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THE MICROSCOPE

FRONTISPIECE.

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X138

PLATE 1.

2

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X1750

3

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

THE

MICROSCOP

AND

ITS REVELATIONS

BY THE LATE

WILLIAM B. CARPENTER, C.B., M.D., LL.D., F.R.S.

EIGHTH EDITION

IN WHICH THE FIEST SEVEN AND THE TWENTY-THIRD CHAPTERS HAVE

BEEN ENTIRELY REWRITTEN, AND THE TEXT THROUGHOUT

RECONSTRUCTED, ENLARGED, AND REVISED

BY THE REV.

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

WITH XXII PLATES AXD DEARLY NINE HUNDRED \VooD EX

LONDON
J. & A. CHUECHILL

7 GREAT MARLBOROUGH STREET

1901

All rights reserved

PREFACE

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 efforts 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. NELSON, 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 PROF. 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-crystallisation, 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
of the microscope.

W. H. DALLINGER.
LONDON : MARCH 1901.

EKUATUM.— Page 333, eleventh Hue from the bottom, read 'Plate IV.' not III.

PBEFACE

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 tin* 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 tar
at least as objectives of the most perfect const ruct ion and greatest
useful magnifying power are concerned — so common that a much
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 mat liemal ical optics involved in the con-
struction of the most, perfect form of the present Microscope have
been very rapid during the last twenty years; and the progress in
the principles of practical construction 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 unique ; 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 apparatus,
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 familial- 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 1ST. 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 ^th inch
or ^th inch achromatic objectives, and especially the ^th inch
and Jyth inch objectives of Powell and Lealand, of N.A. 1'50.
Without comparing the value of the respective lenses, the best
possible results in every 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

Vlll PEEFACE TO THK SK\ KNTI! KIMTP'N

mounting of objects, and a review of tin- uhole Animal. Vegetable,
and Inorganic Kingdoms specially suited for microscopic purpo-e-.
must lie essentially a cyclop.-edic work. This wa> far more po>sible
to one man when Dr. Carpenter began his work than it wa> even
when he issued his last edition. But it is practically impossible
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 1 have heen mo>t iMiierously aided. In
no department, not even that in which for t \\enty years 1 have
been specially at work, have I acted without the cordial interest.
suggestion, and enlightenment afforded by kindred or similar worker.-.
In every section experts have given me their unstinted 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 familiari.-ed me,
more or less, with every department of Microx-opy. and with the
great majority of branches to which it is applied. 1 have therefore
given a common form, for which I take the sole responsibility,
to the entire treatise. The subject might have Keen carried over
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 chapters ; the whole matter of these seven chapters
has been re-written, and two of them are on subject.- not treated in
any former edition. These seven chapters represent the experience
of a lifetime, confirmed and aided by the advice and practical help
of some of the most experienced men in the world, and they may In-
read by any one familiar with the use of algebraic symbol- and the
practice of the rule of three. They are not in any sense abstruse,
and they are everywhere practical.

Jn the second chapter, on The Principles and Theory of Vision
with the Compound Microscope, so much has been done during 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 health and his many obligations forbade this ; and at
length it became apparent that if this most desirable end were to
be secured, 1 must re-study with this object all the monographs of
this author. I summarised them, not without anxiety ; but that was
speedily removed, for Dr. ABBE, with great generosity, consented to
examine my results, and has been good enough to write that he has
'read [my] clear expositions with the greatest interest;' and. after

PREFACE TO THE SEVENTH EDITION IX

words which show his cordial friendliness, 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 greatest satisfaction in
seeing my views represented in the book so extensively and inten-
sively.'

These words are more than generous ; but I quote them here
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 every
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 present 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 method, 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 support 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 endeavours. But
first on the list I must place my friend Mr. E. M. NELSON. Our
lines of experience with the Microscope have run 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 Count CASTRACANE I am indebted for valuable suggestions
regarding the Diatomacea?, to lie used at my discretion ; to Dr.
VAN HEURCK I am also under much obligation for his courtesy 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 PKEFACE TO THE SEVENTH EDITION

lamented Dr. H. B. BRADY, F.R.S., I am under 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
under 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. N. LAXGLEY, of Trinity College,
Cambridge — from both of whom special processes of preparation
for histologies! work were sent.

Mr. FRAXK CRISP, with characteristic generosity, aided me much
by suggestions of special and practical value ; and Mr. JOHX 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 \V. HICKS, F.R.S., Principal of Firth College,
Sheffield, I am indebted for the revision of special sheets ; so also I
owe acknowledgments to Dr. HEXRY CLIFTOX 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 est eemed ; and prominent
amongst these are Professor ALFRED W. BEXXETT, 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. SHEARSOX HYLAXD, D.Sc.

It will be observed that every endeavour has been made to
bring each of the many subjects discussed in this book into conformity
with the most recent knowledge of experts. Many of the section-.
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 Vse of the Microscope and its appli-
ances, as also in those on Diatomacefe, Desmids, Saprophytes,
Bacteria, Rotifera, Acarina, and in the chapters on Microscopic
Geology and Mineralogy. To the same end nineteen new plates
have been prepared and 300 additional woodcuts, many of which
are also new, and for the use of t lie majority of those which are
not so, I am indebted to the Editors and Secretary of the Royal
Microscopical Society.

There certainly never was a time when the Microscope was so
generally used as it now is. With many, as already stated, it is simply
an instrument employed for elegant and instructive relaxation and
amusement. For this there can be nothing but commendation, but it is

PEEFACE 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 purpose
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 accumulation 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 Amateur 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 become 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 used — 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. But 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
venture 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 PAGK

I. ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTIC'S . . 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 .... 753

XIV. FORAMINIFERA AND RADIOLARIA 795

XV. SPONGES AND ZOOPHYTES 855

XVI. ECHINODERMA .... 884

XVII. POLY7IOA AND TUNICATA 904

XVIII. MOLLUSCA AND BRACHIOPODA U19

XIX. WORMS 943

XX. CRUSTACEA . . 957

XXI. INSECTS AND ARACHNID A . ... . . 972

XXII. VERTEBRATED ANIMALS ........ 1017

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

XXIV. CRYSTALLISATION, POLARISATION, MOLECULAR COALESCENCE . 1094

INDEX . 1137

EXPLANATION OF PLATES

FKONTISPIECE

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 i-inch objective of -95 X.A. x 3 projection eye-piece ;
but it was illuminated by a cone of small angle, viz. of Ol N.A., and illustrates
the unadvisability of small 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 rniuute 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 £ of -95 N.A. x 3 projection eye-piece. The illumination was
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 alarnb, 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 one azimuth along the valve.

Fig. (5. A hair of Polyxenus lagurus, a well-known and excellent test object
for medium powers x 490 diams. by apochromatic ^ -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-angled ' 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.

Fig. 8 represents the same structure, x 138 diams., by an apochromatic
i -I'M 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

Fig. 1. The inside of a valve of Pleurosigma angulaturn, 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 angulaturn, 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.1
Photographed with an apochromatic £ l-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 THK 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 WITHOUT THE
EMPLOYMENT OF THE MIRROR

PLATES 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 diatom will be found in the Transactions of the Count// of
Middlesex Natural Hi^loi-ij Society for 1889, Plate I. fig. '2.

EXPLANATION. OF PLATES XV

PLATE VI. (Facing p. 550)

SEXUAL GENERATION OF VOLVO X GLOBATOR. (After Collll)

Fig. 1. Sphere of Volvox globator at the epoch of sexual generation : «.
sperm-cell containing cluster of antherozoids ; «.'-', sperm-cell showing side-
view of discoitlal cluster of antherozoids ; a3, sperm cell whose cluster has
broken up into its component antherozoids ;«', sperm-cell partly emptied by
the escape of its antherozoids; bb, flask-shaped germ-cells showing great
increase in size without subdivision ; b'-, b'2, germ-cells with large vacuoles in
their interior ; b3, 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 oosphere, with dense envelope.

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

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

PLATE VII. (Facing p. 553)

OSCILLAKIACEJE AND SCYTONEMACE^E

Fig. 1. Li/ngbya H'stunrii. Lieb. >. 160.

Fig. 2. Spii-nUna Jcnneri, Ktz. : 400.

Fig. 3. Tolypotliri.r cirrliosa, Carm. : 400.

Fig. 4. OsciUaria insignis, Thw. 400.

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

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

PLATE VIII. (Facing p. 554)
DESMIDIACE.E, RIVULARIACE^, AND SCYTONEMACE.E

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

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

Fig. 3. Euastrum pectinatuin, Breb. (After Kalf s.)

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

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

Fig. G. Xaniliidium aculeatum, Ehrb. (After Ealfs.)

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

Fig. S. R. dura, Ktz. x 400. (After Cooke.)

Fig. 9. Seytonema natans, Breb. : : 400. (After Cooke.)

Fig. 10. Stawastrum hirsutum, Breb. (After Cooke.)

PLATE IX. (Facing p. 580)
DESMIDIACEJE

Fig. 1. Micrasterias cnix-meliteiisis, Ehrb. (After Cooke.)

Fig. 2. Clostcrium setaceum, Ehrb. (After Cooke.)

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

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

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

Fig. 6. Spirot<cnia condensata, Breb. (After Cooke.)

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

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

PLATE X. (Facing p. 5U3)

PLEUROSIGMA ANGULATUM

This is a direct photo-micrograph, taken by Dr. E. 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 lias 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 tinie 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, etc. It is possible also
that it sometimes only exists in a very rudimentary state. The majority of
the students of diatoms agree 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, in 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. Amphiplcnra /irll/n'iilit, Kiitz, 1 and 2, valve resolved into
pearls. Fig. 2 x 2000 diams. Fig. 1 x 3000 diams. Fig. 3. Valve resolved
in stria? at about 2300 diams.

Fig. 4. Ampliipleura Lindheimeri, (ir., x 2-500 diams.

Fig. 5. Pleurosigma cmgulatum, 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 ^ of 1'4 N.A.
The lines being traced upon a cover in crown-glass, the objective of N.A. !•(>
cannot be used here.

Fig. 8. Surirella gemma, Ehrb. x (about) 1 000 diams.

Fig. 9. VanHewtrckiacrassinervis, r.n'b. (Frustulia saxonica, Babh) x 2000
diams.

All the photo-micrographs (except fig. 7) have been done with the new ^-
inch N.A. 1-ttO of MM. Zeiss.

These micro-photographs have been |iio<lueed by sunlight in a monochro-
matic form, the special compensating eye-piece 12, and the Abbe condenser of
N.A. 1-0.

1 ' The Structure of the Valve of Diatoms ' in /// co/W.v »/' tin-
v. xiii.

EXPLANATION OF PLATES xvii

Covers and slides in flint of T72 ; 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. 1-130 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 £th objective, with Lieberkiihn 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 (rnerismopedia). 7. Cocci in packets
(sarcina). 8. Bacterium termo. 9. Bacterium termo x 4000 (Callinger and
Drysdale). 10. Bacterium septicamice. Juemorrhagicff. 11. Bacterium piieu-
monicB cruuposce. 12. Bacillus subtilis. 13. Bacillus muriseplicus. 14.
Bacillus diphtheria. 15. Bacillus typhcsus (Eberth). 16. Spirillum undula
(Conn). 17. Spirillum volutans (Cohn). 18. Spirillum cholera Asiatics
19. Spirillum Obermeieri (Koch). 20. Spirochceta plicatilis (Pliigge). 21.
Vibrio nigula (Prazrnowski). 22. Cladothrix Forsteri (Cohn). 23. Cladotlirix
dichotoma (Cohn). 24. Monas Okcnii (Cohn). 25. Monas Warmincjii (Cohn).
26. Rhabdomonas rosca (Cohn). 27. Spore-formation (Bacillus alvei).
Spore-formation (Bacillus anthracis). 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 Crenothn.i
(Zopf). 32. Involution-forms of Vibrio serpens (Warming). 33. Involution-
forms of Vibrio rugula (Warming). 34. Involution-forms of Clostridium
polymyxa (after Prazmowski). 35. Involution-forms of Spirillum cholera
As'iaticic. 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

xviii EXPLANATION OF PLATES

PLATE XIV. (Facing p. 664)

PURE-CULTIVATIONS OF BACTERIA

Fig. 1. In tin1 tlfpth of Nutrient Gelatine. A pure-cultivation of Koch's
comma-bacillus (Spirillum cholera Asiatics) 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 Metchnikoff.

Fig. 2. On the surface of Nutrient Gelatine. A pure-cultivation of Bacillus
typJiosus 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 surface of Nutrient Agar-agar. A pure-cultivation obtained
from an abscess (Staphylococcus pyogenes aurcus).

Fig. 5. On tlie 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. 7'.»-J!

EOTll KK.K

Fig. 1. Floscularia campanulata.
Fig. 2. Steplianoceros EichJiornii
Fig. 3. Melicerta ringens.
Fig. 4. Pedalion niirum (side view).
Fig. 5. P. mirum (dorsal view, showing muscles).
Fig. (i. Copcus cerberus (side view).

Fig. 7. Philodina 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 Rotifera.

PLATE XVIII. (Facing p. 797)
FOI;AMIMKI.I;A

!-'i^r. 1. Miltoi'nin * iiiiitii/tuii (a and b, lateral aspects).

1 ig. 2. Alreulnni Boscii \<i. lateral aspect; b, longitudinal section).

EXPLANATION OF PLATES XIX

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

Fig. 4. Haliphysema Tumanowiczii, showing the pseudo-polythalamous foot.

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

Fig. 6. Haplophragmium agglutinans (a, lateral aspect ; 6, longitudinal
section).

Fig. 7. H. nanum (a, superior aspect; b, peripheral aspect).

Fig. 8. Tcxtularia gramen (a, lateral aspect; b, oral aspect).

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

Fig. 9a. Pavonina flabclliformis (a, lateral aspect ; b, oral aspect).

Fig. 10. Bulminia spinulosa.

Fig. 11. Chilostomella ovoidea (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. L. sulcata (a, lateral aspect; b, oral aspect).

Fig. 16. Nodosaria raplianus.

Fig. 17. Cnstellaria 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. Amphistegina Lessonii (a, superior lateral aspect ; 6, inferior lateral
aspect ; c, peripheral aspect).

Fig. 25. Nummulites Icevigata (b, lateral aspect ; c, vertical section).

Fig. 26. Portion of Orbitoides nummulitica.

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 o, Plate
XXL, are from ' British Oribatidse,' published by the Eay Society ; tig. 7, Plate
XX., from the 'Journal of the Linnean Society ; ' fig. 4, Plate XXI., 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)

ORIBATID.i:

Fig. 1. Anatomy of Nothrus tlieleproctus (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 oesophagus is seen at the top (the brain having been removed). The
preventricular glands (brown) lie on each side of the ctsophagus. The ventri-
culus is coloured pink ; part of it and the whole of the casca 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. Hoplophora magna (female, lateral aspect, x about 50). The chitin
at the side and the fatty tissue and muscles have been removed. Alimentary
canal pink ; cseca 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 betweeu the legs.

Fig. 3. Tegeocranus 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 tegeocranus ( x about 25), Vigt.
Central Ovary, oviducts with eggs, vagina, and ovipositor.

Fig. 5. The same of Damans geniculatus ( 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. G. Nymph (active pupal stage) of Tegeocramis liericius ( x about 100)
(carrying its cast dorsal skins).

TYROGLYPHID^:

Fig. 7. Hypopial (travelling) nymph of Rhizoglyphus Bobini (ventral
aspect, >: 100).

PLATE XXI. (Facing p. 1010)

Fig. 1. Leiosoina palmicinctum ( 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 nynipha-
skins.

Fig. '6. One of the scales more highly magnified.

CHEYLETIDJE

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

MYOBIIDJE

Fig. 5. Myobia chlropteralis (female, x about 12"n.

PLATE XXII. (Facing p. 1012)

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

GAMASID/E

0

Fig. 2. Camasus terribilis (male, x 30). A species found in moles' nests.

Fig. 3. Frcyana lieteropus (male, x about 95, a parasite of the cormorant).
Fig. 4. Sarcoptcs acabiei (the itch mite, x about 150, adult female).

THE MICEOSCOPB

CHAPTER I

ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

To be the owner of a well-chosen and admirably equipped 'inicro-
scojje, and even to have learnt the general purpose and relations of
its parts 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 principles 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 arc
needful to enable the owner to use either instrument. In the ca>r
of the microscope, for the great majority of purposes to which it is
applied 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 elemeiitary
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 PRINCIPLES OF MICROSCOPICAL 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.1

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
results, although he may have no clear knowledge of trigonometry,
physical optics, nor the mathematical proof of formula?.

All microscopes, whether simple or compound, 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 amjl' /<•///, //,,; 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 vacuum, 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 greater than unity I

Further, the absolute refractive index for any particular suit-
stance will differ according to the colour of the ray of light employed.
The refraction is least for the red. and greatest tin- the violet. The
difference between these refractive values determines what is called
the <//.s-/wr.s'/Yr jxm-i'i- of 1 lie substance.

This will be understood by fig. I. Let 1 ('. a ray of lijrht travel-
ling in ;iir. m. 'ei the "surface A B of water ai the point C. Through
' dra\\ X' .-it right angles to the surface of the water A Ix The
lino X : ' is culled the ,,<>r;i<ii! to the surface A B. The ray I C' \\ill
not continue its p.-ith through the \\ater in a straight line to Q ; but.
because water i> 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
the part I C is called the incident r</?/. and C R the refracted ray.
1 I ' bambi i Mathematical Tab/< ;.

THE LAW OF SINES

3

The angle I C makes with the normal X X7. viz. ION, is called the
angle of incidence ; ;nid the angle R C makes with the normal X' X,
viz. R C X', 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 X-. Thus, when a
ray passes from a rarer to a denser medium it is bent or refracted
towards the normal, and when it passes out of a dense medium into
a rarer one it is bent or refracted airay from the normal.

Further, if the shaded portion of the fig-ure were glass instead of
water, the refracted ray RC would be bent still nearer X'. 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.

1't'iit away from the normal than if it had passed out of water into
air.

The angle of incidence ICX is connected with the angle of re-
fraction RCX' (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 ^.

This law, stated with reference to the figure, would be :

sine ICX

= u — the refractive index or water,
sine R C X

In I C take any point, P, and from P draw P T perpendicular
to NX"'. Similarly in RC take any point, F. and draw FH per-
pendicular to X X'.

B 2

4 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

Now, as sine I C N = I Land sine RCN' = FHi, then, by

PC F C

PT

Snell's law, r= A*.

FC

As any points may be taken in T 0 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 C = E C, and draw the perpendiculars as before, we shall have

K^S

sine I C N = ^ ^ and sine R C X' = * ^, and therefore JL2 = „
-K. C hi C ED

EC

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

JT si „

tlms : -—p. = /.i. K C is cancelled, which leaves = ,<
.hi i) \<] i >

As p 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. Thus, if /.i 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 Avill, 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 therefore of p for a red ray will be less than that of
p' for a. violet ray. As a practical illustration : The refractive in-
dex for a red ray in crown glass is 1-5124 = //, and for a violet ray
is 1-5288 = p', the difference being p' • - ^ = -()164.

The refractive index fora red ray in dense flint glass is 17030
= ju, and for a violet ray is 1 -750 1 = /(', the difference being a' — u
= -0471.

Consequently there will be a greater difference bet ween the bend-
ing of the refracted red and violet rays in the case of dense flint than
in the case of crown glass, the angle of the incident ray with the
normal being the same in either case.

Where air (more correctly a vacuum) is not one of the media,
then the refractive index is called the relative refractive index.

The annual to a. />/(/,«• xnrl'ac,- is always the perpendicular to it;
the normal to a spherical xnrj'ai; is the radius of curvature. The
angle of the incident ray and the angle of the refracted rav are
always measured /'•//// ///>• normal, and not with (he surface.

Fig. 2 a, />. shows the normal* A, 15 ti> both a plane and a
spherical surface, ( ' I >.

In thecase of the spherical surface, \\ is t he cent re olYurvature, E F

PROBLEMS ON REFRACTIVE INDEX

is the incident ray in air, FG the refracted ray in crown glass. Tl it-
angle A FE is the angle of incidence, BFG 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, p ; thus in this particular case :

(Problem) I. :

sin A F E

sin 45C

sin B F G ' ' sin 28°

•707
-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 wrill be clear from
the above that when the
refractive index, absolute
or relative, of a ray from
any first medium is given, g~
the refractive index from
the second to the first may
be found.

Thus, the absolute re-
fractive index LI from air
into glass being given as

O _£. n

-, find

p.f,

the refractive

index from glass into air.

(Problem) II. :

3
2

.

3

JTJ

"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 LI of crown
glass is 1'5, and the ab-
solute refractive index //

of flint glass is l-6 ; find the relative refractive index"/
to flint.

(Problem) III. : „_ / _ Ij6 _ l-QQQ

= ^' :F5 =

The relative refractive index LI'" from flint to crown is determined
by (problem) ii. :

,,'" =- = —. = -938.
u." 1-066

The normakto^a plane and a curved

from crown

6 ELK.MKNTAUY PKIXU 1'LKs ()F MICKOSCOPICAL OPTICS

Let n> m.w supp<»e thiil in fig. '2 tin- ray is travelling in the op-
posite direct inn. (I F in the denser medium will n«>\v l»c the incident
ray. :ind F E in tin- r.-in-r nifdiuin will !«• the rcfrnc-ted ray. Xo\v.
if the ;in^lr |; K <i In- inriviiM-d. the nii^le A F E will also lie in-

!•' 1 1.. :J. -The phenomenon of total reflexion. (Promt i E Nature,'

published by Miifinill

rrease.l in a greater proportion, and the ray I-' K \\ ill a|>|» •oai-h t he
surface F D.

Wlu-ii F E coincides with F D, O F is .sai.l to !>.- ineid..!!! at the
'•/•// iml (inijlc of the medium. When this critical an-le is reached.
none «)C the incident light will pass nut of the denser medium, Imt 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.

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

(Problem) IV. : 11

Sin 6 = = - ='<•):

fJL 1'33

6 = 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

H sin 0 = ;u' sin 0'.

where p. is the absolute refractive index of the first medium, 0 the
angle of the incident ray in it, / the absolute refractive index of
the second medium, and 0' the angle of the refracted ray in it.

The angle <p = 45° of the incident ray in the first medium A F" E

(fig. 2) and p = 1. p.' = _, the absolute refractive indices of both the

_

media, air and glass respectively, being given, find <//, the angle of
the refracted rav in glass.
(Problem) V. 1 :

0. ,, n sin 0 1 x sin 45° 1 x '707 ,71 .
&m*: -7- IT, T5

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) V. 2 :

u' sin a/ 1-5 x sin 28° 1-5 x '741__.7O,,

>M11 t\\ ' ' . _._ — — i \J\J'J.

OI11 (JJ — — .. -,

U.

0 = 45° (found by table).

Now, suppose the A side of CD (fig. 2) is crown glass, M = 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
B FG.

KLEMENTAKY PRINCIPLES <>F MK'KnsCOPICAL OPTICS

(Problem) V. :! :

u sin <& 1-5 < sin 45 I'.") x '7<>7 1-0605

rj • * | _ rt /> o

>>in a = — 7 — = . :;— 7; = ^—5 ^'oOo ;

p' I'D I'D 1'6

0/ = 41Jj° (found by table).

A> a iinal instance. Suppose the ray to lie travelling in the
opposite direction, so that G F is the incident ray and B F G, or
0/ = 41.V. be given, the media being the same as in the last case.
//=]•<) and p.= \-~). iind 0. or the angle of the refracted ray.

(Problem) V. 4 :

_/_sin 0' 1-6 sin 4H° _l-6x"663 1-0608

' 1 1 1 C' ^^— ~ — " -, — 'z ^ ~ ~ ^ ^ ~ */'_'/

II 'I l**^l l-rl

W. 1 *J L fj JL <J

0 = 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.

l>y means of the above pro-
blems and their solutions we
are now able to trace the diver-
gence of a i'<i i/ tltrouyh a jirixnt.

In fig. 5 let ABC repre
sent a prism of very dense Hint
glass whose absolute refractive
indices p! for red light is 1'7.
and p." for blue light is 1'7.">.
Let the refracting angle B A < '
of the prism =50°, and let the
angle of incidence of a ray of
white light 1) E = 45° =0 in
air. //,= 1 . The dotted lines show

<• t.— The geometrical form of the prism, the normals. Then by (problem)
From the 'Forces of Nature.') y_ j f()r ml j^ ^^ J

the angle of refraction 0'.

,_fjL sin 0 Isin 4")° 707

p! "17 ' \'T~ : 41<i '

07== 241° (foun,i by t.,i,i,.).

And for blue light:

p sin 0 1 sin 45° '707

P" 1-75 =l-7^ =

0'/=23|° (found by tabl,.).

Now, for the red ray draw K V (fig. :,). i>4^ to the normal, and
let it meet the other side of the prism A f' in V. At V dra\\
another normal.

On the scale of onr diagram it is not pos>ible to draw two lines

F, one for the red ray and the other tbr the bine, for the} are too

close together, their angular divergence being only :,io. ' Hut by

PATH OF LIGHT THROUGH A PBISM

9

measurement it will be found that E F makes, with the normal at
F, an angle 9' of 257?°, and for the blue ray an angle <p" of 26j°.

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
refraction before determined, thus 50° — 24|° = 25V.

R
B~ C ^r

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

This ray E F now becomes the incident ray on the surface A C ;
and as the angle it makes with the normal at F is known, and as
the refractive indices remain the same. \\t- can. by (problem) v. 2.

find the angles of refraction for each colour.

If we take red light :

u.' sin c/

Sill 0 =

1-7 sin 2-H°

l-7x-43
L

= •732

4>, the angle of refraction =4 7° (found by table).
I f we take blue

Sin </>=

ju/' sin $" 1-75 sin 26^° 1'75 x '442

= •774

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

This dispersion can now be represented in the diagram, 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. Further, 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 I) 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=/z; line F. 1'52395 = /; line C,
l'51535=fi//. Then the dispersive power w

_P!—IL" _ 1-52395 -1;51 535 -Q086_

: „_] —1-5179 _i --:>i7ir

10 ELEMENTARY PRINCIPLES < >F MICROSCOPICAL OPTICS

The values of the same lines for the Hint idass are as follows :
I). 1-7174=,,: F. 1-7:US'.)=M': ('. l-710r>5=/'.

- M

1-7:UH'.> -- 1-7 1 (I");') -0-2434
= --

• ,t - i -1-7174—1

So the dispersive power of the Ilint between the lines C and F is
slightly more than twice that of the crown for the same region of
t he spec) rum. In t he a bo\ e formula the expression p' — ^u" is usually

written c

?
in full it is therefore w =

-

I Caving thus t raced a rav
experimentally through a
prism. our next step is to sho^n

that a cuiii-t'.i- It-it* i.-- nnlti n
en rri'il Jnrni <>j' 1irn xtti-li jH-ixm.^
with their liases in contact.
as is shown in A. tiy. (5.
where the curved line shou.-
the lenticular character and
the shaded elements 1 he two
prisms. A concave lens is in
effect two prisms reversed.
that is. with their apices in
contact, as in l>. til-;, li. \\here.
a»-ain, the cnr\cd line shows
t he form of the lens and the

Pin. (',. Convex and concave FIG. 7. — Proof that a lens maybe considered

lenses are related to the as an assemblage of prisms. (From tin'

I rism. • Fun-rs ui' Nature.')

shaded parts its relation to a pair of prisms. The fact that a lens is.
in effect, as such, but an assemblage of superposed prisms is seen in
fig. 7, the refracting angle of the prism being more acute as the
principal axis is approached, and the deviation being greater as the
angle is more obtuse.

In fig. 8 letOP be the axis in each case; then, from \\hai we
have seen, it is manifest that rays parallel to the axis falling on the
prisms with their bases in contact and acting like a convex lens will
be refracted towards the axis U P. Hut in the other case, uhere
the prisms have their apices together, as in lig. !», act in- as a con
cave lens, the light is refracted away from the axis < > P.

ACTION OF A PAIR OF PRISMS

I I

V

R

H

FIG. 8.— Action of a pair of prisms with their bases in contact on

parallel light.

9. — Action of a pair of prisms with their apices in contact on
parallel light.

12 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

It must, however, be understood that there is a very important
difference between the action of spherical lenses, which is diie to the
different positions of the normals.

In the prisms (figs. 8, 9) the incident stir face A B is a plane ;
and as the normals are perpendicular to it, they must be parallel to
one another, whether neai- the base or near the apex. Thus the
normal at E is parallel to the normal at K ; therefore, whatever
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 ra<lii :
parallelism is therefore impossible, and parallel incident rays will
riot 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 12.

Now, if we study the four following figures, we shall see the
principal,. .action of lenses on light incident on their surfaces. Fig.
•13 shoves that if a radiant is placed at the principal focus of a con-
verging lens, the rays are rendered parallel ; conversely, if parallel
rays fall on a converging lens, they are brought to a principal focus
or point upon the axis.

Fig. 1 4 shows that if a radiant be placed beyond 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-cnu, M \ , .
and diverging meniscus lenses.
(From the ' Forces of Nature.')

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

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 principal focus oa 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 ELEMENTARY PRINCIPLES 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, arid the distances
between this object and the lens and a screen to receive the image
of the object are so adjusted that the image of the object becomes e<jn«l
in 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 between a lens and its principal focus, the
rays on the other side of the lens are still diwyent, arid 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. 1.5, the}' 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 ri/'ttnil conjugate 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 rlrtn.aL1 Parallel rays falling on a concave lens are rendered

FIG. 16. — ' Virtual ' focus of concave lens.

divergent on the other side of the lens, and consequently can never
come to a focus. But if we trace these divergent rays backwards,
as in the dotted lines of fig. 16, we find that they meet in a point,
and this point is called the rirtini/ />rinci/>til focus of the lens.

It will he manifest that since the rays in passing through lenses of
various kinds arc unequally refracted they cannot all meet exactly in a
single local point. This gives rise to what is a most important feature
in the behaviour of lenses, which is known as spherical aberration.

Figs. 17 and IK show the refraction of rays of monochromatic
1 A fro/ iniii^c ca.ii l>c received on a screen, but ft virtual image cannot.

SPHERICAL ABERRATION 15

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

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

FIG. 17. — Spherical aberration.

distance of the focus for any ray passimj throni/h n /«//.$ from the
jn-i/ici/ial focus of that lens.

Thus in figs. 17. 18, the spherical aberration is F F' for the ray>
R2 R2, and F F" for the rays R1 R1, and the difference between the

Fig. 18. — Spherical aberration.

spherical aberration of the rays R1 R1 and that of the rays R2 R2 is
F F" - - F F. which is F' F".

Thus F F' and F F" in (fig. 1 7), cf= - • f/! ; F F' and F F"
in (fig. 18) c/= — 7 • i where cf signifies the distances F F',

«/

F F" respectively, // the distance from the axis where the incident
ray enters the lens, and/" 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. 18 it is t \vice
the radius measured from the point A ; that is, the point Fis distant
fr< mi the lens twice the radius less two-thirds the thickness of the lens.

It will be seen, then, that the amount of spherical aberration is
due to the shape of the lens, and is least in a biconvex lens. when the
radii of curvature are in the proportion of 6 : 1. n-hi-n the more curved
* i/i-face faces the incident light. But when the lens is turned round,
so that the other side faces the incident light, the spherical aberration
reaches a maximum.

It would be well for the student who desires to became 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 : 1

+ !_ M f} _IV

7 >•') (f r'J

where f = principal focal length ; // = semi-aperture ; p = refr.
index ; and r, — ?•', radii.

In an equi-convex of crown, where p, = t , r = — r' = f,

3 /'

In a plano-convex of crown, where ^i = -, — r' = oo, r = -,

7 v2
£ f = — - • --. . Here parallel rays are incident 011 the convex

surface. But when parallel rays are incident on the plane surface.

3 /" 9 i/'2

p = ' , r = oo, -- r' = -, cf= — • -/ ; consequently the sphe-
2 A 2 /

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

When — v' = oo, and /.i = 1'69, the plano-convex becomes the
form of minimum aberration.

v/
= — - • '^,, the parallel i-ays being incident on the more

In a, crossed2 biconvex lens, where - r' = 6 r, and // = -,

15 ?/2

' /'
curved surface.

Formula for finding the principal focus F of a lens equivalent to
two other lenses whose foci &ref,ff and their distance apart d :

1 l 1 A
F==/ /'' ff7

In figs. 5, 8, and 9 we see that when the incident ray D E con-
sists of white light, the colours of which it is composed are unequally
refracted ; the two extremes, 11 (red light) and V (violet light), being
bent in different directions, the other colours lying between them
in their proper order.

This unequal refraction of the different colours takes place in
like manner in spherical lenses, and it is then known as chromatic
aberration.

The effect of this upon the action of a lens is that, if parallel white
light fall upon a convex surface, the most refrangible of its component
rays (which, as we have seen, is the violet) will be brought to a focus
at a point somewhat nearer the lens than the principal focus ; and
the red ray, having the least refrangibility, will be brought to a focus
at a point farther from the lens than its principal focus, which is. in
effect, the mean of the chromatic foci.

1 Encyclopedia Brit. vol. xvii.

a A biconvex lens is said to be 'crossed ' when the radii of its surfaces are in the
proportion of 1 : 6.

HOW ACHROMATISM MAY BE OBTAINED 17

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

The white light, A A", falling on the peripheral portion of the lens,
is so far dispersed or decomposed that the violet rays 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 I), limiting the violet and the red, is termed the longif"
dinal chromatic aberration of the lens.

If the image be received upon a screen placed at C, violet \\ill
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

A

F

FIG. 19. — Chromatic aberration.

predominantly red tint, and will be surrounded by a M'ries 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 focus, 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 unconnected 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 aberration, 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 how achromatism is obtained.

In a prism the amount of dispersion or unequal bending of
R and V (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 different
kind of glass, possessing only half the dispersive power of that in
the figure, but with the angle B A C 50°, as in this case, the separa-

c

1 8 ELEMENTAKY PRINCIPLES OF MICROSCOPICAL OPTICS

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

Then if another prism were made of the same material as that-
assumed in fig. 5, but with only half the refracting angli'. viz. 25°,
the dispersion between R and V would also be but half that repre-
sented. Also a prism having 50° of refracting 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 dispersion reverst -d.
hut refraction also
is neutralised, the
emergent ray being
parallel to the in-
cident ray. Therefore
the equal a nd inverted
system of prisms can
be of 110 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 tact, must
be destroyed without neutralising 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 prism of 25°
angle, composed of glass having twice the dispersive power of the
former. Dispersion will be manifestly destroyed, because it is equal
in amount and opposite in nature to that possessed by the prism 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
] art of the convergence is maintained while the whole of the
dispersion is destroyed.

The spherical lenses which answer to these prisms area cro\\n
biconvex, fitting into a flint plano-concave of double the dispersive
power

It has been pointed out above that all the other colours lie in
their proper order between the rays II and "V (tig. .">). Let us select
one. green, and represent it by (!. Now if G lies midway between
II and V in the prism of 50° of angle, and also between Iv and Y in
the prism of 25° of angle, its dispersion will also be neutralised.
This means that when the dispersion between the three colours in

FIG. 20. — Eecomposition of light by prisms. (From
the 'Forces of Nature.')

ACHROMATIC OBJECTIVES 19

one kind of glass is proportional to their dispersion in the other,
then when any two are destroyed the third is destroyed with them.
This unfortunately 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 Y 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 Y be destroyed. G will
be left outstanding. If a different angle of prism be chosen, so that
R and G are neutralised, then Y 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 irr<iti<>ii<iHti/ of
the spectrum, and the colour or colours left outstanding in a corrected
combination of lenses is known as the sec" //</>//•// spectrum.

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 apochromatic
s\>tem 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 .-ensible
an imperfection in the performance of these lenses under certain
circumstances, which had previously pa.-sed unnoticed, and Andrew
Ross made the important di.M-overy 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 n-'dli or fit/tout a covering of thin glass, an object-
glass which may he 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 direction fall upon
the cover-glass C C (/j = 1'6). On tracing two definite rays, such
as 0 A and O 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 direction parallel to their
first path, and will enter the front lens of the objective at .the
points M and IS".

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

Similarly, by tracing two of the less divergent rays from 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 rays radiating apparently from tn;-, dis-

c 2

20 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

tinct points, X and Y. If there were no cover-glass all the rays would
diverge from O, and then the objective would require to be perfectly
n/t/tntnfic. This word (derived from a = privative, and TrXara'w, to
wander, i.e. free from wandering or error) means, as used by opticians,

FIG. 21. — The effect 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 pre-
sented in fig. 23, so as to focus both these points at once. Here the

FIG. 22. — Aplanatic system.

Fm. 23. — Under-corrected system.

curvature of the surface of the crown lens being increased, the flint
plano-concave is not sufficiently powerful to neutralise all the
>pherical aberration of the crown. As a consequence the peripheral
rays are brought to a focus at F', while the central rays pass on to
F. This is what is meant by 'under-correction' in an Object-glass.

In fig. 24 the reverse condition
is presented, for the incident curve
of the crown lens has been Hattened,
while that of the flint has been
deepened, which increases the cor-
rective power of the Hint, and thus
destroys the balance of the com
binal ion in other directions. The rays passing through the periphery
of the combination will be brought to a focus F', while the central
rays will be focu.ssed at F. This is what is known as over-correction.

FIG. 24. — Over-corrected system.

COLLAR CORRECTION — FOCI OF LENSES 21

An <//>/t( i, atic objective can be made into an under-corrected
objective by (1) ca/i*in</ tJie back lenses of which it is composed
t<> approach the front lens. This is the device of Andrew Ross, and
is now effected1 by means of a special 'collar' arrangement, 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 c«nx'n«j the et/e-pii'fi' f»
approach the objective. This of course is accomplished by the use of
the draw-tube, and must be employed with objectives having rigid
mounts.

Closing lenses, thati 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, tinder-corrects the
objective, that is, gives negative aberration ; while the separation <>f
lenses over-corrects or gives positive aberration.

In using the collar correction1 for a longer body or a thicker
cover-glass the collar adjustment 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 their separation.

In correcting by tube 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 foci 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 is 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).

Formula 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), R radius of curvature for one surface, R' for the other
surface, /j. the refractive index of the medium, then

1 1 /I 1

p + p/=U'-

F=(^-1
1 1 1

FIG. 25A. — Focus of a concave lens.
1 See Chapter V.

22 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

Also, if x is the distance of a focus from F, the principal focus.
and y, the distance of its conjugate from F', the other principal
focus 011 the other side, then

or,

In an equiconvex lens of crown glass if /j = l-f>, F= radius of
curvature. But in a plano-convex lens of crown glass if p = \-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 required. A very approximate deter-
mination of the principal focal length of an equiconvex lens mcasuri'if
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.)

Examples. — Equiconvex lens of crown glass /z=l'5, r=}j, thick-
iiess=|. By above formula F=^. Subtracting from this one-
sixth of the thickness of the lens, we get F=^-4- as the distance
between the focus and the surface of the lens. This is only ^1^ inch
from the truth. If the lens were a sphere it would be necessary to
subtract j- 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, radius=7>, thick -
ness=^, the principal focus on the convex side will be one inch
from the curved surface and on the plane side f inch from the plane
surface.

In an equiconcave lens the foci are virtual and are crossed over ;
thus, the lens in fig. 25A is equicoiicave, 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 crossed over. From the principal focus on the curved
side subtract | of the thickness, and from that on the plane side
subtract the whole thickness of the lens.

Examples. — Equiconcave of dense flint yi/ = l-75, radius= — 7?,
thickness ] . F by formula = — ^ ; subtract from this | of the thick-
ness of the lens, we obtain — 1. which is only rirr inch too short.

i t/ i ~i > '

Planoconcave of dense flint /< = ! '75. radius= — ^, thickness y,
F by formula= — -|, subtract from this the thickness of the lens.
Then F=— YV; this is the focal distance from the plane side. For
the local distance from the curved side subtract § of the thickness,
then F= — f;f:, which is r'4 inch too long.

The principal focus of a combination of two or more lenses, whose

THE FORMATION OF A 'REAL IMAGE' 23

principal foci and distances are known, can be found from the formula

-+ ,= > bv assigning for the value of p the distance of the prin-

P P J "

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 p in the
formula. We have, therefore :

-2V

/ 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 \vay 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

FIG. 26. — 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
aplaiiatic combination. From every point of A B are rays radiating
at every possible angle. Let AF and AH 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, through 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 C. 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 (.!, 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 C I), 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,1 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 part of fig. 26 the two rays, A F and A H, are
traced tln-ough the lens to determine the point C, but in the lower
part of the figure only the ray BK is traced, and the intersection of
this ray by the straight line B I) passing through the optical centre
gives the point D.

2. An image is said to be rirtmil when it cannot be received on
a screen. Fig. 27 shows how a virtual image is formed. The
letters are the same as in the preceding figure, so as to show the
analogy between the two. The fundamental difference between
this figure and the last is that the object A B is placed between P, the
principal focus, and the lens.

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

1 In tin: majority of the preceding diagrams the drawing has represented the
facts accurately ; in this instance they are diagrammatic, the size of admissible illus-
trations making an accurately traced ray impossible.

FOKMATION OF A ' VIRTUAL IMAGE '

image.

and as there is no focal point, there can be no screen
Thus two rays radiating from the point A of the object A B fall on
the lens and are refracted in the directions A F, AH: these are
divergent and will never meet ; but if the human eye is placed near
the lens, so that it can receive the rays F and H, the rays 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. Now if we produce F and H backwards (see the
dotted lines in the figure) we shall find that they intersect at the point
C. As the rays F and H are precisely identical with rays which
would have diverged from the point C had it been an entity, the
retinal image therefore will be an iniaye of a non-existent picture
CD.

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

FIG. 27.— The formation cf a ' virtual image.

The rays A F and A H aiv traced through the

preceding figure.

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 prolongation 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 ELEMENTARY PRINCIPLES <>F 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 1 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 X = - the amplification of one and the same

system varies with the length of /. or the ' distance of vision,' 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. Nevertheless, the 'amplifying jtotrer' of every system is
always the same for both, because the short-sighted and the long-sighted
observers obtain the image of the same object under the same visual
angle, and consequently the 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
.•i long-sighted one, the eye in each case being supposed at the pos-
terior principal focus of the system.

The other generally adopted expression of the power by X =

,

t/

may be put on a somewhat more rational basis than is generally
done by defining 1 he length I (10 inches) not as ' distance of distinct
vision.' but rather as ' distance of projection of the image.' As far
as 'distinct vision' is assumed for determining the amplification,
the valneof X lias no real signification at all in regard to an observer

1 Joiirn. tt.M.8. vol. iv. scr. ii. p. :!is.

AMICI'S USE OF 'IMMERSION' 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 difference 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 Z= 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
accommodation 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
diameter 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 I). 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 immersion 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
front lens. When, on the other hand, rays of the same obliquity
enter the peripheral portion of the lens from water, the loss by re-
llection is greatly reduced, and the benefit derivable from the lar-'e
aperture is proportionately augmented. Again, the

28 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

system' allows of ;i 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 nsed '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 powers sufficiently high, and
apertures sufficiently large, for the majority of the ordinary pur-
poses of scientific investigation, without 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 either direction occasioning comparatively little deterioration in
their performance. But beyond all these reasons for the superiority
of the ' immersion system ' is, as will be presently seen, the fact that
it admits into the lens a larger number 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 modern 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 principles of the construction
of microscopical lenses, and the interpretation 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 lie seen in the following pas;-age. . . .
' 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 sometime presented itself to his mind.' 'The matter
assumed, however, subsequently, a different shape in consequence
of a suggestion made by Mr. John Ware Stephenson, ... of
London, who independently discovered the principle of homogeneous

K>

immersion. -

This method consists of the replacement of water between the
covering glass of the mounted object and the front surface <>t the
object-glass by a liquid having the same refractive and dispersive
power as crown glass. With such a llnid taking the place of air. it

1 On ' Stephenson's System of Homogenous Immersion for Microscopic Objec-
tives' (Abbe), Journ. li.M.S. vol. ii. 1879, p. 257.
-' Ibid.

ABBE'S CONSTRUCTION OF FIRST 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
110 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 efficiently accomplished by the use of homo-
geneous lenses. But in the new aspect in which the problem was
presented by Mr. Stephenson it carried with it new interest to Abbe,
not only as promising to largely dispense with the 'correction collar.'
but also to greatly enlarge the ' numerical aperture,' and therefore
secure a greater resolving power in the objective.

One of the difficulties 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 A bite's calculations based on Mr. Stephenson's sug-
gestion was the construction by Carl Zeiss of a TVth with a N.A.1
of 1'25 of fine ijuality. and still higher promise, and subsequently
of a ^th and a J^th U1- 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 1870.2 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 microscopic image makes the resolving
power of the object-glass dependent upon the diffraction pencils that
are taken up by it.

All this was unknown or unadmitted by those who had previously
sugo-ested oil as an immersion 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.

a Monthly Micro. Jmtrn. vol. iii. p. 303.

30 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

angle. Dr. Royston-Pigott constructed the first aperture table
giving the relative values of dry, water, and homogeneous (nascent
pencil) immersion 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 spherical 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 it
Lealand, of London, very soon made a ^-th inch and a .-^th inch
objective on the homogeneous principle, with numerical apertures
respectively of 1 '38. and during the year 1885 produced lenses of .-in
excellence impossible to any previous system of JLth inch, TVth inch,
and -oVyth 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 must 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 lie of the best
quality, and if the object-glass is of perfect construction and of latest
form (apochromatic, q.r.), results never before attainable can be got
with comparative ease.

With such evidence of advance in the optical construction of
microscopes, dependent apparently on such accessible conditions, the
(jiiestion of what is possible in the future of the instrument no doubt
obtrudes itself; that, however, can only be considered as having
application to the area of our present knowledge and i-esources. It
is impossible to forecast the future agencies which may be at the
disposal of the practical optician. To photograph stars in the im-
measurable amplitudes of space, absolutely invisible to the human
eye, however aided, was hardly within the purview of the astronomers
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 minute in nature which will just as wideh
overstep our present methods of optical demonstration, there can be
little reason to question. B\it it is no doubt true that with 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
improvement of great value, bringing nature more and more nearly
and accurately within our ken and reducing more and more certainly
the interpretation of the most ditlicult textures and constructions
in the minutest accessible tissue to an exact method, is certainly
within our sight and reach. It is not a small matter that the homo
geneoiis lenses \\cre, in a comparatively short period of time, carried
from a N. A. of l"25tol'50; and this carried with it the capacity
theoretically indicated.

NEW VITBEOUS OPTICAL COMPOUNDS 31

High refractive media can greatly reduce the value of even the
wave-length of light, and what is possible in the production 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-micrography, by constantly covering a wider area of applica-
tion with its ever increasingly delicate and subtle methods, is
more penetrating in the revelation of structure than the human
eye.

It may be taken for granted that in the present 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 springs from two causes :
one. as we have just demonstrated, arises from the i-oiiilnnj xji/,r/-irn'
mill rJn-tiii'ittic aberrations, the other takes origin in the n-ant of homo-
geneity, absolute precision of curve, and perfect centering <>ftJut syst>-n/
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 dispersion, 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
attained, the dispersion at different parts of the spectrum being so
greatly disproportional. It has never been possible to unite more
than two different colours of the spectrum. The rest, in spite of all
effort, deviate and form the secondary spectrum, leaving, in the verv
finest lenses, circles of dispersion not to be excluded.

2. The second defect arises in the impossibility of correcting by
means of ordinary crown and flint glass the spherical aberration for
more than one colour. If the spherical aberration be removed as
far as may be for the centre of the spectrum, there remains under-
cnrrectioii for the red, and over-correction for the blue and violet
rays, presenting a want of balance between the chromatic corrections
for the centra] 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

tractive 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 flint 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 chromatic difference
depends, these aberrations could be compensated.

All this was seen and fully demonstrated and set forth by Abbe
as far back as 1876, l and he pointed out that the further perfecting
of the microscope in its dioptrical working was dependent on the
art of glass making ; the production, that is to say, of vitreous
compounds possessing different relations of refractive 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. undertook the
laborkms 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 variety 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
'tijferent parts of the sperf,-»/// 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 proved to be so
unstable in their composition that opticians refrain from using them.
It may be hoped that further experiment and research will greatly
reduce this defect. On the other hand, the kinds of glass which
can be used for optical pin-poses have been so increased in variety
that, while the mean index of refraction is constant, considerable
variations can be given to the dispersion or to the refractive index
while the dispersion remains constant. A high index of refraction
is no longer of necessity accompanied by a high dispersion in
flint glass, but may be retained in crown glass with a low degree
of dispersion.

The practical consequence of this is that both the imperfections

1 Hoffman, A. W., BericJit tiber die wissenschaftlichen Apparate nt/f der Lon-
Internationalen Ausstellung im Jahre 1876.

ADVANTAGES OF APOCHROMATIC OBJECT-GLASSES 33

inalienable from an objective constructed of ordinaiy crown and
flint glass, can be, and have been, eliminated, and the secondary
spectrum 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 two 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 mav lie • dry ' or ' homogeneous,'

*/ • «/ O

we have so great a freedom from colour defect as to admit of their
being designated apochromatic lenses (a=privative ; xpw/i<i=colour i
« 7ro = from, away from ; xP*"V*a=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 inquired curves,
and, .relatively, even the careful selection of lenses not homogeneously
related to each other by a unity of purpose and work 011 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. l>ut.
given such an object-glass — which is the production of a thoroughly
competent practical optician — and its advantages, theoretical ami
practical, are great.

1 . The aperture of the objective can be utilised to its full I'.-'tent.
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 coiicenti'ation of the rays in the image.

2. Increase of magnifying power by means of specially constructed
ejje-pieces is also a most important feature of objectives of this class.
The result of this is that great magnifying powei\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 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

But he has further shown us l that with the best objectives of the
old construction, and with large apertures, the limits of a completely
satisfactory clearness of image are reached when the swper-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 point 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 front 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 peripheral parts, but in the apochromatic
object-glass it is approximately constant for all parts of the opening,
and therefore it allows of correction by the eye-piece, 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, mid 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 measure of the eye-piece magnification, and
at the same time the figures for rational numeration.'-'

From their properties 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 important system, viz. :

1 'On the Relation of Aperture to Power,' Journ. E.M.S. 1883, p. 803.
- ' On Improvements of the Microscope with the aid of new kinds of optical glass '
(Abbe), Join-it. It. M.S. 1887, p. 25 ct scq.

APOCHROMATIC LENSES

35

Apochromatic Objectives.

Numerical

Equivalent

Initial

English
equivalent

aperture

mm.

magnification

inches

1

0-30

24-0
16-0

10-5
155

i

3

Dry series . . •/

0-65

12-0
8-0

21
31

|

1J

0-95

6-0

4-0

3-0

42
63

83

i

(T
i

Water immersion .

l-2o

2-5

100

i

10

Homogeneous

1-30

3-0
2-0
1-5

83
125
167

f

r

16

immersion

1-40

30
20

83
125

*

-h

Compensating Eye-pieces for English Bodies.
248 12 18 27

It is of interest to note that Mosi-s. Powell and Lealand have
since produced a remarkable lens on the same system, having a N.A.
of 1-50, with a, power of jV11 of an inch. Object-glasses are also now
made by other makers, English, European, and American, those
having fluorite in them being termed apochromatic, while others
made "of new kinds of glass are called semi-apochromatic. Senii-
apochromats are being daily improved, so much so that some recent
objectives nearly equal apochromatic objectives themselves.

36 VISION AVITH THE COMPOUND MICROSCOPE

CHAPTER II

THE PBINCIPLES AND THEORY OF VISION WITH THE
COMPOUND MICROSCOPE

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 construction of microscopes. These are distinguished
as simple and compound, 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
11) (server 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 image, 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
l«'st, combined with another lens interposed between itself and the
object-glass, the two together forming what is termed an ei/e-]>iece.
The compound microscope must be the subject of careful and de-
tailed consideration ; but it must be remembered that the shorter
t lie focus of the simple magnifying lens, the smaller must be the
diameter of the sphere of which it forms part ; and, unless its
:i|irr1 ure be proportionately reduced, the distinctness of the image
will be destroyed by the spherical and chromatic aberrations neces-
sarily resulting from its high curvature. 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 older micro-
scopists (among whom Leeuwenhoek should be specially named), on
account of their freedom from the errors to which the compound
microscope of the old construction was necessarily subject ; and the
amount of excellent work done by means of them surprises every one
who studies the history of microscopic inquiry. An important ini-
pi -o\ emeiit 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 are in the proportion of one
to three or nearly so, having their convex sides directed towards

PKINCIPLES 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 perforated diaphragm (or
' stop ') was interposed, and the distance between the lenses \v;is 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.1 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 effected in the com-
pound microscope by the achromatisatioii 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 'Coddiiigton ' 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 1 ). 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 'Coddiiigton' lenses, however, are 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 ' Coddiiigton.' 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

1 Transactions of tlie Society of Arts, vol. xlix.

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

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 on 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 Arc. 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 applied to the enlargement of
minute pictures 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, itc.. 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 pocket loups.
For the ordinary purposes of microscopic dissection single lenses
of from 3 inches to 1 inch focus answer very well. But when higher
powers are inquired, 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 expressly
for this purpose by 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 minute 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 difficult 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 reveals 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-
pear in any English treatise we have seen. Tt has
two achromatic lenses for the objective, and a concave
eye lens. Jt is illustrated in fig. 29.

To remedy the inconvenience of the lens being too

close to the object in all but low powers. Charles

Fn; "'I The C'hevalier, in his 'Manuel du Micrographe' (1839),
Briicke lens, proposed to place above a doublet a concave achro-
matic lens, the distance of which could he varied at
pleasure. The effect of this combination is to increase the masniifvinsr

O «/ O

power and lengthen the focus. Thus arranged, this instrument will

COMPOUND MICEOSGOPE 39

he the most powerful of all simple microscopes, and the space
available for scalpels, needles, &c. will be much greater than
with a doublet alone. The further the concave lens is removed from
the latter, the greater will be the amplification.1 Even in this,
however, Chevalier 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 governed the con-
struction 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 power three to
eight times. The latter power is obtained by lengthening the tube,
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 opera glass, has a very small field.

Compound microscope. — The compound microscope, in its most
simple form, consists of only two lenses, the object-glass and the
eye-ylass, and is a Keplerian telescope adapted for viewing verv near
objects. The former receives the light-rays direct from the object
brought into near proximity to it, and forms an enlarged but inverted
and reversed image at ;i 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, 011 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 power 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 must be altogether abandoned, because objectives are

1 Robin, C., Traite clu Microscope et des Injections, 2nd ed. 8vo. pp. 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 further 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 of 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 t<>
come within the range of the eye-glass. As it thus 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-ylass and the field-glass being together termed
the eye-piece, or ocular. "Various forms of this eye-piece have been
proposed by different opticians, and one or another will be preferred
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 /ftiyi/henian, having been employed
by Huyghens for his telescopes, although without the knowledge of
all the advantages which its best construction renders it capable of
affording. This eye-piece, with others, will be considered in detail
in the chapter (v.) given in part to their consideration ; 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 MICEOSCOPE

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 V.) 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 nan., a (5 mm., and a 4 mm., all dry apochromatics
by Zeiss, and especially with a Jth by Powell and Lea land. It
is, however, a matter of moment and interest to note that with
good, objectives of the ordinary achromatic construction of large
K\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,' 1^ 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 off 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 purpose 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 interposed between 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 Ramsdeii in microscope
work. The compensating eye-piece is also used in conjunction
with the micrometer.

Aperture in microscopic objectives and the principles of micro
SCOpic vision. — It is now of the utmost moment that we should
understand clearly the meaning and importance of 'aperture' in
microscopic objectives, and by that means be led to a perception of
the principles of the most recent and only rational theory of micro-
scopic vision. Within the last twenty-five years this entire subject
has undergone a rigorous and exhaustive reinvestigation by one
of the mo.st competent and masterly mathematical and practical

THE FORMATION 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 supposed
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 principles hitherto quite
unknown 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 telescope. 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. \\ C
would nevertheless remark that visibility of detail in, for example,
the moon depends on the aperture of the telescope ; of course, what
is known as its ' aperture ' is simply estimated by the diameter of
the object-glass, but accuracy appears to require that //sin n = 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 difference, because the sines of small angles such as are
dealt with in the telescope are proportional 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 aperture is wholly wrong in prin-
ciple. The front lens of a ^.-,-in. objective may be no more than the
/roth 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 180°. 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 lirst 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 equal, it was im-

44 VISION WITH THE COMPOUND MICROSCOPE

possible to discern them. In every case tin- 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 true
comparison ; when immersion objectives were introduced — objectives
in which water or cedar oil replaced the air between the objective
and the upper 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 ma.fi m mn 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 inlo
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 vet denote different values, according
as they were in air, water, or oil.

Thus there might be large divergence of aperture in two or
more cases while the angle was identical, and from this the greater
confusion was not only possible but was realised.

A solution of the difficulty was (as we have indicated above)
discovered by Professor Abbe ; and it is to Mr. Frank Crisp's lucid
exposition of Abbe's elaborate monographs that the English student
is immensely indebted.1

The definition of 'aperture' in ils legitimate sense of 'opening'
is shown by Abbe to be obtained when we compare the diameter of

ser.

angled

English Mechanic.

HOW ' APERTURE ' IS OBTAINED 45

the pencil emergent from the objective \\'ith 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 single-\&as 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 proper measure of the rays
admitted ?

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 different.
the amplification of the images is different also, and equal areas of
these images correspond to different areas of the object from which
the rays are collected. Therefore the higher-power lens, with the
same opening as the lower power, will admit a greater number of
rays in all from the same object, because it admits the same number
as the latter from a sm«U<>r portion of the object. Thus, if the focal
lengths of two lenses are as 2:1, and the first amplifies X 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 ca.se

and of -v nim. i11 the second. Inasmuch as the ' opening ' of the

A J^l

objective is estimated by the diameter (and not by the area), the
higher-power lens admits twice as many rays as the lower power,
because it admits the same number from a field of half the diameter,
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 opening and the focal
length, in order to define the same thing as is denoted in the telescope
by the absolute opening.

Consider now the compound objective — the most important 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 ultimate maximum value, which is obviously 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 back 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 011 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 different ; for an immersion system
and a dry system always give the same amplification when the f<ic:d
length is the same.

Thus we arrive at the general 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 same admission requires different
openings in the proportion of the focal lengths, or, conversely, with
the same opening the admission iv 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 difficulty which has
always been involved in the consideration of the apertures of
immersion objectives.

80 long as the angles were taken as the proper expression of
aperture, it was difficult 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, water-
immersion objectives of 9(5° and oil-immersion objectives of 82°
were looked upon as being of much less aperture than a dry objective
of 180°, whilst, in fact, they are all equal, that is. they all transmit
the same rays from the object to the image. Therefore, 180° in
water and 180° in oil are unequal, and both are much larger aper-
tures than the 180° which is the maximum that the air objective can
transmit.

If we compare a series of dry and oil-immersion objectives, and,
commencing with very small air-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 factors 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 80° oil-angle. The inner dotted
circles in the two latter cases
are of the same size as that
corresponding to the 180° air-

Numerieal Aperture

1-52
= 180° oil-angle.

angle.

A dry objective of the full

maximum air-angle of

180°

Numerical Aperture

1-33
= 180° water-angle.

0

©

Numerical Apertuiv

1-00

= 180° air-angle
= 96° water-angle
= 82° oil-angle.

Numerical Aperture

•75
= 97° air-angle.

Numerical Aperture

•50
= 60° air-augle.

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 </////
drv objective is capable of trans-
mitting. Whenever the angle of
an immersion lens exceeds twice
the critical angle for the imnier-
sion-flui'd, 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

FIG. 81. — Relative diameters of the (uti-
lised) back lenses of various dry and
immersion objectives of the same
power (i) from an air-angle of 60° to
art oil-angle of 180°.

48

VISION WITH THE COMPOUND MICROSCOPE

expressed by the sine of half the angle of aperture («) L multiplied
by the refractive index of the medium (??.) in front of the objective,
or n sin «, (n being I'O for air, T33 for water, and 1/5 for oil or
balsam).

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

Let 0 and O* (fig. 32) be the conjugate aplanatic foci of a wide-
angled system ; u, U the angles of inclination of any two rays admitted

1 In the original translation of the papers of Professor Abbe from German into
English the German mathematical symbols have been retained. In the summary of

A

N.A. of objective-ni sin 0-1-5.X -573 = -86.

oi/ ju^

ylass ju =1-5 \

air »*= 1-0 x'60°

u

N.A. of condenser =n* sin »* = l-0x -86=-8G.

ii sin w = l*5 x'573 = '86 = N.A. of objective.

35V oil n = l-

/ ff/ass // =1-5

Angularaperture of
objective = 35° +
35° = 70° in glass,
which is equiva-
lent to the angular
aperture of the
condenser = 60° +
6U°=120° in air.

/a' sill (/>'= 1-Ox -86 = -86 = N. A. of condenser.

B

FIG. Al.— Identity of n sin n (Cermiui math, form) with p. sin </> (English). Also N.A. and

a Titular aperture.

Abbe's theories and demonstrations presented in the following pages the Editor has
scarcely felt justified in altering this, especially as the German form of symbol ob-

RELATIVE APERTURES 49

from the radiant, and u*, U* the angles of the same rays on their
emergence ; then we shall have alw;ivs

sin U* : sin ?'* : : sin U : sin a ;

sin U* sin u*

oi', -==- =— — - = const. = c ;

sin U sin n

that is, the sines of the angles of the conjugate rays on both sides of
;in <(j>l<nmtic system always yield one and the same quotient c. what-
ever rays may be considered, so long as the sMine system and the
same foci are in question.

This proposition holds good for every arrangement of media, and
refracting 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.

"\Vheii, then, the values in any given cases of the expression
n sin u (which is known as the ' numerical aperture ' and expre»ed
by N.A.) has been ascertained, the objectives are instantly compared
as regards their aperture, and, moreover, as 180° in air is equal to
I'O (since « = !•() and the sine of half 180° or 9()° = 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 in our Universities, and is thoroughly understood amongst University men.
But to those unaccustomed to mathematical 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 n ' with 'yu sin <£.'

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 yhiss is equivalent to an angular aperture of
120° in air.

In the figure the upper hemispherical lens represents tlae 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 T5, 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) —

/j.' ro

0' = 60° (from Table I.)

Therefore the ray on emerging from the under surface of the cover-glass will make
an angle of 60C from the normal.

The dotted lines show the path of the ray where the Gernin n xi/mhols arc useil.
n sin u =n* sin u* ;
sin »*=rcsin7*_r-5x-573

n* TO

,,* = 60° (from Table I.)

Xiniierical aperture, 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

50 VISION WITH THE COMPOUND MICROSCOPE

objectives, one a dry objective, the second a water-immersion, and
the third an oil-immersion. These would be compared on the
angular aperture view as, say, 74° air-angle, 85° water -angle, and
118° balsam-angle; so that a calculation must be worked out to
arrive at a due appreciation of the actual relation between them.
Applying, however, 'numerical' aperture, which gives '60 for the dry
objective, -90 for the water-immersion, and 1'30 for the oil-immersion,
their relative apertures are immediately appreciated, and it is seen, for
instance, that the aperture of the water-immersion is somewhat less
than that of a dry objective of 180°, arid that the aperture of the
oil-immersion 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, larger aperture than
a dry objective of the maximum angle of 180° ; so that for any of
the purposes for which aperture is desired an immersion must
necessarily lie 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 compared work in the
same medium, and admit angles of, say, (10°, 90°, 180°, their aper-
tures are not in the ratios of those numbers, but are as -50, '70, and
1-0. The 180°, for instance, does not represent three times the aper-
ture of the 60°, but t'wice 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 (n) and
the sine of half the maximum angle (u) both=l,so that // sin n
= 1 also, whilst with the immersion objective n is greater than 1 (say
1-5 for oil), and the angle « may therefore be much less than in the
case of the dry objective, and yet the value of the expression n sin n
(i.e. the aperture) may be greater than 1 •(').

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 branch
<>f the subject may here be serviceably summarised.

Take, lirst, the case of the medium being the same.

Difference of aperture involves a different <j>iantity of light ad-
mitted to the objective provided all other circumstances are equal.
1 1 ence the question of aperture leads to the consideration of the photo-
metrical equivalent of different apertures or aperture angles. It is
not of the essence of the problem, but it affords an additional illus-
trnl'nm of numerical aperture, and is thus of great service in its
exposition. It is manifest that a pert lire cannot be based on quantity

UNEQUAL INTENSITY OF EMITTED LIGHT

of light alone — more light can always be obtained in the image 1 > y
throwing more upon the object — but 110 increase in the amount of illu-
mination can make a dry lens equal in performance 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 portion 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 (jinmtiti/ of Uyht 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 ,/ sin n
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) Bouguet1 and Lambert2 established
the important fact that
with any surface of inti-
f or in. radiation the inten-
sity of the emitted rays is
not the same in all direc-
tions. The power of emis
siou 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- „

1 „ . \ n •,-,. • FIG. 34. — rhe intensity of emitted rays is not the

sine of the angle ot obliqui- same in ail directions. "

ty under which the ray is

emitted; in other words, in the proportion of the rosins of the angle

of deflection from the perpendicular to the luminous surface under

1 Traite d'Optiqtte snr la Gradation tie la Lumii'-re, 1760.
- Photometria, 1760.

K 2

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 emits 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. 33, hut by fig. 34, the density of the rays decreasing continuously
from the vertical to the horizontal.

< > \ving to the different emission in different directions, the quan-
tities of light emitted by an element in the same medium in cones of
different angle such as w and ?//, fig. 35, are not in the ratio of the
solid cones, as would be the case with equal emission, but in 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 110 contradiction between the measure of the aperture of an ob-
jective (n sin u) and that of the quantity of light admitted by it
the latter being (n sin »)'2.

The simplest experimental proof of the unequal emission in
different 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 the surface, one. n (fig. 36), perpen-
dicular to the line of vision, and the other, b, greatly inclined.
Every infinitesimal surface-element of b sends to the pupil of the eye
a rone of the same angle n', as a similar point of" (the slight differ-
ence of tin- distance from the eye being disregarded). If the in-
tensity of the rays were equal as supposed, the whole area b would
send to the eye the same quantity of light as the equal area a,
since both areas contain exactly the same number of elements. But
the ir/ti,/,'. quantity of light from b would be projected upon a
smaller area of the retina than that from a (as b appears under a
smaller visual angle, being diminished according to the obliquity, or

KADIATION OF LIGHT IN DIFFERENT MEDIA

53

,-is 1 : cos /'•). Consequently, if the assumption were true, b must

appear to be brighter than a, and the sphere would show increashii;

brightness from the centre to the circumference. Close to the

margin the increase ought to be very rapid, and the brightness ;,

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, therefore.

emitted from b within a given small

solid cone u' in an oblique direction

must be less than that which is emit-
ted from a within an equal solid cone

u in a perpendicular direction, and the

intensity of the rays must decrease in

the proportion of 1 : cos iv when the

obliquity •«: 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 angular expression of aperture,
and to prove that, when we are dealing
with one and the same medium only,
the angle is not the sufficient expres-
sion, but that it is the sine of the semi-
inii/li:' 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 u (or the ' numerical ' aperture)
give 1 -0 for the former and 1 -1 7 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
from 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'D (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. 86. — Diagram of a bright
spherical object emitting
light.

54 VISION WITH THE COMPOUND MICROSCOPE

In 1864 R. Cl.'iusius established, by distinguished research, the propo-
sition * that the power 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 : ri2 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 1'5 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 proportion 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
refractiATe indices of the media, viz. as I'O, 1'77, and 2-25.

Thus it is seen that the quantity of lujlit 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 ou the 'product of the sine of the semi-angle and the refractive
index of the medium in H-hich 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 l>e in complete accordance with the expression of aperture.2
We are now prepared 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

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

1 ' Uebei- die Concentration von Wiirme- und Lichtstrahlen &c.' Fogg. Annalen

d. P////.S//,', rxxi. ISIil.

2 Fig. A 2 Drives a good, practical illustration of the relative illuminating power of
objectives of varying apertures, and at the same time affords a simple explanation
nl1 1 he reason why (n sin u)2 is a measure of this illuminating power. Let the circles
A and B represent the backs of two objectives of the same power but of different

'I' i lures; then the radii (' D and E F will represent the angle n sin w (or /j. sin </>)
in ii;_'. A 1 (p. 48, note). Now because the areas of circles are to one another in the

RADIATION IN AIR AND BALSAM

55

A dry objective was therefore supposed to be placed at a disad-
vantage when used upon balsam-mounted objects, its aperture 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 ,-is
large (but no larger) 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

BALSAM

1

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
1 70° (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 1 70° in balsam of fig. 38, and not-
withstanding that a great part of the latter does not reach the

proportion of the squares of their radii, it follows that if we designate the radius by
n sin it (or fj. sin <£), 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 «')'-.

A

Areas
proportioned to

FIG. A 2.— The backs of two objectives of the same power but different 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 ;ible to receive the
whole 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
difficulty 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 difference of the rays which
can be utilised by different apertures, as we have demonstrated
above, the whole question would be of quite subordinate interest.

Another subject requiring some further elucidation here is the
' different angular distribution of the rays 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 different
cones of light. This diameter affords 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 pencil
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 equally enlarged image. 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
for gathering-in rays from luminous objects.

When the admitted pencil 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 different) we see that a
certain solid cone, A, from a radiant is transmitted through a certain
opening. K, and that another solid cone of rays, B, cannot be trans-
mitted through the same opening, a, but requires a wider one, /5,
whilst all other circumstances, except those of the radiant, have
remained the same, we can only conclude that the pencil B must
contain rays which are not contained in A, even if the admitted cone
is not increased in size. For the additional portion (p — a) of the
wider opening. /> conveys rays to the image which are certainly not
coin-eyed by the smaller opening a. From the radiant only can this
surplus come, and the pencil B which requires the additional opening
must embrace mure rays, even if it sJwidd not be of greater angle.

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

EADIATION IN AIK 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 refraction 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 more 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 surplus of new rays, which are never met with in air — that
is, are not emitted ir/ie/i 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 profit.1 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
higher refractive index within
a smaller angle, viz. twice the
critical angle. If in fig. 39
the object is illuminated by an
incident COlie of rays of nearly Fiu. 39.— Comparative compression of
82° within the slide, and the i;8ht ™y* 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° («'), and in oil at nearly 41° (a"), so that the
same rays which in air are expanded over the whole hemisphere 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 dijf ruction is considered, then
Fraunhofer's law shows that the same diffracted beams which in air
1 Jouni. R.M.S. ser. ii. vol. i. p. 303.

VISION WITH THE COMPOUND MICROSCOPE

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 room
for additional beams.'

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

FIG. 40. — Diffracted beams in air.

Fin. 41. — Diffracted beams in 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

a a

170° IN AIR

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 lie 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 1ms not, of course, altered the power of the
objective ; and since the diameter of the emergent pencil is the same
in both eases, 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.

•-, BALSAM

FIG. 44.

denote equal apertures, and t'<jn«l angles in different media denote
different apertures.

That 'immersion' objectives may have gre.-iter apertures than
the maximum attainable by a dry objective is capable of equally
proof by accessible experiment.

If an oil-immersion objective of 122° balsam angle be taken, and
so illuminated that the whole aperture is filled with the incident rays,
and if we use first an object mounted in air, we really find that we

DIFFKACTION RAYS AEE IMAGE-FORMING

59

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
jective, for
lens,
fluid,

FRONT LENS

the
and

glass

geneous.

OBJECT IN AIR
SLIDE.

IMMERSION FLUID
COVER CLASS

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

of the ob-
the front
immersion
the cover -
are nil homo-
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 run 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 lanjer 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 affording
greater facility than the following, viz. : ' Select a good specimen of
Amphi pleura, pdluclda and use oblique illumi-
nation, bringing out clearly the striation.

' On removing the eye-piece, placing the
pupil 011 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 i»ar<jin
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 rav>.'

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.

60 VISION WITH THE COMPOUND MICROSCOPE

demonstrate for 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 I'TrJ, I'oo, and I'D. 'Angular aperture' 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 which 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 practical optics, unverified by
('acts 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 gives
increased advantage to an object-glass manifestly needs careful
study and elucidation. We have but to refer to the best work 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 under 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 apertures 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 thii-ty years ago it presented itself to Professor Abbe as
a problem worthy of most careful inquiry as to why great ' angle '
or obliquity as such gave to objectives an enhanced capacity in the
disclosure of obscure structure. The first step was a consideration
of the grounds on which the theory of the value of angle of aperture
rested. But no such basis was found to exist ; no investigation of
the question had been made. It \vas demonstrated that a pencil of
170° would show minuter structure than one of 80° in the same
medium; and from this a generalisation had been made that upon
the obliquity of i he 'angle' of light depended the delineating power.
// II-KH i ii /,•<•,! ,i » ,,, self-evident /im/H^iiim/ that the formation of th<-
iniiKji' iii l/i" i,iicr<>«<-/>i>i' tank /iluci' in i ri ri/ particular according to
the sonii' ilio/ilrii' Imrx 1>;/ irjiidi htttn/i'n /in- formed in the telescope,
and it \vas tacitly taken for granted that every function of the

'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 production of
tlie microscopic image is wholly and absolutely dependent, not upon
the obliquity of the rays to tlu> nljcct. as it had been so long and so
stoutly maintained, but upon their obliquity to t/t</ tr. >•!.<< of tlic micro-
scope.

Such coarse objects as require only a tew degrees of aperture
to disclose them are dependent on 'shadow effects:' but when
extremely minute and delicate structures are to be disclosed small
apertures are absolutely useless, and mere increase of obliquity of
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 iiidiniiiij the object to the axis of the
microscope; 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
nu'liiKition of tin- 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 cut 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 effective
in any 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 opposed 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
examined with the microscope, which correspond to the different
obliquities, produce the same effects as if they were seen separately
by different eyes, as in the case with the binocular microscope. In
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
different eyes. In microscopic vision there is no difference of pro-
jection connected with different obliquities ; in the binocular micro-
scope there is a diversity of images which are depicted by pencils of

62 VISION WITH THE COMPOUND MICROSCOPE

different obliquities at the object which is -A certain kind of perspec-
tive difference ; 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
opening 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 f< irmation
of an image, such as objectives of lesser aperture cannot form
because their ' openings ' or ' apertures ' cannot admit that ' some-
thing.'

What this is becomes explicable by the researches of Abbe. It
is demonstrated that microscopic vision is sul 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
dioptrical 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 group 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 surface, the shadow will not be
sharp and black, but surrounded by luminous fringes having the
colours of the spectrum, and the centre, where the black shadow of
the wire should be, is a luminous line, as if the wire were transparent.
This phenomenon, as is generally known, is due to the inflection of
the diverging rays on either side of the wire. The inflected rays, in
passing over one edge of the wire, meet the rays inflected by the
other edge and 'interfere.' producing alternate increase and diminu-
tion of amplitude of oscillation or undulatory intensity, and giving
rise to coloured fringes if white light is used, and if homogeneous
light l>e employed giving origin to alternate bands of light and dark,
the centre always being luminous.

Again, if a disc perforated with a very small hole in the centre
lie held in a pencil of diverging light, those undulations which pass

DIFFRACTION PHENOMENA 63

directly through the aperture interfere with those passing obliquely
at the edge of the disc and produce, at certain distances, a dark
spot, at other distances increased brightness, on that part of the
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 luminous waves.

Independently of all experiment, the first principles of undulatory
optics lead to these experimental conclusions. The laws of recti-
linear propagation of the luminous rays of reflection and refraction
are not absolute 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 illustrative the waves of soitnJ, an acoustic shadow i.s
only produced if the obstacle be many times greater than the length
of the sound-waves. If the obstacle is reduced, the waves pass 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 similar
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 multiples 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 diffraction phenomena that Abbe is
enabled to explain the formation of the images of objects containing
delicate stria? or structure, 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 present 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. Dr. Abbe has read
the entire chapter, and, with many generous words besides, relieves
the editor in his consciousness of great responsibility by saving that
he distinctly approves of the 'lively interest and care which (the
present editor) has bestowed on the exposition of his (Dr. AbbeV)
views,' and that he feels ' the greatest satisfaction, in seeing (his)
views represented ... so extensively and intensively.'

But beyond this, an original worker 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. Ablie.
In the expositions of Dr. Abbe's views on the diffraction theory

64 VISION WITH THE COMPOUND MICROSCOPE

of microscopic vision given up to this time, it has been usual to
state that he held and taught that the microscopic image consists of
two superimposed imayes, each having a distinct character as well
as a. different origin, and capable of being separated and examined
apart from each other. 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 n-Jiose elements lie so close together
as to occasion diffraction phenomena can alone be formed, because
these could not be geometrically imaged. So that in the case of an
object with lines closer than the o-^ny of an 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 ; but the second
Mas considered a positive image because it delineates structure, the
parts of which appear self-luminous on account of the diffraction
phenomena which they cause. It was this ' diffraction image ' that
was said to be the instrument of what has so long been known as
the 'resolving' power of lenses.

But Dr. Abbe, with the full light of further investigation and
experience, does not hesitate to modify this explanation. He says :
' I no longer maintain in principle the distinction between the
•l absorption image '' (or direct dioptrical image) and the " diffraction
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 exhaustive 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
l/in/j as the objects are depicted, by means of transmitted or reflected
Hi/lit, whether the objects are of coarse or very fine structure.
Further experiments with a large microscope, having an objective
of about twelve inches focal length, have enabled me to actually
observe the diffraction effect and its influence on the image, viewing
gratings of not. more than forty lines per hid

l

1 Diffraction effects may be observed without a microscope ; they can be easily
demonstrated by observing a lump-flame through a linen pocket-handkerchief or a
tint- gauze wire blind. This can be done readily by placing the eye close to the linen
or wire.

RECENT MODIFICATIONS OF ABBE'S VIEWS 6$

' My present views may lie thus expressed : With coarse objects
the diffracted (bent oft') rays belonging to an incident ray or pencil
are all confined within a very nurron- unyidar space ((round that
incident ray, and do not appear separated from this except witli
objectives of very long focus. The n-hole of such a narrow diffraction
pencil is consequently always admitted to the objective toget/ter with
the direct (incident) beam, whatever may be the direction of inci-
dence, axial or oblique. According to the proposition of p. 7'2 (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 upon 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)
into two parts : (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
of the preparation which is admitted to the objective complete];/.
independently of the limiting action of the lens opening, and hence
the corresponding parts of the object (outlines Arc.) 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 are due to the minute elements are strongly
deflected from the incident beams to which they belong.'1

According to the less or greater aperture of the objective and
the axial or oblique incidence of the illuminating pencil or cone,
tli in 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 apertures and
different illumination, dissimilarity 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 difference is not to be considered one
of jn'iiii'iph, so far as the production 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.2

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 o\ir purpose is a plate
of glass ruled with fine parallel lines. If the name 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 appears 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.

F

66

VISION WITH THE COMPOUND MICROSCOPE

M » ' i I M ' M * « * '

-----•»•••••*

FIG. 48.

central image will be clear and uncoloured, but it will be flanked on
either side by a row of coloured spectra of the flame which are fainter
and more dim as they recede from the centre : fig. 48 illustrates this.
A similar phenomenon may also he produced by dust scattered
over a glass plate and by other objects whose structure contains verv
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 intensity ot light due to difference of phase of vibration.

In the same way in the microscope the diffraction 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 interposed
between the mirror and a plate of ruled lines placed upon the stage.
the appearance shown in tig. 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-diffracted rays, while those
on either side are the diffraction images
produced by the rays which are bent off
from the incident pencil. In homogene-
ous light the central and lateral image-
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 ordinarv
spectrum), forming, in fact, regular spec-
tra, 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 formation of the microscopical image is explained bv the
tact 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 object, and
give rise there to an interference phenomenon (owing to the connec-
tions of the undulations), this interference effect giving the ultimate
image which is observed by the eye-piece, and which therefore
depends essentially on the number and distribution of the diffracted
beams which enter the objective.

Jt would exceed the limits and the object of this handbook to
attempt a theoretical demonstration of the action of diffraction
spectra in forming. the images of fine structure and striation so as
to afford ' resolution.' Those who desire to pursue this part of the

o o o o o

Fn;. i'.t.

DIFFRACTION E X I 'ERIMENTS

67

subject may do so most profitably by the study of the only book in
our language that deals exhaustively with the theory of modern
microscopical optics, viz. the translation of Xaegeli and Schweiidener's
• Microscope 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

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

purpose, and the following experiments will sutlire t > 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 experiment 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 viewed 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. o-2.

Fu;. .">:!.

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 reach the image, the object will appear to be wholly deprived of

68

VISION WITH THE COMPOUND MICROSCOPE

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

This illustrates a case of the obliteration of structure by obstruct-
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.

If a diaphragm such 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 spectra in fig. 50, that row will obviously become identical with
the lower one, and if the theory holds good, we should find tin- 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 doubled
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 way, if we stop off all but the outer spectra, as in fig.
56, the lines are apparently again doubled, ami are seen as in fig. 57.

FIG. 57.

A case of a [>parent creation of structure similar in principle to
the foregoing, though more striking, is afforded by a network of
s»|iiares, such as fig. 5H, having sides parallel to the page, which gives
the spectra, shown in fig. 51), consisting of vertical rows for the
horizontal lines and horizontal rows for the vertical ones. But it
is readily seen that t\vo diagonal rows of spectra exist at right

DIFFRACTION 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, 011 obstructing all the other spectra anil

V

\

FIG. ~>H.

FIG. 59.

allowing only the diagonal ones to pass to the eye-piece, that the
vertical and horizontal lines have disappeared, and two new sets of
lines at i-'njJtt angles to the diagonals have taken their place.

FIG. 00.

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. (51,

X X X X X\
/X X X X XX

\7~
A

\X X X X X

X X

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

An object such as Pleivrosigma n in/nJi/tn n>, which gives six

7O VISION WITH THE COMPOUND MICROSCOPE

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 b a e, another set at right angles to
c af, and a third at right angles to y a d. These three sets of lines
will obviously produce the appearance shown in fig. 63.

A great variety of other appearances may be produced with this
same arrangement of spectra. Any two adjacent spectra with the
central beam (as b c a) will form equilateral triangles and give
hexagonal markings. Or by stopping off all but gee (or bdf) 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 y c or </ f or bf, tvc., 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 produced, all of which cannot
of course represent the true structure.

In practice, indeed, it lias been proved that if the position and
relative 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. angulatum be illuminated by central light transmitted
from an achromatic condenser, and examined by means of a homo-
geneous lens of large aperture, Mr. Stephensoii points out T that
under ordinary conditions it would show, on withdrawing the eye-
piece and looking down the tube, one bright central light from the
lamp with six equidistant surrounding diffraction spectra, produced
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 ]» s
terior lens ; in the stop let six marginal openings be made through
which the diffraction spectra may pass. On examining the image
we find that in lieu of the ordinary hexagonal markings the valve
appears of a beautiful blue colour on a black ground, and covered
with circular spots, clearly defined, and admitting of the use of deep
eye-pieces.

This is precisely what we learn from Abbe that the diffraction
llii-nry involves. In support of this, the philosophical faculty of the
University of Jena had proposed as a question to the mathematical
students the effect produced in the microscope by these interference
phenomena. One problem was that of the appearance produced by
six equidistant spectra in a circle ; they correspond precisely with
the spectra of 1' . (ciujtihtitnii, as accessible to us with our present
numerical aperture ; and the diagram of the diffraction image, de-

1 Jo urn. li.M.S. vol. i. 187s, |> INC,.

PLEUROSIGMA ANGULATOI 71

duced from theory, of what spectra of the given position and inten-
sity 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 photograph of P. aiujnla! inn. 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 bv
Abbe to be an accurate indication by calculation of what image the
given diffraction spectra should produce. An optical glass and
media for ' mounting ' and ' immersion ' of immensely greater refrac-
tive and dispersive indices — at present wholly inaccessible to us—
must, he contends, be found and employed before all the diffraction
spectra of P. auijultil u u* could be admitted to form its absolute and

FIG. 64.

complete ' diffraction 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. aiigxlatnm theory indicated the optical, but not necessarily the
structural existence ! of smaller markings, shown in fig. <>4, bet \\een
the circular spots. These had not been before seen by observers : and
the mathematician who made the calculation (Dr. Eichh* mi) 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 bv careful investigation they

. Al.be'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 structure of the finer
kinds of diatom valves. AVe learn that dissimilar structures will
give identical microscopical in'ni/es when the difference of their
diffractive effect is removed, and conversely si in 'dor i.<trnctt'r<ji* may
give dissimilar images when their diffractive 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 linages of material forms, but only as si</ns of
material differences of composition of the particles composing the
object, so that nothing more can safelv 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.1

It follows, therefore, that the larger the number of diffracted rays
admitted into the objective the greater is the similarity b^tn.-pen the
imaf/e and the object. But carefully observe—

(1) Perfect simtta/rity between these depends always on the ad-
mission to, and utilisation by. the optical combination of the it-hole of
the diffracted rays \\liich the structure is competent to emit.

For the same reason the diffraction fan of isolated corpuscles or
flagella in a clear field must be exactly identical to that of equal -
sized holes or slits of equal shape in a dark background, and theory
shows that there must be a continuous and nearly uniform dissipa-
tion of diffracted light over the whole hemisphere, 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. Such isolated objects can be seen, hotrerer mi/utte the;/ ///'///
hi ; it is merely a question of contrast in the distribution of light, of
good definition in the objective, and of sensibility of the retina.
The diffraction theory 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 flagellum of Tiacteriimi termo only a fraction of a
wave-length in diameter appears as of considerably increased dia-
meter, even with a very wide aperture. The image seen is that of
another thread, the composition of which theory can be employed to

1 See Abbe's note, p. dii. But we cannot pass over in this connection the
remarkable paper in the Joiirn. Qitekctt Club, ser ii. vol. iv. on the ' Sub-stage
Condenser,' by Mr. Nelson. His photomicrographs illustrating the mutable diffrac-
tion effects of the ' small cone ' of oblique illumination, as distinct from a ' solid central
cone,' and the curious ' ghostly ' diffraction images of the former, as distinct from the
immutable diffraction images of the latter, deserve careful consideration. From
p. 125 of the paper this matter is carefully discussed.

SIX EQUIDISTANT SPECTRA AS A DIFFRACTION PROBLEM 73

compute, which would give an exactly similar diffraction fan, but
abruptly broken off at the limit of the aperture. 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 Bacterium 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 01- protoplasmic fibre would be could it be
seen directly without the aid of diffraction.

(2) Whenever a portion 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 <////m-/// structure,
namely, of one the whole of ir/tose diffracted !><~'/IHIS would (if it
physically existed) be represented by the tit/Used 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 ordinary image.
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 twTo points, viz.—

i. ' It is a.n exemplification of the general proposition that the
same object affords different inayes if different portions of the total
diffraction fim 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 hypothetical 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 diffraction
is confined to smaller and smaller angles ; that is, all diffracted rays
of perceptible intensity will be comprised within a narrow cone

74

VISION WITH THE COMPOUND MICROSCOPE

around the direction of the incident beam from which they originate.
In such a case even small apertures are capable of admitting the
n-Jiols. The images of such coarse objects will therefore be always
perfectly similar to the object, and the result of the interference
effect is the same 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 wave-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 expansii n
(the diffracted rays are further apart) ; and when they are reduced

to only a few wave-lengths, not even
the hemisphere can embrace the whole
diffraction effect which appertains to
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. AYith very
minute structures, the diffraction fan
which can be admitted in air. and
even in water or balsam, is only a
greater or less central portion of tin-
whole possible diffraction fan corre-
sponding to those structures, and which
could be obtained if they were in a
medium of much shorter wave-length,
l/nder these circumstances no objec-
tive, however wide may be its aperture,
can yield a complete or t;1rictt</ ximilur
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 from
him relating to the dissimilarity of the
image in the case of incomplete admis-
sion of the diffraction pencil will be of
great value here.

i. 'In the case of regular periodic
structures (i.e. equidistant striae, rows of apertures, 'dots,' and so forth)
the 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 the lines per inch is never changed, provided the direct beam
(i.e. the central maximum of the diffraction fan) in admitted to the
• i/ijrrfiri' i> ml <it least one of the next diffracted rays,, or, in other words,
one of the next maxima of second order. The range of dissimilarity

1- ic;. 05.

DIFFRACTION THEORY UNIVERSALLY APPLICABLE 75

is in this case confined to the proportion between the bright and the
dark interspaces of the striation and to the appearnnce of the con-
tours of the stria\

' If not more than the said tiro rays of the total diffraction fan are
admitted, the dark and the light intervals are always shown of
approximately equal breadth, even if the real proportion of both
intervals differs much from 1:1; and the dark and bright stria? show
always gradually increasing and decreasing brightness; in other
words, want of distinct contours.

'This phenomenon shows the typical picture of every regular
striation for the depiction of which not more than two diffraction
rays can be utilised. For example, Amphipleura pellitdda, or any
other striation which is near to the limit of resolution for 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 centra] 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 must
always involve a loss, and this loss is not confined to the external
(peripheral) 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 oblique
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, except 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 diffraction 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 produce diffraction effects,
observed either by transmitted or reflected light, and being eitl i r
transparent, semitransparent, or opaque.

The value of a = n sin « 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 competent 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, provided 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

VISION WITH THE COMPOUND MICROSCOPE

determined by the fact that no resolution 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, colour as a basis) to be equal to tfirr
the number of undulations in an inch multiplied l>y the numerical
aperture.

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 using the ' sine of halt'
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 aperture 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
determination, since the capacity for appreciating light varies with
different individuals.

If, for instance, we take '43/t 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 1 >e produced.

AIR

SLIDE

Fit;, (id.

This important subject can sraivrlv be considered complete, even
in outline, without a brief consideration, in their combined relations,
of apertures in excess of 180° in air and the special function these
apertures possess.

I. Suppose any object composed of minute elements in regular
arrangement, such as a diatom valve ; and, to confine the considera-
tion to the most simple case, suppose it illuminated by a narrow

APPLICATION OF THE DIFFRACTION THEOIIY

77

axial pencil of incident rays. If this 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 8, fig. 66 (the direct
continuation of the incident rays), and a number of bent-off pencils.
S1? B2, . . . surrounding S.1

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

beam is re<fnc'-J in the exact proportion of n, 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 will represent the
case of the same object in oil.

If now any dry objective (with a given angular semi-aperture u)
is capable of gathering-ill 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, >i, will be
capable of admitting, from within the dense medium, exactly the same
beams (no more and no less), if its angular semi-aperture, r, is less
than u in the proportion :

sin v : sin n = 1 : n.,

or

n sn

= sn

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

For example, a diatom for which the distance of the stria? is 0'6 /.i,
will give the .y??^ bent-off beam of green light (\ = '•>•">/') in air under
an angle of 66'5°. This will be just admitted by a dry objective of
133° (uiijular aperture. In balsam (n = l'.">) the same pencil will
be deflected by 37'5° only, and would lie admitted, therefore, by an
objective of not more than 75° balsam-angle. Again, if the distance
of the lines should be greater, as \-'2p. the .wcm<l deflected beam

1 In figs. 66, 67, and 68 S4 and S6 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.

VISION WITH THE COMPOUND MICROSCOPE

would lie emitted in air under an angle of 66'5°, but in balsam the
tliii-d would attain the same ol)liquity. 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.1

It follows, therefoi-e, 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
retractive 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 »,) 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 would be represented by

A/Ji

sE
1^

FIG. 68.

fig. 66, and the second by 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. l>ut

1 The following are the actual angles represented in the diagrams, viz. :

(Strise^'2-2 ft., wave-length A = '55 /j., medium air » = 1.)
SI = 14° 30'
8., = 30° 0'
8^ = 48° 36'
84=90° (>'.

rise = '2-2 /j., wave-length A = •.">."> /j., medium balsam «=-r5.)
8, =9° 36'
S2=19°28
83 = 30 if

54 = 41° 4*'

55 = 56 liC,'

HOMOGENEOUS VERSUS DRY OBJECTIVES 79

according to the law of refraction, this group, on passing to air l>y
the plane surface of the covering-glass, is spread out — the sines of
the angles being compared — in the ratio of the same refractive index.
Consequently the various diffraction pencils, the first, second. . .
on every 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 pencils 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 In-
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. (36, 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 tliar the number
of the emitted beams is </re«tt>r in balsam than in air in the same
ratio as the refractive index.

A structure the distance of whose elements equals 2'2/< emits in
balsam six distinct beams on each side of the direct beam, but in air
oioijfour (see figs. 66, 67, and 68); the fifth and sixth are completely
lost in air. A dry objective of an angular a | >ert ure closely approaching
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
possesses 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 iltfrartimi 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 precaution 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.

So VISION AV1T1I THE COMPOUND MICROSCOPE

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 'apochromatic' 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 aperture in a striking manner.
For twenty years we have been urging our best English microscope
makers to enlarge the 'angle' of our objectives, and employing
tin 'in from a I -inch to a -Vinch focus. "We have seen them
advance from dry to water immersion, and from this to oil ; from
-^5-inch, a ^--inch, and a ^-inch of IS". A. O95 each, and re-
spectively to water immersions of X.A. 1'04 and then to 'oil
immersions' or homogeneous lenses of X.A. 1'38 for the J--inch
and -Vinch respectively, and ultimately by a ./(T-mch with X.A.
of 1 •">(>: and from that we have progressed to the apochromatic
objectives with compensating eye-pieces.

Xow 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 familiarity — are still in our possession ; what
was revealed by them fifteen, twelve, or ten years ago Ave can
exactly repeat to-day ; and in the general features of the work — in
the broad characteristics of the life histories of the saprophytic
organisms, minute as they are, revision with objectives of X.A. 1'50
and other lenses of the best English and (merman makers, reveals no
positive error, even in the minutest of the details then discovered and
delineated. But the later lenses of great aperture and beautiful
corrections have opened up structure absolutely invisible before.

Thus, for example, a minute oval organism from the Tr,7OI,th to
the -, iMMith of an inch in long diameter was known to possess a
distinct nucleus; the long diameter of this was from the ^th to the
rVth 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 organism were clearly visible and constantly observed : but
of the nucleus nothing could be made out save that it appeared to
share the changes in self-division and genetic reproduction, initi</f«/
liv the organism as a whole. 15 ut by lenses of X.A. 1'50 and the
finest apochromatic objectives of /eiss, especially a most beautifully
corrected 3 mm. and '2 mm., structure of a remarkable kind has
been 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 organism a,s a whole. In short, we have
discovered as much concerning the 'inaccessible' nucleus — which
may be not more than, say, a twelfth of the long diameter of the
whole organism — by means of lou.*zr powers, but greater apertures, as
we were able to find concerning the complete body of the saprophyte
with dry objectives.

But in spite of these facts there is a certain class of even high
power work in biology from which 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
part of the work done in the study of the life history of active
living organisms; and whatever accessories in research on such
subjects be employed, the main path of accurate and well correlated
discovery must be by ultimate and consecutive reference to tin-
changes of the living organism. But we cannot with any certainty
do this with either a water immersion or a homogeneous objective.
With an active organism under investigation, we desire, as far as
practicable, to limit the area of its excursions; a cover-glass of not
more than four-tenths or a quarter of an inch in diameter is large
enough when objectives from a ..^ inch to a -1,, inch are used,
or when the recent 2 mm. objective with 27 eye-piece is employed.
To have oil or water on the top of the cover, 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 reach 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 work of studying consecutively the life history
of unknown organisms, dry objectives will and must be used ; but
in all cases such work must be supplemented by the use of objectives
of great aperture. The details ami relations of minute structure
must be studied in one field, and their general origin and sequences
in another. The latter will be 'continuous,' the former will be
employed as necessity indicates. The diffraction theory of micro-
scopic vision does not invalidate, but in reality, under definable
conditions, directs the employment of 'narrow' apertures. All
depends on the minuteness of microscopic detail. The law has been
enunciated above : the minuter the dimensions of the structural
elements, the wider must be the aperture : the larger the details of
ultimate structure, the narrower the aperture that will suffice. This
is true in regard to objects of every kind ; there is no variation in
the conditions of microscopical delineation.

The men engaged in microscopical research have different aims,
nay, the same worker at different times differs in the object pursued.
•' Ultimate structure ' is not the one consideration of the micro-
scnpist; he often, as indicated above, has to take a comprehensive
view of the whole object or objects of his research, apart from the
most complex and delicate details.

It is folly to suppose that because great apertures have been
proved theoretically and practically to be able to open out minute
structure so perfectly, therefore there is no raison d'etre for small
apertures. Low amplification cannot render distinctly visible de-
tails beyond a certain limit of minuteness, and wide apertures
cannot be utilised unless there is a concurrent linear amplification
of the image which is competent to exhibit to the eye the smallest
dimensions which are by optical law «.• it/tin the reach and grasp 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 an image of much lower amplification.

a

82 VISION WITH THE COMPOUND MICROSCOPE

It will be 'empty amplification,' because there is nothing in the
image which requires so much power for distinct recognition. If
the JIHX-PI- be deficient, aperture will not avail ; if the aperture be
wanting, nothing is gained by high power. If the angular aperture
of the microscope is such that the delineation of fine lines, whose
interspaces are one 'micron,1 is just possible, it is fruitless labour to
increase the amplification beyond what we know to lie .sufficient for
their observation. We potentially differentiate what we are power-
less to see.

Thus it may be inferred from the diffraction theory, as such, that
wide aperture should accompany high amplification, and moderate
aperture 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
fairlv good ~>-inch objective of suitable numerical aperture, and
obtain in its place a, A inch or j1,, inch with scarcely any increase of
numerical aperture, merely for the ease with which amplification is
effected. But it would be well to remember that high amplification
effects nothing unless accompanied by suitably widened aperture.

The circumstances on which what 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.3 Indeed the
greater part of all morphological work is of this kind ; here, then,
in the words of Abbe, 'a proper economy of aperture is of equal
importance with economy of power.' 4

Whenever the depth of the object or objects under observation
is not very restricted, and for the purposes of observation we require
depth dimension, low and moderate powers must be used ; 'and no
greater aperture should therefore be used than is required for the
effectiveness of these powers — an excess in such a case is a real
damage.' 5

Moreover, in biological work — constant application of the iustrn
ment to varied objects — lenses of moderate, aperture and suitable
power facilitate certainty of action and conserve labour. Increase
of aperture involves a diminished working distance in the objective,
and it is inseparable from a rapid increase of sensibility of the
objectives for slight deviations from the conditions of perfect col-
lection . If it lie not iifcessm-;/ 1o encounter the possible difficulties
these things involve, to do so is to lose valuable moments. The>e
difficulties, of course, are diminished bv the use of homogeneous, and

A inirnni is M --,-,-,',-„--, mm. I'iilr JoiU'n. //..1/..S. lss.->, pp. 50'j ami f>2(> ; and

, vol. xxxviii. pp. -2-21, -JJ4. - See p. 83.

. MilicV explanation of the reason of the non-stereo-copic perception of the
given (see pp. '.»:; <-t .sn/.i.

' The Relation of Aperture lo l'o\\er,' Journ. //. .!/".&'. series ii. vol. ii. p. 804.
•'' III id.

PENETRATING POWER IX OBJECTIVES 83

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

Employ the full aperture s<iitul>li> to the power used. This is the
practical maxim ta light in effect by the Abbe theory of microscopic
vision.

It has been suggested that all objectives be made of relative!}'
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,1 that ' scientific work with the
microscope will always require, not only high power objectives of
the widest attainable apertures, but also carefully finished lower
powers of small and very moderate apertures.

We complete this section with a table of numerical apertures,
which will be found on the following page. As already stated, t In-
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 (Clifford's screen) by 80,000.2

The first column gives the numerical apertures from "40 to l-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 (A. = 0'5269/u) 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 depth 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 value 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,' Jvnrii. E.M.S. series ii. vol. ii. p. 309.
- Joufn. B.M.S. (1893), p. 17.

G 2

84

VISION WITH THE COMPOUND .MICROSCOPE

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88 VISION WITH THE COMPOUND MICEOSCOPE

matter was involved in obscurity. The remarkable insight ami
learning of Professor Al>l>e 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
peculiar disproportion in amplification. The linear amplification of
the di'/itlt -dimension is. when both the object and the image arc in
the same medium (air), found to be always equal to the wjunre 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 increasing, over
amplification 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 construction of a three-dimensional image were found. I'.ut
for this the solid object, as such, must be simultaneously visible ;
a single layer of inappreciable depth can convey no conception of
the three space dimensions possessed by the object.

Owing to the disproportions! amplification of the depth-dimensi< m
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 lip of two parts, the limits of which as regards the
depth are determined in a very different manner.

First, the accommodation of the eye embraces a certain depth,
different planes being successively depicted with perfect sharpness
of image on the retina, whilst the eye. adjusting itself by conscious
or unconscious accommodation, obtains virtual images of greater or
less distance of vision. This depth of accommodation, which plays
the same part in microscopical as in ordinary vision, is wholly
determined by the extent of power in this direction possessed by the
particular eye, the limits being the greatest and the least distance
of distinct vision. Its exact numerical measure is the difference
between the reciprocal values of these two extreme distances. The
depth of distinct vision is directly proportional to this numerical
equivalent of the accommodation of the eye, directly proportional
to the refractive medium of the object, and inversely proportional
to the square of the amplification when referred always to the same

PRINCIPLES 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-jj mm.; the calculation for
an object in air would give a depth of vision by accommodation

amounting to

2-08 rum. with 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.
Trans\erse sections of the object which are a little above and below
the exact focal adjustment are seen without prejudicial effects. The
total effect so ol>i«'i n«l is the so-called pf net rut ion or <l>'j>tlt of f<«-,i>.
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.
Tt is found that one minute of arc denotes the limit of sharply
defined vision, two to three minutes for fairly distinct vision, and live
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 which the object is placed, the amplification, 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
ail' will then be—

0-073 mm. for 10 times amplification
0-024 30

0-0073
0-0024
0-00073
0-00024

100

300

1000

3000

l>y 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 sufficient distinctness of vision.

90 VISION WITH THE COMPOUND MICROSCOPE

As the amplification increases the over-amplification of the
depth dimension presents increasingly 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 accommodation-depth diminishes in rapid
ratio, becoming equal to only a small depth of focus ; while when
the magnifying power is greatly increased the accommodation-depth
is a fax-tor of no moment, and we have vision largely, indeed almost
wholly, dependent on depth of focus.

The following table shows the total depth of vision from ten to
M.OOO times:-

Amplification

10

30
100

300
1000

3000

Diameter of
Field

mm.
•25-0

8-3
2-5

0-83
0-25

0-083

Accommoda-
tion Depth

nun.
•2-08

0-23
0-02

0-0023
0-00021

0-00002

Depth of Vision,

Ac commodatiou
Focal Depth Depthi and

Focal Depth

Ratio of Depth
of Vision to
Diameter of
Field

mm.
0-073

0-024
0-0073

0-0024
0-00073

0-00024

mm.
2-153

0-254
0-0273

0-0047
0-00094

0-00026

1

11-6

1

32-7
1

91-6

1

17(5-6
1

266
1

B19

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
spare relations. When with increase of magnifying power the depth
of the image becomes more and more flattened, the images of different
planes stand out from each other move perfectly in the same ratio.
and in the same degree are clearer and more distinct. With an
increase of amplification the microscope acquires increasingly the
property of an optical microtome, which presents to the observer's
eye sections of a fineness and sharpness which would be impossible
to a mechanical section. It enables the observer, by a series of
adjustments for consecutive planes, to construe the solid forms of
the smallest natural objects with the same certainty as he i>
accustomed to see with the naked eye the objects with which it i>
concerned. This is a large advantage in the general scientific u-e
of the instrument; a greater gain, in fact, than could be expected
from the application of stereoscopic observation.

Stereoscopic Binocular Vision. — This subject has been elaborately
considered and partially expounded and demonstrated by Professor
Abbe ; his exposition differs in some important particulars from
that of the original author of this book, but in its present incomplete

STEEEOSCOP1C BINOCULAR VISION . 91

forms it appears to the editor to be the wiser way to allow Dr. Car-
penter's treatment of the subject to stand, and to place below it as
complete a digest of Professor Abbe's theory and explanation of the
same subject as the data before us will admit.

The admirable invention of the stereoscope by Professor Wheat-
stone has led to a general appreciation of the value of the conjoint
me 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 generate with
the like certainty or effectiveness ; and after several attempts,
which were attended with various degrees of success, the principle of
the stereoscope has now been applied to the microscope, with an
advantage which those only can truly estimate who (like the Author)
have been for some time accustomed to work with the stereoscopic
binocular1 upon objects that are peculiarly adapted to its powers.
As the result of this application cannot be rightly understood with-
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 picture 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 that 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 pictures would by no
means correspond. Yet 011 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 coiild
only be otherwise acquired by the sense of touch. ]S"ow if, instead
of looking at the solid object itself, we look with the right and left
eyes respectively at pictures of the object, corresponding to those
which would be formed by it on the retina? 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
there. The stereoscope — whether in the forms originally devised by
Professor Wheatstone or in the popular modification long subse-
quently introduced by Sir 1). Brewster — simply serves to bring to
the two eyes, either by reflexion from mirrors or by refraction
through prisms or lenses, the two dissimilar pictures which would
accurately represent the solid object as seen by the two eyes respec-

1 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 MICROSCOPE

tively, these being tin-own on the two retinae in the precise positions
they would have occupied if formed there direct from the solid
object, of which the mental image (if the pictures have been correct ly
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
a projecting truncated pyramid, 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-

. 69.

bined in the same crossed pictures, their apparent relative distances
are reversed, the remoter being brought nearer and the nearer
carried backwards ; so that (for example) a stereoscopic photograph
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 Wheatstone, which has the effect of reversing
the perspective projections of objects seen through it by the two
eyes respectively ; so that the interior of a basin or jelly-mould is
made to appear as a projecting solid, whilst the exterior is made to
appear hollow. Hence it is now customary to speak of stereosc<>/>t<-
vision as that in which the conception of the true natural relief of an
object is called up in the mind by the normal combination of tin-
two perspective projections formed of it by the right and left eyes
respectively ; whilst by pseudoscopic vision we mean that ' conver-
sion of relief ' which is produced by the combination of two reversed

CARPENTER'S r. ABBE'S VIEW <>K STEREOSCOPIC VISION 93

perspective projections, whether these l>e obtained directly from the
object (as by tlie 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.1

Xo\v it is easily shown theoretically that the picture of any
projecting object seen through the microscope with only the /•/,////-
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
difference between such pictures must increase with the angular
aperture of the objective. This difference may lie 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 satisfactorily 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 ri;/ht half of the objective of a compound microscope
were seen by the r'njJit eye, and that formed by the left half were
seen by the left eye, the resultant conception would be not stereo-
scopic but ps&udoscojnc, the projecting pails being made to appeal-
receding, and ricf i-i'i-sa. 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
I ft and the i-i;//tt 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, or by
that reversal of the two perspective projections formed direct from
the object, which is effected by the pseudoscope. It was from a
want of due appreciation of this principle (the truth of which can
now be practically demonstrated) that the earlier attempts at pro-
ducing a stereoscopic binocular microscope tended rather to produce
a ' pseudoscopic conversion ' of the objects viewed by it than to
represent them in this true relief.

1 For a fuller discussion of this subject see the Author's Mental Physiology,
Sg 168-170.

94 VISION WITH THE COMPOUND MICROSCOPE

In contra distinction to tlil* explanation 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
rdiHon d'etre of ordinary stereoscopic effects 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 layers. 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. 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 to 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 or through the telescope
would be depicted ; nevertheless the brain arranges them so that
the characteristics of solid vision ai-e still presented.

Professor Abbe demonstrates l that in an aplaiiatic system pencils
of different obliquities yield identical images of every plane object,
or of a single layer of a solid object. This is true however large the
aperture may be.

This carries with it, as we have said, a total absence of perspec-
tive and an essential geometrical difference between vision with tin-
binocular microscope and vision with the unaided eye.

An object, not quite flat, as a curved diatom, when observed with
an objective of wide aperture will present points of great indistinct-
ness. This has been by some supposed to arise from the assumption
that there was a dissimilarity between the images formed by the
axial and oblique pencils ; but this is not so. It is wholly expli-
cable by the fact that the depth of the object is too great for the
small depth of vision attendant upon a large aperture.

It will be seen, then, that so long as the depth of the object is
within the limits of the depth of vision, corresponding to the aperture
and amplification in use, \ve obtain a distinct parallel projection of
all the successive layers in one common plane perpendicular to the
axis of the microscope — a ground plan, as it were, of the object.
Manifestly, then, since depth of vision decreases with increasing
1 Jai/ni. U.M.S. series ii. vol. iv. \i\t. '21-'2t.

ABBE ON STEREOSCOPIC VISION 95

aperture, good delineation with these must l>e routined to tl tunic-.'
objects than can be successfiilly employed with objectives of iiarro\\
apertures.

Stereoscopic vision with the microscope, therefore, is due solely
to difference of projection exhibited by the different parallactic dis-
placements of the images of successive layers on the common ground
plane and to the perception of depth, not to the delineation of the
plane layers themselves. For, if there were dissimilar images j ei
rrptible at different planes, the out-of-focus layers must appear COD
fused and no i-ision of depth would be possible.

Xo\v stereoscope vision requires, as shown by Dr. Carpenter, that
the delineating pencils shall lie 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
semicircles representing one of these two arrangements seen in
( > and P, fig. 70.

i n ft % P §

I)r. Abbe's theory is that the only c mdition necessary for r// •///"-
s<-r>i>i<: effect in any binocular system is that these semicircles or
their equivalents should be depicted according to diagram < >. ti-. 70,
and for pseudoscopic 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.

Orthoscopic vision is a 1 \vavs obtained \vheii the right half of the
right pupil and the left half of the left pupil only are employed ;
pseudoscopic vision in the opposite conditions. ' It is quite indif-
ferent whether the effect is obtained with crossing or non-crossing
rays, whether the image be erect, or inverted, or semi-inverted, and
whatever may be components of the optical arrangement.'

The observant reader will perceive that it is at this point that
there is a radical divergence from the interpretation given by Dr.
Carpenter, who, as we have seen above, insisted that orthoscopic
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 r/w.s'.v cm-It
other.

Of course we must keep clearly before 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
rdinary vision. Clearly, then, it is the perspective projections of
this imaye which require to lie compared 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
geometrically demonstrated that right-eye perspective of the ap-
parently solid image is always obtained from the right-hand portion of
the emergent pencils, left-eye perspective from the left-hand portion ;

o

96

VISION WITH THE COMPOUND MICROSCOPE

BINOCULAR MICROSCOPES 97

ami it is quite immaterial, as regards this result, which portion of the
emergent rays is admitted by the right or the left part of the objective.

The manner in which the delineating />i-,H-ils are transmitted
Through the system may be such as to require crossing over of the
rays from the right-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 Xachet binocular (pp. 98, 99) crossing over is required
IK 'cause the inversion of the pencils is not changed by two reflexions.
If the delineating pencils have been reflected an^v/t number 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 practical character was the arrangement of Professor J. L.
Riddell, of Xew Orleans. It was devised in ]*."»] 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. T

A representation of his original instrument is presented in fig. 71,
and the arrangement of the prisms by which the binocular effect was
obtained is shown in fig. 7'2.

It will be seen that the pencil of ray> rmerging from the back
lens of the combination I is divided into two, each half passing re-
spectively into the right and left prisms; the path of the rays is
indicated at «, b, c, d, the object being at o.

To secure coincidence of the images in the field of view for
varying widths between 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 «, fig. 72,
remain always in parallel contact, the inclination of the internal
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 tubes are connected by racks,
one acting above and the other below the same pinion, so thatright-
and left-handed movements are communicated by turning the pinion.

This instrument could only be vised in a vertical position, as
shown in the figure (71). The two prisms in 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
I >hi< icular systems.

1 P. 13.

9s

VISMN WITH THE COMPOUND MICROSCOPE

Nachet s Binocular wa.-> early in the field, but was not a
practical construction on account of the parellelism of its tubes, and
i- not now advocated by its inventor or adopted by opticians ot'anv
country.

Wenham's Stereoscopic Binocular. — All these objections are

overcome in the admirable arrangement devised by the ingenuity of

Mr. Wenham, in I860 (Trans. Microscopi-
cal Soc. of London, vol i. ]S~.S. p. !."»). in
wlio>e 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. 7-!. so
placed in the tube which carries 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 eve-
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. 7,'}, 74, •«.), on its entrance into the
prism, is scarcely subjected to any refrac-
tion, since its axial ray is perpendicular
to the surface it meet.-, ; but within the prism it is subjected to two
reflexions ai // and c, which send it forth again obliquely in the line

1 1. K

FIG. 73. — Wenham's prism
(I860).

FIG. 74. FIG. 75.

^ enhnjn copic Iminrular microscope i IM;U

''"• eye piece of the secondary or left-hand body (fiy. 74.
at its emergence it - axial ray is again perpendicular

WENHAM'S BINOCULAR PKLSM 99

to the surface of the glass, it suffers no more refraction on passing-
out of the prism than on entering it. By this arrangement the
image received by the right eye is formed 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 ri>//it
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 ,-i milled head, as shown in
fig. 75. Now, although it may be objected to Mr. "VVenham'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, (6) by increasing the
tube length of the direct tube ; and (2) that the picture formed 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 emu
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
sufficiently 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. WenhamV
one good and one indifferent 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 for
some length of time together), which results from the convergence
of the axes of the eyes at their usual angle for moderately near
objects ; secondly, that this binocular arrangement 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, the entire cone of rays passing uninterruptedly into it ;
and thirdly, that the simplicity of its construction renders its de-
rangement almost impossible.1

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

IOO

VISION WITH THE COMPOUND MICROSCOPE

Flu. 70.— Riddell's binocular
prisms, as applied by Mr.
Stephenson (1870).

Stephenson's Binocular. A new form of stereoscopic binocular
ha-, been introduced l>v -Mr. Stephenson,1 which has certain dis-
tinctive features, :iii<l ;it 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 Ricldell'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-
M-ription and completion of his binocular.2
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 twro 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-
rangement, the observer looks through
' hem with more comfort. But Mr. Ste-
phen M in'singeniousarrangementisliable
to the great drawback of not being
convertible (like Mr. Wenham's) into
an ordinary monocular by the with-
draual of a prism, so that the use of
this form of it \\ill be probably re-
stricted to those who desire to work
with a binocular when employing high
powers.

I '>ut one of the greatest advantages
attendant on Mr. Stephenson's con-
struction is iK capability of lieing combined with an erectiny
which renders it applicable to purposes for which
""" binocular cannol be conveniently used. |',\- the in-
• '•' I'l'ine silvered mirror, or (still better) of a reflecting

| can !).• no doubt, be might have lur-vly pro-
lusi

I rol. vii.(l!S7'2), P. 107.
l 1.

Fio. 77. — StepliriiM.n's i'ivctiii<_'
i --70).

STEPHENSON'S BINOCULAE

IOI

prism (fig. 77), above the tube containing the binocular prisms,
each half of the cone of rays is so deflected that its image is reversed
vertically, the rays entering the prism through the surface C 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 fif. 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 operations ;
indeed, the value of the erecting binocular consists in its applica-
bility to the picking out of very minute objects, such as J)iatoms,
Polycystina, or Foraminifera,
and to the prosecution of
minute dissections, especially
when, these have to be carried on
in fluid. No one who has only
thus worked monocularlij can
appreciate the guidance derivable
from binocular vision when once
the habit of working with it has
been formed.

Tolles's Binocular Eye-piece.
An ingenious eye-piece has been
constructed by Mr. Tolles (Boston,
U.S.A.), which, fitted into the
body of a monocular microscope,
converts it into an erecting stereo-
scopic binocular. This conversion
is effected 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 centred 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 Weiiham 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

1 See American Journal of Science, vol. xxxviii. (1864), p. 111. and vol. xxxix.
(1865), p. 212 ; and Monthly Microsc. Journal, vol. vi. (1871), p. 45.

FIG. 78. — Stephenson's erecting
binocular (1870).

102

VISK'N VTITH THE COMPOUND MICROSCOPE

Pi

P2

A form of this binocular eye-piece was made by Professor Abbe
with the intimity 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 tin- \Venham
binocular could not well
:i]'ply. the doulilc image
in the right-hand tube
was most conspicuously
apparent, greatly inter-
fering \\ itli the perfection
of the .stereoscopic image.
On this account chiefly it
has not come into general
use. \V e a re 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 affirm that
it furnishes an accurate
solid and withal an erect
image, so that for all the

T

I'h,. 79.— <

's binomial1 microscope (1897).

purposes for \\ hich the use of the binocular is at present desirable it ac-
uliat is Bought, and will be found invaluable for zoologists.

botanists, and enilu-yologists. The microscope is shown in fig. 79,

GEEENOUGH'S BINOCULAR 3IICKOSOOPE

IO-

II

ill!

and has been constructed by means of a combination of Porro prisms
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 through so many prisms, 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 rays of light in passing
from the object to the eye undergo four succes-
sive reflexions at the surfaces of the prisms and
emerge from the last prism with undiminished
intensity. The prisms, it will be seen, have the
effect 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-ylass ; but two complete microscopes,
each having its own objective and eye-piece^,
are simultaneously directed upon the object.
This secures perfect stereoscopic (orthomorphic)
vision, but of course no power higher than
H inch can be employed. The path of the rays
is more clearly seen in fig. 81, giving a diagram FJC;-
by Mr. Nelson with one of the prisms turned
round 90° to make clearer the action of the
prisms on the ray. It is well to note that,
when two of these erectors with a double objective binocular 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 Artificial is ' (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 most advantageous use
when applied either to opaque objects of whose solid forms we are
desirous of gaining an exact appreciation or to transparent 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 iipon 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 by
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 aperture
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

u

80.— Showing th«-
combination of prisms
and the path of the
rays (1894).

104

VISION WITH THE COMPOUND MICROSCOPE

plane of the |iri'ii;ii-:ili(Hi. Mini tin- deilected image by one whose axis
is inclined about a fourth of the angle of aperture.

\\'ith low powers, which allo\\ of a relatively considerable
depth-perspective, tin- slight difference of inclination, which remains

in the latter case, is quite sufficient to
produce a very marked difference in
the perspective of the successive layers
in the images. But with high powers
the difference 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 illumination 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 more 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 trans-
mitted light, it follows that under
these circumstances the difference of
the two images is founded, not on the
whole aperture-angle of the objec-

make the path of the rays clearer, live, but on the much smaller angle

of the incident and directly trans-
mitted pencils, which only allou- of relatively small differences
of inclination of the image-forming rays to the preparation. It is

exident, houever. that when objectives
of short locus and correspondingly large
angle an- used, a considerably greater
diiferentiation of the two images with re-
spect to parallax can lie produced 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>\ one of the pencils.
This kind of double illumination, though
it cannot he obtained by the simple
mirror, can he easily produced by using
with the condenser a diaphragm with two
-'). l'].-"vd in the diaphragm stage under the coii-
We then have it in our power to use, at pleasure, pencils
nai>rower or \\ider aperture and of greater or less inclination

FIG. 81. — Simpler illustration of tin-
path of the ray with one prism
turned through an angle <>t' '.MI t,n

FIG. 82.

o!

POWELL AND LEALAND'S HIGH-POWER BINOCULAR 1 05

FIG. 83.

to — which was origi-

towarcls the axis by making the openings of different width and
different distance apart.

With diaphragms of this form (which can easily he 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
lie 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

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 using 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 suffer ; but if these conditions are satisfied it
yields most striking stereoscopic 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 micrpscopist 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. Tn the place
usually occupied by the Wenham prism, they in-
terpose an inclined plate of glass with parallel sides,
through which one portion of the rays proceeding up-
wards from the whole aperture of the objective paxM-.-
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 ray.s
being the fainter, yet there is marvellously little loss of FIG. 81. (1805.)
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,1 and subsequently by Dr. Schroder,
1 Transactions of the Microsc. Soc. N.S. vol. xiv. (1866), p. 105.

106 VISION WITH THE m.AI POUND MICROSCOPE

for securing binocular vision with the highest powers. We have used
the latter »f these with perfect satisfaction, but all that is required
is .-.t th.- disposal of the student in the arrangement of Powell and

Lealand.

To those who have used these forms of binocular habitually it
has li.-en a frequent source < .f surprise and perplexity that, Although
tlieor.-tically such a form as that of Powell and Lealand's is noii-
stereoscopic, vel objects studied with high powers have appeared as
jfjn relief, ind tin- effect upon The mind of stereoscopic vision has
been distinctly manifest. The Editor was conscious
J _ of tliis for many years in the use of the Powell and

Lealand form, with even the ^Vth of an inch power
of the achromatic construction ; at the time he inter-
preted it as a conceptual effect ; but it always arose
when the pupils fell upon the outer halves of the
Ramsden circles. The explanation, Dr. A. C.
Mercer considers,1 is due to Abbe. Since (fig. 85)
\\ hen the eye-pieces are at such a distance apart that
the Ramsden circles correspond exactly with the
pupils of the eyes, centre to centre, the object appears
tlat. Hut if the eye-pieces lie racked down, so as
FIG. 85. to be nearer together, the centres of the pupils fall

upon the milcr halves of the Ramsden circles and we
have the conditions of orthoscopic effect ; while if they be racked up
so as to be more separated, 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 reader to com-
prehend with intelligibility //"• J>I-!H<-!/>/I>K of theoretical and applied
optics as they relate to the microscope, we believe we shall serve
the higher interests of microscopy, and the wants 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 <i </< n< rnl nut/in,- <i,nl then <m application of the famous
dioptric investigations of (,'nn.^, an eminent (lermaii mathematician,
who, amongst many other brilliant labours in applied mathematics,
expounded the IHH-H ,,/' //,,• /•,//-,/, •/;,,// ,,/' //',//// in tin' case of a co-axial

ni/stem i>/ x/Jit'i-ii-H/ xiirf<«-<>n, Imrnnj ni<><H<i of rur'ninx i-<f /•</<•! i re in-

s li/iiKJ I" In; ' n lln-ui .

Although the assumptions upon which the formula' of (Jauss
are Qol rnincident \\ith the conditions presented by the leiis-
combinations \\hich are employed in the construction of modern
objectives of greal aperture, the results, nevertheless, furnish an
adinii-al)le presentation ,,)' the path of the rays and the positions of

cardinal points, even in the microscope as we know and use it.

\\ <• remember that the microscope is largely used in England

""I America b\ men \\hocaii only einplov it in their more or less

I recessions from professional and commercial pursuits, but who

ten employ ujth enthusiasm and intelligent purpose. .Much

< i . ii. \ ill. ji. p. -271.

DIOPTRIC INVESTIGATION BY GAUSS

IO7

scientific work may be done by such men, and it will promote the
.-icromplishment 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
witli 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 N' (fig. 86) be
the spherical surfaces of a lens
of density greater than air, and
let P R S p 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 meet the perpendiculars
through C and C; in A and A'.

Let C R=r, C' S=»-',2 /*=
index of refraction out of air
into the medium. NN'=fZ, the
thickness of the lens. N R=6,
N' S=//. These may be con-
sidered as straight lines.

Let the equation to
'

P R be

- (1)
RS be

• (2)

1 This figure is greatly exaggerated for
- If either of the curvatures be turned
corresponding r must be changed.

Let the equation to
y — 1, =m'(.r—O N)

in

^akc of clearness.

the opposite direction the sign of

10S VISION WITH THE (.'<LMP<»rM» .MICROSCOPE

y-b'=m'(x-OW) . (3)

Let the equation to Sj» bey- 6'=m//(a;—O 1ST') . . (4)

Prom (2) and (.".)

//_/,=„/' (ON'— ON) = »//'X'X = >//fZ .
Now sin C B. A=;u . sin C R B ;

°

(' A .-iiid (' I! are the v.-ilues of // in equations (1) and ('2)
.'==( ) C ;

.-. C A=& + ™(OC-ON) = /' + m r;

ami >imilarlv CB=6 + ?>(/?';

.-. (ft+mr) sin C A R=/t (6 + m' »•) sin C B R.
Nou CAR, GBR do not in general differ much from each
other. xi that for a first approximation we may consider them to be
equal.

/. k -\--JH r=n(h-\-m' ?•), i.e. u. mf=m — — . f>.

r

= «•; then ju m'=m — b n . . . ((>).
Similarly, sin C' S B'=p . sin C' S A' ;

^ • «n C' B' S=^ - sin C' A' 8 ;
and, as before,

C' B'= / + m" »•', C' A'= // + //// »•' from equations (4) and (:•}) ; •
.'. as before we may lake

Let ~ = "'- tll('n M in' = m" — !>' >•' . (7)

Krom (-)) and ((}) // = /, + '"-'' ",/=A ^ ^ «*

/' \ M / fJ.

this and (7) m"=u m' + b H' (\ - <f "} ™_^«'

V A' / " A*

and t'roin ((}) =„, — }, „ + /, M/ 'i r/ " \ . ;//

/'

Assume

'^A , '7 "' <l U Uf

.—''• ' — ,=.'/-!+ =/, »/ — // -

* /'

t'"'11 I>'—<J /> + /! ///

^ be the coordinates of P, the -point from wliidi the
r.-'\ "i bghl proceeds :

6=Y-«j (X-O.X, :

DIOPTEIC INVESTIGATION BY GAUSS
substituting in (8)

whence

"' = , — , ,^ —

b'=y Y + m (h—u .X-OK) ;
m"=k Y + m (l—k X— O X) ;

A-i/(X-oX)

_/. (x-oxT^ ' *' = fj Y + ^m"~1c ^

Xow sul)stituting in (4) the equation to the retracted ray
becomes

or by (8)

Y

^O X)

--
_ ;, x - O

0')

First: If X be taken such that I— /,' (X -< > X) = 1 . i.e.
X = < > X - LJ=0 E. suppose ;

when
.,-=OV

suppose.

//=Y, or P and p are equally distant from the axis.
Also, if Y = 0. // = 0 ; or if a ray proceed from E, it will after

refraction pass through E'. Also m =

m" _

,.-— - = in", that is,

,.-— -
< — 4- (X — OX)

the ray will be equally inclined to the axis before and after refrac-
tion.

E and E' are called the • principal points.'

du'

/ \ TT> A "V

} -I a

= ( ) XT •
i \ Tr' t'\ "V

k d ti id

n' — a —

du'

/< ("' - ") — d uit,' '
du

l~lj 0-NT/-I M

( > ji< — »J IV

t\ "V/

/.' , <1 II H

n — u — -
t*
d u

(a' — //) — il a n'

Secondly: If m" = 0, or the ray be parallel to the axis after
refraction, we have from (8)

!>•=.— -. m, and the equation to the incident ray become-

K

+ . in = in (x — O X). or // = m ( x -

K \

< > X -

110 VISION WITH THE COMPOUND MICROSCOPE

dvf
i ) X — ( ) X + 7 1

•. when i/ = <>...• = UlN - j- (£«?</

?t — u —

= (.) F. suppose.

If „, = o. or the ray be parallel to the axis before refraction, we

have t'roiii (<s)

/ nd the e( (nation to the refracted ray beconu

/, __ <t 0 — , »'- , «

es

, when „ = ,,. ., = O N' - | = O N' - -

/' — //
= 0 F', suppose.

^ -,n.l F' are called the ' focal points.'

t + d «'

ii — d it

OF/ = ()N" - p („>-<,) -du,
The focal distance -/= 0 F - 0 E = O E' - O F'

_ _

yu (»/ • - ") — dull' /.'

Similarly, it may l»- >lio\vn that if there be two lenses, and sub-
si-i-ipt numbers ivf.-i- t.> tlu- first and second lens respectively, while
E, E', F. F' refer to the entire system, and if

<• =()E, — OE/,

=°

/'•2 ' ''l

~

r, + /x, r._, + <

= OE1+ £Lfe+-*

//._, (^, + ?<-,)
2 »i + A'i r-2 + Sf{ f

\\'c are no\\ prepared to /'•,,,•/• <>nl an I'.i-n in/>l<' <>f lit,- f,'(diss sy
\<\ tiacini; a ray through two or more leii.scs on all axis, showing how
an\ ronpipitc may 1 >e I'ound 1 lirou^h t \vo or more lenses on that axis.1

I;, lemberi] ..... m object, aaid thr iissunifd finulitiuns nt -ionic for whom we
UTI; nut lii'sit.ili' bo 1'ivf.iri- thi^ with the following imtc-- to ivuiind the

ed to certain mathematical expressions.

mi-mi-, iniiint-, \ plane surface of a l<-n^ is considered a spherical surface of
an iiitiniti- i-iuliiis. \M' ininilii-r divided l>y d any number divided byO = o^;

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 .<• // are given. Xo. 1 is
a double convex of crown ^ inch thick, the refractive index /j. being
:;. the radius of the surface A is | and that of B 1 inch. No. 2 lens
is a plano-concave of flint yL inch thick, the refractive index jj. being
?, the radius of the surface 0 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
regard to each lens. These points are called principal points (see
PP', QQ' in fig. 87). When the radii of curvature r and /•'. d,
the thickness, and p{ f.i.2, 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 found.
Thus by applying Professor Fuller's formula* to lens 1 the distance of
P from the vertex A can be determined — seep. 115 (i) — similarly P'
from B — p. 115 (ii). In the same way the points QQ' from C and I)
in lens 2 can be measured off — (v) (vi), pp. 115, 1 Hi.

It must be particularly noticed that in measuring off 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 radius
of IJ, 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 °c.

The following are the rules for the treatment of algebraical signs :

In the multiplication or division of like signs the result is always pi 'us; but if

the signs are dissimilar it is always minus.

In addition, add all the terms together that have a plus sign ; then all the term^

with a minus sign ; subtract the less from the greater and affix the sign of the

greater. Example :

+ S-4 + 2-5=-4.

In subtraction change the sign of the term to be subtracted and then add in
accordance with the previous rule. Example :

-8
+ 2
_ g

An example occurs in the annexed equations (x) and (xi), p. 116, of — + - = +r
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 fj. and d, because when the
principal 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 VJsloN WITH THE COMPOUND MICROSCOPE

their respective lenses, but it dues not follow that they will do so in
every in-tame. In some forms of menisci, for example, they will fall
outside 111" lens ;d together.

\\"itli regard to the focus of the lens it follows the same rule ;
thus, /'in lens 1 is measured to the left from P, and/7 to the right
from P'; similarly in lens 2, /" is measured to the right from Q,
and/"' t.» the left from Q'.

Having determined the focal length of each lens, the distance
bet ween the right-hand principal point of the first lens P' and the
left-hand principal point of the second lensQ must next be found. It
manifestly i> the distance of 15 from P' + the distance B C between
the It-uses. () being at the point 0. Therefore,

P' Q=-21 + -25 = -46 = c.

\\"hen these thr lata have been obtained — that is, the focal

length of each lens, and the distance between them — we are in ;i
position to apply the formulae (ix) and (x), p. 116, to find the principal
I mi) its K and E' of the combination.

In .-.electing 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
+ •947, 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
•<i22 inch to the left from Q'.

<!> also is 1-28 to the left from E, and <pf 1 -28 to the right from E'.

These four points, E E' and (j> <j>', 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 travelling 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 .»-.

LetoM'ijual the distance of the point a: from the focal plane <f>,
•and // the distance of its conjugate from tj/. Then by formula (xiii)

= </;2, and as .- = I inch. .// = = ] -<i384.

This numerically determines the position of the conjugate plane.

If the rays incident on the combination are parallel, then x= oc.

= (». which means that // is coincident with <//.

following is the graphic t hod of finding t he conjugate of

. lig. 87, draw .-. line parallel to the axis to meet E', and

•point vhereit -ts K/dra\\ a I ine t hron-'h X. the point

\\ hen- •/ cuts 1 In- axis, to \Y.

another line tlmm-h .M.the point where 0 cuts
.ami from the point where il meets K draw a

fche axis, cutting the other line in \V. \Y will be the
. \\ Inch was re'piired.

A PRACTICAL EXAMPLE AFTER GAUSS

If it is required to find
the conjugate of a ray pass-
ing through three lenses on
an axis, two of the lense>
must be combined and their
four cardinal points found.
The principal points
and the focal length of the
third lens must then be
calculated, and then com-
bined in their turn bv
formula? (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 fresli
set of cardinal points is de-
termined in this manner
for the three lenses.

So also with four lenses ;
the cardinal points of each
pair being found, they are
combined by the same
formula?, and new cardinal
points for the whole com-
bination of four lenses arc
obtained. Similarly, the
cardinal points of five, six,
or any number of lenses
can be found and the con-
jugate of any point localised.

Finally, no one need be
discouraged by the appear-
ance of the length 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, out.

In lens No. 1, for in-
stance, the numerators of
the fractions are all very
si m pie, and the denomina-
tors of the four equations
are all alike ; so, too, in

^

114 VISION WITH THE COMPOUND MICROSCOPE

tin- equations I'm- No. •_! :mrl in those for both lenses. Further, / is
tin- same .-i-./'"../'"' as /". ami <t>' as (]>. Hence the problem is much
shorter tli.-in it looks.

If the conjugate of a point on the nxis is only required, and if
tin- principal points and foci of each lens have been determined, it
will not be necessary to enter into the further calculation to find E.
K and 0, ijt/, tin- cardinal points of the combination,

The method of procedure is as follows : If ,'• is the given point.
its distance from/, tin- focus of lens No. 1, must first be measured.
Call this distance.,'. Then the distance of o its conjugate from the
other focus./'. supposing lens No. 2 to be removed, can be found by
formula

o x = /-, o = •- .

X

/'-' = -897, x = 1-6");

.'.o='897 = -543.
1-65

'I'll is is the distance from,/' to o.

As the distance from x to/ is positive, the distance between /'
and o is also positive ; so o is to the right off.

lie fore proceeding it will be as well to examine other possible
cases which might occur.

Suppose that x was at the point / then x would equal 0, and
o = x ; that is, o would lie at an infinite distance from /'. If, on
the other hand, the point x was to the right of / x would be nega-
tive, and o would be also negative, because f~ is always positive :
ft would then be measured off to the left of /', and the conjugate
would be virtual. This means that there will lie 110 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/7', and. by applying the same formula, find
the distance of its conjugate from /'", owing to the exclusive effect
of No. 2 lens now replaced. This distance of" may lie found thus;

P'o=P'//+/'o=-947 + -543=1-49;
P' /•"=!" B+BC+Q/"=-21+-25 + l-875=2-335;
I" /" -P'o=o/"=2-335— 1-49 = -845.

Calling this distance ( >, then, by formula // (_) = /" -, we shall find

f" 2 3' 5 15

the distance of // from/'"', which we shall call //. '/= =

(.) "84:0

-\'\ ti. which is posit ive ; therefore // lies 4" 1 1) inches from /"' to the
i ii^li' hand, //^therefore the conjugate of 03, due to the influence
ofl'oth lenses 1 and •_!. Similarly, the conjugate of a.ny point on the
axis mav lie found through any number of len.se>.

.Vo. 1 : l>dtn. Radius A. = -=»•; radius l! = — 1 = r' :

\

thicknes: ,— '^: /'= ' : 1' = principal point mea-

A PRACTICAL EXAMPLE AFTER GAUSS i 1 5

sured from A ; P'= principal point measured from B
3 ] 3

-~~-1E=2_I -2- '— /" — ]— ^ — 1 =_1-
3 ~3 ' ' r' ~ IT" 2

'

( > \ 7 f 7 l 19
/< (u — a) — a u u' = — +/"•= — o = 1'583 •

1 1

- V —

d u' 22 3

"12

= A + -158 . (i)

1 2

P/=B + ^-^-5_?= B +~|= B-^

"12

= B— -21 . /ii)

3

^L p 18
15»- -19
"12

• (iii)
3

y/ = p/_ M _p, L. = p' + !8

12

=P'+-947 . (iv)

9
Lens No, 2 : Data.— Radius C = — 8==/- ; radius 1) = x = /•':

1 8

f0", /"',/'"; thickness = ]0=</; ^ = - ; Q = principal point

measured from C ; Q'=priiicipal point measured from D.

8-l 8--l

., 1 r; g ,( 1 r

~8

64 ^1 8

~75- " 7"> —

>=((+ • 9

75

i 2

, ,r> VISION \vrni THE COMPOUND MICROSCOPE

10
o

'' '' T~\ i "r~\ -^

'~

,. . — u — —

r'^ 1(>

75

= I)_-0625 .

.

- (vi)

8

^ =0-4-— :

O+15 — 04-1-871)

Cvii1!

7.1
_8

' _ — __ O' _

64- 8

75

=Q'- 1-875 . ... (viii)

Until Lenses.— Distance apart = B C =] = -25 ; P' Q = -21 + "25

4

•4(> = r ; /'= focus of No. 1 lens = -947; /'' = focus of No. 2
],.ns= -- 1-875.

F - P = P + = P H

/' + /" -c -947 -- 1-875 — -4(5 -l-:ws

= P---314 . . (ix)

__ Q, c\/" _(), -4(i x - - l-87_5__

./'+./" " -947- -1-875 -- "46

.B- =E-

'''

./'+/'' -. -947 — 1-875 — -46

- !•" + • E' + .

/ + ./" •!• 17 -- 1-875-^46

= E' + _i1;37875 = E' + l-28. . (xii)
•'•'/= V-; // = ^ = 1^1=1-6384 . ... (xiii)

ii;

CHAPTER III

THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

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 oil 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 exhaustive 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. Mayall1 gives what we must consider unanswerable reasons
for looking upon the microscope, ' as we know and employ 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 authors, and
having an apparent or supposed reference to the employment of
magnifying instruments, have been collected and carefully criticise! 1.
with the result that all such passages can lie 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 Cantor Lectures on tlie Microm-ujif. 1S8C, p. 1.

IlS T1IK IHSTt'i;V AND DEVELOPMENT OF THE MICROSCOPE

sun's rays as a burning-glass, and that these were used 1<> produce
ignition; but there is n.» trace of suggestion tliat these refracting
-1,,],,-, could act ax magnifying instruments.

Seneca (• (^ua-t. Nat.' i. <'.. £ ~>) states, however, that 'letters
thon-h small and indistinct are seen enlarged and more distinct
tliniuuli a irlol.e of glass tilled with water.' He also states that
• fruit appears larger when seen immersed in a vase of glass.' But
IK- onl\ concludes from this that all objects seen through water
appear larger t halt t he\ are.

In like manner it could lie shown that Archimedes, Ptolemy,
and others had no knowledge of the principles on which refraction
took place at curyed surfaces.

Nor is there any ancient mention of spectacles or other aids to
\ ision. ( >pt ical 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 110 allusion is made
to even the most simple optical aids ; nor is there any reference to
any si icli instruments by any Greek or Roman physician or author.
In the fifth century of the Christian era the Greek physician Actius
-ays that myopy is incurable; and similarly in the thirteenth
century another Greek physician, 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 depart meni that could riot have been engraved by a
qualified modern engraver by means of unaided vision; and in
reference to some very minute writing which it was stated by Pliny
that Cicero saw, Solinus and Plutarch, as well as Pliny, allude to these
marvels of um -kmaiiship for the purpose of proving that some men
are naturally endoued \\hh powers of vision quite exceptional in
their excellence, no attempt being made to explain their minute
details as the result of using magnifying lenses.

These and many other instances in which reference to lenses
must have been made had the\ existed or been known, are con-
clusive; for it is inconceivable that even simple dioptric lenses, to
say nothing of spectacles, microscopes and telescopes, could have
been known bo the ancients \\ithout reference to them having been
made by many writers, and especially by such men as Galen and
Pliny.

1 '"' (iarlicsl known reference to the myention of spectacles is

in a manuscript dating from Florence in 1 •_!'.»<>. in which the

• I lind myself so pressed by age that \ can neither

1 '""' write without those glasses they call spectacles, lately in-
-'•'•<< advantage of poor old men when their sight
Giordano da Ifivalt,, in 1305 says that the invention

Smitl : L788, li vols. ii. pp. 12, I:;.

A ' LENS ' FROM SAKGON'S PALACE

of spectacles dates back 'twenty years,' which would be about 1285.
It is now known that they were invented by Salvino d'Armato degli
Arnmti, a Florentine, who died in 1317. He kept the secret for
profit, but it was discovered and published be fere his death. But
there is a singular evidence that a lens used for the purpose of
magnification was in existence as early as between 1513 and 1520,
for at that time Kaphael 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 ck»ing years of
the sixteenth century respectively.

It is, of course, known that there is in the 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
excaA^ations of Sargon's Palace at
Ximroud, 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 officials we were
permitted to photograph in various
positions, and we are convinced
that its lenticular character as a dioptric 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 maybe fairly taken as established that there is 110 evidence of anv
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. 88.

FIG. 89.— An Assyrian ' lens ' (?).

120 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

prolialile (lint bv no means certain) that Hans and Zacharias Janssen,
spectacle makers, of Middellmrg, Holland, were the inventors. But
it \\ould appear that the earliest microscope was constructed for
observing olijects bv rellected light only.

At the Loan Collection of Scientific Instruments in London in
|s7fian old microscope, which had been found at Middellmrg, was
>hown. which. Professor Harting considered, might possibly have
been made by the -lansseiis. It is drawn ill fig. 90, and consists of
a combination of a convex object-lens and a convex eye-
lens, \\hich 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 compound 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 b.

It cannot be an easy task — if it be even a pos-
sible one — to definitely determine upon the actual indi-
vidual 01- individuals by whom the compound micro-
scope was first invented. Recently some valuable
evidence has been adduced claiming its sole invention
for Galileo. In a memoir published in 18881 Professor
G. Govi, who has made the ijiiestion a subject of large and continuous
research, certainly adduces c\ ideiice of a kind not "easily waived.

Huygheiis 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 .lanssen 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 liVst microscope to Charles Albert, Arch
duke of Austria; and Sir I). Brewster states, in his 'Treatise' on the
Microscope,' that one of their microscopes which they presented to
Prince Maurice was in 1(517 in the possession of Cornelius Drebbel,
then mathematician to the Court of .lames I., where 'he made
microscopes and passed them oil' as his own invention.'

we are told by Yiviani. an Italian mathematician,
•ife of Galileo,' that -this great man was led to the discovery
the microscope from that of the telescope/ and that 'in 1612 he
senl one fco Sigismund, King of I'oland.'

At' ""u receive evidence through the researches of Govi that
invention was solely due to Galileo in the year 1(510. Professor

FIG. ltd.

' Janssen's '

compound

.

simple microscope' an instrument 'consisting
''•"- or mirror,' and l.y 'compound microscope' one 'con-

il. ii. series ii. • II microscopic composto

V.s. !'i. IV. iss'.l. p. .-71.

DID GALILEO INVENT THE COMPOUND MICKOSCOPE ? 121

sisting of several lenses or a suitable combination of lenses and
mirrors.'

In a pamphlet published 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 until 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 mine 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 vidrrr
iudicares obiectum, adhibito perspicillo, statim certo cognoscas. cssr
hunc Socratem Sophronici filium venientem. sed tempus nos docebit
et quotidians? nouarum rerum detectiones quaiu egregie perspicillum
suo fungatur munere, naui in hoc tota omnis insti'umenti sita est
pulchritudo.

' Audiueram, paucis ante diebus authorem ipsum Excellentissimo
D. Cremonino purpurato philosopho varia narrantem scitu dignissima
et inter caetera. qnomodo ille minimorum aiiimantiuni organa motus,
et senstis ex perspicillo ad vnguem distinguat ; in particular! autem
de quodam insecto quod utrumque habet oculum membraiia crassius-
cula vestitum, qua' tamen septe foraminibus ad instar larvae ferrea?
militis cataphracti terebrata, viam pra?bet speciebus visibilium. En
tibi [so says "Wodderborn to Horky] iiouum argumentuni, quod per-
spicillum per concentrationem radiorum multiplicet obiectu ; sed
audi prius quid tibi dicturus sum : in ca?teris animalibus eiusdem
magnitudinis, vel minoris, quorum etiam aliqua splendidiores habeiit
oculos, gemini tantum apparent cum suis superciliis aliisque partibus
annexis.'

To this Govi adds :-

' I have wished to quote this passage of "VVodderborn textually,
so that the honour of having been the first to obtain from the Dutch
telescope a compound microscope should remain with Galileo, which
the latter called occhialino, and that the glory 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 incontestably was his.'

I turn now to "Wodclerborn's account, published in 1610 (the
date of the dedication to Henry Wotton, English Ambassador at
Venice, is October 16, 1610), which reads thus :-

Till-: JIKfoKY AND DEVELOPMENT OF THE MICKOSCOPE

•I \vill not no\\ attempt to explain all the perfections of this
wonderful occhiale] our sense alone is a safe judge of the tiling >
which concern it. Kut \vhat more can I say of it than that by
pointing a glass to an object more than a thousand paces off, which
docs not even seem alive, you immediately recognise it to be
Socrates, son of SophroniYus, who is approaching? But time and
the dailv discoveries ol' new things will teach us how admirably the
-!a-s does it> \\oi-k. for in that alone lies all the beauty of that
in>iruiiicnt .

• I heard a few day> back the author himself (Galileo) narrate to
the .Most Kxcellent Siguor Cremoiiius various things most desirable
to lie known, and amongst others in what manner he perfectly dis-
tinguishes with hi- telex -ope the organs of motion and of the senses
in the smaller animals; and especially in a certain insect which has
each eye covered I iy a rather thick membrane, which, however, per-
forated \\itli seven holes, like the visor of a warrior, allows it sight.
Here hast thoii u 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 ne\ ertheless 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 compound microscope
in K)10, and the application of it to examine some very minute
animals ; and if he himself neither then nor for many years after
made 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 first

experiments quite forgot the microscope, for in preparing the

'Saggiatore' between the end of 1619 and the middle of October,

I, he spoke thus to Lotario Sarsi Segensano (anagram of Oratio

Grassi Salonense) :—

'I might tell Sar>i 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
\\ill see all the colours distributed in the most minute particles, and
if he will make use of a telescope arranged so that one can see very
near objects, lie \\ill see far more distinctly what I say.'

It will not therefore be surprising if. in !(>•_! i (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 C.-irdinal of S. Susanna, who at first did not

to use them, they were shown to Galileo, who was then

tome, and lie. as soon as he saw them, explained their use, as

writes to Peiresc on .May 24, adding. -Galileo told me

liad invented an occhidle \\hich magnifies things as much

SO that one sees a llv as lar-e as a hen.' '

f Galileo, that he' had invented a telescope which

SO that a tly appears as bio- ;ls a hen,

be referred to the year Kilo, and from the
'<"• amplification by the solidify or volume the

GALILEO'S 'OCCHIALE' 123

linear amplification (as it is usually expressed now) would ha\e
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
;i 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 tin-
possession of I). 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 Signer 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 .Peiresc 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 was the one in
question, for Peiresc, answering Aleandro on July 1, 1624, wrote :—
'But the occIiniJ,' 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 Imperial's letter of September 5, 1624, in which he
thanks Galileo for having given him one in every way perfect, or in
that of Galileo to Cesi of September 23, 1624, accompanying the
gift of an occh latino, 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 owliniVmo 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 T11E HISTOEY AND DEVELOPMENT OF THE MICROSCOPE

Florence on September 1':!. 1624, more than three months after his
depart ure from Home :—

• | ^.|,d your Kxcelleiicy an nrrh/tt/iiw, by which to see close the
smallest tilings, which 1 hope may give you no small pleasure and
ciilcrtainnifnt. as it dues me. I have been long in sending it. because
1 could nut perfect it before, having experienced some difficulty in
finding tin- \\a\- of cutting the glasses perfectly. The object must
lie placed on the movable circle \vliich is at the base, and moved to
see it all. for thai which one sees at one look is but a small part.
And liecaiise the distance between the lens and the object must be
iiio-i exact, in looking at objects which have relief one must be able
to move the g]a-> nearer or further, according as one is looking at
this or t hat part : t herefore the little tube is made movable on its stand
or gnide. as \\e may wish to call it. It must also be used in very
bright, dear \\eather, or even in the sun itself, remembering that the
object must be sufficiently illuminated. 1 have contemplated very
maiiv animals with infinite admiration, amongst which the flea is
nio-i horrible, 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. 8. The little tube is in two pieces, ami
you may lengthen ii 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 I h-ebbel's glass with the two
convex lenses, and have wished to make them pass as his own invention,
vhilst he had always used, and continued to use to the end of his days,
telescopes with a convex and a concave lens without showing that
he had read or in the least appreciated the proposal made by Kepler,

since I {•> | | . to use 1 wo convex glasses in order to have telescopes
with a large field and more powerful and convenient.

In any case it is impossible to form a decided opinion on such a

matter, the data failing; but the very fact Ihal from 1624 onwards

ialileo thought no more of the (H-<-lii<t!i,io (probably because he found

s powerful and lesii useful than the o,r/,W/of I >rebbel), as he

had not occupied himself u it h it or had scarcely remembered it from

year Kilo to L 624, seems sutlicient to show 1hat the occhialmo,

''"• microscope of !6ln. was a small Dutch telescope with two
"'"• convex and one concave, and not a reduced Keplerian

cope like that invented by I Mvbbel jn |r,-j|.

I'b" name of microscope, like thai of telescope, originated with

Academy of the Lincei, and it was Giovanni Faber who invented
a letter of his to Cesi, written April \:\. 162.1. and
the Lincei letters iii the possession of I). 15. llon-
Here is the passage in Faber's letter :-

this more to vour K.xcellencv. Ihal is. that

GALILEO THE INVENTOR OF THE MICROSCOPE IN 1610 125

you will glance only at what I have written concerning the new in-
ventions of Signer Galileo ; if I have not put in everything, or if
anything ought to be left unsaid, do as liest you think. As I also
mention his new occhiale to look at small things and call it micro-
scope, let your Excellency see if you would like to add that, as the
Lvceum 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 Koine who had one. As soon as Signer Kikio'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 u>cd
by ^oddeTborn, perspicillum, •.-ignified at that time, it is clear,' Rezzi
>ays, ' no other optical instrument than spectacles 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 Wodderborn'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, 1611.
If. therefore, the word microscope had not yet been invented, and
if the telesc >pe. or the occhiale as it was then called, was by all
named perspicittum, one cannot see why Wodderborn's perspicillum
cannot have been a cannocchicde (telescope) smaller than the visual
ones, so that it could easily be used to look at near objects, but yet
a citinioccJiiiili' 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
• admiral lilis huius perspicilli,' that is, of the telescope in the iii-t
line, should then have called perspicillum a single lens in the eleventh
line of the same page ? Rezzi's mistake is easily explained, remem-
bering that he had not under his eyes Wodderborn's essay, but only
knew a brief extract reported by Venturi.

It thus appears as in the highest degree probable that (ialileo.
in 1610. was the inventor of the compound microscope ; it was
subsequently invented, or introduced, and zealously adopted in
Holland; and when 1 Mitch invention penetrated into Italy in 1624
< J-dileo attempted a reclamation of his invention (which was undoubt-
edly distinct from that of Drebbel) ; but as these were not warmly
seconded and responded to abroad lie 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 JIISToKY AND DEVELOPMENT OF THE MICROSCOPE

that .-liter tin- vear 100(1. minds having reopened to hope and in-
tellect- In study, there began to dawn some light of science, so that
in I'J7(> a Kranciscan monk, Roger Bacon, of Ilchester, in his ' Opus
.M.-iju-,' dedicated and presented by him to Clement IY., could show
maiiv marvellous tilings, and amongst these the efficacy of crystal
leii-e-. in nrder ID show things larger, and in this wise he says make
nf them -an instrument useful to old men and those whose sight is
weakened, \\lio in such a way will he able to see the letters suf-
licientlv enlarged, however small they are.' As long as no documents
.•interim- to him are discovered, Roger Bacon may be considered the
first inventor of convergent lenses, and therefore of the simple mlcro-
. however small the enlargement by his lenses may have been.
A.-, however, that man of rare genius, the initiator of experi-
mental physics, had brought 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 1 733 by the care of Samuel Jebb, a
learned English doctor.

A Florentine, by name Salvino degli
Armati, at the end of the thirteenth century
(? 12HO) (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 Armati and Spina
really discovered more than Roger Bacon had
discovered ; that is, they found out the use
of converging lenses for long sighted people, an'd of diverging lenses
for short sight. whilst the Knglish 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
keep them lirm in front of the eyes, or to fasten them into two
<-irclc> ""••"It- of metal, or of hone joined by a small elastic bridge
' ' ' l|l(> 11I1S('- Ih'uever it may he, the discovery of spectacles, or,
1 III;IV '"• '•.•died, of the W////-A ,j,;,Tt»<ci>/>,', may be equally divided
Roger Bacon and s.-dvino degli A.rmati, leaving especially to

I he latter t he invent ion of spectacles.

'he earliest known illustration of a simple microscope is given

cartes in his ' Dioptrique ' in Hi:'./: ii-. «»l reproduces it. It

identical with devised by Lieberkiihn a century

Fio. S)l. — Descartes' simple
microscope with reflector
(1687).

' 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 reflector. Descartes apparently devised
another and much more pretentious instrument, but it appears im-
practicable and could never
have existed save as a sugges-
tion. But he appears to have
IK -en 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.

FIG. 93. — Campani's microscope (1660)?

are two small microscopes which
it is affirmed have been handed

down from generation

to gene-

ration since the dissolution of
the Accademia del Cimento in
1667, with the tradition of
having been constructed
(Galileo. They are shown

by

111

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 1642 ; and there is a specially interesting compound

US TJIK IllsTuKV AXI) DEVELOPMENT OF THE MIGEOSCOPE

micron-ope, by (iinseppe Campani, which was published first in 1686,
\\hich is presented in fig. 93 ; its close similarity to 'Galileo micro-
scopes' is plainly apparent, making- it still move improbable that
these cmilil !>•• given a date prior to 1642.

In a journal of the travels of M. de Monconys, published in
I iiii."). there is a description of his microscope which is of much
interest. He .Mates 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
'•i-ht lines; the focus of the third lens, one inch and
eight lines ; and the distance from the eye to the third
lens, eight lilies.

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 lau of Viselius,' it becomes probable in a
very ///<//> </t*i/ree that to him must be attributed
the earliest device of a microscope, with a field-
lens.

In 16(55 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

94 -Hooke's '•.

miiTi>M-i>i>r i lf.C.."ii.

DIVINI'S COMPOUND MICKOSCOPE

129

1

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 spherical 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 whenever
I had occasion to examine the small parts
of a body more 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 movement to the body, of
which he writes : • On the end of this arm
(D, which slides on the pillar C C) wa>
a small ball fitted into a kind of s.icket
F, made in the side of the brass ring ( • .
through which the small end of the tube
was screwed, by me.iiis of which contri-
vance I could place and fix the tube in
whatsoever posture I desired (which for
many observations was exceedingly neces-
sarv), and adjusted it most exactly to any
object.'

It need hardly be remarked that, useful
as the ball-and-socket joint is for mam-
purposes in microscopy, it is not advan-
tageously employed 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 166H 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
and adjustable to four different

lengths by draw-tubes, giving

range

FIG. 95. — Divini's compound
of microscope (1(568).

K

I ;o THE HISTOKV AND DEVELOPMENT OF THE 3IICKOSCOPE

magnification from 41 to 143 diameters. Instead of the usual bi-
convex eye-lens, t\\o plano-convex lenses were applied with their
convex surfaces in contact, by which he claimed to obtain a much
Hatter field. .Mr. M avail found in the Museo Copernicano at Rome
a micro.-cope answering so closely to this description 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.

Cherubin d'Orleans
published, in 1671, a
treatise containing a
design for a micro-
scope, of which fig.
96 is an illustration.
The scrolls were of
ebony, firmly at-
tached to the base
and to the collar
encircling the fixed
central portion of the
body-tube. An ex-
terior sliding tube
carried the eye-piece
above on the fixed
tube, and a similar
sliding tube carried
the object-lens below,
these sliding tubes
serving to focus the
image and regulate
(within certainlimits)
the magnification.
He also suggested a
screw arrangement
to lie applied beneath
the stage for focus-
sing. He devised, or
recommended, seve-
ral combinations of

'roans' compound microscope lenses for the optical

part of the micro-

to combinations of three or four separate lenses
ibjects could be seen erect, which he considered -much to
be preferred.

ented " binocular form of microscope and published

La Vision Parfaite,' in 1677. It consisted of two

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 focus) was ground away to allow the
convergent axes starting from the two eyes to meet at about 16
inches distance flt the common focus. Mechanism was provided 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 diagram (' La Vision Parfaite,' tab. i. fig. 2, p. 80). Accord-
ing to the arrangement of the lenses as shown in the figure a pseudo-
stereoscopic image would have been obtained.

A drawing of this binocular, as known to Zahn, was given in
the first edition of his ' Genius Artificialis ' in 1685 (Fundameii III.
p. 233), and is reproduced in fig. 98.

K 2

132 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

In H>7i! Sir Isaac Newton communicated to the Royal Society

.•I n ite and diagram tiir M reflecting microscope ; we have, however,
no evidence t hat it WMS ever c instructed. But in 1673 Leeuwen-
h<iek lie-Mil tn .send tuthe Royal Society his microsci >pical disci >veries.
Nothing WMS known of the n instruction of his instruments, except
that tliev were simple microscopes. even down to so late a period as
17"'.). We know. h<>\ve\er. that his microscopes were mechanically
rough. ami that optically they consisted of simple bi-convex lenses,
with worked .surface.-. mounted lietweeii 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 t ithe Royal Society ; unhappily, they have mysteriously

disappeared. But Mr. Mav
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, 011 the point of a short
rod, with screw arrange-
ments for adjusting the
object under the lens.

Many modifications of
this and the preceding in-
struments are found with
some early English forms.
but no important construc-
tive or optical modification
immediately presents itself'.
hut some ingenious arrange-
ments are found in the
simple microscopes devised
by Musschenbroek in the

FIG.

Leeuwenhoek's micros,-,, („• (ic,;:;,.

early years of the eighteenth
cent ury.

figured a microscope in his ' Mierographia Nova' in
which optical modification, arise. Divinj had. as was

'ombined two pla sonvex lenses, with theirconvex surfaces

'"'•"i an eye piece: this idea was carried further in l(i(>H

111 optician, who used two pairs of these lenses; Grindl

3°! linl HI addition be used two similar (but smaller) lenses

> manner as an objective. The form of the microscope

'"'I ' from thai ofCherubin d'Orleans (fur. 97), but was

f the application of an external screw.

tonannus modified preceding arrangements by dovisinj;
•bjecl between two plates pressed away from

spiral spring, the focussing being then effected
barrel.'

134 T'1K

AND DEVELOPMENT OF THE MICROSCOPE

Tlii.- >v-tem of focussing was employed in a more practical form
I >y llart.soeker in 1 (V.I4 and was adopted by Wilson in 1702. It
became a very popular tin-in for the microscope in the eighteenth
century.

We are indebted to Uonannus also for originating a horizontal
form <if microscope, \vhich is interesting and which, in a drawing of
the instrument. j> shown to possess a sub-sta<^e rniii /inn ml comleii.^ /•
fitte<l I'-it/i fnriixx'nKj iiri-iinyements for ilium iimtiinj trn,i^^u,-t-nt
ol'jfcts. There was ure.-it convenience in using the microscope in a
horizontal position with a lamp and condenser in the same axis.
'•-pecially as all the compound microscopes previously constructed
had l>eei i employed vertically, or had been directed towards the sky
for purposes of illumination. Remarkably crude as the mechanism
appeai-s. it is a \ery early instance of the use of what has become—
though slowly and late on the continent — a now universally acknow-

H

'•''<•• 1"- Hartsoeker's simple microscope (1694).

ledgeil optical arrangemenl indispensable for the best results, viz a

ompo and condenser fitted with focussing mechanism for illuminating
ransparent objects. The picture of the entire instrument is shown
in d.i,'. KM.

In Eartsoeker's microscope -the lens-carrier A IJ. fii>-. 102 (on

', containing the lens, is serened), screws into the

);" OQ; the thin brass plates Hand K tit within the

SCUl out allowing them to slide,,,, the short pillars

tl»- spiral sprin- pressing them towards C D ;

or an animalcule cage G II (hinged at a 6 to allow

, enclosing the objects between strips of talc),

the P^tes Band F when in position, and the " screw-

•thescrev sockel C hand regulates the focus-

N", tits, on , second "scre^ barrel," L M,

111 the screv -sockel of I K. This arrangement ,,I

HABTSOEKEB'S MICROSCOPE

135

the condenser is better than the plan adopted by Wilson, as it allows
the illumination to be focnssed 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
twro plates there is a rotary multiple
object holder shown in fig. 103A M
IS", the object being inserted in the
apertures in the circumference of
the disc. Focussing is accomplished

FIG. 103 (1702i.

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 rotating wheel of graduated diaphragms
A, C, I), E, placed on the .^idc 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-

JOHN MARSHALL'S

tf:u urulfr y

New Invented

DOUBLEMICROSCOP

For Viewing the

CIRCULATION o

Ma-Jc &.-.Sold bv him a( (Kc Archimedes &s
Goldiu Spcx-lacles in Lud«a(e Street.

\v

'"I 1701 .

HERTEL'S MICROSCOPE

137

FIG. 105.— Hertel's microscope (1710).

138 THE HISTOIJY AND DEVELOPMENT OF THE MICEOSCOPE

scopical construction \\ere here ciiil >i idled. (1) A fine-adjustment
screw !•' i- connected with the sliding socket E, supporting the arm
\>. in \\hich ilit- body-tube is screwed; the focussing could thus be
controlled in ,-i far mure effective manner than by any system pre-
viously applied to ,-i l;irgc microscope-. The previous systems involved
the direct movement of the hody-tube either by rotating in a screw-
socket (a> in Hooke'>) or 1 iv sliding in a cylindrical socket (as in
Divini's and Cherubin's) ; in a few instances the object was moved

'' ''•

i;- M. Joblot's inicni -r,.p(. (1718

lation to the objed lens, but all these plans were more or less

vrith microscopes of large dimensions. Marshall's

"1|H mechanica] improvement, for the object could

luring the actual process of focussing, as the ima<-e

«"Klj in the field. (2) A fork. X X. is here applied

'lamp, 0, on the pillar itself. (3) Hooke's ball-

ww applied to the arm I. is here shifted to

; Pillar, where i, would give the movements of

'microscope instead of to the body tub,- only,

DE. LIEBEEKUHN'S MICROSCOPE

139

as in Hooke's ; the 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-
denser. From the singular 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

FIG. 107. — Lieberkiilm's microscope (1739).

to this time has there been any
trace- of, or reference to, a mirror ;
but in 1716 Hertel employed it and introduced some other consider-
able modifications. The general appearance of the instrument a*
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 'butterfly1 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 t\\<>
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 plate next to the ornamental one
is a concentric rotary stage, of good mechanical
quality. The tube A was called by Joblot
' 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. IS". LieberkUhn devised,
what had been employed in principle by
Descartes a century before,1 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 reproduction
from the earliest drawing known of Lieber-
kiilm'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.

1 See pp. 1-26-7.

FIG. 108 — Culpeper and

Scarlet's microscope (1738).

140 THE H1STOKY AND DEVELOPMENT OF THE MICROSCOPE

the eve being placed behind in tlie direction D at any point the
>iiiide lens or ii coin) ihiatii in might require.

( 'ulpeper mid Scarlet's microscope requires a note, and is illus-
trated in tig. His. It was inappropriately designated a ' reflecting '
inicroM-ope. but this arose merely from the fact that it was the first
Knuli-li 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
M;irtin in his ' Micrographia Xova ' 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 t;d<en. It was called a reflecting microscope because it had
a mirror lilted 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-liarrel ' 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 ba-e
in a line with the optic axis. Fig.
ID'.I 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 mirror, and he furnished
the stage with rectangular movements.

>vas to this maker that the late Professor Quekett was
indebted for an earlj microscope, of which he evidently to the last
thought highly, and which was subsequently purchased by the Royal
Microscopical Society. A drau ing of t his instrument is given in lin.
and should be described in (^ueketfs own words. He says :
stands aboul two feet in height, and is supported on a tripod
the central part or .stem. \\. is of triangular figure, having
, upon which the stage, <), and frame, D, support-
mirror. !•;. are capable of being moved up or down. The
', is three inches in diameter; it is composed of
ner of which contains the eye-piece, and can be
l(.v rack and pinion, so as t(. increase or diminish
power. At the base of the triangular bar is a cradle

Fin. 109. — Wilson's simple microscope
on scroll *t, i IK I. ml fas made by
Adams, 1746).

QUEKETTS MICROSCOPE

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 supplied
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
turned into a horizontal position ; another bar carrying ;i stage and
mirror can be attached by the screw. L N. so ;is to convert it hit > n
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 mav 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 micrometer one, hav-
ing rectangular move-
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 lie
raised or depressed by
rack and pinion ; it is
also capable of removal,
and an apparatus for
holding large opaque.
( >1 >jects, such as minerals,
can be substituted for it.
The accessory instru-
ments are very numer-
ous, and amongst the FlG<130._Martin'S large universal microscope as used
more remarkable may by Quekett (1780J.

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 8, as well as a plate o|'
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 under the magnifying power. The lenses belonging to

142 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

this microscope are twenty-four in number; they vary in focal
length from lour inches to one-tenth of an inch; ten of them are
supplied \vith Lieberkiihns. A small arm, capable of carrying single
lenses, can lie applied at T, and when turned over the stage the in-
strument becomes a single microscope ; there are four lenses suitable
for this purpose, their focal length varying from y^th to TLth of an
inch. The performance of all the lenses is excellent, and no pains
appear to have been spared in their construction. There are
numerous other pieces of accessory apparatus, all remarkable for the
beauty of their workmanship.' '

I'.nij. Martin not only in this way greatly advanced the

;-hanical arrangements of the microscope, but he improved the

optical part. He used a Huvgheniaii eye-piece on the telescope
formula, where the focus of the eye-lens was that of the field-lens
:!. and the distance between them 2; but instead of employing a
-inu'le eve-lens he bi-oke it up into twTo of equal foci, that nearest the
eye being a • crossed ' lens, and the other a plano-convex, the steeper
convex it ies ( >f t hese 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, which possessed several conveniences and improvements. Xot
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 \\ere three designs of microscopes by George Adams, of
London, in 1 7 M> and 1771. \\hich have many points of interest, but
scarcely contribute enough of distinctive improvement to the modern
forms of the microscope to detain us long. That designed in 1771 is
figured in the Adams • M icmgrapliia Illustrata,' and is reproduced
in fig. 111.

In this Lnstrumenl Adams claims to have embodied a number of
improvements on all previous constructions. He applied "two eve-
glasses at A, a third near 15, and a fourth in the conical part between
B and C,' by uhich he increased 'the Held of view and of light ;'
draw-tub. -s were at A and I'., by which these lenses could be separated
more or Less, but the probability is very great that these were
.-imply copied from the improvements of a like kind devised by B.
Martin and described above. He also arranged the object-lenses, or
buttons,' " and /«. to be combined ; seven ' buttons ' were provided,
erspecula |' Lieberkiilms'j highly polished, each having

ignitiei- adapted to the focus of its concavity, one of which is
and the 'buttons' could also be used with 'any
our of these specula ' by means of t lie adapter. //.

-1/-"'-"-" /". :'.nl.''. 1. London, 1H55 8vo

1

THE VARIABLE MICROSCOPE

George Adams Ji06o.rtet

oVf^>

/•"*,

144 THE BISTOBY AND DEVELOPMENT OF THE MICROSCOPE

The body-tube, A B C, with its arm, F (in which it screwed at/),
;m<l stem attachment with the fine adjustment were clearly modified
from :i design which Cuft' 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 noi l>r used unless the fine adjustment was first put out of action
by urn-lamping it. The stage and mirror were adjustable on the stem.
The large ratchet-wheel controlled by the pinion-handle, 8, gave the
required inclination to the stem.

Xos. I 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.
:!] is to receive the glass tube No. 10.' No. 4 was a diaphragm called
a cone, from its conical shape; this was invented 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 extremities of the stage, or in the
socket, R, at the end of the chain of balls No. 6.' No. 6 was an arm
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-magnifier ; No. 9, a sliding arm lens-carrier fitting 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.
A B C, was removed from the ring. K ; 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. Ci. was applied at AV. by the part X.
ami the object held h\ either of the forceps could be turned and
viewed as desired. l-'or dissect ions ,Vc. the stage could be screwed
on at F, and a glass plate applied at T.

One of the best examples of this de.sign 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 l>\ hellebarre was made the subject of a
special report to the 'Academic des Sciences' in June 1777, but
there is nothing in it deserving special consideration in comparison
\\ith contemporary or even anterior forms as bearing upon the evo-
lution r.fihe microscope ; us we now know it. In fact, up to the time
len achromatism exerted so powerful an influence upon the form
jonstruction ofthe instrument, there is no microscope that calls
ier consideration save one— by an English maker named
called .lones • Most Approved Compound Microscope
'and although, in principle, it does not differ from
strument, tig. Ill, H ye1 presented differences of detail.

JONES'S MICROSCOPE

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-
tube 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
:f-(r inch focus, which
on the removal of the
lens-disc can lie
screwed into the
nose-piece.

There were also
designed some inte-
resting forms of re-
flecting microscopes,
to the details of which
we can afford no
space, their influence
having been of no
value in the develop-

JONESS MOST IMPROVED

MlCKOXi-ufE AND /teP.-lR.-lTVS

PIG. 112 (1798).

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 Oassegrainian form was made in 1738 by Smith,
which was, perhaps, the most perfect of the Catoptric forms.

An outline of its construction and the path of the light-beams is

L

146 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

in fig. 1 1 3. It was for examining transparent objects and v,-;is
-imilar to the Cassegrainian telescope, but with an extra long eye-
piece tube to permit the focussing by movement of the eye-lens.
Tin- object was placed at M X ; 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 mixture of foreign rays with those of the object,' otherwise
the instrument gave confused images of distant objects when it was
u>cd as a microscope.

Kven 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
1 iy C'uff that can only be regarded as the precursor of the most com-

A-

FIG. ll:j. — Smith's reflecting microscope (1738).

plete and perfecl of our simple dissecting microscopes : it is shown

A disc of plane glass, C, or a concave, M, was applied,

on the stage of which dissections .vr. could be made; a mirror, I,

was fitted in a -imb.-d 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 horizontal arm. E. sliding through a socket, attached

to a vertical rod, D, sliding and rotating in a socket at the back of

«• pillar for focussing A-c. This motion of the lens over the object

:came very popular and was employed in nearly all microscopes up

mif <>f the establishment of achromatism ;' the last microscope

was that designed b\ Mr. \V. Valentine and made by

'l'1"1 movement in arc lasted much longer, anil

remnant of it is still to be found in Powell's No. I.

••'"1 "" ''"' 'id of the box. within which the instru-

•ked with sundry accessories.

'he discovery of achromatism as applied to microscopic

THE ELSE OF ACHROMATISM

147

object-glasses that we must attribute the strictly 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 far
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 microscope 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 upon 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 Euler, and made comprehensible to workmen by Nicholas
Fuss.' This was translated into (Jennan by Kliigel in 1778, but 110
result of these discussions of the theory of achromatism can be
discovered earlier than 1791, when Frangois 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 HISTORY AND DEVELOPMENT OF THE MICROSCOPE

was composed of two biconvex crown-glass lenses, and a biconcave
Hint lens placed between them.

( '. ( Ihevalier tells us ' that between 1800 and 1810 M. Charles, of
the ' Institiit." Paris, made small achromatic lenses; but they were
too imperfect in lie 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 Amid constructed a
hori/.ontal micro-cope on achromatic principles, which was spoken

well of. But while
up to a very recent
date it was common
to assert that the first
to suggest the plan
of combining two,
thrive, oi' four plano-
convex achromatic
doublets of similar
foci, one above the
other, to increase the
power and aperture,
was Selliguein l«-j:!.
it is now known that
this had been antici-
pated b\ Mar/oli (ch.
v. 353). Selligue's
plan was carried into
execution 1 >y the
Messrs. Chevalier.
The instrument em
bodvinir this plan is

115.

In a report to the
Academic Royale des
Sciences, the well-
known mathema-
tician Fresnel says,
c incerniny this mi-

shown in fig

FIG. 115.— Selligue's arhnmuitic microM-. .|M' i

823

V ''llV^AAAAAAi^ I 1 1 1 ,-> Jill"

uscope. that in comparing the objectives with those of one of Adams's
""" achromatic instruments that un to a magnification of

up

"" hundred times Selligue's was decidedly
that

superior ; but beyond

magnification there was no superiority in the achromatic form.
'"• preferred Adams's form for prolonged observations because
'••<• .-i larger lie],! i |l;m Selligue's.

' chanism ,,(' t|,js mici-oscope was similar t.o the English

shown ai tig. I \-2. The focussing was by rack and

on the stage, the pinion travelling with the stage on

1 draw tubes A and 15. were applied within the

t1"' upper one having a biconcave lens. S, at the

pee, I'.-n-is, ls:;:i, ,,. 86.

MODEL STANDS "FOE ACHROMATIC OBJECTIVES

I4Q

lower end, serving as an amplifier, which was probably the first
application of a ' Barlow lens ' to ,-i microscope.

Illumination for opaque objects was accomplished by a lenticular
prism, P, which was gimballed, and connected with a ring embracing
the body tube.

We learn from Fresnel that the range of magnification was from
40 to 1,200 diameters.
The object-glasses were
composed either of two
doublet systems for low-
power work or of four
doublet systems all
screwed together for
high-power work, and
two oculars were pro-
vided of different 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
Tully the optician, of
London, who at Dr. Gor-
ing's instance had been
working ;it the achroina-
tising of the microscope.
Selligue's is a manifest

modification of one of
the best forms as made
by Adams, Jones, or
Dolloml. 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 >ul i-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, from which so

**. — Lister's achromatic microscope made \>\

150 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

much light was inevitably stopped out by the small diaphragm that
it \vas needful to use iu order to secure a fair image, the objectives
used 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
prohablv 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 Selligne's achromatic
microscope 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 exhibited 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 lie
increased.

Meantime the subject of
achromatism was engaging the
attention of the most distin-
guished English mathematicians.
Sir John Herschel, Sir George
(then Professor) Airy, Professm

Fui. 117.— C. Che\ < limmatic

microscope i i-ii ca 1 *•.; I .

Barlow, 31 r. Ooddtngton, and
several others, worked more or less :,t, the general subject. Cod-
dington alone, however, mutined his attention to the microscope,

• his work was limited to the eyepiece. AKo, for some years,

»eph .). Lister had been earnestly \\orking ex peri mentally and
thematically on the same subject .' and he discovered certain pro-
perties in an achromatic comhinat ion. which \\< -re of importance,
ough they had not been before observed.1 In I-S29 a paper
received and published 1,N the Royal Society,2
<'"• principles it laid down into ractice, Lister was

• btain a coml)ina1ion of lense
.ch. ,. p.

capable of transmitting a
i Trans, Eoy. Soc.fnv is-jii."

FIG. 118.— One of Ross's early microscopes designed by "W. Valentine (1831)

I52 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

pencil of 50° with .-• large corrected field. This paper and its
'suits exerted a very powerful influence on the immediate improve-

resi

rneiit 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 application of achro-
matism to object-glasses.
Mr. A. Ross became
practically acquainted
with the principles of
achromatism as applied
to combinations of lenses
in working with Pro-
fessor Barlow on this
subject, and having ap-
plied Lister's principles
with great success, he
discovered, as we have
already pointed out in
Ch. I.,1 that by covering
i lie object under exami-
nation by a thin film of
glass or talc the correc-
tions Avere disturbed it'
they had been adapted
to an uncovered object :
and we haA'e 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
t lie mechanical as well

' as the optical side of the
microscope. Fig. 118
presents a form of

iV.'iu .-HI extant example which was designed by W.
Val .f \ot t ingliam in March ls:!l and made by Andrew I toss.

P. 20.

IV. Pritchard's microscope n itli
fine ;i<ljii-~tnif]it 1

A ROSS'S ' LISTER ' MODEL

153

The stage is actuated in diagonal directions on either 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. 11 8, but it will
be seen that it has also a curious spiral fine adjustment, which is
plainly an uncovered ' Continental ' form, either adopted in England
from (4. Oberhauser, or it may
have even preceded 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.1

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 reference
to fig. 118. Rectangular me-

Fio. 120. — A Ross microscope fl<s:i;i'.

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 interior method. This instru-
ment dates from 1839, and is 'shown in fig. 120. In 1842 he

1 Trans. Boy. Soc. ls-21).

• '-ope, puivl.iisr.l by R.M. Society in 1841.

PRINCIPAL MODERN STANDS 155

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 lever method as applied to the
nose-piece through the 'cross arm' and brought it to a relatively
high state of perfection. But the full possibilities of this method,
as concerned its sensitiveness, were never utilised by Ross, and it
was Hugh Powell who first published an account of his long level-
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
workmanship were excellent (we 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 -gi^y 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
:idjustment i.s 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 refocnssiiig. The movement usually
required is so slight that the refocussing of the condenser is seldom
required.

James Smith also made an instrument 011 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 h;ive once secured steadiness, the crucial points in a microscope
arc the quality of the fine adjustment, and the delicacy, firmness,
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 endeavour to judge im-
partially from a practical point of view the merits of the principal
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 carefully
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 and objectives our
judgment is given with deliberation and wholly in the interests of
science.

In examining the principal modern microscopes we shall point
out whatever is of absolute importance or relative value ; and the
absence or presence of this in any form provisionally selected is all
that the reader will need to enable him to become convinced of our

Ijfi TIIK HISTORY AND DEVELOPMENT OF THE MICROSCOPE

estimate of the value <>f Mich MM instrnnient. whether the form be
illustrated ill tlic.M- ] ;ii;r> or found ill tlie catalogues of the
makers.

1 Smith's

STEADINESS OF THE MICROSCOPE 157

With this object before us we shall facilitate its attainment by
at once considering what are the essentials of a good microscope.
What are the attributes of the instrument without the possession of
which it cannot meet modern requirements ?

I. Steadiness is absolutely indispensable : this would, in feet,
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 optic 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 ha\e
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 bears 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 promises little
for the instrument as a whole.

What is wanted is the maximum of steadiness with the minimum
of weight. An old plan designed by Cuff, circa 1765, of rotating the
foot below the pillar has been frequently reinvented. It was used
by Adams 1771, by Unss IS 12. by Sidle and Poalk in America 1880,
liy 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 01- horizontal positions. An instance of this form,
made by Andrew Ross in 1 842. 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 either a vertical or inclined position.

Palpably, the mechanical compensation for the difficulty 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 instrument.

II. Next 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 di'tni--tnl>i>. In a first-class instrument this latter should always
lie provided with a rack-and-pinioii motion, and should have a scale
of from two to three inches, divided into tenths or millimetres. This
enables the operator the more accurately to adjust apochromatic ob-
jectives so sensitive, for their best action, to accurate adjustment of
tube-length. In fact, it is always important to remember that ob-
jectives are corrected for a special tube-length ; that is to say, for
the formation of the image at a certain definite distance.

1 Chapter IV.

I5-S TIIK HISTOIIV AND DEVELOPMENT OF THE MICEOSCOPE
"/•''. /iiin-f'.i't'.i; tii-<> I.'in'ls of tube-length : (1) sun. optical and

i,/>lii-iil (uli- li'injlh is measured from the posterior principal
point of the objective to the anterior jn-incipal point of the eye-piece.
Tli'- mechanical //"W<-/////// .should be measured from the top of
tin- mix- intn uhicli the t '\-i --piece fits, and upon which the bearings

Fiii. 1-23.— Old Ross stand (isfji. r.^atin^ fool l.rl,,\\ (\\,- pillar. From the cabinet

"I tin' Koyal Microscopical Snrirty.

• pi'1'-'1 resl to ill.- cud of the no.se-piece into which the
ulijcct i\ i- is scrru cd.

iiatdy ditl'fivnt makers estimate nilM'-li-nyth differently

Perenl points I'n.in \\hich to make their 'measurements.

the matter l»roadly. there are two estimates for tube-

1 IIS(1 : these are the Kn-li>h and tho Continental.

THE 'BODY' OF THE MICROSCOPE 159

What was formerly known as the English standard tube had an
optical length for high and moderate power objectives of ten inches ;
with low powers, however, it was less. The mechanical tube-length
was 8| inches.

Professor Abbe, in constructing his apochromatic objectives foi-
the English body, has taken the mechanical tube-length at 9'8
inches = 250 mni. ; and the optical tube-length 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. ' draiv-
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
I iy a slightly lower eye-piece used at a longer distance or 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 trit/t a short boa;/ tlm n n-itJt « Jong one.
Critical work is carried on in this country to ~1\ 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 J-inch objective with
a small cone is used in place of a 1-inch objective, and an oil im-
mersion jVineh objective with small cone is used to do what a J-inch
would have done. The oil TVinch 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 short
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 course, for Germans 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 HISTORY AND DEVELOPMENT <>F THE MICROSCOPE

|>reci.-iou. A -nod 'coarse adjustment' or primary movable
part of'the instrument is of great impoi-tance. The first requisite is
that tin- body or movable part should move easily, smoothly, hut
uithoiit ' shake ' in the groove or sloi or whatever else it slides in.
\Ye ha\r found in practice that a har shaped like a truncated prism
sliding in a suitable groove acts best and longest. But a bar planed
true and placed in a groove ploughed to suit, it is not enough. The

this brino-s with it a fatal

inevitable friction determines wear, and

Flu. 1-24.— Dia-onal rack and twisted pinion devised in 1881.

All such grooves, which are usually v-sli.-ip.-d. «l,rnM be

so thai by 'tightening up' the v's by

s the bar or limb isagain firmly -rippi>d. Further, the

tor its whole length along the groove but only

fcher ('1"1 ;l1111 '» the middle. IWell introduced these

-' ' •>• 'arse adjustment ' more than 60 years

•usands of instruments in which these principles

"" applied l.av, Keen, by sheer friction wear, soon

'"•'- since then! l!m instruments made bv

FOCrs.SINt; ARRANGEMENTS

161

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 us • 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
Crease' 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, .-aid wherever constant friction is incurred : equally
applicable, too, is the

suggest.

lubricant we
An instrument left
unused in its native
' grease for twelve
months becomes so im-
mobile in most of it>
parts by the hardening
of its ' normal ' lubri-
cant that motion be-
ci niies a peril to its future
if persisted in in that
condition.

If a ' coarse adjust-
ment ' be what it sin mid
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 Sou 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 micro-
scopes. The advantages gained by this method are due to the twist
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. Xelsoii has had made by Messrs. Watson and Sons a still
better form of rackwork. It is what is called a 'stepped' 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. ^\ 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
b.- in action when the bar is racked up, and the other when it

M

IS

[62 THK IIIsmiiY AND DEVELOPMENT OF THE MICEOSCOPE

racked down: so that if the racks are properly placed relatively to
.mi- another • lo.-.- 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
tMi-iu of rack is that any loss of time as the result of wear can be
taken up bv a slight alteration of the position of the second rack.
The ai-rangeineiit is ,-liown in fig. 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 -mall screw. Now the racks are set -ide by side, one being fixed
linallv. The pinion is then made to work freely and smoothly with
this one rack : the >econd 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 ri.idit-hand rack until the smoothest position of action is secured.
The clamping screws are then tightened and the rackwork becomes
fixed; and subsequent irregularity in it is at once corrected by the
-mall screw to which we have referred.

When the best position is found the teeth of the two racks, as
we have stated, will not be ii: aline, 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
•tine 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 principle in the construction of this makes not only inferior,
but for critical purposes absolutely useless, what are otherwise
instrument- of excellent workmanship and real value.

There are two kinds of fine adjustment usually employed :—

i. Those which *///////// i,icn-<>. tin- nose-piece which receives the
objective.

ii. Those which mon- tli<> »•/,<,/<• lin<l,/. or the whole body including
the coarse adjustment.

All constructions of the second das-, formerly proved impracti-
cable, and ••veu pernicious. They inevitably broke down just as 1 lu-
pin-chaser, by practice, began to realise the value of perfect action.
With a large experience of stands of every class, we are obliged to
say that generally uith 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-
side lever and Campbell's differential1 screw and
Watson's Ion-- lev,-]-, to which we shall subsequently refer.

. however, upon the model above referred to. with all its
radical and gla ri ng imperfect i, MI-. thai the majority of Continental

t-oscopes bave been built.

with an extremely line thread, and therefore of extremely
-hallow incision a micrometer -crew in (act —hastobear thestrai<»>/'

< tine adjust nt \va- first suggested by Dr. Goring in

j nui.l.- 1.- : ibout 181

IMPERFECT MODERN MODELS

163

lifting ami lowering the entire weight 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
in :iny 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?

Almost any in-
strument, however
inferior, could be em-
ployed successfully
with a 1-inch object-
ive of ' low angle'' (to
give it what has been
• •ailed 'the needful
penetration' for his-
tological subjects !) to
obtain an image corresponding to a figure in a text-book of, say,
a Malpighiaii corpuscle, or a section of kidney, brain, or spinal cord.
The quality of a fine adjustment is never tinted by these means,
tor, in point of fact, a delicate fine adjustment is not even necessary.
We write 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-hook, 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 all that it touches. But for this there must be poh-nt inliti/ (\vith-

' M 2

FIG. 125.— Eoss-Zeiitmayer model (1878).

1 64 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

nut costliness) in tin- mechanical and optical character of the micro-
scopes commended and approved.

A lo\\ -priced student's microscope of good workmanship and
|,ei-fect design could easily lie devised if the demand for it arose.
Indeed, .piite recently a certain class of students' microscopes have
l>een improved greatly : this has been a concomitant of the science of
haeterio|o-\ . \\ Inch lias compelled the use of the sub- stage condenser.
\\'e lia\e said enough of the value of this instrument in a succeeding
diaptei-. l.ut. until recent years histologists did not use it because it
was not used iii (iermany or with German instruments ! Its present
use, nevertheless, has had the effect of improving the definition
obtained by t he 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
iirinv cases are not even new, and until the introduction of the Jena
glass1 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 only the Continental
model that is deformed by the adoption of this radical error in the
'fine adjustment' with which we are dealing. Even dining the last
twenty 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 sufficiently 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 tine adjustment, is in one solid piece.
If nothing were sacrificed this would be a boon. Formerly, this
m idel was .supplied with a fine adjustment which only moved the
nose-piece, but on a principle which we shall see was wrong, and
from its impel- feet ions il was abandoned, and the solid Lister arm was
cut, and the whole body and its coarse adjustment was pivoted on the
lever of the line adjust ineiil . Thus its normal virtue (a solid limb) was
sacrificed, and a ' line adjust nient .' doomed to failure, was given to it.

A complex roller, a wedge, and a differential screw have in turn
been since employed to redeem this instrument from the failure that
had overtaken it. Part ially. or completely, each has failed. The
differentia] scre'w certainly comes theoretically nearest to success
with this form of instrument. l!ut at the outset this is the case
onl\ where it wholly abandons the lifting and lowering of the body-
bube &c. b\ the action of a ' line adjust meiit .' a ml its motion is only
brought into operation upon the equivalent of a nose-piece.

Tin- form <>f il[ij'< r, nt'ni! screw />,-/»/,//// ,',/ti, nractical operation

. Campbell, of Fetlar, Shetland, was adopted by Swift

v.n iii IX'.H, but had been exhibited in a stand made by Baker

year 1886 at the^uekett Micro. Club.- Its object is to sup-

• apter I.
n (,>.M.C. ser. '2. vol. ii. pi>. -2s:; :lml 287 (1886).

THE FINE ADJUSTMENT

I65

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. Kelson.

It is very simple, and is made by cutting two threads in the
micrometer screw. Fig. 126 will illustrate the exact method. 1) is
the milled head of the direct-acting screw. The upper part, S, of
the screw 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 revolution of ])
causes the screw thread S to move up and down in J> at the rate of

H

luiiiiiHiiiiinmmimnniiii

mm

-H

FIG. 126.— Campbell's
differential screw
fine adjustment
(1886).

Fit;. 127. — Zeiss's usual ' new ' fine adjustment (1880)

twenty turns to the inch, whilst the screw thread T causes the
travelling socket H to move in the reverse direction at the- rate of
twenty-five turns to the inch. The combined effect, therefore, of
turning D twenty revolutions is to raise or lower T, and with it the
body-tube -l-th of an inch, or 1-^-oth of an inch for each revolution.
The spiral spring below H keeps the bearings in clo.se contact.

Of course any desired speed can be attained by proper combina-
tion of the threads : thus 32 and 30 would give ^cfth of an inch
for each revolution, and 31 and 30 would give -g-j^th of an inch.

This screw has provided for the Continental model what Swift's
vertical lever lias clone for the Jackson model ; Mr. Baker, of
Holborn, has adopted it and with very satisfactory results ; for it
has passed through that most crucial of tests for a line adjustment,
its employment in photo-micrography, with excellent results ; and

166 THE HIST"KY AND DEVELOPMENT OF THE MICROSCOPE

\\v hope ili.-it ii may become the general fine adjustment for this
fiirui of microscope in place of the <>1.1 form of direct-acting screw.

In contract and comparison with Campbell's differential screw
we may put tin- principle on which the usual simplified construction
of the line adjustment of the Zeiss stands rests.1 In fig. 127 the
triangular liar < ' i- screwed firmly to the stage ; on it moves a hollow
piece II. \\hich is connected inseparably with the arm A carrying the
tulie. At its uppei- end (_' is cut away for about 15 mm. and B
holloued out at a corresponding place so that space is obtained for j\
spiral spring, This spring bears below against the hollowed-out
part .if I!, its upper end being connected with the projections of the
piece 10 screwed into C. The piece B is closed above by the cap F, in
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
prevent ing over -screwing. The lower end of the screw is rounded off
and bears against the flat surface of a hard steel cylinder let into E.

< 'learly. when worked, the screw remains in the same place.
bearing against C. The female screw, on the other hand, moves over
it. raising and lowering the tube carrier B A connected with it. By
its own weight A B counteracts the rise and thus supplies 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
opinion of one u ho uith practical knowledge would certainly not be
prejudiced against the ('ontinental stand. Dr. H. E. Hildebrand
says that in le.-iching establishments, where as many as two
hundred microscopes may lie used, the weak points of the Continental
stand are soon brought to light. The fine adjustment screw soon
becomes unsteady (an ine\ itable 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. A'c. A'c.. all of which and much more, as we
believe, is the result of the adaptation of a simple and primitive form
to complex appliances for which it was never designed or intended.

It is. however, an admirable characteristic! of the firm of Zeiss,

lli;it while they adhere doggedly t<> the old ( 'ontinental model, they

are continuously putting forth their ingenuity and skill to counter-

\liat are shown to be its defects. In their best usual form the

"I ...... I <>!' the line adjustment is , ,', , inch for each revolution of the

This is undoubtedly too rapid, but it could scarcely be

because, as we have seen, it had the coarse

and lube to lift, and the wear and tear on so fine

in constant use led to rapid failure. But the firm has

luced in 1886, and was it ^-cat improvement on its pre-

bad. ra< B.M.S.J. 1886, i>. 1051.
Mikr. xii. i 1895] ]<]>• 1 '

FlG. 128 (1898).

1 68 THE HISTOl.'Y AND DEVELOPMENT OF THE MICROSCOPE

intioduced a very complex but very remarkable modification of their
line adjustment \\hicli i> intended to obviate both the above defects.
|i is a model ostensibly constructed for photo-micrographie
purpose.-, but if successful will >peedily be applied to all their stands.
'I'lie entire microscope is shown in fig. 128, while a vertical section of
the line adjustment is presented in fig. 129. and a ground plan of
the same in fig. 130. A point which seems to be considered of
importance to some ( lerman microscopists is the provision of a
handle hv means of which the instrument may be readily moved,
and with the provision, of this the usual large milled head controlling
the line adjustment has been displaced. This is shown at H in fig.

Fi<i. I'J'.t iisi.18).

Hut with the accomplishment of this there was a great desire

to bring aboul uhat \\e have >,, nl'ten endeavoured to show was an

indispensable necessity in the beautiful productions of Jena, viz. that

the line adjustment should not have the burden of carrying the

adjustmenl and the tube. They have not succeeded in doing

\\eight of the coarse adjustment and tube is still on

i'i'- micrometer screw. They have diminished the weight

adjustment ha- to support by making the body and

F aluminium. The fine adjustmenl is placed close behind

one, both being fastened quite independently, so that in

THE FINE ADJUSTMENT

169

fact the object holder can be made to receive, and the optical appa-
ratus 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

A. A_

Fiu. 130 (1898).

the micrometer screw M (fig. 129). The pressure of the spiral
spring is in the direction of the axis of the micrometer screw, which
works against a hardened point shown at D2 fixed on the dust-tight
under-cover of H (fig. 128). This < conducting slide ' F (fig. 129)
is firmly screwed to the part 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
1 1 i rect contact with
the hand ; the
turning of the
' micrometer ' or

PIG. 131. — Reichert's new patent lever fine adjustment j 1899).

fine adjustment screw only takes place by means of the motion of
the small milled heads W W (figs. 128 and 130) which work the
endless screw E (fig. 130). This engages the wheel S, which being-
fastened on to the flange of the fine adjustment screw, replaces or

I/O THE HISTORY \-N~I> DEVELOPMENT OF THE MlCKi >S('< >PE

rather supplant.- tin- u>ual milled head ordinarily placed at the top of
II ( li»'. 1'JH). One consequence of this is that the speed of the fine
adju.st meiit is slowed down so much that while Zeiss stands of the

lever fine ndjust ..... nt (1889).

onlj ,,',,11, inch fora revolution of tin- milled head

meter head. thi> form of line adjustment gives

oluticm ofthesmall mille.l heads \VW fis. 128,

THE FINE ADJUSTMENT

I/I

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. More-
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 form, and there are special arrangements made to pre-
vent this delicate construction from being overscrewed either wav.

The mechanical stage of this microscope has some features worthy
of note. It will be seen that the milled heads which work the stage
are on Turrell's plan, but the outer head gives transverse movement
to the stage plate instead of verti-
cal movement. The pitch of tin--
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 altogetln r
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 geared. 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 halfway
between the rack on the right and
the pivot on the left hand side of
the stage.

The stage is concentric with
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 i educing 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 l>\
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. 188. — Swift's patent tint- a
(1881 .

1 72 T1IK H1STOKY .\ND DEVELOPMENT UF THE MICEOSCOPE

levers, h. li. which in turn press the arched piece with its appendix f
on 1u tin' prism support. The principal screw has three threads
to the imllemeTer. which by the lever> is reduced by about one
third. The pointer for reading the micrometer scale on the
milled head is conveniently arranged so that it can be changed to any
iL'iire on the ><-ale. The speed of the adjustment is ., }Tlth inch to
one r<'\-olntion of the milled head.

\\'e may now profitably consider the best forms of fine adjust-
ment thai apply to the Li>ter model, and one of the steadiest and

FIG. 184.— Nelson's model will,
adjustment screw to the left h.-

FIG. 135 (1885;.

•licateofthe.se is that de\i-ed by Messrs. Watson and Sons.

I'lie entire body i> raised or |ov.,-r.-d by means of a milled head fixed

ha\ini.' ;i li;ir«lcn«-d -r,.,.| point, acting on a lever with

aii.l hii/hly poli>h..i| against a point

-li'l'-. in • --tailed fitting, about

hown in fig. 132. Jiy
:l"d li«-ad ih- hard steel I. ... I;, which has its fulcrum

THE FLSE AD.TUST3EEN: 1 7

:

at C. r.ti>e- or lower.- the body with gi-eat smoothness and with -
great delicacy of ^.vrrth inch : :y revolution of the milled head.

and therefore capable of yielding _ —rvice with the highest p
objectives.

"SVe mav now dii-ect oui- attention to the - - .

* • •

into which we have separated riie various kinds of fi: -

viz. that in which the - - -""•-. Ijusti

screw.

v -is one of the new forms of fin-

ment worthy « >f careful trial : it has in it elemei." s of great
It can. how- . iily be ap^ the L>:~r model, and with

adjustment described alx>ve certainly places this f.-iTLi •:•:" mi< : -
l^eyond the danger that soon - _ promi- .ave pi-

extinction as a first-class microscope.1

Th' f (188 - -ound in principle and

ingenious in c:>n-triKti-'U. ui^-i although the patent
modification - of it 1 88s . we believe th- ° rm, wl

makes, to l>e the best. because it only ts - '-iece whil^ -

modification acts on the body-tube.

The earlv form emploved bv Swift

* .L • •

necessity of all > ssful fine adjustments : - - - the

accuracy and perfection of the fitl _ • - -

was done, as >hown in fig. 1- tl _ -

bar. A. to tlu - -d slid: - in V _

at the back of the K.-ly. A I ber s

milled head. F. ts n a vertical Ivut lev- n whicl ~- . K.

fixed to the pi-ism bar bears.

Thei-e is al>o an adjustment _ :iing up the prism bar in

the V-grooves. B B. Siti

with this form of adjustment : while the power to • tighten up " by
means of tl -tan-headed screws enables; wear and tear to be

c.nnptMi-ated. It is obvious that the slowness of the motion is here
c >nnvl'. "iiree factors : (1) the length of the lever. D: (2) the

listaj ' the lifting-stud. E. from the pivot or fiilcnun : and
the pitch of the screw -thread on F.

V -.ufestlv. where a siW-e-lever fine adjustment such as this is
employed it should be. as it now always is, placed on the f<?tf-hand
side of the operator : we can readily focus with the left hand, and
leave the right hand free tor moving the slip and effecting other
adjustments. Ambi-dexterity is not at present a common gift, and
to have the right hand five is important. This was pointed out by

Mr. Nelson when tliis 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
weiv carried right through and a milled head placed on both the
right and the left sides of the body.

\ rlv form of a wvy-oiVtv-aJH/rt - : was em-

% M. «r tf

ployed by Andrew Ross. It was applied to a microscope having a

>*rw. if.Af.S. (18S1)
rm». If.-V.S. (1SSS' p, 120 and ilSsS*) p. 104S, fig. 207

174 T1IK HISTOUY AXD DEVELOPMENT OF THE MICEOSCOPE

movement. It consisted of a lever of the second order inserted
//„• Inr. : mid actuated by a micrometer screw with a milled
head :ii one end. the fulcrum briny at the other, and the nose-
piece between them. This sewed admirably in the days of low-
an-led objectives; but there were two faults belonging to it: one
vs.,. that the tube of the nose-piece had not a sufficient length 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 practical form was invented by Powell. His instrument
also hail a liar movement: but the bar being of relatively great
length, he employed ,i I, r/'t- of tlie first order, the micrometer-screw
being at one e nd. 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 ., ontli 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.
t his 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 posit ion and character will be seen on the right-hand
side of the body of 1 he Smith model, fig. 122. In the light of
what we no\\ need, we are 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 piece tube is so short
that steadine» of movement does not belong to it. It is only that it
was concurrent \\ith t he belief in ' low angles,' and consequent ' pene-
tration ' in object i\ es (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 foregoing \\ e learn that there are three types of micro-
scope model.-, for \\hich a suitable line adjustment has been found.

i. The bar movement model, for which Towell's first order of
level- is I he perfect met hod.

ii. The Lister ...... lei. for which S\\ itt's vertical lever and

Watson's Ion- hori/.oiital lever are the best forms known.

iii. 'I'h.' Continental model, for which Campbell's differential
- the most smooth and delicate device vet suggested, unless
e into consideration the beautiful lever line adjustment of
tleichert.

value of delicacy in t he line adj list meiit call of course

• appreciated by the expert. A tolerable speed may be

adjustment when uncritical images \vith small

••• 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 f 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 secured, 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 lie avoided.

threads to 1 mm.
= four threads to 1 mm.

FIG. i:;c,.— 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 bead in fraction
Model of an iucb

Bausch and Lomb
Beichert (old form)
Zeiss (ordinary) .

Powell . . ^r.th

Baker and Swift (Campbell differential screwJ-J^th
Beichert new patent . . o^th

Swift vertical lever . . . ^th

Watson's long lever ..... ^In1'1
Zeiss's new endless screw arrangement for

photo-micrographic stand . . . ^g^th

IV. The stage of the microscope will next call for considera-
tion. What is known as a mechanic"/ .s-A/i/c must be a part of everv
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 would 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

In- constr

1/6 THK EISTORY AND DEVELOPMENT OF THE MICROSCOPE

mechanical mockery. Better trust to and educate the fingers to
move the object tli.-ui IM- beguiled by any such practically tormenting
delusions. They are simply impossible as accompaniments of a
tirst -class micro>co| ie.

The principle u]ion which alone a perfect mechanical stage can
constructed, so as to work smoothly without ' loss of time,' and en-
dure constant use without failure, must
be the employment of prism-shaped
plates sliding in sprung V-shaped
grooves, ami 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
replacing them, when they should work
smoothly and without shake. Where
the sliding parts are tightened into
easilv fitting and merely ploughed
grooves by pressing the pinion into the
rack, the desirable result of smooth
working and instant responsiveness of
sliding plates to milled heads will not
present itself.

l>ut besides the perfect action of the sliding parts, a perfect
mechanical stage should have equal speed of motion vertical!'/ «ml
horizontally. A common fault is that the speed ot the rack work
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, arid 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 revolutions of the
"ed head.

FIG. 137 (18!)Hj.

in

FIG. i:;s (1898 .

t is most desirable thai I],, piniom «l<nn!,l !„
\ith the movements of the stage, and the mill,-,!

not movable
carrying the
"

/.-• as near t<> >,,,•/, „//,,•</• «« /,oxx/7/A-. The best
Turivll's. devised iii ls::-J. where one (a, screw) is
other (a pinion) passes through it ; this permits both
al the same time \\ith one hand, giving a diagonal

THE MECHANICAL STAGE. HOW TO FOCUS 177

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 affecting
the motion. The cap B covers the ball when fitted together. The
manner in which 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 be larye, not less than 1^
inches in diameter. There ought always to be space enough above
the ordinary slip when it is in position to permit 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 oft' 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 ^th 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 proceed 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 focussing becomes easy and
rapid, a matter of touch, and not of discontinuous procedure to
' discover where the front 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

• M :. - band (1895).

THE MECHANICAL STAGE 179

focus an object in the field with a J-r-inch objective in ten or
twelve seconds.

If a perfect mechanical stage cannot be obtained, take no middle
course, have a, form, well-made plain one with a smoothly 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 purpose 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
mechanic-id stage, and for the same reason.

Mi1. JSTelson suggested a stage of large size, which should have a
IT? or If 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 collaborateurs 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 :d><>
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, marked 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 microscopist in
this relation is, ' Have your mounted slide in a fixed position, but
never clip 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 lie
disengaged and rapid movement 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 four movements in every microscope irhick should be
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 ; '

N2

180 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE
i.ut in practice they ;HT superfluous in the most complete instrument

beyond t hose 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 mechanical .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 .-in attachable stage can be made to work with remarkable
smoothness ; and since some persons have not sufficient delicacy of
touch to move so small and thin an object as a 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
cnii\ eiiient 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 mechanical stage (1894).

moving the slide to assist in rapid focussing. But these are defects
which are rapidly disappearing.

Amongst tlio.se 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

itage, if required, free. The up and down motion is effected by a

milled head l>elo\\- the stage. Th.. lateral movement is produced by

wo endlesi screws engaging in worm-wheels fixed to .smooth rollers.

I'lie lower edge of the slide vests on these, and is kept in gentle

apposition \\itl, them during traverse by a third smooth roller at the

• <-nd of a curved spring as shown in the figure. This is readily

vh.-n changing the object. In its most recent form we

• ; this stage with comforl and pleasure.

?es, made by I'.aker from designs by Mr.

Ill- uhich in its latest form is so arranged

space between the cesi and the spring clip can be

THE MECHANICAL STAGE

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 rack and pinion,
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. th:it 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).

IS2 THE HISTOKY AND DEVELOPMENT OF THE MICROSCOPE

FIG. 142.— Nelson's new mechanical stage (1897).

FIG. 14:!. — Nelson's new mechanical stage (1897).

• IMIII'S nr\v mrrliiuiiral v

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 movement.

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 instrument
of the Continental type, is very carefully made, and the scales

PIG. 145.— Reichert's attachable stage. (About half natural size.) (1892.)

attached are divided to read by means of a vernier to O10 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 in fig. 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-

184 THE H1>T"KY AND DEVELOPMENT OF THE MICROSCOPE

:iMr. for thrv are made by most leading opticians. The last mechanical
stage we illustrated is by Messrs. R. A: J. Beck, which is illustrated
in H-. 117. It ha> vertical rack and pinion and horizontal screw
motion- \\itli graduated finer divisions.

TII .Messrs. ISaiisch and Lomb, however, we are indebted for the
int rod i ict ion of an atiaclialile 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 c mimend. But especially when the illumina-

1 "'• tchable mechanical stage. (3 full size.) (1895.)

lion is daylight, and very critical results are not sought, it will lie

ii-eful, and is admiralilv made.

V. The sub-Stage is scarcely sec-mid in importance in a first-
g microscope to the stage itself. It is intended to receive and

enable us to use in lln- mpsl efficien.1 manner the optical and other

apparatus employed to illuminate thr objects .siiital.lv with the

found needful. Upon this much of the finest

^ i'b the ] lerii microscope depends.

•ompli-l, this a good sub-stage must have rectangular
,anda rack -and pillion focussing adjustment.

THE SUB-STAGE

I85

The vertical 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 continuous
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 moves in jerks, and is liable to spring from the
position intended to be final.

It is not needful that the motion should lie in right lines ;
motion in arcs whose tangents intersect at rii/kt angles are quite as
efficient. A steady, even, reliable motion that will enable a centre
to be 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 under surface of
the slide with sufficient delicacy. But no means should be employed

1 86 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

tor tin's 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
i It -vice fur tin- 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
\vas 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
In- readily understood. It does not interfere with the general

BAUSCH8LOMB OPTICAL CO.

i IG. 14?A. -Attachable sta^'c with diaphragm in the plane of tlie stage. Top
view and cross Motion showing construct i.m of sta;;<> and utta.-lmirnt of iris dia-
phragm.

mechanical arrangements of the sub-stage ; it will be seen that the

milled ||,.;M| A controls a MTC\\ .spindle terminating in a steel cone B.

1 'n rotating A. I', turns, and \\itli ,-i very slow motion forces up (or

the case may l>e) a pinC. inserted in the ba.se plate E of

Tlie iimt imi of (' cai-ries with it Hie condenser. At

;md foi'ming p.-irt of K :,t the back an inner sliding

sagainsl a spring a1 the upper end between bearings F at

liieli are fixed upon the usual racked slide |) of the sub-

THE SUB-STAGE

I87

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 difference 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 spring 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 -^--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 HISTOKV AND DEVELOPMENT OF THE MICROSCOPE

.•UK! from the front in tig. 151. This is brought round at one end
Mt right angles to the front, The fulcrum of this lever is at C, and
it fits under the pin I > \vhich is attached to a dovetailed piece, having
.it tin- back of it enclosed in a metal casing the counteracting spring

FIG. l."o. FIG. 151.

\VaW>n'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
sit 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
linker, which, we are of opinion,
it will be of advantage to the
student to understand. It em-
ploys tin- differential screw, and
JJBt'i by this means obtains a very

i r,-.— Sub-stu-.' line ncljustmrni com- sl(INV 11HA V1 ' "'' |j • '-'"I"' student has
!''«•'' in • Royal1 microscope 0 already understood that the prin-

ciple of this screw is the cutting

breads of a dim-rent - pitch/ one at either end of the screw,
the proportion of one to the other determining the amount of move-
breads found most Miitable for their sub sta-v fine ad-
justmenl were lo.-md 5n to the inch. In fiu. 15;; the screw AC

THE SUB-STAGE

189

has 40 threads to the inch, and works through an immovable fitting,
the thread is discontinued at C, and from C to D a screw having 50
threads to the inch is cut, 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
stern, but it would take 50 revolutions 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, ith

PIG. 153. — Baker's fine adjustment to sub-stage (1888).

of an inch — therefore one revohition raises the fitting E -g-J-jjth 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 here to a com-
paratively recent arrangement known as a stringing sub-stage, which
is, as its name implies, a sub-stage so arranged as to be capable of

190 THE HISTORY AND DEVELOPMENT OF THE MICEOSCOPE

being moved Intertill;/ out of the d.i:is 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 oblique 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 one azimuth, many tacitly assumed
1 hat 1 his 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 I'-") 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',
b\ the latter a similar angle of 161° 23'. These sectors of the cone
oi' lii:hl of <>7° ">' and 80° 41' respectively are in every sense
-oblique illuminators,' and the one more oblique than the other.

Whether or not it is needful or best to use such a sector is
scarcely an open question ; it is manifest that by taking the stop
wit.li its sector away from each condenser and sending in tlie complete
cone of light formed by the condenser, we are still using oblique
illuminators, b-ut the obliquity is in all azimuths.

There can be no doubt that a large aperture in a condenser
provides the microseopist with far greater wealth of resource than an
oblique illuminator in one azimuth can ever give him. A condenser
with an oil angle of 1(51° 23' is much more valuable than even the
semi-angle obtained by a mere section of a luminous cone. The
power 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
obliquity.

Ordinary concentration depends upon the power of the condenser.
If it is required to concentrate the light from the edge of the flame
of a paraffin lamp upon an Amphipleura /><>// >iei<la, the condenser
must lie at least a |tl\ inch or !th inch in power, which will give an
image of the llame nearly the same size as the object. The amount
of I'lijlil which is concentrated upon that object will of course depend
upon the aperture of the condenser. An oblique cone of great in-
tensity is here ulial is needed; the illuminating cone should be
equal and conjugate to that which exists between the object and the
objective.

Nou it is certain that this condition cannot be met by an * oblique

illuminator' of the kind commonly uudersood bv that name; to get

immersion contact . which is of course a sine i/nn m>n, we must employ

a hemispherical button or one greater than a hemisphere-j-placed

in immersion contact \\ith the under surface of t he slide. This may

lie illuminated by a beam from a dry combination, made oblique by

being swung out of the axis. ( Jrauled that the angle

\\hichcan begot \\ith a condenser of great aperture, \\ e

•btain only a port ion. and an attenuated and small por-

Liht given in every, or at will any. azimuth by the con-

den

perfect illumination of an objective, for example,

THE MIRROR 191

a j^th of N.A. 1'4 or 1'5, would be obtained by using a precisely
similar objective as a condenser, 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 ahioh-clas>.
achromatic condenser of great aperture and of homogeneous construc-
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 <?r>,n- n-ith immersion 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-
staye should also be provided with a rack-and-pinion rotary motion ;
that is only really needed in order to use the polariscope. 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 concur*
and from 2| 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 focui-M'il
on the object.

The plane mirror is sometimes found to give several reflexions of
a lamp flame at one time ; we find a very efficient 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 of their
origin is explained in fig. 154. A is the glass surface, B the silver
surface, O the object, and E the eye. In the direction 1, 2, 3 appear
the first three images. No. 1 is from the glass surface. Xo. 2 from
the silver, and Xo. 3 is from the silver and air suri-iri -.

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 HISTORY AND DEVELOPMENT OF THE MICROSCOPE

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 surfaces A and B,
li-. I .">."). hnve ;ni 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, H° 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. 1 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
1 l 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
point is reached where all
the images will be super -
i 1 1 1 p< >sed. All mirrors should
lie so mounted as to admit
of this rotation.

The present Editor is

156. greatly in favour of tlte em-

/iltu/uient of a rectangular

prism CU\ with care and precision. \\> M-t,T l,v this means 'total
reflexion and no double reflexions; and he believes that finer images

111 be obtained by its ans than with the plane mirror. It may

mounted in th.- place o/ the plane mirror— that is to say, the

• mirror may U- as usual in its cell -and in the other cell,

ould have received the pi.-,,,,, nijm,r, the rectangular prism

mounted and be capable of rotation as the plane mirror

would nave 'MTU.

lid, however. l>(. noted il,;,t this applies only when the

B

FIG. 155.

A TYPICAL MODERN STAND

193

FIG. 157.— Powell and Lealand's No. 1 stand (1872)

0

194 THK HISTORY AND DEVELOPMENT OF THE MICROSCOPE

liii'ht is required to In- reflected at ;ni exact right angle. It is of the
greatest service when the microscope is of necessity used in a rigidly
upright i><»it ion.

ft' it !«• used f<>r angles other than right angles, there will lie
refraction as well as reflexion ; and as the necessary decomposition
of the light into a spectrum will accompany the refraction, care must
IK- exercised to see that the rays emerging from the prism are at
ri»-lit angles to those incident to it, and that the areas of the square
faces of the pri.-m 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 ' white clow] Ultnit't natorj
that is, a disc of plaster of Paris, or opal glass with a polished
sin-face. I lut a disc of finely ground glass dropped into the diaphragm-
holder of the condenser will give a precisely similar result.

Mr. A. Michael has, however, pointed out the curious fact that an
n/xin'.scent mirror becomes an inexpensive and excellent substitute
tor a polcvrising prism.

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 every case even, one instrument of
some makers. This would involve simple repetition in the main
features. The reader can compare for himself the microscope of any
given maker from whose catalogue he proposes to select, and can
discover by comparison //* i/ic!>/ci/n' <>/• otherwise, n-ith the (///»'
yivenhere to which it corresponds.

Beginning with the highest types we place first on the list Powell
and Lealand's JV /. This instrument may claim a seniority over
all the foremost instruments, because for nearly fifty years it has
practically remained the same. All its principal features were
brought to their present perfection nearly fifty years ago, while all
other microscopes during this period have been redesigned and
materially altered over and over again. This is no small commenda-
tion, for during thai period, as the reader so well knows, the. aper-
tures of objectives have lieen enormously enlarged, and with this
has come a great increase of focal sensibility. As a result the
majority of the micro-copes of forty years ago are absolutely useless

for tl liject i\ es of to-day, but the focussing a ml stage movements

of 1'ovvell ami Lealand's microscope still hold the first place.

Kig. \~>7 represents the instrument in its monocular form. The

foot of the stand is a tripod in one casting; it has an extended base

(!) inches, forming at once the steadiest and t he lightest foot

any existing microscope. The feet are plugged with cork, and

ien (lie liodv is in a hori/ohial position the optic axis is (as it

should lie) In inche- from t he taMe.

6 coarse adjustment is effected ' >\ a liar, consisting of a inas-

1 metal truncated prism in form, which hears only on a

narrow part at the angles. It extends sufficiently to locus a

POWELL AND LEALAND'S BEST STANI) 195

4-inch objective. The arm which carries 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. Great efforts have been made to accomplish this in
other instruments. The older Ross form from the shortness of the
arm only allowed of a two-thirds rotation, and in the Lister model
many different devices have been tried, the Litest being the placing
of the stage pinions in a vertical position above the stage, which is
an unquestionable error.

The rotation of the stage in the Powell and Lealand model is by
ineaiis of a milled headmost conveniently placed, and the divided
circle is on a plate of silver.1 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
merit (p. 174). The milled head is placed behind the strong pivot
of the arm, where vibration is impossible, and it is in an easy and
natural position for the access of either hand.

The bod i/ may be. with great ease, entirely removed from, the ami',
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,2 or an analysing
prism. 80 also it is of service in low-power photo-micrography.

We have already referred to the stage of this instrument;
but it may be briefly stated that it is large, has complete rotation,
it has one inch of rectangular motion, being graduated to the ruirth
inch for a finder. There is the same speed in the vertical and the
lateral movements, and the pinions do not alter 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
flexure.

The sub-stage has rectangular movements by screw in either direc-
tion, as well as a rotary movement by pinion. The coarse adjust-
ment is by rack work, and njine adjustment is added when desiied.
Fig. 158 illustrates this stage, showing its under side in order to
i-ii.-ible the fine adjustment to lie 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 great smoothness ami accuracy. aKo enabling the
operator to centre with complete precision, while, as we have
already seen (pp. 187 and liMi). 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.
- Chapter V. p. 337.

o '2

196 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

The present Editor lias had oneofthe.se microscopes in constant, and
often prolonged and continuous, use for over twenty years, and the
mo>t delicate work can he 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 \ery best clip that can be used — the pivots of the
mirror, and the carefully X/U-I/IHJ 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 .-( linished 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 be 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 d'etre ; the
size of the tube was such that it
would take in a binocular body a
Iluvidieiiian 2 -inch eye-piece,

having the largest field-glass pos-
sible. The size of this field-glass
depends on two factors.

1. The distance between the
centres of the eyes.

-. The mechanical tube-
length.

In order that the binocular
may suit persons with ' narrow
centres' to their eyes, the dis-
tance between them should not be greater than :H 'inches. The
mechanical tube-length is s:i inches for the standard" tube. When
(lie eye-pieces were -home' in their places in the tubes they just
touched each other, the inner sides of t he binocular tubes being' cut

FIG. 158.— Powell and Lealand's sub stage

with line .nl JM-.I incut i 1882).

away; so under the a hove conditions

a larger fieh

obtained is simply impossible. The size of the
mines Ihe size of the eye piece, and that was made

than

diameter of t he h<>,l\ t ube.

\\i-ely these makers made the tube of the

tubing

Used as

field-glass
' to

is thus
deter-
fix the

same size, so as to have one gau^e of

sub-stage

the

This
a condenser, thus

throughout.

allows a Kellner Or other e\e piece to b,

!•<• liH'ino- the number of adapters.

l-alely this linn ha\e altered their snh-staye tube to a yauge
tnmended by the Royal M irmscopical Society. This involves

lapter where the sub-stage apparatus was adapted to the old
wl"'n an eye piece is used as a condenser; as the size is
large for a binocular.

'- iu its completes) form as left by Andrew Ross,

T. ROSS'S MICROSCOPE

197

except specially ordered 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. Ross
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 Till: HISTORY AND DEVELOPMENT OF THE MICEOSCOPE

ment being slowei

which \v;is without arrangement for rotation; and the mirror was
not. jointed. The model of T. Ross had, as will be seen, a bar move-
n ii -nt, with a foot formed of a triangular plate to which were bolted
two parallel upright plates to carry the trunnions of the microscope.
Tin- 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-
fhan 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 strain
of critical work with im-
mersion objectives of
great aperture. Xe'Ver-
theless the defects of this
stand could have been
readily corrected . "\V i T 1 1
a more extended ba.se. a
better arrangement of
the fine adjustment, 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
a d 1 1 1 irable instrument .

This important firm
were otherwise advised,
however ; and, instead
PIG. 160. -Ross-Zentmayer model (1878). *!" of correcting the error*

of the instrument whose

they had made, they designed an entirely neir, model in which
a Lister limb was substituted tor the bar movement, Fig. lb'0 illus-

I|M~ r"r fthe instrument, from which it will be seen thatthe

also was changed for the worse ; the base was not sufficiently

the hinder part of the font u as too large, so that it

times rocked on four points, because the hinder part was too

surface, in fact. A true tripod will stand firm on an

ible,bu1 this form will not. It is a form liv(|ii«-ntly used by

- and is known as the 'benl daw.' It'is a bad

be, as it has been, easily thrown over laterally. It

THREE GREAT TYPES OF MICROSCOPE 199

s, 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 carried by
the fine-adjustment lever and screw.

This form could not — as it did not — long prevail. Its existence
was ephemeral, and in its place was put a modification of the form
devised by Zeiitmayer, 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
wre 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. 1(55). Jackson also designed the double pillar foot
(%. 161). '

\\ e have already assessed the value of a swinging sub-stage and
found that in our judgment it is at be>t redundant and really
adverse to the accomplishment of the best scientific work.1 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, ivhich were not «t ll«it tiim provided /-//
condensers, this phase of pseudo-illumination was carried to a still
greater and more elaborate development in the production of a <•<///-
centric microscope. This was a Ross- Wenham, known as the rtnlnil
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 t\vo 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 from 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 rackwork

1 P. 188 et ne,j.

FIG. 101 (187«i.

in

163. — Section of bearings
and fittings of pinion.

FIG. 162.— The grand model Van Ileurck. Watbou and Sons (1895).

202 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

i- established liy means of a block of metal which fits upon the
pinion shaft and is pressed or released by means of the two screws
provided ti;r the purpose. This is shown in section in fig. 1(53,
where tlie pinion is P, the anti-friction block X, and one of the
adjusting screws M. The perspective view of the coarse adjustment
showing the adjusting screws is given in fig. 1(U.

The stage can be completely rotated and has mechanical move-
ments on the Turrell principle, both milled heads being on one axis.
The snb-slai^e has a fine adjustment, and the plane mirror is eare-
fullv worked bv hand, while exceptional rigidity for the whole stand
is obtained bv a special system of construction, and the tripod, which
is shod with cork, has a spread often inches.

A high -class stand of distinguished merit is made by the firm of
Uaker of llolhorn. It is illustrated in fig. 165, is made with great

FIG. 164. — Ci'in|.Irtr view of Watson's coarse adjustment (1895).

care and isan instrumenl of precision. It is mounted on a solid tripod
with slotted toes so that it can be firmly clamped to the baseboard
of a photo-micrographic apparatus. The body is mounted on a mas-
sive limb in our piece throughout, 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 line adjustment carries
the body tube only each revolution of the graduated milled head,
bein- e<|ual to the ._,,',,,"' of an inch ; the Campbell differential screw
being employed, and the milled head being placed at the lower end
of the body. The body ran l,e extended to .".(M) mm. and closed to
The mechanical stage is worked on the Turrell method
stationary milled heads working on a common centre commanding

obli<|iieas well as red a ngula r mo\ ement s ; the rectangular movements
\ided silver plates for recording positions: and complete rota-
ion can be secured, either by hand or by rack and pinion, and ran

be a I any point clamped.

sub stage lias rectangular mechanical movements controlled

C. BAKEE S MODEL

2O •

by fixed milled heads, and all fittings .-ire sprung and have adjusting
screws to compensate for wear, and fine adjustment by Campbell diffe-
rential screw which Mi-. Baker adopted for these microscopes in 1 888.

FKJ. 105.— C. Baker's Model (1895;.

Swift and Son formerly made two instruments 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. "NVe have alreaclv seen that

204 TUK HISTORY AND DEVELOPMENT OF THE MICROSCOPE

bv their invention of the vertical lever fine adjustment (figs. 133 and
I :!.")) 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 • l!est " < 'hallenge " M icroscope.' It has a beautifully made coarse
adjustment, the special tine adjustment invented by this firm, a cir-
cular rotat ing 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
rotarv movement for use with polariscope. The stand is a firm
form of tripod, and the mirrors are well worked and mounted on a
double crank.

All 1 he 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 Anici-icnn opticians 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 pernicious 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.

I n later constructions of this form. Tolles first used the mechanical
stage actuated by two pinions vertical to the surface of the stage,
and subsequently adapted by Ross. The line adjustment in this
instrument had the fatal defects characteristic of its form.

Bulloch, another American maker of note, made some modifica-
tions in the Zentma\er model, but they were in the interests of the
winging sub-stage, and. although no doubt ingenious, must pass
with this transient form of the microscope.

A modification of this stand was devised bv Imlloch : it presents
no special point, save the employment of a Cillett condenser with
the diaphragm drum above !/><• lensesl

•\ later development of this form of instrument by the same
maker was made some years after, but the chief difference consists
in the ado). (ion of a stage in which the milled heads stand upon

Stage, which is the reverse of .-in advance. Since, however, the

\ ingim: sub stage form of inst rnmeiit has been ent irely superseded,

•an makers ha\e adopted, with very slight modifications in

i" principle, the Continental stand, \\hich i- made with

ble precision and conscientious care, but still retains its chief

' may therefore be of ser\ ice in consider the principal

cations of the Continental stand, so that they may lie

SWIFT'S BEST CHALLENGE MICEOSCOPE

205

FIG. 166. — Swift's best challenge^ microscope (LSSH.

2O6 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

fairlv compared with equally recent American adaptations of the same
microscope: and then endeavour, after examining instruments of
,-i lower class, to give ;i dispassionate estimate of this model as com-
pared \\ith that of the highest-class English type.

Amongst ( 'out iiiciital 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. 1(57. It is a model of fine
workmanship and has been adapted with singular ingenuity. to the
reception of all their accessory apparatus. The upper body is
inclinable from the vertical to the horizontal position. It is
provided with coarse rack-and-pinion adjustment, and fine adjust-
ment 1>\ mean- 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 \\ith 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 centre, and which can In- rotated about the axis
or entirely swung out.

The circular object stage rotates (not by rack and pinion), but
has centring screws. The aperture in the stage has received a
IIP >re oval form. The rack-and-pinion rectangular movements are
1 Jj-hi. 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 IA stand but the large stand known as 11 n,
an illustration of which is given at fig. 168. Our object in choosing
this instrument is that it combines every essential of the 1 A stand,
and in addition is furnished with the new level- fine adjustment,
invented so reeently by Reichert, and of whose value we have
already given our judgment. It will be seen that on the part of the
body which the line 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 line adjustment makes needful.

A very high-class microscope is made by Leitz of Wetzlar, which,
while it retains the principal features common to all microscopes
based on the Continental model, has yet qualities peculiar to itself,
and obtains by means of workmanship and ingenuity the most ad-
mirable results attainable from the model on which it is based. It
is inclinable \\ith a hinged joint and clamping lever ; and the stage
i- provided \\itha revolving centring table. 'The mechanical stage
i- the 'attachable' one already described, and the adjustment of the
objective is by rack and pinion coarse adjustment, and by a fine
adjustment depending on a micrometer screw provided with a

The draw tube is furnished with a millimetre

sub-stage i> planned on the principle of the Zeiss

will receive the illuminating apparatus as devised

worked \>\ rack and-pinion adjustments, which

FIG. 1(37. — Zeiss's largest and complete stand ils'.loi.

208 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

al>o raise Mini lower the iris diaphragm and provide it with possible
oblique or eccentric movements; and it is furnished with objectives

liert's hir.'i1 stands (lib) with new lever tine adjustment

tittr.l I1MMI'.

FIG, 169.— Leitz's most complete stand (1893).

210 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

and eye-pieces 1 li;ii ghe it magnifying powers ranging from 15 to
I .."inil t hues. This instrument is shown in fig. 1(59, 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 constrained 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
.'l?y in. The sub-stage is provided 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 1(H in. The stage is mechanical, and the circle is divided into
:!t'>( l degrees ; both the horizontal and vertical motions of the stage have
scales read by verniers. The object is fixed on the stage by spring
I i 1 1 i ngs. The fine adjustment has two speeds of motion by two screws,
the one 0'3 mm., the other O'l 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 different chtsxes of
microscopes manufactured by the opticians of Europe and America.
To do this without prejudice and with efficiency it is necessary to
designate the characters which should distinguish each class.

Microscopes placed in Class I. possess—

1. Coarse and fine adjustments.

•2. Concentric rotation of the stage

3. Mechanical stage.

I . .Mechanical suh-staijv.
Class I I .

1. Coarse and fine adjustments.

2. Mechanical stage.

.">. .Mechanical sub-stage.
Class III.

1. Coarse and line adjustments.

2. Plain stage.

.'i. .Mechanical suit-stage.
Class IV.

I . < 'oarse and line adjustments.

-. Plain stage.

'•'>. Sul. stage lit I iny (in. suit sta^e).

/ l| ., -\T

< lass \ .

1. Single adjust men! (coars • line).

'_'. Plain stage.

. \Vilh or without sub stage fitting (no sub-stage).
Tins classification applies also to portable microscopes.

The recent microscopes of the l.est American makers are

erised by the highest quality of \\orkmanship and abundant

• '"" theConl mental mode] is confessedly made t heir foiinda-

EECENT AMERICAN MICROSCOPES

21 I

tii in. In the last edition of this work it was shown that American
opticians made their first-class microscopes with swin^iii"' sub-stages,
and we then pointed out that these were not only without value.

FIG. 169 A 1 11. KID;.

p2

212 TIIK IIJSToKY AND DEVELOPMENT OF THE MICROSCOPE

Imt injurious to the best work po-.-ible to a good instrument. In
the interval tin- swinging .-ub-.-tage lias been given up, even by its
nio.-t ardent ;nlvMc;i1i'> : l>ut ;i1 the same time in the majority of
cases tli -V have abandoned the sub-stage proper and adopted the
(' nitiiic nial condenser titling instead. In fact, the American
opticians have chosen almost exclusively, as the basis of their stands
ot' everv class, the microscope that has been so long in vogue on
the Cont incut of Kurope.

It will sutlice to take examples of the unexceptionally beautiful
work of the two leading opticians of America — The Bauseh and
Lomb Optical Compam and The Spencer Lens Company. An
illustration of the best instrument, known as the ' Crand Model/ of
the former of these opticians is given in fig. 170. It is designated
a ' Continental Micro-cope." 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 appear 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 rotate.-
with centring screws. 'The heads of the centring screw.- 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 other work in the
interval. The mechanical stage is worked by one milled head a*
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 th.
Tolles mechanical stage) being one in which the efficiency of the
mechanism is reduced to its lowe.-t value. We have long advocated
the adoption of Turrell milled heads as employed in Powell's Xo. 1
stand; they giv< the worker power to effect not only rectangular
but diagonal movement.-, and. without displacing the fingers, to
work the stage in all direction.-. We are pleased, as we have
pointed out. to note that the eminent firm of Zeiss have adopted
these in their besl .-land (tig. 1,">'.I).

The sub-stage is compo.-ed oft hive parts, ai-ranged one above the
other. This sub-stage, with the part.- 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 vv it h t he under part of the object slide. The
middle sect ion of the .-lib -tage is movable vertically on the main

sill) stage axis, and carries an A I >he condenser of 1 '20 ~N.A., which

can be swung laterally to the lel'i of the i n.-t rmneiit so as to put it
out of optic..] LI . : but on the other hand it can at will be thrown
I tack into posit ion and placed iii oil contact vv it h t he object slide with-
out altering t he po-it ion oft he upper iii- diaphragm. The third and
lowest section of the sub -tage carries the large iris diaphragm used
below the condenser. Tim- it i- dear that the whole can be used
to^t her. or anv <>i e of the 1 hree sect ions can be worked separately.

214 TIIK

AXD DEVELOPMENT OF THE MICROSCOPE

We note one admirable feature of the mechanical finish of the
mi(Toscope> of this lirin. which is. that they avoid sharp angles and
knifelike edges to all their instruments. This looks a trifle, but the
ii-e of t he 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 mechanic do often break the skin, and are wisely

1 ''"•• ''i'l- l-;ui,rli :ui(l Lomb's sub-stage, separated to show construction.

and happily worked into rounded edges in the instruments of these
distinguished makers, and. we may add. without the slightest loss

"' 'hat appearance of high finish which has al\\a\s been correlative

\\ it h t he manufacl ore of microscopes.

I Cue now look al the No. I -land of t he Spencer Lens Company.
V.. shall tind again that t he model of Oberhatiser

to and the instni n1 is of the (liird class. Thi>

microscope is illustrate, I j,, IJ.T. 170. |, ;s |,eautifully ma<l(\ and
the horseshoe base has a -till longer ( claw ' than those of Bausch,

lurgli Student's ; stand ; H ' (with horseshoe foot 1889,
will , >o\ L893 .

RECENT AMERICAN MICROSCOPES 217

to give the stability required in utilising the hinged joint for
inclination of the body, which stands on a strong uiiial pillar.
The sub-stage is movable by a quick screw ; in other features it
resembles the majority of the microscopes of the type to which it
belongs ; it is, however, distinguished by rounded 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 with an ' attachable ' stage
made by the Spencer Lens Company, having all the advantages of
tlie several forms of these pieces of apparatus already described.

FIG. 174.

il.xlel, No. '2 U89H).

We note with some sin-prise that such accomplished manu-
facturers and opticians have indicated, so far as we can discover, no
advance in their suit-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 adjustment, both of which have been
found of such great practical value in England, and which have been,
as \ve shall shortly show, adopted for the more critical microscopical
work by the Messrs. Zeiss, the leading optical firm of the Continent.

218 TI1K HlSToKY AND DEVELOPMENT OF THE MICROSCOPE

Second-class microscopes are made ill great variety by English

makers.

( >ue df the line-t examples of this class of microseojie at present
brought within the reach of the average student's means is that
knoun as the 'Edinburgh Student's Microscope " H," ' by the firm
of \\atsou .-iiid Suns. It is the most complete of a series of similar
>tauds varving in cost and completeness. It is illustrated in
tig. 1 7'-'>. where it will lie seen that it has the first prime requisite, a
rigid foundation combined with lightness — a- tripod having a spread
of 7 inches --:uid it is also possessed of a well-constructed mechanical
stage uhich is built with the instrument, an advantage over the

best ' atl.-iehable ' stage.

It i- essential! v 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.

Kelonging to this class is an instrument by Baker known as
his .Model. No. '2. It is smaller than the 'A' stand of the same
tvpe and is simplified, but is capable of doing the most refined and
eritic.-d \\ork. It is illustrated in fig. .174. The coarse and fine
adjustments are the same. The mechanical stage has rectangular
movements of one inch; the Turrell 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 as a whole — the thoroughly practical 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.

Pouell and Lealand make an instrument of this class, having
a quality of work not second e\-en to their large stand. It is
illustrated in fig. 17-"i. The tube length is the same, but the stage
and the foot are smaller Than in The large instrument. There is
no rotary movement to the sub stage, and its centring is done by
the crossing of sectors and not lines at right angles; but this is in
no \\a\ a defect. All the movements and adjustments are other-
wise as in No. 1.

liaker. of llolhorn. makes a \er\ admirable and useful instru-
ment of this class known as his D.P.H. microscope, No. 1. It
liasa diagonal rack and pinion coarse movement, a micrometer screw
and lexer tine adjustment, giving a mo\ enient of ._,-}, -r of an inch for
each revolution of the milled head ; a draw-tube, every 10 mm. of
which is engraved with a ting, extending to i!~>u mm. and closing
<" I-"'" mm., thus allowing the use of either Knglish or Continental
objectives; it possessesa mechanical stage giving a movement of 25mm.
in eit her direct [on, graduated to .', mm. : t he milled head of the trans-
verse motion is below the level of the top plate, and as the other is

ovable lar-* culture plates can I >e e\a m ii icd. the distance from
optic ax; s to limb (2| in.) allo\\ in-' of their easy manipulation ; the

BAKEK'S NEW MICROSCOPES

2I9

top plate is provided with three adjustable stops, so that the centre of
a 3 x 1 or 3 x IT? slip is identical with the optic axis when both
the rectangular movements are at the centre of their travel, thus
enabling any desired field to be recorded: the stage clips are

FIG. 175 (1852).

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 DEVELOPMENT OF THE MICROSCOPE

instrument in a horizontal position for photo-mierographic work.
The microscope is illustrated in fig. 176.

A niodilicat inn <>f this instrument was brought out as these
pages arc pissing through the press, which is entitled to rank as a
first-class instrument. It is known as the R.M.8. 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
line adjustment giving a movement of Oil mm. (o45 in.) for each
revolution of the screw, the milled head of which is divided into

PIG. ITti. Inker's D.P.H. shuul No. 1 (16'.»'.i .

ten parts, each division lie ing numliered. It also possesses two draw-
tubes engraved in mm., every tenth numbered, one of which is
provided \\ith rack and pinion adjustment, so thai objectives may
be corrected tor the thickness of the cover glass, &c., by the alteration
of the tube length ; bhese draw- tubes extend to '-!•"><> mm., and close to
-" mm., eit her Knglish or ( 'out mental object i\ es can be used ; this
microscope has a rotating mechanical stage giving a movement of
-•"' nim. (I in.) in either direction graduated to \ mm. (.-'„ in.);
'I'1' milled head of the transverse motion is below the level of the

BAKEK'S LATEST MICROSCOPE

221

top plate, and the othei1 being removable a large flat stage becomes
available if required ; the top plate is provided with three stop-.
adjustable, so that the centre of a 76 mm. x 25 mm. (3 in. x 1 in.)
or 76 mm. x 38 mm. (3 in. x H in.) slip is identical with the optic
axis when both, the rectangular movements are at the centre of their
travel, thus enabling any desired field to be recorded ; the stage

FIG. 177.— Baker's R.M.S. T27 gauge microscope (1900).

clips are mounted on two of these stops, all of which are removable.
J1 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 can be conveniently controlled
without shifting the hand, and it is provided with plane and con-
cave mirrors, and the micm-rope is mounted upon a solid tripod
stand, with a bracket to support the instrument in a horizontal
position for photo-micrographic work.

THE HISTORY AND DEVELOPMENT OF THE MICKOSCOPE

All the fittings are sprung and have adjusting screws to compen-
sate for wear.

Coming now to Third-class microscopes, we note that the dis-
tinguished American firm, Bausch and Lomb, make a very useful
instrument which must be placed in this class. It is intended as

FIG. 178. — Bausch and Lomb's C.A.S. microscope ils'.'i i.

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 lias 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

THIRD-CLASS AMERICAN MICROSCOPES 223

firm can be added. The sub-stage is the new and complete one of
the umker.s, arranged for doing critical work ; the fine adjustment

FIG. 178.\.— Eeichert's 'Austrian' Baugh stand (1899).

is by micrometer screw ; the weight of the body is balanced, the
makers tell us, by a spiral spring which, they believe, subjects the fine

224 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

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 I lie makers h;ive evidence of the durability of the adjust-
ment, as .-il'ter live 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 beautifully made, and is
bv no means an expensive instrument. We illustrate it in fig. 178.

A well-made and remarkable little instrument of the class we
are ei Hindering 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 line adjustment is Reichert's recent patent, giving extreme
delicacy to the movement, and having a movable pointer, /. 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 screw 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 demand, 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 \ve believe him to be the only Continental
manufacturer who has adopted the sprung slots and screws so Liny
used with success by English makers for compensating wear. We
should ha\e siigge>1ed slotting the edges of the stage for sliding
the object -holder or led^e. but \velearnfrom the maker that this
is to be done in all future instruments; all but the smallest stands
Heichert is willing to provide with English pattern sub- stages
titled \\jili centring screws of the standard si/e. and condensers a re
mounted to suit these.

Another instrumenl of the same class and general designation,
made by Messrs. Watson and Sons, and distinguished as ' < !,' is shown
in lig. 1 7U. It is identical in build with the C model, but the
stage is plain, and it has only a tube lilting fora sub-stage appa-
ratus ; the workmanship is of the same order, the movements as
delicate and true, the adjustments as reliable, but 1 he price is only
one half I hat of t he more com | il icat ed form.

Amongst the same class of instruments must be placed another
l'\ Messrs. Swift and Son. Il is kno\\n asan ' Improved "Wale's"
M icroscope.'

Mr. < ieorge Wale, of America. de\ ised in 1S7!> a plan of great
merit fort he stands of microscopes. The • limb ' \\ Inch carries the body
and the stage, instead of being swung by pivots as ordinarily — on
the two lateral Supports (so thai the balance of the microscope is

greatly altered when il is much inclined), has a circular groove cut
on eit her side, into \\ hich lits a circular ridge cast on t he inner side
of each support, as shown in lig. ISO. The two supports, each

WATSON'S '&' MICROSCOPE

225

having its own fore-foot, are cast separately (in iron), 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 ' (1893;.

226 THE HISTOKY AND DEVELOPMENT OF THE MICROSCOPE

are drawn together by a screw, which thus regulates the pressure
iii.-itle liy tin- two ridges that work into the two grooves on the limb.
When this pressure is moderate, nothing can be more satisfactory
than cither the smoothness of the inclining movement or the
balancing of the instrument in all positions; while, by a slight

ls(l- Sv ed ' Wale's ' microscope (1881 and 1883).

tightening of the screw, il can be tinnly fixed either horizontally,

vertically, or at any inclination. The 'coarse' adjustment is made

.1 smooth working rack: the line adjustment is Swift's patent

on ].. 17L> (lig. I.",:.). ,-md the attachable mechanical stage of

firm can be readily added (a> in fig. |so). lmt in the best and

LEITZ'S ENGLISH FOEM OF MICEOSCOPE

227

most complete form of the instrument a large mechanical stage is
fitted, and sub-stage apparatus supplied.

Leitz, of Wetzlar, provides a very useful instrument of the same

FIG. 181.— Leitz's IA stand (1898).

228 TILE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

class, li has a tripod base on the English model, and is a thoroughly
steady Lnstrumenl : it has rack and pinion movement to the coarse
adjustment, and snl> stage; the draw-tube has a mm. scale, andafine
adjustment oft in- usual Continental type, and all the latest adaptations
I'm- sub stage illumination. The instrument in its simplest form is
remarkably low priced, and the more important apparatus can be
;,dded to il 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 line 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, stive in its fine adjustment, this form ap-
proximates somewhat to the Continental model. The fine-adjust-
ment lever is rather 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 >tage over the principal stage; to this the slip is
clipped, and the whole super-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 line
adjustment patented )>\ that firm (q.v.) It was first made from the
designs of Mr. Iv .M . Nelson, and it presented three distinctive
features :—

(1) The milled head of the fine adjustment was placed on the
left-ham' side of t he limb.

(2) The stage was of a horseshoe form, the aperture being
entirely cut out to the front of the stage; and

(•'!) The hod\ tube. \\hich was of standard size, viz. 8| inches.
was made in two pieces, \\hich not only secured portability, but also
permitted the use of hot h long and short, tubes.

Tliis instrument is illnsl rated in lig. I,",."). It was also possessed
of a cheaply made and fairly good centring sub-stage, to carry
1'owell and Lealand's dr\ achroinat ic combination fitted with a turn-
out rotary arm to curry stops. The suh si age was made by adapting
Suift's cent ring nose piece, and providing it, with a rack and pinion
focussing arrangement, as illustrated in lig. 183. There was also a
,'jra dualed stage plate and sliding bar. a plan devised by Mr.
Wright for a Under. The eye pieces were provided with rings, like
IWell and Lealand's, outside the tube to govern the depth which

i should slide into the dra\\ tube, by which means the diaphragm

s in the same place whatever the depth of t he eye-piece employed.

it was constructed to do critical work with the highest

FIG. 182.— Messrs. .R. and J. Beck's third-class microscope (1888).

230 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

<>J ' ilii* instrument ha.- more recently been intro-
duced liy the firm of ('has. Baker, of Holborn, London. It arose in
a suggestion I iy .Mr. Xel-on that thi.- form should be adapted to the
Campbell <1 i[f erential screw fone adjustment, making a good quality
bhird-class microscope. It should be noted that the differential
screw permits of slow action being obtained by means of coarse
tln-i'tiilx; it is therefore very strong. In the ordinary Continental
form of direct -acting fine-adjustment screw, if the motion is slrm\
tlie thread must be fine. Hence in forms where the fine adjustment
is made to lift the body, the differential screw is of great value.

Further, it proved on testing that the Campbell differential screw
was eijual to the most 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-
quent Iv a larger and heavier instrument was made, having a J inch
more of hori/.ontal 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
] ihoto-niicrographic pur-
poses, and no special
detriment 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-
ment, but those which
we have seen have not
given the aperture .suffi-
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
microscope-, which aimed chielly ;,t supplying the student with
relatively inexpensive Instruments, but winch at the same time
should possess ,-dl the ijiialities a nd be capable of receiving all t he
apparatus neei I fill i;,r an eili.-ient use of the microscope. One of the
higher form- arising in this neu departure is the instrument shou n
at fiii. 177. and. \\ith the Campbell screw fitted behind the mirror
lor the line adjustment of the condenser, is a very attractive and
useful microscope. ;md may lie safely recommended to the amateur
and t he -t udent .

T\\o microscopes by Ko— certainly deserve the attention of the
student seeking a reliable in-,t rument belonging to the class we are
• •onsideriiin-. They are both known as - Ros-'s New Bacteriological
Microscope.' The work of this long-established firm.it is needless
to say, is of the very line>t <jnalit\ : and these microscopes are pro-
wit h all the required adjunct- for the work they specify. The
of horse-hoe form; the fine adjustment is sensitive and firm.

FIG. 183. — Ceutriiu nose piece u-t-<l as Mill-stage
(1881).

BOSS'S RECENT MICROSCOPES

231

The principal difference between the two instruments is in their
respective stands. The one shown in fig. 184 gives a wider .spread
to the tripod base than usual, securing greater stability ; but this does
not involve great space in packing, because the hind ' toe ' of the

FIG. 184 — Ross's new (tripod) bacteriological microscope (1898).

tripod is made to fold forward between the two fixed front toes when
not in. use.

The other similar instrument is 011 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 HISTORY 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
-lio\vH in fig. 185, by rotating the pillar upon a reliable fitting at
its base, so th;it absolute steadiness is secured. This is a revival

Fie. 185. — Ross's in \\ bacteriological microscope (1894).

ID <>ld form made in 17(50 by J. ( 'nil', adapted by A. Ross in
and now again used by Ilie same linn (ride fig. 128).

also manufactures an • Kdncational' microscope having
which may fairly V placed in this class. It

MICROSCOPES OF THE FOUKTH 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 perfect
balance for the body, however it may be inclined. This admirably
made instrument is considerably under 51. 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, representing 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, suffi-
ciently so to justify us in
departing from a rule to
point out that with two eye-
pieces, two objectives — a
^-inch and a ^-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 Physio-

FIG. 186. — Ross's educational 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 differential screw actuated
by a large milled head, and capable of work with at least a ^th-inch
objective. It is beautifully swung, and is firm in any 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 HISTORY AND DEVELOPMENT OF THE MICKOSCOPE

Fram ' microx-opr of Mi->srs. Watson and Sons. We illustrate it in
fig. IS'.I. IT i> >tron<i' mid rigid, and its workmanship is of the
order. It has a completely steady tripod foot with a spread

Ki,,. 1 ,-,,._ | icrk's liritish student' nn.-i-oscope (1898).

A STAND BY SWIFT

235

of 7 inches, and its steadiness is unaffected in whatever position the
body may have to be inclined. The coarse adjustment is a diagonal

FIG, 188. — Swift's histological and physiological microscope (1894).

236 THE HISTORY AND DEVELOP3IENT OF THE MICROSCOPE

rack and pinion, while the fine adjustment is the now celebrated
lever employed liy this firm. One revolution of the milled head

FIG. IMI. Widson's 'Fram ' microscope (1898).

• the :;,11,,th of an inch. A.S we have seen (p. 170,
••"U11-'11"1"' is sound in principle. ;md in practice all

FIFTH AND SIXTH CLASSES OF MICROSCOPES 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 instruments of its class.

There is a microscope manufactured by Messrs. Zeiss, known as
' Stand VI. 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 accuiacy and good
quality of workmanship 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 Powell, whose maxim was that a microscope with
only a good coarse adjustment was to be preferred to one having an
indifferent fine adjustment with a sliding tube for the coarse adjust-
ment.

This stand is of cast iron, with a flat tripod, 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 underside of the.-e
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 much 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 'journey-
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 extremely simple microscope of the
fifth class is made by Watson and Sons ; it is illustrated 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 jW-in. objective. The stage is large, the body has a

238 THE HIST"|;Y AND DEVELOPMENT OF THE MICROSCOPE

i
di-.-iu -t ulic. c,-iu be inclined, and it is ;i steady useful microscope. It

c.-iii be obtained complete in a case with one eye-piece for the sum
of -21. 7s. (v/.

FIG. I'.MI. -/..-, 1,1,1 VI. A (1898).

LOW-PRICED STANDS

239

Bausch and Lomb manufacture an instrument \\liieh 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

i|ii;i lit v. .-mil i> thoroughly well made, its object being to meet the
\v;int> iii' M-linnU inn! i-liMiii'iitary workers. "We believe, however,

PIG. 192.— Reichert's stand No ir> (1890).

tor man\ reasons, that it i- better tn rely un an excellent rack and

pinion coarse adjustmenl I'm- siu-h a ])iii-|Mi>f. This instrument is

a> iiii'cliii" • a distinct demand, for though of excellent

RECENT AMERICAN MICROSCOPES

241

workmanship it is sold for twenty shillings. "We illustrate it in

fig. 193.

FIG. 193. — Bauscli and Lomb's lowest-priced microscope (1897).

Reichert, of Vienna, manufactures an instrument 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 HISTORY AND DEVELOPMENT OF THE MICROSCOPE

scopes iidiii' 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.

tary work and school use, and, whilst its finish and \vork a re admirable,
sold for I/.

\ rc:illy beautiful instrument of the same class is made by
•InTl. depilated 'Stand No. I .">,' which is illustrated in lig. li)'2.

It is admirably made, and the maker, as we think, wisely, has thrown

A GOOD LOW-PRICED MICROSCOPE BY LEITZ

243

the best possible work into a spiral rack-and-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

Fit.. 195. — Powell and Lcaland's portable microscope (184<si.

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

K 2 "

-Swiff, porl ible histological microscope (1894).

POKTABLE MICROSCOPES

245

be used 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 earliest and still
the best form of this kind
of microscope was made 1 >y
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-piiiion 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-jj inch, can be

FIG. 197. — Baker's diagnostic travelling
microscope (189(5).

246 THE HISTORY AND DEVELOPMENT 01- THE MICROSCOPE

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
1<H x ~)i x 3i inches, and weighs 6 pounds complete.

Fiii. HIM. -Bausch and Lomb's portable microscope (1898).

mm inal<r> a small useful iust i-iuiH-nt for travelling called
Mln- Diagnostic' microscope, designed by Surgeon -Major Ross,
superintendent, Indian Army Medical Department.
97 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 micrometer
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 8-5 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 been 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 wTith 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 DEVELOPMENT OF THE 3IICKOSCOPE

arranged for use with <-nm jinirnd lenses, has 1 iff -11 devised by employing
tlif binocular of 31i-. Stfpheiison. This iiistriniient is illustrated in
li-. -_!00. It is made by Swift and 8011. The stage may be enlarged
MS a dissecting table, with special rests for the arms. The objective
ami binocular part (if the body remain vertical and focus vertically
b\ a rack-and-pinion coarse adjustment, there being no fine adjust-
ment. The liodies 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
majority engaged in insect dissection or the dissection of very
delicate and minute organisms or organs.

Another type of dissecting microscope has been introduced (as
\ve 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, larva?, 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 effected. 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
different 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.

\Vi- liaxc now to consider the most priiiiiiire 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
pillar (lig. 202), provided with a rack and-pinion movement, and
carrying a jointed arm movable in manv directions by ball-and-
orkft and other joints. />, c, e, but capable of being clamped by
thumb screws or milled heads. <i, 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
1 lie arm makes with its summit, and by using the various joints,
aliii'M any position and elevation may be given to the lens that can

required for the purposes to which it may lie most usefully
:'ppli"d. care being taken in all instances that. 1 lie ring which carries

LENS-HOLDERS

249

the lens should (by means of its joint) be placed horizontally. The
lenses now most suitable for such a holder are those constructed

J.SWIFT &. SON. LONDON

FIG. 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 HISTORY AND DEVELOPMENT OF THE MICEOSCOPE

30 times, .-mil. employed in such ;i stand ;is fig. 202, they are ad-
miral ily 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 Mipport to such a height that when the lens is adjusted
thereto, the eve may be applied to it continuously 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 desirable 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 obsfi-\er. In a suitable position these lenses with their holder

may In- must 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 ariy magnifier mounted on a horizontal

Although the uses of this little instrument are greatly

by its want of stage, mirror, Arc., yet, for the class of pur-

o which it IN suited, it has advantages over perhaps every

er foim that has been devised. Where, on the other hand,

LENS-HOLDERS 2$ I

portability may be altogether sacrificed, and the instrument is to be
adapted to the making of large dissections under a low magnifying
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 >»
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 mav
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 being
n-adily substituted for the other, as nmy 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 inilled-head pinion attached to
a pillar at the further right-hand corner of the stage. The length
of the rack is sufficient to allow the arm to be adjusted to any
focal distance between 2 inches and J 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 micro-
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 MICKOSCOPE

i> fixed to tin- \\ooden l>;isis of the instrument, and places for the
lenso arc made in grooves beneath the hand-supports. The ad-
vantages of this general design have now been satisfactorily de-
mon.--! rated hv tlie 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 (ISi"''. •,

to the hand-rests) may be easily adapted to individual 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

KM,. 204.— Bausch und Lomb's i "Burnt-si 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

special lliey are designed for the instrument. They are

which undoubtedly gi\e a large aplanatic field and fine

definil ion.

the \t-ry Le*1 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
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

r

illustration, and the points of attachment of the other are seen.
The stage has a large opening, 3 x 3| inches, into which can be
placed either a flat brass plate or a glass substitute, or a metal plate
with a half-inch hole in it. Underneath the stage are black and
white screens, which can readily lie turned aside by the use of the

254 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

milled heads, A. The arm, which is focussed by an excellent spiral
rark-\v<>rk adjustment, carries either a Zeiss dissecting microscope,
which, with and without its concave eye-lens, yields six different
[lowers, varying from 15 to 100 diameters, or the arm will receive
the very fine Zeiss-Steinheil simple magnifiers.

The' instrument is provided with a large plane and concave
mirror on a jointed arm. The utility of this simple microscope is
very a\ -eat. 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 instruments, 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 knou-n
as the Continental 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 firm 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 cultivated 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 foi'ty years
may lead to an entire reconstruction of the stand — a whollv new
model — intended to meet all the requirements of modern high-class
work in all departments, and with a line adjustment of the most
relined class. We cannot doubt, if tin's were so, that the same
genius \\hich has so nobly elevated the optical requirements of the
instrument would act with equal succe» on its construction and
mechanism. We ha\e l>een told in the I'rieiidlieM spirit, by one
intended in the Continental stand, and a master in optical
'hat on the < 'nit inent the microscope is • act ually almost
used' in a vertical position. NYvert heless 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 case 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 1'4 N.A. are used with large illuminating cones they become
so sensitive to focal adjustment that the Continental fine adjustment
(the best form of which has hitherto been used by Zeiss) is not
sufficiently 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 through 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
,.g-3-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
services 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 efficient,
microscopy

256 THE HISTORY AND DEVELOPMENT OF THE MICKOSCOPE

At the same time we do not blind ourselves to the fact that
;ni English market for the ' Hartnaek ' model has had very
iiiucli to do with the perpetuation of the errors which that form
contains.

The reason of this it is not difficult to trace. The inductive
nirtliml ;idv;inct'd lint slowly, in practice, upon the professional
activities, and even the professional training, of medical men. The
country \\hirh \\;is 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
empiricism 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 se\ ei it ecu I h rent ii TV. It was not until the early years of this cen-
turv that the modern school of medicine began its beneficent career.
But at that time (//>• 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 philosophical toy, a mere plaything
for the rich iliii'tfu ute. 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
inst rumeiit, all its essentials being more or less completely developed.
.Meanwhile, on the Continent the microscope was regarded by the
I ''acuity 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 in, 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, \\ere easilv 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 invent ion. To this we simply say that it may be
true that their de\elopment has not depended on the immedi«l<-
wants of the microscopist, but was in many cases the result not of
ingenuity so much as of powerful insight and foresight. And how
"('ten have these anticipations been realised! Because early obser-
vations i, fa hislological character (and therefore of a nature to lie
beyond the sphere of the lay amateur) had been successfully made
\\ith a cei-tain form of microscope on the Continent, it was practi-
cally argued that this must be the most suitable instrument for such
a purpose ; luit this was an inference made without knowledge of or
reference to the well known English models.

US carefully examine this instrument. The typical form

hat made hy llailiiack. Seen in its primitive state, we have

catalogues of all 1 he Continental makers — Zeiss, Leitz,

COMPARISON OF CONTINENTAL AND ENGLISH MODELS 257

Heichert, and the rest. It is a non-inclining instrument, with a
short tube on a narrow horseshoe foot, in which steadiness is
obtained by sheer weight. It has a sliding-tube as a coarse adjust-
ment, and a direct-acting screw for the fine adjustment. The stage
is small, and the aperture in it is relatively still smaller, of no service
in reaching 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.
1 nit what are the facts? Let us take an Oberhauser of 1837. ami
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 ^-inch objective properly ;
the fine adjustment works in jerks, and the lateral movement cau>r-
the object to go out of the field. The Powell will now work an
apochromatic of 1'4 N.A. oil immersion with accuracy and precision :
but if a 11 apochromatic oil immersion of 1'4 were placed on the Ober-
hauser it would be at great risk to the objective. Xmv 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 in 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 stih-staije 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 up 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 liea\y
horseshoe foot, steady enough while the instrument is non-inclining,
only needlessly heavy, 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-micrographic outfit the Messrs.
Zeiss practically see this, for they supply (inntld-r foot to u-Jnch the
microscope is clamped. Messrs. Bausch and Lomb tell us that the
foot of their ' B B ' Continental microscope is • //< -iic'ilij leaded to ensure
greater stability.' Sidle and Poalk (1880) and McLaren (1884), and

s

258 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

11 >w IJos-. adopting this foot, employ the added mechanism of the
revolution of the pillar on the foot (an old device) to secure stability
.-it all inclinations ('•!'}<' fig. IS."),].. :2:i2). Surely if the horseshoe
foot were satisfactory for the inclining microscope these modifications
\\utild not have been deemed needful. Besides which we note that
for the same purpose the Continental 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
Held is extremely useful as a finder, but this advantage is completely
lost with the original small Continental tube. That this is seen to
In- a disadvantage would appeal' certain, because the photographic
ni'icroHCojii' iiinild of Zeiss has a la /•</!•/• body-tube ; 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,
is a manifest, if slow, conformity of the primitive model to the
English type, and hardly supports the affirmation 'that (during the
last fortv years) the Continental microscope has closely followed the
wants of the microscopist.'

The if I red -iniiin/ Ki-fi'tr, 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 •"> numerical aperture, especially as a micrometer
screw \\ith 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, ami thus its utility must be wholly
gone.

The iss'.l model has a ne\\ form of line adjust ment, 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
'bat in the new photographic stand made by this celebi-ated
firm, with so extremely delicate a hue adjustment (fig. 12'J), we
I'ave learned through their English representatives that only owe-

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
they may be for class and school purposes, if they use a fine ail just
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 T^th 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 error* 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. Mo>sr>.
Zeiss have profitably departed from this fault by giving to their
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
up 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 bodv
and limb solid with the stage, so that the whole rotates to-
gether.

Practically there is only one point in favour 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.

1. 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 easier to
rotate the illuminating pencil. The only instances in which rotation

b 2

260 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

of the object is important are : (a) WJien 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
si; me rot at in:: 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 can be set at the proper angles, and
then the object rotated without disturbing the relative positions of
the pi-isms.

lint this cannot be done with the arrangement of the Zeiss
model, which rotates body and stage. The firm have, however.
more recently introduced a rotating stage based on the English
model, and we are glad to give our testimony to its admirable
workmanship and perfection of centring. The contention, however,
that \ve think in all friendliness is sustained, is that the charac-
teristic.-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.

(/•]) For photo-micrographic purposes. — In this case, in the Zeiss
si a nd. 1 lie 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 was
regarded on lli<> < 'ont/nciit as a superfluous, if not a foolish, appliance ;
but that prejudice has been killed by the light throAvn on the whole
question by (l)*he 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 focnssed by tic rack-and-phiion movement, but also by
means of « xpiTnil j'n/r tn!j nut iin'iit for bringing out its most delicate
results. lint even a condenser was in use in England in the year
1(591 (vide fig. 101. p. 133), and the best work in England since the
inyeiiiion 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 \\ork without a sub-stage; but.
a permanent centring and focussing sub-stage, into which this optical
arrangement could, amongst others, lit. might be made with half the
labour, ingenuity, and cost. I Jut rather than this, we have in the

less recent forms the c leuser made to slide on the tail-piece, and

to be jammed \vit h a screu .

// //tin therefore m-ltlu-r <•-,•////•///,/ n<n-j'<icnxHin</ //ear; but, striking
as it may apj ear, a >/in/>/i,-ai/iii, \\hich cannol lie used with, and is no
part of, the condenser, /x *n/,fili,;l iii ;, stand not of the most recent,
'"i* ((f coin] aialivdy recent make. /'•//// im'chanical centring and
rack \\oi-k focussing movements ! That is to say, the delicate cent,-,'

THE PURCHASE OF A MICROSCOPE 261

of an optical combination might in that instrument t« kn car* <>f itself,
but a diaphragm aperture must be centred by mechanism and
focussed by rack.

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 arrangement 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. 1(57. the l.-itest and
finest form of Zeiss's best microscope.

There is no fault in the workmanship; it is the best possible.
The design only 'is faulty \ 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
model. To all who study carefully the history of the microscope
and have used for many years every principal form, it will. \ve
believe, be manifest that the present best stand of the best makers
of the Continent is an over -burdened instrument. Its multiplex
modern appliances 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. ue Mibmit
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 purchaser has little knowledge of the instiument, and
does not profess to know what are the indispensable ] arts 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. We 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?' 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 HISTORY AJND DEVELOPMENT OF THE MICEOSCOPE

parts of. -i microscope will commend itself to all workers of large and
broad experience :

1. A coarse adjust incut by rack and pinion.

2. A -ub stage.

3. A fine adjustment.

4. Mechanical movements t<> sub-stage, i.e. focussing and centring.
."">. Mechanical stage.

I). Kack-wnrk to dra\v-tul)e.

7. Finder to stage.

X. Plain rotary stage.

'.I. <; rad nation and rack -work to rotary stage.

10. Fine adjustment to sub-stage.

1 1 . Rotary sub-stage.

]'2. 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 witli a rack-and-
pinion coarse adjustment, -A fine adjustment, but no s«l>-xt<i</<'. Or a
microscope with a coarse adjustment by rack and pinion, a sub-sta^v.
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 importance of all the others is in the order of
their numeration.

A i Hither matter of some significance to the tyro is the relative
value, from the point of view of time consumed, and therefore of
I'rinie cost, in producing the several kinds of microscopes. The
No. 1 stands of li.-ilf a dozen makers may be near the same cost, but
nia\ nevertheless have involved the consumption of very different
<liianiities of the highot class of skilled labour in their production.

.Manifestly the first thing to be lookedat in a microscope making
any pretensions to i|uality is the character of the workmanship ; and
this should carry with it the question how much machine, and how
much hand \\ork mid fitting there is in it. Arcs graduated on
silver, tor example, are very attractive, and with many are most
impressive; but. thev are simplv machine work, and quite inex-
pensive.

In the two great, types of models, the liar movement and the
Jackson limb, the bar movement involves more than double the
actual hand-fitting; while a line adjustment with a movable iiose-
piece takes twice the lilting of one in which the whole body is moved
by the line adjust incut screw. I n the same way a -mechanical xtayK
which is made of machine-planed plates, sliding in a machine-ploughed
groove, is much less costly in t hue and quality of la hour than a hand-
made sprung stage. So a Nnl>-xlti</<' having a movable ring pressed
by tuo screws against a .spring has very far less work, and work of
a louer class, than one with .1 true rectangular centring movement.

It will follow, then, thai a .lackson limbed microscope with no
movable nose-piece, with a machine made mechanical stage and a

movable ring for sub-stage, \\ill not have involved more, perhaps,

than a t hi i (1 of ) |u> skilled work which must lie e\| ended on a well-

SPECIAL MICROSCOPES

263

made instrument of the same size with a liar movement. But if we
compare the range of prices as presented by English and American
makers, we rarely find an equivalent difference in cost.

Then the tyro will lie warned by this not to purchase a pretentious
instrument with a bar movement and mechanical stage for, say, 51.

But if a low-priced
instrument is to be
jmrchased, 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-piece,

and

have a differential
screw fine adjustment,
a large plain sta^r.
and an elementary

centring

sub-stage.

Such an instrument
should be obtained for
51. 10s.

Although not fre-
quently used, it would
lie doing our work im-
perfectly not to refer
to a form of micro
scope devised for
chemical purposes by
Messrs. Bausch and
Lonib. The object of
Prof. E. Chamot, of
the Cornell University,
in inducing these op-
ticians to make this
microscope was,
says, to enable
chemist who
mastered the use of the
microscope ' to employ
the elegant and time-
methods of

he
the
had

FIG. 206. — Microscope for chemical purposes (1897).

saving

micro-analysis,' thus

giving him ability ' to

examine qualitatively

the most minute amounts of material with a rapidity and accuracy

which are truly marvellous, not to speak of the many substances

for which 110 other method of identification is known.'

An illustration 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 i> not provided witli a jointed pillar to secure inclination.
Tin.- coarse adjustment is by ruck and pinion ; the fine, by the usual
micrometer screw of this firm. The stage is circular arid rotates,
lieing provided with centring screws, and its margin is graduated
into degrees for measuring crystal .ingles. Except, for this graduated
circle the stage i> faced with hard rubber. The sub-stage is adjust-
able by means of a quick-acting screw. This is fitted with polarising
apparatus, consisting of a large ISTicol prism 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 pola riser, 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
petrologica] microscope ; it is not intended for petrological or
mineralogical work, it is simply an instrument made at a very
low pi-ice, 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.1 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
diffused daylight or artificial light falling freely upon its surface.
With higher powers an illuminator is used which fits the tube of
the microscope, 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 If) \\ilh regard to the axis of the lube, 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
irreat attention is to be devoted.

As source of light the ' Auer,' a triplex burner, adjustable in

height, may lie recommended;- it is placed .-it a distance of one

metre from the illuminator. The flame is surrounded by an iron

r asbestos cylinder, with only the necessary aperture for illumination

of the object. The source of light should be at exactly the same

\\iththe lens, It, of the illuminator. On removing the eye-

filr <>/>/;/,• und Mechanik, N... 17, I.s:i7.
l I, Reii lifi-t.

SPECIAL MICROSCOPES

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, b, is made to converge, and

Oc

M

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 by the
lenses of the objective itself. The illuminated object sends back a
portion of the light, which passes through the objective and the plate
a, reaching the eye at 0 c.

The object to be examined should have two parallel surfaces, so

266 THE UlSToKY AND DEVKU >P.MEXT ( >E THE MICKOSCOPE

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
\\ itli tin- screws, SS. by \vhich means it may be tilted, and the upper
surface of tin- object made to lie in a truly horizontal plane, which
of course i> m-cr»ary in order to place the entire field in the focus
of the instrument. The stage is a mechanical one, the milled heads,
T'" 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 placed 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 adjusted. 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 preparation of the sample is a>
follows : —

Tl le piece of metal to be examined has two of its sides planed oft"
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 surface 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 \\a.\ is removed, and the surface wiped dry with a
soft cloth. A little oil is next poured over it and allowed to remain

for lil'teeli Illillllte-.

It is then dried again and rubbed on a piece of chamois leather
until it assumes a shiny appearance.

When large piece> of metal are to be examined, small portions
must In- polished by hand and etched as described above.

Ki--. -JUS ,-nid -JlHI are phot omicrogra pi i> taken with this instru-
ment, which are self-explaiiat or\ of the nature of the work it does.

Tank microscopes (a IM> called ai|iiarium microscopes) have, for

certain kind- of work, a value of their own. They may be used

with lou powers outside the glasv ,.r above the water; or the

object glass may be protected by a water tiuht tube outside it, and

with a discol'glas> lixed (also water tight) into that end of the tube

vliich stands below the front lens of the objective, at a proper

for the focus, may then be plunged into the aquarium.

Indeed, the t U be of t lie instrument may be so protected as to work

TANK AND AQUARIUM MICROSCOPES

267

for some depth, and have some range in the water 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.

'*-.

> ..,-Vt'

FIG. 200. — 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 upper milled head. The milled head at the
side, by pressing on a loose plate, fastens the bar securely to the
aquarium.

268 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE

Between the ends of the bar slides an arm carrying a sprung
socket, and the arm can be clamped at any given point of the bar.
Through 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 t his 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 l»e approximately regulated.

The arm of the socket is hinged to allow of the microscope being

, faffs- ^. =r--^gt»(, -=- "a? -~-

<i*F±r r^ .--.T/f.^.r,-v,'7r;?.-:- ••\'-r

FIG. 210

inelined ina plane parallel to the sides of t he aquarium. The loAVer
milled liead clamps the hinge at any desired inclination.

'I 'I ie sockel also rotates on the arm, so thai tlie microscope can be
inclined in a plane parallel to the front of the aquarium. Thus any
point of t he aquarium can be readied.

As an adjunct, and admirable aid to the student of t he lank and

pond, as \\ell as a simple and easy means by which specific forms of

microscopic life may lie found and readily taken, ue call attention

the lank microscope of Mr. <'. Kolisselet. It is illustrated in fig.

- 1 1 and scarcely needs Further description.

< >ne of Zeiss's Steinheil aplaiiatic lenses, to which we ha\e

MR. ROUSSELET'S TANK MICROSCOPE

269

of the
upon

referred, is carried on a jointed arm, which is clamped to the tank,1
the tank being nowhere deeper than the range 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,
arranged upon the body
clamp, as seen
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 tl it-
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 fi ee 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 process glass tanks
are made with melted seams ; these cannot possibly leak, and are to
be preferred to those with the ordinary cemented joints.

1 We prefer 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. — Rousselet's aquarium microscope.

2/0

CHAPTER IV

ACCESSORY APPARATUS

THIS c-h.-i pter on apparatus accessory to the microscope might be
ea>ilv ma ile to occupy the whole of the space we propose to devote
tu 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 describe, and to explain the mode of success-
fully employing, the essential and the best accessories now in use,
neglecting, or only incidentally referring 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 opt Lcians.

I. Micrometers and Methods of Measuring Minute Objects. — It
is of the utmost importance to be able with accuracy, and as much
simplicity as p<»sible, 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 Lucida and Drawing.' 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 l:n/>irlt nc,il<>. both being magnified to the same extent.
The amount 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 •()! inch, the object measures
'he -III inch, and whether \\eareemployinga magnifying power of
a hundred or a thousand diameters is not a factor that enters into
our determination of t he si/.e of 1 he 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-
ment v may at any t hue he made.

In favour of the above method of micro-measurement, it will be

•d (!) t hat im extra apparatus is required. ('I) that it is ext remely
simple, and (.'!) that it is accurate.

MICROMETER EYE-PIECES

271

The most efficient piece of apparatus for micro-measurement is
without doubt 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 parallel
wires, 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 p;iss
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 off at right angles
to the filaments by a scale formed of a thin plate of brass Imving
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 appeai'ed 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. Usual I v
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 yJ^th of an
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 d«mbt, is the one employed
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

2/2

ACCESSORY APPARATUS

convenient l)iit important. because the magnification is not uniform
throughout 'he field. If the power employed is high, in order to
eft'ect tln> .-pan of the great magnification, one wire (the fixed central
one) will lit- in the middle of the field, the other at the margin, and
the comparison will not lie true on account of the unequal magnifi-
cation of the eye-piece throughout the field, whereas if the wire be
placed live 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 gla>s plate with crossed lines, which together with
the eve-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.

FK;. 213.

Fig. _l:i illustrates this inst riiment, complete and in longitudinal
section.

Kach dixision on t!,e edge of the drum corresponds to 0*002 mm.
Wliole turns are counted on a numbered scale seen in the visual
field, .-ind the image may be measured up to H mm.

A modification of this instrument, facilitating both accuracy and
>implicity. was in iS'.Mt devised by .Mr. Nelson,1 of which we think
highly, and of which we give an illustration in fig. 214.

This SCreW micrometer eyepiece differs from tllose of the old

form mainly in two respects : first, the optical part is compensated ;
secondly, the micrometer part uith both webs can be made to
traverse en line, the field of the eye-piece by .-crew motion.

More particularly speaking, the instrument consists oft wo parts :
1 Journ. /.'. M.S. IS;M>. ]., 508.

THE BEST FORM OF MICROMETER EYE-PIECE

2/3

one, a flat rectangular box containing the fixed and movable* webs,
the micrometer screw, and divided head complete; the other part
may be called an ' eye-piece adapter,' with an 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 affects 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 u>ed 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 wel >
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) Different-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 \\ebs
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 scren- ni/ri'nniiti /• n-itli siiccess it should not be inserted,
as the custom has been, like an ordinary eye-piece into the tube of
the microscope, but It shoidd JMV& a Jinn stand quite independently,
preventing 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 01- circnLir foot. The foot in either case only rests on three
I mints ; the upright is capable of telescopic extension by a clamping
tube ; a short tube which takes the eye-piece is fixed to this upright
liv .1 compass joint.

To use 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 -j^ths of an inch of space
between the body-tube and the micrometer tube. It will be now
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 ; hut if it is not so provided 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 -j-gVoth °f an "lcn by means of
a stage micrometer in the focus of the objective ; this was replaced
by a mounted specimen of A nip1iii>1<>nra pellnclda, and he has counted
ninety-six lines in the t 0Voth "^ ;IU 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
iWoiroth °f 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
various 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, where lenses are
(incorrect ed 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 lie made with every set of measure-
ments.

Moreover, the majority of -ta^e micrometers exhibit very con-
siderable discrepancies in the several intervals between the line*:
it is \\ell in the interests of ace u racy to take t he screw value of each
under a high power, liud the value of the average, and then note the
particular space or spaces that may lie in agreement with the average
and always use it. An illustration will make this clear.

provides a stage micrometer of I mm. divided into '1 and

en

a

2;

5
<
H

r :;

- -

X

a
o

w

a

TO OBTAIN THE VALUE OF A MICROMETER INTERVAL 275

•01 . The following are the actual values obtained for each of the '05

divisions, viz. :—

o'4U

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
8-2-",
8-38

8 '38 mean value.

In this instance it will lie seen that the last division. 8'38, agrees
with the mean, and is the best for all future use.1

Having thus obtained a screw-micrometer value for a certain
known interval, the screw-micrometer value for any other objivt
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 n9QK
- = - ='0o85 mm.

8'38

If the answer is required in fractions of an English inch, all that
we need remember is that 1 inch=25-4 mm. ; then

.|i-,
(ii) 8-38 : 6'45::o_° : ..: inch ;

6-45 x -00197 -0127
.<•= -= =-001515 inch.

8-38 s-38

If the stage-micrometer is ruled in fractions of English inches,
then suppose the screw-micrometer value for , nVnth inch = 4-2.">7.
and that for the object=6'45 as before.

(iii) 4-257 : 6-45:: -001 : scinch;

6-45 x -001 .

.«_•=- — ='001515 inch.

4-257

1 In the number yiven 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 off beyond this.

T2

AC C ESSOR Y AP PA R ATUS

1 1' t IK- an>wei • i- required in metrical measurement, then as 1
2.V4 nun..

4-257 : G-45 :: ('001 x 25-4) : .<• mm. ;
li-45x-0254 -1638

(iv)

lii thi> connection it will be as well to give two example> of
scale comparison which are sometimes required. Thus you have a
certain interval on a metrical stage micrometer which you know to
be accurate, and you wish to compare an English stage micrometer
with this -cale in order to find out which particular interval of-j-J^
inch agrees with it. Suppose '05 mm. = 8'38 screw value as above,
t hen all that is nece-sary is to find the point to which the screw
micrometer must be set in order that it may accurately span the
,,,',,,, inch. Take 1 inch=25'4 mm. as before: then -001 inch =
•0254.

(v) -05mm. : -0254mm. :: 8'38 : x screw value;

•0254x8-38 ,__
./'= =4'257 screw value.

•Oo

Conversely, if a metrical scale is to be compared with an accurate
English one where '001 inch=4'257 screw value, then the screw
value for '05 mm. may be found thus: '001 inch='0254 mm.
(vi) '0254mm. : '05mm. :: 4'257 : x screw value;
•05 x 4-257

•0254

- = 8'38 screw value for '05 mm.

A cJii'ti/i substitute for the screw m'cnwweier has been devised by
Mr. f!. Jackson. It consists in having a transparent arbitrary scale

A 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 T(1\)(1 inch or
,',, mm., as the case
may lie. is found in
t erms oft he arbitrary
scale. The value of

I:

i

L,_,

Fli . '2\:>. Jackson's • micrometer.

''"' "''.i1''-' iii Iti'ins of the same scale is also found, and comparison
accordingl. All that need I..- done i.s t,, substitute the terms

: he ai-hit rai-\ scale for screv \alnes in the preceding examples, and

they will meet the ca-e.

ESTIMATING THE EDGES OF MINUTE OBJECTS 277

The arbitrary scale should be capable of movement by a screw,
otherwise the appliance is hardly as accurate as the first method of
micrometry by simple drawing described above.

Of all the methods of micrometry the most accurate is that
performed by photo-micrography. A negative of the object to be
measured is taken, and then, without any alteration in tube- or
camera-length, the magnified image of the stage micrometer is pro-
jected on the ground glass; this is spanned b\- 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 must 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 affecting large and coarse 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 aperture 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 a long scries
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 perhaps 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 aperture 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 Viand 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 adjustment.

II. The Camera Lucida and its Uses. — There are a large number
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 ACCESSORY APPARATUS

'2. Those provided for it when used in H vertical position.

We shall describe what we consider the most practical forms of
each.

In point of antiquity Wollastoris camera lucida 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
uas in some sense supposed to be compensated by the use of lenses.
as seen in the figure ; but the difficulty 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. — Simple camera.

right to be turned to the left, and vice i-<-i-nn. 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, usually made of highly polished steel, which is
placed in the path of the emergent pencil at all angle of 45° to the
optic axis, thus reflecting rays from the image upwards. 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 ray-, 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 mu>t be kept perfect l\ centred to the small reflector.
As there is hut one reflection, the image is inverted, but not trans-
posed. To see the outline of t he image as it is iii the microscope,
i he drawing must he made upon t racing paper, and inverted, looking
•if it as a 1 1 -a n>pareiicy from the wrong side.

There is considerable \ariei \ in the experience of different

microscopists as to the facility with which these t\\o instruments

The difference iii all prohabilitv depends on the

CAMERA LUCIDJE

2/9

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 simplest 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 emergent pencil. The idea
\vas 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 gla>s.
In 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 through 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 effected 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.
Kelson in 1894.1 It takes into
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 transposi-
tion unaltered; therefore all the objects drawn with this camera
are unlike the originals. In illustration place the letter p on
1 Journ. B, M. S. 1895, p, 21 ei seq.

280

ACCESSORY 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 turn this paper round.
hut this object, as drawn by a Beale's camera, will appear "q, and no
turning of the pa pel- can cause it to appear as the original ; it will
only become so when it is viewed as a transparency from the other
side of the paper.

This is, of course, important in many matters with which the
microscopic biologist is concerned.

In many forms of earner;! 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. Kelson consists of a right-angled
prism or small glass mirror 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 reflect them upwards to the eye.

The mirror corrects the transposition, and the neutral-tint the
inversion ; an erect image is therefore seen on tie, 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
modified by screens, not by change of focus in the condense]-, 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

Object on i
1 stage.

1 maur strn through /
the eye- piece. \

Image projected on screen
or on sensitive plate.

MVU through Woll-
aston'S camera.

|IM;IL'I> ]irojecti-i| on taMr
l'1, l"i mirror or right
an.L'lr'l prism, as 'lrvi-,',1

liv c. \V. Cooke.

Image seen through
ground glass.

6

Image seen through
Beale's neutral tint or
Soeimueriug's reflector.

Image seen through Nel-
son's camera.

'I'!"' insf nimrnt referred fco in (/ ) of the above table of inversion

and transposition in microscopic images is a somewhat distinct form

of camera called by Mr. Conrad \V. ('nuke, who devised it in 1865, a

• Micrographic Camera.' The projection of the image is dependent

silvered niin-oi- lixed at lf>". or a riglil -angled "prism. By the

arrangement of this instrument an image can be thrown on a sheet

I paper pluced in a hon'/.ontal position, so that one can readily trace

ABBE'S CAMERA LUCIDA

28l

on the paper the outlines and details of the image with ease and
accuracy ; only it must be remembered that the mirror or prism
erects the inverted image (No. 1 in the above table), but its trans-
position is due to the fact of its not being viewed as a transparency.

This instrument is also useful for the purpose of demonstrating
where two or three persons 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 from the observer.

Coming now to the second group of cameras, there stands first on
the list an instrument 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.
ft. Burch in 1878 (Journ. 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 (S9 fig. 220), is reflected by a large mirror

w

FIG. 220. —Abbe's camera lucida.

in a horizontal direction, 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 horizontal, and vice versa, and it presents the image
precisely as it is seen in the microscope. For the purpose 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 which this one does not so
readily lend itself, which is none the less of great importance ; that is,

282

ACCESSORY APPARATUS

the del crn i i 1 1 i ML;- < it' the magnifying power of objectives. It is manifest
th; it the distance between the paper and the eye of the observer
cannot be MI readily determined in this ease as in those forms of the
instrument where the image of the paper and pencil is seen direct.

The same appai'atus arranged so that the prism casing together
with the mirror may he swung hack 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. (3x2 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 l()-5cin. (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 camera is attached
to the tube by means of the clamping-ring K, and the Abbe double

I CO. '2'21. — Abbe's camera, improved.

prism is ceiilred by means of the screws L and H. The brightness
of the drawing surface and the microscopic image is respectively
regulated by a cap It encasing the prisms, which is provided with a
clear opening and the moderating glasses of varying degrees of
density, and by an eccentric disc I! 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 cpiickly and conveniently be
exchanged for a not her with an a pert i ire of 2 mm.

The prism, together with the moderating glasses, may be turned

aside about thr vertical pin Z into the position indicated by the

lines shown in lig. 222. When the prism is returned to its

original position it is fixed by a catch, which is not externally

visible.

[n the use of a good drawing apparatus (1) the light from the

LATEST FORMS OF ABBE'S CAMERA LUCIDA

283

image must not to any serious extent be weakened by the light from
the drawing material. (2} The image of the drawing paper must

-J-;; i.^i:

s

Fiu. 22-2.— Latest modification of Abbe's camera

re; id i the eye with the least possible intensity and be coaxial with the
microscopic image. (3) There should be an arrangement by which

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 APPARATUS

and capable "f being centred in its horizontal plane. (5) It (should
lit- possible to easily separate the apparatus from the eye-piece and
replace it again in its former position at will. ((5) The image of the
plane of the drawing, and the image of the microscopic object pro-
jected on it. must !>e seen with the apparatus without distortion.
As regards the first two conditions the arrangement of the original
Abbe camera is adopted, viz. two rectangular prisms with the hypo-
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. '2'2'2, 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
purpose* 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, whicli
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 colour, 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 effected 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.

Jn order to pass conveniently from observation through this ap-
paratus to observation through the free eye-piece, the prism with its
diaphragm arrangement can l>e rotated to one side about a vertical
pin Z ; the return of the prism to its central position is marked by
a spring catch. To obtain drawings free from distortion, a drawing
table similar to that described by J )r. Bernhard ought to be
employed.1

This useful instrument has, however, been modified and made
simpler by more than one optical linn. Messrs. Swift have con-
structed a very handy and easily applied form, which is so arranged
'''•it the microscope may be employed with it not only in the
vertical hut also in an inclined position. It is illustrated in fig. 224.

This camera lucida is precisely mi t he same principle as the Abbe
form used lor the same purpose, hut being manifestly less bulky it is
far i IK. re convenient and easier to use. although less efticient for very
• •aivl'id \\ork.

1 Zeitschr.f, wiss. .1/7 /,•>•. \i. US'.MI. pp. -j

ENGLISH AND AMERICAN MODIFICATIONS

285

When this form of camera is used, the paper 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 extremely good and easily applied
modification of the Abbe form is manufactured by
liausch and Lomb. and is illustrated in fig. '2'lo.
The Abbe prism is used as in the large Abbe
drawing camera ; the mirror is reduced in sixe
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 mav be swuny back when not in use. as

i/

shown in the dotted outline. We can testify that

the image off both object and pencil-point are clear.

and this instrument can lie used with most eye-pieces ; but cannot for

complete results be counted equal to the drauing camera of Abbe.

The Editor has used with great facility and suceessa camera devised
by Dr. Hugo Scliriider. and produced by Messrs. Ross. It is figured
at 22(5, and consists of a combination of a right angled prism (fig.
227) A B C. and a rhomboidal prism D E F G. si> arranged that when

Fiu. 2-24. — Swift's
camera lucida on
the Abbe principle.

FIG. -'25. — Bausch and Lomb's modification of Abbe's camera.

adjusted very nearly in contact (i.e. separated hy only a thin stra-
tum of air) the faces B C and T) E are parallel, and consequently
between DE and B E' they act together as a thick parallel [date of
glass through which the drawing paper 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 microscope, 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 I)G. At J a part of

286

ACCESSORY APPARATUS

the r.iv is reflected in the eye by »r<H itn,-?/ reflection in the direction
(lf. I K. ;ind :i p.-n-i transmitted to ,)' on the face A C of the right-
angled prism. Of the latter :i portion is also reflected to K by
ordinary reflection ai J'. Tlie hypotenuse fe.ce A C is cut at such
an angle thai the relied ion from J' coincides with that from J at
t he eye point K , 1 hns utilising the secondary reflection to strengthen
the luminosity of the image. The angle G is arranged so that the
extreme marginal ray II' from the field of the B eye-piece strikes
upon I)G at a point just beyond the angle of total reflection, the
diffract ion Lands at the limiting angle being faintly discernible at
this edge of the field. This angle gives the greatest amount of
liidit hv iinJiiiiii-i/ reflection, short of total reflection.

In use, the microscope should be inclined at an angle of 45°. and
the image tbcussed 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

rainrra llN/ida.

l''n;. 227. — Diagram explaining Schriider's camera lucida.

under the camera. The observer will then see the microscopical
image project eil on 1 he paper, and the fingers carrying the pencil
point will lie clearly in view, the ir/t<>t<.: pupil of the eve being
available for both images, the diaphragm on the instrument bein^
considerably larger than the pupil. The eye may be removed as
often as required, and. if all is allowed to remain wit limit alteration.
I lie drawing may be left and recommenced without the slightest shift-
ing of t he image.

If a \ertical position of the microscope be needful, this may be
done by inclining the table and drawing paper to an angle of 4?>°
either in front or at the side of the microscope. For accurate
drauing. in all a/.imutlis, the drawing paper should of course coin
'•ide \\ ith the plane of the optical image. When the paper is in its
proper position, the limiting circle of the field of the microscope
be projected a- a true circle, but if ot her\\ ise it will appeal-
It is recommended that a circle about the sixe of the
dra\\ n upon t lie paper, and its coincidence with the projected
field compared.

s
z

THE USE OF THE CAMERA LUCID A 287

This camera may be used with a hand-magnifier, or with simple
lenses vised for dissection and other purposes.

With one or other of the foregoing contrivances, every one may
learn to draw an outline of the microscopic image ; and it is extremely
desirable for the sake of accuracy that every representation of an
object should be based on such a delineation. Home 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 tracing 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. When 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 between the size of
the tracing and that of the object is affected 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 paraffin lam}),
seen to the left of plate III., which illustrates the correct method 01
using the camera lucida. This lamp is simple, and is capable oflicini;
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 <• .ndenser 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 ACCESSORY APPARATUS

coloured glass as may tye found needful, and the lamp illuminating
the paper and pencil, and carefully shaded al>ove, is also seen at the
eye-piece end of the body-tube. Often, if the image is too bright.
\\r find that bringing the lamp down to illuminate the paper more
inten>ely suilices I f not, use screens ; the illuminating cone must
not !»• tampered with.

TIT. The Determination of Magnifying Power is an important
and independent branch of this subject. For this purpose, and for
t In- reason given above. Beale's neutral-tint camera1 is eminently
suitable— -indeed, is the best. We can easily and accurately measure
ili,. p.-itli of the ray from the paper to the eye. What is necessary i*
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 l>y an example.

Suppose that the magnified image of two r ,-/,-,- jjths of an inch
division* of the stage micrometer spans njths of an inch on a rule
placed as required ; then

(i) '002 inch : -8 inch :: 1 inch : x power ;

'8x1

,'• — ' =400 diameters;
•002

for it is obvious that under these conditions one inch bears the same
proportion to the magnifying power that , ,MH1ths of an 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 T^- mm. when
projected covers ]sll inch ; then, as there are 25'4 mm. in one inch.

(ii) -02 mm. : (-8 inch x 2.V4) :: 1 : x power;

•8 x 2.r4 x 1

— 10 ](} diameters.

'()_

[fthe reverse is the ca.se. vix. that you have an English stage
micrometer and a metrical scale, then, if the magnified image of two
,,,',,,, ths of an inch spans 18 mm.,

(iii) -002 inch :_*.?_:: \ :X;

7087x 1
x= o =354'3 diameters.

Theahoxe n-Milt.s indicate the combined magnifying power of the
ol>jecti\e and eye piece taken at a distance often inches. The arbi-
trary distance of ten inches is selected as heing the accommoda.tion
distance for normal \ i.sion.

'lie ma--iiifviiig power, bowever, i.s verj dilfei-ent in the case of

i pa

TO FIND THE INITIAL POWER OF A LENS 289

a myopic observer. Let us investigate the case of one whose 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 inclio.
To do this he must cause the objective conjugate focus t<> approach
the eye-lens ; consequently he must shorten his anterior objective
focus. In other words, he must focus his objective nearer the
object. This will have the effect 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 bring>
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 shortening 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.1 In practice
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, ami
refractive indices of the lenses, is required. But a very approximate
determination, sufficiently accurate for all practical purposes, may
be easily made, especially if one has a photo-micrographic camera ai
hand. The principle is as follows :—

Select a lens of medium power — a J-inch is very suitable. Xow.
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 object i\ e. 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 (') ; the quotient will
give the initial power of the lens at ten inches to a very 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. HM">. Article on measurements of magnifying
power of microscope objectives, by E. M. Nekon.

U

290

ACCESSORY APPARATUS

the error introduced by M small displacement of the posterior prin-
cipal torn- d<>e> lint materially amount to much.

There is a further error introduced by the approximation of the
objective to the stage micrometer in order to focus the conjugate at
Mich a distance, but this i> small. We can see, therefore, that this
error tend> to .-lightly increa>e 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
li\ 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 measured. 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.2

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-piece^.

Nose-pieces, although thought to be so. are not a modern idea :
our predeces.-ors 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 fils over the end of the nose-piece, and so keeps the several
objectives which may be inserted in position. It dates, in all proba-
bility, from the end of the seventeenth or the early part of the
eighteenth cent ury.

Hut in the early davs of the microscope rotating discs of objec-
tives. ,-is shoun in fig. -Ji!H (or. perhaps, older still, a long dovetailed

FIG. 228.— Rotating disc of
objectives. Benj. Martin
(circa 1776).

•J-J'.i.— Sliding plate of objectives. Aduins (1771).

slide of objectives, such as tig. 'J-J1.) shows), were frequently
employed.

It is continually desirable to be able to substitute one objective

for aiiot her wit h as little expenditure of t hue and trouble as possible,

to be able to examine under a higher magnifying power the

details of .-in object ol'whicli a general view lias been obtained by

' M< <>1. '•.xxviii. No. 981, ' Optical Till.. I, Mjih. 1,\ I'Yunk

' 178, • Measuri I '• m -.' by( E. M. Nelson.

NOSE-PIECES

291

means of a lower : or to use the lower for the purpose of jiadiny a
minute object (such as a particular diatom in the midst of a slideful)
which we wish to submit to higher amplification. This was con-
veniently effected by the nose-piece of Mr. C. Brooke. \vhich, being-
screwed into the object end of the body of the microscope, carries
two objectives, either of which may be brought into position bv
turning the arm on a pivot. This is shown in fig. 230.

The most generally useful of
all nose-pieces now in use arc
the rotating forms, which enable
one to carry two, three, or four
objectives on the microscope at
one time, and bv mere rotation
each is successively brought
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 Messrs. Powell and
Lealand, and adopted in the forms shown in figs. 231-233. There can

FIG. 230.— Brooke's nose-
piece, as made by Swift.

FIG. -J.".l.

FIG. 23-2.

•I'.y.'i.

be no doubt that for ordinary dry lens work M>me 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
mirror, the other having a coarse 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 drawback to the use of a rotating nose-piece is the extra
weight it throws upon the fine adjustment. As this subject is fully

U 2

292

ACCESSORY APPARATUS

t rented 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 tor 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-ineh. .\-inch, and ^-iiich objective of English make
weigh together 8', «/. without any nose-piece. But Messrs. Watson
and Son have de\ ised and made in aluminium a dust-proof triple
no>e-piece, which, where it is required to be used, reduces the objec-
tion.-- to its employment to their minimum, and not only in greatly
reduced weight, bill in other ways, makes its use more feasible
without strain upon the fine adjustment or danger 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-
tive.- from dust and moisture. Messrs. Watson devised a dust-

-:;1- — ^ Bison's dust-proof aluminium nose-piece.

PlG. •_!;;:>. — Section ,,f Mir alx.vc.

proof arrangement, consisting of an upper and an under disc, having
a spherical curve; to the lower disc are fitted three 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
micrn.se,, pe its axial coincidence is indicated by a, spring catch. The
edge is Covered \\ith a metal rim. making it dust proof.' The weight
"f( ']"' "rd i nary l.rass nose piece is 4;j o/.. ; the \\eight of this one is
Simdar instruments are made by other makers, but the

• lust proof arrange nt and the extreme lightness are. so far as we

enow, characteristic of the instrument of .Messrs. Watson. We
illustrate this nose-piece complete in !i-. :2:U, and in an enlarged

* t • \ — O

< ( • f • I i / 1 1 i iii \ i * .• • ) ' ' .

section in fig. -i:\:>.

r the proper use of a rotating nose-piece the length of the
ive mounts should be so arranged that when the objective is
hanged little focal adjustment \villbe necessary.

.'•'•I! 'lit calotte nose piece for four objectives is made by

CHANGING NOSE-PIECES

293

Zeiss ; this is so arranged that only the optical portion of the objec-
tive is screwed into the nose-pier*-. This plan much lightens it, so
that the nose-piece and the four lenses weigh 3j| oz., or only 1 ox.
more than an English j-inch with a screw collar, and ^ oz. more than
an English ^-inch of wide angle.

A centring nose-jnece 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 co>tly, that Midi a nose-
piece turned upside down, with a turn-out rotating ring for stops, &c.,
fitted below, made a very efficient rectangular centring sub-sta^e
at a small cost. Sub-stages are now quite common and cheap, and
centring nose-pieces are seldom used for any purpose.

Next to the rotating, probably the c//"//'/'".'/ nr>«>'-/iiecv 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, ttc. 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.

Xachet's changing no.M-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 screw. s of
which are -J-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 changer 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 permit the objectives
to be centred to one another : and the second is that they have

294

ACCESSORY APPARATUS

adapters to equalise the length of the objectives, so when a change
of objective- i> made little change of focal adjustment is required.
Ki L:-. _.'!''>. -'.'i7 show the nature of this arrangement. In Kelson's
changing Dose piece a small ring with three studs is screwed on to
the objective; a nose-piece is screwed on the microscope, having
t hree slotsand three inclined planes. Therefore, by placing the studs
into the .-lots and giving the objective a quarter of a turn, the
Muds 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 dispen-es with all extra apparatus.

Three port ions of the thread in the nose-piece of the microscope
it >elfai-e cut away, and also three portions on the screw of the

Pro. 23C. — Xei^sV sliding-objective
changer, with objective in

FIG. 237. — The objective detached from
the body-slide.

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 screu engage.- j ii.-t as i f i he \\ hole ,-crew were there, and
the objective faces up in the usual manner. This plan ill 110 way
injure.- either the microscope or 1 he object ives for use i n 1 he ordinary
way; thus uncut objective.- will screw into the nose-piece, and cut
objectives will screw into an uncut nose piece. This plan is similar
to that employed in dosing the breech of guns, and it was seeing one
Of them in ISS-J which suggested fco .Mr. Nelson to adapt the same
principle bo the microscope. Sub-djueiitly it has been found that
in IHC.'.I Mr. James Vog.-in had proposed much the same plan, only
ing away two portion- instead of three : it is curious that such
an excellent j,!,.;, \Vas allowed to drop.

An "/m///.v/,,,/ nose piece j> th.-it \\hich 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 revolving nose-piece for the purpose 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
performance in a particular azimuth could be immediately noted.
This plan had, however, been previously in use by Professor Abbe
for a similar purpose, but not, as we believe, made public.

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 practical principles.

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 prepai • •< 1
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 termo 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 ACCESSORY APPARATUS

cent res it. t he differences in latitude and longitude maybe noted, and
will give the constants for the correction 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.

.">. 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 3x1 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 centre of a 3 x 1 inch glass slip shall
be f>0, 50.

5. That the division shall be in : —ths of an inch, and the scales
one inch long.

I f 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 pouers. "We have found, for very delicate work,
that \vecould log with advantage between the divisions, thus: say
' long. 41 ;' but if slightly over, but not an estimated half, '41 -f ; ' if
half, '4H ; ' if more than this, but less than 42, it is logged ' -- 42.'
For logging purposes 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 lie used or not at the will of
t he worker.

The other Under u e de.sire to consider is called after its inventor,
and is known as • Maltwood's finder.' x

It consists of a micro-photograph, one square inch in size, divided
into '_', .")()(> little squares, so that each is -^th inch square. Each
square contains t\\o 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 Inr it ; then the figure in the square which most nearly agrees
u |<I' the centre of the field is noted. ( )f course, both the object and
;l"' Malt \\ood finder must lie carefully made to abut against the
stop.

There are two drau backs to this tinder.

I'he divisions are not line enough, so that it is only suitable
for |o\\ |H »\\ers.

'I'he removal of the slide, and its substitution by the Maltwood

i/ oro S . new series, vol. \i. ]sr,s. p. .v.i.

DIAPHRAGMS 297

finder, render it extremely unhandy when using an immersion objec-
tive, all the more so if the condenser 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 several 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 illumination
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 diftractioiial
effects. Therefore it is better
to use a larger aperture
farther 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 FIG. -238.— Zeiss's iris dmplmigm.

which have been much in use

lately, both here and on the Continent, are a mistake for critical
work.1

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 the duty of several. It also
permits of careful adjustment.

The iris diaphragms are so comparatively inexpensive, that they
have superseded for general work and ordinary purposes all others ;
but whatever diaphragm is used it should work easily. Iris dia-
phragms work sometimes so stiffly that the microscope may lie 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 t< > the under side of the metal stage.
Now, if there are neither washers nor a shoulder to the screw, it is
1 Quekett, Micro. Journ. vol. iv. p. 121 et seq.

298 ACCESSORY APPARATUS

more than probable that when the diaphragm is rotated it will
screw up ami jam. The purchaser may easily observe a matter of
this kind. ( Minder diaphragms, which were invented in 1832 by
C. Varley. arc much used . m rhe Continent ; they are also often made
into iris form-. 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 daylight alone. The object of this method
of illumination being to render very translucent objects visible by
increasing the si/.e of the black diffraction bands at their edges, it
is. as before stated, of no use for critical work.

Condensers for Sub-stage Illumination.1- -This condenser is an
absolutely indispensable part 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 atiirm 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 some 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 arrangement 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 microscope 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 simple lenses much of the earlier progress of microscopic
investigation is attributable; and that known as ' Wollaston's
doublet.' devised in lH2'.t. was a decided improvement in all respects.
It consisted of two plano-convex lenses; but this was again improved
by 1 'rite-hard, \\lio 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 achromai ic t imes.

<;<.od results, within certain limits, mav be obtained by means of
the best lYitchard doublets. AVith a ,V,th inch the surface of a
strong I'.idnra scale m.-iy lie seen as a surface svmniet rically scored
Or eii'_:Tav eil ; but t he Kditor has never himself been able to reveal the

mdensi r' tlimujrliout this work is applied to optical appliances for
li.'t is known 8,8 the ' bull's-eye ' is not called a ' comlenser.'

EAELY CONDENSERS 299

'exclamation' marks, and a.s tliis is the experience of the majority
of efficient experts, it may l>e taken that no resolution of these was
accomplished in pre-achromatic 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 were 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 be as perfect <(sthe apparatus for vision, and on this account
I would recommend that the illi'mi x<i1i m/ lens should he pt-rfx-/!,/
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 impossible 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 used 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, bright 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 lie :
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 I). Brewster writes of it with appreciation, saying that
' it pei-forms 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 image of /'/><•
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 l>y
Dr. Wollaston.' I5ut Sir I). Brewster objected to this, contending
that the source nf Injlit itself should lie focussed upon the object, lie
preferred a Herschelian doublet placed in the optic axis of the micro-
scope. But, whilst there is a very clear difference between these
authorities, we can now see that both 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 | 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
from the lens of .-it least five times its focus, so that the difference
lift ween diaphragm focus and • white cloud ' focus, or the focussing
of the image <>f a white cloud upon the object, was not very 'great.
lint l!iv\vsTcr was writing of a name from a saucer of burning spirit
and salt wlini he insisted on the bringing of the condenser to a
focus on the object, and in this he was. beyond all cavil, right.

In IS:!!) Andrew Ross gave some rules for the illumination of
objects in the ' Penny Cyclopaedia.' These were:—

!. 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 lie placed nearlv at the apex of the cone. The object was seen
better sometimes above, and sometimes below, the apex of the cone.
.'!. With lamplight a bull's-eye is to be used to parallelise the
ravs. so that they may lie similar to those coming from a white
cloud.

( )f the old forms of condenser, that devised by Mr. C4illett 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-
phragms ,-ind .stops an excellent one. and it is not clear why it has

fallen into disuse.

It had been thecnst to recommend the n.se of this instrument

racked <-////o- ,i-illii,i in- tr'tlltaiit if.* foCliS. Carpenter employed it
u it hoi it , and (Bucket t within, and one or other of These met hods was
general. Put in the use of good achromatic condensers with high
power work it soon became manifest to practical workers that it is
only when, as Sir David Mrewster pointed out, the source <>i'/i<jhtis

focussed /'// ///'• condenser on the object that a really critical image

i> to be obtained. And .Mr. Nelson readily del istrated this fact

even with the condenser dillett had devised.

I'll'' next condenser of any moment is a most valuable one. and
t it i lies one of t he great modern improvements of the microscope.

I' was an achromatic condenser of !7<>c devised and manufactured

I i'.. -Joli. — (lillctt's condenser, from
on tin- Microscope.'

POWELL AND LEALAND'S CONDENSER 301

by Messrs. Powell and Lealand. We have used this instrument for
thirty-five years on every variety of subject, and we do not hesitate
to affirm that for general and ordinary critical work it is still un-
surpassed. Fig. 240 illustrates this apparatus. The optical com-
bination is a 1th of an inch power, and it is therefore more suitable
for objectives from a ^-th 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 condenser so high a place amongst even
those of our immediate times, it may be well to specify what the
requirements are which a condenser employed in critical work with
high [lowers 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.1 Directly spherical aberration makes itself
apparent the condenser fails ; that is, when, on account of under-
correction, the central rays are 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 practical service it must have a
working distance sufficiently large to
enable it to be focussed through
ordinary slips. It would be an
advantage if all objects mounted
for critical high-power woi'k were
mounted on slips 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 Fi.i. 240,-Powell and Lealand's
combine a large aperture with a condenser,

great working distance the skill of

the optician is fully taxed, for this can only be accomplished (a) by
keeping the diameter of the lenses just large enough to transmit
rays of the required angle and no more ; (/>) by working the convex
lenses to their edge ; (c) by making the flint lenses as thin as
possible.

Now it is due to the eminent firm whose condenser we have
been considering with such appreciation to say that the condenser
referred to (</) 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
readily 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

' This is one of the many expressions which are inevitable to the practical use of
apparatus; it is simply convenient, and means & full cone uf li.yht — a cone with none
of its rays stopped out.

302 ACCESSORY APPARATUS

tor low powers. \Vhen the highest class of work li;is to be done it
;.s- ,„-,-,//'/// tn have condensers nnif<"f 1<> tli<> power of the objective used.

\ ilrv apitchromatic condenser of merit is made by Swift and
Son; it. has a X.A.of O95 and an aplanatic cone approximating

n-'.i± and works witli ease through any object-
slide, but is corrected to do this by thinning
The front lens and setting the front and back
<• liiibinat ions further a part than would be the
case if they were used as an objective. The
lower combination lias a large, clear aperture.
The optical part of this instrument is shown in
lig. '2 H ; we have used it. and find it a tho-
roughly practical and serviceable condenser.

Ki. -'ii -Swift's- Before the introduction of the homogeneous

rlmmiiitio ilMii'.i) con- system, and the production of such great aper-
1 x A 0'95. turns by Powell and Lealand as a 1 •;"> in a ^-th,

a /..tli, 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 snl isequent introduction of the apochromatic system of lenses,
much larger cones were required. To meet this necessity Powell
and Lealand, at the urgent 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
mi 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 -()7, and
for aperture and working distance is. like its dry predecessor, quite
unapproachei 1 .

Messrs. Powell and Lealand have produced an entirely ne\\
condenser, strictly apochromatic, employing a Huorite lens in the
combination, and presenting features in the highest degree desirable.
We find its K.A. to lie (>•'.).>, its focal length long enough for a thick
slip, its aplanatic aperture •'.). We haw found it of the utmost
pra-.'tical value in critical work, and this valuable apparatus has
been greatly increased in efficiency by the application of a device
l»y Mr. E. .M. Nelson, providing it \\ith n correction cnlltir, which
can be used with the utmost ease, no matter in what position the
microscope may lie. It is similar in practice to the correct ion
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, sn that it may lie conveniently reached by the finger. The
whole condenser is represented in lig. iU± and the arm for moving
the correction collar is seen on the right of the optical tube; it
turns at the slightest touch, and the collar moves onlv the back
Icn^ of i he combination, lea\ ing the mount riyfid.

The object of this correct ional movement is primarily to increase

the maximum aplanatic aperture of the condenser ; this is ell'ected

fating the lenses. 1 1' the hack of a wide angled objective be

NELSON'S CONDENSER ' CORRECTION COLLAR

examined when an object is illuminated by the full aperture of the
condenser, the edge of the fiame 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 when the
condenser is racked up.
although the exterior
shape of the illuminated
portion will become
more circular, two dark
patches will appear on
either side of the centre,
showing the ' operation
of the spherical aber-
ration of the condenser.

If under these circum- FlG 242._Nelson,s correction collar to Powell.s
Stances the lenses be apochromatic condenser.

separated bi/ means of

the collar adjustment, the black spots will be close*! u ^,, i/,»f a circular
and evenly ill" initiated disc will appear. This is a distinct optical
g.-iin, and will enable the observer to see more than he could
I lave seen before. Mr. Kelson 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, «) ; these can easily be resolved into
stride with a J--inch objective, but the probability is that these stria*
are not the real structure but rows of minute perforations incom-

FIG. 243.

L anrooiv.

FIG. '244. — Wutsi m's oil-immersion condenser.

pletely resolved (fig. b) ; by using the condenser with the collar
correction these striae 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 record it that it can be with facility exactly resumed
at any time (Journ. K. Micro. Soc. 1895, pt. ii. p. 2:->l-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

power>. 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 bv means <>f a cemented triple back lens, as is shown in the
illustration of tin- optical system in fig. 244. The only flint glass
used in it is the middle of the triple back. The total numerical
aperture is I'.'!.'!, the aplanatie aperture being in excess of T25.
The magnifying power is ] inch, and the clear aperture at the back
of the lens is ,'^ths inch, and it works through a slip of '()7'.\
t hick.

With the front lens removed it is an efficient dry condenser for
medium powers, magnifying Sths inch, with a total N".A. of '56, the

aplanatic aperture being
over -5. It is mounted
like their ' Parachro-
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 5-ths
inch. It is shown here

principally

PIG. 245. — Watson's parachromatic condenser.

mounting,

for the
which is

identical with that used
with fig. 244. The

collar into which the optical part fits carries an iris diaphragm ; on
the diametrical edge of this is engraved a scale showing the 1ST. 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.
iMii 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 principle of an oil-immersion
objectiv< . The X.A. varies from
I ••'!•"> to 1-4, and the aplanatic
cone is about I'.'i 1ST. A., the
\\orking distance being fully •()('>.
We can speak highly of this

instrumenl : it is in our iudir-

" 16 Beck - new acl Mt.ic , •'

,,,,. ment the best condenser ever

made by tin's firm.

Another condenser lias been made recently by the same firm.
A. of l-ii. the maximum obtainable without immersion

SUB-STAGE CONDENSEES

305

contact. Its aplanatic aperture is -9 X.A. 1 '0. We illustrate this

form in fig. 247.

FIG. -247.— Beck's condenser with N.A. TO.

It is with great pleasure that we are able to announce the
production, by the firm of Zeiss, of a ' centi-iit</ oil-immersion achro-
matic condenser' of. X.A. 1-30. This is what we have long desired
to see. and we have used it with admirable results. Jt gives a large
illuminating aplanatic. c-oiie, hence verv oblique illuminatinor rays

rpi , - • J

Hie centring ar-
rangement is the
same as that of the
achromatic conrlen-
><']• of 1 lie same firm
living 1-0 X.A. It
is supplied with an
iris diaphragm of
the most perfect
workmanship, and
the condenser is
focussed not only by
rack - and - pinion
movement, but also
?>>/ means of a special
fine adjustment ; this
is accomplished 1 >y
the aid of a rotating

ring provided with a differential thread, as will IM- seen \>\ examining
the illustration we give in fig. 248. This allows the condenser to
be easily focussed ' at intervals of about ()•()] mm.' 'By means of
this fine adjustment the condenser may lie focussed up to about
1 mm.'

Messrs. Swift and Son make a panachromatic dry condenser
having a X.A. 1 •(".), an aplanatic cone of (HIM, and it works well when
a critical image is desired. It is well corrected for colour; they
also make a panaplanatic oil-immersion of X.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 arliroinutir
condenser (ls;i:i .

306

ACCESSORY APPARATUS

I-'K.. -2 111. I'uker's new achromatic condenser
N.A. 1-0.

iris diaphragm supplied with this condenser is graduated to show
tin- N.A. \\hen greater accuracy is required, but the still more
accurate method of employing fittings with separate discs with their
N.A. marked on them is also supplied by the makers.

A very complete achromatic condenser is now made by Baker of
Eolborn. This condenser is a modification of the well-known Abbe
form, in that the diameter of the component lenses is considerably
smallei : this reduction in the size of the lenses, allowing, as it does, of

greater freedom of move-
ment of the mechanical
stage, has been effected
without in any way de-
creasing its optical effi-
ciency ; on the contrary the
aplanatic aperture lias 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
aplanatic : the diameter
of the back lens is '2'2 mm.

(]-.', in.) and the power of the condenser as a whole is 10 mm. ( p, in.)
with a working distance of 2-5 mm. d1,, in.): with the front lens
removed for low-power work the power is reduced to 20 mm. ( flt- in.),
and the working distance, which is calculated with the lamp liame
at ten inches, is increased to 1O5 mm. (i 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
(l)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. \\ e 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, lint if we have a
suitable object and a perfect objective, it is the strong conviction
of some leading expert- bhat, as we increase the cone in aperture, we
increase the perfect rendering of the image, until the point is reached
\\here the cone from t he condenser is equal to the aperture of the
objective. This ideal can be realised with fine apo- and seini-
apocliromatics up to •'.'> to- I N.A. With the most perfect objectives
of the present day of •."> N.A. and upwards we iind in practice that
iie.M results are obtained uhei i a cone of ] ight is used which, on

the removal of the eye piece, is found to occupy three-quarters of

t he area of t he back lens of t he object ive.

No condenser is sufficiently free from spherical aberration to

• equal to its own aperiure. ( 'ondeiisers are all more

or less under corrected, and consequent Iv focus their central rays at

EFFECTIVE APERTURE OF CONDENSER

307

a greater distance than their marginal ray.-. 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 centre will be beyond the
object.

It is \vell known to those practised in microscopy that, in the
case of a narrow cone from a well-stopped-down condenser — that is.
a condenser used with diaphragms of relatively 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 oi' 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 have their full intensify, 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 mere 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 critical
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.

\Ye may measure the total aperture of a condenser just as we
do that of an objective, viz. by means of Abbe's apertoineter.1 But
the effective aperture cannot be measured in that way; that is to
sav, the aperture of the largest aplanatic cone (or cone free from
spherical aberration) the condenser is capable of giving, cannot be MI
discovered.

To do this, place the condenser in the sub-stage and an objective
on the nose-piece ; focus both 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. Kow move the object just out of tin-

FIG. 250.

FIG. 251.

FIG.

field, remove the eye-piece and examine the back of the object he.
and if the aperture of the aplanatic illuminating cone is < /renter 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 ••">. we know that
the aplanatic illuminating cone cannot be less than •:">. If now we

1 Chapter v.

x -1

308 ACCESSORY APPARATUS

dose the diaphragm MI 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 tlie diaphragm ; thus in fig.
•IT} I it is a trine less than -5 X.A.

In a similar manner the apertures of the other diapliragm open-
ings can be determined.

Now let the diaphragm be opened to the full aperture, and an
objective with a wider aperture, say '95, be used. It will perhaps
be found that before we are able to fill the back of the objective with
liifht b\- 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 appear-
ance shown in fig. 253. The last point before the appearance of the
lilm-h HjintN i/it/icates the largest aplanatic aperture of the condenser, and
i.s III' It mil of the condenser for critical n-ork.1

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
-iven them as full a representation as can be fairly desired. But
there are still some forms that either from their own peculiar value
or their historic import a nee deserve consideration.

A condenser known as t/>p • Webster' was first made in 18(i5. and
is still a very useful one for low powers. 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 price, and on. that account will be
welcome to some workers.

I nits 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 (v///</r,^r/- which has been very lai-gely used in
England and America, and which has secured a great deal of com-
mendation, is 1 hat 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 perfectly frank 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 merit
as to the fad that it has been employed much by those who,
previously ignorant of the value of ,m,/ condenser, have at once
perceived 1 he ei il la i iced value of the results 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 ami pertinaciously taught thai the 'correct'
histological microscope musl be of the llartnack type, and that it
should lie used uith narrou angled dry lenses, perhaps a -ith-inch
focus, and no illumination but that afforded by a small concave
minor, the focal point of which is extremely doubtful or unknown,

'I"' ol.jcctivc and the Condenser.' E. M. NYl-.m. /.'//</. Modi.
184, 1888.

IMPORTANCE OF APLANATIC CONDENSE!! 309

and in practice wholly disregarded. Ko doubt a student instructed
on these lines would be astonished indeed when he exchanged such a
practice for the illumination and improved image afforded by ;ui
Abbe condenser.

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 with those of oilier 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 condensers we oillv Abbe's chromatic condenser.
require the practical testimony of

Professor Abbe to prove ; for /ts>. has since produced an achromatic
condenser of much merit, to which we give consideration 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 Herschelan doublet. This combination is
shown in fig. 254, and the general form of the instrument, as applied
to Zeiss's own microscopes, is shown in fig. 255.

The power of this condenser is low, and its aperture 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 |th inch is ¥Vth inch. Its aplanatic aperture is therefore
only -5. Now, whilst it is a gain of 110 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 top 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 TO the base of the
slide should always lie 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

AC( ESSDRY APPARATUS

of ••") X.A.: for such illumination, in fact, it is perhaps the best
illuminator extant, and shows objects on a dark ground with
sparkling brilliancy, and may be used with polarised light.

A chmiiiiitic condenser, somewhat similar in construction to this,
and of lo\v price, is made by Messrs. Powell and Lealaiid ; 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 \ ields a someuhat larger aplanatic cone. This instrument with
its diaphragms is shown in fig. 256. It is more convenient in form.

FIG. '2~>r>. — AMie's chromatic r.nidciiscr 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 ,",, inch, and that of Abbe's being

1 n; ii"-h. This instrument, of l'o\\ ell's, if fitted in the usual way,

vould l>e now a very ellicient inst rumenl of its kind and quality.

'I'l"- particular quality of oUiniie illumination was in fact still

further advanced by a modified form by the same makers known as

truncated condenser, \\hich gives great obliquity with

'undance of light, but it is as a matter of course very chromatic.

liaphra.-iiis (tig. iT.f,. A) have a central aperture for the

POWELL'S CHROMATIC, ABBE'S ACHROMATIC CONDENSER 3 1 [

pui-po.se of centring, and the movement is made by means of an
outer sliding tube 1>, with a slot at the top in which the arm A fits.
and another arm. B, is placed at the lower end so as to give ready
command of the rotation. This plan allows of the nse of one or t\vo
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. I) is the
optical part of the condenser placed immediately above the dia-
phragms and in oil-immersion 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
diaphragm is screwed to it by a screw in the eccentric hole, shown
in each. It will lie 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 u>e in
high-power photographic
work, but in fact of much
more general utility. It
consisted of a single front
with two double backs, and
it projects a sharp and per
fectly achromatic image of
the source of light in the
plane of the object. Its
power is low, being ^ inch
focus, and it has a total
aperture of I'D. Its great
superiority over the chro-
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 instrument yields a similar
cone of -65. But we have already expressed our pleasure that even
this form has been surpassed by the high quality condenser illustrated
in fig. 257. Like its predecessor, it is large and heavy ; and. with
great 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 | 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 carries a disc of metal to receive the diaphragms,

PIG. 256.— Powell and Lealainl's
chromatic oil condenser (1880).

312

A(( KSSOEY APPARATUS

stops, A:c. (herthi.s is fitted a ring into which screw adapters, which
will allou other condensers to be used on the one mechanism.

The metal disc should have a central aperture as large as the
largest hack lens of any of the combinations to be used with the
mount. It should be thick enough to receive two stops or .dia-
phragm- at a time. This power to alter a diaphragm or stop so as
to secure any required arrangement of apertures and stops without

FIG. '257. — Abbe's achromatic condenser (1888).

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 cmie they cut out. Empirical 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 different numeri-
cal apertures for each may be
marked on either side of the
diaphragm or stop. Memory
cannot fail if we make the
lotrer side of the diaphragm
indicate the apertures for the
lo\\er power condenser, and
vice versa.

We may note that for
dark-ground \\ork. stops

.-honld lie placed close to the back lens of the condenser, and in the

of a diaphragm \\hich is le>s important an inch of distance

should not In- exceeded. This condenser gives dark-ground illuini-

OD \\ it h objectives of 'Ti N . A . : for such illumination it, is one of

' he hot illuminators extant .

FIG. 258.— Baker's fitting I'm- Al.l.i-'s

matic condenser used in English inicro-

A SIMPLE CONDENSEK 313

The iris diaphragm is for general purposes more convenient than
the usual circular plate, Imt it has the drawback of being incapable
of setting to any exact size. A delicate point in an image, caught
with a certain-sized diaphragm, is not regained with ease and cer-
tainty with the iris,1 and may involve much patience and labour;
but a well-made large plate of graduated diaphragms will wholly
remove this difficulty. Moreover, for testing object -glasses it is
supremely important that a metal diaphragm be used, so that the
conditions of illumination may be readily and accurately reproduced.

It may be of service to those who are unable or indisposed to
spend considerable sums upon condensers to state that an excellent
achromatic condenser can be made by placing a Zeiss • aplanatische
Lupeii ' 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 different powers, viz. 1 inch and H inch, and
we can fully testify to their being the most useful hand-lenses for
ordinary work that can be employed. (Treat 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 generally ielt. 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 arranged 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 more perfect instruments, but it
is capable of raising students' and other simple microscopes to a much
higher level.

Without a condenser the microscope is either (by construction)
not a scientific instrument, or it is an instrument 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 for 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 centrally 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 shoulder to hold the diaphragms. stop>. glasses,
etc. Centring gear is not necessary with students' and elemental v
microscopes. The slight displacements due to varying centres of

1 It will be urged that apertures can be exactly reproduced with the iris in
photographic lenses ; why cannot they, therefore, in the case of the microscope ?
The answer is 1 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, i 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.

2 Joiirn. (Jm-kett 3//r. ('/»//, vol. iv. ser. ii.p. 77 (1889), on Zeiss's loup. E.M. Nelson.

314 ACCESSORY APPARATUS

different objectives will with such microscopes prove of no moment
if the sub stage is once for all c.-i n-ftil ly fixed centrally in the axis.

What we require to do is to centre the image of the lamp flame,
as .seen \\ith a low-power lens through the condenser, so that it
-lands 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 l or stop in
this simple mount an internal sliding tube may be used. It will be
a further advantage to have a separate cell to fit into the bottom of
the .sliding tube to receive coloured glasses; a spiral slot-focussing
arrangement mav he added with advantage to this kind of mount,
acting like a pocket pencil. For students' and elementary micro-
scoj e> —still so often and so unwisely without condensers — this is a
most inexpensive and most convenient arrangement.

An epiiome 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).

.'!. A tube carrying the optical combination of the condenser
sliding into (2), with a pin moving in the spiral slot.

I. A long tube carrying the diaphragm and slots sliding into (3).

• ). A cell carrying coloured glasses sliding into the bottom of (4).

1 'o/tdensers require special mounting for use ti-ith the polar i 'scope.
Then at least two ' turn-out ' rotating rings are required to hold
selenites. Swift makes an ingenious inultum in parro mount for
'•i 11 ploying, amongst other things, the condenser with the polariscope.
to which we call attention in describing the polariscope. But we
know of 110 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 apparatus. The outer one receives a
mount of three selenites which revolve, and are placed on ' turn-out'
arms. On t he upper part of this mount of selenites is a screw, which
receives the optical combination of their dry achromatic condenser.
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. Ac. Xow from the mulct- part of the sub-
stage into the inner and revolving ring is fitted the polariser, and
this leaves little to be desired in practice.

^ «' would ad\ise tiie m icrosco] list to avoid condenser mounts
\\hich <-arr\ their own centring movements apart from the sub-
stage. It is with regret that we lind that this plan has been adopted
in Abbe's ne\\ achromatic condenser. It is manifest Iv better to lit
the rectangular movements to the sub stage, and then they become
available for all the apparatus employed with the sub-stage. A plan
vluVh requires il,:,t each piece of sub stage a pparatus which needs
centring should be provided with separate fittings tor this purpose
<"in have nothing to recommend it.

!*ecl or usage of m roscopists a diaphragm means a 1ml,

tjai* •• • PHI! pe g in ih,. illiiplir

•\ ' stop ' is an opaque disc stopping out central <

A COilPAKISON OF CONDENSERS

315

We give below a list presenting the most important 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 X.A. of the
greatest 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 condenser is assumed, for practical
purposes, to be a solid one. so lung 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 periphery 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 lias been ex-
ceeded.

Total

Aplanatic

('uncle-user aperture

aperture

Power

N.A..

X.A.

1. Powell and Lealand's dry achromatic (Is54)

•99

•8

±

5

new formula lls.V.n . -'.I'.)

•8

i

.^t

2. „ ,, „ top lens removed

•5

i

3

3. ., ,, ,, bottom lens only

•24

2
S

4. Swift's -achromatic (1868) . "92

•5

4
10

5. „ ,, top lens removed

•22

6. Abbe's chromatic (3 lenses) (1873) . 1-30

•y

1
.-{

7. ,, ,, top lens removed

—

•3

3

8. Powell and Lealand's chromatic (Abbe's

formula) (1880) . 1-3

•7

1

9. Powell and Lealand's oil achromatic (1886) 1'4

1-1

i

10. „ „ „ ., „ used dry 1-0

•s

i

c;

11. „ „ „ „ top lens removed

—

•4

4
10

12. Abbe's achromatic (1888)

•98

•65

1_

13. ,, ,, top lens removed

—

•28

1

14. Powell and Lealand's low-power achro-

matic (1889)

•83

•'<

:i

15. Powell and Lealand's apochromatic (1891) .

•'..-,

•9

1

16. Zeiss's ' aplanatische lupen,' large field

(Steinheil formula)

—

•32

1

17. Beck's achromatic, dry (1883)

1-0

•9

i

18. „ oil achromatic (1900) 1-4

1-3

i

19. Swift's apochromatic, dry (1892) . -95

•92

i

20. „ panaplanatic, dry (1897) . 1-0

•93

i

21. „ „ oil (1898) . 1-4

1-30

j

22. Watson's panachroniatic, dry (1898) . 1-0

•95

7

23. „ „ oil (1899) . 1-33

1-25

i

24. Zeiss's oil achromatic (1899) T30

—

25. Baker's semi-apochromatic dry (1900)

1-0

•95

~3

The values of the first sixteen and of ISTos. 22, 13, and ~2~> have
been obtained from .actual measurements; the others are from the
estimates of the makers.

The limit given in the table is for the edge of the flame as a

ACCESSORY APPARATUS

source of liirht. When, however, a Dingle point of light in the
iixis is the source, tin- condenser will lie much move sensitive, ami
a Lower value tor tlu- aplanatie aperture than that given in the table
will be obtained. But as a single point of light is seldom, if ever,
practically used in microscopy, it was deemed better to place in
tl,,. tal.le'a practical rather than a theoretical and probably truer

result.

It has been stated that the best dark grounds are obtained when
:, stop is used which is of just a sufficient size to give a suitable dark
Held and no more.

When such a stop 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.

For animalcules in water and ' pond life' generally a stop larger
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
condensers.

To give completeness to this part of our subject it is needful to
refer to the SPOT-LEXS and the PARABOLOID, although they are only
serviceable for very low [lowers, such as 3 -inch to H-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 nil' the central rays for the pin-pose of obtaining a dark
ground upon \\hich the illuminated object lies. Its use is very
lienelicial in lou power work. Large insect preparations are pro-
bably better shown with this device than with any condenser, but
when the moderate powers are Id-ought 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 under the same category.
It consists of a paraboloid of ^lass that relied < to its focus the rays
which fall upon its internal surface. A diagrammatic section of
t his itist rumen! . shou ini: t he course of t he rays through it, is given
in liii. 'J.V.l. t he shaded portion representing the paraboloid.1 The

A |>;ii-;il><>li<- 1 1 1 n 1 1 1 M 1.1 1 . .r was lir-l ilc\iscd liy Mr. \\Vnliaiii, \vln>, however,
• I a si IMT >|M-riilnni tui lln |nir|>oM'. AKont tlic ^amc time Mr. Shadbolt

- I'm- tin- -;llllr pill |M'-.i' iSft-1 Trillin. Micro.

vol. iii. L852, pp. 85, \'.\'1\. Tin- tun |n-iin-i|ili'S arc combined in the ;jl.is-

PARABOLIC ILLUMINATOR

317

parallel rays r r' r'f (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 ground out into
a spherical curve of which F is the centre, the rays in emerging from
it undergo no refraction, since each falls perpendicularly upon the
part 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 .-ingles : 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 8 is attached
to a stem of wire, which passes vertically through the paraboloid

FIG. 260.— Parabolic

illuminator.

FIG. 259.

and terminates in a knob beneath, as shown in iig. 2ti<) ; and by
means of this it may be pushed upwards so as to cut oil' the less
divergent rays in their passage towards the object. It is claimed
that this instrument has great capabilities of giving dark -ground
illumination with lenses of ' wide apertures ; ' 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 condensers it suffers greatly after we pass
the Tf-inch objective, although it does give excellent results with
very low powers such as 1-inch, H-inch, 2-inch, and 3-inch
objectives when employed to illuminate large objects such as whole
insects, because this instrument 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 '

3i8 ACCESSORY APPARATUS

the rays In-fore they pass through the object, and to apply to them,
in soiiii- part of ilit-ir course between the object and the eye, an
«,i<ilii«iii<i iiiediuui. These two requirements may lie provided for
in different modes. The polarise/- may be either a bundle of plates
of thin glass, used in place of the mirror, and polarising the rays by
reflexion : or it may lie a • single image ' or ' Nicol ' prism of Iceland
spar, which is so constructed as to transmit only one of the two
rays into uhicli a beam of ordinary light is made to divaricate by
passing through this sub>tance. 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. tig. 261 ; it i> usually employed in a sub-stage which rotates by a
rack-and-pinion arrangement, so that rotation of the prism is easily
effected. For the (ntnlt/xt'r a second ' Nicol ' prism is usually em-
ploved : and this, tixed in a short tube, may lie 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 mav be
Fie.. :><ii.— Polarising apparatus. worked ill 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 selenitr
between the polariser and the object; and it is advantageous that
this should be made to revolve. A very convenient mode of effecting
this is to mount the selenite plate in a revolving collar, which fits
into the upper end of the tube that receives the polarising prism.
I n order to obtain the greatest variet\ of coloration with different
objects, films of >elenite of different thicknesses should be employed ;
and this may In- accomplished by substituting one for another iii the
re\ol\ in- collar. A stillgreater variety may I ..- obtained by mounting
bhree films, which separate!} give three different colours, in collars
revolving in a frame resembling that in which hand magnifiers are
usualK mounted, (hi- frame hein- fitted into the sub-stage in such
a manner thai either a single .selenite. or any combination of two
or all three together, may IK- brought into the optic axis
be polarising prism (fig. 262). As many as t hi rteeii different
thus be obtained. When the construction of the micro-

POLARISING APPARATUS— RINGS AND BRUSHES

319

scope does not readily admit of the connection of the seleiiite plate
with the polarising prism, it is convenient to make use of a plate of
brass (fill. 2<i:J>) somewhat larger than the glass slides in which
objects are ordinarily mounted, with a ledge near one edge for
the slide to rest against and a large circular aperture into which
a glass is fitted, having a film of seleiiite 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 seleiiite film under polarised light is so
greatly increased by the interposition of a rotating film of mica that
two selenites — red and blue — 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 compact apparatus made by Swift as a general sub-stage

illuminator is useful and commendable,
and is capable of adaptation to most
English microscopes. It is shown in fig.
2<>4. The special advantage of this con-
denser lies in its having the polarising

FIG. 2(53.

prism, the selenite and mica films, the black ground and oblique-
li-ln Mops, and the moderator all brought close under the back lens
• >f 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 (••lourn.
R.M.S.,' 18D2) 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 tc-lr *(•<>!>'•
that is required, and not a microscope. Once this is recognised the
whole matter is simple. As the microsc >pe has to be turned into a
wide-angled polarising telescope, all that is necessary is to screw a lo\\
power on the end of the draw-tul „-. as in 1ig. 2(5."). 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.

120

ACCESSORY APPARATUS

The objective most Miitablc is a 14,[ths of -(55 X.A. ; but a j-th of 71
N.A.. or a J-nl of •(')•") X.A. will do equally well, as the whole of the
liark lens of tlie olijective should be visible through the analysing
• Nicol ; ' the back lens of the objective must not be too large, thus a
.', inch of -li.") X.A. would not do so well. The analysing prism may
lie placed cither where it is in the drawing- or above the eye-piece.
I'rart ically it works very well above the
objective, which is the position, it occupies
in ' ordinarv microscopical outfits.'

For the draw-tube a 2-inch objective
and a 1> or C eye-piece will answer
admirably.

1(''l.— Swill's illiimiiuting and
apparatus.

Fie;. 265.— In this diagram P
is the polarising prism in
the sub-stage, C sub-stage
rondenser. On the stage
.AI mineral. On nose-piece
Ol objective ,',,thsT, I N.A.;
A iinalysing prism. In the
draw-tube, O- objective 2
• >r '•• in. H, Huyghenian
eye-piece.

For setting up the instrument it is better, before screwing the
' "' the end of the draw-tube, to centre the li-ht in the
fficols' being burned so as to give a light field.
the objective in the draw tube, open the sub-stege con-
aperture, and put the mineral on the stage Rack

MONOCHROMATIC ILLUMINATION

321

down the body, so that the objective on the nose-piece nearly
touches the crystal ; then focus with the draw-tube exclusively. The
suit-stage condenser should be racked up close to the under side of
the crystal.

The use of monochromatic light is frequently desirable in micro-
scopic work, especially blue light, although of less moment than
in p re-achromatic days. The usual method of obtaining coloured
light is to pass sunlight through coloured glass, or through a
coloured solution, such as the ammonio-sulphate of copper ; but this
is a most imperfect and unsatisfactory method, and does not give
monochromatic 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. \V. (Jittnrd ;
and when artificial light is
used 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 y^ths deep,
filled with a solution of methyl
and glycerin mixed

FIG. 266. — Gifford screen with an adjustable
stand.

FIG. 2(57.— Gifford' sF-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 screen too opaque; therefore it is
requisite 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 0 . "The importance of this screen cannot be held
too high by the modern microscopist. It makes semi-apochromatic

322

ACCESSORY APPARATUS

objectives equal to real apochromatics, and it sharpens the images
yielded even bv 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 upper
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 given
in fig. 267, which
represents the 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 is
prepared by the Ley-
bold s process, and is
fused at the joints
and never leaks ; a
still simpler and less
expensive means of
making >uch a filter
lias been devised by
Dr. A. Meithe, pro-
fessor of spectral
analysis at IJcilin.
The'liHer consists of
a 1 rough containing
n inch in thickness of saturated solution of acetate of copper
filtered; a variation in the thickness of the troughs or tanks
rable, but the results are excellent.

MICEO-SPECTEOSCOPE 323

Equally perfect monochromatic 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 radiant are passed through a slit, as in fig. 2QS, and
dispersed by a prism of glass, and by means of a second slit any
portion we wish may be selected from the spectrum to be used for
the purpose required.

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,

f

working at ~- (this lens is not shown in the cut). In the parallel
D*O"

beam from this lens and close to it is placed an equilateral prism of

dense flint set at minimum deviation. Close to the prism is placed

f

another Wray 5 x 4 R R. working 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 jVth inch in diameter is cut in the
cardboard screen, through which the required colour is allowed to
pass to the mirror of the microscope, thence to the sub-stage con-
denser. For visual work 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.1 — When 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 mapped them out,
they are known as Fraunhofer lines. The greater the dispersion
given by the multiplication of prisms in the spectroscope, the more
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.' Kirchhoff showed that while the
incandescent vapours of sodium, potassium, lithium, itc. give a
spectrum with 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
a lit >\ving 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 bands, 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 tlie spectroscope and its uses the student is referred to
Professor Rascoe'sLectures <>n SpectrumAnalysiSfOrihe translation of Dr. Schelleii's
Spectrum Analysis, and Hou- to use the Spectroscope, by Mr. John Browning;

Y 2

324

\ ( ' < ' K SSOE Y APPARATUS

cloudy, so that they cannot be resolved into distinct lines by magni-
fication, while too much dispersion thins them out to indistinctness.
N«i\\. it i> I iv 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
• in tin- 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 special modifications. As originally devised by Dr. Sorby
and worked out by Mr. Browning, the micro-spectroscope is con-
st ructed 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
(C C C) in such a manner that the emergent rays, r r, which have
separated by dispersion, leave the prisms in much the same

direction as the immergent ray
entered it. Below the eye-glass,
in the place of the ordinary stop,
is a diaphragm 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

Fi(i. '2(i'.). — Micro-spectroscope.

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
comparisons of such artificial spectra, alike with the ordinary or
natural spectrum and with each other, provision is made for the
formation of a second spectrum by the insertion of a right-angled
prism that cover,-, <,ne half of this slit, and retlects upwards the light
transmitted through an aperture seen on the right side of the eye-
piece. For the production of the ordinary spectrum, it is oiily
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 li-ht reflected from this mirror can be
transmitted, it is only necessary to pbce the slide carrying the
ion or crystalline lilm. or thi- tube containing the solution, in

' ''" ""' lll:lk(' "" ' hange, lesl complications >lmuia uris.-: but we think it
harmonious with analogy to call !l is instrument the snectro-micro-

USE OF THE MICRO-SPECTROSCOPE

325

the frame, D D, adapted to receive it. In either case this second
spectrum is seen by the eye of the observer 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 bands is as important a.s
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
farther apart, their actual positions 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 mirror by which light
from the lamp employed can be
reflected through E I) 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
<ni the graduated
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^lrawn Fl(1. -271.— Bright-line speetro-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.1

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 into 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. I See Jauni. of Roy. Mirrosr. Soc. vol. i. ls?s, p. 326,
and vol. ii. l'.H7l>, p. si. i

io1ro"oth

circle of the wheel.

326

ACCESSORY APPARATUS

:ni<l note the effects of opening and closing the slit by rotating the
screw, (' (lig. -69) ; 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 tin- spectrum to be seen as a broad or a narrow
ribbon. Tin- screw E (or in some patterns two small sliding knobs)

I 10 2,0 3,O -1,O 5,O 6iO 7,O SO 9,0 10O 110 12O 1

mo 1*0

10 iio 20 .110 -iio sio 6io 710 sio ._» o_ J o^_ ijo_ _i2o_ _ay

''l~:lnTlT]]TlT[n'TJIUrTrlTnlTnl~rlTrrlTrTlT7TTlTrTTliiiiliTTTl nli i I 1,1 i. ilniil.ii.l.ini .nl i I '"I 'i I

FIG. '11-1. — Upper half, map of solar spectrum, showing Fraunhofer 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 I). Water tinged with
port wine, madder, and blood are good fluids with which to com-
mence this study of absorption bands.1 As each colour 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
portion only of a larger one,
the prisms are to be re-
moved by withdrawing the
tube containing them ; the
slit should then be opened
Avide, 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 \\ ill lie seen.
m ia Heeled by adjacent ob-
jects. K<>r i irdinary observa-
t ions object ives of from two

fco .-; inch tociis will be found mosl suitable; but for very
minute quantities of material a higlier power musl be employed.
l'"'Vl'n :l single red bluod corpu>c|e may be made to show the

specimens, in small tubes, I'm- the study of absorption-spectra, is
tr. Brown d the directions given in his How /<> imrk witli

• ..refnlly attended to.

FIG

USE OF THE MICKO-SPECTKOSCOPE

327

characteristic absorption bands represented (after Professor Stokes)
in fig. 273. !

For the study of coloured liquids in test-tubes or small cells, the
binocular spectrum microscope, 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 micro-spectroscope. This is an opinion expressed by Dr. Sorby
and other experts, 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

readily understood. The upper part of the instrument s
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. Joiini. vol. vi. 1871, p. !l.

328

ACCESSORY APPARATUS

The Method of using the Micro-spectroscope. — The objects to be
investigated are of t wo 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 preparation 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 . duivk place, for the purpose
of extracting the chlorophyll colouring matter. The concentration
of the solution thus produced, which influences the intensity of the
absorption spectrum and the number and length of the absorption
I lands, 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 sufficiently 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
the microscope or held before the opening of the comparison prism.
as the case may be. For the latter purpose (bringing liquids before

the opening of the comparison prism) a small open trough of glass,
\\itli t\vo parallel glass plates, is very useful. For exact investiga-
tions, however, the trough-flask is preferable. It is a flask whose
t\vo sides, hack and front, are parallel, furnished with a carefully
fitted ground-glass stopper. It should be filled quite full of the
solution ,-ind then laid with its broad side on the stage. It is
especially indispensable when, we wish to study the combination
spectrum of two solutions. In that case two flasks are filled each
with a dill'ereiit solution, and both laid upon the stage, one upon
the other. For the purpose of examining small quantities of
any liquid, a siillicient depth being obtained with very little
material, vertical glass tubes attached to horizontal plates are used,
as proposed by Mr. Sorhy and shown in lig. 276. The narrow tubes
are made of various lengths from seel ions of barometer tubing, in
order to present dill'ereiit thicknesses ,,f ihe contained fluid, the
hroad tuhc I .eing higher <>n one side t haii 1 he ot her. and thus con
Stituting a wedge-shaped cell, which, when filled and closed by a
tliin cover-glass, will present a var\ ing t hickness of lluid for stiid\
and comparison. If the object to he investigated is not a solution,
lll|t ;l preparation of the kind which we commonly employ in micro-
scopic inquiries, we must lirst of all bring it into the locus of the
i. 'I'" do this we must: lirst remove I he tube hearing

ILLUMINATION BY REFLECTION

329

the prisms, 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 pa>t
it and disturb 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
other side push up the shortening apparatus as close as is necessarv.
On the other hand, should the object consist of a number of single
minute grains, which would cause to In- 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 tin: object will
be a little out of focus, some-
what above or below the true
focus. In this way 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 focu>.

Illumination by Reflec-
tion.— Objects of almost every
description will require at
times to be examined and
studied by what is called re-
flected light ; the light in
this case is thrown down upon
the object by various devices,
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 are opaque; the same
diatom, for example, may
often with advantage be ex-
amined with transmitted

light, being transparent, and again by means of an illumination
thrown upon, and reflected up from, 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 powers 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

FIG. 277. — The English form of bullV

330

ACCESSOKY APPARATUS

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
st;ind 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 carries the lens is borne
at the bottom upon a swivel joint, which allows it to be turned in
anv azimuth ; whilst it may be inclined at any angle to the horizon,
liv 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-
/.outal arm may be lengthened or shortened; the lens may be
secured in any position (as its weight is apt to drag it down when

FIG. 277.\.

it is inclined, unless the tubes be made to work, the one into the
other, more >tillly 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.

\ -ond form of the bull's-eye is made by I.eiT/. and is illustrated
l!77\. All the required movements are provided lor, but in a,
different way; the clamping screws are by means of usual milled
heads.

The plane side of the bull's-eye should be turned towards the
object. Some microscopist s like to have their bull's-eye attached to
some p.- ie microscope; but if this is done, care must betaken

ittach it boa li\ed part of t he microscope, ami not to either the

THE USE OF THE BULL'S-EYE 331

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 or 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 aberration is when
its convex side is turned towards parallel or towards the least diverging
rays; consequently, when used by daylight, its pltnic side should be
turned towards the object, and the same position should be given to it
when it is used for procuring converging rays from a lamp, this being
placed four or five times farther off on one side than the object is on
the other. But it may also be employed for the purpose 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 placed 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 somewhat lon-i-r
level, and at a distance of about three inches. The bull's-eye should
be placed between the stage and the lamp, 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 lamp falls on its plane surface at an angle so oblique as to be
almost totally reflected towards 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 tin-
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 require 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.1 so that the source of light may by its means be focussed

on the object. Xeither of these plans will answer for other than low

1 See Join-it. Boy. Microsc. Soi - vol. iii. 1*SO, p. 398.

:>?2 ACCESSORY APPARATUS

«J <J

powers. where there is plenty of room for the light to pass between
the objective ,-ui.l the object. The ingenious use of the bull's-eye
emploxed by .Mr. .lames Smith, as detailed above, increases the possi-
bility of magnilication. luit it needs practice and care. With the
great improvement which has been eifected 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. Nelson has calculated
ami 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 such as will not even yield a minimum amount of
such aberration; also that there is a numerical error in the focal
lengt h of the high-power doublet. He has computed that the spheri-

cal aberration in the Herschel doublets amounts to —'296^-, and

Ji

he gives the following formula for a combination, the spherical

11-

aberrationof \vhichis —"207- ; or 30 per cent, less than in either of

r

those proposed by Sir John Herschel.

Boro-silicate glass, Jena catalogue No. 5 ; yu = l-51 .

£=1=64-0.
fy

1st lens crossed, /•= + 2'359) ,.

s=_ir,-o7Hl(llametei"2'1;

•-'ml lens meniscus , r= + 1'280| ,.

., i0t f diameter I'o.
s= + 3-4o4 )

Distance between the lenses •<>.">. eipiivalent focus 2-0, working
distance or back focus 1 •">.">, total aberration — -1035, clear aperture
2'0, angle 62°. The second (Jauss point of the combination is close
to the posterior .surface of t he 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, (he following rule may be of use.
Halve all the radii and diainet ITS and multiply the results by the local
length that is required. A>"//////r. - 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 »•= + !• 1795,
s= — 7'539, diameter I •().">: multiplying these results by .'!.', \\e
olilnin y=+-H2K, ,s-= — 2t)-3S(i. diameter 3'7. Treat the meniscus
in thi same wav : the lens distance ma v with advantage be kept

•05.

The following hull's eye is not so expensive to manufacture, and
mavoiilliat accounl i>e preferred to the doublet of minimum aber-
ration just described. Its form, though of minimum aberration for
t\\o piano convex lenses, possesses 13 per cent, more aberration than

the fiil-lner. It \\illon fchlS aCCOUnI li"t lie possihlc to olltaill SUcll

an even and unlimken disc of light with this form of bull's eye as
with the Other. The data are as follows.

NELSON'S COMPUTATION

333

Class, boro-silicate, the same as before.

Radii r = + 2'72) ..

, —oo [diameter 2'1 ;

y~ [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
with 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 Ampkipleura pellucida. It
could be very easily obtained with a student's FIG. 278.— Bull's-eye
microscope provided with Nelson's open stage,1 for of good but not the
on this the bull's-eye could be placed against the
edge of the slip without any special apparatus or
fitting.

Another and popular method of -opaque illumination' is bv
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 reflector, as provided with Powell
and Lealand's best stand, is seen in Plate III. The arm of the
side reflector is fixed to an immovable part of the stand, and is thus
unaffected by the racking 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 reflector; 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
speculum is commonly employed, mounted either 011 the objective,
as in Beck's form, fig. 279, or on an adapter, as in Crouch's, shown
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

1 Fig. !:.'.».

best

vised

son.

334

ACCESSORY APPARATUS

n commendable plan, for it increases the distance between the ob-
jective and the Wenhain 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
• list a nee for the focus of objectives of very low power to be shortened
bv the width of such collar, is to be avoided.

The best plan without doubt is to attach the speculum to a fixed

Fici. '279.

FIG. 280.

part of the stand, as is done in the Powell and Lealand, the Ross,
and the Beck stands.

A modification of t/te parabolic reflector was devised by Dr. Sorbij,
and has proved to be very useful in certain investigations, such as
the microscopic structure of metals. It consists of a parabolic
reflector, iii 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 turned out. This arrangement
allows of two kinds of illumination,
oblique and direct, being readilv used,
as the plane reflector is attached to an
arm so that it can be swuiiif out of the

l-'li.. '2S1. — Sorliy's inoilificjitifiii of
lllc p.llMllolic ivtlrrtor.

way when not required, as shown in the figure.

Dr. Sorlty was able to get results in the examination of polished
sections of steel not otherwise attainable.

No opaque illumination, however, has yel surpassed the venerable

Lieberkiihn ; the besl experts freely admit thai the finest critical

1(1 be obtained by this met hod of illumination are secured

'be LieberUiihn. This mode of illuminating opaque objects is

b\ means of a >m.-dl concave speculum reflecting directlv down upon

i a focus the light reflected up to it from the mirror ; it was

LIEBERKUHN — ITS DRAWBACKS 335

formerly much in use, but is now comparatively seldom employed.
This concave speculum, termed a ' Lieberkiilm,' from the celebrated
microscopist who invented it, is made to tit upon the end of the
objective, having a perforation 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 central portion 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 mirror 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 two manifest drawbacks : the first one, that of requiri m/ u
separate Lieberkilhn for each objective, is a difficulty which in the
nature of things cannot be overcome. The radius of the Lieberkiihn

Fir;. 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-
alterations of focus ; for if we employ parallel 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 <lr<nrli<ick has reference to the special n-mj in n-hich
objects have to be mounted 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 width than
is necessary for security.

336 ACCESSORY APPARATUS

H. A stop of paper or varnish should never lie placed behind an

object.

Let everv opaque mount be also a transparent one, since it is
(il'trn most useful to examine an opaque object afterwards by trans-
mitted light. The stop should always be a separate one ; this may
In- a disc on a pin held in the sub-stage, or, what is still simpler, a
piece of model atelv thick • cover' glass, cut to the 3x1 inch size,
oi- rather shorter, should have a small disc of Brunswick black put
on it centrally on the .' turn-table,' J and this maybe placed under
the slide \\hen the Lieberkiihn is to be used. There may be two or
three such slips with stops of different sizes; in this way every
mount may lie examined either with the Lieberkiihn or by directly
transmitted light, and of course by having a larger stop the same
object mav be examined by any kind of reflected light. Many a
valuable preparation has been spoiled by placing a stop 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 gauge of thickness of
slip and diameter of cover-glass were adopted. For the thickness
of the slip, the oiyth of an inch would prove most suitable, and for
the diameter of the cover-glass | of an inch would lie 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 cover-glass being
marked in diamond point 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 coyer-glasses — such as those j\ths of an inch in
diameter are to be wholly condemned. They do not allow the
conditions required by modern microscopy, being adverse to the
employment of oil-immersion lenses in anything like the most
efficient way.

Lieberkuhns can lie used with objectives as high as ^ of an inch
focus of '77 N.A. For higher powers than this a perfectly 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
relied ed on the object .

The light suitable for illumination by Lieberkiihn may be either
the Hat of the lamp Hame. reflected by the plane mirror, or the edge
of the llame, the rays being rendered parallel by a bull's-eye, and
rdled ed from the plane mirror to the Lieberkiihn.

There is one other kind of reflected illumination em-
ployed, produced l.\ the vertical illuminator, which, although it

has been in use for some years, has received an accession of value

fr the employment of immersion lenses. The earliest device for

accomplishing this was invented by Professor H. L. Smith, of
< Iciiev a, I .S.A .

I'he principle of this illuminator is to employ the objective as

1 < 'lupin- vii.

VERTICAL ILLUMINATOR

337

its own illuminator ; which Professor Smith did by means of a
speculum. A pencil of light \v;is admitted from a lateral aperture
above the objective and then reflected downwards upon 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 oblique light obtained
by the milled head,/, A, fig. 284.

Powell and Lealand's method is to fix a piece of glass, >i-orkc<l
flat, at an angle of 45° to the optic axis, with a rotating diaphragm
in front of the aperture admitting the light.

B

FIG. 283.

FIG. 284.

To i(se these instruments the edge of the lamp name should he
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 rover-glass, that which
lias 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 critical angle for glass is totally reflected

z

338 ACCESSORY APPAKATTS

from the innlf-r *x,-f<ice of tin r<,<-<T-<jl(iss. and comes back through
the oil and the objective to the eye-piece and the eye; they are, in
fact, all optically continuous, so that the upper 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 object ive be examined, it will be seen that all
that portion of the back of the objective whose aperture exceeds H)
is brilliantly illuminated. This annulus represents, and is produced by.
the excess of aperture bevoiid the equivalent air angle of 180°, of
\\liich 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 dry-mounted object is in
optical contact with the cover-glass or not. If it be not so it is in-
visible 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 an immersion of I'l IS". A. will have a narrow annulus.
and that of 1 '4 or 1 '5 a broad and still broader one. In this wav.
by practice, a fair approximation to the aperture of an objective max
be obtained.

It is not the absolute si/.e of the annulus, but the relation of the
size of the annulus to that of the whole back, that must be estimated.
Thus jLth of N.A. 1'2 will have as broad an annulus as TVth of
1 -4 X.A.. but the diameter of the back of the g-th is, of course, much
larger than that of the ,'._,tli. and this involves the necessity for a
relative comparison.

Appliances for the Practical Study of Living and other Objects
with the Microscope. — Stage-forceps «nd Vice.--Fm- bringing under
the object-glass in different positions such small opaque objects as
• •an lie conveniently held in a pair of forceps, the stage-forceps (&g.
•JS.")) supplied with most microscopes provide a ready mean:-. These
are mounted by means of a /joint upon a pin which fits into a hole
eil her in 1 he cornel- of 1 he stage it.-clf or in the object -platform ; the
object is inserted by prosing the pili that projects from one of the
blades, u hereby it is separated from the other; and the blades close
again by their (,\\n elasticity, so as to ret ain 1 he object when the
pressure is \\ithdra\vn. P.y sliding I he wire stem which bears the
forceps through its socket, and by moving that socket vertically
upon its joint . and the joint hori/.ontalK upon the pin. the object
maybe brought into the Held precisely in the position required ;
and it may be turned round and round, so that all sides of it
may be examined, by simply Diving a twisting movement to the
\\ife Stem. The other extremity of the stem often hears a. small

STAGE APPLIANCES

339

FIG. 285. — Stage-forcepe.

brass^ box filled with cork, and perforated with holes in its side,
seen in fig. 286 ; this affords a secure hold to common pins, to the
heads of which small objects can be attached by gum, or to which
discs of card. Arc., may be attached, whereon objects are mounted
for being viewed with the Lieberkiihn. This method of mountini;
was formerly much in vogue, but has been less employed of late.
since the Lieberkiihn has unfortunately 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-rice, for the
purpose of holding small
hard bodies, such as
minerals, apt to be jerked
out by the angularmotii >n
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 Heck's Stage-forceps,

disc-holder, fig. 289, or
it may simply drop into
a stage fitting, as in the
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 affords for
presenting them in every
variety of position . The
object being attached by
gum (having a small
quantity of glycerine
mixed with it) or bv
gold size to the surface of a small blackened metallic disc, this i>
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

tj

z 2

FIG. -J.sT.
Three-pronged forceps, screw adjustment.

FIG. -JMS.— The stasje-vice.

FIG. -280.— Beck's disc-holder.

340

ACCESSOEY APPARATUS

socket.1 The supporting plate being perforated by a large aperture,
the object 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
;i\vav bv 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 foi-m 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 upon 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

FIG. 'J'.IO.

minute aipiatic organisms, and of ' cultivating ' such as develop and
multiply themselves in particular fluids. One of the simplest and
..... >t eil'eclive. that of Mr. Botterill, represented in fig. 290, consists
of a slip of ebonite, three indies by one. with a central aperture of
three-fourths of an inch at its underside; this aperture is reduced
by a projecting shoulder, \vhereoii is cemented a disc of thin glass,
which thus forms the bottom of a cell hollowed in the thickness of
i lie ebonite slide. On each side of t his central cell a small lateral cell
communicating \\ith 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

Mimll pair of fmvrp, .nl.ipt.'i] t,, (,,kr up minute object- mu\ IM- lilt. •<! into
the cylindrical In.Mrr in place of a disc.

GROWING SLIDES

341

of water or 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, or (if a greater thickness be required) of a ring of cardboard
or vulcanite. If the fluid be introduced into one of the lateral cells,
and be drawn oft' from the others — either by the use, from time to
time, of a small glass syringe, to be hereafter described, or by
threads so arranged as to produce a continuous drip into one anil
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 attraction.

Dr. Lewis's and Dr. Maddox's growing slides are shown in figs. 291
and 292. Two semicircles of asphalte varnish are brushed on the
slide, one being rather larger than the other, so that the ends of one
half-circle may over-

FIG. 291.

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

fluid 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 {/rowing 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-
quired. 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 could be made
cheaply with thin glass instead of tinfoil.

Dallinc/er and Drysdales Moist tft<t</t' for ContiwMous 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

X X

FIG. 29'J.- -Maddox's growing stage.

342

ACCESSOKY APPARATUS

:i plain -lass 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

Fid. 293. — Dallinger and Drysdale's moist continuous growing stage.

circular aperture. f>. is cut through it, and a thin piece of good glass.
<•. iJ, 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
'I ii '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. Tt is marked in the drawing

y, y, y. The object of this ring is to hold a glass
vessel, fig. 2D4. about If or 2 inches deep. Tt
simply drops in. and the top. a, being slightly
larger than the ring, y, fig. 293, it is prevented
from slipping through.

Let us suppose the stage
to be in its position on the
microscope, and the vessel.
tig. '294. inserted in this
manner into <?, fig. 293. A
piece of good new linen is
now cut to the shape drawn
in (io-. 297, the part ^ being
long enough to reach to the

end of the glass stage, and then al f> I>en1 over, leaving the part in
the \essel. MM-. 291. which is inserted into </. iig. 293. Its posit ion
i- indicated in Iig. 29.'! by 1 he dotted lines. It, It. It. iVc. I5nt before
ii is laid upon the stage a circular aperture, d, fig. 297, is cut nut.
which must lie much larger in diameter than the covering glass
vhich ii is intended to use. \Ve therefore employ small covers.

294.

FIG. 295.

GROWING- STAGE FOR CONTINUOUS WORK

343

The glass with the flap of linen in it is now filled with water,
and the linen is wetted and wrung so as not to drip, 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, /, must be laid on. It will be seen that
there is a broad, clear space between the covering glass and the linen.
We now want to form a chamber into which the object-glass can be
inserted, and which shall enclose a portion of the constantly wet
linen, and be to a very large 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 favourable
circumstances that it rather increases than allows a diminution of
the film of fluid. The manner

effect this is
piece of glass

FIG. 290.

in which we
simple. A

tubing, about IT? 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 ,-i
dra \ving of it; a is the piece
of glass tubing, b 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 sin/ill
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 tliat 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 110 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 a, a, fig. 293 ; b in both figures
stands tor the round aperture in the thick glass : 6, in fig. 296, cor-
responds to the thin glass which 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. ,i, <(, thus making a cell equal to the thickness of the whole
>1age. The linen is "marked in dotted lines in both figures: d,
lig, 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 APPAI{A'1T>

stretched caoutchouc seen at /> in fig. 295, with the object-glass </.
1 it-net rating and tightly tilling up the aperture c in the figure, thus
I'm-ming the moist chamber, ch, ch, by enclosing parts 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 6, fig. 290,
sunk as a cell, by the attachment of the thin glass floor to the under
.-iilf of the stage, as described above, this annular flap of linen over-
hangs, but does not lie upon, the floor 011 which the drop of fluid
with its living inhabitants is placed. This is a great security
against accidental flooding.

It will be seen that the microscope must be vertical ; but there
is no inconvenience arising from this if it be placed on a sufficiently
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 difficulties in
working are few, and can be best discovered and overcome in
practice.

Dr. Dattinger's Thermo-static Starje for Continuous Observations
at H'njli Tan /x'l-nitn-es. — It frequently happens that, either for thepur-

FKI. -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 perspective in fig. 298. At A. a ?> 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 effected by
the milled heads; 15 is a vessel for water with a thermometers
of sufficient delicacy for indicating the temperature; b is a mer-
curial regulator, carefully made, but of the usual pattern; c brings
the gas from the main ; <t conveys as much of the gas as is allowed
to escape from between the fop 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 temperat lire, as is well known, is
a matter of detail depending chielly on the careful use of the mer-
curial screw-plug /and the height and intensity of the burner e.
•\ temperature ,|in'te as accurate as is needed for the purpose
reijuired can be obtained.

•\VAKM 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 required temperature
and regulated.

The manner in which it accomplishes the end desired is as follows :
On the centre of the stage (A) will l>e seen a small cylinder of glass;
this is ground at the end placed on the stage, and covered with a
sort of drumhead of iiidiarulilier at the upper end. By examining
C with 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

B

FIG. 298.

under surfaces of this glass aperture. A glass cup is placed in th<-
jacketed receptacle f (A and C), and this also is filled with water.
A piece of linen is now laid on the stage (A, y) with an aperture cut in
its centre slightly less than the countersunk cell in which the glas*
disc e" is fixed, and a flap from it is allowed to fall over into the gla>s
vessel/(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 aperture than the cell and the opening in the
linen, and consequently a large aiinulus 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 ACCESSORY APPARATUS

aperture, thus forcing the lower ground surface of the cylinder upon
the linen, and making the spare within the closed cylinder practi-
cally air-tight, but still admitting of capillary action in the linen.
Thus the enclosed air becomes saturated.

l!y 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 illuminating 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 Varley, who, in the place of a
level disc of glass for the floor, as well as the top of the ' box,'
bevelled a piece of thick glass and burnished it into the top of the
tube, where it formed the floor of this ' animalcule cage ; ' this
prevented the draining off of the water at the edge by capillary

all rad ion. lint in that form a condenser cannot be used successfully
with it. and therefore a dark ground cannot be employed. But as it
is Rotifera and Infusoria generally that constitute the raison d'etre
for this piece of apparatus, and as a dark ground gives results of
high value-— to say nothing of their beauty — with these forms, it
lo>t much of its value.

.Mi*. 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 outer ring is enlarged sufficiently to allow any
liigli power to focus to the very edge of this glass floor. All object
lying anywhere over the floor can be reached bv 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
object i\c can lie readily arranged with it; and a little practice
enables the operator to employ it for many useful purposes in the
>t udv of • pond life.'

The compressor or compressorium is a more elaborate device,
someuhat of I he xmic kind, but arranged to give the operator
more accurate control over the amount of pressure to which the
object is subjected. .Mr. l!oii>selet has constructed one of very

COMPRESSORS

347

efficient form: we illustrate it in fig. 800, but 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 parallel pressure
between the two glasses, which in delicate work 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 spring raises the
cover-glass, and by an ingenious spring catch it is kept central 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 yearsmade an admirable parallel
compressor, but its weight and cost
were somewhat prohibitive of its use
generally ; the firm have now 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. T\vo
end plates D D slide on to the plate A, and hold the ring B loose 1\
in position, allowing it to be revolved by means of its milled flange,
which projects at E. Within the ring B is screwed a brass disc I-'
which carries the upper thin glass Avhich is attached by the sere \\-

FIG. 300. — Rousselet's compressor.

FIG. 801. — Beck's new compressor.

GG. The screws GG 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 glas>e> are moved towards
or away from one another.

The slides D I) and the ring B, together with the disc F, are
removed for arranging the object on the lower cover-glass, and

ArCKSSOi; Y APPARATUS

when replaced hv revolving the ring at E. any desired amount of
compression may he obtained. The ohject having been arranged,
either side may be examined with equal facility, as the compressor
is reversible.

\Vhen a verv 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 length of the trough, so that it
may be moved freelv 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 bottom edge touching the
inside of the front glass, a small ivory wedge is inserted between
the front glass of the trough and the upper 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,1
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
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 diminution
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.

Mr. Motterill devised a trough which is made of two plates of
vulcanite or metal which screw together, and between them are two
plates of gla». of the proper si/.e. of any desired thickness, kept
a pa i-t liy 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

uith this otherwise convenienl form.

An excellent, though shallow, t rough was made by Mr. (J. <!.
I >' ..... in-, \\hich we illustrate in fig. :!(>:',. The lower plate or trough
1 W.i'.i-li i other elastic metal should not be used, on account of oxidation.

. ;i()2

A SHALLOW TROUGH

349

proper is made of metal, 3 inches long by 1^ wide and about }\}
thick, with an oval or oblong perforation in the centre, and the
under side is recessed, as shown in fig. 303, B. In this recess is fixed,
by means of Canada balsam or shellac, a piece of stout covering glass,
forming 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 or 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, rather 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, ;md at the two top corners 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 I it-
examined is then
placed in the cell, and
may be properly ar-
ranged therein ; the
cover is then lowered
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 sufficient 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 alga?, &c., however, can lie well seen by
placing a drop of the water containing them on an ordinary 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 drop 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 ACCESSORY APPARATUS

moisture tli.-it surrounds their edges bo removed by blotting paper,

llit-v will remain hi place when the microscope is inclined. An
wtmuLar <rll. lluii may In- used either :is ;\ '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 :j inch diameter is "-round, 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
convexit v (the highest part of which should be almost flush with the
general surface 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-oil the
covering glass by capillary attraction, while the furrow serves as an
air-chamber. If the cover 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 lie 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, aquatic insed
larva-, the larger animalcules, or other living objects distinguishable
either by the unaided eye or by the assistance of a magnifying glass
from 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 tidies 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, bv putting
t he linger again on the top of the tube, its contents remain in it
when the t ill ie is lifted out. and may lie deposited on a slip of glass,
or on the lower disc of the aquatic bo.\. or, if too copious for either
receptacle. inav lie discharged into a large glass cell. Ill thus
fishing in jars for any but minute objects, it will be generally found
convenieni to employ t he open -mouthed 1 11 bo (J ; those with smaller
orilices. A. 1>, being employed for ; fishing ' for animalcules, Arc., in
small bottles Or tubes, or for selecting minute objects from the cell
into which the \\ater taken up by the lube (' has been discharged.
It will be found \er\ convenient to have the tops of these last
blown into small tunnels, which shall be covered with thin sheet
indiarubber, or 1op|ed with indiarubber nipples, which bv com-
pression and expansion can then lie regulated witli the greatest
nicet v.

DIPPING TUBES

351

In dealing with minute aquatic objects, and in a great variety
of other manipulations, a small glass si/riiHji>. of the pattern repre-
sented in fig. 305, and of about double the dimensions, will be
found extremely convenient. When
this is firmly held between the fore
and middle fingers, 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 syringes, with
I » ants 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 microscopist can have at his
command. It will also be found
that if a dipping tube with a glass
bulb have an iiidiarubber hollow
ball or teat attached to the top of
it, it will act, for the majority of
purposes, as well as a .syringe.

Forceps. — A iiother instrument
so indispensable to the microscopist
as to be commonly considered an
appendage to the microscope is the
forceps for taking up minute objects ;
many forms of this have been devised, of which 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

FIG. 304.— Dip-
ping tubes.

Fit;. 305.— Glass
syringe.

FK.. 306.

plunged into sea-water, it is better that they should be made of bras*
or of German silver than of steel. since the latter rusts far more
readily; and as they are not intended (like dissecting force] s) 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 essential.

352

ACCESSOEY APPARATUS

however, 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 slipped

aver the milled heads of the coarse adjust -
FIG. 307.— Powell and Lea- , , . , , ,

land's protecting ring for mentj and "7 screwing the small screw home
coarse adjustment. the ring cannot be withdrawn ; but as thcv

are loose upon the milled heads, the latter

cannot be brought into action ; the rings simply 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 counteracting
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 VI.).

353

CHAPTER V

OBJECTIVES, EYE-PIECES, THE APERTOMETEB

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 efficient, 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 gradual ; but there are definite epochs of very
marked and important improvement. Our aim in the study of
objectives is practical, not antiquarian, and we may avoid elaborate
researches on the subject of non-achroniatic lenses and reflect//';/
specula, which have been sufficiently indicated in the third chapter
of this volume. We may also pass over the earlier attempts at
achromatism ; the true history of the modern objective begins from the,
time that its achromatism had been finally worked out.

The first movement of a definite character towards this object
was made, it has been recently shown,1 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
effort to make achromatic lenses, and, through the courtesy of the
President of the Athemeum 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 year
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 award 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 appeai-s small room for
doubt, Marzoli preceded Chevalier in this, as we shall subsequently
see, very practical improvement.

1 Juiinu Roy. Mii: Soc. 1890, p. 4-20.

A A

354 OBJECTIVES, EYE-PIECES, THE APERTOMETEK

It lifts been, however, customary to accredit the first practicable
attempts to achromatise object-glasses to M. Selligues. In 1823
he suggc.-tcd 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 Mould have been, the case had their positions been reversed.1 and.
as \\e 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 conception
of an achromatised combination had been embodied in an initial
manner. Jn 1825M. 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 Avas 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 aplaiiatic foci

or 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
very high probability, applies to the work of

Chevalier, for Selligues' attempt was a blunder

PIG. 808.-T^ly-s achro- apainst the ™™monplace knowledge of his

matic triple. time.

The form of three superimposed similar

achromatic 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 uncemeiited triplet. It was. in fact, a
miniature telescope object-glass, and is illustrated in fig. 308. Two
lenses made on this principle by Tully, having T^ and -j% foci, were
found in practice too thick, and in many ways imperfect; and he
was induced to make another single triplet of -^ focus and 18° aper-
ture, and its performance was said to be nearly equal to that of
the ,V

Subsequently a doublet was placed iii front of a similar triplet of
shorter focus, forming a double combination objective of

aperture. This \\as pronounced to l>c a great advance upon all
preceding combinations, even those which had been produced upon
tin1 Continent.

A note of Lister's at this time upon the objectives of Chevalier

r 1.

LISTER'S DISCOVERY

355

is of interest. He found them much stopped down, and in one
instance he opened the stop and improved the effect. Lister says :
' The French optician knows nothing of the value of aperture, but
he has shown us that fine performance is not confined to triple
objectives ; and in successfully combining two achromatics he has
given an important hint — probably without being himself acquainted
with its worth — that I hope will lead to the acquisition of a pene-
trating l power greater 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 form to the modern microscope, had
been baffled by the difficulties presented by the
problem of achromatism, and had laid it aside
in favour of the reflecting 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
Ill-ought about an important epoch 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 moderate
aperture ; that for the space between these two
points its spherical aberration will be over-corrected, and beyond
them, either way, under-corrected.

Thus, let a,'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 spherical 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

1 ' Penetrating ' meant ' resolving ' power in those clays ; he alludes, therefore, to

increase of aperture.

A A 2

FKJ. :;<>!>. — The two
aplanatic foci of an
optical combination.

356 OBJECTIVES, EYE-PIECES, THE APERTOMETER

from the radiant point, /, h e being a normal to the convex
surface, and / d to the plane one — tinder these circumstances the
aimle of emergence, g eh, much exceeds that of incidence, f d i,
being probably almost three times as great.

It' the radiant is now made to approach the glass, so that the
(•nurse of the ray, f cleg, 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 bear 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 f still approaches the glass, the angle of
incidence continues to increase with the increasing divergence of
the rav, till it will exceed that of emergence, which has in the mean-
while been diminishing, and at length the spherical error produced
by them will recover its original proportion to the opposite error of
the curve of correction. When /'has reached this pointy (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/* 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 experiment sufficiently, Lister1 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 working distance of -11.

One of the immediate consequences of the publication of Lister's
paper 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 progress of improve-
ment was. in consequence, and in comparison with the time imme-
diately preceding, remarkably rapid.

A inlrcn- /iWs began their manufacture in 1831. He was followed
by Hugh IWell iii IK.'U. andin 1H:{{) by James Smith. It is of
more than ordinary interest 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 construction, with the dates of
their production, will give a fair idea of the work of Andrew Ross
in the manufacture of early lenses. He was the earliest of the three

PK1MITIYE FOEM OF LENS COBBECTION

357

English makers, and undoubtedly carried the palm both here and on
the Continent for the excellence of his objectives.

1 inch 14° two doublets, 1832. Made for Mr. E. H. Solly.
18° single triple, 1833.
55° three pairs, 1834. This belonged to Professor Quekett.

P^o • triple front and two double backs 1041 \ , Lister's formula

44°

63° „ „ „ S1842.

74°

Examples of these old lenses are extant and in perfect preserva
tioii, and for correction they are comparable 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 corrected the errors of spherical and
chromatic aberration that the circumstance of cover-
ing an object with a plate of the thinnest glass was
found to disturb the corrections ; that is to say. the
corrections were so relatively perfect that if the ^omb^atian*1t>y
combination were adapted to an uncovered object, Andrew Ross,
covering the object with the thinnest glass intro-
duced refractive disturbances that destroyed the high quality of the
objective.1

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
us by a i-iiich 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 by 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 arrangement was modified by the introduction

1 Vide Chapter I.

FIG. oil. — Primitive
form of lens correc-
tion (1838).

353

OBJECTIVES, EYE-PIECES, THE APEKTOMETEK

of ii sereu 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 pairs are held ; and it is drawn up or down by means
of a ciillar (I1), 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 slit 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

Uncovered.
Covered.

FIG. 312. — Section of adjusting object-glass.

FIG. 813.— Present collar
correction.

of the object-glass is such as to make it suitable for 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 Minder-correction' in the objective, fitted to
neutralise the 'over-correction' produced by the interposition of a

glass cover of ext remest thickness.

This method of collar correction served the purposes of micro-
scopy for upwards of 1 hilly years, but when more critical investiga-
tions were undertaken and objectives had more aperture giA'eii to
them it uas found that the method had two great faults.

The lir.st uas that the 'covered' and 'uncovered' marks were

11 crude. To remedy this, the screw collar was graduated into

litly dixisions. a device introduced by .lames Smith in 1841 so that

THE MODERN 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
front lens while the back remained rigid with the body of the
microscope. The detriment of this arrangement was that in cor-
recting a wide-angled, close-working objective there was a danger of
forcing the front lens through the cover-glass by means of the collar
correction.

Now the arrangement 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. Weuham 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 O18.

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 afforded by his own eye and experience. This is a
very important point, because the interpretation of structure to a
great extent depends on accurate adjustment of the objective, and
it would be folly to suppose 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 work which gives a practical form to such theory. In
fact, it is the test of accurate manipulation that, however the collar
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
correction, while no similar steps were taken on the Continent, is a
sufficient 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 what 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 through the peri-
pheral portion of the lens will be found by experiment with a card
to be brought to a focus at a point on tin- o.-'is nearer the lens than
those passing through the centre. This is under-cwrection, 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 APERTOMETER

it will be surrounded by a coma, and even the portion of the flame
which is in focus will lack brightness. But with the convex side to-
I'-urds tfie 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 011 which an objective is
constructed. They make plain that an over -corrected lens is one
I'-Jiich brings its peripheral rai/s to a longer focus than its central, vide
fig. 24, p. 20. But a cover-glass produces over-correction, therefore
the means employed to neutralise the error is by the under-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.1

Still confining our consideration to the year 1837, we find that a
further improvement was made by Lister, who employed a triple
front combination. This consisted of two cr own 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 drawn from an early ^--inch objective by
Andrew Ross, having bayonet-catch correction adjustment. In 1842
a Tj-inch of 44°, a J-inch of 63°, and a ^--inch of 74° were made
upon 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 1 J-inch focus. When 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 YV-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 primitive order.

This object-glass will divide the podura, marks in a milky field
\vith a full cone, and 1he iield is much curved.

There is also a separating l:]-inch and f-inch which is good
V'hilc thf /(i inch and the ] -inch may be considered fair.

The lenses supplied by Andrew Ross are a good 2 -inch and a

I rider-correction is al-n known as 'positive aberration;' over-correction as
erration.'

TEIPLE BACK COMBINATION 361

fair 1-inch, but we have seen a better than this of about the same
period.

Hugh Powell supplied a 1-inch of good quality, and a ^. 1, £,
jJg-inch fairly good. The apertures of the g- and the ^.-inch are of
course very low.

On the whole it may be said that the corrections are well
1 lalanced 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 i-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 X.A.,
and a T^-inch objective to 135°, or -93 N.A. Of this latter it was
affirmed that it was ' the largest angular pencil that could be passed
through a microscope object-glass.'

In 1850 object-glasses were made with a triple back combination ;
these were attributed to Lister ; but it is also affirmed that they

FIG. 314.— Au early Fn;. 315.— A triple- FIG. 316.— A single-

g-in. combination back combiua- front combination

by A. Boss. tion by Lister (or by Wenham.

Amici ?).

were the previous 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 triple 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, vide1 Journ. R. M. S.,' 1898, p. IQOetseq.
It may be noticed that Tully'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 iiser 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 APERTOMETEE

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
.stria' on A iii]>]iij>h'iir(i pdlaclda were first resolved. Indeed, what is
known as the water-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 Mas
an advance the optical principles of which were certainly not at the
time understood.

In Paris, Prazmowski and Hartnack brought these objectives to
great 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 turn 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
l-23 were reached; and in America, Spencer, Tolles, and Wales
produced some extremely fine lenses of large aperture.

During the year 1869 Wenham experimented with and sug-
gested ] 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
dnidit but this objective would have produced a much deeper im-
pression but for the. fact that it -was in advance of its immediate
time.

Tolles, as we have hinted above, used the duplex front ill the
construction of some of his immersion objectives, and was followed
in this I iv the liest English makers, and. in the case of a celebrated
,'; inch purchased by Mr. Crisp, Tolles was able to reach a balsam
angle of '.Mi0.

At the time that tlie \\.-itei--immersion lenses were being con-
structed by rival opticians with increasing perfection, the great
theory of IVofessor Abbe concerning microscopic vision, the impor-
tance of diffraction spectra, and the relation of aperture to power

M'icro. Joitrii. Vol. I. p. 172.

THE INFLUENCE OF THE DIFFRACTION THEORY 363

was entirely unknown. In the absence of this knowledge wholly
mistaken value was attached to power per se in the objective.

With a focus as short as the ^--inch, it was not uncommon to
find apertures less than 1'2, while objectives of J*, •£$, -^ and even
higher powers, were made with extremely reduced apertures. This
was done in the interests of the common belief that ' power '-
devoid of its suitable concurrent aperture — could do what was so
keenly wanted.

Tins impression, however, was far from universally relied on ;
there were several earnest workers who, without being able to
explain, as Abbe subsequently did. why it was so, still urged 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
reckless 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 oy Wen-
ham, 1869.

FIG. 318. — Combina-
tion for ' homoge-
neous ' immersion
by Abbe.

FIG. 819. — 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 -,\ of O7 N.A. The i-inch objective,
even if a good one, is sure to exhibit spherical aberration, while the
jt- of low aperture will show many minute objects with considerable
clearness, especially if a comparatively narrow 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 grasp details.1

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
microscopic 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

ci ' nst ruction of objectives and eye-pieces, and, as a consequence, has
to some considerable extent given a new character to the entire in-
strument. Its promulgation has indeed inaugurated an entirely
new epoch in the construction and use of the microscope.

The general character and the details of Abbe's theory are given
in the second chapter of this treatise ; but its practical 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 l mm introduced as a logical outcome of the diffraction,
theory of microscopic vision. A formula for a 1-inch objective 011
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.2 It has been, already shown 3 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 ; but as there was a practical identity
between the refractive and dispersive indices of the oil and those
of the crown glass of the front lens, the rays of light passed
through what was essentially a homogeneous 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^-inch, and subsequently, in the same year, a 1-inch
objective, each with a duplex front to work in soft balsam, and with
a N.A. of 1'27. 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, \ve are wholly indebted to Abbe.

The principle was not. nevertheless, so readily and warmly
adopted in Kngland on its first introduction as might have been
.-i i it ici pa ted. This arose partly, however, from the fact that water
immersions had been brought to so high a point of excellence by
.Messrs. Powell and Lea land 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. * P. -21- ;ilso .l,,nni. Ji<>// Min-o. Sor. Vol. II. 1879, p. 257.

•' < 'llM.ptlT I.

THE EXCLUSION OF THE SECONDAKY SPECTBV.M 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 manufacture, and
the anterior front lens was a hemisphere, it appeared that 1ST. A. 1'25
to 1'27 was the aperture limit they could be made to reach.
Messrs. Powell and Lealand, however, by making the anterior front
lens greater than a hemisphere, increased the aperture of a ^-inch
objective to 1'43 KA.

This front, from being greater than a hemisphere, presented
difficulty in mounting ; this was at first overcome by cementing its
plane surface to a thin piece of glas>. which was then fixed in the
metal. Eventually, however, this form of construction was changed
by these makers in a very ingenious manner; so to speak, they
entirely inverted the combination, and accomplished the end by
itxikiix/ the front of flint. By this means they obtained apertures
which have not as yet been equalled by any other makers, reaching
in a 4-, a jV, anfl a ^V a N.A. of 1'50 out of a theoretically possible
aperture of 1'52. Professor Abbe has since, it is true, made ;in
objective with a numerical aperture of 1'63. but this requires the
objects to be mounted and studied in a medium of corresponding
refractive index, and consequently, in the present state of our 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.

\Vehave already pointed out in detail : that it was- the great
defect of the ordinary crown and flint achromatics that two colours
mill/ could be 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 spectrum.

In like manner, it has been shown that it was not possible in the
flint and crown 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,1 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 requirement than any previously attainable having been
manufactured by the Jena opticians.2 Abbe was able to produce
objectives entirely cleansed of the secondary spectrum. From calcu-

1 Chapter I. - Chapter II.

366 OBJECTIVES, EYE-PIECES, THE APEKTOMETEK

lations of a most elaborate and exhaustive kind made by Dr. Abbe,
objectives are made by Zeiss which not only combine three pails of
the spectrum instead of two, as formerly, but are also aplanatic
for two colours instead of for one. This higher stage of achromatism
A I the has called apochromaticm.

A general plan of the coi. struction of an apochromatic objective
as made by Zeiss is shown in fig. 319, which, it will be understood, is
diagrammatic; 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 optical
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 composed, 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
procure 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 lie of a corresponding refractive and dispersive 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
refraction '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 Micro-
scopical Society. This glass has a numerical aperture of l-63; in
a subsequent chapter on the present state of our knowledge as to
the ultimate structure of diatoms we are enabled to present the
results of some of the photo-micrographs produced by its means.
But it may be noted that very much will depend upon the N.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 products.

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. We illustrate it
in fig. 320. The data for this are as follows, viz. :—

O is the object and V its virtual image ; the hyperhemispherical
front is aplanatic for these two points. The scale of the drawing is
an-anged so that, the distance of the vertex A of the front lens to
the object ( > is one inch. The three lenses are made of borosilicate
glass. Xo. ."> in the .Jena catalogue. ju=l-f)l ; and as the reciprocal of

IMMERSION FRONT FOR CONDENSER BY NELSON 367

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

C= + 3-434
D= + 1-280
E=— 15-078
F= + 2-359

Diameters : lens FE = 2'45

DC=2-1

Distance between surfaces : ED='05

CA=-03

FIG. 320. — Nelson's new immersion front for a conden-t-i .

Thickness AB=-683.

Working distance B0='317.

Diameter of the plane surface 1> of front lens=l'192, AO=1'0,
AV=1-51.

The angle 2=62°. and <£ = :>o: 47'; the numerical aperture of
the combination is therefore 1'33 X.A.

Tlie front lens AB is aplanatic ; the spherical aberration of the

V-
next two DC. FE only amounts to - -'214 J-. The back correcting

368 OBJECTIVES, EYE-PIECES, THE APERTOMETER

lens, which might be a triplet, will require to have +'214 V

of spherical aberration to render the whole combination aplanatic.

( hi the whole, and for the purposes of practical and prolonged
biological investigation, it is to the dry apochromatics that we are
most indebted, and from their use we shall derive the largest benefit.

As no subject is really of more importance than a clear under-
standing of the difference 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,
MS ;i popiilar illustration of this important subject.

In fi0". 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
intrrmt'(li;ite zone or the central circle. Let them therefore be
called equilucent zones.

/. 2. 3.

FIG. 321.

If we assign a numerical value for the visual intensity of the
whole spectrum, say 100, made up of the following parts, viz. :—

Red

Orange-yellow
Yellow-green .
Blue

15
40
30
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 eipiilucent /.ones ; consequently 300 will represent the value of
the whole lens, provided the whole of the spectrum is brought to the
saint- focus.

l!y referring to the diagrams we see that in anon-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
-pliei-ica! aberration causes the light which passes through the other
zones to lie brought to other foci, they for all practical purposes migl it-
be Stop | ied out.

In the achromatic- lens we have (lig. 321, 1) in the intermediate
/.one t\vo parts of the .spectrum combined, as 40 + 30=70, and one

ZEISS'S APOCHROMATICS 369

in each of the other zones is also b/-nn</ht to tJte same _/oc»s, say .'!()
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 parts 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.

Recalling the suppositions we have made for the purpose 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
scarcely in simplicity, and it sufficiently 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 powerful position by borrowing achromatism from the
telescope, it has now led the way to the apochromatised state, which
Avithout doubt it will be the work of the optician, in constructing
the telescope of the immediate future, to follow.

We would beg the reader to bear in mind in the purchase of
objectives that. Avhilst 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 ne\ IT have been
constructed, yet it does not by any means folloa- that I><THH>«>. an-
objective is MADE with the Abbe-Schott glass it is therefore a/m-
chromatic ; the secondary spectrum must be removed. a//d tin' *ji//rn'r<>
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 points
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 error 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 pertains only to the higher powers, a correspond-
ing error had to be intentionally introduced into the lower p<>\\ersin
order that the same over-corrected eye-pieces might be available for
use with them.

It appeal's worthy of note in this relation that one of the best
forms for the combination of three lenses is that known 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 APERTOMETER

It is another distinctive feature of the •'! 111111. objective that it
has M. trijilf.i- 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
arc 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.1 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 (5 mm. and a 4 mm. = | inch and j? inch.
have both an aperture of '95.

There are also water- immersions : a 2-5 mm. = -^ inch, with
X.A. I -2."). and t\vo oil-immersions respectively 3 mm. and 2mm.
= ^ inch and ,'.,- 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-iiich, also a 35 mm. or
Uy-inch objective.

With the exception of the 6 mm., 4mm.. and 2'5 rum. object i\ e>.
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 ll it-
very highest order. They present to the most experienced eye unsur-
passed images. They are corrected with a delicate perfection which
only this system, coupled with technical execution of the first order,
can possibly be made to produce. The optical polish, the centring,
the setting, and the brasswork certainly have never been surpassed.

It is a matter also worthy of note that Zeiss's apochromatic
series of objectives are true to their (f<-*ii/in/f!ons as pmvers. The
7j-inch is such, and not a (Ay-inch designated 1-inch. This was
equally true of the early achromatics. 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.
l!ut modem achromatics of fair aperture are always greatly in
excess of their designated power; ^ are nearly ^-inch. A Vineh
of 40° has an initial power of 25, and is a ,',,-inch; -^-inch
objectives are in reality ^-inch ; and ^-iiich objectives of 90° and
upwards have initial powers of 50 instead of 40. which they should
have, SO that they are in reality !ths ; some in fact — by no means
uncommon lia\e an initial power of (50, and are act uallv (lth-inch
objectives.

This is explicable enough from the maker's point of view; it is
far easier to put />nn;-r into an object glass limn <i j><-rtti re . It is

\ltlioiigh the foci of tin- lenses arc expressed iii integers, with the single excep-
tion of the w.iliT immersion 'l''i mm., there are inconvenient <le. im;il fractions in the
initial magnifying power of all the1 series except those of "2'~> ami '2 mm. focus.

HISTOLOGICAL ADVANTAGE OF HIGH POWER 371

easier to make a ^-inch of 100° than a ^ with 100° ; the result is
that low powers with suitably wide apertures are costly.

In the Zeiss apochromatic series of objectives the 24 mm. of '3
X.A. and 1"2 mm. of '65 X. A. may be considered as lenses of ihe
very highest order ; the relation of their aperture to their power is
such that everything which a keen and trained eye is capable nt
taking cognisance of is resolcril idn'ii t/te objective is ///V,V//,,/ ,/
magnification equal to tn-ilr/' times its initial power ; for this purpose
an objective must have 0'26 X.A. for each hundred diameters of
nititbii/t'd magnification. Under these conditions an object is seen in
the most perfect manner possible. In this connection Mr. Nelson
lias suggested1 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 power 10. Then its O.I. is :\V = 30. The
0.1. of the 12 mm. apochromatic of '65 X.A. is (vV°= 31. That of
the £ homogeneous immersion of T4 X.A. is -L±^-= 17. Compare
now these figures with an old water-immersion ^ of Tl X.A. VO'TT*
= 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 -J^ water-immersion
of I'l N.A. had a vast amount of empty magnifying power, while on
the other hand the 24 and 12 mm. will 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 |-of 1-4 N.A. will
stand a higher eye-piece without arriving at an empty magnifying
power than the ^of 1'4 X.A., whose O.I. is ll'O.

As it is more difficult to put aperture into a lens than power, the
O.I. becomes also an index of the money value of a lens. Thus the
j mentioned above that had an initial magnifying power of (50 and
X.A. of "8 ought to be a cheaper lens than a true ^ with an initial
magnifying power of 40 and a X.A. of '9, their optical indices
being 13 and 22 respectively. The limit of combined power for best.
definition with any objective of any given aperture may be found by
multiplying its X.A. by 400. Example : The limit of power for best
definition with a § of -3 X.A. is 120 diameters. The converse rule
may be stated thus: The ideal X.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 X.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 X.A. (= 1-inch) objective, and a 12 compensating eye-piece.
Xote 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.

i Jot/ru. K. M. S. 1S93, p. 12.

B P. 9.

372 DIRECTIVES, EYE-PIECES, THE APERTOMETER

Now change the objective for the 16 mm. '3 N.A. (= J|, but
with the same aperture). Nothing more is to be seen ; the most
dexterous manipulation cannot bring out a single fresh detail ; the
re>olution 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 picture presented
by the f 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, whatever
IIK/H Ite done by the 16 mm. "3 N.A. can be accomplished in an
equally 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 using 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. (= Tj-inch), and an A Zeiss
achromatic of "20 N.A. (= §rds 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 may be 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 which had contracted.

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
by a Zeiss A -2 N.A. magnified 140 diameters. The object of the
photograph is to expose the fallacy which underlies the generally
accepted statement that low-angled glasses are the most suitable for
histological 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
ibe 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
t line, and train the mind to appreciate perspective by means of focal
adjustment .

Jt will be admitted that no clear idea of what an endothelium
cell is can be obtained from fig. 7.

But lig. 8 (frontispiece) represents the same structure slightly
less magnilied (x 1 38) by means of an apochromatic ^ N.A. '65.
Here only the upper surface of the tube is seen ; but the endothe-
lium cells can be clearly t raced, and a sharp definition is given to

HLSTOLOGICAL ADVANTAGE OF LARGE APERTURE 373

every cell. The circular elastic tissue is also displayed, while the
whole image has an increased sharpness and perfection.

Thus, with the objective (A '20 X.A. == frds inch) of lower
aperture, the endothelium cells can.be seen; but when the image is
compared with that of the objective of wider aperture ('65 X.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 greater aperture 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
sufficient dimensions to lie upon the table appealing to the unaided
eye.

We have pointed out in the proper place,1 that although ' pene-
trating power ' varies inversely as the numerical aperture, it also
^al•ies inversely as the square of the power.

Now, from what we know of histological teaching in this country.
\ve 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 j-iiich,' of. perhaps, '65 aperture ;
but by so doing he would have secured only one-third of the pene-
trating power qua aperture, and one-seventh of the penetrating power
1 1 ml 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 poii'tr than is needful.
A J-inch where a Vinch would answer involves loss in many ways,
and would never be resorted to if the aperture of the lense* <>////>/"//<•'/
were as great as the power H.wd legitimately permitted.^

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 hi;/// <i /m //-,.,•. 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 square 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. '(55 X.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 i-inch
focus of 80° (almost certainly a ,',, 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 aperture to 60°, we shall get a fair picture of the
1 Chapter I. - Chapter II.

374 OBJECTIVES. EYE-PIECES, THE APERTOMETER

<li;itoiu — one indeed that was considered critical until that with the
apochromatic was seen. Hut in comparison it is dull and yellowish.
From which it follows that an exceptionally fine achromatic -jA--inch
of r>(>° or •"> N.A. will not suffer comparison of the image it yields
with that of an apochromatic Vinch of '65 N.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.
ranging from 1 inch (24 mm.) to ^ inch (4 mm. '95 N.A.). They
are the most perfect and efficient 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 workers 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 fluorite 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 ,-ire limited with objectives of a very high class, and which
come remarkably near to the best apochromatics. We would
specially call attention (wholly in the interests of students) to No. .'!
(f-inch N.A. O28) at a cost of lf>s. No. 5 is an equally valuable
and admirable objective which is a, 4-inch O77 N.A.. the price of
which is '25s., and it comes so near to an apochromatic as to require
expert judgment to discover that it is not. He also makes a dry
;iV-inch N.A. 0-H7 and a dry f, of -8'2 N.A. at a cost of :!/., which
is a very lo\v price for so good a piece of optical work. Also an
oil-immersion ,',,-inch N.A. 1 •.'!() is sold for:-}/. 1f).s. This glass is
corrected for the long tube, and a similar }\,t\\ N.A. K30 for
-">/. 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 series, all of which we have found
'<) be reliable, and. when examined in numbers, very few indeed
are belou the standard quality. lint such work is so much needed
'hat it is not likely that, \\itl: the glass accessible to all. it will
remain the peculiarity of one make]-; hence we find that Reichert
follows l,eit/ so closely in quality and pi-ice that it is not easv to
distinguish the semi apochromais of one maker from the other.
Reichert's No. :! (>; inch N.A. <)•::<») is 17*.. his 7,v (an admirable
lens) 1 inch N'.A. ()-S7 is \l. 16s. He makes a high-class oil-
immersion ,'.. inch N.A. ]•:!() for s/. And of apochromatic lenses
lie makes a -;-inch N.A. <)•:!<) and a i-inch N.A. <H>r» for 41. each.

AMERICAN OBJECTIVES — EYE-PIECE 375

which, so far as we have seen them (and we have examined
man)7), are excellent. Reichert's semi-apochromatic ^ is also a fine
and useful lens, and his -,'._, -inch apochromatic N.A. 1*30 has
qualities fitting it for use in any kind of research.

But we confess that it is a matter of most pleasant surprise to
us to find that the great American firm of Bausch and Lomb are
putting upon the English market objectives that fairly compete
with the above in the lowness of their price, while their optical
quality and mechanical work are 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, are so low-priced, their correc-
tions and high, quality are beyond all praise. "\Ve would specially
call attention to a §-inch. a ^-inch, and a ^-ineh 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-inimer-
and other, and one of their TV oil-immersions is sold at the

strikingly low figure of

Messrs. Swift and Son are making a large number of objectives,
especially a pochroinats and semi-apochroniats, and they have long
striven to supply the student with high-quality lenses at the lowrst
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 Vinch and the
|--inch. We find that their initial powers are 21 diameters 0'4."> X. A ..
and 40 diameters 0'74 X.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
claim to be amongst the very low-priced lens-s: 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 results, 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 price
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

376

OBJECTIVES, EYE-PIECES, THE APEETOMETEE

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 divisions : (1) positive eye-pieces
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 simple 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 use is negative, and is generally known
as Huyghens's, and sometimes as Campani's. Monconys appeal's 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

Fit i. o'2'2. — Huygheniau eye-piece.

FIG. 323. — Kellner eye-piece.

.'] : 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 different ratio and lens
distance is employed, the fact being that different tube lengths
require different formula1. The general form of a Huyghenian eye-
piece is shown in longitudinal section in fig. 322. This makes a
very convenient form of eye-piece of 5 and 1(1 magnifying power;
but when the power much exceeds this last amount the eve-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
delect, and it may fairly be considered the best ordinary eye-piece.

Another negative eye-piece is that known as the Keliner. or
orthoscopic. This counts of a bi-conve\ field-glass, and an achromat if
doublet meniscus (In-convex and bi -concave) eye-lens. A vertical
section of one so const nil-led is seen in lig. .'!2-'l. These eye-pieces
usually magnify ten times, and the advantage they are supposed to
gi\f consists in a large field of view ; but fcheyare not good in practice
for this very reason ; they lake in a Held of view greater than the

NEW HUYGHENIAN EYE-PIECE

377

objective can stand, and as a rule even the centre of the field will
not bear comparison in sharpness with the Huygheniaii form.

Mi1. Kelson has recently computed and had made a Huyglieiiian
eye-piece on a wholly new formula l 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 power of this eye-piece is 12 ; equivalent focus, -8, corrected
for the English tube (;>=9'5).

Fig. 324 is enlarged twice.

\

Fi«. 3'24. — Nelson's new formula Huyglieniau eye-piece.

Data: Glass, borosilicate crown, ^ = 1'51. r=64'0, Jena cata-
logue No. 5.

Field-lens, biconvex r= + '94) •,.

- diameter 00.
s= — 2'94j

Eve-lens, biconvex r'= + -34) v .,,.

J , ,- diameter '30.

s = — I'Ul )

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, '2(3.

Power, 12 ; equivalent focus, -53, corrected for the Continental
tube (p=6-3).

Data : Glass, same as before.

Field-lens, biconvex ^ + ^51 diameter via.

Eye-lens, biconvex /= =+ -221 diameter -20.

s= — 'bo]

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, '16.

These eye-pieces should enter the tube of the microscope as far
as their diaphragms.

Positive Eye-pieces. — In the early compound microscopes the

i .I.E. M.S. 1900, p. 165.

OIUECTIYES, EYE-PIECES, THE APERTOMETEK

FIG. :

eye-pieces were all positive : that is to say, they consisted of a single
bi-convex eve lens and no field-glass. The definition witS this must
liave lieen most imperfect; the addition of a field-lens, though it
were a l>i-cniive.\ not in the correct ratio of focus nor the theo-
retically best distance, must have been considered a great advance.

In this wav mallei's rested, however, until the theoretically
perfect Huvghenian form was devised. Object-glasses have been
used ,-is eye pieces, a lid all forms of lotips 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 110
results that surpassed a well-made Huy-
ghenian combination ; but the best form of all
of the combinations which have been tried by
us as positive single eye-pieces are the Steiii-
heil triple loups ; a section of one of these is
seen in tig. H25. This combination also forms
one of the best lenses for projection purposes
e\ er const i-ucted. 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
ape-chromatic objectives, of this form of eye-piece.1 In the section
above on objectives we have referred to the fact that these eye-pieces
are over -corrected ; this maybe 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 Huvghenian
eye-piece, this will be blue, being seen through the simple uncor reded
«ye-lens.

There are three kinds of compensating eye-piece as designed by
Abbe. These are :

1 . Searcher eye-pieces.
1. Working ,,
.'{. Projection ,,

1. The searcher forms are negatives of very low power, intended
only for the purpose of finding an object; they consist of a single
lield lens and a doublet eye lens.

Tin' n-nrL'/iiij i'orinsare both posil ive and negative. The eye-piece
lor the long tube has a triplet eye-lens; but the remainder, vi/,. S.
12, IK, and 27, when first introduced, were all positive. The 8 was
subsequently, however, changed for a negative. Having used both.
\\e are glad to learn that it is made now both positive and negative.
It may lie convenient to have the K a negative like the 4. but with
regard to the 12. IK. 27 it is important that, they should be positives.
These positive forms are on a totally new plan, being composed of
a t riple wit h a single piano convex over it : t he diaphragm is. of course,
exterior to the lens (fig. :!2li). With these the definition is of the
&nes1 qualify throughout the field, which has been reduced to about
i) inches They present the admirable condition that with the deeper

1 Chapter 1. p. 88.

HOLOSCOPIC " EYE-PIECES

379

powers the proper position of the e\ c i.s further from the eye-lens
than is the case with those of the Huyghenian construction; which
makes it as easy to use an eye-piece of a- grea^ a power as is or 27
as one of 4 or 8.

The field of these eye-pieces li.-is. as we Relieve, been very wisely
limited to five or six inches. The attempt on the part of English
opticians to give to our eye-pieces fit-Ids reaching eighteen inches is
an error. A microscopic objective with the lowest aperture lias the
field greatly in excess of any other optical instrument ; and to deal
with such eccentrical pencils as must lie engaged by an eye piece
with a field of eighteen inches is a strain not justified l>y what is
gained.

The powers of the working eye-pieces are also arranged in a new
way. The multiplying powers for the long tube are 4. 8. 12, IS. 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 without 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 offer
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
apochroniatic objectives, but they greatly enhance the images given
by ordinary achromatic lenses; and it may be noted that the S. 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
VT. "Watson and Sons, to which they have given the trade name of
' Holoscopic.' AVhal is held to be a very simple method is employed
for rendering them either over- or under-corrected, and therefore
suitable for either apochroniatic or the ordinary achromatic objectives.

This eye-piece is of the Huygheiiian type, but unlike the ordinary
pattern the eye-lens, together with the diaphragm, is mounted in a
tube which slides telescopic-ally in the body of the eye-piece, at the
lower end of which the field-lens is fixed. This is shown in fig. .'127.
When the sliding tube is pushed home as far as it will go, the eye

FIG. 326.— Abbe's

comp e 11 s a t i 11 g
.v c-piece of 12
power.

FIG. 327. — Watson's
holoscopic eye-
piece.

oK.IECTIVES, EYE-PIECES, THE APEETOMETEE

piece is ;iu under-corrected one ;m<l suitable for use with the
ordinary achromatic objectives; by drawing out the sliding tube
and so increasing the distance between The eye and tield 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 are theoretically two distinct advantages with this
eye piece :

(1) It obviates the necessity for being provided with both Huy-
gheiiian 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 appeared 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 tins series of eye-pieces are:—
For the 160 mm. tube length 5, 7, 10. and 14 diameters.

.. „ 250 „ „ 7, 10, 14, „ 20

Kor 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
drawingand 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 tbcussed 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 \\ill perhaps lie of practical utility if we
append a table indicating 1 he focus of the com-
pensating e\ e pieces \\heii used with the lontj and
the sin * r I I m, 1 1/.

Special Eye-pieces. — The most important of
these, the micrometer eye-piece, \\e ha\e already
considered, so far as its application to micrometrv is concerned.1
Its optical character may be properly considered here. Jf.it is a
negative eye piece the micrometer is placed in the focus of the e\ e

: but if a positive Combination, it is placed in the focus of the
Cye piece itself. The KalllSilell form

1 Cliiiptor IV.

•'i<;. :;-J8.— Zeiss's
projection eye-

|iii-rr NII. -J.

described abo\ e is t liorouglily

SPECIAL EYE-PIECES 381

Focus of Eye-pieces for Long Body.

Power

. .

2

4

8

Focus in

mm.

135

67-5

33-7

?i

inches .

5-3

2-6

1-33

12 is 27

22-5 LI 10

•89 -59 -39

Focus of Eye-pieces for Xhort 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

I

Projection Eyepieces. — 2 for short and 3 for long bodies = 90 mm. or 3-/J4
inches ; 4 for short and (3 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.

Tn order that the micrometer may be susceptible 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 positive
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 l^-inch focus ; but it is an inexpensive and a useful
plan to have an additional set of lenses to screw on to the same mount,
so as to make the eye-piece, say. a 4|-inch focus.

Spectroscopic, polarising, goniometer, and binocular eye-pieces
are each treated under their respective subjects.

ftuekett'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, when the magnification is yreat. is to have u dia-
phragm u'ith a small aperture 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 minute
objects by cutting off a large area of light without altering the in-
tensity of what remains, and so makes close oliserv.-ilion more easy.

Diaphragms with a square aperture are fitted into eye-pieces for
the purpose of counting blood-corpuscles in a definite ai-ea. The
hole in the diaphragm must be adjusted for a definite tube length
and for use with a definite objective and used with 110 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 stria1 or lines with oblique light the effect is much
strengthened /'// /il'tr/ny a S'n'nl's <ni<ili/xiii</ prism over the eye-
piece.

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 lie 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. Nothing tests
the quality of an objective so uncompromisingly as a deep eye-piece.
For InU'nnx'ii 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 character,
and even sharpness in the image. Unless the objective be of high
quality, the sharpness 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 large aperture) will bear the deepest
eye-piecing with no detriment. The 24 mm. and the 12 mm. of Zeiss
will suffer 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 bv
their possessors, are simply subjected to a comparison of perform
a nee 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 consideration of a suit-
able test-object, as \\ell as (lie magnifying power and aperture of the,
objective. It is knowing what is meant by a 'critical image,' and
lieing able to discover whether or not a given objective will yield it.
('Irarly all tests of optical instruments, which are not capable of
mi nn-riml expression, must be comparative. Magnifying poiver can
be measured numerically; it is not comparative. In t lie same way
resolr'niij power is mathematically measurable; so is penetrating
I><HI-I-I-. But definition and brillia/iicy <;/' in'ii<j<'. and evidence of
centring, can have no numerical expression; they are consequently
com] arative.

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 perfection of the
test-object shoiild be adapted to the power of the lens. A fairly
correct estimate of the relative performance of lenses of moderate
magnifying power may doubtless be thus made by a competent
( il iserver ; but it is not possible from any comparisons of this kind
To determine what may or ought to be the ultimate limit of optical
performance, or whether any particular lens under 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 comparison can only lie drawn be-
t \\-een objectives of the same magnifying power and aperture. Which
of two or more objectives gives the better image may be readily
enough ascertained by such 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 t hese com-
petitive examinations (which, after all, lead to no better result than
that the observer finds or fancies that one lens performs 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 — affords 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-
ment. It is, however, to be borne in mind that the microscopist, 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 APERTOMETEE

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 complete 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 course of isolated
pencils directed upon different parts or zones of the aperture, and
observing the union of the several images in the focal plane. For
this purpose it is necessary to bring under view the collective action
of each part of the aperture, central or peripheral, while at the same
time the image which each part singly and separately forms must be
distinguishable and capable of comparison 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 larger space than ^ inch. When
two pencils are employed one of these should fall so as to extend
from the centre of the field to ,',. 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 ,',. inch clear between its own
inner margin and the centre of the field. The objective images of
the pencils occupy each a quarter 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 /one on the opposite side of it. between the

,'-, and -,'.. inch (measured from the centre); and the third the

peripheral /one on the same side as the first in fig. .">'2!).

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 clear, colourless picture. Such a
fusion of images into one is, however, prevented 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 coloured 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. 330
coincident images in other parts of the field.

It is to be borne in mind that the errors which are 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 for
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 proceeds, 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 apla-
natic as it can be. This is certainly not true of Abbe's chromatic
condenser or a hemispherical lens. The reason is obvious : the
spherical aberration wholly pi-events 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 lower focal plane
of the illuminating lens must be fitted diaphragms (easily made of
blackened cardboard) 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 aperture (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 found. 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 focal plane of the objective shall be about a fourth
to a sixth part of the diameter of the field of the objective. Holes
having the dimensions thus experimentally found to give the required
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 wo\ud be obtained by putting sections of annular slits
at the lack 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 diaphragm is iixed, 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. \Ye doubt, however, if any sub-stage will revolve with
sufficient accuracv for so delicate a test.

Another plan adopted by Dr. Fripp, 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 ,-iperture, cause the oblique pencils
under 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 provide-d with a, rotating sub-stage,
it is sufficient to mount the condenser 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 removable at will. By rotating
this inner tube the pencils of light will by made to sweep round in

ABBE'S TEST-PLATE 387

the field, and thus permit each part of the central or peripheral zones
to be brought into play. Against the accurate value of this, ayain.
the spherical aberration of an uncorrected condenser 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 collected.

The test-plate consists of a series of cover-glasses, ranging in
thickness from O09 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. Groups of parallel lines are cut through
the films, and these are so coarsely ruled that
they are easily resolved by the lowest powers ;
yet from the extreme thinness of the silver
they also form a very delicate test for objectives
of even the highest power and widest aperture.
The test-plate in its natural size is seen in fig.
331, and one of the circles enlarged is seen in FIG. 332.

fie-

.

To examine an objective of large aperture, the discs must be
tbcussed 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
01- indistinctness of the minute irregularities of the edges. If, after
exactly adjusting the objective for oblique light, central illumination
is used, no alteration of the focus should be necessary 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 <>t
that thickness. On the other hand, if every disc shows nebulous
doubling, or an indistinct appearance of the edges of the lines with
oblique illumination, or if the objective requires a different focal ad-
justment to get equal sharpness with cential 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 /one ; an
alteration of the focus for oblique and central illumination (that is.
a difference of plane between the image in the peripheral and central
portions of the objective) points to an absence of concurrent 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 character 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
secondary spectrum, namely, on one side yellow-green to apple-green,
and on the other, violet to rose. The more perfect the correction of
the spherical aberration, the clearer this colour-band appears.

To obtain obliquity of illumination extending to the marginal
zone of the objective, and a rapid interchange from oblique to
central light, Abbe's illuminating apparatus is manifestly defective
011 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 rule, greater than 180°
in arc (1*00 N.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 cedar 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 necessary data for the estimation
of the spherical and chromatic collections by placing the concave
mirror so far laterally that its edge is nearly 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 character 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 O'Ol 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 term.

I inlistinctiiess 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 means the manner in which an object
of some si/e is defined.

A test for low powers up to ^ of 80° or X.A. -li."") is an object on

OBJECTS FOR LENS-TESTING — APERTOMETER 389

a dark ground. Nothing is so sensitive. For the lowest powers
one of the smaller and more delicate of the Polycistince, because it
takes light well, is good. For medium powers a coarse diatom, a
Tnceratiwm fimbriatum, is excellent; for unless an objective is well
corrected the image will be fringed and surrounded with scattei-ed
light, and the aberration produced by the cover-glass is plainly
manifest, and by accurate correction can be done away.

Error of centring is one of the special defects of objectives
Avhich 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 powers is a hair of Polyxenus layt/rn* mounted in balsam
(frontispiece, fig. 6).

For higher powers nothing surpasses a podura 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 thai
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'.'! X.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 Pol ;/.<•<.' HH* Imjurns, em-
ploying a | illuminating cone.

3. Centring for high powers: by means of podura scale.

4. Definition : with wide-angled oil immersions, ( 'oscinodiscus
(isteromphalns with wide-angled cone obtaining sharp, brilliant,
and clear view of 'secondaries,' or coarse specimen, of Navicula
r/tomboides, 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 ground is a

390

OBJECTIVES, EYE-PIECES, THE APERTOMETER

very severe test, us 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, but the aperture of the objective will have to be
reduced by a stop.

The apertometer. as its name implies, is an instrument for mea-
suring the ii/n 'I'ttire of a microscopic objective. As correct ideas of

aperture 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 controversy on the ' aper-
ture question,' which was in full
operation some eighteen years
since, is not an altogether satis-
factory page in the history of
the modern microscope, and for
many reasons it is well to pass
it unobservantly by. It will
suffice to state that during its
progress an apertometer was de-
vised by R. B. Tolles, of America,
which accurately measured the
true aperture 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 «, n being the refrac-
tive index of the medium, and u
half the angle of aperture.1

The application of this for-
mula to. and its general bearing

upon, the diffraction theory <>!'
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 measure of repeti-
tion in endeavouring to explain the use of this formula in such a
manner thai only a knou ledge of .simple arithmetic will be required

A knowledge of the meaning of the trigonometrical expression ' sine ' is not
necessarj in solving any of the following question-. As the values are all found in
babies, it, is only necessary to caution those \\lio are unacquainted with the use of
' ical i. ililcs to see that they haw the ' natural sine ' and not the 'log sine.'

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 X.A. (i.e. numerical aperture) ?

All that is needful is to find the value of n sine u ; in this case
?i=the refractive index of the medium, which is air, is 1 ; and u,
which is half of (50° = 30° opposite 30° in a table of natural sines,1 is
•-") : sine K. therefore ='5, which multiplied by 1 gives '5 as the
N.A. of a dry objective having (30° of angular aperture.

(ii) What is the X.A. of a water-immersion whose angular
aperture=44°?

n here=l'33, the refractive index of water ; and u, or half 44°,
is 22°. Sine 22° from tables=-375, which multiplied by 1'33 = '5
(nearly), which is the X.A. required.

(iii) What is the X.A. of an oil-immersion, objective having 38V
of angular aperture ?

n the refractive index of oil, which is equal to that of crown
glass, is 1'52; u = 19j and sine u from table=-329, which multi-
plied by V52 = -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 X.A. of "5.

It will be well, perhaps, to give the converse of this method.

(iv) If a dry objective is '5 X.A., what is its angular aper-
ture 'I

Here because n sine «=-5, sine "= the objective being

n '

dry, ?t=l, therefore sine «=-5. Opposite '5 in the table of natural
sines is 30° ; hence «=30°. But as.u is half the angular aperture
of the objective, 2« or 60°=the angular aperture required.

(v) What is the angular aperture of a water-immersion objective
whose X.A. = '5?

•;") '5

Here w-==l'33. n sine u='5 ; sine "• =— = ;, .oq = '37r> :

% 1 " o o

«.=22° (nearly) from tables of sines ; .*. 2?(=44°, the angle re-
quired.

(vi) What is the angular aperture of an oil-immersion objective
of -5 X.A. ?

•5 '5

Here ?i=l'52, n sine «='5 ; sine it=— =,-^-=-329 ;

/ *. JL *J *J

/f=19-}° (by tables of sines) ; and 2« = 38V the angle required.

We may yet further by a simple illustration explain the use of
n sine u.

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 I) ; suppose now that a pencil of light impinges on the surface of
the water at the point where the perpendicular 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

1 Vide Appendix A to this volume.

392 OBJECTIVES, EYE-PIECES, THE APERTOMETER

problem is to find the angle this pencil of light will make with the
perpendicular in the water.

To do this we must remember that n sine u on the air side is
equal to n' sine u' on the water side. Thus on the air side n=l,
?t=30°. and by the tables of sines sine 30° = '5 ; consequently on
the air side we have n sine u=~5.

On the water side /t'=l'33, and ti' is to be found. But as

, . . ,, , n sine n '5 0-c

n sine u = n sine u, therefore sine u = - — = — = '.j/o;

n loo

which (as the tables show) is the natural sine of an angle of 22'
(nearly); consequently /// = 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
perpendicular, would on emerging from the water be bent in air 8°
fwrther a/uoay from the perpendicular, and so make an angle of 30°
with it.

Now if we suppose that these pencils of light revolve round the
perpendicular, cones would be 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 u='5 ; on the oil side n' sine n'=n sine tr ; sine u'=

— =:,- ^ ='329, which (by the tables) is the natural sine of
n I'o2 v J

-°. It follows that the pencil has been bent in the cedar oil 10|°
out of its original course, and a cone of 60° in air becomes a cone of
38^° in cedar oil or crown glass.

Filially, it is instructive to note the result when an incident pencil
in air makes an angle of 90° with the perpendicular : n sine u becomes
unity, and u in water 48|°, in oil 41 c (nearly) ; consequently a cone
of either 971-0 in water, or 82 j° in oil or crown glass, is the exact
equivalent of the whole 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-immersion of 97i° and an oil-immersion of 82^
.•mgiilar aperture.

The last problem that need occupy us is to find the angular
aperture of an oil-immersion which shall lie equivalent to a water-
immersion of 180° angular aperture-.

On the water side- n = 1'33, u = 90°, sine 90° = 1, n sine // =
I •.'>.'{. On the oil side n' = l-f>2 and //.' has to be found.

,. n sine ^(, 1 '•'••'!

A- //sine a = a sine //. therefore sine n = - =

n i-52

= •875; !*' = 61 (nearly) by the- table's; 2"' =122° (nearly),

the angle required.

THE APEETOMETER 393

It thus appears (1) that dry and immersion objectives having
different angular apertures, if of the same eqt< indent aperture, are
designated by the same term. Thus objectives of 60° in air, or 44°
in water, or 38V3 in oil, have identically the same aperture, and are
known by the same designation of '5 X.A.

(2) The penetrating power of any objective is proportional to

•\r~r-, and its illuminating power to (X.A.)2. Therefore, if we

double the X.A. we halve the penetrating power, and inci-ea.se 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 J-inch objective of '8 X.A.
has half the penetrating power of a Vincli of '4 X.A. Neither can
it be said that it has four times the illuminating power. What is
meant is that a ^-inch of -8 X.A. has half the penetrating and four
times the illuminating power of a ^-inch objective of '4 X.A.

But because penetrating and illuminating powers diminish as
the square of the foci, a Viiich objective of -(5 X.A. has four times
the illuminating and nearly four times the penetrating power of a
^-iiich of •(> X.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 '(5 X.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 X.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. Thus it
is not only true that a ^-inch objective of '6 X.A. resolves twice as
many lines to the inch as a ^-inch of '3 X.A., but so also does a
jr-inch of 1'4 X.A. resolve twice, and only twice, as manv as a J-inch
of -7 X\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 numerical
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, EVE-PIECES, THE APERTOMETEB

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 vertical 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 lam}),
is assumed to be in front and on both sides. (Suppose the lens to
be measured is a dry ^j-iiich ; 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

FIG. 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 |i-irt 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
vert ic;il circular portion of the instrument. The images of these
screens c:in be seen in the image of the bright band. These screens
xlmuli! iinir be moved so tlml I //fir edges just touch the periphery of
lli<- Ii,n-Jc 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-
bive and no more.

This angle is now determined by the arc of glass between the

THE USE OF THE APEETOMETEK 395

screens ; thus we get an angle in ytass the exact equivalent of the
aperture of the objective. As the numerical apertures of these arcs
are engraved on the apertometer they can lie read oft' by inspection.
Nevertheless a difficulty is experienced, 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 prefer to designate
it, the limit of aperture, for. curious as this expression 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 for better way of employing
this instrument is to use it in connection, n-ith a <ji-a<ltiated rotor//
staye, the edge of the flame of a paraffin lamp 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 xtayr trill
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 It/ tin- <?<l<i<>. of tin' /i/>i>rture, the arc is then read, and
the same thing is repeated on the other 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 with 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-iiich as well as its angular aperture in air.1

(i) As before, X.A. = it sine u, and n sine u = n' sine ur ;
which means that the aperture on the air side is equal to the aperture
on the glass side ; // = 1 for air ; n' = 1*615, the refractive index of
the apertometer; u' is the mean angle measured, which in this case
is 30° ; and n sine u has to be found.

Now sine 30° = *5 (by the tables) ; w'sine u' = 1*615 x sine 30°
= 1'615 X *5 = *8 = n sine //. = the X.A. required.

(ii) Again, to find iheanyularajH'.rtnn' or 'In. As before. // sine "

, . //.' sine "' 1*61") x ••") _00

= n sine u and sine u = - = _ = *o : zt = •>•>

it 1

nearly (by the tables) ; '2n = 106°. which is the angle required.

(iii) If it be a water-immersion we have to deal with, suppose
the mean angle = 45° = //' : sine 45° = -707 (by the tables) ; n
= 1-33; and «'= 1-615.

•//, sine u = n' sine //' = 1*615 x '707 = 1*14. the X.A. required.

,. ,. . . it' sine u' 1-615 x '707 0/, -mo

(iv) Again, sine u=- - = -8b ; a. = 594/

n 1*33

(by the tables) ; and "In = 11877°, the angle required.

(v) In the case of an oil-iiiuit<'r*i<>n. suppose the mean angle

1 Vide p. 2 ct seq.

396 OBJECTIVES, EYE-PIECES, THE APERTOMETER

= 00° = n ; sine 60° = '866 (by the tables) ; n = 1-52 ; H' = 1-615 ;
•it sine u = n' sine u' = 1*01") x '8(50 = 1-4, which is the X.A.
required.

n' sine u' 1-615 X "86(5

(vi) Again, sine n = = = '92.

?«, 1'52

// = 67° (by the tables), 2n = 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
retractive index is the numerical aperture.

This will l)e found the more accurate 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 purpose 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 N.A. can be read off by inspection without the
necessity of looking out sines or making calculations.

397

CHAPTER VI

PBACTICAL MICBOSCOPY: MANIPULATION AND PRESERVATION

OF THE MICROSCOPE

WITHOUT attempting to occupy 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 recognition of practical facts
if we claim as a definition of microscopy that it expresses, and is in-
tended to carry 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 perfect manner, based alike and equally upon
theoretical knowledge and practical experience.

On this condition a ' microscopist ' means (or at least implies)
one who, understanding 'microscopy,' applies his theoretical and
practical knowledge either to the further improvement and perfec-
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 ' microscopical society ' has a noble raison d'etre, because
it is established, on the one hand, to promote — 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 afforded 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 PRESERVATION OF THE MICROSCOPE

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 provided with some room, or portion of a room,
which he can hold sacred to his purpose. 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 will lie a suitable table.

This should be thoroughly .firm, and it should be rectangular in
xli«/H'. \ round table, if small especially, is most undesirable, as it
offers no support for the arms on either side of the instrument ; and
with prolonged 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 110 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 110 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 dissectiiig-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 rotary nose-piece, especially
where critical work has to be done.

When work is being done in a darkened room there should 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 fiat cylindrical glass shade with a knob on
(lie top. The stand should he suitably arranged to hold two eye-
pieces, three objectives, one condenser, a bottle of cedar-oil (titled
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 liaxe a. diameter greater than nine inches.

Tin' size fin' tin' /"/' ';/ xin-li ii tiililc should be 4^ x 3 feet, and as

MICKOSCOPISTS' WORK-TABLES

399

r>

no work, 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 dejith
of three feet is required for comfortable work AN hen the micro-
scope is set up for drawing,1 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 Jic'njJit of the table.
Ordinary tables, being about 2 ft. 4 in. high, are too low even
for large 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, and not with nests
of drawers on either
side, because with this
particular table it will
be frequently required
that two persons may
sit side by side, which
is only possible with a
clear space beneath.

The accompanying
illustration (fig. .'!.">•"")).
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 above de-
scribed is supposed to
be employed wholly for
general purposes of ob-
servation or research on wholly or partially mounted objects. But
the microscopist who aims at more than this will require an arrange-
ment for dissecting, mounting, and arranging histological and other
preparations, and in some cases a special table for general purposes
of microscopical biology. These are certainly not essentials, especi-
ally if the work done is a mere occasional occupation ; but where
anything like continuity or periodical regularity of occupation with
such work is intended, these will be of great service.

A dissecting and mounting table is indeed of inestimable value to
those who affect 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.

y

FIG. 335. — Microscopist's table.
t (Scale, i inch to 1 foot.)

1. Case for microscope ; 2. Cabinet for objects ;
3. Microscope lamp ; 4. Lamp with shade ;
5. Stand of apparatus ; 6. Book ; 7. Large micro-
scope; 8. Second microscope; 9. Writing pad;
II). Bull's-eye stand ; 11. Light-modifier.

400 MANIPULATION AND PRESERVATION OF THE MICROSCOPE

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
13 drawers may be put, not extending more 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, forceps, pipettes, life-slides, ttc. in the upper one,
and jilii-rs. ruttiiKj i>liers, small shears, files of various coarsenesses
and finenesses, etc. in the other ; on the A side a single drawer con-
taining slips, covers of various thicknesses, bone, tin, glass, and other
cells of all (assorted) sizes, watch-glasses, staining cups or sA/Ax.
lifters (if vised), saw with fine teeth, hones of various shapes, pewter
jilnte, for grinding and polishing glass, etc.. platinum capsule, camera
/>«•/</«. three ' JS"o. 2 'sable brushes (water-colour), etc.

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. 006. — Dissecting and mounting table.

merit 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,
sonic very hard wood finished smoothly, but 'grey.'

The space in the figure immediately in front of the operator
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 thai it may slide firmly 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
he made black on its under side, so as to present a uniform black
xnrfitci-. This is often of great value in certain kinds of work.
Equally useful is a pnrrfi/ n-liiti' unabsorbent surface, and a slab of
ir/iitf /inrci'luiii may be easily obtained of the same size and be made
to lit exact Iv into t he same place.

APPLIANCES FOE DISSECTING TABLE 401

In using this table for dissection the arms have complete rest,
and 1 in the figure would represent the position 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 l to
answer this purpose 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 paraffin, or xylol, glycerine, &c., as well as small bottles
of reagents for botanical or zoological histology, &c.

f) 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-
dude dust.

6 is a stand of cements, varnishes, Arc., 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 R. 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 ^JUs — — £^7 \

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 I'n, :i:',7.-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, employed by this
gentleman, but it will serve admirably for any form of dissecting
instrument.

For the more general purpose of the private laboratory « /tlain.
farm, 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 fitting attached to the main
to supply blowpipe, AV//.sr//.s littrtn'r. A.T.

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 n the longer. This affords a pleasant stream of water for wash-
ing dissections, etc. ; and if the open end. /»' made n-ith a screw, and
have a suitably made piece of tub my fitted to .s-ov/r on to it. this latter
may be attached to an indiarubber 'tube, at t/><- «///<•/• end of u-hich -u-e
may fasten fine ylass nozzles, which will act as wash bottles of the

finest bore, and serve with the finest dissecting work.

5 is a glass trough for waste, with a perforated aperture, 6, con-

Jotirn. R.M.S. new series, l.ssT. )>. r.s-j.

D D

A

402 MANIPULATION AND PRESERVATION OF THE MICROSCOPE

nected with a waste-pipe, through which the waste water, <tc. flows
innocuously away.

3 represents the position of a Thoina microtome, and A, Bare two
well-framed flat slides, which may he drawn out eighteen inches, or

pushed fully in. They
are found at times to
1)e 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 drawers and shelves
for receiving various ap-
paratus and materials.
with larger quantities of
stains and reagents,
hardening, macerating,
and other materials ;
while if a door covers
the whole, the inner side
PIG. 338. — Laboratory table for microscopical work, 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 paper 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 01
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.2 It musi a I \\ays be the case where certain
forms of continuous life stages are employed for prolonged or con-

engraved

FlG. 339. — Tripod for u^inu; microscope in un
upright posit inn.

VII.

Micro. Jaunt, vols. x. to xviii. ; Jo/im. I,'. M.S. vol. iii. p. 1; vol. v.
ii. p. 177 ; vol. vi. p. J.!):j ; vol. vii. p. lNf> ; vol. viii. p. 177.

SPECIAL USE OF MICROSCOPE— UPRIGHT

403

tinuous observations on the development of the minutei forms of
life.

In such cases the table is quite unsuitable, and special stand*
have to be employed that from their form give great stability to the
microscope, and afford the body and head of the observer as much
command and ease in using the instrument in this awkward position
as can be obtained.

This is best done by means of a firmly made tripod, with a V-
shaped piece at the top made to receive the feet of the microscope.
Fig. 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 with metal rods.
A, A is the bed for the tripod 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 lamp.

By this arrangement the body can so place itself as to command
the instrument fully, and there is a.n arrangement at the two sides.
A, A, to receive supports 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 required. The manner of

r> D 2

404 MANIPULATION AND PRESERVATION OF THE MICROSCOPE

using this arrangement is seen in fig. 340. In that case, however,
the whole is employed for the making of a camera lucida drawing
with a oV-inch 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 upon
it, the position of the lamp (partly seen in the immediate fore-
ground to the left), and the relative ease with which the entire
instrument is at the command of the observer, will be manifest.
In order to use the microscope successfully, we must have an

illumination the inten-
sity of which we can
fully rely on. Dtu/lujht
//UN certain, qualities that
iiirolve advantages at
times, and under special
circumstances, in its em-
ployment, but this is the
exception rather than the
/•uli'. What is needed
is a well-made lamp with
a flat name ; this we
should be able to control
with great ease as to
height and distance from
the microscope. ISTo-
thiiig is equal practically
to a 7j-inch or a 1-inch
paraffin 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
paraffin lamp iised with
g< )od ' oil ' has no present
rival. Its illuminating
power should be about

2^ candles, (las 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
f. PI-HI of lamp which will accomplish every purpose. This is a model
arranged by .Mr. Nelson, the drawing of which is given in fig.
.'•41. The lamp hums paraflin and has an ordinary |-inch wick
burner. The reservoir is rectangular and flat, 5^x4x1^; it
serves three distinct purposes: 1st. it will hold sufficient oil to burn
for a whole day; 1'nd, permits the lamp to be lowered near the

PIG. :)11. — Lamp devised by Mr. E. M. Nelson.

NELSON'S LAMP 405

table; 3rd, radiates the heat conducted by the metal chimney, and
prevents the oil boiling. The burner is placed at one angle of the
reservoir to enable the flame 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 powers, 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 ordinary 3 x 1
glass slip in front ; the diameter of the flame-chamber should not
exceed 1^ inch, and the grooves holding the glass slip should project
^ inch from the flame-chamber ; the aperture should be only H 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 stria* and specks common to ordinary
lamp 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.1

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 e< ii
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 pi-event 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 paraffin 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

1 It is very important to remove the metal chimney after use, or at least not to
leave it on when not in use, since the evaporating paraffin gathers round it and causes
undesirable scent when the lamp is again lit. The thinnest slips should be used.

406 MANIPULATION AND PRESERVATION OF THE MICROSCOPE

rod, ]>. is screwed. The square rod carries a socket, 0, with an arm,
D, to which the lamp is attached.

On each side of the burner, and attached to the arm, I), is an
upright rod, G, to one of which the chimney is fixed, independent of

reservoir of the lamp, thus enabling the observer to revolve the
ni-r and reservoir, and ol.iaiii cither the edge or the flat side of
the Maine without altering the posilion of the chimney. The
chimney, I1', is made of thin l»ra». witli t \\< > openings opposite to
other, into \\hich slide 3x1 glass slips of either white, blue, or

LAMP WITH LATERAL MOTION

407

opal glass, the latter serving as a reflector ; but we do not consider
the reflexion here accomplished as other than an error ; it causes
double reflexion and confuses the condensed image.

A semicircle swings from the two uprights, C4, to which it is
attached by the pins, H, placed level with the 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,
in id therefore 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
importance than under ordin-
ary circumstances. The pre-
sent Editor devised a some-
what elaborate apparatus of
this kind, which he always
employs in this kind of ob-
servation.1 But the essential
part of it is only an arrange-
ment 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 right
angles, the centring of the
lamp flame upon the object
is more readily done by means
of motion in the lan>]>. A
very simple form of this lamp
has been made for the Editor
by Mr. Charles Baker, of FIG. 348.

Hoi born ; it is seen in fig.

343, being an ordinary lamp, except 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 reservoir, or at least to have an arrangement by means of
1 MontMji Mirro, Join n. vcl. xv. p. 165,

408 MANIPULATION AND PRESERVATION OF THE MICROSCOPE

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 an

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
!l,me, 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 a,s may
be, and then a little movement in one or both milled heads
bring it accurately 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
.,Hss 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, Ms put into the sub-
staore The lamp is now put into the right position, with a hull s-
eve& 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 it 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. »44.^Edge of lamp flame in cenTrTand °fteil, 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 V., where it will be
observed that the microscope is inclined more towards the horizontal
to suit the observer ; the lamp 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 1 hen treated as is the case (for centring) when the
mirror is used. The first step in the direction of efficiency in the
use of the microscope is to understand the •principles of illumination,
and a knowledge of the various effects produced by the bull's e\e
lies on the 1 hreshold of this.

Having given details as to the forms of l<unp which are of most
service, we assume that a paraffin lamp with Tj-inch wick is used.

If we place the edge of this (lame (K. tig. "'544) in the centre and
exact focus of the Imll's-eye II. A slious the effect of doing so.

If a piece of card were held in the path of the rays proceeding
from P>, the picture as shown at A would not be seen — instead of it
an enlarged and in\erted image of the flame. The image at A is
1 Vide Chapter IV. p. 2'.)s.

J-L

D

THE USE OF THE BULL'S-EYE

409

obtained by placing the eye in the rays and by looking directly 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-
tensity of the light in A depends
011 the focal length of B. The
shorter the focus, the

more in-
tense will be the light.

We are here assuming
thi'oughout that the field lens is
at a fixed distance from the
Inill's-eye B.

But if we move the flame, E
—still central — within the focus
of B, we get the result shown FK; 345._Altered relations between lam

ill D, fig. 345. But by moving flame and bull's-eye.

E without the focus of B we get

the picture H, while K is the picture when E is focussed but 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 parallel 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
different result from
what is aimed at. If
the concave mirror, C,

is to be of any use ill FlG. 346.— Result of placing flame in principal focus

illumination, it must of concave mirror.

be placed so that E is

not at its principal focus, but at its centre of cur rat/ in .

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 name, 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 prin-
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. 347. — Mode of obtaining critical image.

4IO MANIPULATION AND PRESEKVATION OF THE MICROSCOPE

the law, and there can be hut little difficulty remaining in getting

the IM-M results from a condenser.

Fiy. 34* illustrates another method of getting the same result

\V,- may illuminate a condenser with light direct from the flame, as

in fig. 347, or we 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, but 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 IV. But the following
should be carefully considered.

FIG. 348.— Another method of getting
critical image.

Fig. 349 shows a sub-stage con-

denser, 8, and an objective, 0, both focussed on the same point.
The condenser has an aperture equal to that of the objective. Now
if the eye-piece be removed, and we look at the back lens of the
objective, it will be seen to be fall of light, as at 11. 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, .-is in fig. 349, that therefore the field ought to be full of
light. The Held only shows the In-ii/lit image oftheedge of the flame,

o

o

FIG. 349. — Condenser and object-glass

with the same aperture.

FIG, 350. — The same, with the aperture
of the condenser cut down.

and it is /'•// I/ml ulmn' Unit n critinil />i<'tnre can be found. If the
condenser he racked either within or without the focus, the wliol?
I'n'l, I /rill Ix'i-onii' illn niiiii/lr,/ . hut, a,t the same time a far smaller por-
tion of the objective will be utilised. On removing the eye-piece and
examining 1 lie hack lens of t he object ive. pict nres like I), H, fig. 34.").
will be seen I) when within, and II when without the focus.

The condition represented in fig. 349 at R and (_) is the severest
test which can be applied to the microscopic objective; that is to
say, to fill the whole objective \viih light and so test the marginal

Hii'l i-i'iiti-iil /xif/iniiN ill tin' MI mi' linn'.

Kven to obtain the state of illumination known as 'diffused day-

W
EH

H
EH

EH

O

z

s

H

O

H

TO USE DIFFUSED DAYLIGHT

411

light ' with the simple mirror 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
focus from the object. But the manner in which it is usually done,
from want of thought or knowledge, or both, is shown in fig. .'••"> 2.

C

FIG. 351. — Illumination for ' diffused daylight.'

where it is manifest ih;it t lii-re 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 l>e adjusted

It is also important here to note that in dayliyht illumination a

C

FIG. 352. — Erroneous method of arrangement for ' diffused daylight.'

i in- mirror gives a cone of illumination, \\< in fig. •">•">•">. when

is ample sky-room; but a ii-'unJim- arts as a liuiiti/n/ '//'////'A';///'.

In regard to the parallelism of the Jiwt W"/- rdi/x 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 PRESERVATION OF THE MICROSCOPE

which constitiite our daylight fall from every point of the visible
heavens (though with greatly diminished intensity). That is to say,
we have at disposal a light source extending over 180°, 'while the sun
itself extends over a visual angle of bat half a degree. Being thus
surrounded by an illimitable and self-luminous 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 reflexion ; and it

S'

FIG. 353. — Light from the open sky falls upon the mirror in all directions.

becomes equally clear that in order to strike the object the
must al/waysfall obliquely on the 'mirror.

Then it follows from what has been said that the light falling
from the open sky upon a mirror falls in all conceivable directions.
Tims 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 O. The intermediate rays, 2, 3, 4,
5, 6, form the converging Ultnn'nndnnj /><'//cil, with of course an in-
finity of others filling up the spares between.

In other words, every point of a mirror is a radiant of a whole
hemisphere, and this is equollt/ l/'/f K hctlicr the. mirror be. plmn',
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 REFLECTED TO A FOCUS FROM THE OPEN SKY 413

FIG. 354.— With the open sky, light is
focussed at all points.

illumination of which the object is its apex, no matter what the in-
clination or distance of the mirror. 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 ; therefore it practically does make a
difference whether the plane or concave mirror is used, and whether
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 will give
in that particular 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 tin-
lens of the objective when 1 1 it-
eye piece is removed. Fig. 3.55 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. 35(5 illustrates the method of obtaining dark-ground illumi-
nation when the arrangement shown in fig. 347 or 348 does not give
a sufficiently illuminated area even when
the flat of the flame is used. Of coin — '
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 illustra-
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 baa
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 ;>4S.
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 X.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 PRESERVATION OF THE MICROSCOPE

condensers by the method shown in fig. 347. But when the micro-
scope is of necessity used upright the rectangular prism or the plane
mirror must be used, fig. 348.

The arrangement at fig. 356 is sometimes useful for photo-
micrography when it is otheriri.y,> 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.

M

FIG. 356. — Illumination for dark ground (with
stop beneath the condenser).

C

PIG. 357. — Same result with concave

mirror.

Iii 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 lie obtained by the concave
mirror (as shown in fig. 357) instead of the bull's-eye. But this
is a very difficult arrangement, yielding the best results only with
great application and care.

But the supreme folly of using a concave mirror and <i buWs-eye

B

FIG. 858. — Absurdity of vising a b
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 j-hown in fig. 358, where C is (lie eonca\ e mirror and (as before) S

— as will be seen b the

the sub-stage condenser ; this secure* a
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. .'559, which illustrate^
tin' 1',-rnr <>f not liur'uK/ tln> !'</</>> of tin' jhiiin- K iii tin' principal focus
of tke bull's-eye l>. The ravs converge on the condenser S. so that
it will hen me in all probability impossible to focus it on the

DA UK-GROUND ILLUMINATION

415

object. This is a lateral lesson on the value of having the bull's-
eye fixed to the lamp, so that both may be moved together ; and
there should be a notch in the slot or arm which carries 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 give 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 tricerat'nnn (diatom) be taken, and sup-
pose that the objective employed is a §-inch of -2H X.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 conden>er and a tri-
ceralium on the stage and the § 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. 3(50.

FIG. 301.

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.

Xow, 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, as 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 filled with lif/ht, but has instead two crescents of light.

This is a case which frequently repeats itself, but it is of course
not inevitable. The bull's-eye may be more or les.s tilled with liyht.
and may or may not be more nearly centred. In this case \\e have
next to centre the image of the bull's-eye to the triceratium bv
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 liyht. 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 l'<in/> <md bull's-eye must on no

41 6 MANIPULATION AND PRESERVATION OF THE MICROSCOPE

account be altered, and it is understood that the lamp was adjusted
to the picture A in fig. 344 l>y inspection and without the micro-
scope. A very slight movement in azimuth, however, is enough to
effect the desired end (fig. 364), and all that now remains is to open
the full aperture of the condenser and put in the smallest stop ; 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 difference 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 covers the triceratmm.
When the whole aperture of the condenser is opened the size of thnt
disc will not be altered, its intensity only vnft be
increased. When the stop 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 :—
FIG. 364. o. 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 « and y are constants, the oidy way of varying the size of the
dark field is by /3.

In the same way the intensity of the liyht in the disc depends 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.

1 (' 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 ' difficulties
of illumination ' will have practically passed away.

There are tiro /duds of microscopical work — one, the more usual
ami comparatively easy, is the examination of an object to see some-
tit iin/ n-liii'h, is knou-n. The other is the examination of an object
in search of the unknown. Thus some blood may be examined
for ihe purpose of finding a white corpuscle. It matters little
\\liat, is the quality of either the lens or the illumination or the
microscope, or whether the room is darkened or not. because the
observer knows that there is such a thing as a while corpuscle. It-
is quite immaterial as to whether the observer had ever seen one or
not ; so long as lie possesses the knowledge that there is such a thing,
the finding of it, even under unfavourable conditions, will be an
easy i.-isk.

Hut it' the observer has not that knowledge, he may examine

SEARCH WORK — LIGHT AND THE EYES 417

blood many times, under favourable conditions, and yet not notice
the presence of a white ccrpuscle. 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 searching for new things who can
appreciate the enormous difficulty in first recognising a new point.
Therefore, when critical work is undertaken, ''arc should be exercised
to have the conditions as favourable as possible.

When 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 should be ample light on the microscope
table, as it is not at all necessary or desirable that the work should
be insufficiently illuminated. All that i> required is that the lamps
should have shades and be placed at such a height that the direct
ravs do not enter the observer's eye.

If these precautions are taken, several hours' continued work
may be carried on without any injurious effect.

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 particular eye feel
awkward with the other. In continuous work, extending over maii\
months of long daily observation, if the eye has been accustomed to
monocular vision, even with high powers, there is no difficulty
experienced. The effect "f 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 feature
of illumination not due to the light from the edge of a paraffin lamp,
which, as we have stated, is not particularly yellow. Great //<<//// //•-
ness is a sign of imperfect achromatism in an objective. AVe may
with precisely the same conditions find the images yielded by two
objectives of the same power and aperture differ, in so much as one
is yellow and dim and the other white and bright ; 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 bright 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 there 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 PRESERVATION OF THE MICROSCOPE

an error of centring, 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 understood that signal green glass will not yield
monochromatic illumination ; only the Gifford screen or the filter
screen of Prof. Miethe ('/.'•.) or the Nelson spectroseopic arrange-
ment ('/.f.) can be of real service.

Coloured light derived from a polariscope and a selenite is not
monochromatic.

For critical irork. »/'<•!> its test tin/ lenses or forcing out the greatest
resolution with the widest-angled oil-immersion lenses, daylight
illumination is inadmissible.

\Vheii 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 arrange 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 mirror through a light angle. A screen may be placed parallel
to the window which just allows the mirror of the microscope to
project beyond it. This cuts oft' direct light from 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 remember 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 way that the
eye can be brought to the eye-piece in a perfectly natural and com-
fortable manner. The body should also be steadied by resting the
arms on the table.

It is advisable to use the bull's-eye as little as possible; ercn n'itJ>
dark-ground illumination the flat of the flame is preferable, reserving
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 nece>
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 organisms, 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 scarcely be neglected by the searcher for
the unknown. lYotes.-.or Abbedoc* not advise their employment As
in any way final; he says that 'the resulting image produced by
means of ,-i broad illuminating beam is always a mixture of a multi-
tude of partial image* which are more or less different and dissimilar
from the object it>elf;' ,-ind lie does not conceive that there is any
Around for expectation 'that this mixture should come nearer to a
.-Irictly correct projection of the object . . . than the image which

TO DISPLAY OBJECTS MICROSCOPICALLY 419

is projected by ;i narrow 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 other means of tin-
images they present. This is the more a necessity since Mr. Xelsoii
lias been able to obtain the most wonderful results with narrow cones.
' true ghosts ' and ; false ghosts.' the presence of intercostal markings"
in the image of a fly's eye (!). and many complex and false iumge.-,
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.1 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 structure, such as the /» /•
forated membrane, of a diatom, it can be done by annular illumination,
which means the same arrangement as for dark ground, but with a
stop insufficiently large to shut out all the light. This method is not
to be recommended when a structure 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, give-
on slight focal alterations a rariety 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 observation has its
value, and must not be forgotten.

T<> properly display objects under a microscope, is to a certain ex-
tent an art, for it not only demands dexterity in the manipulation
of the instrument and its appliances, but it also requires 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
beginners, to clearly 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 § 011 the nose-piece, and a B 01-
No. 2 eye-piece in the tube.

2nd. Use as a source of illumination the light from a paraffin
lamp with a ^-inch wick.

3rd. Place any suitable object on the stage, and, having focus>ed
it with any kind of illumination, centre it to the field of the eye-
piece.

4th. Place a small diaphragm beneath the sub-stage condenser, or
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 diaphragm should not be central to the

1 Journ. E. M. S , 1*91, p. 90, pi. II.

E E '2

420 MANIPULATION AND PRESERVATION OF THE MICROSCOPE

object on the stage, it must be centred by means of the sub-stage ad-
just ing 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 pel-forming this 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 lie effected by moving the mirror.)

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.

llth. 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 thai 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.

l.'lth. 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 part 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
transmitted 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 forms,
sire seen very well when 'mounted <//•</ <>n corer by means of a j-inch
objective and a Lieberkiihn ; the hull's eye and the plane mirror should
lie used. Home objects are so transparent, or become so transparent
in the medium in which they are mounted, that they will not bear a
large illuminating cone, the brightness of the illumination desi roving
the contrast. It will illustrate this when \\ e recall that dirt on an
«'\v piece which is (jiiite 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
\vith a {-inch objective; for a general view of 'pond life' a Ijj-inch

'CRITICAL' AND UNCRITICAL IMAGES 421

objective with a dark-ground illumination, employing a binocular, is
very suitable. Stained bacteria in tissue are best seen with a l<i !•</<•
cone, as was pointed out by Dr. Robert Koch, and is directly supported
by Dr. Abbe as suitable in his directions for the use of the Abbe
condenser.1 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. 1 and 3 in the frontispiece. Fig. .'! 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 J of -95 X.A.
and No. 3 projection eye-piece ; and it was illuminated by means of
a large solid cone of '65 N.A. from an achromatic condenser.

Fig. 2 is an uncritical image, with all the conditions as above.
s;ive that a cone of small angle, i.e. of O'l, was used for illumination.

The first alteration which thrusts itself upon the eye is the
'foiifilitti/ of the hairs which are in the least degree out of focus.
Hut, further, 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
effect, always, in our experience, present in objects illuminated by
cones of insufficient angle, and it can be easily made to disappear 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 ' stiff 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 are like the
spinous hairs of an insect, and have the usual socket-joint at the

1 Directions for the Use of Abbe's Illuminating Apparatus — a leaflet issued by
Carl Zeiss, 1888.

422 MANIPULATION AND PRESERVATION OF THE MICROSCOPE

ba>e. Iii reality the 'stiff and longisli 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 are important and comparatively large ob-
jects. There appears to l?e considerable probability that this inte-
resting object upon the last ring of the body of the flea, and known
as its • pygidium.' acts as an auditory instrument.1 In the examina-
tion of ordinary 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 use 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 r! will be found as much as they are able to bear — some
indeed will not stand a ]y 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
T> and ^, 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 transmitted 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 iris
diaphragm : let three good wide-angled objectives be chosen, say
I inch, a i-inch. and ]-mch 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
gel the instrument into perfect adjustment. Now close the iris. The
hair will be surrounded by a luminous border, which will give it
a glazy appearance, and its fine point will be blurred out. Now
open the iris until the last trace of that glaziness disappears. The
hair will appear as a different object., its outline being perfectly clear
and sharp. If the eye-piece is removed, about two-thirds of the ob-
jective lu-k \\ill b<> full of light, Now. without disturbing any of
the adjustments, replace the 1 -inch by the i, and it will be found
thai the glaziness or false light will have returned. Let the iris
be further opened until the last trace of it disappears; now, on
examination of t he back of the object ive. 1 wo- thirds of it will be found
full of light, and so on with the j. We call the attention of the
studenl to these facts a^ having a direct hearing upon the question
o] the com pa rat he e fleets of hrge and small illuminating cones, and

icros. Joitm. April i>l. ].s,sr> : ' ry-'iilium oi Flea' (E. M. Nelson).

VARIOUS MODES OF ILLUMINATION— LARGE CONES 423

with no idea of offering opposing opinions to those of Professor Abbe ;
we have 110 direct judgment, but we record these facts as factors in
and for the elucidation 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 appear to suggest itself that this glaziness depends on the
relation of the, aperture of the illwminating <-»DI> to that of the objective
cone. Apochromatic objectives behave precisely 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 aberration in the objective ; another objective must there-
fore be used ; that paleness lias nothing whatever to do with the
glaze or false light mentioned above.

In photo-micrographs 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 rone 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 exposure,
and difficulties of development. If, so far as our experience goes,
a good photo -micrograph is required, these difficulties must be
mastered.

It is hardly necessary to remind the student that in micromcli \
it is essential that the edges of the object should be defined; conse-
quently a large cone must then be employed.

For the examination of Polycystmes. Foraminifera, &c., a binocular
is useful ; illumination may be by a Lieberkiilm 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 Lieberkiilm, and the binocular
should be vised for both.

•Some of this class of objects are best seen under double illumina-
tion ; that is, a dark ground with a condenser and light thrown from,
above with a silver side-reflector, as the Lieberkiilm cannot be used in
conjunction with an achromatic condenser. It is a good plan with
low-power Lieberkiilm work to interpose between the slip and the
ledge a strip of plain glass Vinch wide ; this prevents the ledge
stopping out lit/lit from the Lieberkiihn when it is larger in diameter
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 ' Jourii. R.M.S.' 1896, p. 373,
and the ' Journ. R. M. S.' for 1899, p. 142.

Polarised light used with a condenser is very useful for insect
work. For very low-power work — 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 < Jifibrd's screen with it. With objectives of greater
angle than '6 N.A. it is usually difficult to get satisfactory illumina-

424 MANIPULATION AND PRESERVATION OF THE MICROSCOPE

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 IS". 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 slijt
is thin, the oil inr«ri<il>!t/ r/mx ilon-u n-Jn-n tin- cfnidptisPi' is focussed.

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,
g- inch broad, cemented by
shellac to one edge. This
piece of glass is oiled to
the slip, the ledge being
In xiked over the top of the
slide ; this not onlv pre-
vents its slipping down,
but also keeps the oil from

Slide in situ on thin
>lip with ledge.

Thiii slip of glass with
ledge to place glass
slip with oil contact,
so as to vary the
thickness of a slide.

FIG. :-!(•..-;.

creeping out at the bottom,
which would be the case if the two edges of the glass coincided.1
This is illustrated in fig. 365.

In its proper place we have dealt with the suitable relation of
aperture to power, and have pointed 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 ^lt> inch at ten inches.
Consequently a microscope with a power of 200 should be capable of
showing structure as fine as 5o^nr<i inch. ISTow, 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 i-iiich to enable it to
resolve 50,000 lines per inch. This 2 will be -52 X.A. The inch
objective should have half this aperture, and the } double, and the
g- four times as much, if perfect vision is required ; in other words,
•2<; N.A. for every 100 diameters.3 These ideals have (as we have
before indicated) been realised, notably by the Zeiss apochromatics,
the 1-inch and the i-inch ' resolving everything capable of being
appreciated by the eye when the 12 compensating eye-piece is used.
Tin- j-incli is also a near approach to the ideal, as it has been very
wisely kepi a dry len>. The oil-immersion £-in. of 1 '4 X.A. with
a (') eye piece also attains the ideal. This relation of aperture to

1 i,>. M. C. Journal, November issr,.

'* In reality it will require more, beeause an axial cone is assumed to be used

1 < ibliqlle beam.

'UsJi Mrr/i, </,/,•, vol. xxxviii. 188:-;, No. !>7!>. — E. 51. Nelson.

1 This lens, with an .s eompensat in;_' eye piece, will resolve a Plewosigmn
• liitinii with an axial cone ; this is the lowest power with which it has ever been
done.

THE QUALITIES OF OBJECTIVES 425

power is very significant, and should be carefully pondered by those
who still desire low apertures as the only perfect form of objectives.

It is as well to mention that objectives may lie arranged in two
series — one the 2, 1, ^, £, and £, the other H, f , ^, ±, jV. One of
these series will form a complete battery, as it is unnecessary to
have objectives differing from the next in the serifs by less than
double the power.

The most usual combination is perhaps the 1 and the ^ of one
series, or the f and the ^ of the other. Of these two preference
might rather be given to the latter. The only exception would be
the addition of a L|-mch for pond life.

Eye-pieces should also double the power thus : 5. 10, and 20
(nncompeiisated), 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 N.A.

3. Resolving power.

4. Penetrating power.

5. Illuminating power.

6. Flatness of field.

7. Defining power.

1. Maijnifi/iiKj jxiti-t'i-. — No test is required, as the initial magni-
fying power can be directly measured.

2. Aperture, or N.A. can be directly measured ; 110 test is there-
fore necessary.

3. Resolving power. — A lens illuminated by a large solid axial
cone, when a Gilford's screen is used, should resolve a number of
lines to the inch expressed by its N.A. multiplied by 80. 000. l

4. Penetrating power is the reciprocal of the resolving power of

•VT . . No test needed, but penetrating power varies largely with
JN ..A..

the combined magnifying power, and also with the magnitude of the
illuminating cone used, as already intimated.

5. Illuminating power is the square of the numerical aperture
(N.A.)2. No test is necessary, but the remarks made above in
regard to penetrating 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 : Fol-
low powers a micro-photograph ; for medium and high powers a stage
micrometer.)

7. Defining power depends on (a) the reduction of spherical
aberration, (b) the reduction of chromatic aberration, (c) the perfect
centring of the lenses— by which is meant (i.) the alignment of

1 J.R.M.S. 1893, p. 15.— E. M. Nelson.

426 MANIPULATION AND PRESERVATION OF THE MICROSCOPE

their optic axes, (ii.) the parallelism of their planes. (Hi.) the setting
of their planes at right angles to the optic axis.

De/i/i/iti/ i>(»i-<>i' can only be tested l>y 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
three-fourths of the aperture of the objective.

V/'i-fi Ion- jirni-ers (3-, 2-, and H-iiich). — Wing of Ayrion /;///-
cln'llmn g (dragon-fly).

Loir jtoirffs (1 and ^). — Proboscis of blow-fly. Large diatoms
on dark ground.

M fil in in /iniri'i-s (i, ^j, ^. and low-angled ^). — Minute hairs on
proboscis of blow-fly ; hair of pencil-tail (Polyxenus lagwrus) ;
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 pon-ers (with wide aperture). — Pleumvit/mti formosum ;
\n riculn li/ra in balsam or styrax ; Pletwosigma angulatum dry on
cover ; bacteria and micrococci stained.

///'/// jMH'cra (wide aperture and oil-immersion ^ and jV). — The
secondary structure of diatoms, especially the fracture through the
perforations: Xnrirnln rhomboides from Cherri/field in balsam or
styrax : bacteria and micrococci stained.

Test with a 1 0 or 1 2 eye-piece, and take into account the general
whiteness and brilliancy of the picture.

The podura 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 your object. Care must be exercised to ascertain by
means of vertical 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 experience-
gained by seeing large numbers of objectives.

In the manipulation of the microscope it is not uncommon to
observe the operator rollnnj the milled head of thejitte 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
roll 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
tlie lirst :ind easiest thing mastered by the tyro. We have already
intimated that the fine adjustment should never be resorted to while
the coarse adjustment can be efficiently employed. The focus should
always be found. e\en with ihe highest powers, by means of the
COMI-SC adjustment. It is only a clumsy microscopist who brings his
objective by means of the coarse adjustment near the cover-glass and
looks M! the distance he i.s oil' it cither by the eye or by the aid of a
hand magnifier, and then completes his work with the fine adjust
incut. In iTi'i-t/ <•(/.•«' the focus ought, to be found by the coarse
adjustment , Mini the \\orkingdistMiice should be felt by the linger
tilting the slide gently against the front of the objective. Also the
e\MiiiiiiMl ion of objects for depth of structure with low and medium
powers up to the dry ]- <>r /.inch objective should be performed by

EERORS OF INTERPRETATION 427

the coarse adjustment ; only tin- very finest details, sucli as the podura
'exclamation' marks, require the fine adjustment.

]>evond the correct and judicious use of the microscope and all
its appliances, there is the matter of the elimination of errors f>f in-
terpretation to be carefully considered.

The correctness of the conclusions which the microscopist will
draw regarding the nature of any object from the visual appearances
which it presents 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 upon his knowledge of
the class of bodies to which the particular specimen may belong.
Not only are observations of an;/ kind liable to certain fallacies
arising out of the previous notions which the observer may entertain
in regard to the constitution of the objects or the nature of the actions
to which his attention is directed, but even the most practised ob-
server is apt to take no note of such phenomena as his mind is not
prepared to appreciate. Errors and imperfections of this kind can
only be corrected, it is obvious, by general advance in scientific
knowledge: but the history of them affords a useful warning against
hasty conclusions drawn from a too cursory examination. If the
history of almost nn// scientific investigation were fully made known,
it woidd generally appear that the stability and completeness of the
conclusions filially arrived at had only been attained after many
modifications, or even entire alterations, of doctrine. And it is
therefore of such great importance 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 doctrines as possible. It is due to other truth-
seekers that they should not be misled, to the great waste of their
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 publication of conclusions which may
be at once reversed by other observers 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 /'•//'•/ wv tlcre ,sw;//* 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 learn or too constantly practise. I Jesides t hese
general warnings, however, certain special cautions should be given
to the young microscopist with regard 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 microscopists,
and some of the most serious arise from the use of small cones of
illumination. With lenses of high power, ami especially with those
of large numerical aperture, it very seldom happens that all the
parts of an object, however minute and flat it may be, can be in
focus together; 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 PRESERVATION 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
for actual substance. But the most common error is that which is
produced by the reversal of the lights and shadows resulting from the
refractive powers of the object itself; thus, the bicoiicavity 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 ravs which it occasions; but when they are
brought a little within the focus by a slight approximation of the
object-glass the centres appear brighter than the peripheral parts of
the discs. The student should be warned against supposing that in
all cases the most positive and striking appearance is the truest, for
this is often not the case. Mr. Slack's optical illusion, at silica-crack
slide,1 illustrates an error of this description. A drop of water holding
colloid silica in solution is allowed to evaporate on a glass slide, and
when quite dry is covered 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 present a, very positive and deceptive
appearance of being raised bodies like glass threads. It is also easy
to obtain diffraction-lines at their edges, giving an appearance of
duplicity to that which is really single.

A very important and very frequent source of error, which
sometimes operates even 011 experienced microseopists, lies in the
refractive influence exerted by certain peculiarities in the internal
structure of objects upon the rays of light transmitted through
them, this influence being of a nature 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 lactmce and eanaliculi 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 chamber*
with diverging passages excavated in the solid osseous substance.
When Canada balsam fills up the excavations, being nearly of the
same refractive 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 or in balsam that might chance to contain air-
bubbles, he will be almost certain to be so much more strongly
impressed 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 inquirv will be in regard to
t heir 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
s.-nne education. The best method of learning to appreciate the
ela*s of appearances in question is the comparison of the aspect of
globule* of oil iii water with that of globules of water in oil, or of

Mirronco/iioil Journal, vol. v. 1872, p. 14.

STUDIES IN INTERPRETATION

429

bubbles of air in water or Canada balsam.
be ver readil made b shakin up some oil

This comparison may
with water to which

a little gum has been added, so as to form an emulsion, or by
Dimply placing a drop of oil of turpentine (coloured with magenta
or carmine) and a drop of water together upon a .slide, laying a thin
glass cover over them, and then moving the cover backwards and
forwards several times on the slide. Equally instructive are the
appearances of an air-bubble in water and Canada balsam.

The figures which illustrate the appearance at various points

JB

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, vi/. a diaphragm of
about § of a mm. being placed at a distance of 5 mm. beneath the
stage, and the concave mirror exactly centred.

Air-babbles in n-nt<>.r. — Xo. 1 (fig. 366) represents the different
appearances of an air-bubble in water. 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

43O 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.

( )n 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 numerous.

Air-l>nM>l?x in f'liniiJn l>til-«iiii. — 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
/one 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, with the exception of
some peripheral diffraction rings. On focussing to the centre (B')
or upper part (Cr) of the bubble, we have substantially the same
appearances as in B and C, with the exception of the smaller size
of the central circle.

Fat-globules in water (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 diffraction 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
(C") a small white central disc, brighter than the rest of the field,
and sharply limited by a broad, dark ring which is blacker towards

t lie cent re.

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 ihe objective is brought nearer to the upper pole.

These considerations, apart I'roiii their enabling us to distinguish
between air-bnbUes and fat-globules, and preventing their being
confounded with the histological elements, enable two general
principles to be established, viz. bodies which are of greater re-
fractive power than the surrounding medium have a white centre
\\hich is sharper and smaller, and a black ring which is larger when

' BKOWNIAN ' 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 darker when the objective is lowered.

Monochromatic light. — The same phenomena are observed by
yellow monochromatic light, except that the diffraction fringes are
more distinct, further apart, and in greater numbers than with
ordinary light.

A fat-globule, indeed, seems to be composed of a series of con-
centric layers like a grain of starch. With blue light these fringes
are also multiplied, biit are closer together and finer, so that they
arc not so easily visible.

Yellow monochromatic light, therefore, constitutes a good
means for determining whether the stria- seen on an object are
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 fringe.--.

But there is no source of fallacy, to a certain class of workers, so
much to be guarded against as that arising from errors in the inter-
pretation concerning movements as such, and especially conci'mim/
the movement exhibited by certain /•>'/•// i,iim<t<- />«rlicli>s of matter in
a state of suspension in fluids. The movement was first observed in
the fine granular particles which exist in great abundance in the
contents of pollen grains of plants known as t\w j'ocillu, and which
are set free by crushing the pollen. It was first supposed that they
indicated some special vital movement analogous to the motion of
the spermatozoa of animals. But it was discovered in 1827, by Dr.
Robert 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 affected 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 a?ons. A good illustra-
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 greater; but, triturated finely enough, these also show
the movement, for a long time known, from the name of its dis-
coverer, as Brownian movement, but now more generally called
jwdesis.

The movement is chiefly of an oscillatory nature, but the particles
also rotate backwards and forwards on their axes, and gradually (if
persistently 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 from this movement, and
will make erroneous inferences.

432 MANIPULATION AND PRESERVATION OF THE MICROSCOPE

The movement of the smallest particles in pedesis is always the
most active, while in the majority of cases particles greater than the
-,,11oTpth of an inch arc wholly inactive. A drop of common ink
which has been exposed to the air for 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,1 who has recently
studied this subject, as showing the movement (which he designates
pedesis) extremely well. But 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
v \\.-irm 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 particle moves at any one bound is usually
le>s than .-,,,',,,, th 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 drop of aqueous
fluid that is completely surrounded by oil. and is therefore cut oft'
from all possibility of evaporation; and it has been known to con-
tinue for manv vears in a small quantity of fluid enclosed between
two glasses in an air-tight case; and for the same reason it can
sr;ircelv be connected with the chemical change. But the observa-
tions of Professor Jevons (loc. cit.) show that it is greatly affected
liy the admixture of various substances with water, being, for
example, increased by a small admixture of gum, while it is checked
by an extremely minute admixture of sulphuric acid or of various
saline compounds, these (as Professor Jevons points 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.' for when this is brought to a stand the particles aggre-
gate and sink, so that the liquid clears itself.2

Pedetic motion depends on, that is, is affected by—

1 . The size of the particles.

2. Tin' x/x-cijir i/mi-tti/ of the particles. Metals, or particles of
vermilion, of similar size to particles of silica or gamboge, move much
more slowly and less frequently.

3. Tin1 unfurl' of tin- liquid. No 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, lakes a longtime to settle; whereas, when warm and in
presence of hydrochloric acid, agglomeration soon occurs. Iron pre-
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.

Kill besides the right appreciation of the nature -of pedesis,
there is the utmost caution required in the interpretation of the

1 i(>n,irtfi-li/ Jiinnnil «/ M/ITH, Science, N.H. vol. viii. 1.S78, p. 172.
See ;iU<> tin Ki-v. .1. Delsaulx, ' On the Thermo-dynamic Origin of the Brownitm
ii Monthly Journal of Microsc. Sci. vol. xviii. 1877.

INTERPRETATION OF MICROSCOPIC MOVEMENT 433

rapidity of movement, and kind of movement, which living and motile
forms effect.

The observation of the phenomena of motion under the microscope1
has led to many false views as to the nature of these movements.
If, for instance, swarm-spores are seen to traverse the field of view
in one second, it might be thought that they race through the water
at the speed of an arrow, 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 real
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 horizontal projections of an object of this kind, corre-
sponding to the successive moments of time, appear exactly as if the
movement wrere a true serpentine one. As an example of an appear-
ance of this nature we may mention the alleged serpentine motion
of Spirillum and Vibrio.

Similar illusions are also produced 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 unnoticed.

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 revolve round
a central 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 sufficient to admit of our per-
ceiving whether any rotation does take place. The discovery 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 round the axis. The same holds good also, mutatis mutandis,
of spirally wound threads, spiral vessels, itc. ; we must be able
to distinguish clearly which are the sides of the windings turned
towai-ds 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 Schwendeiier, p. 258 (Eng. edit.).

P F

434 MANIPULATION AND PEESERYATION OF THE MICEOSCOPE

such in small objects. The microscopical 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.
Xelsoii shows that a certain structure in the
remarkable diatom ('Itmacosphenia monilig&ra,
which for a long time has been regarded as inter-
locking teeth, is in reality a spiral pipe.

Moreover, it must not be forgotten 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 represented 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 observer 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 phenomena
proves that when the diffraction spectra of the first order are stopped
out, while those of the second are admitted, the appeai-ance of the
structure will be double the fineness of the actual structure which is
causing the interference.1

FIG. 307. —

in motion.

Fie. 368.

Upon 1hi> 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, li->. :U58, 369. and 370 may be taken to represent

] Sec- Chapter II.

INTERPRETATION OF THE N.A. TABLE 435

n square grating having 25,000 holes per linear inch at the focus of an
objective at P, P D the dioptric beam, P1 P1 diffraction spectra of the
first order, and P2 P2 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 counterpart of the
structure, characteristic of such a group 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 ouli/ n-ill be brougld
to the focal conjugate ; the image, however, will not be materially
affected on that account, as the diffraction elements of the first order
are alone sufficient 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 onl;/ 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 emitting 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 structure 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 applied 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 grating will
appeal- 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 example 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 N.A., or 112° in air. D 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 „ „ „ *0,000

No. 5 .. „ „ 20,000

No. r. „ „ 40,000

It will be understood by the student that the preservation of the
in icroscopp, and its apparatus is a matter that must largely depend
upon 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

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, Arc., must never
lake 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
Itetter than the simple cambric \ve 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
•Iocs or seeks to do are essentials of the successful microscopist.

DUST ON THE EYE-PIECE 437

It may be noted that dust on the eye-piece can be detected in ;r
dim light, and can be discovered by closing the iris diaphragm. 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 condenser, 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 rotate
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

CHAPTEE YI1

PREPARATION, 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 preparation and mounting of objects for the display of
the minute structures thus brought to our knowledge. Xot 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 which 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 tin-
edges, for so little more than the cost of the glass that few persons
to whom time is an object would trouble themselves to prepare
tliem: it being only when glass slides of some unusual dimensions
are required, or \\lien it is desired to construct 'built-up cells,' that
a f'aciliu in cut t i ug glass with a glazier's diamond becomes useful.
The glass slides prepared for use should be free from veins, air-bubbles.
or other (laws, at least in the central part on which the object is
placed : and any whose defects render them unsuitable for ordinary
purposes should be selected and laid aside for uses to which the
working microscopist will find no difficulty in putting them. As

COVERING GLASS 439

the slips vary considerably in thickness, it will be advantageous to
determine on a gauge 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 for the focal point of an
optical combination with great aperture to be fixed readily upon the
plane 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 desirable ; and the third are
to be used for mounting ordinary objects. C4reat care should be
taken in washing the slides, and in removing from them every trace
of greasiness by the use of a little soda or potass solution. If this
should not suffice they may be immersed 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 after wards thoroughly rinsed.
(The same solution may be advantageously used for cleansing cover-
glasses.) Before they are put away the slides should be wiped
perfectly 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 spirit to ensure freedom from
greasiness. Where slides that have been already employed for
mounting preparations are again brought 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 soda?,
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, which 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, clown to the s^th 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 Ion*
powers ; the second, which should not exceed •(>(),"> 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 />!<//! 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 PREPARATION, 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 plates 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 horizontal table of brass, mounted upon a stand, and having
at one end an arc graduated into twenty divisions, each of which re-
presents the 171\y()th of an inch, so that the entire arc measures the
.-,',, th of an inch ; at the other end is a pivot on which moves a long and
delicate level- of steel, whose extremity points to the graduated ai-c,
whilst it has very near its pivot a sort of projecting tooth, which
bears it against a vertical plate 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.
Tims, if the number be 8, the thickness of the glass is '008, or the j-^th
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 remember that, with the exception of objects to which from their
si/.e 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 cover-glass tester is made bv
Zeiss, of .Jena, and illustrated ill fig. 373. It will bo seen that the
measurement is effected by a clip projecting from a box. between the
jaws of \\liicli the cover to be measured is placed; the reading is
yiven l»y an indicator moving over a divided circle on the upper lace
of 'he bo\. The divisions show liundredths of a millimetre, and the
instrument measures to upwards of.") 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-
corded 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 110 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-glass tester.

Fit;. 374. — Mr. J. Ciceri Smith's direct-reading micrometer.

until the required adjustment is obtained, the exact measurement in
decimal parts 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 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

allows the outer thimble to slip. The connection of the spimlle to
the measuring wheels is effected by means of a stop. This takes
into a slot on a sleeve, on which is mounted the thousandths wheel,
which in turn drives the huiidredths and tenths wheels through the
intermediate pinions. These latter have a step-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 purchased 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 triteness 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
everywhere 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
down 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 treat both sides of the covers thus, as one
side generally adheres while the other is subject to the friction.

For cleaning slips ami 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.
I Jut when dry these cloths must not be 'ironed' or smoothed in any
\\ ay, the ' rough-dry ' surface acting admirably for wiping delicate

.-lass.

Varnishes and Cements. —There are three very distinct purposes
for whirl i rriiienis 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 gl;iss covers to the slides or cells containing the object, (2) the
formation of thin 'cells' of cement only, and (.'!) the attachment of
the 'glass plat e '< >r ' tube-cells ' to the slides. The two former of
these purposes are answered by liquid cements or rtt.rnishfts. which
may lie applied without heat; the last requires a solid cement of
gre.-iter tenacity, which can only be used in the melted state. Among
i he many such cements that have been recommended by different
worker^, two or three will be selected by the worker for general

VARNISHES AND CEMENTS 443

purposes, and perhaps three or four for special purposes, and the re-
mainder will be in practice neglected. We do not hesitate to say
that the two cements on which the most complete trust may be re-
posed are jajxmner'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 hard and old. if scraped off it comes away in shreds ; 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 or cement.
Neither appearance nor facility nor cheapness in use should for one
moment weigh against a varnish or cement of known and tested
worth.

Japanner'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 gla--. and when dry it becomes
extremely tough without brittleness. When new it is very liquid
and ' runs ' rather too freely ; 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
thicker 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 naphtha, 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.

Bell's cement is sold by J. Bell and Co., chemists, Oxford Street,
London; they are the sole makers, and retain 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 from 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 reduced by
grinding, &c. Although fresh, soft balsam may be hardened by heating

444 PREPARATION, MOUNTING, AND COLLECT TON OF OBJECTS

it on the slide to which the object is to be attached, yet it may be
preferably hardened en masse by exposing it in a shallow vessel to
the prolonged but moderate heat of an oven, until so much of its
volatile oil has been driven oft' that it becomes almost (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 mounts with glycerin.

_l>rn nxii-ick liliid,- 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. AVe have already stated that Ave 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 keeping small pieces of picked shel-
lac in a bottle of rectified spirit, and shaking it from time to time.
Jt cannot be recommended 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 Avith homo-
geneous lenses. It is a sure protection against the otherwise in-
jurious action of the cedar oil. Hollis's liquid glue may also lie
employed with confidence for this purpose.

Sealing-wax rarnisJt, which is made by digesting powdered
sealing-wax at a gentle heat in alcohol, should neA-er be used as a
cement ; it is serviceable only as a varnish, and resists cedar oil.

Vi'iiici- t///-/><'iir/ i/i' is the liquid resinous exudation of Abies lari.'-.
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 lie reduced by evaporation one-fourth.

This cement is used for closing glycerin mounts. Square covers
are used, nnd \\c lind 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 Kent just the length of one of the sides of the coA-er at
right angles to tin- 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,
mid is instantly brought down upon the slide and the margin of the
cover. The fluid turpentine distributes itself evenly along the cover
and slide and hardens at once. We have no long experience of it,
but from some of its characteristics we are inclined to believe it will
prove a useful cement for this purpose.

Mur'ni^ </lct'. which is composed of shellac, caoutchouc, and
naphtha, is distinguished by its extraordinary tenacity, and by its
power of resisting solvents of almost every kind. Different 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 known in commerce as G K 4. The special value of this cement,
which can only be applied hot, is in attaching to glass slides the glass
01- metal lings which thus form 'cells ' for the reception, of objects
to be mounted in fluid, 110 other cement being comparable 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 preparations
the parts containing special kinds of noteworthy structure. A
very good, black varnish of this kind is made by working 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 hardened 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' (such as foramintfera,
parts of insects, etc.), the Author has found nothing preferable to a
rather thick mucilage of gum arabic, to which enough glycerin 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 ^ 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 water, so that covers may be easily removed (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 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

prevent the varnish applied round its border from running in.
Where a somewhat deeper cell is required, Prof. H. L. Smith
(U.S.A.) suggests the following specially for the mounting of
diatoms. A sheet of thin writing-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
;i 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-clip, 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 deeper 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 Brunswick 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 corners 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
la ill 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 upon a fiat table (the ring being held dowii-
\\ards) will make it so.

Ring-cells. — For mounting objects of greater thickness it i>
desirable to use cells made by cementing rings, either of glass or metal.
1o ihe glass slides, with marine glue. Glass rings of any size, dia-
meter, thickness, and breadth are made by cutting transverse sections
of tliii-k \valle<| lulies. the surfaces of these sections being ground
flat and parallel. Xol only may round cells (fig. 375, A, B) of vari-

MOUNTING IN CELLS

447

B

ous sizes be made by this simple method, but, by flattening the tube
(when hot) from which they are cut, the sections may be made qua-
drangular, or square, or oblong (C, D). For intermediate thicknesses
between cement-cells and glass ring-cells, the Editor has found no
kind more convenient than 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 for 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 be 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
worked over the surface of
attachment by means of a
needle point ; and in this
manner the melted glue
may be uniformly spread,
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
with liquefied glue, the cell
is to be taken up with a
pair of forceps, turned
over, and deposited in its
proper place on the slide; and it is then to be firmly pressed dou n
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

D

FIG. 315. — Glass ring-cells.

448 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

in order 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 are 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 operator desire to proceed at once in
mounting more cells, the slides already completed should be carefully
removed from it. and laid upon a wooden surface, the slow conduc-
tion of which will pi-event 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 cart-fully
cleansed witli 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
weak spirit and a soft
cloth. Incases in which
appearance is not of
much consequence, and
especially in those in
which the cell is to bi-
used for mounting large
opaque objects, it is de-
cidedly preferable not
to scrape off the glue
too closely round the
etlges 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
\\ell to note that all cells cemented with marine glue should be
\\ell -payed,' as the nautical expression is, or well surrounded
with shellac varnish or gold-size as indicated by the nature of
tin- enclosed fluid. Many media, saline fluids especially, work their
wav between the cell and the slide, and at length destroy the marine
glue.

Plate-glass Cells. — Where large shallow cells with flat bottoms are
required (as for mounting ~c>oj>Itt/ti>s, small mwhisce, &c.), tliey may be
made bv drilling holes in pieces of plate-glass of various si/es.
shapes, mid thicknesses (fig. 37tt, A), which are then cemented
to ihe slide with marine glue. 13y drilling two holes at a

C

FIG. 376.— Plate-glass cells.

MOUNTING CELLS

449

A

B

suitable distance, and cutting out the piece between them, any
required elongation of the cavity may be obtained (B, C, ])).

Sunk-cells. — r^his name is given to round or oval hollows, exca
vated by grinding in the substance of glass slides, which for this
purpose should be
thicker than ordinary.
They are shown in fig.
377, A, B, C. Such
cells have the advan-
tage not only of com-
parative cheapness, but
also of durability, as
they are not liable to
injury by a sudden jar,
such as sometimes
causes the detachment
of a cemented plate or
ring. For objects whose
shape adapts them to
the form and depth of
tlie 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
filled either with water

FIG. 377.— Plate-glass sunk-cells.

obscure the image

but as the cavity is

or some other liquid of higher refractive
power, the deflection is so slight as to be practically inoperative.
l!e fore 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 dimensions
not otherwise procurable,

they may be built t<j>
separate pieces of

of

cemented together.

glass
Large

mounting

cells, suitable foi
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
each of its edges, of such

FIG. STS.— Built UP cells.

to be cut off with the diamond from
breadth as shall leave the interior piece

qual in its dimensions to the cavity of the cell that

is desired.
G G

45O PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

This piece 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 upon a pewter plate
and left rough. The perfect construction of large >?<'<';> cells of this
kind, as shown in fig. 378, A, B, however, requires a nicety of work-
manship which few amateurs possess, and the expenditure of more
time than microscopists generally have to spare ; and as it is conse-
quently preferable to obtain them ready-made, directions for making
them need not be here given.

Wooden Slides for Opaque Objects. — Such 'dry' objects asfora-
minifera, the capsules of ntosscs, parts of Inserts, and the like, may
be conveniently mounted in a very simple form of wooden slide (first
devised by the Author and now come into general use), which also
>erves 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 pieces of
'/wiY-black 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 superfluity of it
immediately around the aperture), this is to be laid down upon the
card, and subjected to pressure.1 An extremely neat v cell' will thus
be formed for the reception of the object, as we see in fig. 37!>. tin-
depth of which will be deter-
mined by the thickness of the
slide, and the diameter bv the
size of the perforation : and it
will be found convenient to
FIG. 37',).— Slip made of wood. provide slides of various thick-

nesses, with apertures of diffe-
rent sizes. The cell should always be deep enough for its wall to
rise above the object : hut. on the other hand, it should not be too
deep for its walls to inti-rfeiv with the oblique incidence of the light
upon any object that may be near its periphery. The object, if flat
or small, may be attached by gum-mucilage ; if, however, it be large,
and the part of it to be attached have an irregular surface, it is
desirable to form a ' l>ed to this by gum thickened with starch. If,
on the other hand, it should be desired to mount the object edgeways
(as when the 'month of a fnrnm in iff r is to be brought into view), the
xiili- of the object may lie attached with a little gum to the ii'all of
I lie cell. The complete protection thus given to the object is the
great recommendation of this method. But this is by no means
its onlv convenience. h allows the slides not onlv to range in
the ordinary cabinets, but also to be laid one against or over
another, and to be packed closely in cases, or secured by elastic

1 It will In- found a very convenient |>l.m to prepare a large number of such slides

i once, ;iinl this maybe done in a marvellously short time if the slips of card have

i |in viously cut to the exacl size in a bookbinder's press. The slides, when put

tner, sln.nl. I l» plun .1 in pairs, back to back, and every pair should have each

of lU ends oiubi-Mi-cd by a spi-jn^- !>n--,s ifi;.'. :',s:,i until dry.

TURN-TABLES— FINISHING

451

bands ; which plan is extremely convenient not merely for the
saving of space, but also for preserving the objects from dust. Should
any more special protection be required, a thin glass cover may be
laid over the top of the cell, and secured there either 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 (fig. 380), devised by
Mr. Shadbolt, is almost indispensable to the rnicmscopist who desires
to preserve prepara-
tions that arc
mounted in any
• medium ' beneath
circular covers ; since
it not only serves
for the making of
those 'cement-cells' Fi<;. ?>m>. — Shadbolt's turn-table.
in which thin trans-
parent objects can Itc 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 sei'p-i'<d 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
application of Mr. E. H. fSrittith'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 partial 1\
rotated against the spring and
pushed forward, when the spring
keeps it between the two pins and a third fixed pin, 1), 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 add 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 furnished underneath with a

(r G 2

Fit;. 381.— Griffith's lum-t;iI>lH.

452 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

short bar with which the decentring wheel may be turned, forcing
tlic pin .• i gainst the slide, piishing it as far out of centre as may be
desired. Another improvement is in making the end-pin a screw,
which may he turned down out of the way if desired.

Mounting Plate and Water-Bath. — Whenever heat has to he
applied either in the cementing of cells or 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 upon 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

PIG. o8'2. — Apparatus for preparing mounting media, paraffin, X'c., for imbedding

bv lieat.

supplied by a plate of metal ; and the Author's experience leads
to recommend that this should be a piece of iron 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 bv raising or lowering the ring any desired amount of heat
may lie imparted 1<> it by the lamp or gas-flame beneath. The
iiUantage of a plate of this size and thickness consists in the

temperature which its different parts afford, and in the
slowness of its cooling when removed from the lamp. When many

cells are being cemented at once, it is convenient to have two such
platev. ihal one may lie conling while the other is being heated.

WATER-BATH—SPRING-PRESSES 45 3

It is also needful to have a smaller plate, much thinner, of brass,
having a groove cut in it into which the ordinary 3 x 1 in. mounting
slip can easily slide, but so grooved as to leave a space between a
ledge on each side on which the slip rests, and the main surface of
the brass under the slip. In this way there is always a film of
heated air between the main surface of the heated brass and that of
the glass, giving more facility for rapid 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 required for various
purposes, especially for melting the various mounting media, such
as gelatin, agar-agar, Arc., 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 which
have been devised to combine as large a number of the requirements
of the mounter in one construction as can be conveniently done was
de-vised by Dr. P. Mayer 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
paraffin, and this may be replaced by others for other needful
purposes; 6 and c are half-cylinders with handles for imbedding; (
is a thermometer bent at a right angle ; the hori/.ontal legends 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, Arc. ; ^ is the thermometer for
the water-bath; R is a Reiehert's thermo-regulator. The variation
in temperature is less than 1° C. ; r is the tube in which the gas
and air mix, and f a mica chimney. There is a small independent
and removable water-bath, r, filled with water by means of rubber
tubes attached to lateral openings. It is supplied with a thermo-
meter, t.±, 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 simple lens or
i 1 i ssecting 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, will be found extremely convenient. This,
by its elasticity, a fiords 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, a fiords 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 spring-clip (fig. 384),
si ild 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 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

alteration of the ' American clothes-peg,' which is now in general
use in this country for a variety of purposes, all that is necessary
being to rub down the opposed surfaces of the ' clip ' with a flat file,
so that they shall be parallel to each other 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
fiat side it shall hold the slide parallel to the surface of the table, as
in fig. 383.

FIG. 884. — Spring-clip.

FIG. 3S.">. — Spring-press.

Mounting Instrument. — A simple mode of applying graduated
pressure concurrently 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 instrument 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 dii-ectly applied to the slide from beneath ; and it

Ki<:. :;sCi. — Smith'- in. mill IIIL^ in-t nmirnt.

is attached l>v a stout wire to a handle shown in fig. 3H(j. Close to
this handle there is attached by a joint an upper wire, which lies
nearly parallel to the first., but makes a downward turn just above
(lie 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
t u rn ing on a screw, so as to hring its ivory knob to bear with greater
or less force on the covering-glass. The special use of this arrange-
ment will be explained hereafter.

ARRANGEMENTS FOR DISSECTING

455

Dissecting Apparatus. — The mode of making a dissection for
microscopic purposes must be determined by the size and character
of the object. Generally speaking, it will be found advantageous to
carry on the dissection under water, with which alcohol should be
mingled where the substance has been long immersed in spirit. The
sixe 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 far from its walls nor lie under any great depth of water.
"Where there is no occasion that the bottom of the vessel should !><•
transparent, no kind of dissecting trough is more 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 render 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 colour of this substance enables it to furnish a back-

456 PEEPAEATION, MOUNTING, AND COLLECTION OF OBJECTS

ground, which assists the observer in distinguishing delicate mem-
branes, fibres, etc., especially when magnifying lenses are employed ;
and it is hard enough (without being too hard) to allow of pins being
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 or earthen-
ware troughs are employed, a piece of sheet-cork loaded with lead
must be provided to answer the same purposes. In carrying on
dissections in such a trough, it is frequently desirable to concentrate
additional light upon the part which is being operated on by means
of the smaller condensing lens; and when a low magnifying 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' ordinarily used for stereoscopes.1
Portions of the body under dissection, being floated off 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 lie 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 fluid. For work of this kind no instrument is mon-
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 illustrated in fig.
387, and will be thoroughly suitable for all the work in which it will
lie 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 instruments 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, etc.; the fine
instruments used in operations upon the eye. however, will commonly
lie 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 pair of very fine-pointed scissors (fig. 388).
one leg of which is fixed in a light handle, and the other kept

The e m.i\ lie reeoiumeiided as useful in a great variety of manipulations which
best performed under ;i low magnifying [tower, with the conjoint use of both eyes.

When- H lii'jli | mwer is i led. recourse may lie advantageously had to Messrs.

I'.eeK .; ini I :irlu •uinatic binocular magnifier, which is constructed on the same

iple, allowing t he object to lie brought. \ ery iie;ir (lie e\e>, without requiring any

iiiicoiiit'ui 1;ib!e convergence cif their :i\es.

ANALYSIS OF MOUNTING METHODS 457

apart from it l>y a spring, so as to close by the pressure of the
finger and to open of itself, will be found (if the blades be well
sharpened) much superior to any kind of knives for cutting through
delicate tissues with as little disturbance of them as possible. A
pair of small straight force] is with fine points, and another pair of
curved forceps, will lie found useful in addition to the ordinal y
dissecting forceps.

Of all the instruments contrived for delicate dissections, however,
few are more serviceable than those which the mieroscopist mav
make for himself out of ordinary upprllf-s. These should be fixed in
light wooden handles (the
cedar sticks used for
camel-hair pencils, or the
handles of steel pen-
holders, or small porctl- Fir;. 388.— Spring scissors,
pirte quills will answer

extremely well) in such a manner that their points should not
project far, since they will otherwise have too much 'spring;'
much may lie done by their mere tear'uty action; but if it be desired
to use them as cuttimj instruments, all that is necessary is to harden
and temper them, and then give them an edge upon a hone. It will
sometimes be desirable to give a finer point 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 byre-heating it and plunging it into
cold water or tallow.

Analysis of Methods of Preparation and Mounting which
follow :-

1. Descriptions of microtomes, and knife-kolders and knifp.-
position.

2. Mounting objects in general.

3. Preparation 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.
Plasma tic stains.

Imbedding methods under the following subtitU-
Imbedding methods in general.
The paraffin method.
This last is further subdivided as follows :—

1. Saturation with a solvent.

2. Saturation with paraffin.

PREPARATION, 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 :—
( 'elloidin imbedding in general.
Hardening the mass.
Fixing to microtome and cutting.
Staining and mounting, with description of appropriate serial

section methods.

4. Preparation of hard tissues, under the following titles :—
(Grinding and polishing sections, with descriptions of lathes.
Decalcification.

Desilicification.

5. Sections dealing with

(ft) Vegetable tissues,
(ft) Staining bacteria.

(c) Staining flagella.

(d) Chemical testing.

(e) Preservative media.

(f) Cleanliness, and labelling.

Microtomes are machines devised for the purpose of obtaining
extremely 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 preferable) may he
firmly attached by means of a T-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 01- ' micrometer ' 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 form its cutting bed. At one side is seen a
small milled head, \\hich ads upon a 'binding screw,' whose ex-
tremity projects into the cavity of the cylinder, and serves to com-
press and steady anything that it holds. For this is now generally
substituted a pair of screws, working through the side of the
cylinder, instead of One as in fig. •'!'.)<>. A cylindrical stem of wood.
a piece of horn, whalebone, cartilage, itc., is to be fitted to the
interior of the cylinder, so as to project a little above its top, and is
to he steadied by the 'binding screw ;' it is then to he cut toa level
by means of a sharp knife or ra/or laid Hat upon the table. The
large milled head is next to he moved through such a portion of a
turn as may very slightly elevate the substance to be cut, so as to
make it project in an almost insensible degree above the table, and
thi> projecting part is to he sliced oil' with a knife previously dipped

SECTION CUTTERS

459

iii water or, preferably, methylated spirit and water in equal parts.
An ordinary razor will answer for cutting. The motion given to
its edge should be a combination of drawhiy and press! />;/. " (It will
be generally found that
better sections are made
by working the kuifefroin
the operator than towards
him.) When one slice has
been thus taken off, it
should be removed from
the blade by dipping it into
spirit and water, or by the
use of a camel-hair brush ;
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

FK,. :!s'.(. — Simple microtome.

this amount having been

once determined, the screw shall be so turned as to always produce
the exact elevation required. "Where 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 manner
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 oft' the knife, or floated
away from it, on each occa-
sion, to pi-event it from being
lost among the lamella? of
carrot which are 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 lie placed between two half-cylinders of elder-pith, 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

FIG. 390.— Microtome.

460 PREPARATION, MOUNTING, AND COLLECTION 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 prospective value of which
we can scarcely estimate too highly ; while some of the profonndest
arid 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 principal 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 more or less exhaustive handbooks which the several subjects
have called forth, the fullest account of the subject being that given
in Mr. A. Bolles Lee's 'The Microtomist's Yade-Meciim.' But we
are at the same time convinced that if the student be but rightly
directed as to instruments and the best way of employing them, and
at the same time have the best general processes 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 prescriptions are only satisfactory starting-points
to better methods. "SVe shall therefore describe one microtome
which we believe, on the whole, to be the best, and sufficiently
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, however, to note that extremely tli'm sections

are not the supreme purpose of microtomes. ('• 1 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 successfullv 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 -s-,-n)-jyth of an inch in thickness, or less — only small
sections should be attempted.

Here it may be advisable to state that the standard unit in
microscopy. ;is accepted by the Council of the Royal Microscopical
Society,1 i> the 1()',0tli of a millimetre, which is indicated by the
sign ft. being known a> a micron.

Roy. Micro. Sot • ser. ii. vol. vii. pp. 502, 526 ; Nat. xxxviii. p. 22 1.

THE THOMA MICROTOME

461

The choice of microtomes, English, Continental, and American, is
very large, and high merit is characteristic of many. But one of
these, devised by Thoma and made by Jung of Heidelberg, entered
the field early, having from the first been based on thoroughly
sound practical principles ; and as a result it has been susceptible
of, and has lent itself to, every improvement suggested by the
advancing refinements of this beautiful art 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 Tkoma) microtome is based HJWK thr. moild nf ll'n-pt ; but
that has been immensely expanded in detail. The body of the
instrument consists of three plates, the middle plate, M, and the
side plates, 8 and O, fig. 391. These are fastened to the bottom
plate by screws. »S supports the knife-carriage, M 8, which rests at

Fi<;. B91. — Jung's Thoma microtome.

three points on a planed and polished track; 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 surfaces, the block itself having
weight enough to keep the whole steady, so that at a touch it glides
to and fro with a firmness and precision that could scarcely 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. US, which rests in its place exactly as does the
knife-carriage, M 8.

This plate also bears the scale T//. which, by means of a vernier
on the object-holder, enables the thickness of the section to lie read
off.

The bottom plate is at once a base and a r« ceiver for the dripping
spirit, oil, «\:c.

462 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

For fastening the knife a thumb-screw, 0, fig. 391, serves; hut
in the modified form of the instrument 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 iise of the knife there are several
holes drilled and tapped 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 svstem.

PIG. 392. — The Thoma microtome with the usual zoologist's knife.

knife, however, is also made upon another model, E, fig. 392 ;
it then has a, special holder «, in which it is secured in a conical slit
by the screws ft, ft1, and firmly held.

For deep objects requiring considerable length to cut from, there
are plates provided for elevating the 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
i raiis\ersal position shown in fig. 392 up to and beyond the slanting
position shown in fig. 391. But it provides no means of altering
the till of the blade, that is, of elevating or depressing the back of
the blade relativelv to its edge— a point of considerable importance,
to which we shall return later on. To meet this difficulty, the
maker (It. . I iing. 12 Landhausstrasse, Heidelberg ; his instruments,
as well as price lists, may be obtained from Mr. C. Baker, 244 High
Holhorn. London) supplies wedges to be inserted under the knife-

POSITION OF KNIFE IN SECTION CUTTING 463

holder. These (Neumayer's) wedges, are horseshoe-shaped, so that
they may be slipped round the central screw. They are made in pairs.
one member 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 slipped muJpr the knife-holder
(th i a end towards the operator), and operates to tilt up the back of
the knife.

The sister wedge is then placed ow 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 pairs, of different degrees of bevel.

This simple device is quite sufficient so long as the utmost pre-
cision of section-cutting is not required. For more elaborate work
it is convenient to employ a special knife-holder, which provide- a
means of elevating or depressing the back of the blade by rotating
the blade round its axis. Similar contrivance* have been described
by Dr. Hesse (in the ' Zeitschrift fur wissenschaftliehe Mikroskopie.'
xiv. 1, 1897, p. 13; see 'Journal of the Royal Microscopical Soe.'
1897, p. 441), and by Prof. Apathy (• Zeitschr..' xiv. 2. p. lo. and
'Journal,' 1897. p. 582). This last is rather complicated 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. f>4<>. That of Hesse i> \ery
simple, and ought to be quite .-utlicient where no considerable
change of tilt is likely to be required. It is made by Jung.

Before leaving this part of the subject it appear.- advisable to
consider briefly the question of knife-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 according as to its slant or 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 frequently 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 paraffin method.

As regards tilt: (1) The knife must always 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
them. (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 ' Sitzber. d. med.-naturw.
Section d. Siebenburgischen Museumvereins, Kolozsvar,' xix. 1897,
H. 7, concludes as follows: (1) The knife should always be tilted
somewhat more than enough to bring the under cutting-facet of the

464 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

edge clear of the object. (-) It should in general be less tilted for
hard and brittle objects than for soft ones, therefore, ccet&ris paribtcs,
less for paraffin than for celloidin. (3) The extent of useful tilt
varies (according to the .ingle to which the knife is ground, 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 paraffin sections to roll, and mav
produce longitudinal lifts 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 In-
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 require 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. 303. — Object-holder 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 back over the object before it
is raised. llibbon-cvitting requires a relatively hard paraffin and
less tilt. With celloidin it is very important to avoid insufficient
tilt, as the elastic celloidin. with too little tilt, yields before tin-
knife and is not cut.

The exigencies of sect ion-cutting have given rise to <i (/rent -varicti/
(if ohjirt-ltfiliJi'i-x in this instrument. The simplest is seen in OS,
fig. .'H)l, which is a pair of .jaws clamped by screws and fixed upon
the pivot S£ by the milled head a. At n is the vernier, which indi-
cate^ the position on tin- nun. scale, T//, and t is an agate highly
polished, upon \\hich the micrometer screw HI works to drive forward
the object carrier. ( > S.

'/'//<• Zoological Slutimi nt ,\'</j>li'.x emplovs a holder specially de-
signed for use \vilh paraffin : the object is soldered with paraffin on
1o the cylinder. /> >/. lig. :!<)L'. This is supported on gimbals and may

THE THOMA MICEOTOME

465

In- shifted vertically and horizontally by means of the small screw w,
and is fastened by means of the milled head, m. By the pinion n it
may be displaced over 90°, and as great an inclination can be taken
in a plane perpendicular to this by the supporting metal frames by
means of the pinion p. 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
cylinder a simple pair of jaws with the screw /// to secure objects of
every A-ariety. A. cylinder-holder as in fig. 393 can be placed in
these jaAvs from which the benefits of the Neapolitan holder can be
secured. But fig. 396 shows a still greater improvement which can be
applied to both object-holders, viz. " perpendicular displacement />//
means of a co/j "//</ /><///<>» governing 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 011 the other by tin- rod C: these are joined by the bridge/..

FIG. 394. — Object-holder movable about two horizontal axes at right angles

to each other.

to which a cogged bar is fastened, into Avhich a pinion catches, Avhich
is moved by the lever V, allowing a perpendicular displacement ot Hie
object of 12 mm. At 0 is the millimetre scale on which the perpen-
dicular displacement can be read off by means of the index x.

An object-holder movable about two horizontal axes situated
perpendicularly to each other is seen in fig. 394. These positions
are fixed by the milled heads />'. 1> ; <j shows the jaAvs for holding the
object, into Avhich, however, cylinders like fig. 39r> may be intro-
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 triangular prism 8/. the louer part of which is
furnished Avitli hinges ; on the hinge the screw V moves, which at its
upper 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 8.

n n

466 PREPARATION, MOUNTING. AND COLLECTION OF OBJECTS

Fig. 395 presents an object-holder intended to analyse It //
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, moving in the slot
F F1 ; at 1> and bl the jaws occupy the position common to those of
the ordinary form.

3

FIG. 395. — Object-holder for analysis by diversified section.

( hi tlic circumference of B a spiral is cut. which become.- slightK
visible at (j ; 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 off 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. ::'.H>.— Cylinder for use with jaws.

fixed axis of it lies in this plane, it will only be required that the
screw S be lii-onght into action to obtain wedge-shaped sections of
whatever thickness is required, which will all be made in this axis.

The set of cylinders which may be used with these and other
jaw- i> represented in tig. :!(.)(j : by is the cylinder, CJ the compressing
screw for it. the block W being held in the jaws.

Tin- olji'd */;//,• ii-i(li //.s- n-micr may be sliddeu up the incline by

THE THOMA MICEOTOME

467

hand ; but it is much more accurate to control its movement with
the micrometer-screw. The point of this screw in fig. 392, t, work*
on the polished plane of an agate cone. The clamp on which the
screw is mounted is held firmly in its place by the milled head \V in
Sc/i. It may stretch up as far as O, being refastened by W.

The screw m is so cut that a single rotation moves the slide on
the ,•";",",! mm., which in the inclination of the plane of ] : 20 gives
an elevation of the object of ^'im mm- Tne barrel or drum, K,
situated on the axis of the screw, is divided into fifteen part* ; con
se<[uently the interval of each division corresponds to an elevation of

i

i

mm.

There 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 relieves the eye. This, however, can be
lirought into action or not at the option of the operator.

Besides these object-holders a freezing apparatus can be added
which is simply placed on the object-slide as shown in Hi;.

FIG. o97. — Freezing apparatus for the Thoma microtome.

The freezing is effected by ether-spray. A specially favourable
effect is obtained if the cylinder <j is mica and not glass. A layer of
water freezes in from thirty to thirty-five seconds.

An arrangement of the Thoma for cutting lury 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 1 he screw cf 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 ROCKINO MICROTOME 469

knife, it i.s then fixed in this position by the screw o (scarcely evident
in the illustration).

This done, the spirit-vessel fy> can be arranged in a position
\vhich 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- arrangement is adopted. The spirit-vessel S/» turns
round an axis 011 the column // ; to it is joined the arm L, which
carries in front the fine tube r (connected with tt'), and also the rod
/> ; the latter is movable perpendicularly, and to its lower end a
bridge or grip with two small rollers i and /' is fastened. The rod
}> is so placed that on each side of the metal strip b, screwed on to
the knife-support, there is one of the rollers. By the adjusting-
screws Stf, 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 sti'ip b b b.

The vessel is filled by screwing off the head Z. As the tube r
acts as a siphon, it is necessary, when the cock is turned 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 Ob. 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 _/' 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 quite correct, adjustment is made by
the screw in. By turning the screws S, 8 the holder is fixed.

V is a wheel with cranked axle Ym\ and this by means of a cat-
gut band moves the knife.

For the ra/iid production of ribbons of sections, however, the
instrument par rwllpitcc is the Cambridge rocking microtome. It
is illustrated in fig. ?>99.

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 with 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
lie 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-iron 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
Vs. which rest on a rod fixed to two uprights cast on the base-plate.

47O PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

A horizontal arm projects at light 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

USING THE ROCKING MICROTOME 471

screw passes freely. The bottom ot the boss is turned out spheri-
cally, and into it fits a spherical nut working on the screw. The nut
is prevented from turning by a pin passing loosely through a slot
in the boss. The bottom of the screw rests on a pin fixed in the
base-plate.

It will be seen that the effect of turning the screw is to raise or
lower the end of the horizontal arm, and therefore to move backwards
or forwards the upper pair 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 pivoted systems is 1 in. and
the distance of the screw from the fixed rod is It} in. The thread
of the screw is 25 to the inch ; thus, if the screw is turned once round.

the object to be cut will be moved forward of . or in.

25 <ij 15tt

The turning of the screw is effected 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 supports the screw. This arm is moved backwards and for-
wards by hand or by a cord attached to any convenient motor.
When the arm is moved forward the pawl engages in the milling
and turns the wheel : when the arm is moved back the pawl slips over
the milling without turning the wheel. A stop acting against the
pawl itself prevents any possibility of the wheel 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 varied 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 -34> 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

maximum of '-- of -— of — - , or - of a turn. The practical mini-

.)— -jD Ojr 1UUU

mum thickness obtainable with a good razor is approximately .f-,,,1,,,,,
inch. The values of the teeth on the milled wheel are as follows :-

1 tooth of the milled wheel = ^^^ in. = -000625 mm.

2 teeth „ „ '-^^ in. = -001250 mm.
4 ., „ =I-i_in. = -002-5 mm.

16 „ „ „ ^nr-'no in. = -01 mm.

The movement of the lever which carries the imbedded object is
effected by a string attached to one end of the lever. Tins string
passes under a pulley and is fastened to the arm carrying the pawl.
Attached to the other end of the lever is a spring pulling downwards.
When, the arm is moved forward the teed takes place, the string is
pulled, the imbedded object is raised past the raxor. and the spring
is stretched. When the arm is allowed to move back, the spring
draws the imbedded object across the edge of the razor, and the sec-
tion is cut. The string is attached to the lever by a screw which

472 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

allows the position of the imbedded object to be adjusted, so that at
the end of the forward stroke it is only just past the edge of the
razor. This is mi important 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 perhaps the most prominent advantages of this
instrument: (1) The price is low. ('2) Manipulation is simple.
(3) The work 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 portable.

The above description refers to the original form of the instru-
ment. Later, the Cambridge Scientific Instrument Company have
brought out an improved form, at a higher price. For most
purposes the original form will suffice. The instrument is said by
the makers to cut celloidiii objects; but for this purpose a sliding
microtome will certainly be found preferable.

The Minot microtome, of which a description may lie found in
the 'Journal of the Royal Microscopical .Society.' 1881). p. 143. is a
neat instrument designed, like the Cambridge rocker, for cutting-
ribbons of paraffin-imbedded objects. It is worked on. the sewing-
machine principle, and cuts very rapidly. But its work is not si >
h'ne as that of the Cambridge instrument, possibly 011 account of in-
sufficient compensation in the working parts. This detect is said to
have been satisfactorily overcome in the beautiful instrument, con
stnicted on the same principle, of Reinhold. a description of which
may be found in the journal above quoted, 1893, p. 706. The work
afforded 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 celloidiii sections ; but it is self-
evident that they are not so well adapted 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 description
may be found in the ' Journal of the Royal Microscopical Society,'
1892, p. 703. and that of Gudden and ' others. They are oiily
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
/,(ir wpllnice is Jung's medium-sized Thoina microtome, No. IV., to
\\ lu'cli. if lengthy series of paraffin sections be frequently required, a
Cambridge rocker may conveniently be added.

l!ut it is needful also to describe one or more of the best instru-
ments designed specially for cutting sections by congelation OY freezing
of the imbedding mass. I >r. R. A. Hayes designed an ether freezing
microtome \\ith the object of affording to those who have occasional
need to cut sections of tissues for pathological investigations, ic..
the means of doing s() quickly, conveniently, and accurately, ft is
illustrated in tig. [00. 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>eeii cut by it without difficulty.

It consists of a solid cast-iron base. A. 10 in. X 4^ in., which
rests 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 ordinary razor can be used), the tang
being grasped between two flat pieces of iron, which are pressed
together by a winged nut. C. The razor by this arrangement can
be secured at any desired angle to the direction of its motion to
and fr<>.

The freezing-chamber is formed by a short vulcanite cylinder. I >.
its lower end being screwed 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. I >. and rising from
the base, E, is an ordinary spray, the air and ether being supplied
through tubes, y and H. passing outside through the base. There

FIG. 400. — Dv. Hayes's ether freezing microtome.

is also an opening in the floor of the chamber 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 supply bottle. K. The freezing- chamber is secured to
the top of the micrometer-screw arrangement. Z. which is of the
simplest 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 difficulty.

The method of using the microtome is very simple. The slide
and block, I), 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 answers 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 uorking 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 gum 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 instrument 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 firmly adheres on cooling, or by a simple clamp-
ing arrangement, which can be substituted for the freezing-chamber.
When used in this way large numbers of sections may be cut in series
by attaching to the razor a light support to receive the sections MS
they are cut.

FK;. 401. — Cathcart's freezing microtome.

Another most- serviceMble and admirable, because inexpensive
and ellieient. microtome, especially for freezing purposes, was
devised l>v Mr. Cat heart; and it is now presented in a simplified
and improved condition. The instrument is illustrated in tig. -101.

In this form the clamping arrangements are much more perfect
ihan in the old form; the principal screw and its milled head are
iMrger and more convenient : the freezing- plate is circular, and is
provided with an arrangement for preventing the ether, with \\hich
the freezing is effected, from rcMching the upper side of the plate;
and the instrument JMIOW MI modified that it can be used for ordinary
imbedding MS \\ell MS freezing.

ETHER FREEZING MICROTOMES

475

The increased size of the screw gives a more steady movement
than was possessed by the older and smaller microtome, while the
greater circumference of the screw-head enables an operator to im-
part a finer movement to the screw. The relation between the pitch
of the screw and the circumference of its head is such that if the
edge be moved forward a quarter 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 original instrument the plate was supported on two
pillars, in order 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 pillars and screws i> NO
much reduced that the conducting surface is not greater 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, ^'ith
the instrument are provided the means of preparing paraffin blocks
for imbedding sections.

When it is intended to u*e the microtome for imbedding, the

FIG. 402. — Holder for Cathcart's microtome.

FIG. 403.— Dropping-bottle.

ether spray, spray-bellows, and ether-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 hack-
wards.

Mr. Cat-heart recommends in freezing with this instrument that
a few drops of mucilage (1 part gum to H parts water) lie placed on
the zinc plate, and that a piece of the tissue be cur. of about a quarter
of an inch in thickness, and pressed into the gum : the ether-bottle,
filled with anhydrous methylated ether, is taken and the spray points
pushed into their socket. All spirit must of cour>e 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- bellows briskly until the gum
begins to freeze ; after this, work more gently. Raise the tissue by
turning the milled head, and CUT by sliding the knife along the glass
plates.

4/6 PREPARATION, 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,
Imbedding, Section-cutting, titainmy, 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
preparation. And as also the manipulations of mounting sensu
stricto are in principle the same in both cases, it appeal's advisable
to make the description of the processes of mounting precede that
of the processes of previous preparation ; merely warning the
beginner that in the case of the majority of specimens intended to
illustrate the minute structure of the tissues of either animals or
plants, such previous preparation is a sine qua non.

The manipulations of mounting will alone be described here, the
most useful mounting media being described later on (' Preserva-
tive and Mounting Media ').

In dealing with the small quantities of fluid media required in
mounting microscopic objects, it is essential for the operator to be
provided with the means of transferring very small quantities from
the vessels containing them to the slide, as well as of taking up from
the slide what may be lying superfluous upon it. Where 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 pressure 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 uith fluid, as
much or as little as may be needed is then readily expelled from it
by the pressure of the finger on the cover, the bull) being always
refilled if care be taken to immerse the lower end of the tube before
the pressure is withdrawn. We speak from large experience of the
\alue of this little implement, which is very clean, simple, and use-
ful. Bat the small pipettes now used so commonly for filliny the
stylographie pens, fitted into the centre of a cork and placed in any
wide-mouthed bottle, \\ill lie found to be. though less elegant,
equally useful and much less costly.

Solutions of Canada balsam and yum-dammar in volatile fluids

DHOP-BOTTLES— MOUNTING 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. (Jreat care >hould be
taken to keep the inside of the cap and the part of the neck of the
jar on which it fits quite dean, so as to prevent the fixation of the
neck by the adhesion between these two surfaces. Should such
adhesion take place, the cautious application of the heat of a spirit-
lamp \vill usually make the cap removable. In taking out the
liquid care should be Taken not to drop it prematurely from the rod
—a mischance which may be avoided by not taking up more than it
will properly carry, and by holding it in a horizontal position, after
drawing it out of the bottle, until its point is just over the slip or
cover on which the liquid is to be deposited.

A bottle for use with reagents, enabling the operator to pour out
only the quantity he desires, is in valuable. Small capped and stoppered
bottles, the stoppers of
which are tubes, and the
well-fitting caps of which
prevent evaporation, arc
very valuable for aqueous
and thin fluids. We illus-
trate this bottle in fig. 404.
All that is needful is to
take the bottle, with the
cap off. in the warm hand,
and by slight expansion a
drop or more as required
is exuded. These bottles
are easily procurable.

But we like still better
the small (lerman bottles.
shown in fig. 405, contain-
ing about MO grammes, in

which two deep grooves are cut on opposite sides of the >topper. so
arranged that by giving the stopper half a turn one groove is
connected with a hole in the neck of the bottle : this will be seen at
f(, in fig. 40") ; the air travels down this groove, and by inverting the
bottle the Huid enters the other groove of the stopper and finds its
way to a third groove 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 fiuid
in which they finally are placed into the position in which they are
to be mounted. For very large sections they .-ire probably essential ;
but from personal experience, supported by the most accomplished
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. 40(5. where it will be
seen that one end of the 'lifter' is perforated, for the purpose of
drainage, and the other is plain.

The present writer cannot endorse the recommendation of this

FIG. 404.
Expansion drop-
bottle.

Fin. 405.
German drop-bottle.

47$ PREPARATION, MOUNTING. AND COLLECTION OF OBJECTS

instrument, l>nt. pilfers 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 011 and oft'
the 'lifter' is a needless process. We should, as stated above,
mount on the cover-glass, and this cover should lie the only lifter
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
dissectiiig-knife the clearing fluid in which the section is resting
prior to mount ing is gently disturbed, in a good-sized vessel or
saucer, until the section desired is in its proper position on the
cover. Now lay the cover, section upwards, on fresh blotting-paper,
to take off the superfluous liquid from the free side of the cover, and
then hold the edge of the slip at an angle, more or less a,cute, with
the section towards the blotting-paper, but never suffering the
f'i inner 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 order
that tin- 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
FIG. 4(ti). 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 dry. and being stiffened by the
paratlin may be lifted by means of a. flat camel's-hair brush, or a
scalpel oi- forceps. The manipulations of mounting series of sections
on one slide are described under • Imbedding Methods.'

When the preparation has been previously immersed in aqueous
and is to lie mounted in glycerin, glycerin jelly, or
Warrants' medium, the lies) mode of placing it on the slide is to float
it in a saucer or shallow capsule of water, to place the slide or cover
beneath it, and. when the object lies in a suitable position above it,
to rai.>e the slide or cover cautiously, holding the object in place bv
.1 needle, until it is entirely out of the water; and the small quantity

MOUNTING 479

of liquid still surrounding the object is to be carefully dr;i\vn off by
blotting-paper, care being taken not to touch the object with it (as
its fibres are apt to adhere) or to leave any loose fibres on the slide.
Before the object is covered, it should be looked at under a
dissecting or mounting microscope, for the purpose of improving (if
desirable) its disposition on the slide, and of removing any foreign
particles that may lie accidentally present. A drop of the medium
(liquefied, if necessary, by a gentle warmth) is then to be placed upon
it, and another drop placed on the slip or cover and allowed to spread
out. The cover being then taken up with a pair of forceps 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 cover, a little 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 unfortunately show them-
selves, the cover must be raised at one margin, and a further quantity
of the medium deposited.

If, again, there are no air-bubbles, but the medium does not
extend itself to the edge of the cover, the cover need not lie raised,
but a little may be deposited at its edge, whence it will soon bedraun
in by capillary attraction, imperially if a gentle warmth be applied
to the slide. It will then lie advantageous again to examine the
preparation under the dissecting microscope ; for it will often happen
that an opportunity may thus be found of spreading it better by the
application of gentle pressure to one part or another of the covering-
glass, which may be done without injurious effect either with a stiff'
needle or by a pointed stick ; a method whose peculiar value, when
viscid media are employed, was first pointed out 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 care not to touch the edge of the
cover, as it will be very easily displaced) ; and the remainder may 1>.
washed away with a camel's-hair brush dipped in water, which may
be thus carried to the edge of the cover. The water having been
drawn off', a narrow ring of liquefied glycerin jelly may be made
around — not on — the margin of the cover (according to the suggestion
of Dr. 8. Marsh) for the purpose of fixing it before the cement is
applied ; and when this has set, the slide may be placed on the turn-
table, and the preparation 'sealed' by a ring either of gold-size or
of Bell's cement, which should be carried a little o;w the edge of the
cover, and outside the margin of the ring 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 almovt sure to slide by its own
weight. If glycerin jelly or Fan-ants' medium has been employed.

480 PREPARATION, MOUNTING, 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 completelv
cleansed by a camel's-hair brush dipped in warm water; and. when
quite dried, the slide, placed on the turn-table, may be sealed with
gold-size — any other cement being afterwards added, either for
additional security or for 'appearance.'

It is well in mounting in glycerin jelly to soak the object
previously 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 permanently finish with thin coats of
gold-size.

When, on the other hand, the section or other preparation is to
be mounted in a /•>'«! mxix iii°dit<,in, it must have been previously pre-
pared for this in the modes described further on, which will present
it to the mounter either in some essential oil, or in xvlol or benzol
or the like, or in alcohol. From either of these it may be transferred
to the cover or slide in the manner already described.

The thin sections cut by the microtome, or membranes obtained
by dissection, do not require to be placed in cells when mounted in
any ciscid medium : since its tenacity will serve to keep oft' injurious
pressure by the cover-glass.

Mounting- Objects in 'Natural' Balsam. — Although it is pre-
ferable for histologieal purposes to employ a solution of hardened
balsam. a> directed under 'Mounting Media,' yet as there are nianv
objects for mounting for which the use of the 'natural' balsam is
preferable, it will be well to give some directions for its use. When
sections of hard substances have been ground down on the slides to
which they have been cemented, it is much better that thev should
be mounted without being detached, unless they have become clogged
with the abraded particles, and require to be cleansed out — as is
sometime^ the case with sections of the shells, spines. Arc., of echino-
derms. when the balsam bv 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 chloroform in a capped jar
until the object falls oft' of itself by the solution of its cement. It
may then be thoroughly cleansed by boiling it in methylated spirit,
and afterwards laid upon a piece of blotting-paper to dry. after
which it may be mounted in fresh balsam on a slide, just as if it
had remained attached. The slide having been warmed on the
water bath lid. a sufficient quantity of balsam should be dropped 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 should be broken \\ith the needle-point. The cover having
been similarly warmed, a drop of balsam should be placed on it. and
made to spread over its surface: and the cover should then lie

turned over and let down on the object in the manner already de-
scribed, [f this operation be performed over t he water-bath, instead

MOUNTING— IN BALSAM- IN AQUEOUS LIQUIDS 481

of over the spirit-lamp, there will be little risk of the formation of
air-bubbles. However large the section may be. care should be taken
that the balsam is well spread both over its surface and that of
its cover ; and by attending to the precaution of making it accumu-
late on one side by sloping the slide, and letting down the cover
so as to drive a wave before it to the opposite side, very large sections
mav thus be mounted without a single air-bubble. (The Author has
thus mounted sections of Eozoun three inches square.) In mounting
minute balsam objects, such as <l'utt»ins, poli/ct/stince. sponge-spicules,
and the beautiful minute spines of n/ili'mrida. 110 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 thus 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 force] is 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 spriinj-
clip and the .;/>riit</-j>r?ss prove 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
•ot gasteropoda) which have been prepared by dissection in water or
weak spirit 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 polariscope, are best mounted in natural
balsam, which lias 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 trdl 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 approxima-
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 difficulty in
mounting the object without it. being that after a time the cement
is apt to run in beneath the cover, which process is pretty sure to

i r

482 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

continue when it may have once commenced. "\Vheii cement-cells,
are employed for this purpose, care must be taken that the surface
of the riii"- is perfectly flat; so that when the cover-glass is laid on
110 tilting is produced by pressure 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 sufficient 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 lie examined on the
dissecting microscope that its entire freedom from foreign particles
and from air-bubbles may lie 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
1 IK- 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-paper, especial care being taken to avoid drawing
oft' too much liquid, which will cause the gold-size to run in. It i*
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 suffi-
ciently in an hour or two to hold the cover whilst being ' ringed '
on the turn-table. And it is safer to apply a third coat a day or
two afterwards; old gold-size, which lies thicklv. being then applied
so as to raise the ring to the level of the surface of the cover. As
experience shows that preparations thus mounted, which have
remained in perfectly good order for several years, may be afterward*
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. Enock,1 who put* a
metallic ring of angular section round the outside of the cell,
slightly overlapping 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,
.-mil are usually of sufficient size and substance to allow of air being
entangled in their tissues. This is especially liable to occur where
they have undergone the process of decalcification. which will very
probably leave behind it bubbles of carbonic acid. For the extrac-
tion of such bubbles the use of an air-pui up is commonly recommended ,-
butthe Editor lias seldom found this answer the purpose satisfactorily,
and is much disposed to place confidence in a method lately recom-
mended— steeping the specimen in a stoppered jar filled with freshly
boiled H-iilt-r. uhich has great power of drawing into itself either air
Or carbonic acid. Where the structure is one which is not injured
by alcohol, prolonged steeping in this will often have the same effect.
The next point of importance is to select a cover of a si/.e exactly
suitable to that of the ring, of whose breadth it should cover about

O

wo-thirds, leaving an adequate margin uncovered for the attachment

1 Quekett -limni. second series, vol. i. p. 4(1.

MOUNTING- IN DEEP CELLS 483

of the cement. And the perfect flatness of that ring should then be
carefully tested, since on this mainly depends the security of the
mounting. It is to secure this that we prefer rings of tin or bone,
to those of glass, for cells of moderate depth ; for their surface c;in
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 b;-en
previously reduced to a plane surface, or still better with a good tint
file. If glass rings are not found to be ' true,' they must be ground
down 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 fluid ; 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. When 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
intruders should present themselves beneath the cover, the slide
should be inclined, so as to cause them to rise towards the highest

. . ~

part of its circumference, and the cover slipped away from that part, so
as to admit of the introduction of a little additional fluid by the pipette
or syringe ; and when this has taken the place of the air-bubbles the
Cover may be slipped 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
between 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 effected 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 oft'; for a,s soon as evaporation from beneath the edge
of the cover begins to diminish the quantity of fluid 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 arise from the
object itself are found in the deeper parts of the cell. When all these

i i 2

484 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

have been successfully disposed of, the cell maybe 'sealed' and
' ringed ' in the manner already described.

Preparation of Soft Tissues. — It is impossible in the limited
space at disposal here to do more than give a sketch of the very
elaborate art of histological preparation. The reader who desires to
pursue the subject further will find all necessary information in Mr.
A. Bolles Lee's ' The Microtomist's Vade-mecum' (London: J. it 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
fix its histological elements. Two things are implied by the word
' fixing : ' first, the rapid kill'tny 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
litn-tli'iiiiHj of it to such a degree as may enable it to resist without
further 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 unrecognisable caricature of
the living organism. But if it be first properly killed and slightly
hardened in the proper manner, it may 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 in an appropriate liquid, and leaving it therein until the
desired degree 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 further prepared 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 gradually increasing strength, final dehydration with
absolute alcohol, c/i'nr/in/' (see below) ' with an essential oil or other
clearing agent, and lastly either mounting in balsam or imbedding
in paraffin for the purpose of making sections.'

. nnt>/!>iHt(f is the fixin aent that is most to be recom-

mended for general \\ork. A good formula consists of a saturated
solution in water containing 1 per cent, of acetic acid. The present
uriter adds a little nitric acid, say 1 per cent., which helps to
make the solul ion keep without precipitating. Another good solu-
tion is a saturated soldi ion in alcohol of :">() per cent., or even 70 pel-
cent.. also \\illi addition of 1 per cent, of acetic acid.

Whatever soldi ion is taken, the objects should be removed from
it soon a Her they have become thoroughly penetrated by it. For
sdMimate harden*. very rapidly, and makes tissues brittle if they are

PICRIC AND OSMIC ACIDS 485

allowed to remain too long in it. The objects should lie 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 removal 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 colour. 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 transparent objects is a good guide for
controlling 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 recommended where great power of
penetration is required, as is the case in work with chitiiious
organisms. A saturated solution in water with the addition of
1 per cent, of acetic acid may be taken, or the picro-iiitric acid of
Mayer. This consists of water 100 parts, nitric acid of 25 per cent.
iS"20.,, 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 suffice. 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 pro-
portions have lately been much vised, with good results.

Osmic 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 -j-1^- grin, to 1 grin. It is extremely
volatile. Care should be taken to avoid exposure to the vapours given
off 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 water keeps very badly, as the slightest con-
tamination with any organic dust will cause it to reduce arid precipi-
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

.lS6 PREPARATION, MOUNTING, 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
expose the tissues to be fixed directly to the action of the vapour.
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 there 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 with water before staining, but a very slight wash-
ing will suffice. For staining, methyl-green may be recommended
lor objects destined for study in an aqueous medium, and, for per-
manent preparations, alum-carmine, picro-carmine, or hsematoxylm.'

' 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 Vade-mecum.')

For fixation by solutions, strengths of from TJF to \ per cent, may
lie 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 larva* 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
maybe easily done by putting the objects into a weak solution of
pyrogallic acid or tannin, which will turn them of a fine black.

< tsniic acid stains most fatty substances of an intense black.

Osmic acid is now not so much used in the form of a pure
aqueous solution as in that of the mixture known as liquid of Fie m-
ni i iiij. This consists of 25 parts of 1 per cent, solution of chromic
acid. 10 parts of 1 percent, osmic acid, 10 parts of 1 per cent, acetic
acid, and .")."> of water. This mixture blackens tissues much less than
tlie pure aqueous solution.1

1 Bleaching. —Tissues that have been blackened or browned by osmie or chromic

acid or the like \\\-.\\ often with advantage be bleached by Mayer's chlorine method,

and will thru he found to stain much more readily. — 'Put into a glass tube a few

tals of chlorate of potash, add two or three drops of hydrochloric acid, and as

icon us the green colour of the curving chlorine has begun to show itself, add a few

• uhir centi tics of alcohol of .•>() to 70 per cent. Now put the objects (which 7liust

have |ire\ioiis|\ lieen M.akcd iii alcohol of 70 to 00 percent.) into the tube. They
.it tirst, but e\entiiii.lly sink. They will be found bleached in from a

I an hour to one or two days, without the tissues having suffered.

in obstinate caso should the liquid be warmed or more acid taken.

CLEARING 487

For the very numerous other fixing reagents and mixtures now
in use, and the manner 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 required, be dehydrated and cleared.

Dehydration is performed as follows: — 'The objects are brought
into weak alcohol, and are 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 per
cent, or 50 per cent., then 70 per cent., then U."> 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.

Clearing1. — • The water having been thus sufficiently removed, the
alcohol is in its turn removed from the tissues, and its place taken
by some anhydrous substance, generally an e.-sential oil, which is
iniscible with the material used for imbedding. This operation is
known as clt'iir'nn/. It is very important that the passage from the
last alcohol to the clearing agent be made gradual. This is effected
by placing the clearing medium under the alcohol. A sufficient
quantity of alcohol is placed in a tube (a watch-glass will do, but
tubes are generally better), and then with a pipette a sufficient
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
tin ids 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.
When they have sunk to the bottom (and the wavy refraction-lines
.at first visible round them have disappeared) the alcohol may be
drawn off with a pipette, 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 passage from one fluid to another
applies to all cases in which objects have to 1)6 transferred from a
lighter to a denser fluid — for instance, from alcohol or from water
to glycerine.)' From • The Microtomist's Vade-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 natural 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 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

in the fixed .state. Hence, by penetrating amongst these highly
refractive elements, they render the tissues transparent and clear,
which is the reason of their being called * dealing agents.'

The best clearing agent for general use is oil of cedar /rood. 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 l>t;r</anir>t is useful; it will clear from alcohol of no
more than 90 per cent, strength.

It should be noted that the proper stage for performing minute
dissections in is the one at which the objects have now arrived, a
drop 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 011 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 011 the slide.

Staining. — Good histological stains can in general only be
obtained with properly fixed tissues. But it is possiljle 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
struct! i iv 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 out ' with an appropriate liquid.

Stains for Living Objects (Intra Vitam Stains).— The most
widely used of these stains is methi/len-llue (to be obtained from
Grriibler and Hollborn,1 and not to be confounded with methyl-blue,
which is a totally different dye). Small aquatic organisms (such as.
rotifers, infusoria, small annelids, tadpoles) 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
'"'•'ii attained. Kor if they are allowed to remain longer the
elements that have taken up the dye will begin to yield it up again
to the uater. and the objects may become quite pale again even
though they have not been removed from the coloured water. The
stain is an imperfect one. being most ly confined to certain granules
ot the protoplasm of cells, and taking effect capriciously now on one
tissue and now on another. It is difficult to preserve the stain in a

1 (';i ' '• ' •••'•' ' sche Strasse, Ldp/i- ; or through Mr. C. Baker, 243 High Holborn.

STAINS FOR UNFIXED TISSUES 489

satisfactory manner, as it will not bear mounting in the usual media
without deterioration.

Weak solutions of Bismarck broivn, quinolein-blue, anrfiii-llticJ.,
Congo red, and neutral red (Neutral-roth) may lit- used in the same
way.

Methylen-blue, used as an -infra ritnm 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 i/t/i-n fitxin stains
described in the last paragraph in that they may be applied 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
until they are penetrated by it, then washed with pure water, or,
better, acidified water, and either studied therein or mounted. They
may be permanently preserved 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 Brim'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 are stained almost
as soon as they are penetrated by it. It is, generally speaking, a
nuclear stain, nuclei being stained more rapidly than cytoplasm,
though some kinds of cytoplasm and formed material are stained by
it. It preserves the forms of cells well. It does not overstain,
and requires little washing out. This, if required, is best done wit li
water acidified with acetic acid.

Bismarck brown is also a useful stain for fresh tissues. It m.iv
be used in solution in acidified water, as directed for methyl-gre«n.
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 persons 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 are 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 PREPARATION, 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
lUfin'ons 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 organisms enclosed in thick cliitinous investments,
as is so generally the case amongst the Arthropoda.

The most precise and the safest of the stains of this class are the
alum-carmines — a general term including the divers formula? that
have been recommended under the names of aliim-cnrrnim'.
i-ii, •iiiiih.i in, alum -wlii it ml. One of these will suffice.

/'ortsch's 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 precise nuclear stain, and one with which it is hardly
possible to overstain. It is permanent in balsam and, it is believed,
in aqueous media if not acid. Objects may be left in it for several
hours. They should not be very large, as the stain has no great
power of penetration. Objects containing calcareous elements that
it is desired to preserve must not be treated with this stain, nor
with any other stain containing alum.

Mayers carmaltim is made with carmiiiic acid 1 grm.. 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 hcamalum. — This is made with ha?matein, the essential
•colouring principle of hsematoxylin (obtainable from G rubier and
Hollborn). One grm. of ha?matein 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) either
with distilled water or 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 per cent, of hydrochloric acid. In
this case the acid should be neutralised afterwards by treat inent with
<H per cent, solution of bicarbonate of soda (or other weak alkali).

Passing now to 1 he titcoholic solut ions. (,'r<>-nftt'1trfs alcoholic bor<i.,-
•cuniiini' may IK- recommended as affording a convenient, safe, and
brilliant stain. Dissolve 2 or ."> 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 eijiial volume of 70 per cent, alcohol, allow it to
stand for twenty-four hours, and filter.

Objects are pul into this solution and allowed to remain in it

STAIMN'G ENTIRE OBJECTS 491

until they are thoroughly penetrated (for flays if necessary). They
are then put into alcohol of 70 per cent, acidified with from four to
six drops of hydrochloric acid for every 100 c.c. of the alcohol. The
acid alcohol at once begins to remove the excess of colour from the
objects, which may be seen to give it off in rosy clouds. They
remain in it until the colour no longer comes away f'reelv and they
have exchanged their primitive opaque red coloration for a brilliant
transparent coloration. This may require days (the acid alcohol
should be changed frequently).

The staining is now complete, and the objects are washed in pure
neutral alcohol, cleared 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 (ill per cent, in any case.

This process is an example of what is known as ri'i/ri'^sive or
indirect staining ; the objects are first <>•!•<• ,-t;tiiin<'<l in the carmine
solution, and the excess of stain is then removed to the required
degree in the acid alcohol.

If, as is frequently the case, especially in studies on the
Arthropoda, a still more highly alcoholised stain be desired, Mayer s
alcoholic corliim'nl may be tried. Cochineal in coarse powder is
macerated for several 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 per
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 previously bleached. All
acids should be carefully washed out of the objects before staining.
The stain is permanent in oil of cloves and balsam.

Kleinenberg's Alcoholic Ilmnatoxylin. once very much used, 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 more precise, or of
a richer selectivity.

492 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

The solution known as K&rnschwwz 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 consists
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 sulfur id o.r/jdati,
diluted twentyfold — see the iron-haematoxylin 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, presumably also in aqueous mounting media.
Being a progressive 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. Similar results are also obtained by mordanting
in 2 per cent, solution of tincture of perchloride of iron in 70 per
cent, alcohol, and then treating with 2 per cent, solution of pyro-
gallol in spirit: a process which is applicable to staining in bulk.

/Si'/idtt's iron hcematoxylin is a still more powerful and precise
stain. Sections of material that has been fixed in any way may be
employed. They are ' mordanted ' by soaking for half an hour or
for some hours (as much as twenty-four, if a very strong stain be
required) in liquor ferri sulfur id oxydati, P.G., diluted with one or
two volumes of water.1 They are then well washed, first with
distilled water, then with tap water, 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 ' differentiated.' 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 nuclear
if the differentiation 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 \\ith
those obtained by the inm l«'iiu(to.i-i/Hn JH-OCCXK (if IleiileitJiuiti, 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 ,,f sulphate of iron 80 parts, water 40, sulphuric acid
!.">, and nitric acid Is. The ingredients should be mixed, and give at first a black
liquid which graduallj acquires a red colour. The operation should be performed
out oi doors, or in a rheniica 1 laboratory, as during the process of solution voluminous
nitrous rapours are given off, \\hich would be hurtful to lenses and delicate instru-
1 1 1 1 a

REGRESSIVE STAINING 493

of ferric alum, which can only be obtained from large chemical
works, and does not keep well either in substance or in solution.

Owing to the precision and depth of the stain, preparations
made by this process will bear study with higher microscopic
powers than those made by any other means ; that is to say. it is
certainly found in practice that they will bear notably higher
eye-piecing.

It will be observed that, as with borax-carmine, this is a
1 regressive ' stain. The progress of decoloration, being slow, m.-iv
be controlled under the microscope, and a little practice with this
process may serve a-; an introduction to the art of regressive
staining with safranin and other tar-colours, with which the
progress of decoloration is so rapid that it cannot be controlled
under the microscope.

Safranin is perhaps the most beautiful stain of this class. The
first requisite to success in staining with this colour 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 < > ' Mipplied by ( iriibler and Hollborn is an excellent
one.

The dye is employed in the form of a saturated or at least very
concentrated solution in Avater 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. Good 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 ' differentiation ' of the stain. The
sections are just rinsed with water 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 their colour 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 c<>n*'i inj \ the sections
have become pale and somewhat transparent, 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 In' stoj>i'>fd iiixtmttlf/.'

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
extract slowly a little more colour, and may thus serve to complete
the differentiation in a frequently very 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 cellar oil. or bergamot oil, or xylol, toluol,
or benzol. This being clone, nothing more remains but to add a drop
of xylol-balsam or dammar, and a cover (chloroform 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 safranin, many others of the coal-tar dyes may be used
in the same way : for instance, basic fuchsin (mag&nta), also a red
stain, or yentiaii riolet 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 considered (with the exception of the intra vltam stains)
have been nuclear stains — that is, such as stain nuclei either
exclusively, or at least more energetically than protoplasm or formed
material. In very many cases they perform all that the histologist
requires in the way of rendering 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.

I'irt-ir uclil is a useful one, especially when employed after a
carmine or hsematoxylin nuclear stain. The modus operand/ is as
simple as possible : it consists merely in adding picric acid to the
alcohol employed for dehydrating the objects, and leaving them
therein until the desired intensity of stain is obtained. ' It has the
great 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.'

Li/nns blue (Bleu de Lyon) 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 for
staining sections, in a strong aqueous solution. The objects must,
not remain too long in alcohol after staining.

The dye known as Wasserblau (n-<it<'r-bln<-) 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
process is, first, to stain rather strongly in a concentrated aqueous
solution of the blue, and then for from half an hour to four or five
hours in the safranin, as described above.

Either of these stains will probably be found safer than indii/'i
rur/iii in-, which \\a^ once much employed for similar purposes.

A still more precise and delicate plasma, stain is Sawefuchsin
(also known under the synonyms, or names of brands, of acid
Sawerubin, Fuchsin A', Jfubin S. and others). It is

IMBEDDING- METHODS 495

important not to confound it with basic fuchsin, ;is appears to
liave been done by some writers. For staining- sections a \ per
cent, solution in water may be employed, and allowed to act oil
sections for from 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 hsematoxylin, 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 ordinary 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
sufficiently 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 other 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 separate cell or other anatomical
element, thus giving to the tissues a consistency they could not other-
\\ ise possess, and ensuring that in the thin slices cut from the mass
all the details of structure will precisely retain their natural relations
of position.'

' These ends are usually attained in one of two ways. Either 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 sufficiently 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 sufficient 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 important in order to the production
of thin and undistorted sections. They were content with simply
surroundiny 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 paraffin method, and the collodion or celloidin -itu't/tod. And

496 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

of these, it is the paraffin method that is by far the most usually
employed. It is the most convenient for ordinary work, the
collodion method only presenting points of superiority 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 third 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. Sat in-lit ion tritk a solvent. — The solvents employed are 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
doves. Kone of these are miscible with water, but all of them arc
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 u-mlt'i-
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. Satnriilifni //•//// /mmffin. — 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-point on a water-bath
or in a stove, taking care to keep it protected from vapour of water.
Remove the object from 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 pa ratlin should be
changed for fresh once or twice, so that none of the oil may remain
to contaminate it and render it soft after cooling.

Some persons prefer to bring the objects i/rarfimlli/ from the oil
into the paraffin by passing them through graduated mixtures of oil
and paraffin ; but witli 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
wanning small pieces of paraffin are by degrees added to the
chloroform. So soon as it is seen that no more bubbles are given
oft' from the objects, the addition of paraffin may cease, for that is a
sign that the paraffin has entirely displaced the chloroform in the
objects. This displacement having been a gradual one, the risk of
shrinkage of the tissues is reduced to a minimum.' After this,
however, the whole must be kept warm on the water-bath, at the
temperature of the melting-point of the pure paraffin employed,
until all the chloroform has been driven off from it, as, if even a
trace of chloroform remain in the paraffin, it will render it soft after
cooling. As this is a very long pmces> (it may take days for large
objects), it is frequently better to simply transfer the objects from
the paraffin solution to a bath of pure paraffin.

3. Arranyiuy for cuttimj. — After the objects have been duly
saturated, they are arranged in a suitable position for cutting, and
the paraffin is caused to solidify as quickly as possible. It must n"t
be attoiffd to cool sloirlt/. 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 paraffin 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 paraffin already
soldered to the object-carrier of the microtome, or to a cork or
cylinder of wood fitted into it. This is done as follows :—

' A piece 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 he noted that it is important to melt as little paraffin as
possible at one time, in order that that which is melted may cool
again as rapidly as possible. 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 sliyhtly warmed scalpel. Then fix the blocks to the object -
carrier by means of a heated needle as above described.

For many objects, other methods of arrangement are preferable.
These consist chiefly in causing the paraffin to solidify in a ti/on.lJ of
any desired shape. Pa i»'r ti-<n/>< 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 b //, then along c c' and <1 <?'. taking care to fold
always the same way. Then make the folds .1 A', /I '/!'. C <"J> D' ,
still folding the same way. To do this you apply A c against A a,
and pinch out the line .1 A', and so on for the remaining angles.
This done, you have an imperfect tray with dogs' ears at the angles.

K K

498 PEEPARATION, MOUNTING, AND COLLECTION OF OBJECTS

A

H

To finish it, turn the dogs' ears round against the ends of the box,
turn down outside the projecting flaps that remain, and pinch them
down. A well-made post-card tray will last through several iru-

beddiiigs, and will generally
. work better after having

n A &

been used than when new.
(From Mr. Lee's ' Microto-
mist's Vade-mecum.')

To imbed in such a tray,
or similar receptacle, some
melted paraffin (or other
' mass ') is poured into it ;
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
more melted mass poured on
until the object is covered
by it. Or, the paper tray
being placed on cork, the

<7

— i

B

C

D

a

FIG. 407.

j)' object may be fixed in posi-
tion in it whilst empty by
means of pins, and the tray
filled with melted mass at
one pour. (The pins can be
removed from the mass
when cold.)

In either case, when tin-
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 tig. 408 ;
in this way a suitable
box is formed, and. the
end of the shorter arm
lieiiiir triangularly

Fio. 408. - Type-metal case for imbedding.

en-

larged on! wards. il is closed sufficiently to retain the mass. Placed
in this \vay. with the short arms nearer to or farther from each
other as a less or greater imbedding mass is required, they are set

CUTTING SECTIONS

499

on a plate of glass which has been wetted with glycerin and gently
warmed. The melted paraffin is now poured into this mould and
the object is imbedded in it as described for the paper tray.

Still another plan is to take a common flat 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 fastened 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 lowering its temperature as warm or cold 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, C, which forms a receptacle
for the melted paraffin.

As long as the warm water circulates through
the bottle the paraffin 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 paraffin recep-
tacle or with reference to lines drawn upon the
surface of the bottle. When the cold water is
allowed to enter in place of the warm, the paraffin
congeals rapidly, and may be easily removed as
one piece. The discharge-pipe should open near
the upper surface of the bottle, to draw off 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 pared 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-
earlier for rocking microtomes) — and if the paraffin block
cut into a rectangle, and also set square — that is. with
parallel to the edge of the knife — sections may lie cut in '
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 form 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

Fiu. 40V). — Arrange-
ment for tlie orien-
tation of objects in
paraffin.

be

one edge
ribbons."

500 PREPARATION. 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 furthest
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, arid can thus lie more easily
unrolled.

The rollhty of sections above referred to is an annoying
phenomenon of very frequent occurrence. Its most usual cause is
over-hardness of the paraffin, but it is favoured by excessive
obliquity of the knife, and other circumstances. With large sections

it is not difficult to catch them by the edge
as they begin to mil. and hold them down
with a camel's-hair brush. Or a section-
stretcher may be used.1

If the paraffin 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 position of the knife ; not
only to its obliquity, but also to its tilt, as
explained above

Secondly, if the paraffin 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
effect of exposing the paraffin 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 maniruvres suffice to obtain sufficiently good
sections, the object must lie 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 paraffin employed for imbedding tnvxl In' of n hardness
determined by the temperature <>/' iln- workroom: hard paraffin for a
I'-ni-ni room, soft paraffin for a coldroom. For the Thoma microtome,
• \ paraffin melt in- .-i t I ."i ( '. (or 11:1° F.) gives good results so long as

'Section stretchers are instruments coiisi-,thi";e^-.euti:ill\ of a little metallic roller
suspended over the -.lijcct tn !»• i-nt in -urli a \\ay MS to rest on its free surface with

;i pressure lli.i! can lie delieately ivj'nlaled SO as 1" l>e siil'lieienl !o keep (lie se,

flatwithoul in an\ \va\ hindering ilie knife from gliding beneath it.' They are made in

i MI the most convenient being that of 'Mayer, Andres and Giesbiecht, of which

cription :nnl li-jin-e m;iyl>e found i&tiie Journal oftheRoy. Microscopical Soc.

ISN::. p. :iii;. \,,u that the water flattening process (see below, Flattening) has been

• ion stretchers are no1 so necessarj as they were formerly, and for most

in. iy lie di*.peii-ed with.

FIG. 410.

FLATTENING SECTIONS AND MOUNTING 501

the tempei-ature of the laboratory lies between 15° and 17° C. (59°
and 62° F.) ; though many workers prefer, even with this instrument,
a much harder mass. For microtomes with Jived knives, such as
the Cambridge rocker, harder paraffins may be used than with sliding
microtomes, paraffins 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 order that the sections
may cohere.

Masses of intermediate consistency may be made by mixing a
hard and a soft paraffin. Two parts of paraffin of 50° C. (122° F.)
with one of 36° C. (97° F.) melting-point, give a mass melting at
48° C. (119° F.).

Mixtures of paraffin with vaseline and with various fatty and
other substances have been recommended. They are now generally
abandoned.

5. Flatt'-iii IK/ the section*, mul 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 lie lifted out on a slide or cover-glass slid under
them. The water must not be 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 paraffin.

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 row. Lay the first
row of sections on this streak. Breathe 011 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. l>y 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. (52) to be infallible. Wrap the corner of a clean
cloth round two fingers and rub it with, a piece of chalk. Moisten

5O2 PREPARATION, MOUNTING. AND COLLECTION OF OBJECTS-

it with a drop of water and rub the slide with the chalked part,
then finish with pure water and a clean part of the cloth.

(5. The flattening having been accomplished by either of these pro-
cesses, the sections must now \>Q fixed to the slide or cover before tin-
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 011 the slide (or cover). After they have been got 011 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 affixed 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 paraffin. This will take from half an
hour to three or four hours. When dry the sections will have
assumed a certain horny transparent look. Tli<> paraffin must not be
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
bv 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 affording 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 tubular 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 egg, 50 c.c. of glycerin, and 1 grin, 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 perfect drying. The slides will be sufficiently
evaporated at a temperature of 40° (J. in ten minutes or a quarter of
an hour. And if the evaporation be conducted by waving the slide
to and fro over a (lame, from three to five minutes may suffice. The
paraffin is then melted and removed by xylol or other solvent, as
before. This process lias the advantage over the water process of
greater safety and greater rapidity, but has the disadvantage that
bhe layer of albumen stains obstinately in some plasma stains, thus
producing an inelegant mount.

If the sections be neither rolled norcreased.it is not necessary

CELLOIDIN IMBEDDING 503

to flatten them on water. They may be laid down 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 water serves to
somewhat expand the sections, which, unless cut from extremely
hard paraffin, are generally somewhat compressed by the impact of
the knife.

As soon as the paraffin 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
mamier 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 order 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"! 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 mixture of equal parts ol
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 ether 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 PREPAKATION, MOUNTING, AND COLLECTION OF OBJECTS

They should be soaked first in a thin solution, until thoroughly
impregnated 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 thirl- 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 drop 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 paraffin. A
convenient mould for celloidinis made by taking a cork, and winding
a strip of paper several tilings round one end of it, so as to form a
projecting collar, which is fixed with a pin. Before using this, or
any paper tray, it should be dressed by having the inside painted
with collodion, which is allowed to dry before 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 Jiwrd&ning 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 teaspooiiful of chloroform has been
poured. As soon as the mass has attained sufficient 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 lias 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 cedar
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 mav 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 preserved indefinitely
without change in a stoppered bottle. Cut n'ith <t tin/ /,-////'', the cut
surface will not dry injuriously under several hours. The cutting
<|iiality of the mass is often improved l>\ allowing it to evaporate in
1 he air for some hours.

'The hardening may l»e done at once in the chloroform and cedar
wood mixture, instead of the chloroform vapour, but the latter
process is preferable as giving a better hardening. And clearing may
lie done in pure cedar oil instead of the mixture, but then it will lie

HAKDENING 505

very slow, whereas in the mixture it is extremely rapid.' (From
Mr. Lee's ' Microtomist's Vade-mecum.')

Instead of cedar oil, white oil of thyme may be employed ; and

some workers use glycerin.

TT

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 prdiminai-ii 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 slon- 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 lias attained a consistency such that the ball of
a finger (not 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 proce».

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 be tiyhtly closed, but should be left at least slightly
open.

' To fix the hardened preparation 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 surface first with absolute alcohol, then with ether
(or allow it to dry), place one drop of vert/ 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 chloroform, for a few minutes, in
order that the joint may harden.' (From Mr. Lee's • Microtomist's
Vade-mecum.')

Sections of material prepared in this way are 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 preliminary hardening by alcohol the mass

506 PREPARATION. MOUNTING, AND COLLECTION OF OBJECTS

i^ soaked for a few hours in \vater in order to get rid of the greater
part of the alcohol (the alcohol should not lie removed entirely, or
the mass mav free/e too hard). It is then flipped for a few
moments into gum mucilage in order to make it adhere to the
freezing plate, and is fro/en. The sections are brought into warm
water. If the mass have fro/en too hard, cut with a knife warmed
witli warm water.'

Stain in i / nut! ninniitiity. —The sections are brought 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 glycerin, nothing more is necessary than to add a
drop of glycerin and a cover.

To mount in balsam, dehydrate in alcohol of not more titan 95
per cent., and clear with an oil that does not dissolve collodion, such
as oil of origanum. her^amot oil, cedar oil. or with chloroform or
xylol.

The foregoing relates to single sections. If it be desired to
mount a serie> 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, dear 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 its neighbour at the edges, so that the ether vapour mav
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 should 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 paraffin sections.

For the complicated manipulations involved in the methods of
Weigert, Obregia, and others, which are only necessary in very
special cases, the reader must be referred to Mr. A. Bolles Lee's
'The Microtomist's Vade-mecum.

Grinding and Polishing Sections of Hard Substances. — Sub-
stances which are too hard to he sliced in a microtome — 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
microscopical examination by grinding down thick sections until
they become so thin as to be transparent, (ieneral directions for
making such preparations will lie here given ;' but those special

The fiillnu MIL; directions do not apply to .sv7/rr<///\ substances, as sections of
run only 1)(. prepared by those who possess a regular lapidary's apparatus, and
' specially instructed in the use of it.

GRINDING AND POLISHING SECTIONS 507

details of management which particular substances may require 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 either by a mechanical arrangement such as that
devised by Dr. Matthews,1 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 difficult 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 grinding 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 upon nothing rougher 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 Joiirn. Qiiekett Microsc. Club, vol. vi. LSSO, p. 88.

508 PREPARATION, 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.1

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 fluid. In the former case
the following is the plan to be pursued : — The flattened surface is to
be polished by rubbing it with water on a ' Water-of-Ayr ' stone, or
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.'2
When this has been sufficiently 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 effectually
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 liquefaction, 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
1 1. 1 i-d, which will lie 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 a, void 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 Foraminifera, 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 \<i/. Hint. July ism, 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 lie desired, the slide is gradually heated just suffi-
ciently to allow of the detachment of the mica film and the specimens it carries; and
ii eleaii slide with a thin layer of hardened balsam having been prepared, the mica
lilm is transferred to it with the ground surface downwards. When its adhesion is
• ompleti'. the grinding may he proeeeded with ; and as the mica film will yield to the
stone without the least dithYulty, the specimens, now reversed in position, may ^>e
reduced to requisite thinness.

As the jIuhii'XK of the polished surface is a, mailer of the first importance, that
"i bhe stones themselves si 10 u lil he tested from time to time; and whenever they are
1 '"ind to have heen nibbed down on any one part more than on another, they should
lie flattened on .1 paving-stone with tine sand, or on the lead-plate with emery.

GRINDING AND POLISHING 509

made over the whole. If this be carefully done, even a very large
section may be attached to glass without the intervention of any air
bubbles. If, however, they should present themselves, and they
cannot lie expelled by increasing the pressure over the part beneath
which they are. or by slightly shifting the section from side to side,
it is better to take the section entirely oft", 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 thoroughly saturated (if it be porous) with hard
Canada balsam, it may be readily reduced in thickness, either by
grinding or filing, as before. or, if the thickness be excessive, by
taking oft' the chief part of it at once by the slitting wheel. So
soon, however, as it approaches the thinness of a piece of ordinary
card, it should be rubbed 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 (particular 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 with
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 proportion as the sub-
stance attached to the glass is ground away, the superfluous
balsam which may have exuded around it will be brought into con-
tact with the stone ; and this should be removed 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 thinne^
which is most suitable for the displav 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 verv little
more reduction would destroy the section altogether: and every
microscopist who has occupied himself in making such preparations
can tell of the number which he has sacrificed in order to attain
this perfection. Hence, if the amount of material he limited, it is
advisable to stop short as soon as a i/oml section has been made, and
to lay it aside — ' letting well alone ' —whilst the attempt is heini:
made to procure a better one: if this should fail, another attempt
may be made, and so on. until either success has been attained or
the whole of the material ha.> been consumed : the first section,
however, still remaining, whereas, if the tirsi. like every subsequent
section, be sacrificed in the attempt to obtain perfection, no tract
will be left 'to show what once has been." In judging of the

510 PREPARATION, MOUNTING, 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 line has been given by the
infiltration of some colouring matter, and with any substances
whose particles have a molecular aggregation 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 maybe
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.1

A different mode of procedure, however, must be adopted when
it is desired to obtain sections of bone, tooth, or other finely tubular
structures, un 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, such 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 generally found possible to reduce the sections nearly to the
required thinness by laying them upon a piece of cork or soft wood
held in a vice, and operating upon them first with a coarser and then
with a liner lile. When this cannot safely be carried farther, the
section must be rubbed down upon that one of the fine stones already
mentioned which is found best to suit it; as long as the section is
tolerably thick, the finger may lie u.sed to press and move it: but as

Kneli in Zoologischer Air.-ri//. 13 d. i. p. 36. The Author, having seen (by
I he kindness of Mr. H. N. Moseley) some sections of corals prepared by this process,
le tifj in its complete success.

CUTTING HARD SECTIONS 511

soon as the finger itself begins to come into contact with the stone,
it must be guarded by a Hat slice of cork, or by apiece 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 detract from its appearance,
but prevent the details of its internal .structure from being as readily
made out as they can be in a polished section. This polish may be
imparted by rubbing the section with putty-powder (peroxide of tin)
and water 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
011 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 rock, 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, b, and can also be
rotated in two directions. The object is pressed by the weight, c,
against the steel disc, fZ, which i:? revolved by the wheel, e, acting on
a smaller-toothed wheel on the axis of d.

The second (fig. 412) is intended to be worked by the foot. The

parts a, b, 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, d, is revolved ; h

is part of the cover for the disc, tc prevent the emery flying about.

A box beneath also catches the powder that falls.

(This arrangement is also supplied with fig. 411, though not shown
in the woodcut.) A second wheel at -i, with a cord passing over k,

512 PREPARATION. MOUNTING, AND COLLECTION OF OBJECTS

actuates a vertical spindle, /, which rotates a horizontal cast-iron
plate at m for polishing.

Decalcification. — When it is desired to examine the structure of
the organic matrix in which the calcareous salts are deposited that
ijive hardness to many animal and to a few vegetable structures (such

o «• o \

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 the

FIG. 412.

hard sections.

lime is in the state of carbonate (as. for example. in the skeletons of
echinoderms), the body to l>e deealcifi<-d should be placed in a glass
jar or \vide-mout lied hot tie holding from 4 to (5 o/. 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 process should
be allowed to take place very gradually, more acid being added as
required. When, on the other hand, much of the linie is in the
state of phosphate, as in bones and teeth, the strength of the acid
solvent \\i\\>\ be increased: and for the hardening of the softer parts
«>!' I he organic matrix it is desirable that chromic ;icid should be

DESILICIFICATION 5 1 3

used. In the case of small bones, or delicate portions of large
(such as the cochlea of the ear), a Jj 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 hydrochloric 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 must 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 lamella?. Acid solvents may also be
employed in removing the outer parts of calcareous skeletons, for the
display of their internal cavities (a plan which the Author has often
found very useful in the study of Foramimferci), or forgetting rid of
them entirely, so as to bring into complete view any ' internal cast '
which may have been formed by the silicificatioii of its originally
soft contents. It has been in this mode, even more than by the
cutting of thin sections, that the structure 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.
For the second it is better that the acid should only be strong enough
for the slow 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 carefully coated with paraffin internally, to prevent the
action of the acid used taking place 011 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 are fiercely attacked by this acid, great care must be
exercised in its use ; but small sponges and other similar siliceous

L L

5 14 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 woodv structures of phanerogams 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 should then be steeped for some days in methy-
lated spirit, after which they should again be transferred to water.
The same treatment may lie 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
i lesired to preserve colour, spirit must not be used ; and recourse may
lie 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 rusk, the thick leaves of the water -lily, &c. The tissue
is well soaked in a syrupy solution of gum arabic, and this is then
hardened, either by allowing it to slowly evaporate, or by throwing
it into strong alcohol, or by freezing 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 treated 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. For 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 for staining the saprophytic. pathogenic, and other
sehi/.oniycetes. Some of these stain admirably, but others, especially
the somewhat larger forms, are much altered, arid unless observa-
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.
KYoin each take out with a pipette a small ijiiantitv. and transfer to
a carefully prepared and well-filtered decoction of veal ill a small
glass vessel, al the temperature of the respective putrefactions ; leave
this for half an hour. Then with a tine pipette take out a minute
drop from each vessel and diffuse each drop upon a cover-glass; let

STAINING BACTERIA 515

evaporation go 011 in a warm room for twenty minutes, then fix the
film of saprophytes by means of fairly strong osmic acid vapour ;
float the cover with the surface of bacteria downwards 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
:i moist growing cell examine the living forms, 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, through 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 pot.-ish.
and 200 c.c. of distilled water. On to this float the cover with its
sin-face 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
balsam.

For the same purpose Professor Heiieage Gibbes gives a method
which has proved of great value. Take of rosauilin hydrochloride
2 grms., methylen-blue 1 grin. ; rub them up in a glass mortar. Then
dissolve aniliii 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, Ac., 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 cover-glass on the warm 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 Bacterin in Tissues (Loffler's solution). — To 100 part*
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 diff&i*ential stain hn./ of bacillus tuberculosis which
was devised by MM. Pittion and floux 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 fuchsiii 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.

LL 2

516 PREPARATION, 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 LofHer. A mordant is made as follows : To 10 c.c.
of a 20 per cent, aqueous solution of tannin are added .") c.c. of
cold saturated solution of ferrous sulphate and 1 c.c. of (either
aqueous or alcoholic) solution of fuchsin, methyl-violet, or • Woll-
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 saturated solution
of fuchsin in anilin water (water in which a little aiiilin 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 1890.
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, (/arc 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 a\\av from the slide to which the
test-liquid is applied any loose particles which may He upon it. and
which may lie thus transferred to some other object, to the great
perplexity of the microscopist. For testing inorganic substances the
ordinary chemical reagents ai'e of course to be employed ; but certain
special tests are required in biological investigation, the following
being those most frequenlly required:

a. Solul ion of iodine in water (1 gr. of iodine. :> grs. of iodide <>f
potassium. 1 oz. of distilled water) turns xturi'lt blue and ccUtdose
brown : it also gives an intense brown to albuminoids substances.

ft. C/ilnr imliili' <>/' -.iitr ( Sell ul 1 x.e'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 grins, of iodide of potassium ; then add
0'2 grin, iodine ; shake at intervals till saturated.

This is extremely useful for the detection of pure cellulose. The
zinc chloride converts cellulose into amyloid, which is then turned
blue by free iodine. Wood-cells, cork-cells, the extine of pollen
grains, and all lignified or corky membranes, are coloured yellow.
Starch colours blue, but is rapidly disorganised.

A very weak solution will instantly detect tannin, the cell con-
tents in which it forms a part becoming reddish or violet.

y. Solution of caustic potass 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
\\atcr) gives to cellulose that has been previously dyed with iodine
a blue or purple hue ; also, wrhen mixed with a solution of sugar, it
gives a rose-red hue, more or less deep, with nitrogenous substances
and with bile (Pettenkofer's test).

Sulphuric acid causes starch grains to swell and similarly affects
cellulose.

€. Concentrated nitric acid gives to albuminous substances an
intense yellow.

£. Acid nitrate of mercury (Millon's test) (ten parts of mercury,
ten of fuming nitric acid, and twenty of water) colours albuminous
substances red.

77. 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 tlie
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 all luminous 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

518 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

Canada balsam and dammar, while the latter include 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 mounted in Canada balsam or dammar ; since
they can be at once transferred to either of these from the menstruum
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 off 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-
balsam is now preferred by most mounters. It is made of equal
volumes of xylol and balsam. The natural balsam, however, maybe
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 work with, and hardens quickly.

The following are the principal <«/H<'H;IN media whose \alnehas
been best tested by general and protracted experience:—

a. Fresh specimens of minute protophytes can often lie \ ery well
preserved in distilled water 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.

PRESERVATIVE MOUNTING MEDIA 519

/3. Salt solution, O75 per cent, sodium chloride in water. Use-
ful as a medium for temporary examination, but not for permanent
preservation.

y. White of an eyy. — Simply filter.

8. Syrup in which is dissolved 1 to 5 per cent, of chloral
hydrate, or 1 per cent, of carbolic acid.

e. Liquid of Ripart and Petit. — Camphor water (not saturated).
75 grins. ; distilled water, 75 grins. ; glacial acetic acid, 1 grm. ;
acetate of copper, O30 grm. ; chloride of copper, O30 grm. Maybe
added to preparations stained with methyl-green, which it does not
precipitate, and may be used for preserving either vegetal or animal
tissues.

£. .Fabre-Domerg tie's Glucose Medium. — Glucose syrup 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.

77. 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. Bruris Glucose Medium. — Distilled water, 140 parts; cam-
phorated spirit, 10 parts; glucose, 40; glycerin, 10. Mix the
water, glucose, 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-yreen
included.

i. Gum and Syrup. — Gum-mucilage (B.P.) five parts, syrup 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.

Syrup 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 alreadv
given : — ' Take any quantity of Nelson's gelatin, and let it soak for
two or three hours in cold water, pour off 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 egg. 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 medium 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).' T A
small quantity of absolute phenol may be added to it with advantage.

1 A very pure glycerin jelly, of which the Author has made considerable use, is
prepared by Mr. Rimmington, chemist, Bradford, Yorkshire.

520 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

When 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.

A. For objects which would be injured by the small amount of
heat required to liquefy the last-mentioned medium, the glycerin a i«l
gum medium of Mr. Fan-ants will be found very useful. This is
made by dissolving four parts (by weight) of picked gum arabic 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 preserve 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
mounting 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 employment of glycerin :
first, 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, in proportion as it increases the transparence of
organic substances, it diminishes the reflecting power of their
surfaces, 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 tor 1he preservation of many vegetable
structures.

2. For preserving soft and delicate marine 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 \\a1er the most snilable preservative.

3. For preserving minute vegetable preparations the following
met hod. devised by Iliintsch. is said to be peculiarly etlicient : A mix-
ture is made of three parts ofpnre alcohol. 1 wo parts of distilled water,
and one part of gl vcerin ; and the object, laid in a cement-cell, is
to be covered with a drop of this liquid, and then put aside under a bell-

PRESERVATIVE MOUNTING 3IEDIA 521

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.1

( '«u<id<( liidsani 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-
fully filtered.

l)(iininar. — Dissolve guru-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.

tin in Sti/i-ti,c. — 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 frnstules and the medium in which
they are mounted facilitates the discovery of obscure details. There is
a marked increase of visibility in proportion as the mounting medium
lias a refractive index higher than the object (diatom) mounted.

L\"OW the refractive index of the silex of diatoms is 1'43. But
Canada balsam is 1'52 : hence the 'index of visibility ' in obscure
markings is 9, while styrax by comparison is 15.

Monobromide 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 lapse 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 ring 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 refractive
index not by any means easy to use, devised by Professor H. L.
Smith, there is 110 medium of such high value as that suggested and
ver\ successfully employed by Mi-. J. W. Stephensoii. viz.

Phosphorus. — 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 difficult and somewhat
dangerous to handle 011 account of its spontaneous combustion in
air, and the severe nature of the burns it inflicts. But it is with
slight practice by 110 means an unmanageable medium.

To prepare it, take a 2 -drachm bottle with no contraction for the

1 See the Rev. \V. \V. Spieer's Hand //-book to tltc Collection and Preparation
of Fresh water and Marine AJgce, &c.. pp. 57-59. 'Nothing,' says Mr. Spicer, ' can
exceed the beauty of the preparations of Desmidiacece prepared after Herr Hantsch's
method, the form of the plant and the colouring of the endochrome having under-
gone no change whatever.'

522 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

neck. Make a cylinder of wood that will jn.st fit the inside of the
neck. Fold some filter 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 filter,
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, rapidity, 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
warm 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 011 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 iiidiarubber cap 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-
phorus 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 phosphorus to fill the space between the cover
and the slide ; gently and firmly press 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 or three ring-coatings of gold-size, and filially
be finished with sealing-wax or shellac varnish.

It often is quite impossible to predicate beforehand what preserva-
tive medium will answer 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 (lie specimens from
time to time, so as to judge the condition of each.

Importance of Cleanliness. — The success of the result of any of
Hie foregoing operations is greatly detracted from if. in consequence
of the adhesion of foreign substances to the glasses \\hereon 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 fibres of silk, wool, cotton, or linen, from
the handkerchiefs, ifec., with which the glass slides may have been
wiped ; fibres of the blotting-paper employed to absorb 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 drawer.-,
or a number of bell-glasses upon a flat table, for the purpose of
securing glasses, objects, &c., from this contamination in the intervals
of the work of preparation : and the more readily accessible these
receptacles are, the more use will the rnicroscopist 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 presence of a number, 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 011 which
the number is placed should be as permanent and immovable as the
slip itself. We know of cabinets in which onl// numbers are
marked on slides, and all details are recorded in 'the book.' \\ e
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.

What is wanted is a permanently fixed 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 ground ; at one end the ground
surface may be three quarters of an inch, and at the other end half
an inch. 611 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 principal facts as to the
nature of the object be written and the number of the slide with a

524 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

Faber pencil marked H H H H. On the narrower and opposite
ground surface should be written what the object is mounted in,
how stained, or whence obtained, the date of mounting, itc.

Xow when all this is written take thin covers, cut respectively
1 X% inch and I 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 writing will be
clear and ineffaceable. If the bottom of the trays of the cabinets be
whitened it will render still more easy the instant reading of the
contents of the label.

The grinding of the slips is by no means difficult, 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 (1| 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 emery the finer the surface ; and the finer the surface the
more 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 cabinets 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. Xow 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 M diatomist would have probably a thick ledger for his diatom
collection, whereas aii entomologist would have a 1hin notebook for
his diatoms and a thick ledger for his insects, and so on. The

I looks might lie distinguished from one another by a letter of the

a I | ih; i lid .

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. 1) I). Xo\v a. large inilc.i- iiafi'/innk will be required in
which one line is given to each slide. This notebook contains

COLLECTION OF OBJECTS 525

merely the number 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 porphyry from Peterhead, Aug. 1886. — The quartz
crystals in this section have minute cavities containing a liquid, CO2.
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 j '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 mistake*
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 3 X 1 inches ; but all are not-
some geological and miiieralogical sections, sections of coal, ifec., ;nv
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. L

COLLECTION OF OBJECTS.

A large proportion of the objects with which the microscopist
is concerned is derived from the minute parts of those larger
organisms, whether vegetable or animal, the collection of which does
not require 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

1 It will be understood that there 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 PREPARATION, MOUNTING-, AND COLLECTION OF OBJECTS

of their minuteness, essentially microscopic; and the collection of
these requires peculiar methods and implements, which are, however,
very simple, the chief element of success lying in the knowledge where
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 upon the
light, since they rise to the surface in sunshine, and subside 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 poml-
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 minute. 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 ; (3) a ring or hoop for a
muslin ring-net. When the bottle is used for collecting at the sur-
face, it should be moved sideways with its mouth partly below the
water ; but if it be desired to bring up a sample of the liquid from
below, or to draw into the bottle any bodies that may be loosely
altached 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 u ire ring; the muslin being strained upon it by a ring of
vulcanised indiarubber. which lies in the groove, and which may be
readily slipped oft' and 011. 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 lie furnished with a number
of bottles, into \\hich he may transfer the samples thus obtained,
and none are so convenient as the screw-topped bottles made in all
si/.es 1>\ the York (Jlass Company. It is well that the bottles should

COLLECTING 527

be fitted into cases, to avoid the risk of breakage. When 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.
a flat bottle, as a very valuable piece of apparat us f< >r c< Electing. ' It is
made by cutting a I_j -shaped piece out of a flat and si did piece of india-
rubber, about (i 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 naturally fitting
cork. One or two more, and smaller, bottles can be made with the
remaining indiarubber. 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 difficulty 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
difficult 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 pur poses 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 secured 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, <tc., 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 Yorticellse ; Epistylis, Zoothamiuin, and (Jarchesimn, 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 floseules, 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. Jour)!. ?er. ii. vol. ii. f. 55.

528 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

of the ease with which its leaves can subsequently be placed under the
microscope. Amcharis 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 other
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 middle
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 ; their 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 Asplaiichna, 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. ' l

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 manv which need to be brought up from the bottom
by means of the dredye, 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 every
marine naturalist, whether he concern himself with the microscope
or not, the mode of using it need not be here described ; but the
ust> of the latter for the purposes of the microscopist requires
special management. The net should be of fine muslin, firmly se\\ n
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 stern of a
boat, so as to tow behind it. oi1 it may be fixed to a stick so held in
tlie hand as to project from the x/'/c 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 (if Collecting and Keeping Pond Life for the Microscope,'
from (he 'I'rans. Miiltl/es/'.r \<it. 7//.s7. Soc.

COLLECTING 529

water it contains to drain through it : and should then be turned
inside out and moved about in a bucket of water carried in the
boat, so that any minute organisms adhering to it may be washed
off before it is again immersed. It is by this simple method that
marine tinimfdrnlfs, the living forms of Radiolcwia, the smaller
Mednxr>idx (with their allies ]>eme and Cydippe), Xoctiluca, the
free-swimming larva' of Echinodermata, some of the most curious
of the Timicata. the larva3 of Mollusca, Tm-hellaria. and Annd'cl",
some curious adult forms of these classes, Entomostraca, and the
larva' 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
\\liich has been made of it by qualified observers. It is important
to bear in mind that, for the collection of all the more delicate of
the organisms just named (such, for instance, as echinoderm Ifii-i-n}.
it is essential that the boat should be rowed so slowly that the net
may move </fidl;i through the water. >o as to avoid crushing its .-.oft
contents against its sides. Those of firmer structure (such as the
Entomosi/raca), 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
>teamer. in much more rapid motion.1 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 lie carried by the current produced by the
motion of the vessel through the water, and they will be thus
removed from liability to injury It will also be useful to attach to
tlie 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.

1 In the Challenger Expedition tow-nets were almost constantly kept in use.
not only at the surface, but at various depths beneath it, being attached to a line
which was made to hang 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

530 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

CHAPTER VIII

MICROSCOPIC FORMS 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 <>('
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 proto-
phytes (or simplest forms 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
4 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,1 known in vegetable
physiology as protoplasm, l>ut often referred to by zoologists as

1 Attention was drawn in ls:i"i l>y Dujarclin ithe French zoologist to whom we owe
the transfer of the Foraminifcru from the highest to the lowest place 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 snl>-taiice, to which
lie ga\c tiir name sarcode (rudimentary flesh). In 1*51 the eminent botanist Von
Molil showed that a similar substance forms the essential constituent of the cells of
plants, and termed it protoplasm (primitive plastic or urgani^able material). And in
LSI;:', it was pointed out by 1'mf. Max Kchultze, who had made a special study of the
rhizopod group, that the ' sarcode ' of animals and the 'protoplasm' of plants are
;/i-iil. See his memoir Ui'lin- i1nx Protoplasma der Rhizopoden itnd Pflaiizen-
zellen.

SIMPLEST FORMS OF A^EGETAF.LE LIFE 531

sarcode, has been appropriately designated by Professor Huxley' the
physical basis of life.' In its typical state (such as it presents
among rhizopods) it is a semi-fluid, tenacious, glairy substance,
resembling — alike in aspect and in composition — the albumen (or
uneoagulated ' white ') of an unboiled egg. But it is fundamentally
distinguished from that or any other form of dead matter by two
attributes, which (as being peculiar to living substances) are desig-
nated vital: (1) its power of increase, by assimilating (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
*j>ontaneoas movement, which shows itself in an extraordinary variety
of actions, sometimes slow and progressive, sometimes rapid, some-
times wave-like and continuous, and sometimes rhythmical 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, lias a great power of absorbing
water; but the distinction between these t\vo 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 substance, 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 structural peculiarities, that they are assigned
to the vegetable or to the animal kingdom respectively. It i>
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 approximate 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 n,'</<i//i<- cmnjio main ulrcm! >/ formed
which it takes (in some way or other) into the interior of its body.
oi1, on the other, its possession of the power of prodtiting the organic
coin/tun inlx which it applies to the increase of its fabric, at the
expense of the inonjanic <.•!<' HI<-' at* with which it is supplied by air
and water. The former, though perhaps not an absolute, is a i/eni'i-al
characteristic of the animal kingdom; the latter, but for the exist-
ence of which animal life would be impossible, is certainly the

532 MICROSCOPIC FORMS OF VEGETABLE LIFE- THALLOPHYTES

attribute of the r>'>/ctn/,l>-. "\Ve slmll find that the protozoa
(or simplest animals) are supported 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 tln-lr 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, itc.), and with
those of atmospheric 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 protophytes absorb through their
external surface only, and take in no solid particles of any descri] -
tion. With regard to motion, which was formerly considered the
distinctive attribute of animality, we now know, not merely that
many protophytes (perhaps all. at some period or other of their lives)
possess a power of spontaneous movement, but also that the instru-
ments of motion (when these can be discovered) are of the very same
chai -acter in the plant as in the animal, being little hair-like fila-
ments, termed cilia (from the Latin word I'iliinii. an eyelash), or
longer whip-like Jtayclla, 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 art-
special extensions.

There are certain plants, however, which resemble animals in
their dependence upon organic compounds prepared by other
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, among phanerogams
(flowering plants), with the leafless ' parasites ' which draw their
support from the tissues of their 'hosts.' And it is the case also,
among the lower <'/-'f/>foi/<nns. with the entire group of Frxci ;
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 promoting that process by their
zymotic (fermentative) action. Among animals, again, there are
se\fi-al in whose tissues .-ire found organic compounds, such as chloro-
phyll, starch, and cellulose, which are characteristically vegetable:
but it has not yet been proved that they ijcnr-rati' these compounds
for themselves by the decomposition of (J< >.,.

The plan of organisation recognisable throughout the vegetable
kingdom presents this remarkable feature of uniformity, that the
fabric, alike in the highest and most complicated plants and in tin-
lowest and simplest forms of vegetation, consists of nothing else
than an aggregation of the bodies termed <>f>l/x. every one of which
(sa\e in the forms that lie near the border-ground between animal
and vegetable life) has its little particle of protoplasm enclosed l>v a

THE VEGETABLE CELL 533

casing of the substance termed cellulose — a non-nitrogenous substance
identical in chemical composition with starch. The entire mass of
cells of which any vegetable organism is composed lias been gene-
rated from one ancestral cell by processes of multiplication to br
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 for itself, those produced by the like multi-
plication of the ancestral cell in the oak or palm not only remain 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, flowers, itc.), each of them characterised by specialities
not merely of external form, but of internal structure ; and each
performing actions peculiar to itself, which contribute to the life
of the plant as a whole. Hence, as was h'rst definitely stated by
Schleiden, it is in the ?//<•-/> ixtnri/ <>f tl<>- individual 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- t<'f til. This cell-wall is composed, as long as the cell is in a
living state, chiefly of the substance known as cellulose, one of the
group of compounds called ' carbohydrates,' and bearing the definite
chemical composition CGH10O5. From a physical point of view it
consists of particles or micdlce of cellulose surrounded by water.
In addition to cellulose, recent observations have shown that pectic
.substances enter largely into the composition of the wall 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 apj><>*it <<>/>, that is, by the
constant addition of fresh layers to the inner surface of the cell-wall :
the other that it increases by intussusception, or the intercalation of
fresh particles of cellulose Ket \\een those already in existence. Tin-
results of modern researches tend in the direction of the former
being the more usual 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 i>n<lt>j>l<txi,i (answering to the -endosarc' of rhizopods), or, when
^trongly coloured throughout (as in many "lyce), the endochrome,
consist in the first place of an outer layer of protoplasmic substance
called the ectoplasm, primordial "tricle, 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. 41:1), 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 protoplasm, from which it is chiefly distinguishable by

534 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

the absence of granules; and it is shown by the effects of reagents to
have the albuminous composition of protoplasm. It may thus be
regarded as the slightly condensed external til in of the protoplasmic
layer with which the inner surface of the cell -wall is in contact ; and
it essentially corresponds to the ' ectosarc ' of Am<xl>a or any other
rhizopod. 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, Arc. ; the peculiar body termed the
nucleus ; and chlorophyll corpuscles (enclosing starch granules),
oil particles, <kc. In the young state of the cell the whole cavity
is occupied by the protoplasmic 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
number, 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.
But 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 nucleus 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 l multinucleated cells,'
are not uncommon. They occur, for example, in many algie, in the
' suspensor ' and 'embryo-sac' of the ovule of phanerogams, and in
the ' laticiferous ' tubes. Within the nucleus are often seen one or
more small distinct particles termed itticlroli (fig. 413. A, />), which
can he 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 point.-
the precise function of the nucleus is still unknown, there can be no
doubt of its essential relation to the vital activity oft he cell, at lea>t
in all the higher plants, :dt hough in the cells of some of the lower
eryptogams it has not a.t present been distinguished with certainty
at any stage of their existence. In the nucleated cells which
exhibit ' cyclosis,' it mav be observed that if the nucleus remains
attached to the cell-wall, it constitutes a centre from which 1 he
protoplasmic 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 binary subdivision, which will be presently described, that
the speciality of the nucleus as the centre of the vital activity of the
cell is most strongly 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 colouring
matter is diffused ; and it is by them that the work of decomposing
(J0.2, 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 increase in size until they take the places of the corpuscles
that produced them. 80 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 frequently
destitute of colour ; and the partial solidification of its surface give>
it the character of an 'ectoplasm.' Such individualised mas.-t's 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 nagella, 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 ti-ansverse constriction of the nucleus ; and this con-
striction becomes deeper and deeper, until the nucleus divides itself

536 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

into two halves (fig. 413, B, «, «'). These then separating from
each other, the endoplasni of the parent-cell collects round the two
new centres, so as to divide itself into two distinct masses (C, a, «') :
and by the investment of these two secondary ' endoplasms ' with
cellulose-walls a complete pair of new cells (1), «, «') is formed
within the cavity of the parent-cell. The process of free-cell forma-
tion is always connected, directly or indirectly, with a process of
reproduction rather than of growth, and takes two different forms,
the one occurring in the production of the ' zoospores ' or ; swarm-
spores ' of alga?, 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 endoplasni
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.

PIG. 413. — Binary sithilirisian nf cclh in endo-
sperm of seed of scarlet-runner: A, ordinary
cell, with nucleus 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 endoplasni
(produced by addition of wateri into two sepa-
rate masses round the two segments of original
nucleus ; D, two complete cells within mother-
cell, divided by a partition.

imbedded in its residual

endoplasm, cadi proceeding to complete itself as a cell by the
formation of a limiting wall of eellullose (fig 414). As a 'new
generation' in any phanerogamic plant has its origin in the
fertilisation of a highly specialised 'germ-cell' (contained AVI thin
tin- ovule) by the contents of a 'sperm-cell' (the pollen-grain).
so do we find, among all save the lowest cryptoyains, a provision
for the union of the contents of two highly specialised cells.
the 'germ-cells' being fertilised by the access of motile proto
I'lasmie 1 todies (anthero/.oids), set free from the cavities of the
-pi'i-m -cells ' within which they were developed. Hut although
the sexual process can lie traced downwards under this form into

PROTOPLASM OF THE LIVING CELL

537

FIG. 414. — Successive stages of frei'-cfU /u

in embryo-sac of seed of scarlet-runner; a, a, a,
completed cells, each having its proper cell-wall,
nucleus, and endoplasm, lying in a protoplasmic
mass, through which are dispersed nuclei and cells
in various stages of development.

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 Conjugate and in some fungi in the same light, and to look
upon the ' zygospore,' 1 which is its immediate product, as the
originator (like the fertilised embryo-cell of the phanerogamic seed)
of a ' new generation.'
C4reat attention
has recently been
paid by Strasburger
and others to the con-
stitution of the endo-
plasm and to the
processes connected
with cell-division. On
both 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

cytoplasm ; the portion which constitutes the nucleus is the
nndeoplasm ; that which enters into the composition of the
chlorophyll corpuscles and other allied substances is the chromato-
plasm. Each of these three portions of protoplasm is composed of a
hyaline matrix or hyaloplasm and of imbedded granular structures
or microsomes. A distinct substance, known as nude in, absent from
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 plastic! s. If colourless, they are leucoplasts, &nd

1 The term ' spore ' has been long used by cryptogamists to designate the minute
reproductive particles (such as those set free from the ' fructification ' of ferns, mosses,
&c.) which were supposed— 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 speak) 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 oijspliere, 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, \\ hich have received a great variety of designations, are all to lie
regarded (as will be presently explained) 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 lor 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.]

53§ MICKOSCOPIC FOEMS OF VEGETABLE LIFE— THALLOPHYTES

these are the special seat of the formation of the starch grains. If
coloured they are chromoplasts or ctiromatophores, the origin of the
various colouring matters of the cell ; those which give birth to the
chlorophyll corpuscles being distinguished by the special term chloro-
plasts. Minute bodies termed physodes, endowed with an amn-boid
motion, have been observed within the protoplasm filaments. In
some of the lower plants, at present exclusively in the green alga-,
there are found within the chlorophyll corpuscles homogeneous
proteid substances known as pi/renoids; they are often surrounded
by starch grains.

The division of the nucleus may take place either directly, when
the process is known as 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 nucleus is composed undergoes
a great variety of changes, in the course of which it assumes the
beautiful appearance known as the nuclear spindle, consisting of an
equatorial disc, the nuclear plate, and delicate spindle 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 attracti/xj 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 near the nucleus, one on each side
of it, and imbedded in the cytoplasm. Each centrosphere has in its
centre a body termed the centrosome, composed of one or more small
granules. To follow out all the processes of karyokinesis 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. -11 G). This continuity of protoplasm has
been observed in some seaweeds and other alga-, 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.
En the case of the sensitive plant it is undoubtedly connected with
the remarkalde phenomenon of sensit iveness or irritability displayed
by the leaves.

In the lowest forms of vegetation every single cell is not onl\
capable of living in a state of isolation from the rest, but e\eu
-mally does so; and thus the plant may be said to he n iiin-llnliir.

every cell having an independent •individuality. There are others,
again, in which amorphous masses are made up by the aggregation
o f cells, which, though quite ca pal >le of living independently, remain
attached t o each ot her by t he milt nal fusion (so to speak) of their
gelatinous investments; and there are others, moreover, in which a
adhesion exists bet \\een the cells, and in which regular

CELL-DIVISION

539

plant-like structures are thus formed, notwithstanding 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 different conditions we
shall find to arise out of the mode in which each particular species
multiplies by binary subdivision ; for where the cells of the new pair
that is produced by division of the previous cell undergo a complete
separation from one another, they will henceforth live indepen-

Fi<;. 415. — Division of the pollen-mother-cells of Fritillnrin persica. (From Stras-
burger and Hillhouse's 'Practical Botany,' published by Sonnenschein.)

dently ; but if. instead of undergoing this complete 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 always
takes place in one direction only, a long, narrow filament (fig. 424, D),
or if in two directions only, a broad, flat, leaf-like expansion (C4),
may be generated. To such extended fabrics the term ' unicellular '
plants can scarcely be applied with propriety; since they may be
built up of many thousands or millions of distinct cells, which have

540 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

a 1 /_

no disposition to separate from each other spontaneously.
they correspond with those which are 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 organisms, 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; and fungi, which, not forming chlorophyll for themselves,
depend for their nutriment upon materials drawn from other organ-
isms. Each series contains a very large variety of forms, which,
when traced from below upwards, present gradually increasing com-
plexities of structure :
and these gradations
show themselves espe-
cially in the provisii >ns
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
undiffereiitiated 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 • /.ygospore ' is formed.
The next stage in the ascent is the resolution of the contents
of the male cell into motile bodies (• ant hero/oids '). which, escaping
tVom it, move freely through the water, and lind their way to
ihe female cell, whose contents, fertilised l»y 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 < tents begin to show their characteristically sexual

asped : Imi in the higher they are developed in special organs.
constituting a true -fructification.' This inusl, however, be dis-
tinguished from organs which, though commonly spoken of as the
'fructification,' have no real analogy *\ith the generative apparatus
of flowering plants, their function being merely to give origin to

•^ ^3 a

FIG. 416.-.-Contmuity of protoplasm. (From Vines'*
' Physiology of Plants.' Cambridge University Press.)

STRUCTURE OF PALMOGLCEA

541

f/onidial1 cells or groups of cells, which simply multiply the parent
stock, in the same manner that many flowering plants (such as tin-
potato) can be propagated by the artificial separation of their leaf-
buds. It frequently happens among cryptogams that this yonidial
fructification is by far the more conspicuous, the sexual fructifica-
tion being often so obscure that it cannot be detected without
great difficulty ; and we shall presently see that there are some
thallophytes in which the production of yon ids 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

(ft '!»

Fi<;. 417. — Development of Palmoglcea luarroi'occti.

goiiidial multiplication, under their simplest and most instructive
aspect.

The first of these lowly forms of life to which we call the
attention of the reader is Pdhnoglo&a wacroeocca, Ivtz.,2 one of
those humble kinds of vegetation which spread themselves as green
slime over damp 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 surrounded by a gelatinous envelope ; the cell,
which does not seem to have any distinct membranous wall, is filled
with a granular ' eiidochroine,' consisting of green particles diffused
through colourless protoplasm ; and in the midst of this a nucleus

1 The term go>iids, originally applied to certain green cells in the lichen-crusts
that are capable, when detached, of reproducing the vegetable portion of the plant,
i> used by some writers as a designation of the >n»i-sr.i-unl spores of cryptogams
generally, which it is very important to discriminate from the genitative ' ol'^plieres.
If possessed of motile powers, they are spoken of as ; zoiispores,' or sometimes (on
account of the appearance they present when a number are set free at oncei ;i^
' swarm-spores.' In contradistinction to 'motile ' gonids or ' zoiispores,' those which
show no movement are often termed irxtiinj spores, or hypnospores ', but such may
be either sexual vnxjihcres or non-sexual i/oniils, 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 Pahnogliro are now regarded as belong-
ing to the l)c*uii(l/(t<'f\c, and are included under the genus Mesotcenium. — ED.]

542 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

may sometimes be distinguished, and can always be brought- into
view by tincture of iodine, which turns the ' eiidochrome ' 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 undergoing 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 remain in mutual
contact ; in a yet later stage they are found detached from each
other (I)), 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 separated from
each other (E). Sometimes, however, this is not the case, the
process of sul (division being quickly repeated 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 i/ron-th.
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, vet resumes its vegetative activity whenever
placed in the conditions favourable to it. The conjugating process
commence^ \>\ the putting forth of protrusions from the boundaries
of two adjacent cells, which meet, fuse together (thereby showing
the want of firmness 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, which bursts when the 'zygospore' is wetted: and the
contained cell begins life as a lu'/r (/enerdtim/. speedily multiplying,
like the former ones. l>\ 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 lirst 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 green
granular matter disappearing: and the colour of the conjugated
body changes, with the advance of this process, from green to a light
\ello\\ i>h In-own. When the xygospore begins to vegetate, on the
"thcr hand, a converse change occurs; the oil-globules disappear,
and green granular matter takes their place.

STRUCTURE OF PROTOCOCCUS

543

If this (as seems probable) constitutes the entire life-cycle of
r<ilniogloe-a, it affords no example of that curious 'motile' stage
which is exhibited by most algal protophytes in some stage of their
existence, ami which constitutes a large part of the life -history of
the minute unicellular organism now to be described. Protococdis
pluvialis, Ktz. (GJdamydococcus -plai-lnUs, A. Br.) (fig. 418), which
is not uncommon in collections of rain-water. Xot only has this
protophyte, in its motile condition, been very commonly regarded as
an animalcule, but its different states have been described under
several different 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 arc sometimes red; and their
red form has received the distinguishing appellation of Hwmato-

FIG. 41s. — Development of Protococcus pluvialis.

commonl the red

colouring matter

forms only

coccus.

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 granular point, which has been described by Professor
Ehrenberg as the ci/<>-s]>c>t of these so-called animalcules. It is
quite certain that the red 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 precisely 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 or
green coloured granules are more or less uniformly diffused ; and 1 he
surface of the colourless protoplasm is condensed into an ectoplasm,
which is surrounded by a tolerably firm cell-wall, consisting of cellulose

544 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

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 l>e 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
Palmoglcea, the endoplasm first undergoing separation into two
halves (as seen at B). and each of these halves subsequently developing
a cellulose envelope around itself, and undergoing the same division
in its turn. Thus two. four, eight, or sixteen new cells arc succes-
sively produced ; and these are sometimes set free by the complete
dissolution of the envelope of the original cell ; but they ai-e 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 everv
one forthwith acquires the character 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 into
eight portions, which, being of small size, and endowed with motile
power, may be considered as zoospores. As far as the complete life-
history of Protococcus is at present known, some of these zobspores
retain their motile powers, and develop themselves into the ordinary
'motile' cells; others produce a firm cellulose envelope and become
' still ' cells ; and others (perhaps the majority) perish without am
further 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 earlier period — 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
nagella which seem to be extensions of the colourless protoplasm
layer that accumulates at their base so as to form a sort of trans-
parent beak (H). In this condition it seems obvious that the colour-
less protoplasm is more developed relatively to the colouring matter
than it is in the 'still' cells; and it usually contains 'vacuoles'
occupied onlv l>v clear aqueous fluid, which arc sometimes so
numerous as to take in a large 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 peculiar 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 rendered
more distinct by iodine, and can be made to retract by means of
reagents. The fiagell.i pass through the cellulose envelope, which
invests their base with a sort of sheath, and in the portion that is
within this sheath no movement is seen. During the active life of
the • motile ' cell the vibration of these ilagella is so rapid that they
can lie recognised onlv by the currents they produce in the water
through which the cells are quickly propelled : but when the motion

STRUCTURE OF PROTOCOCCUS 545

become.- slacker the nagella themselves are readily distinguishable,
and they may be made more obvious by the addition of iodine, which,
however, 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 regular binary subdivision (B), whereby a pair
of motile 'eel Is is produced (C),each resembling its single predecessor
in possessing the cellulose investment, the transparent beak, and the
vibratile fiagella, 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 other 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 sometime.-,,
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
lie a more frequent mode of multiplication 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 eiidochrome 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 possessing 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 nagella, thus passing into the ' still '
condition (A) ; and this last transformation may even take place
before they are set free from the envelope within which they were
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
nagella

To what extent Protococcas is an autonomous organism is still
doubtful, but it appears to be more or less closely connected with
many forms of life which have been described, not merely as dis-
tinct species, but as distinct i/ en era of animalcules or of protophytes,
such as Chlamydomonas, Eiiylena, Trachelomonas, Gi/yes. Gonlniti.
I'/iniloriiiii. Botryocystis. Uvetta, Syncrypta, Mnmix. Astasia, Bodo, and
many others. Certain forms, such as the ' motile ' cells I, K, L,
appear in ;i given infusion, at first exclusively and then principally;
they gradually diminish, become more and more rare, and finally
disappear altogether, being replaced by the 'still' form. After
sometime 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 number of ' motile ' cells be transferred from a larger glass into a

N N

546 MICKOSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTEs

.smaller, it will be found, after the lapse of a few hours, that most of
them have subsided to the bottom ; in the course 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 dues in the ' motile.'
What are the precise coj/diti<nts n-liicJt determine the transition
/i/'f/i-fi'n the i stiir and the '•motile'' st(it>'* 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 affected 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
subjected 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, particularly 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, Arc., they may present themselves \\here none had
been previously known to exist ; and there under favourable circum-
stances they may undergo a very rapid multiplication, and may
maintain themselves until the water is dried up, or some other
change occurs which is incompatible with the continuance <>t' 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 Hocculent aggregations on the surface of the water.
This state seems to correspond with the • rest ing-spores' of other
protophytes; and it may continue until warmth, air, and moisture
cause the development oft lie red cells into the ordinary 'still' cells.
green matter being gradually produced, until the red substance forms

PROTOCOCCUS ; CYANOPHYCE.E 547

only the central part of the endochrome. After this the cycle of
changes occurs which 1ms 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 Protococcus, 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 Pal/moglosa ; 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.1

The Cyanophycese or Phycochromacese constitute another group
of lowly forms of vegetable life, distinguished by their blue-green
colour, differing from the Protococcaceae 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 produce zobspores. To the lowest
family of this group, which strongly resemble the Protococcacere,
except in the colour of the cells, the Chroococcacece, belong the genera
Chroococcus, Glosocapsa. Aphanocapsa, Jferismopedia, and many
others, the life-history of which is but very imperfectly known.

The OscittatoriacecB 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 iisually 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 hormoyones. The most abundant genus. O.sv/7-
latoria, has been si > named from the peculiar < iscillatingor 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 X '2

548 MICROSCOPIC 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 2 compares the motion of Xj>i/-n/ina to that of a
growing tendril, and asserts that it is intimately connected with the
growth of the filament. Hansgirg,3 on the other hand, considers 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 rhizopods and other protozoa. Schnetzler 4 describes
the movements in Oscillatoi i« as of six different kinds : (1) rotation of
the thread or of its segments round its axis ; (2) creeping or gliding-
over a solid substratum ; (3) a free-swimming movement in the water :
(4) I'otation or flexion of the entire thread ; (5) sharp tremblings or

concussions ; and (6) a radiating arrange-
ment of the entangled threads. The
movements 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 Scytonemacece (Pis. YII and VIII)
are exceedingly common organisms in
stagnant water, resembling the Oscilla-
toriacete in their blue-green colour, and
in their reproduction by means of

' hormogoiies.'

PIG. 419. — Poitioii of gelatinous
frond of Nostoc.

Nearly allied to the preceding is the
family of Nostocacece, consisting of
distinctly beaded filaments, which, in
the most familiar genus, Nostoc, lie in
firmly gelatinous envelopes of definite
outline (fig. 419). The filaments are
usually simple, though sometimes densely
interwoven, and arc almost always curved
or twisted, often t a king a spiral direction.

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 present 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, Imuever. as they appear to lie: for
they shrink up into mere films in dry weather and expand again
with the first, shower. Other species are not unfrequenl among wet
QlOSS or, OD the surface of damp rocks. Species of A n«l><i>ini and
Aphanizomenon, genera of Xostocacea1. const it ute a large portion of

1 Arc1'. .I///,- ms/,:. Anatomie, lsr>7, ]>. -18.
- Oesterreichische />'<>/. /.I'itscln: IHsti, p. 11.
5 See Uut. Cnilmllihilt, vol. xii. Lss-2, ].. :;r,i.
1 Arch. Sci. Phys. et Nat. 1885, \<. if.i.

CYANOPHYCE.E ; CONJUGATE 549

the bluish-green scum which floats 011 the surface of stagnant water.
Colonies of species of Xostoc and Anabcena are frequently endophy tic
within the cells of Marchantia and other Hepatic*, the prothallia of
ferns, or other aquatic or moisture-loving plants, ^ostoc multiplies,
like the Oscillatoriaceaj, by the subdivision 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-
toriaceae. 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 heterocijsts and resting-spores. The
function of the former is unknown ; the latter develop directly into
new individuals by division in the transverse direction only, with-
out any sexual process.

Resembling the Protococcacese in the independence of their
individual cells are the two groups Desmidiacece and Diatomctcece,
forms of such special interest to the niicroscopist as to require
separate treatment, and a detailed description of which will be found
later on. The JJesmidiacece constitute a group of the family
Conjugate, so called from their mode of reproduction by conjugation,
a process best exemplified in the higher group, the Z//<jtiemacece, 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, Spirogyra), a couple
of star-like discs in each cell (Zygnema), or a single plate running
through it in an axile direction (Mesocarpas). 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 Mesocarpus) the conjugating

5 50 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

t--

cells pour their eiidochromes 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 species of
Spirogyra (fig. 420, B), which are among the commonest and best
known of Conjugatse, the endochrome of one cell passes over entire! \
into the cavity of the other ; and it is within the latter that the
zygospore is formed (C), the two eiidochromes 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.

FIG. 420. —Various stages of the history of a Spirogyra : A, three cells, «, 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 eiidochromes and the
protuberances from the conjugating cells ; C, completion of the act of conjugation,
the eiidochromes 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 are nearly or quite morpho-
logically alike, there must clearly be •&, physiological differentiation,
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 zoi.ispores does not take place among the Conjugatse.

From the composite motile forms of Protococcus the transition
is easy to the group of Volvocineae, an assemblage of minute plants
of the greatest interest to the microscopist, on account both of the
animalcule-like activity of their movements and of the great beauty
and regularity of their forms. The most remarkable example of this
-Tonp is the well-known 1'n/rn.i- t/lo/xtlnr (Plate VI). which is not
uncommon in fresh-water pools, and which, attaining a diameter of
about -,',, or even ..',, 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 .

a

a

b3

West, Newman chromo .

Vol v ox g 1 ob ex t c r ,

VOLVOCINE^E 5 5 l

the water which it inhabits. Its onward motion is usually of a roll-
ing kind ; but it sometimes glides smoothly along, without turning
011 its axis: whilst sometimes, again, it rotates like a top, without
changing its position. When examined with a sufficient magnifying
power the Volvox is seen to consist of a hollow sphere, composed of
a very pellucid material, which is studded at regular intervals with
minute green spots, 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
are due. Within the external sphere may generally be seen from
two to twenty other globes, of a darker colour, 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 coloured particles, which are at first closely aggregated together,
being separated 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, considered 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 ; 1 and the
peculiarity of their history renders it desirable to describe it in some
detail.

Each of the so-called ' monads ' (fig. 421. Nos. 9, 11) is a somewhat
fla.sk-shaped plant-cell, about 3-J(nTth of an inch in diameter, consist-
ing, as in the previous instances, of green chlorophyll granules diffused
through a colourless protoplasm, constituting an endochrome (which
commonly includes also a red spot — 'eye-spot' — of altered chlorophyll),
and bounded by an ectoplasm 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 proceed two flagella (fig. 421, No. 11) ;
and it is invested by a pellucid or hyaline envelope (No. 9, d) of
considerable thickness, the borders of which are flattened against
those of other similar envelopes (No. 5, c, c), but which does not appear
to have the tenacity of a true membrane. It is impossible not to
recognise the close similarity between the structure of this body
and that of the motile encysted cell of Protococciis plurialls (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 hit, great work on the Infusoria (Organismus der
Infusionsthiere, Abtheilung III., Leipzig, 1878), still ranks the Volvocinece among
the flagellate animalcules, to which they undoubtedly show a remarkable parallelism
in structure, the chief evidence of their vegetable nature lying in theif physiological
conformity to undoubted thallophytes.

552 MICROSCOPIC FOKMS OF VEGETABLE LIFE - TIIALLOPHYTES

of the cells which normally detach themselves from one another in
Protococcns. The presence of cellulose in the hyaline substance is
not indicated, in the ordinary condition of Voiron ijlolxitor, hy the
iodine and sulphuric acid test, though the use of ' Schultz's solution '
gives to it a faint blue tinge ; there can be no doubt of its existence,
however, in the hyaline envelope of Volvox a tire us. The tiagella
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 liberated, 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 t-eyptable
in their nature. When the cell is approaching maturity, its endo-
plasm always exhibits one or more vacuoles (fig. 421, No. 9, a, «) of a
spherical form, 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 sl<m
and gradual. This curious action ceases, however, as the cell
arrives at its full maturity ; L 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-
coccua, of a peculiar modification of chlorophyll.

Each cell normally communicates with the cells in nearest
proximity with it by extensions of its own eiidochrome, which are
sometimes single and sometimes double (fig. 421, No. 5, t>, b) ; and
these connecting processes necessarily cross the lines of division
between their respective hyaline investments. The thick' 1 less 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 eiido
chrome to a distance from one another (fig. 421, Nos. '1. '.\. 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 eiidochrome very commonly assume an angular form, and
the connecting processes are drawn out into threads (as seen in No. ~>).
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 I'mlnt'orciix) to be drawn back

1 The existence of rhythmically contracting vacnoles in I Ol rox (though confirmed
by the observations of I' ml'. Stein) is denied by .Mr. Saville Kent (Manual of tile
TnfllSOria, p. 47) ; but it may be fairly presumed that lie lias not looked for them at
the st a •_'<•. of development at- which (heir action was witnessed by Mr. Busk.

PLATE VTI.

Osc icecfi and Scytonemaceee.

We st. New-men cKrom

VOLVOC-USTEJE

553

into the central mass of endochrome ; and they will also retreat 011
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 membrane, without any protoplasmic granules in their

9 **'™'

FIG. 421. — Structure of Volvox globtifoi:

11

interior. It is obvious, then, that an examination of a considerable
number 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

554 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

composed of an aggregation of s< imewhat ; \ ngula r masses of endochrome
(6), 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 appear, the entire aggregation
originated. In the midst of the polygonal masses of endochrome,
one mass («), 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, «), which
come into continuous connection with each other by the coalescence
of processes (/;) 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 the>e
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 pair 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 interposition 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 mass in the relation of the cellulose coat of an ordinary cell
to its ectoplasm, is frequently seen to be marked 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
dear 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 I hat the hexagonal areolse become rounded.
As the primary sphere approaches maturity, the large secondary
i mass, or -.oo*/><>r<i i,</r, \\hose origin has been traced from the
. also advances in development, its contents undergoing
multiplication l>y successive segmentations, so that we find it to
consisl of eight, sixteen, thirl \ two, sixty-four, or still more
numerous divisions, as shown in fig. -1 21, Nos. C>, 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.

T\ _ - . : .1 : -

West.Newrn.aTi cliromo

VOLVO CINE M 555

been produced ; a similar envelope can he easily distinguished, as
shown in Xo. 10, just when the segmentation has been completed,
and at that stage the fiagella pass into it, but do not extend beyond
it ; and even in the mature Volvox it continues to form an invest-
ment around the hyaline envelopes of the separate cells, as shown in
the same figure at Ko. 11. It seems to be by the adhesion of the
hyaline investment of the new sphere to that of the old that the
xM-ondary 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 [dace has not yet been determined.
At the time of the separation the developmental process has gene-
rally advanced as far as the stage represented in Xo. 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 gcniinii>(ircni.s exten-
sion, is the ordinary mode of multiplication in Volvox. taking place
at all times of the ye;ir. except when the sexual generation (now to
be described) is in progress. The mode in which this process i>
here performed (for our knowledge of which we are indebted to
the persevering investigations of Professor Colin) shows a great
advance upon the simple conjugation of twTo similar cells, and
closely resembles that which prevails not only among the higher
algje, but (under some form or other) through a large part of the
cryptogamic series. As autumn advances the Volvo.r, spheres usually
cease to multiply themselves by the formation of zoosporanges, and
certain of their ordinary 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 ylobator (Plate VI, fig. 1) contains
both kinds of sexual cells, so that this species ranks as monoecious ;
but V. aureus is dioecious, 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, rt), in this respect resembling zoosporanges in an early
>t;ige; but their subsequent history is altogether different. The
sperm-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
eiidochrome of the primary cell resolves itself into a cluster of very
peculiar secondary cells (tig. 1, a, a2, fig. 5), each consisting of an
elongated 'body' containing an orange-coloured eiidochrome with a
red corpuscle, and of a long, colourless beak from the base of which
proceeds a pair of long flagella (tigs. (5, 7), as in the antherozoids
of the higher cryptogams. 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, wliich show an active, indepen-
dent movement whilst still within the cavity of the primary cell
(fig. 1, a3), and finally escape by the giving-way of its wall («4),
diffusing themselves through the cavity of the Volvox sphere. The

556 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

germ-ceHs (fig. 1, b, b), on the other hand, continue to increase in
size without undergoing subdivision ; at first showing large vacuoles
in their protoplasm (62, b2), but subsequently becoming filled with
dark-green endochrorae. The form of the germ-cell gradually
changes from its original flask-shape to the globular (b3) ; and it
projects into the cavity of the Volvox sphere, at the same time
acquiring a gelatinous envelope. Over this the swarming aiitherozoids
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 enveloped 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 Palmoylcea, to starch
and a red or orange coloured oil. As many as forty of such oospores
have been seen by (John in a single sphere of Volvox, which thus
acquires the peculiar appearance that has been distinguished by
Ehrenberg by a different specific name, Volvox stdlatus. 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.1

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 affirmed by Dr. Hicks 2 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
zob'sporange, it loses its colour and its regularity <>f form, and

The doctrine of the vegetable nature of Volvox, which had been suggested by
Siebold, Braun, and other German naturalists, was first distinctly enunciated by
1'roi. Williamson, mi tin' b;i sis <>f t,hc history of its development, in the Transaction*
»1 tl/r Philosophical ,S'«r/V/// <>f Mniirln'xirr, vol. ix.

[The most recent and dH.-nlrd accounts of the development of the various forms
of \'nlr,i.r uri- by Klein (Priiit/.f//<'i >n'x -In h I'liiirlicr fur •wissenschaftliche Butonil;
vol. xx. l,ss'.),]>. l;;:;i and Overtoil Botanisches Centralblatt, vol. xxxix. iss'.n, wlu'eli
ilo nut, differ in any material point fruni the description given in the text. See also
lleimelt and .Mm-raVs II, nnlli, >,<!,- nj' Cr///>fn,/(i/i//i- Jlofiut//, p. '2!)'2.— ED.]

Trans, of Mir row. Society, a.s. rol. viii. 1860, p. 99 ; and ijmirt. ./<//</,

Mirr, a.s vol. ii. ISCi-j, p. '.Id.

VOLVOCINE^E ; PALMELLACE.E 557

becomes an irregular mass of colourless protoplasm, containing a
number of brown or reddish-brown grannies, and capable of altering
its form by protruding or retracting 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 which it may come into contact, precisely after
flic 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 observation of the gradational transition
from the one condition to the other, and on the difficulty of sup-
posing that any such bodies could have entered the sphere parasiti-
c-lily from without — that the ' amu-boid ' is really the product of the
metamorphosis of a mass of vegetable protoplasm. This met a
morphosis may take place, according to Dr. Hicks, even after the
process of binary subdivision has commenced. What is the sub-
sequent destination of these amreboid bodies lias not yet been ascer-
tained.1

In other organisms allied to Volro.r, and included in the family
Volvodnece, we find a very interesting and instructive transition
between the various modes of multiplication already described. In
Eitdorina,a common organism in still water, a sexual process similar
to that in Vohw- 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 /ygospore js thus formed
which germinates after a period of rest, reproducing bv binarv
subdivision the original sixteen-celled, mulberry-like Paixlorinn.
We have here, therefore, a true process of conjugation between
motile protoplasm masses, each of which is in itself indistinguish-
able from a zoiispore. A similar process take's place also in
Conferva, Ulotkrix, Hyd/rodlctyon, and a number of fresh-water ali^e
(fig, 422).

Included by many writers under the general term Palmellace38
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 ' amceboids ' has been observed by Mr. Archer in
Stephanosphcera />///r/n//.~<, and is scarcely now to be considered an exceptional
phenomenon.

558 MICROSCOPIC FOKMS OF VEGETABLE LIFE— THALLOPHYTE8

] (articles have little or no adhesion to each other ; or thev 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 Palmoglona and Protococcus; 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 temperature 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 P<ili,i<<l1((.. The second is the condition of Pal-
metto, proper, of which one species, P. cruenta, usually known under
' '

the name of 'gory dew

s 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 afforded by the
Hcematococciis sany.iimus (fig. 423), which
chiefly differs from Palmdla 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 Palmetto, cells seem
occasionally to rupture and diffuse their

PIG. 422.— A, conjugating
microzob'spores of Ulo-
tlirix ; B, megazoiispore
of TJlot li rix, from Vines's
1 Physiology of Plants.'

granular contents through the

gelatinous

stratum, and thus to give origin to a whole
cluster at once, as seen at <°, 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 P<il»t<'U«. are frequently found to con-
tain parasitic growths formed by the extension of other plant -
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
tothem. Sometimes these filaments radiate in various directions from
a single central cell, and must at first he considered as mere exten
sions of this: their extremities dilate, however, into new cells ; and.
when theseare fully formed, the tubulax connections close up, and tin-
cells become detached from each other.1 Of the third condition we

1 This fa,.t, fh'st ma<i,. ,,ui.lic hy Mr. Thwaites (Ami. of Xnt. Hint. 2nd series,
v I1 ::l:;'- ls ""'' "f fundamental importance in the determination of the
real character <>!' this group.

PALMELLACEJE ; ULVACE.E

559

have an example in the curious Palttwdictyon 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 ultimately escape as zocispores. The
alternation between the motile form and the still or resting form,
which has been described as occurring in Protococcus, has been ob-
served in several other forms of this group ; and it seems obviously
intended, 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.1

Notwithstanding the very definite form and large size attained
by the fronds or leafy expansions of the Ulvacese, to which group

FIG. 423. — jHcematococcus sanguineus, in various stages of development ; <(, single
cells, enclosed in their mucous envelope ; I, 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 paivn!
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 (lifters but very
little from that of the preceding group ; and the principal advance-
is shown in this, that the cells, \vlu-n 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 Pal iiii'llin-i'ic are not now regarded by the best authorities as a distinct
family from the Prutococcaceic, and the genus Htei/iatococcus is sunk in Proto-
coccus. — ED.]

560 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

Protococcus, give rise, by tlieir successive subdivisions in determinate
directions, to such 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
(different 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 membranous expansion or thallus thus formed, there is

but little approach to a
differentiation of parts in
the formation of root, stem,
and leaf, such as the higher
algpe 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
bi partition, 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
/oi'ispores. The endochrome
(fig. 425, a) subdivides into
numerous segments (as at
l> and <•). which at llrst are
seen to lie in close contact

\vithin the cell that contains them, then begin to exhibit a kind
of restless motion, and at last escape by the bursting of the
cell-wal.l, and swim freely through the water as /oiispores (d) by
means of their iiagella, each zoospore having become endowed
with either t\v<> or four Ha gel la 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 («) in the manner already described. The walls of the
*'ells which have thus discharged their endochrome remain as
colourless spots on 1 he frond ; somet hues these are intermingled with

/3feV.

s-flaaftssSSsSn*'

i^satf1

U*» -""'tl »•«>» •«'

jSESSssasa:s..«

I"! ' • I

!r« mil <»« •
'

FIG. 4'24. — Successive stages of development
of Ulva.

ULVACE.E

.561

the portions still vegetating in the usual mode ; but sometimes the
whole endochrome of one portion of the frond may thus escape in
the form of zoospores, leaving behind it nothing but a white flaccid
membrane. If the niicroscopist 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
within cells a little more remote from the dividing line, and that a

TTio. 425. — Formation of zoospores in Viva latissima : ci, portion of the ordinary
frond ; b, cells in which the endochrome is beginning to break up into segments ;
f , 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.1

More recent observation has brought out the interesting fact
that hi Ulva 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
' zociganietes ' 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.

1 Such an observation the Author had the good fortune to make in the year 1842,
when the emission of zoiispores from the UlvacetE, although it had been described
t>y the Swedish algologist Agardh, had not been seen (he believes) by any British
naturalist.

O O

562 MICROSCOPIC FORMS 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,
A 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 wounding any
part of the wall. The genus
Vaucheria may be selected as a
particularly good illustration of
this family, its history having
been pretty completely made
out. Most of its species arc
inhabitants of fresh water, biit
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

FIG. 426.— Successive phases of generative variously interwoven. Some
process in Vaucheria sessilis : at A are species form dense green mats
seen one of the ' horns ' or antlierids (a) i -i • a ?

on damp soil in flower-pots, &c.

B

and one of the oogones (&', as yet un-
opened; at B the antherid is seen in
the act of emitting the, antherozoids (e),
of which many enter the opening at the
apex of the oogone, whilst others (d)

The formation of motile gonids
or zoospores may be readily
observed in these plants, the

winch do not enter it, <lis]>la\ their cilin. whole process usually occupying
until they become motionless ; at C the }n\t a Very short 'time. The
orifice of the oogo s closed again by ' ,• ,>,i £i

theformati. Ee cellul *oat around extremity of one of the filaments

the oosphere, thus constituting an obspore. usuallv suells up in the form of

a club, and the endochronic

accumulates in it so as to give it a darker hue than the rest;
a separation of this part from the remain. lev of the filament,
by the interposition of a transparent space, is next seen; a new
envelope is then formed around the mass thus cut oil': and at last
the membranous \\alloflhc investing tube gives way. and the zoo-

SIPHONACEvE 563

spore escapes, not, however, until it has undergone marked changes
of form, and exhibited curious movements. Its motions continue
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 are best seen when their movements have been
retarded or entirely arrested by means of opium, iodine, or other
chemical reagents. The motion of the spore continues for about
two hours ; but after the lapse of that time it soon comes to an
end, and the spore begins to develop itself into a new plant. It has
been observed by 1'nger 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
earlv in the morning. The same filament may give oft' 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, certain 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 anthcrids, which produce antherozoids in their interior ; whilst
the capsule-like bodies (A, 6) are oogones or a/rchegones, each con-
taining a mass of endochrome which constitutes an odspkere that is
destined to become, when fertilised, the original cell of a new
generation. The antherozoids (B. c, d), when set free from the
antherid ff, swarm about the ob'gone b, and, attracted by a drop of
mucilage formed at the mouth of the ob'gone, enter it, one or more
antherozoids becoming absorbed into the substance of the oosphere.
This hitherto naked mass of protoplasm now becomes invested by
an envelope of cellulose (C, ft), which increases in thickness and
strength, until it has acquired such a density as enables it to afford
a firm protection to its contents. While in Vimcji, rin the separate
filaments are so slender as to be scarcely discernible to the naked
eye, the frond of other genera of Siphonacese, mostly natives of
shallow seas in the warmer parts of the globe, attains very large
dimensions. Thus in CoiJimn it is a spongy spherical or cylindrical
floating mass, as much as a foot in length ; in Caulerpa it ha- the
appeai-ance of a branched leaf springing from a stem, which puts
out roots from its under side ; in Acetabidaria it takes a mushroom-
like form with a cap or ' pileus.' a quarter of an inch in diameter,
divided into regular 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-Chaiies l believes that many
fossils generally regarded as Foraminifera are in reality the
calcareous skeleton of alga? 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,2 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 Ckara, the currents
occasionally anastomosing
with each other (fig. 427,
C) . Within about thirty -
six hours 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 off from
the remainder by the
formation of a partition ;
and within this dilated
cell the movement of the
protoplasm continues for

.

FIG. 427. — Development of Aclily a prolifera : A,
dilated extremity of a filament b, separated
from the rest by a partition a, and containing
zobspores in progress of formation ; B, end of
filament after the cell-wall has burst, and
setting free zoospores, a, b, c; C, portion of
filament, showing the course of the circulation
of granular protoplasm.

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 Rendus, vol. Ixxx. 1877, p. 814.

2 [This plant, though, as an inhabitant of water, formerly ranked among Alga;, 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 parasitic on the bodies of living
ii-.li, and causing the very destructive disease to which salmon are liable. — ED.]

ACHLYA; HYDRODICTYON ' 565

quite mature, they are set free by the rupture of its wall (B), and,
after swarming about for a time, develop into tubiform cells resem-
bling those from which they sprang. Each of these zoospores is
possessed of two flagella ; 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 off by a transverse partition.
its contained endoplasm divides into two, three, or four segments,
each of which takes a globular form, and is then fertilised by the
penetration of an antheridial tube which conies off 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 appropriate 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 algje is the ' water-
net,' Hydrodictyon reticulatum, which is found 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
grown, 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 01- 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
dissolution 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
flagella 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 ' hypiiospores ' 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 Protococcus. 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 way
of the enveloping membrane, present the characters of ordinary

566 MICKOSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

/oospores, 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 themselves from the parent-cell ; they then project long
angular processes, so as to assume the form of irregular polvhedra.
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 network that becomes invested with
a gelatinous 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 Pediastnim granulatum.

no more than ^Vrrth of an inch in length ; but in the course of a
few hours they grow to a length of from iVth to ^rd of an inch.

The members of the family Pediastreae were formerly included
in the Desmidiacece ; 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 arc not
made up of two symmetrical halves, and that they are always found
in aggregation, which is not, except in such genera as Hcenedesmus
which connect this group with the Dcsmids, 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 1 he motile spheres of Voiron, and which takes place in

PEDIASTEE.E 567

such a manner that the resultant product may vary greatly in the
number of its cells, and consequently both in size and in form.
Thus in Pediastrum granulation (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 emptied 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 different 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 zousport-s. 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 tin-
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 coiirse 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 which, how-
ever, the arrangement of the interior cells does not follow the
typical plan.1 The formation, of ' microzoospores ' has also been
observed, which have been seen to conjugate.

1 See Prof. Braun on The Phenomenon of Rejuvenescence in Nature, published
by the Ray Society in 1853 ; and its subsequent memoir, --!/.'/« rum Unicellular/nit
Genera nova aut minus cognita, lH5-">.

568 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES-

The varieties which present themselves, indeed, both as to tin
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 jyertusutn, is in reality nothing else than a
young frond of P. <jran ulatum 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, exhibiting the peculiar
surface-marking from which the name of the species is derived, but
composed of no more 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 PciHastnnn : A, P. tctras; B, C, P. Ehrenbergii^
D, P. pci-tnsntn ; E, empty frond of P. granulatum.

them presenting the two-horned shape. So, again, in fig. 429, B and
0, are shown two varieties of Peditistni.m Ehrenberyii, whose frond
is normally composed of sixteen cells ; whilst at A is figured a form
which is designated as P. tetrcis, but which may be strongly suspected
to be merely a four-celled variety of 13 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 groups
by studying 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 offsets from any one .stuck. 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. lull's h:is remarked, 'one pool may abound with individuals of
</cjt'c/>nit or Arthrodesmus incus having the mucro

PEDIA8TEEJE ; CONFEEVACE.E

569

curved outwards ; in a neighbouring pool every 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 from a few primary fronds.' Hence the universality of a in-
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, properly 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 Confervaceae : but they are especially abundant in moving-
water, and they constitute the greater A
part of those green threads which
are to be seen attached to stones,
with their free ends floating in the
direction of the current, in every
running stream, and upon almost
every part of the sea-shore, and
which are commonly known under
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
distributed uniformly 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 usually, 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, «) ; and thus there is
hour-glass contraction across the cavity

FIG. 430.— Process of cell-multipli-
cation in Claclophora glomerata'.
A, portion of filament with incom-
plete separation at a, and complete
partition at b ; B, the separation
completed, a new cellulose parti-
tion being formed at a ; C, forma-
tion of additionallayers of cellulose
wall, <•, beneath the mucous in-
vestment, (Jy and around the
ectoplasm, «, which encloses the
endochrome, l>.

gradually

the subdivision of the
ectoplasm around it
formed a sort of
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 Cladophora
glomerata (a common species), new cells may originate as branches
from any part of the surface by a process of budding, which,
notwithstanding its difference of mode, agrees with that just
described in its essential character, being the result of the sub-

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 considerable 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 conies to be restricted at last to the terminal cell.
The very elongated cells of some species of Confervacea? are
characterised by the possession of a large number of nuclei. They
are multiplied by zoospores, produced apparently indifferently from
any cell of a filament, by free-cell formation. These zoospores arc
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 Confervacere 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.1 The oospore, which is the pro-
duct of the sexual process to be presently described, is filled when
mature 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 gradually 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 repetition of the
like interposition gives to the endochrome that annular arrange-
ment from which lhc plant derives its specific name. This is seen
at No. 9, a, as it present^ itself in the filaments of the adult plant ;
whilst, at ft. in the same figure. \\ e 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 oospheres consists in the aggregation of the
endochrome into definite masses (as seen .-it Xo. 10, a), which soon
become >t;ir-shapcd (as seen at &), each one being contained within a
distinct compartment of the cell. In a somewhat more advanced
stage (as >een at Xo. 11. «), the masses of endochrome begin to draw
1 Aim. des Sci. Nat. ll'ino sor., Bot., torn. v. 1856, p. 187.

SPHJEBOPLEA ANNULINA

5/1

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-

c s** i\ ,-> Vi

';

FIG. 431. — Development mid reproduction of Sphcsroplea.

tioiied masses, which soon afterwards become invested with a firm
membranous envelope, as shown in the lower part of No. 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 first, which extends itself into
stellate prolongations, as seen in No. 13; so that when set free

572 MICROSCOPIC FORMS OF VEGETABLE 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 ob'spores, have their annular
collections of endochrome converted into antherozoids, which, .-is
soon as they have disengaged themselves from the mucilaginous
sheath that envelopes them, move about rapidly in the cavity of their
containing cell (a, b) 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 obgones, 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 obspheres 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 (Edogoniacese resemble Coiifervacece in general aspect and
habit of life, but differ 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 zobspore 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 zobspores are the largest known in any class of alga? ;
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 (Edogoniacece show a curious
departure from the ordinal-}' type ; for whilst the obspheres 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, but are developed within a body termed an
androspore (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 lias come to an end. attaches
itself to the outer surface of an oJigone, or of a cell in close proxi-
mity to an ob'gone, as shoun at No. 1, b ; it then developes into a
verv small male plant, known as a dwarf -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

CEDOGONIACEJE ; CH..ETOPHORACEJE

573

same time an aperture is formed in the wall of the oogone by
which the antherozoid enters its cavity and fertilises its ob'sphere by
becoming absorbed into it. This masstlieii becomes an. ob'spore (No. 3),
invested with a thick wall of its own, but still retains more or less
of the envelope derived from the cell within which it was developed.
The offices of these different classes of reproductive bodies are only
now beginning to lie understood, and the inquiry is one so fraught
with physiological interest, and, from the facility of growing these
plants in aquaria, can lie so easily pursued, that it may be hoped

FIG. 432. — A, Sexual generation of (Edogonium ciliati/in : 1, filament with two
ob'gones 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 ocispore, 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 Chcetophont 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 Chaetophoraceae constitute a beautiful and interesting little
group of confervoid plants, of which some species inhabit the sea,
whilst others are found in fresh and pure water — rather in that of
gently moving streams, however, than in strongly flowing currents.
Generally speaking, their filaments put forth lateral branches, and
extend themselves into arborescent fronds ; one of the distinc-
tive characters of the group is afforded by the fact that the
extremities of these branches are usually prolonged into bristle-

574 MICROSCOPIC FORMS OF VEGETABLE LIFE -THALLOPHYTES

shaped processes (fig. 432, B). As in many preceding cases, these
plants multiply themselves by the conversion of the eiidochrome of
certain of their cells into zocispores, 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 ylomerata,
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 regular intervals whorls of
slender branches, the endochrome of which is deep green, and every
branch ends in a delicate hyaline hair of extraordinary length. The
mode of reproduction of the Cheetophoracece closely resembles that of
the Confervacece.

The Batrachospermeae, 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 Rhodospermece. 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 Cladophora 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-
certed into a beaded filament. Certain of these -branches, however,
instead of radiating from the main axis, grow downwards upon it.
so as to form a closely fitting investment lh,-it seems properly to
belong to it. Some of the radiating In-andie-* grow out into long
transparent bristles, like those of the ( '/«t f»/>/n,i-tn-rti' ; and within
those are produced anthero/oids, which, though not endowed with
the j tower of spontaneous movement, find their way to the oiispheres
contained in oilier parts of the filaments: and by the fertilisation
of tlie ("intents of these are produced the somewhat complicated
fructifications known as n/stocarps, placed in the axils of the

liranclies (llg. I ."..'!).

BATKACHOSPEEME^E,; COLEOCELETACEJE ; CHAEACE.E 575

A very singular relationship, called by some writers an ' alter-
nation of generations,' exists between Batrachospermum and Chan-
transia, a genus of fresh-water alga? previously placed in a totally
different section. This relationship was first described by Sirodot,1
and his observations have since been confirmed by others. The
germinating spores of Batrachospermum put out, under certain
conditions, a. kind of filament, known as n proton erne, which develops
into a Chantransia, a non-sexual form of Batrachospermum, which
can reproduce itself from generation to generation by simple
budding, or by means of non-sexual spores, without producing
sexual organs. Ohantransia is especially found in water where very
little light reaches it. When more exposed to light it undergoes
metamorphosis, and then a branch springs up from the protoneme
which is in every respect a Batrachospermum, bearing true sexual
organs, as above described.
This may then go on repro-
ducing itself, or revert to the
O.hanfransia form.

The Coleochaetaceae are a
small order of fresh - water
Algfe, chiefly represented by
the genus C'oleochcete, which
forms minute discs or cushions
attached to submerged plants,
from -j1-^ to ^ 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-
sexiiallv, bv 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 Rhodospermese, the highest class of Alga?.

Among the highest of the Alga? in regard to the comph-xitv of
their generative apparatus, which contrasts strongly with the general
simplicity of their structure, is the family of Characeae,- 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 \\atcr> are rendered salt by communication with the
sea. They may be easily grown for the purposes of observation in

1 Sirodot, Lcs BatracJiosprrniees, fo. 1884.

2 [Many of the best authorities regard the Characeer, in consequence of their
mode of reproduction, as a group of primary character, of equal rank with the Algee,
and superior to them in organisation. — ED.]

FK;. 433.
Batrachospermum. nionili forme.

5/6 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 Nitella the stem and
branches are composed of simple cells, which sometimes attain
the length of several inches ; whilst in most species of Chara each
central tube is surrounded by an envelope of smaller ones, which is
formed as in Batrachospermum, 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 plate or layer of small cells, which, 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 cijclosis, 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 particles. The produc-
tion of new cells for the extension of the stein or branches, or for
lln> origination of new whorls, is not here accomplished by the
subdivision of the parent-cell, but takes place by tlie method of out-

CHARACE.i:

577

(fig. 434, B, e,f, g. A), which, as already shown, is nothing
but a modification of the usual process of cell-multiplication : in
this manner the extension of the individual plant is' effected with
considerable rapidity. When these plants are well supplied with
nutriment, and are actively vegetating under the influence of light,
warmth, etc., they not unfrequently develop 'bulbils,' which art-
little clusters of cells, filled with starch, that sprout from the side*
of the central axis, and then, falling off, evolve the long tubiform
cells characteristic of the plant from which they were produced.
There are also several other non-sexual ways in which these plant*

FIG. 434. — Nitella flcd'ilis : A, Stem and branches of the natural size : a, b, c, <7, our
whorls of branches issuing from the s-tem; e, f, subdivision of the branches.
B, Portion of the stem and branches enlarged : a, b, joints of stem; c, d, whorls ;
c, f\ new cells, sprouting from the sides of the branches ; g, h, new cells sprouting
at the extremities of the branches.

are reproduced, but they are peculiar among cryptogams in not
producing true spores, either stationary or motile. The CharaceoB
may be multiplied by artificial subdivision, the separated parts
continuing to grow under favourable circumstances, and gradually
developing themselves into the typical form.

The generative apparatus of Cluirticefr consists of two sets of
IK ulies, both of which grow at the bases of the branches (fig. 4."!.").
A, B), either on the same or on different individuals ; one set,
formerly known as -globules,' are really anthwids; whilst the
other, known as 'nucules.' contain the oospheres, and are true
oiigones or archeyones. The globules, which are nearly spherical.

p r

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
portion is principally composed of a mass of filaments rolled up
compactly together. From the centre of the inner face of each
shield a cylindrical cell termed a, manubriu/m projects inwards nearly
to the centre of the .sphere. The antherid is supported on a short

FIG. 485.— Generative organs <>f Cl'iini frui/ilm: A, antherid or globule developed
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, within every one of which an
antherozoid is formed ; in 1), E, and F the successive stages of this formation are
seen ; and at G is shown the escape of tin- mature antherozoids, H.

flask-shaped pedicel, which also projects into the interior. At the
apex of each of the eight manubria is a roundish hyaline cell, called
.1 I'li/iitnl a in. and at the apex of each capitulnm six smaller cells or
' secondary capitula.1 From the centre of each of these secondary
capitula grow four long whip-shaped filaments (C). constituting the
mass already referred to. The number oflhe.se lilaments in each
antherid is about 200, and each of these filaments divides by

CHAKACE.E; DESMIDIACE^E 579

1 ransvcrse septa into from 100 to 200 small disc-shaped cells, which
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
antherozoid, a spiral thread of protoplasm 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 water 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 exterior of the nucule
(A. ]>) is formed by five or ten spirally twisted tubes that give it a
very peculiar aspect ; and these enclose a central sac containing
protoplasm, oil, and starch grains. Each of these tubes consists, in
its lower part, of a very long unsegmented cell; while at its upper
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
pc i -form 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 fertilised germ-cell, or
obspore, gives origin to a new plant after the nucule has remained

dormant through the winter.1

0 .

Among those simple Alga? whose generative process consists in
the 'conjugation' of two similar cells, there are two groups of such
peculiar interest to the microscopist as to need a special notice;
these are the DesmtdiacecK and the Diatomacece. Both of them
were ranked by Ehrenberg and .some other naturalists as animal-
cules ; but the fuller knowledge of their life-history and the more
extended acquaintance with the parallel histories of other simple
forms of vegetation which have been gained during the last twenty
years, are now generally accepted as decisive of their vegetable
nature.

The Desmidiacese 2 are minute plants of a bright green colour
growing in fresh water ; generally speaking, the cells are inde-
pendent of each other (figs. 436-439) ; hut sometimes those which

1 A full account of the CJinracctf will lie found in Prof. Sachs's Text-Bonk <>j'
Botiuty, '2nd English edition, p. '2'.l'2. Various observers have asserted that particles
of the protoplasmic contents of the cells of the Characew, when set free by the
rupture of their cells, may continue to live, move, and grow as independent rhizopods.
But the writer is disposed to think that the phenomena thus represented are rather
to be regarded as cases of parasitism, the decaying cells of Nitclln having been found
by Cienkowski (Beitrfigf zur Kenntniss tier Monaden,in Arch.f. Mikr.Anat. Bd. i.
18(3"), p. '208) to be inhabited by minute, spindle-shaped, ciliated bodies, which seem
to correspond with the ' spores ' of the Myxomycetes,goiog through an ameboid stage,
and then producing a plasinode which, after undergoing a sort of encysting process,
finally breaks up into spindle-shaped particles resembling those found in the Niti'lhi
cells.

- Our first accurate knowledge of this group dates from the publication of Mr.
Ralfs's admirable monograph of the British Desmids in 1.S4.S. Later information in
regard to it will be found iu the section contributed by Mr. W. Archer to the fourth
edition of Pritchard's Infu.wrin, and in Cooke's British Desiiiidy, 1HM7.

r p 2

580 MICROSCOPIC FORMS OF VEGETABLE LIFE — THALLOPHYTES

have been produced l>y binary subdivision from a single parent-
ccll 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 symmetrical halves by a 'sutural line.' which
is sometimes so decided as to have led to the belief that the cell
is really double (Plate VIII, figs. 2. (5). though in other cases
it is merely indicated by a slight notch ; the other feature is the
frequency of projections from the surface, which are sometimes
short and inconspicuous, but are often elongated into spines
(Plate VIII, fig. 6), presenting a. very symmetrical arrangement.
These projections are generally formed by 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 casing, 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. fi) ; but in
other cases its existence is only indicated by its preventing the con-
tact of the cells. Klebs states1 that in Desmids, as in the other
Conjugates, 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
cell 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 interior 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 beautiful 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, vix. : — (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 free
end making wide circular movements ; (3) a circular motion,
followed by an alternate sinking of the free end and elevation of
tin- other end; and (4) an oblique deration 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 line to an exudation of mucilage, and the fir.-4 two to the forma-
tion during Ilie motions of a filament of mucilage by which the
dcsmid i^ temporarily attached to the bottom, and which gradually

nits dc»i Hot. Iii.it. Tubing< n, \^(>, \> '.'•:'••'•.

- llioli.i/isrhi". Crii/i'tiUiltift, 1H85, p. 85S.

PLATE IX.

v . / .•» —

'i W"

Desmidiaceas.

West. Newman chromo.

DESMIDIACE.E 581

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 l»y light, and always move
towards the light.

A ' cyclosis ' may l>e readily observed in many Desinidi«c<;< .
and is particularly obvious along the convex and concave edges of
the cell of any vigorous specimen of Closterium, with a magnifying
j tower 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 carry with them,
from time to time, little oval or globular bodies (A. b) which are put
forth from it. and are carried by the course of the flow to the tran>
parent spaces at the extremities, where they join a crowd of similar
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. 436. — Cyclosis in Closterium luimla : A, cell showing central separation at a,
in which the large particles, b, 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 are kept in a sort of twisting movement
on the inner side («) of the parietal utricle. Other currents are
M-en apparently external to it, which form three or four distinct
courses of particles, passing towards and away from c (as indicated
by the outer arrows). Another curious movement is often to be
witnessed in the interior of the cells of members of this family,
which has been described as 'the swarming of the granules/ from
the extraordinary resemblance which the mass of particles in active
vibratoiy motion bears to a swarm of bees. It is especially
observable in the hyaline terminal portions of the cells of specie-; of
Closterium, as shown in fig. 436, B. This motion continues for
some time after the particles have been expelled by pressure from
the interior of the cell ; and it appears 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 MICROSCOPIC FORMS OF VEGETABLE LIFE — THALLOPIIYTES

When the single cell has come to its full maturity it commonly
multiplies itself by binari/ subdivision ; but the plan on which this
takes place is often peculiarly modified, so a>- to maintain the
symmetry characteristic of the tribe. In a cell of the simple
cylindrical form of those of Desmtdlani (fig. 440), little more is
necessary than the separation of the two halves at the sutur.il 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 symmetrically ; in process of time, how-
ever, the increased development of the sides of the cells which re-
main in. contiguity with each other brings up the smaller projections
to the dimensions of the larger, and the symmetry of the cells is
restored. In Closterlum (fig. 436 ; Plate IX, fig. 2) the two halves of
the endochrome first retreat from one another at the sutural line, and
a constriction takes place irmnd 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
utricle of the two segments ; in this state one half commonly 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 gradually a transparent space is formed,
like that at the opposite extremity, by the retreat of the coloured
layer; whilst at the same time its obtuse 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 Stawrastfrum, the division taking place across the central con-
striction, and each half gradually acquiring the symmetry of the
original. In such forms as Comma-i n m . houever. in which the cell
consists of two lobes united together l>y a narrow isthmus, the divi-
sion takes place after a different method; for when the two halves
of the outer wall separate at the sutural line, a semi -globular protru-
sion of the endochrome i.s put forth from each half; these protru-
sions are separated from each other and from the two halves of the
original cell (which their interposition carries apart) by a nairow
neck; and they progressively increase until they assume the appear-
ance ol'the half-segments of the original cell. In this state, there-
lore, the plant consists of a row of four segments Iving end to end,
t he t uo old one> forming the extremes, and the two new ones (which

DESMIDIACE^E

583

do not usually acquire the full size or the characteristic- markings of
the original before the division occiirs) occupying the intermediate
place. At last the central fission becomes complete, and two bi-
partite fronds 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 hours, is repeated
ere long. The same general plan is followed in Micrasterias ; but
as the small hyaline hemisphere, put forth in the first instance from
each half-cell (fig. 437. A), enlarges with the flowing in of the endo-

A B C

I)

E

F

<~v^T<W-!?->

i

•>- -

mm

FIG. 437.— Successive stages of binary subdivision of Mn-ytiatri-iits d

chrome, it undergoes progressive 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 separation (F) acquires
the characteristic notched outline of its type, being only distinguish-
able from the older half by its smaller size. The whole of this
process may take place within three hours and a half. In
X phcKrozosma the cells thus produced remain connected in rows
within a gelatinous sheath, like those of Desrnidliun (tig. 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 are found nt
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 Con fen-awn-,
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 dehisceiice of the
firm external envelope of each of the conjugating cells, so as to
separate it into two valves (fig. 438, <J,' I) ; fig. 439, C). The
contents of each cell thus set free without any distinct investment
blend with those of the other ; and a zygospore is formed by their
union, which soon acquires a truly cellulose envelope.1 This enve-
lope is at first very delicate, and

is filled with

green and

granular

contents ; by degrees the envelope
acquires increased thickness, and
its contents become brown 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 surfa.ce is sometimes
smooth, as in Closteriwn and its
allies (fig. 439 ; Plate IX, fig. 8) ;
but in Cosmcuriwn it becomes
granular, 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 Desmldiew 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 (tig. 440, I ). K).
Through this tube the entire endochrome of one cell passes over
into the cavity of the other (I)) ; and the two are commingled so as
to form a single mass (E), as is the case in many of the Conjiiyatct;.
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-waif disap-
pears.

FJ<;. 438 — Conjugation of Cosmariiim
liotrytis : A, mature cell ; B, empty
cell-envelope ; C, transverse view ;
D, zygospore with empty cell enve-
lopes.

ever, in many respects different.

1 In crHain species of C'lontrri/nn, as in many of the Ditttomnce(p,i\\e act of
conjugation gives origin to lien /ygospores.

DESMTDIACEJK

585

The subsequent history of the zygospore has been followed out
in the case of Gosmarium botrytis. After remaining at rest for a
considerable time, it germinates by the Inn-sting of the two outer
coats, the protoplasmic contents escaping while still enclosed in the
innermost coat. In this body the protoplasm and endochrome are
already divided into two halves, which contract somewhat, and tin-
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 genera, according to the
method of Mr. Ilalfs (; British Desmidiese '), 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 genera, 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 the>e la>T
particulars that the generic characters are based. The solitary
group presents a similar basis for 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 iii Micrasterhis (fig. 437 ; Plate IX, fig. 1), Cos-marl urn (fig. 43* ;
Plate VTII, fig. 2), and Stnm-nsti-Kin (Plate VI11 : 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 Closteritim striulatuin :
A, ordinary cell ; B, empty cell ; C, two cells in
conjugation, with zygospore.

586 MICROSCOPIC FORMS < >F VEGETABLE LIFE— THALLOPHYTES

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 lie looked for chieflv
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,
commonlv lie at the bottom of

IMC. 440. — Binary subdivision and conjugation of

The larger and heavier species
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 ;
;ii id 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
adhered to it. If. on the
other hand, the bodies of
which we are in search
should be much diffused
through the water, there
is no other course than to
take it up in large quanti-
ties bv the box or scoop

Itrxtiiidiuni cijliudricuni : A, portion of filament, and to separate them hv
surrounded bv gelatinous envelope : B, dividino fi i • '

cell; C, sn.gle cell viewed transversely; D, two* Draining through a piece

cells in conjugation ; E, formation of zygospore.

of linen. At first, nothing
appears on the linen but
a mere stain or a little dirt; but by tne straining of repeated
quantities a considerable accumulation mav be gradually made.
then be scraped oil' with :i knife, and transferred
with fresh water. If what has been brought up
richly charged with these forms, it should be at
bottle; this at first seems onlv to contain

a

This should
into bottles
by hand IK
cnee deposited in a
foul water: but bv

little time, the
the water mav

allowing it to remain undisturbed for

desmids will sink to the bottom,
then he poured oil', to he replacet

most of
a fresh

DESMIDIACE/K : DIATOMACE.^E 587

supply. If the bottles be freely exposed to solar light, these little
plants will flourish, apparently as well as in their native pools ; and
their various phases of multiplication and reproduction may be
observed during successive months or even years. 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 ring-net
may also be advantageously employed, especially if it be so con-
structed as to allow of the ready substitution of one piece of muslin
for another. For. by using several pieces of previously wetted
muslin in succession, a large number of these minute organisms
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 from 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 Diatomacese <>r Bacillariaceae. like the Desmidiacea?, 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 sllf.f, 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 mineral 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 bearing the characteristic
surface-markings is found to remain after its removal by hydro-
fluoric acid. The endochrome of diatoms consists, as in other
plants, of a viscid protoplasm, in which float the granules of
colouring matter. In the ordinary condition of the cell these
granules are diffused through it with tolerable uniformity, except
in the central spot, which is occupied by a nucleus ; round this
nucleus they commonly form a ring, from which radiating lines of
granules may be seen to diverge into the cell-cavity. Instead of
being briyht screen , however, the endochrome is a yellowish brown.

«.

The principal colouring substance appears to be a modification of
ordinary chlorophyll ; it takes a green or greenish-blue tint with
sulphuric acid, and often assumes this hue in drying; but with it i^
combined in greater or less proportion a yellow colouring matter
termed diatomin. 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 other protophytes. A distinct
movement of the granular particles of the endochrome, closely
resembling the cyclosis of the Desmidiacece, has been noticed by
Professor W. Smith in some of the larger species of Diatomacece,
such as Stirii-i'Un biseriata. Xitzschio wn/ftrts. and Campylodiscus
spiral-is, and by Professor Max Schultze in Coscinodiscits, Biddulphia,
and Rhizosolenia ; but this movement has not the regularity so
remarkable in the preceding group.

The name of the class is derived from the ease with which the

5 88 MICROSCOPIC FORMS OF VEGETABLE LIFE — THALLOPHYTES

parts separate from each other. This is well seen in the genus
Diatoma, formed of rectangular individual frustules, where the
arrangement resulting from the principle 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 Grammatophora (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 frustules produced by successive acts of binary subdi-
vision habitually remain coherent 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 cylinder, an aggregation of such
cylinders, end to end. must form a rounded filament, as in Melosira
(fig. 444) ; and, whatever may lie the form of the sides of the
frustules, if they be parallel one to the other a atraight 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
of Licmophora flabellata (fig. 450) ; or the cohesion may be sufficient
to occasion the Viand 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 Meridian circtdare (fig. 448). Many diatoms, again,
possess a stipe, or stalk-like appendage, by which aggregations of
frustules are attached to other plants, or to stones, pieces of wood,
Arc. ; and this may be a simple foot-like appendage, as in Ai-limiiitJifx
longipes (fig. 461), or it may be a composite plant-like structure, as
in Licmophora (fig. 450). (lompJtonema, (fig. 462), and Mnstoylo'm
(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 Desmidiacew, and to the
filaments which sometimes connect the cells of the Palmettacece.
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 Jfastoyloid, (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-
fi/iui. (fig. 442), Pleurosigma (Plate I. figs. 1, 2). Actinocyclus,
Actinoptychus (tig. 467), Arachnoidiscus (Plate Xll), ('«i>i]>i/l<idiscus
(lig. 4r*4J,,sW//v/A/ dig. 45:',), Coscinodiscus (Plate l,figs. :i. 4.'lig.455),
lliin,j>,'ltii , and many others. The solitary discoidal forms, however,
when obl.-iined in their living state, are commonly found cohering
1<> the surface of aquatic plants.

\\ e have now to examine more minutely into the curious struc-
ture of the silicitied casing which encloses every diatom-cell or

s>

DIATOMACE.K 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 through the perpetuation of the
minutest details of that pattern in the specimens obtained from
fossilised deposits. This 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 </ir<.!l>>. and
thus exactly represent a minute box which serves for the reproduc-
tion of the species. This process is known as the encystmeiit, and
is not uncommon, especially amongst the Xaricule<r, frustules being
frequently found amongst them open from the separation of the t\\o
valves, showing the two rings covering each other, 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 ' costa? '
or ' canaliculi ') starting from the outer margin of the valve, and
converging towards the interior of the disc, are rni/s 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
portion is the 'funnel.' the slender part the * stem." The central
portion of the valve inside the internal termination of the ra vs is
the area ; it may be smooth and hyaline, or it may be striate, 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 simple elongated line, it is known
as the raphf m- psendo-raplte.

Dr. 0. Miiller proposes the term epitheca for the overlapping
half-cell of the diatom, the underlapping half-cell being the hypo-
tkeca; for the girdle-bands he proposes the term pleura'.

In describing diatoms, the aspect in which the girdle is turned
towards the observer is known as the ' front ' or * girdle ' view ; that
in which the surface of the valve is turned towards the observer is
the 'side ' or * valve ' view.

It is not coirect to designate the line shown in the front \ ie\v
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 diatom> : for
sometimes each valve is hemispherical, so that the cavity is globular ;
sometimes it is a smaller segment of a sphere resembling a watch-
glass, so that the cavity is lenticular : sometimes the central portion
is completely flattened and the sides abruptly turned up. so that the
valve resembles the cover of a pill-box, in which case the cavity will
be cylindrical; and these and other varieties may co-exist with anv
modificatioiis of the contour of the valves, which may be square,
triangular (fig. 44:2), 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,
4.">(.>). Hence the shape presented by the frustule differs completely
with the aspect under which it is seen. In all instances, the

59O MICROSCOPIC FORMS OF VEGETABLE LIFE — THALLOPHYTES

frustule is considered to present its ' front ' view when its line of
meeting is turned towards the eye, as in fig. 453, B, C; whilst its
'side1 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 increase, the valves separate from
one another ; and by the silicification of the cell-membrane thus left
exposed a pair of hoops is formed, each of which is attached bv one
edge to the adjacent valve, while the other edge is free.1 As will
be presently explained, one of the valves is always older than the
other; and the hoop of the older valve partly encloses that of the
younger, just as the cover of a pill-box surrounds the upper part of
the box itself.2 As the newly formed cell increases in length,
separating the valves from one another, both hoops increase in
breadth by additions to their free edges, and the outer hoop slides
off the inner one, until there is often but a very small -overlap.'
As growth 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 degree 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 surrounding
water may come into communication with the contents of the cell.
Some have believed that they have seen such apei-tures along the
so-called ' line of suture of the disc-shaped diatoms, and at the extre-
mities only of the elongated forms. Ehrenherg, followed bv Kiitzing.
has interpreted as apertures or ostioles the central and terminal
nodules of the Xiirio/li'd. < ' //mbelleff. and similar forms; but this
vie\v is more generally regarded as incorrect. We have, in fact, no
positive demonstration of the existence of special apertures communi-
cating between t lie 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 t \\ o valves,
between which the possibility of such a communication must
necessarily be admitted, or at least the existence of endosmotic and
exosmotic cm-rents 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 XnrirnUi lias been some-
times seen with the valves actually separated.

1 This refers to those diatoms in which the process of biliary subdivision is
possible; but this, as will be seen presently, is not. the rase in many genera.— ED.]

•' This was long since pointed out by Dr. \Vallich in his important memoir on the
' Development and Structure of the Diatom-valve ' i Trii/inncf. of Microbe. Soc, n.s.
vol. viii. 1800, p. 12!>) ; but his observation seems not to have attracted the notice of
diatomists, until in 1S77 he called attention to it in a more explicit manner ( Monthly
M/CI-IIHI-. Jniini. vol. xvii. p. 01). The correctness of his statement has been con-
tinued by the distinguished American diatomist, Prof. W. Hamilton Smith ; but as it
has been cilled in question by M r. .1 . D. Cox (America ii Jo/tnitil of Microscopy,

vol. iii. ls?s, p. 1001, who asserts that in Inthmitt there are three hoops two

attached to the two valves, and the third overlapping them both at their line of
junction tin- Author has himself made a very careful examination of a large series
of specimens of Ixlhniin and liiddnl phia, the result of which has fully satisfied him
of tin. correctness of Dr. Wallich's original description.

DIATOMACE.E

591

markings with which almost every

interesting

111-

441

diatom,
a
has

single

a

The nature of the delicate

diatom frustule is beset has been one of the most
quiries of the students of these forms since the introduction of the
homogeneous, and especially the apochromatic, objectives; and it
cannot be doubted that certain peculiarities of structure have been
demonstrated which were 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 110 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 Difit<>i,i<in--<i', we
represent in Plate I, fig. M, a photo-micrographic image of I'osti-
il^cn-s astei-oui/t/nil us magnified 110 diameters. But in lig.
the areolcn of this diatom are
.-eBn under great magnification
with recent powers. It is
(•.intended that the
although consisting of
siliceous membrane,
double structure, vix. 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 extremely
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 present
object magnified 2,000 diameters.

In Isthmia nen-osa, 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 correspondingly thick ;
but the inner membrane is excessively thin and delicate. The per-
forations are large and irregular in shape 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 perforations.1

Not less interesting is the beautiful form Aulacodiscus Klttonii;
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. <_'. Karop,
Joitrn. Qxekett Club, vol. ii. ser. ii. p. tit'i'.t.

Fi«. 441. -Magnification of ' ultimate struc-
ture ' of Cuscinodisciin aster omphalus,
from a drawing by Messrs. Nelson and
Karop (Juiirn. Qitekftt Club, vol. ii.
ser. ii. p. 2(59).

photo-micrograph of the same

592 MICROSCOPIC FORMS OF VEGETABLE LIFE — THALLOPHYTES

. I. Stm-tii, magnified '2.000 times, is shown in fig. (5 in the same
plate.

The 'beaded' appearance of diatom-valves is so universal in all
those which have been 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. 44-2. -Triceratium favus : A, wide 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 aftbrd 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-
t mately prove to be ; and this is clearly seen
when they are observed with objectives of
sufficient 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 contrary, Amphiplewra pettudda
is extremely variable, and is, as it were, the
torment of microscope-makers and rival
not take into account the variability of
this type, forgetting, in fact, that one A. pettucida maybe extremely
line, and another, being in truth a varietal form, may be nearly as
coarse as X/i riculn rhomboides. The new apochromatic objectives,
and the compensating eye-pieces, both for the eye and for projection.
constructed by /eiss, of Jena, have brought about such progress in
micrography that the image of /'. anyulatwm appears to some minds

Fie;. 443. — Aroolutions in
Ixfhmia iierrosa.

diatom-resolvers, who d<

PLATE X.

PLEUROSIGMA ANGULATUM.

Magnified 4900 diams.

From a Photo-Micrograph by Dr. R. Zeiss taken with the 2 mm. Apocnromatic Objective
N. A. 1.30 and projection eyepiece 4.

Collotype Ptg. Co., 2*2 High Hoi born, W.C.

DIATOMACE.E 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 the
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 probability be considered per-
forations in the silex of the frustule. This is, indeed, placed almost
in the form of a demonstration by the interesting fact that Mr. C.
Haughtoii 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 appears to be formed.
By observing the lines of fracture, 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 srcn 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 Triceratiumfavus (fig. 442) present
a line of fracture which traverses indifferently the hexagonal areola?
and the lines in relief which connect them.

Dr. Tan Heurck has been able to employ the new lens made by
Abbe, having a numerical aperture of 1'63, upon his special subject,
the Diatomacece. He concludes that diatom valves consist of t \\ o
membranes or thin films and of an intermediate layer, the latter
fx'iitij pierced with openings. The outer membrane is delicate, and
may be easily destroyed by acids, friction, and the several proces>e^
of ' cleaning.' When the openings or apertures of this interior portioi i
are arranged in alternate rows they assume the hexagonal form ;
when in straight rows then the openings are square or oblong.

It is, however, due to Mr. T. F. Smith, who worked at this
subject for years, to say that he long maintained this view, and
has presented skilful photo-micrographs in support of his contention,
[n Plate I, fig. 1, we have a photograph of his, showing the inside
f a valve of P. 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. angulatum, showing a different
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. Van
Heurck "has produced some remarkable photo-micrographs, which

9 Q

1 1

594 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

rather confirm these general 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. Van
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 Robert's nineteenth band.

Diatoms, like other organisms already described, are reproduced
by conjugation, and multiply by autofissioii 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 encyst -

ing of the frastnle or
individual diatom, which
implies the existence of
the two valves and of
the double girdle or
zone or connecting ring
projecting from each
valve in a direction at
light 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
offer any explanation.
The fact that in Melosirti
sithflexilis (fig. 444, A)
A Fiu. 444. B and M . rarians (fig.

Melosira xiibflej:i//^. Melosira vnnnns. 444, B) large and small

frustules are seen united

in rows, ought to be sufficient to show that they are dependent not
only on binary subdivision, but also 011 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 reach the smallest possible dimensions.
This has led Pfitzer ' to imagine that when diatoms have readied
their smallest possible dimensions by repeated binary division,
the process of conjugation takes place between them, resulting
in the formation of an rt//./v/x/«>/-«, capable of reproducing two
sporanirial iVustules of considerably larger size, which would again
oive rise, bv fission, to a new series of diminishing frustules.
1 riitfrsiK-lniiir/rti itltt'r Han n. Eiitirii-l.'il iimi <lrr I liici/fn r/ni, 8vo. Bonn, 1871.

PLATE XI.

Fig. 1.

Fig. 2.

Fig. 3,

ttrr

Fig. 7.

,

Fig. 5.

Fig. 8.

Fig. 6.

Fig. 9.

Dr. H. Van Heurck, phM. Collotype Ptg. Co., zSa High Holborn, W.C.

TEST OBJECTS FOR THE MICROSCOPE.

Objective by C. Zeiss, N.A. roo; Eyepiece 12. Monochromatic illumination by sunlight.

DIATOMACEJE 595

until these again reach their minimum size. This theory has.
in the judgment of Count Castracane, deceived many botanists,
from the idea that it was founded on actual observation, and lias
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 regarded 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 thirdly, that there is no mode of reproduction except
by auxospores. That the siliceous walls of diatoms are capable of
distension seems to result from the examples already given of Melosim
sn.bfle.riUs and J/. varians, as also from some other 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 spnrangial frus-
tules ofOrthosira Dickiei,1 where, in the chain of cylindrical frus-
tules of the same diameter, the sporangia! frustule is dilated in its
equatorial axis, but much more so in its polar axis, pushing back the
base of the next cell and furring it to fold itself up 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 guaranteed 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 preparation
of these diatoms in which are a number of sporangia! frustules.
The auxospore theory supposes the fact that all diatoms are capable
of binary subdivision, since the auxospore is understood, according
to Pfitzer, to provide for the progressive decrease 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 genera with unequal valves, as it is universally
acknowledged that the two new valves which are formed in the
process of binary subdivision must stereotype themselves on the old
valves ; and for this reason the process cannot take place in thus,
genera in which the axes cross one another, like Campylodiscus, or
in those in which the two valves, although equal, yet constantly
unite in such a way that the similar parts alternate with one
another, as may be seen in Asteroktmpra. That it is impossible for
binary subdivision to take place in these three classes of forms. i>
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 affords 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-
inidiacece, 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 Eeproduction of Diatoms,' Atti dell' Accad.
Pontif. del Ni/ori, Lhicci, May 31, 1874; and 'New Arguments to prove that
Diatoms are reproduced by means of Germs,' ibid. March 19, 1870.

g Q 2

PLATE XII

ARACHNOIDISCUS JAPONIC is.

.'•

DIATOMACEJS 597

continued connection of the two frustules by its means gives rise to
an appearance of two complete frustules having been developed
within the original (fig. 445, A, C) ; subsequently, 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 l the process of
binary division is continually being repeated ; and a very 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 maybe 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 then-- i>
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

iioes Oil.

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 particular cases.
He contended that he had been able to see in a Podosphenia the
emission of gonids or sporules or embryonal forms, in the same way
in which Rabenhorst saw it in Melosira varians, and O'Meara in
Pleurosigma 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 flagelliforin cilia ; so that
these larger or smaller cysts represented xygo^pores. and some of
them were shown to be zob'zygospores. Castracane had the good
fortune to meet with a number of large and small oval cysts
imbedded in a gelatinous mass, all of them having in the centre t\vc
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 present itself in different forms
and with different peculiarities ; for which reason one ought to
avoid arguing from special cases to general 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 MICROSCOPIC FORMS 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 ;i
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, or the frustule, performs the
function of a sporange. 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,1 among the diatoms
of a marine deposit of the Miocene period, he met with a perfect
frustule of Coscinodiscus punctattis, 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 ha-
also been observed by Comber, Murray, and others.

No one appears at present to have given attention to a circum-
stance described by Castracane 2 in relation to a specimen of Stria ti'lln
unipunctata, 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 alway-
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 granules; thus distinguishing the placochromntic
and the coccochromatic 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 Jlelosirn
varians with its cell-cavity filled with endochrome, not in a condition
of unequal amorphous masses, but of uniform rounded corpiiscles ;
and this demands particular attention, or at least gives good ground
for special research. A diligent examination instituted in these
cases has demonstrated 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 Castracane, he concludes
that this special arrangement of the endochrome must be interpreted
as a prelude to the process of reproduction.

These observations may possibly attracl the attention of some

1 See ' Observations on a Fossil Diatom in relation to the Process of Eeproduc-

,' Mti ili'll' An-iiil. I'lii/f/f. (lei Nuovi Liner/ \\.\\ 17,1885.

2 See 'The Diatoms of ttic Coasts of Istria ami Dalmatia,' Atti delV Accacl,

. i Nuovi Liiicri. April -21 and May 25, 1873.

DIATOMACE^E 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
Raphidea; 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 Pseiidoraphidece ;
while those in which the valves have neither raphe nor its equivalent
are called Gryptoraphidece, 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 Dlatomacew 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 Surirella (fig. 453), the
valves of two free and adjacent frustules separate from each other,
and the two endocl ironies (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 Epithemia (fig. 446, A, B), the
first diatom in which the conjugating process was observed by
Mr. Thwaites,1 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
two 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 undeveloped 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 Histury,'vol. xx. ser. i. 1847, pp. 9, 343 and vol. i.
ser. ii. 1848, p. 161.'

600 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

that are produced by the process of binary division. Hence a very
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.1

This formation of what are termed anxospores — as serving to

augment the size of the

A B IJ 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 ^Mosim
(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 turgida: A, cit.), these also are the
trout view of single frustule ; B, side view of the c ^ • ^ f

same; C, two frustules with their concave surfaces Products of a kind ot
in close apposition; D, front view of one of the conjugation between the
frustules, showing the separation of its valves E, adjacent cells of the Or-
F, side and front views after the formation of the j • j • i •

zygospores. dmary diameter, taking-

place before the comple-
tion of their separation. He describes the eiiclochrome 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, «, I>, c) : around this a ne\v envelope is developed, which mayor
may not resemble that of the ordinary frustules, but which remains
in continuity with them ; and this zygospore soon undergoes binary

See on this subject a valuable paper by Prof. W. Smith ' On the Determination
of Species in the Diatomacece,' in the Quart. Journ. of Microsc. Science, vol. iii.
855, p. 130; a memoir by Prof. W. (ire^orv 'On Shape of Outline as a Specific
('h-u-aeter ot ]>i«f<,i/mceee,' in Traits, of Microsc. Soc. 2nd series, vol. iii. 1855,
p. H>; and the Author's Presidential Address, in the same volume, pp. 44-50; 'On
Na/oicula crassmervis, Frustulia saxonica, and N. rhomboides, as Test-objects,' by
W. H. Dallinger, M,,i,t/,/,/ M „•>•<>. Jowrn. l«7ii, vol. xvii. p. 1; also an Additional
note on the identity of these, by the same Author, ihiil. p. 173.

DIATOMACE.E

60 1

subdivision (No. 3, a, b, 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 reversion 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 Diatomacese are classified by Klebahn under
five different heads, viz. : — (1) Rejuvenescence of a single cell, accom-
panied by an increase in size ; this is the simplest type, and one
of the most common. (2) Two daughter-cells are produced from the
protoplasm of a mother-cell, and from these ,-irise two auxospores
(Achnanthes longipes, Rhabdonema arciiatum}. (3) Two cells lying
side by side cast off 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 creiiulata
Thwaites) : 1, simple filament ; 2, filament developing auxospores ; a, b, c, succes-
sive stages in the formation of auxospores ; auxospore-frustules in successive stages,
ci, b, c, of multiplication.

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 are formed by
the fusion of a daughter-cell from each mother-cell with the
daughter-cell of the other one lying opposite to it ; this is the most
complicated process (Amphora ovalis, Epithemia An/ it.?. l!li<'>/>«-
lodia yibbn, A: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. :dt hough 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 forms, such as the
species of Xartctdd. In those also which are stalked it has been
noticed that if, from any cause, a frustule becomes del ached, it is

6O2 MICROSCOPIC FORMS 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 genera are 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-
nenia, * have been seen to escape from it, and to be prevented from
returning again to it in company with the sister Navlculce. Hence
the genera S<-lii-.i>iu'in«. Jierkc lei/a, and Dickiea must be reunited to
\nricida,; Coccomma, Endonema, and Colletonema to Cymbella ; and
llnnifniiiid'ta to Xitzsch'w. The singular phenomenon of movement
which may be observed in many genera of diatoms — among which
the most singular is that presented by Bacillaria paradoxa (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
-tr.-iight line so far as they meet with no impediment, while the
intervention of obstacles determines a passive change of direction.
The back wa I'd and forward movements of the Navicidce have been
already described ; in Surirella (fig. 453) and Campylodiscus
(fig. 454) the motion never proceeds further than a languid roll from
one side to the other ; and in Gomphonema (fig. 463), in which a
foramen fulfilling the nutritive office 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 authors to the action
of endosrnose 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 result ing 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 must determine a continual change of position
li;icU\\.-irds :md forwards, through the reaction exercised on the
delicate floating frustules.

The principles upon which this interesting group should be classi-
fied cannot be properly determined until the history of the genera-
tive process — of which nothing whatever is yet known in a large
proportion of diatoms, and but little in any of them — shall have
been thoroughly followed out. The observations of Focke2 render it

1 See Castracane, ' Observations on the Genera Homeocladia and ScJtizonema,'
in Aft/ dell' Accad. 1'mi/if. </,•; Nuovi Lmcei, May 23, 1880.

2 According to this observer (Ann. of \«l. IHxf. 2nd series, vol. xv. 1855, p. 237)
Navicula bifrons forms, by the spontaneous fission of its internal substance, spherical
bodies, which, like geminules, give rise to Surirellet microcorct. These by conjuga-

DIATOMACEiE

603

highly probable that many of the forms at present considered as dis-
tinct from each other would prove to be but different states of the
same if their whole history were ascertained. On the other hand,
it is by no means impossible that some which appear to be nearly
related in the structure of their frustules and in their mode of
growth may prove 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 Diatomaceos, 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.1 In each family the frustules may exist under
four conditions : — («.) free, the binary division being entire, so that the
frustules separate as soon as the process has been completed ; (b)
stipitate, the frustules being implanted upon a common stem (fig.

FK;. 448, — Meridian circiilare.

FIG. 449. — Bacillaria 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. lifrons by the same process. He is
only able to speak positively, however, as to the production of N. hi/mils from N.
splendida ; that of Surirella mirrucora from N. bifron-s, and that of N. splendida
from SurireUa inicrocora, 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 VEGETABLE LIFE — THALLOPHYTES

is that of Eunotiece, of which we have already seen a characteristic
example in Epiihemia turgida (fig. 446). The essential characters
of this family consist in the more or less lunate form of the frustules
in the lateral view (fig. 446, B), and in the strife being continuous
across the valves without any interruption by a longitudinal line.
In the genus Eimotia the frustules are free ; in Epithemia they are
very commonly adherent by the flat or concave surface of the con-
necting zone ; and in Hlmantidium they are usually united into
ribbon-like filaments. In the family JSt&ridiece we find a similar
union of the transversely striated individual frustules ; 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 curved instead of
straight, as in the beauti-
ful Meridian circtdare (fig.
448). Although this plant,
when gathered and placed
under the microscope, pre-
sents the appeai-a nee 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
• lays 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-
mentous body covered in this way Is often very elegant,' The frus-
tules of Meridian are attached when young to a gelatinous cushion ;
but this disappears with the advance of age. In the family Licmo-
frfiorvce iilso the frustules are \\ edge-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 A/V/m/////,//-^, instead of itself becoming double

FIG. 450. — Licmojiliuni JI<iln'lUit« .

DIATOMACE^E

605

with each act of binary division of the frustule, increases in breadth,
while the frustules themselves remain coherent, so that a beautiful
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 flabella or fan upon the
summit of the branches, with imperfect flabella1 or single frustules
irregularlv scattered throughout the entire length of the footstalk.
This beautiful plant is marine, and is attached to seaweeds and
zoophytes.

In the next family, that of Fragilariete, 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 stria1
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

FIG. 452.

turned sideways (a), are

Put. 451.

FIG. 451. — Diatoma vulgare: a, side view of

frustule; b, frustule undergoing division,
seen, to be strongly marked FIG. 452. — Grammatophoraserpentina: a, front

by transverse stria3, which tiustuie , o, o, front

. IP • iincl euci views ot divided frustule ; c. frustule

extend into the front View. about to undergo division; d, frustule com-
The proportion between the pletely divided,
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

606 MICROSCOPIC POEMS OF VEGETABLE LIFE— XHALLOPHYTES

next by the genus Nitzschia, 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. siymoidea, 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 named from the elongated staff-like form of
its frustules ; its valves have a longitudinal punctated keel, ;md
their transverse stria3 are interrupted in the median line. The
principal species of this genus is the B. par<«loxay 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
( >f 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. Halts

with some other genera
which agree with them
in the bacillar or staff-
like form of the frus-
tules an din the presence
of a longitudinal keel,
in the sub-family Aite-
schiece, which ranks as
a section of the A'///-/-
rellece. Another sub-
family, SynedrecB, con-
sists of the genus
Synedra and its allies,
in which the bacillar
form is retained, but
the keel is wanting,
and the valves are
but little broader than the front of the frustule.

In the /Snrirellece 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
Xnr!rell(t, in addition to the presence 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 ' ala?.'
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.
>ueli as that of the Mourne Mountains in Ireland (fig. 468, b, c. /•).
In I he 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 peculiar twist or saddle-
shaped curvature (B). It is, in this genus that the supposed ' cana-

Fic. 453. — SurireUa constricta : A, side view ;
B, front view ; C, binary subdivision.

DIATOM ACE.*:

607

liculi ' are most developed, and it is consequently here that they mav
be best studied ; and of there being here really costce, or internally
projecting ribs, no reasonable doubt can remain after examination
of them under the binocular microscope, especially with the ' black -
ground ' illumination. The form of the valves in most of the spec-it -s
is circular or nearly so ; some are nearly flat, 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 costse on the
connecting portions of the frustules, these costse 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

FKJ. 454. — Cainpi/lodisc-us 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 indcjin //<•
in number. In the curious Grammatophora serpentina (fig. 4f>2)
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
inatophora .are very finely striated, and some species, as (/'. subtilissima
and (T. »iarina, are used as test-objects. The frustules in most of
the genera of this family separate into zigzag chains, as in Dtatoma ;
but in a few instances they cohere into a filament, and still more
rarely they are furnished with a stipe. The small family 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 FORMS OF VEGETABLE LIFE— THALLOPHYTES

frustules, which appear in the front view as in Biddulphiece. The
typical form of this family is the Terpsinoe musica, 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 Coscinodiscece are always free, whilst
those of the Jfelosirece 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 frustules are always found detached
(fig. 468, 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 arc
often marked with radiating strife (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 Coscinodiscece 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 Coscinodiscus, 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, ft) especially, the
infusorial earth of Richmond in Virginia, of Bermuda, 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 .1/rAw/v (tig. 444, B). The regularity of the
hexagonal areolation shown by its valves renders them beautiful
microscopic objects ; in some species the areolse are smallest near
the centre, and gradually increase in size towards the margin; in

DIATOMACE^: 609

others a few of the central areolpe are the largest, and the rest are
of nearly uniform size ; while in others, again, there are radiating
lines formed by areolse of a size different from the rest. Most of
the species are either marine or are inhabitants of brackish water ;
when living they are most commonly found adherent 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. Stephenson1 on Coscinodiscus oculus iridis
show that the peculiar ' eye-like ' appearance in the centre of each
of its hexagonal areolae arises from the intermingling of the mark-
ings of two distinct layers, differing 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 carbon.

FIG. 455. — Structure of siliceous valve of Coscinodiscus oculus iridis: 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
phosphorus in bisulphide of carbon, which has a still higher refrac-
tive index. If we suppose a diatom to be marked with com-i- •,<•
depressions, they wouLd act as concave lenses in air. which is le>>
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 nmst 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 oculus 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, arid the framework of its hexagons

O

more readily exhibits its beaded appearance. The lower layer is
nearly transparent, and but little conspicuous when seen in 1 tisulphide
of carbon, except as shown in the figure, when the framework of

1 Montlilij Microscopical Journal, vol. x. 1873, p. ].

R R

6lO MICROSCOPIC FORMS 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 observations
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 upper 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 pro-
truded through them.

The genus Actlnocjjclus l closely resembles the preceding 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 bv
Professor Ehrenberg on minute differences presented by the rays as
to number and distribution ; but since scarcely two specimens c.-in
be found in which there 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 Asterolampru and AsteromphcdubS,
both of which have circular discs of which the marginal portion is
minutely areolated, whilst the central area is smooth and perfectly
hyaline in appearance, but is divided by lines into radial compart-
ments which extend from the central umbilicus towards the periphery.
The difference between them simply consists in this, that in Ast&ro-
lampra all the compartments are similar and equidistant and the rays
equal, whilst in Aster omplial us (PI. 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 ximbilicus.
The eccentricity thus produced in the other rays has been made the
basis of another generic designation, SpatangidiiJ/ni ; but it may be
doubted whether this is founded on a valid distinction.2 These
beautiful discs are for the most part obtainable from guano, and
from soundings in Ir.ipical and antarctic seas. From these we pass
on to the genus Actinoptychus (fig. 456), of which 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 (1$), giving to the
side view (A) the appearance 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

1 The Aiitlinr rnnrtirs with Mr. Ralfs in thinking it pivt'eralile to limit the genus
Ictinocyclus to the forms originally included in it by Kln-eulierj,', and to restore the
f^enus Actinoptychus oi Khivnlier;.', \vliich had lieeii improperly united \viti\Actino-
<7/<7//\ liv Professors Kilt/ing and W. Smith.

- Sec CJreville in <thmr/. -lui/ni. Microsr. Sto'euce, vol. vii. 1859, p. 158; and in
'I'miix. .1/Yovi.sr. Soe. vol. viii. n.s. IftliO, p. lll-J. nnd vol. \. isil-j, p. 41 ; also Wallieh
in the same Transactions, \ol. \iii. iNtill. p. 44.

DIATOIMACEJK

difference of aspect which the different radial 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 Gran
(fig. 467, b, b, &), 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 l>v
its marginal spines, has received from Professor Ehrenberg the dis
tinctive appellation of Ileliopelta (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 are tla1 ; but in the
five which alternate with them
a minute beaded structure is
observable. This may 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 areolatibn is here displayed; and it hence appears pro-
bable that this marking belongs to the internal layer,1 and that the
circular areolation exists in the outer 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 triangles; it is not,
however, nearly 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 areolations 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 respects 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 undulatns.
A, side view ; B, front view.

1 It is stated by Air. Stodder {Quart. Joiirn. Micrnsr. Science, vol. iii. n.s. 1863,
p. 215) that not only lias lie 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.

H E 2

6l2 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLO PHY TES

that are precisely alike. It seems probable, then, that we must
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 Arachnoidiscns (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,1 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 j- or 1-inch objective; or by looking at a
valve as an opaque object (either by the parabolic illuminator,
or by the Lieberkiilm, or by a side light) with a ^-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
Uupodiscece, the members of which agree with the Coscinodisccc -in
the general character of their discoid frustules, and with the ]>i<1-
ilnlphiecu in having areolar processes on their lateral surfaces. In
the beautiful A ulacodi.se H-S these areolations are situated near the
margin, and ai-e connected with bands radiating from the centre ; the
surface also is frequently inflated in a manner that reminds us of
A.ctinoptychus. These forms are for the most part obtained from

guano.

The members of the next family, BiddulpMece, 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 /liddtif />/*!<( (fig. 445) the frustules
have a quadrilateral form, and remain coherent by their alternate
angles (which are elongated into tooth like projections), so as to form
a /ig/.ag chain. They a re marked externally by ribbings which seem
to lie indicative of internal costit- partially subdividing the cavity.
Nearly allied to this is the beautiful genus Tsthmia (fig. 457), in

. M ;<•!•< mi-. Sin-. 1st series, vol. iii. p. 40.

DIATOMACE^

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 b, fig. 457), until they have separated from each other,
but, after such separation, remains for a time round one of the
frustules, 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 Coschwdiscece
and Eupodiscece. Of this family we have
a characteristic example in the genus
Triceratiwm, of which striking form a con-
siderable number of species are met with
in the Berimida 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 011 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 Hippopus and
JIaliotis, 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 triangular 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 quadrantjular \\\\<\ even pentagonal
forms, these being marked as varieties by their exact correspondence
in sculpture, colour, Arc., with the normal triangular forms.1 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 difficult, in fact, to distinguish the
square forms of Friceratium from those included in the genus

1 See Mr. Brightwell's excellent memoirs 'On the genus Triceratium' in
Quart. Jonrn. Microsc. Science, vol. i. 185!:!, p. 245 ; vol. iv. 1856, p. 272 ; vol. vi.
1858, p. 153; also Wallicli in the same Journal, vol. iv. 1858, p. 242 ; and Greville in
Trans. Microsc. Soc. n.s. vol. ix. 1861, pp. 43, 69.

Fi<;. 457. — Isthmia nervosa.

6 14 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

AmpMtetras, which is chiefly characterised by the cubiform shape of
its frustules. In the latter the frustules 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 Biddulphiece is the curious
assemblage of forms brought together in the family C'hcetocerece, some
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-
truin (fig. 459) there are sometimes as many as twelve of these awns,
radiating from each frustule like the spokes of a wheel, and in some

instances regularly bifurcating.
With this group is associated the
genus Rhlzosolenia, of which several
species are distinguished by the ex-
traordinary length of the frustule
(which may be from six to twenty

FIG. 458. — Cltfetoceros WigJi ani/i: ft, front
view, and I, side view of frustule ; c, side
view of connecting hoop and awns ;
(1, entire filament.

FIG. 459. — Bacteriastrum
furcatum

times its breadth), giving it the aspect of a filament (fig. 460),
by a transverse aiinulatiou 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, Salpa>,
Holothuria?, and other marine animals.1

The second principal division (B) of the Diatomace<e consists, it
will be remembered, of those in which the frustules have a median
longitudinal line and ;i central nodule. In the first of the families
which it includes, that of Cocconeidea', the central nodule is obscure
or altogether wanting on one of the valves, which is distinguished as

1 See Bri-jhtwfll in Quart. Jonni. .V/Vmsv. Science, vol. iv. 1856, p. 105; vol. vi.
1858, p. 03 ; Wullic-h in Tnn/s. Mil-rose. Soc. n.s. vol. viii. 1860, p. 48 ; and West, in
the same, p. 151.

DIATOMACE/E

6I5

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 other. The frustules 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,

\

FIG. 460.— Rhizo- FIG. 4Gl.—AcJmanthes

solenia imbri- longipes: a, b, c, d, e,

'•rtta. frustules in different

stages of binary divi-

FIG. 462. — Gomphonemti gemi-
natum: its frustules connected by
a dichotomous stipe.

S1O11.

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 longipes
(fig. 461), which is often found growing on marine alga?. The
difference between the markings of the upper and lower valves is
here distinctly seen ; for, while both are traversed by stria?, which
are resolvable under a sufficient power into rows of dots, as well as

B

6l6 MICROSCOPIC FORMS OF VEGETABLE LIFE- THALLOPHYTES

by a longitudinal line which sometimes has a nodule at each end
(as in Navicida}, the lower valve (a) has also a transverse line form-
ing a sta/iros, 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, «, which
are not yet completely separated the one from the other, but it may
be observed to invest the two frustules b and c, which have not
merely separated, but are themselves beginning to undergo binarv
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 Cymbellece, 011 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-
ginal than central, as we
see in (Jocconema (fio-
468, /).

The Gomphonemecv,
like the Meridiem and
Licmpphorece, have frus-
tules which are cuneale
or wedge-shaped in their
front view (figs. 462, 463),
but are distinguished
from those forms by the
presence of the longi-

tu(h'nal h'^ ™d central
nodule. Although there
are some free forms ill
this family, the greater

part of them, included in the genus Gomphonema, have their frus-
tules either affixed at their bases or attached to a stipe. This stipe
seems 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 carry
apart the peduncles which support them as far as their divergence
• -.•in take place. It is in this manner that the dichotomous character
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 Diatomacece.

Lastly, we come to the large family Nai+iculew, the members
of which are distinguished by the symmetry of their frustules, as
\\ell m the lateral as in the front view, and by the presence of a
median longitudinal line and central nodule in both valves. In the

cjeminatnm, more highly
magnified: A, side view of frustule; B, front view;
C, frustule in the act of division.

&

DIATOMACE.K 617

genus Xactcula and its allies the frustules are free or simply
adherent to each other ; while in another large section they are
included within a gelatinous envelope, or are enclosed in a defi-
nite tubular or gelatinous frond. Of the genus Xavicula 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 Piitnu-
laria (which had been previously applied by Ehrenberg to designate
the striated species), on the ground that its stria? (costa?) 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 strife and costa1. Mr. Slack has
since given an account of the resolution of the so-called costa? of
twelve species of Pi/malar ice into ' beaded ' structures.1 The beauti-
ful genus Stnnroneis. 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 free from stria?, which forms a cross
with the longitudinal band. The multitudinous species of the genus
Xaricnla 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 Navicidce and
rinnularicK. The Polierschiefer, or ' polishing slate,' of Bilm 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 present 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 Savicuhe, ike., which have been consolidated by
heat. The species of Pleurosiyma, 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
Stauroneis 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 Schizonemece, consisting of
those Namculece in which the frustules are united by a gelatinous
envelope, some are remarkable for the great external resemblance
they bear to acknowledged alga1. This is especially the case with
the genus Schizonema, 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
1 Monthly Microscopical Journal, vol. vi., 1871, p. 71.

6l8 MICROSCOPIC FORMS OF VEGETABLE LIFE--THALLOPHYTES

genus Mastogloia, which is specially distinguished by having the
annulus furnished with internal cost* projecting into the cavity of
the frustule, 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 vis 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

PIG. 464. — Schizonema 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 our sys-
tematic treatises are loaded, is a pursuit of far greater real value
than the multiplication of species by the detection of such minute
differences as may be presented by forms discovered in newly
explored localities; such differences 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

DIATOMACEJE

619

any department of natural history, the more does it prove that the
range of variation 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

FIG 465.

Fie. 46U.

FIG. 405. — 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 are
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 Diatomacece are stones of
mountain streams or waterfalls, and the shallow pools left by the

620 MICEOSCOPIC FORMS OF VEGETABLE 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 fact,
most ubiquitous, and there is hardly a roadside ditch, water-trough,
or cistern, which will not reward a seai-ch and furnish specimens
of the tribe.' Such is their abundance in some rivers and estuaries
that their multiplication is affirmed by Professor Ehreiiberg to have
exercised an important 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 afforded by the
observations of Sir J. I). Hooker upon the Diatomacew of the southern
seas ; for within the Antarctic Circle they are rendered peculiarlv
conspicuous by becoming enclosed in the newly formed ice, and by

FIG. 467. — Fossil Diatomaceee, &c., from Oran : a, a, a, Coscinodiscus; b, li, !/,
Actiriocyclus ; c, Dictyochya fibula ; <?, Lithastterianix unliatiis', e, Spongolithis
acicularis ;./,/', (rraininutojjhora paraUela (side view); g, g, Grammatophora
angulosa (front view).

being washed up in myriads by the sea on to the ' pack ' and ' bergs,'
everywhere staining the white ice and snow a pale ochreous brown.
A deposit of mud, chiefly consisting of the siliceous valves of Diato-
macece, 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 comnm
ideation between llie ocean waters and the bowels of a volcano, such

DlATOMACEvE 62 1

us there are other reasons for believing to be occasionally formed,
would account for the presence of Uiatomacete 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
region of higher forms of vegetation ; and were it not for them, there
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 which animal
respiration and decomposition would be continually impartingto them.

Fiu. 468. — Fossil Diatomacea;, &c., from Mourne Mountains, Ireland: a, a, «,
GailloneUu (Melosira)procera and G. gran ulata ; d, d, d, G. biseriata (side view i :
It, b, SurireUci plicata; c, S. craticula', A~, S. caledonica; c, Gomphonema
ymcile; /, Cocconema fusidiiint; g, Tabdlaria vitlgaris', Ji, Piiiinilaria
dactylus ', i, P. nuLiJ/y; I, Synedra iilntt.

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
trou i the intestinal canals of the birds of whose accumulated excre-
ment that substance is composed, those birds having received 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 their living forms have long since disappeared ; for the

622 MICROSCOPIC FORMS 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 Diatomacece, 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 Northern Europe, in winch
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
Gran 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 North 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 Diatomacece those general methods are to be
had recourse to which have been already 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 spread over their surface as delicate velvet—
or depositing themselves as a, filmy stratum on the mud, or inter-
mixed with the scum of living or decayed vegetation floating on the
surface of the water. Their colour is usually a yellowish -brown of a
greater or less intensity, varying 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 fibrous tenacity
which distinguishes other plants; when removed from their natural
position, they become distributed through the water, and a re held in
suspension by it, only subsiding after some little time has elapsed.'
Notwithstanding every care, the collected specimens are liable to lie
mixed with much foreign matter: this may be partlv got rid of by

DIATOMACE.1': 623

repeated washings in pure water, and by taking advantage, at the
same time, of the different specific gravities 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 afford 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. Marine forms must be looked
for upon seaweeds, and in the fine mud or sand of soundings or
dred°'ings ; they are frequently found also, in considerable numbers,
in the stomachs of Holothuria?, Ascidians, and Salpse, in those of the
oyster, scallop, whelk, and other testaceous molluscs, in those of the
crab and lobster, and other Crustacea, 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 animal-
that may fall in his way, rare and beautiful forms having been
obtained from the interior of XoctiJucn. The separation 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 earth is
first to be washed several times in pure water, which should be well
stirred, and the sediment then allowed to subside for some hours
before the water is poured off; since, if it be decanted too soon, it
may carry the lighter forms away with it. Some kinds of earth
have so little impurity that one washing suffices ; 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 effect is produced. When hydrochloric- acid ceases
to act. strong nitric acid should be substituted ; and after the first
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 repeated until no further action
takes place. The sediment is then to be washed 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 DiatomacecK, with the addition, perhaps, of sponge-spicules. The
separation 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 jar of water, and then,
while it is still in motion, pouring 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
poured off: and this process may be repeated three or four times at

624 MICROSCOPIC FORMS 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.1 It sometimes happens that
fossilised diatoms are so strongly united to each other bv 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 usual way. If a very
weak alkaline solution does not answer the purpose, a stronger one
may then be tried. This method, devised by Professor Bailey, has
been practised by him with much success in various cases.2

The mode of mounting specimens of Diatomaceca will depend
upon the purpose 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 brought into view, the fresh diatoms
must be boiled in nitric or hydrochloric acid, which must then be
poured off (sufficient 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 s< >a p, whereby the diatoms will be cleansed
from the flocculent matter which they often obstinately retain.3
After a further washing in pure water, they are to be either mounted
in balsam in the ordinary manner, 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. Jonrn. Min-onc. Science, vol. iii. 18r>r>, 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. Jonrn. of
Microsc. Science, vol. vii. 1859, p. 107, and vol. i. n.s. 1801, p 14:!; and Trans, of
Miri-oKc. iS'ur. vol. xi. 11. s. 18(>3, p. 4. A little book entitled Practical Directions
far Collecting, Preserving, Transport in;/, I'rr/>/</-//i/t,<in</ 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.

3 See Prof. H. L. Smith in Aincr, Joiini. of M/crosroji//, vol. v. ISfSl), p. 'J.r>7.
It, is important that the soap should be free from kaolin, silex, or any other insoluble
matter.

DIATOMACEJE ; PHJiOSPOKE.E 625

ments which have recently been introduced 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 for working, to a far higher point than
we have touched at present, the true structure of such diatoms ,-is
can be made amenable to the powers possessed by our best recent
optical appliances ; and for the leisure of a professional or commercial
man we 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 aggregate ; but, on
account of their minuteness, they cannot be selected and removed
by the usual means. The larger forms, which maybe readily distin-
guished under a simple microscope, may be taken up by a earners-
hair pencil which has been so trimmed as to leave two or three hairs
projecting beyond the rest. But the smaller can only be dealt with
by a single fine bristle or stout sable-hair, which 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.) When the split
extremity of such a hair touches the glass slide, its pails 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 deposited upon a slide damped by the breath. For
the selection and transference of diatoms under the compound
microscope, recourse may be had to some of the forms of ' mechanical
linger' which have been devised by American diatomists.1

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 Melaiaosporece, Rhodosporece,
and Chlorosporece, according as their colouring matter is olive-brown,
red. or green, cannot altogether be retained. Under the head of
fhoeosporecB 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 organs, those set apart for repro-
duction being in the first place separated from those appropriated

1 For a description of those of Prof. Hamilton Smith and Dr. Rezner, see Journ.
of Roy. Microsc. Soc. vol. ii. 1879, p. 951, and that of Mr. Veecler, 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 classification 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 MICROSCOPIC FORMS OF VEGETABLE LIFE- THALLOPH YTES

to nutrition ; while the principal parts of the nutritive apparatus,
which are at first so blended into a uniform expansion or thallus
that no real distinction exists between root, stem, and leaf, are
progressively evolved on types more and more peculiar to each
respectively, and have their functions more and more limited to
themselves alone. Hence we find a ' differentiation,' 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 specialised or limited to particular actions.
But this takes place by very slow gradations, a change of external
form often showing itself before there is any decided differentiation
either in structure or function. Thus in the simple Ulvacece, what-
ever may be the extent of the thallus, every part has exactly the
same structure, and performs the same actions, as every other part,
living for and by itself alone. And though, when we pass to the
higher seaweeds, such as the common Fucus and Lammaria, 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 structure,
the only modification being that the several layers of cells, where
many exist, are of different 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 fi'onds. The cells of the Phceosporece contain a substance
closely resembling starch, and an olive-brown, pigment, which they
share with the Fucacece, known asphyco-phcein QIC fuco-ocanihin. Tin-
group of olive-green seaweeds presents us with the lowest type in
the family Ectoccurpacece, which, notwithstanding, contains some of the
most elegant structures that are anywhere to be found in the group,
the full beauty of which can only be discerned by the microscope.
Such is the case, for example, with Sphacelaria, a small and delicate
seaweed, which is very commonly found growing upon larger algae,
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 peculiar buds or
geinmse known as propagules.

The ordinary mode of propagation of the Pha'osporece is by non-
sexual zoospores ; and these are of two kinds, produced respectively in
unilocular and nrultilocular zoosporanges. The former are compara-
tively large, nearly spherical, ovoid or pear-shaped cells, the contents
of which break up into a large number of zoospores. The multi-
locular zoosporanges have the appearance of jointed hairs, and are
divided internally into a number of chambers, each of which gives
birth to a single zoiispore. The /ofispores from the unilocular

PH.EOSPOKEJ5 ; FUCACE.E

627

sporanges appeal- in all cases to germinate directly, while those from
the multilocular sporanges sometimes coalesce in pairs before ger-
minating. The different families of Phceosporece present a most
interesting gradual transition from the conjugation of swarm-cells
to the impregnation of a female ' ob'sphere ' by male antherozoids.
In Eftocarpus, Giraudia, and Seytosiphon, 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 rest and partially losing its cilia before conjugation
takes place (fig. 469, II). Male sexual organs also occur in the
Sphacelceriacece, but no actual process of conjugation has as yet been
observed. In Cutleria and Zanur-
dinia the differentiation is more
complete. The male and female
swarm-cells are produced either on
the same or on different individuals ;
the latter are much larger 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
' ob'spheres,' being from the first
motionless masses of protoplasm not
provided with cilia, while the an-
therozoids exhibit motility only for
a very short time, and each is pro-
vided only with a single cilium of
unusual length. In the family
Laminariacece, belonging to the
Pkceosporece, are included many of
the largest of the seaweeds, chiefly
natives of southern seas, the frond
often attaining enormous dimen-
sions, and exhibiting rudimentary
differentiation into rhizoids or
organs of attachment, stem, and
leaves. Such are Lessouia, which
grows to a great height and re-
sembles a branching tree with pendent leaves two or three feet
long ; Macroci/stls, where the stalk like base of each branch of the
leaf is hollowed out into a large pear-shaped air-bladder ; N&reocystis,
Laminaria, and others.

In the Fucaceae 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 FUCKS plati/carpus,
the same conceptacles contain both ' antherids ' and ' oogones ; ' in
others these two sexual elements are disposed in different conceptacles
on the same plant ; whilst in the commonest of all, F. vesicnlosus
(bladder- wrack), they are limited to different individuals. When a

s s 2

FKI. 469. — Process of conjugation
in Ectocarpus siliculosiis. (From
Vines's ' Physiology.') I. a-f, the
female zoiispore coming to rest ;
II., the female zoijspore at rest,
surrounded by male zoiispores ;
III. a—c, fusion of male and female
zoiJspores.

628 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

section is made through one of the flattened conceptacles of F.
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 anther ids, 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, imdulatory
motion whereby these
antherozoids are diffused
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 arc
the ooyones, or parent-
cells of the oospheres.
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 fertilisation takes
place. This act consists
in the swarming 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-

Ki<;. 470.— Vertical section of conceptacle of Fucus aphrodite Fuel this takes

l>l<iiyca-rpits\\ne& with filaments, among which lie inpp withf flip

the antheridial cells and the oogones containing l)ltlC

oospheres. 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
rupture so as to give exit to the antherozoids ; and the oiigones also
discharge their oospheres, which, meeting with antherozoids, are
fecundated by them. The fertilised oospores soon acquire a new and
Jinn envelope; and, under favourable circumstances, they speedily
to develop themselves into new plants. The first change is

FUCACEJE

629

the projection and narrowing of one end into a kind of foot-stalk,
by which the oospore attaches itself, its form passing from the
globular to the pear-shaped; a partition is speedily' observable in its
interior, its single cell being subdivided into two ; and by a con-
tinuation of a like process of bipartition, first a filament and then
a frondose expansion is produced, which gradually evolves it.sclt'
into the likeness of the parent plant.

The whole of this process may be watched without difficulty 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 Fucus platycarpus : 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 autheridial cells.

antherozoids, set free from the orange-yellow (male) conceptaclo.
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 subsequent processes of germi-
nation may be watched by means of the ' growing slide.' l The
winter months, from December to March, are the most favourable
for the observation of these phenomena ; but where Fuel abound,
some individuals will usually be found in fructification at almost
any period of the year. This process of fertilisation usually takes
place on fronds 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 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

other species of Fucus, imbedded in the frond, and the ' berries ' of
Sfirgassum bacciferum, 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 group with the
lower algre, especially with the family Coleochcetacece ; such delicate
feathery or leaf-like fronds belong for the most part to the family
Ceramiacea'., some members of which are found upon every part of

Pig. 472. — Arrangement of tetraspores in Carpocaidon mediterraneioii : 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 happen to be so placed
as to be seen at the same view.)

ova- coasts, attached either to rocks or stones or to larger alga>, 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 algae. Hence,
in growing them artificially in aquaria, it is requisite to protect
t hem from an excess of light, since otherwise they become unhealthy.
Various species of the genera Ceraintmn, Griffithsia, CalUthammon,
.n id Ptllota are extremely beautiful objects for low powers when
mounted in glycerin jelly. In many of them the phenomenon to
which we have previously referred under the name of ' continuity of
protoplasm' is very beautifully exhibited. The colour of the red

FLORIDE.E 631

seaweeds is due to the presence of a pigment known as rkodospermin
or phyco-erythrin, soluble in fresh watei', which may be separated in
the form of beautiful regular crystals.

The only mode of propagation which was until recently 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 general substance of the frond, but sometimes
congregate in particular parts or are restricted to special branches.
If the second binary division takes place in the same direction as
the first, the tetraspores are arranged in linear series ; but if its

JF

SJ) Q

FIG. 473. — Nemalion multifidum : I, a branch with a carpogone, c, and pollinoids,
xj> ; II, III, commencement of the formation of the fructification; IV, V, develop-
ment of the spore-cluster ; t denotes the trichogyne, c the carpogone and fructifica-
tion. (From Goebel's 'Outline of Classification.' The Clarendon Press.)

direction is transverse to that of the first, the four spores 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 yonids, analogous rather to the buds
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 tetraspores.
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 MICROSCOPIC FORMS OF VEGETABLE LIFE— THALLOPHYTES

antherozoids, but minute rounded particles, known as pollinoids or
' spermatia,' having no power of spontaneous movement. Some-
times on the same individuals as the antherids, and sometimes on
different ones, are produced the female organs, which curiously
prefigure the pistil in flowering plants. This organ is known as
the procarp, and consists, in its simplest form, 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 carpogone, and one or more sterile cells
which make up the trichopkore, and convey the fertilising substance'
from the trichogyne to the carpogone. Fertilisation is effected 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 pas>»->
down its tube to the trichophore, and thence to the carpogone ; one
of the cells of the carpogone contains the ob'sphere, which, after
fertilisation, breaks up into a number of carpospores ; round these
is frequently formed a hard investment, and this structure is then
known as a cystocarp ; from it the carpospores ultimately csctipc.
and then germinate. In. the true Corallines, which are Floride«-
whose tissue is consolidated by calcareous deposit, not only the
tetraspores, but also both kinds of sexual organ, are produced in
cavities or conceptacles, 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 thi.-.
and consists of two distinct stages. First the trichogyne i* 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 011 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 Ploridese,
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 011 our coasts, there is no branch of
microscopical observation which is more likely to reward the young
investigator with new discoveries.

63;

CHAPTER IX

FUNGI

FUNGI, us already mentioned, differ essentially from algae 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 hypJm .
slender filaments containing protoplasm and a nucleus (except possihlv
in some of the most simple forms), but no chlorophyll and rarely anv
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
fa injus-celhdose. These hyphse maybe 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 hypha?, lying
side by side, constituting a so-called pseiido-pwenchyme, but never a
true tissue. In some families the liyphte 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, ore
very various. Among the latter the most common are by non-
motile spores or gonids, and by zouspores. 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 sporange, or
are detached from the extremity of hypha? by a process of pinching
off or abstraction. From, their extreme lightness they are wafted
through the air in enormous numbers, and thus bring about the
extraordinarily rapid spread of many fungi, such as moulds. The
zoospores are, like those of the lower alga*, 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 FUNGI

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 large number of fungi
no process of sexual reproduction is known. The various 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 forms, which bear no
resemblance to one another, and were long supposed 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 groups. 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
1 >y 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 JJthaliitm septic-urn, 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 spore is a spherical cell (C) enclosed in a
delicate membranous wall ; and when it falls into water this wall
undergoes rupture (I)), 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 atone end, whence a \o\\gflayellmn is put forth, the
lashing action of which gives motion to the body, which may now be
termed a swarm-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 Amoeba, and feeds like that rhizopod upon solid
particles which it engulfs within its soft protoplasm. These swarm-
cells may multiply by bipartition to an indefinite extent ; but after
a time ' conjugation ' takes place between two of these myxamoebce
(H), their substance undergoing a complete fusion into one body (I).

1 [The classification of fungi here adopted is essentially that of De Bary in his
< '/iii/f/ai-atire Morjiholvf/i/ and Bioloijij of the Fungi, Myceiozoa, 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

from which extensions are put forth (J) ; and by the union of a
number of these bodies are produced the motile protoplasmic bodies
known as plasmodes, the ordinary form in which these singular bodies
are known. These continue to grow by the ingestioii and assimila-
tion of the solid nutriment which they take into their substance ;
and, by the ramification and inosculation of these extensions, a
complete network is formed.

The filaments of this network exhibit active uhdulatory move-

FIG. 474. — Development of Myxomycetes : A, plasinode of Didymium scrpitla; B,
successive stages, a, a', b, of sporanges of Arcyria flaua ; C, ripe spore of
Plujsaruni albinn; D, its contents escaping; E, F, G, the swarm-spore first
becoming flagellated, and then amoeboid ; H, conjugation of two amceboids, which,
at I, have fussd together, and, at J,are beginning to put out extensions and ingest
nutriment, of which two pellets are seen in its interior.

merits, which in the larger ones are visible under an ordinary lens,
or even to the naked eye, but which it requires 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. Here and there offshoots of
the protoplasm are projected, and again withdrawn, in the manner of
the pseudopodes of an Amoeba ; while the whole organism, may be
occasionally seen to abandon the support over which it had grown,

636 FUNGI

and to creep over neighbouring surfaces, thus far resembling in all
respects a colossal ramified Amoeba. The plasmodes are often found
to have taken up into them and enclosed a great variety of foreign
bodies, such as the spores of fungi, parts of plants, &c. They are
curiously 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 sclerotes. In a few genera the spores are not contained in
sporanges, but are borne on external supports or sporophores. But
in the great majority of genera the plasmode becomes ultimately
transformed" into sporanges (B, a, a', b) ; 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 inmost genera tubes or threads <>t'
different forms occur among the spores, and constitute the capittitium.
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 through 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 affinity in some respects to the Myxomycetes, and even to the
infusorial animalcules. Their ordinary mode of propagation is by
zoospores bearing one or two cilia, which either germinate directly
or conjugate to produce a restiiig-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 Ustilagineae 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 reproduce 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
generations ; forms previously considered to belong to widely separated
groups being now known to be stages in the cycle of development of
1 he same species. A striking instance of this is furnished by the
well-known and very destructive disease of wheat and other grasses

UKEDINE.E

637

known as 'mildew,' produced by the attacks of the parasitic fungu.s
Pucdnia graminis. It was long ago observed that wheat WMS
especially liable to this disease in the vicinity of barberry bushes ;
and it is now known that a fungus parasitic on barberry leaves, for-
merly known as JEcidium berberidis, is the ' secidiospore ' generation
of the same species of which Puctinia graminis is the ' teleutospoiv '

X

FIG. 475. — Pucdnia gra/ninis. From De Bary's ' Comparative Morphology and
Biology of the Fungi.' (The Clarendon Press.) A, portion of leaf of Berber/*
with young ajcidium; I., section through leaf containing a?cidia : «/», spermo-
gones; a, aacidia opened ; p, peridium ; II., group of ripe teleutospores
bursting through the epiderm e in leaf of Tritic.um i-t'j>e»n ; /, teleutospores;
III., teleutospores t, and uredospores nr; I. slightly magnified ; II. x 190;
III. x 390.

generation. The complete cycle of development of the best known
UredinecB, such as the mildew (fig. 475), is this. The form known as
Pucdnia graminis produces teleutospores, thick-walled spores, borne
usually in pairs, at the extremity of elongated cells known as basids
or stemgmata. Each of these teleutospores gives rise, on germinating
within the tissue of the grass, to a hypha or ppomycele, the terminal
cells of which develop, on slender basids. each a -single spore or

638

FUNGI

sporid. These sporids will germinate only on the leaves of the bar-
berry, where they produce, first of all, a mass of interwoven hyphte
within the tissue, and then the peculiar reproductive bodies known
as ceciclia (fig. 476). The 'a?cidium ' 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 ceddiospores, 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 sjjermogones, smaller spherical or flask-shaped receptacles,
which also eventually break through the epiderm, and are filled with
barren hyphse known as pa/raphyses. Among these are other shorter
hyphse or ' sterigmata,' from the extremities of which are abstricted
narrow ellipsoidal cells, the spermatia. The purpose of these is
unknown ; but they may be male elements which have lost their
function. The secidiospores will germinate only on the leaves and
steins of grasses, either producing the teleutospore-form directly, or

B

ItVJ R^gr^ '-.->•> V>4 1 tf n >(/,.' i

fem

fMf/' i

SCv^XV^j^sb^r-. f .•:< ' ' • ,

FIG. 476. — Mcidimn tussilaginis: A, portion of the plant, magnified ; B, section
of one of the ' tecidia ' with its spores.

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 very 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 Peronosporeae (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 hyphae, are produced the sexual organs, oogones and
anthends. Fertilisation is not effected by means of motile anthero-
xoids, as in other classes of fungi and of algje, but the antherid puts
out a cylindrical or conical tube-like process, the f&rtilisation-tiibe.
The antliri-ids and oiigones are each single enlarged 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

PERONOSPORE.E

639

the oiigone, and discharges into the latter the contents of the
antherid, thus causing its protoplasmic contents or ' ocisphere ' to
develop into the impregnated ' ob'spore.' The further history of the
<>i (spore is singularly different, even in different species of the same
genus. In some it germinates directly into a new mycele ; in others
it breaks 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

B

FIG. 477. — A-G, Cystopus candidus: H, Phytophthora mfestans. A, branch of
mycele growing at the apex, t, with haustoria, It, between the cells of the pith of
Lepidium sativum; B, branch of mycele bearing gonids ; C, D, E, formation of
swarm-spores from gonids ; F, swarm-spores germinating ; G, swarm-spores
germinating on a stomate and piercing the epiderm 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-
sporea? also produce non-sexual spores or <j<>ni<ls, which are borne 011
special branches springing erect from the mycele, the sporophwes or
gonidiophores. A similar difference is exhibited in the further
development of these spores. Either they germinate directly in water
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

germinate

species which are parasitic on living plants, such as Phytophihora
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 weather,

the germinating tube passes through a
stomate, and the mycele is developed with
great rapidity within the tissue 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 germinate on the sur-
face of the plant, sending out processes
which penetrate to its interior, though
otherwise germinating only on cut sur-
faces.

The Saprolegniese are saprophytic or
parasitic fungi, nearly allied to the Pero-
nosporr-cK, and differing from them chiefly
in two points : although organs are
known in many species closely resem-
bling the antlicrids of the Peronosporece,
the act of impregnation has not actually
been observed, the ocispore being, at least
in many cases, apparently produced par-
ihenogeneticallt/, i.e. without impregna-
tion. In some species a single oijspore is
produced within each oiigone ; but more
often the contents of the latter break up
into a number of ob'spores, each of which
gives rise to a mycele, or breaks up into
zoospores. In some genera, e.g. Achlya
(tig. 478), zoospores are also produced in
very large numbers by the breaking-up
of the contents of zoosporanges, special
enlarged cells of the mycele. The well-
known salmon disease is caused by the
attacks of the parasitic Saprolegnia f&rax
on the living flesh of the animal.

The Mucorini are filamentous fungi,
resembling the two last orders in their
vegetative development, but differing in

t heir mode of reproduction. To this family belong some of the most
(•(million moulds which make their appearance on damp or decaying
organic substances. The ordinary mode of i ion -sexual reproduction
is by endogenous spores, produced within a sporange (tig. 479, A).
The sporanges are borne a1 the ends of sporangiophores, long, erect,
imseplated liypha-, springing directly from the mycele or from
tlic original germinating filament. Several other kinds of 11011-

FIG. 478. — Two zoosporanges of
Achlya. From Goebel's ' Out-
Hues of Classification and
Special Morphology.' A, still
closed ; B, open to discharge
the zoospores; a, zoospores
ejected, but still resting ; c,
zoospores which have left
their membrane at b behind
them. Magn. about 800.

MUCOBINI 641

sexual spores occur in the family, including chlamydospores, repro-
ductive cells formed within the ordinary cells of the hyplue. Sexual
reproduction takes place by means of zyyospores (C), but is at
present known only in a few species. Either from ordinary hyphze
or from sporangiophores spring a pair of short branches, the
extremities of which become firmly attached to one another. These

FIG. 479. — B, mycele (three days old) of Pliycomyces ititens, grown in a drop of
mucilage with a decoction of plums ; the finest ramifications are omitted ; g, the
conidiophore of JJ«co>' muceclo in optical longitudinal section; C, a germinating
zygospore of Mucor muceclo ; the gerrn-tube, k, puts out a lateral conidiophore, <y.
In D are conjugating branches, b b, the extremities of which, art, though they have
not yet coalesced, are already cut off 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 nutrient material. A larger or
smaller piece i.s then cut off from each of them by a transverse
wall ; the median cell- wall which separates them disappears, and the
two terminal portions thus cut off coalesce to form the zygospore,

T T

642 FUNGI

which often swells to a considerable size, and its outer coat be-
comes frequently beautifully covered with warts or other protu-
berances. After a period of rest the zygospore germinates, its
inner coat of cellulose bursting through the outer warty and
cuticularised epispore, and developing into the first germinating
filament.

Very nearly allied to the Mucoriiii are the Entomophthorese,
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
motionless 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 breathing-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
Empusa. 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 part 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 afford 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 hyplue. In only a comparatively few species is a
sexual mode of reproduction known ; the special character of the
group is the non-sexual reproduction of ascospores within elongated
sacs or tubes known as asci. These are commonly collected together
in masses ; the collection of hypluv which give birth to the asci i*
known as the hymeniwrn, the mass of tissue enclosing or bearing the
hymenia as the receptacle or fructification. Its form and structure
vary greatly in the different sections of the family. The ascosporo
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 less than four. The asci are usually
surrounded by enlarged club-shaped or sterile hypluv, the

ASCOMYCETES

643

phi/scs. In many Ascomycetes, in addition to the ascospores,
ordinary exogenous spores or conids are produced at the extremity
of sporophores or conidiophores (fig. 480, A). This is the case with
a large number of moulds or mildews, of which the common blue
mould, Penicillinin 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 reproduction

Fief. 480. — Development of Eurotium repens: A, small part of a mycele with the
conidiophove, c, and young ascogones, as ; B, the spiral ascogone, as, with the
antheridial branch, p ; C, the same with the filaments beginning to grow round it
to form the wall of the sporocarp ; D, a sporocarp seen from without ; E, F,
young sporocarp in optical longitudinal section ; w, parietal cells ; /, the filling
tissue (pseudo-parenchymatous) ; as, the ascogone ; G, an ascus ; H, an ascospore.
After De Bary. A, magnified 190, the rest (iOO 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 ascoyone, which constitutes 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 latter 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

FUNGI

germinate directly into a new mycele. The enveloping tissue, together
with the asci, is known as the sporocarp. In a large number of
Ascouiycetes 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 arranged
under three sections : the Discomycetes, in which the sporocarp is
exposed, and is then known as an apothece ; the Pyrenomycetesfin

A.

FIG. 481. — Uoiri/fix li/tssiaiia: 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
enclosed.

In si line Ascomycetes a tendency is exhibited to the formation of
srlrroies. dense hardened masses of interwoven hyph.T. An example
df this is furnished by the structure known as 'ergot,' the sclerote
of a fungus (if tins \<u\*].('/<iri<-t>j>N juir/iiircn . which attacks the ovary

ASCOMYCETES; SACCHAROMYCETES 645

of rye and other grasses. Many species of Peziza have a peculiar
form 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 bassiana (fig. 481), a kind of mould, the
growth of which is the real source of the disease termed musccvrdine
which formerly carried off silkworms in large numbers, 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, however, 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 ah-, 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
Xphcwia takes place in the bodies of certain caterpillars, in ISTew
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 which
the truffle belongs, are formed completely underground.

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 (Torula) cerevisict1.,

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 g-^o^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. When placed in a fermentable fluid con-
taining some form of nitrogenous matter in addition to sugar,1
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 four,
five, or six, which remain in connection with each other whilst the
plant is still growing, but 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

*fi

a b

FIG. 482. — Saccliaromyces cerevisitz, or yeast-plant, as developed during the process
of fermentation : a, b, c, cl, 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 ultimately 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 Torala 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.

1 It appears from the researches of Pasteur that, although the presence of alliu-
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 protoplasm, while the sugar and water supply the carbon, oxygen,
and hydrogen.

SACCHAKOMYCETES ; BASIDIOMYCETES

647

J

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 up in a suitable
liquid (such as an aqueous solution of cane-sugar, with a little fruit-
juice) by sowing in it the spores of any one of the ordinary moulds,
such as Penicillium glaucum, Jfi/cor, or Aspergillus, 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 basids. They
include the largest and
most familial- of our
fungi, such as the genera
Agaricus, Jloletus, Poly-
porus, Lycoperdon, Phal-
lus, &c. They are sapro-
phytes, obtaining their
nourishment from the
decaying vegetable mat-
ter in the soil, stumps
of trees, &c., Ac., among
which the mycele pene-
trates, consisting often
of a dense weft of sep-
tated hypha?, the ' spawn '
of the mushroom. The
aerial portion, known as
the receptacle or fructifi-

FIG. 483.—Agaricus campestris, formation of the
hymeniuin : 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.')

cation, bears either ex-
ternally, as in the case
of the mushroom (fig.
483), or internally, as
in the case of the Lycoperdon, or ' puft-ball,' the fertile portion
or hymenium. On this hymemum project the extremities of special
hypha>, which are swollen into basids ; the non-sexual conids or
basidiospores are formed at the extremity of the basids, usually in
fours, from which they are easily detached, and, from their small
size and great lightness, are readily carried through the air in great
quantities. Tn the JIt/menomycetes, of which the common mushroom

648

FUNGI

(Agaricus 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 into an epiderm. In the family to which the
mushroom belongs, the hynienium is borne at the edge of narrow
gill-like projections or lamella radiating from the apex of the stipe
011 the under side of the pileus. Among the basids are seen other

cells of similar shape and
usually larger size, also the
extremities of special liyplue.
called cystids, the function of
which is obscure. The basi-
diospores vary greatly in
colour in different 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-
spore 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
1809 ' (and now accepted
by the highest authorities).
that instead of constituting
:i special type of Thallo-
phytes, parallel to Alga1 (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 parasitic-ally. As. however, they do not
furnish objects of interest to the ordinary microscopist (the
peculiar density of their structure rendering a minute examina-
tion of it more than ordinarily difficult), nothing more than a

PIG. 484. — Agaricus caiiijimtri*, natural size.
(Prom Goebel's ' Classification and Morpho-
logy of Plants.')

1 Sec lii* memorable work Ucbcr die Alqcntiipen tlrr FlecMengonidien (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 type. 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
spreads itself over the stony surface 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 very 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 hypha*, which dip down into the superficial
layers of the bark of the trees on which they grew, and form by their

FIG. 485. — Lcpto<ii inn scotinum : Vertical section of the 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 hyphse. (From Goebel's ' Classification.']

interweaving a hard crustaceous ' thallus,' in which the gonids are
imbedded, sometimes irregularly, sometimes in definite layers, known
as the yonidial 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 liypha?. and falling,
when dry, into a powder, of which every particle is capable of
reproducing the plant from which it proceeded.

The fructification of 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 former
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
fungus-hyphse, 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 paraphi/ses, so as to form a layer that lies either on the surface

FIG. 480. — Examples of various alga? which are employed as the gonids of lichens :
// indicates always the hypha of the fungus ; g' the gonid : A, germinating
spore, s, of Pliyscia parietina, the germ-tube of which adheres closely to Profo-
coccus viridis ; B, a filament of Scytonema withhypha? of Stereocaulon ramulosus
twined round it ; C, from the thallus of the lichen Phi/sma clialatjanum — a hyphal
branch is entering a cell of the Nostoc filament (gonid) ; D, from the thallus of
the lichen Sy-iialissa symphorea — the gonids are the alga Gloeocapsa; E, from
the thallus of the lichen Cladonia furcata ; the gonids, which are being sur-
rounded by the hypha?, are the cells of Protococc.us. After Bornet. A, C, D, E,
magnified 050; B, 650 times. (From Goebel's 'Classification and Special Mor-
phology of Plants.')

of apotheces, or is completely enclosed within jieritliecrs. Each of the
asci contains a definite number of ascospores, usually eight, which
are projected from the receptacles with some force ; and their
emission, which seems to be due to the different effects 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 ordinary Ascomycetes, is probably the result of a sexual
union which takes place between The male /lollinoids or ' spermatia '
and the female lriclin<n/iie. These pollinoids arc produced within

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LICHENS ; BACTEEIA 65 I

antherids which are often specially designated ' spermogones,' formed
within these cavities, and, when matm-e, escaping in great numbers
from their orifices. Having no power of spontaneous movement,
they must probably be conveyed by the infiltration of rain-water
to a trichogyne 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 fungus-constituent of lichens belongs, in the great majority
of cases, to the Ascomycetes, in a very few to the Basidionrycetes.
The gonids have been referred to a very large number of genera of
algse, among which may be mentioned Protococcus, C/trodcoccus,
Glceocapsa, Palmella, Scytonema, JYostoc, and Chroolepus.

The Bacteria or Schizomycetes. — At the close of this chapter
we place the Bacteria, SchizQmycetes, or fission-fungi. These micro-
organisms have been defined as minute vegetable cells destitute of
nuclei. In spite of the labour which has been bestowed 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 what
is at the same time a remarkable and important group ; 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 Alga? ; 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 gnu I
masses of organic tissue are reduced — and at the same time the
Bacteria, as a whole, have been broadly and comprehensively worked
out, it may be found that both their morphological and physiological
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 saprophytic monads. But as an instance Cercomonas
typica (named by Kent) may be given,1 where the process is
identical. True, the Cercomonas has a conjugating and subsequent
resting stage, after which swarms emerge from spores thus formed.
1 Manual of the Infusoria, i. 259.

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

1 IG. is?, lli'i/ijnitna alba. (From De Bary'sj 'Comparative Morphology of Fungi.')

is .seen ; 11 shows the result, of filaments ha.ving broken up into spores ;
LO shows spores in movement. 1 is magnified f>40 diameters, tin1
ivm.-iiiidn- '.»()() di.-imrtcrs.

Figure IHK shows the growth of the curved and spiral forms

BACTEEIA

655

of the same : A is a group of attached filaments ; B to H show por-
tions of spiral filaments ; C, D, F, to H represent the act of division
into smaller fragments but without motion ; in H the separate
cells are distinctly shown ; E shows the separation of a complete
spirillum form possessed of flagella and capable of great activity.

Bacteria may be, united by some interfusing gelatinous material
in which all notion ceases or is of the most limited kind ; and these
living films, which appear on the surface or suspended in the interior
of putrescent fluids, are
known as ZoogloRce.
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 Piidospom/i*
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-cell is assimi-
lated, gradually reaches
its mature condition.
"What it is that exactly
determines the act of
spore-formation is not
known, but it is probable
that free access to oxygen
constitutes an important
factor.

A chosen illustration
of the endosporous Bac-
tf-rla is Bacillus meyrt-
therium. It was first

observed 011 boiled cabbage leaves, and is considered by De Bary as
an 'exceedingly instructive form.' It is 2'5/i in short 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 2")0 diameters, b two active rods magnified 600 diameters.
•p 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 spores, d to /represent successive stages of a pair of rods in

FIG. 488. — Beggiatoa, alba, curved and spiral
forms. (From De Bary's ' Comparative Morpho-
logy of Fungi.')

656

FUNGI

the act of forming spores, e an hour later than d, and _/' an hour
later than e. The cells which did not contain spores disappeared or
perished. r is a quadricellular rod with ripe spores, t/1 is a five-
celled i-od with three ripe spores placed in a nutrient solution after

several days' desiccation, y- is the
same an hour after ; </3 is the same
after another two hours and a half.
A! is two spores with the walls of
the mother-cells dried and placed in
a nutrient solution ; h.2 is the same
forty-five minutes later ; /, k, I, three
stages of germination of the spore.

Bacillus anthracis and R. snbtili*
are very 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
(From De Bary's 'SpttS % ^ disease in the form of rods

Morphology of Fungi.')

and filaments
5fi to 20/u in
length and 1/u to 1'25/z 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

Y'S

B

l-'ic. 490.— Bacillus antliracis, x 1,200. Blood
corpuscles and liacilli unstained ; from an inocu-
lated mouse. (Friiiikel and Pfeiffer.)

FIG. 491.— A, Bacillus
anthracis; B, B. sub-
tilis. (From De Bary 's
' Fungi.')

spore-formation. At the upper part of the figure twro ripe spores
have escaped. These s]iores on germination elongate and give rise
to new groii])s of n ids and filaments.

BACTERIA

657

B. stibtilis, which is the Bacillus 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

o o

-pores (Tieghem and Cienkovvski).

remaining connected and already greatly increased in size. The whole
represents a magnification of 600 diameters.

II. Arthrosporous forms are reproduced by the separation of
single members from their connection with a group, which then
give origin to new generations. These cells, apparently not differing
from the rest, become larger, with tougher walls ami 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 Lcuconostoc m-esenteroides (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

658

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 typical arthrospores.

The Bacteria behave very variously under the same conditions
i if supply or exclusion of oxygen. The nr-rohlc require free oxygen
in quantity, as e.g. Jl. subtilis ; while in the anaerobic the vital
activities are promoted by its exclusion. But there can be no doubt
that gi-adual modification of either condition will bring about adapta-
tions. Kaegeli has shown that there are forms which usually depend
on oxygen which continue to vegetate when free oxygen ceases.

A B

C D

FIG. 493. — A, Eai/tcriinn frnno, each cell furnished with a single flagellum. Magni-
fied 4,000 diameters. B, C, D, Bacterium liiieoht, each cell when separated
having a flagellum at either end. Magnified 8,000 diameters. (Dallinger.)

Their nutrition is carried on like that of other vegetative forms
devoid of chlorophyll. The actual typical group are without doubt
the saprophytic Bacteria. The relation of the parasitic 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 sc 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

FN;. 404.- -Four individuals of Vilirio rt/r/ttht, each showing flagellum at one or
both ends ; two other individuals, a and &, separating from each other, and draw-
ing out a protoplasmic filament to form their second rlagella. Magnified 2,000
diameters. (Dallinger.)

l>e to put more into the hands of medicine than could be accomplished

I iv any other means.

ll«ct,<>rin in h'l-nio is tlic most universally present and abundant of
the saprophvtic species. It is 1 p to 1 ••")/< long, and 0'5 to O'7/i broad,
usually of dumbbell form. These Martt-ria are usually seen in ' vacil-
lating' movement in their free state ; each cell bears a flagellum at
each end. as 15. I) (tig. -M'->), whilst the double cells bear a flagellum
.•it each extremity. The formation oft he second llagel him takes place
1>\ 1 he drawing out of a filament of protoplasm between two cells
that are separating from each other (as in lig. 1 '.!!.«, !>}. the rupture

SPIEILLA 659

of which gives a new flngellmn to each. Their flagella are so minute
as to be among the most ' difficult ' of all microscopic objects, their
diameter being calculated from 200 measmemeiits by Dallinger

at no more than ._,,,,/, th of an inch.1 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 zooglcea-Sim.. These
germinate into short slender rods, which are at first motionless, but
soon undergo transverse fission, and then acquire flagella.2

The Vibriones may be represented by V. rui/ula, seen in fig. 494.
They are slightly curved rods and threads, from (5/j to 16ytt long, and
varying in thickness from O-.fyt to 2/.<. They have well-marked flagella.
one at each end. They appear in vegetable infusions, causing fer-
mentation of cellulose.

The S/>irillf( are the largest forms in the group, characterised b\

Fio. 49').— A, Spirillum ttndiiht, showing flagellum at each end. Magnified 3,000
diameters. B, Spirillum volutans. Magnified 2,000 diameters. (Dallinger.)

their spirally formed cells and their graceful spiral motion. The\
are fairly represented in fig. 495 by Spirillum undula (A) and
Sjiirillti in i-oliitxiti* (B). The threads of the former are from 1'lu to
l'4u in thickness, and from 9^t to l'2/.i in length. They are intensely
active, and possess a flagellum at either end. They are found in
varying decomposing infusions.

Xj>irilli(iu. rolntfuis was known to and named by Ehrenberg. It
is from 1-5^ to 2'3/n in thickness, and varies from 2-)^ to 30ju or
more in length. It has distinctly granular 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 vor-
tical action visible in the fluid before this spirillum as it advanced.

With the beautifully corrected 6mm. power of Zeiss (apochromatie
dry X.A. 0-95), all but the most difficult of these can be seen in fresh
specimens with relative ease on a (.lark I/round with a 12 or 18 eye-
piece, provided they be examined alive n-ith the flayella in motion.

1 Jouru. of Hoy. Mirr»*c. Sue. vol. i. (1878), p. 175 - Ewart, loc. cit.

u u 2

66o

FUNGI

•'Y 'j£» ' .."-.'im

For the more difficult ones (Jl. ten no and J>. lineola) more careful
arrangements are required. In dried specimens the flagella 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 maybe 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 different forms, but the whole subject needs more
uniform and exhaustive inquiry. The spores of Jl. subtilis retain
their vitality for years if kept in a dry air, while those of
/>. anthracis are stated by Pasteur to remain alive in absolute
alcohol ; l 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 were wholly
killed only after three
or four hours' boiling ; 2
while Pasteur states
that groups of un-
certain spores can
withstand a tempera-
ture of 130° C. There
is. however,

VJ

uncer-

tainty, because a want
of uniformity, in the
results from various
sources • °0° to 25° < '
may be taken as the

average degree of temperature at which these organisms will freely
germinate ; but E. termn. 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.3 When these facts are allowed

PIG. 496.— Flagella of Typhoid Bacilli, x 1,000, stained
by Loffler' s method. (Fninkel and Pfeiffer.l

' (.'harbon et Septieemie,' <'n>nj>t. Unnl. Ixxxv. p. !l!l.
Naegeli, Uiitrm. iihi-r nicili-n- JJ/I::<; 1882, p. \>-2(l.

• \, it seems unquestionable thai amon^ the higher Fungi ' conjugation ' often
es place at a very early stuj,'e <>t j,'ro\vtli, it seems a not very improbable surmise
that the ' granular spheres ' observed by Kwurt In Jim-Hlux and Sj>iriUiim,vr}w]\
seem to correspond with the ' microplasts ' observed by Hay Lankester in his
linrtiTiniii rzt&escews, may be a product of i-onju^ation in' the micrococcus stage of
i he-,,- organisms.

BACILLUS ANTHRACIS

66 1

their due weight, no difficulty can be felt in admitting the action of

Bacteria, Ac., 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 experiments of Pasteur, repeated

and verified in a great 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 conveyed 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

• * t

PIG. 497. — Spore-bearing threads of Bacillus
anthracis, double-stained withfuchsine and
methyleneblue, x 1,200. (Crookshank.)

FIG. 498.— Photograph of a
pure-cultivation of Ba -
cillusantliracis. (Crook-
shank. )

H

\

• splenic fever ' 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 suffering under any of these
diseases may be a focus of infection
to others, for precisely the same
reason that a tub of fermenting beet-
is capable of propagating its fermen-
tation to fresh wort. A most notable
instance of such propagation is
afforded by the spread of the disease FIG. 499.— Bacilli of tubercle in
termed ' pebrine ' among the silk- sputum, 2,500 (from photo-
worms of the south of France, which, f^SSe "Tcrookshank ?arbolised
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

662

FUNGI

first oroke out with violence. It has been shown by microscopic
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^u 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 Pasteur that these corpuscles 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 In-
completely exterminated.

a

PIG. -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 ;i 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
c;ni be .-i successful bacteriologist. But for the methods of the

bacteriological laboratory \ve must refer
this branch of science.1 it being enough

the reader to treatises on
here to remark that the

1 The English student will find an admirald'1 aid in the Text-book of Bacterio-
logy mill 1 njr<-firr Dt^ //.sc.s i Itli «•<!.), by Professor K. Orookshank.

CULTIVATION AND COLONIES

663

employment of nutrient' gelatine, nutrient agar-agar, and other
similar media on glass plates, and in test-tubes (fig. 501), so as by

PIG. 501. — Pure- cultivations of Streptococcus pyogenes : a, on the surface of
nutrient gelatine ; It, in the depth of nutrient gelatine ; f, on the surface of
nutrient agar. iCrookshank.)

^w^a&

&

> • w:m- ;- v" »

^•^••••'^-^t' w
A- ^^-^^^^^"V,,

iMjfet v-, ,,
k

.

' ^ m-P'1 V *•//"• "iT^^^i

W{;:-%^\^-": -"-">

'•'|"_'r " Sf' ''_'],;

FIG. 502. — Colonies of Bacillus antli-aria, x 80 : a, after 24 hours :
b, after 48 hours. (Fliigge.l

inoculation to obtain cultures of specific and isolated forms with
their characteristic appearances, is one of the essential methods

664 FUNGI

(Plate XIV). The inoculated bacteria, instead of moving freely. a>
they would in a liquid medium, are fixed to one spot, where they
develop ' colonies ' in a characteristic manner, showing their own
morphological features (fig. 502). Cleanliness and care, as well as
practice in manipulation, are 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
used 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 work
of this sort may not deal further than to show the right use of the
microscope and its appliances, by which the work of pathological
bacteriology can alone be successfully done.

•

•

-

(

~*&*

665

CHAPTER X

MICROSCOPIC STRUCTURE OF THE HIGHER CRYPTOGAMS

Hepaticse.— Quitting now the algal and fungoid types, and
entering the series of terrestrial cryptogams, we have first to notic
the little group of H?j>atica\ 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-
iiiorpliH. 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 a>i-
tkerids ; while the latter in the first instance bear the female
organs or archegoiies, which afterwards give place to the ni>oru i«j< *.
or spore-cases.1

The green surface of the frond of Marchantia is seen, under a low
magnifying power, to be divided into minute diamond-shaped spaces
(fig. 504, A, «, a), bounded by raised bands (c, c) ; every one of these
spaces has in its centre a curious brownish-coloured body (b, />), with
nil 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 interior, which is bounded
below by a floor (a, f/) of closely set cells, from whose under surface
the rhizoids arise ; at the sides by walls (c, c) of similar solid

FIG. 503. — Froud of Marchantia

niorplia, with gemmiparous concep-
tacles, and lobed receptacles bearing
archegones.

1 In some species the same shields bear both sets of organs ; and in Ma reliant id
androgyna we find the upper surface of one half of the shield developing antherids,
whilst the under surface of the other half bears archegoiies.

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 parenchyme composed of branching rows of cells (f, f) that
seem to spring from the floor, these cells being what are seen from
above when the observer looks down through the central aperture
A just 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
structure of remarkable com-
plexity. Each of these stomates
(as they are termed, from the
(ireek crrof^a, mouth) forms a sort
of shaft (r/), 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 (/) appears to regulate the
aperture by the contraction or
expansion of the cells which
compose it, and is hence termed
the ' obturator-ring.' In this
manner each of the air-chambers
of the frond is brought into coin-

FIG. 504. — Structure of frond of Marchan-
tia polymorpTia : A, portion seen from
;il.ove; a a, lozenge-shaped divisions;
0.0, stomates m the centre or the lozenges;
c, c, greenish bands separating the
lozenges. B, vertical section of the frond, atmosphere, the degree of

munication with the

showing a, a, the dense layer of cellular
tissue forming the floor or the air-
"

external
that

commimicatjoll being

<

regulated

- .

chamber, d, ,7, "the epidermal layer, I, b, by the limitation of the aperture.
forming its roof ; r, c, its walls;/,/, loose
cells in its interior ;,/,«tomate divided per-
pendicularly ; fe, rmgs of cells forming its
wall ;/, cells, forming the obturator-ring,

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 liverwort.

The frond of Marchantia usually bears upon its surface, as shown
in fig. 503, a number of little open basket-shaped yemmiparous ant
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 round or oblong discoidal (/emmce,
each composed 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
formed by the splitting up of the epiderm. as seen at I!, at the
time when the conceptaele and its contents are first making their
wav above the surface. The little genuine are at first evolved as
single globular cells, supported upon other cells which form
footstalks; these sinyle cells, undergoing binary subdivision.

their
evolve

STRUCTURE OF 3IAKCHANTIA

667

themselves into the gemma? ; and these gemma', when mature,
spontaneously detach themselves from their footstalks, and lie free
within the cavity of the conceptacle.
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 well sup-
plied with moisture ; sometimes,
however, they may be found --row-
ing whilst still contained within
the conceptacles. forming natural
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 Marchantin puts forth seem
to have this origin. The very
curious observation was long ago
made by Mirbel, who c-irefully
watched the development of the.-e
<li'iiiin<t\ that stomates are formed
on the side which happens to be
exposed to the light, and that
rhi/oids 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, howevei-. by the
sufficiently prolonged influence of
light upon one side and of darkness and moisture 011 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 Ma/rchantia, 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 exposed 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 which
is composed of a mass of ' sperm-cells,' within which are developed
antherozoids like those of Chora ; the whole being surmounted bv a
long neck that projects through the 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, membranes that

PIG. 505. — Gemmiparous conceptacles
otMcurchantiapoh/morpha : A, con-
ceptacle fully expanded, rising from
the surface of the frond, a. a, and
containing goiiidial gemma- already
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 STRUCTURE OF HIGHER CRYPTOGAMS

connect the origins of the lobes with one another, a set of arc/iegones.
shaped like flasks with elongated necks (fig. 507) ; each of these has
in its interior an ' oiisphere ' 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
through 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' develops itself into a mass of cells en-
closed within a capsule, which is termed a sporange ;
and thus the mature receptacle, in place of arche-
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, spores, each enclosed in
a double spore-membrane ; and elaters, which are
very elongated cells, each containing a double spiral
fibre coiled up 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

Fio. 507.-Archegone «,f Mar- moderate amount of
chantiapolymorpha,in.mcceR- light and warmth, de-
six -e stages of development. velop themselves into

little collections of cells,

which gradually assume the form of flattened fronds; and thus the
species is very extensively multiplied, 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.

Marduintia is the type of the section known as the thalloid
Hepatica?. Another section, the foliose Hepatica?, is represented
by the genus Jungermannia, 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 Icau-.s. This distinct differentiation of
stem and leaves indicates a decided advance in organisation, and
marks the passage from the thattophytic to the cormophytic type of
structure.

FIG. 506.— Elati-i-
and spores of
Marrlianlia.

STKUCTURE OF MOSSES

669

Musci. — There is not one of the tribe of J losses whose external
organs do not serve as beautiful objects when viewed with low powers
of the microscope ; while their more concealed wonders are ad-
mirably fitted for the detailed scrutiny of the practised observer.
Mosses always possess a distinct axis of growth, commonly more or
less erect, on which the minute and delicately formed leaves are
arranged with great regularity. The stem shows some indication of
the separation of a cortical or external portion from the medullary
or central, by the intervention of a circle of bundles of elongated
cells, which seeni to prefigure the woody portion of the stem of

FIG. 508. — Structure of mosses : A, plant of Fiinaria li ijgrometric a, showing, / the
leaves, it the sporanges supported upon the setiw or footstalks .s, closed by the opercule
o, and covered by thecalypter c. B, sporanges of Eimiiyjitra vulgaris, one of them
closed and covered with thecalypter, the other open ; u, n, the sporanges; o, o, the
opercules ; c, calypter ; p, peristome ; s, s, seta?. C, longitudinal section of very
young sporange of Splacknwn ; a, solid tissue forming the lower part of the capsule ;
f, columel ; I, space around it for the development of the spores ; r, epidermal
layer of cells, thickened at the top to form the opercule <> ; j>, t\vo 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 midrib. The leaf usually consists of either
a single or a double layer of cells, having flattened sides by which
they adhere one to another ; they rarely 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
bearing the fructification, and sometimes 011 the midribs of the leaves.
The rhixoids 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
Char a.

JL

g
-

N

-

6/2 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 spores, or gonidial cells, are contained in the
upper part of the sporange, where they are clustered round a central
pillar which is termed the columel. In the young sporange the
whole mass is nearly solid (fig. 508, 0), the space (I) in which the

Frc. 512. — Double peristome of FIG. 513. — Double peristome

Jir/ji//» intermedium. of Cincliduun <t>-<-ti<-inn.

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 sporange is almost entirely occupied by the spores.
These are formed in groups of four by the binary subdivision of the
mother-cells which first differentiate themselves from those forming
the capsule itself. The capsule and seta of mosses together consti-
tute the organ known as the sporoyonp.

The development of the spore into a new plant commences with
the rupture of its firm yellowish-brown outer coat or p.wsporp, and
the protrusion of its cell-wall proper, or eiulosporc, from the
projecting extremity of which new cells are put forth by a process
of outgrowth, forming a sort, of confervoid filament known as the
/>,-»/<>i/f>ti><>. At certain points of this filament its component cells
multiply by subdivision, so as to form rounded clusters or buds, from
c\er\ one of which an independent plant may arise. The Musci,
therefore, present an example of the phenomenon known as alter -
nation ,,f </<'n<'r<itin,tx. The life history of each individual may be

divided into two 'generations:' the s&ewaZ generation or • oophvte,'

SPORANGE OF MOSSES

673

which consists of the leafy pLmt bearing the male and female organs ;
and the non-sexual generation or ' sporophyte,' composed of the
sporogone with its spores, these two generations 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 mi account of the marked differences
by which they are distinguished, the three groups, lL:>j>nti<'<i\
Bryacece (or ordinary mosses), and Sphagnacece, being ranked as to-
gether forming the group of Muscinece.
The stem of ,s'////"///>mw is more dis-
tinctly differentiated than that of
lirt/acfd' 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
through its elongated cells, especially
in the medullary and cortical layer.--,, .so
that if one of the plants be placed dry
in a flask of water, with its rosette
of leaves bent downwards, the water
will speedily drop from this until the
flask is emptied. The leaf-cells of the
Sphagnacea exhibit a very curious de-
parture from the ordinary type ; for
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 their membranous walls
have large rounded apertures, by which
their cavities freely communicate with
nne 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 very young leaf, differ much in appearance from
the others, the peculiarities of both being evolved by a gradual pro-
cess of differentiation. The aiitherids, or male organs, of S]>Ji(i</nin-<-(i
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
••i single globose antherid on a slender footstalk, and they are sur-
rounded by very long branched paraphyses of cobweb-like tenuity.
The female organs, or archegones, which do not differ in structure
from those of mosses, are grouped together in a sheath of deep green
leaves at the end of one of the short lateral branchlets at the side of
the rosette or terminal crown of leaves. The two sets of organs
are always distributed 011 different branches, and in some inMance.s
on different plants. The • sporange ' which is formed as the product
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 lar^e
empty cells, «, a, «, with spiral
fibres, and communicating aper-
tures; and the intervening
bands, b, b, b, composed of
small elongated chlorophyllous
cells.

6/4 MICROSCOPIC 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 groups of four (as in mosses) around a
hemispherical ' columel.' Besides the ordinary spores, however,
the Sphaynacece sometimes develop a smaller kind, the ' microspores,'
formed by a further division of the mother-cells ; the significance of
these is unknown.1 The ordinary spores, when germinating, do not
produce the branched confervoid filament 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 off 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 imbibing
and holding water, the Sphagnacece are of great importance in the
economy of Nature, 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 upon them.
The stem (where it exists) is for the
most part made up of cellular par-
enchyme, which is separated into a
cortical and a medullary portion by the
interposition of a circular series of
fibro-vascular bundles containing true
\v< lody tissue and ducts. These bundles
form a kind of irregular network, from
which prolongations are given off that
pass into the leaf-stalks, and thence
into the midrib and its lateral branches ;
and it is their peculiar arrangement in
the leaf-stalk of the common brake which
gives to the transverse section the mark-
ing commonly known as ' King Charles
in the oak.' A thin section, especially if somewhat oblique (fig. i)15),
displays extremely well the peculiar character of the ducts of the
fern, which are termed xc<il<ii-if<>ri>i from the resemblance of the
regular markings on their walls to the rungs of a ladder. These
bundles of scalar! form ducts or ' tracheids ' are usually surrounded by
sheaths of .sr/ov-//r////y//r, tissue composed of cells the walls of which

FIG. 515. — Oblique section of foot-
stalk of fern leaf, showing
bundle of scalariform ducts.

1 These so-called 'micro pore are iid\\ believed to be spores of a parasitic

funiuis. — En.

STRUCTURE OF FERNS

675

have become very hard and of a deep brown colour. These scleren-
chymatous sheaths are a very conspicuous feature in a transver-M
section of the stem or rhizome of most ferns, and are the principal
agent in giving it strength and solidity.

What" 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 Poli/podium (fig. 516), and in A*i>i<liinii (fig.
5 IS); but sometimes these sori are elongated into bands, as in

FIG. 516. — Leaflet of Poly- FIG. 517. — Portion of frond of Hcemionitis,

podium, with sori. with sori.

the common Scolopendrium (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 Hmnionitis (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 ' 011 the under surface of the fronds ;
but they are frequently covered with a delicate membrane termed
the indusmm, which may either form a sort of cap upon the summit
of each sorus, as in A*/>i<Iinm (fig. 518), or a long fold, as in Scolo-
pendrium and Pteris, or a sort of cup, as in Deparia (fig. 519).
Each of these sori, when sufficiently magnified, is found to be made
up of a multitude of sporaityes, or spore-capsules (figs. 518, 519).
which are sometimes closely attached to the surface of the frond,
but more commonly spring from it by a pedicel or footstalk. The

x x 2

676 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

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 sporange, 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 other, the fissure extending to such a
depth as to separate them completely. In Osmunda (the so-called
' flowering fern ' or 'royal fern ') and Ophioglossum (adder's tongue)
the sporaiiges have no annulus, or one greatly modified. It will
frequently happen that specimens of fern-fructification 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
l>e watched with sufficient attention the rupture of some of the

FIG. 518. — Sorus and indusium of

Aspidituu.

FIG. 519. — Sorus and cup-shaped
indusium of Deparia }iroUjf,'a.

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 aniiuli
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
Scolopendrium, 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 turning back the
I wo 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 niosi valuable training to the young to teach them how
much may be found to interest, when looked for with intelligent
eves, even in the most familiar, and therefore disregarded, specimens
of Nat nrc's handiwork.

The 'spores' (fig. "rJO. A) set tree by the bursting of the spo-
s, usually have a somewhat angular form, and are invested by a

FRUCTIFICATION OF FERNS

6/7

yellowish or brownish outer coat, the exospore, which is marked
very much 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 off of its angles. This is followed by the putting forth
of a tubular prolongation (fig. 520, B, a) of the internal cell-wall or
<•, i<l ospore through an aperture in the outer spore-coat ; and mois-
ture being absorbed through this, the cell becomes so distended as
to bm-st the external unyielding integument, and soon begins to
elongate itself in a direction opposite to that of the first rhizoid. A
production of new cells by subdivision then takes place from itsgrow-

FIG. 520. — Development of prothallium of Pteris serrulata : A, spore set free from
the sporange ; B, spore beginning to germinate, putting forth the tubular pro-
longation a, from the principal cell b ; C, first-formed linear series of cells ; D, pro-
tluillinm 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 ; Ji, Ji, 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 take-
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 prothallium, varies in its configuration in different
species, but its essential structure always remains the same. From
its under sin-face are developed, not merely the rhizoids («, b), which
serve at the same time to fix it in the soil and to supply it with
moisture, but also the antlti-riJs and ((relief/ours, which constitute
the true representatives of the essential parts of the flower of higher
plants. Some of the former may lie distinguished at an early period
of the development of the prothallium (//, //) : and at the time of its
complete evolution these bodies are seen in considerable numbers.

6/8 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

A

especially in the neighbourhood of the rhizoids. Each has its origin
in a peculiar protrusion that takes place from one of the cells of the
prothallium (fig. 521, A, «) ; this is at first entirely filled with
chlorophyll-granules, 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 110
f(A\t/=V>.i 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 aiitherids and an the- formation into a large
rozoids of Pteris sernilata: A, projection of one number of cells known as
of the cells of the prothallium showing the anthe- the antherozoid-wot/,, r
ridial cell b, with its sperm-cells e, within the cavity 77 /• \ i f f 1

of the original cell a. B, antherid completely ce^ (C) ; each Ot
developed ; a, wall of antheridial cell ; e, sperm- again breaks up into
cells, each enclosing an antherozoid. C, anthero- fQvp. c

irv(- .,4- fivo4-

.-. 1-11 • r* i i • • j_ i i-VJllJ. vvllo. 11L/U <LU J.-I- J. Q I

zoid move highly magnified, showing its liM'ge ex- . , , ' . . ,, ,.

tremity a, its small extremity b, and its cilia d, d. provit lei I With cell-walls.

the sperm-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 sj term-
cells themselves burst.
and give exit to their
antherozoids (C), which
execute rapid move

inents of rotation on
FIG. o22. — Archegone of Pteris serrulata : A, as ,

seen from above ; «, a, a, cells surrounding the tnei

base of the cavity; 1>, c, </, successive layers of pendent on the

cells, the highest enclosing a quadrangular orifice. c.j}J.j with which

B, side view, showing A, A, cavity containing the „ . •, •,

germ-cell, « ; B, B, walls of the archegone, made al>e lurnisneu.

up of the four layers of cells, b, c, d, e, and having The (irclieyones are

an opening,/, on the summit; c, c, antherozoids fpwpv mlipv ml

• 1 1 • IT • i i •. -t -i ..-i J.C W C J. Ill lllllll''vTI, CI/JU1VI

within the cavity; </, large extremity; li, vibratile f , ,...

cilia; z, small extremity in contact with the germ- are tOUnd upon a dll-
cell, and dilated. ferent part of the pro-

thallium. Each of them

originates in a single cell of its superficial layer, which undergoes
-nlidivision by a hori/ontal partition. Of the two cells thus produced
the upper gi \cs origin, by successive subdivisions, to the 'neck' of
the archegone, which, \\hen 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 cliininev or shaft. ' II ie lower of the
t\vo first-formed cells becomes the central cell of the archeyone :

axes, j tartly de
long
they

SEXUAL GENERATION OF FEKNS 679

and this again undergoing horizontal subdivision, the lower half lie-
comes the oosphere or germ-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, «). The
oosphere, when fertilised by the penetration of the antherozoids,
becomes the ' embryo-cell ' of a new plant, the development of which
speedily commences.1 In the aberrant group of Ophioglossacece
(adder's-tongue ferns), the development of the prothallium take--
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 grows 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 sufficiently 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 nower-bud capable of developing
itself into a prothallium that may be likened to a receptacle bearing
the sexual apparatus. But this prothallium serves the further pur-
pose of ' nursing ' the embryos originated by the generative 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 fertilisa-
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 tumbler 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 anther-ids, 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 possible 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 Osnnimlft regalis will be found to
afford peculiar facilities for observation of the development of the antherids, which
are produced at its margin.

680 MICROSCOPIC STRUCTURE OF HIGHER CRYPTO<;.\MS

plete in everything but the true generative organs, which evolve
themselves from the detached spores. Here we have, therefore, an
example of alternation of generations 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 aiitherozoid. In mosses, on the other hand, the leafy
plant belongs 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 reproductive
organs. These phenomena are called respectively apospory and
apogamy. The former has been observed especially in varieties of
Athyrium Filix-foemina and Polysticliunt angulare," and is shown by
the production of prothalloid structures bearing antherids anil
archegones on the fronds in the place of ordinary "sori. The latter
occurs not unfrequeiitly in Pier is serrnlata, 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 microscopist. The whole of
their structure is penetrated to such an extraordinary degree
by sile.r, that even when its organic portion has been destroyed Ky
prolonged maceration in dilute nitric acid, a consistent skeleton still
remains. This mineral, in fact, constitutes in some species not le»
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 ;i 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 wind i the prepared epiderm is an
extremely beautiful object : .-uid it is asserted by Sir \\ ISrewster
( whose authority upon this poinl has been generally followed) that
each siliceous particle has a regular axis of double refraction. What
is usually designated as the fruct ilicat i if the K<|iiisetacea? forms a

cone or spike at the extremity of certain of the stem-like brand ie>
(the real stem being a horizontal rhizome) and consists of a cluster
of shield-like discs, each of which carries a circle of sporanyes or
spore-c.-ipsiiles. thai open by longitudinal slits to set free the spore>.
In addition to the spore> each sporange contains a number of elastic
filaments (fig. .Vj:j). called elaters. These are at first coiled up around

EQULSETACEA: ; RHIZOCAKPE/E •, LYCOPODIACE.E 68 1

the sport-, in the manner represented at A, though more closely
applied to the surface; but, on the liberation of the spore, 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 bespread out on a slip of glass under the
field of view, and, whilst the observer watches them, a bystander
breathes 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 return to their previous condition
when the effect of the moisture has passed off. If one of the sporanges
which has opened, but lias not discharged its spores, be mounted
in a cell with a movable cover, this curious action may be exhibited
over and over again. These spores, like those of ferns, develop into a
prothallium ; and this bears antherids and archegones. the former at
the extremities of the lobes, and the latter in the angles between them.
Nearly allied to Ferns, also, is a curious little group of small
aquatic, plants, the Rhlzocarpese (or Pepper- worts), which either
float on the surface or creep along shallow bottoms. These differ

FIG. 523. — Spores of Equisctidit, with their elaters.

from Ferns and Horse-tails in having two kinds of spore, produced
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-
responding 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 (C'lub-mosM-s).
a group 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
Lycopodiece proper the sporanges are all of one kind, and all the
sj »ores are of the same size, each, as in Ophioglossum, giving origin
to a. subterraneous prothallium that develops both antherids and
archegones. The plant 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 SelagineUeoe and
IsocteiK there are (as in the Rhizocarpece) 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 if it//! it 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 Lfpidode/t^ru
and fiigillarifti of the Coal-measures seem to have formed connecting
links between the Vascular Cryptogams and the Phanerogams, alike
in the structure of their stems and in their fructification. For the
Lepidostrobi 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 ' meya
spores.'

Thus, in our ascent from the lower to the higher Cryptogams, we
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 have 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 1 lie • sperm-cells ' being embodied, as it were, in self-moving fila-
ments, the antherozoids, which find their way to the 'germ-cells' by
their own independent movements, and the 'embryo-cell' being
destitute of that store of prepared nutriment which surrounds it in
the 1 rue seed, and supplies the material for its early development .
In the lower < Yyptogains we ha\e seen that the fertilised oiispore
is thrown at once upon the world, so to speak, to get its own living;
lnit in ferns and their allies the •embryo-cell' is nurtured for a
while bv the prothallium of the parent plant. While the true
/•I/II-IH! nclinn < if ili,' species is ell'ected 1 iv the proper generative act,
the multiplication <>j' lln- individual is accomplished by the production
and dispersion of • goiiidial ' spores; and this production, as we have
seen, lakes place at very different periods of existence in the several

ALTERNATION OF GENERATIONS 683

groups, dividing the life of each into two separate epochs, in which
it presents itself under two very distinct phases that contrast
remarkably with each other. Thus, the frond of Marchcmtia,
evolved from the spore and bearing the aiitherids and archegones,
is that which seems naturally to constitute the plant ; but that which
represents this phase in the ferns is the minute Marchantia-lils.e
prothallimn. In ferns, on the other hand, the product into which
the fertilised ' embryo-cell ' evolves itself is that which is commonly
regarded as the plant ; and this is represented in the liverworts and
mosses by the sporogone alone.1 We shall encounter a similar
diversity (which has received the inappropriate designation of ' alter-
nation of generations ') in some of the lower forms of the animal
kingdom.

1 For more detailed information on the structure and classification of the Crypto-
gams generally the reader is referred to Goebel's Outlines of Classification <ui<!
Special Morphology and De Bary's Comparative Anatomy of the Phanerogams
n ml Ft'nts, translations of both of which have been published by the Clarendon
Press ; and especially to Bennett and Murray's Handbook of Cryptogamic P>ot<n///,
published by Longmans (London, 1889).

684 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

CHAPTER XI

OF THE MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

BETWEEN the two great divisions of the Vegetable Kingdom which
are known ; is Cryptogamia and Phanerogamia 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-
ganiia. 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
Gymnosperius (including the Conifercv and Cycadecd) the essential
parts of the generative apparatus are reduced to a condition closely
approximating to that 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-
nioviiig ' antherozoids ' that the contents of the sperm-cell find their
\\.-iv 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 ovule.1 Again
(2), while the ' germ-cell ' or oiisphere 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 011 the parent fabric, but continues for some time
t<> draw from it the nutriment it requires for its development into the
i iiiln-i/o. And at the time of its detachment from the parent the

1 A ver\ remarkable and inteiv iting discovery, for which ue are largely i
to the brilliant observations of twn Japanese botanists, Professors Ikeno and Ilirasc,
lias recently thrown great light on I lie approximation referred to by Dr. Carpenter
between Uie higher Cryptogamia and the lower Phanerogamia. It is now known
that in both the larger groups of (i\ nmo, perms, tlie ('onifene and the Cycadeee, there
are species in \\liich the fertilising liod\ is a motile ant hero/.oid formed \\ithin a
pollen tube, thus romliining (he distinctive modes of fertilisation characteristic of
tin- two gi-eal sections oi the vegetable kingdom. As Dr. Carpenter does not include in
his account of the ' Microscopic Structure of I'li.-nierogainic 1 'hints ' a full description
of the modi! of impregnation in (lowering plant-, the reader is referred, for further
detail . to the mosl recent Text I ..... bs of Botany, or to the Summary of Current Re-
searches in liotany iii the •funrinil i if flu- li. Mii-rt'^cujiiciil Society. — EDITOR.]

STRUCTURE OF PHANEROGA1VIIA 685

matured ' seed ' contains, not merely an embryo already advanced
a considerable stage, but a store of nutriment to serve for 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 oi'ules, which when fertilised and matured become seeds.
are developed from specially modified leaves, which remain open in
Gymnosperms, but. which in all other Phanerogams told together so
a-; to enclose the ovules within an or«r;/. Each ovule consists of a
nucettus surrounded by integuments which remain unclosed at the
apex, leaving open a short canal termed the mlcropijle 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 oosp/«>/-<'
is developed ; in Angiosperms by free-cell-formation, but in
Gymnosperms indirectly after the formation of a 'corpuscle.' which
represents the archegone of Selinjiin-IJn. By a further proce» of
free-cell-formation the remainder of the embry<t->ar comes to be
tilled with cells constituting what is termed the endosperm : 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 other 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. — Xo marked change shows itself in general
organisation as we pass from the cryptogamic to the phanerogamic
series of plants. A large proportion of the fabric of even the
most elaborately formed tree (including the parts most actively con-
cerned in living action) is made up of components of the very same
kind as those which constitute the entire organisms of the simplest
cryptogams. For. although the stems, branches, and roots of trees
and shrubs are principally composed of ivoody tissue, such as we do
not meet with in any but the highest Cryptogams, yet the .special
office 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, are
composed of cellular substance. This tissue is found, in fact,
wherever growth is taking place; as, for example, in the Crowing
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 "v/o>/// structure
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 leave-,
flowers, and certain fruits. All the softer and more pulpy tissue

of these organs is com-
posed of cells, more or les>
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 theparenchy
matous (or pulpy) sub
stance of leaves, which
often forms a distinct
layer known as the
' spongy paren chyme '
immediately beneath the
epiderni 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

A B

FIG. 524 — Section of leaf of Agave, treated with
dilute nitric acid, showing the protoplasmic con-
tents contracted in the interior of the cells ; a,
epidermal cells b, guard-cells of the stomate ;
<•, cells of parenchyme ; if, their protoplasmic
contents.

I-'K;. ,V2."i. Sections of cellular parenrliyme of Aralia, or rice- paper plant
A. transversely t» the axis of the stem ; B, in the direction of the axis.

)|,,.n, ;,iid to liring them into dust- apposition, and then the cavities
of adjacent cell.- are -eparaled by a single partition wall. Fre-
(jiient'ly it happen.- t bat the pressure is exerted more in one direction
than in another, so that the form pre.-ented by the outline of the cell

STRUCTURE OF THE CELL 687

varies according to the direction in which the section is made. This
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-
rally exhibit circular outlines ; whilst, when the section is made verti-
cally, their borders are straight, so as to make them appear like
cubes or elongated prisms, as in fig. 524. A very good example of
such a cellular parenchyme is to be found in the substance known as
• rice paper,' which is made by cutting the herbaceous stem of a
Chinese plant termed Aral'm papyri/era 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
prismatic, as shown in fig. 525. I> ; 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 resistance. Hence their greatest length is nearly
always in the direction of the axis ; but there is one remarkable
exception, that, namely, which is afforded by the •medullary rays'
of exi igenous stems, whose cells are greatly elongated in the horizontal
direction (fig. 547, a), their growth being from the centre of the stem
towards its circumference. It is obvious that fluids will lie more
readily transmitted in the direction of greatest elongat ion. being 1 li it
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 tntiixn-rsfl direction. One
of the most curious varieties of
form which vegetable cells pre-
sent is the stellate cell, repre-
sented in fig. 526, forming the
sp< nigy parenchvmatous substance
in the stems of many aquatic
plants, 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 : FlG- 526- ,<" ' il)nf of s,tellate

, . ,-, • »' 7 7 parenchyme of rusn.

such is the case in A upfiar lutea

(the yellow water-lily), the foot-
stalks of whose leaves contain large air-chambers, UK- 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 variable; for although their diameter is very com-
monly between ^-..th and -,,1M,th of an inch, they occasionally mea-
sure'as much as \j\rth of an inch across, whilst in other instance^
they are not more than J^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 therefore
alwavs be single. It is onlv in older thick-walled cells that a line of

688 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

demarcation becomes obvious in the form of an intermediate 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 their
thickening. This layer very frequently ultimately assumes a muci-
laginous character. Where cells have a rounded outline, it is

O

obvious that intercellular spaces must exist between them ; and a>
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 ; It/siyenons if resulting from the disappearance or
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 alter
their condition ; this is the case
with the pulp of ripe fruits.
such as the strawberry or currant
(the snowberry is a particularly
favourable subject for this kind
of examination), and with the
parenchyme of many Heshy leaves,
such as those of the carnation
(DiantliHs caryophyUiis) or the
London pride (S((.i-ifr«<i«. mn-
brosa). Such cells usually con
tain evident nuclei which are
turned brownish-yellow by iodine.
whilst their membrane is only
turned pale yellow, and in this
way the nucleus maybe 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 bine 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
]><ir'n't<il 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 shrivels up within its cavity, as shown in
fig. -VJ4. It would be a mistake, however, to regard this as a distinct
membrane ; for it is nothing else than the peripheral layer of proto-
plasm. naturally somewhat more dense than that which it include-.
but passing into it, by insensible gradations.

FIG. 527. — Cubical parenchyme, with
stellate cells, from petiole of XiijiJitir
lutea.

It is probable that all cells, at some

stage

or other of their

growth, exhibit, in a greater or less degree of inlensit \ . that curious
movement of ci/c/nxi/j which lias been already described as occurring

CYCLOSIS OF PROTOPLASM 689

in the Characete (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 among these are two, TW//-s//' •/•/'/ x/>ii-<ilis and Anacharis alsi-
nastnun (or Elodea canadensis), which are peculiarly 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
grown in a tall glass jar having at the bottom a couple of inches <it'
mould, which, after the roots have been inserted into it, should be
closely pressed down, the jar being then tilled with water, of which
a portion should be occasionally changed.1 The jar should be freelv
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 lie 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) s > as to bring into view the deeper layer, 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 revived again by gentle warmth ; and it may
continue under favourable circumstances, in the separated fragment,
for a period of weeks, 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 winter
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 -/--inch constructed by Messrs. Powell and Lealand enables the
borders of the protoplasmic current, which carries along the
particles of chlorophyll, to be distinctly defined ; ami this beautiful

1 Mr. Quekett found it the most convenient method of changing the water in the
jars in which Ckara, Valli&neria, &c., are growing, to place 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 STRUCTURE OF PHANEROGAMIC PLANTS

phenomenon may be most luxuriously watched under their patent
binocular.

Anacharis alsinastrum is a water-weed which, having been acci-
dentally introduced into this country many years ago, has since
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 epiderm, but are for the most part composed of two layers of
cells, and these are elongated and colourless 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 necessary being
to take a leaf from 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 for
the examination of the cyclosis in CJiamov in VaUisneria,ihe ^-\w\\
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 power 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 or
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 ^oW11 to -s-^th 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 round the
cell within that period. As in the case of Vallisnrria, 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 are carried round is estimated by Mr. Wenham
at 110 more than ^^yth of an iucl1- When high powers and
careful illumination are employed, delicate ripples may be seen in the
pr< >t< iplasmic currents. !

Cyclosis. houever. is byiio means restricted to submerged plants ;
for it' has been witnessed by numerous observers in so great a variety
< >f 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 l.y
tearing otf with a pair of fine pointed forfeits the portion of the
epiderm from which they spring, care being taken not to grasp the
hair itself, whereliy such an injury would In- done to it as to check
the movement within it. The apochromat ic hair should then lie
1 (jiKirt. Joni'i:. ni' Mii-mxr. Science, vol. iii. ils5."ii, ji. '277.

CYCLOSIS OF PROTOPLASM

691

B

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 line>
along tin- entire length of the cells, as in the hairs from the filaments
of Tr<nli'xc((nti<( rii-i/i iiini . or Virginian spiderwort (fig. 528. A); in
others there are several such cur-
rents which retain their distinct-
ness, as in the jointed hairs of the
calyx of the same plant (15) ; in
i 'fliers, again, the streams coalesce
into a network, the reticulations
of which change their position at
short intervals, as in the hairs of
(ildiicinni Iittfiini ; whilst there
are cases in which the current
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
n iiclf ax (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 are
not so opaque or so strongly
marked as to prevent the move-
ment from being distinguished.
The epiderm may be most readily
torn off from the stalk or the
midrib of the leaf, and must
then be examined as speedilv 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 lie 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 arranged in a -moving train.'
appear as bright lines across the cell; and these lines, 011 being
carefully watched, are seen to alter their relative positions. The
leaf of the common Plaittayo (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

Y Y '2

FIG. 5'28. — Rotation of fluid in hairs of
Tradcscaniia virginica: A, portion of
epiderm with hair attached ; a, />, <•,
successive cells of the hair ; d, cells of
the epiderm ; e, stoniate. B, joints of a
beaded hair showing several currents ;
a, nucleus.

692 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

A

or midrib. It is a curious circumstance that when a plant which
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 rot at ion
is most active, and the IUOM--
meiit is usually quickened In-
artificial warmth, which, indeed.
is a necessary condition in some
instances to its being seen at all.
The most convenient method of
a]>] living this warmth, while the
object is on the stage of the
microscope, is to blow a >tivam
>f air upon the thin gla^s cover

FIG. 529. — Tissue of the testa or seed-coat
of star-anise : A, as seen in section ;
B, as seen 011 the surface.

through

a glass or metal tube

previously heated in a spirit-
lamp.

The walls of the cells of

plants are frequently thickened by deposits, which are first formed
on the inner surface, and which may present very different appear-
ances according to the manner in which they are arranged. In

Fie;. .i:»(l. Section of cheiTv-stnnr,
cutting the cells transversely.

FlG. ">:!1. — Section of eoquilla
nut 111 the direction of the
lung (liametiT of the cells.

its simplest e .million such a deposit forms a thin uniform layer
over the whole internal surface of 1 lie cellulose wall, scarcely detract-
ing at all from its transparency, and chiefly distinguishable bv the
'dotted ' appearance which the membrane then presents (fig. 525. A).
These dots, however, are not pores, as their aspect might naturally
suggest, bu1 are merely points at which the deposit is \\antimr.so

TISSUES OF PHANEROG-AMIA 693

that the original cell-wall there remains unthickened. A more
complete consolidation of cellular tissue is effected by deposits
of sclernr/en (a substance which, when separated from the resinous
and other matters that are commonly associated with it, i.s found
tn 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
through ; 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 sclerenchyme or sclerenchymatous tissue. By a con-
tinuance of the same arrangement as that which shows itself in the
single layer 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 that 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 gritty substance which .surrounds the
seeds and forms little hard 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
fitiri'x. which lie coiled up on the inner
surface of the cells, so as to form a single,
a double, or even a triple or quadruple
spire (fig. 532). Such spiral cells are found
abundantly in the leaves of certain orchi-
daceous plants, immediately beneath the
epiderm, where they are brought into
view by vertical sections; and they may
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 loaf
In an orchidaceous plant named Saccola- of Oncidinm.

f>i/oit (juttattuii 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 are often found upon the surface of the testa or
outer coat of seeds ; arid in Collomla yrandiflora, Salvia verbenaca
(wild clary), and some other plants, the membrane of these
cells is so weak, and the elasticity of their fibres so great, that
when 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 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

peculiar flocculent appearance. This very curious 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
should then be pressed down,
and the box placed upon the
stage, so that the microscope mav
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 ot
water should lie placed on that
part of its internal surface with
which the slice of the seed had been
in contact; and the cover being
rej daced, 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
ilrop of water should be held in its place by capillary attraction,
instead of running down and leaving the object, as it will do if the
glasses l?e 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. 80 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.

Fii;. 5:;5. Granules of starch .is
i under polarised light.

FIG. 534.— Cells of peony filled
with starch.

They are sometimes minute and very numerous, and so closely
packed as to fill the cell-cavity (fig. r>:U) ; in other instances they
;m> "' much larger dimensions, so that only a comparatively smal
"t them are included in anv one cell; while in other

STAKCH-GEAINS 695

cases, again, they are 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 granule when highly magnified exhibits
a peculiar spot, termed the Jiilmn. round which are seen a set of
circular lines that are for the most part concentric (or nearly so)
with it. When viewed by polarised light each grain exhibits a dark
cross, the point of intersection being at the hiluni (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 Nageli
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 formation by the 'apposition of
successive layers also has many advocates. These layers differ in
their proportion of water, the outermost layer, which is tin- mo>t
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 formation 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
•-t arch-grams 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 are permeated by long
branched tubes, constituting the laticiferous tissue. The elements
of this tissue may be either greatly enlarged prosenchymatous cells
or 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 rapidly on exposure. The chemical
composition of the latex varies; it may contain in solution powerful
alkaloids, as in the case of the opium-poppy, or gum-resins. Caou-
tchouc and gutta-percha are the dried latex of tropical trees and
shrubs belonging to several natural orders. Good examples of lati-
ciferous tissue are furnished by the Papaveracea?, of which our
common field-poppy is an example, many Composite such as ihe
dandelion and lettuce, Coiivolvulacea>, Eupho'rbiacese or spurges,
Apocynacese, Moracea? including the mulberry &c.

696 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

Deposits of mineral matter in a crystalline condition, known as
rajtlndes, are not nnfrequently found in vegetable cells, where they
are at once brought into view by the use of polarised light. Their
designation (derived from pact's, a needle) is very appropriate 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 arc-
scarcely less abundant ; thus, instead of bundles of minute needles,
single large crystals, octahedral or prismatic, are frequently met
with, and the prismatic crystals are often aggregated in beautiful
stellate groups. The most common material of these crystals is
oxalate of lirne, 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 .-is
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 acicular raphides consist almost
invariably of oxalate of lime. Some raphides are as long as -^th of
an inch, while others measure no more than -j-^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-
ever, is often so much thinned away as to be scarcely distinguish-
able. Certain plants of the ('tirf/m 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 year-
old, were sent to Kew (lardens from South America, some years
since, it was found neccssarv 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 vegetative processes, being
tor the most part produced by the union of organic acids generated
in the plant with mineral bust's 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
rhubarb. Besides the struct ures ;dre;idy mentioned as affording good
illustrations of different kinds of raphides, mav be mentioned the

pareiichyi >f the lea f of Ai/nrr. .I/or, <'//ni«. Encephalartos, etc.;

the epiderm of 1 lie bull) of the hyacinth, tulip, and garlic ; the bark of
the apple. < '<tNCtiril/(f. < 'i iii-Iiniin. lime, locust, and many other t rees; the
pit 1 1 of i'.ln injii nx. and the testa of l he seeds of . I inii/iiHis and the elm.
A large proportion of the denser parts of the fabric of the higher
plants is made up of the substance which is known as wood/y fibre or
prosenchymatous lisum-. Tin's, however, can only be regarded as a
variety of cellular tissue ; for it is composed of peculiarly elongated

TISSUES OF PHANEROGA3I1A

697

cells (fig. 551). usually pointed at theiv 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 internal deposit, must
possess much greater tenacity than any tissue in which the cells
depart but little from 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 • stringiiiess ' to various
esculent vegetable substances, are commonly known under the
name of Jibro-vascular 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 Jibro-vascular bundles, which are the chief
strengthening elements of such organs as the stem, branches, leaf-
stalks, flower-stalks, etc., are. in the higher
plants, structures of considerable com-
plexity ; in Exogens they consist of three
distinct portions, the ccyfem-portion com-
posed chiefly of the different kinds of
vessels hereafter to be described, a pit loem-
portioii composed of proseiichymatous
tissue and ' sieve-tubes,' and a formative
eam&mm-portion.

A peculiar set of markings seen on
the woody fibres of the Coniferce, and of
some other tribes, is represented in fig.
536 : in each of these spots the inner
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 lenticular
cavity which intervenes between the ad-
jacent cells at this point. There are

varieties in this arrangement so charac-

Fio. 536. — Section of coniferous
wood in the direction of the
trache'ids, showing their
'bordered pits;' «, </, a, me-
dullary rays crossing the
fibres.

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 Coniferce, though not peculiar to that

698 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

order, are known .-is bordered pits, and the elongated cells in which
they occur as trackeids.

All the more perfect forms of Phanerogams contain, in some
part of their fabric, the peculiar structures which are known as
spiral vessels.1 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 maybe single, double, or even quadruple, this last character pre-
senting itself in the very large elongated fibre-cells of Xepenth.es
(pitcher-plant). Such vessels are especially found in the delicate
membrane (medullary sheath) surrounding the pith of Exogvns,
and in the ' xylem-portion ' of the woody bundles of Exogens and
Endogeiis ; thence they proceed to the leal' stalks, through which
they are distributed to the leaves. By careful dissection under the
microscope these fibro -vascular handles may be separated entire ;
but their structure may be more easily displayed by cutting round,
but not through, the leaf-stalk of the strawberry, geranium. A.V.. and
then drawing the parts asunder. The membrane com] losing 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.

Although fluid generally finds its way with tolerable 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
afforded by the peculiar kind of vessels known as ducts, which consist
of cells laid end to end, the partitions between them being more 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,
b, 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. Z37. 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 growth,
the fibre being broken up into rings, which lie sometimes close
together, but more commonly at considerable intervals; such a duct
is said t<> be annular (fig. f>:!7. i). Intermediate forms between the
spiral and annular ducts, which show the derivation of the latter
from 1 1n- former, are \ ery frequent ly to be met with. The spiral* are
sometimes broken up still more completely, and the fragments of the
libre extend in various directions, so as to meet and form an irregular
net \\ork lining the duct, which is then said to lie reticulated. The
continuance of the deposit, however, gradually contract.- the meshes.

1 So long, however, as they retain their original cellular character, and do not
coalesce with each other, these fusiform spiral cells cannot be regarded us having
any more claim to the desi^nat ion of rc.w/,v, than have the elongated cells of the
w ly 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, are
especially met with in parts of most solid structure and least rapid
growth (fig. 537, 3). The scalar (form ducts of ferns may lie re-
garded as a modification of the spiral; but spiral ducts are fre-
quently to lie met with also in the rapidly growing leaf-stalks of
flowering plants, such as the rhubarb. Not unfrequeiitlv. however,
we find all forms of ducts in the same bundle, as seen in fig. i).">7.
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 uiicon-
solidated, can serve for the
conveyance of fluid . Th« -y
are entirely absent in the
1 'oniferce.

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. FIG. r.a?.— Longitudinal section of stem of Italian
The former process is the reed : a, cells of the pith ; b, fibre-vascular
most e'isv -md vields -I bundle, containing 1, annular ducts; '2, spiral

±j-L\jij u v7cuo V«*i|iJ-»i>-i.vrl*lo<l T • j i i i • 1 1 T /• t 11

r „ ducts; 3, pitted ducts with woody fibre ; <•, cells

large amount 01 miorma- Of the epiderm.

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. A: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 most readily
and effectually made with the ' microtome ; ' and there are few parts
of the vegetable fabric which may not be advantageously examined
by this means, any very soft or 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

H r

< W

7CO MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

sections must be made \vitli a sharp knife, the 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, etc., 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 dilute nitric acid or in a mixture of nitric acid and
chlorate of potassa. This last method (which was devised by
Sch nit/) 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, ttc. 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 arc included the stem with its branches, and the root with its
ramifications) always has for the basis of its structure a dense cellular
pareiichyme ; though in an advanced stage of development this
may constitute but a small portion of it. In the midst of the
pareiichyme we generally find nbro-vascular bundles, consisting of
woody 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
arc more correctly term-.-d r.rot/i'ii<>nv (growing on the outside); for
in the former the bundles are dispersed throughout the whole
diameter of thea.xis without any peculiar plan, the intervals between
them being filled up by cellular pareiichyme ; whilst in the latter
tbe\ are arranged 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 l>«rk. These two plans of axis-formation
respectively characteristic of those two great groups into \\hich
Phanerogams are subdivided — namely, the Monocotyledons and the
Dicotyledons \\ill now be more particularly described.

\Vlieu a iransv eise section (fig. .">:!K) of a monocotyledonous stem
is examined microscopically.it is found to exhibit a number of iibro-
vaseular bundles, disposed without anv regularity in the midst of
the mass of cellular tissue, which forms (as it were) the matrix or
basis of the fabric. Each bundle contains two. three, or more large

STRUCTURE OF STEMS

701

ducts, which are at once distinguished by the size of their openings ;
and these are surrounded by woody fibre and spiral vessels, the
transverse 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 gradually more

crowded towards its circumference ; but it frequently happens that the

portion of the area in which they are

most compactly arranged is not abso-

lutely at 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 tibro- 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 e\ in-

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

FIG. 589.— Portion of

time, except at the nodes, leaving section of stem of Wanghie
the stem hollow, as we see in the
whole tribe of grasses. When a vertical section is made of a woody
stem (as that of a palm) of sufficient 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

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 nbro-vaseular
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 110 further additions.
It was from the idea formerly enter-
tained that these successively formed
bundles descend in the interior of the
stem through its entire length until they

,-, reach the roots, and that the stem is thus

fc i<;. 540. — Diagram of the first . . .

formation of an exogenous continually receiving additions to its
stem : a, pith ;&, Z>, bark ; c, e, 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 parts which must
be regarded as the highest form of the development of the axis,
being that in which the greatest differentiation exists. A distinct
division is always seen in a transverse section (fig. 540) bet \\eeii three
concentric areas — the pith, the wood, and the bark — the first ('/) being

dullary rays I left between the
woody bundles d d.

Fit,. 541. — Transverse section of stem of ( 'Ir/nutis : a, pith: l>, 1>, b, woody bundles

r, r, <•. medullary rays.

central, the last (f>) peripheral, and these having the wood interposed
I iet ween them, its circle being mad*' up of wedge-shaped bundles
(il '/). kept apart by the nn-il nllitri/ r<ii/n composed of unchanged cel-
lular tissue (c, c) that pass between the pit li and the hark. The pith
(lii;. ~> 1 I . ") is almost invariably composed of cellular tissue only.
u Inch usuall v pi events (in transverse sect ion) an hexagonal areolation.
When newly Conned it has a greenish hue. and its cells are filled with

STRUCTURE OF STEMS

703

fluid ; but it gradually dries up and loses its colour ; and not im-
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 surrounded by a delicate
membrane, consisting almost entirely of spiral vessels, which is
termed the medullar;/ sheath.

The woody portion of the stem (fig. 541, b, b) is made up of woody
fibres, usually with the addition of ducts of various kinds : these,
however, are absent in one large group, the Coniferci' or fir-tribe
with its allies (figs. 545-548), in which the prosenchymatous cells
or tracheids are of unusually large diameter, and arc 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 Rltamnus (buckthorn), showing
concentric layers of wood.

FJC,. .">43. — 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 different species, the vessels being
sometimes almost uniformly diffused 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 curiously
figured appeal-mice to the transverse section (figs. 542, 54:i). 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 correspond with
that of the years during which the stem or branch has been growing ;
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 110 doubt, however, that such is not the universal rule ;
and that we should be more correct in stating that each layer indi-
cates an ' epoch of 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 for
every crop of leaves there will be a corresponding layer of wood.
It sometimes happens, 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 would
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 interruption to the process -
of vegetation produced by seasonal changes — as bv heat and drought

PICT. 544. —Portion of transverse section of stem of hazel, showing, in the portion
a, b, c, six narrow layers of wood,

in a tree that nourishes 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
KiiK/le one. Thus in a section of ha/el stem (in the Author's posses-
sion), of which a portion is represented in fig. 544, between two
lavers of the ordinary thickness there intervenes a band whose
breadth is altogether less than that of either of them, and which is
vet composed of no fewer than six layers, four of them (c) being very
narrow, and each of the other two (« . b) being about as wide as
these four together. The inner rings of wood, being not only the
oldest, but the most solidified by resinous matters deposited within
their component cells and vessels, are spoken of collectively under
the designation iJtira-'nu'ii 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 colise-
ouentlv designated as dlliii rim in or "sap \\ood.' The line of demar-
cation'I >e I \\ een the two is some! hues very distinct, as in lignum vitfe
and COCOS uood ; 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 alburnum is every year consolidated by internal deposit, and is

STRUCTURE 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

If^1"-^- '.' ' ' ! ~ •-' . l'/.'.^ •!!>!•>•' i i^^'-:\Vrrr4^--t*^tri-T-'^~~''V/:^ '.jfl-iv'U'Vb / 'fff

FIG. 545. — Portion of transverse section of the stem of cedar: a, pith ;
b, b, b, woody layers ; c, bark.

plates of cellular tissue (fig. 541, c, c), not usually extending to any
great depth in the vertical direction. 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 comparison of lrmt/itt«Unal 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 rays seem to run parallel to
each other, i listen d of radiating
from a common centre. They are FIG. 546.— Portion of transverse section of
very narrow ; but are so closely larSe. stem of coniferous wood (fossil),
, i ,1 ,1 i showing part of two annual rings, divided

set together that only two or at fl> f° and traversed by ver£ 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 medullary rays, these are seen as thin plates
(«, «, a) made up of superposed cells very much elongated, and
crossing in a horizontal direction the trachei'ds which lie parallel to
one another vertically. And in the tangential section (fig. 548),

z z

706 MICROSCOPIC STRUCTURE 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

Fio.*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, <i .

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 very limited 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 List (fig. ')4!>)
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 t ra n s \ e i > e
section is shown in fig. 550, and its tan-
gential section in fig. 551, the medullary
rays are seen to occupy a much larger part
of the substance of the stem, being shown
in the transverse section as broad bands
(<i a, a a) intervening between the closely
set prosenchymatous cells, among which
some large ducts are scattered; whilsl in
the tangential section they are observed
to be not only deeper than the preceding
from above downwards, but also to have
much reater thickness. This section

FK,. :> lit. Vertical seel ion
mahogany.

()|.

also gives an excellent view of the ducts.

f> t>. l> /<. \\hieli are here plainly seen to be
formed by the coalescence of large cylindrical cells lying end to end.
In another fossil wood in the Author's possession the medullary rays

STRUCTURE OF STEMS

707

constitute a still larger proportion of the stem. ; for in the transverse
section (fig. 552) they are seen as very broad bands (b, b), alternating

Km. 550.— Transverse section of a
fossil wood, showing the medullary
rays, </, a, a, n,n, it, running nearly
parallel to each other, and the
openings of large ducts in the midst
of the prosenchymatous tissue.

Fij. 551. — Vertical (tangential) sec-
tion of the same wood, showing the
prosenchymatous cells separated
by the medullary rays, and by the
large ducts, I b, b b.

with ]>l:itcs 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.— Tran-verse and vertical -en ions 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 >iuu<>us c<inr>e of the
prosenchymatous cells, instead of looking (as in iig .~>4S) as if they

zz -2

708 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

had forced their way between these cells, which there hold a nearly
straight and parallel course on either side of them. The medullarv
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 bark is usually found to consist of three principal layers :
the external or epiphlceum, which includes the suherous (or corky)
layer ; the middle, or mesophloeum, also termed the ' cellular envelope ; '
and the internal, or endophlceum, which is more commonly known
as the librr.1 The two outer layers are entirely cellular, and are
chiefly distinguished by the form, size, and direction of their cells.
The epiphlceu/m is generally composed of one or more layers of colour-
less 01 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 110 means the product of one
kind 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 tlieir
form, and disposed with 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 liber 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 C!OM-
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 lamina-. The bark is
always separated from the wood by the cambium layer, which is the
part wherein all new growth takes place. This layer seems to con-
sist of mucilaginous 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-vascuiar 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

Tlif term ' lilit-r' is also sometimes applied to the 'phloem-portion' of a fibro-
luimlle. — Ei>.l

STKUCTUEE 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 plan.

.Numerous departures from the normal type are found in particu-
lar tribes of dicotyledons. Thus in some the wood is not marked by
concentric circles, their growth 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 Galycanthus), so that
several woody columns arc 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 stein, of which
fig. 554 represents a transverse section, these cellular plates form

FIG. 554. — Transverse section of the
stem of a climbing plant (Aristo-
lochia ?) from New Zealand.

PIG. 555. — Portion of transverse
section of Arctium (burdock),
showing one of the fibre- vascu-
lar 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 medullary) 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 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

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 disposition of the fibro-
vascular layers which are scattered through nearly the whole of
the fundamental tissue (although more abundant towards its
exterior) in the former case, but are limited to a circle within the
peripheral 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 fibro- 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 epiderm 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 diameter, and becoming progressively thicker as it pa.-.-e>
upwards. 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 dosed.
The open fibre-vascular bundles of exogeus and of gyninosperms
may be stated to consist of three distinct parts : the xylem portion,
which consists chiefly of ducts, of the nature of spiral, annular, in-
pitted vessels, and which is the portion of the bundle nearest to the
centre of the organ; tine phloem or 'bast' portion, which consists
largely of prosenchymatous cells, among which are almost alwav>
sieve-tubes with their sieve plates, and which is the peripheral
portion of the bundle; while between them is the formative cum-
bium, 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 fibre-vascular
bundle lie side by side, as is usually the case, the bundle is said lo
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 sinus. Generally
-peaking, however, the roots of exogens have no pilli. although they
have medullary rays; and the succession of distinct rings is less
apparent in them than it is in the stems from which they diverge.
In the delicate brandies which proceed from the larger root -fibres
a central bundle of \essels will be seen enveloped in a sheath of
cellular substance; and this investment also covers in the end of
the branch, uliich is usually .-omeuhat dilated, and is furnished at
its extremity uitli one or more layers of cells, which are constantly
lieing ihroun oil'. km>\vnas the jiilrnrhir.n or runt-cap. The structure
of the I irandies of the root may be well studied in the common
buckweed, everj floating leaf of which has- a single root hanging down

STRUCTURE OF STEMS AND ROOTS 7 1 I

from its lower surface. The central fibre-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 differentiated from
the surrounding fundamental tissue, known as the bundle- -sherti '/> .
\Ve 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 thorough! v 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 objects, the chief
points to be attended to being the preparation of the stems, &c. 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 it.-,
gum. before being submitted to the cutting process. If the wood
be drv. it should first be softened b\ soaking for a >iifticieiit length

* «/ o o

of time in water, and then treated with spirit, and afterwards with
water, like green wood. Some woods are so little affected 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 a>
that which is cut fresh. When a piece of appropriate length
has been placed in the grasp of the section instrument (wedges of
deal or other soft wood being forced in with it, if necessary for its
firm fixation), a f'e\v 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 moved 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 drawing and. pushing. A little prac-
tice will soon enable the operator 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 flipping the
razor in water so as to float away the slice of wood, a camel-hair
pencil being vised 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 until they be mounted. For the minute exami-
nation of their structure, they may be mounted either in ^eak
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 winch transverse sections
rather thicker 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
until they turn brown, may be mounted with advantage in

712 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

Camilla balsam, and are then very showy 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 process 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
which the organic structure is preserved, he should proceed with it

I

«Mi«Mri

twoBBresf tarn

:;:fMj»

•i!<li-MHMg»..*^^

FIG. 556.— Epiderm of leaf of
Yucca, showing stomates.

:

FIG. 557. — Epiderm of leaf of Indian
corn (Zea Mais), showing stomates.

after the manner of other hard substances which need to be reduced
bv grinding.

Epiderm of Leaves. — On all the softer parts of the higher plants,
save such as grow under water, we find a surface layer differing in
its texture from the parenchyme 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
si ratum (figs. 5(50, f)()2, «, a). The shape of these cells is different in
almost every tribe of plants ; thus in the epiderm of the Yucca (fig.
."•.")()), Indian corn (fig. H")?), Iris (fig. 561), and most other mono-
cotyledons, they are elongated, and present an approach to a
rectangular contour, their margins being straight in the Yucca
and /r/.y, 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

STRUCTURE OF LEAVES

713

like the pieces of a dissected map, as is seen in the epiderm of the apple
(fig. 558, &, 6). Even here, however, the cells of that portion of the
epiderm (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 ordinary epiderm cells of monocotyledons has reference to

FIG. 558. — Portion of epiderm of lower surface of leaf of apple,
with layer of parenchyme in immediate contact with it :
a, a, elongated cells overlying the veins of the leaf; b, fe,
ordinary epiderm-cells, overlying the parenchyme ; c, c,
stomates ; d, d, green cells of the spongy parenchyme,
forming a very open network near the lower surface of the
leaf.

that parallel arrangement of the veins which their leaves almost
constantly 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

A B

FIG. 559. — Portion of epiderm of upper surface of leaf of
Rocliea falcata, as seen at A from its inner side, and at B
from its outer side: 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 011 the side nearest the
atmosphere. This outermost hardened continuous wall of the
epidermal layer of cells is known as the cuticle. The deposit (cutiii)
is of a nature to render the membrane very impermeable to fluids,
so as to protect the soft tissue of the leaf from drying up. In most

714 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

European plants the epiderm consists of but a single row of cells,
which, moreover, are usually thin-walled ; whilst in the generality
of tropical species there exist two, three, or even four layers of
thick-walled cells, this last number being seen in the oleander, the
epiderm of which, when separated, has an almost leathery firmness.
This difference 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 afford
a sufficient protection to the interior structure against the ravs 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 epiderm is presented by
Rochea falcata, which has the surface of its ordinary epiderm (figs.
55!). 5(50, a. a] nearly covered with a layer of large prominent
isolated cells, b, b. A somewhat similar structure is found in
Mesembryanthemum crystallinum, commonly known as the 'ice-plant,'
a designation it owes to the peculiar appearance of its surface,
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 JElceagnus and other shrubs
and herbs are very beautiful objects under the microscope. In

numerous other case-, again,
we find the surface beset with
true /Kih-s, 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. 01- flower-stalk, as in
many kinds of rose, geranium.
.Vc. In other instances, ihe
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
meat 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 I )<'iil :'iu x<-<tli,-d. which a good deal resemble those
within the -iir chambers of the yellow water-lily (fig. 527). The so-
called -glands' 01- -tentacles' of the sundew (Drosera) are not
really hairs, hut out grou t hs of the internal tissue of the leaf, each
penetrated by a libro- vascular bundle.

Fi<;. .">()0. —Portion of vertical section of leaf
of Ifiir/ifii. 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; </, </, cells <>!' the parem-hyim- ;
L, cavity Between the |>arenchynuttous
cells into which the stomate opens.

STRUCTUEE 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 forms of the epidermal cells, hairs,
stomates. itc.. are still marked out in silex, and (unless the dissipa-
tion of the organic matter has been most perfectly accomplished)
arc 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 proportion of mineral matter
exists than that of rice, which contains .some curious elongated cells
with toothed margins. The hairs with which the joafece (chaff-scales)
of most grasses are furnished are strengthened by the like siliceous
deposit ; and in Festtn-n /n-titi-nxix. one of the common meadow-
grasses, the palese 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 arc remarkably
beautiful objects for the polariscope.

In nearly all plants which possess a distinct epiderm, this is
perforated by the minute openings termed stoinuli'* (tigs. 557, 561),
which are bordered by cells of a peculiar form, the guard-cells,
differing from those of the epiderm. and more resembling in character
those of the tissue beneath.
They are further distinguished
by containing a larger number
of chlorophyll-grains than the
ordinary cells of the epiderm.
These guard-cells are usually
somewhat kidney-shaped, and
lie in pairs (fig. 5G1, ?>), with
an oval opening between them :
but by an alteration in their
form, the opening may be con-
tracted or nearly closed. In
the epiderm of Yucca, however,

the opening is bounded bv two

/• n i • i , PIG. 561. — Portion of epiderm of leat ot Lns

of cells, and is somewhal

'•

pairs

quadrangular (fig. 550) ; and a
like doubling of the guard-
cells, with a narrower slit be-
tween them, is seen in the epi-
derm of the Indian corn (lig.
557). In the stomates of no

i/rrniiiii/rit torn from its surface, and
carrying away with it a portion of the
parenchymatous layer iu immediate con-
tact with it: a, a, elongated cells of the
epiderm ; b, b, cells of the stomates ; c, c,
cells of the part-nchyme ; (1, (1, impressions
011 the epidermal cells formed by their
contact ; e, cavity in the parenchyme. cor-
responding to the stomate.

phanerogam, however, do we
meet with any conformation at all to be compared 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 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

there is no distinct epiderm, so there are no stomates. In the erect
leaves of grasses, the Iris tribe, ivc., they arc 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 retained in the system ; whilst they abound most in
those which exhale fluid most readily, and therefore absorb it most
quickly. It lias been estimated that no fewer than 160.000 are con-
tained in every square inch of the under surface of the leaves of
/ft/drangea 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 y?.niifinic« each surface
has nearly 12,000 stomates in every square inch ; and in Yucca each
surface has 40,000. In the oleander, Jltwksia, 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 mouths of these
pits, with the hairs 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 view 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 apposition with the cells

of the upper epiderm (fig.
562, 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 surface
of the leaf, and containing
a large quantity of chloro-
phyll ; these generally pi-ess
so closely one against an-
other that their sides be-
come mutually flattened; and
no spaces are left, save where
there is a definite air-chamber
into which the stomate opens
(fig. 562, e) ; a,nd 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 off (fig. 561 , c, c). This layer, usually peculiar to the upper
surface of lea\cs. is known as the palisade-'parenchyme. Beneath
this first layer of leaf-cells there are usually several others rather
less compactly 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
cpiderm. which the parenchyme only touches at certain points, its
lowest layer forming a sort of network, the so-called spoixjy paren-

FIG. 562. — Vertical section of epiderm and of
portion of subjacent parenchyme of leaf of
Iris germanica taken in a transverse direc-
tion : H, a, rellsof epiderm; b, b, cells at the
sides of the stomates; c, c, guard-cells; d, d,
openings of the stomates; c, e, cavities in the
parenchyme into which the stomates open ;
f,f, cells of the parenchyme.

STRUCTURE OF LEAVES

717

dii/ me (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 parencnyme
not being deeper in one part 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 pareii-
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 (d. 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 peculiar conformation of the leaM •- :
for it' we pull one of them from its origin, we shall find that what
appears 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 under surface is entirely concealed.

FIG. 563. — Portion of vertical longitudinal section of leaf
of Iris, extending from one of its flattened sides to the
other : a, a, elongated cells of epiderm ; &, b, stomata cut
through longitudinally ; c, c, green cells of parenchyme ;
d, (1, colourless tissue, occupying interior of leaf.

The two halves are adherent together at their upper part ; 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 portion 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,
«) these bear a strong resemblance in every respect, save the greater
proportion of their breadth to their length. Another interesting
variety in leaf-structure is presented by the water-lily and other
plants whose leaves float on the surface ; for here the usual arrange-
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 number of air-spaces that give buoyancy to the leaf ;
and these spaces communicate with the external air through the

71 8 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

numerous stomates, which, contrary to the general rule, are here
found in the upper epiderm alone.

The examination of the foregoing structures is attended with
very 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 very little woody structure in
their leaves. In those, 011 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 may 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 medium 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 lo\\
magnifying power, are very striking microscopic objects : and the
interest of the young 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 beauty
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 flower; and among these
he finds abundant sources of
gratification, not merely to his

love of knowledge, but als.» to hi> taste lor the beautiful. The general
structure of the wy^//.s- and /><-lnlx. \\hich constitute the perianth, or
floral envelope, closely corresponds to that of leaves. The petals
seldom contain unchanged chlorophyll; but usually either the
chlorophyll in the petals (and sometimes also in the sepals) is
changed into a solid yellow pigment (cm-ufi n] • or the chlorophyll
has entirely disappeared, and is replaced l>\- a pigment, blue, red,

Ki<;. 5(54.— Cells from [M-t.il <>t
Pelargonium,,

STRUCTURE OF FLOWERS 719

purple, or some other bright colour, antlioi-ijn n , erythrophytt, ite..
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 Pela/rgonium,
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 mam mi 1-
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 compression, occa-
sions the peculiar spots represented in fig. 564. Their real character
maybe brought into view by Dr. Inmaii's method, which consist^
in drying the petal (when stripped 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 distinct!}', 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 (Anagallis arvensis),
that of the common chickweed (Stellar ia 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 somexvhat 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 \vhich
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 afford a good opportunity of
studying that form of ' free-cell-formation ' which seems peculiar to
the parts concerned in the reproductive process, 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 STRUCTURE OF PHANEROGAMIC PLANTS

be made up of ordinary cellular parenchyme 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, anil a thin inti'ii? ; 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 M«rchartt'ta.. (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
Jfi/oscyamus ; 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, primrose, &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 different parts of the wall of one and the same anther.
It seems probable that, as in Hepatica?, 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 effect 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
bring the cell nearer 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 protuberances, 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 absolutely deficient at these points, but is only thinned

POLLEN-GRAINS

721

r%£f&:>*^2^

away. Sometimes the pores are covered by little disc-like piece.-, or
lids, which foil oft' when the pollen-tnl* is protruded. This action
takes place naturally 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, sometime.-* 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 growth will only take place under the natural con-
ditions. I5y treating some pollen-grains, as those of Lil'mm
japonicum. L. rnbrum, or L. <mr<il,i ,n. 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
tul »es may 1 »e ] (reserved almost
unchanged by mounting in
this liquid.

The darker kinds of pollen
may be generally rendered
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
tiside the slide for a time in
a warm place. For the less
opaque pollens the dammar
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, C ichor iacew, Cucurbitacece, Jfalvacece,
and Passiftoreos ; others are furnished also by Convolvulus; Cam-
panula. CEnothera, Pelargonium (geranium). Polj/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 (Malva syl-
vestris) or of the hollyhock (Alt/Ufa 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

3 A

FIG. 565. — Pollen-grains of — A, Altlxea rosea
(hollyhock) ; B, Cobcca scandens ; C, Passi-
flora carulea ; D, Iponitea pnrjiurea.

722 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

upon ;i l>l;ick .surface. They are then, when properly illuminated,
most beautiful objects for objectives of §-, 1-, H-, or 2-in. focus,
especially with the binocular microscope.1

There are, in fact, few more interesting objects for the young
mici'oscopist 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 poplar; or of very irregular outline, 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,
( '//teraria, and many other plants. Sometimes the grains are united
together by delicate threads, as in the Rhododendron and FH<-}IX/<I ;
and this union is much more complete in the Orchidece and Ascle-
/ 1 iinJ >•!'-. where the whole of the pollen in each anther-lobe is glued
together by a viscid substance into a club-shaped y^//* nun, or pollen-
ma». In what are called anemophilous flowers, in which the pollen
is carried through the air by the agency of the wind, the grains are
Miiall. light, dry, and usually spherical ; while in entomophilous
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 hairy
under side of the body of the insect, and thus promote their dis-
persion. The various species of fjpilofi/inn (willow-herb) and
(Enothera (evening primrose) are very favourable objects for ob-
-ervir.g 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 ha.ve a peculiar interest for scientific botanists, but which, in
consequence of the special difficulties that attend the inquiry, are
not commonly regarded as within the province of ordinary micro-
x-'tpists. 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 i.x 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 firs 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 produced, and are deposited as a fine yellow dust,
so strongly resembling sulphur as to be easily mistaken for it. This (supposed)
j> Mi-nil diffusion of sulphur (such as occurred in the neighbourhood of Windsor
in 1x791 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
tin- microscope.

FERTILISATION OF THE OVULE

723

sections made with a sharp razor ; the ovule should not be allowed
to dry up, and the section should be removed 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 tribe are the most
favourable subjects for this kind of investigation, which is best
carried 011 by artificially applying 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 Epipactis,' 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 Ribes nig rum and ruin-urn (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 pollen-tube into
the micropyle may be most easily observed in orchidaceous plants
and in Eu]>hrii«ii<. it being only necessary to tear open with a needle
the ovary of a flower which is just withering, and to detach from the
placenta the ovules, almost every one of which will be found to have
a pollen-tube sticking in its micropyle. These ovules, however, are
too small to allow of
sections being made,
whereby the origin of
the embryo may be dis-
cerned ; and for this pur-
pose, GSnothera (evening
primrose) has been had re-
course to by Hofmeister.
whilst Schacht recom-
mends Lathrwa squam-
aria, Pedicularis palus-
tris, and particularly
Pfd icttlctr is syl/oa t ica .

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 ofthe pink (1 )) is regularly covered with curiously jagged divisions,
every one of which has a small bright black hemispherical knob in its

3 A -2

FIG. 566. — Seeds as seen under a low magnifying
power: A, poppy; B, Amaru nthiin [prince's
feather); C, Antirrhinum inajits (snapdragon);
D, DiantJtns (clove-pink) ; E, Bigiwuia.

724 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

middle ; that of Amaranthus hypochondriacus has its surface traced
with extremely delicate markings (B) ; that of Antirrhinum is
strangely irregular in shape (C), and looks almost like a piece of
furnace-slag ; and those of many Biynoniacea; 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 Eccremocarpus scatter, a half-hardy climbing plant
common in our gardens ; and when its membranous ' whig ' is
examined under a sufficient 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-walls of which cells (those,
namely, which lie in contact with one another) being thickened so as
to form radiating ribs for the support of the wing, whilst the front
and back walls (which constitute its membranous surface) retain their
original transparence, and are marked only with an indication of
spiral deposit in their interior. In the seed of Dictyoloma 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 Sphenogyne speciosa and Lophospermum erubescens, possess wing-
like appendages : bnt the most remarkable development of these
organs is said by Mr. Quekett to exist in a seed of Caloscmthes
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 more 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 Cywnthus 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 mounting for opaque objects : — Anayallis,
AnetJtum graveolens, Begonia, Carum carui, Coreopsis tinctoria,
J)atara. Delphinium, Digitalis, Elatinc. Erica, Gentiana, Gesnera,
Hyoscyamus, Hypericum, Lepidittin, Limnocharis, Li/taria, Lychnis,
Mesembryanthemum, Nicotiana, Oriyanwn onites, Orobanche, Petunia,
Reseda. Saxifraga, Scrophularia, Sedwn, Sem-pervivum, Sllene,
tftcMaria. Sym/phylMm asperrimum, and Verbena. The following
mav be mounted as transparent objects in Canada balsam:
hrosera, Hydrangea, Monotropa, Orchis, Parnassia, Pi/rola, 8axi-
t'raija.1 The seeds of umbelliferous plants generally are remarkable
Cor the peculiar -vittce, or receptacles for essential oil, which are
found in the closely applied pericarp or seed-vessel which encloses

1 A part of these lists havo been derived from the Micrographic Dictionary.

STRUCTURE OF SEEDS

725

them. Various points of interest respecting the structure of the
testa or envelope of seeds, such as the fibre-cells of Cobcea and
Collomia, the stellate cells of the star-anise, and the densely con-
solidated tissue of the 'shells' of the coquilla-nut, cocoa-nut, itc.
having been already noticed, we cannot here stop to do more than
advert to the peculiarity of the constitution of the husk of corn-
grains. In these, as in other grasses, the ovary itself continues to
envelop the seed, giving a covering to it that surrounds the test:i,
and closely adheres to it. The ' bran ' detached in grinding consists
not only of these two coats, but also (as the microscope reveals) 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
microscopist to detect even a small admixture of roasted corn with
coffee or chicory without the least difficulty.1

1 In a case in which the Author was called upon to make such 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
roasted corn.

726

CHAPTER XII

MICROSCOPIC FORMS OF ANIMAL LIFE—PROTOZOA

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

units may be, these are (as among protophtjtes) mere repetitions of one

another, 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 mo-rula 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 primitive stomach, whose wall is

formed of a double membrane, the outer lamella, or ectoderm,1

being derived directly from the external cell-layer of the morula

whilst the inner, or endoderm, is formed by the ' imagination ' of

that layer into the space left void by the dissolution of the central

cells of the 'morula.' This gastrula-stage,2 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 generate and other organs between

1 The terms cpiblust and Itijjiotiliisf 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 appearance at one point
nt an orifice which leads into the central cavity ; this cavity is the original segmenta-
tion cavity of the morula, and not a fresh cavity, as in ' invagimite gastrulae.'

PROTOZOA— PROTOMYXA 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. Thus 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 'nt»rnln-
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 life of the whole. Putting this
important truth into other words, we may say of the Protozoa that
they are either unicellular or unicellular aggregates, while the
Metazoa are multicellular. and their constituent cells have different
functions.

The lowest of the Protozoa, however, like the simplest proto-
phytes, do not even attain the rank of a true cell, understanding
by that designation a definite protoplasmic unit (jplastid), which i^
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 research is continually diminishing the number of forms
devoid of a nucleus, or. at any rate, of a nuclear material scattered
throughout the substance of the plastid ; in retaining, therefore, the
group of non-nucleated Protozoa 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.
Between some of these Monerozoa (as they have been designated
by Professor Haeckel. who first drew attention to them) and the
Myxomycetes or the Chlamydomyxa already described, no definite
line of division can be drawn, the only justification for the separa-
tion here adopted 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 connect »••]
graduation by which they pass into undoubted rhizopods, leave no
doubt of their claim to a place in the animal kingdom.

MONEEOZOA.

A characteristic example of this lowest protozoic type is presented
by the Protomyxa aurantiaca (fig. 567), a marine ' moner ' of an
orange-red colour, found by Professor Haeckel upon the dead shells of
Spirula 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 MICROSCOPIC FORMS OF ANIMAL LIFE — PROTOZOA

itself, together with foreign organisms (ft, c) — such as marine diatoms,
radiolarians, and infusoria — which, having been entrapped in the
pseudopodial network, are carried by the protoplasmic stream into
the central mass, where the nutrient matter of their bodies is extracted,
the hard skeletons being cast out. Neither nucleiis nor contractile
vesicle is to be discerned, but numerous floating and inconstant vacu-
oles («) are dispersed through the substance of the body. After a time
the currents become slower ; the ramified extensions are gradually

FIG. 567. — Profo>»i/,v(i tiiiritntiucd : 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,
showing numerous vacuoles, u, and captured prey, l>, c,

di-awn inwards ; 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 spheres (13), 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 red pear-shaped bodies issue forth
into the water (C), moving freely about by the vibrations offlcigella

PEOTOZOA — PEOTOMYXA

729

formed by the drawing out of their small ends, just as do the
flagellated zoospores of protophytes. These bodies, being without
trace of either nucleus, contractile vesicle, or cell-wall, are to be
regarded as particles of simple homogeneous protoplasm, to which
the designation plastiduhs has been appropriately given. After
about a day the motions cease ; the fiagella are drawn in. and the
plastidules take the form and lead the life of Amcebce, putting forth
inconstant pseudopodial processes, and engulfing nutrient particles
in their substance (1>). Two or more of these amoebiform bodies
unite to form a ' plasmodium,' as in the Myxomycetes ; its pseudo-
podial extensions send out branches which inosculate to form a net-

FIG. 5liH. — Vcnnjii/i < Jin xjiirii</i/i\r, as seen at A, sucking out contents
of Spirogyra-cell', at B in encysted condition, the cyst a 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
si/e of the original. In this cycle of change there seems no interven-
tion of a generative act, the coalescence of the amcebiform plastidules
having none of the characters of a true ' 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 Vampyretta,
of which one form (fig. 568) has long been known in its encysted
condition as a minute brick-red sphere attached to the filaments of
the conjugate Spirogyra ; whilst another (fig. 569) similarly
attaches itself to the branches of Gomphonema. The walls of the

73O MICROSCOPIC FORMS OF ANIMAL LIFE— PROTOZOA

cysts are composed of two membranes, of which the interior gives
the characteristic reaction of cellulose, whilst the softer external
layer is nitrogenous. After remaining some time in the quiescent
condition the encysted protoplasm breaks up into two or four
• tetra spores ' (fig. 569, b, cl) ; these escape by openings in the cyst
(fig. 568, C), and soon take the spherical form, emitting very slender
pseudopodial filaments (figs. 568, I), 569, e) like those of an Actino-
/iJn-i/s. but possessing neither nucleus nor contractile vesicle. In this
condition they show great activity, moving about in search of the
special nutriment they require, drawing themselves out in strings
and fine filaments which tear asunder and again unite to send off
branches and form fine fan-like expansions, and these occasionally
contracting again into minute spheres. When the T". spirot/f/rw 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 Y(im-
/>!ii-f'U(i: In this manner cell after cell is emptied of its contents ;
and the plunderer, satiated with food, resumes its quiescent spherical
form 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 V.
yomphonematis in like manner creeps over the stems and branches of
the Gomphonema (fig. 569. ?), 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 interior, and appropriates its contents.
The valves, when emptied, break off from their support, and are cast
out of the body of the Vampyrella, which soon proceeds to another
(romphonema-ce\\ and plunders it in the same manner. After thus
ingesting the nutriment furnished by several cells, and acquiring its
full size, it passes, like V. spirogyrce, 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 form allied to Vampj/rella — Archerina Boltoni
—which is remarkable for being chlorophyllogenous ; this species
j presents another interesting peculiarity: — 'Groups of ghost-like
outlines corresponding to chlorophyll-corpuscles and their radiant
filamentous pseudopodia., entirely devoid of any substance,' were
observed, and were compared to the numerous cellulose chambers
which are secreted and abandoned by the protoplasm of ('Jilrnnif-
rlomyxa.

Intei-mediate between the foregoing and the ' reticulariaii ' rhizo-
pods to he presently described, is another simple protozoon dis-

LIEBERKUEHNIA

731

covered in ponds in Germany by MM. Claparede and Lachmann,
and named by them Lieberkuehnia Wctgeneri.^ The whole sub-
stance of the body of this animal and its pseudopodial 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 performing a circulatory movement, which
may be likened to the rotation of the particles in the protoplasmic

FIG. 569. — Vtmipi/reUa gomphonematis : A, colony of
Gomphonema attacked by Vtimpyrellce; a, encysted state;
b, b, cysts with contents breaking up into tetraspores, d, d,
seen escaping at e ; at/ is shown a Vampyrella sucking out
contents of Gomphone»ia-ce\ls, the emptied frustules of
which, ff, //, are cast forth. B, isolated Vampyrella creeping
about by its extended pseudopodia.

network within the cell of a Tradescantia. It is a marked peculiarity
of the pseudopodial extension of this type that it does not take place
by radiation from all parts of the body indifferently, but that it

1 Etudes surJes Infusoires et les Bldzopodes, Geneva, 1858-1861. The beautiful
figure of Lieberkuehnia, given by M. Claparede, has been reproduced by the Author
in Plate I. of his Introduction to the Study of the Foraminifera.

732 MICKOSCOPIC FORMS OF ANIMAL LIFE— PROTOZOA

proceeds entirely from a sort of trunk that soon divides into branches
which again speedily multiply by further subdivision, until at last
a multitude of finer and yet finer threads are spun out by whose
continual inosculations a complicated network is produced, which
may be likened to an animated spider's web. The protoplasm is
invested in a very delicate and closely applied envelope. Any small
alimentary particles that may come into contact with the glutinous
surface of the pseudopodia are retained in adhesion by it, and
speedily partake of the general movement going on in their sub-
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 may
be seen, two streams passing
at the same time in opposite
directions ; but in the finest
filaments the current is single
and a granule may be seen to
move in one of them to its very
extremity, and then to return,
perhaps meeting and carrying

with

granule

was
opposite

seen advancing

direction.

that

in the
Even in

FIG. 570.—Lieberkuehnia Wagenerl.

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 Cham. When a granule
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 different 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
pseudopodial 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 ramifications, 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 processes takes place.
Occasionally the pseudopodia are entirely retracted, and all activity
ceases; so that the body presents the appearance of an inert lump.
Hut if watched sufficiently long its activity is resinned, so that it
may be presumed to have been previously satiated with food, which

RHIXOPODA 733

is undergoing digestion during its stationary period. Xo encysting
process has been noticed in lAeberkuehnia ; but Cieiikowsky has dis-
covered that in L. paludosa reproduction 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 stud}-.

RHIZOPODA.

We now arrive at the group of r/ti~npods, or ' root-footed 'animals,
first established by Dujardin for the reception of the Amctba and its
allies, which had been included by Professor Ehrenberg among his
infusory animalcules, but which Dujardin separated from 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
pseadopodia (or false feet), serving alike as instruments of locomotion
and as prehensile organs for obtaining food. According to Dujardin's
definition of this group, the Jfonero^ixi. already described, would b»-
included in it ; but it seems on various grounds desirable to limit the
term Rhizopoda to those Protozoa in which the presence of a nucleus,
the differentiation of an ectosarc (or firmer superficial layer of proto-
plasm) from the semi-fluid vndosnrc. together with the more definite
form and restricted size, indicate a distinct approach to the condition
of true cells. Many different 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,1 based on the characters of
the pseudopodial extensions, have been accepted by more recent
svstematists, it seems best still to adhere to it.

I. In' the fii'st division, Retictdaria, the pseudopodia freely
ramify and inosculate, so as to form a network, exactly as in Lieber-
k i/fli a in. from which they are distinguished by the possession 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 separate 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

1 Natural History Review, 18G1, p. 456; and Introduction to the Study of the
Foraminifera, 186'2, chap. ii.

754 MICROSCOPIC FORMS OF ANIMAL 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 (zodchlorellce) 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 regularity 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 power,
capturing their prey by their extended pseudopodia. The tendency
of modern writers is to separate the Heliozoa. as here understood, into
the two groups of Heliozoa (sens, strict.) and lladiolaria, the latter
being distinguished by the presence of a central capsule or mass of
protoplasm surrounded by a special envelope, the better 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. The third group, Lobosa, contains the rhizopods which most
nearly approach the condition of true cells, in the differentiation 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 lolxttr. that is,
irregular projections of the body, including both ectosarc and endo-
sarc, which are continually undergoing change both in form and
number. The Lobosa 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 formed by many of them, either
by exudation from the surface of their bodies of some material
(probably chitinous) which hardens into a membrane, or by aggre-
gating and uniting grains of sand or other small solid particles,
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. — Tin's type is very characteristically represented by
the genus Ground (fig. 571). some of whose species are marine, and
are found, like ordinary Foraminifera, among tufts of corallines,
algae, CYC. ; whilst others inhabit fresh water, adhering to Confervse
and other plants of running streams. It was in this type that the
presence of a nucleus, formerly supposed tube wanting in lleticularia

GR03IIA

735

generally, was first established by Dr. Wallich. The sarcode-body
of this animal is encased in an egg-shaped, brownish-yellow, chitinous
envelope, which may attain a diameter of from jVth to r\yth 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
orifice, from which, when
the animal is in a state
of activity, the sarcodic
substance streams forth,
speedily giving off 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 pseudopodia
may be given off. By
the alternate extension
and contraction of these,
minute protophytes and
protozoa arc entrapped
and drawn into the in-
terior of the test, where
their nutritive material
is extracted and assimi-
lated ; and if the ' test
(as happens in some
species) be sufficiently
transparent, the indi-
gestible hard parts (such
as the siliceous valves of
diatoms, shown in fig
571) may be distinguished
in the midst of the sar-
codic substance. Bv the

•/

same agency the Gromia

sometimes creeps up the sides of a glass vessel. In the intervals "of

quiescence, on the other hand, the wiiole sarcodic body, except a

film that serves for the attachment of the test, is withdrawn into its

interior.

Another example of the reticularian group is afforded by the
curious little Micnujromia socialis (fig. 572), first discovered by Mr.
Archer, and further investigated with great care by Hertwig,1 which

FIG. 571. — Gromia oviformis, with its
pseudopodia extended.

1 'Ueber 'Microyromia,' in Arcltiv fiir Mikr. Aunt. bd. x. Supplement.

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 remaining 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 orifico, from which the sarcode-bocly extends itself,

FK;. 572. — Microgroiiiia social is '. A, colony of individuals in extended state,
some of them undergoing transverse fission; B, colony of individuals
Koine of them separated from the principal mass) in compact state ; C, D,
formation and escape of swarm spore, seen free at E.

giving off very slender pseudopodia which radiate iii all directions.
A distinct nucleus can be seen in the deepest part 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 tvpe ; but with a peculiar departure

HELIOZOA 737

from the usual method. A T ran -verse con strict ion divides the l>;>dy
into two halves — as shown in two individuals of colony A — each half
possessing its own nucleus and contractile vesicle; the posterior >eg-
ment. which at first lies free at the bottom of the cell, then presses
forwards towards its orifice, as shown at C, 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 irregularly over the pseudopodia of
the colony. But it finally gathers itself together and takes an oval
form ; and either develops a pair of flagella, and forsakes the colony
as a free-swimming mound, or assumes the form of an Actinophrt/s,
moving about by three or tour pointed pseudopodia — probably in
eacli case coming after a time to rest, excreting a shell, and laying
the foundation of a new colony. There is reason 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 former develops a new shell
without undergoing any change of condition.

Heliozoa.1 — The Actinophrys 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 Conferva'
and other aquatic plants, as a whitish-grey spherical particle dis-
tinguishable by the naked eye. from which (when it is brought under
sufficient magnifying power) a number of very pellucid, slender.
pointed rods are seen to radiate. The central portion of the body is
composed of homogeneous sarcode. inclosing a distinct nucleus ; but
the peripheral part has a -vesicular' aspect, as in the type next
to be described (fig. .~>74). This appearance is due to the number
of Lvacuoles' filled with a watery fluid, which are included 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 considerable regu-
larity, is always to be distinguished, either in the midst of the
sarcode body, or (more commonly) at or near its surface ; and
it sometimes projects considerably from this, in the form of a
sacculus with a delicate membranous wall, as shown at fig. •">".'!.
A, cv. The cavity of this sacculus is not closed externally, but
communicates with the surrounding medium — not, however, by any
distinct and permanent orifice, the membraniform wall giving way
when the vesicle contracts, and then closing over again. This alter-
nating action seems to serve a respiratory purpose, the water thus
taken in and expelled being distributed through a system of channels
and vaciioles excavated in the substance of the body, some of the
vacuoles which are nearest the surface being observed to undergo
distension when the vesicle contracts, and to empty themselves
iiraduallv as it refills. The bodvof this animal is nearlv motionles

1 A systematic account of this t;roup is to be found in Dr. F. Schaudiiin's
'Heliozoa,' the first part of the comprehensive J>n* Tliierri'irJi, edited by the
German Zoological Society, Berlin, IS'.x;. M. I'enard's memoir, ' Etudes sur quelques
Heliozoaires d'Eau Douce,' in vol. ix. of tin.1 Ar<:Ji. <lf FSinL, should be consulted.

- A swimming Heliozoon has lately 1>- • • : leso ;1" •() by M. E. Pennrd, who calls it
Myriophrys paradoxa .

:; H

738 MICROSCOPIC FORMS OF ANIMAL LIFE — PROTOZOA

but it is supplied with nourishment by the instrumentality of its
pseudopodia, its food being derived not merely from vegetable par-
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 wit! i
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 particle thus taken captive is introduced into the
body differs according to circumstances. If the prey is large and
vigorous enough to struggle to escape from its entanglement, it may
usually be observed that the neighbouring pseudopodia bend over and

cp

1)

FIG. 573. — Actinoplirys sol: A, figure showing the wide vaeuolated cortical
layer or ectosarc (E) and the fine granulated endosarc (M) ; •», central
nucleus, oa1, axial filaments of pseudopodia; cr, contractile vacuole ; N, food-
mass inclosed in a large food-vacuole. B, a colony of four individuals, after
treatment with acetic acid ; K, 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
escaping, though still inclosed by the inner envelope. (From Biitschli,
after Grenadier, Stein, and Cienkowsky.)

apply themselves to it, so as to assist in holding it captive, and that it
is slowly drawn by their joint retraction towards the body of its
captor. Any small particle nol capable of offering active resistance,
on the other hand, may be seen after a little time to glide towards
the central body along the edge of the pseudopodium, without any
visible movement of the latter, much in the same manner as in flromia.
When in either of these modes the food has been brought to the
surface of the body, this sends mer it on either side a prolongation of

HELIOZOA

739

its own sarcode-substance ; and thus a marked prominence is formed
(fig. 573, A, N), which gradually subsides as the fond is drawn more
completely into the interior. The struggles of the larger animals,
and the ciliary action of Infusoria and Rattfrra, may sometimes be
observed to continue even after they have been thus received into
the body ; but these movements at last cease, and the process of
digestion begins. The alimentary substance is received into one
of the vacuoles, where it lies in the first instance surrounded
by liquid ; and its nutritive portion is gradually converted into
an indistinguishable gelatinous mass, which becomes incorporated
with the material of the sarcode-body, as may be seen by the
general diffusion of any colouring particles it may contain. .Several

FIG. 574. — Actinosphcerium EicJiornii: »i, endosarc ; r, ectosarc ;
c, c, contractile vacuoles.

vacuoles may be thus occupied at one time by alimentary particles ;
frequently 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
digestive process, which usually occupies some hours, is going Un.
a kind of slow circulation takes place in the entire mass of the endo-
sarc with its included vacuoles. If, as often happens, the body
taken in as food possesses some hard indigestible portion (as the shell
of an entomostracan or rotifer), this, after the digestion of the soft
parts, is gradually pushed towards the surface, and is thence extruded
by a process exactly the converse of that by which it was drawn in.
If the particle be large, it usually escapes at once by an opening which

740 MICROSCOPIC FORMS OF ANIMAL LIFE — PROTOZOA

extemporises itself for the occasion ; but if small it sometimes glides
along a pseudopodium from its base to its point, ami escapes from
its extremity.

The ordinary mode of reproduction in Actitiophrys seems to be
by binary subdivision, its spherical body showing an annular con-
striction, which gradually deepens so as to separate its two halves by
a sort of hour-glass constriction, and the connecting band becoming
more and more slender, 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 doubtful, however, whether this junction really involves a
complete fusion of the substance of the bodies which take part in it,

Fie;. 57."). — Marginal portion of Actinosphcerium Eichornii
as seen in optical section under a higher magnifying
power: in, rndosarc ; >; ectosarc ; a, a, «, pseudopodia ;
//, ii, nuclei with nucleoli ; /', ingested food-mass.

and there is not sufficient evidence that it has any true generalise
character. Under these circumstances we must hope that Dr. F.
Schaudinn's preliminary note> of his observations' may soon be
followed by a more detailed account. This author claims to have
demonstrated the fusion of the nuclei of J . sol, and the resemblance
of the course of events to the maturation of the ova of higher
animals is very striking. Certain it is that such a. junction or
• /.ygosis ' may take place, not between two only, but even several
individuals at once, their number being recognised by that of their
contract ile \ esicles : and that, after remaining thu> united for several

1 SI!. \ka,l.. Berlin, isiii;, p. 41).

i >A

741

hours as a colony, they may separate again without having
undergone any discoverable change.

Under the generic name Actinophrys \vas formerly ranked the
larger but less common Heliozoon, now distinguished as Actiiw-
K/>lin>riuni EicJiornii (fig. 574); the pseudopodia are longer and
more numerous ; there are generally a number instead of one con-
tractile vacuole. and there is more than one nucleus. The axis of the
pseudopodia may be seen to be clothed with a layer of soft sarcode
derived from the super-
ficial 01- cortical zone of
the body. Several nuclei
(//. it) are usually to be
seen imbedded in the
protoplasmic ma>s. The
general life-history of
this type corresponds
with that of the pre-
ceding, but its mode of
reproduction presents
some marked peculiari-
ties. In many if not in
all cases it commences.
as first observed bv
Kolliker, with the con-
jugation of two separate
individuals. The binary
segmentation is pre-
ceded by a withdrawal
of the pseudopodia. even
their clearly defined
axis becoming indistinct
and finally disappear-
ing; the body becomes
enveloped by a clear
gelatinous- exudation,
which forms a kind of
cyst ; and within this
the process of binary
subdivision is repeatedly
performed, until the

original single ma^s i>

FIG. 576. — Clathrulina elcgans: A, complete
organism ; B, swarm-spore showing nucleus, »,
and two contractile vesicles near its opposite end.

replaced by a sort of
morula, each spherule of which shows the distinction between the
central and cortical regions, 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 gradually grow into the likeness of their parent.1

1 On the results of the artificial division of Actinosphferium see K. Brandt, T~< 1» r
Actinosphterium EirJinrnii, Halle a'S., 1877; Gruber, Hrrirlite d. Nntiirf. (Irx. zu
Freiburg ijB., lSS(i ; Nnssbaum, Arch. f. M/l'r. Aunt. xxvi.

742 MICROSCOPIC FORMS OF ANIMAL LIFE — PROTOZOA

A large number of new and curious fresh-water forms of thi
type are being frequently brought under notice, of which the Clathru
Una eleyans (fig. 576) may be specially mentioned as presenting ai
obvious transition to the Polycystine type. This has been foun<
in various parts of the Continent, and also (by Mr. Archer J) ii
Wales and Ireland, occurring chiefly in dark ponds shaded fr
trees and containing decaying leaves. Its soft sarcode-body, whicl
is not differentiated into ectosarc and endosarc, is encased by ;
siliceous capsule of spherical form, regularly perforated with ova
apertures, and supported on a long silicified peduncle. The bod;
itself and the pseudopodia which it puts forth through the nper
tures of the capsule seem closely to correspond with those o
Actinophrys. Reproduction 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 hours in a- free condition as an Actinophrys, am
ultimately produces the capsule and stem characteristic of its type
In the second mode numerous small rounded sarcode masses, eacl
possessing a nucleus, are produced within the capsule, in whal
manner cannot be clearly made out ; and every one of these if

enveloped in a firm en
velope, set round witl
short spines, probablj
siliceous. These cyst.-
remain for months with-
in the common capsule
and when the time arrive.-
for their further develop-
ment the sarcode -cor-
puscles slip out of theii
cysts, and escape through
the orifices of the capsule
as flagellated monads oi
oval form (fig. 576, B),
FIG. 577. — Diagrammatic representation of Amoeba eac]l havino" a nucleus

P-;'

VIL

a, near the base of the
flagella, and two con

proteus: E C, ectosarc; E N, eudosarc ; C V, con-
tractile vesicle ; N, nucleus ; P, pseudopodia ;
VIL, villous tuft.

tractile vesicles near its

opposite end. After swarming for some hours in this condition,
they change to the free Actinn/Jtri/s form, and finally acquire the
siliceous capsule and stem of the (Jlathrulina.

Lobosa. — No example of the rhizopod type is more common in
streams and ponds, vegetable infusions, Arc., than the Amoeba
(fig. 577); a creature which cannot be described by its form, for
this is as changeable as that of the fabled Proteus, but may yet be
definitely characterised by peculiarities that separate it from the
two groups already described. The distinction between ' ectosarc '
and ' endosarc' is he-re clearly marked, so that the body approaches

1 See his memoir on Fresh-water Kadiolaria in Quart. Journ. of Microsc. Sci.
n. s. vol. ix. 1809, p. 250.

LOBOSA 743

much more closely in its characters to an ordinary ' cell ' composed
of cell-wall and cell-contents. It is through the ' endosarc ' alone,
E 1ST, that those coloured and granular particles are diffused, on
which the hue and opacity of the body depend ; its central portion
seems to have an almost watery consistence, the granular particles
being seen to move quite freely upon one another with every change
in the shape of the body ; but its superficial portion is more viscid,
and graduates insensibly into the firmer substance of the ' ectosarc.'
The ectosarc, E C, which is perfectly pellucid, forms an almost
membranous investment to the endosarc ; 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 ; l and thus there is no evidence, in Amoeba and its immediate
allies, of the existence of anymore definite orifice, either oral or anal,
than exists in other rhizopods. The more advanced differentia-
tion of the ectosarc from the endusarc of AiiH?l><i is made evident
by the effects of reagents. If an Amceba radiosa be treated with
a dilute alkaline solution, the granular and molecular eiidosarc
shrinks together and retreats towards the centre, leaving the radia-
ting extensions of the ectosarc in the condition of csecal tubes, of
which the walls 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 Colin as possessed by Para-
mecium and other ciliated Infusoria, and with the containing mem-
brane of ordinary animal cells. A ' nucleus,' N", is always distinctly
visible in Amceba, adherent to the inner portion of the ectosarc, and
projecting from this into the cavity occupied by the endosarc : when
most perfectly seen it presents the aspect of a clear flattened vesicle
surrounding a solid and usually spherical 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 rendered 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,' C V, seems also to be uniformly present, though it
does not usually make itself so conspicuous by its external prominence
as it does in Actinophrys ; 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 rounded ;
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 staled by Professor Huxley in the following
admirable sentence : ' Physically the ectosarc might be compared 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 sheet, 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 — PROTO/OA

into contact they scarcely show any disposition even to mutual
cohesion, still less to fusion of their substance. Sometimes the
protrusion -seems to be formed by the ectosarc alone, but more
commonly endosarc also extends into it. and an active current of
granules may be seen to pass from what was previously the centre
of the body into the protruded 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 manner that- ;)n Amoeba moves from place to
place, a protrusion like the linger 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 transference has been accom-
plished, or even before it is complete. The kind of progression thus
executed by an Amveba is described 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
Lachniann that the appearance of rolling is an optical illusion,
since the nucleus and contractile 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 place that the Antn-la en-
counters pai-ticles which are fitted to afford it nourishment ; and it
appears to receive 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 primitive fashion.

It ma}' often be seen that portions of the sarcode-body of an
A/iniftit. detached from the rest, can maintain an independent exist-
ence ; and it is probable that such separation of fragments is an
ordinary mode of increase in this group. When a pseudopodial lobe
lias been put forth to a considerable length, and has become en-
larged and fixed at its extremity, the subsequent contraction of the
connecting portion, instead of either drawing the body towards the
lixed point, or retracting the lobe into the bodv. causes the connect-
ing band to thin away until it separates; and the detached portion
>peedily shoots out pseudopodial processes of its own. and comports
itself in all respects as an independent A •nninba. Multiplication
also takes place by regular binary subdivision. Various observers
have seen phenomena which they have supposed to be evidence of the
formation of • swarm-spores ' or of the development of cysts, but it
must be borne in mind th.--t ,-i large number ol' proto/.oa pass during
the course of their life through amcebiform stages, some of which
mav have been taken as true species of .1 imi'.bd. No sexual act has
been certainly recognised as part of the life-history of Amvt'ba, the
union of two or more individuals, which mav he occasionally wit-
nessed, having more the character of the • /.ygosis r of Actinophrys.
A sarcodic organism discovered 1>\ <!reef. and named by him
I'i'loini/.i-ii /Hiln.Hti'ix (tig. r>78), which spreads over the bottom of
-tagnaiit ponds in the condition of slimv masses of indefinite form,
i Prof. A. M Ivhvanls (U.S.A.) in Monthly Microsc. Journ. vol. \iii.l872, p.2!».

LOBOSA

745

exhibits a further advance upon the A mo.- ban type. The .substance
of its lx»l v. which may be of the size of two millimetres, exhibits ;i
very deal- differentiation between the homogeneous hyaline ectosarc
(J">. a. (1) ami the contained endosarc, which contains such a multi-
tude of spherical vacuoles. ?>, as to have a -vesicular' or frothy
aspect. When it feeds upon the decomposing vegetable matter at
the bottom of the pool it inhabits. it> bodv acquires a blackish hue.
but in other situations it may be colourless. Besides the vacuoles
there are seen in the endosarc a great number of nucleus-like bodies.

FK;. 578.-Pelom.yiKa. palustris: A, as it appears when iu amoeboid
motion; B, portion more highly magnified, showing u.a, the hyaline
ectosarc ; b, one of the vacuoles of the endosarc ; c, rod-like bodii1- i pro-
bably Bacteria) scattered through the endosarc; <?, protruded exten-
sion of ectosarc with endosarc pa^siirj int.. it ; c,/-, nuclei ; /,/, globular
hyaline bodies.

e, e, and also many hyaline globular brilliant bodies. /'._/', which are
regai-ded by (Ireef as germs or swarm-spores developed from micleoli
set free within the general cavity of the body by the bursting of the
nuclei. This creature during the active period of its life moves like
an anueba. either by general undulations of its Mil-face, or by special
pseudopodial extensions, d. After a time, however, its movements
cease, and it looks as if dead : but by the giving way of its ecto-
sarc. a multitude of minute amtebiform bodies break forth, each
having its nucleus and contractile vesicle. The.-e at first live as
, but afterwards pass into a resting state, assuming a spherical

746 MICROSCOPIC FORMS OF ANIMAL LIFE - PROTOZOA

or oval shape, and then, put forth flagella, by which they swim
actively for a time ; later on, they probably settle down to develop
themselves into the parental form.

The Amoeban like the Actinophryan 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, C, 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 regular 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 gravel, shell, &c. cemented together.
In each of these genera 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 Gromia (fig. 571). In each case a de-
tached portion of the sarcodic body will put forth pseudopodia of

. 579. — Testaceous forms of Amoeban rhizopocls : A, Ditfittt/ia
proteiformis ; B, Difflugiu oblonga ; C, Arcella acuminata ; D,
Arcella dentata.

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 amcebans thus inclosed. In Arcella it has been
observed that the pseudopodia of two or more individuals unite hy
bridges of protoplasm, and afterwards separate ; and it seems to be
almost certain that this is a true 'conjugation,' and not a mere
' zygosis.' A remarkable method of reproduction has been observed
by Gruber in E-uglypha 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 covering for the extruded
protoplasm ; in about an hour 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 constricted; the anterior portion passes into the daughter-
cell. Here we have the remarkable 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 anivebans have been recently discovered, which
form tests of remarkable 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 formed by
exudation from their own bodies or have been picked up from the
surface over which the animals crawl. There can be no doubt of
this kind, however, in regard to the Qt/adnda symmetrica repre-
sented in tig. 580 ; the sarcode-body is here encased in a pear-shaped
test, of glassy transparence, made up of a great number of square
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 protoplasm. In the
posterior part of the body is seen a large clear spherical nucleus,
with a distinct dark nucleo-
lus ; and in front of this are
contractile vesicles, usually
two in number.1

Coccollths and Cocco-
spheres. — This would seem
the most appropriate place
for the description of certain
peculiar little bodies found
very extensively diffused 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, •>)
which Dr. Wallich in 1860
found aggregated in the
spherical masses which lie
designated as _' coceospheres ' FlG_ 5so.-Q/t(lJntla symmetrica, with

(3). Regarding the gelati- extended pseudopodia,

nous matrix in which they

were imbedded as a new type of the Mon&rozoa described by Haeckel,
having the condition of an indefinitely extended plasmnditim. Pro-
fessor Huxley proposed to designate it by the name Bathybvus,
indicative of its habitat in the depths of the sea ; and this idea was
accepted by Haeckel, whose representation of a living specimen of
J3athi/bius, with imbedded coccoliths, is given in fig. 581, 3. The
observations made in the ' Challenger ' Expedition, however, have,
not confirmed this view ; the supposed Bathybius being a gelatinous

1 See especially the admirable work of Professor Leidy on the fresh-water
rhizopods 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.

-MICROSCOPIC FORMS OF ANIMAL LIFE— PROTOZOA

precipitate, consisting of sulphate of lime, slowly deposited in water
to which strong spirit lias been added. Whatever be their nature.1
coecoliths and coccospheres are bodies of great interest; since their
occurrence 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 former are round or oval discs, having
a thick strongly refracting 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, -j). 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

Fi(i. ~>.sl. — C'urrdlith.-i and Coccosplieres: 1, 2, 7, cyatholiths seen
obliquely; 3, coccosphere with imbedded cyatholiths; 4, coccoliths im-
bedded in supposed protoplasmic expansion ; ;">, discolitli seen in front
view; 6, cyatliolith seen in front view, showing (1) central corpuscle, (2J

.irranular zone. i::i t runs parent outer zone; 8, 9, discoliths seen edgewise.

convex side; so that when viewed edgewise they present the appear-
ances shown in tigs. -s. .'/. Their length is ordinarily between ^/OTT^'I
:|II(I .-, ,>'iM,tn of an inch ; but it ranges from oyVoth to -j-iooo^1- J^11'
largest are commonly free, but the smallest are generally found im-
bedded among heaps of granular particles, of which some are probably
discoliths in an early stage of development. The 'cyatholiths, also,
which have the general appearance of a cup and saucer, have, when
full grown, an oval contour, though they are often circular when
immature. Thev are convex on one face and flat or 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, 3) surrounding an oval thick-walled
central corpuscle (/). in the centre of which is a clear space some-

1 Messrs. Murray .MM! l>Iurkm.in h;t\r, ill ;i pvi'lmiinarv notice ( I'roc. Hoi/. Sue.
London, Ixiii. IM'.IS, p. -jCiih, -ii^i-sti'd that the ( 'ni-n^plicracea- are unicellular AlfTiv.

SPOROZOA 749

times divided into two. The zone (i>) immediately surrounding
the. central corpuscle is usually more or less distinctly granular,
and sometimes hns an almost bead-like margin. The narrower
outer zone (:j) is generally clear, transparent, and structureless,
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. .•.-*. 7). Each con-
sists of a lower plate, shaped like a deep saucer or watch-glass ; of
a smaller upper plate, which is sometimes fiat, sometimes more or
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 from about ji^^tli to s,,1,,,^]] of an inch, those of
.,,,',,,, of an inch and under being always circular. It appears
from the action of dilute acids upon the coccoliths that thev 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 strong! v
refracting character; and there remains an extremelv delicate,
finely granular membranous framework. When treated with iodine
thev are stained, but not very strongly, the intermediate sub
stance being the most affected. Both discoliths and cyatholiths are
completely destroyed by strong hot solutions of caustic potass or
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
7,';ilth of an inch. What is their relation to the coccoliths, and
under what conditions these bodies are formed, are questions whereon
no positive judgment can be at present given.

SPOBOZOA.

The term Sporozoa was applied by Leuckart to a group of
protozoic animals of which the well-known (iregariiiida, theCoccidi-
idea, the Hsemosporidia, the Myxosporidia, and the Sarcosporidia ' are
the chief divisions. They are especially characterised by the peculi-
arities of their mode of reproduction, in which a period of encystation
(which may or may not be preceded by conjugation) is succeeded by
the breaking up of the contained protoplasm into a large number of
small 'spores,' the products of which become intracellular parasito.

The (Jregarinida lead a parasitic life, and may often be met with
in the intestinal canal or other cavities of earthworm, insei-ts, tvc..
and sometimes in that of higher animals. An individual Gregarina
essentially consists of a large single cell, usually more or less ovate
in form, and sometimes attaining the extraordinary length of tn-o-
t/iii-ds <>f «i> inch.- 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. Blancliuvd in J3nU. Sin-, /.mil. France, x. p. '244.

2 See Prof. Ed. Van Beneden on <-! ri'i/n ri/m i/ii/n i/tm it'nuutl in the intestinal
canal of the lobster) in (Jinirt. Juiini. Mi<-,-<ia<-. Sci. n.s. \<>1. x. 1.870, p. 51, and vol.
xi. p. '24-2.

75O MICROSCOPIC FORMS OF ANIMAL LIFE — PROTOZOA

circular row of booklets, closely resembling that which is seen on
the head of Tfenia. 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
.•ipparatus 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
a pellucid nucleus ; and when, as often happens,

is generally seen

Fi<;. 58-2. —Cyst of Monocystis <i gilts, the Gregarinid of the earthworm
(750 diams.), showing ripe chlamydospores and complete absence of
any residual protoplasm in the cyst. (After Professor Ray Lankester.)

the cell undergoes duplicative subdivision, the process commences in
;i constriction and cleavage of this nucleus. The membrane and its
contents, except the nucleus. ;m> 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 oftbe body. The cell itself may undergo contraction, and
consequent change in form, which may, or may not. lie accompanied
by locomotion ; circular constrictions may extend along the body;
or Ilie cell may bend on itself and again straighten out. By Van
Ueneilen the contractility of the cell is localised in a layer of the

SPOKOZOA

751

ectoplasm, the so-called ' myocyte ' which he has found to consist of
a layer of contractile fibrils. When the process 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 b, c, f/, ?, fig. 583) into forms very
like those of Naviculce, and a cyst more advanced, and greatly
magnified, is shown in fig. 582. These ' pseudo-navicellse ' 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 Gregarinse, first passing through an

Fiy. 583. — Gregarina S<enuriilis, from testis of Tubifex riviilnruin,
two adults uniting: ((.succeeding stage; b, encystation stage; in c
and d the contents are seen breaking up; in e the characteristic
pseudo-navicellar form has been acquired by the spores. (After
Klilliker.)

amreba-like stage. A sort of ' conjugation ' has been seen to take
place between two individuals whose bodies, coming into contact
with each other by corresponding 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 completely fused
together. But as the products of this ' zygosis ' are the same as that
of the ordinary encysting process, there seems no sufficient reason

752 MICROSCOPIC FORMS OF ANIMAL LIFE — PROTO/OA

for regarding it. like the • conjugation ' of protophytes, as a true
generative act.

Tlie (Joccidia (tig. 5H4) are Sporozoa which look like minute ova.
and which are found resting within the cells of their hosts ; the young,
developed from spores, are 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 liver of
vertebrates. Some parasites found in the blood (Hsemamoebidae),
such as Drcj'xinidinni rci mini in. 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 malaria.1 Among

FIG. 58i. — ( 'ni'i-iili n in nrii'in'ii/i' (Leuckart) from theliverof the rabbit :
ft, cyst just formed ; /<, condensed contents, the outer envelope has
disappeared ; r, contents divided into four sporoblasts ; d, the sporo-
blasts have become rounded and clearer internally; e and/, formation
of the falciform germ ; r/ and fe, spores more highly magnified -17 from
the wide, // from in front.

the Myxosporidia is (jlnyen, the cause of the silkworm disease. The
Sarcosporidia are only known from the striped muscular 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 observed by Rainev and others, and wrongly
regarded as the cause of the cattle plague, are sarcocystids which
live in the muscular fibre of mammals.

1 More and more interest is lirini: taken in this subject, and sonic; of the results of
recent researches are of .u'reat interest and importance. Malaria appears to lie due to a
Hsemamoebid which develop?- m .^na.ts of t he ^cnus Aim/t/n-lcs ; when they arrive in the
human subject, they appear as minute aiixebulse which live in or on the red blood
corpuscles; they give rise to sporocytes which multiply indefinitely, or to sexual
•jametorytes which undertake their sexual functions as soon as they enter the
stomach of ;_:iiats. See Moss and Fielding Ould. (J/nni. Jonni. Mia: Sci. xliii.
(1900) p. 571, and a very interesting ' Nuie on the Morphological Significance of the
Various 1'hases nf I I;emam<ebida'.' l>y M. Hay Lankester, torn. cit. )i. ">81. The
student, should also consult M. A. l.aUie's ' Heclierches Zoolo^i(]lies, CytologiqueS
et, Iliolo^iipies siir les ( 'occidies,' in !,<•//. /.iiul. Expi'n: 1890, p. 517 ct net]., and 1'r.
NVasielewski's H/HH <> . ,<-ii I; >i mi i\ .lena, I.S'.M;. A detailed bibliography will b(^ found in
I'rof. <!. Schneidi'miihrs l>n l',-i>/ii:.nri/ nix Krn nk/ic.itaerrnfffn; Leipzig, 18!)8. Tin-
\ ai'i'ins Memoirs of Grassi , Laveran, and I .c>jei- may be profitably studied.

753

CHAPTER XIJI

ANIMALCULES— INFUSORIA AND EOT IF K 1 1 I

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 applied, 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 discerned 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, zoophytes, minute crustaceans, larva' of worms,
molluscs, &c.. 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 from the application of the principle 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 overthrow 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 lon\ and the other of comparatively Ji/ti/Ji
organisation. On the lower group he conferred the designation of
Polygastrica (many-stomached), in consequence of having been led
to form an idea of their organisation which the united voice of t In-
most trustworthy observers now pronounces to be erroneous ; and
as the retention of this term must tend to perpetuate the error, it-
is well to fall back on the name Infusoria, or infusory animalcules,
which simply expresses their almost universal prevalence in infusions
of organic matter. To the higher group Professor Ehrenberg's
name Rotifrra or Rotntorin is, 011 the whole, very appropriate, as
significant of that peculiar 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 group, however, includes many members in which
the ciliated lobes are so formed as not to bear the least resemblance
to wheels. In their general organisation these ' wheel-animalcules'
stand at a much higher level than the unicellular Infusoria, but it

3 c

754 MICROSCOPIC FORMS 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 treat of them in connection with one another ;
since the microscopist continually finds them associated together,
and studies them under similar conditions.

X I . - 1 X FUSORI A .

This term, as now limited by the separation of the Hkiz
on the one hand, and of the Uotifera on the other, is applied to a
far smaller range of forms than was included by Professor Ehren-
berg under the name of ' polyga stric ' animalcules. For a large
section of these, including the 2)esmidiacece, Diatomacece, Voli'ociufci',
and many other protophytes, have been transferred by general
(though not universal) consent to the vegetable kingdom. And
it is not impossible that many of the reputed Infusoria may be but
larval forms of higher 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 apparently consisting of but a single cell,
though its parts are often so highly differentiated as to represent
(only, however, by way of analogy) the 'organs' of the higher
animals after which they are usually named.

Among the dilate* Infusoria, which form not only by far the
largest, but also the most characteristic division of the group, there
is probably none save such as are degraded by parasitic habits
which has not a mouth, or permanent orifice for the introduction
of food, which is driven towards it by ciliary cm-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 cesophageal 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 aggregated, during their passage down
the oesophagus, into minute pellets, each of which receives a special
investment of firm protoplasm, constituting it a dit/t'stire vexic/f
(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 network
of canals channelled out in the 'ectosarc;' Avhile their rhythmical
action resembles that of the circulatory and re*])iratori/ apparatus
of higher animals. There is ample evidence, also, of the presence
of a specially contractile modification of the protoplasmic substance.
having the action (though not the structure) of muscular fibre:
and the manner in which the movements of the act ive free-swimming
Infusoria are directed so as to avoid obstacles and find out passage-

INFUSORIA 755

seems to indicate that another portion of their protoplasmic suh-
stance must have to a certain degree the special endowments which
characterise the nfi-i-i-m* systems of higher animals. Altogether, it
may he said that in the ciliate Infusoria tin- lift1 of iJtP sinyl? c<-U
finds its highest expression.^

Before proceeding to the description of the ciltutfi Infusoria.
however, it will he of advantage to notice two smaller groups — the
flagellate and the suctorial — which, on account of the peculiarities
of their structure and actions, are now ranked as distinct, and of
whose 'unicellular' character there can be no reasonable doubt.
since they are, for the most part, 'closed' cells, scarcely distinguish-
able morphologically from those of protophytes.

Flagellata. — Our knowledge of this tribe has been greatly aug-
mented in recent years, not only by the discovery of a great variety
of new forms, but still more by the careful study of the life-historv
of several among them. The umini^s. properly >o 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 very
remarkable relation to sponges. The Mmm* /o/x. long familiar
to microscopists as occurring in stagnant waters and infusions of
decomposing organic matter, is a spheroidal particle of protoplasm,
from 7>Tl\njth to -5-o1Tnyth of an inch in diameter, enclosed in a delicate
hyaline investment or ' ectosarc.' and moving freely through the
water by the lashing action of its slender flagelluin, whose length
is from three to five times the diameter of the body. Within the
body may be seen a variable number of vacuoles ; and these are
occasionally occupied by particles distinguishable by their colour.
which have been introduced as food. These seem to enter the body.
not by any definite mouth (or permanent opening in the ectosarc).
but through an apertiire that forms itself in some part of the oral
region near the base of the llagellum. In some true

neither nucleus nor contractile vesicle is distinguishable, but in
the majority a nucleus can be clearly seen. The life-history of
several simple Monddime presenting themselves in infusions of
decaying animal matter (a cod's head "being found the most pro-
ductive material) has been studied with admirable perseverance

1 The doctrine of the unicellular nature of the Infusoria has been a subject of
keen controversy amongst zoologists from the time when it was first, definitely put
forward by Von Siebold (Lehrbuch der vergleich. Attnt. Berlin, 1S4.">) in opposition
to the then paramount doctrine of Ehrenberg as to the complexity of their organisa-
tion, which had as yet been called in question only by Dujardin (H/xt. Nut. flea
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 protozoic simplicity of the reproductive and de-
velopmental processes 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 Metazoa, which has been
specially and forcibly insisted on by Haeckel (' Zur Morphologie der Infusorieii,'
Ji'iiiiiscJif Zeitsclir. Bd. vii. 187o). An excellent summary of the whole discussion
was given by Professor Allman in his Presidential Addiv.-s 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

a c -2

756 MICKOSCOPIC FORMS OF ANIMAL LIFE

and thoroughness by Messrs. Dallinger and Drysdale, of whose im-
portant observations a general summary will now be given.1

The present Editor adopts the lead of Dr. Carpenter, in
arranging 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 ISTostocacese and the Bacteria.

There are some reasons for looking at the saprophytic monad
forms as a possibly degraded 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 011 the
destructive ferment. But whei-e great masses of tissue are decom-
posing, the presence of the larger monad forms is certain and in-
evitable ; and by them, accompanied by the Bactei'ia, the processes
of fermentative rotting are carried to the end.

It is their morphology which points to the Flagellata, and we
should incline to consider them a degenerate, and by degeneration
specialised form of the Flagellata if they — about eight or nine dis-
tinct forms in this latitude — belong properly to the Flagellata at
all.

The simplest of the>e organisms is represented in fig. 1, Plate
XV. A. It has been named by Saville Kent Jfoiias DnUimjrri.
and has by comparison a simple life-history. As it is with the
entire group, 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
organism. It has a long diameter of about the ^^th of an 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. -1. This is at once
accompanied by an apparent 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. o. This
becomes longer, as at 4, and attains the length of two flagella as at
5. when the two dividing halves approach and mutually dart from
each other, snapping the connecting fibre of sarcode in the middle,
so that two perfect forms are set free, as in (\ 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 rapid geometric ratio.

But this is an exhaustive process vitally, for after a period vary-
ing from eight to ten days there always appear in the unaltered
and unchanged field of observation normal forms, with a remarkable
ditlluent or amoeba-like envelope, as seen in figs, s and '.». A. These

1 See their successive papers in the Monthly Microsr. Joiirn. vol. x. 187:-!,
pp. r,:;, '.Mf. ; \ol. xi. 1874, pp. 7, <>!), 97; vol. xii. 1874, p. 2(51 ; and vol. xiii. 1875,
p. lK.r) ; and I'mrrnl. J!o//. N»r. vol. xxvii. 1878, p. 33*2. But especially for the latest
n-ulK with recent objectives, Journ. Roy- Micro. Sor. vol. v. 1885, p. 177 ; vol. vi.
!>. I'.):!; vol. vii. p. ISfi ; vol. viii. p. 177.

^er deLainat.

LIFE HISTORIES OF SAPROPHYTES.

A SHttth,Li-ir London.

MONAS 757

sometimes swim and sometimes creep, amoeba-like, by pseudopodia ;
but directly the diffluent sarcode of one touches that of another
they at once melt together, as in dg. in, A. This leads to the rapid
approach of the oval bodies of the two organisms, as in fig. 11, B,
resulting in their fusion, as in figs. 12, 13, 14, and a still condition
of the sac (fig. u) for a period of not less than six hours ; when it
bursts, as seen in fig. 15, pouring out an immense host of exquisitely
minute H/I»,-I^. as shown in fig. i:>. These are opaque or semi-opaque,
but by observation upon them at a temperature of 65° to 70° Fahr.,
they in the course of thirty minutes become transparent, elongate,
as in figs, id and 17, and. continuing to grow, assume the conditions
and sizes represented in figs, is and ID; and we were able to trace
them through all their changes of growth from the spore into the
adult condition, as at fig. •_><}, 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 more complex morphological history. It is seen in its normal
form in fig. ], C. It has but one tiagellum, and, as we believe, on
fhat account lias a much more restricted power of movement. Jt
is from the -,,,',,,,1^1 to the 7 ,,'„,, 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, a^
in fig. c,. Then the circumference of the flattened sphere twist-,
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. s ; 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, although much smaller 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 periods 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 near-
est the flagellum. These forms rapidly attached themselves to the
normal forms, as seen in fig. n, which resulted in a blending of the
two' as they swam together, until 'either was melted into Other ;'
and a still sac, shown in fig. i->, resulted.

This remained from thirty to thirty-six hours absolutely inert ;

'but at the expiration of that time it burst, as seen in fig. 13", 1), 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. Tt

75$ MICKOSCOPIC FORMS OF ANIMAL LIFE

was only after much effort that we at last. by keeping the finest of our
lenses near the mouth of the empty sac, were able to discover, where
before nothing was visible, the appearance of minute specks, which
I x -came larger and larger, growing as seen in 14. 15, Hi. 17, until
the adult size was reached, as at is. and by the act of multipartition
on the part of one of these, watched from its first disclosure by the
microscope, we were able to re-enter the cycle of its life-history.

The third form, which we may here consider fully, so as to present
a good group of histories typical in their presentation of the morpho-
logy of the whole of the monad-saprophytes as we at present know
them, is given in E and F, Plate XV.

The monad lias been named by »S. Kent Dallingeria Drysdali.
The form more recently and completely studied by Mr. Dallinger—
with all the advantages derived from trained experience, and under
objectives of the highest quality and greatest 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 nagellum about half as long again as the body. From
the shoulder-like projections behind this (//, c.) arise two other long
and fine flagella, which are directed backwards. The sarcode-bodv
is clear, and apparently structureless, with minute vacuoles dis-
tributed through it ; and in its hinder part a nucleus (d) is dis
t inguishable. The extreme length of the bodv is seldom more than,
the -.foVoth of an inch, and is often the ,; ,nyoth. This monad swims
with great rapidity, its movements, which are graceful and varied,
being produced by the action of the fiagella. which can not onlv
impel it in any direction, but can suddenly reverse its course or check
it altogether. But besides this free-swimming movement, a very
curious ' springing' action is performed by this monad when the de
composing organic matter of the infusion is breaking up, the process
of disintegration being apparently assisted by it. The two posterior
tiagella 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 forward to its horizontal position, to
lie again drawn back by the spiral coiling of the anchored flagella.
This monad multiplies bv longitudinal fission, the first stage of
which is the splitting of the anterior nagellum into two (fig. •_', a. f>).
and a movement of the nucleus (c) towards the centre. In the course
of j'rniii thirty to si. ft// *v<v///<7.s- the fission extends down the neck (fig.
.% a) ; a line of division is also seen at the posterior end (c). and the
nucleus (?>) shows an incipient cleavage. En a few seconds the
cleavage-line runs through the whole length of the body, the separa-
tion being widest posteriorly (fig. 4,^); and in from one to four
minutes the cleavage becomes almost complete (fig. 5), the posterior
part of the body, with the two hah es (« and b) of the original nucleus,
being now quite disconnected, though the anterior parts are still
held together bv a transverse band of sarcode, as seen in fig. *i, which
continues to rapidly elongate, as in tig. 7. and becomes the length of
t\\o side tiagella. as in tig. s. The forms then approach and rapidly
recede from each other, snapping the cord, as ill figs. '.) and lo. Ill
this \\ay (/'•» forms exist instead of one : and each of these almost illl-

3IONADS 759

mediately enters upon ami passes through the same process of fission,
which from first to last is completed in from four to seven minutes;
and being repeated at intervals of a few minutes, this mode of multi-
plication produces a rapid increase ill the number of the monads.

Such fission does not, however, continue indefinitely, 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 lateral fiagella and the great
development of the nucleus, and afterwards in the formation of a
transverse granular band across the middle of the body (fig. n. 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 fively and vigorously about,
shown in fig. IL', generally for from thirty-five to forty-five minutes.
Gradually, however, a ' fusion ' of the two bodies and of their re-
spective nuclei takes place, the two trailing fiagella of the ; springing '
form being drawn in (fig. in. F) : and in a short time longer the two
anterior flagella also disappear, and all trace of the separate bodies is
lost, the nuclei vanish, and the resultant is an irregular amoeboid mass
(fig. 14). which gradually acquires the smooth, distended, and ' still '
condition represented in fig. 14. «. This is a cyst filled with repro-
ductive particles of such extraordinary minuteness that, when
emitted from the ends of the cyst (tig. l.">. a) after 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 } articles, when con-
tinuously watched, are soon observed to enlarge and to undergo
elongation (figs. 16, 17. is, 19, -20). and within two hours after their
emission from the sac the anterior fiagellum, and afterwards the two
lateral fiagella (fig. H»), can be distinguished. Slight movements thei

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. L>I). though still only one-half the
length of its parent: but this it attains in another hour, and the
process of multiplication by fission, as already described, commences
very soon afterwards. 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
regarded (as in protophytes) in the light of a true generative process ;
and it is interesting to observe the indication of sexual distinction
here marked by the different states of the two conjugating individuals.
There is every reason to believe that tJie entire life-cycle of this monad
lias thus been elucidated ; and it will now be sufficient to notice the
principal diversities observed by Messrs. Dallinger and I >rysdale in
the life-cycles of the other monadine forms which they have studied.
The bi-jiaijellate or 'acorn' monad of the same observers (identi-
fied by Kent with the r<>lt/t<>ni<( m-i'lln, of Ehreiiberg) presents some
remarkable peculiarities in its mode of reproduction. Its binary
fission extends only to the protoplasmic substance of its body, leaving

760 MICROSCOPIC FORMS OF ANIMAL 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 Ehrenberg 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 accumulation of
granular protoplasm, giving to that region a roughened acorn-cup-
like aspect ; the bursting of the projection, while the creature is
actively swimming through the water, sets free a multitude of
indefinitely shaped granular fragments, 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 up of the proto-
plasm. It is, like the previous process, non-sexual or gounUnl, the
true generative process consisting here, as in the preceding cases, in
the •conjugation' of two individuals, with the usual results.

The hooked nion/id (Heteromita nin-i-nfita, Kent) is another bi-
fiagellate form, usually ovate with one end pointed, and from ¥IjLn_th
to ToYM|frh of an inch in length, being distinguished from the pre-
ceding by the peculiar character of its nagella, 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 backwards, 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
fiagellum. Multiplication takes place by transwrxi' li*>ion. and con-
tinues uninterruptedly for several days. A difference then become
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 latter
being. MS 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 particle*,
which are set free by the rupture of the cyst, and of which each is
usually furnished with a single fiagellum, by whose lashing move-
ment it swims freely. These germs speedily attain the size and form
of the parent, and then begin to multiply by transverse fission, thus
completing the 'genetic' cycle.

The culi/cii/i' monad of the same observers (Titruiiiitxs rostriitu*.
I'eiiv) has M length of from ,,,',, >th to j^^-th of an inch, and a com
pressed body tapering backwards to a point. Its four flagella (which
constitute it* generic distinction) arise nearly together from the
llattened front of the body, and it* swimming movement is a grace-
ful gliding. Near the base of the fiagella are a pair of contractile
vesicles, and further behind is a large nucleus. Multiplication takes
place by longitudinal fission, which is preceded bv a change to a semi-

MONADS 761

aimeboid state. This gives place to a more regular pear-like form,
the four flagella issuing from the large end ; and the fission commences
at their base, two pairs being separated by the cleavage-plane. The
nucleus also undergoes cleavage, and its two halves are carried apart
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 vibratory movement, and 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, likeaii Anueba (devouring all
the living and dead Bacteria in its neighbourhood), occurs previously
to • conjugation ; ' and this takes place between two of the amceboid
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 liagella 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 moi-e
without any violent splitting or breaking up. sets free innumerable
masses of reproductive particles. These under a- magnifying power
of 2. ">()() diameters can be just recognised as oval granules, which
rapidly develop themselves into the likeness of their parents, and
in their turn multiply by duplicative fission, thus completing the
• genetic ' cycle.

One of the most important researches thus ably prosecuted by
Messrs. Dalliiiger and Prysdale 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 se\eral forms now described indubitably led to the
(•inclusion that all the ad alt forms, as well as all those which had
reached a stage of development in which they can be distinguished
from the reproductive granules, are utterly destroyed by a tempera-
ture of i ;")( >° Fahr. But, on the other hand, the reproductive granules
emitted from the cysts that originate in 'conjugation' were found
capable of sustaining a Jiuid heat of 220°. and a dry heat of about
o()° more, those of the Cercomonad surviving exposure to a dry heat of
.'!i)0° Fahr. Tin- is a fact of the highest interest in its bearing on the
question of' spontaneous generation.' or abiogenesis; since it shows
that germs capable of surviving desiccation mav be everywhere diffused
through the air, and may, on account of their extreme minuteness
(as they certainly do not exceed ^-^Wijth °f :in inch in diameter),
altogether escape the most careful scrutiny and the most thorough
cleansing processes ; while (2) their extraordinary power of resisting
heat will prevent these germs from being killed, either by boiling, or
by dry-heating up to even 300° Fahr.1

Beyond these facts others of some importance, as well as a new

1 Descriptions of the special apparatus used by Messrs. Dallinger and Drysdale
in their researches will be found in Moiitltly Micros. Jotini. vol. xi. 1874, p. 97 ; ibid.
vol. xv. 1S70, p. 105; and Pmrci-il. lioij. Soc. vol. xxvii. 1878, p. 343.

762 MICEOSCOPIC FOKMS OF ANIMAL LIFE

saprophytic organism l of special character. 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 observations tin' in}n- homogeneous
objectives, especially in their apochromatic form, have been succes>-
fully employed in enlarging the area of knowledge.

The present Editor has gone carefully over the greater part of the
work, revising all the critical points with the best apochromatic ob-
jectives, and the homogeneous forms of achromatics with an aperture
of 1'50 and with a clear demonstration of the immensely greater ease
with which the work could have been done had these lenses been used
in the original investigation.

But the easily accessible proof of this is given in the work done by
I )r. I >allinger upon the ituclt-HN of the nucleated forms of these monads.

Briefly to present the facts, we may recall the part taken in the
net of fission in the form last described (Ilnlli tnji'rin l)fi/sd<di). It
will be seen by reference that it appeared to us that the /x/cli'/'s fol-

*/ l_ l

lowed tin' i>roc(-£srx inaugurated b;/ tin' xunintic sarcode. That in fact
it was a passive participator in the act of fission. This is all that
can be made out to-day by the very lenses originally employed.

But by the employment of a /..th inch and ..^th inch homo-
geneous of X.A. I'HO by Powell and Lealand. and an apochromatic
of /oth inch N.A. 1'40 by the same firm ; and also by the use of
the beautiful 3 mm. and '1 mm. X.A. 1 "40 of Zeiss (apochromatic).
it can be seen with comparative ease ilmf it i* in tin1 nn.cli'ns tlmt
nil the activities of the body arc (iriijinnt/'/l .

This may be followed from a study of Plate XVI. Fig. 1, A.
represents the nucleus of the form drawn at fig. 1. E, Plate
XV. In long diameter it is of an average length of -rth °f au

inch ; hut instead of being a darkly refractive object, as seen with
the objectives used twelve years ago. it is with the present lenses.
freed from chromatic and spherical aberration, a body in the monad
undergoing no process of change, an oval globule with a complicated
plexus-like involution throughout its substance, as seen in fig. n. A.
I Mate 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
tiagellum. as in fig. i>, E, Plate XV ; for by observing the nucleus
we discover,. before any change has begun in the body-substance.
that the plexus in the nucleus has condensed itself on cither sidi- "/
///*• nucleus, as in fig. i? o, A. Plate XVI. A clear space is left at c,
and no change has taken place in the body-sarcode, «. a. a. But.
shortly an incision takes place in the nucleus, as at </. fig. 2. and
this is immediately fitllinn-if by the incision f in the body-sarcode,
and the process goes on simultaneously in nucleus and body, as in
tig. :'., until the division of the nucleus is completely effected, and the
total severance of the bodv follows.

But as soon as the nucleus is ilir'«ti'<l , the plexus, which has been
during division, as in fig. :;, condensed over /HI ft of each dividing
half, at once distributes it self evenly again, as in tig. (i. A. and re-
mains so until another change is inaugurated in the form to which
the nucleus belongs.

1 -Inii ni. nf J{di/(il Micron. Soc. vol. v.

A

XV]

4

a

f

B

/

i

•

A. S Huth,L'th^ 1

STUDIES OF THE NUCLEUS IN SAPROPHYTIC ORGANISMS.

SAPROPHYTIC LIFE-HISTORIES 763

Xot less remarkable is this in the conjugation of the same form.
With the old lenses \ve could only discover that the end of a
series of fissions had been reached by the change which came over
the entire body of the terminal form seen in fig. 11, E. Plate XV.
But now. before the amoeboid state preceding the assumption of
the condition shown in fig. 11 takes place, it can be seen that the
nucleus undergoes remarkable change, for it passes from a highh
refractive plexus-like condition into a large milky structureless stair,
and in this condition blends with one of the ordinary forms whose
nucleus is of the ordinary type. The first result of fusion is seen in
fiy. 4. A, Plate XVI. showing only the greatly 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
when the blending is more perfect there is a diffusion of this
central or iiucleolar body through the substance of the whole, as in
fig. 5, A.

In B, Plate XVI. the nucleus only, separate from the body of
the organism known as Tctrn mitus i-ostr(iti>x. is shown as we can
reveal it with recent (-Serman and English apochromatic objectives.
This entire organism is relatively large, and its nucleus will average
in long diameter the iTnrooth <>f an inch.

Hence it affords a still better means of study. Xow this
organism divides by fission for a very considerable time, hut at length
many forms become anueboid — acting precisely as an a nice ha. but
retaining traces of their primal form. In this state two of them
blend, and as a result a sac of spore is formed from which a new
generation arises.

\\ e could with the old objectives determine nothing more than
the fact that the anm>boid form had supervened ; 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 aimrboid slate is certain to
supervene.

This is even more striking in the growth of the germ. It attains
a certain size in growth, and then there is an arrest of all enlarge-
ment. This we had long ol (served in the earlier observations. But
now with apochromatic object-glasses it has been demonstrated that
this arrest of outward growth is only the signal for an internal de-
velopment. Fig. I , B, Plate XVI, shows the condition of the nucleus
when there is an apparent pause in its growth. 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 mature body of the monad
until fission is to he inaugurated, when the change seen in fig. :>,
followed by the changes and deeper division seen in figs. 4. 5, 6, 7,
and s, ensue, and after the state of the nucleus seen in fig. 4 has
been reached, the division of the entire body begins.

It thus appears that a form of Jcaryokinesis takes place in the
nucleus of even such lowly forms as these, and that it is the nucleus
that is the seat of their intensest vitality.

A large series of more complex forms of flagellate Infusoria
has been brought to our knowledge by the researches of the late

764

MICROSCOPIC FORMS OF ANIMAL LIFE

Professor James -Clark (U.S.A.),1 followed by those of Stein, Saville
Kent,2 and Bergh. In some of these a sort of collar-like extension of
what appears t<> be the protoplasmic ectosarc proceeds from the anterior

extremity of the body (fig. 585. d). forming a kind of funnel, from the
bottom of which the flagellum arises ; and by its vibrations a cur-
rent is produced within the funnel, which brings down food-particles
to the 'oral disc ' that surrounds its origin while the ectosarc seems
softer than that which envelops the rest of the body. Towards the
base of the collar a nucleus (?/) is seen; while near the posterior
termination of the body is a single or double contractile vesicle (cr).
The body is attached by a pedicel proceeding from its posterior
extremity, which also seems to lie a prolongation of the ectosarc.
These animalcules multiply by longitudinal fission : and this, in
some cases (as in the genus M<niiixi<j<i}, proceeds to the extent of a
complete separation of the two bodies, which henceforth, as in the

• irdinary Monadina,
live quite independ-
ently of each other.
But in other forms, as
Codosiga, the fission
does not extend througl i
the pedicel, and the
twin bodies being thus
held together at their
bases, and themsehv-
undergoing duplicative
fission, clusters are pro-
duced which spring
from common pedicels
(fig. 5H6) : and by
the extension of the
division down the
pedicels themselves,
composite arborescent
fabrics, like those of
zoophytes, are pro-
duced.

In another group a structureless and very transparent horny
calyx, closely resembling in miiiiatiire the polype-cell of a Gampanu-
Ifiria, forms itself round the body of the monad, which can retract
itself into the bottom of it; and in the genus Salpingceca both
calyx and collar are present. In some forms of this group imilti-
plication seems to take place, not by fission, but by gemmation;
.-Hid, as among hvdroid polypes, the i/i'iitnni'- may either detach
themselves and live independently, or may remain in connection
with their parent stocks, forming composite fabrics, in some of which
the calyces follow one another in linear series, while in others they

cr

FIG. 585. — Single zo
collar ; ti. nucleus ;

iid
cv,

>t' Codosiga innbellata : rl,
double contractile vesicle.

1 Sec bis memoirs in Ann. \'nl. I/ixf. si-r. '•'•, vol. xviii. 18(50; op. cit, ser. 4, vol. i.
18G8 ; vol. vii. 1S71 ; and vol. ix. 1s7'2.

- Sec his Mninni! ,,/ fl/r Infusoria, issn-su, 2 vols. and 1 vol. of plates.

FLAGELLATA 765

take on a ramifying arrangement. While some of these composite
organisms are sedentary, others, as Diiiohri/oii. are free-swimming.

Two solitary flagellate forms. Anthophysa axid Anisonema, inay
be specially noticed as presenting several interesting points of
resemblance to the peculiar type next to be described, the most
noticeable being the presence of a distinct mouth and the possession
of two different motor organs — one a comparatively stout and stiff
bristle, of uniform diameter throughout, which moves by occasional
jerks, and the other a very delirate tapering flagellum. which is
in constant vibratory motion. If. as appears from the observa-
tions of Biitschli, the well-known Astfisia — 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 — lias a true mouth for the

FIG. 586. — Godosiga uinlielhttu : Colony-stock, springing from single
pedicel tripartitely branched.

reception of its food, it must be regarded as an animal, and sepa-
rated from the Enylena (with which it has been generally associated),
the latter being pretty certainly a plant belonging to the same
group as Volvox.1

There can be 110 longer any doubt that the well-known Noct-iluca
i/nliaris — 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 'unicellular' Flagellatrt . This animal, winch is of sphe-
roidal form, and has an average diameter of about ^-tli of an inch,
is just large enough to be discerned by the naked eye when the water
in which it may be swimming is contained in a glass jar held up to

1 See the memoir by Prof. Biitschli in Zeitschriftf. WixwtiM-h. Zool. Bd. xxx.,
of which an abridgment (with plate) is given in Qn/ui. Jinn-n. Micros. Sci. vol. xix.
1«79, p. 63.

;66

MICROSCOPIC FORMS OF ANIMAL LIFE

the light ; and its tail-like appendagCj whose length .-iliout equals
its own diameter, and which serves as an instrument of locomotion,
may be discerned with a hand-magnifier. The form of Xoctiluca, is
nearly that of a sphere, so compressed that while 011 one aspect (fig.
f>87, A) its outline when projected on a plane is nearly circular, it
is irregularly oval in the aspect (B) at right 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,1 originates ; whilst it deepens down at the base
of the tentacle to the mouth, e. Along the opposite meridian then-
extends a slightly elevated ridge, c. which commences with the
appearance of a bifurcation at the end of the atrium farthest from

B

:• ••-::.:, "\*&^\ >%^C% ••'•:,-

i$:;:^i§lli5^?-:^::

••• /A A \ •••.,:

•

FIG. 587. — Noctilt/c/i miliaris as seen at A on the aboral side, and at
B on a plane at right angles to it : a, entrance to atrium ; I/, atrium ;
a, superficial ridge; d, 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,
duplicature of wall; h, nucleus. (Magnified about Hit diameters.)

the tentacle : this is of firmer consistence than the rest of the body,
and has somewhat the appearance of a rod imbedded in its walls.
The mouth opens into a short (esophagus, which leads directly down
to the great 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 from its Hoor there arises a long

The organ here termed ' tentacle ' is commonly designated ffiifjel/iun ; while
what is here termed the flagellum is spoken of by most of those who have recognised
it as a i-iliinii. 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 Xotlil/icit . and the latter as the true homologue of the flagellum of
the ordinary Flagellata. It is curious that several observers have been unable to dis-
co1 er the so- called ci 1 nun . which \vas lirst noticed by Krohn. Professor Huxley sought
for itin at least fifty individuals without success ; and out of the great number which
lie afterwards examined lie did not 'jet a. dear \ lew of it in more than half a dozen.

NOCTILUCA

767

lti ID. which vibrates freely ill its interior. The central proto-
plasmic mass sends ofl' in all directions branching prolongations of
its substance, whose ramifications inosculate ; these become thinner
and thinner as they approach the periphery, and their ultimate
filaments, coming into contact with the delicate membranous body
wall, extend themselves over its interior, forming a protoplasmic-
network of extreme tenuity (fig. 588). Besides these branching
prolongations, there is sent oft' from the central protoplasmic mass a
broad, thin, irregularly quadrangular extension (fig. 587, B. /*), which
extends to the superficial rod-like ridge, and seems to coalesce with
it: its lower free edge h.-is a thickened border; whilst its upper
edge becomes continuous with a plate-like striated structure, y. wliich
seems to be formed by a peculiar duplicature of the body-wall. At
one side of the protoplasmic mass is seen a spherical vesicle. J>. of

PIG. 588. — Portion of superficial protoplasmic reticulation formed
by ramification of an extension « of central mass. (Magnified
1,000 diameters.)

about T.-j'-g^ths of an inch in diameter, having clear colourless
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 nude us.

The particles of food drawn 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 from the surrounding protoplasm, and thus consti-
tute extemporised digestive vesicles. These vesicles soon find their
way into the radiating extension* 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 number
and position are alike variable ; sometimes only one or two are to
be distinguished : more commonly from four to eight can be seen;

;68

MICROSCOPIC FORMS OF ANIMAL LIFE

and even twelve or more are occasionally discernible. The place of
each in the body is constantly being changed by the contractions of
the protoplasmic substance, these in the first place carrying it from
the centre towards 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 lias discharged any indigestible material it may
have contained, which is got rid of through the mouth. Every part
of the protoplasmic reticulation is in a state of incessant change,
which serves to distribute the nutrient material that finds its way
into it through the walls of the digestive vesicles ; but no regular
ci/closis (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 central protoplasmic mass, and in its extensions
as is shown in the centre of fig. 587. There is no contractile vesicle.
The peculiar 'tentacle' of Xoctiluca is a flattened whip-like fila-
ment, gradually tapering from its base to its extremity, the two
flattened faces being directed respectively towards and away from
the oral aperture. When either of its flattened faces is examined.it

Fi<; . 589. — Pair of digestive vesicles of Nocfiltica lying in course of exten-
sion of central protoplasmic mass, a, to form peripheral reticulation,
b, and containing remains of Alga-. (Magnified 4HO diameters.)

shows an alternation of light and dark spaces, in every respect
resembling those of striated muscular fibre, except that the clear
spaces are not subdivided. l?ut when looked at in profile, it is seen
that between the striated band and the aboral surface is a layer of
granular protoplasm. The tentacle slowly bends over towards the
mouth about five times in a minute, and straightens itself still more
slowly, the middle portion rising 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 forming the
dark bands produces the bending of the filament; whilst, when
this relaxes, the filament is straightened again by the elasticity of
the granular layer.

The extreme transparence of Xoctili'ca renders it a particularly
favourable subject for the study of the phenomena of phosphorescence.
When the surface of the sea is rendered luminous by the general
diffusion of Xoctiltica , they may be obtained by the tow-net in un-
limited quantities ; and when transferred into a jar of sea-water,
tliev soon rise to the surface, where they form a thick stratum. The
slightest agitation of the jar in the dark causes an instant emission of

NOCTILUCA 769

their light, which is of a beautiful greenish tint, and is vivid enough
to be perceptible by ordinary lamp-light. This luminosity is but of
an instant's duration, and a short rest is required for its renewal. A
brilliant but short-lived display of luminosity, to be followed by its
total cessation, may be produced by electric or chemical stimulation.
Professor Allman found the addition of a drop of alcohol to the water
containing specimens of Xoctiluca. on the stage of the microscope,
produced a luminosity strong enough to be visible under a half-inch
objective, lasting with full intensity for >everal seconds, and then
gradually disappearing, lie was thus able to satisfy himself that
the special seat of the phosphorescence is the peripheral protoplasmic
reticulation which lines the external structureless membrane.

The reproduction in this interesting type is effected in various
ways. According to Cienkowskv. even a small portion of the proto-
plasm of a mutilated Xoct'dnca will (as among rhizopods) reproduce
the entire animal. Multiplication by fission or binary subdivision,
beginning in the enlargement, constriction, and separation of the two
halves of the nucleus, has been frequently observed. Another form
of non-sexual reproduction, which seems parallel to the 'swarming
of many protophytes, commences by a kind of encysting proce->.
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 >phere. 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 • genimules ' are formed, each consisting
of a nuclear particle enveloped by a protoplasmic laver. 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 breaking up into individuals ;
whilst, more commonly, the genimules detach themselves one bv one,
the separation beginning at the margin of the disc, and proceeding
towards its centre. The genimules 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 afterwards make their appearance. A process of 'conjuga-
tion' has also been observed, alike in ordinary Xoctilacw and in their
closed or encysted forms, which seems to be sexual in its nature.
Two individuals, applying their oral surfaces to each other, adhere
closely together, and their nuclei become connected by a bridge of
protoplasmic substance. The tentacles are tin-own off, the two bodies
gradually coalesce, and the two nuclei fuse into one. The whole
process occupies about five or six hours, but its results have not been
followed out.1

1 Noctili/ca has been the subject of numerous memoirs, of which the following
are the most recent: Cienkowsky, Arch f. micros. Anat. Bel. vn. 1871, p. 181, and
Bel. ix. 1873, p. 47; Allman, Quart. Juu.ru. Microsc. Sci. n.s. vol. xii. 1872, p. 327 ;
Robin, Journ. de I'Anat.et de Phijsiol. torn. xiv. 1878, p. .586; Vigiial, Arch. ,lr
Phi/sioL set. ii. torn. v. 1878, p. 415; Stein, Der Organismus dcr Iiifusionsthiere,
iii.2, 1883; and Biitschli, Morplwl. JaJirbuch.x. 1885, p. 529. For the group of
which it and the Mediterranean genus Leptodiscits (Hertwig ) are the representatives,
Haeckel has suggested the name CystaflugrUata.

3 D

770

MICROSCOPIC FORM* OF ANIMAL LIFE

The name Cilio-flayellata and the definition of the group must
both be altered, now that Klebs and Biitschli have shown that what
was regarded as cilia in the transver.se grooves of their bodies is
really a flageUum ; the name to be used is Dinaftayellata.1 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 Peridirtium observed
by Professor Allnian in 1854 was present in such quantities that
it imparted a brown colour to the water of some of tlie large ponds
in Phoenix Park, Dublin, this colour being sometimes uniformly
diffused, and sometimes showing itself more deeply in dense clouds,
varying in extent from a few square yards to upwards of a hundred.
The animal (fig. 590, A, B) has a form approaching the spherical,
with a diameter of from laVo^1 to -^^th of an inch, and is
partially divided into two hemispheres by a deep equatorial furrow,
ft. whilst the nagelluin- bearing hemisphere. A. has a deep meridional
groove on one side, 6, extending from the equatorial groove to the
pole, the nagelluin taking its origin from the bottom of this vertical

<

FIG. 590. — Peril! / iii it in nlici'i'inii/ in : A, B, front and buck view.- ;
C, encysted stage; D, duplicative subdivision.

groove, near its junction with the equatorial. The members 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 chromatophores 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
observed ; but a large nucleus, sometimes oval and sometimes horse-
shoe-shaped, seems always present. The Peridinia multiply by
transverse fission (fig. 590, D), which commences in the subdivision
of the nucleus, and then shows itself externally in a constriction of
the ungrooved hemisphere, parallel to the equatorial furrow. They
pass into a quiescent condition, subsiding towards the bottom of the
water, and the loricated forms appear to throw off their envelopes.
Then- is rea.son to believe that conjugation obtains in certain cases:
Glenodinium i-in/inni has been observed 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-\\ater.- but the most remarkable marine

1 Or, more rnnvctly, Dinomastigophora.

- Si-f F. Sc-liiitt, ' Die Peridineen dcr Plankton Expedition,' Ergebn. Plankton
1. ./•/„ il. 1,^.1:.. 170 pp. and 27 pis.

CERATIUM 771

forms of the cilio-flagellate group belong to the genus C&ratiwm (fig.
i)91), in which the cuirass extends itself into long horny appendages.
In the Ceratlum tripos (1) there 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.
< me 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.1

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 litile flattened,

1 See Allmaii in Quart. Microsc. Journ. vol. iii. 1855, p. 24 ; H. James-Clark in
Ann. Xi/t. Hist. ser. iii. vol. xviii. 1866, p. 4-29 ; Bergh, MorplwL Jahrbiich. vii. 1881.
P. 177, and Vanhliffen, Zool. Anzeig. xix. 1896, pp. 133-4.

3c 2

772

MICROSCOPIC FOBMS OF ANIMAL LIFE

tapering gradually from the base to the point. Their size is ex-
tremely variable, the largest that have been observed being about
5^oth of an inch in length, and the smallest about is^-orjth. When
in motion each filament appears to bend from 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 affected in
succession with this motion, the appearance of progressive waves
following one another is produced, as when a cornfield is agitated
by successive gusts. When the ciliary action is in full activity,
however, little can be distinguished saA'e the whirl of particles in
the surrounding fluid ; but the back stroke may often be perceived,
when the forward 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. 5il'2. — A, Kcrona sihtrus: a, contractile vesicle; b,
mouth ; c, c, animalcules swallowed by the Kerona, after
having themselves ingested particles of indigo. B,
Paramecium raiidatiini: <i, ft, contractile vesicles;
b, mouth. The dotted lines 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 clearly seen. Their action has
been observed to continue for many hours, or 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 ziiospores of many
protophytes. and also clothe the free internal surfaces of the respi-
ratory and other passages in all the higher animals, including man
(our own experience thus assuring us that their action takes place,
not only without any exercise of irill, but even without conscious-
ness), it is clear that to regard animalcules as possessing a ' voluntary '
•control over the action of their cilia is altogether unscientific.

CILIATA 773

In the ciliated Infusoria, the differentiation of the sarcoclic sub-
stance into ' ectosarc ' or cell-wall, and ' endosarc ' or cell-contents,
becomes very complete, the ectosarc possessing a membranous
firmness which prevents it from readily yielding to pressure, and
having 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 interior of the ectosarc), in other cases lying in the
midst of the endosarc. In many Ciliata a distinct ' cuticle ' or
exudation-layer may be recognised on the surface of the ectosarc;
and this cuticle, which is studded with regularly arranged markings
like those of Diatomacea1, seems to be the representative of tin-
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 ' lorica ' that completely invests
it ; and there are other cases in which it is so prolonged and
doubled upon itself as to form a sheath resembling 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 ingestioii 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) regular, parallel, fine stria? may be
distinguished, and as this striation is also distinguishable in the
eminently contractile foot-stalk of Vorticella1 (fig. 593, B) there seems
good reason to regard it as indicating a special modification of proto-
plasmic substance, which resembles muscle in its endowment*.
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 effected 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 around the oral

1 On the morphology of the Vorticellinse see Biitschli, Marphol. Jaltrli. xi.
1>. 553.

774

MICROSCOPIC 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 month, sup-
plies the means in this group of Infusoria both for progres-
sion through the water and for drawing alimentary particles into
the interior of their bodies. 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. Some 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 bristle-like firm-
ness, and, instead of keeping
themselves in rapid vibration, are
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 Trichoda lynceus (fiy.
597, P, Q). In Ckilodon and
JTassitla, 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 iiud subsequent drawing together
(somewhat after the fashion of the tentacula of zoophytes) than of
reducing them by any kind of masticatory process.' Some, like
OfHiliiin., are entoparasit ie. and have no mouth; a form allied to
O/>(/l/ti(i, (Anoplophrya o/v//A//,lS) ]jvt.s in the blood of J «•////>•
Kiftniticits; other entoparasites. such as TricJinii i/ni/tlni in the ' white
ant,' still possess their mouth. The curious contraction of the foot
stalk of the Vortici'lld (fig. 59:!), again, is a movement of a different
nature, Iteing due to the contractility of the tissue that occupies
the interior of the tubular pedicle. This stalk serves to attach tin-
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

KIG. 503. — Group of ]'miici'Jfn iirJnilifera
showing, A, the ordinary form ; B, the
sa,nie with the stalk contracted ; C, the
same with the bell closed ; D, E, F, suc-
cessive stages of tissiparous multiplica-
tion.

CILIATA 775

food, with its cilia in active vibration, the stalk is fully extended.
If, however, the animalcule should have drawn to its mouth any
particles too large to be received within it, or should be touched by
any other that happens to be swimming near it, or should be
'jarred ' by a smart tap on the stage of the microscope, the stalk
suddenly contracts into a spiral, from which it shortly afterwards
extends itself again, into its previous condition. The central cord,
to whose contractility 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 special irritability
of muscle, being instantly called into contraction (according to the
observations of Kiihne) by electrical excitation. The only special
1 impressionable ' organs 1 for the direction of their actions with the
possession of which Infusoi'ia can be credited are the delicate
bristle-like bodies which project in some of them from the neighbour-
hood of the mouth, and in Stfutor from various parts of the surface.
The red spots seen in many Infusoria, which have been designated
as eyes by Professor Ehrenberg, from their supposed correspondence
with the eye-spots of Rotifera, really bear a much greater re-
semblance to the red spots which are so frequently seen among
protophytes. R. Hertwig, who seems to have successfully defended
himself against the strictures of Professor Vogt, has described a
vorticellid — Erythropsis ay His — as having a pigment-spot which
cannot but be regarded as a rudimentary eye : Metschnikoff, who
thinks that JErythropsis is an Acinetan, found a similar form with a
similar eye near Madeira ; and Harker observed that if light be
allowed to fall on a part only of a colony of Oph/ridiwm versatile all
the members soon congregate to the illuminated portion.2

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 appears in many instances to send prolongations
across it in different directions, so as to divide it into chambers of
irregular shape, freely communicating with each other, which may
be occupied either by protoplasm, or by particles introduced from with-
out. The alimentary particles which can be distinguished in the
interior of the transparent bodies of Infusoria are usually proto-
phytes of various kinds, either entire or in a fragmentary state.
The Diatomacea? seem to be the ordinary food of many ; and the
insolubility of their loricce enables the observer to recognise them
unmistakably. Sometimes entire Infusoria are observed within the
bodies of others 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 particular kinds of
them are attracted by particular kinds of aliment ; the crushed
bodies and eggs of TSntomostraca, for example, are so voraciously

1 The term 'organs of sense.' implies a consciousness of impressions, with which
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.

2 These results are confirmed by the observations of R. Franze; see Zeitsclir.
iviss. Zool. Ivi. 1893, pp. 138-64.

7/6 MICROSCOPIC FORMS OF ANIMAL LIFE

consumed by the Coleps that its body 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 conscious 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 through the water containing the animalcules a few
particles of indigo or carmine. These may be seen to be carried 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 globular 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 Noctiluca, 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.
however, is by 110 means universal, 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 aftei-
a time (first yielding up their nutritive materials), generally by a
distinct anal orifice, but sometimes by the mouth. When the
pellets are thus moving round the body of the animalcule, two of
them sometimes appear 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 polypinum, which may
be recommended for the study of the processes 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 protoplasm ; 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.

( 'oiitractile vesicles (fig. 592. a, a), usually about the size of the
' vacuoles,' are 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 rhythmical movements of contraction and dilatation at
tolerably regular intervals, being so completely obliterated, when
emptied of their content.-. a> to lie quite undistinguishable, and
coming into view again as they are refilled. These vesicles do not
change their position in the individual, and they are pretty
constant, both as to si/.e and place, in different individuals of the
same species; hence they are obviously quite dim-rent in character
from the • \acuoles.' In l'(ir<i im-ci n in t here are always to lie observed
two globulai vesicles (fig. .")'.)'J. 15. a. (/). each of them surrounded by

/'////. Tmnx. 1894. B. pp. 355-83.

CILIATA 777

.several elongated cavities, arranged in a radiating manner, so
as to give to the whole somewhat of a star-like aspect, and
the liquid contents are seen to be propelled from the former into
the latter, and vice rersa. Further, in iStentur. a complicated net-
work of canals, apparently in connection with the contractile
vesicles, has been detected in the substance of the ' ectosaiv.' and
traces of this may be observed in other Infusoria. In some of the
larger animalcules it may be distinctly seen that the contractile
vesicles have permanent valvular orifices opening outwards, and thai
an expulsion of fluid from the body into the water around it is
effected by their contraction ; in some vorticellids the contractile
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 appears likely that
their function is of a respiratory and depuratory nature ; and that they
serve, like the gill-openings of fishes, for the expulsion of water
which has been taken in by the mouth, and which has traversed the
interior of the body.

Of the reproduction of the ciliated Infusoria our knowledge
though imperfect has advanced. As h.i- been well said by Mr.
Adam Sedgwick,1 'the more recent work of Biitschli and Maupas
[has] shown that in their reproduction these animals resemble other
Protozoa ; that is to say, that the whole bodv participates in the
reproductive fission, that the parent disappears in the offspring, and
that special conjugating cells of the nature of ova and spermatozoa are
not formed. Maupas - especially, by following the history of the
individual resulting from conjugation, has definitely established the
fundamental distinction between conjugation and reproduction, and
has thrown a flood of light upon the meaning of the whole phenomenon
of conjugation.' The best evidence is that of Gruber, 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 Chllodon cucidlalus (fig. 595), it has been supposed to
occur in either direction indifferently. But it may fairly 1 >e questioned
whether, 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 Ehreiiberg. no fewer than
268 millions might be produced in a month by the repeated sub-
divisions of a single Paramecium. "When this fission occurs in
Vorticella (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 (_'odo*i<jn

1 Student's Tr.rtlool; of Zoology, 1898, p. 20.

2 See particularly his memoirs, in vols. vi. and vii. of the second series of the Arch.
Zool. EJ-JJCI: 1888-9.

77s

MICROSCOPIC FORMS OF ANIMAL LIFE

(fig. 586) by the like process of continuous subdivision. Another
curious result of this mode of multiplication presents itself in
the family Ophrydlna, masses of individuals which separately resemble
certain Vorticellina 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 o£ Xostoc, 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 from the surface of their bodies,
instead of at once becoming completely isolated. From a comparison of
the dimensions of the individual Ophryda, each of which is about nrgth
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 eiyht mil-
linns of them if closely
packed ; and many
times that number
must exist in the

larger

masses, even

FIG. 504.— Reproduction of Infiisttritt.

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 central portions occupied by water only.
Hence we have, in such clusters, a distinct proof of the extra-
ordinary extent to which multiplication by duplicative subdivision
may proceed without the interposition of any other operation.
These animalcules, however, free themselves at times from their
gelatinous bed, and have been observed to undergo an ' encysting
process ' corresponding with that of the Vorticellina. The chemical
composition of this jelly or xiiocytium has been investigated by
Halliburton, who finds that it resembles vegetable cellulose in its
general properties, but differs from it and agrees with the form of
cellulose manufactured by the Tunica ta in being less easily converted
into sugar.

Many, perhaps all. ciliated Infusoria at certain times undergo an
• •iict/xtiit<j /trocess, resembling the passage of protophytes into the ' still'
condition, and apparently serving like it as a provision for their pre-
servation under circumstances which do not permit the continuance
of their ordinary vital activity. Previously to the formation of the
cy>t. the mo\ ements of the animalcule diminish in vigour, and
gradually cease altogether; its form becomes more rounded; its
oral aperture closes; and its cilia or other filamentous prolonga-

CILIATA

779

tions are either lost or retracted, as is well seen iu Vorttcella
(fig. 596, A). A new wreath of cilia, however, is developed near
the base, and in this condition the animal detaches itself from its

B

E

FIG. 395. — Fissiparous multiplication of Chilodon cucullulus '. A,
B, C, successive stages of longitudinal fission (?) ; D, E, F, succes-
sive stages of transverse fission.

stem and swims freely 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- Fl(i- 596.— Encysting process in Vorticella .

i i ,1 xtoma : A. full-grown individual in its eiic\ -

O17GT l/-*l-ir»' 1^10^- I \0 TMO

state ; «, retracted oval circlet of cilia ; u, nucleus :
c, contractile vesicle ; B, a cyst separated from it-,
stalk ; C, the same more advanced, the nucleus
broken up into spore-like globules ; D, the same
more developed, the original body of the Vorticella,
(I, having become sacculated, and containing many
clear spaces ; at E, one of the sacculations having
burst through the enveloping cyst, a gelatinous
mass, e, containing the gemmules is discharged.

ever long may be the
duration of its imprison-
ment, it emerges with-
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 tin-
cyst, come forth as spontaneously moving spherules. Each of the.se
soon increases in size, develops a ciliary wreath within which a mouth

7 80 MICROSCOPIC FORMS OF ANIMAL LIFE

makes its appearance, and gradually assumes the form of the T-i'icli(>-
iJ'nia yrand'niella of Ehrenberg. It then develops a posterior wreath
of cilia and multiplies by transverse fission ; each half fixes itself by
the end on which the mouth is situated, a .short stem become.- de-
veloped, and the cilia-wreath disappears. A new mouth and cilia-
wreath then form at the free extremity, and the growth of the stem
completes the development into the true vorticellan form.1 In
Trichoda lynceus, again, the ' encysting process ' appears subservient
to a like kind of metamorphosis, the form which emerges from the
cy.-t 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,2 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 O.ri/tricha. This posses.-e.-
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 midst of the
endosarc without any surrounding vesicle ; and sometimes even an
animalcule of the same species, but in a different stage of its life. i>
seen in the interior of one of these voracious little devourers (B). In
this phase of its existence the Trichoda undergoes multiplication 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 for
some time in this condition, the contents of the cyst become clearly
separated from their envelope ; and a space appears on one side, in
which ciliary movement can be distinguished (I). This space
gradually extends all round, and a further discharge 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 effete residue 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). Tin- body thus discharged (N) does not differ
much in appearance from that of the Oxytricha before its encyst -
iiient (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 ; numerou- very stiff bristle-

1 Evert -;, r///<'i-xiti'/iiDiffi'it an Vorticella neltulifcra, quoted by Professor Allmaii,
/>.><•. fit.

ales ilr.\ fir/. Xnl. ser. iii. tome xix. 1858, p. 10!).

CILIATA

781

like organs are developed, on which the animalcule creeps, as by
Ifgs, over solid surfaces; the external integument becomes more
consolidated on its upper surface, 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 round
and round with great rapidity, so as to describe a soi-t of inverted
cone whereby a current is brought towards the mouth. This latter
form had been described by Professor Ehrenberg under the name of
Asjndisca. It is very much smaller than the larva, the difference
being, in fact, twice as great as that which exists between A and

FIG. 507. -Metamorphoses of Trirlwda li/nceus : A, larva (Oxytricha) \ B,a
similar larva after swallowing the animalcule represented at M; C, a very
liii'Kf 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 ; G, 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 Aspidisca, 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 preceding. How the
Aspidisca-form in its turn gives origin to the Oxytricka-form
has not yet been made out. A similar 'encysting process' has
been observed to take place among several other forms 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 unlikely 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,
>ince they have been found to endure desiccation in this state,
although in their ordinary condition of activity they cannot be dried

/82 MICROSCOPIC 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

€/ O

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 ;
In it 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 ISTussbaum and Maupas make the
limit less than two years.

Gruber has recently reinvestigated the process of conjugation in
the Infusoria : he finds that the nucleolus of each becomes a striated
spindle, and approaches 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 doubt 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 gemmlparous multiplication — the putting forth of a bud
fi-om 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 Butschli and Engelmaiin, 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 after the expulsion of the broken-up nuclear
structures the characteristic 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, l, A, B, C), by means of which
it imbibes nourishment until the formation of its mouth permits it to
obtain its supplies in the ordinary \vay : whilst others maintain these
acinetiform bodies to be parasites, which even imbed themselves in
the substance of the Infusoria they infest.1

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 entire life-history what are to be accounted really distinct

can he no doubt that Stein was wrong in his original doctrine that the
fully il<-\ doped Arinetina are only transition stages in the development of Vorti.-
i-i-ll/'i/ii and other ciliated Infusoria. But the balance of evidence seems to the writer
tn be in favour of his later statement, that the bodies figured in fig. 594, i, are
really infnsovian embryos, and not parasitic Acinetas.

SUCTOKIA 783

forms. And the differences between them, consisting chiefly in the
shape of their bodies, the disposition of their cilia, the possession of
other locomotive appendages, the position of the mouth, the presence
of a distinct anal orifice, and the like, are matters of such trivial
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 worthy of the attention of microscopists. who can
scarcely be better employed than in tracing out the sequence of its
phenomena with similar care and assiduity to that displayed by
Messrs. Dallinger and Drysdale in the study of the Monadina. ' In
pursuing our researches,' say these excellent observers, 'we have
become practically convinced of what we have theoretically assumed
—the absolute necessity for prolonged and patient observation of
the same forms. Competent optical means, careful interpreta-
tion, close observation, and time are alone capable of solving the
problem.'

Suctoria. — The suctorial Infusoria constitute a well-marked
group, all belonging to one family, Acitiftina, 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
Vot ticettinte, but also to filamentous Algse, stems of zoophytes, or to the
1 K idies of larger animals. Their nutriment is obtained through delicate
tubular extensions of the ectosarc. which act as suctorial tentacles
(fig 598), the free extremity 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 first 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 drawn nearer to that of its captor. Instead, however,
of being received into its interior like the prey of Actlnophrys, 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 rapid stream, indicated by the
granules it carries, sets along the tube, and pours itself into the
interior of the Acineta-body. Solid particles are not received through
these suctorial tentacles, so that the Aci'tiffina cannot be fed with
indigo or carmine; but, so far as can be ascertained by observation
of what goes 011 within their bodies, there is a general protoplasmic
ci/closis without the formation of any special • digestive vesicles.'
The better known forms of this group aretranked under the two genera
Acineta and I'lido/iJn-i/a, which are chiefly distinguished by the
presence of a firm envelope or lorica in the former, while the body

784

MICROSCOPIC FORMS OF ANIMAL LIFE

of the Litter is naked. In one curious form, the Opliryodendron, the
suckers are borne in a brush -like 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 genm/n ,-ire 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 surface be-

FIG. 598. — Suctorial Infusoria: 1, Conjugation of Podoplirya1
quadripa/rtita ; 2, formation of embryos by enlargement and sub-
division of the nucleus ; 3, ordinary form of the same ; 4, Podo-

comes clothed with cilia; and, after a short time, it detaches itself
and swims away — comporting itself subsequently like the internal
embryos, whose production seems the more ordinaiy 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 embryos are put in motion within the
body of the parent (fig. 598, :>), from which they afterwards escape
by its rupture. In this condition (a) they swim about freely, and
.-.rein identical with what lias been described by Ehrenberg as a

1*No\v called, after Biitschli, Tokoj>1/rya,on account of its mode of reproduction ;
see his 1'i-iilozoa, p. lit'JH.

REPRODUCTION OF INFUSORIA

785

<listinct generic form, Megatricha, And, according to the observa-
tions of Mr. Badcock,1 these Megatricha-farras multiply freely by
self-division. After a short time, however, they settle down upon
filamentous Alga? or other supports, lose their cilia, put forth suctorial
tentacles (which seem to shoot out suddenly in the first instance
but are afterwards slowly retracted and protruded with a kind of
spiral movement), and assume a variety of amcebiform shapes (fig.
599, l, 2, 3), some of them corresponding to that of the genus
Trichophrya,. 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.
But with the return of warmth their development recommences, a

FIG. 599. — Immature forms of Podophrya quadripartita : 1, Amoe-
boid state (Trichophrya of Claparede and Lachmann) ; 2, the
same more advanced ; 3, incipient division into lobes.

footstalk is formed, and they gradually assume the characteristic
form of Podophrya quadripartita. A regular • conjugation ' has been
observed in this type, the body of one individual bending down so
as to apply its free surface to the corresponding part of another,
with which it becomes fused (fig. 598, l) ; but whether this always
precedes the production of internal embryos, or is any Avay prepara-
tory to propagation, has not yet been ascertained.2

1 Jouni. of Roy. Microsc. Soc. vol. iii. 1880, p. 563.

- The Acinetina were described both by Ehrenberg and Dujardiii ; but the first
full account of their peculiar organisation was given by Stein in his Oi-guttismus der
Infustoiistliierclien. 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 VorticeUincp and other ciliate Infusoria; this doctrine he long
since abandoned. Much information as to this group will also be found in the
beautiful Etudes sin- les Infmoires et les Rhizopodes of MM. Claparede and Ltich-
maun, Geneva, 1858-01.

3

786

MICROSCOPIC FORMS OF ANIMAL LIFE

SECTION 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. Daclay, who lias made a study of the

I >

II

FIG. 600. — lititifn vulffaris,as seeiiat H. with bhe wheels drawn in, and
nt A with the wheels expanded: It, eye-spots; /-, wheels; il, antenna;
c, jaws and teeth ; /, alimentary canal ; .'/, cellular mass inclosing it ;
//, longitudinal muscles; /, /, tulies of water-vascular system; /,-,
young' animal ; /, cloaca.

I lot item of the Bay of Xaplcs. stated that in 1H1M. •")() species were
known from tlie Baltic, 13 from the Mediterranean. H from
elsewhere, hut 32 of these occur also in fresh water. The vast
majorily 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 tile leaves and steins of water-plants, some-
limes creeping on Alga-, on which some are parasitic. l somet hues

1 Compare particularly (lie interesting O!>MT\ at ions ol' 1'rof. W. Rothert in vol.ix.
1890, of the Zoolog. •Itihi-li'i'n-hrr (Aljth. Svstemut.), \i\\ (<~-2-li->.

KOT1FERA 787

swimming freely through the water. They are met with also in
gutters on the house-top, in water-butts, on wet moss, grass, anrl
liver-worts, in the interior of Volco.f globator ami Y<nn']n ,-'m . in vege-
table infusions, on the backs of Sntomostraca, in the viscera of slugs,
earth-worms, and XaiaJfx. and in the body-cavities of Si/mi/itn
in fact, in almost every place where there are moisture and food.
The Avheel-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 projections of the bodv
whose margins are fringed with long cilia ; and it is the uninterrupted
succession of strokes given by these cilia, each row of which nearly
retnrns (as it were) into itself, that gives rise by an optical illusion
to the notion of ' wheels.' The disposition 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 tin-
mouth, to conduct it through the alimentary canal, and to enable the
animal to swim.

The great transparence of the Rotifera 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 mav
be soft and flexible, or membranous and of very varying degrees of
-t illness, or even, of an inflexible substance capable of resisting the
action of caustic potash. In this latter condition it is called silorica.
The greater number of the Rotifera have an organ 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 anv
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, however, in letting go their hold and moving through tin-
water in search of a new attachment, and may therefore be con-
sidered as perfectly free. The sessile species, in their adult stage,
on the other hand, remain attached bv the posterior extremity to tin-
spot oil which they have at first fixed themselves, and their cilia are
consequently employed for no other purpose than that of creating
currents in the surrounding water. In considering the internal struc-
ture of Rotifera we shall take as its type the arrangement which
it presents in Ili-ncli'tuim* i-uhi>n* (tig. (iOl). a common large and
handsome animal, and one that bears the temporarv captivitv of
a compressorium remarkably well.

Its vase-shaped lor/ex is hard and transparent : open in front to
allow the protrusion of the head, and closed behind, except where a
small aperture permits the passage of the foot. The anterior
dorsal edge bears six sharp spines, and the ventral edge has a
wavy outline. The //>;ad is shaped like a truncated cone, with the
larger end forward, is rounded at each side, and carries on its front,
surface three protuberances (#/>). covered with stout vibrating 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 buccat
funnel. At the bottom of the buccal funnel is the innxtn.i- (/".'•)• a

3 E •>

;88

MICROSCOPIC FORMS OF ANIMAL LIFE

muscular bulb containing the jaws or trophi (ti). These latter are
hard, glassy bodies consisting of two hammer-like pieces called
•ni'illei (fig. 602) and a third anvil-piece called an incus. Each
m'lllett'S (ms) is in two parts — the manubrium (mm), or handle,
and the imcm (us), of five finger-like processes, which unite to

PIG. 601. — Bracltioiuix riihciix: sj>, styligerous prominences civ, coronal
wreath ; ts, tactile styles ; «., dorsal antenna ; a', a', lateral antenna? ; I HI,
longitudinal muscles; as, oesophagus; oy, ovary; 0111, ovum; <•/, germ;
/•/, vibratile tags; i, intestine;/, foot; t, toes; c/n, brain ; r, eye; mx,
imistax ; ti, trophi; f/g, gastric glands; .s, stinnac-h : h; longitudinal
canals; rv, contractile vesicle ; <•!, cloaca;./}/, foot-gland. (After Dr. Hudson.)

form the hummer's head. The Incus (is), or anvil, is tin-mod of two
|irism-shuped bodies, or ratni (rs), pointed at their free ends, and
.it t.-idicd .'il thoir In-oad ends to a thin plate called the/WZr/-///// (./'^O-
which, seen vont rally or dorsallv. looks like ;i rod. These vui-ious
parts are connected by muscular fibres, and so .-u-tod on by muscles

EOTIFERA

789

attached to themselves, and to the interior of the mastax. that the
niici rise, and fall at the same time that the rami open and shut.
The food is torn by the nnci, crushed by the rami. and then pass's
between the latter down a short oesophagus (o?) into the stomach (s).
This has thick cellular walls, and is lined with cilia, especially at its
lower third, which is often divided by a constriction from the upper
part, and is often so different in its shape and contents as to merit
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 (o.y), and contractile vesicle (cv}. Just above the mastax.
and sometimes just below it. on the oesophagus, are what are sup-
posed to be salivary glands : while ttached to the upper end of
the stomach are two gastric ylaiids^
((/(/), often possessing visible ducts.
There are two further glands (fy)
in the foot, which is itself a prolon-
gation of the ventral portion of tin-
trunk below the aperture of the cloaca.
These foot-glands secrete a viscid sub-
si a nee 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

FIG. 602.— Malleate type of jaw.

*«, malleus {'^uncus

(mm, manubrium.

<rs, ramus.

is. incus -

(fin, fulcrum,
acting on the fluids of the

lorica can be readily seen, and these
parts are driven out again by the
pressure of transverse muscular fibres
body.

On either side of the body is a tortuous tube commencing in a
plexus in the head and running down to open on the coittraclili-
vesicle (cv]. These tubes bear little tags (vt), each of which appears
to contain a vibrating cilium. The real structure of these bodies i-
uncertain, and the use of the whole apparatus is much disputed ;
but the tags are possibly very minutely ciliated funnels, their 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.1

There is a bilobed nerrot's ganglion (,'/") between the buccal
funnel and the dorsal surface. Above it is the eye (e) — a refracting
sphere on a mass of crimson pigment. From the ganglion pa»
nerve-threads to a dorsal antenna (a) and to two lateral ant<'u K«> («')
on either side of the dorsal surface. These latter organs are rocket-
headed terminations of the nervous threads, and have each a bundle
of fine hairs passing through a hole in the lorica. The dorsal

1 But see Dr. Hudson's Presidential Address, Jou rn. of the 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.

790

MICROSCOPIC 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 ganglion, 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 seta' :it
the free extremity can, by invagination, be drawn within the tube.

The ovary is large and its germs are conspicuous. The animal is
( iviparous 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 arc
smaller, more spherical, and produce males ; and ephippial eggs (fig.
603), with thick cellular coverings, often ornamented with spines.
These latter can be dried completely without losing their vitalitv.
and so, lying buried in the mud of dried-up ponds, preserve the
species for next yaar.

FIG. C03.
Ephippial egg.

p-—

FIG. 604. — Male : <?, eye ; ?c, longi-
tudinal canals ; rt, vibratile tag;
cv, contractile vesicle ; ss, sperm-
sac ; 2}, penis ; /, foot ; fg, foot-
gland.

The male (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 idinti'ittiir;/ irnct of any kind, but it has a
nervous .syWr/// similar to that of the female, a red e\c. and antenna'.
Its c.rcretori/ and nin^'ulnr systems .-m- also of the female pattern.
The only other internal organ is a large nj><',-iit-*t/<- (**) ending at its
lower extremity in a protrusile. ciliated, hollow penis (y>), whose
outlet holds the position of the aims in the female: that is. on 1he
dorsal surface, at the liase of (lie fool.

The luil it'ei-a have been divided l>y I )r. 1 1 udson and 31)-. P. II. ( iosse1
into four orders, according to their powers of locomotion. These a re :
1. KiiizoTA (////• rnnfi't/). Fixed when adult.

1 'I'll/ I,'i>/i/'i'i-u, or WJteel-animalcules. Longmans, issii. It should be added
that Dr. ri;i,ti', in IS'.MI (/.film-In: f. iviss. /.»i>l. xlxi.), has suggested a division
according to the p:iiivil or unpaired charMi-lcr of tin-

ORDERS OF ROTIFERA 791

2. BBELLOIDA (the leech-like). That swim with their ciliary
wreath, and creep like a leech.

3. PLOIMA (the sect-worthy). That only swim with their ciliary
wreath.

4. SCIHTOPODA (the skippers). That swim with their ciliary wreath
and skip with arthropodous limbs.

The order Rhizota contains two families, chiefly differing from
each other in the position of the month, which in the Flosculariida
(figs. 1 and 2, Plate XVII) is central, lying in the body's longer axis,
but in the j) fell 'cert idee (fig. 3, Plate XVII) 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 from 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 faecal pellets also regularly
deposited.

The second order, Bdelloida (fig. 7, Plate XVII), while
having many points in common with the Jfelicertidce, have
a foot peculiarly their own. It has several false joints Flfi- 605-

that can be drawn one within the other like those of a "

• /> i ante n n a

telescope. The corona consists of two nearly circular discs, jn tube.

each surrounded with a double row of cilia, and both of

these can be withdrawn into an infolding of 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 capo. It can swim, however, in the usual
fashion, with its ciliary wreath. All the species of this order can,
under proper circumstances, be dried up into balls, which will retain
their vitality for even years, though in a state of utter (lustiness.
This is due to their secreting round their bodies (after ha vi i ig dra \vn in
both head and foot) a gelatinous covering which retains the body-fluids
safe from evaporation.1 This proces.s takes some time, so that if
an attempt is made to dry them on an ordinary 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 Ilotifera 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, Pliihiia. is divided into a loricate and an illoricate
group, which are not, however, ven .-.harply .separated; as in some
cases the outer layer of the skin is. though horny, yet thin and
flexible. Jjrac/doit/ix ritbens (fig. 601), which has already been fully
described, is a good type of the L/n-tcata and Copeus cerberus (fig. (5,
Plate XVII) of the Illoricata. Most of the species of this order have

1 See Davis in Monthly M/rroxr-. Jmn-ii. vol. ix. 1863, p. 207 ; Slack, at p. 241 of
same volume ; and the report of a discussion on the subject at the Royal Microsco-
pical Society, Jniini. «f lloi/n! Microsc. Soc. Lss7, p. 179.

792 MICROSCOPIC FORMS OF ANIMAL LIFE

a forked jointed foot, the fork being formed of two toes varying
greatly in size and shape, but all secreting the viscous fluid already
mentioned. The great majority of the Rotifera belong to the
Plo'ima.

The fourth order, Sdrtopoda, contains but one family, Pedal ion i>.'n ,
and has only two genera. Pedalion and Ifp.'-arthra. and the latter of
these has but one known species, the former only two. Pedal i<m
(figs. 4, .5, 8, Plate XVII) is an extraordinary creature. Its internal
organs are on the usual rotiferous plan, but its body bears no fewer
than six hollow limbs, ending in plumes like those of the Arthrojx.nl a.
and worked by pairs of opposing muscles which traverse their entire
length. These limbs are arranged round the body, some on the
dorsal, some on the ventral surface, and all running parallel to the
body's longer axis. In Hexarthra, 011 the contrary, all the limbs
are on the ventral surface, and are arranged radiatingly. There is
no foot in either Rotifer ; but in Pedalion there are two ciliated
club-like processes at the posterior extremity, rising above the
dorsal surface and secreting a similar viscous fluid to that secreted
in the toes of other Rot! f era.

This strange creature 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 Kile in 1853 ; their arthropodous limits
give them a strong resemblance to a Nauplius larva, and make it
probable that the nearest relations of the Rotifera are the ART mm -
PODA;1 at any rate, there is more probability in this suggestion than
in that of Professor Hartog that thev are allied to the Pilidium-

O t/

larva of Nemertine worms.2

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; Dujardiii,
Sistoire naturelle des Zoujilii/tes infusoires, Paris, 1841; Pritchard, History of
Infusoria, 4th ed. London, i.861 (a comprehensive repertory of information) ;
Stein, Der Organismus der Infusionstliierc, Leipzig : Erste Abtheilung, 185'J ; Zweite
Abtheilmig, 1867 ; Dritte Abtheiluiig, Halite I. 1878. Saville Kent's Manual of tlie
Infusoria, 1880-1 ; and Professor Biitschli's Protozoa (1880-1) in the new edition of
Bromi's Thierreichs. For the Rhizopoda and Infusoria specially see Clapurede
and Lachmauu, Etudes sur les lufn.^iircs ct les Rliizopodes, Geneva, 1858-(il :
Cohn, in S/i'hiilil mid A"n///7,T/ '.v Zeitschrift, 1851-4 and 1857; Lieberkiilin, in
Mailer's Arcliiv, 1850, and Ann. of Nat. Hint. '2nd ser. vol. xviii. 185(1; Engelmami,
/.nr -\ntit rtj/ xr/ti<-///r der Infusionsthiere, 1862; and Professor Biitschli's Studien
•iili/T die Coiijtif/itfioi/ ilcr Infusorien &c.., 1876. For the Rotifera specially see
Leydig, in Siebold itiul Kdlliker's Zrit.iclirift, Bd. vi. 1854; Gosse on Melicerta
ringens, in Quiirt.-lniirii.nl Microsc. Si-i. vol. i. 1853, p. 1 ; on the Manducatory-
Organs of Rotifera,!'////. Trim*. 1856; Huxley on Laciiiularia socialis in Trans.
Microsc. Soc. ser. ii. vol. i. 1853, p. 1 ; Colm, in Siebold und Kolliker's Zcitsclrrift,
Bde. vii. ix. 1856, 1858; Dr. Moxon, Trans. Linn. Soc. isiii ; Karl Eckstein, Siebold
/mil Kiil/iL; I-'K Zeitschrift, ls.s3; Bourne, l;»t<t, ra, in the 9th edition of tlieEnci/-
c/ii/n,'i/in Briliinnica ', Joliet, ' Monographie des ifclici'i-tt's.' Arcliiv. zool. exper.ser.
ii. torn. i. p. 131 ; and Plate, Jencme/N Zeitsckr. 'six. p. 1. Tin Hufit'cra, or Wheel-
iiiiiin<ilriilrx, by Hudson and Gosse, Longmans, l.s.xn. This has been usefully sup-
jilcinentcd l>y Mr. C. F. Rousselet in two papers entitled 'List of New Rotifers since
ls,s'.»,- in Journ. I!. .l/7r/Y/.vr. Soc. IS1.):;, ]>p. 45(l-,s, and 'Second List,' &c. in the
saint; journal for 1897, pp. 10-15. The bibliographical lists appended by Mr. Ilousse-
Ic-t will be found of much service, as since the publication of the work of Messrs.
Hudson and (lussc there lias been a ^reat re\i\al animiL;- the students of this Lrroup.
Mr. Slack's Murnl.s <</' Ponrl Lifr, '2nd edit. (London, 1*71), contains many interest

I > ervationa on the habits of Infusoria and Rotifera.

2 See his remarks on the relation of ihe Hot il'era to the Trochophore, in I!< j*. Brit.

PLATE XVII .

1 f

'.

ilrl JSil

West, Newman chromo

Typical Rotifers

793

APPENDIX TO CHAPTER XIII

THE preparation and preservation of Rotifers well extended as in life to
serve as type specimens is now possible, and the following is an outline of
Mr. C. F. Rousselet'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 Rotifers is to isolate the animals
by transferring as many as may be available by means of a very
fine pipette to a fresh watchglass full of perfectly clean water until all
[(articles of foreign matter have been eliminated. This is necessary
because when the animals are dead these particles adhere to the cilia of
the Rotifers, from whence it is very difficult to remove them. In the case
of fixed Rotifers, such as Melicerta, Lirnnias, 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 1 the narcotic. The great difficulty with
Rotifers 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 narcotic has been found in the following
mixture :

2 per cent, solution of hydrochlorate of cocaine . 3 parts

Methylated spirit .... . . 1 ,,

Water ... . . . 6 „

The Rotifers 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, ai'n T
three or four minutes another dose of two or three drops of the narcotic
is added, and then repeated again if necessary until it is seen that the
animals 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 Rotifers vary so much in their behaviour imder
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 Rotifer '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. 830, and compare with them the suggestion of Dr. Plate in Zeitm-Jir. f.
wiss. Zvol. xlix. (1889), pp. 1-41.

794 -MICROSCOPIC FORMS 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 Rotifers 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 2^ 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.1

1 More detailed particulars iu the treatment of the various species and in
mounting in cells will be found in Mr. Rousselet's papers on the subject, particularly
those of March 1895 and November 1«)8, in the Joiim. of tJ/>' <t>n, •!;<•(( Micr. Club,
vol. vi. pp. 5-13, and vol. vii. pp. 03-97.

795

CHAPTER XIV

FOEAMINIFEEA AND EADIOLABIA

RETURNING now to the lowest or rhizopod type of animal life
(Chapter XII), we have to direct our attention to two very remarka bit-
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 retain their form and intimate structure
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 110 other protection
than a membranous envelope. In the second group, the fiadiolaria,
the skeleton is always silicious and may either be composed of dis-
connected spicules, or may consist of a symmetrical open framework,
or may have the form of a shell perforated by numerous apertures,
which more or less completely incloses the body. The Forctminifera
probably take, and always have taken, the largest share of any animal
group in the maintenance of the solid calcareous portion 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
Radwlaria do the same, though in far less measure, for the silex.
And both extract from sea-water the organic matter uiiiversallv dif-
fused through it, converting it into a form that serves for the nutri-
tion of higher marine animals.

Sl'( TION I. FORAMINIFERA.1

The animals of this group belong to that r<;tienl:ii-ii< n form 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 alwavs

Fora t

1 For the earlier literature yonsult Jlr. C. D. Sherbom's 'Bibliography of the
rauiinifern, recent and fossil^ from 1565 to 1888,' London, 1888.

796 MICROSCOPIC FORMS OF ANIMAL LIFE

present.1 Tin- shells of Foraiuinifera are. for the most part, poli/-
thalamous, or many-chambered (Plates XVIII and XIX), often so
strongly resembling those of Nautilus, Spirula, and other cephalopod
molluscs, that it is not surprising that the older naturalists, to whom
the structure of these animals was entirely unknown, ranked them
under that class. But independently of the entire difference in the
character of the animal bodies by which the two kinds of shells are
formed, there is a most important distinction between them in regard
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 larger one, the animal of a nautiloid foraminifer has a
composite 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 earl i
of the chambers, however 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 ' p< >ly
thalamous ' forms, therefore, and the monothalamous or single
chambered, of which we have already had an example in Groin in.
and of which others will be presently described, is simply that,
whereas any buds produced by the latter detach themselves to form
separate individuals, 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 plan on which the gemmation takes place will
be the configuration of the shelly structure produced by the seg-
mented bod}'. Thus, if the bud should be put forth from the
aperture of a Layena- (Plate XIX, fig. 12) in the direction of tin-
axis of its body, and a second shell should be formed around this
bud in continuity with the first, and this process should be succes-
sionally repeated, a straight rod-like shell would be produced, whose
multiple chambers com mi in irate 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
peduncle or • stolon ' of the same material, could now project itself
or draw in its food. The successive segments may be all of the
same si/.e. or nearly so, in which case the cut ire rod will approach
1 he cylindrical form, or will resemble a line of beads; but it often
happens thai each segment is somewhat larger than the preceding

1 Dr. ScliMiidiini (Zfitxrln: ./'. wiss. /.mil. lix. !«!».">, |>. 191) has traced tin-
details of nui- 1 c Mi1 (In i -inn in Cn/cil/iliii /lo

Plate XVIII.

• • ^ '•
fc.~.

--

s , :•

m

'^M$

*m

'-^"»x

•>:&^*-:

""Si'

.
gtfS

.3|«^ »4.:i

•.. S i^-

si?-

u^ili

^.'.i:->;:'.

m

mv2:

X=SS/

v-^

A.T.H

A TYPICAL GROUP OF FO ! 1F.ERA I to 11 .

Edwin Wilson, Cambridge

FORAMINIFERA

797

(fig. 16), so that the composite shell has a conical form, the apex of
the cone being the original segment, and its base the last one formed.
The method of growth now described is common to a large number
of Foraminifera, chiefly belonging to the sub-family Nodosarince, ;
but even in that group we have every gradation between the recti-
lineal (fig. 16) and the spiral mode of growth (fig. 22) ; whilst in
the genus Peneroplis it is not at all uncommon for shells which com-
mence in a spiral to exchange this in a more advanced stage for the
rectilineal habit. When the successive segments are added in a
spiral direction, the character of the spire will depend in great degree
upon 'the enlargement or non-enlargement of the successively formed
chambers; for sometimes it opens out very rapidly, every whorl
being considerably broader than that which it surrounds, in con-
sequence of the great excess of the size of each segment over that of
its predecessor, as in Peneroplis, fig. 606 ; but more commonly there i>
so little difference between the successive segments, after the spire
lias made two or three turns, that the breadth of each \vhorl scarcely
exceeds that of its predecessor, as is well seen in the section of the
'la, represented in fig. 624. An intermediate condition is

FKI. G0(i. — Foraminifera: — Peneroplis and Orbiculina.

presented by Rotalia, which may be taken as a characteristic type of
a very large and important group of Foraminifera, whose general
features will be presently described. Again, a spiral may be
either ' nautiloid ' or ' turbinoid,' the former designation being
applied to that fonn in which the successive convolutions all
lie in one plane (as they do in the Nautilus), so that the shell
is ' equilateral ' or similar on its two sides; whilst the latter is
used to mark that fonn in which the spire passes obliquely round
an axis, so that the shell becomes • inequilateral,' having a more
or less conical form, like that of a snail or a periwinkle, the first-
formed chamber being at the apex. Of the former we have charac-
teristic examples in Polystomella (Plate XIX, fig. 23) and Xoiiionina ;
whilst of the latter we find a typical representative in Rotalia
Jjeccarii (fig. 22). Further, we find among the shells whose increase
takes place upon the spiral plan a very marked difference as to the
degree in which the earlier convolutions are invested and concealed
by the latter. In the great rotaline group, whose characteristic
form is a turbinoid spiral, all the convolutions are usually visible.
at least on one side (fig. 17) ; but among the nautiloid tribes it more
frequently happens that the last-formed whorl encloses the preceding

798 MICROSCOPIC FORMS OF ANIMAL LIFE

to such an extent that they are scarcely, or not at all. visible
externally, as is the case in Cristellnria (fig. 17), Polystdmella (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 Glohiyerina (figs. 20, 21, Plate XIX) there are
usually only four; and in Valvvlnia, the regular number is only
three. Thus we are led to the SiseriaZ arrangement of the chambers,
which is characteristic of the textnlarian group (fig. s, a, b, and '.».
Plate XVIII), in which we find the chambers arranged in two rows,
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. GOl.—DlsrorliiiHi i/fiili/ilnrix i ItoKitlii/tt i-ftrians, Schultze),
with its pseiulopodia extended.

»i±J. A. which shows a ' cast ' of the sarcode-body of the animal. ( >n
the other hand, we find in the nautiloid spire a tendency to pass
(by a curious transitional form to be presently described) into the
ci/clic'tf mode of growth ; in which the original segment, instead of
budding forth on one side only, developes (ji'/mi/n all round, so thai
a riny of small chaml)ers (or ehamberlets) is formed around the
primordial chamber, and this in its turn surrounds itself after t lie
like fashion with another ring; and by successive repetitions of the
same process the shell conies to have the form of a disc made up of
a great, number of concentric rings, as we see in Orbitolites (tig. tiO'.l)
and in ( 'i/rt<x-/i//ii'tiH (fig. (\'27).

These and other differences in the j>l<in <>/' <//-i»i-(// were made by

ace I

18

1 1 • •

.' j|

0 , . <•

\

A.T. Hollick liLh. .vm Wilson, C

ATYPICA] rpon ;RAMIK i . °6.

FOBAMINIFERA 799

M. d'Orbigny the foundation of his classifie&,tion 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 Foraminifera. For it has come to be generally admitted that
'plan of growth ' is a character of very subordinate importance
among the Foraminifera, so that any classification which is primarily
based upon it must necessarily be altogether unnatural, those
characters being of primary importance which have an immediate
and direct relation to the physiological condition of the animal,
and are thus indicative of the real affinities of the several groups
which they serve to distinguish. The most important of these
characters will now lie noticed.1

Two very distinct types of shell structure prevail among ordinary
Foraminifera — namely, the porcellanous and the Jii/olinf or ritri-unx.
The shell of the former, when viewed by reflected light, presents an
opaque-white aspect which bears a strong resemblance to porcelain ;
but, when thin natural or artificial laminae of it, are 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' type often present the appearance
of being perforated with foramina, yet this appearance 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 extreme! \
common in the shells of the ' porcellanous' Foraminifera, one of the
most frequent forms of it being a regular alternation of ridges and
furrows, such as is occasionally seen in Jfiliola, but which is an
almost constant characteristic of Peneroplis (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 tvpe. on the
other hand, the proper shell substance has an almost glassy trans-
parence, which is shown by it alike in thin natural lamellae and in
artificially prepared specimens of such as are thicker and older. It
is usually colourless, even when (as in the case with many Rotaliince)
the substance of the animal is deeply coloured; but in some few
species, such as Globigerina ruln-a. 7\i/inr</f//li,tt/ rwn. and Polytrema
iiihinn-i',1 ni. 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 with tubular i'>prf<>i-<iti»its. which pass directly,
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 minute as only to lie 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 tin' Foraminifera, published by the Ray Society, l>< which work he would
refer such of his readers as may desire more detailed information in regard to it.

SOO MICROSCOPIC FORMS OF ANIMAL LIFE

power, as is shown in figs. 632, 633. When they are very numerous

and closely set, the shell derives from their presence that kind of
opacity Avhich is characteristic of all minutely tubular textures
whose tubuli are occupied either by air or by any substance having
a refractive power different from that of the intertubular siibstance,
however perfect may be the transparence 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 Nummulites (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 peculiar vitreous lustre they exhibit when
light is thrown obliquely on their surface. In shells formed upon
this type we frequently find that the surface presents either bands
or spots which are so distinguished, the non-tubular bands usually
marking the position of the septa, and being sometimes raised into
ridges, though in other instances they are either level or somewhat
depressed ; whilst the non-tubular spots 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 perforations which are common
in the rotaline type, and the minute tubuli which are characteristic
of the nummuline, there is such a continuous gradation as indicates
that their mode of formation, and probably their uses, are essen-
tially the same. In the former, it has been demonstrated by actual
observation that they allow the passage of pseudopodial extension*
of the sarcode-body through every 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 purpose in the latter, since,
minute as they are, their diameter is not too small to enable them
to be traversed by the finest of the threads into which the branching
pseudopodia of Foraminifera are known to subdivide themselves.
Moreover the close approximation of the tubuli in the most finely
perforated iiummulines makes their collective area fully equal to
that of the larger but more scattered pores of the most coarsely per-
forated rotalines. Hence it is obvious that the tubulatlon or non-
tubulation of foraminiferal shells is the key to a very important
physiological difference between the animal inhabitants of the t\v<>
kinds respectively ; for whilst every segment of the sarcode-body in
the former case gives oft* pseudopodia, which pass at once into the
surrounding medium, and contribute by their action to the nutrition
of the segment from which they proceed, these pseudopodia are
limited in the latter case to the final segment, issuing forth only
through the aperture of the last chamber, so that all the nutrient
material which they draw in must be first received into the last seg-
ment, and lie transmitted thence from one segment to another until
it reaches the earliest. With this difference in the physiological con-
dition of the animal of these two types is usually associated a further
very important difference in the conformation of the shell — viz.

FOEAMINIFEEA SOI

that whilst the aperture of communication between the chambers
and between, the last chamber and the exterior is usually very small
in the ' vitreous ' shells, serving merely to give passage to a slender
stolon or thread of sarcode from which tin- succeeding segment may
be budded off, it is much wider in the ' porcellanous' shells, so ,-is 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 porcelldnons and the vitreous
series respectively, which frequently bear a close resemblance to
each other in form, there are certain other well-marked differences
in sti'iictiirr. which clearly indicate their essential dissimilarity.
Thus, for example, if we compare Orl>it»1ites (fig. 609) with Ct/clo-
clypens 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 appeal- little
difference between them. But a minuter examination shows that
not only is the texture of the shell 'porcellanous' and non-tubular
in Orbitolites, 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 having 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 lamirue 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 outgrowths, 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. ho\\e\er. 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 (.'i>rnnx]>ir« 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 hitter portion of the spire is often very much flattened
out, as in /Vm-/v/y///x. 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 utillatiix' 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 examining, in the first

3 F

802 MICROSCOPIC FORMS OF ANIMAL LIFE

instance, the form which has been designated as Spiroloculina. 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. Xow 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 MS 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 011 one side than on the
other. Miliolw thus modified (tig. 1, PI. XVIII) have received the
names of Quinqueloculina 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 Siloculina, 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 face>
the observer. It is very common in milioline shells for the external
surface to present a ' pitting,' more or less deep, a ridge-aiid- 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 marked 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 illustration of the tendency to dimorphism
amongst the Foraminifera has been observed by MM. Munier
Chalmas and Schlumberger ! in the .structure of the shells of this
-roup. They find that while two forms, which they distinguish as
form A and form B, are similar externally they differ in internal
structure, form B having its initial chamber much smaller than that
of form A. and this ' microsphere ' is followed by a larger number 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 general ions. The observations of the French naturalists referred
to open out a new field of inquiry, and one which is enjoying the
attention of seseral \vorkers in this department of research. -

1 11/i/lr/in iS'oc. Geol. ser. iii. vol. xiii. j>. '27:',.

3 Gf. J. J. Lister in Phil. Trans. l:;r, B (IS'.c.i, p. 401, and F. St-haudimi, ' Ueber
den Dimorphismus der Foraminifereii,' S.B. Ges. Natitrf. Jlciiin, l.s!ir>, \>. 87.

PEXEROPLIS; ORBICULINA 803

Reverting again to the primitive type presented in the simple
spiral of Gor/tnHj>/r<>. we find the most complete development of
it in Paterojdis (fig. 606), a very beautiful form, which, although
not to be found on our own 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 ti-aversing each of the septa, and giving passage to threads
of sarcode that connect the segments of the body. At <i is shown
the 'septa! plane' closing in the last-formed chamber, with its single
row of pores through which the pseudopodial filaments extend them
selves into the surrounding medium. The surface of the shell,
which has a peculiarly ' porcellanous ' aspect, is marked by close 1\
set strife that cross the spaces between the successive septal bands;
these markings, however, do not indicate internal divisions, and are
due to a surface-furrowing of the shelly walls of the chambers. This
type passes into two very curious modifications, one having a spire
which, instead of flattening itself out, remains turgid, like that of a
NoMtilus, having only a single aperture, 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 arrou ^haped 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
circular septal plane, into which both /V// '/•>/////* and l)<-n<l,-itiiin
tend to elongate themselves.

From the ordinary nautiloid multilocular 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 Orfiif/'lnti/ (fig. 606).
The relation of this to the preceding type will be best understood
by an examination of its earlier stage of growth ; for here we
see that the shell resembles that of l'<'ii>'r<>j>lis in its general form.
but that its principal chambers are divided by 'secondary septa'
passing at right angles to the primary into ' chamberlets ' occupied
l>y 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 chamberlets
of successive rows alternate with one another in position ; and the
pores of the principal septa are so disposed that each chamberlet of

3 F 2

804 MICROSCOPIC FORMS OF ANIMAL LIFE

any row normally communicates with two chamberlets in each of the
; ii 1 jacent rows. The later turns of the spire very commonly grow com-
pletely over the earlier, 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 vise to the peculiar shape represented
in fig. 606, in the illustration on the extreme light, which is the
common aduncal type of this organism. But sometimes even at an
early age the growing margin extends so far round 011 each side
that its two extremities meet 011 the opposite side of the original
spire, which is thus completely inclosed by it ; and its subsequent
growth is no longer spiral but cyclical, a succession of concentric
riti.ys 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 spired 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 general
arrangement, such as will be more fully described hereafter in

o »/

Orbitolites. The largest existing examples of this type are far sur-
passed in size by those which make up a considerable part of a
Tertiary limestone on the Malabar coast of India, whose diameter
reaches 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 larger speci-
mens are found in the Tertiary 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 chambers
into ' chamberlets.' The chamberlets of each row are connected wil h
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
tin- 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
Orbitolites; a type which, long known as a very abundant fossil in
the earlier Tertiaries of the Paris b;isin, has lately proved to be
sr.-ircelv less abundant in certain parts of the existing ocean. The
largest, recent specimens of it. sometimes attaining the si/.e of a
shilling, have hitherto been obtained onlv from the coast of New
lloll.-md. tin' Fijian reel's, and various other parts of the Polynesian
Archipelago; but discs of comparatively minute si/e and simpler
organisation are to be (bund in almost all foraminiferal sands and
dredgings from the shores of 1 he warmer regions of the globe, being

ALVEOLINA

805

especially ^abundant in those of some of the Philippine Islands, of the
Ked Sea. of the Mediterranean, and especially of the ^-Egean. When
such discs are subjected to microscopic examination, they are found
(if uninjured by abrasion) to present tin- ,-t nu-ture represented in
lit--. <><)i». where we set' on the surface (by incident light) a number

a
3

-K

5

.
-

e ^

IE!

.
_j

c3

oo
o

of rounded elevations, arranged in concentric zones around a sort of
nucleus (which has been laid open in the figure to show its internal
structure) ; whilst at the margin 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

MICROSCOPIC FOEMS OF ANIMAL LIFE

light through them ; but in those which are too opaque to be thus
seen through, it is sufficient to rub down one of the surfaces upon a
stone, and then to mount the specimen 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
ling ; 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 chamberlets of the latter ; so that the
chamberlets of each zone alternate in position with those of the zones

FIG. 609. — Orbitolitcs. Ideal representation of a disc of complex type.

internal and external to it. The radial passages 1'ioiu the outermost
ammlus 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' («), usually somewhat pear-shaped, that com-
municates by a narrow passage with a much larger 'circumambient
chamber' (f>), 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

1 Although the above may be considered the typical form of the Orbitolite, yet,
in a very large proportion of specimens, the first few /ones are not complete circle*..
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 cijcHcal. Iiithe Orbiin-
liti-x it«l ii'ii \ lig. Gil), brought up from depths of 1,500 fathoms or more, the ' nucleus '

OBBITOLITES

807

The idea of the nature of the living occupant of these cavities
which might be suggested by the foregoing account of their arrange-
ment, is fully borne out by the results of the examination of
the sarcode-body, which may be obtained by the maceration in
dilute acid (so as to remove the shelly investment) of specimens of
Orbitolites 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, b, by a narrow foot-
stalk or stolon ; and this circumambient segment, after passing almost
entirely round the primordial, ha-; 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 FlG> 610.— Composite animal of simple type of Orlito-
ill other cases (as in lites compJanata: — rt, central mass of sarcode;
the example before us) 6, circumambient segment, giving off peduncles, in

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 suit-segments.

The radial extensions of the outermost zone issue forth as
pseudopodia from the marginal pores, searching for and drawing in
alimentary 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 Spiroloculina, the growth after-
wards becoming purely concentric.

which originate the concentric zones of sub-segments
connected by annular bands.

8o8

MICROSCOPIC FOEMS 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 probable 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 differentiation of parts 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 endowments of the segments is shown by the fact — of which

o «/

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 Orbitolites italica, Costa, sp. (U. toiiiiissiiita, 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 history of this type is its
capacity for developing itself into a form which, whilst funda-
mentally the same as that previously described, is very much more
complex. Tn all the larger specimens of Orl>i1»Ht<'x \ve observe that
the marginal pores, instead of constituting but a single vow. form
many rows one above another; and, besides this, the ohamberlets
of the two surfaces, instead of being rounded or ovate in form, are
usually oblong and straight-sided, their long diameters lying in a
radial direction, like those of the cyclical type of Orlicnl/na. When
a vertical section is made through such a disc, it is found that these
oblong chambers constitute two xn/i<'>jici<il layers, between which

ORBITOLITES

809

are interposed cobni>,t<ir 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, d d, lying side by side, so as to form part of an aimulus, but
each of them disconnected from its neighbours, and communicating
. only by a double footstalk with the two annular ' stolons,' a a' ', b bf,
which obviously correspond 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 specimens, which ought never to be passed by when it can possibly
be appealed to, furnishes these very singular results: 1st. 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 simple type, whilst
the outer and later- formed part has developed itself upon the complex ;
2nd, that although the last-mentioned circumstance would 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

FIG. G12. — Portion of animal of complex
type of Orbitolites complanata:
a a', b b', the upper and lower rings of
two concentric zones ; c c, the upper
layer of superficial sub-segments, and
<l <J, the lower layer, connected with the
annular bands of both zones ; e e and
e' e', vertical sub-segments of the two
zones.

8 10 MICROSCOPIC FORMS OF ANIMAL LIFE

out two or more tiers of radiating threads), more frequently the
simple prevails 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 lower 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.1

In what manner the reproduction of Orbitolites is accomplished,
we can as yet do little more than guess ; but from appearances
sometimes presented by the sarcode-body, it seems reasonable to
infer that yemmules, 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 preceding family, and espe-
cially in the genus Miliola, we not unfrequently find the shells en-
crusted with particles 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 Foraniinifera in which the true shell is constantly
and entirely replaced 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 arenaceous ' 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 related to each other, so as to form a series of their own.
And it is further remarkable that while the deep-sea dredgiiigs
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
variety of which far exceed all previous conception. These have
been systematically described by Mr. H. B. Brady, F.R.S.,2 whose
researches have led him to believe that the long-established division

1 For further information on the subject of Orbitolites see the Author's account
of the genus in the reports of H.M.S. Challenger. Mr. H. B. Brady in his ' Challenger '
Jir/ini-/ i [i. -2-2 1 1 describes a remarkable allied type from the Southern Ocean —
Keramosphtzra Murrayi — in which the test is spherical, and the chambers are
arranged in concentric layers.

2 See his important report on the Foraminifera dredged by H.M.S. C'lm/li'tii/i-r
(1884), illustrated by lid plates. A large number of deep-sea forms has lately been
described by Dr. A. Goes, from the dredgiiigs of the Albatross; see Bull. Slits. Comp.
Zool. xxix. (1896).

GLOBIGERIXA 8 1 1

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 Textula/riidce, 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 pellets,
varying in size from a large pin's head to a large pea, formed
of an aggregation of sand-grains, minute foraminifers, Arc., 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 blending.- 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-
rli'r.a. 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 O4 inch in
diameter, and the 'cervicorn 'to nearly O5 inch in length.1 A much
larger form was found by Mr. Brady among the dredgiiigs made
in the Faroe Channel (see his '"Challenger'1 Keport,' p. 242);
Syringammina 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 support its own weight when taken out of the water.

Later on another large and interesting <>!"' belonging to
the same group was obtained by Mr. Wood-Mason, late of the
Indian Museum, from the Bay of Bengal.2 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 are ranged by Mr. H. B.
Brady 3 (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. xvi. 1870, p. 221.

2 Ann. (Did Mufj. Xut. Hist. 1889, ser. vi. vol. iii. p. 293, woodcuts.

3 See his ' Notes' in Quart. Jo/tru. <>t AZVcrm-.s. ,SV/'. n.s. vol. xix. 1879, p 20, and
vol. xxi. 1881, p. 31.

812

MICROSCOPIC FORMS 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 the more 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 circular mouth (fig. 613, a, b,
c). This type, which occurs in extraordinary abundance in certain
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 (Saccdnvmina Carter!) is.

-

FIG. 613. — Arenaceous Foraminifera : a, Saccammina xjihu rim ; b, the same
laid open ; c, portion of the test, enlarged to show its component sand-
grains ; d, Pilulina Ji'jfrrymi; e, portion 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 I'-ilnlt/ia, from
its resemblance to a homoeopathic 'globule' (fig. 613, d, 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 ,V. xj>lurrir« consult L. Rhmnbler, in vol. Ivii. of Zeitschr.
f. wins. Zonl.

ARENACEOUS FOEAMINIFEKA

313

stance of which very fine sand-grains are dispersed. This ' felt ' is
somewhat flexible, and its components do not seem to be united by
any 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 >hould 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 Mfirsipdlri eloni/(it<t
(fig. 614, d), on the other hand, is somewhat fusiform in shape. ;ind
has its two extremities elongated into tubes, with a circular orifice
at the end of each. The materials of the ' tests ' differ remarkably
according to the nature of the bottom whereon they live. When

Fi<;. 014.— Arenaceous Foraminif era : u, li, upper and lower aspects of
/iJimgiiiiinn globigeriniforme', c, Hormosinu f/lolmJi/frn : </, Miir
elongata ; e, terminal portion, and/', middle portion of the same, enlarged ;
g, Thitrtinimina papillata ; /(, 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 xmd-gi-ains ; hut when
they are brought up from a bottom on which sand predominates,
the larger part of the ' test ' is made up of sand-grains mid 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 length) are entirely made up of sponge-spicules laid side by side
with extraordinary regularity. The genus Rhabdammina (Sars)
resembles Saccammina in the structure of its 'test.' which is com-
posed of sand-grains very firmly cemented together ; but the grains

8 14 MICROSCOPIC FORMS OF ANIMAL LIFE

are of smaller size, and they are so disposed as to present a smooth
surface internally, though the exterior is rough. What is most
remarkable about this is the geometrical regularity of its form,
which is typically triradiate (fig. 615, c), the rays diverging at equal
angles from the central cavity, and each being a tube (d) with an
orifice at its extremity. Not unfrequently, however, it is quadri-
radiate, the rays diverging at light angles ; and occasionally a fifth
ray presents itself, its radiation, however, being generally in a
different plane. The three rays are normally 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 forms in which there is no
vestige of a third ray, but merely a single straight tube, with an
orifice 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 dredgiiigs 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 Hcdiphysema, which, from constructing 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
character.1

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 great 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 structure, 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 ' plaster ' with which the exterior
of the test is smoothed off, so as to present quite a polished surface.
It is remarkable that the cement contains a considerable quantity
of oxide of iron, which imparts a ferruginous hue to the ' tests ' in
which it is largely employed. The forms of the Litiinllne ' tests '
often simulate in a very curious way those of the simpler types of
t he vitreous series. Thus, the long spirally coiled undivided sandy
tube of Ammodiscus is the isomorph rf S}>irtUina. In the genus Haplo-
phragmium (fig. ()]4.n./>. and Plate XVIII, fig. C>) we have singular
imitations of t he < ilobigerine. Rotaline, and Nonionine types ; and in

1 See Mr. Seville Kent in Aim. <>f ^,«t. ll/\f. ser. v. vol. ii. 1878; Professor Ray
in (Jtnti-t. Jonni. Micninr. ,sv/'. vol. xix. 1878, p. 17C> ; and Prof essor Mobius's

ii run Mti /tri/i/ix,

AEENACEOUS FORAMINIFERA

8l5

Thurammina papillata (fig. 614, g) a not less remarkable imitation of
the Orbuline. This last is specially noteworthy for the admirable
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, A),
whilst there are at intervals nipple-shaped protuberances, in every one
of which there is a rounded orifice. A like perfection of finish is seen
in the test of Hormosina glcibulifera (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 Xodosarine type, their
tests being sometimes constructed with the regularity characteristic
of the shells of the true Nodosaria, Plate XIX. ic,. whilst in other

FIG. 615. — Arenaceous Forarninifera : c<, I/, exterior and sectional views of
Slieophax sabulosa ; c,Rhabda mmina abyssm-itm ; d, cross section of one
of its arms; e, Rheophax scorpiurus ', /, Hormosina Carpenti ri.

cases the chambers 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 very coarse, but small Foraminifera are often worked up with
them (fig. 615, e). A straight, many-chambered form of the same
genus (fig. 615, a, b) is remarkable for the peculiar finish of the neck
of each segment ; for whilst the test generally is composed of sand-
graiiis, as loosely aggregated as those of which the test of Astrorhiza
is made up, the grains that form the neck are firmly united by fer-
ruginous cement, forming a very smooth wall to the tubular orifice.
The highest development of the 'arenaceous' type at the present
time is found in the forms that imitate the very regular naxtiloid

8i6

MICROSCOPIC FORMS OF ANIMAL LIFE

shells, both of the ' poi-cellanous ' and the ' vitreous' scries : and the
most remarkable of these is the Cyclwrwmina concellatn (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 divided into chambers
by partitions formed of cemented sand-grains (b). a communication
l>et \veen these chambers being left by a fissure 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 chandler into passages excavated in its thick external wall,

FIG. 616. — Gyclammina ctnirrl/nfti, showing at a, its external aspect ;
b, its internal structure ; c, a portion 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
sand-grains, as shown at c. It not ^infrequently 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 pas>ages simply running from the chamber-
cavity into the thickness of its wall, and having (so long as this is
complete) no external opening. This • labyrmthic- ' structure is of
great interest, from it.v relation not only to the similar structure
of the large fossil examples of the same type, but also to that which
is presented in other gigantic fossil arenaceous forms to he presently
described.

Although some of the nautiloid Lilnnln -are among the largest
of existing Koraminifera. having a, diameter of fK! inch, they are
mere dwarfs in comparison with two gigantic fossil forms, whose

PAEKEKIA

8I7

structure has been elucidated by Mr. H. B. Brady and the Author.1
Geologists who have worked over the Greensand of Cambridgeshire
have long been familiar with solid spherical bodies which there
present themselves not unfrequently, varying in size from that of a
pistol-bullet to that of a small cricket-ball ; and whilst some regarded
them as mineral concretions others were led by certain appearances
presented by their surfaces to suppose them to be fossilised sponges.
A specimen having been fortunately discovered, however, in which
the original structure had remained unconsolidated by mineral in-

FIG. 617.— General view of the internal structure of Pnrkeria : In the hori-
zontal section, Jl, I-, F', 7* 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 processes ; E, the aspect of a
section taken in a radial direction, so as to cross the solid lamellae and their
intervening spaces ; c\ c"1, c7', c ', successive chambers of nucleus.

filtration, 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. "VV 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 concentrically 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 1'nrkfi'in and Loftiisia' 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 Hydroida (see the memoir by Professor Alleyne Nicholson, published
in 1886 by the PalsBontographical Society).

3 G

SiS

MICBOSCOPIC FOEMS OF ANIMAL LIFE

chain berlets, instead of being arranged in successive rinys 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 ehamberlet is itself penetrated by extensions
of the cavity into its substance, as in the Cyclammirial&st described ;
and these passages are separated by p-ulitions very regularly built
up of sand-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 ;i wall is seen in
fig. 617 to close in the last-formed series of chamberlets. But these

\v;ills have the same ' labyrinthic'
structure 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, c1, c2, c3, c4, sometimes
forming a spiral, and in one in-
stance returning upon itself.

PIG 618.-Portion of one of the lamellae But the outermost chamber en-
of Parkena, showing the sand-grains or , , , .

which it is built up, and the passages larges, and extends itself over the
extending into its substance. whole ' nucleus,' very much as the

' circumambient ' chamber of the

< h-bitolite extends itself round the primordial chamber ; and radial
prolongations given off 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
Mrcol'nia (fig. 608), and attaining a length of three inches, lias been
described by Mr. H. B. Brady (loc. cit.} under the name Loftusia, after
its discoverer, the late Mr. \V. K. Loft us. 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 arc accustomed to regard as
necessary to (lie manifestations of conscious life. Suppose a human
mason to lie put down 1>\ t he side of a pile of stones of -various shapes
and sizes, and (obetold to build a dome of these, smooth on both
surfaces, without using more than the least possible (plant it v of a
very tenacious but very cost I y cement ;u holding the stones together.
If he accomplished this well, he \\ould nceh e credit for great in
'el licence and skill. Vet this is exactly what I hoe lit 1 le • jel 1\ -.pecks '

VITREOUS FOEAMINIFERA 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 same *<ni<l ij Inttom one species picks up the. coiy/wv quartz-
grains, unites them together with a ferruginous 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 Jim-r
grains and puts them together with the same cement into perfectly
spherical ' tests ' of the most extraordinary finish, perforated with
numerous small pores disposed at pretty regular intervals. Another
selects the inumti'st sand-grams and the terminal portions of spoiige-
spicules and works these up together — apparently with no cement
at all. but by the mere 'laying' of the spicules — into perfect white
spheres, like homoeopathic 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 tin-
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
.s//'7As 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, SpiriUiiui 1ms a minute, spirally convoluted, undivided
tube, resembling that of Cornuspira, but having its wall somewhat
coarsely perforated by numerous apertures for the emission of pseudo-
podia. The ' monothalamous ' forms of this growth mostly belong to
the family Lagenidx. which also contains a series of transition forms
leading up gradationally to the ' polythalamous ' nautiloid type. In
Layerta (Plate XIX, figs. !:>, 13. 14, 15) the mouth is narrowed and
prolonged into a tubular neck, giving to the shell the form of a micro
sropic flask ; this neck terminates in an exerted lip. which is marked
with radiating furrows. A mouth of this kind is a distinctive
character of a large group of many-chambered .-hell*, of which each
single chamber bears a more or less rlo,-e iv.-emhlauce 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, Tims the shell of X<><1</-
*«/•!<(. (Plate XIX, fig. !(>) 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 ( 'i-inliJIt/,-in (fig. 17)
we have a similar succession of chambers, presenting the characteristic
radiate aperture, and often longitudinally ribbed, disposed in a
nautiloid spiral. Between Xodnsnrin and Cristellaria, moreover,
t here is such a gradational series of connecting forms as shows that
no essential difference exists between these two types, and it is a fact
of no little interest that some of the simpler of these varietal forms,

3 .; 2

820 MICROSCOPIC FORMS 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 backAvards in geological time at least as far
as the Permian epoch. In another genus, Polymorphina, 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.
Grlobigerinida. — Returning once again to the simple ' monothala-
mous ' condition, we have in Orbulina — a minute spherical shell that
presents itself in greater or less abundance in deep-sea dredgings,
from almost every region of the world — a globular chamber with
porous walls, but destitute of any general aperture, the office of which
is served by a series of larger pores scattered throughout the wall of
the sphere. It has been maintained by some that Orbtdina is really
a detached generative segment of Globigerina, with which it is
generally found associated. The shell of Globigerina consists of an
assemblage of nearly spherical chambers (fig. 619), having coarsely

FIG. 619. — Globigerina biiUoi/lcs as seen in three positions.

porous walls, and cohering externally 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, mav not communicate directlv 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 extraordinary abundance in which it occurs
at great depths 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 l.i'UO to •_>.(»()() 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. ('»-<)); and this thickening is shown by examination of thin
sections of the shell to be produced by an exogenous deposit around
the original ehainlier wall (corresponding with the 'intermediate
skeleton' of the more complex types), which sometimes contains
little flask-shaped cavities filled with sarcode — as was first pointed
out by the late Dr. Wallich. 15ut the sweeping of the upper waters

GLOBIGERINA

821

of the ocean by the ' tow net,' which was systematically carried on
during the voyage of the ' Challenger,' brought into prominence the
fact that these waters in all but the coldest seas are inhabited by
floating Glvlnyeruice, whose shells are beset with multitudes 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 sarcodic substance of the body exudes through the pores
of the shell, forming a flocculent fringe around it ; and this extends

FIG. 620. — Globigt-rina conglobuta (Brady): «, b, c, bottom specimens ;

(7, section of shell.

itself 011 each of the spines, creeping up one side to its extremity,
;md 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 ;nlded 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 GlobigeriB.se
of the upper waters. Avhich (he affirmed) only live at or near the sur-
face, and which, when they die, lose their spines and subside. The

822

FORMS OF ANIMAL LIFE

Author, however, from his own examination of the Globigerina ooze,
is of opinion that the shells forming its surface-layer 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

pass their whole lives at
depths of at least 2,000
fathoms is proved, in regard
to those forming calcareous
shells, by their attachment
to stones, corals. Arc. ; and
in the case of the arena-
ceous types by the fact that
they can only procure on tin-
bottom the sand of which
their ' tests ' are made up.

A very remarkable type
has recently been discovered
adherent to shells and corals
brought from tropical seas.

to which the name
teria has been given. This
may be regarded as a highly
developed form of Globi-
gerina, its first formed por-

Fio. 621.-Gloli!/<;-hi«, as captured by tow-net tion living 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 A vst ilmle into
which they all open forms a sort of vent whose orifice is at the apex
of the cone, and is sometimes prolonged into a tube that proceeds
from it ; and the external wall of this cone is so marked out by
septal bands that it comes to bear a strong resemblance to a minute
/lulu H n.x (acorn-shell), for which this type was at first mistaken. The
principal chambers are partly divided into chamlierlels by incomplete
partitions, as we shall find them to be in Ko;oun. The presence of
sponge-spicules in large (|uantity in Ilie chambers of many of the
best preserved examples of this type was for some time a source of
perplexity ; but this was explained by the lale Professor Max
iSchiilt/.e,1 who .showed hou the pseudopodia of this rhi/opod have
t he habit, like those of Hcdiphysema, of taking into themselves >ponge-
spicules. which they draw into the chambers, so that 1 hex liecome
incorporated with the sarcode-body. It should he added that Pro-
1 Archiv f. \n/iirt/i'nc!i. \.\i\. 1st;:-!, p. 81.

TEXTULARIA

323

lessor Schultze, with whom Mr. H. J. Carter,1 Mr. H. B. Brady,2 .-mil
Dr. Goes3 are in agreement, regard Carpenteria as allied to Polytrcum.
Some interesting observations have been made by Professor Mb'bius 4
on a large branching and spreading form of Ccvrpenteria which he
recently met with on a reef near Mauritius, and to which he has given
the name of C. r/n^i/tidodendron.

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 Textidarian and the Rotaluni. 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 Te.rtc-
laria (Plate XVIII, fig. 9) the chambers are arranged biserially
along a straight axis, the position 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

A B

FIG. 622. — Internal silicious casts representing the forms of the segments of
the animals of, A, Te.rt t//n r/n ; B, Rotalin.

' internal casts' (fig. 622, A) as exhibit the forms and connections of
the segments of sa re-ode by which the chambers were occupied during
life. In the genus Btdiinina the chambers are so arranged as to form
a 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 ; but in this, as in the preceding type, there is an extraordinary
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 Te.ftxlni-initi'. is entirely replaced by a sandy test, that
some systematists prefer to range this group among the Arenacea.

In the Jlotiilittii series the chambers are disposed in a turbinoid
spire, opening one into another by an aperture situated on the lower

1 Amuda <(iid Mug. Nat. Hist. ser. iv. vols. xvii. xix. xx.

2 'Challenger' Report.

5 K. Svenska Vet. Handlings; xix. No. 4, p. 94.

4 See his Foram inife ra vim Mn // ritius, 1880, plates v. vi.

824

MICROSCOPIC FOEMS OF ANIMAL LIFE

FIG. 623. — Tinoponts laculntits.

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 Discorbina. The early form of Planorbulina is a
Rotaline spire, very much resembling that of Discorbina ; 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 regular 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 Textula/ria) 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 Gypsum
present great diversities of shape,with
great constancy in their 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-water
dredgings on some parts of the Australian 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
OrbitoUna, some of which attain a very large size. Globular Orbito-
liiin:, which appear to have been artificially perforated and strung
as 1 leads, 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-
caling with each other like those of Tinopwus, and differs from that
genus primarily in its erect and usually branching manner of growth
and the freer communication lief ween its chambers. This, again, is
of special interest iu relation to h\>~ut>n, showing that an indefinite
zuophytic mode of growth is perfectly compatible with truly fora-
miniferal st met ure.

In Ji'iiliilin, properly so called, we find a marked advance towards
fhe highest type of foraminiferal structure, the partitions that

ROTALIA

825

long

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, are
thickened by an extraneous deposit or ' intermediate skeleton,' which
sometimes forms radiating outgrowths. This peculiarity of conforma-
tion, however, is carried much further in the genus < '/ilcnriiHi. 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-

to the ' intermed i ; 1 1 < •
skeleton ' b, which is quite
independent of the cham-
bered structure a ; and this
is nourished by a set of
i -1 1 1 als containing prolonga-
tions of the sarcode-body
which not only fin-row the
surface 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 foramiiiifer 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, affords
a valuable clue to its meaning in the gigantic fossil organism
Eozooti canadense. The resemblance which Calcarina bears to the
radiate forms of Tiitnj>nrns (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 Cdlcur'nni continue to be
added on the same plane, those of 'rimi/im-i/* are heaped up in less
regular piles.

Certain beds of Carboniferous limestone in Russia are entirely
made up, like the more modern Nummulitic 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 relationship 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 Hotalia Schroefericma near
its base and parallel to it, showing, «, a, the
radiating interseptal canals ; b, their internal
bifurcations ; c , a transverse branch ; d, tubulated
wall of the chambers.

826

MICROSCOPIC FORMS OF ANIMAL LIFE

elongated extensions, which correspond to the ' alar prolongations '
of other spirally growing Foramiiiifera, 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
Nummuline shell that Alveolina, bears to Orltictilhia ; and this view
of its affinities is fully 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 tabulation of its component shells as to
prevent him from confidently affirming it, yet the appearances he
could distinguish were decidedly in its favour. And having since
received from Dr. 0. 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 Ftisi/liitu limestone.

preservation, he is able to state with certainty, not only that
is tubular, but that its tabulation is of the large coarse nature that
marks its affinity rather to the Ro1«!i,x' than to the N'ummtdnn'
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
greater 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 tlie completeness of the
wall that surrounds each segment of the body (the septa briu-
generally double instead of single), the density and line porosity of
the shell-substance, and the presence of an 'intermediate skeleton,'

POLYaTOMELLA 827

with a 'canal system ' for its nutrition. It is true that these cha-
racters are also exhibited in the highest of the Eotaline series, whilst
they are deficient in the genus Amphistegina, which connects the
Xumnmline series with the Rotaline ; but the occurrence of such
modifications in their border forms is common to other truly natural
yroups. With the exception of Amphistegina, all the genera of this
family are symmetrical in form, the spire being iiautiloid 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 Amphistegina there is a reversion to the
Rotalian type in the turbinoid form of its spire, as in the characters
already specified, although its general conformity to the Nummuline
type is such as to leave no reasonable doubt as to its title to be
placed in this family. Notwithstanding the want of symmetry of
its spire, it accords with Ope n-n linn and Xnnniudit<:s 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 chandlers, 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.1

The existing ymniiiti'ini'lfe 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 development of the ' canal system ' anywhere to be
met with, is common 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.
Xo such openings-, however, exist, the only communication which
the sarcode-body of any segment has with the exterior being
either through the fine tubuli of its shelly walls or through the
row 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. 62(5) of the sarcode-body and
canal system of the large P. crtitic/'lnta of the Australian coast as
may sometimes be obtained by the same means from dead shells
which have undergone infiltration with ferruginous silicates.2 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 Joiies,
see the Author's Introduction to the Study of the Foraminif era (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 Griinsand und seine Erliiuterung des

828

MICKOSCOPIC FORMS OF ANIMAL LIFE

we see that the segments of the sarcode-body are smooth along their
anterior edge b, &', but that along their posterior edge, a, they are
prolonged backwards into a set of ' retral processes ; ' 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, cl,
passing through 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 symmetrical. At d,
dl, d'2 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, e1, e2, 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 ; b, bl, smooth
anterior margin of the same segment ; r, cl, stolons connecting successive
segments, and uniting themselves with the diverging branches of the meri-
dional canals; d, dl, d-, three turns of one of the spiral canals; e, cl, e-,
three of the meridional canals; /, /',/•', their diverging branches.

divide the segments; whilst from each of these there passes off
towards the surface a set of pairs of diverging branches,/', fl,f'2. 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 fur-
rows between the retral processes of the next segment. These canals
appear to be occupied 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,

organischeii Lebens,' in Abhandlungen drr kiinigl. Akad. tier WissenscJiafteti,
Berlin, 1855. It was soon afterwards shown by the late Vrofessor Bailey (Quart. Jam '» .
Microsc. Sci. vol. v. 1857, p. 83) that the like infiltration occasionally takes place in
recent Foraminifera, enabling similar ' casts ' to be obtained i'mm them 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 ' Kxpedit ion, the most notable from
the coast of Australia.

POLYSTOMELLA

829

with the stolon processes connecting the successive segments of the
latter, as seen at c1. 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 layei1 that forms the proper walls of the chambers, ami

/,
/&&,

••>/$/{%':&
. •&:'•/;• .;"•-:-.

FIG. 627. — Cycloclyi)eus— 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 tin- spire. The substance of this 'boss'
is traversed by a set of straight canals, which pass directly from the
spiral canal beneath, towards the external surface, where they open
in little pits, as is shown in Plate XIX, -23, the umbilical boss
in P. ci'ispa, however, being much smaller in proportion than it

FIG. 6'28. — Operculina laid open to show its internal structure : «, marginal

general distribution of which is seen in the septa e, e', the lines radiating
from c, c point to the secondary pores ; g, g, non-tubular columns.

is in P. craticulata. There is a group of Foraminifera to which the
term Xoiiionina is properly applicable, that is probably to be con-
sidered as a sub-genus of Pob/stoineU<i. agreeing with it in its general
conformation, and especially in the distribution of its canal system,
but differing in its aperture, which is here a single fissure at the
inner edge of the septa 1 plane, and in the absence of the ' retral

§30

MICROSCOPIC FORMS OF ANIMAL LIFE

processes' of the segments of the sarcode-body, the external walls of
the chamber^ being smooth. This form constitutes a transition to
the ordinary Nummuline type, of which Poli/stomeUa is a more aber-
rant modification.

The Nummuline type is most characteristically represented at the
present time by the genus Operculina, which is so intimately united
to the true Numtniulite 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
Peneroplis, so as to be very much broader than the preceding. The
external walls of the chambers, arching over the spaces between tin-
septa, are seen at It, 1> ; and these are bounded at the outer edge of

Fin. &iQ.— (.'<ilrnriin< laid ..pen t" show its internal structure : «, chambered
portion; l>, 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 inner to the outer surface of the chamber-walls
without division or inosculation (fig. <5o2), is traversed by a system
of comparatively large inosculating 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 ,-it ,-.,-;
while the 'alar prolongations' of those c.-ivities over the surface of

the preceding whorl are shown at c', c'. The chambers are separated

1>V the septa <f, <l, '/. formed of i \\o lamina- of shell, one belonging
bo each chamber, and having spaces between them in which lie the

• intersepfal canals,' whose general distribution is seen in the sepia
marked e, ''. and whose smaller branches are seen irregularly divided
iii the sepia (/'. </' , whilst in the seplum </" one of the principal
trunks is laid open through its whole length. At the approach of
each septum to the marginal cord of Hie preceding is seen the
narrow fissure which const it iiles the principal aperture of communi-

NUMMULITES 831

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
from 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 Xummulite ; but. as
in that genus, not only are the septa themselves composed of vitrei nis
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 y, y . Similar columns
of non-tubular substance, of which the summits 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 sufficient to characterise distinct species, were it not that
on a comparison 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 represented at the present time
by small and comparatively infrequent examples, was formerly de-
veloped to a vast extent, the Nummulitic limestone, chiefly made up
by the aggregation of its remains (the material of which the Pyramids
are built), forming a band, often 1,800 miles in breadth and frequent ]\
of enormous thickness, that may be traced from the Atlantic shore-
of Europe and Africa, through Western Asia to Northern India and
China, and likewise over vast areas of Xorth America (iig. (KiO).
The diameter of a large proportion of fossil Xummulites ranges
between half an inch and an inch: but there are some whose
diameter does not exceed /^th of an inch, whilst others attain the
gigantic diameter of 4^ inches. Their typical form is thai of a
double-convex lens ; but sometimes it much more nearly approaches
the globular shape, whilst in other cases it is very much flattened ;
and great differences exist in this respect among individuals of what
must be accounted one and the same species. Although there aie
some Xummulites which closely approximate Operculince in their
mode of growth, yet the typical forms of this genus present certain
well-marked distinctive peculiarities. Each convolution is so com-
pletely invested by that which succeeds it. and the external wall or
spiral lamina of the new convolution is so completely separated from
that of the convolution it incloses by the 'alar prolongations' of its
o-u n 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 Xum-
mulite through the median plane, which may often be accom-
plished simply by striking it on one edge with a hammer, the opposite

832

MICROSCOPIC FORMS OF ANIMAL LIFE

edge being placed on a firm support ; or, if this method should not
succeed, by heating it in the flame of a spirit-lamp, and then throw-
ing it into cold water or striking it edgeways. ISTummxdites usually
show many more turns, and a more gradual rate of increase in the
breadth 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 characteristic

B

FIG. 630. — A, piece of Niiuniuditic limestone from Pyrenees,
showing Nummulites 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 Nintnnidites Icevigata : a, margin
6f external whorl ; b, one of the outer row of chambers ; c, c, whorl invested
by a ', (I, one of the chambers of the fourth whorl from the margin ; e, <•',
marginal portions of the inclosed whorls ; /, investing portions of outer
whorl ; g, </, spaces left between the investing portion of successive whorl* ;
h, h, sections of the partitions dividing these.

examples of the genus ; thus in some, as J'. distant, they keep their
own separate course, all tending radially towards the centre: in
others, as .T. /trr!</nt«, their partitions inosculate with each other, so
as to divide the space intervening bet \veen each layer and the next
into an irregular network, presenting in vertical section the appear-
ance shown in fig. 6,'U ; whilst in .A', yarfinsciixis they are broken

NUMMULITES

833

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 wholes, yet there is evidence that

FIG. 632. — Portion of a thin section of Nitinniiilitr.* l^-r/ijutu taken in the
direction of the preceding, highly magnified to show the minute structure
of the shell : ", a, portions of the ordinary shell-substance traversed by-
parallel tubuli ; b, b, portions forming the marginal cord, traversed by
diverging and larger tubnli ; c, one of the chambers laid open ; d, <1, il,
pillars of solid substance not perforated by tubuli.

the segments of sarc;>de which they contained were not cut ofl' 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
piiss directly from one surface to the other. These tubes are shown.
as divided lengthwise by a vertical section, in fig. 632, «, a ; whilst
the appearance they present when cut across in a horizontal section
is shown in fig. 633, the

- « !

•''"'I.

transparent shell -substance

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, 6, b), the tubes are larger,

and diverge from each other

at greater intervals ; and it

is shown by horizontal sections FIG. 633.— Portion

that they communicate tVcdv

with each other laterally, so

as to form a network such a.s

seen at f>. h. fig. 634. At

of horizontal section of
Nummulites showing the structure of the
walls and of the septa of the chambers :
«, a, a, portion of the wall covering three
chambers, the punctations of which are the
orifices of tubuli ; b, 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 (1.

IS

certain other points, d. d, d,
fig. 632, the shell -substance
is not perforated by tubes, but
is peculiarly dense in its texture, forming solid pillars which seem
to strengthen the other parts ; and in Nummulites whose surface.-,
have been much e.\|>< >>,•<! to attrition, it commonly happens that the
pillars of the superficial layer, being harder than the ordinary shell-
substance, and bein»- consequently less worn down, are left as

3 H

§34

MICROSCOPIC FORMS OF ANIMAL LIFE

PIG. 634, — Internal cast of two of the cham-
bers of NiDiiiinilifc.'i atrintti, with the
network of canals, It, in the marginal
cord communicating with canals passing
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
aperture, but is more com-
monly formed by the more or
less complete coalescence of
several separate perforations,
as is seen in fig. 631, b. 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
chambers. The canal system
o£ Nummulites seems to bear-
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. b, b), while from
these may be seen to proceed the lateral branches (c, c), which, after
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 inosculating 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 Nummuline type
is presented in the genus
Heterostegina (fig. 635), which
bears a very strong resemblance
to Orb ic nil art in its plan of
growth, whilst in every other
respect it is essentially dif-
ferent. If the principal cham-
bers of an OjH>rci<li>trt were
divided into chainberlets by
secondary partitions in a direc-
tion transverse to that of t he-
principal septa, it would be
converted into a Ifeterostegina,

just as a /'I'tiero/tHs would be converted bv the like subdivision into
an Orbiculina. Moreover, we see in Heterosteyimi. as in (Jrbictttlttu,
a great tendency to the opening out of the spire with the advance of

FIG. 635.—Hcti'ri»,h'i/iii,i.

NUMMULITES

835

age ; so that the apertural 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 Orbiculina, 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 Foraniinifera, some speci-
mens of its discs in the British Museum having a diameter of two
and a quarter inches. Notwithstanding the difference of its plan
of growth, it so precisely accords with
the ISTummuline 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 Cyclocli/jn'iix 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 lamella1
in the central portion of the disc
than they do nearer its edge, that
new lamella? must be progressively
added to the surfaces of the disc
concurrently with the addition of new
rings of chamberlets to its margin.
These lamella?, however, are closely
applied one to the other without any
intervening spaces ; and they are all pIG. (536.— Section of Orlnt<
traversed by columns of non-tubular Fortisii, parallel to the surface,
substance, which spring from the j£
septal bands, and gradually increase layer.
in diameter with their approach to
the surface, from which they project in the central portion of the
disc as glistening tubercles.1

The IsTummulitic limestone of certain localities (as the south-west
of France, Southern (iermaiiy, Xorth-Ea stern India, Arc.) contains a
vast abundance of discoidal bodies termed Orlitoides (fig. 630, B),
which are so similar to Nummulitesas to have been taken for them,
but which bear a mueh closer resemblance to Cydoclypens. 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 fourpemiy-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 '
(Nachr.Ges. Gottingen. 1891, p. .11 1 is chiefly based on palaoutological considerations.

3 H 2

836

MICROSCOPIC FORMS OF ANIMAL LIFE

two different kinds of structure are usually seen, in it, one being
composed of chamberlets of very definite form, quadrangular in some
species, circular in others, arranged with a general but not constant
regularity in concentric circles (figs. 636, 637, b, b) ; the other, less

a b

FIG. 637. — Portions of the section of Orbitoides Fortisii, shown in fig. 63G,
more highly magnified : «, superficial layer ; b, median layer.

transparent, 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 punctatioiis may be observed, which seem to be

a

a'

FIG. 638. — Vertical section of Orbitoides Fort in/ i, 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 Cycloclypeus ; 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-
berlets of the median layer in Orbitoides
seem to correspond very closely with
those which have lieeii already described

as ^.evailim? ill Gvdodyp&US, the most
f ,, p. ,. •' . «Ar. ', . ~,

satisfactory indications to this effect

lieiny furnished by the silicious ' internal

,,,sTs- ,(> ,„. lf '^th iu tvi-taili Green-

.
sands. winch afford a model of the sar-

code-body of the animal. In such a

fragment (fig. (j.'S'.t) \ve recognise the c-hainlierlets ol' three successive
zones, a, a', a", each of \vhich seems normally to communicate by
one or two passages \\ith the chamlierlets of the /one internal and
external to its own; whilst between the chaml>erlets of the same

b'

, b'

FIG. 639.— Internal cast <>f p<>r-
tionof median plane ,,f Or/,/-
towes Jforfom, snowing, at
a a, a' a', a", a", six chambers
of each of tlnve zones, with
their mutual communicatiions;
and at o o, o b,b /< . IMTI .....
; ...... ilar 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 spiral canals of the Nummulite, and of which the ' internal
casts ' are seen at b b, b' 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 known to contain organic remains ; and the
determination of its real character may be regarded as one of
the most interesting results 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

PIG. 640. — Vertical section of Eozoon cn/tnJi ityc, showing alternation of
calcareous (light) and serpentinous (dark) lamellce.

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 considerable 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 FORMS OF ANIMAL LIFE

who at once recognised its foraminiferal nature,1 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 serpentinous 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 pei'iods, 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.2 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 minernl origin,3 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 m.-iy
be specially named), but also by geologists who have specially
studied the micro-mineralogical structure of the older Metamorphic
rocks,4 the Author feels justified in here describing Eozoon as
he believes it to have existed when it originally extended itself as
an animal growth over vast areas of the sea-bottom in the Laurentian
epoch.

Whilst essentially belonging to the Nummuline 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 Fora-
minifera in its other characters. For in its indeterminate zoophytic
mode of growth it agrees with Polytrema in the incomplete sep;n-;i
tion of its chambers ; it has its parallel in Carpentaria ; 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 succession

1 This recognition was due, as Dr. Dawson has explicitly stated in his original
memoir (Quart. Jtnirtt. <>i' '.'<«/. .S'nr. vol. xxi. p. 541, 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 mineral ogical relations of Euziion, as well as of its organic structure, in
a small book entitled Tin- Dnirn nf Life.

- For a fuller account of the results of the Author's own study oi Eozoon, and of the
basis on which the above reconstruction is founded, see his papers in Quart. Joiini.
nl' < ii ni . Sac. vol. xxi. p. 59, and vol. xxii. p. '219, and in the Intellectual Observer,
vol. vii. 1865, p. 278; and his 'Further Researches' in Ann. nf .Y<</. ///.s/. -June ]S7I.

" Sec the memoirs of Professors King and Knwiiey in Quart. Jo/ir/i. nf CcoL Soc.
vol. xxii. p. 185, and Ann. of Xut. llixl. .May 1*74.

1 Among these the Author is permitted to mention L'rofessor Geikie, of Edinburgh,
who has thus studied the older rocks of Scotland, and ProfessorBonney of London, who
has inii.de 11 like sludy of the Cornish and oilier Serpentines. By both these eminent
authorities lie is assured that they have met with no purely mineral structure in the
least resembling Eozdon, either in its regular alternation of calcareous and serpen-
I i nous lamellie, or in the1 dendril ie 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 unalile to eoneei\ cof any inorganic agency by which
such a structure could have been produced.

EOZOON

839

of 'storeys' of chambers (fig. 641, A1. A1, A-, A2), 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 5, b. These septa are traversed by passages of communication
between the chambers which they separate, resembling those which,
in existing types, are occupied by stolons connecting together the
segments of the sarcode-body. Each layer of shell consists of two
finely tubulated or ' Nummuline ' lamellae. B, B, which form the
boundaries of the chambers beneath and above, serving (so to spe;ik)
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 ' Xum-
muline' layer (fig. 643)
are usually filled up (as
in the Nummulites
of the ' Nummulitic
limestone ') by mineral
infiltration, so as in.
transparent sections to
present a fibrous ap-
pearance ; but it for-
tunately 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 waviiiess to
that of the crab's claw.
The thickness of this
interposed layer varies

FIG. 641.— Portion of the calcareous shell of

canadense as it would appear if the serpentine
that fills its chambers were dissolved away : A1, A1,
chambers of lower storey opening into each other
at «, a, but occasionally separated by a septum,
b, b ; A2, 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, D, connecting' the chambers of
different storeys, and penetrated by the arbores-
cent systems of canals, E, E, E.

considerably in differ-
ent parts of the same mass, being in general greatest near its
base and progressively 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. These canals take their origin,
not directly from the chambers, but from irregular lacinni1 or
interspaces between the outside of the proper chamber-walls and
the 'intermediate skeleton,' exactly as in <~'(ilcnriiifi. the exten-
sions of the sarcode-bodv which occupied them having apparently
been formed by the coalescence of the pseudopodial filaments that
passed through the tubulated lamella3.

840

MICKOSCOPIC FORMS 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 up by the silicious infiltra-
tion which has taken the place of the original sarcocle-body, as in the
cases already cited, and thus when a piece of this fossil is subjected
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
inn/lei in hard serpentine of the soft sarcode-body which originally
occupied the chambers, and extended itself into the ramifying 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.

& ' 1 •£&&r-

FK;. 642. — Decalcified portion of Eosituii ^•(^nadense shell, showing the ser-
pentinous iiitfi-iinl cast of the chambers, canals, and tubuli of the original,
presenting an exact model of the animal substance which originally filled
them.

\\'c 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 direc-
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-bodv
that originally passed into them. But this is not all. In specimens
in which the Xummuline 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 serpen
linons residuum), that layer is represented by a thin white film
covering the exposed surfaces of the segments : the superficial aspect

EOZOON

841

of which, as well as its sectional view, is shown in fig. 642. And
when this layer is examined with a sufficient magnifying power 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 ' asbestiforin 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
T0p00th of an inch in diameter, are the ' internal casts ' of the tubuli
of the Nunimuline layer (a precise parallel to them being presented in
the ' internal cast ' of a recent Amphistegina which was in the Author's
possession) ; and their arrangement presents all the varieties which
have been mentioned as existing in the shells of (^//•/•/•ntli///. Thus

'

FIG. 643. — Vertical section of a portion of one of the calcareous lamellae of
Eozoon cana dense : a <t, Numuiuliiie layer perforated by parallel tubuli,
which show a flexure along the line «' «'; 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 Xummuline layer.

these delicate and beautiful silicious fibres represent those pseudo-
podial threads of sarcode which originally traversed the minutely
tubular walls of the chambers ; and a 'precise 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 upper part of the ' decalcified' specimen shown in fig. 642
it is to be observed that the segments are confusedly heaped together
instead of being regularly 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 later
growth of types whose earlier increase takes place upon some much

842 MICROSCOPIC FORMS 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 Laurentians, 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 from its solu-
tion in ocean water, in the same manner as do the polypes by whose
growth coral reefs and islands are being upraised at the present time.

An elaborate work, ' Der Ban des Eozuoii Canadense ' (1878),
has lieen recently published by Professor Miibius, of Kiel, in which
the structure of Eozoon is compared with that of various types of
Foraminifera, and, as il differs from that of every one of them, is
atlirmed not to be organic at all, but purely mineral. Upon this the
Author would remark, thai if the validity of this mode of reasoning
be admitted, «//// fossil whose structure does nol correspond with that
of some existing type is to be similarly rejected. Thus the S'troiint
ln/xn'ii of Silurian and Devonian rocks, which some palaeontologists
regard as ;i coral, others as |>olv/.o;irv. others as a calcareous sponge,
and others as a foraminifer. would not be a fossil at all, because it

EOZOON 843

differs from every known living form. Yet the suggestion that it is
of mineral origin would be scouted as absurd by every palaeontologist.
Again it is urged by Professor Mb'bius that as the supposed canal
system of Eo^oon 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 reply — (1) that
a prolonged and careful study of this 'canal system,' in a great
variety of modes, with an amount of material at his disposal many
times greater than Professor Mb'bius 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 differ from each other, its yeneral conformity to them being
such as to satisfy Professor Max Schultze (one of the ablest student*
of the group) of its foranriniferal character ; and (3) that not only
does the distribution of the canal system of Eo^oo/t differ in certain
essential features from every form of mineral infiltration hitherto
brought to light, but that canal systems in no respect differing from
each other in distril'i>li<nt are occupied by different minerals; a fact
which seems conclusively to point to their pre-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 Mb'bius altogether ignores.

The argument for the foranriniferal 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 other hypothesis accounting at once for the
whole assemblage of facts.1

Collection and Selection of Foraminifera. — Many of the Fora-
minifera attach themselves in the living state to i-ea \\vrds. zoophytes.
Arc. ; 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

1 The above account of Eozoon is allowed to stand as Dr. Carpenter's name has
become so intimately connected with the view, HOW not commonly held, that the body
has an animal origin. It may be noted that Prof. J. W. Gregory, who has had an
opportunity of examining the so-called Tudm- -pi-cimeu of Eozoon, communicated
to the Geological Society, on March 11, ls91, a paper, of which the following is an
abstract : —

After careful examination of all the slides and figures, and after consideration of
Sir W. Dawsoii'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 Euziioii, or any , reasons for regarding the calcite bands as the
'intermediate skeleton* of a foraminifer. There are points in Sir W. Dawsoii's
figure which might pass as ' stolon passages,' but they appear very different in a
photograph, and the specimen 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 011 negative evidence alone; for, though the rock is much contorted, the
twin lamellae 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. Selwyii and Vennor in the Huronian.

844 MICROSCOPIC FORMS OF ANIMAL LIFE

much larger numbers, however, from the sand or mud dredged up
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 marked 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 our 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, etc., 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 Ihunj specimens from the
oyster-ooze for the examination of their 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 diffused through the liquid, while the
coarser 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 lie
exposed for some hours to the heat of an oven, and be turned over
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 Arc. subside. Another
method, devised by Mr. Legg, consists in taking advantage of the
relative si/es of different kinds of Foraminifera and of the substances
that accompany them. This, \\liich is especially applicable to the sand
and rubbish obtainable from sponges (which may. be go1 in large quan
tity from the sponge-merchants), consists in sifting the \\hole aggre-
gate through successive sieves of wire-gauze, commencing with one

"I ten wires to the inch, which will separate large e\t raneous particles,
and proceeding to those of twenty, forty, seventy, and a hundred
wires to the inch, each (especially that of seventy) retaining a much
nf Microsc. Soc. ser. ii. vol. ii. 1854, p. 1!).

COLLECTING FOEAMINIFERA 845

larger proportion of foraminiferal shells than of the accompanying
particles ; so that, a large portion of the extraneous matter 1 >eing thus
got rid of, the final selection becomes comparatively easy. Certain
forms of Foraminifera are found attached to shells, especially bivalves
(such as the Chamidce) 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 under some appropriate
form of single microscope, a fine camel-hair pencil, with the point
wetted IK 'tween the lips, being the instrument which may lie most con-
veniently 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 tlie\ .-ire to be viewed
by transmitted or by i-pffrcted 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 lattei- 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 sideways 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 their 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 necessary to make extremely
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
sep.-n-ate 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 ripe experience of Mr. F.
Chapman :

Perhaps the foraminiferous clays are the most satisfactory for
those who desire to collect foraminifera. Ordinary clays require to
be slowly and thoroughly dried, broken into small pieces 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 very little
water, and more water should then be added so as to clean.se 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 the fossiliferous
deposits :

• - "Weathered surfaces of carboniferous limestone and seams of clay
in the joints of it.

Clay from the lias formation.

(!ault 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.

SECTION II. — RADIOLARIA

It has been shown that one series of forms belonging to the
rliizopod type is characterised by the radiating arrangement of their
rod-like pseudopodla, suggesting the designation Heliozoa or ' sun-
animalcules ; ' and that even among those fresh-water forms that do
not depart widely from the common ActvnopJwys there are sonic
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 Radiola/ria ; 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 thai pr< >duced 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, such 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).
Nothing, however, was known of the nature of the animals thai
formed them until they were discovered and studied in the living slate
by Professor J. Miiller,1 who established the group of luidiolaria,
including therein, with the Polycystina of Ehrenberg, the Acanllm
///'•//•/////, first recognised by himself, and the T/xifi/^irnf/n \\hich had
lieen dis3overed by Professor I luxlcv. Not long afterwards appeared
the magnificent and ' epoch-making ' work of Professor EEaeckel;9

1 ' Ueber die Thalassicollen, Polyeystinen, und Acanthometren des Miltel-
meeres,' in Abhandlungen der /,-<inii//. Al;ml. <!/•>• IIV.w/w//. ;:n lii-rlin, 1858, and
separately published ; also 'lYber die ini Ilufen von Messina beohuehteten Poly-
i -I/H! < urn,' iu ilii! Miiinilxlii'i'n-li/r of the Berlin Academy for 18">.r>, pp. 071-(i7(i.

-' l>n liiiiUnln rim (Rhizopnda Hadiarial, Berlin, 18(>'2. This pvaf \vork has
l;itc] v been followed by a gigantic monograph published in the ' Challenger ' Reports,

EADIOLARIA

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
corpuscles. 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 inner central capsule is also the special

\

PIG. 644. — Fossil Radioluriu from Barbadoes : a, Poilocijtiin n/ifni; It,
Bhabdolithus sccptrtim ; c,Lychnocaniumfalciferum ; d, Eucyrtidium
titbiilus; c, Fliisl rclln concentrica;f,Lychnocanium Inccrn/i ; g,E>ic//r-
tidiuiii eh'fjdiis; ll, Dictyospyris rlnfl/n/s ; /, F.in-i/rtiiliinn M<ni<i<>lfu /•! ;
k, StcjrfiditalitJtis spinescens ', I, S. notluxn : ///, Lithni-i/i-lin m-rlJiin; /i,
si/lri/ni ; <>, Podocyrlix i-utJm nmfn ; j>, Rhabdolithuspipa.

organ of reproduction, for it is the intracapsular protoplasm, with
the nuclei imbedded in it, which serves for the formation of flagellate
spores ; the outer capsule has the special office of protecting and
providing nourishment for the cell.1 The pseudopodia radiate in all
directions (fig. 645) from the deeper portion of the extracapsular
sarcode ; they have generally much persistency of direction and very

which extends over 1,800 pages, and is illustrated by 140 plates. In it are described
4,318 species, of which 3,508 are new to science.

1 The structure of the central capsule of Aulacantha has been carefully worked
out by W. Karawaiew, in Zool. Aiizeig. xviii. (1895), p. 286 and p. ~2'.>:\.

848

MICROSCOPIC FORMS 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 alga?, marine infusoria, etc.), 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 Zooxanthellce, as K. Brandt has proposed to
call them, so often seen in these cells, are not confined to Radiolaria,

A B

\

FIG. 645. — Polycystina : A, HaUomnni Injuh-ir ; B, Pterocaniiim, with animal.

for they are found also in Actinia? and various other invertebrates;
nor are they always present in examples studied ; they are now com-
pletely recognised ' as alga' 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 Radiolariit skeletal structures are developed in the
.samxle-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 /-ti>/ it/tin;/ spines, 3, or of
comliiiiat ions of these, 4, 5. But in many cases the .skeleton consists
only of a lew scattered spicules ; and this Ls especially the case in
certain large composite loi-ms or ' colonies ' (fig. <>.VJ). which may

K. i;i,mclt, Verhandl. Physiol. Gesellsch. Berlin, 1881-82, p. 22 ;
Milth. /.<!<,!. Stat. Neapel, iv. [>. 191; P. Geddes, Nature, \\\. \>. ::ii:'..

POLYCYSTINA

849

B

'

consist ot'as many as a thousand zooids aggregated together in various

forms, discnidal. cylindrical, spheroidal, chain-like, or even iiecklace-
like. The 'colonies ' seem to be produced, like the multiple segments
of the 1 todies of Foraminifera, by the non-sexual multiplication of a
primordial zo'oid ; but whether this multiplication takes place by
fission, or by the budding oft' 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 sonic cases with a rod-
like crystal, has been observed in many radiolarians ; but of the mode
in which they are produced, and of their subsequent history, very
little is at present known. Viitil the structure and life history of
the animals of this very interesting type shall have been more fully
elucidated, no satisfactory classification of them can lie trained : and
nothing more will be here attempted than to indicate xnne of the
principal forms under which the radiolarian type presents it-elf.1

Discida. — Among the
beautiful silk-ions struc-
tures which are met with
in the radiolarian sand-
stone of Barbadoes (fig.
i)44) there is none more
interesting than the ske-
leton of Astl'OiillltH (fig.

648). in which we have a
remarkable example of
the range of variation that
is compatible with con-
formity to a general plan ^ Y^i -
of structure . As i n < tther
forms 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 le>- completely
filling 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 <-' 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 Considerable attention has been given to the question of the classification of
the Rarliolaria by Haeckel and by R. Hertwig, JfiKtisch* Denkschr. ii. 1H79, p. 129.

FIG. 646. — Polycystina : A, Poiloryrtis Schoin-
burgkii', B, TXhopalocanium ornatnm.

850

MICROSCOPIC FORMS OF ANIMAL LIFE

the difference in the -nnmber of rays is probably of no more account in
these lo\v forms of animal life than it is in the discoidal diatoms.
< >ther discoidal forms, showing a like combination of radial and
circumferential parts, are represented in figs. 649 and 650. and also
in fig. 644, e, m.

^€;mt%

;p'f& -(;yi); (?fv'v"»

^;^f@:^t'

•A..<-^

^* «Wr

Y'

FIG. 647.— Various forms of Kadiohu-ia (after llacckd) : 1, Etlniiosphcrra
r., />//<iuii/ili<>)-(i ; '2, ActiiKiiiinni iiii'niii' ; ;!, Accinthometrd xiphicantha \
I, A.rnclinospli(BTa (>H</<«-«i<tltn ; .">, Cladococciis riniiniilin.

Elltosphserida. In tlii- i--i-ou|) the siliciou* shell i> splicroid.-il.
and is formed u-itliin the capsule; and it is not traversed bv radii,
although prolongations of 1 he shell often extend themselves radially

POLYCYSTIXA

85I

outwards, as in C'ladococcas (fig. 647, o). 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 ns of the wonderful concentric spheres carved in ivory by

FIG. IU8. — Varietal modifications of Astromma.

the Chinese. One of the most common examples of this group is
the Haliomma Hiunholdtii (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. — Perichla/mydium pr&textiim.

FIG. C")0. — St/jlodyctya gmcilifi.

the capsule. This shell may. as in the preceding, be a simple sphere
composed of an open silickms 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, which sometimes

3 i"2

852

MICROSCOPIC FORMS OF ANIMAL LIFE

extend themselves to several times the diameter of the shell, and
ramify more or less minutely, as in J rachnospkcera (tig. <>47. -t). But
more frequently the shell opens out at one pole into a form more or
less bell-like, as in Podocyrtis (fig. (346. A, and fig. 644, a. o). Himjxiln-
canium (fig. 646, B), and Pterocaniwm (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 Etici/rtidii' in
(fig. 644, d, y, /). The transition between these forms, again, proves
to be as gradational, when many specimens are compared.1 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 radiate in all directions from a common
centre (fig. 645, A). The soft sarcode-body is spherical in form, and
occupies the spaces left between the bases of these spines, which
are sometimes partly inclosed (as in the species represented) by
transverse projections. The 'capsule' is pierced 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 le>s
tenacious protoplasm, resembling that
of which the pseudopodia are composed.
One species, the Acanthometra eclt'm-
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 TTy\yirths of an inch, is very
common on some parts of the coast of
Xorway. especially during the preva-
lence of westerly winds; and the
Fw.65l.—H(iU<j»ni>(t Humlohliii. Author has himself met with it abun-
dantly near Shetland, in the floating

brown masses termed inti<lr<'. by the fishermen (who believe them
to furnish food to the herring), which consists mainly of this
Acanthometra mingled with Entomostraca.

Phaeodaria. — Among the most important of the Jhidinlaria
collected by the 'Challenger' are the comparatively large (as much as
I mm. in diameter) single-celled forms which are remarkable for the
constant presence of large dark brown granules, which an- scattered
irregularly round the central capsule and cover the greater part of
its outer surface. The nucleus is large, the capsular membrane is
always double, and is pierced by one or more large 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 structure of which the group is subdivided. -

Collozoa. — To this group belong those remarkable composite
forms which, exhibiting the characteristic radiolariaii type in their

1 The general plan of structure of the I'oliji-ijuthiii, and the signification of their
immense variety of forms, were alily discussed \>\ Mr. \Valhcli in the. Trans, of ihe
iiTiiKr. Sue. H.S. vol. xiii. lH(i.r), p. 75.
- On reproduction in this group, cf. A. Borgert, Ziiol. Anzrig. xix. (IMOfi), p. 307.

RADIOLAEIA

853

individual zooids, arc aggregated into masses in which the skeleton
is represented only by scattered spicules, as in Sphcerozoum (fig. (352)
and Tlialassicolla, Tliese ' 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 rounded bodies, of very variable
size and shape, but usually either globular or discoidal. Externally
they are invested by a layer of condensed sarcode. which sends forth
pseudopodial 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 arc scattered a great number of
oval bodies resembling cells having a tolerably distinct membraiiiform
wall and a conspicuous 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 arc 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-
ternal portion of each
mass is composed of an
aggregation of large
vesicle-like bodies im-
bedded in a softer sar-
codic substance.1

From the researches
made during the 'Chal-
lenger ' Expedition, it

i. — Sjil/ti rOZOUm orail/unirr.

appears that the Radiolaria are very widely diffused 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, burl also at considerable depths. Their silicious skeletons
accumulate in some localities (in which the calcareoiis remains of
Foraminifera are wanting) to such an extent as to form a 'radio
larian ooxe ; ' 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 rock, which is said to attain a thickness of
1,100 feet, or a similar rock of yet greater thickness in the Xicobar

1 See Professor Huxley (to whom we oweour first knowledge of these forms) in .4?m.
Nat. Hist. ser. ii. vol. viii. 1851, p. 433: also Prof essor Miiller, of Berlin, in Quart. Ju urn.
.l//Vn>.sr. Set. vol. iv. iNoli, p. 7'2, anil in his treatise Ueber i/ie ThaldssieaUeii, Pul//-
i-i/x/itie>i, uncL Acanthometren des Mittelmeeres, the magnificent work of Professor
Ha.eckel, Die Badiolarien, and the monograph by K. Brandt, published in tlie Fauna
mid Flora ties Golfes ran Xea/iel, 1885, 'Die koloniebildeuden Eacliolarien
( Sphserozoeen) des Golfes von Neapc-1.'

854 MICROSCOPIC FORMS OF ANIMAL LIFE

Islands. 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) their
solid forms then become much move 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 pm-pose of display,
although minute details of structure can be better made out when
they are viewed as tvanspavent objects with higher powers. Many
of the more solid forms when exposed 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 may then be mounted
on a black ground and illuminated either with a side condenser or
with the parabolic speculum. ISTo 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

1 For a fuller description of the fossil forms of this group see Professor Ehrenbeix'V
memoirs in the Monatsbericlite of the Berlin Academy for 1846, 1847, and 1850 ; afso
his Microgeologie, 1854 ; and Ann. <>f Nat. Hint. vol. xx. 1847. The best method of
separating the Polycystina from the Barbadoes sandstone is described by Mr. Fur-
long in the Quart. Jotiru. of Mir ruse. Sri. ii.s. vol. i. 1801, p. (54.

855

CHAPTER XV

SPONGES AND ZOOPHYTES

1 . 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 li';id an independent
existence, but combine to form an organic whole, various parts
undertaking various functions. Of these Metazoa the simplest ex-
amples are to be found among SPONGES. The determination of the
real 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 properly 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,1 but it may now be unhesitatingly affirmed that a
sponge is an aggregate of protozoic units only in the sense in which
all Metazoa are confposed of cells ; some of these cells have a. striking
resemblance to the collared Flayellata (fig. 585), whilst others re-
semble Amoebae (fig. 577). These iinits 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 forms of
sponges, however, this framework is altogether absent : in other* it
is represented only by calcareous or silicious ' spicules.' which are
dispersed through the sarcodic substance (fig. 654, 13); in other*,
again, the skeleton is a keratose (horny) network, which may lie
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 what may be regarded as the highest type* of
the group, the silicious component of the skeleton increases, and the
keratose diminishes until the skeleton consists of a beautiful silicious
network resembling 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. JMincliiii's essaj
dj i ' The Position of Sponges in the Animal Kingdom' in Science Papers, i. (n.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 lui~ sug
gested the use of the group-name Parazoa. See also Trcul/^r on 7. '"mini/ /j^ vol. ii.
London, 1900.

856

SPOXGES AND ZOOPHYTES

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 <mttr,i,-<Ix \>\
repeated ramification, sending their zooid-bearing branches to
meet the water they inhabit, the surface of the former extends
itself inwards, 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, b, b) with which the surface a. a of the living sponge
is beset lead to incurrent passages that open into chambers lyiiii^
beneath it (c. c), and open into the ' ampullaceous sacs.' or, as thev
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
excurreiit canals, which, uniting into largerjtrunks, proceed to the

oscvla or projecting
vents, d. 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
*IG. 658.— Diagrammatic section of a sponge: «, a, nxvo-pn • -.11, 1 fvmn +11Q
superficial layer; I, inhalant apertures or pores ; r, c, OX^8en > andtiom the
flagellated chambers; fl, exhalant oscule ; e, deeper manner in which
substance of the sponge. coloured particles ex-

perimentally diffust •( I

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 product of its digestion of them
to the general sarcodic mass.

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 long slender 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 sponge. As was tirst shown by
Professor C. Stewart, sensory organs, formed of groups of cells
>\ith long projecting filaments, are to he seen on the surface of

lll:lll> s| ges. Any one of these a imrlx .ids. again, detached from

the mass, may lay the foundation of a ne\\ ' colony.' Jn the
aggregate mass produced by its continuous segmentation certain
globular clusters are distinguishable, each having a cavity in

SPONGES 857

its interior ; and the amceboids that form the wall of this cavity
Income metamorphosed into collared flagellate cells whose flagella
project into it. Thus is formed one of the characteristic ' ampul-
laceous sacs.' which, at first closed, afterwards communicates with
the exterior, on the one hand by an incnrrent passage, and on the
other with the excnrrent canal-system leading to the oscula. ]!<•
sides this reproduction by • microspores,' there' is another form
of non-sexual reproduction by macrospores. 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 mam
protophytes. 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 am<rboids. 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 1'<>tcti.>-.
becoming 'sperm-cells.' and developing spermatozoa by the meta-
morphosis of their nuclei: while others become -germ-cells.'
developing themselves by segmentation (when 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 . l

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 a low 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 altogether

*/ •

imbedded in the fibres, or are implanted into them at their base- ;
but smaller and simpler sponges, such as (,'r<tntia. have no horny
skeleton, and their calcareous spicules are imbedded in the general
substance of the body. Sponge-spicules are much more fre
oiiently silicious than calcareous ; and the variety of forms pre-
sented by the silicious spicules is much greater than that which
we find in the comparatively 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 are sometimes pointed at both ends, sometimes at one only;
one or both ends may be furnished with a head like that of a

1 See Chapter V. of ilv. Saville Kent's Man mil nfthf Iitfi<nt»-iu, and Chapter V. of
vol. i. of Mr. Balfour's Comparative Embryology, as well as Professor Haeckel's im-
portant work on the Calcareous Sponuc--.

858

SPONGES AND ZOOPHYTES

pin or may carry 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-

B

rn.c.

Fin. (>">-]. — A, section through I'lnilcrllin r< 'nl iJtiLriiin, var. connexida, taken at right
angles t,ci the surface, to show the- arrangement of the parts of a. sponge : /i, pores
on the snrt'iiee |e:nlin;j to if, tlie inlialiinl canals, then to the flagellated chambers,
/'<•, Mild thence to the exhalant CMluils, /'f, to o, the oseiila, in the derniMl meinlirane.
dm. I'}, nion- highly magnified view of (lie internal portion ichoanosome) of
Axiliella paradoXd • li'.M) : me, so-called mesodennal cells. Other letters MS in A.
(After Ridley and Bendy.)

SPONGE-SPICULES

859

...It'

vals, giving them a jointed appearance.1 The more recent authorities
on Sponges, such as Professor Hollas and Messrs. Ridley and Dendy,
have recognised that in the present state of our knowledge the spicules
which are ordinarily found in silicious Sponges belong to one of two
groups, which, as they differ considerably in size, may be called
megascleres (or, more correctly, megaloscleres) and microscleres. It is
to the definite arrangement of the former 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 110 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

at

L..I

FIG. 655. — Structure of the chela of Mo-
naxonid Sponges: 1, tridentate anisochela
from in front; la, from the side; 2, la,
front and side views of a palmate isocliela ;
/, /', tubercle; at, at', anterior tooth or
palm ; U , It', lateral tooth or palm ; s, shaft ;
/, timbria. (After Ridley and Dendy.)

single axis, but the growth

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 there are

three axes and three rays : but in some sponges, such as Teiius's

flower-basket, the growth is along both directions of the axes, so

that while there are three axes there are six rays, or the spicules

are hexactiiiellid. In others, such as Ceodia and the Lithistid

Sponges, there are four axes, whence such forms are called tetraxonid.

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
Spongiada? ' in Phil. Trans. 1858, pp. 279-332; and in his Monograph of tlie
British S/m/ti/iaihr, published by the Kay Society. The Calcareous Sponges have

, and Messrs. Ridley and Dendy, Report on the ' Challenger' Monax-
onida, pp. xv-xxi.

860 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 (dragmata) ; 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 lie 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
chela'. They describe these scleres (fig. 655) as having a more or
less curved shaft (s). which bears at each end a variable number of

sharply recurved processes (at. at', 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
X9oo HI, 6 n projection, which is generally so trans-
parent as to be with difficulty made out.
The shaft itself is frequently drawn out
at the side into wing-like processes or
fimbriae (/). If the two ends of the

FK ;.<;,->(;. — Amsochels? of Clado- . ^ , , , .„

rliizt, lu versa, showing », the spicule are equal, we have isochelce ; if un-
nucleus of the mother-cell of equal, anisochelcB. The stellate micro-

the spicule from in front « sc\eves nmy ])e spiml have a shaft with
and from the side. o. x 300. . ,J , ' . . . , , „ . ,

(After Ridley and Dendy.) spniose whorls, or a cylindrical shaft with

a toothed whorl at either end. The

spicules of sponges cannot be considered, like the rap/tides 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. 65(>).
and the special cells which produce them are distinguished as silico-
blasts ; in this there is first developed a central organic thread
around which concentric layers of silica or chalk are laid down.1

There is an extremely interesting group of Sponges in which the
horny skeleton is entirely replaced by a silicioits 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 Euplectella of the Manila
seas — which was for a long time one of the greatest of /oological
rarities, but which now. under the name of ' Venus's ilower-basket .'
is a common ornament of OUT drawing-rooms — is one of the most
characteristic examples." Another example is presented by the

1 For a compendious statement »f the <-h;ir;i<-irrs of sponge-spicules see pp. 82— 4
1. 1' Mr. A. Si-d-wick-'s Stiiilrnfx Text-book of Zoology,Ijondion, l.s'.is.

- The structure and ai rangement of tin- soft |>;n-t- of Eujjlcctclla u^ergillinn have

SPONGES 86 I

tloltenia Car j)e uteri, of which four specimens, dredged \\p Iroin a
depth of 5:>0 fathoms between the Faroe Islands and the north of
Scotland, were among the most valuable of the 'treasures 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 Eujdn'tdln. 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 regular sex-
radiate spicules, there is a remarkable variety of other forms, which
have been fully described and figured by Sir Wyville Thomson.1
One of the greatest features of interest in this Holtenia is its
singular resemblance to the Ventricufotes of the Cretaceous formation.
Subsequent investigations have shown that it is very widely diffused,
and that it is only one of several deep-sea forms, including .some
of singularly 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 Barbadoes
after a storm. This Dictyocalyx puvniceus has the shape of a mush-
room, the diameter of its disc sometimes ranging to a foot. A small
portion of its reticulated skeleton is a singularly beautiful object
when viewed with incident light under a low magnifying power.

With the exception of the genus S pong ilia and its allies, all
known sponges are marine, but they differ very much in habit of
growth. For whilst some can only be obtained by dredging at con-
siderable depths, others live near the surface, whilst others attach
themselves to the surfaces of rocks, shells, lire, between the tide-
marks. The various species of (irantia 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 water 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 ;L-inch objective on some
of the cells of the gelatinous substance scraped from the interior of
the bag ; or they may be seen /•// situ by making \ vry thin trans-
verse sections of the substance of the sponge. It is by such sections
alone that the internal structure of sponges, and the relation of
their spicular and horny skeletons to their fleshy substance, can be
demonstrated. They are best made by the imbedding process. In
order to obtain the spicules 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. l{i>//<il S<>f. <;/' Eilin Imryli, xxix.
p. 661.

: See his elaborate memoir in PJ/il. Truns. 187<l. iinrl his Depths of the Sea,
1872 p. 71.

862

SPONGES AND ZOOPHYTES

T

reagents. The latter method is preferable, as it is difficult to free
the mineral residue from carbonaceous particles by heat alone. If
(as is commonly the case) the spicules are silicious, the sponge may
be treated with strong nitric or nitre-muriatic acid, until its animal
substance is dissolved away ; if, on the other hand, they are cal-
careous, 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 preferable
011 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.1

11. ZOOPHYTES (COZLEXTERA)

Under the general designation Zoophytes it will be still con-
venient to group those animals which form composite skeletons or

4 polyparies ' of a more or less
plant-like character, associating
with them the Acalej>/<8, which
are now known to be the ' sexual
zooids' of polypes, but excluding
the Pohpoa on account of their
very different structure, not-
witlistandiiig their zoophytic
forms and habits of life. The
animals belonging to this group
may be considered as formed
upon the primitive gastrula
type, their gastric
(though sometimes
itself almost indefinitely) being
lined by the original endod&rm,
and their surface being covered
by the original ectoderm, and
these two lamella? not being
separated bv the interposition
of any body-cavity or ccelmit.
It is a fact of great interest
that although the product of
the development of a iimrnln. is
here a distinctly individualised
polype, in which several mutu-
ally dependent parts make up
a single organic whole, yet these parts still retain much of their
independent protosoic 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 amo-boid cells, which send out pseiidopodial

1 A complete and valuable handbook to the Sponges has been published by
Dr. G. (.'. Yosmaer as vol. ii. of I'.ronnV A"/./x.sry/ /mil (>nl>»t nffcn des TMerreichs,
:ig, 1HS7. Compare also the article by Professor Sollas in the ninth edition of the

cavity
extending

FIG. 657. — Longitudinal section of the body
of a hydra killed in full digestion : ec,
ectoderm; en, endoderm ; >i/]>, muscular
processes; d, a diatom; /, food. (After
T. J. Parker.)

CCELENTEKA 863

prolongations into its cavity (fig. 657) by whose agency (it may be
pretty certainly affirmed) the nutrient material is first introduced
into the body-substance. This process of ' intracellular digestion '
was first noticed by Professor Allman in the beautiful hydroid polype
Myriothela ; ! the like has been since shown by Mr. Jeftery Parker
to be true of the ordinary Hi/dm ; - and Professor E. Ray Lankester
has made the same observation upon the curious little Medusa (I/mnm
codiuiii), which lives in fresh-water tanks in this country, whither
it has undoubtedly been introduced ; while the observations of
Ivrukenberg have shown that a similar process obtains among the sea-
anemones.3 (It may be mentioned in this connection, that Metschni-
koft' has seen the cells which line the alimentary cannl of the lower
planarian worms gorging themselves with coloured food-particles,
exactly in the manner of Aimrn/f and the liver-fluke, and that a
number of larva- are known to obtain their nourishment in the same
way.4) The second ' survival ' of protozoic independence is shown
in the extraordinary power posse»ed by [fi/Jra. Actinia. Arc. of
reproducing the entire organism from a mere fragment. This great
division includes the two principal groups the HYDROZOA and the
ACTIXOZOA, the former comprehending the Poli/pes, 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 ; whilst 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 Ifi/dra, or fresh-water
polype, a very common inhabitant of pools and ditches, where it is
most commonly to be found attached to the leaves or stems of aquatic
plants, floating pieces of stick, etc. Two species are common in this
country, the //. viridis or green polype, and the //. rnlyaris, which
is usually orange-brown, but sometimes yellowish or red (its colour
being liable to some variation according to the nature of the food
on which it has been subsisting) ; a third less common species, the
If.fH.sca, is distinguished from both the preceding by the length of
its tentacles, which in the former are scarcelv as long as the body,
whilst in the latter they are. when fully extended, many times longer

Eiici/rlojH/'did Britannica ; the ' Challenger' Ih-jiartx by Professor Schulxe, Messrs.
Ridley and Dendy, Polejaeff, and Sollas ; and the numerous memoirs of Professors
O. Schmidt and Schulze. More recently important additions to our knowledge of
Sponges have been made by Prof. Yves Delage and Monsieur E. Topsent in the
Arch. Ziiol. Exper. ct (+<•//. 189-2-5, and by Dr. O. Maas in the Mitth. Zl',,,1. Shit.
Neapcl, x. and elsewhere.

1 Phil. T ru tin. 1875, p. 55-2. It should lie noted that the late Professor Claus
called attention to the ingestion of foreign bodies by anueboid cells of Moiioplnjen
ill 1874. See his Sell ri th-n Zi'inl. Inhnltx iWien, 1874), p. :!0.

- PI-IH-. »f lioij. Sue. vol. xxx. issil, p. C,l.

" Qi/dii. Joi/rn. Mirmsr. Sri. n.s. vol. xx. 1880, p. 371.

1 Consult an interesting article on 'Intercellular Digestion,' by Metschnikoff, in
Ecinic Scientifique, ser. iii. vol. xi. p. (is:;.

864

SPONGES AND ZOOPHYTES

(fig. 658).' The body of the Hydra consists of a .simple bag or sac,
u liich may lie regarded as a stomach, and is capable of varying its
shape and dimensions in a very remarkable 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
ylohe, 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 arranged with great
regularity around the orifice.
The body is prolonged at its lower
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 • iirticating organs' will
be described hereafter; at pre-
sent it will be enough to point
out that this apparatus, repeated
many times on each tentacle, is
organ a great prehensile power,

FIG. 658. — Hydra f iiitca, with a young bud
at b, and a more advam-ed bud at c.

to the

doubtless intended to "

the minute filaments forming a rough surface adapted to prevent
the object from readily slipping out of the grasp of the arm. whilst
the central spicule or * dart is projected into its substance, probably
conveying into it a poisonous fluid secreted bv a vesicle at its base.

1 On the specific characters of Hi/dm consult Haacke, Ji-nainchi: Zritschr. xiv.
p. l:'.:!; and Jickdi, /.iiol. Anzcig. v. p. 4!H.

•' To this intermrdiiitc' layer, Mr. G-. C. Bourne applies the term niesof/Jten. For an
arc-omit of its variations and structure among the Cu;leiitera, and a discussion of its
limnology with the mesoderm of higher Meta/.oa, see his essay on Fitngin in vol.
xxvii. of the' (th<ii>-/. Jmirii. Mirt'uai-. St'i. n.s.

HYDROZOA

865

The latter inference is founded upon the oft-repeated observation
that if the living prey seized by the tentacles have a body destitute
of hard integument, as is the case with the minute aquatic worms
which 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 polype's embrace.
The contractility of the
tentacles (the interior
of which is traversed by
a canal that communi-
cates with the cavity
of the stomach) is very
remarkable, especially
in the Hydra fttsca,
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 support from
animals whose activity,

as compared with its

i • i .L f

own slight powers ot

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. 80011, 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. — Ga/nvpanulafia gelatinosit.

866

SPONGES AND ZOOPHYTES

of the stomach ; but this would not seem to he properly regarded as
anal, since it is not used for the discharge of such exuviae ; it is
probably rather to be considered as representing, in the Hydra, the
entrance to that ramifying cavity which, in the compound Hijdrozoa,
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,
b, 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 before quitting its parent ;
and as many as nineteen young
Hijdr(t' in different stages of
development have been seen thus
connected with a single original
stock (fig. (5(50). 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 extraordinary power which
one portion possesses of repro-
ducing the rest. Into whatever
number of parts a Ili/dra 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 process, the fecundating apparatus, 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 apparatus forming a conical projection just
beneath the arms, while the female ovary, or portion of the body-
substance in which the ovum is generated, has the form of a
knob protruding from the middle of its length. It would appeal-
that sometimes one individual Hydra develops only the male cysts
or sperm-cells, while another develops only the female cysts or ovi-

•mi

FIG. 660. — Hi/dni fused in gemmation: a,
mouth ; b, base ; c, origin of one of the
buds.

HYDROZOA 867

sacs ; but the general rule seems to be that the same individual
forms both organs. The fertilisation of the ova. however, cannot
take place until after the rupture 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 April and July. According to Ecker, the eggs of H. vir'ulix
produced early in the season run their course in the summer of
the same year ; while those produced in the autumn pass the winter
without change. When the ovum is nearly ripe for fecundation
the ovary bursts its ectodermal covering, and remains attached by
a kind of pedicle. It seems to be at this stage that the act of
fecundation occurs ; a very 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 that may be more suitable to its wants ;
its changes of place, however, seem rather to be performed under the
influence of liykt, towards which the Hydra seeks to move itself, than
with reference to the search after food.1

The compound Ht/droids may be likened to a Jft/dra whose
gemma', instead of becoming detached, remain permanently connected
with the parent : and as these in their turn may develop gemm:e
from their own bodies, a structure of more or less arborescent
character, termed a. pnlijparu, 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 proceed from the base of
each, along the stalk and branches, to communicate with 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 cortical layer, and thus giving rise
to fabrics of various forms. Between these a muscular coat is some-
times noticed. The fleshy tube, whether single or compound, is called
a cd'.nosarc, and through it the nutrient matter circulates. The
' zo'oids,' or individual members of the colony, are of two kinds : one
the polypite, or alimentary zb'oid, resembling the Hydra in essential

1 A very full account of the structure and development of Hydra has been
published by Kleiiienberg, "f whose admirable monograph a summary is given by
Professor Allman, with valuable remarks of his own, in Quart. Jnurn.Microsc. Sci. n.s.
vol. xiv. 1874, p. 1. See also the important paper by the late Mr. Jeffery Parker
already cited. On the chlorophyll corpuscles of H. ririilix consult Brandt, Mittli.
ZSol. Stat. Neapel, iv. p. 191 ; Hamann, ZiJol. Aii,~fi/j. vi. p. MOT; and Lankester
Quart. Jonni. Mirror. Sri. n.s. xxii. p. '2'29.

3K1>

868 SPONGES AND ZOOPHYTES

structure, and more or less in aspect ; the other, the gonozooid, or
sexual zooid, developed at certain seasons only, in Ituds of particular
shape.1

The simplest division of the Hydroida is that adopted by Mr.
Hincks,2 who groups them under the sub-order Athecnta and Thecata,
the latter being again divided into the Tkecaphora and the Gymno-
chroa. In the first, neither the ' polypites ' nor the sexual zooids
l>ear true protective cases ; in the second the polypites are lodged in
cells, or, as Mr. Hincks prefers to call them, calycles, many of which
resemble exquisitely formed crystal cups, variously ornamented, and
sometimes furnished with lids or opercula ; in the third, which con-
tains the Hydras, there is no polypary, and the reproductive 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, but more commonly the zoophyte is dioecious. 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 Medusce (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 plan-tiler, closely resembling ciliated
Infusoria, first move about within the capsule, and then swim forth
freely 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 forth around it ; and the body increases in length
and thickness, so as gradually to acquire the likeness of one of the
parent polypes, a t'ter 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 Medusa-
of these polypes (fig. (363) belong to the division called 'naked-eyed.'
on account of the eye-spots usually seen surrounding the margin of
the bell at the l>asr of the tentacles.

A characteristic example of this production of medusa-like
'gonozooids' is presented by the form termed SyncOryne X((rsii (fig.

1 A useful li^ti nl the principal terms used in describing liydroids, u ith definitions,
will be t'liuud cm \>\>. l(i and 17 of Professor Allmun's lu/xni mi the Hi/droida (Pin-
i of Ilic CLu/li HI/IT.

ll//// i-diii Zi'i'/ili//// ,v, lsi;s.

DEVELOPMENT OF HYDROZOA

869

661) belonging to the sub-order Athecata. At A is sho^n the ali-
mentary zooid, or polypite, with its tentacles, and at B the succes-
sive stages a, b, c, of the sexual zooids, 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 Medusae of the genus Si/ncori/ne (as now restricted) have the
form named Sarsta 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 remain fixed, and
mature their products while at-
tached to the zoophyte.1 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 which the goiiozooids
are produced in the common
Hydra, as .-dread v described, and
that of Syncoryne. In Tubn-
laria the goiiozooids, though
permanently attached, are fur-
nished with swimming bells,
having four tubercles repre-
senting marginal tentacles. A
common and interesting species,
Tnbularia imHrixii., receives its
specific name from the infre-
quency with which branches are
given off from the stems, these f< >r
the most part standing erect and
parallel, like the stalks of corn,

upon the base to which they are Fj(; 661._Development of Medusa-buds

in Si/i/i'iii-i/iir Sa/rsii: A, an ordinary
polype, with its club-shaped body covrn :< I
with tentacles ; B, a polype putting forth
medusoid gemmae ; a, a very young bud ;
b, a bud more advanced, the quadran-
gular form of which, with the four
nuc-h'i whence the cirrhi afterward*
spring, is shown at d; c, a bud still
more advanced.

zoo-
grows

attached. This beautiful
phyte, which sometimes
between the tide-marks, but is
more abundantly obtained l>v
dredging in deep water, oft en
attains a size which renders it
scarcely a microscopic object. it>
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 for

1 Hincks, op. cit. p. 49.

870 SPONGES AND ZOOPHYTES

a recurrence of the operation, this, as observed by Mr. Lister,
being repeated at intervals of eighty seconds. Besides the foregoing
movement, a regular flow of fluid, carrying with it solid particles of
various sizes, may be observed along the whole length 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 beiieatn ; and this exuviation and
regeneration may take place many times in the same individual.1

It is in the families Campanulariida and Sertulariida (whose
polyparies are commonly known as ' corallines ') that the horny
branching fabric attains its completes!, development, not only afford-
ing an investment to the stem, but forming cups or cells for the
protection of the polvpites, as well as capsules for the reproductive
gonozooids. Both these families thus belong to the sub-order Thecato.
In the Campanulariida the polype-cells are campaiiulate or bell-
shaped, and are borne at the extremities of ringed stalks (fig. (359, 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 Hydra ; and the
mode in which they obtain their food is essentially the same. Of
the products 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
Campanularia as in Tubularia, the circulation being most vigorous
in the neighbourhood of growing parts. It is from the ' ccenosarc '
(fig. 659, f) contained in the stem and branches that new polype-
Inn Is (?>) are evolved ; these carry before them (so to speak) a portion
( >f 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 reproductive capsules or ' gonothecse : (r?) is
exactly similar, but their destination is very different, AVithin
them are evolved, by a budding process, the generative organs of
the zoophyte ; and these in the Campanulariida 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 polypes; or, in cases in which
the medusoid structure is less distinctly pronounced, may not com-
pletely detach themselves. 1ml (like the flower-buds of a plant) expand
one after another at the mouth of the capsule, withering and drop-
ping oH'af'tei- they have matured their generative products. In the
Sertulariida, on the oilier hand, the medusan conformation is wanting,
as the gonozooids are always fixed ; the reproductive cells (tig. lilii!.^).
which were shown by Professor Kdward Korlies to lie really meta

1 The British '1'iilniln riiiln I'orni Mir subject of a most complete ,-niil l>r;iutiful
monograph )>y the Lite Professor Allmim, published \,\ the K;i\ Society.

COLLECTING ZOOPHYTES

871

morphosed branches, developing in their interior certain bodies which
were formerly supposed to be ova, but which are now known to be
' medusoids ' reduced to their most rudimentary condition. Within
these are developed — in separate gonothecte, sometimes perhaps 011
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 first instance into
ciliated ' gemmules,' or planul;e. which soon evolve themselves into
true polypes, from every one of which a new composite polypary
may spring.

There are few parts of our coast which Avill 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
up by the waves from the
deeper waters, especially
a ffcer a storm . Many kinds,
however, can only be obr
tained by means of the
dredge. Of the remarkable
forms dredged by the ' Chal-
lenger ' mention can only
be made here of the gigantic
Tubulariaii — Monocaulus —
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 Streptocaidxs j>nlc/ier)-inms, in which by the twisting of
the stem the ultimate ramules are thrown into ' a graceful and
beautiful spiral.' For observing them during their living state, no
means is so convenient as the zoophyte -trough. In mounting com-
pound Hydrozoa, as well as Polyzoa, it will be found of great
advantage to place the specimens alive 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 liquid may be
withdrawn, and replaced by (Joadby's solution. Dearie'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, portion 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.1
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 fully 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. When 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 forms 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 polarised 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 Zoophytes and the medusoid Acalejilia (or -jelly-fish '). \Ye
now know that the small free-swimming medusoids belonging to

See Mr. J. W. Morris in Qinni. Joiirn. of Mirronc. .SV/'. n.s. vol. ii. 180:2, \>. 110.

'i'fi ilr llinlo/fic, vi. ]). 115.

JELLY-FISHES

8/3

the ' naked-eye ' group, of which Thaumantias (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

FIG. (563. — A, Thaumantias pilosella, one of the ' naked-eye ' Medusas : «, a,
oral tentacles ; b, stomach ; c, gastro- vascular canals, having the ovaries,
il (/, on rither side, and terminating in the marginal canal, e e. B, TJm/i-
inantiim Ewliwhaltzii, Haeckel.

budded oft', endowed with independent organs of nutrition and
locomotion, whereby they become capable of maintaining their own
existence arid of developing their sexual products. The general con-
formation of these organs will lie understood from the accompany-
ing figure. Many of this group are very beautiful objects for

874 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 be 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 Jfedusfe1 or ACA-
LEPH.E which are commonly known as •jelly-fish ' is essentially simi-
lar ; for their progeny have been ascertained to develop themselves
in the first instance under the polype form, and to lead a life which
in all essential respects is ziiophytic ; their development into Medusre
taking place only in the closing phase of their existence, and then
rather by gemmation from the original polype than by a metamor-
phosis of its own fabric. The huge lihizostoma found commonly
swimming round our coasts, and the beautiful Chrysaora remarkable
for its long ' furbelows ' which act as organs of prehension, are oceanic
acalephs developed from very small polvpites, 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 ' plaiiula,' of rather oblong form,
very closely resembling an intusory animalcule, but destitute of a
mouth. One end soon contracts and attaches itself, however, so as
to form a foot ; the other enlarges and opens to form a mouth,
four tubercles sprouting 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-tuba, which is the polype stage of
the Chrysaora 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 110 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 goiiozooids, 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 rouleau of coins ;
a sort of fleshy bulb, « (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 beende
tached ; as many as thirty or even forty have thus been produced in
one specimen. The constrict ions then gradual Iv deepen, so as to divide
the cylinder into a, pile of saucer like bodies, the division being

1 See Professor Clans, Untersuchungen nl>rr<lii Organisation undEntwickelu/ng

I/IT Mi-ihtxrii, I'lM-'nr :iiid 1 jfi| >/.i-, ls,s:',, and Miss Ida H. Hyde, ' Entwickelungs
t'ini^er Scyplionii'duson,' in /.cit^t-h r. t. WISS. /.i'ml. Iviii. p. .r>:!l.

REPEODUCTIOX OF ACALEPHS

875.

most complete above, ami the upper discs usually presenting 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 supposed rudimentary 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 before the detachment of the topmost disc, this
circle disappears, and a new one is developed at the summit of the
bull) which remains at the base of the pile. At last the topmost
and largest disc begins to exhibit a sort of convulsive struggle ; it

<*

'i<;. 004. — I, two Hi/il rir tii/iif [Scyphistoma-siage] of ('//mica
<-iipiU(it<i, with tw<> hi, in undergoing fission (jStfrofeiZa-stage).
II, a and b of fig. I three days later. In o the tentacles are
developed beneath the lowest of the Ejtlnjrcr, from the stalk
of the Strobild, 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 Medusa1. 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 member of which may in its turn bud off a pile
of Medusa discs.

The bodies thus detached have all the essential characters of the
adult Jfeditsce. Each consists of an umbrella -like disc- divided at
its edge into a variable number of lobes, usually eight ; and of a

SPONGES AND ZOOPHYTES

stomach, which occupies ;i considerable proportion of the disc, and
projects downwards in the form of a proboscis, in the centre of which
is the quadrangular mouth (fig. 6(55, A, B). As the animal advances
towards maturity 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 Medusje are very voracious, and grow rapidly, so as to attain

FKI. <ii;r>.— Development of Chnjsaora from Hydra tuba: A,
i!H ached individual viewed sideways, and enlarged, showing
the proboscis a, and b the bifid lobes; B, individual seen from
above, showing the bifid lohes of the margin, and the quadri-
lateral mouth ; C, one of the bifid lohes still more enlarged,
showing the rudimentary eye i?) 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 < '//"""' ;|1I(1 Chryscwne, which are common all
round our coasts, often have a diameter of from six to fifteen inch.- :
while Rhlzostoma sometimes reaches a diameter of from two to three
feet. The quantity of solid matter, ho\\ ever, which their fabrics con-
tain is extremely small. It is not until adult age has been attained
that the generative organs make their appearance, in four chamber-
disposed around the stomach, which are occupied by plaited mem-
branous ribbons containing sperm-cells in the male and ova in tin-
female, and the embryos evolved from the latter, when they have
been fertilised by the agency of the former, repeat the extraordinary
cycle of phenomena which has been now described, developing them-
selves in the first instance into hydroid polype*, from 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,1 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 origin to a form which does not re-
semble itself, but returns to the form A, from which B itself sprang.
It was early pointed out, however, by the Author2 that the term
' alternation of generations ' does not appropriately represent the
facts either of this case or of any of the other cases grouped under
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 yem rut'ire or sexual act. whilst in the other it is
by a process of <ieiinn«tifni or budding. Thus the Jfed/isc' 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 zoophytic phase of life being the most conspicuous in
such Thecattt as Campanulariida and Sertulceriida, 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 said the Tubipora
of warmer 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 Gotynt/ln or sea-tans. A third order. /'II</<IN«, consists of fossil
corals, whose stony poly paries are intermediate in character between
those of the two preceding. And lastly, the Ctenophora, free-swim-
ming gelatinous animals, many of which are beautiful objects for
the microscope, are by some zoologists ranked with the Actinozoa.4

Of the Zoantliaria. the common Actinia or ' sea -anemone ' may
lie taken as the type, the individual polypites of all the composite
fabrics included in the group being constructed upon the same model/'
In by far the larger proportion of these zoophytes, the liases of the

1 See his treatise on Tin- Alternation of Gent'i-ationx, a translation of which ha.-
been published by the Ray Society.

2 Brit, a ml For. Med. CJih: Review, vol. i. 18i8, p. I'.t-J ,-t seq.

5 Compare Huxley, Atiafoii/i/ of Iiit'crfrlmtfcil Animals, p. l:-Ju ; and Balfour,
Comptirath't' Einl>ri/i>h>gi/, i. p. 151.

1 Professor Haeckel, led by the study of Cteinir'ni ctenophora, associates the
Ctenophora with the Hydrozoa (Sitzungsber. Jcn«incJic GcscllscJicift, May 16, 187(.l).

s On the anatomy of Actinia and its allies, see O. and B. Hertwig's monograph
in vols. xiii. and xiv. of the JauiiscJie Zeitst-lirift.

878 SPONGES AJND ZOOPHYTES

pol vpites, 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 01- lamella'
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 Fumjla, or ' mushroom-
coral ' 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 Cavyo-
j/////llif(. a like solitary anemone of our own coasts, which is scarcely
distinguishable from an Actinia by any other character than the
presence of this disc, and also on the surface 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
very beautiful objects for low powers, the former 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-hydrogeii microscope. An exceedingly use-
ful method of preparing sections of corals has been devised- by 1 )r. ( ! .
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 permeation they are taken out and dried
slowly until the masses become quite hard. These masses may now
be cut into sections with a fine saw and rubbed down tin a whetstone
in the ordinary manner ; after staining, the sections may be 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.1

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 Medusa1, 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 >eleded in preference) be
cut off, and be subjected to gentle pressure between the two glares
of the aquatic box or the compressorium. multitudes of little dart-
like organs will be seen to project themseh es from its surface near
its tip : and if the pressure be gradually augmented, many additional
darts will every moment come into view. Xot only do these organ>
present different forms in different species, but even in one and the
same individual very strongly marked diversities are shown, of
\\liich a few examples are given in fig. (500. At A, B, (.'. 1) is
shown the appearance of the ' filiferous capsules.' whilst as yet the

1 Sec Zi'idloii/xc/irr Air:r/ffi •/•, i. p. :;<',; and I'm*-. Z*"},il. Sin-. London, Isso, p. -jj.

ALCYONAEIA

879

thread lies coiled up in their interior; and at E, F, G, H are seen
a few of the most striking forms which they exliil.it when the thread

These thread-cells are found not merely in

or dart has started forth.

the tentacles

the external

tinozoa, but also in the

merits which lie in coils

and other parts of
Ac-
fila-
within

integument

>f

long

the chambers that surround 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 lie 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
from its base ; and when there 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 capsules, in
their unprotected state, are about
:1i'M,th of an inch in length : while
the thread or dart, in Corynactis
Allitiaiiin, when fully extended
is not less than ^th of an inch,
or thirty-seven times the length
of its capsule.1

( )f the Alcyonaria a character-
istic example is found in the Alc;/-
ot/ium digitatum of our coasts ; F
a lobed sponge-like mass, covered
with a tough skin, which is com-
monly known under the name of
•dead-man's toes,' or by the
more elegant name of ; mermaid's
this is first torn from the rock to
contracts into an unshapely mass.

fl\

\J

!(;. 666. — Filiferous capsules of Acti-
iin;<iin : A, B, Cnri/i/nft/x Allmanni',
C, E, F, (',in/f>i>li/jiri,i XmitJiii; D, G,
ri'tiHNtfoi'itts ', H, Ai'tiit/u fiin-

fingers.

When a specimen of
which it lias attached itself, it
whose surface presents

nothing

1 See Mr. (iosse's Xnt/irulisfa llnniltli'x on tin' ]")>•/•<
h, ' IVljrr ilen Buu u.s.w. dt-r Nesselkapseln

Const, and Professor
r 1'olypen nnd Quallen,' in

Abhandl. X/itur/r. I'rfcinn ~.n Hamburg, Band v. ISKI;. On the relations of stinging
cells to the nervous system, >-»•(• l)v. v. LcndcntVld, tjnurt. Jn/ini. <>f Mictpse. Set. n.s.
xxvii. p. :-!'.):-!. On the stinging cells of Co-lentHni ^fnei-ally, see X. Iwanzoff in Bull .
Sor. Mosrow, 1896, pp. '.15 and:-!-2::.

SSo

SPONGES AND ZOOPHYTES

but a series of slight depressions 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 />/////"• or filaments, fringing
each margin, and arching onwards ; and with a higher power these
pinme 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, unshapely masses, rudely notched at their
rdu'es.' (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 polype:-, which
are thus brought into free communication with each other, a cha-
racter that especially distinguishes this order. A movement of fluid
is kept up within these canals (as may be distinctly seen through

A B

PIG. 667.— Spicules of Alcijoniniii
and Gorgon id.

FKI. 668. — A, spicules of Gorc/onia yuttnta
B, spicules of Murireu elongata.

their transparent bodies) by means of cilia lining the internal surfaces
of the polypes ; but no cilia can be discerned on theii- external sur-
faces. The tissue of this spongy polypidom is strengthened through-
out, like that of sponges, with mineral spicules (always, however, cal-
careous), which are remarkable for the elegance of their forms ; these
arc disposed with great regularity around the bases of the polypes,
and even extend part of their length upwards on their bodies. In
the Gorgonin or sea-fan, whilst the central part of the polypidom is
consolidated into a lioruy 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. (5(57. <>68,
sometimes colourless, but sometimes of a beautiful crimson, yellow,

CTENOPHOEA

881

( >r purple. These spicules are best seen by black-ground illumination,
especially when viewed by the binocular microscope. They are, of

course, to be sepa-
rated from 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 al\\:iy-
possess an oi-guiiu
basis, as is proved
by the fact that
when their lime is
dissolved by dilute
acid a gelatinous-
lopking residuum is
left which preserves
the form of the
spicule.

The Ctenopkora,
or ' comb-bearers,'
are so named from
the comb-like ar-
rangement of the
rows] J of tiny

FIG. 669. — 1. Etij>/;i/:-iiiiii>< stationis, with its tentacles
extended, about twice the natural size: -in, mouth; c,
ctenophoral plate ; t, tentacular apparatus. (After Chun.)
2. Diagrammatic view of Hurmipltora plumosa, seen
from the aboral pole : c, as before ; tv, tentacular vessel ;
PPt polar plates. (After Chun.)

FIG. 070. - Bi mi-
Forskalii, show-
ing the tubular
prolongations of
the stomach.

' paddles ' by the movement of which the bodies of these animals are
propelled. A very beautiful and not uncommon representative of

»> L

882 SPONGES AND ZOOPHYTES

this order is furnished by the Cydippe^ j) ileus (compare fig. 669), very
commonlv known as the Seroe, which designation, however, properly
appertains to another animal (fig. 670) of the same grade of organisa-
tion. The body of Cydippe 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 pole to pole like meridian lines. These bands
are seen with the microscope to be formed of rowTs 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 iridescent
colours. In addition to these ' paddles ' the Cydippe 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
withlateral 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 Medusa; (fig. 664) ; and this
leads to the true stomach, which passes towards the opposite pole,
near to which it bifurcates, its branches passing towards the polar
surface on either side of a little body which has every appearance of
being a nervous ganglion, and which is surmounted externally by a
fringe-like apparatus that seems essentially to consist of sensory
tentacles.2 From the cavity of the stomach tubular prolongations
pass off 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
mouth. 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,3
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 pores situated in the hollow of the fringe, one on either side of the
inT\ons ganglion. The Author, however, has not been able to satisfy himself of the
existence of such excretory pores in the ordinary Ci/<lippe or Bcroi', although he has
repeatedly injected their whole alimentary canal ami its extensions, and has atten-
tively watcher! the currents produced by ciliary action in the interior of the bifurcat-
ing prolongations, which current^ always appear to him to return as from ciecal
extremities. He is himself inclined to believe that this arrangement, has reference
solely to tin nutrition of the nervous ganglion and tentacular apparatus, which lies
imbedded 'so to speak) in the bifurcation of (lie alimentary canal, so as to be able
to draw its supply of nutriment direct from that cavity.

5 On the anatomy of BeroS, seeTSimex, Ziiologische Studien <u/f Cu/iri. T. Uebcr

J3f-rni: lira/ nn, Leipzig, ]K7:i.

CtELENTEKA 883

body are effected by the like agency of paddles arranged in meridional
bands. These are splendidly luminous in. the dark, and the lumi-
nosity is retained eyen by fragments of their bodies, being augmented
by agitation of the water containing them. All the Ctenophora are
reproduced from eggs, and are already quite adyanced in their deve-
lopment by the time they are hatched. Long before they escape,
indeed, the}- 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 (.'.'/<? ijtpe are then
only short stumpy protuberances. By Ccelopla/na and Ctenopla/na
the Ctenophora appear to be allied to the Planarian Worms.1

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 History < >f Britinli Zi'xiplnjtes ; Professor Mihie-
Edwards's ' Recherches sur les Polypes,' and his 'Histoire des Corallaires ' fin the
SH ites I'l Biiffnii ), Paris, 1857 ; Professor Van Beneden, ' Sur les Tubulaires ' and ' Sur les
Campanulaires,' in Mem. <!/• I'Acad. Roy. tie Bn/xcUes, torn, xvii., and his ' Recherches
snr 1'Hist. Xat. des Polypes qui frequentent les Cotes de Belgique,' op. eit. torn,
xxxvi. ; Sir J. G. Daly-ell's Ban- <DK! Bcui/trkable Animals of Scotland, vol. i.;
Trembley's Mini, jumr servir <'< /'l/i^tnire d'un genre dc Polype d'eau donee; M.
Hollard's 'Monographic du Genre Actinia' in Ann. des Sci. Nat. ser. iii. torn. xv. ;
Professor Max Schultze, 'On the Male Reproductive Organs of Ca/m/panularia genicu-
1 <i tii ' in <t>nn rt. Juunt. Mirr. Sci. vol. iii. 1855, p. 59; Professor F. E. Schulze's memoirs
on Gordylophora lacustris, Leipzig, 1871, and on Syncoryne, 1873 : Professor Agassiz's
beautiful monograph 011 American Medusae, forming the third volume of his Contri-
butions to tlie Natural History of the United States of America', Mr. Hincks's
Brit tali Hiidroid Zoophytes ; Professor Allman's admirable memoirs on Cordylophora
and Mi/r/othela in the Phil. Trans, for 1853 and 1875; Professor Lacaze-Duthiers's
Hist. Xat. du Corail, Paris, 1864, and his essays on the Development of Corals, in
vols. i. and ii. of the Archives tie Zool. experimentale; Professor J. R. Greene's
Manual of the Sub-kingdom Ccelenterata, which contains a bibliography very com
plete to the date of its publication, and the articles ' Actinozoa,' ' Ctenophora,' and
' Hydrozoa ' in the supplement to the Natural History Division of the English Cyclo-
pedia. The Ctenophora are specially treated of in vol. iii. of Professor Agassiz's
Contributions to the Natural History of the United States. See also Profe^or
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. X. Moseley, ' On the Structure of
a Species of Millepora,' in Phil. Trans. 1*77, p. 117. and ' On the Structure of the
Sty last eridce,' ibid. 1878, p. 4'25 ; and on the Acalephce, Professor Haeckd's Britriige
zur Naturgeschichte der H.y drained use n ; the masterly work of the brotlu-r^ Hrrtwig,
Das Nervensystem nnd die Sinnesorgane der Mcditsen, 1878 ; and the memoir of
Professor Scha'fer, ' On the Nervous System of Anrelia anrita,' in Phil. Trans. 1878,
p. 563. Of later treatises Professor Ray Lankester's article on Hydrozoa, in the 9th
edition of the Encydopcedia Britannica; the 'Challenger' Reports of Professor
Allman on the Hydroida I Plumulariidte only), Professor Haeckel on (lie Medusa?,
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 monographs by Dr. A. Andres on Actinia; and by Dr. C. Chun on Ctenophora,
published in the Finuia n ml Flora ill's Golfi-x mn Nfupi'l, should be consulted.
Dr. Chun has made some progress with a general account of the Co-lentera in Bronn's
' Thierreich,' Bd. ii. Abth. ¥2. On fresh- water Medusa?, see Mr. R. T. Giinther
in Quart. Joitru. M/rj-. Sri. xxxvi. p. ^84.

1 See Korotueff, Zritm-Jir. f. //-/'x.s. Z<"ml. xliii. p. -J4'J, and Dr. A. "Willey Quart.
Journ. Mii-r. Sri. xxxix. p. :;-j:i.

:j r. 2

884

CHAPTER XVI

ECHINODEBMA

As we ascend the scale of animal life, we meet with such a rapid
advance in complexity of structure that it is no longer 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), Asterias (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 their structure which furnish microscopic objects of
such beauty and interest that they cannot by any means be passed
by ; while the study of their embryonic forms, 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 nearly every member of this class that the ordi-
nary microscopist finds most to interest him. This attains its highest
development in the Echinoidea, in which it forms a box-like shell or
' 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 lie very long and slender
rods. The intimate structure 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 areolct or interspaces freely communicating with each
other (figs. 671, 672). These ' areolre,' and the solid structure which
surrounds them, may bear an extremely variable proportion one to
the other; so that in two masses of equal si/.c the one or the other
may great! v predominate; and the texture may have either a re-
markable lightness and porosity, if the network be a very open one,
like that of fig. 671, or may possess a considerable degree of com-
pactness, if the solid portion lie strengthened. Generally speaking,
the different layers of this network, which are connected together
by pillars that pass from one to the other in a direction perpendicu-

STEUCTUEE OF ECHINOLDS

885

lar to their plane, are so arranged that the perforations 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 arrange-
ment, it comes to pass that the plates of which the entire ; test ' is
made up possess a very 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

. 671.— Section of shell of Echinus
showing the calcareous network of
which it is composed: <t a, portions
of a deeper layer.

FIG. 67'2. — Transverse section of cen-
tral portion of spine of Heterocen-
t rot us, showing its more open net-
work.

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
sufficiently 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 shown
in fig. 674.

The most beautiful display of this reticulated structure, however,
is shown in the conformation of the ' spines ' of Echinus, Cidfiris, &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
Echinus (the small spines of our British species, however, being
exceptional in this respect) or its immediate allies, we see it to be

386

ECHIXODEIOIA

made up of a number of concentric layers, arranged in a manner that
strongly reminds us of the concentric lings of an exogenous tree

FK;. C7i5. — 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

appear) 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.
(572) ; and this is bounded
by a row of transparent
spaces (like those at a «',
b b'. r c', &c., fig, 07.-.).
which on a cursory in-
fection might be sup-
posed to lie \ did, but are

Ki,.. (J74.— On, o\ bhe segments of the calcareous found on closer exnmina-
skeleton of an ambulacral disc of

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
ol' every layer. Their solidity becomes very obvious when we

SPINES OF ECHINOIDS

887

either examine ;i section of a spine whose substance is pervaded (as
often happens) with a colouring matter of some depth, or when we
look at a very thin section by black-ground illumination. Around
the innermost circle of these solid pillars there is another layer of
the calcareous network, which again is surrounded by another circle
of solid pillars ; and this arrangement may be repeated many times,
as shown in fig. 675, the outermost row of pillars forming the
projecting 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
serves not merely to hold down the cup upon the tubercle over which
it works, but also by its contractility to move the spine in any required
direction. The increase in size of the spine appears to be due to the
protoplasmic substance which fills up the spaces in the open network
of the spine and other 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 ;i transverse section

^m^m^-^
^ki&k^.

FIG. 675. — Portion of transverse section of spine of Heterocentrotus

ni/i in iniUtitns.

will depend in part upon 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 growth, notwithstanding
that, in the club-shaped spines, this terminal portion may be of con-
siderably larger diameter than the liasal ; and in any intermediate
part of the spine, so many layers will be traveix-d 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 or socket which works
over the tubercle of the shell.

Their combination of elegance of pattern with richness of colour-
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 Heterocentrotxs
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 lividus are pre-eminent) ; but for exquisite

888

ECHINODERMA

neatness of pattern there are no spines that can approach those
of Echinometra (fig. 673). The spines of Stomopneustes variolas-is
are also remarkable for their beauty. No succession of concentric
layers is seen in the spines of the British Echini, probably be-
cause (according to the opinion of the late Sir J. G. Daly ell) 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 irregularities
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

Pi(i. 676. —Transverse section of a spine of (innioriilni-in

which shows that the prickles on the spine are formed, not by the
crust only, but also by the inner reticular tissue. (Prom 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 clearly shown by a longitudinal section.1 The spines
of Ci<liii-ix present a marked departure from the plan of structure
exhibited in Echinus ; for not only are they destitute of concentric
layers, but the calcareous network which forms their principal
substance is incased in a solid calcareous sheath perforated with
tubules, which seems to take the place of the separate pillars of the
Kcliini. This is usually found to close in the spine at its tip also ;

Sec the Author's description of such reparations in the Moiit/ili/ Microscopical
<ln>n mil, vol. iii. 1870, p. -2 '2 5.

SPINES ; PEDICELLARI^E

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, where 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 Jeffrey Bell, who has brought forward x evidence
to show that if two spines of different sizes be taken from two
examples of Cidaris metularia, 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. — Spiue of Sputangus.

of Spatanyus (fig. 677) and the innumerable minute hair-like pro-
cesses attached to the shell of Clypeaster are composed of the like
regularly reticulated substance ; 2 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 Pedicdlarice (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.3

1 Joiini. Boy. Micnjxr. Soc. 1884, p. 845.

; A number of rare spines are described and figured by Prof. H. W. Mackintosh
in vols. xxvi. (p. 475 1 and xxviii. (pp. 241 and -259) of the Trans. Boy. Irish Academy.

0 Prof. Alex. Agassiz has shown the relations of the Pedicellaria? to the spines.
Much information regarding the various forms of these curious bodies will be found
in Professor Perrier's memoir in the Ann. He. Nat. (5), vols. xii. and xiii. ; Mr Sladen's

Spo

ECHINODERMA

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 differs 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,1 however,
have fully demonstrated that the appearances which have suggested
this comparison are to be otherwise explained, the plan of structure
of the tooth being essentially the same as that of the shell, although
greatly modified in its working out. The complete tooth has some-

' ri-/'c"

3*

- •

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 by
the relative hardness of its elementary parts; a, the clear condensed axis;
b, the body formed of plates; c, the so-called enamel; tl, 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
part 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 portion, resembling the shell in texture; at c c, the enamel.

u hat the form of that of the front tooth of a rodent, save that its
concave side is strengthened by a projecting ' keel,' so that a trans-
vei'se 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. (578, 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

in Ann. and Mag. Nat. Hist. (5),vi. p. 101 ; and M. Foettinger's paper in vol. ii.
p. 455 of the Archives de Biologic.

1 See his memoir, ' On tlie Structure and Growth of the Tooth of Echinus,' in
l'//il. Trans, fox 1861, p. :JS7. See also (Jiesln-echt, 'Der feinere Ban der Seeigel-

x.iihue,' ;l/o;y///. JiiJirlllcJt, vi. p. T'.l.

CALCAREOUS TISSUE 891

convex surface of the tooth (c, c, c) is covered with a firmer layer,
which has received the name of 'enamel.' 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 substance 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 preserved with its attached soft parts in alcohol, or (which is
preferable) by examining the base of the tooth of a fresh specimen,
the minuter the better. The lengthening of a tooth below. a> it
is worn away above, is mainly effected 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. tSalter 'secondary
plates.' Another set of appendages termed ' flabelliform 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 further 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 lacuna?,
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 <>/>]iii!i-<>i<I<;i
('sand-stars' and 'brittle stars') have the same texture as tlmse of
the shell of Echinus. And this
presents itself, too, in the spines or
prickles of their surface when
these (as in the great (,'oninster
pq'uestris or 'knotty cushion-star')
are large enough to be furnished
with a calcareous framework. An
exam] tie of this kind, furnished by
the Astrophyton, is represented in
fig. 679. The spines with which Pm 679 Calcareous 1)lat,, aml elaw
the arms of the species ofOpMothrix of AstnmJii/tdii.

(' brittle star ') are beset are often

remarkable for their beauty of conformation ; those of <>. penta-
pJvyllwm, one of the most common kinds, might serve (as Professor
E. Forbes justly remarked), in point of lightness and beauty, as

892 ECHINODERMA

models for 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 Ophiothrix clearly 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
Ophiothrix may be mounted in balsam as a transparent object with
scarcely any grinding down ; and it is then seen that the basal por-
tion of the tooth is formed upon the open reticular plan characteristic-
of the ' shell,' whilst this is so modified in the older portion 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 up of a calcareous network
closely resembling that of the shell of the Echinus. This is extremely
well seen, not only in the recent Pentacrinus asterins, a somewhat rare
animal of the West Indian seas, but also in a large proportion of
the fossil crinoids, 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 spines of Echinida),
yet their organic structure is often most perfectly preserved.1 In
the circular stems of Encrinites the texture of the calcareous net-
work is uniform, or nearly so, throughout ; but in the pentangular
Pentttcnni a certain figure or pattern is formed by variations of
texture in different parts of the transverse section.2

The minute structure of the shells, spines, and other solid parts
of the skeleton of Echinoderma can only be displayed by thin
sections made upon the general plan already described in Chapter VII.
But their peculiar texture requires 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 pi-event 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 line saw. and be rubbed on a Hat
file until it is about as thin as ordinary card, after which it should
!»• smoothed on one side by friction with water on a Water-of-Ayr

1 The calcareous skeleton even of living Keliiu.Mlernis hasa crystalline a-;.; relation,
as is very obvious in Hie more solid spines of Erliiiiotm 1 1 \> , &c. : for it is difficult, in
sawingthese across, to avoid their tendency to cleavage in the oblique plane of
calcite. And the Author is informed hy Mr. Sorhy that the calcareous deposit which
tills up the areol;,. of tln> fossilised skeleton has always the same crystalline sj
with the skeleton itself, as is shown not, merely l>y the uniformity of their cleavage,
lint hy their similar action on polarised li.^'ht.

2 See fitfs. 7-1-711 of the Author's memoir »\\ 'shell structure' in the Report oj

li Association,

PREPARING SPINES 893

.stone. It should then, after careful washing, be dried, first on white
blotting-paper, afterwards by exposure for some time to a gentle
heat, so that no water may be retained 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 saturate its substance throughout. The right
degree of hardness is that at which the balsam can be with difficulty
indented by the thumb-nail ; if it be made harder than this, it is
apt to chip off 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 network become clogged with them. If. when
rubbed down nearly to the required 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 lie observable, or some minute air-bubbles should
show themselves between the glass and the under surface, it is de-
sirable to loosen the specimen by the application of just enough heat
to melt the balsam (special care being taken to avoid the production
of fresh air-bubbles) and to turn it over so as to attach the side
last polished to the glass, taking care to remove or- to break with
the needle point any air-bubbles that there may lie in the balsam
covering the part of the glass on which it is laid. The surface now
brought upper-most is then to be very carefully ground down,
special care being taken to keep its thickness uniform through every
part (which may be even better judged of by the touch than by the
eye), arid to carry the reducing process far enough, without carrying-
it too far. Until practice shall have enabled the operator to judge
of this by passing his finger over the specimen, he must have con
tiriual recourse to the microscope during the latter stages of his
work ; and he should bear constantly in nrind that, as the specimen
will become much more translucent when mounted in balsam and
covered with glass than it is when the ground surface i> exposed, he
need not carry his reducing process so far- as to produce at once the
entire trarrslucence 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 sufficient to produce air-bubbles or to
loosen the specimen from 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 reduce it further-, care being-
taken to harden to the proper degree the balsam which lias been
newly laid on.

If a number of sections are to be prepared at once (which it is
often useful to do for the sake of economy of time, or- in order to
compare sections taken from different parts of the same spine), this
may be most readily accomplished by laying them down, when cut
off by the saw, without any preliminary preparation save the blow-
ing of the calcareous dust from their surfaces, upon a thick slip of

894 ECHINODERMA

well covered with hardened balsam ; a large proportion of its
surface may thus be occupied by the sections attached to it, the
chief precaution 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 upon a flat piece of
grit (which is very 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 sufficiently 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 slip.
special care being taken to pi-ess them all into close contact with it.
They are then to be very carefully rubbed down, until they are
nearly reduced to the required thinness ; and if, on examining them
from time to time, their thinness should be found to be uniform
throughout, the reduction of the entire 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 brush 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 usual manner. 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 hardened 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 forming 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 very
little studied, Mi-. Stewart being the only microscopist who has given
much attention to it,1 but it is well worthy of much more extended
research.

It now remains for us to notice the curious and often very beau-
tiful structures which represent, in the class Jfolothurioidea, 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 iinune.ss of 1 heir envelopes ; and ex-
cept inir in the case of the various species which have a set of cal-
careous plates, disposed around the wall of the pharynx, we do not find
among them any representation, that is apparent to the unassisted
eye. of thai skeleton which constitutes so distinctive a feature of the

1 Set- Ills iiifiuoir in ilic L/Kitrtin Triinxiii-tiniix, xxv. p. oG5 ; see :ilso Bell,
. Eoy. Microsc. Soc. lss-2. ].. -l-ll.

HOLOTHUEIAN SPICULES

895

class generally.1 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 re-
ticulated structure, which are set with greater or less closeness in

ii;. 680. — Holotlmrioidea : I, ,SY/r//o/;/rs Ki'/crstciitii ; rt, calcareous
plate of same; b, c, calcareous plates of Holothuria vagabundq ;
r7, the same of H. iuJni/i/lis; t>, the same of H. botcllus; f, of H.
in: cj, of H. ediilis.

the substance of the skin. Various forms of the plates which thus
present themselves in Holothuria are shown in fig. 68(>.2 In the
Xf/ii(i,])ta, one of the long-bodied forms of this order, which abounds
in the Mediterranean Sea, and of which two species (the A', iligitata

B

A

FK;. 6.S1.— Calcareous skeleton of Si/ntijitii : A, plate imb.Yldeil in
skin ; B, the same, with its anchor-like spine attached ; (', anchor-
like spine separated.

and ,S'. uiha'i-ens) 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 toot, in the

1 For an account of a very remarkable form see Moseley ' On the Pharynx of an
unknown Holothurian, of the family Dendrochirotse, in which the calcareous skeleton
is remarkably developed,' Qiuirt. Jaiini. Mimw. Sfi. n.s. xxiv. p. -2 .">.•;.

- For figures of the spicules of British Holothurians, see Bell, Catalogue <if tin-
Britixli Echinoderms, London, 1S9-2, pis. i.-vi.

•" 'On the spicules of Si/iittjitii, together with some general remarks on the archi-
tecture of Echinoderm spicules,' consult R. Semon, Mitth. Ziiul. Sfu/. \cupcl, vii.
p. 27'2. An excellent summary of our knowledge of the spicules of Holothurians is
given by Prof. Luclwig in his volume in Broim's TMerreich, pp. ;!5-()l.

896 ECHINODEE.MA

manner shown (in side view) at B. The anchor-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 Chiridota, the integument of which is entirely destitute of
' anchors,' but is furnished with very remarkable wheel-like plates ;
those represented in fig. 682 are found in the skin of Chiridoto
violacea, a species inhabiting the western pai'ts of the Indian Ocean.
These ' wheels ' are objects of singular beauty and delicacy, 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 inner margin of their ' tires/ There can be
scarcely any reasonable doubt that almost every member of this cl MSN
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 which they are respectively
distinguished are most useful in the determination of genera and
species.1 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 drv. are

FIG. 682. — Wheel-like plates from skin of

Cliiriilutit violacea most advantageously mounted

in the same medium, by which

their transparence is greatly increased. All the objects of this clas.x
are most beautifully displayed by the black-ground illumination, and
their solid forms are seen with increased effect under the binocular.
The black-ground illumination applied to vert/ 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 binocular than it can be with the ordinary microscope.2

Echinoderm Larvae. — We have now to notice that most remark-
able set of objects furnished to the microscopic inquirer by the larval
states of this class ; for our first knowledge of which we were in-
debted to the painstaking and widely extended investigations of
Professor J. Miiller.-'' All that our limits permit is a notice of two of
the most curious forms of these larva- by way of sample of the won-

1 No systematic account of a species of Holothurian can lie regarded as complete
which does not contain an account of the form of its spicules, when these are present.
Figures of various forms will he found in Professor Semper's Rr-iseii im Arcliipel tier
PhiltppiHcii : HdldtJtnrieii, Dr. Theel's ' CJuiUrngcr ' Reports, and the memoirs of
Professors Bell, Lnclwij;, and Selrnku.

- It may be here pointed out that the reticulated appearance is sometimes de-
ceptive, what seems to !><• xiiliil network liein;_; in many instances a liollinv 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 l>y Mr. A. Agassiz, in vol. ix. of the Memoirs oftJic
Museum t>f Comparative Zoologr.

LARVAL ECHINODERMS

897

derful phenomeiiii which hi.s researches brought to light, and to which
the attention of microscopists who have the opportunity of studying
them should be the more assiduously directed, as even the most deli-
cate of these organisms have been found capable of such perfect
preservation as to admit of being studied, when mounted as pre-
parations, even better than when alive. The larval zijoids have, by
secondary adaptations to their mode of life, acquired a type quite
different from that which characterises the adults; for instead of ,-i
radial symmetry they exhibit a Itifatfral, the two sides being pre-
cisely alike, and each having a ciliated fringe along the greater part
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 zb'oid is always situated.
The external forms of these
larva?, 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.

FIG. 683. — JBipiitiinriti <mt< i-ii/rm, or larva
of star-fish : (/, mouth; a', oesophagus; b,
intestinal tube and anal orifice; <••, furrow
in which the mouth is situated ; d <7', Iji-
lobed peduncle; 1, 2, 3, 4, 5, (>, 7, ciliated
arms.

One of the most remarkable
forms of Echinoderm larva? is
that which has received the
name of Bipinnarla (fig. 688).
from the symmetrical arrange-
ment of its natatory organs. The mouth (<(). which opens in the
middle of a transverse furrow, leads through an oesophagus, «' , 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 are fringed
with cilia; and the posterior part 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 mouth of the star-fish ; for the

:: M

898 ECHINODERMA

oesophagus of the hitter enters on what is to become 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 Bipimmriaii 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 species 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 Ophiwroidea, 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 ' mulberry-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-four
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 hoiirs 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 («). In this condition the embryo swims freely
111 rough the water, being propelled by the action of the cilia, which
clothe the four angles of the pyramid and its projecting arms, and
which arc sometimes thickly set upon two or four projecting lobes
(f) ; and it has received the designation of /'//'/>•"*. The mouth is
usually surrounded by a soH of proboscis, the angles of which are
prolonged into four slender processes (y, y,y. .'/). shorter than the four
outer legs, but furnished with a similar calcareous framework.

The first indication of the production of the young Kchinus from
it* • plutens' is given by the format. ion of a circular disc (fig. (iH4.
A, c) on one side of the central .stomach (/>): and this disc soon
presents live prominent tubercles (15). 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 tubules the rudiments of spines are seen to protrude
( I )) ; 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 suckers and spines
rapidly increases. The
calcareous framework 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-

B

584. — Embryonic development of Ecliintts: A
Pluteus larva at the time of the first appearance
of the disc : a, mouth, in the midst of the four-
pronged proboscis ; b, stomach; c, Echinoid disc:
(1, d, (1, cl, four arms of the platens-body; r, cal-
careous framework ; /', ciliated lobes : //, </, c/, ff,
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,
<7, and spines, x, projecting considerably from the
surface. (N.B.— In B, C, and D, the Pluteus is not
represented, 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 aspect of an Echinoid animal, gradually
in size, multiplies the number of its plates, cirrhi. and

3 M 2

augments

900 ECHINODEBMA

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.1

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 Mi1. Balfour s
' Comparative Embryology,' and in Professor A. Lang's ' Jahrbuch
der vergleichenden Anatomic,' which has been translated into English.2
In collecting the free-swimming larva? 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
1 >y Mr. Percy Sladen : — ; For killing and preserving echinoderm /o'oids.
I have come to prefer either osmic acid or the picro-sulphuric mix-
ture of Kleineiiberg 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 zb'oids remain in it for one or two hours; then wash
them thoroughly in 70 per cent, spirit, until 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 living zboids 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, ami
proceed as in the other case.'

One of the most interesting to the microscopist of all Echino-
derma is the Aittedon3 (more generally known as Comatula), or
• feather-star' (fig. 685), which is the commonest existing representa-
tive of the great fossil series of Crh>oidea, 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 off 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-

1 Abbreviated development, in which there is no free-swimming larva, is now
known to be more common than \\ a,s .mcc supposed: among Holothurians Citcit
nutria i-rored, among Ophiuroids OpJliacantha riri/Hint, ;iml among Kchinoids
I li'iiiinn/ff curi'i-inixiia 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, !>/<•
EcMnodermenlarven <l< >• I'lmiktoii Expedition .Kiel and Leipzig, 1898), in which
there is M. systematic revision of the Echinoderm larva1 already known.

3 See the Author's ' Researches <>n the Structure, Physiology, and Development
of \nfr</on mxarcnn,' Tart I., in I'Jiil. Trans. 18(5(5, p. (571.

ANTEDON

901

siderable activity, but it exerts this power chiefly 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 pentacriiioid
stage. The pentaerinoid larva l — first discovered by Mr. J. V.
Thompson, of Cork, in 1823, but originally supposed by him to be a
permanently attached Crinoid — forms a most beautiful object for the
lower powers of the microscope, when well preserved in fluid, and
\ie\\ed by a strong incident light (fig. 686,3) ; and a series of specimens
in different stages of development shows most curious modifications
in the form and arrangement of the various component pit.-i-c-
of its calcareous skeleton. In its earlier stage (fig. 686, l) the
body is inclosed in a
calvx composed of
two circles of plates,
namely, five basa/*.
forming a sort of
pyramid whose apex
points downwards, and
is attached to the
highest joint of the
stem ; and five orals
superposed 011 these,
forming when closed
a like pyramid whose
apex points upwards,
but usually separating
in give pass; i go to the
tentacles, of which a
circlet surrounds the
mouth. In this con-
dition there is no
rudiment of arms. In PIG.
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 basal* supported on the
top of the stem ; r1, the circle o^first radials, now interposed between
the basals and the orals, and alternating with both ; between two
of these is interposed the single aval plate a ; whilst they support
the second and the third raJials (r~, r;J'), 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
re>ted 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

1 The pentaerinoid larva; of Aiitedon have been found abundantly (attached to
seaweeds and zoophytes) at. Millport, on the Clyde, and in Lamlash Bay, Arran : in
Kirkwall Bay, Orkney; in Lough Straiigforcl, near Belfast, and in the liay of Cork ;
and at Ilfraconibe and in Salrombe Bav, Devon.

GiSr>. — Aiitcdu)! (Comatula), or
seen from its aboral side.

9O2

ECHINODEKMA

beginning to enlarge, to form the centro-dorsal plate (-2, c rf), from
which are beginning to spring the dorsal cirrhi (cir) that serve to

Fm. C>«8. — Pentacrinoid larva of Aittcdoit. 1. Skeleton of early peiitacrinoid,
under black-ground illumination, showing its component plates : l>, It,
basuls, articulated below to the highest point of the stem; rl, •/•', first
radials, between two of which is seen the single anal plate, a ; »•-, second
radials ; r"', third radials, giving off the bifurcating arms at their summit ;
i>, <>, 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 tig. '•'<
in order to bring the mouth and its surrounding parts into view : b, b,
luisals; /•', /•'-', /-"', first, second, and third radials; n, anal, now carried
upwards by the projection of the vent, v ; o, u, orals; cii; d<.>r>:il cirrlii,
dr\ dojied from the highest joint of the stem.

ANTEDON 903

anchor the animal when it drops from the stem ; this supports the
nasals, on which rest the first radials (>'}) ', whilst the anal plate is
now lifted nearly to the level of the second radials (r'2) by the
development of the anal funnel or vent to which it is attached. The
oral plates are not at first apparent, 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, o, 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 basals are at first concealed by the
great enlargement of the centre-dorsal (which finally extends so far
as to conceal the first radials 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 develop-
ment by a free-swimming ' larval zb'oid ' or pseudembryo, which was
first observed by Busch, and has been since carefully studied by
Professors Wyville Thomson } and Goette.2 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 Z/aminaria), 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.3

1 ' On the Development of Antedon rosacens' in Phil. Trans, for 1805, p. 518.

2 Archiv f. tnikrosk. 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 linijal Soc/cti/ 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 Anatomic der
Crinoideen ' (Leipzig, 1877), forming part of his Morphologische Stuilien nit Ki-liiim-
(Jennen, is the most important. Those who wish to carry further their study of the
Crinoidea should consult the two monographs by Dr. P. Herbert Carpenter in the
' Challenger' Reports.

904

CHAPTER XVII

POLYZOA AND TT'XICATA

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 were
the two groups to be now otherwise dealt with, and as. finally, there
is no real inconvenience or impropriety in discussing them in one
chapter, it is proposed to continue, with this word of warning, the
original arrangement of the Author. Some members of both these

o o

Croups are found on almost every coast, and are most interesting
objects for anatomical examination, as well as for observation in the
living state.1

Polyzoa. — The group which is known under this name to many
British naturalists (corresponding with that which by Continental
zoologists is designated 7>V//o~w) was formerly ranked 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
/.ix/unci'la (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
laver 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 oritice. 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
understood from the examination of a characteristic example, such as
the LIII/H iirnlii /-I'/ir/tx. which is shown in the state of expansion at A.
fig. uX7. and in the state of contraction at I \ and < '. The mouth is

i KHV .1 L I general account see IM-. Manner in vol. ii. of the ('amln-iili /»• \nlnral

History, l*;ir,.

POLYZOA

905

an

surrounded by a circle of tubular tentacles, which are clothed bv

vibratile cilia ; these tentacles, in the species we are considering,

vary from ten to twelve in number, but in some other instances they

are more numerous. By

the ciliary investment

of the tentacles the

Poly /COM. are at once dis-

tinguishable from those

hydroid polypes to

which they bear a

superficial resemblance,

with which they

at one time con-
founded ; and accord-
ingly, while still ranked
among zoophytes, they
were characterised as
citiobrachiate. The ten-
taciila are seated upon
an annular disc, which
is termed the lopJm-
/>//<>,•''. and which forms
the roof of the visceral
or perigastric cavity ;
and this cavity extends
itself into the interior of
the tentacula,1 through
perforations in the lo-
phophore, as is shown at
J), tig. 687, representing
a portion of the ten-
tacular circle on a
larger scale, a n, being
the tentacula. b b their
internal canals, c the
muscles of the tentacula.
il the lophophore, and c
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, b,
which is separated from
the oesophagus, d, by a
valve at c ; and this oeso-
phagus opens into the
stomach, «?, which occu-
pies a considerable part of the visceral cavity. (In the Bowerbankia

' Tin's communication between the tentacular anil visrcnil cavities is denied by
Dr. Vigelius, who has recently made a careful search for it.

FIG. 687. — Structure of Layuncula repvnx(\-M\

den). A, pi >lyi>ide ex [landed ; B, polypide retracted ;
C, another view of the same, with the visceral
apparatus in outline, that the manner in which it
is doubled oil itself, with the tentacular crown and
muscular system, may lie more distinctly seen :
a a, tentacula; b, pharynx; c, pharyngval valve:
iJ, oesophagus ; <-, stomach; /', it> pyloric orifice;
(/, cilia on its inner surface ; /;, biliary follicles lody < •< 1
in its wall ; /, intestine ; k, particles of excremen-
titious matter ; /, anal orifice; ///, testis; 11, ovary;
o, ova lying loose in the perivisceral cavity; j), out-
let for their discharge ; q, spermatozoa in the peri-
visceral cavity ; /•, s, t, it, r, n-, ,r, muscles. D, por-
tion of the lophophore more enlarged: a (^tenta-
cula; It b, their internal canals; c, their muscli--, :
J, lophophore ;

its retractor muscles.

906

POLY/OA AND TUN1CATA

and some other Polyzoa a muscular stomach or gizzard for the tri-
turation of the food intervenes between the oesophagus and the
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 rudimentary digestive gland. This, however, is
more obvious in some other members of the group. The stomach is
lined, 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, f. 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 circulating
apparatus here exists ; but the liquid which fills the cavity that sur-
rounds the viscera 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-
FK;. 688. — Cells of P<i!//,-:oti -. A, Mttxt/</<i/>/i<ini lit/ml- .,]jv flowm0' over them
inanni ; B, Cnbrilina fiqularis', C, Umbonula • .„.

verrucosa. Jne Production of

iji-nimce 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 latter cast- there 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 /.iioid, but it constitutes the chamber
surrounding the digestive viscera, which organs have their origin
in a. thickening of the lining membrane that projects from one side
of the cavity into its interior, and gradually shape> itself into the
alimentary canal with its tentacular appendages. Of the produc-
lion of gemma- from the polypides themselves the best examples are
furnished by the Flnstro' and their allies. From a single cell of tin-
Klustrse five such buds may he sent oil', which develop themselves

POLYZOA 907

into new polypides around it ; and these in their turn produce 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 their vitality,
whilst the edges are in a state of active growth.1 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 mature 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, are 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 c?ecal 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/>, beneath the tentacular circle.

These creatures possess a considerable number of muscles, by
which their bodies may be projected from their sheaths, or drawn
within them ; of these muscles, r, s, t, u, i>, w, x, the direction and
points of attachment sufficiently 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 projection and expansion of the animal, on the con-
trary, appear 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 separate movements seem to be produced. At
the base of the tentacular circle, just above the anal orifice, is a small
body (seen at 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 are performed by so many of them simulta-
neously as to indicate the existence of some connecting agency : and
such connecting agency, it is affirmed by Dr. Fritz Miiller,2 is fur-
nished by what he terms a ' colonial nervous system.' In a Set-'m-
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. Join >i.
Microsc. Sri. xxiii. p. ">!(>. Embryonic fission has been observed by Harmer in f'ri.tin
and Lichenopora.

- See his memoir in Wici/nmnn's Archil', LstiO, p. 311, translated in Quart. Jonr/i.
nf Mirrnnr. Sci. n.s. vol. i. 1861, p. 300 ; Piev. T. Hincks's 'Note on the Movements of
the Vibracula in Caberea boryi, and on the supposed common Nervous System in
the Polyzoa,' Quart. Jtnirii. Microsc. Sri. xviii. p. 1.

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 histologies! experience maintaining that what he regards as
nerve-fibres are only connective tissue.

Of all the Polyzoa of our own coasts the Membraniporidce, or
'sea -mats' (Fltt-stra, Membranipora}, are the most common; these
present flat 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 <>ii both sides, and it was calculated by Dr. C4rant 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 110 fewer than 18,000 polypides. The want
of transparence in the cell-wall, however, and the infrequency with
which the animal projects its body far beyond the mouth of the cell,
render the species of this genus less favourable subjects for micro-
scopic examination than are those of the Sowerbankia, a polyzooii
with a trailing stem and separated cells like those of Laguncula, which
is very commonly found clustering around the base of masses of
Flustr;v>. 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. Farre, who discovered it in 1837, and subjected it to a far more
minute examination than any polyzoon had previously received ; 1
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 sufficient 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 withdrawn 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 lilin. 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 crown 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
\\ith all its polypides expanded.2 Several of the fresh-water Polyzoa
are peculiarly interesting subjects for microscopic examination, alike

1 Sec liis memoir 'On the .Minute Structure of some of the Higher Forms of
Polypi,' in the Phil. Trans, for is:;?, p. ::*?.

- Mr. .1. I, (DMAS lias detected c;llc;ireoll-. -.plcllles ill Ale 1/0111(11)11)1 f/C Idtl HOSII III,

Mud t'mds tha.t theviu-e more .1 1 mi i. l;i nt in older than in younger colonies, SeeProceed-

hnjx of tin' I.irt'r/Kinl di-n/iii/irii/ Society, v. p. '241.

GROUPS OF POLYZOA 909

011 account of the remarkable distinctness with which the various
parts of their organisation may be seen and the very beautiful man-
ner in which their ciliated tentacula are arranged upon a deeply
crescentic or horseshoe-shaped lo/tjiojiliurr. By this peculiarity the
fresh-water Polyzoa are distinguished from the marine : and they,
with the marine Rhcibdopleura, may be further distinguished by the
possession of an epistome, or moveable process above the mouth,
whence Professor Allman calls them the Phylactolcemata, as com-
pared with the others, which are Gymnolcemata, or have no epistome.
The cells of the Phylactolcemata are 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 l'ri*t«t<-llf( the poly/oary is un-
attached, so as to be capable of moving freely through the water.1

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. Cheilostomata, 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 011 our own coast, not-
withstanding their wide differences in form and habit. Thus the
polyzoaries of some (as Flustra) are horny and flexible, whilst those
of others (as Eschar a and Retepora) are so penetrated with calcareous
matter as to be quite rigid ; some grow as independent plant-like
structures (as Euaula and Gemellaria), whilst others, having a like
arborescent form, creep over the surfaces of rocks or stones (as
ffippothoa) ; and others, again, have their cells in close apposition,
and form crusts which possess no definite figure (a> i- the case with
Lepralin and Mpmbranipora). II. The second order, Cyclostomata,
consists of those Polyzoa which have the mouth at the termination of
tubular calcareous cells, without any movable appendage or lip (fig.
689). This includes a comparatively small number of genera, of which
Crisia and Tnbitlipora contain the largest proportion of the specie-
that occur on our own coasts. III. The distinguishing character of
the third order, Gtenostomata. is derived from the presence 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 F«rrt'lln and Bowerbankia),
sometimes fleshy and coalescent (as in Alcyonidium). IV. In the
Entoprocta^ which are represented by Lo.rosonia and /W/(W/m«,
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 inverted bell, like

1 See Professor Allman'sbeautifulMo«o;r/rap/; of the British Fresh-water Polyzoa,
published by the Bay Society, 1857; and J. Jullien, ' Monographic des Bryozoaares
d'eau douce,' Bull. Soc. Zool. tie France, x. p. 91.

910

POLY/OA AND TUNICATA

that of Vorticella (fig. 593). As the Polyzoti altogether resemble
hydroid zoophytes in their 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.1

A large proportion of the Cheilostomata 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

(tig. 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 ' immersed ; ' but
in the genera Buyitla 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
Fro.689.-A, portion otBicellaria ciUata, en- f bi d ^ th ;

larged ; B, one of the ' bird s head processes of

Buffiiln ni-ifiilaria, more highly magnified, and able, like its lower jaw ; the

seen in the act of grasping another. 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 ' Recherelies sur les Bryozoaires de la cote
d'Ostende ' in Mem. <!<• I'. [cm/. Hoi/. <lc BrnxcUfx, torn, xvii.; Mr. G. Busk's
('nt«l<>(/iir <>f tin- Mariiii J'<>l//.-;oi< ui tin- < 'oil ret ion of the British Museum; Mr.
Hincks's liritiali Marine Polyzud, 1880; and Nilsclie, ' Beitriige zur Kenntniss dor
Bryozoen,' in Zeitschrift f. //v.s.s. '/.iiol. 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 tin- Structure and Development of Loxosoma ' and ' On
the Life-history of PedicelUna,' in vols. xxv. and xxvi. of the Qitaii. Journ. of
Mieroxe. ,SV/.; .1. Harrois, ' Ueelierehes sur I'Kniliryologie des Bryo/.oaires,' Lille, 1877,
ii.nd oilier nii-nioirs ; W. J. Vigclius, ' Morphologische rntcrsuciumgen fiber Flustra
\lciiihnvnaceo-truncata,1 Biolog. Ci-nl rnlhlntl . ui. p. 70">, and Bijdragen tut <!<•
l)irr];n ml<\ \i. For a general a,i-c(iunl sec L'rolVssor K;i\ Lankester's article ' Polyzoa,'
ill the Dtli edition of the Encyclopedia l!n/<i 11 n ii'n , and Dr. Mariner's work already
referred (,,.

AVICULARIA AND VIBRACULA 911

tactile organ, being brought forwards when the month 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 worms 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 poi-tion of the polyzoary of
Hinj nil 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 Pedicellariag of
Echini, to remove extraneous particles that may be in contact with
the surface of the polyzoary. The latter would seem to be the func-
tion of the i-l/irnrula, which are long bristle-shaped organs (fig. 688.
A), each one springing at its base out of a sort of cup that contains
muscles by which it is kept in almost constant motion, sweeping
slmvly and carefully over the surface of the polyzoary, and removing
what might be injurious to the delicate inhabitants of the cells when
their tentacles are protruded.1

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 Chonlnfa. As the noto-
chord is always restricted to the hinder part of the body, the
Tunicata may be called Urochordata. In all (except, perhaps,
Appendicularia) 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 stimctures 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 Salpi<l<i and other floating species
which are chiefly found in seas warmer than those that surround our
coast, and the curious Appendiculcvria to be presently noticed, they
are rooted to one spot during all but the earliest period of their lives.
The larger forms of the Ascvlinn group, which constitutes the bulk
of the class, are always solitary : not propagating by gemmation,
except in the case of the Clavelinid;v. Although of special importance

1 See Mr. G. Busk's ' Remarks on the Structure and Function of the Avicuiarian
and Vibracular Organs of Polyzoa' in Traits. Hit- rose. Soc. ser. ii. vol. ii. 1854,
1'. 'Jt>; and Mr. A. W. Waters, 'On the use of the Avicuiarian Mandible in the Deter-
mination of the Cheilostomatous Bryozoa,' Join-it, lioij. Mir rose. Sor. CJ), v. p. 774.

912 POLYZOA AND TUNIC ATA

to the comparative anatomist and the zoologist, this group does not
afford much to interest the ordinary microscopist, except in the pecu-
liar actions of its respiratory and circulatory apparatus. In common
with the composite forms of the group, 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
orifice, and a large proportion of it is caused to pass through the
fissures, by the agency of the cilia with which they are fringed, into
a surrounding chamber, whence it is expelled through the atriopore,
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 Poly/oa
affords a more beautiful spectacle. It is peculiarly remarkable in one
species that occurs on our own coasts, the Gorella parallelogramma*
in which the wall of the branchial sac is divided into a number of
areolfe, 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 direction
of the circulation. The heart, which lies at the bottom of the
branchial sac, has its one end connected with the principal trunk
leading to the body, 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 *v>
temic trunk to the body at large ; 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 from
the body generally, and being propelled to the branchial sac. After
this reversed course has continued for some time another pau>e
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 ; but in the solitary Gorella paralldogramnwt (a species
very common in Lamlash Bay, Arraii), 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 compound Ascidian* 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 from
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 we have a characteristic
example in Amcvroucium prolif&rum, of which the form of the com

1 Sec Alder in Ann. of \nt. Hint. SIT. iii. vol. xi. INIJ:!, i>. ir>7: and Hiuimrk in
• Iniini. Linn. .S'nr. ix. p. 333.

TUNICATA

913

posite mass and the anatomy of a .single individual are displayed in
fig. (590. Its clusters appear almost completely inanimate, exhibiting
no very obvious movements when
irritated ; but if they be placed
when fresh in sea-water a slight
pouting of the orifices will soon be
perceptible, and a constant and
energetic series of currents will be
found td filter by one set and to lie
ejected by the other, indicating that
all the machinery of active life is
going on within these apathetic
1 todies. In the family P<>1 i/cli ni<l/i
to which this genus belongs the
body is elongated, and may be
divided into three 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 are lodged. At the summit
<>f the thorax is seen the oral orifice.
c. which leads to the branchial sac e;
this is perforated by an immense
number of slits, which allow part of
(lie water to pass into the space
between the branchial sac and the
muscular mantle. At k is seen the

vV

ft

Fit;. 690.— Compound mass of Amarouciumproliferum with the anatomy <>f a
single zb'oid: A, thorax; B, abdomen; C, post-abdomen; r, oral orifio- :
»', branchial sac ; /, thoracic blood-vessel ; /, atriopoiv ; i', projection over-
hanging it ; ./', ncr\-(uis ganglion; /r, (esophagus; Z, stomach surrounded by
dige-tivc tnliuli ; in. intestine ; it, anus opening into the cloaca formed by
the mantle; o, heart; o', pericardium ; y<,ovarinm; ;/, egg ready to escape ;
7, testis ; /•, spermatic canal ; r', termination of this canal in the cloaca.

(esophagus, which is continuous with the lower part of the pha-
ryngeal cavity ; this leads to the stomach, /, which is surrounded
by glandular follicles ; and from this passes off the intestine, rn. which
terminates at n in the vent A current of water is continually

3 x

914 POLYZOA AND TUNIC ATA

drawn in through the mouth by the action of the cilia of the bran-
chial sac and of the alimentary canal ; a part of this current 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 pharyiigeal 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 forth
together by the atrial orifice, i. The long post-al tdomen 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, r . At the very bottom of the post-abdomen we find the
heart, o, inclosed in its pericardium, o'. In the group we are now
considering 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 cellulose, which generally ranks as a vegetable product.
The mode in which new individuals are developed in this mass is by
the extension of stolons or creeping stems from the bases of those
previously existing ; and from each of these stolons several buds may
be put forth, every 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 occupies the centre. This shortening is still
more remarkable, however, in the family of Botrylliaiis, whose
beautiful stellate gelatinous incrustations are extremely common upon
seaweeds and submerged rocks (fig. (591). The anatomy of these
animals is very similar to that of the A'nidroutium already described ;
with this exception, that the body exhibits no distinction of cavities.
all the organs being brought together in one, which must be eon
sidered as thoracic. In this respect there is an evident a pproximation
towards the solitary species.1

This approximation is still closer, however, in the ' social ' Asci-
diaiis, or Clavellinidce, in which the general plan of structure is
nearly the same, but the zooids are simply connected by their stolons
instead of being included in a common investment; so that their
relation to each other is very nearly the same as that of the poly-

1 For more special information respecting the runi/xi/i ml Axr/il/tniN see espe-
cially the admirable monograph of Professor Milne-Edwards on that, group ; Mr. Lister's
memoir, ' On the Structure and Fund inns of Tnlmlar and Cellular Polypi, and of

.Ysridiii',' ill the Phil. Trillin. Is:;] ; and the article ' Tunicata,' by L'rnfessorT. Rupert
Jones, in the Cyclo/tii'if/d «{ Amituni// <in<! I'lu/niolwjij. More recent, authorities
.ire cited on p. 91*.

s

TUNICATA

915

pules of Laguncula, the chief difference being that a regular cir-
culation takes place through the stolon in the one case, such as has
110 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 Perophora, first discovered by Mr. Lister,
which occurs not unfrequently on the south coast of England and in
the Irish 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 transparence of
its tunics, not only enable the movements of fluid within the body to
be distinctly discerned, but also allow the action of the cilia that
border the slits of the respiratory sac to be clearly made out. This
sac is perforated with four rows of narrow oval openings, through
which a portion of the water that enters its oral orifice escapes

FIG. 6V) 1. — BotrylltiK riohtreim; A, cluster on the surface of a Fucus ;
B, portion of the same

into the space between the sac and the mantle, and is thus dis-
charged immediately by the atrial funnel. Whatever little j articles,
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 lod»e
somewhere on the sides of the sac, and then travel horizontally until
they arrive at that part of it down which the current 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

3 N 2

916

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 pax-^a^o
between the oval slits, whilst others are first distributed to the
stomach and intestine, and to the soft surface of the mantle. All

these reunite 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 i>
ligature ;

interrupted by a
and the stream which re-
turns from the branchial
sac and the viscera is then
poured into the posterior
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
parent, presents some phe-
nomena of much interest
to the microscopist which
alone can be noticed here.
After the ordinary repeated
of the yolk,
mulberry ma»
is produced, a sort of ring-
is seen encircling its central

si.

'.t'J. Diagrammatic longitudinal section of

\wiilin showing the heart, the blood-vessels,
the branchial sac, the alimentary canal, &c.
from the left side : br.si., branchial siphon :
ut.*/.. atrial siphon; t., test: ///., mantle;
iir.K.. branchial sac: /i.hr., peri branchial
cavity; r/., cloaca; ll.g , nerve ganglion;
fit., tentacle; f/L, neural gland; «?.«., ceso-
phageal aperture ; nf., stomach; /..intestine:
/•., rectum; a., anus; o.v., genital organs ;
g.d., genital ducts; /(., heart: I-.NJI., cardio-
jplanchnic vessel; r.t., vessel to the test;
/.//., terminal knob on vessel in test,; v.t'.,
vessel from the. test; r..\/., vessel to tin1
stomach itc. ; V.HI., vessel to the mantle;
r. in'., vessel from the mantle: d.V., dorsal
vessi.'l : //•., transverse vessel n| branchial
sac- : /./'., tine longitudinal vessel of branchial
sac; .y/., stigmata, of branchial sac; V.V.,
\enii-al vessel; ln-.r., branchio-cardiac

vessel; v//. /</•., spia.ncl lii-anchial vessel.

(After Prof, llerdman. I

segmentation

whereby a '

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

tin- cloaca, or soon after it has escaped from the vent, its envelope
bursts. ;nid the larva ex-apes, and in this condition it presents very

TUNICATA 9 1 7

much the appearance of a, tadpole, the tail being straightened
out. and propelling the body freely through the water by its lateral
strokes. The centre of the body is occupied by a mass of liquid yolk,
and this is continued into the interior of three prolongations which
extend themselves from the opposite extremity, each terminating in a
sort of sucker. 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 important 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 entire 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 very 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
lieart is first seen on the third day, and the formation of the branchial
and anal orifices takes place on the fourth, after which the ciliary
currents are 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 except ion of some species "I
Molgula.1

This larval condition is represented in a very curious adult free
swimming form, termed Appendicularia, which is frequently to In-
taken with the tow-net on our 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 t he body dif-
fers grea I ly from that of the Ascidians. its plan being much simpler ;
in particular, the pharyngeal sac is entirely destitute of ciliated
branchial fissures opening into a surrounding cavity ; but two canals,

1 The study of the development of AuriiliniiK derived a new interest and im-
portance from the discovery, made by Kowalevsky in 1N<;7, that their free-swimming
liirvic present a most striking parallelism to vertebrate embryos, in exhibiting the
beginnings of a spinal marrow and a notochord ; thus bridging over the gulf that was
supposed to separate them from Invertebrata, and (when taken in connection with
the curious Ascidian affinities of Ain/ihioxus, 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 dcr
einfachen Ascidien ' in Mi' in. St. Pi'tersb. Acdd. Sri. torn. x. 1867, and the abstract
of it in Quart. Jot/ni. Mir rose. Sri. x. ii.s. 1870, p. 5!) ; also Professor Haeckel's History
of Creation, ii. pp. 1;V2, '200. Further information will be found in chap. ii. of vol.
ii. of the late Professor Balfour's < '/uir/ni i-nt/vr Embryology, and an application of the
facts of development to the philosophy of the subject in Professor Ray Lankester's
liri/riii'rtitioit (London, 1880).

918 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 are 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 canal >.
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 or
Haus (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 lias
met with this appendage ; but it has been since seen by Allmaii.
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

1 For details in respect to the structure of Appendicularia, see Huxley in Phil.
Trans, for 1851, and in Quart. Joiirn. of Microsc. Sci. vol. iv. 1856, p. 181 ; also
Allman in the same journal, vol. vii. 1859, p. 86; Gegenbaur in Siebohluiirl Kolliker's
Zfitnchrift,~B&. vi. 1855, p. 406; Leuckart 's Zoologische Untersuchungen, Heft ii.
1S54 ; Fol's ' Etudes sur les Appendiculaires ' in Arcliiv. Zool. exper. torn. i. 1872,
p. 57 ; the three memoirs by H. Lohmaim published in 1896. For the Tunicata
generally, see Professor T. Rupert Jones in vol. iv. of the Cyclop, of Anatomy
mill Physiology, Professor Herdman's article, 'Tunicata,' in the 9th edition
of the Encyclopcedia 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 Jon rnal of flu-
l./nnean Society, vol. ix. p. 309. Great additions to our knowledge have been
made by Professor Herdmaii, whose reports on the forms collected by H.M.S.
C/inUnii/er should be consulted, and by Professors Van Beneden and Julin (see espe-
cially their memoirs in the Archives de Biolur/ir). See also Roule, ' Reeherrhes sur les
Ascidies simples des cotes de Provence,' Ann. Mn^i-nni Mctrxi-iUrx. ii. ; Seeliger,
'Die Entwickelungsgeschichte der Socialen Ascidien,' Jenaische Zeitschr. xviii.
p. .V2<s : Salensky, ' Neue Untersuchungen iiber die embryonale Entwickelung dei-
Salpen.' Mittli. /.;><>!. Slut. \'rn/ifl, iv. pp. !)0, 327; and I'lianin, ' Die Arten des
(lattmig I iiJin/n in im (loli'e von Neapel,' in the Fauna nml l-'lnrn <l<s Golfesvon
\i'n/ir/, \. The above titles by n<> means exhaust the list of recent important memoirs
on 1'n niriitii. but the researches «\' Caullery, Metcalf, Pixon, and Seeli^iTare beyond
the scope of this work. The last-named has commenced a systematic account of the
L;ron]i in Uronii's Thirrri'irh.

919

CHAPTER XVIII

MOLLUSC A AND BBACHTOPODA

THE various forms of 'shell-fish,' with their 'naked' or shell-less
allies, furnish a great abundance of objects of interest to the micro-
M-opist, 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 LAMELLJBRANCHIATA) and BRACHIO-
PODA, in both of which classes the shells are ' bivalve,' while the animals
differ from each other essentially in general plan of structure ; (2)
the structure of the tongue 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 Mollusca. — 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 presents certain very remarkable variations in some of the
groups of which the molluscous series i.s composed. We shall first
describe that which may be regarded as the characteristic structure
of the ordinary bivalves, taking as a type the group of Jfftr</arltacetK,
which includes the Melectgrina or '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 int<'rn«l, which lias a pearly
or • nacreous" aspect, and is commonly of a lighter hue.

The structure of the outer layer may be conveniently studied in
the shell of riiiixt., in which it commonly projects beyond the inner,
and there often forms lamina? sufficiently thin and transparent to
exhibit its general characters without any artificial reduction. If a
small portion of such a lamina be examined with a low magnifying
power by transmitted 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
<>f an assemblage of segments of basaltic columns (fig. 696). This
outer layer is thus seen to be composed of a vast number of prisms,
having a tolerably uniform 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 are arranged perpendicularly (or
nearly so) to the surface of the lamina of the shell ; so that its thick-
ness is formed 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 until it possesses a high degree of transparence, the

prisms being then seen (fig.
693) to be themselves com-
posed of a very homogeneous
substance, but to lie sepa-
rated 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 just
as perfectly as did the
original shell (fig. 694), its

hexagonal divisions bearing a strong resemblance to tlie 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 are frequently
seen to be marked by delicate transverse stria? (fig. 696) closely re-
sembling those observable on the prisms of the enamel of teeth, to
which this kind of shell-structure may be considered as bearing a
very close resemblance, 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-
dinallv. ;md will be seen to
lie more or less regularly
marked by the transverse
stria? just alluded to. It
sometimes happens in re-
cent but still more com-
monly iu fossil shells, that
1 lie decay of the animal
membrane leave.x the con

lained 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 lie marked by the like striations.
which, when a sufficiently high magnifying po\\er is used, are seen
to be minute grooves, apparently resulting from a thickening of the
intermediate wall in those situations. These appearances >eem l>est
accounted for by supposing that each is lengthened by successive

FK;. 694. — Membranous basis of the same.

STKUCTUEE OF SHELLS

921

additions at its base, the lines of junction of which correspond with
the transverse striation ; and this view corresponds well with the
fact that the shell-membrane not unfrequently shows a tendency to
split into thin lamina1 along the lines of striation. whilst we occa-
sionally meet with an excessively thin natural lamina lying between
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 entire
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

Df . . . L . FIG. 695.— Section of the shell of Pinna

in / tuna mgmna) this colour in the direction of its prisms.

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 carbonate of lime in the
shells of Pinna and its allies has been long familiar to coiicholo-
gists, and regarded by them as the result of crystallisation. When

FIG. 69(5. — Oblique section of prismatic shell-substance.

it was first more minutely investigated by Mr. Bowerbank1 and the
Author,2 and was shown to be connected with a similar arrangement
in the membranous residuum left after the de-calcification of the shell -
substance by acid, microscopists generally3 agreed to regard it as a
' calcified epidermis,' the long prismatic cells being supposed to be
formed by the coalescence of the epidermic cells in piles, and giving

' On the Structure of the Shells of Molluscous and Conchiferous Animals,' in
Trans. Mir >•<>.•«•. i'oc. ser. i. vol. i. 1344, p. 123.

2 ' On the Microscopic Structure of Shells ' in Reports of British Association for
1*41 and 1847.

•• See Mr. Quekett's Histologicul Catalogue of the College of Surgeons'
iind his Lrrtiin-s on Hivtoliit///, 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 interpretation, the Author being now disposed to agree
with Huxley l 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,' 2 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 3) 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 Meleagnna or ' pearl-oyster '
is carefully examined, it becomes evident that the lines are produced
by the crop ping out of laminae of shell situated more or less obliquely
to the plane of the surface. The greater the dip of these laminae, the
closer will their 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 present 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
wearing away of the edges of the animal laminae, whilst those of the
hard calcareous laminae 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 110 more than -^-Jg-^th of an inch apart. But
\\licn the nacre is treated with dilute acid, so as to dissolve its cal

1 See his article, ' Tegumentary Organs,' in < 'i/rlo/wiHd of Anato/i/i/ mid
Physiology, supplementary volume, pp. 489-492.

- Theperiaxfrni-iim is the yellowish-brown membrane covering the surface of
many shells, which is often (but erroneously) termed their epidermis.

•''Phil. TnuiK. 1<S14, p. 397.— The late Mr. Barton (of the Mint) sure ..... led in
producing an avlilicial irideM-rnre on metallic buttons by drawing closely approxi-
mating lines with a diamond point upon the surface of the steel die by which they
were st click.

STEUCTUEE OF SHELLS

923

careous portion, no such repetition of membranous layers is to be
found ; on the contrary, if the piece of nacre be the product 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 sui-fa.ce of
the shell of Mi'b'mji'ina, 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 mussels, Unio and A no-
don ; but these are usually less remarkable for their pearly lustre ;
and, when formed at the edge of the valves, they may be partly or
even entirely made up of the prismatic substance of the external
layer, and may be consequently altogether1 destitute of the pearly
character.

In all the genera of the Mo/rgo/ritacece we find the external layer

• - *• ^a^^i'^LdaJh'-^V -W A / "^rff1 "" J -IF

Pi(i. C!)7. — Section of nacreous lining of shell of
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 diionidce (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 prismatic
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 Ostreacece (or oyster tribe), also, the greater
part of the thickness of the shell is composed of a 'sub-nacreous"
substance, representing the inner la\er of the shells of Margaritacese,
its successively formed lamina?, 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 prismatic substance distinguished by its

924

MOLLUSCA AND BBACHIOPODA

brownish-yellow colour. In these and some other cases a distinct
membranous residuum is left after the decalcificatioii of the prismatic
layer by dilute acid ; and this is most tenacious and substantial
where (as in the Jfargaritacete) there is 110 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 undergoing abrasion; thus it is found pretty generally in
Ghama, Triyonia, and >Solen, and occasionally in Anomia and /Vr/r//.
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 usually ex-
tremely hard. This hardness ap-
pears to depend upon the mineral

arrangement of the carbonate of

PIG. 098.— Section of hinge-tooth of
M//H arenaria.

lime ; for whilst in the /
and ordinary inict-miis layer this
has the crystalline condition of
rti/'rlte, it can be shown in the hard
shell of PJiolxs to have tlie arrange
meiit of arragonite, the difference
between the two being made evi-
dent by polarised light. A very
curious appearance is presented 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 iravellite. Approaches to
this curious arrangement are seen in many other shells.

There are several bivalve shells which almost entirely consist of
what may be termed a sub-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 i.s the case, for example, with most of the
/'rrflni/fti- (or scallop tribe), also with some of the Jft/til<t<-<><f (or
mussel tribe), and with the common OyxAr. In the internal layer
of by far the greater number of bivalve shells, however, there is not
llie least approach to the nacreous aspect; nor is there anything
that can be described as definite structure: and the residuum
left after its decalcilication is usually a structurele— -basement
membrane.'

The ordinary account of the mode of growth of the shells of
bivalve Mollusca — that they are progressively enlarged by the depo-
sition of new laminae, each of \\ liirh is in contact with the internal
surface of the preceding, and extends beyond it doe> not express
the whole truth ; for it takes no account of the fact that nn»t shells
are composed of two layers of very different texture, and does not

SHELLS OF LAMELLIBEANCHS

925

specify whether both these layers are thus formed by the entire
surface of the 'mantle' whenever the shell has to be extended, or
whether only one is produced. An examination of tig. 699 will
clearly show the mode in which the operation is effected. This figure
represents a section of one of the valves of Unio occiclens, 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, traversing 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 lamina?, separated by the lines <i «' . ?> b', c c',
Arc. 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 newT
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 : n, b, c, successive formations of the outer
prismatic layer ; a', b', c', the same of the inner nacreous layer.

The number of such lamina}, therefore, in the oldest part of the shell
indicates the number of enlargements which it has undergone. The
outer or prismatic layer 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 overlapping another
except at the lines of junction of two distinct formations. When the
shell has attained its full dimensions, ho\\e\er. new laminae of both
layers still continue to be added, and thus the lip becomes thickened
l>y 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 in the Oyster, with this differ-
ence, that the successive layers have but a comparatively slight
adhesion to each other.1

The shells of Terebratulce and of most other Brachiopods are
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, ft), which are arranged with such obliquity

1 The most important recent work on the shells of Lamellibranchs is that of
the lately deceased F. Bernard ; see Bull. Hoi-. Gt'ul. Frni/i-f, vols. xxiii. and xxiv.

926

MOLLUSCA AND BEACHIOPODA

tliat their rounded extremities crop out upon the inner sin-face of the
shell in an imbricated (tile-like) manner (a). All true Terebratulidce,
both recent and fossil, exhibit another very remarkable peculiarity ;
namely, the perforation of the shell by a large number of canals,

A

B

FIG. 700. — A, internal surface, a, and oblique section, b, of shell of ]]'tilrUici>nia
a a \t rii I is ; B, external surface of the same.

which generally pass nearly perpendicularly from one sin-face to the
other (as is shown in vertical sections, fig. 701), and terminate inter-
nally by open orifices (fig. 700, A), whilst externally they are covered

by the periostracuin (B) . Their
diameter is greatest towards
the external surface, where
they sometimes expand sud-
denly, so as to become trum-
pet-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.,dd). Hence the
diameter of these canals, as
shown in different transverse
sections of one and the same

shell, will vary according to
701 —Vertical sections of shell of Wuhl- f ., ", . ,

k,La. austraUs, showing at A the canals th'' Pari ^ ^ thu-kness which
opening by large trumpet-shaped orifices the section happens to tra-
on the outer surface, and contracting at verse. The shells of different
il, il into narrow tubes ; and showing; at TJ e c j T> 7 •

a bifurcation of the canals. sPecies (lt perforated Brachio-

pods, however, present very

striking diversities in the size and closeness of their canals, as shown
by sections taken in corresponding 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 interior is filled with a fluid containing minute
cells and granules, which, from its corresponding in appearance with
the fluid contained in the great sinuses of the mantle, may perhaps

SHELLS OF BRACHTOPODA

927

be considered to be the animal's blood. Of their special function in
the economy of the animal it is difficult to form any probable idea ;
but it is interesting to remark (in connection with the hypothesis of
a relationship between Brachiopods and Polyzoa) that they seem to
have their parallel in extensions of the perivisceral cavity of many
species of Flustra, Eschara, Lppralia, Arc., into passages excavated in
the walls of the cells of the polyzoarv. Professor Sollas ' 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 regarding the processes as organs which are sensitive to
luminous impressions.

In the family Shynchonellidce, which is represented by only
six recent species, but which contains a very large proportion of

FIG. 702.

FIG. 703.

FIG. 704.

FIG. 702.— Horizontal section of shell of Terebratula buJlata (fossil, Oolitn.
FIG. 703. „ „ Megerlia lima (fossil, Chalk).

FIG. 704. „ „ Spiriferina rostrata (Triassic).

fossil Brachiopods, these canals are almost entirely absent : so
that the uniformity of their presence in the Terebratulidce, and their
general absence in the Rhynchonettidce, supply a character of
great value in the discrimination of the fossil shell.-, belonging
to these two groups respectively. Great caution is necessary.
however, in applying this test ; mere surfaa nt«rl. •/'////* r,//t,//>t be
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 Spiriferidce and Strophomenidce, 011 the
other hand, some species possess the perforations, whilst others are
destitute of them ; so that their presence or absence t/ierc serves only
to mark out subordinate groups. This, however, is what holds good
in regard to characters of almost every description in other depart-
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 Proc. Eoy. Dublin Soc. v. 318.

- For a particular account of the Author's researches on this group see his memoir
011 the subject, forming part of the introduction of Mr. Davidson's Monograph of thf

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 greater number 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 render 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 appearances 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 ' poreellanous ' shells are com-
posed of three layers, all presenting the same kind of structure, but
each differing from the others in the mode in which this is disposed.
For each layer is made up of an assemblage of thin lamina? placed
side by side, which separate one from another, apparently in the
planes of rhomboidal cleavage, when the shell is fractured ; and, as
was first pointed out by Mr. Bowerbank, each of these laminn3 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 intersect 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 Cyprcca 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 Holiest/on in that of the
elephant.

The principal departures from this plan of structure are seen in
I'litflla, Chiton, Haliotis, Turbo 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 ritfus,
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
some\\lial hexagonal in form, and sometimes (juite transparent,
whilst in other instances it presents an appearance closely resembling
that delineated in fig. (398. In the epidermis of the mantle of some
species of Ilori*. 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

L''(ixm'l ]!riii-//in/>iii/ii, published liy the Palaeontographical Society. A very
remarkable example of the importance of the presence or absence of the perforation-
in distinguishing shells whose internal structure shows them to be generic-ally dif-
ferent, whilst from their external conformation they would be supposed to be not
(inly (ji tirrini/li/ but ,s/»T///Yr///// //Irittictil, will be found in the Ann. Nil/. Iliaf.
ser. iii. vol. xx. 1867, 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 dilute 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, and 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
Umax.

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 interesting objects for the polariscope. Much valuable informa-
tion may also be derived from the examination of the surfaces pre-
sented by 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 Spir-ula does not present any peculi-
arities that need here detain us. The rudimentary shell or scpinstnin
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 laminse.
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 lamina? have been detached from them, the lines
of junction being distinctly indicated upon these. By this arrange-
ment each layer is most effectually supported by those with which

3 o

930

MOLLUSCA AND BEACHIOPODA

it is connected above and below, and the sinuosity of the thin
intervening laminae, 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 curious 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 for
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.

Palate of Cephalophorous Molluscs. — The organ which is
sometimes referred to under 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)
FiG.705.-PortionofthelefthalfofthepaJate t go as

of Helix liorteit.'iis, the rows of teeth near " L ,.

the edge separated from each other to show to form a nearly flat surtace.

their form. Qn 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
1 uwing 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 (Jastropods. such as the
snail (HeUx) and slug (L/nii/.>-). in which the number of plates in
each row is very considerable (figs. 70-1, 706), amounting to 180
in the large garden slug (Limn.'- HHLI-UHHS) ; whilst the latter prevails
in many marine Gastropods, such as the common whelk (lim-cin mn
itiiiliitu in), 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 tin- number of
rows of teeth vary greatly in different species. ( Jem-rally speaking,
the tube of the terrestrial < lastropods is short, and is contained
entirely within the nearly globular head; but the rows of teeth
being closely set together are usually very numerous, there being
frequently more than 100, and in some species as many as lu'Oor
170; so that the total number of teeth may mount up. as in //<•//. >•
pomatia, to 21,000, and in L'm>«.<- inn,<-linnx to •_'(>. 800. The trans-

PALATES OF GASTKOPODA

931

PIG. 706. — Palate of Hyalinia 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 ill 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 i-dgt^.
whilst those of the lateral are
large and serrated. The palate
of Trochus zizyphinus, 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 Ihdiotis.
in which there is a central band
of teeth having nearly straight
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
differences also present themselves among the different species of
the same genus. Thus in Doris pilosa 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 central band is the

3 o 2

w-

FIG. 707. — Palate of TrocliiiK zi,~i/pliinus.

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
this 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.1 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 animal;-
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-
num (whelk) and its allies is
used by these animals as a file,
with which they bore holes
FIG. 708.— Palate of Doris tuberculata. through the shells of the molluscs

that serve as their prey; this

they are enabled to effect 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 fa>t
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

' I H.H!.'

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. \'nf. Hi*/, ser. ii, vol. x. 1852, p. 413.

DEVELOPMENT OF MOLLUSC A

933

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 F
object, and the analysing prism is made to
rotate.1

Development of Molluscs. — Leaving to

the scientific embryologist the large field of study that lies open to
him in this direction,2 the ordinary microscopist will find much to
interest him in the observation of certain special phenomena of

709.— Palate of Buccl-
num undattun as seen under
polarised light.

which a general account will be here given.

Attached to the gills of

fresh-water mussels (Unto and Anodon) there are often found in the
spring or early summer minute bodies which, when first observed,
were described as parasites, under the name of (.ilnclii,li,i, Inir 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
Anodon, 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. In
this stage of its existence no internal organ is definitely formed,
except the strong 'adductor' muscle (a ad) which draws the valves
together, and the long, slender byssus-filament (by) 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

1 For additional details on the organisation of the palate and teeth of the
Gastropod molluscs, see Mr. W. Thomson in Cyrluji. Aunt, and Pltysiol. vol. iv.
pp. 1142, 1148, and in Ann. Xnt. Hist. ser. ii. vol. vii. p. 86; Professor Troschel, Da*
Gebiss der SchnecJcen, Berlin, 1856-79; A. Riicker, ' Ueber die Bildung der Kadula
bei Helix jjomt/tia,' Bericlit oberhess. Gesellscli. Giessen, xxii. p. 209 ; P. Geddes, ' On
the Mechanism of the Odontophore in certain Molluscs,' Trans. Zb'ol. Soc. x. p. 485.

2 See Balfour's Comparative Embryology, vol. i. chap. ix. More recent text-
books of embryology, such as that of Professor Korschelt and Heider, need not here
be specifically cited.

934

MOLLUSCA AND BRACHIOPODA

branchia? and of oral tentacles ; but 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.1

In certain members of the class Gastropoda, 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
nidameiitum. The nature of this envelope, however, varies greatly ;
thus, in the common Limncnus staynalis, or ' water-snail,' of our ponds
and ditches it is nothing else than a mass of soft jelly, about the size
of a sixpence, in which from fifty to sixty eggs are imbedded, ;md
which is attached to the leaves or stems of aquatic plants ; in the
Sucdnum w/ndatum, or common whelk, it is a membranous case,

at/

F!(T. 710. — A, Glochidium immediately after it is hatched : ail, ad-
ductor ; sh, shell ; by, byssus-cord ; s, sense-organs. B, the same
after it has been on the fish for some weeks : b>\ branchiae ; am ,
auditory sac; f, food; ft. ad and p. ad, anterior and posterior
adductors; al, inesenteroii ; tut, mantle.

connected with a considerable number of similar cases by short stalks.
so as to form large globular masses which may often lie picked up on
our shores, especially between April and June ; in the Pwrpura
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
gem-rally (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) arc packed closely
together in the midst of a jelly-like substance, tin's t ube bring disposed
in coils of various forms, which are usually attached 10 sea \\crds or
/iioplivtes. The course of development, in the first and last of these
instances, may be readily observed from the very earliest period down

- I. In- I!i-v. \V. Houghton, ' On the Parasitic Nature of the Fry of the A)u>-
il,inln cygnea,' in (Jimrl. Journ. Microsc. Sci. n.s. vol. ii. 1801, p. 162, and especially
Balt'our, <>i>. <•//. ]>p. 'J-Jii-'J-j;;. On tin; embryonal byssus-gland of A noilnnhi, see
J. Carrierc, j/iiu/ui/. Inneiff. vii. |>. II.

DEVELOPMENT OF DOEIS

935

to that of the emersion of the embryo, owing to the extreme trans-
parence of the nidamentuni 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 others, and so on until a morula, or mulberry -
like mass of minute yolk-segments, is produced (fig. 711, A-F),
which is converted by ' imagination ' into a ' gastrula,' whose form

FIG. 711. — Embryonic development of Don's bilamellata : A, ovum, consist-
ing of enveloping membrane, a, and yolk, 1> ; B, C, D, E, F, successive
stages of segmentation of yolk ; G-, 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 ; cl, foot ; g, hard plate or operculum. attached to it ;
Ji, stomach ; ?', intestine; in, n, masses (glandular?) at the sides of the
(Esophagus; o, heart (?); s, retractor muscle (?) ; f, situation of funnel;
v, membrane enveloping the body ; ,r, auditory vesicles ; y, mouth.

936 MOLLUSC A AND BBACHIOPODA

is shown 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 direction : this
movement is due to the cilia flinging a sort of fold of the ecto-
derm termed the velum, which afterwards usually gives origin to a
[tail- of large ciliated lobes (H-L, c) resembling those of Rotifers.
The velum is so little developed in Limnceus, however, that its
existence was commonly overlooked until recognised by Professor
Ray Lankester,1 who also has been able to distinguish its fringe of
minute cilia. This, however, 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 cilia (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,x) 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 afterwards 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 hinder
part and gradually extending itself until it becomes large enough to
inclose the embryo completely, when this contracts itself. The
ciliated lobes are best seen in the embryos of Nudibranchs ; and the
fact of the universal presence of a shell in the embryos of that group
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,servingalso
to bring food to the mouth, which is at that time unprovided with
the reducing apparatus subsequently found in it. The same is true
of the embryo of Lymnceus, save that ils swimming movements are
less active, in consecpaence of the non-development of the ciliated
lobes ; and the currents 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-

1 Sec his valuable ' Observations MM the Development of LimiuFiiH stayim/ix and
nn the curly stages of other Mnllusca ' in (J/iurt. •Imi rn. •.V/Vni.sr. Sri. October 1874;
and 'On the Developmental History of the Molhiseu, ' Pliil. T rn >ix. ls7-~>. See also
Lereboullet, ' Ttcrheivhes sur le De'\ elnppemenl <lu Lirrmex>,' in Ann. ili'n Sri. ,\n/.
/.not. 41' serie, torn, xviii. p. 47.

DEVELOPMENT OF PUKPUKA

937

tion. The disappearance of the cilia 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
fresh water alone for some time, without vegetable matter of any
kind, the gastric teeth are very imperfectly developed, and the cilia
are still retained.1

A very curious modification of the ordinary plan of development
is presented in Pnrpnra lapilliis, and it is probable that something
of the same kind exists also in Hucci/ittm, 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 which 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 Buccinuni) seems to
be as follows. Of those 500 or
600 egg-like bodies, only a small
part are fertile ova, the remainder
being unfertilised eggs, the yolk
material of which serves for the

nutrition of the embryos in the FIG. 712.— Early stages of
later stages of their intracapsular
life. The distinction between
them manifests itself at a very
early period, even in the first
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 such 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,

_ embryonic

development of Purpiint lajjiU/ts: A,
egg-like spherule ; B, C, E, F, G, suc-
cessive stages of segmentation of yolk-
spherules ; D, H, I, J, K, successive
stages of development of early embryos.

1 See Trans, Microsc, Soc. ser. ii. vol. ii. 1854, p. 93.

933

MOLLUSCA AND BKACHIOPODA

this border 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 cavity, 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 buried
within this as only to be discoverable 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 different

FIG. 713. — Later stages of embryonic development of PiirjJitra lapilhis.
A, conglomerate mass of vitelline segments, to which were attached the
embryos u, !>, c, <1, c. B, full-sized embryo in more advanced stage of
development.

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 sirallon- the yolk segments of the conglomerate mass,
and capsules will not unfrequently be met with in which embryos
of various sixes, as «, b, c, d, e (fig. 713, A), are projecting from its
surface, their difference of size not being accompanied by advance in
development, but merely depending upon the amount of this ' supple-
mental' yolk which the embryos have respectively gulped dcwii.
For during the time in which they are engaged in appropriating this
additional supply of nutriment, although they increase in sice, yet
they scarcely exhibit any t>l her 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. 7\'2. K. So soon as this
operation lias heen completed, houever. .-Mid the embryo hasallained
its full bulk, the evolution of its organs takes place MTV rapidly; the

DEVELOPMENT OF PURPUEA 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 Kotifera, 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 part 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 are as yet far
from complete) the capsule thins away at its summit and the embryos
make their escape from it.1

It happens not imfrequently 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 ciliated lobes with scarcely
the rudiment 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
development, others remain of unusually small size, without being-
deficient in any of their organs; and others, again, are more or less
completely abortive — the supply of supplemental yolk which they
have obtained having been too small for the development of their
viscera, although it may have afforded what was needed for that of
the ciliated lobes, eyes, tentacles, auditory vesicles, and even the
foot — or, on the other hand, 110 additional supply whatever having
been acquired by them, so that their development has been arrest i < 1
at a still earlier stage. These phenomena are of so remarkable a
character that they furnish an abundant source of interest to any
microscopist who inav happen to be spending the months of August
and September in a locality in which the I'ln-fmra abounds ; since,
by opening a sufficient number of capsules, no difficulty need be
experienced in arriving at all the facts which have been noticed in
this brief summary.2 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 Pn rjni r</, 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 Piayjiira, see Professor Haddon, ' Notes on the Development of the
Mollusca,' Quart. Ju/ini. M/crosr. Sri. xxii. p. 3t>7.

- Fuller details on this subject will be found in the Author's account of his re-
searches in Trillin. Mirror-, tioc. ser. ii. vol. iii. 1855, p. 17. His account of the
process was called in question by MM. Koren and Danielsseu, who had previously
given an entirely different version, of it, but was fully confirmed by the observations
of Dr. Dvster. 'See Ann. Xat. Hist. ser. ii. vol. xx. 1857, p. 1C. The independent

940 MOLLUSCA AND BRACHIOPODA

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 through 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 general phenomena of ciliary motion 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 (Mytilus)? as they are also on those of the Anodon
or common ' fresh- water mussel ' of our ponds and streams. Nothing
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, upon 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 afford a general view of this spectacle ; but a much
greater amplification is needed to bring into view the peculiar 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 effect 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 afforded by
the Diatomacece, Infusoria, &c. which it carries in with it.

Organs of Sense of Molluscs. — Some of the minuter and more
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 ring; 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 \<-ri/in<i jl/i ri<it//ix i Miiller's
Arcltiv, 1857, p. 109, and abstract, in Aim. of Xut. Hisf. 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 Purpura. The subject lias a^ain been recently studied with pv.it
minuteness by Selenka, -\ / cili rl/i n /! ischfS AfrJlil' fit >' Zfiiiloi/ii', 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-ORGANS OF MOLLUSCA 941

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.1 Professor H. N". Moseley made the
interesting discovery that many of the CMtonidce 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 011 the back of Onchidium, which resemble
the vertebrate retina in having the optic fibres inserted into the front
aspect of the layer of rods.2 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 Hlixj. 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 touched 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 part into which it has been drawn back. The structure
of the eyes and the curious provision just described may easily be
examined by snipping off 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 mt»t
Gastropods (fig. 711, K, x), 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 small 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 appeai-ance 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 interesting to watch the action of the

1 See Mr. S. J. Hickson on ' The Eye of Pecten ' in Quart. Journ. Microsc. Sci.
vol. xx. ii.s. 1880, p. 443, and K. E. Sclireiner, 'Die Augen bei Pecten und Lima,'
Bergens Mus. Aarbog, 1896, no. 1.

2 See Professor Moseley ' On the Presence of Eyes in the Shells of certain Chitonidae
and on the Structure of these Organs,' in Quart. Journ. Microsc. Sci. xxv. p. 37.

942 MOLLUSCA AND BEACHIOPODA

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.1

Chromatophores of Cephalopoda. — Almost any species of cuttle-
fish (.SV///V/) or squid (Loliyo) will afford the opportunity 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 different 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 Avhich it may be
in proximity. The alternate contractions and extensions of these
pigment-cells, or clirnnifitophnivs. 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 sufficiently 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 Weichthiere,' ZeitscJir. fiir triss. Zool. xxvi. p. 227.

- For further information regarding the chromatophores see an essay by Dr.
Klemensiewicz in the Sitzungsberichte of the Vienna Academy, vol. Ixxviii. p. 7,
and Krukenberg, Vrrgl. pliysiol. Sfudiaii, 1880.

The following works and memoirs on the Mollusca generally may be consulted by
the student: S. P. Woodward, A Munnul nf tin' Mollusca, :!rd ed. London, 1875;
Keferstein, in Bronn's Klassfii inn/ ( >ri in/in t/i'ii </rs Tl/irrn'/i'l/s; the article 'Mollusca,'
by Professor Ray Lankester, in the Itth edition of the Enci/chipaJia Hr/tnit/i/i ,/ :
M. P. Fischer's Manuel de Conchyliologie, Paris, 1881-87; and the Rev. A. H.
Cooke's volume in the Cainbriili/i- \ntiirul History; as well as the numerous reports
on the Mollusca collected by H.M.S. ( '/m/lnnjer.

943

CHAPTER XIX
WORMS

UXDER 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 pi'ogress of research, group after group may be expected to
be removed. Among others there are included in it the Entozoa or
intestinal worms, the Kotifera or wheel-animalcules. Turb'ellaria, 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 already
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 absorption of the juices of these, thus
bearing a striking analogy to the parasitic Fungi.1 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 apparatus. Thus
in the common Tcvnia (' tape-wi >rm '), 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 generative apparatus,
the male and female sexual organs being so united in each as to
enable it to fertilise and bring 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 menscJiUcJien Parasite n, of which a second edition is now in course
of publication ; of this the first portion has been translated into English by
Mr. W. E. Hoyle.

944 WORMS

had been previously ranked as a distinct group. These are not
found, like the preceding, in the cavity of the alimentary canal of
the animals they infest, but always occur in the substance of solid
organs, such as the glands, muscles, etc. They present themselves to
the eye as bags or vesicles of various sizes, sometimes occurring
singly, sometimes in groups ; but upon 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 Ccemirus, which is developed in
the brain, chiefly of sheep, where it gives rise to the disorder known
as ' the staggers.' Now, in none of these cystic forms has any
generative apparatus ever been discovered, and hence they are ob-
viously to be considered as imperfect animals. The close resemblance
between the ' heads ' of certain Cysticerci and that of certain Tcenice
first suggested that the two might be different 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-like
order — of which the common Ascaris may be taken as a type ; one
species of this (the A . lumbricoides or ' round worm ') is a common
parasite in the small intestine of man, while another (the Oxyuris
vermicularis or ' thread-worm ') is found rather 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.1 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 Gordius or tin- • hair- worm,' are parasitic for the greater part of

1 See particularly the various recent memoirs of Van Beneden and of Boveri, based
on a study of Ascaris meyaloccpliala.

NEMATODES AND TREMATODES 945

their existence, but leave their host for the purpose of maturing
their generative products ; in these later stages the Gordius is fre-
quently found in 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 A nguillulce are little eel-like worms,
of which one species, A. fluviat His, is very often found in fresh water
amongst Desmidice, (Jonfervce, &c., also in 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 affected with the blight termed the
'cockle; ' another, the A. ylutin-is (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 Vibriones)
are being developed gradually assumes the appearance of a black
peppercorn ; and if it be divided the interior will be found 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 cottony substance seems to the eye to consist
of bundles of fine fibres closely packed together ; but on taking out
a small portion, 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 Anyuillulce 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. ylutinis, 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 foregoing orders of Entozoa, the Trematode group,
which is more closely allied to the Cestoda than to the ISTematodes,
must be named ; of this the Distoma kepaticum, or ' fluke,' found
in the livers of sheep affected 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 Planariw (fig. 714) ; and also for the curious

3 P

946 WORMS

phenomena of its development, several distinct forms being passed
through between one sexual generation and another. These have
been especially studied in the Distonia, 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 larvae, 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 having 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 Distonia, the peculiar ' habitat ' of
which would render them, useless.1

Turbellaria. — This group 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 Planariaii
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 sufficiently 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 (6) 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, IJi/a/'t. Joiirn. Micrimi'-. Set. xxiii. p. 1 ; and R. Leuckart, Archiv fur Natur-
tjescli. xlviii. p. 80. On its anatomy, see Dr. F. Sommer, Zeitschr. fur iviss. Zool.
xxxiv.

PLANARIA

947

oviducts (&, &), which
dilatation (/) at their

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 impregnating the ova of the other, seems to be gene-
rally necessary. The ovaria, as
in the Eiitozoa, extend through
a large part of the body, their
ramifications proceeding from
the two
have a

point of junction. The Pla-
,<«ri(i ! 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 Phtnariti-
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
comparable to the ' thread-cells ' of zoophytes.2

Animlata. — 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 members of which inhabit freshwater or live

1 See Balfour's Con/jianitive Embryology, vol. i. pp. 159-162.

- For further information regarding the TurbeUaria consult Dr. L. Graff's article
on Planarians in the 9th edition of the Encyclopedia Brit an >i ica, and his magnifi-
cent Monographic der Tiirbi-Jlciritleii, Leipzig, 1882; A. Lang, Die Polycladen,
Leipzig, 1884; P. Hallez, Contributions <'i n/i^tnire naturelle des TurbeHarirx,
Lille, 1879. On transverse fission, see Bell, Jottni. Hot'. Microsc. Soc. (2),vi. p. 1107.

3 p'2

FIG. 714. — Structure of Pohjcelis levi-
gatus (a Planarian worm) : a, mouth,
surrounded by its circular sucker ; b,
Imccal cavity ; c, cesophageal orifice ;
d, stomach ; e, ramifications of gastric
canals ; /, cephalic ganglia and their
nervous filaments ; g, </, testes ; 1i,
vesicula seminalis ; /, male genital
canal ; k, k, oviducts ; /, dilatation at
their point of junction; ui, female
genital orifice.

very possibly

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, 011 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 Serpula, Terebella, Sabel-
laria, (fee.) have their bodies inclosed
by tubes, either formed of a shelly
substance produced from, their own
surface, or built up by the agglutina-
tion of grains of sand, fragments of
shell, tfcc. ; : 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 faeNereida .
or simply bury themselves in the
sand, as the Arenicola or ' lob-worm.'
In these respiratory 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

iu. t±'O. — v_;iiuuiii'Uiiii: uj[jijciji-u>iju» ui . -.

Terebella concMlega : a, labial ring; Species to occupy the space that

b, b, tentacles ; c, first segment of intervenes between the outer sur-

the trunk ; <7 skin of the back; e face of the alimentary caiial and

pharynx; f, intestine ; ff. longitudinal ,, n /? ,1 i •,

muscles of the inferior surface of the the "iner wall of the body, and to

body ; h, glandular organ ; i, organs pass from this into canals which

of generation ;j, feet; M, branchiae; often ramify extensively ill the
/. dorsal vessel acting as a respiratory . J

heart; m, dorso-intestinal vessel; respiratory organs, but are never
, venous sinus surrounding oesopha- 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 Terebella we find a distinct provision for the

FIG. 715. — Circulating apparatus of

gus ; ;;', inferior intestinal vessel;
o, o, ventral trunk ; p, lateral vascular
branches.

1 For an interesting account of the formation of these tubes see Mr. A. T. Watson's
paper in Jaitrn. Iloij. Micr. Snc. 1890, p. 685.

DEVELOPMENT OF WORMS 949

aeration of both fluids ; for the first is transmitted to the tendril-
like tentacles which surround the mouth (fig. 715, b, b), whilst the
second circulates through the beautiful arborescent gill-tufts (k, Jc}
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 different members of the class. In
the observation of the beautiful spectacle presented by the respiratory
circulation 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
different forms of teeth, jaws, etc. 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 ofl
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.1 The number of segments pro
gressively increases by the interposition of new ones between the
caudal and its preceding segments; the various internal organs
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 Annelid, 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 riot under observation,
this larva 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 having 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
whilst 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.1

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 larva? are often to be obtained at the
same time and in the same manner as small Medusa? ; 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 (tig. 716), bears a
strong resemblance in many particulars
to the ' bipinnariaii ' 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 ' loplm-
phore' of certain Polvzoa. The mouth
(e) is concealed by a broad but pointed
hood or ' epistome ' («), 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 (b) is surrounded by a
circlet of very large cilin. 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
•Actinotrocha branchi- itself together, so as to bring the anal
ata : a, epistome or hood; b .° , ' ,

anus; c, stomach; d, ciliated 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 stomach (c) art-
kept in a continual whirling movement by the agency of cilia, with
which its walls are clothed.2 An even more extraordinary departure
from the ordinary type is presented by tin- larva which has received
the name Pilidium (fig. 717), its shape being tliat of a helmet, the

1 For further infunnation on this subject see Balfour's Comparative Embryology,
vol. I. cluiji. xii. and the meiiinirs there cited.

'Ueber I'lliilium und Actinotrocha' in Midler's Archiv, lur.s, p. -jici. For
more recent observations upon the latter creature, see Balfour's Comparative
Embryology, vol. i. |.p. •J1.i!i-:j(i-j; and a paper on 'The Origin and Significance of the
Metamorphosis of Actinotrocha,' \>\- Mr. E. B. Wilson (of Baltimore), in Quart.
•Inuni. Microsc. .S'r/. April 1881.

LAKV.E 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 Miiller, has
been since ascertained to be the larva of some species of the Nemer-
tine worms, which belong to the division Anopla, a group in which
there are no stylets to the proboscis.1

Among the animals captured by the tow-net the marine
zoologist will not be unlikely to meet with a worm which,

FIG. 717. — Piliclium gyrans . A, young, showing at a the alimentary
canal, and at b the rudiment of the Nemertid ; B, more advanced
stage of the same ; C, newly freed Nemertid.

although by no means microscopic 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 Tomopteris, 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 ' Uiitersuchungen iiber niedere
Seethiere ' in Midler's Arcliiv, 1853, p. 569 ; and Balfour, op. cit. p. 165. The Author
has frequently met with Pilidium in Lamlash Bay.

952

WOEMS

an inch 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 development of Tomopteris onixc/foriii/s : A, portion
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 perivisceral cavity and its caudal prolongation ; D,
ciliated canal, commencing externally in the larger and smaller rosette-like
discs, «, b ; E, one of the pinnulated segments, shelving 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 Tomopteris, showing at a a the larval antenna' ; b b, the incipient
long antenna- of the adult ; c, d, i; f, four pairs of succeeding pinnulated
L-ments, followed by bifid tail.

TOMOPTEEIS

953

ance of a ' hammer-headed ' shark ; behind these there is a pair of
very long antennae, 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 antennae, a ganglionic mass, the component
cells of which may lie clearly distinguished under a sufficient mag-
nifying power, as shown at F ; seated upon this are two pigment-
spots (b, b), 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 situated 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 oesophagus 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. C. 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. K"o 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 fig. E, b, arid on a larger scale
at fig. D ; each of these orifices (D, «, b) is surrounded by a sort of
rosette, and the rosette of the larger one («) 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 appeal- to be
distinct, ova being found in some individuals and spermatozoa in
others. The development of the ova commences in certain ' germ-

954 WORMS

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 outer wall of the
body at the third and fourth segments. The male reproductive
organs, on the other hand, are limited to the caudal prolongation,
where the sperm-cells are developed 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 aperture 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 through 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 differs 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 antenna?. 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 antenna?, «, «, 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 larvae having eight or ten segments this is developed
into a second pair of antenna? resembling the first ; and the animal
in this stage has been described as a distinct species, T. quadricornis.
At a more advanced age, however, the second pair attains the
enormous development shown at B, and the first or larval antenna1
disappear, the setigerous portions separating at a sort of joint (G, a,
«), whilst the basal projections 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 uncommon,
and the microscopist can scarcely have a more pleasing object for
study.1 Its elegant form, its crystal clearness, and its sprightly,
movements render it attractive even to the unscientific

1 See tin- memoirs of the Author mid M. Claparede in vol. xxii. of the Linnean
Ti'inistirfioiiN ;m<l the authorities there referred to; also a memoir by Dr. F.
Ve'jdovsky in Zritxi-lirift f. U'/ss. ZooJ. Bd. xxxi. 1878.

NAIS 955

observer ; whilst it is of special interest to the rnorphologist as one
of the simplest examples yet known of the Annelid type.

To one phenomenon 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.1

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 respiratory organs, and
the thinness of the general integument appears to supply all needful
facility for the aeration 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 oft' 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 peculiar 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 posterior
segment behind the line of constriction before its separation from

1 See his memoirs on the Annelida of La Manche in Ann. lies hid. Nat. ser. ii.
Zool. torn. xix. and ser. iii. Zool. torn. xiv. ; and Professor Mclntosh in Nature,
xxxii. p. 478.

956 WOKMS

the anterior.1 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 haiid-
magnifier, 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 aperture 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 row 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 Oligochaeta,' Report Brit.
Assoc. 1885, p. 1096.

- Among the various sources of information as to the anatomy and physiology of
the Annelids the following may be specially mentioned : the ' Histoire Naturelle des
Anneles Harms et d'Eau douce ' of M. de Quatrefages, forming part of the Suites a
Buffon; the successive admirable monographs of the late Professor Ed. Claparede,
Recherches Anatomiques sur les Annelides, Turbellaries, Opalines et Gregarines,
observes dans les Hebrides, Geneva, 1861; Recherclies Anatomiques stir les Oligo-
chetes, Geneva, 1862 ; Beobaclitimc/eniiber Anatomic und Entwiclcelungsgeschichte
wirbelloser TJiiere an der Kiiste von Nonnandie, Leipzig, 1863 ; and Les Annelides
Chetopodes du Golfe de Naples, Geneva, 1868-70 ; the monograph of Dr. Ehlers, Die
Borsf.enwurmer (Annelida Chatopoda), 1864-68. With the exception of Professor
Macintosh's article in the Encyclopedia Britannica, and the various articles on
' Worms' in the Cambridge Natural History, which 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, Mclntosh, and St. Joseph are especially to be consulted ;
Hatschek, Kleinenberg, and Saleusky have written the most important contributions
to our knowledge of development ; Benham, Bergh, Bourne, Eisig, Meyer, Perrier,
and Whitman have, among others, added to our knowledge of their anatomy and
morphology.

957

CHAPTER XX

CRUSTACEA

PASSING to the division of Arthropods, in which the body is
furnished with distinctly articulated or jointed limits, some of which
• are always modified to servf 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 uell-knowii 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 transparent
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 form 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.1 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

CRUSTACEA

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
antenna? 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 (b) situated in the

FIG. 719. — Anvmothea pycnogonoides : a, narrow oesophagus ;
b, stomach ; c, intestine ; (I, digestive Cffica of the foot-jaws ;
e, e, digestive c-eca of the legs.

midst of the thorax, from 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;tjcal 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 ca?cal prolongations are continually
executing peristaltic movements of a very curious kind ; for they
contract and dilate with an irregular alternation, so that a flux and
reflux of their contents is constantly taking place between the
central portion and its radiating extensions. The perivisceral space
I>H ween the widely extended stomach and the walls of the body and

PYCNOGONIDA ; ENTOMOSTKACA 959

limbs is occupied by a transparent liquid, in which are seen floating
a number of minute transparent corpuscles 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, whenever the caecum of
any one of the legs undergoes dilatation, a part of the circum-
ambient liquid will be pressed out from the cavity of that limb,
either into the thorax or into some other 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 quantity of light through them, and to give to this
light such a quantity as shall sharply define the internal organs.1

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 mouth-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 marked difference 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 Lophyropoda, or 'bristly-footed' tribe, a small number of
legs not exceeding five pairs have their function limited to locomotion,
the respiratory organs being attached to the parts in the neighbour-
hood of the mouth ; whilst in the Sranchiopoda, < >r ; 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 Apvs
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 Pycnogonida 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 AmcJuiida adapted to a marine
habitat than as Crustacea. See Balfour's Comparative Embryology, 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,'1
1881, and his ' Nouvelle Etude sur les Pycnogonides,' in Art-hires d? Zool. Ex per. ix.
p. 445 ; and Professor G. 0. Sars's report in the Zoology of the Norwegian 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 Brcmchipus) and sometimes
with equal facility on the back, belly, or sides (as is done by Arte/nix
Sdlina, 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, Ostracodd.
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 Cypris, which
is a common inhabitant of pools and streams ; this may be recognised
by its possession of two pairs of antenna?, 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 support 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 pass 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 antenna? and legs,
or walking upon plants 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 swimming) amongst Conferva-
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.1

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,
tlu- 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 l)tt/>lnii«, 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 <>f the Linnean Society of London; compare also Zenker,
' Monographic <ler Ostracoden,' ArcJiiv fur Nature/, xx. 1854. Glaus has an essay on
the dtvflnpment of Cypris, Marburg, 1868 ; see also Dr. Brady's' Challenger Report.'

EXTOMOSTKACA

961

to the tre.sh water, 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 Cy there is most
abundant, whilst others inhabit the open ocean, and must be col-
lected by the tow- net. The body of the Ci/dops is soft and gela-
tinous, and it is composed of two distinct parts, a thorax (fig. 720, a)
and an abdomen (£), 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 antenna? (c), possessing
numerous articulations and
furnished with bristly ap-
pendages, and another small
pair (d) : it is also furnished
with a pair of mandibles or
true jaws and with two
pairs of ' maxilla?,' 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 (/'. _/') 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
egg - capsule (B) ; within
which the ova, after be- FIG. 720.— A, temale of Cyclops quadricornis :

ing fertilised, undergo the ?>b°^'> b>iail'< c' anterma; Aaatemnla; a
°,. ,, ., v , ieet ; t, plumose setse of tail. B, tail, with

earlier stages ot their de- external egg-sacs. C, D, E, F, G, successive
velopment. The Cyclops is stages of development of young,
a very active creature, and

strikes the water in swimming, not merely with its legs and tail
but also with its antenna?. 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 young, are
brought to its mouth to be devoured.1

The tribe of Branchiopoda 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 antenna?, 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 G-. S. Brady's Monograph of the free and
semi-parasitic Copcpoda 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.

3Q

962 GEUSTACEA

this is the Daph/nia j>/tlex, which is sometimes called the ' arborescent
water-flea,' from the branching form of its antennae. 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, Phyttopoda, 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 genera Apus and Nebalia 1
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
Apus 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, are 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
antenna? 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
stagnates 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
antenna1, whence it has received the name of Cheirocephalus ; but

1 Professor Glaus has pointed out the relations of Nebalia to the Malacostraca, or
higher division of the Cruslarea, and h;is sn^ested for the group which they re-
present the name of Leptostnit-ii. See the Zeitschr. t'iir wiss. ZdoZ. 1872, p. 828 ;
Claus, Untersuchungen zur Erftiwlnint/ tin- i/<'ii<'<//«<//*<'//rt/ (inuicllar/e des
('i-iixtnr-rrii-S//xt<-ii/N, "SVieu, 1*7(5, as \vcll as ' Ueber den Organismus der Nebaliiden
die systematische Stellung der Leptostraken,' in Arl>. Zoo/. Inst. Wien.\m.
(1889), pp. 1-148, ir, |,|s. ; but a different view is taken by Professor G. O. Sars in his
on tin1 (' linlli'iiijt'i- 1'hyllocarida.

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 B>'<ni<-Jti/>tis or Cheirocephalus is certainly the most beautiful and
elegant of all the Entomostraca, being rendered 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-green and bright
red of its prehensile antennae, 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 Arfemia salina, or ' brine-
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 peculiar mode in which their generative function
is performed, and in their tenacity of life when desiccated, in which
last respect they correspond with many Rotifers. 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-
plete desiccation, although they will preserve their vitality in mud
that holds the smallest quantity of moisture; but their eggs are
more tenacious of life, and there is ample evidence that these will
become fertile oil being moistened, after having remained for a long
time in the condition of fine dust. Most Entomostraca. ton. are
killed by severe cold, and thus the whole race of adults perishes
every winter ; but their eggs seem unaffected by the lowest tempera-
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 prevail 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 species 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 intervals. 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 CEUSTACEA

single fertilised female of the common Cyclops <i>nid i-lcortils may be
the progenitor in one year of 4,442,189,120 young.1

The eggs of some Entomostraca are deposited freely in the water,
or are carefully attached in clusters to aquatic plants; but they are
more frequently carried for some time by the parent in special
receptacles developed from the posterior part of the body ; and in
many cases they are retained there until the young are ready to
come forth, so that these animals may be said to be ovo- viviparous.
In Daphnia the eggs are received into a large cavity 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 Daplinlu may be seen with a dark
opaque substance within the back of the shell, which has been called
the c/i/ii/>/ limit, from its resemblance to a saddle. This, when cart--
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,2 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 ephippial 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 layers 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 ciuious provision obviously affords protection to
the eggs which are to endure the severity of winter cold; and an
approach 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
Lord Avebury), 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
1 he ephippial eggs .seem to have the same power of continuing the

1 For ai i interest in;4 arc-mint of tin- parthenogenetic development of Apus and its
see llir sixth of Von Siebold's lif// n'ii/r zur farthenogenesis der Arthropoden
I.eip/i.u'.lHyi).

• \n .irmiint of the two Methods of Reproduction in Daphnia, and of the
Striu-tnri- of tin- Kpliippium,' In 1'liil. '1'nuix. 18,'>7, p. 70. On the ' summer-egg ' of
J.)n/>/ini<i see Lebedinsky, Ziinl. Anzeig. xiv. p. 149.

ENTOMOSTRACA 965

race by non-sexual 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-
M-rssing 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 process is very commonly
repeated at intervals of a day or two) presenting some new parts,
and becoming more 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 conferva?, 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
off 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 which elapses between the nioultings of the adult,
these, in Daphnia, 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
setre 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.1

Forming part of the entomostracous group is the tribe of
suctorial Crustacea,2 which for the most part live as parasites upon
the exterior 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 Xnctori«- undergoing such extraordinary changes in their

1 For a systematic and detailed account of this group Dr. Bairn's Natural
History of the British Entomostraca, published by the Ray Society in 1849, must
still be recommended. The numerous essays by Professor Glaus should also be
consulted.

- It is now generally recognised that these should be placed with the Copcpoda,
which may be divided into the Eucopepuila and the Brancliiura; the former are
divisible into the Grnathostomata, most of which are non-parasitic, and have been
already described under Copepoda, and the SipJionostomata, of which Lrriurn is an
example.

966 CRUSTACEA

pi-ogress 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 Branchiwra may be
specially mentioned the Argulus foliaceus, which attaches itself to
the surface of the bodies of fresh-water fish, such as the stickleback,
and is commonly known under the name of the ' fish-louse.' This
animal has its body covered with 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, cylindrical ap-
pendage, terminated by a curious sort of sucking-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 braiichiopods, are chiefly adapted for
swimming ; and the tail, also, is a kind of swimrneret. 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 ca?cal
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 crecal prolongations of the
stomach of the Planar la (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 Lernaia, 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 appears 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, C. D).
which they much resemble ; and only attain the adult form after a
series of metamorphoses, in which they cast off their locomotive
members and eyes. It is curiou^ 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 sup|M»e
the two to belong to the same family, much less to the same specie*.
but Cor the study of their development.1

Kroin the parasitic suctorial Crustacea the transition is not

1 As the Ljroiip of suctorial ( 'nist area is interesting rather to the professed
uaturali-t than to the amateur microscopist, even an outline view of it would be on-
suitable t« the i in -.-,01 it work ; and the Author would refer such of his readers ;is may
desire to study it to the e\eel|rni treatise liy Dr. Haird already referred to. Of the
numerous recent essays and memoirs those of Prot'e^.,1 Glaus should by all means
he e, HI siil ted. Mr. P. \V . lias-en Smith, Staff-surgeon It.N., h.is iii the last few years
published several interesting papers.

CIRBIPEDIA

967

really so abrupt as it might at first sight appear to the group of
Cirripedia, 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 departure 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 Bala/nus balanoides : X, 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,1 with
the extensions and rectifications which they have subsequently
received from others (especially Mr. Spence Bate2 and Mr. Dar-
win 3), show that there is no essential difference between the early
forms of the sessile Cirripeds (BalnniiJn' or 'acorn-shells') and of the
pedunculated (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 Researches, No. IV. 1830, and Phil. Trans. 1835, p. 355.

2 ' On the Development of the Cirripedia' in Ann. Nat. Hist. ser. ii. vol. viii.
1851, p. 324.

3 Monograph of the Sub-Class Cirri/u'dia, published by the Ray Society.

968 CRUSTACEA

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 protrusion of a large and strong anterior pair of
prehensile limbs, provided Avith 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
.•idhesion ; 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 their cast skins, often collected by
the tow-net, are well worth mounting.1

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 Decapods in its most complete form consists
of three strata, 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 Phyllosomata
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 pagurus 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
pails of the shell is chiefly contained, may lie easily brought into
view by grinding a way 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 diameter>.
driving a strong light through it with the achromatic condenser:

1 Y:ilii;ililr details as to the structure of this group will \^ found in Dr. P. P. C-
li<"'kV rc|Hii-|, 011 the Cirripeds collected \>\ H.3I.S. C'liaUengcr. Compare, also,
M. Niissl.iimn, Anatomische Studien, 13.nm. IS:MI.

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 tin- 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 strongly 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 apparent
walls of the areola? are merely translucent spaces from which the
tubuli are absent, their orifices being abundant in the intervening
spaces.1 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 portion 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.2

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 egg 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 inacrurons (long-tailed) or to
the lii-iicjii/iii-nii.s (short-tailed) division of the group; and the forms

1 The Author is now quite satisfied of the correctness of the interpretation put by
Professor Huxley (see his article, ' Teguuientary Organs,' in the Cyclop. Anui. anil
Phtjs. vol. v. p. 487), and by Professor W. C. Williamson ( ' On some Histological
Features in the Shells of Crustacea ' in Quart. Journ. Micrnsc. St.-i. vol. viii. 1860,
p. -'S) upon the appearances which he formerly described {Report of British Asso-
ciation for 1847, p. 128) as indicating a cellular structure in this layer.

2 Consult Braun, ' Ueber die histologischen Vorgange bei der Hautuug von
Astacus fluviatilis,' Arbeit. ZiioL I/t*t. Wiirzbin-g, ii. p. 121.

970

CRUSTACEA

of these larva? are so peculiar, and so entirely different from any of
those into which they are ultimately to be developed, that they were
considered 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 < 'n /•<•/' mis mcenas (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 metamorphosis 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 Carr/ntm nm mis : A, first or Zoea
stage ; B, second or Megalopa stage ; C, third 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 era 1 is. 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 more 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 Mvgalojxi. when it was sup-
posed to be a distinct type. In the next stage, ('. we find the
abdominal portion reduced to an alniosl rudimentary condition, and
In-lit under the body ; the thoracic-limbs are more completely adapted
for walking, save the first pair, which are developed into c/tflce or
|iincers ; and 1 he little creature cut irely loses the active .swimming
habits which it originally possessed, and takes on I he mode of life
peculiar to the adult .'

[n collecting minute Crustacea the ring net should be used for

1 On tin' mc'tann'i-plii^es of Crustacea and Cirripcdia, wee especially the Unter-
iibi-i- Cnixtiirct'ii of Professor Clans, Vienna, 1876. A number of

COLLECTING CRUSTACEA 971

the fresh-water species, and the tow-net for the marine. In localities
favourable for the latter the same ' gathering ' will often contain
multitudes of various species of Entomostraca, accompanied perhaps
by the larva? of higher Crustacea, echinoderm larva?, annelid larvae,
and the smaller JlcJus" . 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 general 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 larva? 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. Mailer's Facts
<i ml Ari/itmoit? fur Darwin (London, 1869). The work of Reicheiibach on the
Development of the Crayfish is contained in vol. xxix. of the Zeitschr. f. wiss. Zool.
p. 123, 1877, and vol. xiv. of the Abhandl. Senckenlerg. Naturf. Gesells. 1886. See
also the essay, by W. K. Brooks, On the Development of Lucifer, in Phil. Trans.
1882, p. 57. Mr. F. H. Herrick's memoir on the American Lobster (Bull. U.S. Fish.
Comm. xv. [1895] ) contains matter of much interest. Professor Sars's fully illustrated
monograph of the Crustacea of Norway is being steadily and rapidly published.

9/2

CHAPTER XXI

IX SECTS AND ABACHXIDA

THERE is no class in the whole animal kingdom which affords to the
rnicroscopist 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 different
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 a fiord 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 proportion
may be got out, examined, and mounted Avith very little skill or
trouble. Take, for example, the common house-fly ; its eyes may
be easily mounted, one as a transparent, the other as an opaque
object; its antennae, ;dt hough not such beautiful objects as those of
many other Diptera, are still well worth examination ; its tonyue or
' proboscis ' is a peculiarly interesting object, though requiring some
care in its preparation ; its spirctcles, 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 /'•///'/. examined in a living specimen
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 areolatioii of surface,
when examined by light reflected from its surface at a particular
angle; its foot has M 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 structure
and physiology of it - sesewaZ 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.' 1 1 ence, in treating of this department
in such a \\ork as 1 he present., the Author labour* under i \\eembnrrcts
des richesses ; for, to enter into such a description of the parts of the
structure of insects most interesting to the rnicroscopist as should

1 See Mr. Low iicV valnabli treatise »n I'/tr Anatomy and Physiology of tin-
, INTO; second edition ls:il-4.

MOUNTING INSECTS 973

be at all comparable in fulness 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 source* 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.1

A considerable number of the smaller insects — especially those
belonging to the orders Coleoptera (beetles). X<:nrc>j>trrit (dragon-fly,
May-fly, &c.), Hymenoptera (bee. wasp, Arc.), and Dipter<i (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, applicable to small and
large insects alike, may be found in various text -books of ento-
mology. There are some, however, whose translucen.ee 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 buy 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 larva? of the Diptera and Xeuro-
ptera, which are so transparent that their whole internal organisa-
tion can be made out without dissection, arc 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 /nnr>jiinit/i (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 inject 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 beauty, 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 larva?

1 An excellent introduction to the study of insects will be found in The Structure
and Life-history of the Cockroach, by L. C. Miall and A. Denny (London, 1886).
See also Dr. D. Sharp in the Cambridge Xat/traJ History.

974 INSECTS AND AKACHNIDA

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, ifcc., 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 appropriately commence
with their integument and its appendages (scales, hairs, etc.). The
body and members are closely invested by a hardened skin, which
acts as their skeleton, and affords 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 animal substance termed chitine,
as well as in some cases by a small quantity of mineral matter. It
is in the Coleoptera 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 lamin.se 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 lamella? 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 particular angle, even when not discernible in
transparent sections. The integument of the common red mil
exhibits the hexagonal cellular arrangement very distinctly through
out; and the- broad flat expansion of the leg of the drahrn ('sand
wasp') affords another beautiful example of a distinctly cellular
arrangciiH'iil of the outer la yer of the integument. The inner layer,
however, which constitutes the principal part of the thickness of the
horny casing of the beetle tribe, seldom exhibits anv distinct organi-
sation, though it may l>e usually separated into several lamella\
which are sometimes traversed 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 or that of hairs more or less
approaching the cylindrical shape, or some form intermediate be-
tween the two. The scaly investment is most complete among the
Lepidoptera (butterfly and moth tribe), the distinguishing character
of the insects of this order being derived from the presence of a
regular layer of scales upon each side of their large membranous
wings. 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 off from its surface. Each scale seems to be composed
of two or more membranous lamella-, often with an intervening

O

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 Curculio
imperialis ('diamond beetle'), seems to be a purely optical effect,
depending either (like the prismatic hues of a soap-bubble) on the
extreme thinness of the membranous lamellae, or (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 sufficiently magnified, very 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 frequently differ a good deal on the several parts of the wings
and of the body of the same individual, being usually more expanded
on the former and narrower and more hairlik'e on the latter. A
peculiar type of scale, which has been distinguished by the designa-
tion plumule, is met with among the Pieridw, one of the principal
families of the diurnal Lepidoptera. The 'plumules' are not flat,
but cylindrical or bellows-shaped, and are hollow; they arc attached
to the wing by a bulb at the end of a thin elastic peduncle that
differs in length in different species, and proceeds from the broader.
not from the narrower end of the scale ; whilst the free extremity
usually tapers off 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 Li/crf/ndc' by the
' battledore' scales to be presently described.1

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, 814.

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
degree to which the angular aperture 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 Jfr/r/>//n M< ,/,--
laus (fig. 723), the ordinary and ' battledore ' scales of the Polyom-
iiminx A ri/i'-S (figs. 724, 725), and the scales of the Lepismu saccharin a
(fig. 726), which are now only used for testing objects of low or
medium power, were the recognised tests for objects of /////// 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 apochroinatic 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 Podwra
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
Jforpho Menelaits(&.g.7Z3). 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. 72:!). probably on the in-
ternal surface of one of the membranes. The large scales of the Poly-
nia main* A <•</»* (-azure blue ' butterfly) reseml »le 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 si rial ion. When one of these
scales lies partK over another, the effed of the optical intersection
of the t \\ose1s of ribs at an oblique angle is to produce a set of
interrupted striations (A), very much resrniMing 1 hose of the I'odura
scale. The same butterfly furnishes smaller scales, which are coin-

FIG. 723.— Scale of Morpho
Menelaus.

SCALES

977

in only termed the ' battledore ' scales, from their resemblance in
form to that object (fig. 724, «). These scales, which occur in the
males of several genera of the family Lyccenidce, and present a
considerable variety of shape,1 are marked by narrow longitudinal
ribbings, which at intervals seem to expand into rounded 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. Anthony 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.2 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? ; 3 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 T/ii/f»ntti/-n. but

FIG. 724. — Scales of Polyommatus Argus
(azure blue) : a, battledore scale ; //,
interference striae.

FK;. 725. — Battledore scale of
Polyommatus . l/y/».s (azure
blue).

now separated by Sir John Lubbock,1 on account of important
differences in internal structure, into the two groups Collembola and
true T1ii/x«ii;n'r<. Of the former of these the Lepismidce constitute
the typical family ; and the scale of the common Lepisnut, saccha-
or ' sugar-louse,' 3 very early attracted the attention of

1 See Watson, Joe. cit.

- ' The Markings on the Battledore Scales of some of the Lepidoptera ' in Monthly
Microm-opicul Journal, vol. vii. 1872, pp. 1, 250.

~' See ' Proceedings of the Microscopical Society,' op. cit. p. 278.

4 Bee his MonograpJi of the Collembola and Thysanura, published by the Ray
Society, 1*72.

•"' This insect maybe 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 pressing this gently on its body.

3 R

978

INSECTS AND ARACHNIDA

microscopists 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)1 upon the intersection
of two sets of stria?, representing the different structural arrange-
ments of its two superficial membranes. One of its sin-faces (since
ascertained by Mr. Joseph Beck 2 to be the under or attached
surface of the scale) is raised, either by corrugation or thickening,
into a series of strongly marked longitudinal ribs, 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 or 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 different angles produces
most curious effects 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
r<»hira ; but where the crossing is
transverse 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 different surfaces,' has been
confirmed by the careful investigations of Mr. Moorhouse.3 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 effect of diffraction. In the scale of a type
nearly allied to J.<-/>ixii/<i. the .l/m-///7/.s /><>l///>r></(t, the very distinct
ribbing (fig. 727) is produced by the corrugation of the under 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 1o structure between the superposed

1 'I'lr Arlrii/iKi//,- ,l//Y/-iisrn/«', p. 50.
S,-c Ins appendix l<> Sir ,|i>liu Lulilmek's Mni/oi/i-n/i/i.

\I«ii!lilii Miri-iisrii/iiriit .1(1111-11,11, vol. xi. 1874, p. 13, iiiicl vol. xviii. 1877, p. :'>!•
•' [lil, I. vol. i.\. 187:!, p. G:J.

FK;. 726.— Scale of
saccharina.

SCALES

979

membranes, probably a deposit on the interior surface of one or both
of them.1

Although the Poduridce and Lepismidce now rank 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 Poduridce are found
amidst the sawdust of wine-cellars, in garden tool-houses, or near
decaying wood, and derive their popular 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 particularly
well developed in the species now designated Lepidoc/jrtus curri-
collts, which furnishes what are ordinarily 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), arid are therefore by no
means of uniform value as tests. The
general appearance of their surface, 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 modei'ate 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 ; hut this is much more
apparent in other scales from the same
species (tig. 729), as well as in tlir
scales of various allied types, which were
carefully studied by the late Mr. II. 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
sin-face 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 Lepisina ; but more or less
distinct indications of radiating corrugations often present them-
selves. The appearance of the interrupted 'exclamation marks'
Mr. J. Beck considers to be due 'to irregular corrugations of the
outer surface of the under membrane, to slight undulations on the
outer surface of the upper membrane, and to structure between the
superposed membranes.' It has, indeed, been stated by Mr. Joseph

1 See Mr. Joseph Beck in Sir ,T. Luhliock's M<»i«</mj>h, p. 255.

; Trail*. Mirrayf. Sue. n.s. vol. x. IHtl-j, p. <s:;. See also Mr. Joseph Beck, in the
;i|i|»'iiclis in sir John Lnblmck's Monograph, And in Monthly MicroscopicalJournal
iv. p. 125S.

3 R 2

Fin. I'll. -Scale of Much/Us
polypoda .

980

INSECTS AND ARACHNIDA

Beck that the scales of a lepidopterous insect belonging to the genus
Mormo, which under a low power present the watered-silk appear-
ance seen in the Podura scale, under a -1 in. ohj. show the 'exclama-
tion markings/ whilst under a r\r in. obj. they exhibit distinct ribs
from pedicle to apex, thus showing in one scale how the appearances
run from one into the other.1

The hairs of many insects, and still more of their larv;e, are
very interesting objects for the microscope on. account of their
branched or tufted conformation, this being particularly 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 siu-e to exhibit

FIG. 728. — Test, scales of "Lepidocyrtus ciirvi-
cuUis : A, large strongly marked scale ; B,
small scale, more faintly marked.

FIG. 729. — Ordinary scale
of Li'/iiilucyrttis curvi-
collis.

strong prismatic colours if the chromatic aberration should not have
been exactly neutralised ; and that of the larva of a Dermestes
(commonly, but erroneously, termed the ' bacon-beetle ') was once
thought a very good test of defining power, and is still useful for
this purpose. It has a cylindrical shaft (fig. 730, B) with closely set
whorls of spiny protuberances, four or five on each whorl: tin-
highest of these whorls is composed of mere knobby spines ; and the
hair is surmounted by a curious circle of six orsevenlarge filaments,
attached by their pointed ends to its shaft, whilst at their free e\
tremities they dilate into knobs. An approach to this structure is
seen ill the hairs of certain M i/i-/n/i<><h (centipedes, galley-worms, A*c.),
of which an example is shown in fig. 7.'!0, A; but a beautiful photo-

1 Jniini. It'"//. MicniHf. Soc. \«\. ii. 1S71I, p. Nil). On tlic siibji-ct. generally

Dr. A. Spuler's 'Beitrag zur Kenntiiiss des feinerenBaues . . . der Fliigelbedeckung
dcr Schmetterlinge,' in Ziiul. Julirb. Aunt. viii. should lie consulted.

HAIRS

981

micrograph of the hair of Polyxenus lagiirus, of the family Poltj-
desmidcK (order Chilognatha), is given in fig. 6 of the frontispiece.
This is one of the finest test objects for medium powers. Other
minute forms of this class are most beautiful objects under the
binocular microscope on account of the remarkable structure and
regular arrangement of their hairs.

In examining the integument of insects and its appendages
parts of the surface may be viewed either by reflected or transmitted
light, according to their degree of transparence and the nature of
their covering. The beetle and butterfly tribes furnish the greater
number of the specimens suitable to be viewed as opaque objects ;
and nothing is easier than to mount portions of the elytra of the
former (usually the most showy parts of their
bodies), or of the wings of the latter, in the
manner described in Chapter VII. The tribe
of CurcuMonidce, 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 affords, the most
remarkable in this respect being the well-known
Curcitllo imperial/is, or ' diamond beetle ' of South
America, parts of whose elytra, when properly
illuminated and looked at with a low power,
show like clusters of jewels flashing against a
dark velvet ground. In many of the British
CiwculionidcB, 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 pit 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 objects 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, are rendered less attractive by this
treatment; and in order to aM-rrtain whether it is likely to improve
or to deteriorate the specimen, it is a good plan first 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 or dry. according as the turpentine increases or
diminishes the brilliancy of the scales on the spot to which it was
applied. Portions of the wings of Lepidoptera are best mounted a.-*
opaque objects without any other preparation than gumming them
flat down to the disc of the wooden 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
Mili-i',j<u<l ; B, hair of
Dermestes.

982

INSECTS AND ARACHNIDA

by softening such

portions it is well to choose those which have the brightest and
the most contrasted colours, exotic butterflies being in this respect
usually preferable to British ; and before attaching them to their
slides care should be taken to ascertain in what position, with the
arrangement of light ordinarily used, they are seen to the best ad-
vantage, and to fix them there accordingly. Whenever 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 horny cases of perfect insects of various orders, but also of
those of their pupa?, 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
parts in potash and viewing them in fluid. The
scales of the wings of Lepido-
ptera iV.e. are best transferred
to the slide by simply pressinga
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. Home 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
of the methods. Hairs, on the other
best mounted in
balsam.

Parts of the Head.— The

of insects, situated upon the upper and outer part of the head,
are usually very conspicuous organs, and arc 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
si -c n is to be the predominating ' idea ' in the conformation of arti-
culated animals; for each of the large protuberant bodies which we
designate as <m <>i/r, 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 const ructed upon the same general
plan as those of insects, though their shape and position are often
very peculiar. The individual ocelli are at once recognised 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 l>\ very regular divisions either into hexagons

Fm. 731. — Head and compound eyes
bee, showing the ocellile* in nitit on

-idc, A, and displaced on the other, B •
a, a, a, stemmata ; b, b, ant<-nn;r.

EYES

983

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 ascertain the number of ocelli

in each ; compound eye.' In the two A

eyes of the common fly there are

as many as 4,000 ; in those of the

cabbage-butterfly there are about

17,000; in the' dragon-fly 24,000;

and in the Mordella beetle 25,001).

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- FIG. 732.— Diagram of a section of the

rate, elongated hypodermic cells,
some of which are continuous with
fine branches of the optic nerve ;
these may be called retinal cells.
The next stage in complication is
seen when the.sc last combine to form
groups, ' retinuhe ; ' the sensitive
cells may become divided into two
regions, an outer one, which is
' vitreous ' and refractive in functi< in,
while the inner part remains sensi-
tive ; the corneal surface may In-
come 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 differentiated 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

composite eye of Melolontha vul-
t/iiriij (cockchafer) : a, facets of the
cornea; I, transparent pyramids
surrounded with pigment ; c, fibres

of the optic nerve ;
optic nerve.

trunk of the

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 upon the head, the

combined action of the entire aggregate will probably afford but

Fit.. 7o8. — Part of the compound eye
of Ph )•//'/"'"•" ; the retinal cells are
seen to be united into a retinula (/•)
which is differentiated into a rhab-
dom (m) posteriorly; cc, crystalline
cone; /, facet of compound eye :
2->g, pigment. (After Grenadier.)

984

INSECTS AND AEACHNIDA

a single image, resembling 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 vertebrate eyes. It is to Professor S. Exner,
of Vienna, that we are indebted for the striking though simple
results. He has been engaged for 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 photograph might be ta,ken 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 Lainpjjris splettdiduld,
(fire-fly), in the position
in which it would he 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

l''ni. 734. — Image of a window with the
letter R on one of its panes, and a church
beyond, taken through the compound eye
of Liiiiijii/ris splendidula,&m.A magnified
120 diams.

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 eves, the im-
portance of the result obtained by the ingenuity and skill of Professor
Exner is great, giving us a ne\\ start on solid ground. The mathe-
matics of the question are fully discussed by Exner in a memoir, to

EYES

985

which the student must be referred for complete information.1 The
kind of image formed by the compound eye has long been a matter
of discussion amongst physiologists.2

The process of taking the photo-micrograph copied in fig. 734
was this : The eye of the Lftmpyi'is was carefully dissected out from
the head, the retina and pigment removed with a fine camel-hair
pencil, and the back of the eye immersed in a mixture of glycerin
and water, possessing a refractive index of 1'346; this was already
known to be the refractive index of the blood of the Lcmvpyris. The
whole was placed upon an ordinary cover-glass, this being fixed by
its edges to a slide or object-carrier with a circular aperture cut in
it, as in fig. 735, 0 ; a is the slide with an aperture less in diameter

D

FIG. Too. — Diagrammatic illustration of the method by which
the image in fig. 7M4 was photo-micrographed.

than the cover-glass b cut through it; c is the fluid-medium of
«= 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 d, being, as in the normal state, in air. If the eye

1 Sitzitngsber. Akcul. U7s.srH.sr7/. U7>K, Bd. xcviii. (1889), pp. lo, 143 ; also / '
Physiologic i1/ir facettirten A/njm r<»t Krdjsen uiul Insecte/t (Leipzig mid \Vicn,
1891).

- A critical history of the discussion will be found in Chapter VII. of Sir J.
Lubbock's Senses of Animals (London, 1S88), and in Dr. D. Sharp's Annual Address
to the Entomological Society of London, 1888 (1889). See also Mr. A. Mallock in P >•<><•.
Jini/. ,S'of. Loud, vol. Iv. p. S5. 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, Untersuchungtm uber clan SeJiuri/nn <l>-r
ArUiropoden &c. (Gottingen, 1879) ; Carriere, Die Sehorgane der Thiere &c. (Munich
•and Leipzig, 1885); Hickson, ' The Eye and Optic Tract of Insects,' Quart. Jonni.
Micruar. ,SY/. xxv. p. 215; Lankester and Bourne, 'The Minute Structure of the
Lateral and Central Eyes of Scorpio and Limulus,' Quart. Jo urn. Microsc. Sci.
xxiii. p. 177 ; Lowne, 'On the Compound Vision and the Morphology of the Eye in
Insects,' Trans. Linn. Sac. (2), ii. p. 389 ; Patten, ' Eyes of Molluscs and Arthropods,'
Mitth. ZiioL Stat. X,'<ij,rl, vi.

986 INSECTS AND ARACHNIDA

be now 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 3) (fig. 73f>). where
e, f represent the image, h the cornea with its ' lenses ' y, 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'-f) 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 diagrammatic-ally
at E (fig. 735), where / 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 1 is the sensitised
[date 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 from
it (most of them slight) in the different 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 fiuid or ' aqueous
humour.' In other instances, again, this space is occupied by a
double-convex bod}*, 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, &c. Besides
their ' compound ' eyes, insects usually possess a small number of
'simple' eyes (termed ocelli or st^nnnntu} seated upon the top of the
head (fig. 731. «, a1 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 optic nerve. Such ocelli ;i Ti-
the only visual organs of the larva' 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 inserts
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 Imlli eyes when they approach
each other so as nearly or quite to meet (as in fig. 731), whilst the
latter will best display otic when the eves are situated more at the
sides of the head. For the minuter examination of the ' rorneiiles,'
however, these must be separated from the hemispheroidal mass
whose exterior they form by prolonged maceration, and the pig-
ment must be carel'nllv washed awav bv means of a fine camel-hair

i «/ *J

brush from the inner or posterior surface. In flattening them out
upon the gl.-i^s slide one of two things must necessarily happen:
either the margin must tear when the central portion is pressed

ANTENNJE

987

down to a level, or. the margin remaining entire, the central por-
tion must lie thrown into plaits, so that its corneules overlap one
another. As the latter condition interferes with the examination
of the structure much more than the former does, it should be
avoided by making a number of slits in the margin of the convex
membrane before 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 lie cut very thin,

and should lie mounted in Canada balsam. The following are s e

of the insects whose eyes are best adapted for microscopic pre-
parations ; Goleoptera, Cicindela, Dytiscus, Melolontha (cockchafer).
Lucanus (stag-beetle) ; Orthoptera, Acheta (house and field crickets).
Locusta ; Hemiptera, Notnnecta (boat-fly); N&wroptera, Libellula
(dragon-fly), Agrion ; Hymenopt&ra,- "VespidsR (wasps) and Apida-
(bees) of all kinds ; Lepidoptera, Vanessa (various species of), Sphinx
ligustri (privet hawk-moth), Bombyx (silkworm moth and its allies) ;
Jjijitfra, Tabanus (gad-fly), Asilus, Eristalis (drone-fly), Tipula (crane
fly), Musca (house-fly), and
many others.

The anteiriiu'. which are
the two jointed appendages
arising from the upper part
of the head of insects (fig.
731, b 6), present a most
wonderful variety of confor-
mation in the several tribes
of insect.-*, 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 afford
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 may pre-
vent us from clearly discerning this relation. Thus among the
Coleoptera we find one large family, including the glow-worm, fire-
fly, skip-jack i\rc., distinguished by the toothed or serrated form of
the antennae, and hence called Serricnru'ut ; in another, of which the
burying-beetle is the type, the antenna' are terminated by a club-
shaped enlargement, so that these beetles are termed Clavicorni" ;
in another, again, of which the Hyd/rophilus, or large water-beetle,

FIG. 736. — Antenna of Mi-ln/<>ittli<i
(cockchafer).

988

INSECTS AND ARACHNIDA

is an example, the antenna? are never longer, and are commonly
shorter, than one of the pairs of palpi, whence the name of Palpi-
cornid is given to this group ; in the very large family that includes
the Lucani, or stag-beetles, with the Scwrabcei, of which the cockchafer
is the commonest example, the antenna? terminate in a set of lea Hike
appendages, which are sometimes arranged 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 Lametticornia ; whilst another large family is
distinguished by the appellation Longicornia, from the great length
of the antenna?, which are at least as long as the body, and often
longer. Among the Lepidoptera, again, the conformation of the
antenna? frequently enables us at once to distinguish the group to
which any specimen 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 longer
than to specify such as are most interesting to the microscopist :
Coleoptera, Brachinus, Calathus, Harpalus, Dytiscus, Staphylinus,
Philonthus, Elater, Lampyris, Silpha, Hydrophilus, Aphodius,
Melolontha, Cetonia, Curculio, Kecrophorus ; Ort/tnptera, Forficula
(earwig), Blatta (cockroach) ; Lepidoptera, Sphiiigida? (hawk-moths),
and Noctuina (moths) of various kinds, the large 'plumed ' antenna-
of the latter being peculiarly beautiful objects under a low magni-
fying power ; Diptera, Culicida? (gnats of various kinds), Tipulidse
(crane-flies and midges), Tabanus, Eristalis, and Aluscida? (flies of

various kinds). All the
larger antenna?, 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 antenna? 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-
tenna? being thus preserved
together when not too
to be discovered in the

PIG. 737. — Minute structure of leattike expan-
sions of antenna of Mi Inluiill/n : A, their in-
ternal layer ; B, their superficial layer.

large.

A curious set of organs is
antenna? 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, sometime.-, nearlv 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 deter-
mined ; whilst to its deepest part a nerve-fibre may be traced. The
expanded lamella? of the antenna- of .l/<'/u/<>, >//><> present a great dis-
play of these cavities, which are indicated In tig. 7-'>7. A. by the

THE MOUTH OF INSECTS 989

small circles that beset almost their entire area ; their form, which
is very peculiar, can here be only made 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 trouble than that
of bleaching the specimen to render it more transparent.1

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 part reduced to a
small number of types or plans, which are characteristic of the dif-
ferent orders of insects. It is among the Goleopfera, 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
[•resent the typical conformation of the mandibulate mouth, which is
adapted for the prehension and division of solid substances; and this
consists of the following parts : 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 manduca-
tion ; 2, a second pair of jaws, termed maxilla', 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 labrum ; 4. a lower lip or labiwm ; 5, one or two pairs
of small jointed appendages, termed palpi, attached to the niaxillse,
and hence called maxillary palpi ; <>, a pair of labial /talpi. The
labium - is often composed of several distinct parts, its basal portion
being distinguished as the mentinn or chin, and its anterior portion
being sometimes considerably prolonged fin-wards, so as to form an
organ which is properly designated the lit/tila-. but which is more
commonly known as the ' tongue.' though not really entitled to that
designation, the real tongue being a soft and projecting organ which
forms the floor of the mouth, and which is only found as a distinct
part in a comparatively small number of insects, as the cricket. This
ligula is extremely developed in the./fy kind, in which it forms the
chief part of what is commonly called the 'proboscis' (fig. 739) ; 3

1 See the memoir of Dr. Hicks, ' On a new Structure in the Antenna? of Insects,"
iu Trans. Linn. Soc. xxii. p. 147; and his 'Further Remarks' at p. 383 of the
same volume. See also the memoir of M. Lespijs, ' Snr 1'AppareiI auditif <les Insectes,'
in Ami. fir* Sri. X<tt. ser. iv. Zool. torn. ix. p. 258 ; and that of M. Clapaivde, ' Sur les
pretendus Orgaiies auditifs des Coleoptt-res lamellicornes et aiitres Insectes,' in Aim.
des Hci. Nat. sfa. iv. Zool. torn. x. p. 236. Dr. Hicks lays great stress mi the 'bleach-
ing proces^ ' as essential to success in this investigation, and he gives the following
directions for performing it: Take of chlorate of potass a drachm, and of water ,-i
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.
Chlorine is thus slowly developed, and the mixture will retain its bleaching power
for some time. For an account of Herr F. Euland's observations see Joiirn. lioij.
Micr Soc. I.ss8, p. 723.

- The labium and the labial palps are, morphologically, a second pair of maxillae
which have undergone more or less fusion of the basal parts along the median line.

•"' 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 parts of this wonderful organ, and for minute descriptions of them, the

990

INSECTS AND ARACHNIDA

and it also forms the 'tongue' of the bee and its allies (fig. 73ft).
The ligula of the common fly presents a curious modification of the
ordinary trachea! structure, the purpose of which is not apparent ;
for instead of its tracheae 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, but are terminated by a set of arches that pass
from one to another (fig. 739, B).1 In the Diptera, or two-winged
flies generally, the labrum, maxilla?, mandibles, and the internal
tongue (where it exists) are converted into delicate lancet-shaped
organs termed setce, which, when closed together, are received into

a hollow on the upper 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 proboscis. Frequently, how-
ever, two or more of these organs
may be wanting, so that their
number is reduced from six to
four, three, or two. In the
Hymenoptera (bee and wasp tribe)
the labrum and the mandibles
(fig. 738, 6) much resemble those
of mandibulate insects, and arc
used for corresponding purposes ;
the maxilla? (c) are greatly elon-
gated, and form, when closed, a
tubular sheath for the ligula, ox
• tongue,' through which the
honey is drawn up; the labial
palpi ('7) also are greatly de-

Fm. 788. — Parts of the mouth of Ajtis
iHt'U/Jicd (honey-bee) : a, mentuni ; 5,
mandibles ; r, maxilla.- ; <1, labial palpi ;
e, ligula, or prolonged labium, com-
monly termed the ' tongue.'

drawn up
also are

veloped. and fold together, like
the maxillie, so as to form an
inner sheath for the ' tongue ; '
while the ' ligula ' itself (e) is a

long tapering muscular organ, 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 i'roboseis of the Blow-rly,' in
Mniithhi MiiTdM-. •linini. i. p. 331, and to Mr. Lowne's treat ise on Tl/e Anatomy
<t ml Physiology <//'//«• Jiln/r-fl//.

'• . \rcordlng to Dr. Anthony (Monthly Mii't'iixi'ii/iirnl Junrii. vol. xi. p. 24'2), these

P eudo tracheae' aiv Mictorial 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, tin- edge-, of this lissnre being formed by the alternating series of 'ear-like

appendages ' ronnreted with the terminal ' arches,' the closing together of which

converts the pseudo-tracheae into a. complete tube. Dr. Anthony considers each of

I In e ear Like appendages to be a minute sucker, 'either for the ail I lesion of the fleshy

in-, or for tin- imbibition of fluids, or perhaps for both purposes.' The point is

\\ell worthy of further investigation.

MOUTH-PAKTS <>F INSECTS

991

'vl\//:/<?7"

FKI. 7SO. — A, tongue of common Hy: a, lolics nf ligula; ?>, portion inclosing
the lancets, formed by the metamorphosis of the maxillaa ; r, maxillary
palpi. B, a portion of some of the pseudo-tracheae more highly magnified.

992

INSECTS AND ARACHNLDA

great distance beyond the other parts of the mouth ; but when at
rest it is closely packed up and concealed between the maxilhe. ' 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 te accumulate upon it
and to ascend along its upper surface, until it reaches the orifice of
the tube formed by the approximation of the maxilla? 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 Lepidoptera, 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 maxilla' are immensely
elongated, and are united together along the median line to form

the haustellum, or
true ' proboscis,'
which contains a
tube formed by the
j unction 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 over 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 A-ni-])/to)i-y.r, one of the
Sphinyidce, 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 opposite side. Each half, moreover, mav
lie ascertained to contain a trachea or air-tube, and it is probable,
from the observations of Mr. Newport, that the sucking up of the
juices of a flower through the proboscis (which is accomplished with
greal rapidity) is effected by the agency of the respiratory apparatus.
The proboscis of many butterflies is furnished, for some distance from

FIG. 740. — Haustellum (proboscis) of Vanessa.

PAETS OF THE BODY 993

its extremity, with a double row of small projecting barrel-shaped
1 ><><lies (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 other 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 suffice.

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 'muscular
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 grasshoppers,
crickets, and locusts, the nature of whose food causes them to require
powerful instruments for its reduction.1

The circulation of blood may be distinctly watched in many of
the more transparent larva?, 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
thoracic 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 lie followed.2 The current enters the 'dorsal
vessel' at its posterior extremity, and is propelled forwards by the
ci infractions 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 either
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

1 The student who desires to carry further the study of the digestive ;q>|niratus
should consult Professor Plateau's memoir, ' Recherches sur les Phenomenes de
la Digestion chez les Insectes,' .!/< '///. Ai-inl. lioij. tie JJrlyiqiu; xli.

2 On the blood-tissue of insects consult Mr. W. M. Wheeler in vol. vi. of the
American journal Psyche.

:-i s

994 INSECTS AND AKACHNIDA

of the J^jilii-mi'i-ti niiu'ijiiixtfi (day-fly), the extreme transparence of
which render* 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-currents
seem rather to pass through channels excavated among the tissues
than through vessels with distinct Avails. In many aquatic larva-.
especially those of the Culicidce (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 freely communicates at
each end with the dorsal vessel. This condition strongly resembles
that found in many Annulata.1

The circulation may be easily seen in the wings of many insect*
in their />n/>« state, especially in those of the Neuroptera (such as
dragon-Hies and day-flies), which pass this part of their lives under
water in a condition of activity, the pupa of A//rion pnf.lla. 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 from the tracheal system
of the body : and it is in a space around the trachea that the blood
maybe seen to move when the hard framework of the nervure itself
is not too opaque. The same may be seen, however, in the wings < if
pupa- of bees, butterflies, Arc., which remain shut up motionless in
their cases; for 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
brought into view by inclosing it (without water) in the aquatic
box. and pressing down the cover sufficiently to keep the body at rest
without doing it any injury.

The ri'x/iii-tttnri/ <i />/><ii'<it>i* 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 limy of a
\rrtebrated animal or the gill of a mollusc, but bv the introduction
of aii- into every part of the body, through a system of minutely
distributed tracheae, or air-tubes, which penetrate even the smallest
and mo.st delicate organs. 'Thus, as we have seen, they pa>s into
t he lui nxti'll a in . or 'proboscis, of the butterfly, and they are minutely

1 See the memoirs on Coretf/zra plumicomis, by Pn>t'c-sM>r i;\mri- Jones, in Trims.
Microsc. Soc. n.s. vol. \\. ISC.T, p. '.ill ; by I'rotessorK. Kay Lankester, in tin- Pai/uliir
Science Revieio for October 1865 ; and by Dr. A. Weismann, in /.dtxcJn-.t'- n-ixa. Zi'>«l.
I'.d. <• \ i. | '. \''. On th<- rnvnhttni'x system o! insects n insult ( '. nilicr, ' U'eber den pr<>
pulsatorisrlien Appavat iler Iiisecten,' .\n-li.fiir niil'r. Aunt. i\. p. 1'_>'.I.

.RESPIRATORY APPARATUS

995

distributed in the elongated labimn or • tongue ' of the fly (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. :md
connected with each other by a transverse canal'

in every segment ;

these trunks communicate, on the one hand, by short wide passages
with the ' stigmata,' • spiracles,' or ' breathing pores ' (a), through

1*1,1 • I I't.T , 1 *1 ^ 1

whilst they give oft branches

lischarged

greatly
being

which the air enters and is (
to the different segments,
which divide again and
again into ramifications of
extreme minuteness. They
usually communicate also
with a pair of air-sacs (/>)
which is situated in the
thorax; but the size of
these (which are only found
in the perfect insect, 110
trace of them existing in
the larva;) varies
in different tribes,
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 oniee : for within
the membrane that forms
their outer wall an elastic-
fibre winds round and
round, 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, however.

there is another membranous wall to the air-tubes, so that the spire
winds between their inner and outer coats. When a portion of one
of the great trunks with some of the principal branches of the
trachea! system has been dissected out, and so pressed in mounting
thar 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 very
beautiful appearance, resembling that of watered silk, is produced

3 s 2

PIG. 741. — Tracheal system of Xt/m i \vatn--
scorpioni : </, head ; b, first pair of legs ; c, first
second pair of wings ; e,
/', tracheal trunk; tj, OIK-

of thorax ;

second pair of legs ;

of the stigmata; /;, air-sac.

996

INSECTS AND ARACHNIDA

by the crossing 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
tracheal 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 encli
margin of the under sur-
face. In most larv.-e.
nearly every segment is
provided with a pair, bur
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 spiracle>
varies greatly in regard to
complexity in different 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

v «/

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-
re.Mvnt growths from the
border of the spiracle, as
in the common fly (fig.
743). or in the I ti/tiscti.-; :
or it may be a membrane
perforated with minute
holes, and supported upon
a framework of bars that
is prolonged in like manner
from 1 lie thickened margin
of 1 he aperture (fig. 744),
as in the larva- of the
Melolontlta (cockchafer). Not iinfrei|iient ly the centre of t he aper-
ture is occupied by an impervious disc, from which radii proceed
to its margin, as is well seen in the spiracle of Ti/mln (craiie-
lly).1 Ill those aquatic larva- which breathe air \\ e often find one

1 Consult Landois and Thiele, ' Der,Tracheenverschlusa lu.'i den Insecti-n,'

m 'Itrifl f. WISS. /.lint, xvii. p. 187.

FIG. 742.— Portion of a large trachea of Dytiscus,
with some of its principal branches.

FIG. 743. — Spiracle of common tly.

RESPIRATORY APPARATUS 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 larvas of the gnat tribe may frequently be observed
in this position.

There are many aquatic larva3, however, which have an entirely
different provision for respiration, being furnished with external leaf-
like or brush-like appendages into which the trachea? are prolonged, so
that by absorbing air from the water that bathes them they may con-
vey this into the interior of the body. We cannot have a better example
of this than is afforded by the larva of the common Uphemera (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 cluster's of delicate hairs which appear to
minister to the same function. In the larva of the Lil>cUnl<i
(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 h.-i thing this surface is ejected
with such violence that the body
is impelled in the opposite direc-
tion ; and the air taken up by its
trachea? is carried through the
system of air-tubes 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 trachea? in
the intestinal folds. FlG 744._Spiraele of larva of

The main trunks of the tracheal cockchafer.

system, with their principal ramifi-
cations, may generally be got out with little difficulty by lay ing-
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 trachea?, to whose
position these branches will serve as a guide. Mr. Quekett recom-
mended the following as the most simple method of obtaining ;i
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 trachea?
may then be well washed with the syringe, and removed from 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 out in the position best adapted for
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 ARACHMDA

liy mounting them in fluid (weak spirit or Goadby's solution), using
a shallow cell to prevent pressure. The finer ramifications of the
trachea! system may generally be seen particularly 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
trachea? full of air (whereby they are much better displayed), if care
be taken to use balsam that has been previously 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 pair of fine scissors ; they should
be mounted in glycerin jelly when their texture is soft, and in
balsam when the integument is hard and horny.

Wings. — These organs are essentially composed of an extension
of the external membranous layer of the integument over a frame-
work formed by prolongations of the inner hornv layer, within
which prolongations trachea? are nearly a 1 \vay.s to be found, whilst
they also include channels through which blood circulates during
the growth of the wriiig and for a short time after its completion.
This is the simple structure presented to us in the wings of Nem-f>-
ptera (dragon-flies, etc.), Hymenoptera (bees and wasps), Diptera
(two- winged flies), and also of many Homoptera (Cicadce and Aphides) ;
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 wings 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 Dipt era such
reunions are rare, especially towards the margins of the wings, and
the areolse are much larger. Although the membrane of which
these wings are composed appears perfectly homogeneous when
viewed by transmitted light, even with a high magnifying power,
yet when viewed by light reflected obliquely from their surfaces
an appearance of cellular areolation is often discernible ; this is well
seen in the common fly, in which each of these areoLe 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 A/>Iii<le.*} exhibit when thus viewed, it is con-
venient to hold the wing in the stage -forceps for the >ake of giving
it every variety of inclination : and when that position has been
found which best displays its most interesting features, it .should In-
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 \\ing should be raised a little above it, its
• stalk ' be.ii ig held in the proper posit inn by a little cone of soft wax,
in the apex of which it may be imbedded. The wings of most
llymenoptera are remarkable for the peculiar apparatus by which

WINGS : S< IUND-OKG-ANS 999

tliose of the same side tire connected together, so as to constitute in
flight but one large wing ; this consists of a row of curved booklets
on the anterior margin of the posterior wing, which lay hold of the
thickened and doubled down posterior edge of the anterior wing.
These booklets are sufficiently apparent in the wings of the common
bee, when examined with even a low magnifying power ; but they
HIV seen better in the wasp, and better still in the hornet. 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
Tim-ilia- or ' clothes-moths,' form very beautiful opaque objects fol-
low powers, the most beautiful of all being the divided wings of
the Fissipennia or •plumed moths/ especially those of the genus
Pterophortis.1

There are many insects, however, in which the wings are more or
less consolidated by the interposition of a layer of homy 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 ForfictiM&ce, 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 Orthoptera (grasshoppers,
crickets, etc.), although not by any means so solidified as those <>f
Coleoptera, contain, a good deal of horny matter ; they are usually
rendered sufficiently transparent, however, by Canada balsam to be
viewed with transmitted light ; and many of them are so coloured
MS to lie very showy objects (as are also the posterior fan-like wings)
for the electric or gas microscope, although their large si/e and ihe
absence of any minute structure pi-event them from affording much
interest to tin- ordinary mieroscopist. 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 'tympanum,' or drum, which is
a space 011 each of the upper wings, scarcely crossed by veins, but
bounded externally by a large dark vein provided with three or four
longitudinal ridges; and of the : file ' or 'bow,' which is a transverse
horny ridge in front of the tvmpanum, 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 texture as those of the
Orthoptera, and possess about the same value as microscopic objects.
differing considerably from the purely membranous wings of the
Cicadce and Aphides, which are associated with them in the order
In the order 'Hemtptera, to which belong various kinds

1 Compare the recently published memoir by 31. Baer, ' Ueber Ban imd Farben
der Pltigelschuppeii bei Tagfaltern,' in Zeitschr.f. n-iss. ZiJol. Ixv. (1898), pp. 50-0",,
as also M. von Li 11 ile ji on the development of the markings, pp. 1-.~>I) of the same volume.

1000 INSECTS AND ARACHNID A

of land and water insects that have a suctorial mouth resembling
that of the common bug, the wings of the anterior pair are usuallv
of parchmeiity consistence, though membranous near their tips, and
are often so richly coloured as to become very beautifxil objects
when mounted in balsam and viewed 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 species are by
no means so interesting, and the aquatic kinds, which, next to the
bed-bugs, are the most common, always have a dull brown or almost
black hue ; even among these last, however, of which 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 smell, whilst, when developed upon the palpi and other organs in
the neighbourhood of the mouth, it ministers to the sense of taste.1

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 fioG? but that
number is subject to reduction ; and the vast order Coleoptera is
su) (divided into primary groups, according as the tarsus consists of
five, four, or three 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 sin >\vs the upper surface made resplendent
by its je\vel-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 liis memoir, 'On a new Organ in Insects,' in Journ. Linn. Hoc. vol. i. 1850,
!>. 1 :•('>; liis ' Further Remarks on the Orpins found on the Bases of the Halteres
and Wings of Insects,' in Tnni^. L/i/ti. Sor. xxii. p. Ill; and his memoir, 'On
certain Sensory Organs in Insects hitherto uude^criliecl,' in Trans. Linn. Soc.
xxiii. p. ISO. Compare also the interesting memoir of Weinland, in Zeitsclir. /•
/ri.ia. Zdol. li. i t.s'.io , pp. :;.-»-K;I>, r> pis.

- See, however, Professor Huxley i \inlt. of J n rrrti ln'iitr Animals, p. 348), who,
rding the ' pnlvillus ' of the cockroach as a joint, find-- the numher to he six.

FEET

IOOI

termed pidvilli (fig. 745) ; and these are beset with numerous hairs,
each of which has a minute disc at its extremity. This structure is
evidently connected with the power which these insects possess of
walking over smooth surfaces in opposition to the force of gravity ;
yet there is still considerable uncertainty as to the precise mode in
which it ministers 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 proved by the
fact that the feet of flies continue to hold on to the interior of an
exhausted receiver; whilsi.
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 pulvilli, as when w<-
remove a piece of adhesive
plaster by lifting it from
the edge or corner. Fliesare

often found adherent to window-panes in the autumn, their reduced
strength not being sufficient to enable them to detach their tarsi.'
A similar apparatus 011 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 (ft) is extremely large, and is furnished with strong
radiating fibres ; a second (b) 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 communications to the Quart. Joitru. Microsc. Sci. vol. ii.
1854, p. 158, and vol. iii. 1855, p. 312. Sec also Mr. Tuffeu West's memoir ' On the
Foot of the Fly,' in Trims. Linn. Soc. xxii. p. 393; Mr. Lowne's Anatomy of
tin- Bloir-flir, H. Dewitz in Zodlof/ixrJirr An:;eiye>; vi. p. 273; and G. Simmt-r-
maclier in ZeitscJir.f. u-iss. Zi'xil. xl. p. 4*1.

FIG. 745. — Foot of flv.

IOO2

INSECTS AND AEACHNIDA

structure, the large suckers being furnished, like the hairs of the
lly s foot, with secreting sacculi, which pour forth fluid through the
tubular footstalks that carry the discs, whose adhesion is thus
secured ; whilst the small suckers form the connecting link between
the larger suckers and the hairs of many beetles, especially (Jurcv-

The leg and foot of the J)yti,scus, 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
Imriiy claws; but each of those of the other segments, which are
termed 'pro-legs,' is composed of a. circular series of comparatively
slender curved booklets, by which the caterpillar is enabled to cling
t<> the minute roughness of the surface of the leaves. Arc., on which
it feeds. This structure 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

•••>*v$

is

" '5K-*r*t' •• ' - ^s;<* -I- c Jo1* j?\ T

^sa*^^^ 'U^j^ftWi

-^M«*

FIG. 746.— A, foot of Dytiwti.s, showing its apparatus of suckers : <t, !>, large
suckers ; c, ordinary suckers. B, one of the ordinary suckers more highly
magnified.

penultimate segments of the abdomen (the eighth and ninth ab-
dominal segments of the larva) into a peculiar organ, which in one
dixision 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. Arc. : the latter of the
sa\\ Hies, gall Hies, ichneumon-Hies. A-c. These tuo sets of instru-
ments are not so unlike in structure as they are in function.-' The

1 See Mr. Lownc, 'On tin- so < -n I led Sucker.-, < if I h/tiwiix and the Pulvilli of Insects,"
in Maiillilii Mil-row. Joiini. \. p. 267.

See Kraepelin, ' 1 ; nt.erstiehuugt'M iilier den B;iu, Mechanismus uu<l Entwicke-
lungsgeschichte der bienenartigen Thiere,' in /.i-itwlir. j. II /.s.s. /.Hoi. xxiii. p. 'Js'.i:
. 'Ueber l!;ui HIM! Kntwickrlung <les St:i< -liels und der Legcselieide,' op.-cit.

STINGS AND OVIPOSITORS IOO3

• sting' is usually formed nt';i pair of darts, beset with barbed teeth
at their points, and furnished at their roots with powerful muscles,
whereby they can be caused to project from 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 near 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 Ichnewmonidce), is simply a long tube, which
is inclosed, like the sting, in a cleft sheath. In the gall-flies
(Cynipidce) 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 lias a peculiarly
irritating effect upon the vegetable tissues, occasioning the production
of the ' galls." \\ Inch are new growths that serve not only to protect the
larva?, but also to afford them nutriment. 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 are often
found upon the surface of oak-leaves are extremely beautiful objects
for the lower powers of the microscope. In the Tenthredinidce, or
' saw-flies,' and in their allies, the Siricidw, the ovipositor is furnished
with a still more powerful apparatus for penetration, by means of
which some of these insects can bore into hard timber. This c. msist -
of a pair of 'saws' which are not unlike the 'stings of bees. Arc..
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 arch formed
by the terminal segment of the body. When a slit has been made
by the working of the sa\vs they are withdrawn into this sheath ;
the ovipositor is then protruded from the end of the abdomen (the
body of the insect being ciirxed downwards), and. being guided into
the slit by a pair of small hairy feelers, there deposits an egg.1
.Many other insects, especially of the order J)ij>l<'i'((. 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 larva* will obtain appropriate nutriment. A remarkable example

xxv. p. 174 ; and ' Ueber Ban und Entwickeliini: des Stachels iler Ameisen,' op. cit.
xxviii. p. .V_!7.

1 The above is the account of the process .uiven by Mr. J. W. Gooch, who has
informed the Author that he lias repeatedly verified tin- st itcmeiit formerly made hy
liim (Science G'«w/<, 1-Vb. 1, 1873', that the et^'s are deposited, not, as originally
stated l>y Reaumur, by means of a tube formed by the coaptation of the saws, but
through a separate ovipositor, protruded when the saws have been withdrawn.

1004

INSECTS AXD AEACHNIDA

of this is furnished by the gad-fly (Tabamts), whose ovipositor is
ci imposed 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 kind of inflammation is set up there,
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-

FIG. 747. — Various i"j-s, chiefly of the Mallophaga (Anoplura).

parts are 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
1hem would be unsuitable to the present work, a reference to a
copious source of information respecting one of their most curious
I'e.-it nres. and to a list of the species that afford good illustrations.
must hen- sullice.1 The c<j</s of not only the class Insecta, but of

1 See the memoirs of M. Lacaze-Dutkiers, ' Sur 1' Armure (icnitaledes Insectes,' in
Ann. it i-n Sri. _\nf. ser. iii. Zool. tomes xii. xiv. xvii. xviii. xix. ; and M. Ch. Robin's

EGGS

IO05

many of the minuter forms of the class Arachnida, 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 are
objects of great beauty, on account of the regularity of their form
and the symmetry of the markings on their surface (fig. 748). The
most interesting belong for the most part to the order Lepidoptera ;
; uid there are few among these that are not worth examination,
some of the commonest (such as those of the cabbage butterfly,

Fici. 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
/•////'/'/), the privet hawk-moth (>'/'// ni.<- fi</H^tri). the small tortoise-
shell butterfly (Vanessa, trrttcce), the meadow-brown butterfly (Hip-
parchla janira), the brimstone- moth (Runt in crntu i//itn\ and the
silkworm (Boml>i/.f mart) may be particularly specified; and, from
other orders, those of the cockroach (Blatta oriental-is), field-cricket
(A<-/"'tif campestris), water-scorpion (Xi'jm ,-/</i/tt/-«), bug (('tin ./•
cow-dung fly (Scatophaga s/cri'n, •«/•/(/). arid blow-fly

Memoire sin- hs'Objets qui pcuri'iit i'trr rtiiisi'iTt's en Prijxirntiinis iiiiri-i>m-<>/ii<fti,'x
(Paris, l.S5(i), which is peculiarly full in the enumeration of the objects of inti-iv-i
afforded bv the class of Insects.

1006 INSECTS AND ARACHNID A

(Jfusca vo/i//fi>t'Hf,).1 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 .sonic
of the best binocular effects.

The remarkable mode of reproduction that exists among the
A/)/tidi's must not pass unnoticed here, from its cm-ions connection
with the non-sexual reproduction of Entoiaostraca and llotifera. 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 summer.
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 period. 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 true
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 A'

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 rather
to the ' gemination ' of other animals and plants than to their sexual
'generation;' for it is favoured, like the gemmation of Hydra, by
warmth and copious sustenance, so that by appropriate treatment the
viviparous reproduction may he 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
performance 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
pel-led female in 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
ill-ones; and others in the ordinary cells, which become workers or
neuters. It has long been known thai 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 unimpregnatecl queens, occasionally lay

1 Compart- K. Li-nckarl, in ArrJiir /. \inil. 18.">:!. p. '.Ml. ' I'eliertlie Micr.ipyle unil
ilcn feint-Hi Bau der Schalenhaut bei den Insectcm-iem,' mid A. Rr;mtlt, I'l-hcr iltm
i'.i unil seine Iti/ilinn/n/i'i/ti , Leip/i^, ls?s.

- 'On the Agamic liepn »l net inn .mil Morphology of l/////.s' iii Trans. Linn.
Soc. sxii. p. I'.i:;. Kor observation-. on American Aphides see various paper-- 1>\
Mr. C. M. NY ( -I'll in Pi/scJit and other American journals.

DEVELOPMENT < >!• INSECTS IOO/

eggs from which eggs none but drones are ever produced. From careful
microscopic examination of the drone eggs laid even by impregnated
queens. Siebold drew the conclusion that they have not received the
fertilising influence of the male fluid, which is communicated to the
queen-eggs and worker-eggs alum- ; so that the products of sexual
generation are always female, the males being developed from these
by a process which is essentially one of gemmation.1

The embryonic development of insects i> 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 larva is produced within the egg. and that b\
which the imago or perfect insect is produced within the bodv 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 micro>copist. The following summary of the
history of the process in the common blow-fly, however, will pro-
bably be acceptable. A ijastraln with two membranous lamella?
having been evolved in the first instance, the outer lamella very
rapidlv 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 lamella^, 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 mam- 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 volk :
of Purpura) having thus been laid up within the body of the
larva, it resumes (so to speak) its embryonic development, its JIMSMI-.
into the pupa state, from which the imago is to come forth, involving
a degeneration of all the larval tissues; whilst the ti»ues 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 Dlptera and other insect.- whose larva- are unfurnished
with legs, their head and thorax being newly formed from 'imaginal
discs,' which adhere to the nerves and trachete of the anterior
extremity of the larva : - and. strange as this assertion may seem.

1 See Professor Sidiold's memoir, On True Parthenogenesis in M'-H/a m/i! I'ces,
translated by W. S. Dallas (London, 1857) ; and his Beitraffe .in- l'n rH/mni/, >

dcr Artli.ropoili-ii (Leipzig, 1871A

- See his ' Entwickelung der Dipter

jteren ' in Zeitschrift f. II i >>-•>. Zijnl. xiii. and xiv. :

Mr. Lowiie's Anatomy of the Bloir-tfy (1st ed.), pp. 6-9, li3-121 ; and A. KowuW^ky,
' Beitriige zur Kenntaisder Naehembryonalen-Entwickelung dur 3Iu-riden,' Zeitschr.
f. Wiss. Ziiol. xliv. p. 542.

IOOS INSECTS AND ARACHNIDA

it has been confirmed by the subsequent investigations of Mi-.
Lowne.1

The Arachnida, or scorpions and pseudo-scorpions, and the J/-<Y-
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 applicable
to these, but have special application in that group known as the
Acarina, consisting of the mites 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
1 laving eight legs instead of six. The A<:«i-imt 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 familial' to every 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 placed
under a magnifying power sufficiently low to enable a large number
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
different substances ; and some of these are extremely destructive.

The Aciirina are the smallest of the .1 rllir<>i>»,l.(i, and are specially
well fitted for microscopical examination ; indeed, with the exception
of the Ixodidce (including the Aryos'um'), which attain a substantial
si/.e, particularly 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 in hot countries than in Europe. 3Lmv
species make beautiful objects for the microscope, and may be well
preserved, the hard-bodied specimens in balsam without heat or
pressure, the soft-bodied in glycerin or glycerin jelly : e.g. the
nymphs of Leiosoma palmacinctu/m, Tegeocranas cepheiformis, T.
dentatus, and the adults of (Uyciplmgns plwnig&r and <!. jn/lniifi-/-
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-tree-.
on fallen oak-wood, in the fodder in stables, and on cellar-walls.
Many of the T>i>mhiiHi<l<> and HydrachnidcK also are very beautiful :
and the Dermaleichi. especially the males, and such creatures as
M i/f>l>i<t, Listrophorus, AT., are extremely curious. With the excep-
tion of the Phytoptidce, all Ai-nr'mti in the adult stage have eight
legs and the constriction between cephalo-thorax and abdomen. is
Car less marked than in insects and spiders — in manv genera it is
wholly lost. The sexe> are distinct and often very different from
each other; the reproduction is oviparous or ovo-viviparous — pos-
sibly in rare and exceptional instances viviparous. The ova are
usuallv elliptical or oval : in those which have a hard shell a
curious stage known as the • deiitov ium exists; as the egg increases
in si/.e the shell splits into two symmetrical halves, which remain
attached to the lining membrane, but are widely separated, the

]{< 'I'm 'in •!• slmiild tic ni.nlr in Professor liiitschli's observations in Morpliol.
li, xiv. p. 17<i, JIM |»L-. Viirlt/liiiw's ]I.I|HT in Arbeit. Zool. Ziinl. lust. Wilrz-
ix. p. 1.

PLATE XX

Ac anna .

West, .Newman cliromo

MITES IOO9

membrane becoming the external covering in the space left. The
eggs of the so-called stone-mite (Petrobia lapidum) 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 Crate-
rium pyriforme ; they are good microscopical objects. The larvae
of all Acarina, except Phytoptus 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 Orthopt&ra ; it
usually undergoes several ecdyses. In many species of the Oribatidce
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, Tyroglyplii, &c.
the nymphs usually greatly resemble the adults ; in the Oribatidce
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 Gfamasidce
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.1 A large oral tube is formed by the ankylosed
maxilla' and probably upper lip and lingua. Up the centre of this
tube the mandibles pass freely ; they are very long and chelate ;
the first 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 whollv 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 ©ribatidce, Tetranychus, the Sarcoptidce,
&c. the mandibles are also chelate, but of two joints only, shorter,
more powerful, and not capable of such great protrusion. In the
Hydrachnidce, Trombidiince, &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 maxilla'
are large toothed crushing organs in the Oribatidce ; they are very

1 Jon >-u. de I'Anat. et dc la PJn/siol. Kobin, May 1870.

3 T

10 10 INSECTS AND AEACHNIDA

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, Myobia, &c. they are anky-
losed to the lip ; in the PJvytopti Nalepa is of opinion that they are
needle-like piercing organs, but these may well be the maxill?e. In
some predatory forms, as Cheyletus, Trombidium, &c.. they assume
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 species 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
(Demodex) to seven (some Trombidiidce and Gamasidce) ; five is the
most usual number. They are terminated by a sucker as in the /v//--
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 Jfyobia has
the claw of the first leg flattened out so as to form a broad lamina,
which curls round the hair and presses it against a chitinous peg on
the tarsus ; Myocoptes has a similar arrangement on the third leg.
Both these genera contain species which are parasites of the mouse,
and easily obtained. In the Oribatidoe, Tyroglyphi, &c. the legs
are all strictly walking organs ; but in Cheyletus, 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
heteropus, 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 different sternal skeleton on the two sides ; the
strangest fact is that it is not always the same side that is thus
developed ; it is usually the left, but occasionally the right. The
integument of the Acarina is almost always soft in the immature
forms ; in the adults it is hard and chitinised in the Oribatidce and
most Gamasidce ; partly so in the Txodidce ; 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 palum-
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 common. In Glydphagus pltimiger they are
elegant plumes ; in some Sarcoptidce, e.g. Symbiotes tripilis, some of
the simple setiform hairs are three times the length of the body :
in the Trombidiidce 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 arc ihcv that C/tei/letus
and some Gamasids, which are predatory and rapture such active
creatures as Tln/f<«ii«i-i<hr, are entirely eyeless, and trust to the
tactile sense only. Haller was of opinion that certain specialised

'V -

'•'.tiC Nevnr.a-.-v

MITES IOIJ

hairs had an auditory function. In the Ixodidw a singular drum-
like structure in the first leg has been considered by Haller and
others to be the hearing organ ; while in the Oribatidce 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-
niata. The Gama&idce, Oribatidce, Tyrogl>/plii<l<i', Sarcoptidce, A:c.
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 Trombidiidce 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 (esophagus, 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 ])<inn< a* i/i'iiif/ifiiftis, the oesophagus 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 ctecal 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 Gamasula; 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 Oribatidce they are absent, their function
being apparently performed by supercoxal glands. The Tyro-
glyphidce, Savcoplulce, Phytoptidfr, Arc. are without special respira-
tory organs ; the Oribatidce and some r i-»ii<nTtt have simple uii-
branched trachea3, much in the same condition as those of r<jri/>ut</x.
The other Gainasidw, the Trombidiidce, Cheyletidce, Ixodidtf. AT.
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 Mipra-
cesophageal and smaller suboesophageal 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
round 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 I.rodes, first by Kramer and afterwards by "Winkler and
Claus ; 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 Arachnida; thus in female Or ibatidce they consist of a central
ovary, with an oviduct springing from near each end, in which the

3 T-2

1 01 2 INSECTS AND ARACHNIDA

eggs are matured ; the oviducts both 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 maybe found in most Gamasidce,
Hydrachnidce, etc., but without the ovipositor. Spermathecae are
often found in the Gamasidce, T-i/roglt/phidce, &c., and accessory
glands frequently accompany the vagina in almost all families. The
male system varies greatly, but is frequently constructed on similar x
lines, preserving somewhat of the ' ring ' form.

The principal families into which the Acarina are divided 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.
Pteroptus and Dermanys&us, 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 with the external
air, but have a long tubular peritreme in the chitin 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 opportunity offers, attach them-
selves to animals by sinking their long serrated rostral projection
into the skin, have a single ventral stigma on each side, com-
municating 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 Art/fix!'/'"
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 iu the acetabula of the
legs. The pseudo-stigmata (hearing organs) of this family have
been before referred to. Oribatida are \egctahle feeders, living in
moss, lichen, fungus, dead wood, under hark of trees, ive.. and some
few species on aquatic plants. They are \\idelv dislrilmted from
the arctic regions to the equatorial. If<>j>l<>iilin,-<t has the power of
withdrawing the legs wholly within the carapace, and then shutting
down the cephalo-thorax against the abdomen, s,> us to close the
opening, when it appears like a ehitinons hall ; from this power it
ha> been called tin- vho\-mite.' The >e\es have not any external

difference.

The Trombididdce are a large and varied group, mostly predatory

PL, AT Hi X.XJ.1.

Ac arm a.

MITES 1013

and with soft, often velvety skins, frequently of scarlet and other
brilliant colours. The large Trombidium Jiolosericum 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. Bryobia is an allied genus found in great numbers on ivy
&c. in gardens and is a beautiful object. The hexapod larva? of several
species of Trombidium 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 Leptus
autwmnalis, and are known in England as the 'harvest-bug,' and
in France as the rouget. The Bdellidce are also included in this
family ; some authors also include the Cheyleti, which, however, seem
to need a separate family, having many curious characters, including
the dorsal position of the male organs.

The Hydrachnidw, 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
fft/drachnidce, but are crawling, not swimming creatures, and are
found in fresh water ; but the Halicaridce, which either constitute
a sub-family of, or are closely associated with them, are marine,
and are much found among Hydrozoa, on which they probably
prey.

The parasitic Myobiidce 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 Acarina, swarming in countless numbers and
devouring hay, cheese, drugs, growing plants and roots, Arc. ; the
genus Gflyciphagus contains many singular and interesting forms, as
G. platygaster and G. Kramer i, 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 Hypopus 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 itc.

The Sarcoptldce are divided into two great sub-families, the Sar-
coptim<>, or itch-mites, of which the well-known Sarcoptes scabiei of man
(Plate XXII, fig. 4) is the type, and the Analyesince, or bird-parasite
mites ; all have soft bodies with finely striated cuticles. Sarcoptes

10 1 4 INSECTS AND ARACHNID A

scabiei is a, minute creature of almost circular form, the female of
which burrows under the epidermis, causing the disease. The mite 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 (Dermaleichi) 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 structures. The species are not always parasitic on one
bird only ; often the same species may be found on numerous birds,
while several 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 Phytoptidce 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 different in other respects, is
Demodex folliculorum, which is found in the sebaceous follicles of
the human 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 AGO/TUB will often be found within.
Similar parasites exist 011 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 niicroscopist by the
spider tribe 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 crustaceans 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 mandilmlate. and is less
complicated than that of the mandibulate insects. The respiratory
apparatus is not trachea 1, 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 a (lord a large surface to the air.

In tin- .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
toot is furnished, have their edges cut into conib-like teeth, which
appear to he used 1 >y the animal as cleansing instruments, and in
many cases lor the manipulation of the silk of their snares. ]>u1 a
lea I ure deserving study by the microscopist is the physical cause of
the exquisite sensitiveness of these 'feet.' By resting these upon a

SPIDERS

IOI5

FIG. 749. — Foot, with comb-like claws, of the
common spider (Epeira).

trap-line of silk carried to her den, she can, by a veritable telegraphy,
discover instantly, not only the fact that there is prey upon her
snare, but 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 communicating with the main lines of the snare,
she can discover in an instant the presence and position of her prey,
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 open 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 scaffolding 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 crowned 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 110 doubt that there is a difference in the silk secreted
by different glands, and its appropriate employment is a part of
the skill of the spicier.

It is certain that the silken threads of a snare are of two kinds ;

B

FIG. 750. — Ordinary thread (A) and viscid
thread (B) of the common spider.

IOl6 INSECTS AND AEACHNIDA

(1) that which rapidly 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,1 and the
habits of spiders offer a good scope for industrious study in the field.2

1 See the work of Kishinouyi in Joitrn. Coll. Sci. Imp. Univ. Japan, vol. iv.

2 See particularly 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.

IOI/

CHAPTER XXII

VEE TEBBA TED AN IMA L S

WE are now arrived at the highest division of the animal kingdom,
in which the bodily fabric attains its greatest development, not ouly
as to completeness, but also as to size ; and it is in most striking
contrast with the class we have been last considering. 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 appropriate course to give under this head a
sketch of the microscopic characters of those primary 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 complete development in this group.1

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 protoplasmic substance, univer-
sally diffused 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,2 the smallest living ' elementary part ' of every organised

1 This sketch is intended, not for the professional student, but only for the
amateur microscopist 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 Sydenhani Society; the
translation of the 4th edition of Professor Frey's Histology anil Histo-Chemistry of
Man; the ' General Anatomy ' of the 10th edition of Quant's Anatomy, 1893. by
Professor Schiifer; and the Atlas of Histology, by Dr. Klein and Mr. Noble Smith.

2 Professor Beale's views are most systematically expounded in his lectures On the
Structure of the Simple Tissues of the Human Body, 18C1 ; in his How to ivork
ivith the MnTiifi<'nj)e, 5th edition, i860 ; and in the introductory portion of his new

1 01 8 VEETEBEATED ANIMALS

fabric is composed of organic matter in two states : the protoplasmic
(which he termed germinal 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 ' elementary 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
different 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 tin-re is no proper
distinction between ' cell-wall ' and ' cell-contents.' This condition,
which is characteristically exhibited by the nearly globular colourless
(•or/iuscles 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 differential/ion 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 7'////.s/f/. ii/icnt Anatomy, lsr.7. Tlic principal results
of the inquiries of (rerman histolo^ist * on this point are well stated in a paper by
Dr. Duffin on ' Protoplasm, mid the Part it plays in the Actions of Living Beings' ill
(Jinn-/. Jim f/i. 'Mimixr. Sri. n.s. vol. iii. Isc.:',. p. -J.'il. The Author feels it necessary,
however, to express his dissent from Professor P.eale's \ lew* in one important particular,
viz. liis denial of ' vital ' endowments to I he ' formed material ' of any of the tissues ;
--ince it ^i-enis In him illogical to designate ei nit rartile milM-nlar (Hire (for example)

as ' dead,' merely because it has not the power of self-reparation.

CELLS 1019

commonly replaced more or less completely by some special product
(such as fat, in tlie cells of adipose tissue, or hemoglobin 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 exhibit in
one view the successive stages of the process.1 Whether 'free ' cell-
multiplication ever takes place in the higher animals is at present
uncertain.

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 different 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 up, 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 fibre-cartilage, in which the so-called ' inter-
cellular substance ' is often, as fibrous as tendon. The difference
between the two types, in fact, seems essentially to consist in this,
that, whilst the segments of ' germinal matter ' which 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 ' lacuna? ' 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 appeal1 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 Zellbildiiny und Zrl/fl/ril inn/, 1880.
See also Dr. Klein's ' Observations on the Structure of Cells and Nuclei' in Quart.
Jouru. Mil-rose. Sci. n.s. vol. xviii. 1878, p. 315, and vol. xix. 1879, pp. 125, 404 ; and
chap. xliv. of his Atlas of Histology. The numerous essays of Flemming, in the
Art-It ic /. inikr. Aunt, from 1875 to 1890 ; Gruber, on the Nucleus of Protozoa, in vol. xl.
of the Zeitschr.f. Wiss. Zool.; and Carney, in La CcUi/Ir, 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 ASCII ris megalocephala in Bull. Acad. Roy. Sci. Selg. xiv. pp. 215-95.

1020 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 pulp, 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,1 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 completely differentiated into
solid bone and medullary cavity as it is in the higher Vertebrata.
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 presented to us in the lower Vertebrata. the whole
thickness is usually more or less • cancellated.' i hat i>. divided up
into minute medullary cavities. When we examine, under a low
magnifying power, a liiiiijilii'liniil »-ction of a lung In me. or a section

' This term is used in its most ;_;eneral scrt-^c, as inchuliiii: not "tily the proper
internal skeleton, Init also the hanl parts pvotertinu- the extovior of the bod}-, which
form the dermal skeleton.

STRUCTURE OF BONE

IO2I

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 with the central cavity, aiidaie 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 apply-
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 in wards,
or in the direction of the
centre of the system of
lings, and the other out-
wards, or in the direction
of its circumference ; and
by the inosculation of the
tubules (or caiialiculi) of
the different rings with
each other a continuous
communication 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 ' caiialiculi ' 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 lacuna? into communication with the walls of the blood-
vessels.

The minute cavities or lacuiw from which the caiialiculi proceed
(fig. 752) are highly characteristic of true osseous tissue, being never
deficient in the minutest parts of the bones of the higher Yertebrata,
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 effect, dependent (like the black-
ness of air-bubbles in liquids) upon the dispersion of the rays by the
highly refracting substance that surrounds them. The size and
form of the lacuna? differ considerably in the several classes of Yer-
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

\3

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 lamellae ; 2, the same, with
the lacuiise and caiialiculi ; 4, portion of the
lamelhe parallel with the external surface.

IO22

VEETEBEATED ANIMALS

microscopic examination of even a minute fragment of it. The
following are the average dimensions of the lacuna?, in characteristic
examples drawn from four principal divisions, expressed in fractions
of an inch :—

Man .

Ostrich
Turtle .
Conger-eel

Long Diameter
1-1440 to 1-2400
1-1333 „ 1-2250
1-375 „ 1-1150
1-550 , 1-1135

Short Diameter
1-4000 to 1-8000
1-5425 „ 1-9050
1-4500 ,. 1-5840
1-4500 „ 1-8000

The lacume of birds are thus distinguished from those of mmn-
mals by their somewhat greater length and -smaller breadth, but

they differ still more in the
remarkable tortuosity of their
canaliculi, which wind back-
wards and forwards in a vcrv
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 lacuna? of the latter
are remarkable for their angularity of form and the fewness of their
radiations, as shown in fig. 75o, 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
lacume in any bone do not bear any relation to the size of the animal

FIG. 752.— Lacuiire of osseous substance
a, central cavity ; l>, its ramifications.

FIG. 753. — Section of the bony scale of Lr/i/,/,i^/,-:is : a, show-
ing the regular distribution of the lacunae and of the connecting
canaliculi ; b, 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 ill the
smallest lizard now inhabiting the earth. Hut 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 • perennibraiichiate '
I'.atrachia. the extraordinarily large size of whose blood-corpuscles
will be present 1\ not iced.

TEETH 1023

Long Diameter SJiort 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

Lepidosiren . 1-375 „ 1-494 1-980 „ 1-2200

Pterodactyle . 1-445 „ 1-1185 1-4000 „ 1-5225 '

In preparing sections of bone it is important to avoid the pene-
tration of the Canada balsam into the interior of the lacuna? and
caiialiculi, 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 fi>st 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 placed on the balsamed surface of
the slide, and being overlain by the balsamed cover, such 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 quick Iv
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.2 The study of the ossein or organic basis (if 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 differences 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 structure 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

1 See Professor J. Quekett's memoir on this subject in the Trans. Microsc. Soc.
ser. i. vol. ii. ; and his more ample illustration of it in the Illustrated Catalogue of
the Histological Collection in the Museum of the Royal College of Surgeons,
vol. ii.

2 Some useful hints on the mode of making these preparations will be found in
the Quart. Journ Microsc. Sci. vol. vii. 1859, p. 258.

1024

VEKTEBBATEB ANIMALS

is surrounded by a. system of tubuli (fig. 755), which radiate into
the surrounding 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 continued 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

; '

\ \4o<rV •• X^..i--j'/7i/>. .A1 P"J<Ii J*K

liHI

-»• /

FIG. 754. — Perpendicular section of
tooth of Lamna, moderately en-
larged, showing network of me-
dullary canals.

Fie. 755. — Transverse section of por-
tion of tooth ofPristis,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
diameter at their largest part averages 1(nVorith of an inch; their
smallest branches are immeasurably fine. The tubuli in their course
present greater and lesser undulations ; the former are lew in number,
but the latter are numerous; and as they occur at the same part of
1 he course of several cont iguous 1 ubes t hey give rise to the appearance
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. r\
tending into them from the central pulp.

T\\o other substances, one of them harder and the other softer

TEETH

1025

than dentine, are frequently found associated with it ; the former is
termed enamel, and the latter cementum or crusta petrosa. The ena,ni>'l
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 ^g^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 prisms,
the form of which usually ap-
proaches the hexagonal. The
course of the enamel prisms is
more or less wavy, and they
are marked by numerous trans-
verse strife, resembling those

of the prismatic shell-sub-
stance, and probably origina-
ting in the same cause — the

coalescence of a series of shorter

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, a),

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-

FIG. 756. — Transverse
Myliobates (eagle
opaque object.

section of tooth
ray), viewed as

of
an

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 110 continuous layer of
enamel over the crown. This

arrangement provides by the unequal wear of these three
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 011 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

O »-*

3 u

Fn;. 757.— Vertical section of human molar
tooth: a, enamel; b, cementum or crusta
petrosa; <; dentine or ivory; <?, osseous
excrescence arising from hypertrophy of
cementum ; e, pulp-cavity ; /, osseous lacunae
at outer part of dentine.

sub-

IO26

VEHTEBEATED ANIMALS

in the embryonic tooth ; and he has further shown that it is much
more frequently present than used to be supposed. The cementum,
or cruftta petvosu, has the characters of true bone, possessing its dis-
tinctive stellate lacuna? 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
resembles 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 surface, until worn away at the crown..1

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 texture, 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
partially 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 different 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

FIG. 758.— Portion of skin of sole, viewed as an texture and composition;
opaque object. 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. Arc. and other • epi-
dermic appendages." In nearly all the existing fishes the scales are
flexible, heing but, little consolidated by calcareous deposit : and in
some specie.- 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

n*

I ', • . S t '}*/&'$ • (.'••'• "

1 The student i-, ivconmiciidi'd to consult Mr. C. S. Toiucs's Manual of Dental

Anatoinn, Human mid < 't<iii[iti i-ti/i rr.

•SCALES OF FISHES

IO27

beneath it. or by tearing off the entire thickness of the skin and
looking for them near its under surface. This is the case, for
example, with the common eel, and with the viviparous blenny ; of
either 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, however, the posterior extremity of each scale
projects obliquely from the general surface, carrying before it the
thin membrane that incloses it, which is studded with pigment-
cells ; 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 their
posterior extremities with comb-like teeth, fig. 759), when dried
with its scales in sittt, is a very beautiful opaque object for the low
l»>\vers of the microscope (fig. 758), especially with the binocular
arrangement. Care must be taken, however, that the light is made
to glance upon it in the most advan-
tageous mariner, 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, appear not to be cells (as they
might readily be supposed to be), lint con-
cretions of carl K mate 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 ,

. . .. FIG. 759. — bcale of sole, viewed

is placed behind it, and the analysing as a transparent object.

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 carp, 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 lamiiiie of a structureless trans
parent substance like that of cartilage; the outermost of these
lamina1 is the smallest, and the size of the plates increases pro-
gressively from without inwards, so that their margins appeal- on the
surface as a series of concentric lines ; and their surfaces are thrown
into ridges and furrows, which commonly ha.ve a radiating direction.
The inner layer is composed of numerous lamina? of a fibrous

1 See his elaborate memoirs, ' On the Microscopic Structure of the Scales and
Dermal Teeth of some Ganoid and Placoid Fish,' in Phil. Trans. 1849 ; and ' Investi-
gations into the Structure and Development of the Scales and Bones of Fishes,' in
Phil. Trans. 1851.

3 u2

IO28 YERTEBRATED ANIMALS

structure, the fibres of each lamina being 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
differ 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 difference, 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 yqiioid 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 (tig.
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 ycii'oe, 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 regular 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 placoid 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 under various 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 Ihe sharks (to which tribe belongs the 'dog-fish
of our own coast) the scales have mure 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
lacuna1. These tooth-like scales are often so small as to be invisible
to the naked eye ; but they a re 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 -hown by the assistance of polarised light.

HAIR IO2t)

A like structure is found to exist iu the ' spiny ra ys ' 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 sharks 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.1

The scales of reptiles, the feathers of birds, and the hairs, hoofs,
mills, claws, and horns (when not bony) of mammals are all epi-
ili'i-tnic 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
difficulty 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 globular 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
manner — 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 period of the growth of the feather, with
the soft pulp, only the shrivelled remains of which, however, are
found within it after the quill has ceased to grow. i

Although the hairs of different mammals differ 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 JenaiscJie Zeitschrift, and vols. ii. and v. of the
Morplioloy. Jahrbitch. A condensed summary of our knowledge, from the more
recent standpoint, will be found in Dean's FisJirs, Liring and Fossil (New York,
1805), pp. 23-0.

1030

VERTEBKATED 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
other 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

B

C

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 med ull a ry substance is made
up. The hairs of the bat tribe
are commonly distinguished by
the projections on their surface,
which are formed by extensions
of the component scales of the
cortical substance: these an-
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 stein ((.!). In the hair of 1 lie peccary (tig. 763)
t he cortical envelope sends inwards a set of radial prolongations, the
interspaces of which are occupied 1 > y the polygonal cells of'1 he medul-
lary substance; and this, on a larger scale, is the structure of the
' (piills ' of the porcupine, the radiating partitions of which, when seen
through the more transparent parts of the cortical shfath, give to

FIG. 762.— A, small hair of squirrel ; 1:5, large
liair of squirrel; C, hair of Indian liat.

HAIR

1031

FIG. 703. — Transverse section
of hair of peccary.

the surface of the latter a fluted appearance. The hair of the ornitho-
rhynchus is a very curious object ; for whilst the lower part of it
resembles the fine hair of the mouse or squirrel, this thins away and
then dilates again into a very thick fibre, having a central portion
composed of polygonal cells, inclosed in
a flattened sheath of a brown fibrous
substance.

The structure of the human hair is
in certain respects peculiar. \Vhen its
outer 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 (C, 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 hair.
The constituent fibres of the substance, which are marked out by
the delicate striae 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

A B

FIG. 764. — Structure of human hair : A, external surface of the shaft, show-
ing the transverse strife 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.

(•ells ; but this ; medullary ' 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 diffused through its substance, but partly also to the
existence of a multitude of minute air-spaces, which cause it to

IO32 VERTEBBATED 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 several 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.1

The stems of 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 pith-like substance
which that sheath includes. In small feathers this may usually be
made very 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 barbs, which are sometimes found to be composed of single
liles of pear-shaped cells, laid end to end ; but in larger feathers
it is usually necessary to increase the transparence of the barbs,
especially when these are thick and but little pervious to light,
either by soaking them in turpentine, mounting them in Canada
lialsam. 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 proximal side have (hick turned-up edges in their median
port ion; ,-is the two sets of barbules that spring from two adjacent
barbs cross each other at an angle, and as each hooked barlmlc of
one locks into the thickened edge of several barbules of the other,
the harbs are connected \ ery lirmly. in a mode very similar to that

1 On the minute structure ol hair, consult Grimm's Atlas der menscJtlichen mid

llnnn (Lulir, 1SSI, It,., v. ith a prefer l.y \V. \Valdeyer).

HORNS, HOOFS, CLAWS

1033

in which the anterior and posterior wings of certain hymenopterous
insects are locked together. Feathers or portions of feathers of
1 lirds distinguished by the splendour of their plumage are 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 Vin 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 rema.rka.ble
character.1

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 remarkable 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
presented in the appendage
borne by the rhinoceros upon
its snout, which in many
points resembles a bundle of
hairs, its substance being-
arranged in minute cylinders
around a number of separate
centres, which have probably
been formed by independ-
ent papilla? (fig. 765). When
transverse sections of these
cylinders are viewed by polar-
ised light, each of them is
seen to be marked by a cross. PIG. 765. — Transverse section of horn of
somewhat resembling that of rhinoceros viewed by polarised light.

starch-grains: and the light

and shadow of this cross are replaced by contrasted colours when
the selenite plate is interposed. The substance commonly but erro-
neously termed irhalebone, 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 R. S. Wray, ' On the Structure of the Barbs, Barbuk-s, and Barbicels of a
typical Pennaeeou* Feather,' in the Ibis for 1887, p. 4'2t>.

1034

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.

Blood. — 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-
puscles. These are of two
kinds: the -red' and the
' white ' or • colourless.' The
red present, in every instance,
the form of a flattened disc,
which is circular in man and
most mammalia (fig. 767), but
is oval in birds, reptiles (tig.
FIG. 766.— Red corpuscles of frog's blood: 766), and fishes, as also ill a
aa, their flattened face; b, particle turned few mamma]s (all belonging to
nearly edgeways ; c. colourless corpuscle ; , i "u \ T

d, red corpuscles altered by diluted acetic the camel tribe). hi the one
acid. 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, fr tj ff t f pressm.e in
represented at «, as they are seen when

rather within the focus of the microscope ; breaking them up. Lhe red
and at b, as they appear when precisely in corpuscles in the blood of

oviparous Vertebral a are dis-
tinguished by the presence of a

central spot or nucleus; this is most distinctly brought into view
by treating the blood-discs with acetic acid, which causes the nucleus
to shrink and become more opaque, whilst rendering the remaining
portion extremely transparent (fig. 766. </). lly examining un-
altered red corpuscles of the frog or neut under a sutliciently high
magnifying pouer the nucleus is seen to be tra\er>ed bv a network
"I filaments, which extends from it throughout the ground sub-
stance of the corpuscle, constituting an int racellular reticulation.
The red corpuscles of the blood of mammals, however, possess no
distinguishable nucleus, the dark spot which is seen in their centre
(lig. 767, A) being merely an effect of refraction, consequent upon
the double coiicaxe form of the disc. When these corpuscles are
treated with water, so that their form becomes first flat and then

BL( JOD-COEPUSCLES

1035

double convex, the dark spot disappears ; 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 sufficiently thick stratum of blood
drawn from 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 th<> 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 corpuscles is not
altogether uniform in the same blood ; thus it varies in that of man
from about the q.7,1,TT)-th to the -., sVoith of an inch. But we generally find
lliat there is an average size, which is pretty constantly maintained
among the different individuals of the same species ; that of man may

be stated at about

of an inch.

The following table l exhibits

Man .
Dog .
Whale
Elephant
Mouse

Golden eagle

Owl

Crow

Blue-tit .

Parrot

Turtle .
Crocodile
Green lizard
Slow-worm
Viper

Perch

Carp

Gold-fish

MAMMALS

1-3200 Camel . . 1-3254, 1-5921

1-3542 l Llama . . 1-3361, 1-6294

1-3099 Javan chevrotain . 1-12325

1-2745 ' Caucasian goat . 1-7045

1-3814 ! Two-toed sloth 1-2865

BIRDS

1-1812, 1-3832
1-1830, 1-3400
1-1961, 1-4000
1-2313, 1-4128
1 INKS, 1-4000

Ostrich .

Cassowary

Heron

Fowl

Gull

REPTILES AXD BATRACIIM

1-1231, 1-1882
1-1231, 1-2286
1-1555, 1-271:;
1-1178, 1-2666
1-1274, 1-1800

Frog

\V;iter-newt
Siren
Proteus .
Arnphiuma

FISHES

1-2099, 1-2824 Pike
1-2142, 1-3429 Eel.
1-1777, 1-2&24 Gymnotus

1-1649, 1-3000
1-1455, 1-2800
1-1913, 1-3491
1-2102 1-3466
1-2097, 1-4000

1-1108, 1-1S21

1-8014, 1-1246

1-420, 1-760

1-400, 1-727

1-345, 1-561

1-2000, 1-3555
1-1745, 1-2842
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 a 1 .-.<>
fig. 768.) Thus it appears that the smallest red corpuscles known
are those of the Javan chevrotain (Tragnlus javanicus), whilst the
are those of that curious group of Batrachia (frog tribe) which

1 These measurements are chiefly selected from those given by Mr. Gulliver in
his edition of Hewson's W'vrks, p. '2:>(5 ft seq.

1036

VERTEBRATED ANIMALS

O

that of man,
the former case

retain the gills through the whole of life ; one of the oval blood-discs
of the Proteus, 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.1
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 white or ' colourless : corpuscles are more readily distinguished

in the blood of batrachians
than in
being in

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
difference 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; a,_ elephant; 3, musk-deer; movecl a little OH the slide,
I. droineilar\ ; ... ostrich : t>, pigeon ; 7, humming-
bird; 8, crocodile ; ;>, python; 10, proteus ; 11, so as to cause the red cor-
perch; 12, pike ; i:-i, 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 different species of vertebrated ani-
mals, the size of the white is extremely constant throughout, their dia-
meter being seldom much greater or less than .t-,MM,th of an inch in the
warm-blooded classes and -, -,',,,, th in 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 vrr\ ink'i-estiiif,' account "f (lie 'Structure ol I he Red Corpusrlrs of tin-
iin 1 1 •/iftirfi/l/t>ii ' has ] tc.cn uiven by Dr. II. D. Schmidt, of New Orleans, in
the Jonni. Roy. Mir rose. Soc. vol. i. 187H, pp. -''7, !>7.

BLOOD-COEPUSCLES 1037

Thus, in their early state, in which 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-
tinuous 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 clearly 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
granules 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 Amcebcu.

When thus moving they pIG. 769.-Altered white corpuscle of blood
engulf particles which lie an hour after having been drawn from the
in their course — such as finger,
granules of vermilion that

have been injected into the blood-vessels of the living animal — and
afterwards eject these in the like fashion.1 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 important observation that the
immunity of certain animals to certain diseases appears to be due to the power that the
white corpuscles possess 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 place more rapidly, and to a greater extent than the growth
and multiplication of the Bacillus, the converse being also true ' (see A. de Bary,
On 1-li/rti'i-iii, English edition, p. 130). The importance of phagocytes is becoming
more and more recognised by the pathologist.

1038 VERTEI5RATED ANIMALS

unless 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, which
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
•upon this, but with its edge just
touching the edge of the drop : for
the blood will then be drawn in by
FIG. 770.-«, blood-cell of a~frog m capillary attraction, so as to spread
the act of engulfing a rod of in a uniformly thin layer between
Bacillus anthracis, observed in the the two glasses. Such thin films

ES^iSLSE.falSSS: ™y be p— ed in ^^ fate

later. (After Metschnikoff ; highly by applying a cover glass and ce-
magnifieil.) meriting 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 sufficiently different from each other, and from those of man,
to allow the nature of any specimen to lie pronounced upon with a
high degree of probability.1

Simple Fibrous Tissues. A very beautiful example of a tissue of
I liis kind is furnished by the membrane of the common fowl's egg;
which (as may be seen by examining an egg 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
oilier forming that lining to it which is known as the nii'iiiln'<ni<i

1 This is a matter which has given vise t<> mm-li discussion union;.; r\]>orts. See
Proc. Imer. Mi< •>: Hoc, xiv. (1898), i>i>. ltl-1'jti

FIBROUS TISSUE

1039

•jni/i/iitinis. Tin- latter may be separated by careful tearing with
needles and forceps, after prolonged maceration in water, into several
matted lamella? resembling that represented in fig. 771 ; and similar
lamella* may be readily obtained from the shell itself by dissolving

v-m-< ?%&t*~3ar-Q&i>%3(
'.^•:^ - ^^tkj&Zszq

rs^jsgg^. 4-41 • ,. ;• >^a
^P^W^ "\.-x-y ; '"**g

jgpyCfig^

FIG. 771. — Fibrous membrane
from egg-shell.

FIG. 772. — Wliite fibrous ti
from ligament.

au.-iy its linn- by dilute acid. The simply fibrous structures of the

body generally. however, belong to one of two very definite kinds of

tissue, the ' white ' and the ' yellow,' whose appearance, composition.

and properties are very different. The white

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 ^-^th

of an inch, which are 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 their direction; and they have a pern-

liar tendency to fall into undulations, when it is

attempted to tear them apart from each othei

(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 bringing into view certain

oval nuclear [(articles of 'germinal matter.'

which are known as 'connective tissue cor-

FIG. 773.— Portion of
young tendon, show-
ing the corpuscles
of ' germinal matter,'
with their stellate
prolongations, inter-
posed among its fibres.

pu>des.' These are relatively much larger, and

their connections more distinct, in the earlier

stages of the formation of this tissue (fig. 77-J>).

It is perfectly inelastic ; and we find it in such

parts as tendons, ordinary ligaments, fibrous

capsules, itc. whose function it is to resist tension without yielding

to it. It constitutes, also, the organic basis or matrix of bone ; for

although the sul istance which is left when a bone has been macerated

sufficiently long in dilute acid for all its mineral components to be

1040

VERTEBHATED 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 lamella?, in which, if suf-
ficiently thin, the ordinary structure of a fibrous membrane can be
distinguished. The yellow fibrous tissue exists in the form of long,
single, elastic, branching filaments, with a dark decided border ;
which are 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 -uVoth and nroinft'h 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 ' ligameiitum nuchai' of quadrupeds,

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 areohc 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 which no distinct fibrous
arrangement is visible. The proportion of the t\vo 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 larger 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.
A-c.. binds together the fat cells into minute masses (tig. 7SD). these
into large ones, ami 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
mechanical fund ion. there is good reason to regard the -connective

FIG. 774. — Yellow fibrous tissue from liga-
mentum nuchre 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 affords
them ; and these must be torn asunder
with needles under the simple micro-
scope, until the fibres are separated to
a degree sufficient 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 effect
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 Fi,-,. 77.~>.— \\-\-\ ical section of skin
into two principal layers : the cutisvcro
or 'true skin,' which usually makes up
by far the larger part of its thickness,
and the 'cuticle,' ' scarfskiii,' or <'/>i-
dermis, which covers it. At the mouth,
nostrils, and the other orifices of rlie
open cavities and canals of the body.
the skin passes into the membrane that
lines these, which is distinguished MS
the mucous membrane, from the pecu-
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, itc. are lined by membranes of a different kind ; which, as
they secrete only a thin, serous fluid 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 thincuticular
layer, which, as it differs 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-
dula? of various kinds ; and in the skin we also find abundance of
nerves and lymphatic vessels. !ts well as, in some parts, of hair

3 x

of finger : A. epidermis, the

surface of which shows depres-
sions « «, between the emi-
nences b b, on which open the
perspiratory ducts s ; at in is
seen the deeper layer of the
epidermis, or stratum Malpighii.
B, cittis rcr«, in which are im-
bedded the sweat-glands d,
with their ducts r, and aggre-
gations of fat-cells/; g, arterial
twig supplying the vascular
papilhe p ; f, one of the tactile
papillse with its nerve.

1 042

VERTEBRATED ANIMALS

follicles. The general appearance ordinarily presented by a thin
vertical section of the skin of a part furnished with numerous sensory
jmpillce 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, supplied with loops of blood-
vessels from the trunk, (/, and a tactile papilla, t, with its nerve
twig. The spaces between the papillae are filled up by the soft
' Malpighian layer,' in, 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 upon the surface, which presents
alternating depressions, a, and elevations, b. The distribution of
the blood-vessels in the skin and mucous membranes, whieh is one
of the most interesting features in their 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 protective, the supply of blood-vessels is
more scanty.

Epidermic and Epithelial Cell-layers. — The epidermis or ' cuticle '
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 are continually wearing off 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 off 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. When 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 iniwosn m .
FIG. 776.— Cells from the pig- The change in form is accompanied by a
mm tn in nit/mm of the eye: change in the chemical composition of the

< teue, whirl, seemstobeduetothemetamor-

nucleus. pliosis of the contents of the cells into a

horny substance identical with that of which

hair, horn, nails, hoofs. Are. arc composed. Mingled with the epi-
dei-mic cells we find others which secrete colouring mailer instead
of horn ; these, which are termed •pigment-cells.' are especially to
l>e noticed in the epidermis of the negro and other dark races, and
are most distinguishable in the Malpighian layer, their colour ap-
pearing to lade as they pass towards the Mirfaee. The most remark-
able development of pigment-cells in the higher animals, however,
is on the inner surface of the ehoroid coat, of the eye, where they
ha\e a very regular arrangement, and form several layers, known as

EPIDERMIS

1043

the pujinentum uti/runi. When examined separately these cells are
found to have a polygonal form (fig. 776, ft), 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,cc). 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 epidermis may be
examined in a variety of ways. If it be
removed by maceration 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
race, the pigment cells will be very distinctly
seen. This under-surface of the epidermis
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 arrangement from tail of tadpole : a a
,.,. , , i • j.1 j_i • i simple forms of recent

which is shown on a large scale in the thick origin; b b, more complex
cuticular covering of the dog's foot, the sub- forms subsequently as-
jacent papillae being large enough rn lie dis-
tinctly seen (when injected) with the naked

eye. The cellular nature of the newly forming layers is best seen
I iy 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 epidermic growths from the
surface of papillae enlarged by hypertrophy.

The epithelium may be designated as a delicate cuticle, covering
all the free intfritnl 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

1044

VEETEBEATED ANIMALS

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 membrane. Their shape varies
greatly. Sometimes they are broad, flat, and scale-like, and their
edges approximate 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 parts has all the characters of an epithelium. In other cases
the cells have more of the form of cylinders, 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
• prismatic1." On the other hand, if the surface on which it rests be
convex, the bases or lower ends of the cvlinders become smaller than

FIG. 778.— Detached epithelium-cells:
a, with nuclei //, and nucleoli c,
from mucous membrane of the
mouth.

FIG. 779. — Ciliated epithelium :
a, ivucleated cells resting on
their smaller extremities; l>,
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 (tig. 77'.'). Ciliated epithelium is
found upon the lining membrane of the air-passages in all air-
breathing Vertebrata ; and it also presents itself in many other
situations, in which a propulsive' power is needed to prevent an ac-
cimiulal ion of mucous or ot her secret ions. < hying to the very slight
attachment that usually exists between the epithelium and the
membranous surface \\hei-eon it lies, there is usually no difficulty
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 vigour: 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 examination, 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 afford, 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 propei- 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 watery fluid, that their contents are
retained without the least transudation,
although these are quite fluid at the tem-
perature of the living body. Fat-cells, when
tilled 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 high refractive power;
but if, as often happens in preparations that
have been long mounted, the oily contents

should have escaped, they then look like any oilier 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 crystals shoot-
ing from a ceiiti-e, 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 tissiies of the
muscles, joints, &c. Small collections of fat-cells exist in the deeper
layers of the true skin, and are brought 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

pose tissue: a a, t';i! fells;
l> b, tibres of areolar
tissue.

1046

VERTEBRATED ANIMALS

in thin cells, so as to take oft' the pressure of the glass cover from
their surface, which would cause the escape of the oil-particles. No
method of mounting (so far as the Author is aware) is successful 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
an ' intercellular substance.' 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-
ferent stages of the growth of any
one. Thus in the cartilage of the
FIG. 781.— Cellular cartilage of external ear of a bat or mouse (fig.

mouse s ear.

781), the cells are 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 evidently
formed by a process of ' binary subdivision.' The substance of these

cellular cartilages is entirely
destitute of blood-vessels,
being nourished solely by
imbibition from the blood
brought to the membrane
covering their surface. Hence
they may be compar-ed, in
regard to their grade of or-
-anisation, with the larger
alga?, which consist, like
them, of aggregations of cells
held togei her by intercellular
substance, without vessels of
f FIG. 782.— Section of the branchial cartilage of any kind, and are nourished
tadpole: a, group of four cells, separating bv imbibition through their
from each other; b, pair of cells in apposi- ', , « rpi

tion ;<-,-, nuclei of cartilage-cells; rf, cavity wh°le Surface. There are
containing tluve cells (the fourth probably many cases, however, in
'"'llind)- which t he st met ureless inter-

cellular substance is replaced

by bundles of fibres, sometimes elastic, but more commonly 11011-
elastic; such combinations, which are termed ///'/-"-cartilages, are
interposed in certain joints, wherein tension as well as pressure has
to be resisted ; as. tor example, between the vertebra1 of the spinal
column and the bones of the pelvis. In examining the structure
of cartilage nothing more is necessarv than to make verv thin

GLANDS IO47

sections, preferably with the microtome. These sections may be
mounted in weak spirit, Goadby's solution, or glycerin-jelly ; but
in whatever way they are mounted, they undergo a gradual change
by lapse of time, which renders them less fit to display the cha-
racteristic features of their structure.

Structure of the Glands. — The various secretions 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 they are to secrete ; and they then seem to deliver
it up, either by the bursting or by the melting away of their walls,
so that this product may be poured forth from, 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 presents 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 FlG- 78S.-Ultimate follicles

„ . , •, . of mammary gland, with

make sections ot these organs thin enough their 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 are 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

VERTEBRATED ANIMALS

are well displayed in injected preparations. For such a full and
complete investigation of the structure 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 displayed 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 structure of the
' muscular fibre ' is very different 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, composed of minuter fibres bound
together by delicate filaments of connective
tissue. By carefully separating these Ave may
obtain the ultimate muscular libre. This fibre
exists under two forms, the xli-iali'it and the
784. — Fasciculus non striated. The former is chiefly distinguished
of striated muscular by the transversely striated appearance which

fibre, showing at a the ^ presents (liir. 7S;">), and which is due to an
transverse strui-, and ,,J ,•' i- i , i

at/, its junction with alternatlon "' ''K1'1 and (1:ll'l< *p:l(>t's along its
the tendon. whole extent; the breadth and distance of

these stria1 vary, houever, in different fibres.

and even in different parts of the same libre. according to their
state of contraction or relaxation. Longitudinal stria* are also
frequently visible, which are due to a partial separation between
the component fibrillse into which the fibre may be broken up.
\\ hen a libre of this kind is more closely examined, it is seen to be
inclosed within a delicate t ubular sheaf h. which is i (iiite distinct on

\>

MUSCLE

1049

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 sarcoleinma, is
not perforated by capillary 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-

PIG. 785.

Striated muscular fibre, separating
into fibril !;!•.

of an

whilst in
inch, and

' teasing

the

the

with

tremesalso are wider; thus

its dimensions vary in the

frog from y^-oth to j^^th

of an inch, and in the skate from ^th to

human subject the average is about ^^jtl:

extremes about ^-^th and ^-^th.

The substance of the fibre, when broken up by
needles, is found to consist of very minute fibrilla3. 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 fibrilla? 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 <v///y<ox///'o// 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. When 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 stripe, 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 tbe

FIG. 786.— Diagram of

striated fibrilla.

IO5O VERTEBRATED 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 strife is closer in the
contracted state, while the breadth (representing the thickness) of
the fibre is correspondingly increased.1 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 fibrilla?. It ranges itself, from
the modern microscopist'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 frog ; 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 fibrilhe is rendered more distinct by
such treatment. The fibrill;e 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 great facility into their component fibrilla?, are
readily obtainable from the limbs of crust; i< v.-i .-md of insects; and
their presence' is also readily distinguishable in the bodies of worms,
even of very low organisation ; so that it maybe regarded as charac-
teristic of the articulated series generally. On the other hand, the
molluscous classes art1, 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 J*ecten. Its presence seems related to energy and rapiditx
of movement, the non-striated presenting itself where the move-
ments are slower and feebler hi their character.

The 'smooth' or unit striated form of muscular fibre, which is

1 (Jinn-/. Jouni. Jl/"/Vn>.sr. ,SVv. n.s. \\i. p. 307. More recent views will be found in
.Mr. C. K. iU.ai'shiillV |iii|>er in vol. \\viii. of the same journal, and in the memoirs
cited by him. The subject is one which will doubtless lon^r occupy the attention of
the

MUSCLE : NERVE

IO5I

especially found in the walls of the stomach, intestines, bladder, and
other similar parts, is composed of flattened bands whose diameter is
usually between auVoth anc^ T^Vo^h °f an 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 unlike ' woody fibre ' in shape (fig. 787, a a) ; each
distinguished, for the most part, by the presence of a long staff-
shaped nucleus, b, 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

FIG. 787. — Structure of non-stria ted
muscular fibre : A, portion of
tissue showing fusiform cells a a,
with elongated nuclei b 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
wTell as by their contractile endowments.

Nerve-substance. — Wherever a distinct nervou.-, system can be
made out, it is found to consist of two very different forms of tissue,
namely, the cellular, which are the essential components of the
ganglionic centres, and tia.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

VERTEBBATEI) ANIMALS

vesicles. The cells, which do not seem to possess a definite cell-wall,
are, for the most part, composed of a finely granular 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 grey 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
Schwami,' 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 pressure ; 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 tube is then found to
be occupied by a transparent substance
which is known as the ' axis cylinder ; '
and there is reason to believe that this last,
which is a protoplasmic substance, is the
essential 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 different nerves, being
sometimes as great as T-!ll(,th of an inch.
and as small in other instances as -loijonth.
In many of the lower iiivertebrata, such as
Meilnxii and Cotnatulce, we seem i'ullv

- iur. iu».— "ureuiuuous nerve- • , •/» i i T • i • ^ • ^

fibres, from olfactory nerve, 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
nervous character, \\hich has been questioned by some anatomists,
seems established beyond doubt.

The ultimate distribution of the nerve-librcs is a subject on
\\lnch there has been great divergence of opinion, and one which can
only be successfully investigated by observers of great .-> pcriencc.

789.— Gelatinous

NEKVE-F1BKES

1053

The Author lielieves that it may be stated as a general fact, that in
both the motor and the sensory nerve-tubes, as they approach 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 fibrilhe, 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 fibrilla? 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 papillary
apparatus, which in the skin may be readily made out in thin
vertical sections treated with solution of soda (fig. 790). It was
formerly supposed that all the cutaneous papillae 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-fibre>.
must have for their special
office 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, c c) of the skin
is occupied by a peculiar
• axile body,' which seems
to be merely a bundle of
ordinary connective tissue.
whereon the nerve-fibre
appears to terminate. The
nerve - fibre.- are more
readily seen, however, in
the • fnngiform ' papilla? of

the tongue, to each of which several of them proceed ; these bodies.
which are very transparent, maybe well seen by snipping off minute
portions of the tongue of the frog; or by snipping off the papilla}
themselves from the surface of the living human tongue, which can
be readily done by a dexterous use of the curved scissors, with no
more pain than the prick of a pin would give. The transparence
of these papilla? also is increased by treating them with a weak
solution of Mida. Nerve-fibres have also been found to terminate
on sensory surfaces in minute 'end-bulbs' of spheroidal shape and
about (i i1, (Fth of an inch in diameter, each of them being composed
of a simple (niter 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 ' Pacmian corpuscles,' which are
best seen in the mesentery of the cat. and are from -j'-.-.th to njth of

Fin. 790. — Vertical section of skin of finger, show-
ing the branches of the cutaneous nerves, a, b,
inosculating to form a plexus, of which the ulti-
mate fibres pass into the cutaneous papillae, c c.

IO54 VEKTEBRATED ANIMALS

an inch long, seem to be more developed forms of these ' end-
bulbs.'

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 best
seen in the Hyla 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 much more 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 (j?l<iti IKHIX
fibres are found in the greatest abundance in the sympathetic nerves;
and their characters may be best studied in the smaller branches of
that system. So for the examination of the ganglioriic 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 orbit of the eyes of fishes, with the ophthalmic ganglion and
its branches, which may be very readily got at in the skate, and of
which the components ma\ be separated without much difficulty,
form one of the most convenient objects for the demonstration of the
principal 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 must 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 aide histologists
who have devoted themselves to it.1

Circulation of the Blood. -One of the most interesting spectacles
Iliat the microsciipist: can enjoy is that which is furnished by the

1 For further information i-c^'iinlin^ (he nervous system the memoir of F. N'aiisen
on 'The St.rnetniv and Combination of the Eistologica] I 'lie incuts of the Centra]
Nervous System' in Mermen's .~\rnxr/i n/.\ AttTSberetning for ISSli llss?:, l>. till, should
be COB lilted. \n excellent summary of the more valuable modern methods of
staining m-r\e Ml ires and cells \\ as u'iven in IS'.I'J to the Knyal Microscopical Society

by Dr. C. K. Heevor. Sec their Journal, is-.iti, p. M>7.

CIRCULATION OF BLOOD 1 05 5

circulation of the blood in the capillar;/ 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 through
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 f 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 can. be laid over the central aperture. The
spreading out of the foot over the aperture is to be accomplished
either by passing 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 troublesome, 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 \\ill 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 is brought
into view on the adjustment of the focus (a power of from 75 to 100
diameters being the most suitable for ordinary purposes), provided
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 uii-
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 few7 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. When 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 pass 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) towards their trunks («•) ; 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 diameters is employed, the visible area is of
course greatly reduced ; but the individual vessels and their contents
b b

a

FIG. 791. — Capillary circulation in a portion of the web of a frog's foot :
a, trunk of vein ; b, 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 very rapid ra.te along the centre of each
tube, the' white ' corpuscles, which are occasionally discernible, move
slowly in the clear stream near its margin.

The circulation may also be displayed in the tout/lie of the froi>-
by laying the animal (previously chloroformed) on its bade, with it*
head dose to the hole in the cork-plate, and. after securing the body
in tin's position, drawing out the tongue with the forceps and tixiiiu
it on the other side of the hole with pins. Ho, again, the circula-
tion may be examined in the /tun/*- where it alford-. ;i spectacle of
singular beauty — or in the /m'w///,-/-// of the living frog l>v laving
open its body and drawing forth either organ, the animal having
pi-e\ iously been made insensible l>y chloroform. The t(((fj)olf of the
frog, when siitlicieni ly young, furnishes a good display ofthe capillary
circulation in its tail : and the dilliculty of keeping it ipiiet during

CIRCULATION OF BLOOD 1057

the observation may be overcome by gradually mixing some warm
water 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.1 The larva of tin' n-(ifrr-n<'n-t. when it can be obtained,
furnishes a most beautiful display of the circulation, both in it-
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'-
action. The circulation may also be seen in the tails of small fish,
such as the minnfm- or the stickleback, by confining these animals in
tubes, or in shallow cell.-, 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 maybe seen
upon the yolk-bag 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
zoophyte-trough. The store of yolk which the yolk-bag supplies
for the nutrition of the embryo not being exhausted in the fish (a>
it is in the bird) previously to the hatching of the egg. this bay
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 distributed over it in copious streams.
partly that it may draw into itself fresh nutritive material, and
partly that it may be subjected to the aerating influence of the
surrounding water.

The tadpole serves, moreover, for the display, under proper
management, not only of the capillary, but of the general circulation ;
and if this be studied under the binocular microscope, the observer
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 bv
-ills alone, and having its circulating apparatus arranged accord-
ingly; but as its limbs are developed, and its tail becomes relatively
>hortened, 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. l )
and at the bases of these, concealed by opercula or gill-flap.-

1 A special form of live-box for the observation of living tadpoles ^e., contrived
by Prof. IP. E. Schulze, is described and figured in the Qinu-f. Joiin/. M/crosc. Sci.
n.s. vol. vii. 1867, p. 201.

3 Y

1053

VERTEBRATED ANIMALS

resembling those of fishes, are seen the rudiments of the intern nl
gills, which soon b3gin to be developed in the stead of the preceding.

FIG. 792. — Circulation in the tadpole.

1. Anterior portion 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 ; b, opercuhun ; c, remnant of external gill
on the right side, turned in.

:i. Advanced tadpole, showing the course of the general circulation: a, heart;
/>, branchial arteries; c, pericardium; d, internal gill; c, first or cephalic trunk;
/', branch to lip; #, branches to head ; h, second or branchial trunk; /, third trunk,
uniting with its fellow to form the abdominal aorta, which is continued as the caudal
artery, A', to the extremity of i the tail; I, caudal vein; in, kidney; n, vena cava ;
a, liver ; />, vena porta> ; if, sinus \ enosus, receiving the jugular vein, ;-, and the ab-
dominal veins, /, n, as also the branchial vein, v.

4 The branchial circulation on a larger scale : A, B, C, three primary branches of
. the branchial artery ; <i, cartilaginous arches ; b, additional framework ; c, c, twigs of
branchial artery; dtf, rootlets of branchial vein.

."). Origin of the vessels of the internal gills, g, from the roots of those of the
e -.lenuil.

6. The heart, systemic arteries, pulmonary arteries and veins, and lungs, in the
adult frog, the heart being turned up in Hie right-hand figure, to show the junction
of the pulmonary veins and their entrance into the left auricle.

CIRCULATION IN TADPOLK 1059

The external gills reach their highest development on the fourth or
fifth day after emersion ; and they then wither so rapidly (whilst
being at the same time drawn in by the growth of the animal) that
by the end of the first week only a remnant of the right gill can be
seen under the edge of the operculum (-2, c), though the left gill
(b) is somewhat later in its disappearance. Concurrently with this
•change the internal gills are undergoing rapid development ; and
the beautiful arrangement 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-
parence 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 paler ; 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 vis 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
arterial trunks which arise on each side from the truncus arteriosus,
b, the first, or cephalic, e, is distributed entirely to the head, running
first along the upper edge of the gill, and giving off a branch, f, 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, A, 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 trunk. 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 Y 2

1060 YERTEBEATED ANIMALS

the spine, where it meets its fellow to form the abdominal aorta, i,
which, after giving ofl' branches to the abdominal viscera, is con-
tinued as the caudal 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, p.
known as the jtortid fcin. which distributes it through the substance
of the liver, a, as in man ; and after traversing that organ it is dis-
charged by numerous fine channels, which converge towards the
great abdominal trunk, or rc/n/ <•/</•<!, n, as it passes in close proximity
to the liver, onwards to the simis venosus, q, or rudimentary auricle
of the heart. This also receives the jinjular nin. 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, n. meet and pour their blood direct into the
sinus A-enosus ; and into this cavity is also poured the aerated blood
returned from the gill by the branchial 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, which 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 pei'iod 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 circulation 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 <>t
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 line
camel-hair pencil the lung may now be turned out, so as to enable
the operator to see a large part of it by I rimxniittnl light. Unpin
the frog and place him on a slip of glass, and then transmit the
light through the exerted portion of lung. Remember that tlieluui;
i> very elastic, and is emptied and collapsed by very slight pressure.
Therefore, to succeed \\ith this experiment, the lung should be
t. niched as little as possible, and in the lightest, manner, with the
liru.sh. If the heart is acting feebly you will see simply a trans-
parent sac. shaped according 1 o the qua nt it v of air-bubbles it may

happen to contain, hut \ oid of red vascular ity and circulation. Hut

INJECTED PREPARATIONS

IO6l

should the operator succeed in getting the lung well placed, full of
air, and have the heart still beating vigorously, he will see before him
a brilliant picture of crimson network, alive with the dance and
dazzle of blood-globules, in rapid chase of one another through the
delicate and living lace-work 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. <;.

Injected Preparations. — Next to the circulation of the blood in
the living body, the varied distribution of the capillaries in its
several organs, as shown 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, FIG. 79:;.— Transverse section of small bites
from those who have made it tine of rat, showing the \illiinsilu,.

a business to produce them

than are likely to be prepared by amateurs 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. 794. — Section of the toe of a mouse: <i, n, u, tarsal bones; b, digital artery;
c, vascular loops in the papilla? forming the thick epidermic cushion 011 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

1 See especially the article 'Injection' in the Micrographic Dictionary; M,

IO62

VERTEBKATED ANIMALS

Many anatomical parts, when well injected and mounted, become
objects of both interest and instruction. This is the case with the
villi of the 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 effectiveness 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

FIG. 797. — Distribution of capil-
laries iu 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 siuface 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 circulating apparatus, however, does the
disposition of the capillaries present more points of interest than it
does in the respirators organs. In bony fishes the respiratory sin-face

Hi. I, hi's work, /'» Mifi-(>xi-«ii<- <•/ i/r.\ J njfrfniiin : I'rof. H. Frey's tre;iti-e, !>ns Mikro-

"clntik; Dr. lieale's Hitir iu «-<>rk with the Micro-

und die mikroskopische '!''•<

scopt ; I lie llninllinitl,- /a /In- ]'/i//s/t>I<if//<-(!/ Ltilniritturij ; and Rutherford's and
Schiller's treatises on Pnictit-nl Ilintalar/i/.

RESPIRATORY ORGANS

1063

is formed by an outward extension into fringes of gills, each of which
consists of an arch with straight lamina? hanging down from it, and
every one of these lamina? (fig. 799) is furnished with a double row
of leaflets, which is most minutely supplied with blood-vessels,
their 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 their
surface, like those of molluscs
and of the larva of the water-
newt, the necessity for such a
mode of renewing the fluid in
contact with them being super-
seded by the muscular 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 Vertebrate,,
the great extension of surface
which is effected ill the latter FIG. 799. — Two branchial processes of the

gill of the eel, showing the branchial
lamellae : A, portion of one of these pro-
cesses enlarged, showing the capillary
network of the lamella?.

by the minute subdivision of
the cavity not being here neces-
sary. In the frog (for example)
the cavity of each lung is un-
divided ; its walls, which aiv
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 trachea!
cartilage form a network over
the interior, that its surface is
depressed into sacculi whose
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 which the blood comes into relation with the air ;
but each air-cell has a capillary network of its own, which lies
011 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 same

FIG. 800. — Interior of upper part of

lung of frcg.

1064

VERTEBKATED ANIMALS

general arrangement prevails ; but the cartilaginous reticulation
of its upper part projects much farther into the cavity, and incloses
in its meshes (which are usually square, or nearly so) several layers
of air-cells, which communicate, one through another, with the
general 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 large 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

FIG. 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.

Fi<;. 802. — Arrangement of the capillaries on the walls of the air-cells of

the human lung.

which lias its own bronchial tube (or subdivision of the windpipe)
.-nid its «\vn system of blood-vessels, which have very little com
inimical ion with those of other lobules. Kach lobule has ,-i central
easily, which closely resembles that of a frog's lung in miniature.
having its walls strengthened by a net \\ork of cartilage derived from
1 he bronchial tube, A, in the inter.-. paces of which are openings le.-ul
ing to sacculi in their substance. l.>ul each of these cavities is .sur-
rounded by a solid plexus of blood-vessels, which does not seem to lie
covered by any limiting membrane, but which admits ail1 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 very favourable to the complete and rapid aeration of the blood
they contain.1 In the lung of man and mammals, again, the plan of
structure differs from the foregoing, though the general effect of it is
the same. For its whole interior is divided up into minute air-cells,
which freely communicate with each other, and with the ultimate
ramifications of the air-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 million*.

1 On the respiratory organs of birds, see Campana, La Respiration tli's Oiseai/x,
Paris, 1875.

io66

CHAPTER XXIII

APPLICATION OF THE MICROSCOPE TO GEOLOGICAL

INVESTIGA TION

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 earth ; 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.' 1 The observations in this paper
were based upon the microscopical examination of thin sections of
rocks and minerals ; still, although Dr. Sorby was the first to apply
this manner of investigation to such objects, the first to suggest and
arrange the method of preparing thin sections appears to have been
William Xicol. 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 Cordier (in
1815) was able to determine the constituent minerals of many rocks
by the study of the powder under the microscope ; a procedure which
Fleuriaii de Bellevue had previously recommended in 1800, and
which is still found valuable for certain purposes. Seven years before
Dr. Sorby's paper appeared, the (Icrman scholar Oschat/ exhibited
a series of thin sections of minerals and rocks and drew attention
to their important bearing upon structural studies, but the collection
\v;is ivg.-mled more as a curiosity than as a scientific achievement.3

That paper, however, gave an enormous impetus to geological
research, and this, iu the hands of English and (ierman students,
led to the growth of a 'micro-petrology.'

In order to examine minerals and rocks, sections must be pre-
pared Iliin enough to permit of the use of transmitted light; for

f. Journ. (,'rol. Sat: vol. xiv. 1H58, pp. 4-":!-."iO(K
- Observations on /•'<«.•,// Vegetables, 'EidinoMigh s^ad London, 1831.

' The history nf I he ;ipplic;ition of the microscope to peolnuy has lieen sketched
liy !•'. Xirkel in iik paper l>ir i'.i iifiiln-n ni/ t/r.v Mikroskajm in ilitu mi ncralogiscli-
i- Xtmliini/, l,eip/i;4, 1881.

MICROSCOPIC SECTIONS OF EOCKS 1067

this purpose they should be from about y^th to r,,1,,,^'1 °f ;in
thick.

A chip about an inch square is struck or cut oft' the specimen to
be studied. One surface of this is then ground down on a flat cast-
iron plate with emery and water. This grinding may be done either 1 >\
hand or by means of a machine specially constructed for this purpose
(Chap. VII).1 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 difficult part 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 point appears to be that in which large air-bubl >1< -s
force themselves through the viscous mass.

A small quantity of the warm balsam is poured upon the slab of
glass, and the smooth surface of the rock -fragment, being pressed into
the balsam, is held down upon the glass till the balsam hardens. The
slab is then examined from its under side to see that no air-bubblc.s
have been included between the glass and the stone. Should they he
present in any quantity, the whole process must be repeated. When
the balsam has quite hardened, the other side of the fragment is
ground down with coarse emery and water on the iron plate. Upon
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 optical work. This transference is effected

1 F. G. Cuttell (61 Camden Eoacl, N.W.), T. Riley (18 Bumfoot Avenue,
Fulham, S.W.), and J. Rhodes, Museum of Geology, Jermyn Street, S.W., prepare
good sections ; and the principal petrological opticians can generally recommend
efficient operators. Voigt and Hochgesang (Giittingen, Rothe Sir. 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 beyond 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 possible
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
beyoml ; it is too thick when the bar cannot be distinctly seen.

IO68 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
wire 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 for a moment over the spirit flame and laid
upon the section. Gentle pressure is then applied 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
or dei- 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.2

In the application of the microscope to petrological and minera-
logical research the employment of polarised light is constantly re-
quired, and various means and appliances are needful for its most
advantageous application, 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.3

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 miiieralogical
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.4

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 maker*) in place of the usual sliding bar.
l!elow the stage, \vhich has neither sliding nor rotatory movements.

• [nnales <1> Chimie <•/ <!<• rin/xii/i/e (5), xx. p)>. :>r>-2— 1:'.'2.

Tschermak's ;UVy/r/vi/m//,s<7/r mnl L'l-tmi/r. Mitt. Bel. v. 1SH2, p. 33.
•"• y\r. .}. Swift, of Tottenham Court Road, Mr. AVatson, of Holborn, London, and
Messrs. Henry Crouch, Limited, make suitable instruments. Those constructed by

/>•! . of .len;i ; N ache), lit 1'aris ; Voigt and I lnrliL:v-.:i MI'. d Gottingen; Fuess, of

and 1 l:irl nark. of l'i>l-,ilam, can ;I!M> lie recom ..... in lei I.
1 The instrument is protected liy letter- putent.

PETIiOLOGICAL MICROSCOPE

1069

is mounted the polariser, B. capable of independent rotation like the
analyser, and upon the tube of the polariser is mounted a toothed

B

Fiu. 803. — Swift's petrological microscope.

wheel of the same size as that upon the analyser ; this wheel years
into a wheel carried by a tube which forms a telescopic extension of

10/0 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

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. Now, in the microscopes
usually constructed for petrologies! 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-thre;ids in
the eyepiece, as the centring of the objective is scarcely ever so
perfect as not to produce some displacement ; and, if the centring
be adjusted so as to be perfect for one objective, it is likely to be
faulty for another. (By a small crystal is meant a crystal under the
, ,,',,- ,-,-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 which 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 marking 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 I\
in position.

The great value of the instrument is in the facility with which
studies in convergent light can be performed, (t is a slide fitted
with a double convex lens which may be used for showing the
optical figures of crystals, and H is a similar slide carrying a lens
and a diaphragm of small aperture used for showing optical pictures
i n minute crystals. The polariser 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 upon the stage. The achromatic con-
denser, A, shown at the foot of the figure, also works in conjunction
with the sliding lei is. A, when the highest angular:! pert lire is required.1

1 In the latest made instruments a new achromatic convergent system is intro-
duced over the polariser. It, jjivcs a N.A. of TOO, and an aplanatic cone O'il'2. When
used as an immersion condenser, these are increased respectively to ri'2 and 1-05.

It is littrd with an iris diaphragm placed above the polarising prism. A milled

collar actuates the focussing of the lower portion of the condenser. The fine adjust-

i'. nl i- the different i.il screu lorm. which is stifliciently delicate and accurate to

determine the refract i\ e index of minerals l>y the difference between the focus taken

CORRODED CRYSTALS 1 07 I

When convergent light is required the slide on the .stage and
either G or H are pushed in, and the eyepiece covered with the
analyser B'. The optical figures of the crystal then appear with
almost ideal clearness. 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 perfect 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
determining by his unaided eye with the use of simple chemical tests
the mineral components of rocks of coarse texture, the case is
different with those of extremely fine grain ; still more with such as
present an apparently homogeneous, compact, or glassy character.
The study reveals facts of the most striking significance, and wel-
come light has been thrown upon the question of the order and
method of formation of rock constituents.1

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 before the arrival
of the fluid mass at the surface of the earth. Such crystals are
usually 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 formed under high
pressure, for the pressure lowers the melting-point of most substances.
Accordingly, as the pressure is relieved upon the lava getting at or
near the surface, the crystals which are floating in the fused mass
at the time are liable to become corroded or redissolved. Again, some
subterranean change may produce a distinct rise in the temperature
of the mass, or an access of heated water may increase the solvent.
power of the molten portion. Instances of corrosion from one or
more of these causes are numerous. The quartzes of the quartz-

through the substance and 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 purpose of show-
ing optical pictures in minute crystals of various sizes.

1 The reader is referred to the following works treating of the microscopical charac-
ters of minerals and rocks : — F. Fouque et Michel Levy, Mintralogie micrographique,
Paris, 1878; E.Hussak, Anleitung zumBcstimmendergesteitisbihlendeii Mineralien,
Leipzig, 1885; E. Kalkowsky, Elemente der Lithologie, Heidelberg, 1886; A. V.
Lasaulx, Elemente der Petrographie, Bonn, 1875, and Einfiihruity in die Gesteius-
lehre, Breslau, 1886 (also edition in French) ; Levy et Lacroix, Les Mineraux r/Vs
Roches, Paris, 1888; F. H. RoaenbMsch, MikrosJcopische Physiographie, 2nd edition,
vol. i. ' Die Mineralien ' (translated into English by Iddings), vol. ii. ' Die miissigen
Gesteine ; ' Hulfstabellen zur mikroskopischen Mvneralbestimmung in Gestenn n
(translated into English by F. H. Hatch); and Elemente der Gesteinlehre, 1898;
F. Rutley, The Study of Socks, 3rd edition, 1884, and Rock-f<ir»i ing Minerals, 1888 ;
J. J. H. Teall, British Petrography, 1888 ; F. Zirkel, LeltrbucJi der Pe trograj >///<•,
2 vols. 2nd edition, 1803; Basaligesteine, Bonn, 1870 ; Die miltroskopische Beschaf-
fenheit der Mineralien und Gesteine, Leipzig, 1873 ; Microscopical Petrography
(U.S. Geol. Exploration of 40th parallel), Washington, 1876 ; A. Harker, Petrology
for Students, 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 voluminous that it is practically impossible 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.

1 072 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

porphyries have this corroded appearance ; whilst the porphyritic
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 usually
small in size, the microscope being frequently required for their
detection and determination.

A glass is sometimes produced in the last stage of consolidation.

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. (ienerallv
speaking, these products are present in two stages of development.
The less perfectly developed forms of these are known as crystallites.
They occur in a variety of forms — hair-like, spherical, etc. — 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 constituent.- and their mutual
interference the usual tests tail ; in other cases they may lie desig
nated embryonic crystals.

The bodies belonging to the higher stage of de\ elopment are ca lied
microlites or microliths (fig. &Q5). They dill'er from the crystallite-
in possessing the internal structure of 1 rue crystals and in acting on
polarised light. The position of the microlites with 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

STRUCTURES OF CRYSTALS

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 rocks nn'crolites are collected into more or
less spherical masses, exhibiting a radial structure, 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 from 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.1

Masses of molten material mav, however, consolidate at a con-

«/ '

siderable depth beneath the surface of the earth ; in such cases the
distinction bet ween the first and second period >
of crystallisation is not generally so well
marked .

A crystal is, in one respect, like an
organism — it is affected 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 rendered another great service, inasmuch as
it has enabled the petrologist to draw conclusions as to the physical
condition of the fused mass or magma 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 fused mass become entangled, which on
cooling remain in a glassy condition, or ' become stony, so as to
produce what may lie called glass- or stone-cavities.' 2 When formed

1 This subject is discussed in Quart. Joan/, (irol. Soc. 1885 (Presidential
address).

2 Sorby, Quart. Journ. Geol Soc. 1858, p. 24-2.

Fm. 800. — Augite showing
zonal structure. (After
Zirkel.)

r ._ -- =i — :

_ - _:

--_--

ir <BB- ~— TT — - _ " __ " - _ "_ »irr_

- - -^r- Hir- _ . _ ^^

:
"

1 1 •_"l~Ti ; ~ - ~_^ ;• H ~_~— - ..

- - * - - " -

H 1- -Z-- ~"~

_

, - -

r " . T-~ - _ .u-^. -frroer a&

_ - - _ -- - _-. .- i

We tsTi? d«h ki-Wi«?r^o more T-oevoiH^

~ r.vfc?- i-rc^-_ii' "- ?oai»? ac«ea.t5?ii-

--

-
-

-

1076 THE MIO - ?E IN GEOLOGICAL INVESTIGATION

salt slowly evapoi-ating. Restoration of the broken angles first takes
place : then deposition goes on over the whole exposed sin face, in
per:- . crystalline continuity, so as To change a broken

fragment into a definite crystal. A similar process frequently takes
place in limestones which are not absolutely }>nre. x times this
•adary deposit is carried so far on the gi-ains of a clean sandstone
that the interstici - ' :y filled up and the rock is converted

into a quartzite.

By the mi. - mination of volcanic- dust o1. - - .t is

possible to determine the constitution ol the igneous mass whose
eruption gave rise to such material. Thus the a.shes and dust which
fell at various places after th- _ I Krakatoa eruption in ISS.'iwere
found to belong to an acid lava, a pyroxene andesite.'-'

Further, glacial boulders can be satisfactorily identified with rocks

- : '.y a microscopical examination of their thin s g Thus

X - _ian rocks have been shown to occur as boulders in the

Eastern (Amnties. while Swedish and Finnish
rocks are common in the drift of Xorth
(.Termany and Saxony.

"We now come to the discussion of the
metamorphism to which all rock--. — - ..re
liable. The metamorphism caused by atmo-
spheric agencies results in decomposition and
disintegration. The constituents are. of
course, very differently affected, but rapidity
of disintegration demands the decomposition
of one of the principal constituents. Such a

with much quan nstituent is felspar, which decomposes under

posited on the surface the influence of water charged with carbonic
(After Dr. Sorby.) :lcid illto 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 affected by these
agencies, and hence are to be found in all clays and sands.3 At
greater depths from the surface disintegration is replaced by the
formation of new, especially hydrous, nrinerals. Thus serpentine
is formed from olivine, and sometimes from suitable varieties of
augite or hornblende ; chlorite from biotite : epidote from suitable
ruinenils. 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 ha.s

J E. Wethered, Qitart. Joitrn. Geol. Soc. xlviii. (1892), p. 877.

1 See J. Murray and A. Renard on 'Volcanic Ashes and Cosmic Dust' in Xature,
vol. xxix. p. 585 : also J. W. Judd, Krakatoa Report, published bv the Roval

M. Hutching, however, is of opinion that rutile is produced as a

•condary mineral in certain slates, though he would not dispute its occurrence as

ated <-}e01. Mag. 1890, p. 264). A series of papers bearing on the subject

which - published since that date in the same periodical are all worthv of

iidv.

SOS. — Sand-grain (A)

•

'

the ]><>r\ •' repUt. /

.-Trt*linf:, giUx-. '•.• rnij>-.

junctio
;\i\<\ l>>
in tl -'allio o,

'

• ner in
probably with

The in*
in flue nee on I
. ounding T..

-
ijrri--

.
nev.

I
.
I
rnev. -aet-

.

. as w are

formed fro. Occasion. ; e as to

es into a brownish glass.

TV- roved most useful in studying question*

.ic metamorphism. or that dne to "earth -stresses/
>n hy movement has .sometimes been .so great as to
obi:' - or even wholly, the original structure of a rock.2

The intense pressures must produce some elevation of tempera-
ture and increase the solvent action of water, so that the original
constituents of the rock are destroyed, partiallyj 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
crashing 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.3 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 conjoir.
produce important changes in rocks, many of which can now be
identified.

1 Bonney, Quart, fourn. Geol. Scr. xliv. (1888 •. p. 11.

2 Tresca, ' Flow of Solids,' Proc.. Inst. Mech. Eiitj. 1878, p. 301.

3 A minute hydrons mica, often called aericite, ^eems to form i-eadilj in an
argillaceous rock nnder preaaure. The ailky-Iooking -ilate<< itn which the name
phyllite is restricted by aome authors! are largely composed of it.

1078 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

The optical methods now in use enable the petrol ogist to determine
the constituents of rock-masses with great success. The colour
of the mineral in transmitted light, the crystallographic outlines,
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. Arc.

Very important service has been rendered by the microscope in
the study of the phenomena known as optical anomalies. There
exist a large number of minerals which show in thin sections optical
properties 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 cause amorphous bodies,
like glass, and crystals belonging to the regular system to become
double-refracting, and a uniaxial crystal becomes biaxial lay 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 crystal-
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
PIG. 809.— Leucite showing twin- traces of twin-lamella? (fig. 809). This
striation under crossed nicols. anoma]y was for a loni, time inexplicable.
(After Zirkel.) .in ~/ . , , , *\

till Ivlein 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 hi.- 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 posses-ed
the optical anomalies described above.

The rela.tion between optical characters and chemical constitu-
tion has received some degree of attention, and in the case of the
felspar group lias been accurately determined. Only the ' quantitative'
portion of the subject can be deal) uith here, and we must abstain
from the discussion of those mineral.- \\hose microscopical appearance
lead- the trained petrologisl to draw qualitative conclusions. |!\
employing convergent light, a slice of a mineral, cut in the right
direction, can he examined and an 'optical picture' obtained.

r ii <lcs< vi|iti,,n ,,(' the so-called ' Erhitzungs-Mikroskop,' sec Groth's PJiysi-

Krystallographie, l,rip/i^, issr,, p. r,:;i.

OPTICAL EXAMINATION OF MINERALS 1079

Inferences may be drawn from the presence or absence of this on the
surface of easiest cleavage in a flake. In a slice from a rock the
minerals may be cut in any direction, and are often too small for
proper study ; nevertheless important inferences may be drawn from
the shadows seen to sweep over them as the stage is rotated between
crossed nicols.1 Even if only parallel rays be used, with the ordinary
apparatus, minerals often may be identified with practical certainty
from their optical characters. Minerals of the regular system, like
colloids, being isotropic,2 produce 110 effect on the polarised rays, and
thus remain dark between crossed nicols. So do all slices cut from
a uniaxial mineral perpendicular to the principal axis (that of
symmetry), for they are isotropic to light passing in that direction.
The same property exists in all biaxial minerals in two directions
(called the optic axes). But in passing through 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 position 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 moiioclinic 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
liasal 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 moiioclinic by its straight extinction.3 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.4 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 illustrations, F. Fouque and M. Levy, Minera-
logic Micrograph i-qiie, 1879, pp. 101-3. Also F. Rutley, Rock-forming Minerals, p. 84.
; That is, having the ether equally elastic in all directions.

3 Obviously, more than one observation is needed, because, as intimated above, a
moiioclinic mineral, if cut in certain directions, also gives straight extinction.

4 Levy, Determination drs Felspatlis (1894) p. 31. Summaries of results will be
round in Rutley, Hock-forming Minerals, pp. 204, 221, and Cole, Aids in Practical
Geology (see ' Felspar' for the references).

IOSO 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
i-ock 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 isomorphous mixtures of albite (Na2(Al2)Si6O16) and
anorthite (Ca(Al2)Si2O8), 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, appeal- to stand out conspicuously on the slide. When 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 lias been already mentioned. It
is not seen in colourless minerals, or in slices so cut as to be isotropic
in the plane at right angles to the path of the transmitted 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, but 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, but 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, quart/., when it occurs
in a granite, usually gives lii»-|i lints, but in a trachyte they are
rather Imv. At first the student must lie cautions in drawing
inferences ('mm polarisation tints, but after a certain amount of
practice he may do this with more confidence, t hough lie will rely more'
on the 'quality' than on the 'quantity' of the colour. For in-
stance, f hough lioth aiigite and olivine usually alt'ord rich colours, an

EXA3IINATION OF MINERALS !O8l

experienced eye can generally tell the difference, for the latter
appeai-s move diaphanous than the fovmev. 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 fragmeiital, 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.1 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 reflect light. Next,
put on the polariser and examine pleochroisni ; and lastly, insert 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,2 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 structure 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 ha've been. It has been employed
by Professor Orville Derby in the determination of the pre-
sence of monazite in Brazilian sands.3 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.

- This method can also be used to enhance a weak pleochroisni.

" Ann-ru-mi J/i/i n/n! <if ,Sr/V//rc, vol. xxxvii. 1880, p. 109.

IO82 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

characteristic of that element. The test afforded by studying the
colour of the flame when a small fragment is acted upon by the blow-
pipe is often valuable — but this, of course, hardly forms part of
microscopy.1

The discovery of the presence of foreign inclusions in all minerals
has led to a remarkable 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 plus 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 mineral and that of its inclusions, the so-called
' heavy solutions ' being employed for the separation.2 Most satis-
factory 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.3 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 adjunct to petrology is to be found in micTO-
chemistry.4 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 evaporate 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 permeable to
aviilin blue, fuchsin, itc. In the case of nepheline the colouring
matter cannot be washed out, and hence • staining' proves a delicate

Whei-e such a course is possible, minute pieces of the question-
able minerals should be isolated and treated singly. There sire two

1 It was suggested by Professor S/abd and is well described in G. A. J. Cole, Aids
in I'rtift/riil (!f'<ifut/i/, I "art ii. ch. viii.

2 For their mode of preparation see Knseubiisrb, M~ikr<>nk<i//isrl/r Physiographic,
p. 201) rf HI'I/. (Knglisli edition by Iddings.)

3 Neues Jahrbuch fur Miiicrd/ni/ir, \T. l>d.. ii. iss-t, p. 172.

4 The following works ca.n be consulted on t.bi-, Mibjert : F,. lioricky, I'.lrnirnti'
1 1 HIT ii i' ii I- ii chemisch-mikroskopischen Mi //mil u ml (Irxtri/ixd/iu />/*<• .Prague, ls?7 :
T. H. I icluvns, M^i/.-riK-lirii/ixi'/if Mi'tlnuJrn ,:///• Miner alanalyse, Amsterdam, 1881;

llanshofer, Mikroskopische liri/rl /niii'//, r.i-aunschwei^, ISS.'i ; Klenieiit et Renard,

Reactions microchimigues <'< 1-rin/n/i.r. >Vc.. l'.ni\elles. issi; ; Rosenbusch, Mikro-
kopische /'////•,/><.//•<//>///>, vol. i. 1.SS5, pp. l(.l."i-'2:;s iKn^lisb edition by Iddings) ;
I1', llutley, line/,- furi/iinij Miitfrii/x, liondon, isss. A iisi't'ul summary of a number
o!' mil loi-lienncal investigations is j,'i\eu by ('. A. McMalion, Miner dlog. Mn ijn -:i in \
vol. \. )). 7'.l.

PALEONTOLOGY 1083

methods in use for testing such particles micro-chemically. The
first is that proposed by Boricky, who employed pure hydro-fluo-
silicic acid (H2SiF6). which attacks almost all rock-forming minerals.
The mineral particle is placed upon a glass object-holder protected
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 crystals
of the silico-fluorides of the metals present in the mineral. The
nature of the crystals may then be determined microscopically.

The second method is that proposed by Behrens. and mostly
follows the usual method of chemical analysis. The isolated particle
i> 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 O0005 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. Generally 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 every 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 lamina? 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
lialxim. 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-
don o us stems may be discovered in such lignites in the utmost
perfection; and the peculiar modification presented bv conifer<niN
wood is also most distinctly exhibited. As we go back, however,
through the strata to the Secondary period, we more and more rarely
meet with the ordinary dicotyledonous structure ; and the lignites of

1084 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

the earliest deposits of these series are, almost universally, either
gymnosperms ' or palms.

Descending into the paleozoic 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. G. Williamson have shown that
they formed a series of connecting links between Cryptogam ia.
and flowering pla,nts, being obviously allied to Equisetacece, L//CO-
podiacece, &c., in the character of their fructification, whilst their
stem-structure foreshadowed both the 'endogenous ' and ' exogenous '
types of the latter.2 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 •.syw/v/m/w, of gigantic L/ycopodiacece of the
Carboniferous flora.3

Lime-secreting algre are now known to have often played an
important part in the formation of calcareous rocks. Those
organisms called coccolitlis and rhabdoliths, which though so
minute are important constituents in chalk and some other lime-
stones, are referred to these plants (? to the class /•Vrv/vVAv), and a
tiny tubular organism naiiu'd ( lirvaiu-lla which occurs in various
palaeozoic and later limestones is now generally regarded as an
alga. According to Mr. E. Wethered 4 it plays an important part
in the formation of pisolitic and oolitic grains. Moreover
calcareous alga?, such as Lithothanmion. arc sometimes important
constituents in Tertiary limestones, as lor instance in the Leitlia-

1 Under this head arc included (lie CycadecB, along with the ordinary Conifcrce,

or pine and lir trilie.

Scr his I nr 1 1 loirs on the coal-plant s published i i 1 1 1 ir Vi >1 11 nil's i if ( lie PlliL TrClllS.,
which arr now beiu^' continued by Dr. D. 11. Scott.

I "i notes iipo thuds to lie employed in inakiii'j preparations of coal, see

Rutlcy, ,S7/(r/// ,,f Bocks, 1SSI, p. 71.

1 Quart. Jowrn. Geol. Soo. slvi. > 1890), p. -270, xlviii. p. ;',77, xlix. p. 236.

MINUTE ORGANISMS AS ROCK-MAKERS 1085

kalk of Europe. They have also been identified in rocks of Secondary
and even of Palaeozoic age. It is an admitted rule in geological
science that the past history of the earth 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 dry land, we can entertain no
doubt that this silicious deposit originally accumulated either at the
bottom of a fresh-water lake or 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 lapse 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
Foraniinifera. some of them entire, othei's broken up into minute
particles (as in the case of the Fasulina limestone of the Carboni-
ferous period, and the Xnnmn/Titie 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
remains 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 parts of the shores of Australia, as Dr. Carpenter was
informed by Mr. J. Beete Jukes, thus affording the exact parallel to
the stratum of Orbltolites (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. Williamson1 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-
tomacea? (silicious), eight species of Foraminifera (calcareous), and a
miscellaneous group of objects (fig. 810). consisting of calcareous and
silicious spicules of sponges and G'orgot/iir. 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 Diatomacese, 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 Globigerince ; and this fact, first
determined by the examination of the small quantities brought, up
by the sounding apparatus, has been fully confirmed by the results of

1 Memoirs of the Manchester Litcranj and Philosophical Society, vol. vii.

1086 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 whose thickness cannot
be even guessed at. ' Under the microscope.' says Professor "WYville
Thomson l 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 oryanisrns in Levant, mud: A, C, D, silk-ions
spiculcs of '/>/////«.; B, H. spiculcs of (ifmTni; ]•}, calcareous spiculc of
< : rxiili/i ; F, G, M, O, portions of calcareous skeleton of l-'.rhinoflermatu \
I, calcareous spicule of i •<>!•</, mln ; K, L, N, silicions spiculcs of s
P, portion of prismatic layer of shell of PiiDia.

of f'rag-

of entire shells of Globigerina l>nllt>i(1<>«. large and small, an . ..,

incuts of such shells mixed with a quantity of amorphous ndcaivoii>
matter in fine particles. ,-i little line sand, and nianv spirule,-. pm-1 ioio
of sjiiciiles, and shells of Hml'inlii ,•',«. .-, |',.\\ spicules of spmiges. and a

few frustules of diatoms. Below the surface-layer the sediment lie
comes gradually more compact . and a slight grey colour, due proliaUv

i The Depths of the Sea, y. no. s(.<- also \'n//,if/r ,,i choUnnjo-, di. iii., and
ger Reports, e&peci&Ui Deep Sea Deposes (Murraj and

MINUTE ORGANISMS AS ROCK-MAKERS

I08;

to the decomposing organic matter, becomes more pronounced, while
perfect shells of Globigerina almost disappear, fragments become
smaller, and calcareous mud, structureless, 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 Globigerina ; 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.
Now the resemblance which this Globiyerina-m\.u\. when, dried, bears

FIG. 811. — Microscopic organisms in chalk from Gravesend : a, b, c, d,
Texinlaria . globulosa : <•, e, e, h'otnliii aspera : f, Textularia »<•///, ;,ta;

ifi l/<:ras; )

to chalk is so close as at oner to suggest the similar origin of the
lattev ; and this is fully confirmed by microscopic examination. For
many samples of it consist in great part of the minuter kinds of
Foraminitera. especially Globigerince, whose shells are imbedded in a
mass of apparently amorphous particles, many of which, nevertheless,
present indications of being the disintegrated fragments of similar
shells, or of larger calcareous organisms. In the chalk of some
localities the disintegrated prisms of Pinna, or of other large shells
of the like structure (as fnoceramus), form the great bulk of the
recognisable components ; whilst in other cases, again, the chief part
is made up of the shells of f 'i/t/n-rina. a marine form of entomo-
stracous crustacean. Different specimens of chalk vary greatly in

I08S THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

the proportion which not only the distinctly organic remains bear to
the amorphous residuum, but also the different 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 Gflobigerina-m\id, that the amorphous constituent of chalk like-
wise is the disintegrated residuum of foraminiferal shells, or at any
rate of some small calcareous organism. But, further, the Globig&rinar
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 extraordinarily close resemblance, alike in structure

FIG. 812. — Microscopic organisms (chie&j foraminifera) in chalk from Meudoii,
seen partly as opaque, and partly as transparent objects.

and in external form, to the Y<-»tric(dites which are well known as
chalk fossils, as to leave no reasonable doubt that these also were
silicious sponges living on the bottom of the cretaceous sea. Finally
(as was lirst pointed out by Dr. Sorby) the coccoliths and eocco-
*pheres at p resent found on the sea-bottom are often to be discovered
l>v the microscopic examination of chalk.1 All these correspondences
show that the formation of chalk took place under conditions
essentially similar to those under which the deposit of Globiyerina-
irmd is being formed over the Atlantic sea-bed at the present time.
In examining chalk or other similar mixed aggregations, whose

1 'On the Organii Origin of the so-called "Crystalloids" of Chalk' in Ann. Nat.
Hist. ser. iii. vol. viii. iMIil, pp. l!l:!-'200. Murray and Knurd, I >ii /> ,SV<i Deposits
(Challenger Jlrporta), p. 257.

CHALK, FLINT, AND CHERT 1089

component particles are easily separable from each other, it is de-
sirable to separate, with as little trouble as possible, the larger and
more definitely organised bodies from the minute amorphous particles ;
mid the mode of doing this will depend upon whether we are operat-
ing upon the large or upon the small scale. If the former, a quantity
of soft chalk should be rubbed to powder witli water by means of a
soft brush ; and this water should then lie proceeded with according
to the method of levigation already directed for separating the
Diatomacece. It will usually be found that the first deposits contain
the larger Foraminifera, fragments of shell. iVrc.. and that the smaller
Foraminifera and sponge-spicules fall next, the fine amorphous par-
ticles remaining diffused through the water after it has been standing
for some time, so that they may be poured away. The organism*
thus separated should be dried and mounted in Canada balsam. If
the smaller scale of preparation be preferred, as much chalk sera} MM!
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 lew 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 structure of flints such
chips as may be obtained with a hammer will commonly serve very
well, a clear translucent flint being first selected, and the chips that
are obtained being soaked for a short time in turpentine (which in-
rivases their transparence) ; those which show organic structure,
whether sponge-tissue or xanthidia, are to be selected and mounted
in Canada balsam. The most perfect specimens of sponge-structure,
however, are only to be obtained by slicing and polishing.

The study of thin slices of flint and chert during late years
has thrown much light 011 their origin and on the structure of
fossil sponges. Spicules are often found to be extremely abundant
as in the chert (Upper Greenland) from the quarry by Ventnor
station (Isle of Wight), where they can be detected by the naked eye.
The radiolaria from the Tertiary marl of Barbadoes have long been
known to mieroscopists, but these organisms more recently have been
detected in cherts. In Britain such cherts have been described
from the Ordovician rocks of Mullion Island. Cornwall, and of south
Scotland, and the Carboniferous of south-west England.1

There are various 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 ; 2 and the
presence of these may be largely recognised, by the assistance of the
microscope, in sections of calcareous rocks of various dates, whose
other materials were fragments of corals, crinoid-stems, or the shells
of molluscs. In the formation of the Coralline Crag (Tertiary) of the
eastern coast of England, polyzoarie.s had the greatest share ; but

1 On the former subject see G. J. Hiude, British 3I«scinn Catalogue of Fossil
Sponges; on the latter, the same, Quart. Journ. GeoL Soc. vols. xlvi. xlix. li.

- For illustrations of fossil foraminifera, see Carpenl er, Introduction t<> Study of
Foraminifera (Ray Society), and the publications of the Palaeontographical
Society: Cm a Foraminifera (T. Rupert Jones, &c.) : Carbni/lferoKn and Permian
l-'iii-dininifi ni (H. B. Brady). The series also contains volumes upon the Crag
Polyzoa and various small Entomostraca of different ages.

4A

1090 TE:-: mci - :z :y -i AL

E i riary Iime>- : which Paris is chiefly btiilt consists almost

f the s - I is thus known as mill

(mi - - - - !:. the -- ~tratum of nurumiilitic 1:

- - ' - - -

:x»scopr - - thai 'le matrix in which the 1 _

- :-e nomimi- ~ - - ioibe- - hself composed of oomrm'n ••-

rng sL - f 1 -•.-.- _ •- "ler with minuter

r Lfer Sii _ sms, with fragments of crin

moflusca. cent. .re abundantly p: - in the

- Ln this " those of S _ _

• »--.«,

Europe, as 5 in I - ther I :c- lime-

- - sequent charu 3 red the

- . : • . - . >tituents in 1st _ - ble. T:_'>

_ - • is of Russia 1 ads f limestone of

- 3 rram fifteen inches- to ir .\nd

frequentlv rej - ~ -rough a pth of two hundred :

liic-h are ahnost entirely composed of the
_ - Again, - - f the Carl :rix>us

lim- - eland which have undergone le st. -~ rrbance can be

plai: - nofn_ - - - - - -

-le reir - : Fr-raminif-rifi. Poly: _ fcs of corals.

~. unfrequently har pens, - of this liL -

^e^e ai L to be loaded with • n_i

us kin«is. particularly Foraminif : which

~ • - present ~. - lUtiful pol

knowTi - -corals.

Mention has been already made of P: fess Elrenberg's
remarkable disco very that a large proportk>: - - - ' st : the
yr**H sand* which present themselves in various stratifie<i deposits,
from the Silurian period to the Tertiary era. and in that ci
the TTpper Greensand. is composed of the casts of the interior of
minute shells of Foraminifera and Moflusca. the shells themselves
having entirelv disappeared. The mineral material of these casts
has not merely filled the chambers and their communicating

• — . - * LH.S uL>> 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 entirelv of

• internal casts " of existing Foraminifera.1

It is. however, in the ca.-* of the teeth, the bones, and the dermal
-

me? most apparent : since their structure presents so many

characteristics which are subject to well-marked variations in their

*-veral classes, orders, and families that a knowledge of these

-icters frequently enables the microscopist to determine the

:?r Report* : Deep Sta Dcpaut* < UnrraT and "Rfnintl. p- 3~8.

-. - - -•--:- :-:.::-:/:-

DETEIOII: - - :. . . ;-. .

nature of even the ij. .mentary specimen.-. J* was

th that * ability of * terminal

clear by the lab J

following may b~ .
fon. "

elia: - - - . _ ~ . either to the ( -

Bed £ stone of thi* . . I whos- :~ -

-uch that T . JTicr

se from - .- - - :

the only L • - - .

practical imp f the i

mi". - - - t under.: no reasonable

.hope of coal couLi - -rmination of the

B iiic remains whic. ~ . but nnfortun; :

few and fragment . hicfa

- G -

ed, Proml - . _

- -

- _• --her wifh their form.

- - .- -

tha* - _ i -au-

i-ian T- in which

-• -t would

have been considered as
Red : but micro-

ic examination of

. intimate structure
unmistakably proved

them to belong -
genus of fi-sh-r-
dus) which Ls exclusively
palaeozoic, and thus de-
cided that the formation
must be Old Red. So.
again, the microseopi<
examination of certain frs
_::- di^-losed A

ff t-

FIG. 813. — See!

(shown
that ha.
and the
teeth I
certain

been discovered in the

> ascertained to exist in cer
Keupersandstein : "

that the "Warwickshire and "W

Mieof

icture
^th

~.

•^Lch these
*e ahnost
Hies were

--

to which these teeth belonged,
characters merely, to the oni
clear that they were gigantic
points of relationship to Cft
which shows a similar, thonsh

-

(the Australian

•

tmettire)

external

t is now*

4 A 2

1092 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

The researches of Professor Quekett on the minute structure of
bone l have shown that from the average size and form of the lacunae,
their disposition in regard to each other 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 Professor Quekett.
who informed him, on microscopic evidence, that they might certainly
be pronounced reptilian, and probably belonged to an animal of the
tortoise 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 011
considerations derived from the very imperfectly preserved external
forms of these fragments, was called in question by some other
palaeontologists, who thought it more probable that these bone-;
belonged to a large species of the extinct genus Pterodactylws, 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, Avas at that time known ; and the characters
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 from that of every known bird, that no one Avho placed
much reliance upon that evidence could entertain the slightest doubt
on 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 Avas finally set at rest, and the
value of the microscopic test triumphantly confirmed, by the discovery
of undoubted pterodactyle bones of corresponding and even of greater
dimensions in the same and other chalk quarries.

The microscopic examination of the sediments no\\ in course of
deposition on various parts of the great oceanic area, and especially
of the large number of samples brought up in the ' Challenger 'sound-
ings, has led to this very remarkable conclusion — that the detritus
resulting from the degradation of continental land-masses is not
carried far from their shores, being entirely absent from the bottom
of the ocean-basins. The sediments there found were not of
organic origin, hut mainly consist of volcanic i/Ma-ix and of clay that
seems 1o have been produced by the disintegration of masses of very

See ins iiiciiiciir (Hi the ' Comparative SI nictmv nt I'.mie ' in the Trans. Microsc.
er. i. \ol. ii.; and the ('<itti/n(/in- uft/n- Ilinfii/uf/ii-ii/ Miixritiii uf fhr R<>>/. Coll.

i,i' Surgeons, >»1. ii.

OllIGIN OF OCEANIC AREAS 1 093

vesicular lava, which, after long floating and dispersion by surface-
drift or 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 or coral atolls,
this almost universal absence of any trace of submerged continental
land over the great oceanic area affords strong confirmation to the
belief that the sedimentary rocks which form the existing land were
deposited in the neighbourhood of pre-existing land, whose degrada-
tion furnished their materials ; and suggests that the original
disposition of the great continental and oceanic areas was not very
different from what it now is.1 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
<|tiestioiis 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. Soy, Geog. Soc. July 1879 ;
and for detailed results ' Preliminary Report of Cruise of " Challenger " ' (Wyville
Thomson), Proc. Roy. Soc. vol. xxiv. (1876) p. 463, and ' Challenger ' Reports (Murray
and Renard), Deep Sea Deposits, p. 327.

1094

CHAPTER XXIV

MICROCRYSTALLISATION. OPTICAL PROPERTIES OF CRYSTALS.
MOLECULAR COALESCENCE. MICRO-CHEMICAL ANALYSIS.

ALTHOUGH by far 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 recent years to the
use of the microscope in elucidating the internal structure of
crystalline substances, whether 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
internal structure of the crvstal. This homogeneity of structure

«/ O v

consists in 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
nients of points in space have been investigated by Bravais, Sohncke.
.nid others,1 and on classifying them according to their symmetry
they fall into thirty-two classes identical with the thirty-two known
t-v\ stalline systems. These thirty-two types of structure differ in
i heir symmetry, and this difference is expressed in the symmetry of
the external form ; the external form, however, is very liable to
< list i >i -1 inn. in consequence of a lack of uniformity in the conditions
prevailing during the growl li of the crystal, and so is at best but an
untrustworthy guide to the symmetry oft he internal .structure. The
^>|it ical properties of the solid structure, also themselves expressions
of the symmetry, and consequently of the crystalline system, are
no! 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 structures in accordance with the symmetry

1 Sec A. SrliiM'iillirs, KriinlnU^i/ati'iiif n ml l\ i->ixtullnt rn<-t u r, Lrip/ij,', 1S1H.

FORMATION OF CRYSTALS

1095

of arrangement of the structural units gives rise to the phenomena
of double refraction, circular polarisation, pleochroism, &c., observed
with crystalline bodies. The important results to be anticipated
from the microscopic examination of crystalline preparations such as
rock sections, etc., was pointed out by H. C. Sorby in 1858 ; the micro-
scopic methods as at present applied to pure crystallography have
been fully described by P. Groth l and by Th. Li ebisch,2 whilst their
Applicability to the identification of the crystalline constituents of
rocks has been exhaustively treated by H. Rosenbusch.3

The study of crystalline materials in such minute crystals as are
appropriate subjects for observation by the microscope is not only
t\ very interesting application of its powers, but is capable of
affording some valuable hints to the designer. This is particu-
larly the case with crystals of snot/:, which belong to one of the
* hexagonal systems,' the basis of every figure being a hexagon of
six rays ; for these rays ' become incrusted with an endless variety
of secondary formations of the same
kind, some consisting of thin lamina •
iiione, others of solid but translucent
prisms heaped one upon another, and
others gorgeously combining lamina'
and prisms in the richest profu-
sion,' 4 the angles by which these
figures are bounded being invari-
ably 60U or 120°. Beautiful ar-
borescent forms are not imfrequently
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 other crystalline rocks, also of
•agate, aragonite. piedmontite, the zeolir.es, and other minerals, are
very beautiful objects for the polariscopo.

The actual process of the formation of crystals may be watched
under the microscope with the greatest facility, all that is necessary
being to lay on a slip of glass, previously warmed, 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 upper edge, where the pro-
portion of liquid to solid is most quickly reduced by evaporation, and
Avill gradually extend downwards. If it should go on too slowly,

FJI;. 814.— Crystallised silver.

1 Physikalische K r ij st all a g rapine, Leipzig, 1895.

'-' Grumlriss der physikalischen Erystallographie, Leipzig

1896.

Microscopical Physiography of the Rock-maJcing Minerals, London, 1895.
4 Glaisher on ' Snow-crystals in 1855,' Quart. Journ. Micros/-. Sci. vol. iii. 1855,
1>. 179. See also C. A. Hering, Zcita.f. Krijst. Bd. xiv. 1888, p. 250.

1096 MICROCEYSTALLISATION, ETC.

or should cease altogether, whilst a large proportion of the liquid
still remains, the slide may be again warmed, so as to re-dissolve
the part already solidified, after \drich 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 l 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 dark ground, by
the use of the spot lens, the paraboloid, or any other form of black -
grouiid 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,2
who observed that precipitates first separate in the form of very
minute liquid globules, and that these subsequently coagulate to
form an undoubtedly crystalline precipitate. Later investigation
of the subject by Fraiikenheiin, and then by Vogelsang,3 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 very 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 crystallites. The
latter subsequently arrange themselves in rows as mar<j«rH <•.-<.
several of which then amalgamate, forming longulites, and the
process of aggregation proceeds until at last the crystalloids — the
first product in which the structure of the crystal itself is traceable
—are obtained. The separate existence of so many transition forms
has been disputed, notably by Behrens ; 4 but their mention serves
the purpose of indicating that the formation of crystalline bodies is
really an operation of considerable complexity.

Upon the temperature 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 centres 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 5 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 M,>Jr/;it/(tr/i/i//iiil>, 2 vols. Leipzig, 1888 anil 188!).

- /'or/,/. Ann. lid. xlvi. is:;;), p. -j.-.s. z ]),<• Kr//xt(iUitr)i, Bonn, 1875.

1 Die Kri/Ntitl/i/ni, Kiel, 1S7-I.

•' See hi-; p:iper ' < )n the Crystallisation at various temperatures <>f tin1 Double
Salt, Sulphate <>!' IMa^nesiii ami Sulphate of '/AM,' in Quart. Joi/rn. Microsc. Sci. u.s.
vi. pp. 1I-S7, 177. See also H. N. Draper on 'Crystals for the Miero-polariseope,'
in Intellectual OLscrrrr, vol. \i. lsc.r>; p. 437.

OPTICAL PROPERTIES OF CRYSTALS

1097

fig. 81(3. Mr. Slack has shown that a great variety of spiral and
curved forms can be obtained by dissolving metallic salts, or salicin,
santonin. Arc., 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 drv upon a slide;
the result of which will be the production of a complicated series of
cracks, many of them curvilinear. When a group of crystals in for-
mation tend to radiate from a centre, the contractions 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 rings, when it
is illuminated with Powell and Lealand's modification of Professor

FIG. 815. — Radiating crystallisation of santonin.

Smith's dark-ground illuminator for high powers, and viewed with
a gth 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.1 Very 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. O. Leh-
nianii 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
following 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 the preparation of Crystals for the
Polariscope,' in Monthly Microsc. Journ. v. p. 50.

1098 MICKOCRYSTALLISATION, ETC.

the development of colour. The substances marked d are distin-
guished by possessing the curious property termed pleochroisin.
which was first noticed by Dr. Wollastoii and carefully investigated
by Sir D. Brewster. This property, to which was previously applied
the misnomer dichroism, consists in the exhibition by these crystals
of colours varying with the direction in which they are examined :
thus, the cube-shaped crystals of magnesium platinocvanide 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 passing
into the crystal is decomposed, are absorbed selectively — that is to
say, the crystalline medium absorbs light of certain colours from the
one polarised ray, whilst absorbing quite differently coloured com-
ponents from the second ray. Pleochroic substances are most easily

FIG. 816. — Spiral crystallisation of copper sulphate.

recognised by the fact that they change in colour when rotated mi
the microscope stage in plane polarised light — namely, when only on<'
Xicol prism is interposed between the eye .-md the lamp. It not
nnfrecpieiitly happens that a remarkably beautiful specimen of
crystallisation develops itself which the observer desires to keep
for display. Tn order to do this successfully, it is necessary to
exclude the ail- ; and Mr. \Yarrington recommends caslor oil as the
best preservative. A small quantity of this should be poured on
ihe crystallised surface, a gentle warmth applied, and a thill glass
cover then laid upon the drop and gradually pressed down; and
after the superfluous oil has been removed from the margin a coat
of gold-si/.e or ot her varnish is to be applied. Although most of the
objects furnished by vegetable and animal structures, which are
advantageously shown bv polarised light, have been already noticed
in their appropriate places, it will be useful here to recapitulate the
principal, with some additions.

SALTS FOR CRYSTALLISATION

1099

Alum

Ammonium Borate
Chloride

Hydrogen Tartrate
Nitrate
Oxalate
Oxalurate
Phosphate
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

Cinchonidine
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
Uric 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

Polvpidoms of Hydrozoa

Spicules of Gorgonias

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

I IOO

MICKOCBYSTALUSATION, ETC.

in the ordinary forms of ciystallisable 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,1 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
carbonate, which progressively increase in diameter at the expense of
an amorphous deposit which at first intervenes bet ween them, two
such spherules sometimes coalescing to produce • dumb-bells/ whilst
the coalescence of a larger number gives rise to the mulberry-like
body shown in fig. 817, b. The particles of such composite spherules
appeal1 subsequently to undergo rearrangement 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 <i. <>
are gradually produced. The
structure of these, especially
when examined by polarised
light, is found to correspond
very closely with that of tin •
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 concretionary 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 already described being here insufficient to solidify
it. Ill the external layer of an ordinary eggshell, on the other
hand, the concretions ha\e enlarged themselves by Ilie progres-
sive accretion of calcareous particles, so as to form a continuous
layer, which consists of',-i series of polygonal plates resembling those
of ;i tessellated pavement. Ill the solid 'shells' of the eggs of the

See hi* treatise ' On the Mode of Formation of the Shell* of Animals, of Bone,
and of se\eral other structures, liy a process of Molecular Coalescence, demonstrable

Main artificially formed product*,' lM.r>.s; and his 'Further Experiments and
Observations ' in <t>//m-t. Journ. Microsc. Sci. n.s. vol. i. Lsoi, p. •2:',.

FIG. 817. — Artificial concretions of
carbonate of linn-.

HARTIXG'S CALCO-GLOBULINE I IOI

ostrich and cassowary this concretionary layer is of considerable
thickness : and vertical as well as horizontal sections of it are very
interesting objects, showing also beautiful effects of colour under polar-
ised light. And from the researches of Professor AV. 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 contemporaneously pursued by
Professor Hartiiig, of Utrecht, who, working on a plan funda-
mentally the same as that of Mr. liainey (viz. the slow precipitation
of insoluble calcium salts in the presence of an organic 'colloid'),
lias not only confirmed but greatly extended his results, showing
that with 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 strong resemblance to calcareous
structures hitherto known only as occurring in the bodies of animals
of various classes. The mode of experimenting usually followed by
Professor Harting was to cover the hollow of an ordinary porcelain
plate with a layer of the organic liquid to the depth of from 0-4 to
0'6 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 very gradually dissolved, the two substances may come slowly
to act upon each other, and may throw down their precipitate in the
midst of the ' colloid.' The whole is then covered with a plate of glass,
and left for some davs in a state of perfect tranquillity ; when there
begins to appeal- at various spots on the surface minute points re-
flecting light, which gradually increase and coalesce, so as to form a
crust that comes to adhere to the border of the plate ; whilst another
portion of the precipitate 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 different character : so Thai
in the same experiment several very distinct products are generally
obtained, each in some particular spot. The length of time requisite
is found to vary with the temperature, 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
line employed. When these concretions are treated with dilute
acid, so that their calcareous particles are wholly dissolved out,
there is found to remain a basis substance which preserves the form
of each ; this, which consists of the ' colloid ' somewhat modified, is
termed by Harting calco-globuline. Besides the globular concretions
with the peculiar concentric and radiating arrangement obtained
by Mr. Rainev (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 Alcyonama, and the very similar spicules in the
mantle of some species of Doris. 3. Lamella- of • prismatic shell-
sul (stance/ which are very closely imitated by crusts formed of
flattened polyhedra, found on the surface of the 'colloid. 4. The
spheroidal concretions which form a sort of rudimentary shell within

I 1 02 MICBOCKYSTALLISATION, ETC.

the body of Lima. i\ 5. The sinuous lamella1 which intervene between
the parallel plates of the ' sepiostaire ' 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 away
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 wyhich, in some cases, a little sodium phosphate
was mixed.1 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 throw much light on the process of
ossification and tooth formation. The connection of these results
with the work of Vogelsang (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.2

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 various reagents, have been applied by numbers of
workers to the identification of the constituents of a material by the
aid of chemical reactions, the results of which are 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,3 and a somewhat similar, although naturally less com-
prehensive, scheme has been given by the same author for the
identification of organic compounds.4 The analytical methods are
intended primarily to serve for identifying the components of a
material available only in small quantities ; but in many cases the
micro-chemical method is more 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 appearance as a result of the addition of
different reagents are then noted, and from the information thus
obtained a knowledge of the constituents of the original substance is
deduced. A \eryiniportaiit application of micro-chemical analysis

1 See Prof, Hill-tin;-:'- Hi-i-l/i-ri-ln-x ilr Mor/>J/i>/<><//r Ni/>if!ti'fiiinr mir la i>i-n<l ill-Hun
iir/i/irirl/i' ilr I/ /I i 'I i/ II rn Far in il t it ins ( 'illrii/i'i s 1 IK irt/nii I ij lit'*, /itllil i<'rs /nir/' . \cildfmie

l,'tii/<i/<- N&erlandaise dcs .S'r/rmv.s-, Amsterdam, ls7'2 ; and (jiun-t. Jonni. Microsc.
Sri. xii. ]). lls.

- Sec his tri-utisr Oil till' Illjllli Hi'/' / if ('ullnii/s il/nui ('rt/sfilllii/r l-'i'nil Illllf

•a, I loudon, I.STII.

1 \.rileitung zur mikrochemisclitn AHU/I/HI-, Hamburg, isn.'i.
1 Mikrorlit ini^i'hi'ii Ainili/^i der organischen \'rr/ii>nl/ni(/rn, Hamburg, IS'.l.'i.

MICKO-CHEMICAL ANALYSIS 1103

IIMS been made in connection with the detection of poisons, nnd by
;i judicious combination of microscopical with chemical research.
the application of reagents may be made effectual for the de-
tection of poisonous or other substances in quantities far more
minute than have been previously supposed to be recognisable.
Thus it is stated by Dr. Wormley l that micro-chemical analysis
enables us by a very few minutes' labour to recognise with un-
erring certainty the reaction of the f-o^Vo~oth part 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 quantities of substances, the true nature of which
could only be otherwise determined in comparatively large quantity,
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
\vhich the use of the microscope affords. Thus it has been shown that
by the careful sublimation of arsenic 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 maybe 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,2 &c.
And subsequently it was shown 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 other substances in very minute quantity
can be accomplished with a facility and certainty such as wrrc-
formerly scarcely conceivable.

1 Micro -chemistry of Poisons, New York, 18.">7.

Wyiitfi- Blyth, 1'uisun-?, their Effects and Detection, London, is'.i.v

APPENDICES AND TABLES

USEFUL TO THE MICKOSCOPIST

4 B

1 107

APPENDIX A

TABLE OF NATURAL SINES

o

0'

15' = i°

30' = i°

45'=J°

O

0'

15' = i3 30' = i°

45' = f°

0

•0000

•0044

•0087

•0131

46

•7193

•7224

•7254

•7284

1

•0175

•0218

•0262

•0305

47

•7314

•7343

•7373 -7402

o

•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 1 67

•9205

•9222

•9239

•9255

22 -3746

•3786

•3827

•3867 1 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 -'.1681

•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

1-0000

45

•7071

•7102

•7133

•7163

Hote. — 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,

Sin 3n°- Perpendicular j
hypotenuse 1

4n2

IIOS APPENDICES AND TABLES

APPENDIX B

TABLE OF REFRACTIVE INDICES

DUUStiHillUC

Water

-LVeliiU

;uvt: iiiutx

1-334

54-7

Saliva ........

Sea-water
Human blood

• ME

. M E

1-339
1-343
1-354
1-457

t/^x f

Ether (60° Fahr.)

. M u

1-357

84-9

Albumen

. M D

1-350

—

Absolute Alcohol . . . . . .

. M D

1-364

58-6

Oil of Ambergris ......

. M E

1-368

—

Salt (sat. sol.)

. M E

1-375

—

Fluor Spar

. M*>

1-4338

97-3

Diatom Silex ... ...

. M D

1-434

—

Spermaceti

. M D

1-503

—

Bees-wax .......

. M D

1-553

—

Oil of Olives (sp. gr. 0-913) ....

. M D

1-476

54-7

Borax

. MD

1-515

60-6

Naphtha

. M E

1-475

—

Oil of Turpentine (sp. gr. -885) .

. M D

1-474

46-5

Oil of Linseed (sp. gr. -932) ....

. M l>

1-485

—

Castor Oil

. MD

1-490

—

Chloride of Tin . . ...

M L>

1-503

—

Oil of Cinnamon ......

. M D

1-619

14-3

Oil of Cedar

. MD

1-510

—

Gum Arabic

. M E

1-512

—

Dammar

M D

1-520

—

Oil of Cloves

M D

1-533

—

Sugar

. M D

1-535

—

Felspar

. H D

1-764

—

Cedrene

. M D

1-539

—

Canada Balsam

M D

1-526

41-5

Oil of Fennel

M U

1-544

Bock Crystal

. M D

1-545

70-0

Bock Salt (sp. gr. 2-143) ....

. U U

1-555

—

Nitro-benzene ......

. MU

1-558

—

Styrax

. jU ])

1-582

—

Meta-cinnamene ......

. /u D

1-597

29-8

•Quinidine

. /u u

1-602

24-1

Benzylaniline ......

. M l}

1-611

—

Methyldiphenylamine .....

. M i>

1-616

— .

Balsam of Tolu

. ME

1-618

—

Bisulphide of Carbon

. M l->

1-630

18-3

•Oil of Cassia

1 -57S

17-0

Quinoliiu!

. M !>

1-633

— .

Tourmalin (ordinary ray) ....

. M 1'

Mills

—

Krcasote .......

. M 1'

1-538

29-9

r<'!n>lrum

. M i>

1-457

15-3

Phenyl-thiocarbimide .....

. M 1'

I -65.1

18-7

[celand Spar (ordinary ray) .

. M •"

1-657

49-0

USEFUL TO THE MICROSCOPIST 11 09

Substance Refractive Index

Monobromonaphthalene . . . . /j. D 1-658 19'9

Piperine and Balsam ...... /j. D 1-657

Naphthyl-phenyl-ketone ..... /j. D 1-669 17'6

Bromide of Antimony . . (approximately) /u D 1-680

Piperine ...... " . n D 1-681 9'88

Methylene di-iodide ...... n D 1-743 21-2

Sulphur in methylene di-iodide . . . . /x D 1-778

Zircon ..... . p. D 1-950

Carbonate of Lead . . /u D 1-81 to 2-08

Borate of Lead ....... /u A 1-866

Phosphorus in methylene di-iodide (equal weights) /t D 1-944 17'1

Sulphur (melted) . ..... '. /* E 2-148

Phosphorus ...... . ^ D 2-224

Diamond (sp. gr. 3-4) ...... /j. D 2-47

Chromate of Lead ...... /u. D 2-50 to 2-97

Realgar (artificial) ...... /* E 2-549

Substance Refractive Index

Crown /j. D 1-51 to 1'56 59'0 to 46-0

Plate M D 1-516

Extra Light Flint . . . . ^ D 1-541 49"2

Light Flint /t D 1-574 41-0

Dense Flint /J.D 1-622 36-5

Extra Dense Fluid . . . . /j. D 1-650 34'2

Double Extra Dense Flint . . . /* D 1-710 30-0

/Boro-silicate Crown . . . /j. D 1-51 64-0

Phosphate Crown . . . //. D 1-51 to 1-56 70-0 to 67-0

Barium Silicate Crown . . ^ D 1-54 „ 1-60 59'0 „ 55-0

Boro-silicate Flint . . . /J.D 1-55 „ 1-57 49-0 „ 47-0

Borate Flint . . ./ID 1-55 „ 1-68 55'0 ,. 33-0

Barium Phosphate Crown . . /* D 1-58 65-2

Wery heavy Silicate Flint . . /*. i> 1-963 19-7
Glass of Antimony . . . ./to 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 three times its dispersion.

03

C
o

1-5

I IIO APPENDICES AND TABLES

APPENDIX C

TABLE OF ENGLISH MEASURES AND WEIGHTS, WITH THEIR

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 = 5£ yards = 5-029196 Metres.

Chain = 4 poles = 2-011678 Decametres.

Furlong = 10 chains = 2-011678 Hectometres.

Statute Mile = 8 furlongs = 5,280 feet = 1-609343 Kilometre.

Geographical Mile = 6,087'23 feet . = 1-855386 „

Knot = (3,080 feet = 1-853182

SUPERFICIES

Square Inch = 6*45159 Square Centimetres.

•00645 Milliare.

„ Foot = 144 Sq. Inches = -92903 „
„ Yard = 9 „ Feet = 8-36126 Milliares.

•83613 Centiare.

Perch = 30^ „ Yards = 2-52928 Declares.
Eood = 40 Perches . 10-11712 Ares.

Acre = 4 Eoods . . 40-46849 „

Square Mile = 258-99836 Hectares.

VOLUME

Cubic Inch = 16-387 Cubic Centimetres.

„ Foot. . =1728 Cubic Inches = 2-83168 Centisteres.
„ Yard. . =27 „ Feet 7-64553 Decisteres.

CAPACITY
Apothecaries'

Minim, 77^ . . . -05919 Cubic Centimetre or Millilitre.

Drachm, f 5 = 60 71^ = 3-5515 „ Centimetres or Millilitres.
Ounce, f 5 =8 £5 28-4123 „ „ = 2-84123 Centilitres.

Pint, O . = 20 f 5 = 568-245 „ „ = 5-68245 Decilitres.

Gallon, C = 8 O •• 4-54596 „ Decimetres, Millisteres, or Litres.

Imperial

Gill =142-061 Cubic Centimetres = 1-42061 DecUitre.

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 :i-('.;5677 Decalitres.

Quarter = 8 bushels = 2 90942 Hectolitres.

USEFUL TO THE MICEOSCOPIST

mi

Grain, gr. .
Scruple, 3 .
Drachm, 5 •
Ounce, 5

WEIGHT

Apothecaries'

= 6-479892 Centigrammes.

. . = 20 gr. = 1-29598 Gramme.
= 33= 60 gr. = 3-88794 Grammes.

= 8 5 = 480 gr. = 3-11035

Decagrammes.

Avoirdupois

Grain, gr =

Drachm, dr = 27'34375 gr. =

Ounce, oz. . . . =16 dr. = 437'5 gr. =
Pound, Ib. . . . =16 oz. = 7000 gr. =

Stone, st. ... = 14 Ib

Quarter, qr. . . = 28 Ib = 12-70059

Hundredweight, cwt. = 4 qr = 50-80235 = '50802 Quintal.

Ton =20 cwt = 1-01605 Tonne.

1 Ib. Avoirdupois = -822857 Ib. Troy or Apothecaries'.
1 Ib. Troy or Apothecaries = 1-21527 Ib. Avoirdupois.

6-479892 Centigrammes.
1-77185 Gramme.
2*83495 Decagrammes.
4-5359243 Hectogrammes.
6-35029 Kilogrammes.

TABLE OF METRIC MEASURES AND WEIGHTS, WITH THEIR

ENGLISH EQUIVALENTS

The metre was originally intended to be the TTrWtriTinJth 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 length of a definite standard in Paris.

LENGTH

Micron, i.e.
Millimetre .
Centimetre .
Decimetre .
METRE . .

Decametre .

Hectometre

Kilometre

Millimetre .
= 3^ Centimetre
= 3^ Decimetre .
= tV Metre . .
= Unit ....

= 10 Metres . . .
= 10 Decametres .
= 10 Hectometres .

Milliare .
Centiare .
Deciare .
Are = Unit
Hectare .

= -00003937 Inch.
= -03937

•39370

= 3-93701 Inches.
= 3-28G84 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.

VOLUME

Millistere . . = 1 Cubic Decimetre = 61-0239 Cubic Inches.

Centistere . . =10 „ Decimetres = 610-239 „ „

Decistere . . = 100 „ „ 3-531476 „ Feet.

Stere = Unit . = 1 „ Metre 1-30795 „ Yard.

Decastere . . =10 „ Metres = 13-07954 „ Yards.

Hectostere . . = 10 Decasteres = 130-7954

II 12

APPENDICES AND TABLES

CAPACITY

Millilitre = Cubic Centimetre = -007039 Irnpr. Gill.

Centilitre = 10 Cubic Centimetres -07039 „

Decilitre = 100 „ „ -7039

Litre . =Millistere =1-7598 ,. Pint.

Decalitre = 10 Litres = 2-19975 „ Gals.

Hectolitre = 10 Decalitres = 2-74969 „ Bush.

Kilolitre = 10 Hectolitres = 1 Stere = 1 Cubic Metre = 3-43712 „ Qvs.

Milligramme

Centigramme

Decigramme

Gramme . .

Decagramme

Hectogramme

Kilogramme

Myriagramme

Quintal . .

Tonneau

WEIGHT

= i1^ Centigramme

= j3^ Decigramme

= 3^ Gramme

:Unit

•• 10 Grammes

= 10 Decagrammes

. 10 Hectogrammes

= 10 Kilogrammes

10 Myriagrarnrnes

10 Quintals

Avoirdupois
•01543 Grain.
•15432 „
1-54324
= 15-432356 Grains.

5-64383 dr.
= 3-5274 oz.

2-204622 Ib.
= 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 measured by Rogers,
who found it equal to 39-370155 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 MIOROSCOPIST

III3

CONVERSION OF BRITISH AND METRIC MEASURES

Computed by Mr. E. M. Nelson from the New Coefficient obtained by Order of

the Board of Trade in 1896.

LINEAL.
Metric into British.

fj.

ins.

1

•000039

2

•000079

3

•000118

4

•000157

5

•000197

6

•000236

7

•000276

8

•000315

9

•000354

10

•000394

11

•000433

12

•000472

13

•000512

14

•000551

15

•000591

16

•000630

17

•000669

18

•000709

19

•000748

2O

•000787

31

•000827

32

•000866

23

•000906

24

•U00945

25

•000984

26

•001024

27

•001063

28

•001102

29

•001142

30

•001181

31

•001220

32

•001260

33

•001299

34

•001339

35

•001378

36

•001417

37

•001457

38

•001496

39

•001535

4O

•001575

41

•001614

42

•001654

43

•001693

44

•001732

45

•001772

46

•001811

47

•001850

48

•001890

49

•001929

6O

•001969

6O

•002362

70

•002756

8O

•003150

9O

•003543

1OO

•003937

2OO

•007874

30O

•011811

4OO

•015748

5OO

•019685

60O

•023622

7OO

•027559

8OO

•031496

900

•035433

1OOO

( = 1 mm.)

mm.

ins.

mm.

ins.

1

•039370

51

2-007876

2

•078740

52

2-047:.' l.;

3

•118110

53

2-086616

4

•157480

54

2-125986

5

•196851

55

2-165356

6

•236221

56

2-204726

7

•275591

57

2-244096

8

•314961

58

2-283467

9

•354331

59

2-322837

10

•393701

60

2-362207

11

•433071

61

2-401577

12

•472441

62

2-440947

13

•511811

63

2-480317

14

•551182

64

2-519687

15

•590552

65

2-559057

16

•629922

66

2-598427

17

•669292

67

2-637798

IS

•708662

68

2-677168

19

•748032

69

2-716538

20

•787402

70

2-755908

31

•826772

71

2-795278

32

•866142

72

2-834648

23

•905513

73

2-874018

24

•944883

74

2-913388

25

•984253

75

2-952758

36

1-023623

76

2-992129

27

1-062993

77

3-031499

28

1-102363

78

3-070869

29

1-141733

79

3-110239

SO

1-181103

8O

3-149609

31

1-220473

81

3-188979

32

1-259844

82

3-228349

33

1-299214

83

3-267719

34

1-338584

84

3-307089

35

1-377954

85

3-346460

36

1-417324

86

3-385830

37

1-456694

87

3-425200

38

1-496064

88

3-464570

39

1-535434

89

3-503940

40

1-574805

90

3-543310

41

1-614175

91

3-582680

42

1-653545

92

3-622050

43

1-692915

93

3-661420

44

1-732285

94

3-700791

45

1-771655

95

3-740161

46

1-811025

96

3-779531

47

1-850395

97

3-818901

4$

1-889765

98

3-858271

49

1-929136

99

3-897641

SO

1-968506

decim. ins.

1 3-9370113

2 7-8740226

3 11-8110339

4 15-7480452

5 19-6850565

6 23-6220678

7 27-5590791

8 31-4960904

9 35-4331017

1 metre 3-2808428 ft.

1-09361425 yd.

1 1 14

APPENDICES AND TABLES

British into Metric.

ill.

mm.

in.

mm.

in.

mm.

1

25-399978

£

2-309089

&

•298823

2

50-799956

1

2-116665

JL

9O

•282222

3

76-199934

12

10-583324

1
95

•267368

4

101-599912

TSJ

14-816654

i

10O

•254000

5

126-999890

8

23-283313

•169333

6

152-399868

X

1-953844

ISO
20()

•127000

7

177-799846

a.

14

1-814284

£3^0
50

•101600

8

203-199824

1

15

1-693332

30O

•084667

9

228-599802

i

1-587499

_1
35O

•072571

10

253-999780

T5_

4-762496

1
4OO

•063500

11

279-399758

lef

7-937493

_1

450

•056444

1 ft.
lyd.

304-799736
914-399208

7

¥
P

11-112490

14-287487
17-462485

_1

50O

55 O

1
60O

•050800
•046182
•042333

in.

nun.

ft

20-637482

1
C5O

•039077

§

12-699989

15
16

23-812479

Too

•036286

8-466659

i
17

1-494116

1

750

•033867

3

16-933319

¥

1-411110

1
800

•031750

i
4

6-349994

19

1-336841

1
85O

•029882

3
4

19-049983

1
20

1-269999

1

90O

•028222

I

5-079996

1
21

1-209523

I

950

•026737

2

10-159991

22

1-154544

5~

15-239987

1
23

1-104347

in.

M

4

20-319982

24

1-058332

Tooo

25-399978

^

4-233330

A

1-015999

i

2000

12-699989

5
6

21-166648

£

•846666

1
3000

8-466659

f

3-628568

B

•725714

1

ilH li'.

6-349994

3-174997

i

40

•634999

1

^, u<H I

5-079996

|

9-524992

£

•564444

1

4-233330

tiOOO

8

15-874986

A

•508000

1
Yooo

3-628568

7
£

22-224980

A

•461818

i

¥006

3-174997

9

2-822220

i

80

•423333

i

2-822220

iJOOO

£

2-539998

1
65

•390769

1

10000

2-539998

S

10

7-619993

1
70

•362857

1
15000

1-693332

7

17-779985

?

•338666

1

1-269999

1O

1:0000

9

22-859980

M

•317500

1

1-015999

10

^5000

USEFUL TO THE MICROSCOPIST

III5

TABLE FOB THE CONVERSION OF FRACTIONAL PARTS OF AN
ENGLISH INCH INTO METRICAL LINEAR MEASURE.

1 -:-

mm.

1 -4-

!
Micra. 1 -^

Jlicra.

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

in6

APPENDICES AND TABLES

Lines
per inch

Lines
in mm.

Lines
per inch

Lines
in mm.

Fractions
of an inch

M

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
85,000
90,000

3,150
3,346
3,543

25,399-98
50,800
7fi onn

Lines in /j.
1
2

Q

150,000
160,000
170,000

•169
•159
•149

95,000
100,000

3,740
3,937

1 D,wUU

101,600
127,000

o

4
5

180,000
190,000

•141
•134

110,000
120,000

4,331

4,724

152,400

6

200,000
250,000

•127
•1016

130,000

5,118

177,800
or»Q f)nr\

7

300,000

•0847

140,000
150,000

5,512
5,906

-iUO,<iUU

228,600
254,000

9
10

350,000
400,000

•0726
•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 MICROSCOPIST III/

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 amplification of the
photo-micrograph being known.

Example : In a photo-micrograph of a diatom amplified 735 diarns.
12 dots can be counted in '3 of an inch. At what rate per inch is the
structure in the diatom ?

magnifying power >; number counted
ace counted

= 29,400 per inch.

space counted
735 x

•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.

735 x 12

•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.

735 x 12 x 25-4

- = 32,004 per inch.
/ mm.

IIl8 APPENDICES AND TABLES

APPENDIX D

COMPAEISON OF THE SCALES OF FAHRENHEIT'S, THE
CENTIGRADE, AND REAUMUR'S THERMOMETERS

THESE three thermometers are graduated so that the range of temperature
between the freezing and boiling points of water is divided by Fahrenheit's
scale into 180° (from 32° to 212°), by the Centigrade into 100° (from 0° to
100°), and by that of Reaumur 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
Reaumur's ; a degree of the Centigrade is equal to 1'8 of Fahrenheit's, or
to '8 of Reaumur's; and a degree of Reaumur'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 Reaumur's,
subtract 32 and multiply the remainder by f for the Centigrade, or f for
Reaumur's.

To convert degrees of the Centigrade or Reaumur's into Fahrenheit's,
multiply the Centigrade by f , or Reaumur's by f , as the case may be,
and add 32 to the product.

EXAMPLE

Let F, C, and R = the number of degrees Fahrenheit, Centigrade, and
Reaumur respectively. Then

- -

5 4

r_5 (F-32). r_5R

~9~ IT

R_4(F-32). -R-4C.

-Q- "5"!

F = C + R + 32.

This last formula is of use, because in England thermometers are
usually graduated in Fahrenheit and Centigrade. Reaumur 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 Reaumur and Centigrade. Fahrenheit can be found
by adding Reaumur and Centigrade and 32. — Example : If the thermometer
reads 8 Reaumur and 10 Centigrade, the Fahrenheit will be

8 + 10 + 32 = 50 F.

USEFUL TO THE MICROSCOPIST 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 curve A = r, and that of B = s, t = thickness of the lens
and p. the refractive index. Then

(i)

r—s r—s

Example explaining the method of treating the signs : First, it should
be particularly noticed that ah1 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 = 1 ; then by (i)

A c _ 2x1 _2_ = ?. BC_ -3x1 ^^-3 _ _3
~2-(-3)~2~T3~5' 2-(-3) 2 + 3 5'

The point C is measured, therefore, to the right hand from A, and to
the left from B. In a plano-concave let r = — 2, s = GO , and t = 1 ; then

— 9 V 1 i-f^ V 1

A ri _ _ =0' B f1 = ' = = 1 (\\

~ -2- oo~ ~ -2-00 -oo "

c is therefore coincident with A.
The principal points D and E may be found thus :

A D = - . — ; B E = - . -ll- (ii)

H r — s p. r — s

1 3

Example : In a meniscus r = — 3, s = — 2, t = — , and u = ; concavities

4 2

facing the left hand.

-3 l _? -?

1 _°_'j 2 "4 24231 ..

3 ' -3-(-2) 3 ' -8 + 2 3 ' -1 3 '4" 2
2
D is measured J inch to the right from A.

-2.1- -I

BE i i. 2 _2_ _ 2 1 1

•D -t' = q O ^TA — Q ' Q , O ' Q • O : " " * *• '

o — o — ( — £,) 6 — o + Z o iJ o

2

E is measured 3 inch to the right from B.
If the meniscus is turned round so that its convexities face the left

hand, r = 2, s = 3, t = -, u. = - ;

4 2

1 2 ' 4 21 I

Similarly B E = — -. Both are therefore measured to the left. The-
2

1 1 20 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 :

A T^ ^* • ~R T71 ^ f"*\

= M(r-S)-^-l)' %(r-8)-*0*-l)

Plano-convex Lens. — Let /=the principal focal point and y = ihe
semi-aperture ; then if parallel rays are incident on A, the plane side of
the lens, r = GO, and by (ii) B E = 0. The principal point is therefore at
the vertex B, and the focal length

= _i; E/=B/ (iv)

The spherical aberration

o

Thus when p. =• ,

ft

2

.._«£

If the parallel r&ys are incident on the convex side A, s = oo ,

B E = - - (ii), and the focal length
P

B/= _?!--* ...... (vi). E/= * ..... (vii)

ft-1 M /^-l

The spherical l aberration

, , K(fi-2) +2 y- . ....

8/= "270^1? '/ ...... (vm)

"When n = 1'516 (plate glass)

S/= -1-1^ ........ (viii)

When p. = 1'62 (flint glass)

8/= --8042^ ........ (viii)

To find the radius of a plano-convex lens, the ref. index and focus
E/ being given :

l) .......... (vii)

To find the radius of a plano-convex lens, the ref. index, the thickness,
and the focus B/ being given :

P-

A plano-concave lens follows a plano-convex ; f will be negative,
which shows that the focus is virtual. Concaves being thin, t is usually
neglected.

Equi-convex and equi-concave generally :

Equi-convex more accurately:

1 lloalli's Geometrical Optics, 1887.

USEFUL TO THE MICROSCOPIST I I 2 I

Equi-convex more accurately :

Spherical 1 aberration

In an equi-convex lens when /* = 1 516

8/ = - 1-618 £ ....... . . . (xii

To find the radius of either an equi-convex or equi-concave lens.
generally, the ref. index and the focus B/ being given :

.......... lix)

To find the radius of an equi-convex lens, the ref. index, the thickness,
and the focus B / being given :

Bi-convex and bi-concave, generally :

E_/= . (xii1.

Correction for thickness :

Bi-concave t may be neglected B/=E/ practically.

Bi-convex more accurately, and converging and diverging menisci :

-

When the light is travelling from right to left

r t
v < (/j. — 1) - + s }•
A/'= L. £ _ (xiv)

Spherical aberration :

V- •-«?? -T---i^! ' ' «">

Example : Let r = 2, s = - 3, * = 1, and /x = ^ ; then by (xivl
1 Heath's Geo/iteti icnl Of tics, 1887.

4 c

I I 22 APPENDICES AND TABLES

B/ =

-3i

2 ' \2 ' 3 /

(!-'){

2 r 3^ (3-l\V 1 V2 + 3 * 2>>
~U ^ 2V 2 '3V

-3x *

3 _ 5 _ «,!

1 14 V V

2' 3 3

A /' - 9^

7 V

1 144

:iii) 3

19 x 4. X 9'"> 19 fi 4
3 /• 1Z 1 Ao

(xiv)

1 14 I '

2' 8 3

r\

Similarly

By (xii) and (xiii) „.,

o o

2X

This is larger by ^-g inch than the result obtained by (xiv).
The following is an example worthy of note. Suppose

t

Thus let r = 5 -. s = 5, t = 1, /* =|.

2 2

5C--— "l - —

Then by (xiv) B/==_i— 2_-= — = -310.

2 \2~sJ 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 :

The focus of a sphere measured from its surface :

Bf=l~I (SYi)

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/= 11 . . . ... (iv)

When p. = 1'5 the focus of a sphere measured from the surface = A the
radius.

The focus of a hemisphere measured from the plane side = 1^ 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 1 1 23

.
'/

For boro-silicate glass /* = 1-51 ; r = -5898/; ands = -3'7b'9/; (xvii)

S/= -1-042 £ .........

For flint glass /* = l'G2 ; r = -653/; and s = - 12-06/; (xvii)

(xv)

Critical angle. — Let 6 be the critical angle for a ray passing out of a
denser medium into a rarer one.

Then sin 6 = ......... ixviii)

When p = 1-333, 6 = 48° 30^'; M = , 0 = 41° 48i ; M = l-52 0 = 41° 8f ;

A

^ = 1-62, (9 = 38° 7'.

Let /be the principal focus, and j; = the distance from the object to
the optical centre of the lens, p' = the distance from the optical centre of
the lens to the conjugate image.

Then p--, *=•; f -

-, =• --,

P ~f P "/ P + P

Let v be the distance from the object to f, and w be the distance from
/ on the other side of the lens to the conjugate image.
Then

v = p-f\ w=p'-f\ p = v+f; p' = w+f; and vw=f~; v=*~

f'~
w = J- ..................... (xx)

V

If o be' the size of the object and i the size of its conjugate image

= of=op' of =o(p'-f) .

~

* ~"

_/• ~f

p-f f

if ^i(p -f) .

f w p' p'-f f
_op'_f(i + o). „,_*¥_/(*+<>) .

P — ' . -- ; - > 1J ~ >

^ ^ o o

of if f ip op' ow

t; = -^; w=-*-; /=^-±- =_i- .... (xxi)

/. o ^ + o ^ + o ^

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 ?

+ o) = -5(1-5 + -08) = 9.

,., _

-03

Therefore p' = 25- inches, the distance required ...... (xxi)

*J

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 ?

4 4
the size of the diatom required.

1 .••_>

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.

f ow -01 x 60 -6 1

/=T=^4-=^4=4 ....... (XX1)

To find F, the eqiiivalent focus of two lenses in contact :

=

where / is the focus of one lens and /' 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

o

combined focus F = -6, p-=^', find their radii (see figs. 4, 6, 8, and 9).

Then/=2/'.

2// _2.f

"

_

'+/' :;./" 3

/ = ^ = L8 = -9; and/=2/' = l-8 . . . . (xxii)
•i ^i

r = (^ = 1) /= /3 - l\ 1-8 = -9 ; similarly r --- '45 . . (vil i
The focus for three lenses follows that for two, thus :

which may be written — = 2 -..

5 ./

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_ - , iin

f+f'-a

More accurately, let D E be the principal points of the first lens and
I ' l-y those of the second, A B and A' B' being the respective vortices,
il the distance from E to D' ; then G and G', the principal points of
the combination, are :

F= f+f-i ........

I ' i- nii'iisurod from o)ie of the principal points of the combination. An
:iiplc will be of interest. Let parallel rays fall on the convex face of
tin field lens of a lluyghenian eyepiece; find their focus.

Let /, the focus of the field lens = 3, and that of the eye lens /' = !;

USEFUL TO THE MIGROSCOPIST 1 1 25

/j = -, and the distance between the surfaces, that is B A', = 1-8 ; t the

thickness of the field lens = ; and t' that of the eye lens = - • A D = 0

10 20

(ii); BE= -!=--2 (ii), Similarlv A' D' = 0; B' E'= -- = - l iii) •
/j. fi 10

1 = 2. Now

2x1

G' = E'-

3 + 1-2

F= 3x1 3

3+1-2 2

We see, therefore, that the equivalent focus is 1A 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 i inch to the right from E'. Now as
E' is T^j inch to the left of B, the plane surface of the eye lens, it follows
that F, the focal point, is T\ 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
G' from which that focus was measured was 9 ft. 11^ inches from the
objective, which would give i 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 ^ 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

G = D+ 2 * 8 = D + 3 . . (xxiv)

o + 1 — 2

G 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 8 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 « = the diameter of the eye-spot, then

*-§-"' ....... <*"*>'

1 Journal B.M.S.

1126 APPENDICES AND TABLES

« = 2(N.A.)<£; N.A. = -i- (xxix)'

If X = the number of waves per inch of light of a given colour, L the
limit of resolving power of any objective with an illuminating beam of
maximum obliquity is

L = 2X(N.A.) (xxx)

But with a solid .7 axial cone and white light the resolving limit is
equal to the N.A. multiplied by 70,000. When Gilford's screen is used, or
photography employed, the limit is raised to the N.A. multiplied by 80,000.

The aberration for non-parallel rays. — It is a little more troublesome
to find the aberration of rays other than parallel, but if the following
instructions are carefully attended to the problem merely becomes the
simplification 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) :

o f 2 f
a=-£--l (xxxi); « = !--£ (xxxii)

Next find x by (xxxiii) or (xxxiv) :

- 1 . . . (xxxiii) ; x — 1 -

r

Then find co by (xxxv) :

co = , -- s x~ + 4i(fjL •+ 1) a, x 4- (3/x + 2) (jfj. — 1) a~ •+• — — > (xxxv)

8/x(/x -I)/3 L/i — 1 n — 1J

1111, v-> t

p'=f~ T5~+ -(X~a)~~A~f2 + <»y~ ' • • (XXXVI)

The aberration 8 P'= -coP'2?/2 . . . (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 111

p/==7~p' Q/=r/+~Q ' ' ' ^Xlx^

for the first lens, - = - - _ + co y- . . . (xxxvi)

for the second lens, -_= — +: + co' y- . . . (xxxvi)
Q ./ Q

for both lenses, — = _ + - — - + (co + co') y~ . (xxxviii)

^e J J

Therefore, for 'ii lenses, — n = 2 - - - + 2 co y" . (xxxviii)

Q ./

The aberration 8 Q' = - 2 co Q'2y- and 8 F = - 2 co F-y- . (xxxix)

Example : Two plano-convex lenses of equal foci have their convex
surfaces in contact (tig. 7) ; find the aberration for parallel rays. Then

3 . /• ,v

f-r.

the first lens r = 00 ; therefore x = -1 (xxxiii); P = oo ; therefore

a = - 1 (xxxi) ; and co -- .., ............. (xxxv)

'

For the second lens « r. ; therefore rc = l (xxxiv); -= — - ; there-

y /
1 Journal 11. M.S.

USEFUL TO THE MICROSCOPIST II2/

13 20

fore a, = - 3 (xxxi) ; &>' = — (xxxv) ; o> + a/ = 2 a> = -— ;

20
/= 2 F (xxii) ; therefore 2 <u = — _— ;

90 "F2?/2 f» ?/-

">-inF;-Hr '"-*>

This is half the aberration of an equi-convex lens (fig. 1) of the same
focal length as the combination where

V--S-1? ; • ""

If the front lens of the combination be turned round so that its convex
surface faces the incident light the aberration is

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

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 f . 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 § F.

I j til

i i

VJ

FIG. 1. FIG. 2. FIG. 3.

Fig. 1. An equi-convex, r = F ;

8F= -l'6|-2= -'173 (xi)

F

Tjl

Fig. 2. A plano-convex, '' = 5-;

8F= -l-l«2£= --121 (viii)

7 7
Fig. 3. A crossed convex, r = — F ; s = - -F (xvii )

1 — -J

SF= -1-07^,- -'111 (xv>

F

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 = =j- • • (xxii)

8 F = - -833 |J = - '087 ...... (xxxix)

F

Fig. 4. The same, only 2/=/ ;

8 F = - 1-611 |T = ~ '16S (xxxix)

F

1128

APPENDICES AND TABLES

Fig. G. The same, only/=2/';

8F = --5 y. = --052
F

Fig. 5. The first lens inverted, /=/';

8F= - -416 y~ = - -043 .

(xxxix)

(xxxix)

Fig. 8. The same, only 2/=/' ;

F = --623^ = --065 (xxxix)

F

Fig. 9. The same, only/=2/';

SF= --376|= --039 (xxxix)

FIG. 4.

II

FIG. 5.

PIG. 0.

I i

FIG. 7.

I I

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, p. = 1'516, for parallel rays similar in arrange-

5 f'
ment to fig. 9 is when /= -£-.

The Aplanatic Meniscus. — A 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 =fj. 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 aplauatic 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 formal ;e for finding r and P' when P is given are :

r--£l

P'= -

(Xl)

uad those for finding r and P when P' is given are :

p/ p/

r = -i P=-f-

. . (Xli)

USEFUL TO THE MICEOSCOP1ST I I 29

An excellent combination, suitable for a bull's-eye, can be made of
boro-silicate glass, refractive index 1-51, j/ = 64'0

1st lens ' crossed r = + ^2-359 | diamilter 2>1

2nd lens 1 meniscus r = + 1-280 , diameter ^

Distance between lenses, '05 ; equivalent focus, 2'0 ; back focus, T55 :
total aberration, - 'lOo ; 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 fig. 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
I) to point p' with minimum aberration.

o

Let |8 be the coefficient of -4- in formulae v, viii, xi, and xv, then for

parallel rays in each particular case the lateral aberration = ^L $ . . (xlv)

1 if3
Diameter of least circle of aberration = ~ ^ 3 (xlvi)

J

f\ i)

Distance of least circle of aberration from focus = - *L ft . (xlvii)

When the rays are not parallel

(xlv) = (ap'y3 (xlvi) = —to p' y3 (xlvii) = -a>p'~y~

It is interesting to uote that-'- = "2, (fj. — l)t (xlviii)

3 ir
Therefore, when w= _., . =t.

2' /

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
held close to the lens,

w=l + y- .-. (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 (six) 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^>' the normal focus ; then/ will be the focus of the spectacles
required.

In both cases p 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 p. be the refractive index of a mean ray (D line nearly) for a
certain material, nv that for a blue ray, and \ir that for a red ray ; the dis-
persive power of the material is ^ — (*?• this is usually written — ^, or w.

fjT1 f*-1

The formula for achromatism is

d/t 1 «M' 1_Q.
' -

•or -or'

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 :

'"'

Ifp is large,/ in the denominator 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 p - f, d is the sum of the foci.

Formula relating to Spherical Mirrors

Let^p == one focus, p' - its conjugate,/ = principal focus, and r
= radius of curvature ; then in concave mirrors

Pr . .,'_ Pf . f- PP' . _r.

~ p ~;/-"

>/•
r = --,; r = 2/; 4= ...... (X1X

P+P P /

To find^) interchange^)
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

p

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 /j. ; thusr= — 2/(vii).

Let y = the semi-aperture ; then the spherical aberration

«/= -5 • £ ..... (v) or (viii)
B /

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 mav
converge aplanatically to another point p', must be elliptical, having p
and p' for its foci.

USEFUL TO THE MICROSCOPIST 1131

Formula rein f ing to Prisms

Lot i == the refracting angle of the prism, <p the angle of incidence on
the first surface, <£' the angle of refraction at the first surface, -\|/ the angle
cf incidence in the prism at the second surface, and \// the angle of re-
fraction on emergence ; then the total deviation

D = <£ + ^'-i; 0' + ^ = i ill,)

When the ray passes through the prism symmetrically the deviation
is at a minimum : (p = \//, <p' = ty = -, and

a

I

sin •

(liii)

by which formula the refractive indices of media can be found, because
both i and D are capable of accurate measurement.

Formulae relating to Conic Sections
Ellipse.— Let A = major axis; a = minor axis. Then
Focus = A

a

Parabola. — Let A = height ; a= - base. Then

2i

o

Focus = — — • ......... (Iv)

4 A.

Hyperbola. — Let A = major axis ; a = minor axis. Then

Focus = - ...... (lyi)

2

Works consulted: — Coddington, Camb. 1830; Parkinson, Carnb.; 'Ency-
clopaedia Britannica' ; ' Journal 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 I 32 APPENDICES AND TABLES

APPENDIX F

EXAMPLES USEFUL TO THE MICBOSCOPIST

Square £ inch . . . 10-08061 square millimetres.

>> A » • • • 6-45159 „

,, A ». • • • 4-48027 „

„ Tfo „ . . . -06452 „

= 04515-9 „ n
» ... 645-159

Square centimetre = 15-50006 square T^ inch.

,, millimetre =15-50006 „ T^ „

„ 100 /x =15-50006 .. TuVtr „

„ 10 M -15500 „ „ „

„ /x = -00155 ., „ „

Multiples of the above may be found by multiplying the values given
by the square of the multiplier.

Thus, square T*ff inch = ^ x 4 ; the square of4=-4x4 = 16, and 6-45159
x 16 - 103-2254 square millimetres, the answer required.

Cubic £ inch .... 32'00589 cubic millimetres.

,, T'TJ „ .... 16-38702 „

„ A „'•••• = 9-48323 „

„ .... -01639 „ .,

.... =16387-02 „ M

Cubic centimetre = 61-0239 cubic ^ inch.

„ millimetre = 61-0239 ., Tfo „

„ 100 , a = 61-0239 „ T^Tr „

„ 10 p. -061023 ., „ „

„ /x -0000610-23 .. „ „

Multiples of the above may be found by multiplying the values given
by the cube of the multiplier.

Thus, 2 cubic millimetres : 2 cubed =2x2x2 = 8, and 61-0239 x 8
= 488-1912 cubic ^-J^ inch, the answer required.

Areas of Circli'*

£ inch diameter = 1-22718 sq. x\ inch = 7-91726 sq. millimetres
A „ „ -78539810 „ „ „ 5-00706 .,

,', „ „ -545415 „ „ „ :i-51879 „

,,',0 „ „ -78540 ,,ifo „ -(»:)067 „

= 50670-6 „ /t

= -78540 ,,' „ 506-7

1 millimetre diam. = -78539816 sij. mm. = 12-17372 sq. T^-0- inch.

100 ^ .... 7854-0 „ ,/ = 12-17372 „

10/i. . . . . = 78-54 „ „ -T2174 „

^ = '7854 „ „ = -0012174 ,

I'SEFUL TO THE MICROSCOPIST

1133

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 = Tf ^ inch, then the square of 3 being
1) and -7854 x 9 = 7-0686 sq. ^ inch and -05067 x 9 = -45603 sq. millimetre,
is the area required.

Volumes of Spheres

16-75835 cubic millimetres.
8-58024

in. diameter

= 1-02266 cubic TV inch =
= -52360

A „

"
TTJTfTf )) »

1 mm. diam.
100 n „
10 u

4-96543
•00858

•30301 .,
•52360 ..
•52360 „

•52360 cubic mm. = 31-952 cubic
= 523600-0 „ n =31-952 „

523-60 „ „ -03195.,

•52360 = -0000310:,

inch.

^Multiples of any of the above follow the preceding example of cubic
measures. Thus, if the diameter of the sphere = 30 /i, then the cube of £
hoing 27 and 523-6 > 27 = 14137'2 cubic p. and '03195 x 27 = '86265 cubic T5V?T
inch, is the volume required.

1 1 34 APPENDICES AND TABLES

APPENDIX G

USEFUL NUMBERS AND FORMULA

Paris line = -088813783 inch.

Cubic foot of water weighs 62*2786 Ib. avoirdupois at 62° Fahr.

Cubic inch of water weighs 252-286 grs. at 62° Fahr.

Gallon of water weighs 10 Ib. avoirdupois at 62° Fahr.

1 gallon = 277-27384 cubic inches.

Cubic foot of sea water weighs 63-96 Ib.

Weight of sea water = 1-027 weight of fresh water.

1 inch of rainfall = 100 tons per acre.

Pressure of water in Ib. 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 A/height.

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 fathoms 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 Ib. per sq. foot = 0-002288 (velocity in feet per
second)2.

Areas and Volumes

Area of triangle = base x £ 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 | height.

Velocity of light = 186,377 statute miles per second.1

Wave-length of yellow light = inch.

•iolUO

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.

2 ~. v/S= 8-025 V/S'.

AritJuin Ural Progression

A, first term ; B, last term ; S, sum ; d, difference between terms ; •«,
number of terms.

1 Latest determination by Prof. Newcoinb, of Washington.

USEFUL TO THE MICROSCOPIST 1135

Geometrical Progression
m, multiplier or divisor.

B 0 B in — A

A= . -B = A in (" 'i>. b =

m (" m — 1

Properties of Circles and Spheres <<T.

77 = 3-141592653589793 + log TT = 0-4971498727

77- = 9-8696 A/?7= 1-77245 =-31831

77

1 = -10132 - = -56419 - = -785398

77~ A/77

- = -5236 A/2 =1-41421

Circumference, C. Area, A. Radius, r. Diameter, d. Volume, V.

Surface, S.

x?3 / '

/-If) T * O 1'' \T y7

6 ' ~TT'

,. - degrees in arc x. area of circle
Area of sector of circle = — .

Length of arc = number of degrees x -017453 r.

Unit of circular measure = 57°'2957S.

Side of square of equal area to a circle = r \/TT.

Side of inscribed square /• \/2.

Area of ellipse = ^ major axis x i minor axis x 77.

Volume of spheroid = polar axis x (equatorial axis)- -.

6

Number of Threads per inch in Whitworth's Standard Screws
Sizes ^ . . No. of threads 150 Sizes . No. of threads 20

„ A • 80

^ CiO

i: B'l ' " »

„ * • . 40

., ^j . . ,, 32

,,. • »

I . . „ 16

A • 14

A - . „ 12

,,

Convenient Approximations for rapid Calculations

6 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 = J inch.

1 pole = 5 metres ; 1 furlong = 2 hectometres.

5ft = -nW inch ; T fa inch = £ mm. ; TuoWo incl1 = ? M-

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 / 5 (ounces) ; 4 litres = 7 pints ; 2 grammes = 31 grains ;

I 136 APPENDICKS AND TABLES USEFUL TO THE MICROSCOPIST

4 grammes = 15 (drachm) (apothecaries') ; 7 grammes = 4 dr. (drachms)
(avoirdupois).

5 kilogrammes = 11 Ib. (avoirdupois).

50 kilogrammes = 1 cwt.

NoberVs 19 Band Test Plate
Band Lines per inch Band Lines per inch

1 11259-5

15 90076-1

19 112595-1

5 33778-5

10 61927-3

Difference between each band = 5629-75.

Nobert's last 20 Band Test Plate
Band Lines per inch Band Lines per inch

1 11259-5

5 56297-6

15 168892-7

20 225190-3

10 112595-1

Difference between each band = 11259'u.

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-ECH*,,; ,-;**-», l-^K-^l,

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 = 20i 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 Microscopical Soi-irtifs (./•"/ //r.v

Substage 1-527 inch = 38-786 mm.

Eyepieces. No. 1 -9173 „ -_|:;-:500 „

„ 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 d'o not, how-
ever, supply standard screw gauges. Full particulars regarding the
screw will be found in the 'E.M.S. Journal,' IS'.u;. p. :i89.

INDEX

ABB

A

Abbe (Prof.), 011 amplifying power of lens,
26 ; on homogeneous immersion, 28 ;
011 improvement of optical glass, 32 ;
on classification of eye-pieces, 34 ; on
principle of microscopic vision, 43, 44,
45; on definition of aperture, 44; <>n
aperture, 48 note ; on radiation, 57 ;
on angle of aperture, 60, 61, 62 ; on
diffraction, 63-75 ; on ' intercostal
points,' 73; on 'penetration,' 82; on
over-amplification, 90; on stereoscopic
vision, 90, 94 ; on ' aplanatic system,'
94 ; on orthoscopic effect, 95 ; on Rains-
deii's circles, 106 ; on solid cones of
light, 418

— his stereoscopic eye-piece, 102 ; camera
lucida, 281-284 ; apertometer, 307,
390-89(5 ; 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 note; chromatic,
31 ; spherical, 31, 301, 306, 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 zociids of polypes, 862 ;

relationship to hydroids, 872 ; develop-
ment of, 874 ; medusan phase of, 877

Acanthometra xipliicantha, 850; eclii-
noides, 852

Acanthometriiifi, 846, 852 ; central cap-
sule of, 852

Acarina, eggs of, 1004-1006 ; anatomy of,

ACT

1009-1012 ; larva? of, 1009 ; nymph of,
1009; integument of, 1010; legs of,
1010 ; eyes of, 1011 ; classification of,
1012

Accommodation, of the eye, 88 ; depth, 89
Aretabularid, 563; pileus of, 563
Acetic acid, as a test for miclei, 517
Acheta, 987

— cwnvpeatris, eggs of, 1005

Aclilya, zouspores of, 564 ; oospores of,
565 ; zoosporanges of, 640

— proli fera, 563 and note, 563
AcJmanthece, characters of, 615
Ac-hnanthes, frustules of, 588, 615 ; stipe

of, 588, 615 ; ' stauros ' of, 616 ; struc-
ture of frustule, 615

Aclinantltes longipes, 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 pyciiogonids, 959

— doublet, Fraunhofer's, 148 ; meniscus,
376

— lenses, Charles's, 148 ; Marzoli's, 353 ;
Selligue's, 354

- objectives, 19, 32 ; Tally's, 354 ; Wen-
ham's, 361, 362 ; cover slips for use
with, 439

— oil condenser, Powell and Lealand's,
310, 311

Achromatism, 17, 19, 150 ; in photo-
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

Art inia, reproduction from fragments,
863
en ml nla, thread-cells of, 879

- crassiconiis, thread-cells of, 879
Aftinocijclus, 588, 610, 620
Actinouuua inennc, 850, 851
Actinoplirys, 846

— form of Microgromia, 736

- sol, 737-742
Actiuopti/clnts, 588, 610, 611

•1 D

H38

INDEX

ACT

ActinosplicErium Eichornii, 741
Actinotrocha, 950
ACTINOZOA, 863, 877-Hw:;
Actius, on myopy, 118
Actuarius, on myopy, 118
Adams' variable microscope, 142; non-
achromatic 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; 'Continental1
type, 162 ; Swift's vertical side
lever, 16?, 173, 174; Campbell's
differential screw, 164, 165, 175,
230 ; Zeiss's, 166, 175 ; Reichert'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

.^Ecidiospore generation of Pitccinia,

637
JEcidium berberidis, relation to Puc-

cinia, 637

— tussilaginis, 638

JEthaUum septicum, plasmode, of, 634
Affnricus, 647

— campestris, 648
Agate, 1095

Agave, leaf of, 686 ; raphides of, 696
Agrioii, 987

— pulchellum, wing of, as test for defi-
nition, 426

— pueila, 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 Fiisiilhui, 826;

in Niuinnidites, 8'27, 831; of Calca-

rina, 830
Albite, 1080
All luminous substances, tests for, 516,

517

Alburnum, 704, KM
Alcohol, as solvent for resins, A:c., 517;

as hardening agent, 4)S4
.\/i-i/onaria, 877, 879; spines of, imi-
tated, 1101

Alcyonian, resembled by poly/onn, llnis
.\/i-i/iniii/iii»i; 908 ; polyxoary of, 909

- f/i'/iitii/'Osinii, calcareous spicules in,

908 note
llcyonium i/ii/ituf/m/, HT.\; spiculeg of,

SSI I

Alder, on branchial sac of Cor/'lln, 912

note
Alexander, on mynpy and prcsbyopy, 118

ANA

ALG.E, preparation of, 514 ; included
under general term of ' thallophytes,'
540 ; symbiotic in radiolarians, 848

- lime secreting, 1084

Algal constituents of lichens, 650

Alkaloids, micro-chemical examination of,
1103

Allman's experiments on luminosity of
Noctiluca, 769

Allman, on Polyzoa, 909 ; on the ' Hau>
of Appendieularia, 918; on Myri<>-
thela, 863 note

Aloe, raphides of, 696

Alternation of generations in Batraclio-
spermniti, 575 ; in Fungi, 634; in ferns,
680; in Medusa, 877

Althaea rosca, pollen-grains, 721

Alveolina, 804 ; resembled by Loftusia,
818; resembled by Fusulina, 825

AmaranthacetB, pollen-grains, 721

Ainnrantliiin hypochondriacus,&eeAs of,
724

. J iiiurouciiim 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

Ammodiscus, 814

Ainmothea pycnogonoides, 958

Amoeba, 733, 742-747, 1018

.4.wce&rt-phase of Monns, 756

Amoeba proteus, 742
- rii/liosa, experiments on, 743

Aincelxe, cells of sponges resembling,
855

Amoeboid phase of Tetramitits, 761

Amphibians, plates in skin of, 1026

Amphioxus, affinities with ascidians, 917
note

Ampliipleura pelluclda, with oblique
illumination, 59, 75 ; resolution of, 85 ;
markings measured, 274 ; markings
on, 592

Amphistegina, 827; internal cast of,

841

Amphitetras, 614
Auipliiuina, red blood-corpuscle (jf, licii;

AinjiJ/uni/.r, haustellium of, 992

Amplification, SS-'.H)

— linear, 26, Ml); of images, 45
Ampullaceous sacs of sponges, 856, 857
Anaboena, 548

Anacluu'is, 5-js

— alsinastri/ni, eyelosis in,(;s;t ; habitat.
690

A>iai/n//it, rMphides nf, ('.'.M'.; seeds of,
724

iiri'fiinin, petals of, 719
Anal plate of .\ nti-tlmi, '.Mil!
An<ilii<-xin<r, 1013, 1014
Analyser, 318
Analysing nose-piece, -J'.M

INDEX

1139

ANA

Analysis, micro-chemical, 1102 ; method

of, 1102

A/iartijihidete, 599
Anchor-like plates of Synapta, 895
Andalusite, 1077
Androspore of (Edogonium, 572
Anemones, 803. See ACTIXOZOA
Anemophilous flowers, 722
Anetlmm graveolens, seeds of, 724
Angle, extinction, 1079

• of incidence, 3 ; of refraction, 3 ;

of aperture, 61
Angles of aperture, air, balsam, oil,

water, 83-87
Angstrom's law for the absorption of

light rays, 323
Angnillnla acefi, 945

— fiuviatilis, 945

— glutinis, 1)45
Anguillulce, 045
Angular aperture, 395

— of dry objective, 391 ; of oil immer-
sion, 391
- of aperture, resolution dependent

on, 44

- of water immersion, 391
Angular distribution of rays, 56 ; grip,

61 ; semi-aperture, 77
An-gitliferce, characters of, 613
Animal kingdom, two divisions of, 727
Animalcule cage, 346
Animalcules, 753. See ROTIFERA, Infu-
soria, RHIZOPODA, &c.
Animals and plants, differences between,

530

Anisochelse of sponges, 860
Anisoneina, 765
Annual layers in trees, 704
Annular cell, Weber's, 350

— ducts of Phanerogams, 698

- illumination and false images, 419

— illumination for examining perforated
membrane of diatom, 419

Annulata (Annelids), larvae 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

Amiulus of sporange of fern, 676

A notion, pearls in, 923; gloclu'dia of,
933 ; for observation of ciliary motion,
940

Anomia, prismatic layer in, 924

Aiuiphi (Nemertines), 951

Anoplophrya circidans, 774

Anorthite, 1080

Antedon, food of, 771; pentacrinoid
larva of, 900-902 ; pseudembryo of, 903

Antennas of insects, 987; preparation of,
988, 989 note

Antherid of Vaucheria, 563; of Churn,
577, 578; of Fncacece, 627, 628; of
Fl<»-itlt'<<-, (531 ; of Peronosporeee, 638 ;
of Marclitiiitiu, 6(55, 6(57 ; of mosses,
(570 ; of Spnagnacece, 673 ; of ferns,
677 ; tapetal cells in, 678

APU

Antherozoids, 536, 540; of Volvox, 555;
of Vaucheria, 563; of Sphceroplea,
572; of (Edogonium, 572; of Batrti-
chospermum, 574; of Chnra, 579; of
PhcEosporece, 627; of Fucacece, 628;
of ferns, 678; of Rhizocarpece, 681

Anthers, 719

Anthony (Dr.), on pseudo-tracheie of fly's
proboscis, 990 note

A nthopli t/sa , 765

Antirrhinum umjus, seed of, 724

Apertometer, 195, 390 ; Abbe's, 307, 394;
Tolles', 390 ; use of, 394

Aperture, in microscopic objectives, 33,
42-50, 60; how obtained, 45 ; Abbe on
definition of, 45, 48 note

— angular, 49 note, 53, 395

- numerical, theoretical maximum of,
30

- 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, 390

— relation of, to power, 82, 83, 3(53 ; as-
certained by vertical illumination, 338

- table, Royston-Pigott's, 30

- numerical, table of, 84-87
Apertures, relative, 49
Aphanizomenon, 548
Aphanocapsa, 547

Ajiliides, wings of, 998, 999 ; agamic re-

production in, 1006
Aphodius, antennas of, '.is*
Apida, 987

Apia meUijicn, mouth-parts of, 990
Aplanatic system, 20,23

— objective use of, 21

— cone, 307

— aperture, 309, 315

— foci, Lister's discovery, 355
Apochromatic objectives, 19, 30, 33,

35, 80, 258 ; advantages of, 33, 34 ;
objective, Zeiss's, 365-371 ; dry, 3(5* ;
comparison of, with chromatic and
achromatic lenses, 368 ; homogeneous
objectives, value of, in study of
monads, 762 ; objective, use with
various test scales, 97(5

— condenser, Powell and Lealand's, 302
Apochromatism, 366, 369

laticiferous tissue of
695

Apogamy in ferns, 680
Apospory in ferns, 680
Apotheces of lichens, 650, 651
Apparent creation of structure, 6S
Ajijii'inlii-ularia, 911, 917; pharyngeal
sac of, 917 ; tail of, 917 ; notochord, 918 ;
'Hans' of, 018

Apple, raphides in bark of, 696
Apposition, growth by, 533
- mode of growth of starch, 695
Apus, 959, 962; parthenogenesis of, 964
note
i-ii ncrifortnis, carapace of, 9(52

4 D 2

1 140

INDEX

AQTT

Aquarium microscopes, 260-269 ; J. W.

Stephensoii's, 267-268 ; Rousselet's,

268-269

Aquatic microscope, 147
ABACHNIDA, 1008

— eggs of, 1005 ; related to Pycnogonida,
959 note; reproductive organs of, 1011

Arachnoidiscus, 588, 612

Araclinosplicera obligacantJia, 850, 852

Aragonite, 1095
- in shell of Pholas, 924

Aralia pajji/rifera, parenchyme of, 687

Araneiila, 1008

Arcella, 746

Archegoiies of Vaueheria,563', of.Cha/ra,
577,578; of Marcliantia, 665, 668; of
mosses, 670, 671 ; of Spliiignacece,
673; of ferns, 677, 678; of Lyco-
podiece, 681 ; of Rhizocarpeee, 681

Archer, on amcebiform phase of Stepha-
nosplicEra, 557 note ; on desmids, 579
note ; classification of, 585 ; on
ClatJintl/Hd, 742 note

Archeriitti Bultoni, 730

Arctium, stem of, 709

A rcyria flava, sporanges of, 635

Arenacea, 810-814

Arenaceous character of Tetctulariniee,
823

— Foraminifera, varying size of particles
in test of, 818

— test of Foraminifera, 810
Arenicola, 948

Areolae of frustule of Coscinodiscus, 591

Areolar connective tissue, 1040, 1045

Argas, bite of, 1012

Argasidce, 1012

Argonauta, 929

Argosince, 1008

Argulus fotiaceus, 966

Aristolochia, stem of, 709

' Aristotle's lantern ' of echinids, 890

Aristotle on myopy and presbyopy, 118

Arsenic, micro-chemical analysis of, 1103

A Hernia, 962

— salina, movement of, 960 ; habitat of,
963

Arteries, 1056

ArtJwodesnms incus, 568

ABTHBOPODA, 957-1016; smallest of, 1008 ;
eye of, 983

- limbs of Pednlion compared with
those of, 792

Arthrosporous Bacteria, 657

Artificial light, 417

'Artificial lightning,' C,s--!

Ascarix lumbricoides, '.144

Asci of Ascomycetes, QiZ; of lichens, 650

Ascidians, diatoms in stomach of, ti!4,
623; solitary, 911; branchial sue of,
912, 913, 915 ; circulation in, 912, 91.-) :
compound, 912 ; cloaca of, 913; stolon-
of, 914; bibliography of, 914 nott :
social, 914 ; general structure of, 916;
di'\-c'lo|nni.|ii ol', '.Mt', : t;iil|iolc of, !H7 ;
Efinities with AmpTiioxus, 917 note

Asclepiadeee, pnllhmnu «!', 722

BAG

Ascogoneof Ascomycetes, 643; of lichens,.
651

Asriniii/f-etes, 642-645; as fungus-con-
stituents of lichens, 651

Ascopores of Ascomycetes, 642; of
lichens, 650

Asellus aqiiaticus, ciliated parasite in
blood of, 774

Asilu-s, eye of, 987

Aspergilli/s, fermentation by, 647

Asphalte for cells, 446

— varnish, 443

Aspidisca, a phase in development of

Triclioda, 781

Aspidimii., 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
Asteromphal/us, 610
Astromma, 849
Astrophyton, spines of, 891
Astrorhiza, 811, 815
Astrorhizida, 812
Atliecata, 868
Atlnirium Filix-fceinina, apospory inv

680

Atrium of Noctiluca, 766
Auditory vesicles of Mollnsca, 941
Audouin, on ' muscardine,' 645
Augite, 1072 ; zonal structure in, 1073
Aulacodiscus, 612

— Kittonii, markings on, 591
- Stnrtii, markings on, 592

Autofission of diatoms, 594

Auxospore, 594-601

Avanturine, 1095

Avicularia of Poli/zoa, 910, 911

' Awns ' of Clicetocerece, 614

Axile body of tactile papilla, 1053

Axinella paradoxa, 858

Axis cylinders of nerve-tube, 1051, 1052'

Bacill <n-i<i me of Kiitzing, 587
Bacillaria varudoxa, movements of,,

602, 606

Bacilli, form of, <;.">:;
Bacillus, ' granular spheres ' of, 660 note
— mitliracis, 656; spores live in abso-
lute alcohol, 660

- megaterium, 655

— of anthrax, 10:17 note

- of tuberculosis, modes of staining,.
515, :,it;

- mil>tiUx, <;.")<;, 6.17, 6.-.H ; spores of, 660
' Bacon-beetle,' 980

Bacon (Roger), inventor of simple micro-
scope, 126

liacti-r'ui, use of large and small cones in
r\:i,mining, 421; photo-micrographs,
423; as test for definition, 426; staining,
514-516; (see Schizonii/cetes], 651;
M (Unities to A/t/ic, 651; to Flagellata,

INDEX

BAG

•651; to Nostocacfcc, 652; movements

of, 652; mode of multiplication, <i.V2 ;

forms of, 653 ; classification of, 055 ;

nutrition of, 658; flagella of, 659;

germinating power of, 660 ; spores of,

660

Bacteriastnmi furrutum, 614
Bacteriology, 662
Bacterium lineola, compared with

Cercoinonas, 651
— lineola, 658

- termo, flagellum of, 72, 658; move-
ment of, 653 ; zoligloea of, 659 ; germi-
nation of, 660

Bailey, on internal casts of Fomn/ini-
fe.ru, 828 note

Bailey's method of isolating diatoms,
024

Baker, on Cuff's microscope, 142

Baker's microscopes. 202, 218-221, 230,
246, 251 ; mechanical stage, 181 ; sub-
stage, 188, 189 ; achromatic condenser,
306 ; fitting for condenser, 312

- lamp, 407
BalanidcB, 967

Bcihtuus balanoides, 967; disc of, 968

Balsam angle, 50, 7H
— refractive index of, 77

Banksia, 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 Cirripi <li<i

Basals of Antedon, 9ol

Basidiomijcetes, 647 ; as fungus consti-
tuent of lichens, 651

Basidiospores of Basidiomycetes, 647 ;
of Hymenomycetes, 047

Basids of Piiccinia, 637 ; of Basidiomy-
cetes, 647

Bast, 710

Bat, parasite of, 1012; hair of, 1030;
cartilage in ear of, 1046

' Bathybius,' 747

Batrachia, red blood-corpuscles, 1035;
lungs of, 106'3

BatrachospermecB, 574

BatracJiospermum •moiiiliforini', 575

- protoneme of, 575

' Battledore scale ' of Lijccenidfe, 975

Bausch and Lomb's microscopes, 212-214,
222, 239, 247, 252; mechanical stage,
183, 184 ; sub-stage, 212 ; chemical
microscope, 263 ; camera lucida, 285 ;
objectives, 375

Bdella, maxillary palps of, 1010

Bdellidie, 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 ;

SIR

condensers, 304, 305 ; side reflector,
333 ; vertical illuminator, 337 ; disc-
holder, 339 ; compressor, 347 ; rings
for locking coarse adjustment, 352 ;
objectives, 375 ; lamp, 405-407 ; achro-
matic binocular magnifier, 456 note ;
disc-holder for examination of Furn-
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 Coleoptera

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
Beueden (Ed. Van), on Gregarina gi-

gantea, 749 note ; on movement of

gregarines, 750
Benzol, uses of, 517
Bergh, on Flagellata, 764
' Bergmehl,' 622
Berkeley a, 602
Bermuda earth, 608, 611
Beroii, collecting, 529

- Forskalii, ,881, 882

— ovatus, Eimer on, 882 note
Bicellaria ciliata, 910

Biconvex lens, formulas relating to, 21
Biddulphia, 612

— cyclosis in, 587 ; chains of, 588, 596 ;
structure of frustule, 590 mite

Biddulphietz, character of, 612

Biflagellate monad, 759

Bignonia, seed of, 724

Bignoniacea, winged seeds, 724

Biloculina, 802

Binary subdivision of cell, 535, 536

Binocular eye-piece, Tolles', 101 ; Abbe's,
102

Binocular magnifier, Beck's achromatic,
456 note

Binocular microscope, 61, 97

- Riddell's, 96 ; Nachet's, 98 ; stereo-
scopic, Weiiham'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 ; Rousselet's portable, 245 ; Ste-
phenson's for dissection, 248, 401, 456 ;
spectrum microscope, 327

Biology, 530

' Bipimiaria,' resemblance of Actiim-
trocha to, 95C

BipiniKiriti ustrrigera, 897

Birch, pollen-grains of, 722

BIRD, parasite of, 1012, 1014 : lacuna? in
bone of, 1022 ; epidermic appendages
of, 1029 ; red blood-corpuscles of, 1034,
1035 ; lungs of, 1064

1 142

INDEX

BIE

Bird's egg, concretions on shell imitated,
1102

' Bird's head processes,' see Aviculnrin,
910

Bismarck bi'own, 489

Bivalves, structure of ligament in, 10 10

' Black dot,' Nelson's, 277

' Bladderwrack,' 627

Blanchard (R.), on Sporozoa, 749 note

Blatta, antenna? of, 988

— orient alis, eggs of, 1005
• Blenny, scales of, 1027

Blood, colourless corpuscles, 1018; 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-corpuscle, relation of size to that

of bone lacuna?, 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, 420

— development of, 1007 ; ' imaginal discs '
of, 1007

' Blue mould,' 643

Boflo, 545

Body of the microscope. 157

' Bog-mosses,' 673

Boletus, 647

Bombyx, 987
- mori, eggs of, 1005

Boiiannus's microscope, 132; his hori-
zontal microscope, 133, 134 ; his com-
pound condensers, 298, 299

Bone, 1020 ; structure of, 1020-1023 ;
preparation of, 1023 ; matrix of, 1039 ;
decalcification of, 512

Bones, fossilised, 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

BotryUus violaceus, 915

Hiitryoctjsfis, 545

lintrijtix huHshmn, 645

Botterill's growing slides, ;;io; his
zoophyte trough, 348

Bouguet, on uniform radiation, .".I

Bowerbunk, on sponge spicules,,s5:i note :
on slruetiire of molluscaii shells, '.i-JI,
928

Bowi-rliiiii/ti/i, gi/.xard of, 905; stem of,
'.MIS; poly/oaries of, 909

' Box-mite,' 1012

Brachinns, antenna- <if, 9N.s

Brachionus rubens, 7^7-790,791; male
of, 7:io

BBACHIOPODA, shells <.f, 919, 925 927 :
relation oi shell to mantle, 926 ; jftini-

ties tn I'nl/1 oa, '.''-! 7

Bract ipod vomi'j of, (.M'>9

BUT

Brady (H. B.),on Fora n> in if era, 810; on-
arenaceous Foraminifera, 811 ; 011
affinity of Ca/rpenteria-, 823

Brady and Carpenter, on fossil LituolcB,.
817

' Brake-fern,' 675. See Aspidiuni

Bran, 725

Branchiie of annelids, 948, 949

Branchiopoda, 959 ; divisions of, 961

BrancMpus, movement of, 960

- stagnaUs, 962, 963
Branchiura, 965 note, 966

Brandt (K.), on artificial division nf
Actinosphcerium, 741 note ; on
zooxanthellft-, 84<s

Braun, on Pediastrnm, 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 Triceratium, (i!3 iiotr :
on Chcetocerece, 614 »«//•

Brilliancy of image, 382

' Brimstone moth,' eggs of, 1005

Brine shrimp, 960, 963

'Brittle stars,' 891. See Opliiiiroidca

Brooke's nose-piece, 291

Brownian movement, experiments, 431,
432

Browning's bright-line spectro-micro-
meter, 325

Briicke lens, 3H

Brunswick black as a black ' ground,'
444 ; for cells, 446

Uri/iicea, 673

Bryobia, 1013

Bryony, cells of pollen-chambers, 720

Bnjozoa, 904. See POI.YZOA

Bri/um intermedium, peristome of,.
072

Bubbles in cavities of crystals, 1074

BncciiiiiHi, 937

- initliit/tiii, palate of, 930, 932,933;
nidamentum of, 934

Buchner's experiment on spores of

Bacteria, 660
Buckthorn, stem of, 70:'.
Bug, mounting medium for, 973
Bugula, polyzoary of, 909
— itr'n-nlnria, 910, 911
Built-up 'cells,' 419
Bulbils of NitrUct, 577
Bulloch's modiliratiiin of /entmayer's

microscope, -0 1
Bull's-eye, :!00 ; use of, 1-529-383, 407-

120; with high power, :'<:',! ; for use in

study of saproplivtic organisms, 833;'

I'ouell and Zealand's, :!•"•:'•
Bull's-eye stand. 248
Bundle-sheath, 711
r.iii-docK. item ofj 70'i
Biitschli, on mouth of Antnu/ii, 765; on

I 'ortici-lhr, l'i'.\ null
— and Engelmann, on conjugating vorti-

cellids, 7H2

INDEX

1143

BUT

Butterflies, wing of, 9114
Butterfly. See Lepidoptera

Cabbage-butterfly, eye of, 983 ; number
of facets in, 983 ; eggs of, 1005

Caberea Soryi, vibracula of, 907 not,'

Cabinet for slides, 523 ; arrangement of,
523

Cactus, cells of pollen-chambers, 720

Cactus senilis, raphides of, 696 ; brittle-
ness of, 696

Cacitmnria crocea, development of, 900
note

Calamites, 1084

CaJatlns, antennae of, 988

Calcarina, 825, 830 ; compared with
Eozoon, 838

Calcispongice, spicules of, 859

Calcite in shells, 92-4

Calco-globuline, 1101

Callitnamnion, 630

Cnliisaiithes indica, winged seed of, 724

Calotte diaphragms, 297

Cahjcanthus, bark of , 709

Calycine monad, 760

Calycles of hydroids, 868

Calypter of mosses, 671

Calyx of Flagellata, 764

Cambium, 710
- in Exogens, C>'.)7

— layer, 708

Cambridge rocking microtome, 4611

Camera lucida, 277 ; Soemmeriiig's, 278 ;
Wollaston's, 278; Amici's, 279; Nel-
son's, 279, 280 ; Beale's, 279, 2,ss ;
Cooke's, 280, 281; Abbe's, 281-284;
Swift's modification of Abbe's, 2.S t ;
Bausch and Lomb's modification of
Abbe's, 285 ; Schroder's, 285, 286

Campaiii's microscope, 128 ; eye-piece, 376

Campanula, pollen-grain of, 721

( 'ampanularia,81Q

— gelatinosa, 865
Campanulariida, «70 ; zoijphytic stage

of, 877

Campbell's differential screw, 162, 164,
165, 174, 202, 230

Campylodiscits, 587, 588, 595; move-
ments of, 602 ; structure of frustule,
606

— eh/pens, 607

— sjiiralis, 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 Ctilrtirhia, 825; of
Polystomella, 827; of Nummulites,
,S27 '

Caiialiculi of bone, 1019, 1021
Cancellated structure of bone, 1020
/!•<•>• pagurus, skeleton of, 968

CBL

Canna, starch-grains of, 695
Cannocchiale, 125
Capacity of object-glass, 382
Capillaries, 1056. 1062
Capillitium of Myxomycetes, 636
Capsule, central, of Radiolaria, 847
— of mosses, 670 ; of Pnrpura.,934
— silicious, of Clathrulina, 742
Carapace of Copepoda, 960 ; of Clado-

cera, 961
Carbon bisulphide as a solvent for oils,

&c., r,17
Carboniferous epoch, vegetation of, 681

- limestone, 1090
Carches-ium, collecting, 527
CarciiiKS nurnas, metamorphosis of,

970

Carnation, parenchyuie of, 688
Carnivora, arrangement of enamel in,

1025

Carp, scales of, 1027
Carpenter (H. P.)j on crinoids, 903 note
Carpenter (W. B.), on stereoscopic vision,
90-93 ; on classification of Fora/mini-
fera, 799 ; on Eozoon, 838 ; on alter-
nation of generation in Medusas, 877 ;
on the so-called excretory pores of
CtenopJiora, 882 note ; on development
of Antedon, 903 note ; on structure of
molluscan shells, 921
Carpentaria, 822; mode of growth com-
pared with Eozoon, 838

- rhaphidodendron, 823
Carpogone of Floritlecr, 632 ; of Ascomy-

cetes, 643

Carpospores of Floridece, 632
Carrot, seeds of, 724
Carter (H. J.), on affinity of Carji/'iit/'i'/ti ,

823

Cartilage, 1046 ; mounting, 1047
('urn in rnnii, seeds of, 7'JI
Caryophyllia, lamella? of, 878

- Smithii, thread-cell of, 879

Cased rilla, raphides of, 696

Cassowary, egg-shell of, 1101

Castracane, on beaded structure of di-
atoms, 593 ; on Pfitzer's auxospores,
595 ; on sporangia! frustules of
diatoms, 595 ; on reproduction of di-
atoms, 597 ; on diatoms, 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

Caiderpa, 563

Cauterisation by focussing the sun's rays
(Pliny), 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 Polijzoa, 904

Celloidin imbedding method, 503-506

- staining and mounting, sections, 506
Cell- sap, 534

1 144

INDEX

CEL

CHE

'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, 440 ; 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
zob'cytium of Opforydium, 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

•Cementum of teeth, 1025, 1026

Centipedes. See Myriopoda

Central capsule of Radiolaria, 734

Centring, 382, 389

Centring nose-piece, 293 ; as sub-stage,
230

Centro-dorsal plate of Antcdon, 902

Ceplialolitliis sylvi-na, 847

Cephalophorous mollusca, palates of,
930-933

CEPHALOPODA, 929

— organs of hearing in, 941 ; chrumato-
phores of, 942

CeramiacetB, 630
Ceramium, 630
Ceratium, 771
-fnrcd, 771

- tripos, 771
Ceratodus, 1091

Cercoinonas typica, compared with Bac-
teria, 651

Cereals, seeds of, starch in, 694
Centra vinula, eggs of, 1005
Cestoid, 943

Cetonia, antennae of, 988
CJicetoccretr, affinities of, 614

— ' awns' of, 614

— occurrence in marine animals, 614
Ghcetoceros WigJiainii, 614

< '//</ tophoracees, 573

' Clul'f-scales,' silex in, 715

Chalk, microscopic constituents of, 10N5,

1087 ; resemblance to GJobi-f/rriini

ouzo, 10.S7 ; mode of preparation for

examination, 108«
('In/ inn, prisniiit.ie layer in, 924
Chamberlets in l-'<>r<in//i/ij'rr<i, 7'.».s, ,sn:i,

804; of Parkrriit, 817; in Fusiiliini,

825; of Cyclocfypeus, 835
Chct >ii/<lt<', J''or<tn/iiiil'ir<i ;i,tt;i<-hed to

shells of, 845

Changes of form of white corpuscles, 1037

Chantrdnsia and Batracho8permum,575

Chara, 576, 669 ; antherozoids of, 667

CharacecB, 575-579

Charles's achromatic lenses, 148

Cheese-mite, 1008, ]013

Cheilostomata, characters of, 909 ; ex-
amples of, 910

Cheirocephaliis, 962, 963

Chemical tests for biological 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

Cheijleti, 1013

CheyletuitB, tracheae of, 1011

Cheyletits, hairs of, 1010; legs of, 1010;
mouth parts of, 1010

Chickweed, petals of, 719

Chicory, adulteration of, 725 note

Chilodon, mouth of, 774

— citciiMulus, biliary division of, 777, 779
( 'hilognatha, 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

< 'hlamydomonas, 545
Chlamydomyxa, affinity with Moiiero-

zoa, 727

Ghlamydospores, of Murorini, 641; of
gregarines, 750

Chloral hydrate as a preservative me-
dium, 519

Chloroform, uses of, 517

Chlorophyll corpuscles, 534, 535

Chlorosporece, 625

CHORDATA, 911. See VERTEBKATA

Choroid coat of eye, pigment-cells in,
1042

Chromatic, comparison of, with achro-
matic and apochromatic lenses, 368

— aberration, 1(5, 17, 31

- condenser, Abbe's, 308, 309 ; 311, 312 ;
385

- Powell and Lealand's, 801-303

— correction, test for, 388

— dispersion, diminished by Huyghens'
objective, 42

Chi-'jmatophores of Periiliiiiinti, 770 ; of

Cephalopoda, 942
( Inromatoplasm, 537
Chroiicocnicftr, ehanu-ters of, 547

< '/iroiiriii-i-iifi, 547 ; as gonid of lichen, ('>.">!

< '//i-i""t/r/>ns, as gonid of lichen, (I">1
<'/tr//Ni«>r<t, 874, 87(i ; development of,

876

INDEX

1145

CHY

Chyle, corpuscles in, 1037

Chijtridiacete, 636

Cicada, wings of, 998, 999

Cichoriacece, pollen-grains of, 721

Cicindela, 987

Cidaris, spine of, 885, 888

— metularia,mo&e of formation of spines
in, 889

Cienkowski, on decaying cells of Nit ell a,
579 note ; on parasitic plasmode in
Nitella, 579 note ; on reproduction of
Noctiluca, 769

Cilia, 532, 1044 ; of Infusoria, 771 ; use
of, in Ci1i<ita,7T3\ of TurbeUaria, 94(3

Ciliary action, 772

— motion on gills of MoHusca, 940

— movement in protophyes, 535
Ciliata,771-7U3; ciliary action of, 772,

774 ; ' shield ' of, 773 ; lorica 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 aciiietiform young of, 782 note

Ciliate Infusoria, general structure of,

754

Ciliated epithelium, 1044
Ciliobrachiate zoophytes, 905
Cilio-flageUuta, 770
Cilium of Noctiluca, 766 note
Cimex lectularius, eggs of, 1005
Cinchona, raphides of, 696
Cinclidium arcticum, peristome of, 672
Cineraria, pollen-grains of, 7'22
Cineritious matter, 1052
Circulation in ascidians, 912, 915
- of blood, 1054
Circumambient chamber in Orbitolitcs,

806

Cirrhi of Cirripedia, 968
Cirripedia, 967
Cladocera, 961
Cladocorcns ri»iiii(il/n, 851
Cladon/a fitrcatu, 650
( 'ladophora //l<»i/rr<it(t, 574 ; cell division

of, 569, 574

Cladorhiza ii/rfrsn, 860
Claparede and Lachmami, on Lieber-

kuehnia, 731 ; on ' rolling ' movement

of Amoeba, 744

Clark (James), on Flaycllata, 764
Clastic rocks, 1075
t'liitJii-iiliitu e.legans, 742
Clausius on emission of light, 54
CliivrJinidie, gemmation of, 911 ; stolons

of, 914

Clnriceps p/trp/t ri'ii, 644
< '/iiricorn/ii, antennae of, 987
Claws, 1029, 1033
Clay, 1092

Cleanliness, importance of, 522
' '/fin fit is, stem of, 702

COL

' Closed ' bundles, 710

Closterium, cyclosis in, 581 ; ' swarming
of granules ' in, 581 ; binary division
in, 582; two zygospores in, 584 noti-\
zygospore of, 584 ; form of cell, 585

Clostridia, form of, 653

' Clothes-moth,' 999

Clove-pink, seed of, 7'2:;

' 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

Cobcea, testa of seeds of, 725

— scandens, pollen-grains of, 721
Coccidia, 752

CoccidiidcE, 749

Coccidium oviforme, 752

Coccoliths, 747-749 ; in chalk, 1084, 1088

Coccoiieidece, characters of, 614

Cocconeis, 615

Cocconenia, 602, 616

— fusidium, 621

Coccospheres, 747-749 ; in chalk, 1088
Cockchafer, antennae of, 974. See Melo-

lontlia

' Cockle ' in wheat, 945
Cockroach. See Blatta
Cocoa-nut, 725
- shell of, ( Ui:;
Cocos-wood, 704
Coddingtoii lens, 37
Codiutn, 563

Codonella, silicious shell of, 773
Codosiga umbellata, fission of, 7t'>4 :

arborescent colonies of, 765
CCKLENTERATA, 862-883 ; bibliography of,

883; permanent gastrula- stage of, 726

— See ZOOPHYTES
Cceloplana, 883

('..•iiosarc, of hydroids, 867, 870

Ccenurus, 944

Colin, on sexual generation of Volrva.-,
555 ; 011 movements in Oscillator ia , 548 ;
on reproduction of Sphceroplea, 570

ColeocJicstacece, 575 ; zoospores of, 575 ;
trichogyiie of, 575

ColeocJuete, 575

Coleoptera, 973 ; dermo-skeleton of, 974 ;
scales of, 975 ; elytra of, 981 ; eyes of
983, 987 ; antenna' of, 1)87, 988 ; month-
parts of, 989 ; wings of, 999 ; leg of,
1000

Coleps, food of, 776

Collar correction, 35s

Collared cells of sponges, 856

'Collars' of FlageUata, 764

' Collateral ' bundles, 710

Collection of microscopic objects, appara-
tus for, 526-529

CoUembola, 977

CoUetonema, 602

Collins's condenser with rotating sub-
stage, 386

Colloiuia testa of seeds of, 725

1 146

INDEX

COL

CoUomia grandiflora, spiral fibres in

seeds of, 693
Collozoa, 852
Colonial Acinetlna, 784
Colonies, in Codosiya, 764 ; of Radiola-

rians, 849; of Polijzoa, 9'24
Columel of Sphagnaceee, 674
Comatula, 900, 901; nerves of, 1052
' Comb-bearers,' 881. See CtenopJwra
Comrnensalism, in lichens, 650
Compensating eye-pieces, 34, 273, 878
Composites, laticiferous tissue of, 695
Compound condenser, sub-stage, 134

- microscope, construction of, 39 ; in-
vention of, Govi on, 120

Compression of light rays, 57

Compressor, Rousselet's, 346 ; Davis' s,
347; Beck's, 347

Compressorium, 346

' Concentric ' bundles, 710

Conceptacles of Fitcaceee, 627 ; of Mar-
cJtantici, 666

I'liin-liifrr/i, shell of, !(l!l

Concretionary spheroids, 1100

Condensers, 190, 298-316

Kellner eye-piece used as, 196 ;
Gillett's, 204, 300 ; Hartsoeker's, 298 ;
Bouannus's compound, 298, 299 ;
Powell and Lealand's, 301, 302, 310;
apochromatic, 302; Swift's, 802, 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

< '(ii/f/'iTd, 557

< 'i/iifc.rvacece, 569, 570; binary division
of, 569 ; zoospores of, 570 ; resemblance
of Melosircee to, 608

Con/err, f, 945, 960

Conical epithelium, 101 1

Conids, of Ascomycetes, 643 ; of Baxiilia-
//it/ri'tcs, 647

Cuiiiferce, 684; woody cells of, 697

Coniferous wood fossilised, 705, 1083

( 'onjugatce, affinities of, 549

< 'on jugate foci, 13 ; focus, 24 ; image, 24
( 'onjugating cells, 540
Conjugation, a sexual act, 537
Conjugation of Mesoi-fir/mx, 5-19; of

S/i/nii/i/i-iii 549; of I'lo/lii'/.r, 557; of
Wydrodictyon, 5i>5; of Desmidiacece,
.'•si; of diatoms, 599 ; of P/uros/iorecB,
627 ; of Mi/.r<»ni/rrteN, 6;M ; of . I rcelln ,
716; (zygosis) of (1 rei/a ri utc, 751 ; of
Ili-li-rniiiitii, 700; of Tetramitus, 761 :
of \<, i-l iliifii , 769; of (ili'iiniliiiiii in,
770; of r»<l<>/> h ri/d, 785; of Ciliata,
782: <>f Forticella, 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 Actinophrys, 737; of

Microgromia, 737; of Amoeba, 743;

of Infusoria, function of, 754; in

Flagellata,764; of P<irnu/eeiitm,n6;

of Ciliata, 776; of Stentor, 111; of

Botifera, 789
Convergence of light, 18
Convergent light in petrology, 1070, 1078
Conversion of relief in spectroscope, 92
Convol/uulacece,].a,iicifeiouB tissue of, 695
('iii/raJf/tlim, pollen-grains, 721
Copepoda,96Q', classification of , 965 note
Copeus cerberus, 791
Copper sulphate, crystallisation of, 1096
Coquilla-nut, 725

— section of, 692

Coralline crag, microscopic constituents

of, 1089
Corallines, 960

— conceptacles of, 632 ; ostiole of, 632

- (sertularids), 870

Corals, section of hard and soft parts,
510

- red, 877 ; stony, 878 ; mushroom, 878
CoreUn parallelogramma, branchial sac

of, 912

Coreopsis tinctoria, seeds of, 724
Cork, 708

Corky layer of bark, 70S
Cormophytic type, 668
Cormorant, parasite of, 1010
Conieules of arthropod eyes, 983
Corn-grains, husk of, 725
Cornuspira, 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 ; English,

357 ; Continental, 359
Corroded crystals, 1071
Corrosive sublimate, as a fixative, 484
Corynactis AUni/nnii, thread-cell of, 879
CoscvnodiscecB, ohanu-tiTs of, 608
Cusriiitiilixrus, 588, 620

- cyclosis in, 587 ; frustules of, 589, 590 ;
markings on frustule of, 591 ; areolse
of, 591

asterompJialus, 591 ; for testing lcnsr->.

s, fossil, with embryonal
form, 598
Cosnni rin in, division of, 582; form of

cell, 5.S5

iinl njtix, zygospore of, 584
Cosmic dust, 1098

INDEX

1147

cos

CYP

Costfe of Campylodiscus, 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
Int//n///i, 590 note

Crab, 957 ; metamorphosis, 969 ; blood-
corpuscles of, 1038 ; ' liver ' of, 1047

Crabro, leg of, 974

Crane-fly. Sec Tipula

Cniteriii in pi/r/furnir, 1009

Crayfish, 957 ; young of, 969

Creation of structure by diaphragms, 68

Crib ril in a tit/nl<ii-/x, 906

Cricket, gizzard of, 993 ; wings of, 999 ;
sound-producing apparatus, 999. See
Acheta

('r/iiiiii/i-ii, skeleton of, 892; larva of,
898

Crisia, 909

Crisp (F.i, 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,
333

' Crow silk,' 569

Crown glass, refractive index of, 5 ; com-
position of, 32

Crusta petrosa of teeth, 1025, 1026

CRUSTACEA, 957-971

- larvae of, collecting, 529
CRUSTACEA, suctorial, 965

— collecting, 970 ; preserving, !)71 ; com-
pound eyes of, 982 ; pigment-cells of,
1043 ; ' liver ' of, 1047 ; concretionary,
spheroids in shells of, 1100

CBYPTOGAMIA, 530-6.S3

- preparation of, 514 ; structure of, 532-
535 ; reproduction of, 535-549 ; litera-
ture, 683 ; passage to PHANEROGAM i \,
684 and note

Gryptoraphidea, 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
Crystallites, 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-

scopical structure of, 1075 ; optical pro-
perties and chemical constitution,
1078, 1079 ; as microscopic 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
Ctfini ri<i ftenopliora, 877 note
Ctenoid scales, 1028

Ctenophora, 877, 881, 883; excretory

pores of, 882 note
Ctenostoinatit, characters of, 909
Cucurbitacece , pollen-grains of, 721
Cuff's micrometer, 142 ; microscope, 142
Citlicidte, antennas of, 988; larvae, blood

of, 994
Curculio, antennre of, 988

— imperial is, scales of, 975; elytra of,
981

Cure ii lion id IP, 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 Goni«^l<i

Cuticle, 1041, 1042

— of leaves, 713; of CHiata, 773
'Cutin, 71:-!

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

Cf/niiicii capillatu, ephyra? of, 874,875;
scyphistoma of, 87.") ; strobila, 875

('//.iiifhttN iiihiin; seed of, 724

Cyatholiths, 748, 749; artificially r. re-
duced, liol

( '//'•<"/'•«", 684

('//ens, raphides of, 696

( 'i/i-l(tniiiiiii<i ,-(i>/r, 'lli/tn , 816, 818

Cyclical mode of growth in shell of
Foraminifera, 798

Cycloclypeus, 829 ; shell of, 7'.'*

— compared with Orliif,i!iff>i, 801, 835
Cycloid scales, 1028

Cyclops, eye of, 960 ; larva, 968

— qiHidriconiis, 961 ; number of off-
spring of, 964

Cyclosis, 534 ; in Chara, 576 ; in des-
mids, 5sl ; in Dinfo/inicece, 587; in
Phanerogam cells, 688 ; in plant hairs,
690; infiieberJcuehnia, 732; in Ac iue-
liini, 783

Cf/rlostoniufd (Polij;:o(t), characters of,
'909

Ci/ilippe, collecting, 5'29

— /i/'l, -us, 882

Ci/ii/bcUa, 602

Cymbellece, affinities of, 616

( '//ni/iidce, ovipositor of, 1003

Cyprcea, shell of, 928

Cypris, 960

1148

INDEX

CYS

Cyst, of Protococcns, 544, 551 ; of Proto-
"HIIJJCU, 728; of Clathntlina, 742; of

gregarines, 751 ; of Dallingeria, 759 ;

of Polift oma, 760

Cystic Entozoa, relation to cestoids, 944
Cysticercus, relation to cestoids, 944
Cystids of Hymenonvycetes, 648
Cystocarp of Floridde, 632 ; of Bafra-

chospermum, 574
Cyst op us candid us, 640
Cythere, 960, 961

('//tl/i'i-iii/i, 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, 756-7C3 ; on effects of tempe-
rature on monads, 761
Dallinger (W. H.), on S/i rii-n!<t, &c., as

test objects, 600 note ; on nucleus of

monads, 762

Dallinger' s thermo-static stage, 344-346
DalUni/rria Dri/sdali, life-history and

structure of, 758 ; nucleus of, 762
Dalyell I J. G.), on Hydra tuba, 874
Damans genioulatus, proventriculus of,

1011
Dammar, as a preservative medium, 518 ;

as a mounting medium, 521
Dandelion, laticiferous tissue of, 695 ;

pollen-grains of, 721
DtijiJiiiia, eye of, 960; eggs of, 964;

ephippial eggs of, 964
Diiphnia pulex, 962
Darwin (Charleston Cirripedia,967
Datura, seeds of, 724
Davis, on desiccation of Rut if era, 791

note
Dawson (W.), on foraminiferal nature of

EozoSn, 837

' Day-fly.' See Ephemera.
'Dead-man's toes,' «?'.». See . I Ici/tnii n in
Dean's medium for mounting insects,

973
De Bary, on fungi, etc., 634 note ; on

potato-disease, 640 ; on alternation of

generations in ferns, 680
Decalcification, 512 ; of echiiiodenns,

512 ; of bones, 512 ; of teeth, 512 ; of

Foraminifera, 513 ; of Eozoihi, 513
lii'i'iijiin/a, !)57 ; exoskeletoii of, '.His;

macrurous, !t09 ; brachyurous, lit'i'.i
Decomposition, produced by llae/eria,

661

— of rock-masses, 1076
l>eiining power. 425; tests for, 426
I ii'linilion of image, 382
I i' •jem-ratkiii in T/ni/eafa, 911
I )cliydration, 4*7
I ii-llcbiirrc's microscope, 144
1 >rl/, In ni n i //, seeds of, 72 1
Ddliuili X, le^s of, 1010
- fullieillnrillll, 1014

DIA

De Monconys, his compound microscope,

128
De ndritina, 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 Verfebnifa, 10'26
DermaJeicJii, 1008, 1014
Derma IIIJSSHS, 1012

- larva of, 1009
Denuestes, hair of larva, 980
Descartes' simple microscope with reflec-
tion, 126

Desiccation of rotifers, 791

Desiderata in a microscope, 261-263

Desilicification, 513

DESMJDIACE/E, 549, 579-587; connection
with Pcdiaatrece, 566; sutnral line of,
580 ; cellulose envelope, 580 ; mucila-
ginous sheath, 580 ; primordial utricle,
580 ; endochroire, 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

- Hiintzsch's glycerin method of pre-
serving, 520

Desmidiece, 945

— conjugation of, 584 ; zygospore of, 584

l>< •uniiili/iDi, binary division, 582; fila-
ments of, 583

Desmids. See DesmidiacecB

Deutovium of Acarina, 1008

Deutzia scabra, 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,' it7">

Dianthus, seed of, 723

caryophylltBUs, parenchyme of, (588

Diaphragm, 261, 297, 306, SO.s, 310, 312,
313, 314

with two openings for double illumina-
tion, lot

- Zeiss's iris, 297; calotte, 297 ; in eye-
pieces, 376-379, 381; for use in test-
ing object-glasses, :'.*.">, :;sr,

- in Tally's microscope, 149
Diatoma, 588 ; frustules of, 588, 605

ntlatire, chains of, 605
DIATOM U-K.V., 549, 587-625

- perforated membrane of, examined
with annular illumination, 419 ; mode
ot examination of, 419 ; mounting, 481 ;
silicious coat, refractive index of, 521 ;
stipes of, 588 ; beaded appearance, 592 ;

INDEX

1149

DIA

markings of, 593 ; binary division
of, 594-597 ; reproduction of, 594-601 ;
placochromatic, 598 ; coccochroraatic,
598 ; conjugation of, 599 ; zj-gospores
of, 599 ; gonids of, 599 ; movements of,
liOl ; 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-
tliiirice, &c., 614, 623

Diatoms. See DIATOMACE*

Dichroisrn, 1098. See PLEOCHROISM

lUrldea, 602

Dicotyledonous stems, fossilised, 1083

DICOTYLEDONS, 700 ; stem of medullary
rays of, 702 ; epiderm of, 712

Dictyocalyx pumiceus, 861

Dictyochya fibula, 620

Dic.tyocysta, silicious shell of, 773

Dictyoloma peruviana, winged seed,
724

Dictyospyris clathms, 847

Dicti/ota, oijspheres of, 627

Didemnians, 914

Dlflijmium serpula, plasinode of, 635

Differential screw, Campbell's fine ad-
justment, 162, 164, 165, 174, 202, 230

Differential staining, 493

Differentiation of cell, 533

Difflngia, 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, 6 1 ;
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, 70, 78 ;
bands, 277 ; phenomena, Abbe's experi-
ments, 434 ; ghost, 435

Digestive vesicles of Cilia fa, 776

Digitalis, seeds of, 724

Dimorphism in Foraininifent, 802

Dinobryon, 765

Dinqflagellata, 770

Dinomastigophora, 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 ; aiiteunfe of,

988 ; mouth-parts of, 990 ; wings of,

998 ; ovipositor of, 1003 ; imaginal discs

of, 1007

Direct division of nucleus, 538
' Directive vesicles ' of egg of

937
Disc-holder, Beck's, 339

DYT

Discida, 849

Discoliths, 748, 749 ; artificially produced,.

1101
Jt/xrurliina, 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 apparatus, 455

— microscope, Greenough's binocular,
248 ; Stephenson's binocular, 248 ;
Huxley's, 251; Zeiss's, 251, 253
Bausch and Lomb's, 252

Distance of projection 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 desrnids,
582

— artificial, of ActinospJieerium, 741 note

— of naiads, 955
Dobie's line, 1049
Dog-fish, scales of, 1028

D'Orbigny, on plan of growth of Fora-

ininifera, 799
Doris, spicules in mantle, 928, 929 ; nida-

mentum of, 934 ; eggs of, 942 ; spines

of, imitated, 1101
- bilameUata, development of, 935-

937

— ji/iosa, palate of, 931

— tuberculata, palate of, 931

Double illumination, Stephenson's me-
thod, 105

Doublet, Wollaston's, 36, 153
Dragmata, of sponges, Ntio
Dragon-flies, wings of, 998
Dragon-fly, facets in eyes of, 983

— Sec Libflliila
Drapamaldia yloinerata, 574
Draw-tube of microscope, 157
Drebbel's modification of Keplerian

telescope, 121

Dredge, 528

Drepanidium ratian/ui, 752

Drone-fly. See Erixtuli*.

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

Dudrcniiiii/a, fertilisation in, 632 ; ferti-
lising tubes, 632

Dajardin, on ' sarcode,' 530 iiutc

— separates Ai/ueba from Infusoria,
733

Duiining's zoophyte trough, 348
Duramen, 704

Dwarf-male of CEilar/niti/nii, 572
Di/tiscus, eye of, 987 ; antennas of, 988 ;

spiracle of, 996 ; trachea of, 996 ; foot

of, 1001, 1002

1 150

INDEX

EAE

E

Earth-stresses, 1077
Earwig. See Forfcula
Eccremoci'irpns smber, winged seeds of,

724
Echinoderm larvre, collecting, 900 ;

preparing, 900 ; mounting, 900

— skeletons in mud of Levant, 10S5
ECHINODEKMATA, larvie of, collecting,

529
-- 884-903; skeleton of, 884, 891, *92,

894 ; spines of, 885-889, 891 ; pecli-

cellarife of, 889; teeth in, 890, 892;

preparation of skeleton spines, Arc.,

892 ; internal skeleton, 894 ; larvae of,

896

Echiiioderms, decalcification of, 512
Echinoiileii, skeleton of, 884 ; spines of,

885 ; pedicellaviae of, 889 ; larva of,

898; direct development in, 900 note
Echinomet ra , spine of, 886, 892; colour

of spines, sss
Echinus, shell of, 885, 886; spines of,

us;, ; teeth of, 890

— liridtvi, coloured spines of, 887
Ectocarpacete, 626

Eciocu-rpits silicnlosus, conjugation of,
627

Ectoderm, 726

Ectoplasm, 535

Ectoprocta, 909

Ectosarc. 534; in Rhizopoda, !'•>'•'< :
experiments on, 743; of Ciliata, 773

Edentata, cement in teeth of, 1026

Edible crab, metamorphosis of, 970

Edwards (A. M.), on supposed 'swarm-
spores' of Amiebit, 744

Eel, scales of, 1027

' Egg without shell,' concretionary sphe-
roids in, 1100

Egg-capsule of Cyclops, 961

Egg-sacs of Lerneea, 966

Egg-shell membrane, 1038

Eggs of Sepiola,Doris,94£\ of Acnrina,
1005 ; of insects, 1005

Ehrenberg, on eye-spot in Pi-utocoeriiv,
543 ; on Volvox, 551 ; on structure of
frustules, 590; on rapidity of repro-
duction of I'n nni/ci-iii in, 111; on
internal casts of Foraminifera, N27
note ; on fossil lladioln rin, ,s.".4 no/e

Eld 'iit/nnn, raphides in pith of, 696;
peltate scales of, 714

Elastic ligament of bivalves, structure of,
Hill)

1-lluter, aiitcnnii' of, 9ss

of Mnrc/innlin, 668; of Eqni-
, 680

l-'.liitiiie, sri'ds of, 724

I'Jdrr, pith of, 6H7

Ellis's aquatic microscope, 147

Klin, niphidrs of, 69(i

i'J ml en ciininlenttin, cyclosis in, 6*9

Elytra of Coleoptera, 981, 999

Kmlii-xo ot Phanerogams, 72:!
cell of fern, development of, 679

EPI

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
Einpu.ua mnscce, 642
Enamel of teeth, 1025

- of teeth of Echinus, 891

— on ganoid scales, 1028
Enee/i/iitlnrfos, raphides of, 696
Encrinites, 892

End-bulbs, 1053

Eudochrome, 533; of Pul mai/lcea, 541;

of Spirofftjnt, 550; of Volvox, 551,

552, 554 ; of desmids, 580
Endoderm, 726
Endogenous spores of Mucorini, 640

- steins, 700-712
Endogens, spiral vessels of, 698
Endonema, 602
Eiidophloeum, 708
Endoplasm, 533

Endosarc, 533; in Rhizopoda, 733; of

Ciliata, 773
Endosperm, 685
Eiidospores of mosses, G72 ; in ferns, 677 ;

of Volvox, 556 ; of Hymenomycetes,

648

Endosporous Bacteria, 655
Enock's metallic ring for mounting, 4S2
Entomophilous flowers, 722
EntomophthoreoB, 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, i ..... ;

— collecting, 529

- Rotifera upon, 787
Entomostracan eggs as food of Ciliata,

775

Entoproctit, 909
"

Entozoa, 943

Eolis, nidameiitum of, 934

Eozoiin, 837 ; mounting, 481 ; mode of
growth of, compared with that of
Polytrema, 824; canal system com-
pared with Culi-iiriiia, 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

Epc'int, foot of, 1015 ; silk threads of,

1015
Ephemera, branchiae of larva, '.'H7

— margindtn, larva of, 973; circulation
of blood in larva of, '.)'.< I

Ephippial eggs of lint /fern , 790
Ephyrii- of ( 'i/n iin'ii, .S75 ; of Clin/siim ,/,

876

Epiblast, 726 note
K|iiilrrm of IIMVCS. 712
1 .pidermic appendages, 1029

INDEX

II5I

EPI

Epidermis, 1041, 1042 ; method of ex-
amining, 1043

Epidote, 1076

l-:/iilobium, emission of pollen-tubes, 722

l\/iipactis, pollen-tubes of, 7'23

Epiphlceum, 708

Epispore of Miicorini, 642

Epistome of Polyzoa, 909; of Actino-
troclia, 050

Epistylis, collecting, 527

Epithelium, 1043, 1044

Epithemia, conjugation of, 599; zygo-
spores of, 599

— turgid a, 604
Equiconcave lens, 22
Equilucent zones of light, 368
l''.ijiiini-tiici'ii', 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 ; antennae of, 988

Error of centring, 389

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

Etltmosplufra siplionopliora, 850, 851
Eucalyptra vulgar is, 669
Eucope2)of1a, 965 note
Eucyrtidiuiii elegans, 847, 852

— Mongol fieri, 847

- tubulus, 847

Eudorina, sexual process of, 557

Euylena, 545, 765

Euglijpha- alveolata, reproduction of,
746

Euler's microscope, 148

Euler on achromatic microscopes, 147

Eunotia, 604

Eunotiecp, characters of, 604

Enp]n»'bi«ce(e, laticiferous tissue of, 695

Eiipltrasia, micropyle of, 723

EuplecteUa ii.i/>rrgiUt{»i, 860 note

Eupoilisrece, characters of, 612

Eurotiitm repcns, 643

Evening primrose, emission of pollen-
tubes, 722

' Exclamation markings ' on scales, 978

Excretory organ of Botif era, 789,790

Exner (S.i, on the image in eye of
Lanijii/rin, 984

Exogenous stems, 700

— stem, structure of, 708

— and endogenous stems contrasted, 709,
710

Exogens, fibre-vascular bundles, 697,
698 ; medullary sheath of, 698 ; spiral
vessels in, li'.is

Exoskeletoii of decapods, 968

Exospores of mosses, 672 ; of ferns, 677 ;
of Hymenomijcetcs, 648

FER

Extinction, straight, 1079

— angle, measurement of, 1079

Extine of pollen-grains, 720 ; markings
on, 720

Eye, accommodation of, 88
-of Pecten, 940 ; of Onrhidii/iii, 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, 43, 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,
37(5 ; 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, 378 ; negative,
376, 377 ; positive, 377 ; solid, 378 ;
searcher, working, 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, 41'.i

Farravits' s medium, 47s, 520 ; for mount-
ing insects, 973
Farre (A.), on structure of Polygon, 90s

note

Furrelhi, 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 Actinophrys, 738 ;

hi sponges, 856
Feet of insects, 1000-1002 ; of spiders,

1014

Felspar, decomposition of, 1076, 1077
Felspar rock, effect of dynamic meta-

morphism on, 1077
Felspars, zonal structure in, 107: 1
'Female' plants of Polijtriclui in, 671
Fermentation of alcohol by yeast, 646 ;

by Penirillinm, Mucor, &c., 647
- putrefactive, 661
Fermentative action of Ftn/cji, 532
Ferns (see Fili<:i's}, 674 ; in coal, 1084
Fertilisation of Phanerogams, 722
Fertilisation-tubes of Peronosporece, 638
Fertilising tube of Dudresnaya, 632

115:

INDEX

FES

Festuca praten&is, paleoe 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 Moiiconys, 128, 376 ; by Hooke, 128,
376

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, antennae of, 987

' Fire-fly,' 984, 988. See La»ipi/riii

Fish, circulation in tail of, 10.37 ; 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, 1034, 1035 ; pigment-
cells of, 1043 ; muscle fibre of, 1049 ;
gills of, 1063

Fission in Lielrr/.'/irh nin, 733; of
Monas, 756; of Minionii/ii, 704 ; of
Codosiga, 764 ; of plaiiarians, 947

Fititiijit'iines, wings of, 999

Fixation, 484-487

Fixing agents : alcohol, 484 ; corrosive
sublimate, 484 ; osmic acid, 485 ; picric
acid, 485

Flabella of Lir/itn/ilion/, 605

Flagella, 532; of Bacteria, 652, 658, 659

Fl(i</,'ll,itf,, 755-771

- experiments 011, 761 ; nucleus in, 762;
karyokinesis in, 763; colonial t'nrins,
764

— collared, resembling cells of sponge*,
855

Flagellate chambers of sponges, 856,
857

Fhrjelll i .\ ..rli/lll'll, 766 tllitr

Flat bottle for collecting, 527

Flatness of Held, 425

Flea, presumed auditory organ of, 422;

FOR

hairs on pygidiuni of, as a test, 421

mounting medium for, 973
1 Flesh,' 1048
Flint, derivation of, 622

- glass, refractive index of, 5 ; disper-
sive power of, 10 ; composition of, 32

- implements found with Orbitolincp,
824

Flints, preparation of, 1089

Floral envelope, 718

Floridea, 630-632 ; affinities of, 630

FlosculariadcB, 791

Floscules in confinement, 528

' Flowering fern,' sporanges of, 676

' Flowering plants,' 684. See PHANEBO-

GAMIA

Flowers, 718-723 ; Iiiman's method of
prepaiation, 719

' Flowers of tan,' 634

Fluid inclusions in crystals, 1074

' Fluke,' 945

Fluorite lenses for apochromatic objec-
tive, 35, 366

Fluatra, 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, vai'ious instructive organs to be ob-
tained from, 972 ; eye of, facets in,
983 ; proboscis of, 989 ; circulation in
wing of, 994 ; spiracle of, 996 ; areolae
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 Navicidu and Sitrirella, 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
Fontinal/is antipyretica, 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 mounting, 450 ;
method for sectionising, 508 note ; de-
calcificatioii of, 513 ; structure of, 795 ;
chamberlets in, 798, 803, 804, 806 ;
cyclical mode of growth in, 798 ; plans
of growth, 798, 804 ; porcellanous
shells, 79!i ; vitreous shells, 799; tubu-
latioii of shell in, 799, 800 ; rotaline
type, 800; nunmiuline type, 800; in-
termediate skeleton of, 801 ; canal sys-
tem of, 801 ; Porcelhniea, 801 ; fos-
silised forms of, 801, 804, 812, 824, 837;
dimorphism in, 802 ; secondary septa
in, 803; Arriiarea, 810; sandy iso-
mov|ilis, 814; nodosarine type, 815;
I'i/ri'd, 819; internal casts, 823, 827
n«ti'\ nummuline series, 826; alar
prolongations, ,s;ui, ,s:'.l ; interseptal

INDEX

1153

FOR

canals, 830 ; marginal cord in, 830, 834 ;
collecting, 843 ; method of separating
from sand .etc., 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 Sertulariidn,
870

Forceps, 351

— slide, 453

- stage, 339

For fie ula, antennee of, 988
Forficulid(E, 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

- Cijpnihr, 960

- Fomii/inifcra, 801, 824-826

- Lituolce, 816

- Racbiolaria, 846, 854 note

— Saccammina, 812

— sponges, 1089

- wood, 705, 706

Fossilised Foraminifera (EozoGri), 837

— wood, sections of, 712
Fragilaria, 605

Fragilariees, 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, 407

— microtome, Hayes's, 472 ; Cathcart's,
474

— imbedding by, 505

Fresnel, on Selligue and Adams's micro-
scope, 148 ; on. range of magnification,
149

Freyatia heteropus, legs of. 1010

Fripp's method of testing object-glasses,
386

Frog, blood- corpuscles of, 1034, 1035;
muscle fibre of, 1049 ; papillae 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 Phceospcrea, 626
Fructification, goiiidial, 541 ; sexual, 541
- of thallophytes, 540 ; of Ascomycetes,

642 ; of lichens, 649 ; of mosses, 670 ;

of ferns, 675; of Equisetctceee, 680
Frustules of Diatomacece, 588; shapes

of, 588, 589 ; structure of, 589, 590

note ; girdle, 589 ; ostioles in, 590 ;

markings on, 591 ; character of, as basis

GEL

of classification, 602 ; of Coscinodiscus,

609

Fncacea, 627 ; conceptacles of, 627
Fuchsia, pollen-grains of, 722
Fucits, 626

Fucus platycarpits, 627, 628
- vesiculosus, 629
Fulgoridie, wings of, 999
FuiKiria hygrometrica, 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

Futigia, lamellae of, 878

Fungiform papillae, 1053

Fungus-cellulose, 633

Fusion in DaUiiii/i'riu, 75!)

Fuss's description of a microscope, 1 47

Fusulina, 825, 826, 1090

-F«s«?/H«-limestone, 825,

G
Gabbro, 1095

— fluid inclusions in, 1074
Gad-fly, ovipositor of, 1004

— See Tabanus
Gaillonella procera, 621

— granulata, 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 occhiale, 122, 124 ; his
microscope, 127

' Gall-flies,' ovipositor of, 1003

Galley-worms. See Mijriopfula

Gamasula, legs of, 1010; integument of,
1010; Malpighian vessel of, 1011;
heart of, 1011 ; tracheae of, 1011 ; cha-
racters of, 1012 ; reproductive organs
of, 1012

<!<uini>>ux 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 Ccelenterata,
720 ; formation of, 726 note ; of zoo-
phytes, 862; of Gastropoda, 935; of
blowfly, 1007

Gauss's optical investigations, 106-110;
his dioptric investigations, 106-110 ; his
system, practical example of, 111-1J6

Gelatinous nerve-fibres, 1052
- in sympathetic, 1054

4 E

IIS4

INDEX

GEM

GRE

GemeUaria, polyzoary of, 909

Genimee of Marchantia, 666, 667; of
Salpingceca, 764; of Suctoria, 784 ; in
Foraminifera,798; of Polyzoa,9Q6

Gemmation and shape of shell in Fora-

minifera, 796
Gemmules of Noctihica, 769 ; of sponges,

857

Gent ia IK/, seeds of, 724
Geodia, spicules of, 859, 1086
Gephyrean worm, 950
Gem ilium, glandular hairs of, 714 ; cells
of pollen-chambers, 720 ; pollen-grains,
720

Germ-cells of Volvox, 555 ; of MarcJtan-
tiu, 668 ; of mosses, 671 ; of ferns, 679 ;
of Phanerogams, 685 ; of sponges, 857 ;
of Hydra, 866

'Germinal matter,' 3018; of fibrous tis-
sue, 1019 ; of dentine, 1020
Gesneria, seeds of, 724
Ghostly diffraction image, Nelson on, 72

note
Gibbes (Heneage , on. staining Bacteria,

515

Gifford's screen, 321
Gill (C. Haughton), on the ' dots ' of

Naviculd, 593
Gillett's condenser, 204, 300
Gills of tadpole, 1057, 1059
Giraudia, conjugation of, 627
GirvaneUa, 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 cinctnin, conjugation of,

770

Globigerina, shell of, 798 ; mud, 811 ;
pseudopodia of, 821 ; mode of life of,
821 ; Wyville Thomson's views on, 821 ;
Carpenter's views on, 822
Globigerina btilloidcs, 820 ; in the ' ooze,'
1086

conglobata, 821
— ooze, 820, 1085 ; resemblance to chalk,

1087

- r nbra, colour of, 799
Globigerine shell, sandy isomorph of,

814

(i/oliii/rriniild, H20
Globule of ('/HI r<i, 577, 578
Globalises, 1096
( IWhidia of Anodun, 983
(l/d'iK'd/iKtt, 547 ; us gonid of lichen, 651
< Hi >\v-\v< >i-m, 9S 1 ; antenna- of. 9SM
Glue and lidiiry miicnt, 444
<lliit.cn of grass seeds, 725
Olyccrin, as preservative medium, 51S,
020; llii'nt/sch's method, 520; lieale's
method, 520

Glycerin-jelly, Lawrence's mounting in,
480, 519 ; solvent for CaCO-, 520 ; for
mounting insects, 973 ; for mounting
cartilage, 1047

Gl/ycipTiagus Kramcri, 1013

- '"Htlmifer, 1008

— platygaster, 1013
-plumiger, 1008 ; hairs of, 1010

Gnathostonidtd (Crustacean), 965 note
Goadby's solution for mounting cartilage,

1047

Goes (Dr.\ on affinity of Carpenteria, 823
Goette, 011 development of Antedon, 903
Gold size, 443

Gomphonenia, stipe of, 588, 616; move-
ments of, 602; attacked by Vampyrella,
730

— geminatum, 616; stipe of, 616

- ijr/icile, 621

GomjjJionemece, characters of, 616
Goniaster equestrix, spines of, 891
Gonidial cells, 541

- fructification, 541

- layer of lichens, 649
Gonidiophores of Peronosphorece, 639
Gonids, or non-sexual spores of Crypto-
gams, 541 note; of Vaucheria, 562;
of PocLosphenia, 597 ; of Floridcce,&'&\ ;
of Fit ncji, 633 ; of Peronosporece, 639

Goniocidaris florigera, spine of, 888
Gonium, 545

Gonothecffi of Campanulariida, 870
Gonozoidof hydroids, 868; of Syncoryne,

869; of Tubular ia, 869
Gonozoids of Sertitlariidd, 870
Gordius, 944, 945
Gorgonia, spicules in, 929

- guttata, spicules of, 880
Gorgonife, 877 ; spicules of, in mud of

Levant, 1085
Goring (Dr.), on magnification of objects,

43
' Gory dew,' due to PalmeUa cruenta,

558
Govi, on invention of microscope by

Galileo, 120

Graduated rotary stage, 395
Grcummatophora, chains of, 588, 607

— angulosa, 620
- marina, 607

Grammatophora -parallela, 620

NiT/iriifhid, 607

militilissima, 607
Granite, 1095

— fluid inclusions in, 1074
Grant in, S57, 861 ; spicule of, 1086
Grasses, nodes of, 701 ; silex in epiderm

of, 715 ; palese of, 715 ; seed of, 725
Grasshopper, gizzard of, 993 ; wings of,

999

Green glass for softening light, 321, 417
< Irccnsunds, microscopic constituents of,

1090

Gregarina, characters of, 749; move-
ment of, 750

, in lobster, 749 note
, 751

INDEX

"55

GRB

Gregarhiida, 749

Gregory (J. W.), on Eozoon, 843 note

Gregory (W.), on species of diatoms, 600

note
Greville, on Kpatangidium, 610; on

Tfii'i'i-iif/i/in, 618 note
Grey matter, 1052
Griffith's turn-table, 451
GriffitJisia,63Q
Grinding sections of hard substances,

506

Grindl' s microscope, 182
Gt-omia, 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, 341
Guard-cells, 715
'Gulf-weed,' 630
Gum and glycerin, 520 ; and syrup, as a

preservative medium, 519

- imbedding for vegetable substances,
514

— arabic, formula, 445 ; for freezing, 505

— resins, latex of, 695

— styrax, as a mounting medium, 521 ;
index of refraction, 521

Gyges, 545
GynutocJ/roa, 868
Gymnolcemata, 909
Gymnosperms, fossilised, 1084

- generative apparatus in, compared

with Cryptogams. 684
Gypsina, 824

H

Haddon,on budding in Poly zoa, 907 note
Haeckel (E.), on Badiolaria, 846 ; on

Hydrozoiin affinity of GtenopJiora, 877

note
and Hertwig, on classification of

radiolarians, 849 note
Hfsmanicebidce, 752 and note
Hcematococcns, red phase of Proto-

coccus, 543
— sangitineus, 558
Haematoxylin, solutions, 491, 492
Heemionitis, sori of, 675
Hcemosporidia, 749
Haime (Jules), on development of Tri-

cnoda, 780
' Hair-moss,' 671
: Hair-worm,' 944
Hairs of leaves, 714; of insects, 980; of

Acarina, 1010; of mammals, 1029
HaUcaridce, 1013
Haliomma Hwniboldtii, 851
- lujstrix, 848
Haliotis (diatom), 613

- (mollusc), shell structure of, 928 ;

palate of, 931
Haliphyseina, 814; spoiige-spicules in,

822

HET

Haller, on auditory organs olAcarinci,

1010

Halteres of Diptera, 1000
Hand-magnifier, Brewster's, 37
Hansgirg, on movement of OsciUato-

riacete, 548
Hiintzsch's glycenn method for desmids,

520
HaplopJiragmiivm, 814

/ih ib ii/i 'fin i for me, 813
Hardening agents, 484-487 ; corrosive
sublimate, 484 ; alcohol, 484 ; osniic
acid, 485 ; picric acid, 485
Hardy's flat bottle for collecting, 527
Harpalus, antennas of, '.INS
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 Scoloprn-

<1 riinn

' Harvest-bug,' 1013
' Haus ' of Append icularid, 918
Haustellate mouth, 992
Haustellium, 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 Gastropoda, 941 ;

in Cephalopoda, 941
Heart of ascidians, 912 ; of Acarina, 1011
Heartsease, pollen-tubes of, 723
' Heart-wood,' 704
Heating-bath, Mayer's, 453
HeUopelta, 588, 611
Heliosoa, characters of, 734 ; examples

of, 737-742

Helix pomatia, teeth of, 930
— Jiortensis, palate of, 930
Heller's porcelain cement, 521
Helmholtz on aperture, 47
Hemiaster cavernosiis, development of,

900 note
Hemiptera, eyes of , 987 ; wings of , 999 ;

suctorial mouth of, 1000
Hensen's stripe, 1049
Hr/iidicce, 665; thalloid, 668; foliose,
668 ; elaters of, compared with spiral
cells, &c., of pollen-chamber, 720
Hcrbivfiri'i, 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 Microgromia, 735

note; on Actinia, 877 note
Heterorottrottis, spine of, 885
- iiiauui/iUatii.s, spine of, 887
Heterocysts of Nostoc, 549

4 E2

1156

INDEX

HET

Heterowita itnrinata, life-history of, 760

Heterostcgi)ia, 834

Heurck (Van1, on markings of diatoms,

593

He.i-firtJ/ra, 792
Hicks, on amcebifonn phase of Volvox,

556 ; on preparation of insect antennae,

989 note; on structure of halteres and

elytra, 1000
HiniantidiiiHi, 604
Hippa re liia janira, eggs of, 1005
Hippopus, 613
Hlppotlioa, 909
Holland's triplet, 37
Hollis's liquid glue, 444
Hollyhock, pollen-grains of, 721, 722
HolotJn/ria botelliis, plates of, 895

- t'd/tlis, plates of, 895

— inlutbilis, plates of, 895

— vagabunda, plates of, 895
Solothunts, diatoms in stomach of, 614,

623

Holofliurioidea, skeleton of, 894; pharyn-
geal skeleton of, 895 note ; plates in
skin of, 895 ; preparation of calcareous
plates, 896 ; abbreviated development
in, 900 note

Holtenia Carpenteri, 861

HoineocJadia, 602

Homogeneous, word first applied to
lenses, 30

- immersion, 364 ; Abbe's combination,
365

- immersion lenses of Powell and Lea-
land, 30 ; of Zeiss, 29

— objectives, value of, in study of monads,
762

- system, 28

Homo2)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

- compound microscope, 128
Hooked monad, 760

Hooker (J. D.), on diatoms of antarctic

circle, 621

Hooklets on wings of Hi/ini'iiupf, rn, 999
Hoplothora, 1012

- maxillaj of, 1010

Hormogones of Om-il/nf :>r/,imf, 547 ; of
Rivulariacece, MM : of t>i-///tiiii'inniT«',
548 ; of AW or, 54!)

Hormosina <il<>l>uUfrra, 813. 815
( 'arp> ni' r/, ,xl5

Hornblende, 1077

corroded rr\stalsof,1072 ; pleochroism

Hornet, wing of, '.»!>!•; sting of, 1003

Horns, 10211, In:;:;

Horny substances, chemical treatment

"f, 517
' Horse t.-ilK,' t;sn. See TSquisetacece

Hosts of par.lsit ie pliillts, .'>:!•_!

' I. • - fly. \, c .M.

HYP

Hudson, on the functions of contractile

vesicle of rotifers, 789 note
Hudson and Gosse, on classification of

rotifers, 790
Human blood-corpuscles, 1034

- hair, 1031

Husk of corn-grains, 725

Huxley, on the ectosarc of Amoeba, 743
•note; on coccoliths, 747; onSathybius,
747 ; on Collozoa, 853 note; on struc-
ture of molluscan shells, 922 ; on pul-
villusof cockroach, 1000 note; on agamic
reproduction of Apliis, 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 Foniiiinnfrni, 799

Hi/aUnia cell aria, palate of, 931

Hyalodiscits subtilis, 608

Hyaloplasm, 537

Hydra, collecting, 527 ; intracellular
digestion in, 863 ; thread-cells of, 864 ;
structure of, 864 ; reproduction of, 866 ;
gemmation of, 866
-- fusca, 863, 865

- viridis, 863, 867

- vulgar is, 863

'Hydra tuba ' of CJrrysaora, 874, 876

Hijdraclmidce, 1008 ; 'mandible of, 1009;
eyes of, 1011 ; reproductive organs
of, 1012 ; characters of, 1013

Hydrangea, number of stomates in. 716 :
seeds of, 724

Hydrodictyon, 557, 566
— reticulatum, 565

Hydroida, classification of, 868

Hydroids, compound, 867 ; structure of,
867 et seq. ; Meiliisce of, 868 ; planulse
of, 868, 871 ; habitats of, 871 ; ex-
amination of, 871 ; mounting, 871 ;
polariscope with, 872 ; preservation of,
872

Hyilropldlus, antennas of, 987, '.ISH

H//<lrozoa, 863-877

Hydruzoa and marine mites, 1013

Hyla, nerves of, 1054

Hymenittm of Ascomycetes, 642 ; oilin\:-
diomycetes, 647; of Hymenomycetes,
648

1 1 i/nii'iioiiiyceti's, 647; pileus of, 648;
stipe of, 64*

Hijmenojrtera, 973; eyes of, 987; mouth-
parts of, 990 ; wings of, 998 ; sting of,
1002, 1003; ovipositor of, 1002, 1003

Hi/oscyamus, spiral cells of pollen-
chambers of. 720; seeds of, 724

ll/f/ifl'/i-lllli, seeils of, 724

Hyphie oi fungi, 633

ll\ pnosiiore of H///i I'm! it-t i//»i, 565
Ilypnospores, nieanilig of, 541 note
Hypoblast, 72(1 //

ll\ pojiinl stage of Tyrogtyphidce, 1013
n, 1013

INDEX

H57

K'E

' Ice-plant,' epiderm of, 714
Ichneumonidce, ovipositor of, 1003
Illuminating power, 425

- power of objectives, 54; compared
with penetrating power, 393

Illumination for dissection, 401

- for opaque objects, 149

- oblique, 190, 191, 388

— of objects, Ross on, 300 ; monochro-
matic, 321-323 ; Gifford's screen for,
321 ; Meithe's filter for, 322 ; Nelson's
apparatus for, 323 ; by reflection, 329-
338 ; opaque, 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 cause 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; Beck's,
337 ; for examination of metals, 337

Image, real, 14 note ; virtual, 14 note, 376 ;
conjugate, 24; inverted conjugate, 24;
absorption or dioptrical, 64 ; diffrac-
tion, 64 ; negative, 64 ; positive, 64 ;
solid, 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 C Haifa, 775
Incidence, angle of, 3
Incident ray, 2
Incus of Rot if era, 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

IRI

Inflection of diverging rays, 62
Infusoria, 754-785 ; as food of Actino-

phrys, 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
Inoceram us, portions of shell of, in chalk,

1087

INSECTS, 972-1007
— mounting media for, 973 ; integument

of, 974 ; tegumentary appendages of,

974 ; scales of, 975-980 ; hairs of, 980 ;

parts of head, 982; eyes, 982-987;

antennae of, 987 ; mouth-parts of, 989 ;

circulation of blood, 993 ; alimentary

canal, 993; wings of, 994, 998-1000;

trachea; 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 ; egg*

of, 1004 ; agamic reproduction of, 1006 ;

embryonic development of, 1007 :

' liver ' of, 1047

- parasitic fungi in, 642, 645

— parts of, wooden slides for mounting,
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 Foraininifera,
801 ; of Globigerinida, 820 ; of Calca-
rina, 825; of Rotalia, 825 ; of Niimmu-
lites. 826 ; of Eozoon, 839

Internal casts of Textularia, 823; of
Botalia, 824; of Ei>.:oun, 840 ; of wood,
1083 ; of shells in greensand, 1090

Interpretation, errors of, 427

' Interseptal canals ' of Calcarina, 830

Intestine, cells of villi in, 1044

Intine of pollen-grains, 720

Intracellular digestion in zoophytes, 863

Intussusception, 533

- mode of growth of starch, 694
Invagination, 726

Iiivertebrata, blood corpuscles of, 1038

Inverted conjugate image, 24

Iodine, as a test for starch, &c., 516

Ipomcea pnrpurca, pollen-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 i/crntanica, epiderm and stomates of,

715, 716

H58

INDEX

IKK

Irrationality of spectrum, 19, 305

Isochelee of sponges, 860

Isoetece, 082

Isotropism, 1079

IntJnnia, chains and frustules of, 588, 612 ;
structure of frustules, 590 note ; divi-
sion of, 596

— nervosa, (513

— areolations in, 592
Italian reed, stem of, 699
'Itch-mites,' 1013
Ivory, 1024

Txodes, heart of, 1011

Ixodidce, 1008 ; integument of, 1010 ;

auditory organ, 1011 ; tracheae of, 1011 ;

characters of, 1012

Jackson's modification of Ross model,
199 ; his eye-piece micrometer, 270

Janssen (H. and J.), inventors of first
microscope, 120 ; their compound
microscope, 120

Jars, capped, for Canada balsam, 477

Jelly-fish. SeeAcalephce and MeduscB

Jones's compound microscope, 144, 145

Jungermannia, 068

Jung's (Thoma's) microtome, 401-469

Kaolin, 1070

Karop, his fine adjustment to sub-stage,
187

Karop and Nelson on fine structure of
diatoms, 591 note

Karyokinesis in monads, 703

Kellner's eye-piece, 42, 370 ; as a con-
denser, 196

Kent (Saville), on contractile vacuoles of
Volvox, 552 >iote ; on Flctffellata, 704

Keplerian telescope, Drebbel's modifica-
tion, as a microscope, 121

'Keramosphcera Murrayi, 810 /m/r

Keratose network of sponges, 855 ; pre-
paration of, 857

Kidneys of Vertcbrata, 1047

King-crab, 957

Kirchner, on the ob'spores of Volvox,
55.6

Klebahn, on formation of auxospores of
diatoms, 001

Klebs, on mucilaginous sheath of des-
inids, 580 ; on movi'incut, of desmids,
580

— and Biitschli, on the 'cilia 'of JJino-
Jtii(/i-tliitn, 770

Klein, on Volrn.r, 5.~><; i/otr

Knife, special, for microtome, -402

Koch's method of sectionising corals,
878

Kowalevsky, on development of uscidiiins,
1)17 note

Krukenberg,on digestion in sea -anemones,
863

LEG

TL\\i7,mg,o\\Pahnodictijon, 559; on struc-
ture of frustules of diatoms, 590 ; his
classification of diatoms, (in:!

Labarraque's fluid for bleaching vege-
table substance, 514

Labels, permanent, 523

Labyrmthic structure of Cyclam uiiini.
810 ; of Parker ia, 818

Labi/rititJioclon, tooth of, 1091

Lacunse 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
Lagenida, 819
Laguncula, 906, 908, 950

— stolon of, 904 ; polypides of, compared
with Clavcllinidce, 914

- repcns, anatomy of, 904, 905
' Lamellae ' of corals, 878

— of Hymenomycetes, 04*
Lamellibranchiata, shell of, 919
Lamellicornia, antennae of, 988
Laminaria, 626, 627
Lauiinariacecf, 027

Lamna, tooth of, 1024

Lamp, Nelson's 404 ; Beck's 406 ;

Baker's, 407
JJampyris, antenna- of, 988

— splendiditla, photograph through eye
of, 984

Land-crab, young of, 969

Laukester (E.Ray), on Bacteria, 652; on
movement of gregarines, 750 ; on
Hcfiuamcebidce, 752 note; on intra-
cellular digestion in Limnocodium,
863

Lantern-flies, wings of, 999

Lapis lazuli, 10'.)")

Larva of Echinodcrmata, 896; of As-
teroldea, 898 ; of Echinoidea, 898 ; of
Oplimroidea, 898 ; of Crinoidea, 900 ;
of ascidians, 910 ; of fly, 1007 ; of
Acarina, 1009

Latex of Phanerogams, 695

Lathrcea squarnaria, embryo of, 723

Laticiferous tubes, free-cell formation in,
534

- tissue of Phanerogams, 695
Laurentian rocks, 837, 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, 71 >s

Leech, D.IO

Leeuwenhoek's simple microscope, 132

Legg's method of selecting Foraminifera,
844

Legs of insects, 1000, 1002; of Acarina,
1008, 1010

INDEX

U59

LEG

e, seeds of, 685
Leiosoma palmacmctum, 1008 ; hairs of,

1010
Leitz's microscopes, 206, 227

- bull's eye, 330

— objectives, 374

Leiis, spherical, 12 ; biconvex, 12, 13 ;
plano-concave, 13; diverging meniscus,
13 ; plano-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 Sargoii's palace, 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
Lepidiiun, seeds of, 724
Lepidocyrtiia t-n rvicollis, scales of, 979
Lepidodendra, 682, 1084
Lepidoptera, scales of, 975, 976 ; wings

of, 981, 999 ; scales of, mounting, 981,

982 ; eyes of, 987 ; antennae of, 988 ;

mouth-parts, 992 ; eggs of, 1005
Lepidosteus, bony scale of, 1022, 1028
Lepidostrobi, 682

Lepisma saccJiarina, scales of, 976, 977
Lepismidce, 979
LepraUa, 909; mode of growth in, 904 ;

extension of perivisceral cavity of,

927

Leptodiscns (ally of Noctil/K-u}, 769 note
Leptogoniuni scot inum, 649
Leptothrix, form of, 653
Leptus autumnalis, 1013
Lerncea, 965 note, 966
Lessonia, 627

Lettuce, laticiferous tissue, 695
Leucite, mineral inclusions in, 1075 ;

anomalies in, 1078
Lever of contact, Ross's, for testing

covers, 440
Libelhtla, eye of, 983, 987; respiratory

apparatus of larva, 997 ; wings of,

998

Liber, or inner bark, 708
LICHENS, 648-651 ; fungus-constituents

of, 651
LicmopJiora, stipe of, 588, 604 ; flabella

of, 605

-flabeUata, 588, 604
Licmophorece, 616

— characters of, 604 ; vittte of, 604
LieberJcuelniia, movement of, 732

— paludosa, 733
- Wagner i, 731

LOM

Lieberkiihn's microscope, 139 ; his specu-
lum, 334-336

'Ligamentumnuch.se,' structure of , 1040

Light; refraction of, 2; recompositioii
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 note ; cone of, 190 ; monochromatic,
321, 417, 418 ; intensity of, necessaries
for, 4 Hi

— convergent, in petrology, 1070, 1078

Ligiiified tissue, test for, 517

Lignites, 1083

Lignum vitce, wood of, 704

Lilac, pith of, 687

LiJiiun, experiments with pollen-grains
of, 721

' Lily-stars," 900. See Criiiniili-a

Limax maximus, palate of, 930

- shell of, imitated, 1102

- nifus, shell structure of, 928
Lime, raphides of, 696

— secreting Algae, 1084
Limestone, metamorphism of, 1077

- rocks, 1084, 1085

LimncBiis stagnates, nidamentum of,
934 ; velum of, 936

Limnocaridce, characters of, 1013

LimnocJiaris, seeds of, 724

Limnocodiiim, intracellular digestion in,
863

Limpet. See Patella

Limnlits, 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

Listrophorus, 1008

Lithitnti'i-ifiriis fiiiliiitus, 620

Lithistid sponges, spicules of, .w.)

Lithocyclia ocellus, 847

LitJiot'hanniion, 1084

Litiiola, S14

Lituolce, large fossil forms of, 816

Litiiolida, 814

Live-box, 346

Liver, 1047

Liver-cells, 104S

' Liverworts,' 605. See Hcjui/ini'

Lobosa, characters of, 734; examples of,
742-747

Lobster, 957 ; metamorphosis of, 969

' Lob-worm,' 948

Loculi, of anthers, 720

Locust, gizzard of, 993; ovipositors of,
1004

Locusta, eye of, 987

Loftusia,818

Loligo, pigment-cells of, 942

Lomas (J.), on calcareous spicules in
Alcyonldium, 908 note

1 1 60

INDEX

LON

' London Pride,' parenchynie of, 688
Longicornia, antennas of, 988
Longulites, 1096
I;opliophore of Polyzoa, 905, 950 ; of

fresh-water Polyzoa, 909
Lo^thopxs, collecting, 528
Lophospermv/m erubesceiis, winged seed

of, 724

LopliyTOpodat 959
Lorica of Ciliata, 773 ; of Acineta, 783 ;

of Hot if era, 787
Loup-holders, 248
- for tank work, Steinheil's, 268
Loups, Reichert's, 38 ; Steinheil's, 38, 378 ;

Steinheil's aplanatic, 248 ; Zeiss's, 268
Louse, mounting media for, 973
Loven, on classificatory value of palates

in Gastropoda, 982
Loxosoma, lophophore of, 909
Lubbock, on Thysami/ra,971 ;on Poilnra

scale, 979

Lucanus, eye of, 987 ; antennas of, 988
Luminosity of Noctiluca, 765 ; of Cteno-

phora, 883 ; of annelids, 955
Lungs, circulation in, 1056, 1062-1065
Lychnis, seeds of, 724
Lychnocanium falciferum, 847
— luccntd, .S47

Lycoperdon, 647 ; hymenium of, 647
LycopodiacecB, 681 ; in coal, 1084
Lycopodiecf, 681
Ly minds, collecting, 527
Lymph, corpuscles, 1037
Lysigenous spaces in Phanerogams, 688

M

Maceration of vegetable tissues, 700 ;

Schultz's method, 700 ,

MachiUs polypoda, scale of, 978
Machines for cutting hard sections, 511,

512

Macrocystis, 627
Macrospores of Polytoma, 760; of

sponges, 857

Macrurous -DeeftpocZa, young of, 969, 970
Madder, cells of pollen-chambers, 720
' M.a,<ire,' Acanthometra, occurring in, 852
Madrepores, 878
Magma, 1073
Magnetite, 1072
Magnification, range of, of Selligue's

microscope, 149
\ 1. 1 unifying power, testing of objectives,

425 ; determination of, 288
Mahogany, size of ducts of, 699 ; stem of,
706

M iiliirnxtnird, 968
'Male' plants of l'nl i/t ricliiuii, 671
Mallei of lioi ifc ra , 788
Mallow, pollen -'i-ii ins of, 721, 722
Malpighian vessel of Gnui/iniil,,', 1011
- layer of skin in mammals, Id 12
- bodies in verlelinite kidney, 1047
Mult wood's finder, 2116
Mii/rti Ki/lrrxti-ix, pollen-grains of, 721

MEG

MalvacecB, 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

Maiidibulate mouth, 989

Manganese concretions, 1090

' Mantle ' and growth of shell in Molhtsca,
925

Marcltant in, 665-668 ; archegones of, G65,
668 ; stomates of, 666 ; elaters of, 668

— androgyna, 665 note

- pofymorpha, 665-668
Margaritacece, 919 ; nacreous layer of,

922 ; prismatic layer of, 923
Margarites, 1096
' Marginal cord ' of Operculina, 830

— oiNummiilites, 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

Ma/rsvpella 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 Rotifera, 787

MastigopJiora Hyndananni, 906

Mastogloia, stipe of, 588, 619 ; gelatinous
sheath of, 588, 619 ; development of,
597 ; range of variation in, 618
— lanceolata, 619

- Sin it hi i, 619

Matthews' s method of sectionising hard

substances, 507
Mayall, on history of microscope, 117 ;

on Divini's microscope, 130
May all's removable mechanical stage, 183
M nyer's heating bath, 453
' Meadow-brown,' eggs of, 1(H).">
' Measly pork,' due to Cysticercus, 944
' Mechanical finger ' for selecting di-
atoms, 625

- movements of the stage in Lister's
(Tully's) microscope, 14!>

- stage, 17"'

Turrell's, 176 : Watson's, 177;
Nelson's, 179, 181; /eiss's, 179, 183;
Swift's, L80; Allen's, 180 ; MayaU's re-
movable, 183; Reichert's, 183 ; Bauseh
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 E,xogens, 698 ; of

dicotyledons. 703
Medusa of fresh-water, 863
Mcdusce, mounting, 448 ; of Hydroids,

868 ; naked-eyed, 868 ; development

of, 874 ; alternation of generations in,

877 ; nerves of, 1052
Medusoids, collecting, 529
Megalopa, 970
Megaloscleres, 859
Megasphere of certain Fora mini/era,

802
Megaspores of RhizocarpetB, 681 ; of

carboniferous trees, 682 ; of Isoetece,

682 ; of Selaglnellcte, 682
Megatherium, teeth of, 1026
Megatriclta of Ehrenberg, a phase in

development of Suctoria, 785 ; Badcock

on, 785
Megazoiispores of Ulothrix,557i of Uh<«,

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 ; antennae 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

- subflexilis, 594, 595

- varians, 594, 595; endochrome of,
598

Melosirece, characters of, 608 ; resem-
blance to Confervacete, 608

Membrana putaminis, 1032

Membranipora, 908, 909

Membranvporidte, 908

Mercury nitrate as a test for albuminous
substances, 517

HeridiecE, 604, 616

— characters of, 604
Meridian circulars, 588, 604
Merismopedia, 547

' Mermaid's fingers,' 879. See Alcyo-

•niuin
Mesembryanthemum, seeds of, 724

- crystallinum, epiderm of, 714
Mesucarpus, conjugation of, 549; zygo-

spore of, 550

Mesoglcea of Hydra, &c., 864 note
Mesophlceum, 708
Metal case for imbedding, 498
Metamorphism, dynamic, 1077
Metamorphism of rock-masses, 1076,

1077 ; of limestones, 1090
Metamorphosis of Lerucea, 966 ; of

Cirripedia, 967 ; of Mai a cost mm,

969
Metazoa, 727, 855

MIC

Meteorites in oceanic sediments, 1093
Metschnikoff, on acinetan character of
Erythropsis,775; on intracellular di-
gestion, 863 ; on phagocytes, 1037 note
Mica, 1077

Michael's (A.) opalescent mirror, 194
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 Myxcnn i/cctcs, (J3G
Microgroinia 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 note

Micropyle in ovule, 685 ; of Euplirasia,
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 ; preservation of,
436

- Galileo's, 127 ; Campaui's, 128 ; Prit-
chard's, with Continental fine adjust-
ment, 153; Ross's 'Lister' model,
153 ; Powell's (H.), 155 ; James
Smith's, 155

— achromatic, 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 ; Greenough's,
102,250; Powell and Lealand's, 105;
Cherubin d'Orleans', 130; 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 ; path 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; Cul-
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

I 1 62

INDEX

MIC

Microscope, concentric, 191, 199

- dissecting, Greeuough's, 102, 250 ;
Stepheiisoii's binocular, 248; Baker's
(Huxley's), 251 ; Bausch and Lomb's
(Barnes), 252 ; Zeiss's, 253

- horizontal, Bonarmus's, 134; Arnici'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, 36, 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 ; Eoss'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
Microscopic and macroscopic vision, 62

— determination of geological formations,
1090

— dissection, single lenses for, 38

- investigation of rocks, &c., 1066

- vision, principles of, 43
Microscopical optics, principles of, 1
Microscopist'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 certain Fora in i/iif<-rtt,802
Microspores of SpJiagnacecs, 674 ; of
Rhizocarpece, 681 ; in carboniferous
trees, 682 ; of Isoetea, 682; of Xrl<i</i-
11,'l/rtr, 682; of Pobjtoum, 760; of
sponges, H57

Microtome, 458-475 ; Ryder's, 401; sim-
ple, 458-460; Thoma's (Jung's), 461-
Hl'.t; freezing apparatus for, 467; Mi-
not's, 472; Strassor's, 472 ; Guddrn'.-,,
472

— Cambridge rocking, 169-472 ; advan-
tages of, 172

I'rrry.ing, Hayes's, 472 ; minimum
thickness of sections with, 473 ; Cuth-
cart's, 474

WON

Microzoospores of Ulothrix, 557 ; of

Ulra, 561 ; of Hydrodictyon, 565
' Mildew,' 637. See Uredinece
Miliola, shell of, 799; encrusted with

sand, '810
Miliolee, 802

Miliolida, 801 ; in limestone, 1090
Miliolina, 802

Milioline Foraminifera, fossils of, 801
Miliolite limestone, 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 pi'ism, 192
Mites, 1008. See Acarina

Mobius, on mineral nature of Eosoiin, 843
Mohl (Von), on protoplasm, 530 note
Moist-stage, Dallinger and Drysdale's,

341-344

Molecular coalescence, 1099-1102
Molgula, development of, 917
MOLLUSC A, larvie 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 ; ciliatioii 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
MonaiUuce, life-histories of, 755-763 ;
saprophytic, affinities of, 756 ; effect
of temperature on, 761 ; nucleus in, 762
Monads, 755. See Monadince
Mono s, 575

- Dallingeri, life-history of, 756

- lens, 755

Moniixonida, spicules of, 859
Monazite, 1081

Monconys (De) devises microscope with

field-lens, 12s
Monerozoa, 727-733
Monocanlus, 871
Monochromatic light, 321, 417, 431

— illumination, means of obtaining, 417,
418

MONOCOTYLEDON'S, 700 ; stem of, 700 ;

nodes of, 701 ; epiderm of, 712
Monocotyledonous stem, fossilised, 1083
Monocular, Powell and Lealand's, 194,

195

INDEX

1163

MON

SITX

Monocystis (if/U/n, cyst of, 750
Monophyes, digestion in, 863 note
Monosiga, fission of, 764
Monothalamous Fora in inifera, 796
Monotropa, seeds of, 724
Moracece, laticiferous tissue of, 695
Mordellu beetle, eye of, facets in, 983
Mormo, scales of, 980
Morplio Menelatts,sca,les of, 976
Morula of higher animals compared with

' multicellular ' Protozoa, 726
Morula of Gastropoda, 935
Moseley (H. N. ', 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, 1053

Motorial end-plates, 1053

' Moulds,' 640, 643

Moults of EntoiniiHtrarit, 964, 965

' Mountain-flour,' 622

Mounted objects, keeping, 523; labelling,

r>23 ; arrangement of, 524
Mounting plate, 4."i2

- 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;
Polycystina, 481 ; sponge-spicules,
481 ; chitinous substances, 481 ; palates
of gastropods, 481 ; sections of horns,
&c., 481; Lepidoptera 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 ; Ripart 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 naphthaliii, 521 ;
phosphorus, 521

Mouse, hair of, 1030-1031 ; cartilage in

ear of, 1046

Mouse's intestine, villi of, 1062
Mouth, suctorial, of Hemiptera, 999

- of Acarina, 1009
Mouth-parts of insects, 989
Movement, interpretation of, 431-434

— of LieberJcuehnia, 732; of Amceba,
744; of Dallingeria, 758; of plana-
rians, 946 ; of Artemia, 960; oi Bran-
cJiipns, 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

Mil cor, fermentation by, 647
— inucedo, 641
Mncoi-ini, 640 ; spores of, 640; epispores-

of, 642
Mucous membrane, 1041 ; capillaries in,

1062
Mud of Levant, microscopic constituents

of, 1085

Mulberry, laticiferous tissue of, 695
Mulberry-mass, 726
Miiller (J.), 011 the Badiolaria, 846 ; on

larva of Nemertines, 951
Midler's (Fr.) 'Common Nervous Sys-
tem' in Polyzoa, 907 and note
Multicellular organisms, 726
Multiplication of Palnioglaa, 541; of

Protococcits, 543; of Volvox, 555 ; of

Pahnclla, 558; of Bacteria, 652; of

Microgromia, 736; of Amoeba, 744;

of Dallingeria, 758; of Hetcnuniln.

700; of Tetrandtiis, 760; of Nn<-til m-n,

769; of Peridinium, 770 ; of Suctorin,

784; of Ciliata, 111
Multiplying power of eye-piece, 290
Munier Chalmas and Schlumberger, on

dimorphism of Fora m in if era, 802
Munier- Charles, on certain fossil Fora-

minifera, 564

Miiricea elongata, spicules of, 880
Musca, eye of, 987 ; antennae of, 988
- vomitoria, eggs of, loin;
' Muscardine,' 645
Musci, 670-674
Muscinetz, 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, 1030
Musschenbroek's simple microscope, 132
Mussels. See Unionidce and Mytilcn e&
Mya (trrnaria, hinge tooth of, 924
Mycele of Fungi, 633 ; of Dstilaginece,636
Mijcetozoa, 634
Mi/Iiobates, tooth of, 1025
Myobia, 1008 ; legs of, 1010 ; maxillre of,

1010

Mi/obiMtc, 1013
Mi/ocoptes, legs of, 1010
' Myophan-layer ' of Vorticella, 773
Myopy, 118

MyrioplnjUum a good weed to collect, 527
MYKIOPODA, hairs of, 980
Myriotliela, intracellular digestion in,

863

MytilacecB, sub-nacreous layer in, 924
Mytilus, for observation of ciliary motion,

940

Myxauiwbce, 634
Myscogastres, 634

Myxomycetes, 579 note, 634; develop-
ment of, 634, 636 ; spores of, 634, 636 ;
swarm-spores of, 634 ; affinity with
Monerozoa, 727
Myxosporidia, 749, 752

1164

INDEX

NAG

N

Nachet, on ' immersion system,' 27 ; his
binocular, 97, 98, 99 ; his changing
nose-piece, 293

Nacreous layer in molluscaii shells, 919,
922, 924

Naegeli and Schwendener, on microscopi-
cal optics, 67

Niigeli's theory of formation of starch,
695

Nails, 1029, 1033

Nais, 955

Naphthaliii, monobromide of, as a mount-
ing medium, 521 ; refractive index of,
521

Narcissus, spiral cells of pollen-chambers
in, 720

Nassula, mouth of, 774

Nauplius, compared with Pedalionidce,
792

Nautiloid shell of Foraminifera, 797

Nautilus, 929

Navicula, 590, 597, 617 ; markings on,
593 ; cysts of, 597 ; zygospores of, 597 ;
zoiJzygospores of, 597

- bifrons, presumed relation to Stiri-
rella microcora, 602 note

- in chalk, 1087

- li/>'(i, as test for definition, 426

- rJtoniboides, markings on, 592 ; as test
for definition, 426

Naviculece, 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 ', 011 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 mici'o-
meter eye-piece, 272 ; his ' black dot,'
277; his plan for estimating edgt's ..t
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

\'i-iiintii>ii multifidum, 631

NYnmtodes, desiccation of, 945

Nematoid worms, 944

Ni-incrtino lar ,a, '.)."> 1

Nepii, tracheal system, it 95 ; wings of,
1000

- I'll Illl t I'll, C.L'gS of, 100.")

Nept'iit In ^, spiral fibre-cells of, f'.'.ts
\, i-fidce, 948
Ncreocystis, 627

NTJC

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 striie, 381

Nicotiana, seeds of, 724

' Nidameiitum ' of Gastrojioila, 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
XitzscliiecE, 606

Noctiluca, collecting, 529 ; tentacle
(flagellum) of, 766, 768 ; cilium of, 766
note ; protoplasmic network of, 767 ;
reproduction of, 769

- miliaris, 765-769
Noctuina, antennae of, 988
Nodes of monocotyledons, 701
Nodosaria, 819
Nodosarince, 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 Lealaiid'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
Nosema bo»ibi/cis, 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 Appen-

dicularia, 918

Notouecta, 987 ; wings of, 1000
Nucellus, 685
Nuclear stains, 491-494

- spindle, 538 ; plate, 538
Nuclein, 537

Nucleoli, 534
Nucleoplasm, 537

INDEX

1165

NUC

DOS

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 ; initiative
action in monads, 762

— and cell division, 1019 note
Nucule of Chara, 577, 579
Nudibranchs, nidamentuni 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
Nummulme layer of Eozoon, 840

- plan of growth, Parker and Rupert
Jones on, 827 note

Xi!>/i»ntlini(lff, H2(>
XHimwidites, 826, 827, 831

- dint (t us, 832

— garansensis, .s:;2

— Icevigatct, 832

- striata, internal cast of, 834

- tubuli in shell of, 800
Nummulitic limestone, 831, 835, 1085,

1090
Xii/i/uu- lutea, parenchyme, 687 ; stellate

cells of, 687
Nymph of Acarina, 1009 ; of Oribatidte,

1009

O

Oak, size of ducts in, 699
- galls, 1003

Oberhiiuser's spiral fine adjustment, 153

Object-glass of compound microscope,
36, 39 ; of long focus, 40 ; of short
focus, 40 ; capacity 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 Thorna's (Jung's) mi-
crotome, 464, 465, 466

— changer, Zeiss's, 293

Objectives, achromatic, 19, 32 ; aplanatic,
19 ; apochroniatic, 19, 30, 34, 80 ; cor-
rected, 20, 21 ; immersion, 28, 34, 58 ;
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 -life-histories, 81 ; penetrating
power of, 83, 393 ; sliding plate with,
290; rotating disc with, 290; of wide
aperture, 369 ; of small aperture, ex-
amiiiation 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; Selligue'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

- apochroniatic, 366, 370, 371-375

- homogeneous immersion, introduction
of, 364

- ' semi-apochromatic,' 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

Oblique illumination, 190, 191, 387

— illuminator, 190

Obliteration of structure by diaphragms,
68

Occhiale, Galileo's, 122, 123

Occhialino, Galileo's, 121, 124

Oceanic sediments, microscopic examina-
tion of, 1092

Ocelli of planarians, 947 ; of insects, 982,
986

Ocellites of compound eye, 982

Ocular, 40, 375 ; spectral, 327

GEdogoniacece, 572

(Edogonium ciliatum, 573

(Enothera, pollen-grain, 721; emission
of pollen-tubes of, 722 ; embryo of, 723

Oil for immersion lenses, suggested by
Aniici, 29

- of cedar-wood, for immersion objec-
tives, 29

Oil-globules, 429-431

Oil-immersion, 29

- objectives. See Objectives, oil-
imniersion

Oils, solvents for, 517

Okeden, on isolation of diatoms, 624
note

Oleander, epiderni of, 714; stoinates of,
716

Oliviiie, corroded crystals of, 1072

Oitchiditnn, eyes of, 941

Oncidium, spiral cells of, 69:!

Onion, raphides of, 696

Oijgones of Vaucheria, 563; of SpJ/i* ro
plea, 572; of CEdogonium, 572; of
Chara, 577; of Fucacece, 627, 628; of
Peronosporece, 638

Oolitic grains, 1084

Oiiphyte in ferns, 680

Oospheres, use of the term, 537 note ; of
Volvox, 556; of Vaiiclieria, 563; of
Splia -fn/i/, -a, 570; of (Edogo)iitim,~)~i-l:
of Cham, 577; of Plueosporece, 627 :
of Fucacece, 628 ; of Marchantia, 668 ;
of ferns, 679

u66

INDEX

oos

Oiispores, 540; of Volvox, 556; of Vau-
cJ/eria, 563; of Achli/a, 565; of
Kphceroplea, 572 ; of CEcLogonium, 573 ;
of Chara, 579; of Fticaceee, 628

Ooze, Globigerinq, organisms in, 811,
813, 820 ; compared with chalk, 1085

Opalescent mirror as a substitute for
polarising prism, 194

0 [Ml in n, 774

Opaque illumination by side reflector, 333

- mounts, 336

' Open ' bundles, 710

OpercitUna, 830 ; and Nummulites com-
pared, 834

Opercule of mosses, 671

OjiJiiai-antl/a ricij/ara, development of,
900 note

Ophioglossaceee, development of pro-
thallium of, 679

Ophioglossum, sporanges of, 676

OphiotTvrix pentaphyllum, spines of,
891 ; teeth of, 892

OpJiiitrida, mounting, 481

OpJiin raided, skeleton of, 891 ; spines of,
891 ; teeth of, 892 ; larva of, 898 ;
direct development in, 900 note

Ophrydia, quantities of, 777

OijJiri/diuni, cellulose in zoocytium of, 778

- versatile, effect of light on, 775
OpTvryodendron, 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 Anted on, 901
Orbiculina, 803, 804, 808

— compared with Heterosteginu, 834
Orbitoid.es, 835

- and Cycloclypeus compared, 835

- Fortisii, 836
Orlitolina, 824

Orbitolince, occurring with flint instru-
ments, 824
Orbitolites, 804-810

— shell of, 798 ; range of variation in,
810 ; structure of Parkena resembling,
817 ; deposits of, 1085

— and GycloClijpeus compared, SOI

— coni/iln initii, animal of, 807-80'.'

— italincn, 800 note, 808
ti'it /linn/ma, 808

Orbiilina, 820

Orbuline shell, sandy isomorph of, 815

< )rr///</t'n", pollinium of, 722

Orchids, mycropyle of, 723

O/rA/.v, pollen-tubes of, 72:5 ; seeds of,
724

Organised structure and living action,
580

< (rgans, .".:!:;

• Organs df sense' in ('iJiata, 775 note

nrilxit i<l<r, nyni|ih of, 1009; mouth-parts
of, KM ill; legs of , KH(l; int.'guinent of,
1010; auditory organ, loll ; reprodnc-
ti\i UI-'JMII .. lull; siipi'i-roxal glands

PAL

of, 1011 ; tracheae of, 1011 ; characters

of, 1012
Orienting small objects for sectionisiiig,

499

Orie/aniimonites, seeds of, 724
Ornithorhynchus, hair of, 1031
Orobanche, seeds of, 724
Ortlwptera, eyes of, 987 ; antenna; of,

988; wings of, 999 ; nymph of, 1009
Orthoscopic effect, 95 ; with Ramsden's

circles, 106

— eye-piece, 376

Ortlwsira Dickiei, sporangial frustule

of, 595

Oscillator/a, 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 corallines, 032

Ostioles of Naviculacece, 590; of Cym-
belleee, 590

Ostracoda, 960

Ostreacece, shell of, 923

Ostrich, egg-shell of, 1101

Otoliths compared with artificial concre-
tions, 1100

- of Molliisca, 941
Ovarium of Polyzoa, 907
Over-amplification, 88
Over-corrected objective, 20
Over-correction, 358-360
Overtoil, 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, 86(5

Oxytricha, a phase in development of

Trichoda, 780
Oxyuris vermicularis, 944
Oysters, shell of, 923

Pacinian corpuscles, 1053
Palaeontology, use of microscope in, 1083
' Palate ' of Gastropoda, 919, 930 ; classi-

ricatory value of, 932 ; preparation of,

9:>2 ; viewed with polariscope, 933 ;

bibliography, 933
Palese of grasses, silex in, 715
Palisade-parenchyma of leaves, 716
Palm, stem of, 701
Palmclla, as gonid of lichen, 651
— crttentfi, 5 5s

Piihiicllacece, 557 ; frond of, 558
Palmodictyon, 559; zoiispores of, 559
Pahnoghea wacrococca, life-history of,

541, 542
Palpicornia, antennas of, 9ss

INDEX

I 167

PAL

Paludina, infested by Distoma, 940
Pancreas, 1047
Pandorinu, 545

— morum, generative process of, 557:
swarm-spores of, 557

Piijiaccracece, laticiferous tissue of, 695

Paper-cells, 446

Parabolic illuminator, 816; speculum,

;;:;:-!; reflector (Sorby's), 334
Paraboloid illuminator, 316
Paraffin, solvents for, 496

- imbedding method, 496-503

- for imbedding, melting point of, 500

- mounting, sections, 501

— cells, 446

Paramecium, Colin'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

Purkeria, 817; a possible Stromato-

i, seeds of, 724
Parthenogenesis, 1007 note
- in Saprolegnice, 640
Pass/flora ccerulea, pollen-grains of , 721
Passiflorecf, pollen-grains of, 721
Paste-worm, 945
Pasteur's solution for growing yeast, 646

iiati", 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

' Ptibriue ' in silkworms, 661
Peccary, hair of, 1030
Pecten, prismatic layer in, 924 ; pallial

eyes of, 940 ; fibres of adductor muscle,

1050

Pectinibrancltiata, 937
Pectinidce, sub-nacreous layer in, 924
Pedalion, 792
Pedalionidee, 792
Pedesis, 431 ; experiments in, 432
Pediastrece, 566 ; affinities of, 566
Pediastrum, zoospores, 567 ; micro-
zoospores, 567

- Ehrenbergii, 568

— ffranulatum, 566-568
jii't'tiisiini, 568

— tetras, 568

PedioellarisB of echinids and asterids,

889

Pedici'Uina, lophophore of, 909
Pedicularis palustris, 723
— sylvatica, embryo of, 723
Pedunculated cirripeds, 967

PHA

Pelargonium, petal of, 719 ; pollen-grain,

721

Pelomyxa palustris, 744
I'l'in'i-opUs, 801

— variation in shape of shell in, 797;
shell of, 799 ; varietal forms of, N03

Penetrating power, 4'2.">

— in objectives 83 ; of objective, com-
pared with illuminating power, 393
Penetration, 38, 82, 83
Pt'iiiciHium, fermentation by, 647

— glaucum, 643

Pentncrinus asterius, skeleton of, 892

Pcntatoma, wings of , 1000

Penny, starch in cells of, 694

' Pepperworts,' 681

Perception of depth, 94

Perch, scales of, 1028

Perforated shells of Brachiopoda, 926

Perforationof shell in Font ininift-ra, 799,

800

Perianth, 718

Perichlamydium pnrtr.i-tiun, 851
J'l-riiliitin, 770. 771
Peridinium uberrimum, 770
Perigone of mosses, 670
Periodic structures, 74
Periostracum of molluscan shells, 922 ;

of brachiopod shells, 926
Peripatus, tracheae of, 1011
Peritheces of lichens, 650
PeronosporecE, 638-640
Perophora, respirator}- sac of, 915 ; cir-
culation of, 915

' Perspicillum,' Wodderbprn's, 125
Petals, 718

Petrobia lapidum, eggs of, 1009
Petrological microscope, Swift's, 1068
Petrology : micro-spectroscope in, 1081 ;

micro-chemistry in, 10 is 2
Pettenkofer's test, 517
Petunia, seeds of, 724
Peziza, botrytis-form of, 645
Pfitzer, on reproduction of diatoms, 594
Phezodaria, 852
PJuposporete, 625-627
Phagocytes, 1037 note
Phakellia ceiitilcibrnni, 858
Phallus, 647

PHANEKOGAMIA, 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 etseq.; fibro-
vascular bundles, 697 ; root, structure
of, 700 ; epiderm of leaves, 712-718 ;
flowers of, 718 ; pollen-grains of, 719 ;
fertilisation of, 722 ; ovules of, 722 ;
seeds of, 723
Phanerogams. See PHANEEOGAMIA

n68

INDEX

PHI

PJiilnnthits, antennae of, 988
Phloem, 710
- of Exogens, 697
Pliolas, shell of, 924
Phoronis, 950
Phosphorescence of sea, due to Noctiliira,

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

PJn-yganea, eye of, 983
Phycocyanin in Ghrodcoecaceee, 547
Phyco-erythriii, 631
Phycomyces nitens, 641
Phycophsein, 626
PJiylactolcemata, 909
Phyllite, 1077 note
PJ/yllupoda, 962
P]iyU<>niiu/i/ta, skeleton of, 968
Ph ysartnn album, development of, 635
Physciu parietina, 650
Plujama cltrilaf/niitiiit, 650
Phytrlr/tJids, endosperm of seed of, 693
Phytophthora iiifestans, 639, 640
Phytopti, mouth-parts of, 1010
PhytoptidcB, 1008; characters of, 1014
PJiytoptiis, larva of, 1009
Picric acid, 485
Picro-carmine, 489
Piedmontite, 1095
Pieridee, scales of, 975
Pigment-cells of cuttles, 942 ; of ver-
tebrate skin, 1042 ; of fishes, 1043 ; of

Crustacea, 1043

Pigmentum nigrum, of eye, 1043
Pike, scales of, 1028
Pileorhiza, 710
Pileus of Acetabularia, 563
Pilidiiun gyrans, 950
Pilulina Jeffreysii, 812
Pimpernel, petals of, 719
Pines, pollen-grains, showers of, 722 note
I 'in/id, structure of shell of, 919-922;
prisms of shell of, in Globigerina ooze,
1086 ; prisms of, in chalk, lo.sT

ii/i/rina, colour of shell of, 921
I'/ininltirid, C>17
— dactylux, i'i-jl
- nobilis, 021
Finns canadensis, 41:;
Pipette, 351, 470
I 'Iscilitlu'i- irr.iins, 1084
Pistil, 722

Pitcher-plant, spirit 1 fibre cells of, 698
Pith. im-aM^cim-nt of, 700, 702
J'ittcil duct-, ii[ I'liaiiiTiiuiuii-,, f.'.l'.l

Pli id scales, 1(12*

ase felspar, KINO
Planaria, stomach of, '.» it;

POL

Planarice, 946; movement of, 946; fis-
sion of, 947 ; ocelli of, 947 ; intracellular
digestion in, 863

Planarians. See Planarice

— allied to Ctenophora, 883
Plano-concave lens, 13
Plano-convex lenses, 13, 15, 22, 37
Planorbulina, 824

Pldiitdgo, 'Plantain,' cyclosis in, 691
Plants and animals, differences between,

531

Planulffi, 868

Plcninlaria hexas, in chalk, 1087
Plasmode in cells of Nitella, 579 note ;

of JEtlialium, 634; of Myxomycetes,

635
Plasmodium of Protomyxa aurantiaca,

729

Plastid, contrasted with cytode, 727
Plastidules, flagellated, of Protomyxa,

729
Plates, calcareous, of Holotlnirioiilcn ,

895

Pleochroism, 1078, 1098
Pleochroism, variations of, 1080
Pleurosigma, 588, 617

— diffraction image of, 71

— angulatiun, 69-71; as test for defi-
nition, 426; markings on, 592, 593

— formosum, as test for definition, 426

— Spencerii, sporules of, 597

Pliny, on cauterisation by focussing sun's

rays, 117 ; on sight, 118
Ploima, 791, 792
PhinidteUa, collecting, 528
' Plumed-moth,' wings of, 999
Plumule of Pieridee, 975
Plutarch, on myopy, 118
Pluteus larva of echinoids, 897-899
Podocyrtis cothurnata, 847

- mitra, 847, 852

- ScJiomburgkii, 849, 852
Podophrya quadripartite/,, 784; imma-
ture form, 785

— elongata, 785
I'liilosphenia, 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,
Ki;i7; 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 note

Polarising apparatus, 317-319 ; condenser
for, 314 ; Swift's illuminating and, 319

Polarising prism, substitution of opales-
cent mirror for, 194

1 Polierschirlc]-,' (117

INDEX

Il6q

POL

Polishing ground sections, 511

— sections of hard substances, 506

- -slate, 617

- -stones, 508, 617

Polistes (wasp>, with attached mould,

64'2
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
Polliniura of orchids and asclepiads, 722
Pollinoids of Floridece, 632 ; of lichens,

650

Polyaxial spicules, 859
Polycelis levigatus, 947
PolyclinidcE, 913
Polycystina, 846, 851
PoifycystimcB, as test for low powers,

389 ; mounting, 481
Polydesmidfe, 981
' Polygastrica,' Ehrenberg's erroneous

views 011, 753

Polygointin, pollen-grains of, 721
Pofymorphina, 820
Polyommatus Argus, scales of, 976
Polyparies of zoophytes, 862
Polypary of hydroids, 867
Polypes, 863. See Hydrozoa
Polypide, of Polyzoa, 906 ; formation of

buds from, 907
Polypidom of zoiiphyte, 904
Polypite, of hydroids, 867
Polypoduim, sori of, 675
Polyporus, 647

Polystichuin angulare, apospory in, 680
Polystomella, shell of, 797

- craticulata, 827, 829

- crispa, 827, 829
Polythalamous Foraminifera, 796
Poli/toma uvella, life-history of, 759
Pofytrema, 824 ; mode of growth com-
pared with EozoiJn, 838

- miniaceum, colour of, 799
Polijtrichum commune, 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 ; gemnife 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 Brachiopoda, 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

PKI

Porcellanous shells of Foraminifera,
799; of Gastropoda, 928

— and vitreous Foraminifera, difference
in, 799-801

Porcupine, hair of, 1030

Pores of sponges, 856

Porpliyra, 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 ;
Bausch and Lomb's, 247

Portwnus, 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, 30 ; fluorite lenses,
35 ; high-power 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 ;
protecting ring for coarse adjustment,
352 ; water-immersion objectives, 362,
364 ; ^pinch 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 Vertebrata, 1017
Primordial cells, 535, 536

- utricle, 533 ; of desmids, 580 ; of Pha-
nerogam cells, 688

- chamber in Foraminifera, 798; of
Orbitolites, 806

Primrose, cells of pollen-chambers, 720
1 Prince's feather,' seed of, 723
Principle of microscopic vision, 43
Principles of microscopical optics, 1
Pringsheim, on generative process of

Pandorina, 557; on VniirJ/fria, 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 F

I I JO

INDEX

PKI

Prism, Nicol's, 318 ; Nicol's analysing,
for resolving striae, 381; Abraham's,
401

- refracting angle of, 9, 18
Prismatic epithelium, 1044

- layer in molluscaii shells, 919-925

- layer of shells compared with snamel,
920, 1025

— shell-substances imitated, 1102
Prisms, recomposition of light by, 18
Frist is, 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 Floridece, 632
Projection eye-piece, 380
Promycele of Puccinia, 637
Prosenchymatous tissue, 696
Proteus, red blood-corpuscle of, 1036
Prothallium of SphcCgnacecB, 674; of
ferns, 677 ; of Equisetacece, 681 ; of
Rhizocarpece, 681; of LycopocLiaceee,
681
Protococciis, as goiiid of lichens, 051

- pluvialis, 543-547 ; life-history of,
543 ; multiplication of, 544 ; zouspores
of, 544 ; mobile and still forms of, 545-
547 ; encysted, 551

Protuuiijxu aiiraiifincii, 727-729
Protonenie of Batrachospermum, 575
Protophytes, 530, 651, 726

— mounting, 518; mode of nourishment
of, 532 ; movement by cilia and con-
tracting vacuoles of, 535

Protoplasm, 530 ; vital attributes of, 531 ;
continuity of, 538, 630; of Rhizopaila,

733 ; of Noctiluca, 707
Protoplasmic substance in Vertebrata,

1017

PROTOZOA, 726-785
- mode of nourishment of, 532

' Pseudembryo' of Antedon, 903

Pseudo-iiavicelhe, 751

Pseudo-parenchyme of Vnmji, 633

Pseudopodia of Protoi/ii/xa, 728; of
Vampyrella, 730; of LieberkiteJvnia,
731; of Rhizopofla, 733; of Rcti,-/<-
laria, 734 ; of Heliozoa, 734 ; of Lobosa,

734 ; of Gromia,T35 ; of Mi/-rnt/ninti(i,
736; of Act inoplirys, 738; of Aiitwbn,
743; of Arcella, &c., 746 ; in Amoeba
phase of monad, 757 ; of Eozoiin, 841 ;
of Globigerina, 822; of Radiolatia,
847 ; of endoderm cells in zoophytes,
862

Pseiiilorti/iliiiti'ii', 599

1 'seudoscope, Wheatstone's, 92

Pseudoscopic effects, 95

— effect with Ramsden's circles, 106

- vision, 92
Pseudo-scorpions, 1008

I' < Mild stiumata of Orilmfiilce, 1011,

1012
Pseudo-tracheee, on tly's proboscis, 990

note

RA.Y

' Psorosperms,' 752

Pteris, sori of, 675 ; iiidusium of, 675

— serrulata, apogamy in, 680
Pterocaniwm, 852
Pterodactylus, bones of, 1092
Pterophorus, wings of, 999
Pteroptus, 1012

Ft Hot a, 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

Ptirpura, method of examination of egg-
capsules of, 939 ; supplemental yolk of,
938, 1007

— lap III us, nidamentum of, 934 ; develop-
ment of yolk-segments of, 937

' Puss-moth,1 eggs of, 1005
Pyc>iogoni(la,yoT; related to A rarlinida,

959 note

Pijrohi, seeds of, 724
Pyroxene, andesite, 1076

Q

symmetrica, 747
Quartz-porphyries, 1072
Quartzite, 1077
Quekett (E.), on Martin's microscope,

140 ; on production of raphides, 696 ;

on preparation of tracheae of insects,

997 ; on minute structure of bone,

1092

' Quills ' of porcupine, 1030
Quinquelociilina, 802

R

Radials of Anteclon, 901

Radiating crystallisation, 1097

Radiation of light in different media, 53-
58 ; in air and balsam, 55-57

Radiolaria, collecting, 529; fossilised
forms of, 846, 854 note ; central cap-
sule of, 847 ; skeleton of, 848-854 ; zoii-
xanthellffi in, 848 ; bibliography of, 853
— colonies of, 848 ; distribution of, 853-
854 ; mounting, 854

Radiolariau, shells in ' ooze,' 1086

Rainey, on presumed cause of cattle
plague, 752 ; 011 molecular coalescence,
1100

1 tails, 011 British desmids, 579 note;
classification, 585; on 2V// :x<-/ii<t and
Bacillaria, 606

Ramsdeii circles, 106

Ramsden's' screw micrometer eye-piece,'
272 ; positive eye-piece, 42, 378, 380

Hn/il/i'ilrce; 599

Raphides of Phanerogams, 696 ; of plants
and sponge-spicules compared, 860

Rays, scales of, 1028

INDEX

II7I

REA

Reagents, mode of labelling bottles, 402
Real image, 14 note; formation of, 23, 24

— object image, 375
Record/position of light by prisms, 18
Red ant, integument of, 974

— blood-corpuscles of Vetebrata, 1034;
size of, in various Vertebrata, 1035;
relative sizes of, in various Vertebrata,
1036

- coral, 877

— corpuscles, flow of, 1056

' Red snow,' due to PalmeUa 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 index, 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 naphthalin, 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
Actinosph cerium, 741 ; of C la thru Una,
742; of Euglypna, 746; of sponges,
857; of Campanulariida, 870; sexual,
of Polyzofi, 907 ; agamic, of Entomo-
fitraca, 963 ; agamic, of Apliiilrn, \.rc.,
1006

Reproductive organs of Acarina, 1011

REPTILES, lacunae in bone of, 1022 ;
cement in teeth of, 1026 ; plates in skin
of, 1026 ; epidermic appendages of,
1029 ; red blood-corpuscles of, 1034,
1035; muscle-fibre of, 1049; lungs of,
1063

Reseda, seeds of, 724

Residuary secondary spectrum, 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
Mete nutcosum, 1042
Retrpora, calcareous polyzoaries of, 90'.i
Reticiihiria, 733; characters of, 733; ex-
amples of, 734-737

Reticulated ducts of Phanerogams, 698
Retinula-, n.s:;

EOS

Revolving nose-piece, Nelson's, 295
Rezzi, 011 invention of compound micro-
scope, 125

Rlialidammina, 813
— aliijssorum, 815
Rlialtdolitlius pi pa, 847

— sceptrum, 847

Rhabdoliths, in chalk and limestone,

1084

Rhabdom, 983
Wi<il>d«plri<r<i, 909
li'lui limns, stem of, 703
Rlieopliax sahulosa, 815

— scorpiiiriis, 815
Rhinoceros, horn of, 1033
RliizocarpecB, 681
Rhizoids of mosses, 669
Rhizome of ferns, 675
RHIZOPODA, 733-747

— protoplasm of, 531 ; ectosarc of, 534 ;
skeletons of, 795 ; sarcode of, 101S ;
pseudopodial network of, 1053

Rliizosolenia, 614

— cyclosis in, 587
Rltizostoma, 874, 876
RMzota, 790, 791

Rhododendron, pollen-grains of, 722
Rlindospermece, 574
Rhodospermiii, 631
Rhodosporece, 625
Rhopalocanium ornatum, 859
Rhubarb, stellate raphides of, 696 ; spiral

ducts of, 699

RlupiclioiiellidcB, shell structure of, 927

Ribbons of sections, 464, 469

Ribes, 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

Rivulu fiiifC(c, hormogon.es of, 548

Roach, scales of, 1028

Rurlicii fiilrntii, epiderm of, 714

Rock, ground-mass of, 1072 ; fluxion-
structure of, 1073

— sections, method of examining, 1081
Rocks, method of making sections of,

liniil-1068 ; metamorphism of, 1076
Rodents, hair of, 1030
Root of Phanerogams, structure of, 700

ft St'l].

Root-cap, 710

Hiixtilinit rctriiiiix, 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

1 172

INDEX

EOS

SCH

Ross's (Andrew) ' Lister ' microscope, 153
Ross, model, 197

- and Co.'s microscopes, 196, 230-233 ;
camera lucida, 285, 286

Ross-Jackson model, 199
Ross-Wenham's radial microscope, 199
Ross-Zentmayer model, 198
Rotalia, 824 ; intermediate skeleton of,
825

— aspera, in chalk, 1087

- Beccarii, shell of, 797

— Schroeteriana, 825
Rotaliaii series, 823
Rotaliimr, colour of shell, 799
Rotaline shells-of Formninifera, 797

— shell, sandy isomorph of, 814
Rotating disc of objectives, 290
Rotatoria, 753. See ROTIFEEA
Rotifer vidgaris, 787

ROTIFERA, collecting, 527; keeping alive,
528 ; a food of ActimopTvrys, 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 preservation 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

Rugosa, 877

Rumia 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

Sabellaria, tubes of, 948
Sable, hair of, 1030
Saccammina in limestone, 1090

- Cartcri, 812

- NfiJ/erica, 812

Saccharomyces cerevisice, 645
SaccJia/romycetes, 645; zymotic action

of, 645; endospores of, 646
,S, i, -i-ii luliiiiiii t/i/ftiif/iii/, spiral cells of,

698

Sachs, on CJiara, 579 unfr
Sago, starch-brains of, (1 '.).">
Salivary glands, 1017
Salmon, scales of, 1028
— disease, (140

HII//H, \ (li;i.1uins in stomach of, 614, 623
Salpidce, 911

Sal/pmgceca,ca,ljxof, 764

Salt solution as a preservative medium,

519
Salter (J.), on the ' teeth ' of Echinus,

890
S alvia verbenaca, spiral fibres in seeds

of, 693

Sand-grains surrounded by silica, 1075
'Sand-stars.' See Ophiiiroidea
' Sand-wasp,' 974

Sandy isomorphs (Fora minif era), 814
— tests of Lituolida, 814
Santonine, crystallisation of, 1096
Sap-wood, 704
Saprolegnia, alliance with Acldya, 564

note

Saprolegniie, 640
Saprophytic, Bacteria, 658

- fungi, 633, 642, 647
Sarcocystids, 752

Sarcode, 530 note, 531 ; of Rldzopoda,

733

Sarcolemma, 1049
Sarcoptes scabiei, 1013
Sarcoptitlte, mandibles of, 1009; maxill»

of, 1010; hairs of, 1010 ; legs of, 1010 ;

characters of, 1013
Sarcoptince, 1013
Sarcos'poridia, 749
Sargassum bacciferum, 630
Sarsia (Medusa of Synconjne), 869
' Saw- flies,' ovipositor of, 1003
Saxifraga, seeds of, 724

- nmbrosa, parenchyme of, 688
Saxifrage, cells of pollen-chambers, 720
Scalariform ducts of ferns, 674 ; as modi-

fied spiral ducts, 699
' Scales,' covering epiderm of leaves, 714 ;
of Elceagnus, 714

- of Lepidoptera, 975, 976 ; of CoJeo-
ptera, 975 ; of Curculio imperialis, 975 ;
of Lyccenidce, 975, 977 ; of Pierichr,
975 ; as tests for objectives, 976 ;
of insects, markings of. 976 ; of
Thysanura, 977 ; on wing of Lepido-
pte'm, 999 ; of fishes, 1026 ; of reptiles,
1026, 1029

Scallops. See Pecten

Scarabcei, antennae of, 988

' Scarfskin,' 1041

Scatophaga stercoraria, eggs of, 1005

Scenedesmus, megazob'spores of, 566

Schists, 1077

Schizogenous spaces in Phanerogams,

(ISM '

Si-/// •;i»iii/cctes, 051-064
Schizonema, 602, 617
<;,, rillii, 618

- gelatinous sheath of, 588, 617
SrJii."iini'inece, character of, 617
Schnetzler, 011 movement of Oscillatoria,

548
Schott (Dr.) and the improvement of

object-glasses, 32
Seln-mlei- on binocular vision, 105; his

camera lucida, 285

INDEX

H73

SCH

Schultz's method of macerating vege-
table tissues, 700
Schultze (Prof. Max), on identity of

' sarcode ' and ' protoplasm,' 530 note ;

on cyclosis in Diatomacece, 587 ; on

affinity of Carpenteria, 823
Schulze (Prof. F. E.), on soft parts of

Euplectella, 860 note
Schweiidener, on lichens, 648
Scirtopoda, 791, 792
Scissors, spring, -157
Sclerenchyme of ferns, 674
Sclerogen, 693
Sclerotesin Fungi, 633 ; of Myxomycetes,

636
Scolopendrium, 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
Scytoneinacece, 548 ; hormogones of, 548
Scytosiphon, conjugation of, 627
Sea-anemone. See Actinia
Sea-anemones, intracellular digestion in,

863

Sea-fans, 877. See GorgonicB
' Sea-jellies,' 853
Sealing-wax varnish, 444
' Sea-mats,' 908. See Flustra and Mein-

branipora

Searcher eye-pieces, 378
' Sea-slugs.' See Doris, Eolis
1 Sea-urchin,' 884. See Echinus
Sea-weeds, 625-632
— continuity 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, 469 ; 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

Sedtun, 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

Selaginelleo', 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

SIL

Sempervivum, 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 Foruinin/fera, 796, 803,

804
Serialaria, presumed nervous system in,

907

Serous membrane, 1041, 1042
Serpula, tubes of, 948
Serricornia, antenna; of, 987
Sertularia cupressina, 871
Sertulariida, gonozoijids of, 870 ; zoo-

phytic stage of, 877
Sessile cirripeds, 967
Seta of Tomopteris, 953
' Sewage fungus,' 653
Sexual fructification of Tliallophytes,

540

— generation of Volvox, 555
Shadbolt, on structure of A rachnoidiscus,

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 Ret ic id aria, 733; of
Microgroinia, 736

— silicious, of Dictyocysta, Codonella,
773

- of Foraminifeni, 796-801; of Lamel-
libranchiata, 919 ; of Brachiopoda,
919

Shellac cement, protection 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, 773

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
SigillaricE, 682, 1084
Silene, seeds of, 724

ii74

INDEX

SIL

Silex in JSquisetacecB, 680 ; inepiderm of

grasses, 715

Silk glands of spiders, 1015
' Silk-weeds,' 569
' Silkworm,' eggs of, 1005
Silkworm diseases, 045, 661
Silplia, antennae of, 988
Simple magnifier, 37

— microscope, 248
Sines, law of, 3

SiphonacecE, 562-564 ; Munier-Charles

011 fossil forms of, 564
Siphonostomata, 965 note
SiricidcF, ovipositor of, 1003
Sirodot, on alternation of generations in

Batrachospermum, 575
Skate, muscle fibre, 1049
Skeleton, dermal, of Vertebrata, 1026 ;

fossilised, 1090

- fibrous, of sponges, 857

— silicious, of Heliozoa, 734 ; of Radio-
laria, 846

— of sponges, 855 ; of zoophytes, 862 ;
of EcJmoidea, 884; of Asteroiilrn,
891; of Ophiuroidea^Qlioi Crinoi</<'n,
892; of Holotlntrioiilea,894.; of Aiitc-
don, 901 ; of Vertebrata, structure of,
1020

Skin, 1041 ; pigment-cells in, 1042 ; capil-
laries in, 1062
Skip- jack, antenme of, 987
Slack, on the costas of Phunilnria, 617
Slack's optical illusion, 428
Slide-forceps, 453
Slides, glass for, 438
Slides for cultures, 340, 341

- Seiler's solution for cleansing, 439
Sliding-plate of objectives, 290
Sloths, fossil, teeth of, 1024

Slug. See Umax

Slug's eye, 941

Slugs, Eotifera in, 7*7

Smell, organ of, in insects, 1000

Smith's Cassegrainian microscope, 145,
146

Smith (H. L.), on Tolles' binocular eye-
piece, 101 ; his vertical illuminator,
336; on classification of diatoms, .V.rj

Smith (James), his microscope, 155 ; on
use of bull's-eye witk 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 Diatomacece,
587; 011 species of diatoms, 600 >inlr\
on habits of diatoms, 619

Smith (W. H.), on structure of frustules,
590 note; on movements of diatom-,,
602

Snuil, 930; eye of, 941. See Hrli.r
- muscle of odontophore, 1050

Snake, lung of, KM;:;

Snapdragon, seed of, 723

Snell's ' Law of Sines,' 49

Snow, crystals of, 1095

SPH

Suowberry, pareiichyme of fruit of, 688

Snowdrop, pollen-grains of, 722

Soda, caustic, action on horny substances,
517

Soemmeriiig's simple camera, 278

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, delineation of, 88 ; correct
appreciation of, 88

- vision and oblique illumination, 61
Sollas, on sponges, 855 note ; on the ex-
tensions of the perivisceral cavity in
Poltjzoa, 927

Sorby (H. C.), on microscopic structure

of crystals, 1066
Sorby's parabolic reflector, 334
Sorby-Browiiiiig's micro-spectroscope,

323

Soredes of lichens, 649
Sori of ferns, 675
Sound-producing apparatus of crickets,

999

Spatangidium, 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, parabolic, 333 ; Lieberkuhii's,

334-336; in Smith's illuminator, 336
Spencer Lens Company's Microscopes,

214, 215
Spermathecas of Gamasidce, 1012 ; of

Tyroglypliidce, 1012

Spermatia of Puccini a, 638 ; of lichens,650
Sperm-cells of TJiallophi/tes, 536; of

Volvox, 555 ; of ferns, 678 ; of sponges,

857; of U //ih;i, soo; of Polyzoa,()01
S|ii'niiogoni>s of Puccinia, 638 ; of

lichens, 651
Sphacelaria, 626

, (126

in caterpillars, 645

annitUiia, 570-572
,S'////,/ t;>-:osma, rows of cells in, 583
Sph&rozoum <>r<i<iiniare, 853
Sphagnacets, 073
K/I/HII/IIIIIII, leaf of, 673
Hji/n-noffi/iie fijii'ciiimi, winged seed of, 724
Spherical aberration, 14, 15, 31, 299, 301,
306, :;s7

— diminished by Huyghens' objective,
42

INDEX

1175

SPH

STA

Spheroidal concretions of carbonate of

lime, 1100

Sphiiujidtr, antennas of, 988
Spliliix, eye of, 987; antenna of, 988

— ligitstri, eggs of, 1005
Spicules of alcyonarians, 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 tracheae of insects, 995

Spiriferidce, perforation in shells of,

927

Spiriferina rostra tti, shell of, 927
SpiriUina, 819

— sandy isomorph of, 814
Spirillum, movementi of, 433; granular

spheres of, 660 note

- innhild, 659

— volntans, movement of, 652, 653, 659
Spirit, dilute, as a preservative medium,

518

K/iirorhcete, 653
Spvrogyra, 549, 550 ; attacked by Vampy-

rella, 730
Spirolina, a varietal form of Peneroplis,

803

Spiroloculina, 802
Spirilla, 929

— shells of, bearing Protomyxa, 727
Spirulina, movement of, 548
Splarlinum, sporange of, 669

Splenic fever due to Bacillus antliracis,

656, 661
Spouge-spicules, 857-860

— mounting, 481

— in Carpenteria, 822 ; in mud of Le-
vant, 1085

Sponges, 855-862 ; skeleton of, structure
of, 855, 856 ; reproduction of, 857 ;
habitat of, 861 ; preparation 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, Q65, 668;

of mosses, 671 ; of Spliugnarece, 673 ;

of ferns, 675; of Eqitisetacecc, 680
Sporangia of Lycopodiacecr in coal, 1084
Sporangiophores of Mitcorini, 640
Spore, use of the term, 537 note

Spores of Nostoc, 549 ; of Myxomycetes,
634, 636 ; of Peronosporece, 639 ; of
Bacteria, 655, 657, 660; of Mar-
chantia, 668 ; of mosses, 670 ; of ferns,
676 ; of ferns, method for studying
development of, &1S note; of Equise-
taceif, 680 ; of Lycopodiece, 682 ; of
gregarines, 751 ; of Monas DalUngeri,
757 ; of Lycopodiacece in coal, 1084

— different kinds of, 541 note

— resting, of CluetoplioracetF, 574
Spends of Ustilni/iin'ir, 636; of Puccinin ,

638

Sporocarp of Ascomycetes, 644
Sporogone of mosses, 672
Sporophores of Myxomycetes, 636 ; of

Peronosporece, 639; of Ascomycetes,

643

Sporophyte in ferns, 680
Sporozoa, 749-752
Sporules of Melosiru, 597; of Pleuro-

sigma, 597; of Podosphenia, 597
Spot-lens, 316
Spring-clip, 453

- press, 453

— scissors, 457

' Spring-tails,' 979. See Podnridce

Squid, 942

Squirrel, hair of, 1030

Stag-beetle, antennae of, 988

Stage, horse-shoe, Nelson's, 179, 228 ; of
the microscope, 175-184 ; qualities
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 ; Reichert'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, 344-34(1

— Turrell's, 176 ; Watson's, 177 ; Zeiss's,
179 ; Tolles', 204

— -vice, 339

' Staggers ' of sheep, due to Ccenurus,

944

Stahl, on movement of desmids, 581
Staining, 4«s

— regressive, 491

— B(«-ti'ri«, 514-51(5

- flagella, 516

Stains, intra-vitam, 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-hasmatoxylin, 492 ; ' Kernschwarz,'

1 176

INDEX

STA

SUP

492 ; safranin, 493 ; acid-fuchsin, 494 ;

basic-fuchsin, 494 ; Lyons blue, 494 ;

picric acid, 494 ; water-blue, 494 ;

thioiiin, 494
Stanhope lens, 37
Stanhoscope, 38
Sta/phylinus, antennae 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
Staurastrum, 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 Acinetina, 785 note

Steinheil's loups, 38; 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 BryacecB, 673 ;
of Spliagnacece, 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

Stentor, collecting, 527 ; contractile
vesicle of, 774 ; impressionable organs
of, 775 ; conjugation of, 7S2

Stej/hanoceros, collecting, 527 ; in con-
finement, 528

StephanoUthis spinescens, 847

- nodosa, 847

Stephen wsphcera pluvialis, amoeboid

phase of, 557 note
Stephenson, on Pleurosiyma ungulatum,

70 ; on ' intercostal points," 73

- his suggestion on homogeneous im-
mersion, 28

— on Coscinodiicus, 609
Stephenson's stereoscopic binocular, 100;

its erecting arrangement, 101, 102; as

a dissecting microscope, 248, 456 ; his

tank microscope, 267
Stereocaulon ramulosus, 650
Stereoscope, 91 ; Brewster's modification

of, 91
Stereoscopic binocular, Wenham's, 98 ;

for study of op;<, jue objects, 103-105

- eye-piece, Tolles's, 101 ; Abbe's, 102

- vision, 90-97
Sterigmata of Puccinia, 637

Sterile cells of Volvox, 555
Stichopus 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 Hcm,ophorat

604; of Gomphonema, 616
Stolon of Foraminifera, 796; of Eozooii,

839; of Laguncula, 904; of ascidians,

914

Stomach, follicles of, 1047
Stomates, 715

- of Marchantia, 666
Stomopneustes 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

Stop, introduction of, 37; in the eye-
piece, 42 ; use of, 312, 316

' Straight extinction,' 1079

Strawberry, parenchyme of fruit, 688

Streptocaulus pulcherrimus, 871

Striated muscle, 1048 ; size of fibres in
different groups, 1049

Striatella unipunctata, 598

Striatellece, characters of, 607

' Strobila' of Cyanea, 875

Stromatopora, doubtful character of,
842

Stromatoporoids, 817 note

Stropliomenidce, perforations in shells of,
927

Styloilyctya gracilis, 851

Suberous layer of bark, 708

Sub-nacreous layer in molluscan 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
note; 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

Cwrculionidce, 1002
Suctoria (Protozoa), 783-785

- (Crustacea), 965, 966

' Sugar-louse,' 977. See Lepistna
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

1177

SUP

THA

Supplemental yolk in Purpura, 938, 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

Surirellete, 606

Suspensor of ovule of Phanerogams, 534

Sutural line of desmids, 580

Swarm-spores, 536 ; meaning of term,
537 note; of Pandorina, 557; of Cut-
leria, 627; of Clatlirnlina, 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, 302, 305 ; condenser for
polariscope, 314 ; microspectroscope,
325 note ; objectives, 375 ; petrological
microscope, 1068

Symbiosis in lichens, 650

Synibiotes tripilis, hairs of, 1010

Symbiotic algae in radiolarians, 848

Sympathetic nerves, 1054

Syiuphytiun asperrimum, seeds of, 724

Synalissa symphorea, 650

Synapta digitata, ' anchors ' of, 895

— inhcerens, ' anchors ' of, 895
Synapt(E, rotifers in, 787
Syncoryne Sarsii, gonozob'ids of, 868
Syncrypta, 545

Synedra, 606

Syrmgammina, 811

Syringe for catching minute aquatic

objects, 351

Syrup, as a preservative medium, 519
- and gum, as a preservative medium,

519

Tabamis, eyes of, 987; ovipositor of,
1004

Tabellaria vulgaris, 621

Table of numerical apertures, 84-87
- for microscopists, 398-402; for dis-
secting and mounting, 399

Tactile papillse 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

Tannin, test for, 517

Tapetal cells in fern antherid, 678

' Tape-worm,' 943

Tardigrada, desiccation of, 945
Tarsonemidce, 1013
Taste, organs of, in insects, 993, 1000
Teeth, decalcification of, 512

- fossilised, 1090

- in palate of Helix, 930 ; of Limax,
930 ; of Buccinuni, 930 ; of Mollusca.
930

- preparation of, 1023 and note

- of Echinus, 890 ; of Ophiothrix, 892 ;
of Vertebrata, 1023

— of elephant, Rolleston on enamel in,
928; of BJiodent ia, Tomes on enamel
in, 928

Tegeocranus cepheiforinis, 1008

- dentattts, 1008

Tegumentary appendages of insects, 974
Telescope, Barker's Gregorian, 145
Teleutospore generation of Puccinia,

637
Temperature, effect of, on various

monads, 761
Tendon, 1019

Tentacle of Noctilitca, 766, 768
' Tentacles ' of Drosera, 714 ; of Suctoria,

785 ; of Hydra, 864 ; of annelids, 949
Tenthredinidce, ovipositor of, 1003
Terebella, tubes of, 948; gills of, 949

— conchilega, 948
Terebratula bullata, shell of, 927
Terebratulce, shells of, 925, 926
Terpsinoe musica, 608
Terpsinoea, character of, 607
Tertiary tints in crystalline bodies, 1097
Tessellated epithelium, 1044

Test of Gromia, 735; of Arcella, 746;

of Difflugia, 746
Testa of seeds, 725
Testaceous amcebans, 746, 747
Testing object-glasses, 381 ; diaphragm

for use in, 385 ; Fripp's method, 386 ;

Abbe's method, 384-387
Test-plate, Abbe's, 387
Tests, sandy, of Lituolida, 814
Tethya, spicules of, 1086
Tetramitus restrains, life-history of

760 ; nucleus of, 763
Tetranychi, 1013
Tetranychiis, mandibles of, 1009
Tetraspores of Floridece, 631 ; of Vam-

pyrella, 730
Text ill aria, 823

— aculeata, in chalk, 1087

— ylobulosa, in chalk, 1087
Textularian form of shell, 798

- series, 823
TextulariidcE, 811

Textxla finite, arenaceous character of,

823

TJialassicolia, 846, 853
Thallophytes, 530-632
Thallophytic type, passage to cormo-

phytic, 668
Thallus of Ulva, 560 ; of Phteosporece,

626 ; of lichens, 649
Tliaumantias Esclischoltzii, 873

- pilosella, 873

INDEX

THE

' Theca ' of mosses, 671
Thecaphora,868

TJiecata, 868, 870

— zoophytic stage of, 877
Thermo-regulator, Reichert's, 453
Thermostatic stage, Dallinger's, 344-

346

Thoma's (Jung) microtome, 461-469

Thompson (J. Vaughan), on pentacrinoid
larva of Antedon, 901 ; on Cirripedin,
967

Thomson (Wyville), on development of
Antedon, 903

Thread-cells of Ciliata, 773 ; of Hydra,
864 ; of ZoautJidrid, 878, 879 ; of pla-
narians, 947

' Thread-worm,' 944

Threads of spiders' webs, 1015

Thura/mmina papillata, 815

Thwaites, on conjugation of Epithemia,
599 ; of Melosira, 560

Thysanura, scales of, 977

Ticks, 1008. See Acarhui

Tineidce, wings of, 999

Tinoporus baculatits, 824

Tipula, spiracle of, 976 ; eye of, 987 ;
antennae of, 988

Tolles' binocular eye-piece, 101 ; his me-
chanical stage, 204 ; his immersion
objectives, 362, 364 ; his apertometer,
390

Tomes (Charles), on teeth, 1025

Tomopteris 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
Tracheae of insects, 994; of Acarin«,

1011
Trache'ides of ferns, 674 ; of conifers, 698,

703

Trachelomonas, 545
Tradescantia virginira, cyclosis in

hairs of, 691
Tragulusjavanicus, red blood-corpuscle

of, 1035

Trematodes, 945
Triceratium, 588, 61:!

— as test for illumination, 415, 4 Hi

— favus, 593, 613

— fiiniirint ii in, as test for medium
powers, 389

Tnrhocysts of Ciliata, 773

7 'rirliodd h/tice.us, crawling of, 774; re-
production of, 780, 781

TricJtodina grandinella, a phase in de-
velopment of Vorfifftld, 78(1

Trichogyne of Colcochcete, 57")
- of Fl»ridi'ii\ (i:;-2 ; in lichens, li.'O

Trichonympha, 77 1

Trichophore of Fl«i-iilnr, 632

• UNG

Trichr>phrija,& phase in development of

Suctoria, 785

Trigonia, prismatic layer in, 924
Triloculina, 802
Triple-backed objectives, 361
Triplet, Holland's, 37
Triplex front to objectives, 370
Tripoli stone, 617
Trochus zizi/2j]n>nis,-p&l&te of, 931
Trombicliiclte, 1008, 1009; legs of, 1010 ;

hairs of, 1010 ; eyes of, 1011 ; tracheae of,

1011 ; characters of, 1012
Trombidium, maxillae of, 1010 ; larvae of,

1013

— liolosericiun, 1013
Trophi of Ii of if em, 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 rivnloriim, gregarine of, 751
TuMpora, 877

Tnbularia, gonozooids of, 869
— indivisa, 869
Tubuli in Nummulites, 827 ; of dentine,

1024

Titbulipora, 909
Tulip, raphides of, 696
Tully's (Lister's) achromatic microscope,

149 ; his live-box, 345 ; his triplet, 354 ;

his achromatic objective, 354
' Tunic ' of Tunicata, 911
TUNICATA, 904, 911-918 ; zoological posi-
tion of, 911; bibliography of, 918;

'liver 'of, 1047
Turbellaria, 946,947

- larvae 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 lamellae in leucite, 1078
Tylcnclius tritici, 945
Tympanum of cricket, 999
TyrogtypM, nymph of, 1009; legs of,

1010

Tyrogl/yphidce, reproductive organs of,
1012 '; characters of, 1013

U

Ulothrix, conjugation of, f>">7
Ulva, 560, 561
I'li-orea, 559-561
Umbelliferous plants, seeds of, 724
Unibonula verrucosa, 9(i(i
Under-corrected objective, 20, '21
Under-correction, 355-360
Unger, on the zoospores of Vaucheria,
563

INDEX

II/9

TJNI

Unicellular plants, 538

Unio, pearls in, 923 ; glochidia of, 933

— occidens, formation of shell in, 925

U)iioni(lce, nacreous layer of, 923

Unit i standard) for microscopy, 460

Ureclinece, 636-638 ; alternation of gene-
rations in, 636

Uredo-form of Pticcinia, 63s

Uredospores of Puccinia, 638

Urinary calculi and molecular coales-
cence, 1102

Urine, micro-chemical examination, 1103

Urochordata, 911

Uropoda, tracheae of, 1011

' Urticatiug organs.' See Thread-cells

Ustilaginece, 636

Uvella, 545

Vacuoles in vegetable cell, 534

- contractile, in protophytes, 535 ; of
Volvox, 552

- of Actvnophrys, 737
Vagine of mosses, 671

Vallisneria, habitat, 689; mode of de-
monstration of cyclosis, 689, 690
V«lpitli>ia, shell of, 798
Vampyrella, 729, 730

— gomphonematis, 729

— spifoyynr, 729

Vanessa, eye of, 987 ; haustellium of, 992

- urtic(p, eggs of, 1005
Variation, range of, in Astroinma, 849
Varley's live-box, 346

Varnish, test for, 443 ; asphalte, 443

Varnishes, 442-445 ; sealing-wax in alco-
hol, 444 ; red, 445 ; white, 445 ; various
colours, 445

' Vascular Cryptogams,' links with Pha-
nerogams, 682

Vascular papillae of skin, 1042

VaucJieria, 562, 563

- Rot if era 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

Ventricnlites, 861, 1088

Venus' flower basket, 859, 860 ; spicules
of, 860

Verbena, seeds of, 724

Vertebrata, 1017-1065; bone of, 1020;
teeth of, 1023 ; dermal skeleton of,
1026 ; blood of, 1034 ; red blood-cor-
puscles, 1034 ; white blood-corpuscles,
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 ; respiration,
1063

WAR

Vertebrated animals, 1017. See Verte-
brata

Vertical illuminator, 336-338 ; how to
use, 337 ; for examination of metals,
337 ; for ascertaining ' aperture,' 338

I'l'spidcr, eye of, 987

Vibracula of Poh/zoa, 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 Poli/soa,

905 lltiti

Vine, size of ducts of, 691)
Viohi tricolor, pollen-tubes of, 723
Violet, cells of pollen-chamber, 720
Virginian spider- wort, cvclosis in, 691
Virtual image, 14 note, 24, 25, 376
Vision, depth of, 88, 89, 90; stereoscopic,

89

Visual angle, 27
Vitrea (Fora minif era), 819
Vitreous cells (arthropod eye), 983

— optical compounds, 31

— shells of Fora minif era, 799

' Vittse ' of LicmophorecB, 604 ; of seeds

of umbellifers, 724
Vocal cords, structure of, 1040
Vegan's changing nose-piece, 294
Volcanic ashes and dust, microscopical

examination of, 1076
Volvocineee, 550-557
Volvox associated with Astasia, 765

- vegetable nature of, 556 note ; anioe-
biform phase of, 556 ; Eotifera 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, 554-5-">li

VorticeUa, 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

Wahllieimia aust rails, 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-lichens, 649

Wallicli, on structure of diatom frustule,
590 note ; on Triceratitun, 613 note;
on CJicetocerece, 614 note', on cocco-
spheres, 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

WAR

Warmth, mode of applying, for cyclosis,

692

Wasps, wings of, 998, 999 ; sting of, 1003
Water, refractive index of, 3, 7

— distilled, for mounting Protophytes,
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

1 Water-mites,' 1013

' Water-net,' or Hyrlrodictijon, 565

Water-of-Ayr stone, 508

Water-scorpion, 995. See Nepa

'Water-snail.' See Limneens

Water-vascular system of Teenier, 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

Wavellite in My a, 924

Web of spiders, 1015

Weber's annular cells, 350

Webster condenser, 308

Weismann, on development of Dijitcra,
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 Cha'tocerece, 614 note

' Whalebone,' 1033

Wheat, starch-grains of, 695

Wheatstone's stereoscope, 91 ; his pseudo-
scope, 92

'Wheel-animalcules,' 753, 786. See

EOTIFEEA

Wheel-like plates of Chirod 'ota, 896

' Wheels ' of Motif era, 787

Whelk. See Buccinum

' White ant,' ciliate parasite of, 774

White blood-corpuscles of Vertebrata,

1036 ; flow of, 1056
- fibrous 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 notr ;

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

ZOO

Wings of insects, 998-1000; of Ptero-
phorus, 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 ; his camera
lucida, 278

Wood, arrangement of, 700, 702 ; concen-
tric rings of, 703 ; fossilised, 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

stomates in, 715, 716

Z

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 apochro-
matic objective, 366-374 ; his water-
immersion, 370 ; his apochromatic, for
resolving diatom markings, 592 ; his
apochromatic for study of monads, 702

Zeiss-Steinheil's loups, 249, 268

Zentmayer's microscope, 204 ; swinging
sub-stage in, 204

Zeolite, 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

Zob'chlorelUe of Heliozoa, 734

Zob'cytium of Opfyrydium, chemical com-
position of, 778

INDEX

IlSl

zoo

Zoiiglcea of Beggiatoa, 653
Zoogkeee, 655, 657
ZOOPHYTES, 862-883

— cells for mounting, 448, 449

— non-sexual reproduction of, 1006
Zoophyte troughs, 348-350
Zob'sporaiige of Volvox, 554, 555
Zoosporanges of Phceosporece, 626
Zob'spores, 536 ; of Protococcus, 544, 545 ;

of Pahnodictyon, 559 ; of Ulva, 560 ;
of Vaitcheria, 562 ; of Achlija, 565 ;
development of, 565 ; of Hi/drodicti/on,
566; of Confervacece, 570; of (Edo-
goniinii, 572; of CJuetophoracece, 574 ;
of Coleoch&tacecf, 575 ; of Phceosporece,
626; of Fungi, 633; of radiolarians,
849
Zodthamium, collecting, 527

ZYM

Zooxanthellae in radiolarians, 848
Zob'zygospores of Navicula, 597
Zukal, on movement of Spirulina, 548
Zygnemacece, characters of, 549 ; habitats

of, 549 ; conjugation of, 549
Zygosis in Actvnophrys, 740 ; of Amoeba,

744 ; of gregarines, 751
Zygospore, 537 ; formation of, 540 ; of

Hydrodictyon, 565; in Destnldiacece,

584, 585
Zygospores of Palmoglaea, 542 ; of Meso-

carpus, 550 ; of Spirogyra, 550 ; of

Pandorina, 557; of Ulva, 561; of

Navicula, 597; of diatoms, 599; of

Mucorini, 641

Zygote of Glenodinium, 770
Zymotic or fermentative action of Fungi.

633

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