby's parabolic, 334
Refracted ray, 2

Refracting angle of a prism, 9, 18
Refraction, 57

— angle of, 3

— of light, laws of, 2, 3

— by plane surface, 3, 4 ; by curved sur-
face, 5 ; by prisms, 8, 9 ; by lenses, 10-
25

Refractive isidex, absolute, 2 ; of water,
3 ; relative, 4, 5 ; of crown glass, 5 ;
of flint glass, 5 ; of balsam, 77 ; of gum
styrax, 521 ; of Canada balsam, 521 ;
of monobromide of naphtlialin, 521 ; of
phosphorus, 521

of silicious coat of diatoms, 521

— indices of air, of cedar oil, of water, 60
Reichert's loups, 38 ; his lever fine ad-
justment, 171 ; his microscopes, 206,
210, 224, 241, 242, 264-266 ; his objec-
tives, 374, 375 ; his thermo-regulator,
453

Reindeer, hair of, 1030

Reproduction in Actinophrys, 740 ; of
Act inosphcerium, 741 ; of Clathrulina,
742 ; of Etiglypha, 746 ; of sponges,
857 ; of Camjiamtlariida, 870 ; sexual,
of Polyzoa, 007 ; agamic, of Entomo-
straca, 963 ; agamic, of Ajihides, &o.,
1006

Reproductive organs of Acarina, 1011

Reptiles, lacunae in bone of, 1022 ;
cement in teeth of, 1026 ; plates in skin
of, 1026 ; epidermic ajjpendages of,
1029 ; red blood-corpuscles of, 1034,
1035 ; muscle-fibre of, 1049 ; lungs of,
1063

Meseda, seeds of, 724

Residuary secondary sfiectrum, 365

Resins, solvents for, 517

Resolving power of objectives, 83, 425

of object-glasses, 44 ; of lenses, 64 ;

of objective and numerical aperture,
75, 393

Respiration of insects, apparatus of, 994

Respiratory organ of spiders, 1014

Hete mucosum, 1042

Betepora, calcareous polyzoaries of, 909

Heticularia, 733 ; characters of, 733; ex-
amples of, 734-737

Reticulated ducts of Phanerogams, 698

Eetinulte, 983

Revolving nose-piece, Nelson's, 295
Rezzi, on invention of compound micro-
scope, 125
Bhabdammina, 813

— ahyssoriim, 815
BJiabdolitJiKS pipa, 847

— sceptrnm, 847

Rhabdoliths, in chalk and limestone,

1084
Rhabdom, 983
Bhabdopleura, 909
Bliamnus, stem of, 703
Bheophax sabulosa, 815

— scorpiurus, 815
Rhinoceros, horn of, 1033
BhisocarpecB, 681
Rhizoids of mosses, 669
Rhizome of ferns, 675
Rhizopoda, 733-747

— jjrotoplasm of, 531 ; ectosarc of, 534 ;
skeletons of, 795 ; sarcode of, 1018 ;
pseudopodial network of, 1053

Bhizosolenia, 614

— cyclosis in, 587
Bhizostoma, 874, 876
Bhizota, 790, 791

Bhododendron , pollen-grains of, 722
Bhodospermece, 574
Rhodospermin, 631
Bhodosporecs, 625
BJiopalocanium ornatum, 85''.
Rhubarb, stellate raphides of, 696 ; spiral

ducts of, 699

BhynchonelUdcB, shell structure of, 927

Ribbons of sections, 464, 469

Bibes, pollen-tubes of, 723

Rice, silicified epiderm of, 715

' Rice-paper,' 687

Rice- starch, 695

Riddell's binocular microscope, 97

Ring-cells, 446-448

Rivalto (Giordano da), on invention of
spectacles, 118

Bivulariacea, hormogones of, 548

Roach, scales of, 1028

Bocheci falcata, epiderm of, 714

Rock, ground-mass of, 1072 ; fluxion-
structure of, 1073

— sections, method of examining, 1081
Rocks, method of making sections of,

1066-1068 ; metamorphism of, 1076

Rodents, hair of, 1030

Root of Phanerogams, structure of, 700
et seq.

Root-cap, 710

Bosalhia varians, 798

Rose, glandular hairs of, 714

Ross (Andrew), on correction of object-
glass, 19-21 ; his early form of achro-
matic microscope, 152 ; mechanical
movements of his stage, 153 ; his fine
adjustment, 153, 173 ; on illumination
of objects, 300 ; his arrangement for
locking coarse adjustment, 352 ; his
achromatic objectives, 356-358 ; his
lever of contact for testing covers,
440

4 F 2

11/2

INDEX

EOS

Ross's (Andrew) ' Lister ' microscope, 153

Ross, model, 197

— ■ and Co.'s microscopes, 196, 230-233 ;

camera lucida, 285, 286
Eoss-Jackson model, 199
Eoss-Wenliam's. radial microscope, 199
Eoss-Zentmayer model, 198
Botalia, 824; intermediate skeleton of,

825

— aspera, in chalk, 1087

— Beccarii, shell of, 797

— ScJiroeteriana, 825
Eotalian series, 823
Rotaliince, colour of shell, 799
Eotaline shells-of Foraminifera, 797

— shell, sandy isomorph of, 814
Rotating disc of objectives, 290
Botatoria, 753. See Rotifeba
Botifer vulgaris, 787

EoTiFEEA, collecting, 527; keeping alive,
528 ; a food of Actinojphrijs, 739 ; de-
scription of, 786-792 ; habitats of, 786 ;
structure of, 787-790 ; mastax of, 787 ;
lorica of, 787 ; contractile vesicle of,
789 ; males of, 790 ; eggs of, 790 ; clas-
sification of, 790, 791 ; desiccation of,
791 ; bibliography of, 792 ; preparation
and i^reservation of, 793, 794 ; wheel
apparatus of, compared with velum of
gastropods, 936, 939 ; winter eggs of,
964 ; non-sexual reproduction of, 1006

Rotten-stone, 617

' Round worm,' 944

Rousselet's binocular portable micro-
scope, 245 ; his tank microscope, 268 ;
his compressorium, 346; his live-box,
346 ; his method of preparing rotifers,
793

Royston-Piggott constructs first aperture
table, 30

Bugosa, 877

Buinia cratcegata, eggs of, 1005

Rush, stellate tissue in, 687

Rutile in clastic rocks, 1075 ; a secondary
mineral in slates, 1076 note

Ryder's microtome, 401

S

Sabellana, tubes of, 948
Sable, hair of, 1030
Saccammina in limestone, 1090

— Carteri, 812

— spTierica, 812
Saccharoniyces cerevisice, 645
Saccharomycetes, 645 ; zymotic action

of, 645; endospores of, 646
SaccolcMum guttatum, spiral cells of,

693
Sachs, on Cliara, 579 note
Sago, starch-grains of, 695
Salivary glands, 1047
Salmon, scales of, 1028

— disease, 640

SalpcB, diatoms in stomach of, 614, 623
Salpidce, 911

SCH

Salpingceca, calyx of, 764

Salt solution as a preservative medium

519
Salter (J.), on the 'teeth' of Echinus,

890
Salvia verbenaca, spiral fibres in seeds

of, 693
Sand-grains surrounded by silica, 1075
' Sand-stars.' See Ophiuroidea
' Sand-wasp,' 974
Sandy isomorphs {Foraminifera), 814

— tests of Lituolida, 814
Santonine, crystallisation of, 1096
Sap-wood, 704

Saprolegnia, alliance with Acklya, 564
note

— ferax, 640
Sapirolegnice, 640
Saprophytic, Bacteria, 658

— fungi, 633, 642, 647
Sarcocystids, 752

Sarcode, 530 note, 531 ; of Bhizopoda,

733
Sarcolemma, 1049
Sarcoptes scabiei, 1018
Sarcoptidce, mandibles of, 1009 ; maxillge

of, 1010; hairs of, 1010; legs of, 1010;

characters of, 1013
Sarcoptince, 1013
Sarcosporidia, 749
Sargassum bacciferum, 630
Sarsia (Medusa of Syncoryne), 869
' Saw-flies,' ovipositor of, 1003
Saxifraga, seeds of, 724

— umbrosa, parenchyme of, 688
Saxifrage, cells of pollen-chambers, 720
Scalariform ducts of ferns, 674 ; as modi-
fied spiral ducts, 699

' Scales,' covering epidermof leaves, 714 ;
of Elceagnus, 714

— of Lepidoptera, 975, 976 ; of Coleo-
ptera, 975 ; of Curculio imperialis, 975 ;
of LyccenidcB, 975, 977 ; of Fieridce,
975 ; as tests for objectives, 976 ;
of insects, markings of, 976 ; of
Thysaniira, 977 ; on wing of Lepido-
ptera, 999 ; of fishes, 1026 ; of reptiles,
1026, 1029

Scallops. See Pecten

Scarabcei, antennae of, 988

' Scarfskiu,' 1041

Scatophaga stercoraria, eggs of, 1005

Scenedesmus, megazocispores of, 566

Schists, 1077

Schizogenous spaces in Phanerogams,

688
Scliizomycetes, 651-664
Scliizonema, 602, 617

— Grevillii, 618

— gelatinous sheath of, 588, 617
SchizonemecB, character of, 617
Schnetzler, on movement of Oscillatoria,

548
Schott (Dr.) and the improvement of

object-glasses, 32
Schroder on binocular vision, 105 ; his

camera lucida, 285

INDEX

II73

SCH

Schultz's method of macerating vege-
table tissues, 700
Scliultze (Prof. Max), on identity of

' sarcode ' and ' protoplasm,' 530 )iote ;

on cyclosis in DiatomacecB, 587 ; on

affinity of Carpenteria, 823
Schulze (Prof. F. E.), on soft parts of

Euplectella, 860 note
Schwendener, on lichens, 648
Scirtopoda, 791, 792
Scissors, spring, 157
Sclerencliyme of ferns, 674
Sclerogen, 693
Sclerotesin Fungi, 633 ; of Myxomycetes,

636
Scolopendritom, indusium of, 675 ; sori

of, 675 ; sporanges of, 676
Scorpions, 957, 1008
Screw-collar adjustment, 358
Scrophularia, seeds of, 724
' Scypliistoma ' of Cijanea, 875
Scytonema, as gonid of lichen, 651
Scytonemacece, 548 ; hormogones of, 548
Scytosiphon, conjugation of, 627
Sea-anemone. See Actinia
Sea-anemones, intracellular digestion in,

863
Sea-fans, 877. See Gorgonice
' Sea-jellies,' 853
Sealing-wax varnish, 444
' Sea-mats,' 908. See Flustra and Mem-

branipora
Searcher eye-pieces, 378
' Sea-slugs.' See Doris, Eolis
' Sea-urchin,' 884. See Echinus
Sea-weeds, 625-632

— coiitinuity of protoplasm in, 538, 630

— red, 630

Secondary spectrum, 19, 31 ; overcome

by Abbe's objectives, 365
Section lifters, 477 ; cover-glass as, 478

— mounting, 477, 501, 506

Sections, ribbons of, 464, 409 ; of hard
substances, 506 ; of bones, 506, 510 ;
of coral, 506, 510 ; of enamel, 506 ; of
fossils, 506 ; of shells, 506 ; of teeth,
506, 510 ; of hard and soft substances
together, 510 ; of Phanerogam tissues,
699

Seditjn, pollen-grains of, 721 ; seeds of,
724

Seeds, 685, 723

Segmentation of Gastropoda egg, 935 ;
of annelid body, 948

Seiler's solution for cleaning slides, 439

Selaginella, archegone of, homology of,
685

SelaginellecB, 682

Selenite plates, 318

— blue and red, 319

— stage, 319

■ — with mica film, 319

Selligue's achromatic microscope, 148,

150 ; objectives, 354
Semi-apochromatic objectives, 35 ; of

Leitz, 374; of Reichert, 374 ; of Swift,

375

Semper vivwyn, seeds of, 724

Seneca, on magnifying by water, 118

Sense, organs of, in Mollusca, 940

Sensory nerves, 1053

— organs of sponges, 856

Sepals, 718

Sepia, pigment-cells, 942

Sepiola, eggs of, 942

' Sepiostaire ' of cuttle-fish, structure of,

924 ; imitations of, 1102
Septa in shell of Foraminifera, 796, 803,

804
)SeriaZarif<., presumed nervous system in,

907
Serous membrane, 1041, 1042
Serpula, tubes of, 948
Serricornia, antennae of, 987
Sertularia cupressina, 871
Sertulariida, gonozociids of, 870 ; zoli-

phytic stage of, 877
Sessile cirriiDeds, 967
Seta of Tomopteris, 953
' Sewage fungus,' 653
Sexual fructification of Tliallophijtes,

540

— generation of Volvox, 555
Shadbolt, on structure oi ArachnoidisGUs,

612
Shadbolt's turn-table, 451
Shadow effects, 61
Shark, dentine of, 1023
Sharks, scales of, 1028
Sheep-rot, 945
Shell, bivalve, of Ostracoda, 960

— calcareous, of Seticularia, 733 ; of
Microgromia, 736

— silicious, of Dictijocijsta, Godonella,
773

— of Foraminifera, 796-801 ; of Lamel-
Ubranchiata, 919 ; of Bracldopoda,
919

Shellac cement, jprotection against cedar

oil, 444
' Shell-fish,' 919. See Mollusca
Shells of Mollusca, nacreous layer of,
919, 922, 923, 924 ; prismatic layer of,
919, 920, 921 ; colour of, 921 ; an ex-
cretory product, 922 ; sub-nacreous
layer of, 923, 924

— of Brachiopoda, 925 ; periostracum
of, 926 ; perforations of, 926

— of Gastropoda, structure of, 928

— of Cirripedia, 968

' Shield ' of Ciliata, 7T3

Shrimp, concretionary spheroids in skin

of, 1100
Shrimps, skeleton of, 969
Side reflector, 333

— lever, short, fine adjustment, 174
Swift's vertical fine adjustment,

173
Siebold, on agamic reproduction in bees,

1006
Sieve-plates, 710

Sieve-tubes, 710 ; in Exogens, 697
SigillaricB, 682, 1084
Silene, seeds of, 724

II74

INDEX

Silex in Eqioisetacece, 680 ; in eiDiderm of

grasses, 715
Silk glands of spiders, 1015
' Silk-weeda,' 569
' Silkworm,' eggs of, 1005 ;
Silkworm diseases, 645, 661
Sili^ha, antennae of, 988
Simple magnifier, 37

— microscope, 248
Sines, law of, 3

SiplwnacecB, 562-564 ; Munier-Charles

on fossil forms of, 564
Sii^hoiiostomata, 965 note
SiricidcB, ovipositor of, 1003
Sirodot, on alternation of generations in

Batracliospermum, 575
Skate, muscle fibre, 1049
Skeleton, dermal, of Vertehrata, 1026 ;

fossilised, 1090

— fibrous, of sponges, 857

— silicious, of HeUozoa, 734 ; of Haclio-
laria, 846

— ■ of sponges, 855 ; of zoophytes, 862 ;
of Eclinoidea, 884; of Asteroidea,
891 ; of 02}hiuroidea, 891 ; of Crinoidea,
892 ; of Solothurioidea, 894 ; of Ante-
don, 901 ; of Vertehrata, structure of,
1020

Skin, 1041 ; pigment-cells in, 1042 ; capil-
laries in, 1062

Skip-jack, antennsB of, 987

Slack, on the costse of Pinniilaria, 617

Slack's optical illusion, 428

Slide-forceps, 453

Slides, glass for, 438

Slides for cultures, 340, 341

— Seller's solution for cleansing, 439
Sliding-plate of objectives, 290
Sloths, fossil, teeth of, 1024

Slug. See Limax

Slug's eye, 941

Slugs, Botifera in, 787

Smell, organ of, in insects, 1000

Smith's Cassegrainian microscope, 145,

146
Smith (H. L.), on ToUes' binocular eye-

Xjiece, 101 ; his vertical illuminator,

336 ; on classification of diatoms, 599
Smith (James), his microscope, 155 ; on

use of buU's-eye with high powers,

331 ; his achromatic lenses, 356 ; his

separating lenses, 360 ; his mounting

instrument, 454
Smith (T. F.), on markings of diatoms,

593
Smith (W.), on cyclosis in Diatomacecs ,

587 ; on species of diatoms, 600 note ;

on habits of diatoms, 619
Smith (W. H.), on structure of frustules,

590 note ; on movements of diatoms,

602
Snail, 930 ; eye of, 941. See Helix
— muscle of odontophore, 1050
Snake, lung of, 1063
Snapdragon, seed of, 723
Snell's'Lawof Sines,' 49
Snow, crystals of, 1095

j Snowberry, parenchyme of fruit of, 688

Snowdrop, pollen-grains of, 722

Soda, caustic, action on homy substances,
I 517

Soemmering' s simple camera, 278
I Sole, scales of, 1026, 1027, 1028

Solen, prismatic layer in, 924

Solid cones of light for minute observa-
tion, 419

— eye-pieces, 378

— image, 95

— objects, dehneation of, 88; correct
appreciation of, 88

— vision and oblique illumination, 61
SoUas, on sponges, 855 note ; on the ex-
tensions of the perivisceral cavity in
Polyzoa, 927

Sorby (H. C), on microscopic structure

of crystals, 1066
Sorby's parabolic reflector, 334
Sorby-Browning's micro-spectroscope,

323
Soredes of lichens, 649
Sori of ferns, 675
Sound-x^roducing apparatus of crickets,

999
Spatangidiinn, 610
Spatangus, spines of, 889
' Spawn ' of mushroom, 647
Spectacles, invention of, 118
Spectra, diffraction, 67

— artificial, 324
Spectral, ocular, Zeiss' s, 327
Spectro-micrometer, bright-line, 325
Spectroscope in micro-chemical opera-
tions, 1103

Spectroscopic test, 324
Spectrum, 19 ; irrationality of, 19

— binocular, microscope, 327

— map, 325

— natural, 324

— of dark lines, 323 ; of bright lines, 323
Speculum, x^arabolic, 333; Lieberkuhn's,

334-336; in Snaith's illuminator, 336
Spencer Lens Company's Microscopes,

214, 215
Spermathecse of Gainasidce, 1012 ; of

TyroglypliidcE , 1012
Spermatia oiPuccinia, 638 ; of lichens, 650
Sperm-cells of Thallophytes, 536 ; of

Volvox, 555 ; of ferns, 678 ; of sponges,

857 ; of Hydra, 866 ; of Polyzoa, 907
Spermogones of Puccinia, 638 ; of

lichens, 651
Sphacelaria, 626
Spliacele, 626

Sphceria in caterpillars, 645
SphcBroplea annulina, 570-572
SphcBrozosnia, rows of cells in, 583
Sphcerozoum ovodimare, 853
Sphagnacea, 673
Spihagnum, leaf of, 673
Sphenogyne speciosa, winged seed of, 724
Spherical aberration, 14, 15, 31, 299, 301,

306, 387
diminished by Huyghens' objective,

42

INDEX

II75

Spheroidal concretions of carbonate of

lime, 1100
Sphingidce, antennte of, 988
Sphinx, eye of, 987 ; antenna of, 988

— ligustri, eggs of, 1005
Spicules of alcj'onarians, 880

— of sponges, 857 ; their names, 859-860

— silicious, of sponges, 857

— calcareous, of sponges, 857
Spiders, 1008, 1014-1016; microscopic

objects furnished by, 1014 ; spinning-
apparatus, 1015

Spindle fibres, 538

Spinnerets of spiders, 1015

Spiny lobster, metamorphosis, 969

Spiracles of insects, 995, 996

Spiral cells in Phanerogams, 693 ; mode
of preparation of, 694

— crystallisation, 1096

— focussing arrangement for projection-
lens, 380

— vessels of Phanerogams, 698 ; obser-
vation of, in situ,, 719 ; of plants com-
pared with trachea of insects, 995

Spirit erid(s , perforation in shells of,

927
Spiriferina rostrata, shell of, 927
Spirillina, 819

— sandy isomorph of, 814
SpiriUum, movement of, 433 ; granular

spheres of, 660 note

— undiila, 659

— volutans, movement of, 652, 653, 659
Spirit, dilute, as a preservative medium,

518
SpirocTicete, 653
Spirogyra, 549, 550 ; attacked by Vampy-

rella, 730
Spirolina, a varietal form of Peneroplis,

803
Spiroloculina, 802
Spirula, 929

— shells of, bearing Protomyxa, 727
Spirillina, movement of, 548
Splachnum, sporange of, 669

Splenic fever due to Bacillus anthracis,

656, 661
Sponge-spicules, 857-860

— mounting, 481

— in Carjienteria, 822 ; in mud of Le-
vant, 1085

Sponges, 855-862 ; skeleton of, structure
of, 855, 856; reproduction of, 857;
habitat of, 861 ; iDreparation of, 861 ;
bibliography of, 862 note

— fossil, 1089
Spongilla, 861
Spongolithis acicularis, 620
Spongy parenchyma of leaves, 716
Spontaneous generation, 761
Sporange of Fungi, 633; of Myxomy-

cetes, 636 ; of Marchantia, 665, 668 ;

of mosses, 671 ; of Sphagnacece, 673 ;

of ferns, 675 ; of Eqiiisetacece, 680
Sporangia of Lycojoodiacece in coal, 1084
Sporangiophores of Mucorini, 640
Spore, use of the term, 537 note

Spores of Nostoc, 549 ; of Myxomycetes,
634, 636 ; of Peronosp>orece, 639 ; of
Bacteria, 655, 657, 660; of Mar-
chantia, 668 ; of mosses, 670 ; of ferns,
676 ; of ferns, method for studying
development of, &19 note; of Eqiiise-
tacece, 680 ; of Lycop)odiecB, 682 ; of
gregarines, 751 ; of Monas Dallingeri,
757 ; of Lycopodiacece in coal, 1084

— different kinds of, 541 note

— resting, of ChcetophoracecE, Bli
Sporids of Ustilaginece, 636 ; of Puccinia,

638
Sporocarp of Ascomycetes, 644
Sporogone of mosses, 672
Sporophores of Myxomycetes, 636 ; of

Peronosporece, 639 ; of Ascotnycetes,

643
Sporophyte in ferns, 680
Sporozoa, 749-752
Sporules of Melosira, 597 ; of Pleuro-

sigma, 597 ; of Podosphenia, 597
Spot-lens, 316
Spring-clip, 453

— press, 453

• — scissors, 457

' Sj)ring-tails,' 979. See Poduridce

Squid, 942

Squirrel, hair of, 1030

Stag-beetle, antennse of, 988

Stage, horse-shoe. Nelson's, 179, 228 ; of
the microscope, 175-184 ; quahties
needful in a, 177 ; concentric, rotatory
motion of, 179 ; in the ' Continental '
model, 259 ; graduated rotary for use
with apertometer, 395

— attachable, simple form, 180 ; Swift's,
180 ; Allen's (Baker's), 181; Eeichert's,
183 ; Bausch and Lomb's, 183, 184 ;
Mayall's, 183 ; Zeiss's, 183 ; Beck's, 184

forceps, 338

— -micrometer, 270, 274, 288, 290

— moist, 341

— -plate, glass, 340

— thermostatic, 344-346

— Turrell's, 176 ; Watson's, 177 ; Zeiss's,
179 ; Tolles', 204

vice, 339

' Staggers ' of sheep, due to CcBnurus,

944
Stahl, on movement of desmids, 581
Staining, 488

— regressive, 491

— Bacteria, 514-516

— flagella, 516

Stains, intra-vitani, 488, 489

— for unfixed tissues, 489

— for fixed tissues, 490, 491

— nuclear, 491-494

— plasma, 494, 495

Stains, solutions of, methylen blue, 488 ;
Bismarck brown, 489 ; Congo red, 489 ;
methyl-green, 489 ; neutral-red, 489 ;
alcoholic borax-carmine, 490 ; alum-
cochineal, 490 ; carmalum, 490 ; hsema-
lum, 490 ; alcoholic cochineal, 491 ;
iron-liEematoxylin, 492 ; ' Kernschwarz,'

iiy6

INDEX

STA

SUP

492 ; safranin, 493 ; acid-fuchsin, 494
basic-fuchsin, 494 ; Lyons blue, 494
picric acid, 494 ; water-blue, 494
tliionin, 494

Stanhope lens, 37

Stanlioscope, 38

Staphylinus, antennse of, 988

Star-anise, tissue of testa of, 692 ; testa
of seeds of, 725

Starch, tests for, 517 ; formation of, 694

— grains, 534, 535 ; mode of growth,
694; hilum of, 695; in Canna, 695; in
potato, 695 ; in wheat, 695 ; in rice,
695

' Star-fish,' 891. See Asteroidea
Statospore of Protomyxa, 728
Staurastruin, binary division of, 582 ;
form of cell, 585

— dejectum, 568
Stauroneis, 617

' Stauros ' of Achnanthes, 616

Steenstrup on alternation of generations,
877

Stein, on affinities of Volvox, 551 note ;
on contractile vacuoles of Volvox, 552
note ; on Flagellata, 764 ; on Nocti-
luca, 769 note ; on Aciyietina, 785 note

Steinheil's loups, 88 ; his combination of
lenses, 38 ; his aplanatic loup, 249 ; his
loup for tank work, 268 ; his formula
for combination of lenses, 316 ; his
triple loups, 378

Stellaria, seeds of, 724

-^ media, petals of, 719

Stem of mosses, 669 ; of Bryacece, 673 ;
of. SphagnacecB, 673 ; structure of, in
Phanerogams, 700 ; of Phanerogams,
development of, 709 ; treatment of, for
examination of their structure, 711

Stemmata of insects, 986 ; of spiders,
1014

Stento7\ collecting, 527 ; contractile
vesicle of, 774 ; impressionable organs
of, 775 ; conjugation of, 7.S2

Stephanoceros, collecting, 527 ; in con-
finement, 528

Steplianolithis spinescens, 847

— nodosa, 847

Step)hanosplicBra ^:)Zi(.DiaZis, amoeboid

phase of, 557 note
Stephenson, on Pleurosigma angidatum,

70 ; on ' intercostal points,' 73

— his suggestion on homogeneous im-
mersion, 28

— on Coscinodiscus, 609
Stephenson's stereoscopic binocular, 100;

its erecting arrangement, 101, 102 ; as

a dissecting microscope, 248, 456 ; his

tank microscope, 267
Stereocaulon ramulosjis, 650
■Stereoscope, 91 ; Brewster's modification

of, 91
Stereoscopic binocular, Wenham's, 98;

for study of opaque objects, 103-105

— eye-piece, Tolles's, 101 ; Abbe's, 102

— vision, 90-97
Sterigmata of Puccinia, 637

Sterile cells of Volvox, 555
SticTiopus Kefersteinii, 895
Stick-net for marine work, 529
Stickleback, parasite of, 966 ; circulation

in tail of, 1057
Stigmata of insects, 995, 996
' Stinging hairs ' of nettle, 714
Stings of insects, 1002, 1003
Stipe of diatoms, 588 ; of Licmojjhora,

604 ; of Gomjjhonema, 616
Stolon of Foraminifera, 796; of Eozoon,

839 ; of Laguncula, 904 ; of ascidians,

914
Stomach, follicles of, 1047
Stomates, 715

— of Marcliantia, 666
Stomojyneustes variolaris, spines of, 888
Stone-cavities in crystals, 1073
Stone-mite, eggs of, 1009

Stones of fruit, preparing sections of,
699

— of stone fruit, constitution of, 693
Stone-wort, 576

Stony corals, resembled by polyzoaries,
904

Sto]3, introduction of, 37; in the eye-
piece, 42 ; use of, 312, 316

' Straight extinction,' 1079

Strawberry, jjarenchyme of fruit, 688

Streptocaulus piidcherrimus, 871

Striated muscle, 1048 ; size of fibres in
different groups, 1049

Striatella imipunctata, 598

Striatellece, characters of, 607

' Strobila ' of Cyanea, 875

8tromatopo7'a, doubtful character . of,
842

Stromatoporoids, 817 note

Stropliomenidce , perforations in shells of,
927

Stylodyctya gracilis, 851

Suberous layer of bark, 708

Sub-nacreous layer in moUuscan shells,
923, 924

Sub-stage, 184-191, 262; Nelson's fine
adjustment to, 185 ; Powell and Lea-
land's, 186; Karop's fine adjustment
to, 187; Watson's, 187; Baker's, 188;
centring nose-piece used as, 230

' Sub-stage condenser,' Nelson on, 72
7iote ; compound, 134

— illumination, 298-316

— simplest form of, 313
Succulent plants, stomates in, 716
Sucker on legs of Sarcoptidce, 1010
Suckers on foot of Dytiscus, 1001 ; of

Curcidio7iidce, 1002
Sitctoria {Protozoa), 783-785

— {Crustacea), 965, 966

' Sugar-louse,' 977. See Lepisma
Sulphuric acid, as a test, 517
' Sun-animalcule,' 737
' Sundew,' glands of, 714
Sunk-cells, 449
Super-amplification, 33
Super-stage, see attachable mechanical
stage, 180

INDEX

I 177

StJP

Supplemental yolk in Purpura, 988, 939,

1007
Surirella, 588, 606; conjugation of, 599 ;

zygospores of, 599 ; movements of, 602 ;

frustule of, 606
— - biseriata, cyclosis in, 587

— caledonica, 621

— constricta, 606
^ craticula, 621

— plicata, 621
Surirellece, 606

Suspensor of ovule of Phanerogams, 584

Sutural line of desniids, 580

Swarm-spores, 536 ; meaning of term,
587 note ; of Pandorina, 557 ; of Cut-
leria, 627 ; of Clathrulinci, 742 ; pre-
sumed, of Pelomyxa, 745

Sweat-glands, 1042

' Sweetbread,' 1047

Swift's side-lever, 162 ; vertical side-
lever fine adjustment, 173, 174 ; attach-
able stage, 180 ; microscopes, 203, 224,
228, 233; portable microscope, 245;
condenser, 802, 305 ; condenser for
polariscope, 314 ; microspectroscoxDe,
325 note ; objectives, 375 ; petrological
microscope, 1068

Symbiosis in lichens, 650

Symbiotes tripilis, hairs of, 1010

Symbiotic algge in radiolarians, 848

Sympathetic nerves, 1054

Symjphytum asperriniiim, seeds of, 724

Synalissa symphorea , 650

Synapta digitata, ' anchors ' of, 895

— mJicBrens, ' anchors ' of, 895
SynaptcB, rotifers in, 787
Syncoryne Sarsii, gonozooids of, 868
Syncrypta, 545

Sijnedra, 606

Syringamviina, 811

Syringe for catching minute aquatic

objects, 351
Syrup, as a preservative medium, 519

— and gum, as a preservative medium,
519

Tahanus, eyes of, 987; ovipositor of,

1004
Tabellaria vulgaris, 621
Table of numerical apertures, 84-87

— for microscopists, 898-402 ; for dis-
secting and mounting, 399

Tactile papillas of skin, 1042 ; nerve to,
1053

Tadpole, pigment-cells of, 1043; circu-
lation in tail of, 1056 ; general circula-
tion in, 1057; blood-vessels of, 1059,
1060

— of ascidians, 917

Tadpole's tail, epithelium of, 1044

Tcenia, 943

Tank microscopes, 266-269

Tarmin, test for, 517

Tapetal cells in fern antherid, 678

' Tape-worm,' 943

THA

Tardigrada, desiccation of, 945
Tarsonemidw, 1013
Taste, organs of, in insects, 993, 1000
Teeth, decalcification of, 512

— fossihsed, 1090

■ — in palate of Helix, 930 ; of Limax,
930 ; of Buccinum, 930 ; of Mollusca,
930

— preparation of, 1023 and note

— of Echinus, 890 ; of Ophiothrix, 892 ;
of Vertebrata, 1028

— of elephant, Rolleston on enamel in,
928 ; of Bhodentia, Tomes on enamel
in, 928

Tegeocranus cepheiformis, 1008

— dentatus, 1008

Tegumentary aj^pendages of insects, 974
Telescope, Barker's Gregorian, 145
Teleutospore generation of Puccinia,

637
Temperature, effect of, on various

monads, 761
Tendon, 1019

Tentacle of Noctiluca, 766, 768
' Tentacles ' of Drosera, 714 ; of Suctoria,

785 ; of Hydra, 864 ; of annehds, 949
TenthredinidcB, ovipositor of, 1008
TerebeUa, tubes of, 948 ; gills of, 949

— conchilega, 948
Terebratula bullata, shell of, 927
TerebratulcB, shells of, 925, 926
Terpsinoe musica, 608
TerpsinoecB, character of, 607
Tertiary tints in crystalline bodies, 1097
Tessellated epithelium, 1044

Test of Groviia, 735 ; of Arcella, 746 ;

of Difflugia, 746
Testa of seeds, 725
Testaceous amoebans, 746, 747
Testing object-glasses, 381 ; diaphragm

for use in, 885; Fripp's method, 886;

Abbe's method, 384-887
Test-plate, Abbe's, 887
Tests, sandy, of Lituolida, 814
Tetliya, spicules of, 1086
Tetramitus rostratus, life-history of

760 ; nucleus of, 763
Tetranychi, 1013
Tetranyclius, mandibles of, 1009
Tetraspores of FloridecB, 681 ; of Vam-

pyrella, 730
Textularia, 823

— aculeata, in chalk, 1087

— globulosa, in chalk, 1087
Textularian form of shell, 798

— series, 823
TextulariidcB, 811

Textularinice, arenaceous character of,

828
ThalassicoUa, 846, 853
Thallophytes, 530-632
Thallophytic type, passage to cormo-

phytic, 668
Thallus of Ulva, 560 ; of Phaosjwrem,

626 ; of Hchens, 649
Thaumantias Eschscholtzii, 878

— pilosella, 878

1 178

INDEX

' Theca ' of mosses, 671
Thecaphora, 868
Thecata, 868, 870

— zoophytic stage of, 877
Thermo-regulator, Reichert's, 453
Thermostatic stage, Dallinger's, 344-

846

Thoma's (Jung) microtome, 461-469

Thompson (J. Vaughan), on pentacrinoid
larva of Anteclon, 901 ; on Cirripedia,
967

Thomson (Wyville), on development of
Anteclon, 903

Thread-cells of Ciliata, 773 ; of Sydra,
864 ; of Zoantharia, 878, 879 ; of pla-
narians, 947

' Tlnread-worm,' 944

Threads of spiders' vs^ebs, 1015

Thura'mmina papillata, 815

Thwaites, on conjugation of Epithemia,
599 ; of Melosira, 560

Thysanura, scales of, 977

Ticks, 1008. See Acarina

Tineidce, virings of, 999

Tinoporus bacidatus, 824

Tipula, spiracle of, 976 ; eye of, 987 ;
antennse of, 988

ToUes' binocular eye-piece, 101 ; his me-
chanical stage, 204 ; his immersion
objectives, 362, 364 ; his apertometer,
390

Tomes (Charles), on teeth, 1025

Toniopiej'is onisciformis, 952, 953 ; de-
velopment of, 954

— quadricornis, 954

' Tongue' of Gastropoda. See Palate

' Tortoiseshell butterfly,' eggs of, 1005

Torula cerevisicE, 645

Total reflexion, 6, 7

Tourmaline, pleochroism in, 1078

Tow-net, 528

Tow-nets of Challenger Expedition, 529

note
Tracheas of insects, 994; of Acarina,

1011
Tracheides of ferns, 674 ; of conifers, 698,

703
Trachelomonas, 545
Tradescantia virginica, cyclosis in

hairs of, 691
Tragidus javanicus, red blood-corpuscle

of, 1035
Trematodes, 945
Triceratium, 588, 613

— as test for illumination, 415, 416

— favus, 593, 613

— fimbriatum, as test for medium
powers, 389

Trichocysts of Ciliata, 773

Trichoda lynceios, crawling of, 774; re-
production of, 780, 781

Trichodina grandinella, a phase in de-
velopment of Vorticella, 780

Trichogyne of Coleochcete, 575

— of FloridecB, 682 ; in Hchens, 650
Triclionympha, 774
Trichophore of Floridece, 682

Trichoplirya, a phase in development of

Suctoria, 785
Trigonia, prismatic layer in, 924
TrilocuUna, 802
Triple-backed objectives, 361
Triplet, Holland's, 37
Triplex front to objectives, 370
Tripoli stone, 617
Trochus zizyphinus, palate of, 981
TrombidiidcB, 1008, 1009 ; legs of, 1010 ;

hairs of, 1010 ; eyes of, 1011 ; tracheae of,

1011 ; characters of, 1012
Tronibidium, maxillae of, 1010 ; larvse of,

1013

— holosericum, 1013
Trophi of Botifera, 788
Truncatulina rosea, colour of, 799
' Tube-cells,' cements for, 442
Tube-length, English and Continental,

158, 159
Tuberculosis, bacillus of, 661 ; methods of

staining, 515, 516
Tubifex rivulorum, gregarine of, 751
Tubipora, 877
Tubitlaria, gonozooids of, 869

— indivisa, 869

Tubuli in Niunniulites, 827 ; of dentine,
1024

Tuhulijio^'a, 909

Tulip, raphides of, 696

Tully's (Lister's) achromatic microscope,
149 ; his live-box, 345 ; his triplet, 354 ;
his achromatic objective, 854

' Tunic ' of Tunicata, 911

TuNicATA, 904, 911-918 ; zoological posi-
tion of, 911 ; bibliography of, 918 ;
'liver 'of, 1047

Turbellaria, 946, 947

— larvEe of, collecting, 529
Turbinoid shell of Foraminifera, 797
Turbo, shell structure of, 928
Turkey-stone, use of, 508 ; constituents of,

617
Turn-table, Shadbolt's, 451; Griffith's,

451
Turpentine, uses of, 444, 518
Turrell's mechanical stage, 176
Twin lamelliB in leucite, 1078
TylencJms tritici, 945
Tympanum of cricket, 999
Tyroglyphi, nymph of, 1009; legs of,

1010
Tyroglyphidce, reproductive organs of,

1012 ; characters of, 1013

U

Ulothrix, conjugation of, 557
Ulva, 560, 561
TJlvacecE, 559-561
Umbelliferous j)lants, seeds of, 724
Umbonula verrucosa, 906
Under-corrected objective, 20, 21
Under-correction, 355-360
Unger, on the zoospores of Vaucheria,
568

INDEX

1 1 79

Unicellular plants, 538

Uiiio, pearls in, 923 ; glochidia of, 933

— occideiis, formation of shell in, 925

TJnionidce, nacreous layer of, 923

Unit ( standard) for microscopy, 460

UredinecE, 636-638 ; alternation of gene-
rations in, 636

Uredo-form of Puccinia, 638

Uredospores of Puccinia, 638

Urinary calculi and molecular coales-
cence, 1102

Urine, micro-cliemical examination, 1103

Uroclwrdafa, 911

TJropoda, tracliete of, 1011

' Urticating organs.' See Thread-cells

TJstilaginece, 636

Uvella, 545

V

Vacuoles in vegetable cell, 534

— contractile, in protophytes, 535 ; of
Volvox, 552

— of Actinojphrys, 737
Vagine of mosses, 671

Vallisneria, habitat, 689; mode of de-
monstration of cyclosis, 689, 690
Valvulina, shell of, 798
Vainpyrella, 729, 730

— gomphonematis, 729

— spirogyrce, 729

Vanessa, eye of, 987 ; haustellium of, 992

— urticce, eggs of, 1005
Variation, range of, in Astromnna, 849
Varley's live-box, 346

Varnish, test for, 443 ; asphalte, 443

Varnishes, 442-445 ; seaHng-wax in alco-
hol, 444 ; red, 445 ; white, 445 ; various
colours, 445

' Vascular Cryptogams,' links with Pha-
nerogams, 682

Vascular papilla of skin, 1042

Vaucheria, 562, 563

— Botifera in, 787

' Vegetable ivory,' endosperm of, 693

Vegetable substance, preparation of, 514 ;
gum-imbedding for, 514 ; bleaching of,
514

Veins of vertebrates, 1056

Velum, in gastropod larva, 936

Venice turpentine cement, for glycerin
mounts, 444

Ventriculites, 861, 1088

Venus' flower basket, 859, 860 ; spicules
of, 860

Verbena, seeds of, 724

Vertehrata, 1017-1065; bone of, 1020;
teeth of, 1023 ; dermal skeleton of,
1026; blood of, 1034; red blood-cor-
puscles, 1034 ; white blood-corijuscles,
1036 ; fibrous tissues, 1038 ; skin, mu-
cous and serous membrane, 1041 ; dis-
tribution of ciliated epithelium, 1044 ;
fat, 1045 ; cartilage, 1046 ; glands of,
1047 ; muscle, 1048 ; nervous tissue,
1051 ; circulation, 1054 ; resjjiration,
1068

WATl

Vertebrated animals, 1017. See Verte-
bra ta

Vertical illuminator, 336-338 ; how to
use, 337 ; for examination of metals,
337 ; for ascertaining ' aperture,' 338

Vespidce, eye of, 987

Vihracula of Polyzoa, 910, 911

Vibrio, movement of, 433

— rugula, 659

' Vibriones,' as applied to certain nema-
todes, 945
Vibriones, form of, 653, 659
Vigelius, on tentacular cavity of Polyzoa,

905 note
Vine, size of ducts of, 699
Viola tricolor, pollen-tubes of, 723
Violet, cells of pollen-chamber, 720
Virginian spider-wort, cyclosis in, 691
Virtual image, 14 oiote, 24, 25, 376
Vision, depth of, 88, 89, 90 ; stereoscopic,

89
Visual angle, 27
Vitrea (Foraminifera), 819
Vitreous cells (arthroi^od eye), 983

— optical compounds, 31

— shells of Foraminifera, 799

' Vittas ' of Licviojolio^'ecE, 604 ; of seeds

of umbelhfers, 724
Vocal cords, structure of, 1040
Vogan's changing nose-piece, 294
Volcanic ashes and dust, microsc oi^ical

examination of, 1076
VolvocinecB, 550-557
Volvox associated with Astasia, 765

— vegetable nature of, 556 note; amoe-
biform j)hase of, 556 ; Botifera in, 787

— aureus, cellulose in, 552; starch in,
552

— globator, 550-557 ; flagellate affinities
of, 551 note ; contractile vacuoles in,
552 ; endochrome of, 552 ; development
and reproductive cells of, 554-556

Vorticella, foot-stalk of, 773 ; contrac-
tion of foot-stalk, 774, 775 ; fission of,
777 ; encystment of, 778 ; classification
of, 782 ; gemmiparous reproduction of,
782; conjugation of, 782

— microstoma, 779

W

WaldUeimia australis, shell of, 926

Wale's model, 224; his limb, 224; his
coarse adjustment, 226 ; his fine ad-
justment, 226

Wallflower, pollen-grains of, 722

Wall-Hchens, 649

Wallich, on structure of diatom frustule,
590 note ; on Triceratium, 613 note ;
on Ghcetocerece, 614 note; on cocco-
siDheres, 747 ; on Polycystina, 852 note

— his plan for sectioning a number of
hard objects, 508 note

' Wanghie cane,' stem of, 701

' Warm-stage ' for observing blood-cor-
puscles, 1034

ii8o

INDEX

WAE

Warmth, mode of applying, for cyclosis,

692
Wasps, wings of, 998, 999 ; sting of, 1003
Water, refractive index of, 3, 7

— distilled, for mounting Protopliytes,
518

— milfoil, collecting, 527
Water-angle, 50
Water-bath, 452
Water-boatman, wings of, 1000
'Water-fleas,' 959, 962
Water-globules in oil, 429, 430
Water-immersion objectives, 362 ; Zeiss's,

370

Water-lily, leaf-structure of, 717 ; cells
of pollen-chambers, 720

' Water-mites,' 1013

' Water-net,' or HydrocUctyon, 565

Water-of-Ayr stone, 508

Water-scorpion, 995. See Nepa

'Water-snail.' See Limn ce us

Water-vascular system of Tcsnia, 943

Watson's microscopes, 199-202, 218, 224,
234, 237 ; coarse adjustment, 161, 202 ;
fine adjustment, 162, 172, 174, 175;
mechanical stage, 177 ; sub-stage, 187 ;
nose-piece, 292 ; condensers, 303, 304 ;
objectives, 375 ; eye-pieces, 379

Wavelhte in Mya, 924

Web of spiders, 1015

Weber's annular cells, 350

Webster condenser, 308

Weismann, on development of Diptera,
1007

Wenham, on binocular vision, 105 ; on
cyclosis of Vallisneria, 690

Wenham's suggestion of homogeneous
immersion, 29 ; his stereoscopic bino-
cular, 98, 99 ; his prism, 98; his para-
boloid, 316-317 ; his achromatic objec-
tive with single front, 361 ; his duplex
front objective, 362

West, on ChcEtocerece, 614 note

' Wlialebone,' 1033

Wheat, starch-grains of, 695

Wheatstone's stereoscope, 91 ; his pseudo-
scope, 92

'Wheel-animalcules,' 753, 786. See

EOTIFERA

Wlieel-like plates of Chirodota, 896
' Wheels ' of Botifera, 787
Whelk. See Bioccinuni
' White ant,' ciliate parasite of, 774
White blood-corpuscles of Vertebrata,
1036; flow of, 1056

— flbrous tissue, 1038-1041

— of egg, as a preservative medium, 519
Whitney's directions for examination of

frog's circulation, 1060
Wild clary, spiral fibres of, 693
Williamson (W. C), on Volvox, 556 note ;

on structure of fish-scales, 1027 ; on

structure of coal-plants, 1084
Willow-herb, emission of pollen-tubes,

722
Wing of Agrion, circulation in, 994
Winged seeds, 724

Wings of insects, 998-1000; of Ptero-
phoi'us, 999 ; venation of, in Neuro-
ptera, 998

Wodderborn, on Galileo's invention of
compound microscope, 121, 125

Wodderborn's ' perspicillum,' 125

Wollaston's doublets, 36, 153 ; hiscam.era
lucida, 278

Wood, arrangement of, 700, 702 ; concen-
tric rings of, 703 ; fossihsed, 705, 1083

Wooden slides for opaque objects, 450

Woody fibre, 696

— tissue of ferns, 674

Working eye-pieces, 378

Worms, 943-956

X

Xylem of Exogens, 697, 698, 710
Xylol-balsam as a preservative medium,
518, 521

Yeast, 646 ; fermentation due to, 646
Yellow cells, in Actinice, 848 ; in radio-

larians, 848
— fibrous tissue, 1039, 1040
Yolk-bag of young fish, circulation on,

1057
Yucca, epiderm of, 712 ; guard-cells of

stoniates in, 715, '716

Zanardinia, swarm-spores of, 627

Zea Mais, epiderm of, 712 ; stomates of,
715

Zeiss's oil-immersion objectives, 29 ; his
eye-pieces and objectives, 34 ; his
photographic microscope, 178, 257, 258 ;
his mechanical stage, 179, 183; his
latest microscope, 206, 237 ; his dis-
secting microscope, 248, 253 ; his apla-
natic loup, 249, 268 ; his calotte nose-
piece, 292 ; his sliding objective
changer, 293 ; his iris-diaphragm, 297 ;
his spectral ocular, 327 ; his apoehro-
matic objective, 366-374 ; his water-
immersion, 370 ; his apochromatic, for
resolving diatom markings, 592 ; his
apochromatic for study of monads, 762

Zeiss-Steinheil's loups, 249, 268

Zentmayer's microscope, 204 ; swinging
sub-stage in, 204

Zeohte, 1095

Zinc, chlor-iodide of, as a test, 516

— cement. Cole's, 445 ; Zeigler's, 445

Zoantharia, 877

Zoea, 970

Zonal structure in crystals, 1073

Zoochlorelloe of Heliozoa, 734

Zoocytium of Ophrydium, chemical com-
position of, 778

INDEX

II8I

zoo

Zooglcea of Beggiatoa, 653
Zoogloeffi, 655, 657
Zoophytes, 862-883
— cells for mounting, 448, 449
— • non-sexual reproduction of, 1006
Zoophyte troughs, 348-350
Zoosporange of Volvox, 554, 555
Zoosporanges of Phceosporecs, 626
Zoospores, 536 ; of Protococcus, 544, 545 ;
of Palmodictyon, 559 ; of Ulva, 560 ;
of Vauclieria, 562 ; of Achhja, 565 ;
development of, 565 ; of Hydrodkti/on,
566; of ConfervacecB, 570; of CEdo-
gouium, 572; of Clicetoplioracece, 574 ;
of Coleochcetacece, 575 ; of PhcBosporece,
626 ; of Fungi, 633 ; of radiolarians,
849
Zootliamiuin, collecting, 527

Zooxanthellse in radiolarians, 848
Zoozygospores of Navicida, 597
Zukal, on movement of Spirulina, 548
ZygiieviacecB, ch&vactevs oi, 549; habitats

of, 549 ; conjugation of, 549
Zygosis in Actuiophrgs, 740; of Amoeba,

744 ; of gregarines, 751
Zygospore, 537 ; formation of, 540 ; of

Hydrodictyon, 565 ; in Desmidiacece,

584, 585
Zygospores of Pahnoglcea, 542 ; of Meso-

carpus, 550 ; of Spirogyra, 550 ; of

Pandorina, 557 ; of tllva, 561 ; of

Navicida, 597; of diatoms, 599; of

Mucorini, 641
Zygote of Glenodinium, 770
Zymotic or fermentative action of Fungi,

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College Dispensary, Glasgow. Crown
Svo, with 100 Plates (230 drawings), 15s.

Convergent Strabismus, and its

Treatment ; an Essay. By Edwin^
HOLTHOUSE, M.A., F.R.C.S., Surgeon,
to the Western Ophthalmic Hospital.
Svo, 6s.

Refraction of the Eye :

A Manual for Students. By Gustavus
PIartridge, F.R.C.S., Surgeon to the
Royal Westminster Ophthalmic Hospital,
Tenth Edition. Crown Svo, with 104.
Illustrations, also Test-types, &c., 6s.

By the same Atithor.

The Ophthalmoscope : A Manual

for Students. Third Edition. Crown Svo,.
with 68 Illustrations and 4 Plates. 4s. 6d.

Methods of Operating for Cata-
ract and Secondary Impairments-
of Vision, with the results of 500 cases.
By Major G. H. Fink, H.M. Indian
Medical Service. Crown Svo, with 15.
Engravings, 5s.

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Atlas of Ophthalmoscopy.

Composed of 12 Chromo - lithographic
Plates (59 Figures drawn from nature)
and Explanatory Text. By Richard
LiEBREiCH, M.R.C.S. Translated by
H. RosBOROUGH SwANZY, M.B. Third
Edition, 4to, 40s.

Glaucoma :

Its Pathology and Treatment. By
Priestley Smith, Ophthalmic Surgeon
to, and Clinical Lecturer on Ophthalmo-
logy at, the Queen's Hospital, Birming-
ham. 8vo, with 64 Engravings and 12
Zinco-photographs, 7s. 6d.

Eyestrain

(commonly called Asthenopia). By
Ernest Clarke, M.D., B.S Lond.,
Surgeon to the Central London Ophthal-
mic Plospital, Surgeon and Ophthalmic
Surgeon to the Miller Hospital. Second
Edition. 8vo, with 22 Illustrations, 5s.

Diseases and Injuries of the
Kar. By Sir William B. Dalby,
F.R.C.S., M.B., Consulting Aural Sur-
geon to St. George's Hospital. Fourth
Edition. Crown 8vo, with 8 Coloured
Plates and 38 Wood Engravings. los. 6d.

By the same Author.

Short Contributions to Aural
Surgery, between 1875 and 1896.

Third Edition. 8vo, with Engravings, 5s.

Diseases of the Ear,

Including the Anatomy and Physiology of
the Organ, together with the Treatment of
'the Affections of the Nose and Pharynx,
which conduce to Aural Disease. By
"T. Mark Hovell, Aural Surgeon to
the London Hospital, and Lecturer on
Diseases of the Throat in the College.
Second Edition. 8vo, with 128 Engrav-
ings, 2 IS.

A System of Dental Surgery.

By Sir John Tomes, F.R S., and C. S.
Tomes, M.A., F.R.S. Fourth Edition.
Post 8vo, with 289 Engravings, i6s.

Dental Anatomy, Human and
Comparative: A Manual. ByCHARLES
S. Tomes, M.A., F.R.S. Fifth Edition.
Post 8vo, with 263 Engravings, 14s.

Dental Caries: an Investigation
into its Cause and Prevention.
By J. Sim Wallace, M.D., B.Sc,
L.D.S.R.C.S. 8vo, 5s.

Dental Materia Medica, Phar-
macology and Therapeutics. By

Charles W. Glassington, M.R.C.S.,
L.D.S. Edin. ; Senior Dental Surgeon,
Westminster Hospital ; Dental Surgeon,
National Dental Hospital; and Lecturer on
Dental Materia Medica and Therapeutics
to the College. Crown 8vo, 6s.

Dental Medicine :

A Manual of Dental ^lateria Medica and
Therapeutics. By Ferdinand [. S.
Gorgas, M.D., D.D.S., Professor of
the Principles of Dental Science in the
University of Maryland. Sixth Edition,
8vo, 1 8s.

A Manual of Dental Metallurgy.

By Ernest A. Smith, F.I.C, Assistant
Instructor in Metallurgy, Royal College
of Science, London. With 37 Illustra-
tions. Crown 8vo, 6s, 6d.

A Practical Treatise on Mecha-
nical Dentistry. By Joseph Rich-
ardson, M.D., D.D.S.' Seventh Edition
revised and Edited by George W.
Warren, D.D.S. Roy. 8vo, with 690
Engravings, 22s.

A Manual of Nitrous Oxide

Anaesthesia, for the use of Stu-
dents and General Practitioners.

By J. Frederick W. Silk, M.D. Lond,,
M.R.C.S., Ancesthetist to the Royal
Free Hospital, Dentar School of Guy's
Hospital, and National Epileptic Hospital,
8vo, with 26 Engravings, 5s.

Skin Diseases of Children. By

Geo. H. Fox, M.D., Clinical Professor
of Diseases of the Skin, College ot
Physicians and Surgeons, New York.
With 12 Photogravure and Chromographic
Plates, and 60 Illustrations in the Text,
Roy. 8vo, I2S. 6d.

A Handbook on Leprosy.

By S. P. Impey, M.D.,M.C., late Chief
and Medical Superintendent, Robben
Island Leper and Lunatic Asylums, Cape
Colony. With 38 Plates and Map,8vo, 12s.

Diseases of the Skin

(Introduction to the Study oQ. By
P. II. Pye- Smith, M. D., F.R.S.,
F.R.C.P., Physician to, and Lecturer on
Medicine in, Guy's Hospital. Crown
8vo, with 26 Engravings. 73. 6d.

A Manual of Diseases of the

Skin, with an Analysis of 20,oco con-
secutive Cases and a Formulary. By
Duncan L. Bulkley, M.D., New
York. Fourth Edition, Roy. l6mo, 63. 6d.

Cancerous Affections of the Skin.

(Epithelioma and Rodent Ulcer.) By
George Thin, M.D. Post 8vo, with
8 Engravings, 5s.

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Pathology and Treatment of

Ringworm. Svo, with 21 Engravings,

t;s.

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i\

Tne Operative Surgery of Malig-
nant Disease. By Henry T. Butlin,
F.R.C.S., Surgeon to St. Bartholomew's
Hospital. Second Edition, with I2 En-
gravings. Svo, 14s.

By the same Aittlior.

Malignant Disease (Sarcoma
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Sarcoma and Carcinoma :

Their Pathology, Diagnosis, and Treat-
ment. Svo, with 4 Plates, Ss.

Cancers and the Cancer Pro-
cess : a Treatise, Practical and Theoretic.
By Herbert L. Snow, M.D., Surgeon
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Ringworm and some other
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Svo, 5s.

The Diagnosis and Treatment of

Syphilis. By Tom Robinson, M.D.,
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Second Edition, Crown Svo, 3s. 6d.

By the same Author.

The Diagnosis and Treatment

of Eczema. Second Edition, Crown
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Illustrations of Diseases of the
Skin and Syphilis, -with Re-
marks. Fasc. 1. with 3 Plates. Imp.
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Selected Papers on Stone, Pros-
tate, and other Urinary Dis-
orders, By Reginald Harrison,
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pital. Svo, with 15 Illustrations. 5s.

Chemistry of Urine ;

A Practical Guide to the Analytical
Examination of Diabetic, Albuminous,
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ings, Svo, 7s. 6d.

Clinical Chemistry of Urine
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MuNN, M.A., M.D. Svo, with 64
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By Sir Henry Thompson, Bart., F.R.C.S.
Diseases of the Urinary Organs.

Clinical Lectures. Eighth Edition.
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Practical Lithotomy and Litho-

trity; or, An Inquiry into the Best Modes
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Calculous Disease, and the Use of

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Electric Illumination of the

Bladder and Urethra, as a Means
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and St. Peter's Hospital for Stone. Second
Edition. Svo, with 54 Engravings, 6s, 6d.

By the same Atithor.

Tumours of the Urinary Blad-
der. The Jacksonian Prize Essay of
1S87, rewritten with 200 additional cases,
in four Fasciculi. Fas. I. Royal Svo. 5s.

Also.

Ulceration of the Bladder,
Simple, Tuberculous, and Malig-
nant : A Clinical Study. Illustrated,
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Also.

The Cardinal Symptoms of

Urinary Diseases : their Diagnostic
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36 Illustrations. 8s. 6d.

Atlas of Electric Cystoscopy.

By Dr. Emil Burckhardt, late of the
Surgical Clinique of the University of
Bale, and E. Hu rry Fenwick, F. R. C. S. ,
Surgeon to the London Hospital and St.
Peter's Hospital. Royal Svo, with 34 Col-
oured Plates, embracing 83 Figures. 21s.

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Urinary and Renal Derange-
ments and Calculous Disorders.

By Lionel S. Eeale, F.R.C.P.,
F.R.S., Consulting Physician to King's
College Hospital. Svo, 5s.

Male Organs of Generation

(Diseases of). ByW. H. A. Jacobson,
M.Ch. Oxon., F.R.C.S., Assistant
Surgeon to Guy's Hospital. Svo, with
88 Engravings. 22s.

The Surgical Diseases of the

Genito - Urinary Organs, in-
cluding Syphilis. By E. L. Keyes,
M.D., Professor in Eellevue Hospital
Medical College, New York (a revision
of Van Buren and Keyes' Text-book).
Roy. 8vo, with 114 Engravings, 21s.

Diseases of the Rectum and

Anus. By Alfred Cooper, F.R.C.S.,
Senior Surgeon to the St. Mark's Hos-
pital for Fistula ; and F. SwiNFORD
Edwards, F.R.C.S., Senior Assistant
Surgeon to St. Mark's Hospital. Second
Edition, with Illustrations. Svo, 12s,

Diseases of the Rectum and

Anus. By Harrison Cripps, F.R.C.S.,
Assistant Surgeon to St. Bartholomew's
Hospital, &c. Second Edition. Svo,
with 13 Lithographic Plates and numer-
ous Wood Engravings, 12s. 6d.

By the same Author.

Cancer of the Rectum.

Especially considered with regard to its
Surgical Treatment. Jacksonian Prize
Essay. Svo, with 13 Plates and several
Wood Engravings, 6s.

Also.

The Passage of Air and Faeces
from the Urethra. Svo, 3s. 6d.

Syphilis.

By Alfred Cooper, F.R.C.S., Con-
sulting Surgeon to the West London and
the Lock Hospitals. Second Edition.
Edited by Edward Cotterell,
F.R.C.S., Surgeon (out-patients) to the
London Lock Hospital. Svo, with 24
Full-page Plates (12 coloured), iSs.

On Maternal Syphilis, including

the presence and recognition of Syphilitic
Pelvic Disease in Women. By John A.
Shaw-Mackenzie, ^LD. Svo, with
Coloured Plates, los. 6d.

A Medical Vocabulary :

An Explanation of all Terms and Phrases
used inthe various Departments of Medical
Science and Practice, their Derivation,
Meaning, Application, and Pronunciation.
By R. G. Mayne, M.D., LL.D. Sixth
Edition by W. W. Wagstaffe, B.A.,
F.R.C.S. Crown Svo, IDS. 6d.

A Short Dictionary of Medical

Terms. Being an Abridgment of
Mayne's Vocabulary. 64mo, 2s. 6d.

Dunglison's Dictionary of

Medical Science : Containing a full
Explanation of its various Subjects and
Terms, with their Pronunciation, Accentu-
ation, and Derivation. Twenty-second
Edition, much enlarged. By RicharD'
J. DuNGLisoN, A.M., M.D. Super-
royal Svo, 30s.

Terminologia Medica Poly-

glotta : a Concise International Die
tionary of Medical Terms (French, Latin,
English, German, Italian, Spanish, and
Russian). By Theodore Maxwell,
M.D., B.Sc, F.R.C.S. Edin. Royal
Svo, 1 6s.

A German-English Dictionary of

Medical Terms. By Sir Frederick
Treves, K.C.V.O., Surgeon to the Lon-
don Hospital; and HuGO Lang, B.A.
Crown Svo, half-Persian calf, 12s.

A Handbook of Physics and

Chemistry, adapted to the require-
ments of the first Examination of the
Conjoint Board and for general use. By
Herbert E. Corbin, B.Sc.Lond., and
Archibald M. Stewart, B.Sc. Lond.
With 120 Illustrations. Crown Svo, 6s. 6d.

A Manual of Chemistry, Theo-
retical and Practical. By William
A. TiLDEN, D.Sc, F.R.S., Professor of
Chemistry in the Royal College of Science,
London ; Examiner in Chemistry to the
Department of Science and Art. With
2 Plates and 143 Woodcuts, crown 8vo,.
los.

Chemistry,

Inorganic and Organic. With Experi-
ments. By Charles L. Bloxam^
Eighth Edition, by John Millar
Thomson, F.R.S., Professor of Chemistry
in King's College, London, and Arthur
G. Bloxam, Head of the Chemistry
Department, The Goldsmiths' Institute,
New Cross. Svo, with nearly 300 Illustra-
tions, iSs. 6d.

By the same Author.

Laboratory Teaching;

Or, Progressive Exercises in Practical'
Chemistry. Sixth Edition. By Arthur
G. Bloxam. Crown Svo, with 80
Engravings, 6s. 6d.

Watts' Organic Chemistry.

Edited by William A. Tilden, D.Sc,
F.R.S., Professor of Chemistry,. Royal
College of Science, London. Second
Edition. Crown Svo, with Engravings,

I OS.

LONDON: 7, GREAT MARLBOROUGH STREET.

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13

Practical Chemistry

And Qualitative Analysis. By Frank
Clowes, D.Sc. Lond., Emeritus Pro-
fessor of Chemistry in the University
College, Nottingham. Seventh Edition.
Post 8vo, with 10 1 Engravings and
Frontispiece, 8s. 6d.

Quantitative Analysis.

By Frank Clowes, D.Sc. Lond.,
Emeritus Professor of Chemistry in the
University College, Nottingham, and J.
Bernard Coleman, Assoc. R. C. Sci.
Dublin ; Professor of Chemistry, South-
West London Polytechnic. Fifth Edi-
tion. Post Svo, with 122 Engravings, los.
By the savie Authors.

Elementary Quantitative Ana-

lysiSi Post Svo, with 62 Engravings,
4s. 6d.

Also.

Elementary Practical Chemistry
and Qualitative Analysis. Third
Edition. Post Svo, with 68 Engravings,
3s. 6d.

Qualitative Analysis.

By R. Fresenius. Translated by
Charles E. Groves, F.R.S. Tenth
Edition. Svo, with Coloured Plate of
Spectra and 46 Engravings, 15s.
By the same Author.

Quantitative Analysis.

Seventh Edition.

Vol. I., Translated by A. Vacher.

Svo, with 106 Engravings, 15s.
Vol. II., Translated by C. E. Groves,

F.R.S. Svo, with 143 Engravings,

20s.

Inorganic Chemistry.

By Sir Edward Frankland, K.C.B.,
D.C.L., LL.D., F.R.S., and Francis
R. JAPP, M.A., Ph.D., F.LC, F.R.S.,
Professor of Chemistry in the University
of Aberdeen. Svo, with numerous Illus-
trations on Stone and Wood, 24s.

Inorganic Chemistry

(A System of). By William Ramsay,
Ph.D., F.R.S., Professor of Chemistry in
University College, London, Svo, with
Engravings, 15s.

By the same Author.

Elementary Systematic Chemis-
try for the Use of Schools and.
Colleges. With Engravings. Crown
Svo, 4s. 6d. ; Interleaved, 5s. 6d.

Valentin's Practical Chemistry
and Qualitative and Quantita-
tive Analysis. Edited by W. R.
Hodgkinson, Ph.D., F.R.S.E., Pro-
fessor of Chemistry and Physics in the
Royal Military Academy, and Artillery
College, Woolwich. Ninth Edition. Svo,
with Engravings and Map of Spectra,
9s. [The Tables separately, 2s. 6d.]

Practical Chemistry, Part I.
Qualitative Exercises and Analy-
tical Tables for Students. By J.
Campbell Brown, Professor of Chemis
try in Victoria University and University
College, Liverpool. Fourth Edition.
Svo, 2s. 6d.

The Analyst's Laboratory Com-
panion ; a Collection of Tables and
Data for Chemists and Students. By
Alfred P3. Johnson, A.R.C.S.I.,
F.LC. Second Edition, enlarged, crown
8vo., cloth, 5s., leather, 6s. 6d.

Volumetric Analysis :

Or the Quantitative Estimation of Chemi-
cal Substances by Measure, applied to
Liquids, Solids, and Gases. By Francis
Sutton, F.C.S., F.LC. Eighth Edi-
tion. Svo, with 116 Engravings, 203.

Commercial Organic Analysis :

a Treatise on the Properties, Modes of
Assaying, Proximate Analytical Examina-
tion, &c., of the various Organic Chemi-
cals and Products employed in the Arts,
Manufactures, Medicine, &c. By Alfred
H. Allen, F.LC, F.C.S. Svo.

Vol. I. — Introduction, Alcohols, Neu-
tral Alcoholic Derivatives, &c.,
Ethers, Vegetable Acids, Starch
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Fats, Glycerin, Nitro - Glycerin,
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tial Oils, Resins and Camphors,
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Colouring blatters, Inks. Third
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Vol. III.— Part HI. Vegetable Alka-
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14

.1. 4- A. CHURCHILLS RECENT WORKS.

Cooley's Cyclopaedia

of Practical Receipts, and Collateral In-
formation in the Arts, Manufactures, Pro-
fessions, and Trades : Including Medicine,
Pharmacy, Hygiene and Domestic Eco-
nomy. Seventh Edition, by W. North,
M.A. Camb., F.C.S. 2 Vols., Roy.Svo,
with 371 Engravings, 42s.

Chemical Technology:

A Manual. By Rudolf von Wagner,
Translated and Edited by Sir William
Crookes, F.R.S., from the Thirteenth
Enlarged German Edition as remodelled
by Dr. Ferdinand Fischer. 8vo, with
596 Engravings. 32s.

Chemical Technology ;

Or, Chemistry in its Applications to Arts
and Manufactures. Edited by Charles
E. Groves, F. R.S., and William
Thorp, B.Sc.
Vol. I. — Fuel and its Applica-
tions. By E. J. Mills, D.Sc,
F.R.S., and F. J. Rowan, C.E.
Royal 8vo, with 606 Engravings, 30s.
Vol. II. — Lighting by Candles
and Oil. By W. Y. Dent, J.
McArthur, L. Field and F. A.
Field, Boverton Redwood, and
D. A. Louis. Royal 8vo, with 358
Engravings and Map, 20s.
Vol. III. — Gas Lighting, By
Charles Hunt. With 2 Plates
and 292 Engravings. Royal 8vo, i8s.

Technological Handbooks.

Edited By John Gardner, F.I.C,
F.CS., and James Cameron, F.I.C.
Brewing, Distilling, and Wine

Manufacture. Crown 8vo, with

Engravings, 6s. 6d,
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Printing. With Formulae. Crown

8vo, with Engravings, 5s.
Oils, Resins, and Varnishes.

Crown 8vo, Mith Engravings. 7s. 6d.
Soaps and Candles. Crown 8vo,

with 54 Engravings, 7s.

Chemistry an Exact Mechanical
Philosophy. By Fred. G. Edwards.
Illustrated. 8vo, 3s. 6d.

The Microscope and its Revela-
tions, By the late William B. Car-
penter, C.B., M.D., LL.D., F.R.S.
Seventh Edition, by the Rev. W. H.
Dallinger, LL.D., F.R.S. With 21
Plates and 800 Wood Engravings. 8vo,
26s. Half Calf, 30s.

Methods and Formulae

Used in the Preparation of Animal and
Vegetable Tissues for Microscopical Ex-
amination, including the Staining of
Bacteria. By Peter Wyatt Squire,
F.L.S. Crown 8vo, 3s. 6d.

Encyclopaedia Medica, under

Watson, M.B., M.R.CP.E. About
now ready.

The Quarterly Journal of Micro-
scopical Science. Edited by E. Ray
Lankester, M.A., LL.D., P\R.S.; with
the co-operation of Adam Sedgwick,
M. A., F.R.S., W. F. R. Weldon, M.A.,
F.R.S., and Sydney J. Hickson, M.A.,.
F.R.S. Each Number, IDS.

The Microtomist's Vade-Mecum:

A Handbook of the Methods of Micro-
scopic Anatomy. By Arthur Bolles
Lee, Assistant in the Russian Laboratory
of Zoology at Villefranche-sur-Mer (Nice).
Fifth Edition. 8vo, 15s.

Photo-Micrography

(Guide to the Science of). By Edwari>
C. BousFiELD, L.R.C.P. Lond. 8vo,
with 34 Engravings and Frontispiece, 6s..

A Treatise on Physics,

By Andrew Gray, LL.D., F.R.S., Pro-
fessor of Natural Philosophy in the
University of Glasgow. Vol. I. Dyna-
mics and Properties of Matter. With
350 Illustrations, 8vo, 15s.

An Introduction to Physical

Measurements, with Appendices on
Absolute Electrical Measurements, &c.
By Dr. F. Kohlrausch, Professor at
the University of Strassburg. Third
English Edition, by T. H. Waller,.
B.A., B.Sc, and H, R. Procter, F.LC.^
F.C.S. 8vo, withgi Illustrations, I2s. 6d,

Tuson's Veterinary Pharma-
copoeia, including the Outlines of
Materia Medica and Therapeutics.
Fifth Edition. Edited by James Bayne,
F.C.S., Professor of Chemistry and
Toxicology in the Royal Veterinary
College. Crown 8vo, 7s. 6d.

The Veterinarian's Pocket Re-
membrancer : being Concise Direc-
tions for the Treatment of Urgent or
Rare Cases. By George Armatage,
M.R.C.V.S. Second Edition. Post 8vo,

Chauveau's Comparative Anat-
omy of the Domesticated Ani-
mals, revised and enlarged, with the Co-
operation of S. Arloing, Director of the
Lyons Veterinary School. Edited by Geo.
Fleming, C.B., LL.D., F.R.C.V.S.,
late Principal Veterinary Surgeon of the
British Army. Second English Edition..
8vo, with 585 Engravings, 31s. 6d.

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LONDON: 7. GREAT MARLBOROUGH STREET.

INDEX TO J. & A. CHURCHILL'S LIST.

Allen's Chemistry of Unne, II

Commercial Organic Analysis, 13

Armatage's Veterinary Pocket Remembrancer, 14
Auld's Pathol. Researches, 2

Barnes (R.) on Obstetric Operations, 3

on Diseases of Women, 3

Beale (L. S.) on Liver, 6
Slight Ailments, 6

Urinary and Renal Derangements, 12

Beale (P. T. B.) on Elementary Biology, 2

Beasley's Book of Prescriptions, 5

Druggists' General Receipc Book, 5

Pharmaceutical Formulary, 5

Bell on Sterility, 4

Bentley and Trimen's Medicinal Plants, 5
Bentley's Systematic Botany, 5
Berkart's Bronchial Asthma, 6
Bernard on Stammering, 7
Berry's (Jas.) 'Thyroid Gland, g

(R. J.) Regional Anatomy, i

Bigg's Short Manual of Orthopaedy, 9
Birch's Practical Physiology, 2
Bloxam's Chemistry, 12

Laboratory Teaching, 12

Bousfield's Photo-Micrography, 14
Bowlby's Injuries and Diseases of Nerves, 9

Surgical Pathology and Morbid Anatomy, 9

Brockbank on Gallstones, S
Brown's (Haydn) Midwifery, 3
Ringworm, 11

(Campbell) Practical Chemistry, 13

Bryant's Practice of Surgery, 8

Bulkley on Skin Diseases, 10

Burckhardt's (E.) and Fenwick's (E. H.) Atlas o

Cystoscopy, 11
Burdett's Hospitals and Asylums of the World, 2
Butler-Smythe's Ovariotomies, 4
Butlin's Operative Surgery of Malignant Disease, 11

IMalignant Disease of the Larynx, 11

Sarcoma and Carcinoma, 11

Buzzard's Diseases of the Nervous System,. 7

Peripheral Neuritis, 7

Simulation of Hysteria, 7

Cameron's Oils, Resnis, and Varnishes, 14

• Soaps and Candles, 14

Carpenter and Dallinger on the Microscope, 14
Cautley's Infant Feeding, 4
Charteris' Practice of Medicine, 6
Chauveau's Comparative Anatomy, 14
C'hevers' Diseases of India, 5
Churchill's Face and Foot Deformities, 9
Clarke's Eyestrain, 10
Clouston's Lectures on Mental Diseases, 3
Clowes and Coleman's Quantitative Analysis, 13

Elmntry Practical Chemistry, 13

Clowes' Practical Chemistry, 13

Coles on Blood, 6

Cooley's Cyclopaedia of Practical Receipts, 14

Cooper on Syphilis, 12

Cooper and Edwards' Diseases of the Rectum, 12

Corbin and Stewart's Physics and Chemistry, 12

Cripps' (H.) Ovariotomy and Abdominal Surgery, 9

Cancer of the Rectum, 12

Diseases of the Rectum and Anus, 12

Air and Faeces in Urethra, 12

Cripps' (R. A.) Galenic Pharmacy, 4
Cuff's Lectures to Nurses, 4

Culling worth's Short Manual for Monthly Nurses, 4
Dalby's Diseases and Injuries of the Ear, 10

Short Contributions, 10

Dana on Nervous Diseases, 7
Day on Headaches, 8
Domville's Manual for Nurses, 4
Doran's Gyncccoiogical Operations, 3
Druitt's Surgeon's Vade-Mecum, 8
Duncan (A.), on Prevention of Disease in Tropics, 5
Dunglison's Dictionary of Medical Science, 12
Edvv'ards' Chemistry, 14
Ellis's (T. S.) Hum.an Foot, 9
Encyclopaedia Medica, 14

Fagge's Principles and Practice of Medicine, 6
Fayrer's Climate and Fevers of India, 5
Fenwick (E. H.), Electric Illumination of Bladder, 11
Tumours of Urinary Bladder, ii

Ulceration of Bladder, II

Fenwick (E. H.), Symptoms of Urinary Diseases, it
Fenwick's (S.) Rledical Diagnosis, 6

Obscure Diseases of the Abdomen, 6

■ Outlines of Medical Treatment, 6

Ulcer of the Stomach and Duodenum, 6

The Saliva as a Test, 6

Fink's Operating for Cataract, g

Fowler's Dictionary of Practical Medicine, 6

Fox (G. H.) on Skin Diseases of Childien, 10

Fox (Wilson), AtlasofPathological Anatomy of Lungs, e

Treatise on Diseases of the Lungs, 6

Frankland and Japp's Inorganic Chemistry, 13
Eraser's Operations on the Brain, 8
Fresenius Qualitative Analysis, 13

Quantitative Analysis, 13

Galabin's Diseases of Women, 3

Manual of Midwifery, 3

Gardner's Bleaching, Dyeing, and Calico Printing, 14

; Brewing, Distilling, and Wine Manuf., 14.

Glassington's Dental Materia Medica, 10
Godlee's Atlas of Human Anatomy, i
Goodhart's Diseases of Children, 4
Gorgas' Dental Medicine, 10
Gowers' Diagnosis of Diseases of the Brain, 7

Manual of Diseases of Nervous System, 7

Clinical Lectures, 7

Medical Ophthalmoscopy, 7

Syphilis and the Nervous Sj'stem, 7

Granville on Gout, 7

Gray's Treatise on Physics, 14

Green's Manual of Botany, 5

Vegetable Physiology, 5

Greenish's Materia Medica, 4

Groves' and Thorp's Chemical Technology, 14
Guy's Hospital Reports, 7
Habershon's Diseases of the Abdomen, 7
Haig's LTrlc Acid, 6

Diet and Food, 2

Harley on Diseases of the Liver, 7
Harris's (V. D.) Diseases of Chest. 6
Harrison's Urinary Organs, 1 1
Hartridge's Refraction of the Eye, 9

Ophthalmoscope, 9

Hawthorne's Galenical Preparations of 6. P., 5
Heath's Injuries and Diseases of the Jaws, 8

Minor Surgery and Bandaging, 8

Operative Surgery, 8

Practical Anatomy, i

Surgical Diagnosis, 8

Hedley's Therapeutic Electricity, 5
Hellier's Notes on Gynaecological Nursing, 4
Hewlett's Bacteriology, 3

Hill on Cerebral Circulation, 2
Holden's Human Osteology, i

Landmarks, i

Holthouse on Strabismus, 9

Hooper's Physicians' Vade-Mecum, 5

Horton-Smith on Tj-phoid, 7

Hovell's Diseases of the Ear, 10

Human Nature and Physiognomy, 14

Hyslop's Blental Physiology, 3

Impey on Leprosy, 10

Ireland on Mental Affections of Children, 3

Jacobson's Male Organs of Generation, 12

Operations of Surgery, 8

Jellett's Practice of Midwifery, 3

Gynaecology, 3

Jessop's Ophthalmic Surgery and Medicine, 9.
Johnson's (Sir G.) Asphyxia, 6

■ Medical Lectures and Essays, 6

(A. E.) Analyst's Companion, 13

Journal of Mental Science, 3

Kellogg on Mental Diseases, 3

Kelynack's Pathologist's Handbook, i

Keyes' Genito-Urinary Organs and Syphilis, 12

Kohlrausch's Physical Measurements, 14

Lancereaux's Atlas of Pathological Anatomy, 2

Lane's Rheumatic Diseases, 7

Langdon-Down's Mental Affections of Childhood, 3

Lawrie on Chloroform, 4

Lazarus-Barlow's General Pathology, i

Lee's Microtomist's Vade Mecum, 14

Lewis (Bevan) on the Human Brain, 2

Liebreich (O.) on Borax and Boracic Acid, 2

Liebreich's (R). Atlas of Ophthalmoscopy, 10

\,Co7itinned on the next page.

LONDON: 7, QREAT MARLBOROUGH STREET.

Index to J. & A. Churchill's List — continued

Cucas's Practical Pharmacy, 4

MacMunn's Clinical Chemistry of Urine, 11

Macnamara's Diseases and Refraction of the Eye, 9

-Macnamara's Diseases of Bones and Joints, 8

McNeill's Epidemics and Isolation Hospitals, 2

Malcolm's Physiology of Death, 9

Marcet on Respiration, i

Martin's Ambulance Lectures, 8

Maxwell's Terminologia Medica Polyglotta, 12

Maylard's Surgery of Alimentary Canal, 9

Mayne's Medical Vocabulary, 12

Microscopical Journal, 14

Mills and Rowan's Fuel and its Applications, 14

Moore's (N.) Pathological Anatomy of Diseases, i

Moore's (Sir W.J.) Family Medicine for India, 5

— — • Manual of the Diseases of India, 5

Morris's Human Anatomy, i

Anatomy of Joints, i

Moullin's (Mansell) Surgery, 8
Nettleship's Diseases of the Eye, 9
Notter's Hygiene, 2
■Ogle on Tympanites, 8
Oliver's Abdominal 'Tumours, 3

Diseases of Women, 3

Ophthalmic (Royal London) Hospital Reports, 9

Ophthalmological Society's Transactions, 9

Ormerod's Diseases of the Nervous System, 7

Parkes' (E.A.) Practical Hygiene, 2

Parkes' (L.C.) Elements of Health, 2

Pavy's Carbohydrates, 6

Pereira's Selecta e Prescriptis, 5

'Phillips' Materia Medica and Therapeutics, 4

Pitt-Lewis's Insane and the Law, 3

Pollock's Histology of the Eye and Eyelids, 9

Proctor's Practical Pharmacy, 4

fye-Smith's Diseases of the Skin 10

Ramsay's Elementary Systematic Chemistry, 13

Inorganic Chemistry, 13

Richardson's Mechanical Dentistry, 10
■Richmond's Antiseptic Principles for Nurses, 4
Roberts' (D. Lloyd) Practice of Midwifery, 3
Robinson's (Tom) Eczema, 11

Illustrations of Skin Diseases, 11

Syphilis, II

Ross's Diseases of the Nervous System, 7
■St. Thomas's HospitaJ Reports, 7
Sansom's Valvular Disease of the Heart, 7
■^Scott's Atlas of Urinary Deposits, 11
Shaw's Diseases of the Eye, 9
"Shaw-Mackenzie on Maternal Syphilis, 12
Short Dictionary of Medical Terms, 12
Silk's Manual of Nitrous Oxide, 10
'Smith's (Ernest A.) Dental Metallurgy, 10
Smith's (Eustace) Clinical Studies, 4

Disease in Children, 4

WastingDiseasesof Infants and Children, 4

Smith's (F. J.) Medical Jurisprudence, 2

bmith's (J. Greig) Abdominal Surgery, 8
Smith's (Priestley) Glaucoma, 10
Snow's Cancer and the Cancer Process, 11
■ Palliative Treatment of Cancer, 11

Reappearance of Cancer, 11

Solly's Medical Climatology, 8
Southall's Materia Medica, 5

Squire's (P.) Companion to the Pharmacopoeia, 4

London Hospitals Pharmacopeias, 4

, Methods and Formulae, 14

Starling's Elements of Human Physiologj-, 2

Sternberg's Bacteriology, 6

Stevenson and Murphy's Hygiene, 2

Sutton's (J. B.J, General Pathology, i

Sutt_on's(F.) Volumetric Analysis, 13

Swain's Surgical Emergencies, 8

Swayne's Obstetric Aphorisms, 3

Taylor's (A. S.) Aledical Jurisprudence, 2

Taylor's (F.) Practice of Medicine, 6

Thin's Cancerous Affections of the Skin, 10

Pathology and Treatment of Ringworm, 10

on Psilosis or " Sprue," 5

Thompson's (Sir H.) Calculous Disease, 11

-— Diseases of theUrinaryOrgans,ii

Lithotomy and Lithotrity, 11

Stricture of the Urethra, 11

Suprapubic Operation, 11

Tumours of the Bladder, 11

Thome's Diseases of the Heart, 7

Thresh's Water Analysis, 2
Tilden's Manual of Chemistry, 12
Tirard's Medical Treatment, 6
Tobin's Surgery, 8
Tomes' (C. S.) Dental Anatomy, 10
•Tomes' (J. and C. S.) Dental Surgerj-, 10
Tooth's Spinal Cord, 7

Treves and Lang's German-English Dictionary, 12
Tuke's Dictionary of Psychological Medicine, 3
Tuson's Veterinary Pharmacopoeia, 14
Valentin and Hodgkinson's Practical Chemistry, 13
Vintras on the Mineral Waters, &c., of France, 8
Wagner's Chemical Technology, 14
Wallace on Dental Caries, 10
Walsham's Surgery : its Theory and Practice,
Waring's Indian Bazaar Medicines, 5

Practical Therapeutics, 5

Watts' Organic Chemistry, 12

West's_(S.) How to Examine the Chest, 6

Westminster Hospital Reports, 7

White's (Hale) Materia RIedica, Pharmacy, &c., 4

Wilks' Diseases of the Nervous System, 7

Wilson's (Sir E.) Anatomists' Vade-Mecum, i

Wilson's (G.) Handbook of Hygiene, 2

Wolfe's Diseases and Injuries of the Eye, 9

Wynter and Wethered's Practical Pathology, i

Year-Book of Pharmacy, 5

Yeo's (G. F.) Manual of Physiology, 2

N.B.—J. ^- A. Chm'chilVs larger Catalogue of about 600 works on Anatofny,
Physiology, Hygie?ie, Midwifery, Materia Medica, Medicine, Surgery, Chemistry.,
Botany, S^x. Sj-c, with a complete Index to their Subjects, for easy refer ence^
will be forwarded post free on applicatiofi.

America. — J. ^ A. Churchill bemg in constant commiimcation zvith
-various publishing houses in America are able to conduct negotiations
favourable to English Authors.

LOADON: 7, GREAT MARLBOROUGH STREET.

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